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5/08 K:IPrqIAoc&PresICIRC/Date0m.indd

 

DIPOLAR ENERGY TRANSFER IN RHENIUM(I) POLYPYRIDYL-
BASED DONOR-ACCEPTOR ASSEMBLIES

By

Troy Elvin Knight

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PI-HLOSOPHY
Chemistry

2009

ABSTRACT

DIPOLAR ENERGY TRANSFER IN RHENIUM(I) POLYPYRIDYL-
BASED DONOR-ACCEPTOR ASSEMBLIES

By
Troy Elvin Knight

Dipolar energy transfer is a through-space mechanism that occurs
when a donor emission dipole non-radiatively couples to an absorptive
dipole in the acceptor. In 1946, Theodor Forster published the theoretical
framework for dipolar (Forster) energy transfer in solution. The classic l/R6
distance dependence of the dipolar energy transfer rate has been widely used
as a “spectroscopic ruler” to determine the distance between fluorescently
labeled donor and acceptor complexes. In addition to the 1/R6 distance
dependence, the through-space orientation (K2) between the donor and
acceptor transition moment dipoles also affects the observed rate of energy
transfer, and plays a particular role in the energy transfer efficiency in
covalently attached donor-acceptor systems that are unable to sample all
possible orientations in solution. The majority of the research presented in
this dissertation involves understanding and quantifying the dependence of

2

K on F firster energy transfer in covalently attached transition metal-based

donor-acceptor assemblies.

The donor-acceptor systems discussed throughout this work are
comprised of ReI polypyridyl donor molecules covalently attached to various
first-row transition metal complexes through an organic-based linker
molecule, and are referred to as MRex (where M represents a first-row
acceptor metal center and X = 2 or 3). The lowest energy excited-state of
the ReI donor moieties involves a metal-to-ligand charge transfer transition
(3MLCT). This 3MLCT state is located on an isolated portion of the ReI-
based donor moiety, which simplified the location of the donor emission
dipole and in turn aided in the determination of the through-space orientation
between the donor emission and acceptor absorption transition moment
dipoles in the MRex complexes. The acceptor moieties are all M(acac)3
complexes that possess charge-transfer (M = Fem) or ligand-field (M = Cu",
Crm, Col", and Ni") acceptor states. Varying the nature of the acceptor
transitions permitted fundamental investigations of the dependence of
Forster energy transfer on through-space dipolar orientation and on the size
of the transition moment dipoles involved in the energy transfer event. The
synthesis and characterization, photophysical properties, and dipolar energy

transfer behavior in the MRex complexes will be presented and discussed.

Copyright by
Troy Elvin Knight
2009

Dedicated to my two grannies: Uva Nell Handley and Lillie Belle Knight.
I hope I live up to the potential that both of you always saw in me.

ACKNOWLEDGEMENTS

To begin the acknowledgements I must first give a short back story on
my road to the chemistry department at Michigan State. I began my
graduate work at The University of Alabama at Birmingham where my
research focused on the photophysical properties of transition metal
compounds, which also happens to be the focus of the McCusker group.
During my third year at UAB my research advisor was unfortunately denied
tenure, and I was presented with some unexpected choices, either move to
another group at UAB (which I did not want to do) or transfer to another
university. My former advisor always spoke very highly of Jim and his
science and suggested I look into joining the McCusker group. After
contacting Jim, I decided to transfer to MSU and continue my focus on
inorganic spectroscopy. 1 have to say it was one of the best choices I have
ever made and I would do it all over again if given the chance. I have
learned so much fiom Jim and I want to sincerely thank him for teaching me
how to think and act as a scientist. Achieving the high scientific aptitude
that Jim possesses will be daily goals of mine as I continue my career.
Thanks Jim. “SCIENCE!”

The second round of people I would like to thank is my family -

Danny and Jeanie Knight and Treva Hadley. Dad, you are the hardest

vi

 

 

working person I have ever known and I want to thank you for being an
excellent example for me through out these years. The constant drive you
have to better yourself educationally and professionally has had a profound
influence on me. I hope I can one day know as much about chemistry as you
know about electricity. Mom, you are the best mother a person could ever
ask for and I truly thank you for being there for me throughout these years.
You always gave me your best everyday while growing up and I am truly
gratefiJl for that. I know I also inherited my aptitude for math and science
from you, which I am very thankful for as well. Treva, thanks for being my
sister and putting up with me all of these years! I have watched you grow
into a great person, a caring social worker, and an excellent mother to Drew.

Next, I want to give a huge thanks to my fiancé Dr. Nicole Torres.
Nicki, we have survived graduate school and writing our PhD theses
together, which if we can do those things means we are really meant for each
other! Over the past 3+ years I have watched you grow into a very skilled
organic chemist and an excellent scientist, and I am so very proud of you
and your accomplishments. I also love the fact we can bounce scientific
ideas off each other at night after a long trying day in the lab. I know our
postdoctoral training together at UNC will be filled with many discoveries

and great times. Lastly, you are the most caring partner a person could hope

vii

 

for and I very thankful to have found you. I love you more than you will
ever know.

Lastly, I would like to thank the McCusker group for all of the great
times we have spent together in the synthetic lab, laser lab, group meetings,
and hanging out at the bar. All of you are doing great science and I know
you will all have very productive scientific careers. I look forward to seeing
everyone at various meetings and conferences. A few of you have
contributed positively to my graduate career in various capacities and
deserve particular mention: Dr. D. G. (Dong Guo), Dr. Monica Soler, Uncle
Rick Fehir, U. J. (Joel Schrauben), Johnny 0. C. Kouzelos, and Kate
McCusker. Graduate school would have been much different without all of

you. Thanks!

viii

TABLE OF CONTENTS

LIST OF TABLES ........................................................................................ xii

LIST OF FIGURES ....................................................................................... xv
Chapter 1. Introduction to Forster Energy Transfer Theory and Transition

Metal-Based Donor-Acceptor Systems ........................................ 1

1.1 Introduction to Forster Energy Transfer Theory ............................... 1

1.2 Transition Metal-Based Donor/Acceptor Complexes ....................... 9

1.3 Contents of Dissertation .................................................................. 14

1.4 References ....................................................................................... 21

Chapter 2. Quantifying Forster Energy Transfer in Charge Transfer Based
Donor-Acceptor Complexes: FeRe3 Multinuclear Assemblies .. 26

2.1 Introduction ..................................................................................... 26
2.2 Experimental Section ...................................................................... 32
2.2.1 Synthesis and Characterization ............................................ 32
2.2.2 Physical Measurements ....................................................... 38
2.3 Results and Discussion .................................................................... 46
2.3.1 Synthesis and Characterization ............................................ 46
2.3.2 Single-Crystal X-ray Structures .......................................... 48
2.3.3 Electrochemistry .................................................................. 53
2.3.4 Electronic Absorption Spectroscopy ................................... 55
2.3.5 Steady-State and Time-Resolved Emission ........................ 59
2.4 Mechanistic Considerations ............................................................ 65
2.4.1 Electron versus Energy Transfer Quenching ....................... 65
2.4.2 Dexter versus Forster Energy Transfer ............................... 68
2.4.3 Quantifying Forster Energy Transfer .................................. 71
2.4.4 Rate Constant Calculations: Modeling Solution Phase
Energy Transfer Dynamics .................................................. 82
2.5 Geometry Optimization Calculation ............................................... 89
2.6 Conclusions ..................................................................................... 99
2.7 References and Notes .................................................................... 101
Chapter 3. Orbital-Specific Energy Transfer: CuRez Assemblies .............. 109
3.1 Introduction ................................................................................... 109
3.2 Experimental Section .................................................................... 115
3.2.1 Synthesis and Characterization ......................................... 115
3.2.2 Physical Measurements ..................................................... 122

ix

3.3 Results and Discussion .................................................................. 127

3.3.1 Synthesis and Characterization ......................................... 127
3.3.2 Single-Crystal X-ray Structures ........................................ 131
3.3.3 Electronic Absorption Spectroscopy ................................. 137
3.3.4 Steady-State and Time-Resolved Emission ...................... 141
3 .4 Mechanistic Considerations .......................................................... 151
3.4.1 Electron versus Energy Transfer Quenching .................... 151
3.4.2 Spectral Overlap Analysis: State-Selective Energy
Transfer ............................................................................. 1 56
3.5 Conclusions ................................................................................... 171
3.6 References and Notes .................................................................... 174
Chapter 4. Confirming F orster Energy Transfer Involving Ligand-Field
Acceptor States: CrRe3 Complexes ........................................ 184
4.1 Introduction ................................................................................... 184
4.2 Experimental Section .................................................................... 188
4.2.1 Synthesis and Characterization .......................................... 188
4.2.2 Physical Measurements ..................................................... 193
4.3 Results and Discussion .................................................................. 196
4.3.1 Synthesis and Characterization .......................................... 196
4.3.2 Electronic Absorption Spectroscopy ................................. 198
4.3.3 Steady-State and Time-Resolved Emission ...................... 202
4.4 Mechanistic Considerations .......................................................... 208
4.4.1 Electron versus Energy Transfer Quenching ..................... 208
4.4.2 Dexter versus F brster Energy Transfer ............................. 214
4.4.3 Comparison with the CuRez Assemblies ........................... 216
4.5 Conclusions ................................................................................... 218
4.6 References and Notes .................................................................... 220
Chapter 5. Spin Dependent Energy Transfer: CoRe3 and NiRe2

Complexes ................................................................................. 223
5.1 Introduction ................................................................................... 223
5.2 Experimental Section .................................................................... 229
5.2.1 Synthesis and Characterization .......................................... 229
5.2.2 Physical Measurements ..................................................... 231

5.3 Results and Discussion for [Co(pyacac)3(Re(bpy)(CO)3)3]-

(OTf)3 ............................................................................................ 234
5.3.1 Synthesis and Characterization .......................................... 234
5.3.2 Electronic Absorption Spectroscopy ................................. 235
5.3.3 Time-Resolved Emission ................................................... 238

5.3.4 Spin-Dependent Dipolar Energy Transfer ......................... 239
5.4 Results and Discussion for [Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2]-

(OTf); ............................................................................................ 244
5.4.1 Synthesis and Characterization .......................................... 244
5.4.2 Single-Crystal X-ray Structures ........................................ 245
5.4.3 Electronic Absorption Spectroscopy ................................. 249
5.4.4 Steady-State and Time-Resolved Emission ...................... 251
5.4.5 Mechanistic Considerations: Dexter versus F 6rster .......... 256
5.4.6 Spin Dependent Dipolar Energy Transfer ......................... 259

5.5 Conclusions ................................................................................... 265
5.5 References ..................................................................................... 267
Chapter 6.‘Future Work ............................................................................... 270
6.1 Overall Goals ................................................................................. 270
6.2 Goal 1: Monitoring 3MLCT Relaxation in Re(bpy,)(CO)3 ........... 270
6.2.1 Background ........................................................................ 270
6.2.2 Dicyano Derivatives of CuRez and BeRez ..................... 272
6.2.3 Synthesis and Characterization .......................................... 274
6.2.4 Electronic Absorption Spectroscopy ................................. 274
6.2.5 Time-Resolved Emission ................................................... 276
6.2.6 Proposed TRIR Experiments ............................................. 277

6.3 Goal 2: Additional Derivatives for the CoRe3 Series ................... 279
6.3.1 Background ........................................................................ 279
6.3.2 Proposed CoRe3 Derivatives ............................................. 281

6.4 References ..................................................................................... 284

xi

Table 2-1.

Table 2-2.

Table 2-3.

Table 2-4.

Table 2-5a.

Table 2-5b.

Table 2-5c.

Table 2-6.

Table 2-7a.

Table 2-7b.

LIST OF TABLES

Crystallographic Data for [F e(pyacac)3(Re(bpy)(CO)3)3]-
(OH), (2) and [A1(Pyaca0)3(Re(bPY)(C0)3)3](OTfls (5)50

Selected Bond Distances (A) and Angles (deg) for [Fe-
(pyacac)3(Re(bpy)(CO)3)3](OTD3 (2) and [A1(pyacac)3(Re-
(bpy)(CO)3)3](OTf)3 (5) ...................................................... 51

Electrochemical and Infrared Data for Complexes 1-6 ...... 55
Photophysical Data of Complexes 1-6 ................................ 65

RDA (A), e)T (°), (9D (°), oA (°), and K2 values determined
from the internal dimensions of complex 2 using
donor/acceptor interactions A:D, B:D, and C:D ................. 78

RDA (A), eT (°), @130), (9A (°), and K2 values determined
from the internal dimensions of complex 2 using
donor/acceptor interactions AzE, BE, and C:E .................. 79

RDA (A), OT (°), OD (°), 6)», (°), and K2 values determined
from the internal dimensions of complex 2 using
donor/acceptor interactions A:F, B:F, and C:F ................... 80

Calculated Forster rate constants for [Fe(pyacac)3(Re(tmb)-

(CO)3)3](OTf)3 (1): [Fe(pyacac)3(Re(bpy)(CO)3)3](OTf)3 (2),
and [Fe(pyacac)3(Re(deeb)(CO)3)3](OTf)3 (3) .................... 84

Calculated Fbrster rate constants (s") for complexes 1-3 at
all donor-acceptor interactions occurring at A:D, B:D, and
C:D ...................................................................................... 85

Calculated Forster rate constants (3") for complexes 1-3 at
all donor-acceptor interactions occurring at AzE, BE, and
C:E ....................................................................................... 86

xii

Table 2-7c.

Table 2-8.

Table 2-9a.

Table 2-9b.

Table 2-9c.

Table 2-10.

Table 2-1 1.

Table 3-1.

Table 3-2.

Table 3—3.

Table 3-4.

Calculated Forster rate constants (3") for complexes 1-3 at

all donor-acceptor interactions occurring at A:F, B:F, and
C:F ....................................................................................... 87

Fen-Re distances and FeOz/acac dihedral angles for the
optimized and single-crystal X-ray structure data for
complex 2 ............................................................................ 92

RDA (A), OT (°), OD (°), (9A (°), and K2 values determined
from the optimized geometry of complex 2 using
donor/acceptor interactions A:D, B:D, and C:D ................. 93
RDA (A), aT (°), aD (°), (aA (°), and K2 values determined
from the optimized geometry of complex 2 using
donor/acceptor interactions AzE, B:E, and C:E .................. 94

RDA (A), OT (°), OD (°), @A (°), and K2 values determined
from the optimized geometry of complex 2 using
donor/acceptor interactions A:F, B:F, and C:F ................... 95

Calculated Fbrster rate constants for [Fe(pyacac)3(Re(bpy)-
(CO)3)3](OTf)3 (2) from geometry optimization
calculations .......................................................................... 97

Calculated Forster rate constants (3") for the optimized
structure of complex 2 at all donor-acceptor interactions... 98

Crystallographic Data for [Cu(pyacac)2(Re(bpy)(CO)3)2]-
(OTflz (3) and [BeCPyacaC)2(Re(tmb)(C0)3)2l(0T02
(6) ...................................................................................... 134

Selected Bond Distances (A) and Angles (deg) for [Cu-
(pyacao)2(Re(bpy)(C0)3)2](0Tf)2 (3) and [Be(pyacaC)2(Re-
(tmb)(CO)3)2](OTf)2 (6) .................................................... 135

Photophysical Data for [M(pyacac)2(Re(bpy’)(CO)3)2](OTf)2
(complexes 1-10) ............................................................... 150

Electrochemical Data and Calculated Electron Transfer
Driving Forces (AGF‘T) for Complexes 1-10 ..................... 154

xiii

Table 3-5.

Table 3-6.

Table 4-1.

Table 4-2.

Table 5-1.

Table 5-2.

Calculated Spectral Overlap Integrals and Energy Transfer
Rate Constants for Complexes 1-5 .................................... 161

Calculated Orbital Composition of the Lowest-Energy Spin
Allowed Absorptions of Cu(phacac)2 in CHzClz
solution .............................................................................. 165

Photophysical Data of Complexes 1-6 .............................. 208

Calculated Overlap Integral Values (J) and Energy Transfer
Rate Constants for the CrRe3 and CuRez Complexes ....... 218

Crystallographic Data for [Ni(pyacac)2(THF)2(Re(bpy)—
(CO)3)2](OTf)2 ................................................................... 247

Selected Bond Distances (A) and Angles (deg) for
[Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2](OTf); .................... 248

xiv

Figure 1-1.

Figure 1-2.

Figure 1-3.

Figure 1-4.

Figure 1-5.

Figure 1-6.

Figure 1-7.

Figure 2-1.

Figure 2-2.

LIST OF FIGURES

Simplified energy level diagram illustrating resonance
between the donor (D) emission and the acceptor (A)
absorption transition dipoles ................................................. 3

Spectral overlap plot showing energy conservation between
an arbitrary donor emission and acceptor absorption
spectra .................................................................................... 5

Definitions of the angles used for calculating the orientation
factor (K2) between the donor emission (m) and acceptor
absorption (rA) transition moment dipoles ............................ 7

Range of values possible for the orientation factor (K2)
between the donor emission (blue vectors) and the acceptor
absorption (red vectors) transition moment dipoles .............. 8

Illustration of spherically-shaped donor (D) and acceptor (A)
complexes that possess an isotropic through-space
interaction between their respective transition moment
dipoles ................................................................................... 9

Simplified energy level diagram for 2nd and 31rd row (16
polypyridyl complexes ........................................................ 11

General structure of the FeRe3 assemblies discussed in
Chapter 2. The FeRe3 donor-acceptor complex shows the
thermalized 3MLCT excited state of the ReI donor moiety
utilized in all of the donor-acceptor complexes discussed in
this dissertation .................................................................... 13

Polypyridyl derivatives coordinated to the ReI metal
centers .................................................................................. 32

Drawing of the cations of [F e(pyacac)3(Re(bpy)(CO)3)3]-

(0T03 (2, mp) and [A1(PyacaC)3(Re(bPY)(C0)3)3](0T03 (5,
bottom) obtained from single-crystal X-ray structure

determinations ..................................................................... 52

XV

Figure 2-3.

Figure 2-4.

Figure 2-5.

Figure 2-6.

Figure 2-7.

Figure 2-8.

Figure 2-9.

Electronic absorption spectra of [M(pyacac)3(Re(bpy’)-
(CO)3)3](OTf)3 assemblies, where M = FeIII (black traces) or
A1111 (blue traces) acquired in CH2C12 solution .................... 58

Electronic absorption spectrum of Fe(phacac)3 acquired in

- CH2C12 solution at 298 K .................................................... 59

Corrected steady-state emission spectra for [M(pyacac)3(Re-
(bpy’)(CO)3)3](OTf)3 assemblies in deoxygenated room-
temperature CHzClz solution, where M = A1111 (blue traces)
and FeIII (red traces) ............................................................ 61

Left. Nanosecond time-resolved emission data for the AlRe3
model complexes: 4 (tobs= 2260 i 100 ns), 5 (tobs= 560 i 30
ns), and 6 (robs = 235 i 20 ns). Right. TCSPC data for the
corresponding FeRe3 complexes: 1 (robs = 450 i 30 ps), 2
(1:01,S = 755 i 40 ps), and 3 (rob, = 2.5 i 0.1 ns). Both spectra
were measured in deoxygenated room-temperature CH2C12
solution ................................................................................ 64

T 0p. Nanosecond time-resolved differential absorption
spectrum of [Al(pyacac)3(Re(tmb)(CO)3)3](OTf)3 (4) in
room temperature CH2C12 solution (Apump = 355 nm). Bottom.
Time-resolved absorption data for [Fe(pyacac)3(Re(tmb)-
(CO)3)3](OTf)3 (1) in room-temperature CHzClz solution at
Apmbe = 700 nm following ~100 fs excitation at Apump = 400
nm ........................................................................................ 68

Overlay of the emission spectra of [Al(pyacac)3(Re(tmb)-
(C0)3)3](0Tf)3 (4, blue), [Al(pyacac)3(Re(bpy)(CO)3)3]-
(0T03 (5, green), and [Al-(PyacaC)3(Re(deeb)(C0)3)3](OTfls
(6, red) with the electronic absorption spectrum of

F e(phacac)3 .......................................................................... 70

Through-space interactions of the Rel-based 3MLCT

emission dipoles (blue) with the Fem-based 6LMCT
absorption dipoles (red) ....................................................... 75

xvi

Figure 2-10.

Figure 2—1 1.

Figure 2-12.

Figure 2-13.

Figure 3-1.

Figure 3-2.

Figure 3-3.

Figure 3-4.

Figure 3-5.

Figure 3-6.

Single Rel-based emission dipole moment spatially
interacting with the three 6LMCT absorption dipole
moments .............................................................................. 76

Point-dipole approximation used for calculating the through-
space distances (RDA) and orientation factors (K2) needed for
quantifying F orster energy transfer theory in the FeRe3
assemblies ............................................................................ 77

Energy level diagram depicting the excited-state dynamics
of the FeRe3 assemblies ....................................................... 89

Drawing of the geometry optimized structure of
[Fe(pyacac)3(Re(bpy)(CO)3)3](OTf)3 (2) ............................ 91

Structures of the CuRez and BeRez multinuclear
assemblies........- .................................................................. 114

Drawing of the cation [(Be3(OH)3(pyacac)3)((Re(bpy)-
(CO)3)3)](OTf)3 obtained from single-crystal X-ray structure
determinations ................................................................... 13 1

Drawings of [Cu(pyacac)2(Re(bpy)(CO)3)2](OTf)2 (3, Top)
and [Be(pyacac)2(Re(tmb)(CO)3)2](OTf)2 (6, Bottom)
obtained from single-crystal X-ray structure
determinations ................................................................... 1 36

Electronic absorption spectra of [M(pyacac)2(Re(bpy’)-
(CO)3)2](OTf)2 assemblies in CH2C12 solution, where M =
CuII (red traces) and Be" (blue traces) .............................. 139

Left. Electronic absorption spectrum of Cu(phacac)2 in
showing the higher energy charge transfer and organic-based
transitions. Right. Electronic absorption spectrum of a
concentrated solution of Cu(phacac)2 showing the two mid-
visible ligand-field bands. Both spectra were acquired in
CH2C12 solution ................................................................. 141

Systematic titration of a CHzClz solution of Cu(phacac)2
with (NEt4)(CF3SO3) (2 - 40 eq.) ...................................... 141

xvii

Figure 3—7.

Figure 3-8.

Figure 3-9.

Figure 3-10.

Figure 3-11.

Corrected steady-state emission spectra for [M(pyacac)2(Re-
(bpy’)(CO)3)2](OTf)2 where, M = cu" (red traces) and Be"
(blue traces). All spectra were acquired in room-temperature
deoxygenated CH2C12 solution .......................................... 144

Left. Nanosecond time-resolved emission data for the BeRez
model complexes in room-temperature deoxygenated
CHzClz solutions: 6 (robS = 1990 :t 100 ns), 7 (tot,s = 645 i 30
ns), 8 (robs: 540 i 30 ns), 9 (robS = 110 2!: 10 ns), and 10 (rob,
= 250 :i: 20 ns). The data were acquired by monitoring at the
emission maximum of each compound. Right. Time
correlated single-photon counting (TCSPC) emission data
for the CuRez complexes in room-temperature deoxygenated
CHzClz solutions: 1 (1'1: 14.9 :1: 0.7), 2 (121: 8.1 i 0.4), 3 (n
= 8.2 i 0.4), 4 (1:1= 5.6 i 0.3), and 5 (111: 5.0 d: 0.3) ........ 148

Illustration of oxidative (eq. 2a) and reductive (eq. 2b)
electron transfer (kET) quenching processes out of the Rel-
based 3MLCT excited-state to produce Cu1 or Cum as photo-
products, respectively ........................................................ 153

Overlay of the emission spectra of [Be(pyacac)2(Re(tmb)-
(C0)3)2](0T02 (6, 131111316), [B6(PyacaC)2(Re(dmb)(C0)3)2]-
(0T02 (7, blue), [Be(pyacaC)2(Re(bpy)(C0)3)2](0T02 (8,
green), [Be(pyacac)2(Re(dc1b)(CO)3)2](OTf); (9, orange),
and [Be(pyacac)2(Re(deeb)(CO)3)2](OTf); (10, red) with the
electronic absorption spectrum of Cu(phacac)2 (black
trace) .................................................................................. 157

A. The ground state absorption spectrum of Cu(phacac)2 in
CHzClz (black) fit with a series of three Gaussians (G1 (solid
trace), G2 (dotted trace), and G3 (dashed trace)). B and C.
Overlay of the emission spectra of [Be(pyacac)2(Re(tmb)-
(C0)3)2](0T02 (6, Purple), [Be(Py3030)2(Re(dmb)(CO)3)2]-
(0T02 (7, blue), [Be(PyacaC)2(Re(bpy)(C0)3)2](0T02 (8,
green), [Be(pyacac)2(Re(dclb)(C0)3)2](OTflz (9, orange),
and [Be(pyacac)2(Re(deeb)(CO)3)2](OTi); (10, red) with G2
and G3, respectively .......................................................... 163

xviii

Figure 3-12.

Figure 3-13.

Figure 4-1.

Figure 4-2.

Figure 4-3.

Figure 4-4.

Figure 4-5.

Figure 4-6.

The four lowest energy transitions of Cu(phacac)2
determined from a TD-DFT calculation plotted against the
ground state absorption spectrum of Cu(phacac)2 in CHzClz
(black) ................................................................................ 166

Drawings of the orbitals involved in the lowest-energy spin
allowed transitions of Cu(phacac)2 based on a TD-DFT
calculation ......................................................................... 1 68

One electron-orbital description of the 4A2 —> 4T2 electronic
transition in Crm. This diagram depicts the electronic
configurations from which the multielectronic wavefimction
corresponding to the 4A2 and 4T2 are derived .................... 186

Structure of the [M(pyacacl)3(Re(bpy’ )(CO)3)3](OTi)3
assemblies (where M= Cr or A!III and bpy’-— — —tmb, bpy, or
deeb) .................................................................................. 188

Electronic absorption spectra of [M(pyacac)3(Re(bpy’)-
(CO)3)3](OTf)3 assemblies, where M = CrIII (red trace) and

(blue trace). All spectra were acquired in room-
temperature CH2C12 solution ............................................. 200

Electronic absorption spectrum of Cr(phacac)3 acquired in
room-temperature CHzClz solution ................................... 201

Corrected steady-state emission spectra for [M(pyacac)3(Re-
(bpy’)(CO)3)3](OTf)3 assemblies, where M = A1111 (blue
traces) and CrIII (red traces). All six spectra were acquired in
deoxygenated room-temperature CH2C12 solutions following
excitation at 355 nm (complexes 1, 2, 4, and 5) and 400 nm
(complexes 3 and 6) .......................................................... 204

Corrected steady-state emission spectra for [Cr(pyacac)3(Re-
(bpy)(CO)3)3](OTf)3 (2) monitored over the course of 4 hrs
in room-temperature CHzClz solution. The various emission
traces are represented as follows: the initial spectrum (black
trace), after 1 hr (red trace), after 2 hrs (blue trace), after 3
hrs (green trace), and after 4 hrs (orange trace) ................ 205

xix

Figure 4-7.

Figure 4-8.

Figure 4-9.

Figure 4-10.

Figure 4-1 1.

Figure 5—1.

Figure 5-2.

Figure 5-3.

Time correlated single-photon counting (TCSPC) data for
the CrRe3 complexes: 1 (tot,S = 6.8 i 0.1 ns), 2 (robS = 4.8 d:
0.2 ns), and 3 (t'= 7.0 a: 0.3 ns) and r2 = 190 a 15 ns). All
data were collected in deoxygenated room-temperature
CHzClz solution ................................................................. 207

Steady-state emission spectrum of Cr(phacac)3 in 4:1
EtOH/MeOH at 82 K following excitation at 375 nm ...... 211

The black trace is the corrected steady-state emission spectra
of [Cr(PyacaC)3(Re(bPY)(CO)3)3](0Tf)3 (2) in 411
EtOH/MeOH at 82 K following excitation at 375 nm. The
red trace corresponds to the intensity of the emission
spectrum scaled by the relative absorbances of the Rel-bpy
and Crm(pyacac)3 chromophores ....................................... 212

Ground-state absorption spectrum of [Cr(pyacac)3(Re(bpy)-
(CO)3)3](OTt)3 (2) in 4:1 EtOH/MeOH solution at 298 K
(black trace), and the excitation spectrum of 2 in 4:1
EtOH/MeOH at 82 K monitoring at item = 795 nm (red
trace) .................................................................................. 213

Overlay of the emission spectra of [Al(pyacac)3(Re(tmb)-
(C0)3)3](0Tf)3 (4, blue), [Al(pyaca0)3(Re(bpy)(C0)3)3l-
(OTD3 (5, green), and [Al(pyacac)3(Re(deeb)(CO)3)3](OTi)3
(6, red) with the electronic absorption spectrum of
Cr(phacac)3. Data were acquired in room-temperature
CHzClz solution ................................................................. 216

Illustration of total spin angular momentum conservation in
the energy transfer quenching of a 3MLCT excited-state by

02 ....................................................................................... 225
Structure of the CoRe3 assembly ....................................... 227
Structure of the NiRe2 assembly ........................................ 228

XX

Figure 5-4.

Figure 5-5.

Figure 5-6.

Figure 5-7.

Figure 5-8.

Figure 5-9.

Figure 5-10.

Electronic absorption spectra of [Co(pyacac)3(Re(bpy)-
(CO)3)3](OTf)3 (red trace) and [Al(pyacac)3(Re(bpy)-
(CO)3)3](OTf)3 (blue trace) acquired in room-temperature
CHzClz solution ................................................................. 236

Top. Electronic absorption spectrum of Co(phacac)3
showing the higher energy charge transfer and organic-based
transitions. Bottom. Electronic absorption spectrum of a
concentrated solution of Co(phacac)3 showing the mid-
visible 1A] —> 1T1 ligand-field transition. Both spectra were
acquired in room-temperature CH2C12 solution ................ 237

Left. Nanosecond time-resolved emission data for [Al-
(pyacac)3(Re(bpy)(CO)3)3](OTf)3 (robS = 530 i 20 ns). Right.
Nanosecond time-resolved emission data for [Co(pyacac)3-
(Re(bpy)(CO)3)3](OTf)3 (tot,S = 595 :i: 20 ns). Both traces (A1
and Co) were acquired in deoxygenated room-temperature
CHzClz solution monitoring at the emission maximum (565
nm). The solid red lines correspond to fits to mono-
exponential decay models for both complexes ................. 239

Overlay of the emission spectrum of [Al(pyacac)3(Re(bpy)-
(CO)3)3](OTf)3 (blue trace) with the electronic absorption
spectrum of Co(phacac)3 (red trace) ................................. 241

Top. Spin conservation diagram illustrating the spin-allowed
energy transfer pathway between the 3MLCT excited-state
of the ReI donor and the CrIII acceptor. Bottom. Spin
conservation diagram illustrating the spin-forbidden
pathway between the 3l\fl_.CT excited-state of the ReI donor
and the CoIII acceptor ........................................................ 243

Drawing of the cation [Ni(pyacac)3(THF)2(Re(bpy)(CO)3)2]-

'(OTt); obtained from single-crystal X-ray structure

determination ..................................................................... 249

Electronic absorption spectra of [Ni(pyacac)2(THF)2(Re-
(bpy)(CO)3)2](OTf)2 (red trace) and [Be(pyacac)2(Re(bpy)-
(CO)3)2](OTf)2 (blue trace) acquired in room-temperature
CH3CN solution ................................................................. 250

xxi

Figure 5-11.

Figure 5-12.

Figure 5-13.

Figure 5-14.

Figure 5-15.

Figure 5-16.

Figure 6-1.

Electronic absorption spectra of Ni(phacac)2 acquired in
room-temperature CH2C12 solution ................................... 251

Corrected steady-state emission spectra for [Ni(pyacac)2-
(THF)2(Re(bpy)(CO)3)2](OTf); (red trace) and [Be(pyacac)2-
(Re(bpy)(CO)3)2](OTf)2 (blue trace). Acquired in
deoxygenated room-temperature CHZCIZ solution following
excitation at 355 nm .......................................................... 253

A. Corrected emission spectra of [Ni(pyacac)2(TI-IF)2(Re-
(bpy)(CO)3)2](OTt)2 in CH2C12 solution over a period of 27
hours (Aer = 355 nm) exhibiting a steady increase in
emission intensity. B. Corrected emission spectra of
[Ni(pyacac)2(TI-IF)2(Re(bpy)(CO)3)2](OTf)2 in THF solution
over a 5 hour time period (lax = 355 nm) showing a constant
emission intensity over the course of data collection ....... 254

Left. Nanosecond time-resolved emission data for [Be-
(pyacac)2(Re(bpy)(CO)3)2](OTt)2 (robs = 530 i 30 ns). Right.
TCSPC data for [Ni(pyacac)2(TI-IF)2(Re(bpy)(CO)3)2](OTf)2
(121: 5.0 :l: 0.4 ns and 12 = 25 :i: 3 ns). Both traces (Be and Ni)
were acquired in deoxygenated room-temperature CH2C12
solution monitoring at the emission maximum (565
nm) ......................................................... 256

Overlay of the emission spectrum of [Be(pyacac)2(Re(bpy)-
(CO)3)2](OTf)2 (blue trace) with the electronic absorption
spectrum of Ni(phacac)2 (red trace) .................................. 259

Top. Spin conservation diagram illustrating the spin-allowed
energy transfer pathway between the 3MLCT excited-state
of the ReI donor and the pseudo-octahedral NiO6 acceptor.
Bottom. Spin conservation dia am illustrating the spin-
forbidden pathway between the MLCT excited-state of the
ReI donor and the pseudo-D2,l NiO4 acceptor .................... 262

Structures of [Cu(pyacac)2(Re(dcnb)(CO)3)2](OTf); and
[Be(pyacac)2(Re(dcnb)(CO)3)2](OTf)2 .............................. 273

xxii

Figure 6-2.

Figure 6-3.

Figure 6-4.

Figure 6-5.

Figure 6-6.

Electronic absorption spectra of [Cu(pyacac)2(Re(dcnb)-
(CO)3)2](OTf)2 (red trace) and [Be(pyacac)2(Re(dcnb)-
(CO)3)2](OTf)2 (blue trace) acquired in room-temperature
CH3CN solution ................................................................. 275

Left. Nanosecond time-resolved emission data for [Be-
(pyacac)2(Re(dcnb)(CO)3)2](OTf)2 (robs = 110 d: 10 ns)
acquired in room-temperature deoxygenated CH2C12
solution (Apump = 400 nm; )‘Iprobe = 625 nm). Right. Time
correlated single-photon counting (TCSPC) emission data
for [Cu(pyacac)2(Re(dcnb)(CO)3)2](OTi)2 acquired in room-
temperature deoxygenated CH2C12 solution (11mm, = 400 nm;
limb, = 625 nm) ................................................................. 277

Overlay of the emission spectra of [Al(pyacac)3(Re(tmb)-
(C0)3)3I(OTf)3 (blue), [A1(pyacac)3(Re(bpy)(CO)3)3](OTf)3
(green), and [Al(pyacac)3(Re(deeb)(CO)3)3](OTf); (red) with
the electronic absorption spectrum of Cr(phacac)3. Plot is
reproduced from Figure 4-12 ............................................ 280

Left. Steady-state emission spectrum of [Al(pyacac)3(Re-
(dclb)(CO)3)3](OTf)3 in room-temperature CHzClz solution
(rpm, = 375 nm); Am = 595 nm for 3MLCT —> 1A1
emission. Right. Nanosecond time-resolved emission data
for [Al(pyacac)3(Re(dclb)(CO)3)3](OTt)3 (tob,= 120 :t 10 ns)
acquired in deoxygenated room-temperature CHzClz
solution (Apump = 415 nm; Apmbe = 595 nm) ........................ 282

Overlay of the emission spectra of [Al(pyacac)3(Re(bpy)-
(C0)3)3](0Tf)3 (blue), [A1(PyacaC)3(Re(dclb)(C0)3)3](0T03
(green), and [Al(pyacac)3(Re(deeb)(CO)3)3](OTf)3 (red) with
the electronic absorption spectrum of Co(phacac)3 .......... 283

Images in this dissertation are presented in color.

xxiii

Chapter 1: Introduction of Fiirster Energy Transfer Theory and

Transition Metal-Based Donor/Acceptor Systems.

1.1 Introduction to Fiirster Energy Transfer Theory

The initial descriptions of through-space energy transfer phenomena
were introduced in the early 1900’s by the Nobel prize winning physicist
Jean Baptiste Perrin. He was the first to theorize that energy between two
closely spaced molecules in solution can be transferred by the interaction of
their oscillating electric dipoles, as long as a resonance condition was
satisfied between the two dipoles.1 This seminal work of Perrin ultimately led
to the development of the well known fluorescence resonance energy
transfer theory, or more commonly referred to as Fbrster theory. Ever since
Theodor Fdrster first introduced his seminal paper in 1948,2 F firster energy
transfer theory has become one of the most ubiquitous photophysical
processes and has been used in biology, chemistry, and physics. The UK6
distance dependence between two interacting dipoles has been widely
utilized as a spectroscopic ruler3'7 to determine the distance between donor
and acceptor molecules, and has been shown to correspond very closely with
distances determined from structural data.8 Examples of various areas

impacted by Forster theory include measurements of distances between

fluorescently labeled donor and acceptor molecules attached to proteins,9
determination of the conformational dynamics of RNA,10 and improving the
light-harvesting capabilities of solar energy conversion devices.”15

In general, excited-state energy transfer processes involve the transfer
of excited-state energy from a donor molecule (D) to an acceptor molecule

(A), with the process beginning with the donor molecule absorbing a photon,

and can be represented by equation 1-1:

 

D+A+hv——>D*+A "W >1D+A* (1-1)

Forster energy transfer is a through-space mechanism that occurs
when a donor transition moment dipole non-radiatively couples to an
acceptor transition moment dipole.”l7 The dipolar nature of this interaction
gives rise to a R'6 (R = distance) dependence, allowing this mechanism to be
operative over very long donor-acceptor separations. More specifically,
Fbrster energy transfer involves the non-radiative coupling of donor
emission dipoles with energetically matched acceptor absorption dipoles and

is represented in Figure 1-1.

 

 

 

 

 

A

 

 

Dipole-Dipole
Energy Transfer

 

 

 

 

 

 

 

D l A
V M

Figure 1-1. Simplified energy level diagram illustrating resonance
between the donor (D) emission and the acceptor (A) absorption
transition dipoles.

 

 

 

 

It can be seen from Figure 1-1 that the initially produced electronic excited
states of the donor complex (Di) non-radiatively relax to the lowest energy
vibronic level of D*, which can radiatively couple to various ground state
vibrational levels of the donor (D) (blue, green, and red transitions). These
emissive dipoles can then interact with energetically matched absorptive
transitions of the acceptor molecule to produce acceptor excited states (A‘),
which may be thought of as a virtual photon exchange between D’ and A.
The transitions initiated in the acceptor molecule are equivalent to the
acceptor absorbing a photon directly to generate A*.

The picture of the two-state coupling shown in Figure 1-1 is a

minimal model and represents the interaction of molecules in the gas phase.

Donor emission and acceptor absorption transitions in solution interact with
the surrounding medium and are usually observed as broad featureless
spectra. Forster’s ingenious contribution to dipole-dipole energy transfer
theory was his realization that coupled transitions in solution can be
quantified by the spectral overlap of the donor emission spectrum with the
acceptor absorption spectrum, which are both easily accessible experimental
observables.18 The area of spectral overlap, which represents resonance
matching of donor emission and acceptor absorption transitions, is shown
qualitatively in Figure 1-2 (black lines). Forster’s spectral overlap model
depends on small electronic communication between the donor and acceptor
molecules (the so-called “weak coupling” regime),19 which is usually
experimentally observed by the linear combination of the electronic
absorption spectra of separated donor and acceptor molecules corresponding
to the absorption spectrum of the intact donor-acceptor assembly.
Specifically, the definition of the “weak coupling” regime given by F firster
relies on two characteristics: 1) equilibration of the solvent bath with D“
occurs on a much faster time-scale than the excited-state energy transfer
dynamics, and 2) the interaction of the donor and acceptor molecules with

the solvent is much greater than the electronic coupling between them.

 

Donor Emrssron
uondrosqv Jotdooov

 

 

 

Figure 1-2. Spectral overlap plot showing energy conservation between
an arbitrary donor emission and acceptor absorption spectra.

As mentioned above, Forster theory has a direct connection to
parameters that can be accessed experimentally. The energy transfer rate
constant (km) is described by the Forster equation given in equation 1-2,20

_ 90001n(10)K2(I)DJ
EnT 1287T5774NATDR6 (1'2)

 

where K2 is the dipole orientation factor, (DD is the radiative quantum yield of
the donor, 1] is the refractive index of the solvent, NA is Avogadro’s number,
ID is the excited-state lifetime of the donor, R is the donor-acceptor
separation, and J the spectral overlap integral. This latter term, which
essentially quantifies the resonance condition necessary for dipole-dipole
coupling, can be evaluated from the spectroscopic properties of the system

according to equation 1-3,

 

Ho] D(V)8A(V)— adv (13)

C

where Fe is the (normalized) emission spectrum of the donor and EA is the
absorption profile of the acceptor in units of molar absorptivity. From
equation 1-2, knowledge concerning the geometric positions of the donor
emission and acceptor absorption transition dipoles, which is known as the
orientation factor (K2), must also be elucidated to accurately model excited-
state kinetics. The orientation factor relates the angle between the donor
emission and the acceptor absorption transition dipoles relative to each other
and the angle the donor and acceptor transition dipoles each create with the

donor-acceptor connection line, and is given in equation 1-4:

= (cosG')T — 3cos®Dcos® A )2 (1-4)
where @1- is the angle between the donor transition dipole moment (rD) and
the acceptor transition dipole moment (rA), and OD and @A are the angles
between the donor-acceptor connection line (RDA) and the donor transition
dipole moment and the acceptor transition dipole moment, respectively. The

angles are defined in Figure 1-3.

 

 

GD ,5 (,9
®Tf (N DA”

/ RDA /
YD YA

Figure 1-3. Definitions of the angles used for calculating the
orientation factor (K2) between the donor emission (rD) and acceptor
absorption (rA) transition moment dipoles.

 

The values for K2 can range from 0 to 4 depending on the relative angles
between the two transition dipoles, with K2 = 0 being the orthogonal
interaction between the two dipoles, K2 = 1 being the parallel interaction, and
K2 = 4 being the highest possible interaction with both dipoles aligned along

the donor-acceptor connection line (Figure 1-4).21

K2=0 hoe...-

‘3»

K _ 1 000.00

K2=4 {3.1—coco.“

Figure 1-4. Range of values possible for the orientation factor (1(2)
between the donor emission (blue vectors) and the acceptor absorption
(red vectors) transition moment dipoles.

An isotropic value has also been determined for the dipolar orientation
factor (K2 = %), and represents the average of all possible dipole-dipole
through-space interactions between donor and acceptor species in solution.22
The isotropic value of 2/3 can be thought of as the donor emission and the
acceptor transition moment dipoles existing within two spherically shaped
molecules in solution (Figure 1-5), with these two systems possessing
unrestricted movement within all three degrees of freedom (x, y, and z). The
isotropic value works well for bimolecular donor-acceptor systems in

solution such as lanthanide ions,23 or systems that behave like spheres that

rapidly diffuse through all possible orientations,24 but this simplification can

introduce a significant amount of error between the theoretical and observed
energy transfer rate constants with covalently attached donor-acceptor
systems that are unable to sample all degrees of fieedom.”26 Covalently
attached donor-acceptor complexes, such as the Rel-based compounds
discussed throughout this dissertation, negates the possibility of freely
rotating donor and acceptor molecules and thus rules out the possibility of
using K2 values of 2/3. Theoretically modeling and quantifying the orientation
factor in covalently attached donor-acceptor systems is one of the overall

themes of the work presented in this dissertation.

 

Figure 1-5. Illustration of spherically-shaped donor (D) and acceptor
(A) complexes that possess an isotropic through-space interaction
between their respective transition moment dipoles.

1.2 Transition Metal-Based Donor/Acceptor Complexes

Developing artificial energy-conversion systems is arguably the most

important goal in the area of donor-acceptor photochemistry. Mother Nature

has incorporated transition metal ions into various areas of the energy
conversion sequences of photosynthesis,”30 which has led many researchers
to follow in her footsteps and develop transition metal-based artificial
photosynthetic systems.”32 Two major design lessons that can be taken
from natural systems are 1) the initial light absorption step must produce a
charge separation in order to generate high energy reagents that perform
useful chemistry, such as excited state energy transfer or an electron transfer
cascade and 2) spatial control of the excited-state energy flow must be
achieved in order to deliver the useful energy to certain areas of the
supramolecular array.33 Metal-based donor-acceptor assemblies consisting of

III -
have received

d6 polypyridyl complexes of Rel, Ru", Os", and Ir
considerable attention in this area due to possessing long-lived (nanosecond)
charge-separated excited states in the form of metal-to-ligand charge transfer
transitions (MLCT), relative stability of those charge transfer excited states,
and well documented ground and excited-state electronic properties.”42

The ground and excited state electronic structure of d6 polypyridyl
complexes deserves some focus in the section, due to the charge transfer
excited states of ReI polypyridyl complexes utilized as the donor electronic

states throughout this entire dissertation. Figure 1-6 shows a simplified

energy level diagram for 2nd and 3rd row (16 polypyridyl complexes.43 The

10

absorption of a photon initially produces a 1MLCT excited state (t2g —-> at
(polypyridyl)), which then undergoes extremely fast relaxation (~ 100 fs)
through intersystem crossing and Vibrational cooling dynamics to the lowest
energy 3MLCT excited state.44’45 It is out of this thermalized 3MLCT state
that all of the radiative and energy transfer dynamics occur.46'52 The lifetime
of the 3MLCT state in these complexes exist in the nanosecond to
microsecond time regime,53'58 which is a timescale very amenable to
performing useful excited-state chemistry processes and also to probe with
typical photophysical instrumentation.

ll

1MLCT

3MLCT

 

 

 

Nuclear coordinate (Q)

Figure 1-6. Simplified energy level diagram for 2nd and 3rd row (16
polypyridyl complexes.

11

One example of the Rel-based supramolecular assemblies (FeRe3)
discussed in this dissertation and possessing a charge transfer excited state in
the donor moieties is shown in Figure 1-7. Forster-type energy transfer
reactivity can be envisioned to occur out of the thermalized 3MLCT state of
the Re1 donor complex to produce an absorptive transition in the acceptor,
and in the FeRe3 example the acceptor states are LMCT (ligand-to-metal
charge transfer) transitions associated with the F e(acac)3 core (acac =
acetylacetonate) (Figure 1-7). The charge transfer nature of the 3MLCT
donor emission and the LMCT acceptor absorptions in the FeRe3 complexes
induce the movement of charge within the individual donor and acceptor
moieties, and these electronic transitions have been modeled as the blue
(donor) and red (acceptor) vectors in Figure 1-7. The assignment of a
specific excited state quenching mechanism depends strongly on the redox
properties of the metal and the ligands, the strength of electronic coupling
mediated by the bridging ligand (ie. whether the excited electron resides on
the bridging or peripheral ligands), and the energy of the lowest energy
3MLCT state of the donor.59 The structural and electronic characteristics of
the FeRe3 assemblies (Figure 1-7), along with the other Rel-based
polynuclear assemblies reported throughout this work, are very amenable to

Féirster energy transfer quenching of the ReI donor state.

12

o
\\ ///°

c\ \\
R mn--R§<CEO
/

 

Figure 1-7. General structure of the FeRe3 assemblies discussed in
Chapter 2. The FeRe3 donor-acceptor complex shows the thermalized
3MLCT excited state of the ReI donor moiety utilized in all of the
donor-acceptor complexes discussed in this dissertation. A single ReI
donor excited state is shown (blue vector) interacting with all three
acceptor transitions (red vectors) due to a single Rel photon absorption
event per assembly.

13

1.3 Contents of Dissertation

The research contained in this thesis has focused on developing a
more in depth understanding of energy transfer dynamics in transition metal-
based donor—acceptor assemblies. Specifically, the structural and electronic
properties of the metal-based donor and acceptor complexes were designed
to study and quantify dipole-dipole (F orster) energy transfer processes in
covalently-attached assemblies. Polypyridyl complexes of ReI were chosen
as the donor moieties due to possessing an isolated, relatively long-lived
3MLCT excited-state, which simplified the geometric location of the
emission transition dipole moment of the donor and allowed for a
quantitative application of Fbrster theory. The various sections of this thesis
represent investigations into particular aspects of Fbrster energy transfer
theory undertaken by altering the charge and spin-state of the acceptor metal
centers. Specific areas of focus include quantifying through-space dipole-
dipole orientation (K2) in charge transfer-based donor-acceptor complexes,
utilizing ligand-field transitions to uncover orbital specific energy transfer
reactivity, and investigating the spin-state dependences of dipolar energy
transfer.

The first section, Chapter 2, focuses on the synthesis, characterization,

and photophysical properties of a family of chromophore-quencher

14

complexes with the general formula [Fe(pyacac)3(Re(bpy’)(CO)3)3](OTt)3
(where bpy’ = 4,4’-5,5’-tetramethyl-2,2’-bipyridine, 2,2’-bipyridine, or 4,4’-
diethylester—2,2’-bipyridine). The FeRe3 complexes contain a high-spin FeIII
acceptor core coordinated through three 3-(4-pyridyl)-acetylacetone ligands
to three Re(bpy’)(CO)3 donor moieties. Steady-state as well as time-
resolved emission and absorption measurements acquired in room-
temperature CHZCIZ solutions indicate quenching of the Rel-based 3MLCT
excited state when compared to isostructural A1Re3 model compounds.
Electron transfer was found to be thermodynamically unfavorable for all
three Rel-containing systems: this fact coupled with the absence of optical
signatures for the expected charge-separated photoproducts in the time-
resolved differential absorption spectra and favorable spectral overlap
between the donor emission and acceptor absorption profiles implicated
dipolar energy transfer from the Rel-based excited-state to the high-spin F eIII
core as the dominant quenching pathway in these compounds. The charge-
transfer-based donor emission (3MLCT) and acceptor absorption (6LMCT)
states were modeled as the movement of charge along particular vectors
within the coordination environment [3MLCT: Re"(bpy') —> Rel(bpy);

6LMCT: Fem(acac) —-> F e"(acac+)]. Metric details obtained from single-

crystal X-ray structural data allowed for a quantitative application of F firster

15

energy transfer theory by systematically calculating the donor-acceptor
separation (RDA) and spatial orientation of the charge transfer-based donor
and acceptor transition moment dipoles (K2). The results of this analysis and
agreement with experimentally derived energy transfer rate constants will be
discussed.

The unambiguous assignment of Fb’rster energy transfer as the
dominant quenching pathway in the F eRe3 assemblies, led to the design of
structurally similar complexes that possess acceptor metal centers with low-
intensity ligand-field absorptions that overlap with the Rel-based donor
emission spectra. The goal of the research contained in Chapter 3 was to
examine if dipole-dipole energy transfer could operate in systems that
possess very small transition dipole moments. Chapter 3 focuses on the
synthesis, characterization, and photophysical properties of a family of
donor-acceptor complexes with the general formula [Cu(pyacac)2(Re(bpy’)-
(CO)3)2](OTi)2 (where bpy’ = 4,4’-5,5’-tetramethyl-2,2’-bipyridine, 4,4’-
dimethyl-2,2’-bipyridine, 2,2’-bipyridine, 4,4’-dichloro-2,2’-bipyridine, or
4,4’-diethylester-2,2’-bipyridine). The CuRez complexes contain a d9 CuII
acceptor core coordinated through two 3-(4-pyridyl)-acety1acetone ligands to
two Re(bpy’)(CO)3 donor moieties. Steady-state as well as time-resolved

emission measurements acquired in room-temperature CH2C12 solutions

16

indicate quenching of the Rel-based 3MLCT excited state when compared to
isostructural BeRez model compounds. Electron transfer was found to be
significantly endothermic for all five CuRez complexes; this fact, coupled
with the ca. 10 A donor-acceptor distance and favorable spectral overlap
between the 3MLCT emission profile and ligand-field absorptions of the CuII
center implicated dipolar energy transfer as the dominant quenching
pathway in these compounds. Gaussian deconvolution of the ground-state
absorption spectrum of Cu(phacac)2 (phacac = 3-phenyl-acetylacetonate)
allowed for a differential analysis of the spectral overlap between the donor
emission spectra with the two observed ligand-field absorption bands of the
Cu" ion. The rate of energy transfer was found to increase with increasing
overlap for coupling to the lower-energy ligand-field band, consistent with
expectations fi'om Ft'irster theory. These results were supported by time-
dependent DFT calculations on Cu(phacac)2, and indicated preferential
coupling to a particular ligand-field transition of the Cu" center. These
results will be discussed in terms of an orbitally-specific energy transfer
process occurring in the CuRez series.

Observation of emission fi'om an acceptor moiety provides
unequivocal proof of an excited state energy transfer mechanism. Although,

the structural and electrochemical properties of the FeRe3 (Chapter 2) and

17

CuRez (Chapter 3) complexes rule out electron transfer as a viable
quenching mechanism, the goal of the work contained in Chapter 4 was to
introduce an acceptor metal center that emits in response to quenching the
Rel-based 3MLCT excited state. Chapter 4 focuses on the synthesis,
characterization, and photophysical properties of a series of chromophore-
quencher complexes with the general formula [Cr(pyacac)3(Re(bpy’)-
(CO)3)3](OTf)3 (with bpy’ defined above). The CrRe3 complexes contain a
d3 CrIII acceptor core coordinated through three 3-(4-pyridyl)-acetylacetone
ligands to three Re(bpy’)(CO)3 donor moieties. Steady-state as well as time-
resolved emission and absorption measurements acquired in room-
temperature CH2C12 solutions indicate quenching of the Rel-based 3MLCT
excited state when compared to isostructural AlRe3 model compounds.
Acquisition of the excitation spectrum of [Cr(pyacac)3(Re(bpy)(CO)3)3]-
(OTf)3 at km, of the Crm-based 2E —> 4A2 emission in fiozen EtOH/MeOH
(4:1) solution reproduced it’s ground-state absorption spectrum, which
proves that all of the photons absorbed by [Cr(pyacac)3(Re(bpy)-
(CO)3)3](OTt)3 results in Crm-based emission and unequivocally proves an
energy transfer mechanism. A comparison between the nature of the ligand-
field acceptor states in the CrRe3 and the CuRez complexes, along with their

effect on dipolar energy transfer reactivity will also discussed.

18

In Chapter 5, the Rel-based compounds - [Co(pyacac)3(Re(bpy)-
(C0)3)3](0Tf)3 and [Ni(PyacaC)2(THF)2(Re(bPY)(CO)3)2](0T02 - were
synthesized in order to investigate the spin-state dependence of dipolar
energy transfer. The first section of Chapter 5 contains the synthesis,
characterization, and photophysical properties of a CoRe3 assembly that
possesses a low-spin (16 Col" metal center ('Al, S = 0). Steady-state and
time-resolved emission measurements acquired in room-temperature CHzClz
solution revealed no quenching of the Rel-based 3MLCT excited state, with
the observed excited state relaxation behavior comparable to an isostructural
AlRe3 compound. Favorable spectral overlap between the Rel-based 3MLCT
emission spectrum and the 1A] —+ 'T, ligand-field absorption profile of the
Co"I ion was observed, suggesting dipolar energy quenching should be
active in the CoRe3 assembly. Determination of the total spin angular
momentum (ST) values possible in the CoRe3 complex revealed that the 'A,
—-> 1T1 acceptor transition represents a spin-forbidden relaxation pathway for
the 3MLCT donor state. This suppressed energy transfer reactivity between
the 3MLCT donor and the CoIII core is compared with the results received in
Chapter 3 on a structurally similar CrRe3 complex the exhibits significant
quenching of the Rel-based 3MLCT state. The CrRe3 system possesses

identical overall charge and structural characteristics with the CoRe3

19

complex, but simply differs in the spin-state of the acceptor metal center.
The results on the CoRe3 system highlight the ability of controlling excited
state quenching reactivity by simply varying the spin-state of the reagents.
The second section of Chapter 5 will focus on the synthesis,
characterization, and photophysical characteristics of [Ni(pyacac)2(THF)2-
(Re(bpy)(CO)3)2](OTf)2, referred to as the NiRez assembly, which contains a
pseudo-octahedral NiII (d8) acceptor metal center. Determination of the total
spin angular momentum (ST) in the NiRez assembly showed the ligand-field
absorption of the Ni06 core (3A2 —> 3T2) to be a spin-allowed relaxation
pathway for the Rel-based 3MLCT donor state. Comparison of the steady-
state emission of [Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2](OTf); in THF and in
CHzClz solutions at room-temperature revealed an on/off 3MLCT quenching
mechanism, with the dissociation of the axially coordinating THF molecules
in CH2C12 solution generating a NiO6.x (x = 1 or 2) complex and subsequent
spin-forbidden energy transfer pathway.

Future directions of the research are included as a sixth chapter.
Various MRen (n = 2 or 3) compounds that have been synthesized and
characterized, proposed complexes and photophysical experiments, and the
scientific questions the molecules will address comprise the content of

Chapter 6.

20

1.4 References

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(2)
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Van Der Meer, B. W.; Coker, G. 1.; Chen, S.-Y. Resonance Energy
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DeMember, J. R.; Filipescu, N. J. Am. Chem. Soc. 1968, 90, 6425.

Wang, X. F.; Herman, B., Eds. Fluorescence Imaging Spectroscopy
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Thomas, D. D.; Carlsen, W. F.; Stryer, L. Proc. Natl. Acad. Sci. U. S.
A. 1978, 75, 5746.

Dale, R. 13.; Eisinger, J.; 13., B. w. Biophys. J. 1979, 26, 161.

Furue, M.; Ishibashi, M.; Satoh, A.; Oguni, T.; Maruyama, K.; Sumi,
K.; Kamachi, M. Coord. Chem. Rev. 2000, 208, 103.

Grosshenny, V.; Harriman, A.; Hissler, M.; Ziessel, R. J. Chem. Soc,
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Mukhopadhyay, S.; Manda], S. K.; Bhaduri, S.; Armstrong, W. H.
Chem. Rev. 2004, 104, 3981.

Ferreira, K. N.; Iverson, T. M.; Maghlaoui, K.; Barber, J .; Iwata, S.
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Stryer, L. Biochemistry; 4th ed.; W.H. Freeman and Co.: New York,
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22

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Balzani, V.; Scandola, F. Supramolecular Photochemistry; Horwood:
Chichester, U. K., 1991.

Alstrum—Acevedo, J. H.; Brennaman, M. K.; Meyer, T. J. Inorg.
Chem. 2005, 44, 6802.

Fabbrizzi, L.; Poggi, A., Eds. Transition Metals in Supramolecular
Chemistry; Klewar Academic Publishers: Dordrecht, 1994.

Balzani, V.; Ceroni, P.; Juris, A.; Venturi, M.; Campagna, S.;
Puntoriero, F.; Serroni, S. Coord. Chem. Rev. 2001, 219, 545.

Baranoff, E.; Barigelletti, F.; Bonnet, S.; Collin, J. P.; Flamigni, L.;
Mobian, P.; Sauvage, J. P. In Photofunctional Transition Metals
Complexes; Springer-Verlag Berlin: Berlin, 2007; Vol. 123, p 41-78.

Blan, Z. Y.; Sumi, K.; Furue, M.; Sato, S.; Koike, K.; Ishitani, O.
Inorg. Chem. 2008, 4 7, 10801.

Easun, T. L.'; Alsindi, W. Z.; Towrie, M.; Ronayne, K. L.; Sun, X. 2.;
Ward, M. D.; George, M. W. Inorg. Chem. 2008, 4 7, 5071.

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23

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Chem. A 2004, 108, 2363.

Damrauer, N. H.; Cerullo, G.; Yeh, A.; Boussie, T. R.; Shank, C. V.;
McCusker, J. K. Science 1997, 275, 54.

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Wrighton, M.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998.

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Vléek, A.; Busby, M. Coord. Chem. Rev. 2006, 250, 1755.

Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin
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Lees, A. J. Chem. Rev. 1987, 87, 711.
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Dattelbaum, D. M.; Omberg, K. M.; Schoonover, J. R.; Meyer, T. J.
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Dattelbaum, D. M.; Martin, R. L.; Schoonover, J. R.; Meyer, T. J. J.
Phys. Chem. A. 2004, 108, 3518.

Worl, L. A.; Duesing, R.; Chen, P.; Della Ciana, L.; Meyer, T. J. J.
Chem. Soc., Dalton Trans. 1991, 849.

24

(59) Brown, W. R.; O'Boyle, N. M.; McGarvey, J. J.; Vos, J. G. Chem.
Soc. Rev. 2005, 34, 641.

25

Chapter 2. Quantifying Fiirster Energy Transfer in Charge Transfer

Based Donor/Acceptor Complexes: FeRe3 Multinuclear Assemblies.

2.1 Introduction

Elucidating the mechanism of excited-state reactivity is a necessary
first step for understanding and ultimately manipulating complex photo-
induced chemical processes."4 Accordingly, numerous fundamental studies
of excited-state dynamics have been reported in the literature. Assemblies
based on d6 polypyridyl complexes of Rel, Ru", and OsII have garnered
particular attention due to relative stability of their excited states, well
documented ground and excited-state electronic properties, and the ability
one has to tune these properties through synthetic means. Both electron and
energy transfer processes have been the subject of intense scrutiny. As a
result, much has been learned about the factors that govern both of these
types of excited-state reactions in transition metal-based systems.5'14

The two most widely occurring energy transfer mechanisms are
electron superexchange (Dexter)15 and through-space dipole-dipole coupling
(F brster).16 Dexter energy transfer is subject to a distance dependence that
falls off as exp(-2R) due to its reliance on orbital overlap. As such, it is

usually relegated to covalently linked systems in which the donor and

26

acceptor are in close proximity (e.g., _<_ 5 A)”22 Fbrster transfer is a
through-space mechanism that occurs when the donor emission dipole non-
radiatively couples to an absorptive dipole in the acceptor.”25 The dipolar
nature of this interaction gives rise to a shallower R'6 dependence, allowing
this mechanism to be operative over much longer distances. Féirster-type
reactivity is therefore usually dominant in systems that place the lowest
energy excited-state on an electronically isolated portion of the donor or
26-29

between metal centers that are separated over long distances.

The F6rster energy transfer rate constant (km) is described by the

 

equation given in equation 2-1,30
_ 90001n(10)rc2<I>DJ
EnT 12871'5774NA2'DR6 (2-1)

where x2 is the dipole orientation factor, (1),, is the radiative quantum yield of
the donor, 77 is the refractive index of the solvent, NA is Avogadro’s number,
I'D is the excited-state lifetime of the donor, R is the donor-acceptor
separation, and J is a spectral overlap integral that essentially quantifies the
resonance condition necessary for dipole-dipole coupling. This latter term
can be evaluated from the spectroscopic properties of the system according

to equation 2-2,

27

 

Ho] D (V)8A (V)€1V -

0

(2-2)

where Fo is the (normalized) emission spectrum of the donor and BA is the
absorption profile of the acceptor in units of molar absorptivity. From
equation 2-1, knowledge concerning the geometric positions of the donor
emission and acceptor absorption transition dipoles, which is known as the
orientation factor (K2), must be elucidated to accurately model excited-state
kinetics. The orientation factor relates the angle between the donor emission
and the acceptor absorption transition dipoles relative to each other and the
angle the donor and acceptor transition dipoles each create with the donor-

acceptor connection line, and is given in equation 2-3:

2
= (cosG')T — 3cos®Dcos® A) (2-3)
where @T is the angle between the donor transition dipole moment and the
acceptor transition dipole moment, and @D and @A are the angles between

the donor-acceptor connection line and the donor transition dipole moment
and the acceptor transition dipole moment, respectively. Although equations
2-1, 2-2, and 2-3 constitute a complete description of the rate of dipolar
energy transfer, quantifying the donor-acceptor distance (R) and the

orientation factor (K2) can be quite difficult in metal-based donor-acceptor

28

complexes, due to R typically being approximated as the metal-metal
separation and K2 usually assigned as 273.3140 Taking R as the metal-metal
distance is a reasonable assumption when structural (data are not available,
but in the point-dipole approximation of Fbrster theory this may or may not
accurately reflect the relevant distance in systems comprised of donor and
acceptor states that are charge-transfer in nature. In addition, the orientation
factor of K2 = 2/3 typically invoked represents an isotropic value for species

sampling all possible angular distributions.“

While appropriate for
bimolecular energy transfer processes, this approximation may not be
reasonable given the rotational barriers that likely exist in covalently
attached donor-acceptor complexes. The resulting ambiguities that can arise
concerning these two variables often lead to a large variance between
experiment and theory.

Moore et a1. recently applied F firster theory in conjunction with
molecular modeling calculations utilizing naphthalene and anthracene,
which possess known directional emission and absorption transitions,
covalently linked through a ZnII containing macrocycle that holds the
separation distance constant.”43 The studies revealed that the minimized

conformers, with quantitatively determined donor-acceptor distances and

orientation factors, very accurately describe the observed Forster energy

29

transfer kinetics when all of the possible conformers were taken into
account. The donor-acceptor systems to be discussed in this chapter possess
known donor-acceptor separations and directional electronic transitions as
well, which utilize a ReI polypyridyl based 3MLCT state as the donor and
F em tris-acetylacetonate based 6LMCT (ligand-to-metal charge-transfer)
states as the acceptor. The current systems also have moderate spectral
overlap between the donor emission and acceptor absorption profiles, which
was instrumental in systematically studying and solely modeling the donor
emission quenching dynamics as Fdrster energy transfer.

Specifically, the synthesis, structure, and photophysical properties of a
series of isostructurally related molecules are reported, with the general
formula [Fe(pyacaC)3(Re(tmb)(C0)3)3](0T03 (1), [Fe(pyacaC)3(Re(bPY)-
(C0)3)3](0Tf)3 (2). and [F6(PyacaC)3(Re(deeb)(C0)3)3](0T03 (3) (Where
pyacac = 3-(4-pyridyl)-acetylacetonate, tmb = 4,4’-5,5’-tetramethyl-2,2’-
bipyridine, bpy = 2,2’-bipyridine, deeb = 4,4’-diethylester-2,2’-bipyridine,
and OTf = CF 3SO3'), and with the entire series referred to as the FeRe3
assemblies. The Fe"I metal centers are covalently attached to three fac-
Re(bpy’)(CO)3 (bpy’ = tmb, bpy, and deeb) (Figure 2-1) moieties through
three pyridyl-acetylacetonate bridging ligands. It was observed that the Re]-

based 3MLCT excited-states for complexes 1, 2, and 3 are significantly

30

CllleIICth in the presence of the Fe111

metal center relative to structurally
analogous AlRe3 analogs. The AlRe3 model systems with the general
formula [A1(PyacaC)3(Re(tmb)(C0)3)3](0T03 (4), [A1(pyacaC)3(Re(bPY)-
(C0)3)3l(0T1)3 (5). and [A1(pyacaC)3(Re(deeb)(C0)3)3](0T03 (6) were
synthesized in order to investigate the excited-state dynamics of the
Re(bpy’)(CO)3 moieties in the absence of the emission quenching dynamics

incurred by the F eIII

metal center. Varying the bpy’ attached to the ReI metal
center permitted a systematic investigation of the donor-acceptor spectral
overlap by modifying the energy of the 3MLCT excited-state, with the
emission quenching rate constant corresponding with the amount of
observed spectral overlap. in addition, the single-crystal X-ray structure
data for complex 2 enabled very accurate calculations of the RDA and K2
interactions within this system, with the same values applied to complexes 1
and 3 by assuming that variation in the polypyridyl substituents causes
minor altering of the internal dimensions within this family of compounds.
The rigid structure of these systems also enabled a very accurate modeling
of the solution phase F orster energy transfer kinetics by assuming the RDA
and K2 values remain unchanged from those calculated fiom the X—ray

structural data, with the theoretical rate constants accurately reproducing the

experimentally observed values. Lastly, one of the major hurdles in

31

quantifying Ftirster energy transfer is accurately determining the through-
space separation and orientation factor relating donor and acceptor transition
dipoles, and the work in this chapter will show that the unique structure of
the FeRe3 analogs allow for a very accurate determination of these two

variables.

 

Figure 2-1. Polypyridyl derivatives coordinated to the ReI metal
centers.

2.2 Experimental Section
2.2.1 Synthesis and Characterization

General. All solvents used were purified and dried according to
previously reported methods.44 Spectroscopic grade CH2C12 was used for all

photophysical measurements; the solvent was dried under CaHz reflux until

32

no water was detected by 1H NMR and degassed using freeze-pump—thaw
techniques. 3-(4--pyridyl)-2,4--pcntanedione,45 Al(pyacac)3,45 Re(tmb)(CO)3-
(on),46 Re(bpy)(CO)3(OTt),46 Re(deeb)(CO)3(OTf),46 and fac-
[Re(bpy)(CO)3(4-Etpy)](PF6)47 (4-Etpy = 4-ethylpyridine) were prepared
following literature procedures. 3-phenyl-2,4-pentanedione was purchased
from TCI America. Elemental analyses and F T-IR data were obtained
through the analytical facilities at Michigan State University. Mass spectra
were obtained through the analytical facilities at The University of South
Carolina.

Tris(3-(4-pyridyl)acetylacetonato)iron(III), Fe(pyacac)3. The
synthesis of this compound has been reported previously by a different
method.48 Amounts of 70.1 mg (0.432 mmol) of FeCl3 and 230 mg (1.30
mmol) of pyacac were dissolved in 30 mL of THF and stirred for 6 hrs.
Sodium tert-butoxide (125 mg, 1.30 mmol) was then added to the reaction
flask and the solution was stirred overnight. The reaction mixture was
filtered over celite to remove excess salt and the solvent removed under
vacuum. The product was recrystallized from CHzClz/hexanes (1:1 v/v).
Yield: 131 mg (52%). Anal. Calcd for C30H30N306Fe: C, 61.65; H, 5.17;

N, 7.19. Found: C, 61.49; H, 5.13; N, 7.07.

33

Tris(3-phenyl-acetylacetonato)iron(III), Fe(phacac)3. The synthesis
of this compound has been previously reported by a different method.48
Amounts of 86 mg (0.53 mmol) of F eC13 and 277 mg (1.57 mmol) of phacac
were dissolved in 30 mL of THF and stirred for 6 hrs. Sodium tert-butoxide
(149 mg, 1.57 mmol) was then added to the reaction flask and the reaction
mixture was stirred overnight. The solution was then filtered over celite to
remove excess salt and the solvent removed under vacuum. The product
was recrystallized from CH2C12/hexanes (1:1 v/v). Yield: 238 mg (78%).
Anal. Calcd for C33H3306Fe: C, 68.16; H, 5.72. Found: C, 68.11; H, 5.79.

[Fe(pyacac)3(Re(tmb)(CO)3)3](OTf)3 (1). An amount of 230 mg
(0.360 mmol) of Re(tmb)(CO)3(OTf) was dissolved in 75 mL of hot THF,
after which 70 mg (0.12 mmol) of F e(pyacac)3 was added and the solution
purged with argon for 20 min. The reaction mixture was then fit with a
condenser and stirred under argon for 3 days in hot THF in the dark, after
which a dark red solution formed along with an orange precipitate. The
precipitate was collected and the filtrate was concentrated under vacuum to
yield additional orange solid. The combined precipitates were dissolved in
CHzClz, filtered through celite, and the solvent removed under vacuum. The
product was recrystallized several times from CHzClz/pentane (1:1 v/v).

Yield: 155 mg (52%). Anal. Calcd for Cg4I-I73N9F9024S3FeRe3: C, 40.70; H,

34

3.17; N, 5.08. Found: C, 40.36; H, 3.23; N, 4.86. IR (KBr, cm'l): 2031 s,
1918 s, 1614 m, 1566 s, 1448 m, 1365 m, 1263 s, 1155 m, 1032 s, 638 m.
MS: [ESL m/z (rel. int.)]: 677.3 (70) {[Fe(pyacac)3(Re(tmb)(CO)3)3]}3+,
1090.5 (23) {[Fe(pyacac)3(Re(tmb)(CO)3)3](OTf)}2+, 2330.1 (1)
{[Fe(pyaeac)3(Re(tmb)(C0)3)31(OTf)2}”-
[Fe(pyacac)3(Re(bpy)(CO)3)3](OTf)3 (2). Amounts of 70 mg (0.12
mmol) of Fe(pyacac)3 and 210 mg (0.365 mmol) of Re(bpy)(CO)3(OTf)
were dissolved in 25 mL of THF and flushed with argon for 20 min. The
reaction was stirred in the dark for 3 days at room temperature. An orange
solid precipitated out of solution and was collected and washed with
hexanes. The solid was then dissolved in CH3CN, filtered through celite,
and the solvent removed under vacuum. The product was recrystallized
several times from acetonitrile/ether (1:1 v/v). X-ray quality crystals were
obtained by slow diffusion of ether into an acetonitrile solution of the
compound. Yield: 166 mg (60%). Anal. Calcd for C72H54N9F9024S3FeRe3:
C, 37.37; H, 2.35; N, 5.45. Found: C, 37.00; H, 2.23; N, 5.27. IR (KBr, cm"
I): 2033 s, 1922 s, 1568 m, 1446 m, 1367 w, 1261 m, 1159 m, 1032 m, 771

m, 638 m. MS: [ESL m/z (rel. int.)]: 621 (100)

1[Fe(pyacac)a(Re(bpy)(CO)a)3113*. 1006 (18) {1Fe(pyacacb<Re(bpy>-

(CO)3)3](OTD}2+9 2161-1 (1) {[F6(PyacaC)3(Re(bpy)(C0)3)3](0T02} ”-

35

[Fe(pyacac)3(Re(deeb)(CO)3)3](OTf); (3). Amounts of 29 mg (0.050
mmol) of Fe(pyacac)3 and 108 mg (0.150 mmol) of Re(deeb)(CO)3(OTf)
were dissolved in 25 mL of THF and flushed with argon for 20 min. The
' reaction was stirred in the dark for 4 days at room temperature. The solution
was then filtered over celite and the solvent removed under vacuum to give a
red solid. The product was recrystallized several times using
CH2C12/pentane (1:1 v/v). Yield: 65 mg (48%). Anal. Calcd for
C90H73N9F9036S3FeRe3°3CH2C12: C, 38.00; H, 2.79; N, 4.15. Found: C,
38.00; H, 2.93; N, 4.32. IR (KBr, cm'l): 2036 s, 1923 s, 1732 s, 1566 s,
1462 m, 1263 s, 1153 m, 1032 s, 767 m, 638 m. MS: [ESL m/z (rel. int.)]:
765.4 (75) {[Fe(pyacac)3(Re(deeb)(CO)3)3]}3+, 1222.6 (19) {[Fe(pyacac)3-
(Re(deeb)(CO)3)3](OTf)}2+, 2594.2 (1) {[Fe(pyacac)3(Re(deeb)(CO)3)3]-
toms)”.

[Al(pyacac)3(Re(tmb)(CO)3)3](0T1)3 (4). An amount of 224 mg
(0.360 mmol) of Re(tmb)(CO)3(OTf) was dissolved in 75 mL of hot THF,
after which 66 mg (0.120 mmol) of Al(pyacac)3 was added and the solution
purged with argon for 20 min. The reaction mixture was then fit with a
condenser and stirred under argon for 3 days in hot THF in the dark, after
which a dark yellow solution formed along with a yellow precipitate. The

precipitate was collected and the filtrate concentrated under vacuum to yield

36

additional yellow solid. The combined precipitates were dissolved in
CHzClz, filtered through celite, and the solvent removed under vacuum. The
product was recrystallized several times from CHzClz/pentane (1:1 v/v).
Yield: 185 mg (64%). Anal. Calcd for C34H73N9F9024S3A1Re3°CH2C12: C,
. 40.55; H, 3.17; N, 4.95. Found: C, 40.37; H, 3.12; N, 4.95. IR (KBr, cm'l):
2031 s, 1918 s, 1612 m, 1585 s, 1452 m, 1396 m, 1263 s, 1155 m, 1032 s,
"638 m. MS: [ESI, m/z (rel. int.)]: 667.7 (65) {[AIQJyacac)3(Re(tmb)-
(CO)3)3]}3+, 1076.1 (25) {[Al(pyacac)3(Re(tmb)(CO)3)3](OTf)}2+, 2301.3 (1)
1lAl<pyacacb<Re<tmbxc(3)3131(orbsl"‘1
[Al(pyacac)3(Re(bpy)(CO)3)3](OTf)3 (5). Amounts of 70 mg (0.126
mmol) of Al(pyacac)3 and 210 mg (0.365 mmol) of Re(bpy)(CO)3(OTf)
were dissolved in 25 mL of THF and the solution flushed with argon for 20
min. The reaction was stirred in the dark for 3 days at room temperature. A
yellow solid precipitated out of solution and was collected and washed with
hexanes. The solid was then dissolved in CH3CN, filtered through celite,
and the solvent removed under vacuum. The product was recrystallized
several times from acetonitrile/ether (1 :1 v/v). X-ray quality crystals were
obtained by slow diffusion of ether into an acetonitrile solution of the
compound. Yield: 187 mg (65%). Anal. Calcd for C72H54N9F9024S3A1Re3:

C, 36.90; H, 2.39; N, 5.53. Found: C, 37.51; H, 2.35; N, 5.13. IR (KBr,

37

cm”): 2033 s, 1920 s, 1585 s, 1446 s, 1398 s, 1263 s, 1159 s, 1029 s, 771 m,
638 3. MS: [ESL m/z (rel. int.)]: 611.6 (100) {[Al(pyacac)3(Re(bpy)-
((20).)3113“. 991.9 (30) {[Aloyacac)3(Re(bpyxCOh).1(0T012*. 2132.9 (1)
{[A1(Pyacac)3(Re(bPY)(C0)3)3](0T02} 1+~

[Al(pyacac)3(Re(deeb)(CO)3)3](OTf)3 (6). Anounts of 28 mg of
Al(pyacac)3 (0.050 mmol) and 108 mg of Re(deeb)(CO)3(OTf) (0.150
mmol) were dissolved in 25 mL of THF and flushed with argon for 20 min.
The reaction was stirred in the dark for 4 days at room temperature. The
solution was then filtered over celite and the solvent removed under vacuum
to give an orange solid. The product was recrystallized several times from
CHzClz/pentane (1:1 v/v). Yield: 60 mg (44%). Anal. Calcd for
C90H73N9F9036S3A1Re3: C, 39.82; H, 2.90; N, 4.64. Found: C, 39.54; H,
2.85; N, 4.48. IR (KBr, cm'l): 2036 s, 1930 s, 1734 s, 1585 s, 1448 s, 1273 s,
1151 m, 1032 s, 768 m, 638 m. MS: [ESL m/z (rel. int.)]: 755.7 (42)
{[Al(pyacac)3(Re(deeb)(CO)3)3]}3+, 1208.1 (10) {[Al(pyacac)3(Re(deeb)-
(CO)3)3](OTD}2+: 2565-3 (1) {[A1(pyacaC)3(Re(deeb)(C0)3)3](0T02} ”-
2.2.2 Physical Measurements

X-ray Structure Determinations. Single-crystal X-ray diffraction
data for complexes 2 and 5 were acquired at the X-ray facility of Michigan

State University. Diffraction data were collected on a Siemens SMART

38

diffractometer with graphite-monochromatic Mo K01 radiation ()1 =
0.71073A). Data were collected at -100 °C by using an Oxford Cryosystems
low temperature device. Crystallographic data are summarized in Table 1;
selected bond distances and angles are listed in Table 2. Lattice parameters
were obtained from least-squares analyses and data were integrated with the
program SAINT .49 The integration method employed a three dimensional
profiling algorithm and all data were corrected for Lorentz and polarization
factors, as well as for crystal decay effects. The absorption correction
program SADABS50 was employed to correct the data for absorption effects.
The structures were solved by direct methods and expanded using Fourier
techniques. All structure calculations were performed with the SHELXTL
6.12 software package.51 Anisotropic thermal parameters were refined for all
non-hydrogen atoms. Hydrogen atoms were localized in their calculation
positions and refined by using the riding model. Further details concerning
the structure determinations may be found in Supporting Information.

Cyclic Voltammetry. Electrochemical measurements were carried
out in a Nz-filled drybox (Vacuum Atmospheres) using a BAS CV-SOW
electrochemical analyzer. A standard three-electrode arrangement was
utilized consisting of Pt working and counter electrodes and a Ag/AgNO3

reference electrode. 'Measurements were carried out in either CHzClz or

39

CH3CN solutions that were 0.1 M in NBu4PF6. The choice of solvent was
dictated by the potential window required to observe a given redox couple;
in cases where a compound could be examined in both solvents, differences
in the observed potentials were found to be within experimental error of
each other. Potentials are reported versus the ferrocene/ferrocenium couple,
which was used as an internal standard, and listed in Table 3.

Electronic Absorption and Steady-State Emission Spectroscopies.
Extinction coefficients for all compounds were acquired in CHzClz solutions
using a Hewlett-Packard 8452A diode array spectrophotometer. Steady-state '
emission spectra were acquired using a Spex F luoromax fluorimeter and
corrected for instrumental response using a NIST standard of spectral
irradiance (Optronic Laboratories, Inc., OL220M tungsten quartz lamp).52a
Spectra were acquired on samples dissolved in thoroughly degassed CH2C12
under optically dilute conditions (o.d. ~ 0.1) and sealed under an argon
atmosphere in 1 cm path length quartz cuvettes.

Radiative quantum yields ((1),) were determined relative to fac-

[Re(bpy)(CO)3(4-Etpy)](PF6) (<1)r = 0.18 in CHzClz).47 Quantum yields were

calculated according to equation 2-4,

 

(2-4)

2

(Inn Aun ) un
(Dunk =(Dstd k/ k 77 k
(Istd /Astd) flstd

40

where (Dunk and (1),“, are the radiative quantum yields of the sample and the
standard, respectively, Iunk and 1,“, represent the areas of the corrected
emission profiles for the sample and the standard, Am and Astd are the
absorbance values of the sample and the standard at the excitation
wavelength, and flank and nstd correspond to the indices of refraction of the
sample and standard solutions (taken to be equal to the neat solvents).
Excitation wavelengths were 355 nm for the bpy and tmb analogs and 400
nm for the deeb analogs. The corrected excitation spectrum of fac-
[Re(bpy)(CO)3(4-Etpy)](PF6) in CHzClz overlaid well with the compound’s
absorption spectrum over the range of wavelengths examined (355 - 400
nm), implying that the radiative quantum yield for fac-[Re(bpy)(CO)3(4-
Etpy)](PF6) does not vary significantly over this spectral window. The
reported value of (I)r = 0.18 was therefore used for determining the radiative
quantum yields at both A.,x = 355 nm and 400 nm.

Radiative quantum yields are not being reported for complexes 1-3
due to the presence of an emissive impurity. The source of the impurity was
traced. to a small amount of dissociated complex present in solution, most
likely generated by water in the CHzClz solvent. Despite our best efforts at
drying the CHzClz, there was an unacceptably large variance in repeated

measurements of (1),. Due to the difference in time scales associated with

41

excited-state decay between those emissive fragments and the intact
assembly, the presence of these impurities does not significantly affect the
kinetic analyses of these systems.

Values for the zero-point energy gap (E00) of the Rel-based 3MLCT
excited states were determined by fitting the emission profiles of complexes
4-6 based on the approach described by Claude and Meyer.53 Wavelength
data were converted to energy units following the correction of Parker and
Rees;54 the best fit was determined by Visual inspection of the results of a
least-squares minimization routine.

Time-Resolved Emission Spectroscopy. Nanosecond time-resolved
emission data for the A1Re3 model complexes 4-6 were collected using a
NszAG-based laser spectrometer that has been described previously.52a
Data were acquired at room temperature in thoroughly degassed CH2C12
solutions having absorbances of ~0.1 at the excitation wavelength (ltcx = 355
nm for complexes 4 and 5 and 420 nm for complex 6). Samples were sealed
under an argon atmosphere in 1 cm path length quartz cuvettes. The decay
traces correspond to an average of 250 shots of the signal probed at the
emission maximum of each compound.

Picosecond time-resolved emission data for the FeRe3 complexes 1-3

were collected using a time-correlated single photon counting (TCSPC)

42

apparatus that has been described previously.55 Data were acquired in
thoroughly degassed CH2C12 solutions having absorbances of ~0.1 at the
excitation wavelength (lax = 370 nm for complexes 1 and 2 and 430 nm for
complex 3). Samples were sealed under an argon atmosphere in 1 cm path
length quartz cuvettes. Each reported decay trace corresponds to a signal
average of six data sets, with each data set resulting from ca. 1 hr of data
acquisition time. The decay traces for all three complexes manifest a small
baseline offset within the ca. nanosecond data acquisition window due to the
presence of the impurity mentioned above; this was incorporated into the
kinetic model. Data were fit using the OriginPro 7.5 software package.56
Time-resolved Absorption Spectroscopy. Nanosecond time-
resolved absorption measurements were collected using a NszAG-based
laser spectrometer that has been described previously.52 Data were acquired
on thoroughly degassed CH2C12 solutions in 1 cm path length sealed quartz
cuvettes. Sample absorbance was ~0.6 at the excitation wavelength of 355
nm. Excitation energies at the sample were ca. 2 mJ per pulse; all data were
checked for linearity with respect to pump power, and steady-state
absorption spectra were acquired before and after data acquisition to ensure

the integrity of the sample. Differential absorption spectra were generated

43

by plotting the amplitudes of fits of the decay kinetics acquired as a function
of probe wavelength.

Sub-nanosecond time-resolved data were obtained for complex 1
using a femtosecond time-resolved absorption spectrometer that is described
elsewhere?2b Samples were dissolved in CH2C12 in an Ar atmosphere drybox
and placed in 1 mm path length quartz cuvettes with absorbance values of
~0.6 at the excitation wavelength (Apump = 370 nm). Single-wavelength
kinetics data were collected at Aprobe = 700 nm following ~100 fs excitation
at 370 nm with a pump power of ca. 4 pl Single-photon excitation was
confirmed by the linearity of the signal response with respect to pump
power. Acquisition of full spectra for complex 1 was hampered by the
formation of a photoproduct that precipitated over the course of several
hours of data acquisition. Nevertheless, the data were sufficient to establish
that identical kinetics are observed across the entire visible probe window;
this will be elaborated upon in the discussion section. All data were fit using
programs of local origin.

Flirster Energy Transfer Rate Calculations. Calculations of energy
transfer rates were carried out based on eqs 1-3. The variables of the Fiirs’ter
equation are presented in the Introduction section of this chapter, with the

following briefly describing how the donor-acceptor separation (RDA) and

44

the angles used to solve the orientation factor (K2) were defined for the
F eRe3 family. Donor-acceptor distances and angles were measured using
the single-crystal X-ray structure data of complex 2 with the Diamond 3.1d
crystal structure and visualization software.57 The donor-acceptor separation
was ascertained by a simple through-space distance measurement between
the interacting points of origin of the donor and the acceptor (donor-acceptor
connection line). OT was acquired by measuring the angle generated
between the Re(bpy) donor plane versus the interacting FeOz acceptor plane.
OD was acquired by measuring the angle the donor-acceptor connection line
creates with the vector drawn from a donor origin to the Re1 metal center.
(9A for acceptor positions D and E (Figure 2-11) were obtained by measuring
the angle the donor-acceptor connection line creates with the vector drawn

- . 11
from an acceptor orlgln to the Fe I

metal center. (9A for acceptor position F
(Fenl center) was measured by evaluating the angle the donor-acceptor

connection line creates with the vector drawn from the Fe111 metal center to

the center of the oxygen atoms on the acac portion of the bridging ligand.

45

2.3 Results and Discussion

2.3.1 Synthesis and Characterization. Our interest in these systems was to
investigate energy and/or electron transfer processes in structurally well-
defined polynuclear charge-transfer assemblies. The choice of using FeIII
and ReI was based on the well known MLCT-based reactivity of Re1 and the
propensity for FeIII to act as both an energy and electron acceptor.”63 The
utilization of the M(pyacac)3 core (M = FeIII and Aim) as a ligand for
Re(bpy’)(CO)3(OTf) allowed the pyridyl group to displace the weakly
coordinating triflate anion and generate the tri-cationic FeRe3 and AlRe3
complexes. The formation of the tetranuclear assembly was facilitated by
the low steric crowding afforded by the roughly 120° separation of the three
pyacac ligands.

The ESI-MS data for complexes 1-6 in acetonitrile solution are
consistent with the formation of the desired FeRe3 and AlRe3 assemblies.
For example, complex 2 shows peaks corresponding to [Fe(pyacac)3(Re-
(bpyXCOhhrt [Feoyacacb(Re(bpyXCOhhltort)”, and [Fe(pyaeae)3-
(Re(bpy)(CO)3)3](OTf)2'+. X-ray quality crystals for complexes 2 and 5
were generated by diffusion of ether into an acetonitrile solution of the

complexes over the course of approximately one week; this in turn provides

additional evidence for the general robustness of complexes 1-6 in solution.

46

Nevertheless, the Fem-containing complexes 1-3 were found to be
susceptible to slight decomposition in the presence of water. As mentioned
in the Experimental Section, this was established via single-photon counting
emission spectroscopy through the detection of a highly emissive, long-lived
excited-state species. Since FeIII compounds are non-emissive, this impurity
is most likely a Re-bpy-containing fragment resulting from displacement of
the pyacac ligand by adventitious HZO. This decomposition pathway is not
too surprising given the lability of high-spin FeIII and its propensity to bind
H20, but the extremely low intensity of the TCSPC signal corresponding to
this species indicates that this represents a very minor component in
solution.

The v(CO) stretching bands of the Re(bpy’)(CO)3 moieties in
complexes 1-6 (Table 2-3) were assigned based on previously reported data
for fac-[Re(4,4’-X2-bpy)(CO)3(4-Etpy)](PF6) complexes.64 The
characteristic spectral profile consists of two very intense peaks. The broad
band at lower energy corresponds to two overlapping transitions assigned to
the A'(2) and A" modes (CS symmetry), whereas the sharper, higher energy
band is assigned as A'(l).65 The fact that the carbonyl frequencies observed

for each F eRe3/A1Re3 pair are virtually identical is indicative of minimal

47

I .
11 metal centers 1n

direct electronic communication between the Re1 and Fe
the ground states of the FeRe3 assemblies.7

2.3.2 Single-Crystal X—ray Structures. X-ray quality crystals were
obtained for [F e(pyacac)3(Re(bpy)(CO)3)3](OTt)3 (2) and [Al(pyacac)3(Re-
(bpy)(CO)3)3](OTt)3 (5). The two complexes are isostructural and crystallize
in the triclinic space group P1. Crystallographic details are given in Table 2-
1 with selected bond distances and angles for the two complexes listed in
Table 2-2. The coordination environments about the central metal ions in
both complexes (Figure 2-2) are distorted octahedra with six oxygen atoms
from the acac groups comprising the coordination sphere. In the case of
complex 2, the Fe-O bond distances of ca. 1.98 i 0.02 A are consistent with
high-spin F em and compare favorably with other structurally characterized
examples of F em-acac systems.48 Significantly shorter metal-oxygen bonds
(ca. 0.1 A) are observed for complex 5: such differences are to be expected
given the smaller ionic radius of All".48 The structures of the Re(bpy)(CO)3
moieties in both molecules are unremarkable, exhibiting the pseudo-C3v
coordination environment common to Re1 complexes in this class.”69 Not
surprisingly, the metric details of the Re1 fragment are insensitive to the

identity of the central metal ion: bond distances, and angles associated with

the primary coordination sphere as well as within the bipyridyl ligand itself

48

are identical within experimental error for the FeIII and Al111 complexes.
Slight differences are noted in the Rel—core metal distances, with complex 5
being uniformly shorter by ~O.1 A due mainly to the decrease in metal-
oxygen bond lengths relative to complex 2. Despite these minor differences,
there is considerable structural homology between the two complexes,

III
1

underscoring the appropriateness of using A as a structurally and

electronically benign replacement for Fem in this system. Efforts to obtain
X-ray quality crystals for complexes 1, 3, 4, and 6 have thus far been
unsuccessful, however, substituent changes on the periphery of the bipyridyl

ligand will have little effect on the basic structural features in these systems.

49

Table 2-1. Crystallographic Data for [Fe(pyacac)3(Re(bpy)(CO)3)3]-
(0T1): (2) and [A1(PyacaC)3(Re(bpy)(C0)3)3](0T03 (5)-

 

 

2 5

formula C72H54N9024F983FCR63 C72H54N9024F 983A1Re 3
MW 2310.87 2282.00
cryst syst triclinic triclinic
space group Pl-bar Pl-bar
T/K 173(2) 173(2)
a/A 13.626(2) 13.534(2)
b/A 17.676(3) 17.590(3)
c/A 19.889(3) 19.809(3)
011° 89.494(3) 89.076(3)
B/° 82.518(3) 81.890(3)
y/° 71 .451(3) 71.053(3)
V/A3 4500.1 (1 1) 4413.4(12)
Z 2 2
Dc/g cm'1 1.717 1.717
20max 47 46
reflns measured 33672 36985
independent 12741 12190
reflns
observed reflns 6347 6354

. [I > 20(1)]
u(Mo Koo/cml 4.342 4.278
Rim 0.0918 0.0899
R1a 0.0675 0.0666
wR2b 0.1373 0.1564
GOF 1.007 1.044

 

aRl = 2111:.)- chll/ZIFOL waz = [2w(F02 — F,2)2/2w(1=,2)2]“2, w
)+ (aP)2 + bP], where P = [F02 + 2Fc2]/3,

1/[o 2(F,

Table 2-2. Selected Bond Distances (A) and Angles (deg) for
[Fe(pyacaC)3(Re(bpy)(C0)3)3](0T03 (2)21nd [A1(PyacaC)3(Re-
(bPY)(CO)3)3l(OT1)3 (5).

 

 

2 5

Bond Distances (A)
Fe(1)- 0(1) 1.992(6) Al(1)-O(1) 1.877(10)
Fe(1) — 0(2) 1.970(7) Al(l) — 0(2) 1.861(10)
Fe(l) — 0(6) 1.997(6) Al(1) — 0(6) 1.871(11)
Fe(1) — 0(7) 1.963(6) Al(1)— 0(7) 1.894(11)
Fe(l)—O(11) 1.981(7) Al(1)—O(11) 1.883(12)
Fe(1) — 0(12) 1.986(6) A1(1)— 0(12) 1.852(10)
Re(1)—N(1) 2.232(7) Re(1)—N(1) 2.215(12)
Re(2) —N(4) 2.218(7) Re(2) -N(4) 2.225(12)
Re(3) —N(7) 2.206(9) Re(3) —N(7) 2.191(14)
Fe(l)mRe(1) 9.88 Al(1)mRe(1) 9.76
Fe(1)---Rc(2) 9.88 Al(1)---Re(2) 9.74
Fe(1)«-Re(3) 9.78 Al(1)---Re(3) 9.66

Bond Angles (deg)

0(1) — Fe(1) — 0(2) 85.4(3) 0(1) — Al(1) — 0(2) 90.0(4)
C(23) - Re(1)— N(1) 92.1(4) C(23) — Re(1)— N(1) 93.0(6)
C(21) — Re(1)— N(1) 178.7(4) C(21) — Re(1)— N(1) 177.9(7)

aplane lu-plane 2 280.1 aplane lee-plane 2 79.0
bplane 100-plane 2 82.5 bplane 1°»plane 2 81.0
cplane 1°»plane 2 68.8 _ cplane louplane 2 69.2

 

aPlane 1 is defined by atoms 0(1), 0(2), C(1), C(2), C(3), C(4), C(5);
plane 2 is defined by atoms N(1), C(6), C(7), C(8), C(9), C(10). bPlane
1 is defined by atoms 0(6), 0(7), C(24), C(25), C(26), C(27), C(28);
plane 2 is defined by atoms N(4), C(29), C(30), C(31), C(32), C(33).
cPlane 1 is defined by atoms O(11), 0(12), C(7), C(48), C(49), C(50),
C(51); plane 2 is defined by atoms N(7), C(52), C(53), C(54), C(55),
C(56).

51

t ‘
cs4 1 .'3
Rel ‘ C9 C10 C5] C5 7‘ '- ‘
* p . cr ' csz ‘ N7 S,
‘ N1 , ‘ C2 012 . 50 "9, C55 .1

O
u.
S
9.3 a
. I a
D O
NO‘S
‘UI r—
O
N
A
.01

Figure 2-2. Drawing of the cations of [Fe(pyacac)3(Re(bpy)(CO)3)3]-
(010. (2. top) and [A1(pyacaC)3(Re(bPY)(C0)3)3](0T03 (5, bottom)

obtained from single-crystal X-ray structure determinations. Atoms are
represented as 50% probability thermal ellipsoids.

52

Figure 2—2 (cont’d)

'.“ ‘T
5‘ ‘3. _ Re3
— \
. . C5 7
‘ 'e 4 C5 “C55
. 3" C9 C56
. 1 '4 ..
a? . 10 C5 C7 052
s \ C6 4 C 2‘ a 49
5‘ l o . C3 '4 .1 2 ‘ 51
é.‘ C7 ‘ 50
3 C 5- 012
Cl‘ V‘ l]
020 07
C2 C27
C2 " '- C28

2.3.3 Electrochemistry. The electrochemical properties of complexes 1-6
were examined using cyclic voltammetry; the data are given in Table 2-3.
The availability of the Al”1 model complexes greatly simplifies assigning the
features observed for all six complexes due to the redox-inert nature of this
ion. Accordingly, the single reduction waves seen for complexes 4-6 can be

immediately ascribed to the bipyridyl ligand of the Re moiety in each case.

53

The positive shift in potential across the series is consistent with the more
electron withdrawing nature of the substituents as one progresses from the
methyls of tmb (4) to the diethylester groups in complex 6. Similarly, the
oxidation waves seen for all three complexes are easily assigned to the
Rel/Rel1 couple. The influence of the bipyridyl substitutents are apparent in
these data as well, with the more electron deficient ligand giving rise to the
most positive oxidation potential for the Re center. The results are all
consistent with what has been observed for complexes of the general form
fac-[Re(4,4'-X2-bpy)(CO)3(4-Etpy)](PF6) previously reported in the
literature.46

Given these assignments, the reductions at ca. -0.9 V observed for

"1 center.70 It can be seen

complexes 1-3 are clearly associated with the Fe
that modification of the bipyridyl ligand has no discernible influence on the
redox properties of the central Fe ion. In a similar vein, the ligand reduction
and ReI oxidation potentials of all three Fe-containing compounds are

essentially identical to what was observed for the AlIII

analogs. These data
are indicative of (relatively) weak electronic coupling between the central

metal ion and the peripheral chromophores.

54

Table 2-3. Electrochemical and Infrared Data for Complexes 1-6.

 

electrochemical potential (V) v(CO) (cm-l)a

complexes 13,x (Rel/u) E,,d(1=e“"“) 1:.red (bpy'o’) A'(1)A",A'(2)
FeRe3

 

 

tmb(1) +1.47"~c .0.91‘Le -1.84b 1918 2031
bpy(2) +1 .41b -0904 -1594 1922 2033

deeb (3) +1 .60b -0.93d - 1 .20d 1923 2036
AlRe3

tmb (4) +1.37b -1.82" 1918 2031
bpy (5) +1.42b 4.57"!f 1920 2033

deeb (6) +1.54b 119" 1930 2036

 

aMeasured as pressed KBr pellets. 1:,Measured in CH3CN solution.
°Potential in CHzClz solution is +1.41 V. dMeasured in CHzClz solution.
ePotential in CH3CN solution is -0.86 V. fPotential in CH2C12 solution
is -1.57 V.

2.3.4 Electronic Absorption Spectroscopy. The electronic absorption
spectra of the FeRe3 (1-3) (black traces) and AlRe3 (4-6) (blue traces)
analogs have been acquired in room-temperature CHzClz solutions and are
shown in Figure 2-3. Transitions associated with these complexes were
assigned based on previously reported analyses of ReI polypyridyl and
F e(phacac)3 absorption spectra. ReI polypyridyl complexes exhibit a well
known lMLCT (t2g ——> 71* (bpy’)) transition occurring from approximately

330 to 430 nm depending on the substituents of the bpy’ ligand,“72 with the

55

absorption profiles of the AlRe3 compounds solely exhibiting these lMLCT
based characteristics. Amax for the 1MLCT excited-state reflects the electron
donating/withdrawing ability of the bpy’ substituents, with Amax for
complexes 4, 5, and 6 occurring at 344 (8 = 18,500), 364 (8 = 11,500), and
394 (a = 14,300) nm, respectively. As the substituents become progressively
more electron donating (e.g., H for bpy and CH3 for tmb) this feature
systematically shifts to the blue and begins to overlap with the ligand-based
absorptions in the ultraviolet (Figure 2-3). The AlRe3 analogs allow for
analysis of the ground-state absorption behavior of the Re1 donor moieties
without overlapping with the Fem-based charge-transfer transitions
contained in the FeRe3 analogs.

The presence of Fe"I in complexes 1-3 gives rise to a new, broad
absorption feature on the low-energy side of the Rel-based charge transfer
band. Figure 2-4 shows the absorption spectrum of Fe(phacac)3 which
exhibits two strong transitions centered at 370 and 460 nm assigned as 6A.
—+ 6MLCT (t2g —-> 71*(acac)) and 6A1 —+ 6LMCT (7t (acac) —> tzg) transitions,
respectively.73’74 In complexes 1-3 the higher energy MLCT absorption is
obscured by the more intense 1A1 —> 1MLCT band of the ReI chromophore,
but the low energy tail of the 6A, ——> 6LMCT transition can be seen extending

out to approximately 600 nm. The linear combination of the corresponding

56

AlRe3 analog with Fe(phacac)3 is also shown in Figure 2-3 (red trace), which
sufficiently models the absorptive characteristics of the FeRe3 analogs. The
additive property of these constituent species reiterates the minimal ground-

"I

state electronic interaction between the ReI and Fe metal centers observed

from the infrared data.

57

A

W03

lvi

Molar Absorpt

Molar Absorptivity (8)

Molar Absorptivity (8)

 

30000‘
25000
20000-
15000-
10000-

5000-

 

 

 

0..

 

25000;

20000 \

15000r \

10000-

5000~

 

o .

 

 

 

 

40000‘
35000
30000-
25000r
20000-

15000- ‘

10000-
5000-

 

 

  

 

 

O '- L l 1 l
350 400 450 500 550 ‘ 600

Wavelength (nm)

Figure 2-3. Electronic absorption spectra of [M(pyacac)3(Re(bpy’)-
(CO)3)3](OT1)3 assemblies, where M = FeIII (black traces) or AllII (blue
traces). All spectra were acquired in room-temperature CHzClz
solution. A. [F e(pyacac)3(Re(tmb)(CO)3)3](OTf)3 (1) and [Al(pyacac)3-
(Re(tmb)(C0)3)3l(0Tf)3 (4). 13- [Fe(pyacaC)3(Re(bPY)(C0)3)3](0T03
(2) and [A1(Pyacae)3(Re(bp>')(C0)3)3l(OTf)3 (5)- C- [Fe(pyacae)3(Re-
(deeb)(C0)3)3l(0Tf)3 (3) and [A1(PyacaC)3(Re(deeb)(C0)3)3](0T1)3 (6)-
The dashed red lines in all three graphs correspond to linear
combinations of the molar absorptivity (8, (M'l, cm'1)) profiles of the
AlRe3 complex with that of Fe(phacac)3.

58

 

4" 6000-
5000
4000-
3000-
2000-
1000

 

 

 

Molar Absorptivity (M 1cm

0
l

 

350 400 450 500 550 600 650
Wavelength (nm)

Figure 2-4. Electronic absorption spectrum of Fe(phacac)3 acquired in
room-temperature CHzClz solution.

2.3.5 Steady-State and Time-Resolved Emission. Emission spectra for the
FeRe3 and AlRe3 complexes were obtained in room—temperature
deoxygenated CH2C12. The AlRe3 complexes represent an ideal structural
model for the dynamics associated with the Re1 based 3MLCT emission, due
to the inability of engaging in quenching dynamics incurred by the F eIII core
in the FeRe3 systems. The emission profiles are given in Figure 2-5, and
show the emission intensity for the F eRe3 complexes are significantly
attenuated compared to the AlRe3 model systems. The spectral profiles
correspond to previously reported photophysical studies of ReI polypyridyl
systems, with the emission originating from the 3MLCT —-> 1A1

phosphorescence.75 The emission maximum for the tmb (526 nm), bpy (566

nm), and deeb (624 nm) derivatives reflect the electron donating and

59

withdrawing behavior of the polypyridyl ligands. The radiative quantum
yields ((1),) of 4 ((1),: 0.51), 5 ((1).: 0.16), and 6 (CI),= 0.07) were determined
relative to [(bpy)Re(CO)3(4-Etpy)](PF6) ((1)r = 0.18 in CHzClz) (Table 2-4),
and are comparable to the reported values for the corresponding
mononuclear ReI polypyridyl derivatives.46

The (1), values for complexes 1-3 are analytically unreliable due to a
small emissive impurity generated by dissociation of the complexes. The
source of the dissociation was discovered to be residual amounts of water
contained in the CHzClz, which generates the highly emissive fac-
[Re(bpy’)(CO)3(pyacac)] complex. The drying and distilling procedures that
were employed were exhausted until no water was detectable by ]H NMR,
but despite our best efforts small traces remained in the solvent. It should
also be noted that the extent of dissociation was small over the course of the
radiative quantum yield determinations, but a steady increase in the emission
signal for the three systems was observed when monitored over several

hours.

60

 

 

 

 

 

 

 

 

 

1 A
‘3
Q)
.5
78‘
O
5
3‘
E
a
E
A 'l B
"U
Q)
.5
i
O
5
r?
(I)
C:
a
E.

l C
6
O.)
.5
"‘8
O
5
.2»
'a
C
8
.E

.500 600 700 800

Wavelength (nm)

Figure 2-5. Corrected steady-state emission spectra for [M(pyacac)3-
(Re(bpy’)(CO)3)3](OTt)3 assemblies, where M = AlIII (blue traces) and
Fe‘" (rcd traces). A. [Al(pyacac)3Re(tmb)(CO)3])3](OT1)3 (4) and [Fe-
(PyacaC)3(Re(tmb)(C0)3)3](0Tf)3 (1) 13- [A1(pyacaC)3Re(bP)/)-
(C0)3])3l(0Tf)3 (5) and [Fe(pyacaC)3(Re(bPY)(C0)3)3](0T03 (2) C-
[Al(pyacac)3(Re(deeb)(CO)3)3](OTf), (6) and [Fe(pyacac)3(Re(deeb)-
(CO)3)3](OTf)3 (3). The spectra were acquired in CHZCIZ solutions at
room temperature following excitation at 355 nm (complexes 1, 2, 4,
and 5) and 400 nm (complexes 3 and 6).

61

Additional details concerning the excited states of the A1Re3 and
FeRe3 complexes were obtained through nanosecond emission lifetime and
time-correlated single photon counting (TCSPC) measurements. The
nanosecond emission decay lifetime (robs) in room-temperature CHzClz
solution for complexes 4, 5, and 6 are shown on the left side of Figure 2-6,
and could be fit to single-exponential models with rob, = 2260 :l: 100, 560 :l:
30, and 235 :l: 20 ns (Table 2-4), respectively. The corresponding radiative
and non-radiative decay rate constants are, k, = 2.3 i 0.1 x 105 s'1 and knr =
2.2 d: 0.1x105 s'1 for 4, k, = 2.9 i 0.13 x105 s'1 and k... = 1.5 a 0.07 x106 s'1
for 5, and k. = 3.0 :t 0.13 x105 s'1 and 1r... = 4.0 a 0.17 x106 s" for 6. As
with the quantum yields, the observed excited-state lifetimes and rate
constants are all consistent with the assignment of 3MLCT —> 1A1 emission .
reported previously for the mononuclear ReI polypyridyl derivatives.46’76 The
kinetics reveal that the reduction in quantum yield is due primarily to an
increase in the non-radiative decay rate for 3MLCT relaxation (km) as
opposed to significant variations in radiative coupling to the ground state
(Table 2-4).

The lack of emission signal observed for the FeRe3 systems during
nanosecond lifetime experiments prompted the employment of time-

correlated single photon counting (TCSPC) methods. Plots of the TCSPC

62

data obtained in room-temperature CH2C12 solution for the FeRe3 complexes
are shown on the right side of Figure 2-6. All three decay traces could be fit
with single-exponential models with robs = 450 i 20 ps (kobS = 2.3 :l: 0.13 x
109 s"), 755 r 40 pS (km. = 1.3 a: 0.06 x 109 s"), and 2.50 a 0.1 ns (km. = 4.0
:t 0.09 x 108 S”) for complexes 1, 2, and 3, respectively (Table 2-4). The
signal-to-noise ratio is relatively poor owing to a combination of virtually
complete quenching of the Rel-based 3MLCT states coupled with radiative
rate constants that are on the order of 105 s']. The time constants of the A1m
model complexes are several orders of magnitude shorter than what was
observed for the Fe"I complexes, indicating that excited-state relaxation in

[II

complexes 1-3 is dominated by reaction with the Fe core. The rate constant

for the reaction can be given by equation 2-5,
k q (FeRe3) = k

(FeRes) " kobs(A1Re3) (2'5)

obs

where the values of kr and km for the 3MLCT excited state of a given FeRe3
assembly are taken to be equivalent to the corresponding AlRe3 model
complex. Given the extensive quenching of the 3MLCT state as indicated by
the time constant for decay in complexes 1-3, the observed lifetimes

effectively correspond to the quenching time constants in all three cases.

63

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

4 100» 1
5 ,.
E g. 80~ 1‘
b v
7,; .E‘ 60-
:: g . 1
8 8 ‘ ll 1 1‘ l1
5 £40 i.
l
l
0 l 1 I 1 20 1 1 I I I
0 3000 6000 9000 1200015000 0 1 2 3 4
Time (ns) Time (ps)
50—
5 2
A 40— 50? [
> ”a? ‘ ‘
40
E 30- 3 i .
r? a 30- .
r: 20 a
3 a) _
E E 20 I‘ll
10— "‘ l
10
0 l I l 1 1 I 0 1 r I r 1
0 300 600 900 120015001800 0 1 2 3 4
Time (ns) Time (ps)
50" 6 30%
A 40r; 25%
> 1 7,;
E 30 2 gzoor
E‘ i‘ 9150-
8 20:1 a
E 1 a 100
._ O , c:
l " b—t
l 50;
0'1 1 1 1 1 0 1 1 1 l 1 I I 1 I
O 300 600 .900 1200 1500 0 2 4 6 8 10 12 14 16
Time (ns) Time (ns)

Figure 2-6. Left. Nanosecond time-resolved emission data for the
AlRe3 model complexes: 4 (robs: 2260 i 100 ns), 5 (robs: 560 d: 30 ns),
and 6 (rob, = 235 d: 20 ns). Right. TCSPC data for the corresponding
FeRe3 complexes: 1 (robS = 450 i 30 ps), 2 (1:01,s = 755 :t 40 ps), and 3
(rob, = 2.5 d: 0.1 ns). All data were collected in deoxygenated CH2C12 at
room temperature. The red solid lines correspond to fits to single-
exponential decay models.

64

Table 2-4. Photophysical Data of Complexes 1-6.

 

E00 k k

 

complexes AW (cm")a (I), k0,,(s‘1) (><105s")d (x106s")e
FeRe3

tmb(1) b b c 2.3i0.1x109 f f
bpy(2) b b c 1.3:l:0.1x109 f f
deeb(3) b b c 4.0i0.1x108 f f
AIR83

tmb(4) 526 19,900 0.51 4.4i0.2x105 2.3101 0.21001
bpy(5) 566 18,700 0.16 1.8i0.1x106 2,910.1 1,510.1

deeb (6) 624 16,800 0.07 4.3 :l: 0.2 x 106 3.0 :1: 0.1 4.0 a 0.2

 

aZero-point energy difference between 3MLCT excited state and ground
state based on spectral fitting analysis. bThis value is expected to be
identical to the corresponding AlIII complex. °Values are not quoted due
to the presence of an emissive impurity. See text for further details. dk,
= kob,*(1),. ekm = kob, - k,. fExcited-state decay kinetics dominated by

kobs-

2.4 Mechanistic Considerations.

2.4.1 Electron versus Energy Transfer Quenching. Both electron and
energy transfer processes can be envisioned to occur out of the Rel-based
3MLCT excited state. Based on the presence of Fem in the ground states of
these complexes, electron transfer would proceed as an oxidative quenching

. . 11
reaction to produce a Ren/FeII charge separated spe01es. ReI —> Re

65

”I —-+ FeII reduction potentials for complexes 1-3 (Table 2-

oxidation and Fe
3), along with the zero-point energy gaps of the 3MLCT states (E00)
determined from fits of the emission spectra of the corresponding A1Re3
analogs (Table 2-4), were used to determine the thermodynamic driving
force for photoinduced electron transfer.”78 These calculations revealed that
electron transfer is thermodynamically unfavorable for complexes 2 and 3
(~0 and +0.45 eV, respectively), and only slightly exothermic in the case of
complex 1 (-0.1 eV). The magnitude of AGET for complex 3 is prohibitively
large, particularly given the observed rate constant of nearly 109 s]. In the
case of complex 2, electron transfer at room temperature is
thermodynamically feasible; however, the fact that the quenching rate is
only a factor of ~3 faster than what is observed for complex 3 suggests that
both of these complexes are reacting via similar mechanisms, i.e., energy
transfer.

The fact that electron transfer is predicted to be exothermic in the case
of complex 1 prompted further study. We therefore carried out time-resolved
absorption measurements on [F e(pyacac)3(Re(tmb)(CO)3)3](OTf)3 (1) in an
effort to identify whether a charge-separated species was being formed upon
1A1 —+ lMLCT excitation. The transient absorption spectroscopy of ReI

pOlypyridyl complexes has been described by a number of workers.”86

66

Their excited-state spectra typically consist of a moderately intense feature
in the ultraviolet corresponding to absorptions of the polypyridyl radical
anion, as well as a transient bleach due to loss of the ground-state 'AI —->
lMLCT absorption. Additional absorptions can also be observed toward the
red edge of the visible spectrum that are usually ascribed to bpy' transitions
of the excited state species. Consistent with these expectations, the
differential absorption spectrum of [Al(pyacac)3(Re(tmb)(CO)3)3](OTf)3 (4)
(Figure 2-7, top) exhibits transient absorptions at 370 nm and 760 nm that
we assign to tmb-based transitions. The hallmark for an excited-state
electron transfer process in complex 1 would be a wavelength-dependence in
the observed kinetics, namely the loss of the tmb' features coupled with a
persistence of the ground-state 1A] —> 1MLCT bleach. Instead, what we
observe is complete ground-state recovery at all probe wavelengths with a
time constant that is within experimental error of what was measured via
time-resolved emission spectroscopy (Figure 2-7, bottom). We have
recently discussed the notion that this observation does not necessarily rule
out a sequential electron transfer process.87 In the present case, however, the
driving force for charge-recombination would place the back-reaction deep
enough into the inverted region such that the relative rates necessary to

satisfy this condition are not likely to be realized.

67

 

0.12 -
0.10 ~
0.08 r
0.06 r
0.04 - 13,}
0.02 - 5194831113 5

0.00 — .................................................

Change in Absorbance

 

 

 

400 500 600 700 800

 

0.0014
0.0012
0.0010 -
0.0008 -
0.0006 —
0.0004 -
0.0002 -
0.0000 3 -------- . -------- . ------- ------:-

0 200 400 600 800 1000 1200

Time (ps)

 

Change in Absorbance

 

 

 

Figure 2-7. Top. Nanosecond time-resolved differential absorption
spectrum of [Al(pyacac)3(Re(tmb)(CO)3)3](OTf)3 (4) in room-
temperature CH2C12 solution. The spectrum was constructed from the
amplitudes of fits to single-exponential decay kinetics at each probe
wavelength following excitation at 355 nm. Bottom. Time-resolved
absorption data for [Fe(pyacac)3(Re(tmb)(CO)3)3](OTt)3 (1) in room-
temperature CHzClz solution at Aprobe = 700 nm following ~100 fs
excitation at Awmp = 400 nm. The red solid line corresponds to a fit of
the data to a single-exponential decay model with rob, = 400 :t 30 ps.

2.4.2 Dexter vs Ftirster Energy Transfer. In light of these observations,
the most likely explanation for 3MLCT quenching in all three of the F eRe3

assemblies is excited-state energy transfer. As discussed in the Introduction,

68

the Dexter mechanism requires orbital overlap between the donor and
acceptor involved in the energy transfer. The X-ray structure data for

111

complex 2 shows a Rel-«F e separation of nearly 10 A, a value that lies at

the limit of what is typically considered for an exchange-based process.”90

"1 subunits

The lack of significant electronic coupling between the ReI and Fe
is also supported by the similarities in v(CO) frequencies of the FeRe3 and
AlRe3 analogs, as well as the fact that the absorption spectra of the FeRe3
assemblies can be represented in terms of a simple linear combination of its
constituents (Figure 2-3). The lack of electronic communication denoted by
the structural and spectroscopic data greatly attenuates the plausibility of a
Dexter energy transfer mechanism for the excited-state quenching reactivity.

The applicability of the Forster mechanism is supported by the
moderate degree of spectral overlap that exists between the Rel-based
3MLCT emission and the Fem-based 6A1 —) 6LMCT absorption for the FeRe3
assemblies:91 this is depicted graphically in Figure 2-8. The overlap between
the 3MLCT —> 'A1 emission of the Alm-containing tmb (4) (blue trace), bpy
(5) (green trace), and deeb (6) (red trace) analogs with the 6A1 —> 6LMCT
absorption of Fe(phacac)3 is shown. In particular, the plot shows the amount

of spectral overlap is greatest for the highest energy 3MLCT excited-state

corresponding to the most electron donating group (tmb) and smallest for the

69

most electron withdrawing group (deeb). The area ascertained from the
product of the normalized emission profile of an AlRe3 analog with the
extinction coefficient spectrum of the corresponding FeRe3 complex was
used to calculate the reported spectral overlap values. The magnitude of
spectral overlap (J) also corresponds with the observed rate constants for
energy transfer with J values of 1.0 x 10'14 (kENT = 2.3 x 109 s'l), 7.6 x 10']5
(kENT = 1.3 x 109 s"), and 1.2 x 10'15 (1n;NT = 4.0 x 108 s") M'cm3 for the tmb
(1), bpy (2), and deeb (3) analogs, respectively. It is quite clear that the
overlap requirement in these systems is satisfied, which strongly implicates
F0rster transfer as the dominant quenching mechanism in the FeRe3

complexes.

 

1.!
l
.—-

 

Absorbance (normalized)

 

<2
0
(pozgeunou) [(1181191111 uolssrulg

 

 

400 500 600 700
Wavelength (nm)

Figure 2-8. Overlay of the emission spectra of [Al(pyacac)3(Re(tmb)-
(C0)3)3](OT1)3 (4. blue), [A1(PyacaC)3(Re(bPY)(CO)3)3](0Tf)3 (5,
green), and [A1(pyacac)3(Re(deeb)(CO)3)3](OTt)3 (6, red) with the
electronic absorption spectrum of Fe(phacac)3. Data were acquired in
room-temperature CH2C12 solution.

70

2.4.3 Quantifying Fbrster Energy Transfer. The spectral properties of the
FeRe3 family of complexes combined with the structural rigidity of the
system provides a rare opportunity to quantitatively apply F 0rster theory and
compare calculated rates with those obtained experimentally. In addition to
the overlap factor alluded to above, the rate of energy transfer is also
sensitive to the relative orientation of the donor and acceptor transition
dipoles (eq 2-3). An accurate determination of this quantity can be quite
challenging. In one noteworthy example, Fleming and coworkers utilized
TD-DFT (time-dependent density functional theory) to enable them to
visualize the transition dipoles of peridinin, which in turn provided them
with tremendous insights into the role geometry plays in facilitating energy
transfer from the singlet excited state(s) of that system.92 Unfortunately, the
complicated electronic structures of transition metal containing systems do
not easily lend themselves to a similarly detailed analysis, so more
approximate methods must usually be employed.

An excellent example of this is that of Harriman and coworkers, in
which F0rster energy transfer dynamics in Ru11 and Osll polypyridine donor-
acceptor complexes bridged by a rigid Spiro-based spacer moiety were
investigated.93 In their approach, energy-minimized structures were

calculated for each molecule with the donor and acceptor transition dipole

71

moments modeled along the six respective Ru—N and Os—N bond vectors.
Using this geometric picture, calculations of donor-acceptor separations
(RDA) and orientation factors (K2) afforded a theoretical F0rster rate constant
that agreed very closely with the experimentally observed values. We have
taken a similar approach for analyzing the FeRe3 family of complexes, albeit
with slight differences in the physical description of the system. For
example, we have chosen to approximate the donor and acceptor transition
dipoles as bisecting the local C2 axes of the bpy’ and acac ligands as
opposed to them lying coincident with the metal-ligand bond vectors. In
addition, the point of origin for the transition moment dipoles has been
modeled to originate solely from the ligands involved in the donor and
acceptor transitions. These choices are based largely on Density Functional
Theory (DFT) calculations on [Ru(bpy)3]2+ by Daul94 and Gorelsky‘)5 that
suggest the lowest energy excited-state (3MLCT) is localized on the bpy
ligands. In addition, Meyer and coworkers have shown that the majority of
the amplitude of the thermalized 3MLCT wavefunction for complexes of the
form fac-[Re(4,4’-X2bpy)(CO)3(4-Etpy)](PF6) (X = CH3, H, and COzEt) is
concentrated within the 11* levels of the bpy’ ligand and less so along the

Re—N bond vector.65

72

In light of these reported computational results, Figure 2-9 is drawn to
depict the Rel-based 3MLCT emission dipoles as blue arrows and the Fem-
based 6LMCT absorption dipoles as red arrows. The emission dipoles of the
three Rel donors (Rel, Re2, and R83) relative to the Fe111 center are
presented simultaneously for clarity, but only a single ReI moiety is
considered involved during an excited-state quenching event due to the low
excitation photon flux used for the steady-state and time-resolved emission
measurements (Figure 2-10). Figure 2-10 shows a single Rel-based emission
dipole moment spatially interacting with the three 6LMCT absorption dipole
moments. The donor-acceptor distances (R1, R2, and R3) and the three angles
used in the K2 equation (OT, OD, and CA) are also shown, and were evaluated
for each donor-acceptor dipolar interaction using the single-crystal X-ray
structure data of complex 2. The Rel, Re2, and Re3 donor dipoles were
examined individually relative to the three possible 6LMCT absorption
dipoles, and an RDA and K2 value was calculated for each case. The charge-
transfer nature of the donor and acceptor transitions makes it difficult to
place an exact point of origin for each, so distance and orientation factors
were calculated for a range of possible loci for both the donor and acceptor

(Figure 2-11). Figure 2-11 specifically illustrates the point-dipole model

utilized for calculating the donor-acceptor distances and angles, with the

73

through-space interaction of three different donor origins at a single Rel
emission dipole with the three 6LMCT absorption dipole origins located on
each FeOz triangle of the acceptor. The points A, B, and C, were chosen to
systematically vary the location of the 3MLCT wavefunction in order to
investigate the origin of the donor dipole. The MLCT nature of the donor
emission makes it difficult to place an exact point of origin on the bpy
ligand, so donor-acceptor distances and orientation factors for the three
points (A, B, and C) along each of the donor moieties (Rel, Re2, and Re3)
interacting with the 6LMCT transition dipoles were investigated. In
addition, Figure 2-11 also shows the acceptor origin was systematically
varied by placing it between the oxygen atoms of each acac ligand (points
D1, D2, and D3), at the center of the three F e02 triangles (points E1, E2, and
E3), and at the Fem metal center (point F), again due to the charge-transfer
nature associated in this case with the acceptor absorption dipoles. The
6LMCT state can be envisioned as occurring fiom either the non-bonding
electrons of the oxygen atoms or from the delocalized 7: electrons of the acac
ligands, and we feel these three points of origin adequately represents the
charge-transfer character of the acceptor with geometric considerations of
both the FeIII metal center and acac ligand. This approach yielded a total of

81 donor-acceptor interactions (27 for each ReI fluorophore), each being

74

defined by specific RDA and K2 values that were evaluated based on the
single-crystal X-ray structure data of complex 2. A complete list of the
values of RDA and K2 used in our analysis can be found in Tables 2—5a, 2-5b,

and 2-5c.

 

R R

Figure 2-9. Through-space interactions of the Rel-based 3MLCT
emission dipoles (blue arrows) with the Fem-based 6LMCT absorption
dipoles (red arrows).

75

 

--..-_..--..,
I
I
I
/
I

 

Figure 2-10. Single Rel-based emission dipole moment spatially
interacting with the three 6LMCT absorption dipole moments. Also
shown are the donor-acceptor distances (R1, R2, and R3) and the three
angles used in the K2 equation (OT, OD, and 6),.) evaluated from the
single-crystal X—ray structure data of complex 2.

76

 

 

Figure 2-11. Point-dipole approximation used for calculating the
through-space distances (RDA) and orientation factors (K2) needed for
quantifying Fbrster energy transfer theory in the FeRe3 assemblies.

77

Table 2-5a. RDA (A), OT (°), OD (°), (9A (°), and K2 values determined from
the internal dimensions of complex 2 using donor/acceptor
interactions A:D, B:D, and C:D.

 

R(ll) R(12) R(13) R(22) R(21) R(23) R(33) R(31) R(32y

 

8.46
77.81
84.68
154.1
0.213

10.73
35.87
91.22
46.84
0.729

10.41
56.34
86.53
61.42
0.219

8.40
80.28
86.30
158.7
0.122

10.65
35.52
92.54
49.62
0.810

10.41
54.45
87.17
59.73
0.257

8.38
60.72
87.26
147.2
0.372

10.89
63.14
86.28
28.49
0.079

9.70
39.82
95.31
82.65
0.646

 

8.64
77.81
77.07
147.5
0.604

10.75
35.87
85.24
51.00
0.427

10.53
56.34
80.30
60.52
0.093

8.55
80.28
78.49
150.9
0.479

10.63
35.52
86.36
53.34
0.490

10.51
54.45
81.05
58.47
0.114

8.3 1
60.72
82.96

142
0.606

10.82
63.14
83.97
23.37
0.026

9.44
39.82
92.33

87.5
0.598

 

8.99
77.81
69.69
141.4
1.050

10.93
35.87
78.93
54.9
0.230

10.81
56.34
74.03
60.25
0.021

8.82
80.28
71.49
144.1
0.884

10.74
35.52
80.59
57.16
0.300

10.73
54.45
75.28
57.58
0.03

8.48
60.72
75.78
136.4
1.050

10.95
63.14
78.56
19.35
0.012

9.42
39.82
85.98
92.27
0.603

 

78

Table 2-5b. RDA (A), OT (°), (99 (°), OA (°), and K2 values determined from
the internal dimensions of complex 2 using donor/acceptor
interactions AzE, B:E, and C:E.

 

R(ll) R(12) R(13) RQZ) R(21) RQBL R(33) R(31) R(32)

 

A
RDA
9r
@1)
®A

K2

9.13
77.81
86.25
156.1
0.153

10.25
35.87
89.49
49.78
0.629

10.08
56.34
87.05
65.02
0.239

9.08
80.28
87.74
160.3
0.079

10.19

35.52
90.84
52.76
0.707

10.07
54.45
88.04
63.29
0.287

9.00
60.72
88.53
149.7
0.309

10.25
63.14
87.85
30.44
0.126

9.64
39.82
92.49
86.91
0.601

 

B

RDA

9.26
77.81
79.17
149.9
0.488

10.31
35.87
83.21
54.12
0.363

10.20
56.34
80.63
64.06
0.116

9.18
80.28
80.52
153.1
0.371

10.21
35.52
84.4
56.63
0.426

10.15
54.45
81.63
61.94
0.141

8.89
60.72
84.84
144.9
0.503

10.15
63.14
85.15
25.01
0.049

9.44
39.82
89.41
91.87
0.591

 

9.57
77.81
72.16
144.1
0.913

10.53
35.87
76.71
58.12
0.199

10.47
56.34
74.16
63.69
0.037

9.42
80.28
73.95
146.7
0.743

10.37
35.52
78.42
60.56
0.268

10.36
54.45
75.65
60.95
0.049

9.02
60.72
78.13
139.6

0.92

10.26
63.14
79.33
20.71
0.005

9.47
39.82
83.06
96.63
0.656

 

79

Table 2-5c. RDA (A), OT (°), OD (°), GA (°), and K2 values determined from
the internal dimensions of complex 2 using donor/acceptor
interactions A:F, B:F, and C:F.

 

R(l 1) R(12)

R(13) R(22) R(21)

R(23)

R(33)

R(31)

R(32)

 

9.80
77.81
87.61
22.13
0.009

9.80
35.87
87.61

127
0.784

9.80
56.34
87.61
1 11.2
0.359

9.76
80.28
88.98
18.25
0.014

9.76
35.52
88.98
123.8
0.712

9.76
54.45
88.98
1 12.9
0.363

9.63
60.72
89.63
28.16
0.223

9.63
63.14
89.63
147.4
0.219

9.63
39.82
89.63

88.8
0.589

 

9.90
77.81
81.01
28.00
0.041

9.90
35.87
81.01
122.5
1.130

9.90
56.34
81 .01
112.2
0.535

9.83
80.28
82.30
24.98
0.038

9.83
35.52
82.30
119.8
1.030

9.83
54.45
82.30
114.3
0.558

9.49
60.72
86.49
32.62
0.112

9.49
63.14
86.49
153.1
0.379

9.49
39.82
86.49
83.77
0.560

 

10.17
77.81
74.35
33.5
0.215

10.17
35.87
74.35
118.4
1.430

10.17
56.34
74.35
112.7
0.750

10.03
80.28
76.11
31.09
0.200

10.03
35.52
76.11
115.8
1.270

10.03
54.45
76.11
1 15.4
0.793

9.58
60.72
80.21

37.6
0.007

9.58
63.14
80.21
157.8
0.853

9.58
39.82
80.21
79.08
0.451

 

The analysis we have carried out is predicated on two critical
assumptions: (1) that the variations in substitutents on the peripheral
bipyridine group do not significantly alter the metrics relevant for dipolar
energy transfer (thereby allowing us to use the X-ray structure of complex 2

as the basis for analyzing all three FeRe3 assemblies), and (2) that the

geometry of each compound in solution is essentially unchanged from that

80

determined by solid-state X-ray crystallography. The major influence of the
bipyridyl substituents will be to shift the electron density associated with the
excited state according to the electron donating/withdrawing ability of the
group. This assertion is supported by structure minimizations which revealed
virtually identical bond distances and angles for all three complexes.96 Given
the distribution of anchoring points on the bipyridyl ring that we are
evaluating, we believe this first issue is being adequately addressed. In terms
of solution versus solid-state geometry, the only significant degree of
freedom in these systems is rotation along the Re—N (pyridine) bond. We
expect there will be some barrier to this motion, but it is unlikely to afford
the same average angle in solution as found in the solid state. Even though
this represents a possible difference in the structure of the compound
between what we measure in the solid-state versus what exists in solution, an
analysis of this motion revealed that the relative distances and orientations of
the donor and acceptor transition dipoles (and therefore RDA and K2) do not
change over the entire 360° that the system can sample. Therefore, while in
principal using a solid-state structure to model geometric properties in
solution can be problematic, the particular structural aspects of the F eRe3

family of complexes makes such a comparison very straightforward.

81

2.4.4 Rate Constant Calculations: Modeling Solution Phase Energy
Transfer Dynamics. A given ReI donor can couple to any of the three Fe-
pyacac acceptor dipoles: the energy transfer process in these compounds can
thus be described in terms of three parallel reactions. The overall rate
constant for such a kinetic model is given by a linear combination of the rate

constants for each reaction pathway as shown in equation 2-6,

kRe(ll) + kRe(lz) + kRe(13) = kRel (2‘63)
kRe(zl) + kRe(22) + ch(23) = kReZ (2'6b)
kRe(31) + kRe(32) + kRe(33) = ch3 (2'69)

where kRe(ij) corresponds to the rate calculated for the ith Re donor coupling
to the jth Fe—pyacac acceptor; each of the km“) values derives from the
average of the nine possible donor-acceptor vectors defined by the point-
dipole origins depicted in Figure 2-11. For example, kg,” represents the
RelzLMCTl dipole-dipole interaction that is the average of the nine rate
constants calculated for each of the point dipole origins along the Rel donor
and the covalently attached FeOz triangle (A:Dl, AzEl, A:F, B:Dl, B:El,
B:F, C:Dl, C:El, and C:F) (Figure 2-11). Analogous procedures were
applied to the Rel :LMCT2 and Rel :LMCT3 interactions, with all three total
interactions summed to yield kRel. ‘ This same analysis was applied to the

Re2 and Re3 donor moieties to yield km and kRe3, which were then

82

averaged to give an overall theoretical rate constant (<kT>) from our model.
Table 2-6 lists all nine kRer) (Re#:LMCT#) values for complexes 1-3 along
with their <kT> values. It can be seen that there are variations in the
calculated rates of energy transfer within each group of interactions owing to
slight geometric differences at each Re-bpy site. Nevertheless, despite the
lack of a quantitative picture of wavefunctions for the donor and acceptor
charge-transfer states, the level of agreement obtained between experiment
and theory — less than a factor of three across the entire series — is quite good
and further supports our assignment of F brster transfer. The complete list of
calculated rate constants for complexes 1-3 are given in Tables 2-7a, 2-7b,

and 2-7c.

83

Table 2-6. Calculated F0rster rate constants for [Fe(pyacac)3(Re(tmb)-

(C0)3)3](0Tf)3 (1), 1FC(PyacaC)3(Re(bPY)(CO)3)3](0T1); (2).
and [F e(pyacac)3(Re(deeb)(CO)3)3](OTf)3 (3).

 

 

Interaction"l complex 1b complex 2 complex 3b
kr (8") kr (s') kr (s‘)

Re1:LMCT1 3.6 x108 3.3 x108 5.5 x107

Rel :LMCT2 2.9 x 108 2.7 x 108 4.4 x 107
RelzLMCT3 1.3x 108 1.2x 108 1.9x 107

1rRel (s")° 7.8 x 108 7.2 x 108 1.2 x 108
Re2zLMCT1 3.1 x 108 2.8 x 108 4.7 x 107
Re2zLMCT2 3.1 x 108 2.8 x 108 4.6 x 107
Re2:LMCT3 1.4 x 108 1.3 x 108 2.2 x 107

er,2 (s‘)6 7.6 x 108 6.9 x 108 1.2 x 108
Re3:LMCT1 1.2x108 1.3 x 108 1.8x 107
Re3:LMCT2 3.9 x 108 3.6 x 108 5.9 x 107
Re3zLMCT3 5.3 x 108 4.9 x 108 8.0 x 107

km (s")c 1.0 x 109 9.8 x 108 1.6 x 108
<k1‘>(s")d 8.5 x 108 8.0 x 10" 1.3 x 108
km, (s‘) 2.3 x 109 1.3 x 10" 4.0 x :108

 

aDonor-acceptor through-space interaction as defined in the text and in
Figure 2-11. bRDA and K2 values derived from the single-crystal X-ray
data for complex 2 using the geometries outlined in Figures 2-9 and 2-
10. A complete list of calculated rate constants for complexes 1-3 can
be found in Tables 2-7a, 2-7b, and 2-7c. cRate of energy transfer
calculated according to eqs 2-6. dOverall rate of energy transfer given
by (kRel '1' kReZ + ch3)/3-

84

 

 

 

 

 

 

 

 

 

 

 

 

Table 2-7a. Calculated Fbrster rate constants (8") for complexes
1-3 at all donor-acceptor interactions occurring at
A:D, B:D, and C:D.
Donor complex R(ll) R(12) R(13) kp (Rel)
tmb (1) 2.9 x 108 2.4 x 108 8.6 x 107 6.2 x 108
A bpy(2) 2.7x 108 2.2x 108 8.0x 107 5.7x 108
deeb (3) 4.4 x107 3.6 x 107 1.3 x 107 9.3 x 107
tmb(1) 7.3 x 108 1.4x108 3.4x 107 9.0x 108
B bpy(2) 6.7x 108 1.3x 108 3.2x 107 8.3x 108
deeb(3) 1.1x108 2.1x107 5.2x 106 1.4x 108
tmb(1) 1.0x 109 6.8x 107 6.6x 106 1.1x109
C bpy (2) 9.2 x 108 6.3 x 107 6.1x 106 9.9 x 108
deeb(3) 1.5x 108 1.0x107 1.0x106 1.6x 108
R(22) R(21) R(23) 1rF (Re2)
tmb(1) 1.7x 108 2.8x 108 , 1.0x108 5.5 x 108
A bpy(2) 1.6x108 2.6x 108 9.4x107 5.1x108
deeb (3) 2.6 x 107 4.2 x 107 1.5 x 107 8.3 x 107
tmb (1) 6.2 x 108 1.7 x 108 4.2 x 107 8.3 x 108
13 bpy(2) 5.7x108 1.6x108 3.9x 107 7.7x 108
deeb (3) 9.3 x 107 2.6 x 107 6.4 x 106 1.3 x 108
tmb(1) 9.4x 108 9.8x 107 9.9x 106 1.0x109
C bpy(2) 8.7x 108 9.1x 107 9.1x106 9.7x 108
deeb(3) 1.4x108 1.5x107 1.5x106 1.6x108
R(33) R(31) R(32) k..- (Re3)
tmb(1) 5.4x 108 2.4x 107 3.9x108 9.5 x 108
A bpy(2) 5.0x 108 2.2x107 3.6x 108 8.8x 108
deeb(3) 8.1x107 3.6x106 5.9x 107 1.4x108
tmb(1) 9.2x 108 8.1x106 4.2x 108 1.3 x 109
B bpy(2) 8.5x 108 7.5x 106 3.9x108 1.2x109
deeb(3) 1.4x108 1.2x106 6.4x 107 2.1x108
tmb(1) 1.4x109 3.5x 106 4.3x 108 1.8x 109
C bpy(2) 1.3x109 3.2x106 4.0x 108 1.7x109
deeb (3) 2.1x108 5.3 x 105 6.5 x 107 2.8 x 108

 

85

Table 2-7 b. Calculated F0rster rate constants (3") for complexes
1-3 at all donor-acceptor interactions occurring at
AzE, B:E, and C:E.

 

Donor complex

R(ll)

R(12)

R(13)

kp (Re1)

 

A

tmb(1)

bpy(2)
deeb (3)

1.3x108
1.2x108
2.0x 107

2.7x 108
2.5 x 108
4.1x107

1.1x108
1.1x108
1.7x107

5.1x108
4.8x 108
7.8x 107

 

tmb (1)

bpy(2)
deeb (3)

3.9x108
3.6x108
5.9x107

1.5 x 108
1.4x108
2.3 x 107

5.2 x 107
4.8 x107
7.8 x 106

5.9 x 108
5.5 x 108
9.0x 107

 

tmb(1)

bpy(2)
deeb (3)

6.0x 108
5.5 x 108
9.0 x 107

7.3 x 107
6.8x107
1.1x107

1.4x107
1.3x107
2.1x106

6.9x 108
6.3 x 108
1.0x108

 

tmb(1)

bpy(2)
deeb (3)

R(22)

R(21)

R(23)

1rF (Re2)

 

7.1x 107
6.5x 107
1.1x107

3.2 x 108
2.9x 108
4.8 x 107

1.4x108
1.3x108
2.1x107

5.3 x 108
4.9x 108
8.0x107

 

tmb(1)

bpy(2)
deeb (3)

3.1x 108
2.9x108
4.7x 107

1.9x108
1.7x108
2.9x107

6.5 x 107
6.0 x 107
9.8 x 106

5.7 x 108
5.2 x 108
8.6 x 107

 

tmb(1)

bpy(2)
deeb (3)

5.3 x 108
4.9x108
8.1x 107

1.1x108
1.0x108
1.6x107

2.0 x107
1.8 x 107
3.0x 106

6.6x108
6.1x108
1.0x108

 

tmb(1)

bpy(2)
deeb (3)

R(33)

R(31)

R(32)

kp (Re3)

 

2.9 x 108
2.7 x 108
4.4 x 107

5.5 x 107
5.0 x 107
8.2 x 106

3.8x 108
3.5x108
5.7x107

7.3x 108
6.7x108
1.1x108

 

tmb(1)

bpy(2)
deeb (3)

5.1x108
4.7x108
7.7x107

2.3 x 107
2.1x 107
3.4x106

4.2 x 108
3.9x108
6.3 x 107

9.5 x 108
8.8x108
1.4x108

 

tmb(1)

bpy(2)
deeb (3)

8.6x 108
7.9x 108
1.3 x 108

2.2 x 106
2.0x106
3.3 x105

4.6 x 108
4.2 x 108
6.9 x 107

1.3 x 109
1.2x109
2.0x108

 

86

 

 

 

 

 

 

 

 

 

 

 

 

Table 2-7c. Calculated Fbrster rate constants (8‘) for complexes
1-3 at all donor-acceptor interactions occurring at
A:F, B:F, and C:F.
Donor compound R(l 1) R(12) R(13) kp (Re1)
tmb(1) 5.1x106 4.4x 108 2.0x108 6.5 x 108
A bpy(2) 4.7 x 106 4.1 x 108 1.9 x 108 6.0 x 108
deeb(3) 7.7 x 105 6.7 x 107 3.1 x 107 9.9 x 107
tmb(1) 2.2 x 107 6.0 x 108 2.9 x 108 9.1 x 108
B bpy(2) 2.0 x 107 5.6 x 108 2.6 x 108 8.4 x 108
deeb(3) 3.3 x 106 9.1 x 107 4.3 x 107 1.4 x 108
tmb(1) 9.7x107 6.5x108 3.4x108 1.1x109
C bpy(2) 9.0x107 6.0x108 3.1x108 1.0x109
deeb(3) 1.5 x 107 9.8 x 107 5.1x107 1.6 x 108
R(22) R(21) R(23) kF (Re2)
tmb(1) 8.1x 106 4.1x108 2.1x108 6.3x 108
A bpy(2) 7.5 x 106 3.8x108 1.9x108 5.8x 108
deeb(3) 1.2 x 106 6.2 x 107 3.2 x 107 9.5 x 107
tmb(1) 2.1x107 5.7x108 3.1x108 9.0x108
B bpy(2) 2.0 x 107 5.3 x 108 2.9 x 108 8.4 x 108
deeb(3) 3.2 x 106 8.6 x 107 4.7 x 107 1.4 x 108
tmb(1) 9.9x107 6.3 x 108 3.9x108 1.1x109
C bpy(2) 9.1x107 5.8 x 108 3.6x108 1.0x109
deeb(3) 1.5 x 107 9.5 x 107 5.9 x 107 1.7 x 108
R(33) R(31) R(32) 1rF (Re3)
tmb(1) 1.4x108 1.4x108 3.7x 108 6.5 x 108
A bpy(2) 1.3 x 108 1.3 x108 3.4x108 6.0x108
deeb(3) 2.1 x 107 2.1 x 107 5.6 x 107 9.8 x 107
tmb(1) 7.7 x 107 2.6 x 108 3.8 x 108 7.2 x 108
B bpy(2) 7.1 x 107 2.4 x 108 3.6 x 108 6.7 x 108
deeb(3) 1.2x107 3.9x107 5.8x107 1.1x108
tmb(1) 4.5 x 106 5.5 x 108 2.9 x 108 8.4 x 108
C bpy(2) 4.2 x 106 5.1 x 108 2.7 x 108 7.8 x 108
deeb(3) 6.9 x 105 8.4 x 107 4.4 x 107 1.3 x 108

 

87

The results of this study allow us to construct a comprehensive picture
of the excited-state energies and dynamics for these FeRe3 systems (Figure
2—12). The lefi side of Figure 2-12 is an energy level diagram for the
relevant electronic states of the Rel-bpy’ chromophore, along with kinetic
pathways associated with the various excited-states. Initial population of the
IMLCT excited-state is followed by rapid intersystem crossing (kisc) to the
3MLCT excited-state.”98 The thermalized triplet state can then undergo
radiative (k,) and non-radiative (km) transitions to the 'Al ground-state, or
can be quenched by the Fem core via Fdrster energy transfer (kENT)- The
right side of Figure 2-12 shows the electronic structure of the F e(pyacac)3
core, which contains charge-transfer (6LMCT) and ligand-field electronic
excited-states that are thermodynamically accessible from the 3MLCT
manifold of the Re chromophore. Dipolar energy transfer results in the
formation of a 6LMCT excited state within the Fe(pyacac)3 core, followed by

111

non-radiative relaxation to the 6A1 ground-state of the Fe moiety.

88

E:
IMLCT::~~-l(.i§9~10_l3 S'_1

‘Ei:kvib ~1012 S.1
:E kEnT ~108-109 s'1_

,,.r""3MLCT

 

 
 

hv 1r,~105 s'l

  

[Xx ~ 6 -1 —I6LMCT
. km 10 s . . . .

(N on-radlatlve relaxatlon 5
via ligand-field manifold

to 6A1 ground-state)

 

Rel-bpy' F eIH(pyacac)3
chromophore core

Figure 2-12. Energy level diagram depicting the excited-state
dynamics of the FeRe3 assemblies. The rate constants for lMLCT —>
3MLCT intersystem crossing and vibrational cooling within the 3MLCT
state are based on the work of Vléek and co-workers (cf. 85), whereas
the other time constants represent approximate values for complexes 1-
6.
2.5 Geometry Optimization Calculation

As mentioned in the previous section, one of the more notable aspects
of the FeRe3 series are the rigid building blocks (i.e., stiff pyacac bridging
ligands and coordination environments) that constitute these supramolecular
complexes. These structural features allowed the crystal structure of
Complex 2 to be utilized in calculating the donor-acceptor distances (RDA)

and orientation factors (1(2) for all three analogs based on the geometries

defined in Figures 2—9, 2-10, and 2-11. The ability to directly use the single-

89

crystal X-ray data to model solution phase structures and by extension the
solution phase energy transfer dynamics is a unique property of the FeRe3
series (Table 2-6).

A potential drawback of this approach is the effect crystal packing
forces may have on the internal structure of complex 2. From the X-ray
structural data of complexes 2 and 5 (Figure 2-2), it can be seen that the
pyacac bridging ligands are slightly bent away from the expected linear
geometry. This structural variation represents a possible error in the donor-
acceptor distances (RDA) and orientation factors (1(2) calculated from the
model given in Figure 2-10. In order to investigate if this possible source of
error has affected the theoretical energy transfer rate constants, a gas phase
geometry optimization calculation of complex 2 was performed using the

- 99,1
Gauss1an 03 00

software package to determine if any differences exist
between the solution phase and confined crystal structure geometries.
Figure 2-13 shows the optimized geometry of complex 2, along with the

ill

. . 1 .
observed llnear connectlon between the Fe and Re metal centers Vla the

relaxed geometry of the pyacac bridge.

90

 

Figure 2-13. Drawing of the geometry optimized structure of
[Fe(pyacaC)3(Re(bPY)(C0)3)3l(0T03 (2)-

111"-Re1 distances and the dihedral angles

Table 2—8 compares the Fe
between the F e02 and acac planes for the three F em/ReI donor-acceptor arms
for the optimized and solid-state structures of complex 2. From Table 2—8,
the dihedral angles between the FeOz triangles and the corresponding acac
planes are close to zero in the optimized geometry and are bent away from
planarity in the solid-state, which indicates crystal packing forces are
affecting the internal geometry of the F eRe3 complexes. The bent geometry

also decreases the Fen-Re distances by 0.2 — 0.3 A compared to the

optimized geometry. The elongation of the metal—metal distances do

91

represent a source of error in the F6rster calculations that were based on the
X-ray structural data of complex 2, so an identical analysis of the donor-
acceptor distances (RDA) and orientation factors (1(2) for the optimized
geometry of complex 2 was performed. The analyses and labeling system
for the various donor and acceptor point dipoles and through-space
interactions are identical to the system used for the single-crystal X-ray data
outlined in section 2.4.3. The complete list of RDA and K2 values calculated
using the optimized geometry of complex 2 can be found in Tables 2-9a, 2-
9b, and 2-9c.

Table 2-8. Fen-Re distances and F eOz/acac dihedral angles for
the optimized and single-crystal X-ray structure data for complex 2.

 

 

Optimized single-cgstal
FeOz/acac (1) (°)a 2.70 15.60
FeOz/acac (2) (°)21 2.80 10.75
F eOz/acac (3) (°)a 3.39 23.04
Fen-Rel (A) 10.080 9.881
Fe-"Re2 (A) 10.081 9.879
F e-"Re3 (A) 10.092 9.775

 

aDihedral angle between the FeOz triangle and corresponding
acac planes.

92

Table 2-9a. RDA (A), OT (°), OD (°), (9A (°), and K2 values determined from
the optimized geometry of complex 2 using donor/acceptor
interactions A:D, B:D, and C:D.

R(l 1) nag RQ3L R(ZZJ RQI) R(23) R(33LR(31) R(32)

A
RDA
9T
(")0

8.89
81.41
73.84
174.5
0.961

11.26
33.77
68.82
52.56
0.030

11.19
52.50
80.69
55.21
0.110

8.88
87.15
74.40
172.0
0.720

11.16
35.29
76.14
55.78
0.170

1 1.27
50.65
76.00
51.18
0.032

8.89
81.50
74.05
174.4
0.938

1 1.28
35.11
69.07
51.20
0.022

11.20
52.55
80.97
54.54
0.112

 

9.28
81.41
67.12
167.7
1.661

11.73
33.77
63.81
47.77
0.003

1 1.42
52.50
75.23
60.84
0.056

9.26
87.15
67.59
164.6
1.328

1 1.49
35.29
70.74
56.59
0.074

1 1.60
50.65
70.66
51.92
0.001

9.27

81.50
67.33
167.5
1.630

11.75
35.1 1
64.08
46.33
0.008

1 1.43
52.55
75.52
60.23
0.056

 

9.82
81.41
60.42
161.6
2.417

12.32
33.77
58.52
43.87
0.089

11.81
52.50
69.30
65.87
0.031

9.796
87.15
60.84
158.4
1.984

1 1.96
35.29
65.02
57.65
0.019

12.07
50.65
64.99
52.98
0.017

9.81

81 .50
60.62
161.4
2.379

12.33
35.11
58.79
42.35
0.109

11.81
52.55
69.59
65.33
0.029

 

93

Table 2-9b. RDA (A), OT (°), OD (°), OA (°), and K2 values determined from
the optimized geometry of complex 2 using donor/acceptor
interactions AzE, B:E, and C :E.

R(ll) R(12) R(13)

9.65
81 .41
74.19
174.9
0.928

10.81
33.77
71.53
55.75
0.088

11.19
52.50
77.70
58.52
0.076

R(22) R(21)

9.63
87.15
75.01
172.7
0.671

10.75
35.29
75.83
59.13
0.193

R(23) R(33)

10.81
50.65
75.75
54.31
0.041

9.64

81 .50
74.41
174.8
0.904

RQI)

10.82
35.11
71.78
54.33
0.073

R(32)

10.78
52.55
77.96
57.82
0.076

 

10.02
81.41
68.01
168.6
1.564

1 1.23
33.77
66.18
50.64
0.004

11.07
52.50
72.10
64.26
0.043

9.99
87.15
68.74
165.8
1.220

1 1.09
35.29
70.22
59.87
0.094

1 1.15
50.65
70.18
54.99
0.003

10.01
81.50
68.24
168.4
1.531

11.24
35.1 1
66.43
49.13
0.001

1 1.07
52.55
72.36
63.64
0.042

 

10.55
81.41
61.70
162.9
2.276

11.78
33.77
60.56
46.42
0.034

11.52
52.50
66.13
69.32
0.032

10.51
87.15
62.36
159.9
1.840

1 1.57
35.29
64.33
60.83
0.034

1 1.63
50.65
64.32
55.93
0.009

10.53
81.50
61.92
162.7
2.238

11.78
35.11
60.81
44.84
0.048

11.51
52.55
66.39
68.77
0.030

 

94

Table 2-9c. RDA (A), 9r (°), (99 (°), @A (°), and K2 values determined from
the optimized geometry of complex 2 using donor/acceptor
interactions A:F, B:F, and C:F.

R(ll) R112) R(13) R(22) R(21) R(23)

10.40
81 .41
74.49
4.70
0.423

10.40
33.77
74.49
120.8
1.543

10.40
52.50
74.49
117.9
0.969

10.38
87.15
75.54
6.81
0.482

10.38
35.29
75.54
117.3
1.345

10.38
50.65
75.54
122.3
1.070

R931 M31) R(32)

10.40
81.50
74.73
4.82
0.409

10.40
35.11
74.73
122.3
1.538

10.40
52.55
74.73
118.6
0.974

 

10.77
81.41
68.78
10.58
0.843

10.77
33.77
68.78
126.2
2.170

10.77
52.50
68.78
112.1
1.035

10.73
87.15
69.73
13.22
0.926

10.73
35.29
69.73
116.6
1.644

10.73
50.65
69.73
121.7
1.393

10.75
81 .50
69.02
10.78
0.823

10.75
35.11
69.02
127.8
2.180

10.75
52.55
69.02
112.7
1.047

 

11.27
81.41
62.82
15.96
1.365

11.27
33.77
62.82
130.8
2.981

11.27
52.50
62.82
107.1
1.023

11.22
87.15
63.69
18.77
1.462

11.22
35.29
63.69
115.8
1.945

1 1.22
50.65
63.69
120.8
1.730

11.26
81.50
63.05
16.18
1.341

11.26
35.11
63.05
132.4
3.012

11.26
52.55
63.05
107.6
1.040

 

Table 2-10 lists all nine kRe(ij) (Re#:LMCT#) values and the <kT>
value calculated for the optimized geometry of complex 2 based on the
analysis outlined in section 2.4.5. The <kT> value calculated from the

single-crystal X—ray structure of 2 is also listed in Table 2-10 for

comparison, with the optimized and solid-state geometries yielding <kT>

95

values of 9.6 x 108 s'l and 8.0 x 108 s", respectively. The optimized
geometry does give a closer energy transfer rate constant to the observed
value of 1.3 x 109 s", but only by a small factor considering the point-dipole
approximations assumed in the rate calculations (RDA and K2). These results
confirm the applicability of single-crystal X-ray data to structurally well-
defined donor-acceptor complexes and particularly for the donor-acceptor
separations and orientation factors used in the energy transfer rate
calculations in the FeRe; family. Due to the structural homology of the
FeRe3 complexes, geometry optimization calculations were not performed
on complexes 1 (tmb) and 3 (deeb). The complete list of calculated rate

constants for complex 2 are given in Table 2-11.

96

Table 2-10. Calculated Forster rate constants for [Fe(pyacac)3(Re-
' (bpy)(CO)3)3](OTf)3 (2) from geometry optimization

 

 

calculations.
Interactiona complex 2b
kT (3-1)

Rel :LMCTI 6.8 x 108

RelzLMCTZ 2.1 x 108

RelzLMCT3 1.1 x 108

kRel (S-l)C 1.0 x 109
Re2:LMCT1 1.8 x 108

Re2:LMCT2 5.6 x 108

Re2:LMCT3 1.4 x 108

km,2 (s")° 8.8 x 108
Re32LMCT1 2.2 x 108

Re3zLMCT2 1.1x 108

Re3zLMCT3 6.6 x 108

kRe3 (54).: 9.9 x 108
kT (s")d 9.6 x 108
kT (s'l) (X-ray) 8.0 x 108
k,.,,(s") 1.3 x 10"

 

aDonor-acceptor through-space interaction as defined in the text
and in Figure 2-11. bRDA and K2 values derived from the
minimized gas phase structure using the geometries outlined in
Figures 2-9 and 2-10. A complete list of the calculated rate
constants for complex 2 can be found in Table 2-10. °Rate of
energy transfer calculated according to eqs 2-6a, 2-6b, and 2-6c.
dOverall rate of energy transfer given by (kRel + km + kRe3)/3.

97

Table 2-11. Calculated Forster rate constants (8") for the
optimized structure of complex 2 at all donor-
acceptor interactions.

 

 

 

 

 

 

 

 

 

 

 

Interaction R(l 1) R(12) R(13) kp (Re1)
A-D 9.0 x 108 6.8 x 106 2.6 x 107 9.3 x 108
B-D 1.2x109 6.1x105 1.2x107 1.2x109
C-D 1.3x109 1.2x107 5.3x 106 1.3x109
A-E 5.3 x108 2.6x107 2.2x107 5.8x108
B-E 7.2x108 9.1x 105 1.1x107 7.3 x 108
CE 7.7 x 108 5.9 x 106 6.4 x 106 7.8 x108
A-F 1.6x108 5.6x 108 3.6x108 1.1x 1o9
B-F 2.5x 108 6.5x108 3.1x108 1.2x109
C-F 3.1x108 6.7x108 2.3x 108 1.2x109

R(22) R(21) R(23) kF (Re2)
A-D 6.8 x 108 4.1x107 7.3 x 106 7.3 x 108
B-D 9.8x108 1.5x107 8.7x104 1.0x108
C-D 1.0x109 3.0x106 2.5 x 106 1.0x109
A-E 3.9x108 5.8x107 1.2x107 4.6x 108
B-E 5.7 x 108 2.3 x 107 6.2 x 105 5.9 x 108
GE 6.4 x 108 6.5 x 106 1.7 x 106 6.5 x 108
A-F 1.8x108 5.0x108 4.0x 108 1.1x109
B-F 2.8x108 5.0x108 4.2x108 1.2x109
C-F 3.4x108 4.5x 108 4.0x108 1.2x109

R(33) R(31) R(32) kF (Re3)
A-D 8.8 x 108 4.8 x 106 2.6x107 9.1x 1o8
B-D 1.2 x 109 1.4x106 1.2 x 107 1.2 x 109
C-D 1.2 x 109 1.4x107 5.0x106 1.2 x 109
A-E 5.2x108 2.1x 107 2.2x107 5.6x 108
B-E 7.1x108 2.5 x 105 1.1x107 7.2x 108
GB 7.6 x 108 8.3 x 106 6.0 x 106 7.7 x 108
A-F 1.5 x 108 5.7 x 108 3.6x108 1.1x109
B-F 2.5x 108 6.5x108 3.1x108 1.2x109
C-F 3.1x108 6.9x108 2.4x108 1.2 x 109

 

98

2.6 Conclusions

The synthesis, structures, and photophysical properties of a series of
donor-acceptor complexes based on Rel-bipyridine donors and Fem-acac
acceptors have been described. Steady-state and time-resolved emission
spectroscopies indicated that the strongly emissive Rel-based 3MLCT
excited-state was significantly quenched when compared to model

1" center had been replaced by All". The

complexes in which the Fe
favorable overlap between the donor emission and acceptor absorption
profiles coupled with a ca. 10 A donor—acceptor separation, unfavorable
driving forces for electron transfer, and the absence of features characteristic
of charge separation in the transient absorption spectra allowed for an
assignment of F firster (dipolar) energy transfer as the dominant excited-state
reaction mechanism. The well-defined structural aspects of this system
permitted a quantitative geometric analysis of the dipole-dipole coupling
giving rise to the observed dynamics. The calculated energy transfer rate
constants differed from the experimental values by less than a factor of
three, a level of agreement that is significantly better than what is typically

encountered. In addition to providing quantitative support for F 6rster

transfer in this system, this study also demonstrates the degree of accuracy

99

that can be achieved if the metric details concerning dipole-dipole

interactions can be explicitly described.

100

2.7 References and Notes

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(100) Geometry optimization was performed with the UB3LYP functional
and LANLZDZ basis set.

108

Chapter 3. Orbital-Specific Energy Transfer: CuRez Complexes

3.1 Introduction

The development of molecular assemblies that can serve as photo-
active components of optical systemsl'9 requires exquisite control of their
photophysical properties. This level of control, in turn, can only be achieved
if the nature of the ground and excited states of the constituents can be
elucidated.”12 Polypyridyl complexes of Re], Ru", Os", and IrIII have
received considerable attention in this regard due to their synthetic
accessibility as well as the structure-function correlations that have emerged
from several decades of research on their photo-induced properties.”"24 As a
result, compounds within this class have been utilized in a variety of settings

ranging from fundamental studies of electron donor/acceptor interactions”32

33-36

to artificial photosynthetic systems and, most recently, as catalysts for

water splitting reactions.37'40

Recent work from our own group focused on energy transfer
dynamics in a tetranuclear assembly consisting of three ReI polypyridyl
donors surrounding an Fem(acac)3 moiety.“ Emission from the Rel-based

Ill

3MLCT excited state was efficiently quenched by the Fe core; dipolar

coupling between the ReI fluorophore and absorptions associated with the

109

F eIII center was quantitatively established through a detailed metrical
analysis of the relative orientations of the donor/acceptor transition dipoles.
Energy transfer rate constants on the order of 109 s'] were observed in this
system, driven in part by the charge-transfer nature of the optical transitions
involved. It occurred to us that this basic compositional motif could provide
a convenient platform for examining additional fundamental aspects of
dipolar energy transfer. The notion of coupling to weakly absorbing
acceptors was of particular interest in light of other work we have carried out
on energy transfer involving spin-coupled di-iron(III) complexes.”43
Ligand-field states are intriguing candidates in this regard, due to their low
oscillator strengths (8 ~ 10 — 100 M'lcm") while at the same time
representing nearly ideal manifestations of the point-dipole approximation
central to Forster theory.

Low energy ligand-field states of first-row transition metal complexes
have been implicated as energy acceptors in a number of studies involving
multinuclear assemblies.“58 Recently, Ford and coworkers have shown that
ligand-field states of Cr111 in trans-Cr(cyclam)X2+ (X = C1" or ONO')
complexes quench the photoluminescence of CdSe/ZnS core/shell quantum

dots (QDs) via a F 6rster energy transfer mechanism.”60 The investigation

showed that highly absorbing QDs act as antennae which in turn

110

photosensitize the weakly absorbing Crm-based 4A -—> 4T ligand-field states
of an attached Cr(cyclam)(ONO)2+ complex to trigger the release of NO;
potential applications of this water soluble system include the delivery of
bioactive agents to specific physiological targets. Other recent examples of
metal-centered acceptor states come from the work of Ward and coworkers,
who have demonstrated significant quenching of MLCT type emission by f-f
acceptor states in Ln(III)-containing donor/acceptor systems.“65 The
shielding of the 4f electron shell of the Ln(III) ions by the outer core 5s and
5d electrons allow lanthanide coordination complexes to retain atomic-like
absorption and emission profiles,66 resulting in line-like spectra that allow
for straightforward assignments of acceptor states that energetically overlap
with the 3MLCT emission profiles. These studies constitute excellent
examples of metal-centered transitions involved in excited-state energy
transfer, albeit with some ambiguity as to the mechanism of energy transfer
operative in these systems. The clear-cut assignment of a dipolar
mechanism in the FeRe3 series mentioned above led us to design structurally
analogous complexes that contain ligand-field states possessing favorable
spectral overlap with the Rel-based 3MLCT emission spectra. The current
study thus replaces the 6LMCT acceptor states of the FeRe3 series“ with

ligand-field (d-d) states in an effort to quantitatively explore energy transfer

111

using an acceptor that represents a nearly ideal manifestation of the point-
dipole approximation central to Féirster theory.

The synthesis, structure, and photophysical properties of a new family
of trinuclear CuRez chromophore-quencher complexes having the general
form [Cu(pyacac)2(Re(bpy’)(CO)3)2](OT1)2 (where pyacac = 3-(4-pyridyl)-
acetylacetonate and bpy’ = 4,4’-5,5’-tetramethyl-2,2’-bipyridine (tmb, 1),
4,4’-dimethyl-2,2’-bipyridine (dmb, 2), 2,2’-bipyridine (bpy, 3), 4,4’-
dichloro-2,2’-bipyridine (dclb, 4), and 4,4’-diethylester—2,2’-bipyridine
(deeb, 5), and OTf = CF3SO3') are reported. The CuII metal centers are
covalently attached to two fac-Re(bpy’)(CO)3 (bpy’ = tmb, dmb, bpy, dclb,
and deeb) moieties through two pyridyl-acetylacetonate bridging ligands
(Figure 3-1). Variations in the substituents on the bipyridyl groups allowed
for systematic energetic tuning of the emission profile of the Rel-based
3MLCT state relative to the absorption profile of the CuII center. Emission
from the ReI fluorophores in complexes 1, 2, 3, 4, and 5 was found to be
significantly quenched in the presence of the CuII metal center relative to
structurally analogous BeRez analogs. The BeRez model systems (Figure 3-
1) with the general formula [Be(pyacac)2(Re(tmb)(CO)3)2](OTf)2 (6),

[Be(PyacaC)2(Re(dmb)(C0)3)2](OTfiz (7), [136(PyacaC)2(Re(bPY)(C0)3)2]-

(0102 (8), [136(Pyaca0)2(Re(d01b)(CO)3)2](0T02 (9), and [36(PyacaC)2(Re-

112

(deeb)(CO)3)2](OTf)2 (10) were synthesized in order to investigate the
excited-state relaxation kinetics of the Re(bpy’)(CO)3 moieties in the
absence of the emission quenching dynamics incurred by the CuII metal
center. The confluence of data acquired on the CuRez systems (donor-
acceptor separation, redox properties, and spectral overlap analyses)
unequivocally establishes dipolar energy transfer as the dominant quenching
mechanism. Time-dependent density functional theory (TD-DFT)
calculations on the Cu11 core - Cu(phacac)2 (phacac = 3-pheny1-
acetylacetonate) - in a dichloromethane solvent bath revealed the orbital
composition and transition energy of the Cu11 ligand-field acceptor states.
The unique electronic properties of the Cu11 acceptor coupled with the
computational work have allowed identification of an orbitally-specific
pathway for energy transfer, a result that illustrates the role of relative

transition dipole orientation operating at the atomic-orbital level.

113

" ‘0 [1,, m“ 0' ‘ — ’4’" /
y; ‘ H-co
or? co

oc co
c‘ .2
-Re—
cl 8
—N N
R2 \/
R1 R,
R1 R1721"
oc go a; N—
oc— —12e— —N \ <"'O\Be-““‘°"‘> _N—1Re/—co
Ia, — “'O/ ‘0" \ ’ s'\
—N /N_ 00 co

R1 R1

1 (Cu) and 6 (Be): R, = R2 = CH3 2 (Cu) and 7 (Be): R, = CH3, R2 = H
3 (Cu) and 8 (Be): R, = R2 = H 4 (Cu) and 9 (Be): R, = Cl, R2 = H
5 (Cu) and 10 (Be): R, = COzEt, R2 = H

Figure 3-1. Structures of the CuRez and BeRez multinuclear
assemblies.

114

3.2 Experimental Section
3.2.1 Synthesis and Characterization

General. All solvents used were purified and dried according to
previously reported methods.67 Spectroscopic grade CHzClz was used for all
photophysical measurements and was dried under CaHz reflux until no water
was detected by 1H NMR. Solvents for both steady-state and time-resolved
emission measurements were thoroughly degassed using freeze-pump-thaw
techniques. 3-(4-pyridyl)-2,4-pentanedione,68 Cu(pyacac)2,69 Cu(phacac)2,7O
Be(pyacac)2,7' Re(tmb)(CO)3(OTf),72 Re(dmb)(CO)3(OTf),72 Re(bpy)(CO)3-
(orr),72 Re(dclb)(CO)3(OTf),72 Re(deeb)(CO)3(OTf),72 and fac-
[Re(bpy)(CO)3(4-Etpy)](PF6)73 (4-Etpy = 4-ethylpyridine) were prepared
according to literature procedures. 3-phenyl-2,4-pentanedione was
purchased fiom TCI America. Elemental analyses and F T-IR data were
obtained through the analytical facilities at Michigan State University; mass
spectra were obtained through the analytical facilities at The University of
South Carolina.

[Cu(pyacac)2(Re(tmb)(CO)3)2](0T1); (1). An amount of 99 mg
(0.24 mmol) of Cu(pyacac)2 was dissolved in 40 mL of hot THF, after which
300 mg (0.475 mmol) of Re(tmb)(CO)3(OTf) was added and the solution

was purged with argon for 20 min. The reaction mixture was then fit with a

115

condenser and stirred under argon for 24 hours in hot THF in the dark, after
which time a blue-green solid formed. The precipitate was collected,
washed with dry THF, and recrystallized from acetonitrile/ether. Yield: 129
mg (32%). Anal Calcd for C56H52N6F6OI6SZCuRe2: C, 40.06; H, 3.12; N,
5.00. Found: C, 40.19; H, 3.30; N, 4.79. IR (KBr, cm'l): 2030 s, 1925 s,
1902 3,1614 m, 1575 s, 1417 m, 1261m, 1157 m, 1032 s, 639 m. MS:
[ESI+, m/z (rel. int.)]: 690.5 (62) {[Cu(pyacac)2(Re(tmb)(CO)3)2]}2+, 1530.1
(2) {[Cu<pyacac)2(Re<tmb)(CO)3)2](on)}“i
[Cu(pyacac)2(Re(dmb)(CO)3)2](OTf); (2). An amount of 77 mg
(0.19 mmol) of Cu(pyacac)2 was dissolved in 40 mL of hot THF, after which
219 mg (0.369 mmol) of Re(dmb)(CO)3(OTf) was added and the solution
was purged with argon for 20 min. The reaction mixture was then fit with a
condenser and stirred under argon in hot THF for 24 hours in the dark, after
which a light green solid formed. The mixture was allowed to cool to room
temperature and hexanes were slowly added to precipitate additional green
solid. The solid was collected, washed with dry THF, and recrystallized
from acetonitrile/ether. Yield: 132 mg (44%). Anal Calcd for
C521-I44N6F6016SzCuRe2: C, 38.48; H, 2.73; N, 5.18. Found: C, 38.22; H,
2.75; N, 5.27. IR (KBr, cm'l): 2030 s, 1927 s, 1915 s, 1619 m, 1573 s, 1419

m, 1259 m, 1159 m, 1031 m, 638 m. MS: [ESI+, m/z (rel. int.)]: 662.4 (60)

116

{[Cu(pyacac)2(Re(dmbXCO),)2])2”: 1474(4) {[Cu(pyacac)2(Re(dmb)-
(COthOTofli

[Cu(pyacac)2(Re(bpy)(CO)3)2](OTf); (3). An amount of 80 mg
(0.19 mmol) of Cu(pyacac)2 was dissolved in 40 mL of hot THF, after which
220 mg (0.382 mmol) of Re(bpy)(CO)3(OTf) was added and the solution
was purged with argon for 20 min. The reaction mixture was then fit with a
condenser and stirred under argon in hot THF for 24 hours in the dark, after
which time a light green solid formed. The precipitate was collected,
washed with dry THF, and recrystallized from acetonitrile/ether. X-ray
quality crystals were obtained from slow diffusion of ether into an
acetonitrile solution of the compound. Yield: 255 mg (65%). Anal Calcd
for C43H36N6F6OI6SZCuRe2: C, 36.79; H, 2.32; N, 5.36. Found: C, 36.94; H,
2.43; N, 5.25. IR (KBr, cm'l): 2033 s, 1913 s, 1604 w, 1575 s, 1419 m, 1259
m, 1160 m, 1031 m, 772 m, 638 m. MS: [ESI+, m/z (rel. int.)]: 634.4 (100)
{[Cu(pyacac)z(Re(bpy)(C0)3)2])2“: 1417.8 (5) {[Cu(pyacac)2(Re(bpy)-
(coxhkOTorfi

[Cu(pyacac)2(Re(dclb)(CO)3)2](OTf); (4). An amount of 73 mg
(0.18 mmol) of Cu(pyacac)2 was dissolved in 40 mL of hot THF, after which
227 mg (0.352 mmol) of Re(dclb)(CO)3(OTf) was added and the solution

was purged with argon for 20 min. The reaction mixture was then fit with a

117

condenser and stirred under argon in hot THF for 24 hours in the dark, after
which a dark green solid formed. The mixture was allowed to cool to room
temperature and hexanes were slowly added to precipitate additional dark
green solid. The precipitate was collected, washed with dry THF, and
recrystallized from acetonitrile/ether. Yield: 195 mg (65%). Anal Calcd for
C43H32N6C14F6016S2CuRe2: C, 33.82; H, 1.89; N, 4.93. Found: C, 33.69; H,
1.99; N, 4.61. IR (KBr, cm"'): 2033 s, 1933 s, 1574 s, 1468 m, 1257 m,
1156 m, 1031 m, 840 w, 638 m. MS: [ESI+, m/z (rel. int.)]: 703.3 (30)
{[Cu(pyacac)2(Re(dc1b)(CO)3)2]}2+, 1555.6 (1) {[Cu(pyacac)2(Re(dclb)-
(corxkOani

[Cu(pyacac)2(Re(deeb)(CO)3)2](OTf); (5). An amount of 74 mg
(0.18 mmol) of Cu(pyacac)2 was dissolved in 40 mL of hot THF, after which
255 mg (0.354 mmol) of Re(deeb)(CO)3(OTf) was added and the solution
was purged with argon for 20 min. The reaction mixture was then fit with a
condenser and stirred under argon in hot THF for 24 hours in the dark, after
which time a light green solid formed. The precipitate was collected,
washed with dry THF, and recrystallized usng CHzClz/hexanes. Yield: 200
mg (61%). Anal Calcd for C60H52N6F6024S2CuRe2°CH2C12: C, 38.15; H,
2.79; N, 4.30. Found: C, 37.90; H, 2.62; N, 4.66. IR (KBr, cm'l): 2035 s,

1926 s,1732 m, 1573 m, 1408 m, 1323 m, 1263 s, 1154 m, 1031 m, 767 m,

118

638 m. MS: [ESI+, m/z (rel. int.)]: 778.5 (100) {[Cu(pyacac)2(Re(deeb)-
((30).)2112‘: 1706.1 (3) {1Cu<pyacac)2<Re(deeb)(CO)3>21<0To}“1

[Be(pyacac)2(Re(tmb)(CO)3)2](OTf); (6). An amount of 67 mg
(0.19 mmol) of Be(pyacac)2 was dissolved in 40 mL of hot THF, after which
233 mg (0.369 mmol) of Re(tmb)(CO)3(OTf) was added and the solution
was purged with argon for 20 min. The reaction mixture was then fit with a
condenser and stirred under argon in hot THF for 24 hours in the dark. The
volume of the mixture was reduced to ~10 mL, and hexanes were slowly
added to precipitate a yellow solid. The yellow solid was collected, washed
with dry THF, and recrystallized from acetonitrile/ether. X-ray quality
crystals were obtained from slow diffusion of ether into an acetonitrile
solution of the compound. Yield: 135 mg (45%). Anal Calcd for
C56HszN6F6OI6S2BeRe2: C, 41.40; H, 3.23; N, 5.17. Found: C, 41.53; H,
3.44; N, 5.00. IR (KBr, cm“): 2031 s, 1916 3,1613 m, 1575 m, 1417 m,
1261 m, 1155 m, 1031 m, 847 m, 638 m. MS: [ESI+, m/z (rel. int.)]: 663.2
(86) {[Be(pyacac)2(Re(tmb)(CO)3)2]}2+, 1475.5 (2) {[Be(pyacac)2(Re(tmb)-
(C0)3)2](OTf)} ”-

[Be(pyacac)2(Re(dmb)(CO)3)2](OTf); (7). An amount of 69 mg
(0.19 mmol) of Be(pyacac)2 was dissolved in 40 mL of hot THF, after which

227 mg (0.3 82 mmol) of Re(dmb)(CO)3(OTt) was added and the solution

119

was purged with argon for 20 min. The reaction mixture was then fit with a
condenser and stirred under argon in hot THF for 24 hours in the dark. The
volume of the mixture was reduced to ~10 mL, and hexanes were slowly
added to precipitate a yellow solid. The yellow solid was collected, washed
with dry THF, and recrystallized from acetonitrile/ether. Yield: 123 mg
(41%). Anal Calcd for C52H44N6F6016S2BeRe2: C, 39.82; H, 2.83; N, 5.36.
Found: C, 40.00; H, 2.94; N, 5.47. IR (KBr, cm'l): 2031 s, 1915 s, 1620 m,
1576 s, 1418 m, 1260 m, 1159 m, 1031 s, 847 m, 638 m. MS: [ESI+, m/z
(rel. int.)]: 635.2 (100) {[Be(pyacac)2(Re(dmb)(CO)3)2]}2+, 1419.4 (2)
{[Be(pyacac)2(Re(dmb)(CO)3)2](0T1)1“i

[Be(pyacac)2(Re(bpy)(CO)3)2](OTf); (8). An amount of 72 mg (0.20
mmol) of Be(pyacac)2 was dissolved in 40 mL of hot THF, after which 228
mg (0.396 mmol) of Re(bpy)(CO)3(OTf) was added and the solution was
purged with argon for 20 min. The reaction mixture was then fit with a
condenser and stirred under argon in hot THF for 24 hours in the dark, after
which time a crystalline yellow solid formed in the yellow reaction solution.
The mixture was allowed to cool to room temperature and the yellow
precipitate and yellow solution were separated by filtration. The precipitate
was collected, washed with dry THF, and recrystallized from

acetonitrile/ether. ESI-MS and single crystal X-ray analysis of the yellow

120

crystalline material revealed the formation of a Be3Re3 hexamer that
contains a previously reported 36303 core”76 that will be discussed in
section 3.3.1. The volume of the yellow reaction solution was reduced to
~10 mL, and hexanes were added to produce additional yellow solid. The
solid was collected, washed with dry THF, and recrystallized from
acetonitrile/ether. Yield: 31 mg (10%). Anal Calcd for C43H36N6F6016S2-
BeRez: C, 38.12; H, 2.40; N, 5.56. Found: C, 38.25; H, 2.61; N, 5.39. IR
(KBr, cm'l): 2033 s,1919 s, 1604 m, 1576 s, 1419m, 1261 m, 1161 m, 1031
m, 849 m, 771 m, 636 m. MS: [ESI+, m/z (rel. int.)]: 607.1 (83)
{1Be(pyacac)2(Re(bpy)(C0)3>2112+

[Be(pyacac)2(Re(dclb)(CO)3)2](OTf); (9). An amount of 66 mg
(0.18 mmol) of Be(pyacac)2 was dissolved in 40 mL of hot THF, after which
234 mg (0.363 mmol) of Re(dclb)(CO)3(OTf) was added and the solution
was purged with argon for 20 min. The reaction mixture was then fit with a
condenser and stirred under argon in hot THF for 24 hours in the dark, after
which time a dark yellow solid formed. The mixture was allowed to cool to
room temperature and hexanes were slowly added to precipitate additional
solid. The dark yellow solid was collected, washed with dry THF, and
recrystallized from acetonitrile/ ether. Yield: 199 mg (66%). Anal Calcd for

C43H32N6C14F6OI6S2BeRe2: C, 34.94; H, 1.95; N, 5.09. Found: C, 35.06; H,

121

2.22; N, 4.81. IR (KBr, cm'l): 2035 s, 1919 s, 1575 s, 1467 m, 1258 s, 1158
m, 1030 m, 846 m, 755 w, 638 111. MS: [ESI+, m/z (rel. int.)]: 676.0 (100)
{[Be(pyacac)2(Re(dc1b)(CO)3)2]}2+, 1501.1 (1) {[Be(pyacac)2(Re(dclb)-
(C0)3)2](OTf)} ”-

[Be(pyacac)2(Re(deeb)(CO)3)2](OTf); (10). An amount of 60 mg
(0.17 mmol) of Be(pyacac)2 was dissolved in 40 mL of hot THF, after which
240 mg (0.334 mmol) of Re(deeb)(CO)3(OTi) was added and the solution
was purged with argon for 20 min. The reaction mixture was then fit with a
condenser and heated under argon for 24 hours in darkness. The volume of
the mixture was reduced to ~10 mL, and hexanes were slowly added to
precipitate an orange solid. The orange solid was collected, washed with dry
THF, and recrystallized using acetonitrile/ether. Yield: 136 mg (45%).
Anal Calcd for C60H52N6F6OZ4S2BeRe2: C, 40.02; H, 2.91; N, 4.67. Found:
C, 39.63; H, 2.92; N, 4.45. IR (KBr, cm-l): 2035 s, 1925 s, 1732 m, 1576
m, 1464 m, 1323 m, 1263 s, 1155 m, 1031 m, 849 m, 767 m, 638 m. MS:
[ESI+, m/z (rel. int.)]: 751.3 (79) {[Be(pyacac)2(Re(deeb)(CO)3)2]}2+, 1651.6
(2) {[BetpyacaoxRemeebxc013>21(0T01‘fi
3.2.2 Physical Measurements

X-ray Structure Determinations. Single-crystal X-ray diffraction

data for complexes 3 and 6 were acquired at the X-ray facility of Michigan

122

State University. Diffraction data were collected on a Siemens SMART
diffractometer with graphite-monochromatic Mo K01 radiation (A. =
0.71073A). Data were collected at -100 °C by using an Oxford Cryosystems
low temperature device. Crystallographic data are summarized in Table 1;
selected bond distances and angles are listed in Table 2. Lattice parameters
were obtained fi'om least-squares analyses and data were integrated with the
program SAINT.77 The integration method employed a three dimensional
profiling algorithm and all data were corrected for Lorentz and polarization
factors, as well as for crystal decay effects. The absorption correction
program SADABS78 was employed to correct the data for absorption effects.
The structures were solved by direct methods and expanded using Fourier
techniques. All structure calculations were performed with the SHELXTL
6.12 software package.79 Anisotropic thermal parameters were refined for all
non-hydrogen atoms. Hydrogen atoms were localized in their calculation
positions and refined by using the riding model. Further details concerning
the structure determinations may be found in Supporting Information.

Cyclic Voltammetry. Electrochemical measurements were carried
out in a Nz-filled drybox (Vacuum Atmospheres) using a BAS CV-SOW
electrochemical analyzer. A standard three-electrode arrangement was

utilized consisting of a Pt working electrode, graphite counter electrode, and

123

 

a Ag/AgNO3 reference electrode. Measurements were carried out in CH3CN
solution that was 0.1 M in NBu4PF6. Potentials are reported versus the
ferrocene/ferrocenium couple, which was used as an internal standard.
Electronic Absorption and Steady-State Emission Spectroscopies.
Extinction coefficients for all compounds were acquired in room-
temperature CHzClz solution using a Varian Cary 50 UV-Visible
spectrophotometer. Steady-state emission spectra were acquired using a
Spex Fluoromax fluorimeter and corrected for instrumental response using a
NIST standard of spectral irradiance (Optronic Laboratories, Inc., OL220M
tungsten quartz lamp).80 Spectra were acquired on samples dissolved in
thoroughly degassed CHzClz under optically dilute conditions (o.d. ~ 0.1)
and sealed under an argon atmosphere in 1 cm path length quartz cuvettes.
Radiative quantum yields ((1),) were determined relative to fac-
[Re(bpy)(CO)3(4-Etpy)](PF6) ((1)r = 0.18 in CH2C12).73 Quantum yields were

calculated according to equation 5-1,

_ (I) (lunk /Aunk) (flunk )2

unk — s d
t (Istd /Astd) ”std

 

(5-1)

where (Dunk and (1),“, are the radiative quantum yields of the sample and the
standard, respectively, lunk and 1,,d represent the areas of the corrected

emission profiles for the sample and the standard, Am and A5“, are the

124

absorbance values of the sample and the standard at the excitation
wavelength, and nlmk and nsrd correspond to the indices of refi'action of the
sample and standard solutions (taken to be equal to the neat solvents).
Excitation wavelengths were 355 nm for the tmb, dmb, and bpy analogues,
375 nm for the dclb analogues, and 400 nm for the deeb analogues. The
corrected excitation spectrum of fac-[Re(bpy)(CO)3(4-Etpy)](PF6) in CH2C12
compared favorably with the compound’s absorption spectrum over the
range of wavelengths examined (355 - 400 nm), implying that the radiative
quantum yield for fac-[Re(bpy)(CO)3(4-Etpy)](PF6) does not vary
significantly over this spectral window; the reported value of (I), = 0.18 was
therefore used for determining the radiative quantum yields at 71,, = 355 nm,
375 nm, and 400 nm. Values for the zero-point energy gap (E00) of the Re]-
based 3MLCT excited states were determined by fitting emission profiles
based on the approach described by Claude and Meyer.81 Wavelength data
were converted to energy units employing the correction of Parker and
Rees;82 the best fit was determined by visual inspection of the results of a
least-squares minimization routine.

Time-Resolved Emission Spectroscopy. Nanosecond time-resolved
emission data for the BeRez model complexes 6-10 were collected using a

NszAG-based laser spectrometer that has been described previously.80

125

Data were acquired at room temperature in thoroughly degassed CHzClz
solutions having absorbances of ~0.1 at the excitation wavelengths.
Samples were sealed under an argon atmosphere in 1 cm path length quartz
cuvettes. The decay traces correspond to an average of 250 shots of the
signal probed at the emission maximum of each compound.

Picosecond time-resolved emission data for the CuRez complexes 1-5
were collected using a time-correlated single photon counting (TCSPC)
apparatus that has been described previously.83 Data were acquired in
thoroughly degassed CH2C12 solutions having absorbances of ~0.1 at the
excitation wavelength. Samples were sealed under an argon atmosphere in 1
cm path length quartz cuvettes. Each reported decay trace corresponds to a
signal average of six data sets, with each data set resulting from ca. 30
minutes of data acquisition time. The decay traces for all five complexes
manifest a small baseline offset within the ca. nanosecond data acquisition
window due to the presence of a small amount of an emissive impurity
identified as fac-[Re(bpy’)(CO)3(pyacac)] based on their similarities to the
long-lived kinetics observed for the corresponding BeRe2 model complexes.
Decay traces for all five Cu-containing complexes were therefore fit with bi-

exponential kinetic models by fixing the long-lived component to the

126

lifetime of the corresponding BeRez complex. Data were fit using the
OriginPro 7.5 software package.84

DFT Calculations. The Gaussian 0385 software package was used for
all calculations. The nuclear coordinates used for Cu(phacac)2 in the
calculations were based on the single-crystal X-ray structure reported by
Carmichael et al.70 The geometry optimization was performed with the
UB3LYP functional and LANLZDZ basis set. The effect of CH2C12 solvent
on the optimization was modeled using the polarizable continuum model
(PCM);86'89 no symmetry restrictions were imposed. Time-dependent
density functional theory (TD-DF T) calculations were performed with the
UB3LYP functional and the 6-311G** basis set, also incorporating CHzClz
using the PCM model. All calculations used tight convergence criteria,90
and assumed a molecular charge of 0 and a ground-state spin of S = 1/2. The

molecular orbitals were generated using GaussView.”

3.3 Results and Discussion

3.3.1 Synthesis and Structural Characterization. Our interest in
developing these systems was to investigate dipole-dipole energy transfer
processes in structurally well-defined assemblies that possess weakly

absorbing acceptors. The choice of using ReI and CuII was based on the well

127

known MLCT-based reactivity of ReI and the low oscillator strength
absorptions associated with CuII ligand-field states. The utilization of the
M(pyacac)2 core (M = CuII and Be") as a ligand for Re(bpy’)(CO)3(OTf)
allowed the pyridyl group to displace the weakly coordinating triflate anion
and generate the di-cationic CuRez and BeRez complexes. The formation of
the binuclear assembly was facilitated by the low steric crowding afforded
by the 180° separation of the two pyacac ligands, along with the propensity
of both metal ions to form 4-coordinate complexes.

The ESI-MS data for complexes 1-10 in acetonitrile solution are
consistent with the formation of the CuRez and BeRez assemblies. For
example, complex 3 shows peaks corresponding to [Cu(pyacac)2(Re(bpy)-
(CO)3)2]2+ and [Cu(pyacac)2(Re(bpy)(CO)3)2](OTf)1+. X-ray quality crystals
for complexes 3 and 6 were generated by diffusion of ether into an
acetonitrile solution of the complexes over the course of approximately one
week; this in turn provides additional evidence for the general robustness of
complexes 1-10 in solution.

As mentioned in the synthetic procedure of [Be(pyacac)2(Re(bpy)-
(CO)3)2](OTf)2 (8) (section 3.2.1), a crystalline byproduct consisting of a
Be3Re3 hexamer was formed during the course of the reaction. The Be3Re3

complex was confirmed by elemental analysis, ESI-MS, and single-crystal

128

X-ray structural analysis. Yield: 39 mg (15%). Anal Calcd for C72H57N9F9-
OZ7S3Be3Re3: C, 37.06; H, 2.46; N, 5.40. Found: C, 36.67; H, 2.79; N, 5.21.
MS: [ESI+, m/z (rel. int.)]: 628.6 (100) [(Be3(OH)3(pyacac)3)((Re(bpy)-
(C0)3)3)l3+, 1017-5 (20) [(Bea(0H)3(PyacaC)3)((Re(bpy)(C0)3)3)](0102+.
The structure of the complex is given in Figure 3-2, and shows the core of
the complex consisting of three Bell ions bridged by three hydroxo groups
that form the Be3O3 core of the molecule, along with the Be3O3 core
connected through three pyacac ligands to three Re(bpy)(CO)3 moieties that
contain pseudo-C3,, symmetry commonly observed in ReN3C3 coordination
environments. Synthesis procedures and single-crystal X-ray structural data
for derivatives of the Be3O3 core - Be3(OH)3L3 (L = bis-chelate complex) -
complexes have been reported in the literature.74 The geometry of the Be3O3
core and the Be—O bond distances of [(Be3(OH)3(pyacac)3)((Re(bpy)-
(CO)3)3)](OTf)3 are very similar to these reported systems. The first steps in
the synthesis of the Be(pyacac)2 starting material needed to synthesize
complex 8 is to make an aqueous solution of pyacac and pyridine and then
add Be(SO4)2 to the alkaline mixture, which could possibly generate both
Be(pyacac)2 and the Be3(OH)3(pyacac)3 complex needed to form the
[Be3Re3](OTf)3 system. ' Direct probe MS measurements on solid

Be(pyacac)2 and in THF solution exhibited only m/z = 361.4 expected for

129

Be(pyacac)2, and not m/z = 606.6 expected for the Be3(OH)3(pyacac)3
complex, which rules out the possibility of Be3(OH)3(pyacac)3 being formed
during the synthesis of Be(pyacac)2. In addition, the formation of the
crystalline Be3Re3 hexamer was only observed during the synthesis of
complex 8 and not for the four other BeRe2 compounds (6, 7, 9, and 10).
This behavior suggests the most likely reason for the formation of the
[(Be3(OH)3(pyacac)3)((Re(bpy)(CO)3)3)](OTf)3 cystalline material is due to
the THF solubility differences between this complex and the four other
possible Be3Re3 derivative comprised of tmb, dmb, dclb, or deeb. The four
other Be3Re3 complexes were not observed in the ESI-MS analysis, which
suggests that either they were never formed in the reaction solutions of
complexes 6, 7, 9, and 10 or were removed during the purification

(recrystallization) processes.

130

 

Figure 3-2. Drawing of the cation [(Be3(OH)3(pyacac)3)((Re(bpy)-
(CO)3)3)](OT1)3 obtained from single-crystal X-ray structure
determinations.

3.3.2 Single-Crystal X-ray Structures. X-ray quality crystals of
[CU(Pya080)2(Re(bPY)(C0)s)2](0T02 (3) and [Be(pyacaC)2(Re(tmb)(C0)3)2]-
(OTDZ (6) were obtained following diffusion of ether into acetonitrile
solutions of the complexes. The complexes both crystallize in the
monoclinic space group P2(1)/c. Crystallographic details are given in Table
3—1 with selected bond distances and angles for the two complexes listed in

Table 3-2. The coordination environment about the central metal ion in

complex 3 (Figure 3-3, Top) is formed from the four oxygen atoms of the

131

acac ligands. The Cu—O bond distances of ca. 1.917 :t 0.002 A and a 0.0 A
deviation of the Cu" ion from the O4 mean-plane are both consistent with
pseudo-D2,, square-planar CuII and compare favorably with other structurally
characterized examples of Cull—acac systems.69’70’92’93 The structure of
complex 3 also shows Cu-—-O distances of 2.702 and 2.545 A corresponding
to axial interactions of the CF3SO3' counter ions with the Cu11 core. The
axial Cu---O distances are notably longer than typically observed for
tetratragonally distorted CuO6 complexes such as [Cu(HzO)6]2+,94'96
suggesting that the two Cu—--O associations are at best weakly covalent
metal-ligand interactions; spectroscopic evidence indicates that these
interactions are not retained in solution (Vida infra). As expected,
significantly shorter metal-oxygen bonds (ca. 0.3 A) are observed for the
tetrahedral coordination environment of complex 6 (Figure 3-3, Bottom) due
to the smaller ionic radius of Be".71 The internal geometries of the
Re(bpy)(CO)3 and Re(tmb)(CO)3 moieties in complexes 3 and 6 exhibit the
pseudo-C3,, coordination environment common to Re1 complexes in this
Classm"00 and are also insensitive to changes in the substituents of the
p01ypyridyl ligand. Differences are noted in the Rel-«MII distances, with

cOrnplex 6 being uniformly shorter by ~0.3 A, which is again due to the

SII‘laller BeII ion and the concomitant decrease in metal-oxygen bond lengths

132

relative to complex 3. Despite these minor differences there is considerable
structural homology between the two complexes, underscoring the

appropriateness of using BeII as a structurally and electronically benign

replacement for Cu11 in these systems.

133

Table 3-1. Crystallographic Data for [Cu(pyacac)2(Re(bpy)(CO)3)2](OTf)2
(3) and [Bemyacac)2(Re(tmb)(C0)3)2l(0T1)2 (6)-

 

 

3 6
formula C43H36N6F6016SZCUR62 C 56H52N6F 601682BCR€2
MW 1566.9 1624.6
cryst syst Monoclinic Monoclinic
space group P2(1)/c P2(1)/c
T/K 173 (2) 173 (2)

a/A 10.3213(12) l8.7639(3)
b/A 24.532(3) 16.9382(3)
c/A 12.1608(14) 21.0542(4)
a/° 9O 90

B/° 104.027(2) 106.241(1)
'y/° 90 90

V/A3 2987.3(6) 6424.5(2)
Z 2 4

Dc/g cm" 1.833 1.696
20max 50 136.4
reflns measured 291 10 72385
independent reflns 5253 1 1601
observed reflns 4827 10856
M(Mo Koo/cm" 4.556 8.63

Rim 0.026 0.041

R1 3 0.019 0.048
wR2b 0.0443 0.132
GDP 1 .005 1 .050

 

 

aR1 = ZIIFOI- |F,||/2|F,|. wa2 = [Zw(F02 — F.2)2/2w(F,2)2]“2, w =
14620202) + (21>)2 + bP], where P = [F02 + 2F,2]/3.

134

Table 3-2. Selected Bond Distances (A) and Angles (deg) for [Cu(pyacac)2-
(Re(bpy)(C0)3)2l(OTf)2 (3) and [B6(pyacaC)2(Re(tmb)(C0)3)2l-

 

 

(OTflz (6)-
3 6
Bond Distances (A)
Cu(1)—0(1) 1.920(2) Be(1)—O(1) 1.618(9)
Cu(1)—0(2) 1.914(2) Be(1)—O(2) 1.590(9)
Cu(1)—O(1A) 1.920(2) Be(1)—O(6) 1.601(9)
Cu(1)—O(2A) 1.914(2) Be(1)—O(7) 1.610(10)
Re(1)-—N(1) 2.168(2) Re(1)—N(1) 2.220(5)
Re(1)—N(2) 2.170(2) Re(1)—N(2) 2.156(6)
Re(1)—N(3) 2.214(2) Re(1)—N(3) 2.161(5)
Re(1)—C(22) 1.918(4) Re(1)—C(25) 1.921(7)
Re(1)—C(23) 1.927(4) Re(1)—C(26) 1.922(7)
Re(1)—C(24) 1.915(4) Re(1)—C(27) 1.913(7)

Cu(1) 0(8) 2.702
Cu(1) O(8A) 2.545

Cu(l)°"Re(1) 9.707 Be(1)°“Re(1) 9.466
Cu(1)-“Re(1A) 9.707 Be(1)°-'Re(2) 9.416
Bond Angles (deg)

O(1)—Cu(1)—O(2) 92.0(8) O(1)—Be(1)—O(2) 105.6(5)
O(1)—Cu(1)—O( 1 A) 180.0(13) O(1)—Be(1)—O(6) 108.8(6)
N(1)—Re(1)—N(2) 7502(9) N(2)—Re(1)—N(3) 75.3(2)
C(22)—Re(1)—N(3) 95.22(11) C(26)—Re(l)—N(l) 92.0(2)
C(23)—Re(1)—N(3) 175.8(5) C(25)—Re( 1 )—N( 1) 176.9(3)
aplane 1'°°plane 2 87.414 cplane 1°"plane 2 60.448
bplane 1mp1ane 2 87.414 dplane 1-«p1ane 2 75.75

 

aPlane 1 is defined by atoms 0(1), 0(2), C(1), C(2), C(3), C(4), C(5);
plane 2 is defined by atoms N(3), C(6), C(7), C(8), C(9), C(10). bGiven
by the atoms that define the two planes in the adjacent pyacac ligand.
°Plane 1 is defined by atoms 0(1), 0(2), C(1), C(2), C(3), C(4), C(5);
plane 2 is defined by atoms N(1), C(6), C(7), C(8), C(9), C(10). dGiven
by the atoms that define the two planes in the adjacent pyacac ligand.

135

. ' . 02A 1 O1 . 024
. u \
R 1A - ‘- '
.9 fi N3A ’ fl ‘ N3 '. v Re1 023
01A 02 , . :.

 

Figure 3-3. Drawings of [Cu(pyacac)2(Re(bpy)(CO)3)2](OTf)2 (3, Top)
and [Be(pyacac)2(Re(tmb)(CO)3)2](OTDZ (6, Bottom) obtained from
single-crystal X-ray structure determinations. Atoms are represented as
50% probability thermal ellipsoids.

136

3.3.3 Electronic Absorption Spectroscopy. The electronic absorption
spectra of complexes 1-10 were acquired in room-temperature CH2C12
solution and are given in Figure 3-4. Rel polypyridyl complexes exhibit a
1A, —> lMLCT (t2g —-> 7:1 (bpy’)) transition in the range of 330 to 430 nm

depending on the substituents of the bpy’ ligand,“102

with electron donating
and withdrawing groups shifting Xmax for this feature to the blue and red,
respectively. In the present series, the absorption maximum systematically
shifts from ca. 350 nm for the tmb-containing complexes (1 and 6) to ca. 400
nm in the Rel-deeb analogues (5 and 10). The presence of charge transfer
transitions associated with the CulI core in the ultraviolet (Figure 3-5, Left)
gives rise to differences in this region between these adducts and their
corresponding Be'l analogues. Nevertheless, the general similarities among
these spectra, particularly with regard to the 1A, —-> 1MLCT absorption of the
Rel-bpy’ chromophores, is suggestive of minimal electronic coupling
between these groups and the CuII center in complexes 1-5.

In addition to the Rel-based charge-transfer transitions, the CuRez
complexes also possess ligand-field transitions associated with the square-
planar CuO4 core. The low-energy tails of the charge-transfer transitions

associated with the Re1 chromophores largely obscures these absorptions in

compounds 1-10, however, the electronic absorption spectrum of

137

Cu(phacac); allows for an examination of these mid-visible d-d bands
(Figure 35, Right). The two peaks at 530 nm (e = 47 M'lcm'l) and 652 nm
(8 = 50 M'lcm'l) arise fiom four spin-allowed ligand-field transitions
corresponding to dxz—> dxy, dyz—> dxy, dzz—> dxy and dx2 _y2—> dxy
based on the polarized crystal spectrum of Cu(phacac)2103 and the coordinate
system shown in Figure 3-5 (Right). This absorption spectrum, which
accurately reflects the optical properties of the CuHOa core of complexes 1-5,
will therefore provide the basis for the spectral overlap analysis to be
described later.

It should be noted that the structure of complex 3 shown in Figure 3-3
reveals two CF3SO3' counter ions interacting with the Cu11 core that could in
principle yield 5- or 6-coordinate CuII metal centers for the five CuRez
complexes (1-5) in solution. These interactions would be expected to alter

the absorption profile of the CuII core,104

thereby negating the utility of the
Cu(phacac); spectrum as a surrogate for the acceptor in the CuRez series.
Systematic titration of a CHzClz solution of Cu(phacac); with
(NEt4)(CF3SO3) (2 - 40 eq.) did not cause any discemable change in the

absorption spectrum of Cu(phacac); (Figure 3-6). This result indicates that

the triflate group is not associating with the CuII center in solution, thereby

138

validating the use of the Cu(phacac)2 ground-state absorption spectrum in

spectral overlap analyses of the CuRez assemblies.

 

50000

40000

300004

20000-

10000

 

 

 

60000-
50000-
4000O~
30000~
20000-
10000-

 

 

 

Molar Absorptivity (M 10m!) Molar Absorptivity (M'lcm'l)

 

 

300 400 500 600
Wavelength (nm)

Figure 3-4. Electronic absorption spectra of [M(pyacac)2(Re(bpy’)-
(CO)3)2](OT1)2 where, M = Cu" (red traces) and Be'1 (blue traces). All
spectra were acquired in CHzClz solution at room temperature. A.
[Cu(pyacac)2(Re(tmb)(CO)3)2](OTf)2 (l) and [BeQayacac)2(Re(tmb)-
(CO)3)2](OT1)2 (6). B. [Cu(pyacac)2(Re(dmb)(CO)3)2](OTf)2 (2) and
[BC(PyacaC)2(Re(dmb)(C0)3)2](OTflz (7)- C- [CUQDyacaC)2(R6(bPY)-
(C013)2](OTf)2 (3) and [BC(pyaca0)2(Re(bp>')(C0)3)2](0102 (3) D-
[Cu(pyacac)2(Re(dclb)(CO)3)2](OTf)2 (4) and [Be(pyacac)2(Re(dclb)-
(C0)3)2](OT1)2 (9). E. [Cu(pyacac)2(Re(deeb)(CO)3)2](OTf)2 (5) and
[Re(py acac)2(Re(deeb)(CO)3)2](OTDZ (10).

139

Molar Absorptivity (M'lcm'l)

Molar Absorptivity (M"cm")

Molar Absorptivity (M'cm'l)

Figure 3-4 (Cont’d).

 

50000
40000
30000
20000

10000

 

 

 

 

 

60000
50000
40000
30000
20000
10000

 

 

 

 

 

 

60000
50000
40000
30000
20000
10000

 

 

 

 

300 400 500 600
Wavelength (nm)

140

 

 

 

 

 

 

 

 

“‘5 r" 60- v

0 E

“'3 20000 _0 50 ‘

3‘ E 40) °\c-/ 1‘

E 15000 g. o/ \

‘é- ~55 30

8 10000- g:

“g g 20~

5 5000 B 10_

O .—r

2 0" 1 1 1 1 1 x g 0L 1 1 1 J_ 1 ‘
300 350 400 450 500 550 500 600 700 800 900

 

Wavelength (nm) Wavelength (nm)

Figure 3-5. Left. Electronic absorption spectrum of Cu(phacac)2
showing the higher energy charge transfer and organic-based
transitions. Right. Electronic absorption spectrum of a concentrated
solution of Cu(phacac)2 showing the two mid-visible ligand-field
bands. Both spectra were acquired in room-temperature CHzClz
solution.

 

 

0.07 ~
0.06
0.05
0.04 ~
0.03
0.02 -
0.0] ~
0.00 -

 

 

Absorbance

 

 

 

 

 

500 600 700 800 900
Wavelength (nm)

Figure 3-6. Systematic titration of a CHzClz solution of Cu(phacac);
with (NEt4)(CF3SO3) (2 - 40 eq.)

3.3.4 Steady-State and Time-Resolved Emission. Emission spectra for the
CuRez and BeRez complexes were obtained in room-temperature

deoxygenated CH2C12 solutions and are shown in Figure 3-7. The spectral

141

profiles for all of the compounds correspond well to previously reported
photophysical studies of ReI polypyridyl systems, with the emission
originating fi'om the 3MLCT —> 1A, transition.'05 Emission maxima for the
tmb (524 nm), dmb (550 nm), bpy (568 nm), dclb (600 nm), and deeb (620
nm) derivatives reflect the expected trend in zero-point energy of the
3MLCT state based on the electron donating and withdrawing behavior of
the substituents on the polypyridyl ligands. Likewise, the radiative quantum
yields ((1),) for complexes 6-10 (Table 3-3) are all comparable to the reported
values for the corresponding mononuclear ReI polypyridyl derivatives.72

The data on complexes 1-5 reveal that the emission intensities for the
Cull-containing compounds are significantly attenuated compared to the
BeRez model complexes. Unfortunately, radiative quantum yields for
complexes 1-5 were found to be analytically unreliable due to a small
amount of an emissive impurity present in solutions of these compounds.
Cu(phacac); was determined to be non-emissive in room-temperature
CHZCIZ solution: the impurity is therefore likely due to the presence of fac-
[Re(bpy’)(CO)3(pyacac)] generated by displacement of the pyacac ligand by
residual amounts of H20 contained in the CH2C12. The drying and distilling
procedures that were employed were exhausted until no water was

detectable by 1H NMR, but the low concentration of the CuRez compound

142

used for the emission measurements means that even trace amounts of H20
could be sufficient to generate a small amount of dissociated species. A

similar problem was encountered in our previous study of FeRe3

assemblies;4 as in that system, the presence of this emissive component

proves to be inconsequential for the ensuing analysis.106

143

 

 

 

 

1 A
:2?
VJ
8
a
H
S:
.2
(I)
.E
:2
LL]
n
V A I
1 B
r?
m
3':
8
:2
1—4
x:
.2
m
.59.
E
LL]
0

 

 

 

 

500 600 700 800
Wavelength (nm)

Figure 3-7. Corrected steady-state emission spectra for [M(pyacac)2-
(Re(bpy’)(CO)3)2](OTf)2 where, M = CuIl (red traces) and Be11 (blue
traces). All spectra were acquired in room-temperature deoxygenated
CHzClz solution. A. [Cu(pyacac)2(Re(tmb)(CO)3)2](OTDZ (1) and [Be-
(pyacac)2(Re(tmb)(CO)3)2](OTf); (6). B. [Cu(pyacac)2(Re(dmb)-
(C0)3)2](0Tf)2 (2) and [BC(PyacaC)2(Re(dmb)(C0)3)2](OTflz (7)- C-
[CU(PyacaC)2(Re(bPY)(C0)3)2](OTf)2 (3) and [Be(PyacaC)2(Re(bPY)-
(CO)3)2](OT1)2 (8). D. [Cu(pyacac)2(Re(dclb)(CO)3)2](OTf)2 (4) and
[Be(pyacac)2(Re(dclb)(CO)3)2](OTf)2 (9). E. [Cu(pyacac)2(Re(deeb)-
(C0)3)2](0T1)2 (5) and [Be(PyacaC)2(Re(deeb)(C0)3)2](0T1)2 (10) The
spectra were acquired following excitation at 355 nm (complexes 1, 2,
3, 6, 7, and 8), 375 nm (complexes 4 and 9), and 400 nm (complexes 5
and 10). For each plot, the emission profiles have been normalized
with respect to the absorbance of each sample at its excitation
wavelength. The relative magnitudes of the signals are therefore
accurate representations of their relative intensities. The emission
corresponding to the CuReg analogues are due largely to the presence
of Re(bpy’)(CO)3(pyacac) in solution. See main text for further details.

144

Figure 3-7 (cont’d)

 

Emission Intensity

C

 

 

~
I

Emission Intensity

 

 

 

 

 

 

 

 

.2
{I}
c:
B
E.
c:
.2
.2
E
Lu
0 l _L 1 L
500 600 700 800
Wavelength (nm)

Quantitative information concerning emission quenching by the Cu" center
in complexes 1-5 was obtained via time-resolved emission spectroscopy.
Data for the BeRez model complexes could be fit to single-exponential

decay kinetic models; emission traces and values for the observed decay

145

rates for all five BeRez complexes are given on the left side of Figure 3-8
and Table 3-3, respectively. As with the quantum yields, the observed
excited-state lifetimes are consistent with an assignment of 3MLCT —+ 1A,
emission.”107 The kinetics reveal that the reduction in quantum yield across
the series is due primarily to an increase in the non-radiative decay rate for
3MLCT relaxation (km) as opposed to significant variations in radiative
coupling to the ground state. The only deviation from this trend is the
smaller value of km for complex 10. This is most likely due to the additional
conjugation present in the 71 system of the deeb ligand, which allows for
increased delocalization of the excited electron relative to other members of
the series. This increased delocalization is expected to give rise to a smaller
net displacement of the ground- and excited-state potential energy surfaces,
which in turn leads to a decrease in the rate of non-radiative decay.73 ’80’81’108

Analogous measurements on the nanosecond time-scale for the CuRez
complexes failed to reveal any signal beyond that of the trace impurity
mentioned above. Consequently, time-correlated single-photon counting
(TCSPC) was employed to measure the excited-state lifetime of the Cu"-
containing complexes (1-5). A plot of the TCSPC data obtained for

complexes 1-5 in deoxygenated CHzClz solution is shown on the right side

of Figure 3-8; data for all five CuRez complexes (1-5) are listed in Table 3-3.

146

The observed time constants for excited-state decay for complexes 1-5 are
all significantly larger than the corresponding BeRez model complexes,
ranging from a factor of ca. 20 in the case of complex 4 to more than two
orders of magnitude for complex 1. The signal-to-noise ratio for the TCSPC
data is relatively poor owing to a combination of significant quenching of
the Rel-based 3MLCT states coupled with radiative rate constants for

emission on the order of 105 8".

Nevertheless, these observations clearly
indicate the presence of a very efficient quenching process stemming from a

reaction between the Rel-based 3MLCT excited state and the Cu11 core of the

CuRez assemblies.

147

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

50-
6 .5 ‘ 1
{3,2000-1
A 40- 8,
> a» l
"—1 I500 ',
E 30- E” l
G)
'E E 1000—1
I: 20' g 1
(D
‘a ,2 l
"" 10» 3, 500~
E ‘
0 I I; I 1 I I I I m 0 L A; I I L I I l I
0 2 4 6 8 10 12 14 0 10 20 30 4O 50 60 70 80
Time (ns) x103 Time ("8)
40- 7 A1000- 2
U)
> 30- V f
E a? .
v g 600-‘
.‘E‘ 20—; 3 {
E E 400-
8 g 1
c: - _
H 10 § 200.4
E
0 1‘4 1 1 I I I l m 0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 5 10 15 20 25 30 35 40
Time (ns) x 103 Time (ns)

Figure 3-8. Left. Nanosecond time-resolved emission data for the
BeRez model complexes in room-temperature deoxygenated CH2C12
solutions: 6 (rob, = 1990 i 100 ns), 7 (robs = 645 :t 30 ns), 8 (rob, = 540 i
30 ns), 9 (robs: 110 i 10 ns), and 10 (robs: 250 :t 20 ns). The solid red
lines correspond to fits to single-exponential decay models. The data
were acquired by monitoring at the emission maximum of each
compound. Right. Time correlated single-photon counting (TCSPC)
emission data for the CuRez complexes in room-temperature
deoxygenated CHzClz solutions: 1 (1'1: 14.9 i 0.7), 2 (131: 8.1 :t 0.4), 3
(1:1: 8.2 i 0.4), 4 (11: 5.6 i 0.3), and 5 (1,: 5.0 i 0.3). The data were
acquired by monitoring at the emission maximum of each compound.
The solid red lines correspond to fits to bi-exponential decay models; in
each case the second component (12) corresponds to emission from
trace amounts of Re(bpy’)(CO)3(pyacac) present in solution. I; was
taken to be the emission lifetime of the corresponding BeRez model
complex.

148

Intensity (mV) Intensity (mV)

Intensity (mV)

40

30

20

4O

30

20

50

40

30

20

Figure 3-8 (cont’d)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

_ 8 A
U)
a.
3
.2
U)
s:
- B
l .2
s:
_‘ .2
.3.
E
I I I | I I L I m 0 I I I I I I l I J_I
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 5 10 15 20 25 30 35 40
Time (ns) x 103 Time (ns)
_ 9 A
(I)
a.
1‘ 8
.2
(I)
a:
_ 53
I E.
a
- .2
.8
B
I I I I I l 1 1 LL] 0 I 1 I I l 1 1 l 1
0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40
Time (ns) Time (ns)
' 10 A 100- 5
U)
8
' v 80-
.2
- 8 60-
8
.5
- 1:: 40-
.2
- .8 20-
E
m 0 I I I I I 4| I I L
0 250 500 750 1000 1250 ‘0 5 IO 15 20 25 30 35 40
Time (ns) Time (ns)

149

Table 3-3. Photophysical Data for [M(pyacac)2(Re(bpy’)(CO)3)2](OTf)2
(complexes 1-10).

 

 

E00 krc knrd,e

complex Mum) (cm'l)a (I)r k(,1,s(s")b (X105 5") (><106 s'l)
tmb (1) f f f 6.7 r 0.3 x 107 f 67 a 4
tmb (6) 524 19,900 0.63 5.0 :1: 0.3 x 105 3.2 a 0.2 0.19 i .01
dmb(2) f r f 1.21:0.1x108 f 120:1:6
dmb (7) 550 19,200 0.25 1.6 i 0.1 x 106 4.0 :1: 0.2 1.2 a 0.1
bpy(3) f f f 1.2ao.1><108 f 120i6
bpy(8) 568 18,750 0.18 1.9:r0.1><106 3.4102 1.6i0.1
dclb(4) f f f 1.8:t0.1><108 f 180110
dclb (9) 600 17,650 0.03 8.8 a 0.5 x 106 2.6 :1: 0.1 8.5 a 0.4
deeb (5) f f f 2.0 a: 0.1 x 108 f 200 i 10

deeb

(10) 620 16,900 0.07 4.0 a 0.2 x 106 2.8 i 0.2 3.7 a 0.2

 

aZero-point energy difference between 3MLCT excited state and ground
state derived from a spectral fitting analysis. bError bars on kob,

represent

the

standard deviation (20) of five

independent

measurements. ckr = kob,*(I),. dk,,,(CuRe2) = kobs(CuRe2) — k,(BeRe2).
ekm(BeRe2) = kobs(BeRe2) — k,(BeRe2). fThis value is not quoted due to
the presence of an emissive impurity, but is expected to be identical to
the corresponding BeII complex. See text for details.

150

3.4 Mechanistic Considerations

3.4.1 Electron versus Energy Transfer Quenching. Time-resolved
emission data, while clearly revealing the presence of an excited-state
reaction, provides no information as to the mechanistic origin of that
reaction. Indeed, both electron and energy transfer processes can be
envisioned to occur out of the Rel-based 3MLCT excited state. Based on the
presence of Cu" in the ground states of these CuRe2 complexes, electron
transfer (kET) could in principle proceed either by oxidative or reductive

quenching to produce Cu1 or Cu!II

as photo-products (Figure 3-9). In order
to determine if electron transfer is thermodynamically viable in our system,
the electrochemical properties for the CuRez as well as the BeRez analogues
were measured. Cyclic and differential pulse voltammograms for complexes
1-10 were recorded in CH3CN solution; the data are listed in Table 34.109
The availability of the Bell model complexes considerably simplifies
assigning many of the electrochemical properties of all ten complexes due to
the redox-inert nature of this ion. Accordingly, the single reduction waves
seen for complexes 6-10 can be immediately ascribed to the bipyridyl ligand
of the Re moiety in each case. The positive shift in potential across the

series is consistent with the more electron withdrawing nature of the

substituents as one progresses from the methyls of tmb (6) to the diethylester

151

groups in complex 10. Similarly, the oxidation waves seen for complexes 6-
10 are easily assigned to the Rel/Re" couple. The influence of the bipyridyl
substitutents are apparent in these data as well, with the more electron
deficient ligand giving rise to the most positive oxidation potential for the
Re center. The results are all consistent with what has been observed for
complexes of the general form fac-[Re(4,4'-X2-bpy)(CO)3(4-Etpy)](PF6)
previously reported in the literature.72

In addition to the Rel-based features, complexes 1-5 should also
exhibit redox chemistry associated with the central CuIl ion, which prompted
the investigation of the electrochemical properties of Cu(phacac)2. The
electrochemical properties of Cu(acac)2 and related complexes in CH3CN
solution have been previously reported,”0’m with the Cu11 metal center
observed to undergo highly irreversible reductive (Cuu —-> CuI) and oxidative
(CuII —> Cum) processes. Cu(phacac); exhibits a wave at -1.48 V in CH3CN
solution which can be assigned to the CuII ——> CuI reduction; this potential
shifts to and at -1.73 V in CHzClz. The CuII —> Cum oxidation occurs at
+1.31 V in CH3CN and lies outside the solvent window of CHzClz (i.e., >
1.5 V). Although the Cu" potentials are masked in certain cases by the Rel-

and polypyridyl-based processes, the assignments reported in Table 3-4 for

152

the CuRez series are validated by their close correspondence to the data
acquired on the BeRez systems.
The ReI -—> ReII oxidation potentials for complexes 1-5, Cu11 —+ CuI

- 11
reductlon and Cu —+ Cu"I

oxidation potentials of Cu(phacac)2, along with
the zero-point energy gaps of the 3MLCT states (E00) determined from fits of
the emission spectra of the corresponding BeRez analogs (Table 3-3) were
used to determine the thermodynamic driving force for photoinduced
electron transfer.”2’“3 The values of AGET listed in Table 3-4 reveal that
electron transfer is significantly uphill for both oxidative and reductive
quenching processes in all five CuRez complexes. Significantly, these
reactions become more endothermic in CHzClz (the solvent used for the
photophysical measurements) due to the shifts in the Cull-based redox

processes indicated above. These electrochemical data therefore effectively

rule out electron transfer as a viable quenching mechanism for the CuRe2

series.
km (bpy')Re"-Cu'
ox.

(bPY')Re'NCu" —> (bpy")Re"-Cu“ —C
red. .- l m
kETZ (bpy )Re NCu

Figure 3-9. Illustration of oxidative (kg—n) and reductive (kg-r2) electron
transfer quenching processes out of the Rel-based 3MLCT excited-state
to produce Cu1 or Cu"I as photo-products, respectively.

153

Table 3-4. Electrochemical Data and Calculated Electron Transfer Driving
Forces (AGET) for Complexes 1-10.

 

AC‘ETox ACIETred

 

 

electrochemical potential (V) (eV)a (eV)b
complex B... (Re"”)° E... (bpy'w'f
tmb (1) +1.30 -1.83 +0.56 +0.67
tmb (6) +1.28 -1.81 -—- ---
dmb (2) +1.31 -1.71 +0.66 +0.64
dmb (7) +1 .31 -1.68 --- ---
bpy (3) +1.36 -1.61 +0.77 +0.60
bpy (8) +1.34 -1.57 --- ---
dclb (4) +1.41 -1.38 +0.95 +0.50
dclb (9) +1.44 -1.34 m ---
deeb (5) +1.51 -1.23 +1.14 +0.44
deeb (10) +1.50 -1.19 --- ---

 

on (Cull/"If Ered (Cum)
Cu(phacac); +1.31 -1.73d (-1.48)c

 

 

aCalculated using the AGETox equation given in Ref. 113 and using
the Cu" —+ CuI reduction in Cu(phacac)2 in CHzClz as the reduction
potential of the acceptor. bCalculated with the AGETm, equation
given in Ref. 113 and using the Cu" —> CulIl oxidation in
Cu(phacac); in CH3CN as the oxidation potential of the acceptor.
cMeasured in CH3CN solution. dMeasured in CH2C12 solution.

Based on the analysis presented above, the most likely explanation for
3MLCT quenching in all five of the CuRez assemblies is excited-state energy

transfer. The two most important mechanisms are electron superexchange

154

(Dexter)114 and dipole-dipole coupling (Féirster).115 Dexter energy transfer is
subject to a distance dependence that falls off as e'2R due to its reliance on
orbital overlap. As such, it is usually relegated to covalently linked systems
in which the donor and acceptor lie in close proximity and are electronically
coupled. F6rster transfer is a through-space mechanism that occurs when
the donor emission non-radiatively couples to an absorptive feature of the
acceptor. The dipolar nature of this interaction gives rise to a shallower R'6
distance dependence, allowing this mechanism to be operative over much

. 1
longer dlstances.l 6

The X-ray structure data for complex 3 shows a
Rel-"CuII separation of nearly 10 A, a value that lies at the limit of what is
typically considered for an exchange-based process.46" 17" '9 The similarity in
the charge-transfer region between the absorption spectra of complexes 1-5
and their respective BeRez analogues, as well as their electrochemical
properties, suggests minimal electronic exists coupling between the Re1 and
Cu'1 subunits. This point is amplified by the computational results of Meyer
and coworkers which reveal that the majority of the amplitude associated
with the thermalized 3MLCT wavefunction for complexes of the form fac-
[Re(4,4’-X2bpy)(CO)3(4-Etpy)](PF6) (X = CH3, H, and COzEt) is

concentrated within the 75 levels of the 4,4’-X2bpy derivatives and less so on

the 4-Etpy ligand.120 This situation decreases the possibility of the pyacac

155

bridging ligands in the CuRez series imparting significant electronic
coupling between the ReI and Cu" metal centers. This combination of the
structural and electronic characteristics of the CuRez series make it unlikely
that Dexter-type exchange is playing a dominant role in the energy transfer
dynamics of these systems.

3.4.2 Spectral Overlap Analysis: State-Selective Energy Transfer. The
classic equation describing the Ffirster energy transfer rate constant (kEnTr) is
given in equation 3-3,m

_ 90001n(10)x2k,J
5”" 1287:5774NAR6 (3‘3)

 

where K2 is the dipole orientation factor, kr is the radiative rate constant of
the donor, J is the spectral overlap integral, 1] is the refractive index of the
solvent, NA is Avogadro’s number, and R is the donor-acceptor separation.
The spectral overlap integral (J), which essentially quantifies the resonance
condition for energy transfer, can be evaluated fiom the spectroscopic

properties of the system and is given in equation 3-4,

 

(3-4)
where F0 is the normalized emission spectrum of the donor and 8A is the
absorption profile of the acceptor in units of molar absorptivity. A plot of the

156

emission spectra of complexes 6 through 10 along with the visible
absorption spectrum of Cu(phacac)2 is shown in Figure 3-10. It can be seen
that the systematic shift in the emission maximum across the series results in
a modulation of the overlap between the donor and the acceptor. As
mentioned previously, the absorption spectrum of the CuII acceptor consists
of a ligand-to-metal charge transfer feature in the blue/near—ultraviolet
region, as well as ligand-field transitions in the mid-visible; the substituent
changes on the bpy ligands serve to tune the resonance between the 3MLCT

emission of the Rel-bpy’ fluorophore and these various acceptor state(s).

 

_.
1
_‘

Absorbance (normalized)

 

 

 

O
O

 

(pozneuuou) misuaiul uoissiwg

#—
N

212 210 1'8 16 14

Energy (x103 cm")

Figure 3-10. Overlay of the emission spectra of [Be(pyacac)2(Re(tmb)-

(C0)3)2](OTf)2 (6, purple), [136(pyacaC)2(Re(dmb)(C0)3)2](0T02 (7,

blue), [BC(Pyaca0)2(Re(bpy)(C0)3)2](0102 (8, green), [Be(PyacaC)2-

(Re(dclb)(CO)3)2](OTf)2 (9, orange), and [Be(pyacac)2(Re(deeb)-

(CO)3)2](OTf)2 (10, red) with the electronic absorption spectrum of
Cu(phacac)2 (black trace).

157

The Forster model defines a proportionality between the spectral
overlap integral and the rate of energy transfer subject to variations in the
donor-acceptor distance, the intrinsic radiative lifetime of the donor, as well
as the relative orientation of the coupled transition dipoles (1(2). In viewing
the series of complexes 1 through 5 as a whole, several of these variables
can be accounted for explicitly as a means of examining trends in the rate of
energy transfer. The availability of the BeRe2 model complexes affords
values of kr for each compound in the series. We can therefore write an
expression for the rate of energy transfer normalized for the rate of radiative

decay as equation 5a:

%£=C'K2'J (3-5a)

r

where C represents the collection of constants from the Féirster equation
(including the R'6 donor-acceptor distance term). Our previous study on
Fem-containing assemblies“ demonstrated that one could use metrical data
afforded fiom single-crystal X-ray structures in conjunction with reasonable
estimates for the positions of the relevant transition dipole vectors to
calculate K2 explicitly. In the present case, the metal-localized nature of the
d-d States on the Cun center makes this approach more difficult (vide infra),

. . 2
however, we can 1nvertth1s process and calculate values for K based on our

158

experimental data. Values for kEnTr/kr, the overlap integral (Jmml), as well as

K2 for compounds 1-5 are listed in Table 3-5. These numbers reveal the
surprising result that the value of K2 is not constant across the series. Using a

donor-acceptor distance of 10.37 A (a value corresponding to the distance
between the Cu11 center and the midpoint of a vector that bisects the N-Re-N
bond angle“), K2 for compound 1 is found to be 2.8 whereas for compound 5
the corresponding value is 5.7; this latter result exceeds the maximum value
of K2 = 4 calculated for head-to-tail parallel transition dipoles.l '5
The underlying reason for the apparent disconnect between the
predictions of eq 3-5a and the results obtained for compounds 1 through 5
can be understood by examining the normalized rates of energy transfer
across the series in a slightly different manner. For any two members of the
series i and j, we can write equation 3-5b for the relative change in the rate
of energy transfer as a function of spectral overlap:
1w 2
kr 1' _Cj"‘j'Jj

kw, _ Cr K124: (3-5b)

 

 

 

 

The isostructural nature of compounds 1 through 5 implies that, to a

reasonable degree of precision, the donor-acceptor distance is essentially

159

constant across the series.‘22 Cancellation of these terms affords equation 3-

[kEnVJ
k, . K? J

_ _.l =__ _1'
[kE"%J Kf J1. (3-50)

This expression indicates that the relative rates of (kr-normalized) energy

5c:

transfer should correlate with the ratio of their spectral overlap integrals
weighted by a proportionality constant equal to the relative magnitudes of
their orientation factors: dipolar coupling involving the same two states (i.e.,
the 3MLCT state of the Re-bpy’ moieties and the Cu11 acceptor) should
therefore yield a slope of 1 since their orientation factors will cancel. Using
the data for compound 1 as our im reference state we find that sz/Kzl
increases from 1.18 for compound 2 to 2.03 for compound 5. These data
confirm the trend suggested by the individual K2 values listed in Table 3-5,
namely that the orientation factor associated with energy transfer in this
system is different in one region of the spectrum versus another.
Specifically, our analysis reveals a net increase in the rate of energy transfer
as the donor emission comes into resonance with the low-energy portion of

the CulI absorption profile.

160

Table 3-5. Calculated Spectral Overlap Integrals and Energy Transfer Rate

Constants for Complexes 1-5.

 

 

complex Jtotala JGla Joza 1038l kEnT (x 108 S'l)b kEnT/krc K2d
tmb (I) 4.29 0.127 2.26 1.90 0.67 208 2.8
dmb (2) 5.14 0.046 2.07 3.03 1.18 295 3.3
bpy (3) 5.52 0.027 1.92 3.57 1.18 347 3.7
dclb (4) 6.68 0.0045 1.29 5.39 1.71 658 5.7
deeb (5) 7.09 0.0014 0.97 6.12 1.96 700 5.7

 

aReported in units of 10'16 M'Icm3 . bkEnT = kob,(CuRe2) — kob,(BeRe2)
(Table 3-3). 0Values for k, were taken from the corresponding BeRez
complex (Table 3-3). dCalculated based on eq. 3-5 for R = 10.37 A.

In order to determine whether there are, in fact, differential
contributions fiom the excited-state manifold of Cu" to the energy transfer
dynamics of this system, we deconvolved the absorption profile of
Cu(phacac)2 into a sum of Gaussians as a means of examining each portion
of the compound’s excited-state structure individually (Figure 3-11A). As
expected, the spectrum is well-described by three bands corresponding to the
high-energy charge—transfer feature (G1) and two, lower-intensity bands
assigned to ligand-field transitions (G2 and G3).'03 Using this fitted
spectrum, the total spectral overlap integral (how) can be apportioned
according to its various components (where 1m, = J G] + J02 + J03) (Figure 3-
113 and Figure 3-11C). The data in Table 3-5 reveals that the charge-

transfer state contributes minimally to the overall spectral overlap, with

161

contributions ranging from a high of ca. 3% in the case of compound 1 to
less than 0.02% in compound 5. This is a clear, quantitative indication that
the ligand-field terms are the dominant acceptor states in this system. As the
emission spectrum is tuned toward the red the total spectral overlap
increases, however, the contribution from the higher energy ligand-field
transition(s) decreases fi'om 53% in compound 1 to 14% in compound 5. At
the same time, fractional overlap with the G3 component increases from
44% to 86% of the total. Since the value of sz/Kzl from eq 3-50 is observed
to increase across the series, we can infer fiom this analysis that the rate of
energy transfer from the 3MLCT state of the Re-bpy’ moiety is intrinsically
faster when coupling to the G3-component of the Cu11 absorption profile as
compared to G2. This conclusion is further supported by noting that the ratio
of orientation factors as defined by eq 3-5c yields the same value of 2.03 for
compounds 4 and 5, implying that 135/184 = 1. Inspection of the data in Table
5 reveals an increase in Jm, for compound 5 versus compound 4 but
comparatively little change in the relative contributions of G2 and G3. In
this context, the variation in K2 across the entire series can be interpreted in
terms of differences in the orientation of the transition dipoles associated
with G2 and G3: once changes in spectral overlap are isolated to G3 (i.e.,

compounds 4 and 5), the proportionality factor in eq 3-5c collapses to unity.

162

 

  
        

 

 

Molar Absorptivity (M'lcm'l)
OJ
C

--;_ ___ ...........

22 20 l8 l6 14 12
Energy (x103 cm'l)

 

 

 

 

 

O
(pazrleuuou) Atsruoiul uorssttug

 

Molar Absorptivity (normalized)

22 20 118 1'6 14 12
Energy (x103 cm'l)
Figure 3-11. A. The ground state absorption spectrum of Cu(phacac)2
in CHzClz (black) fit with a series of three Gaussians (G1 (solid trace),
G2 (dotted trace), and G3 (dashed trace)). B and C. Overlay of the
emission spectra of [Be(pyacac)2(Re(tmb)(CO)3)2](OTf)2 (6, purple),
[Be(pyacac)2(Re(dmb)(CO)3)2](OTDZ (7, blue), [Be(pyacac)2(Re(bpy)-
(C0)3)2l(0Tf)2 (8, green), [B6(PyacaC)2(Re(dC1b)(C0)3)2](0T02 (9,

orange), and [Be(pyacac)2(Re(deeb)(CO)3)2](OTDZ (10, red) with G2
and G3, respectively.

163

Figure 3-1 1 (cont’d)

 

 

 

 

 

 

Molar Absorptivity (normalized)
(pozneuuou) misuatul uorsstuig

22 20 1 8 16 14 12
Energy (x103 cm'l)

The analysis presented above clearly points to a substantive difference
in the nature of the excited states associated with the two observed
absorption features of the Cull core in terms of their ability to engage in
energy transfer from the Re-bpy’ fluorophore. We therefore carried out a
time-dependent DFT calculation on Cu(phacac)2 in CH2C12 solution in order
to obtain a more detailed theoretical description of these states. The
transition energies, oscillator strengths, and orbital compositions of the four
lowest energy excited states that are predicted from this calculation are listed
in Table 3-6. The low oscillator strengths (fca,c = 0.0000) calculated for all
four transitions are consistent with the Laporte-forbidden nature of d-d
absorptions. The calculated transition energies along with the absorption
spectrum of Cu(phacac)2 acquired in CH2C12 solution are plotted in Figure 3-

12. The results of the calculation are in reasonably good agreement with

164

experiment, which we take as an indication of the general validity of the

wavefunctions comprising these four absorptive features.

Table 3-6. Calculated Orbital Composition of the Lowest-
Energy Spin Allowed Absorptions of Cu(phacac)2 in

CH2C12 8011111011.

 

E (cm'l) fcalca

orbital transitions (Ci)b

 

Excited State 1c 18,831 0.0000

Excited State 2C 18,703 0.0000

Excited State 3d 16,280 0.0000

Excited State 4d 14,778 0.0000

860 H 1080 (0.17)
870 H 10813 (0.12)
900 H 1088 (0.23)
9613 H 1080 (0.94)
1008 H 1080 (0.11)

8613 H 1088 (0.23)
870 H 1088 (0.31)
970 H 1080 (0.92)

860 H 1080 (0.42)
870 H 1080 (0.33)
900 H 1088 (0.23)
1000 H 1080 (0.85)

840 H 1080 (0.17)
950 H 1080 (0.62)
1070 H 1080 (0.75)

 

aCalculated oscillator strength. bc, corresponds to the
coefficient for the specific orbital transition in the
calculated absorption feature. The contribution of each
component to the total wavefunction is given by c,.
cExcited states 1 and 2 comprise the G2 transition of the
deconvolved spectrum in Figure 3-11A. dExcited states 3
and 4 comprise the G3 transition of the deconvolved

spectrum in Figure 3-11A.

165

 

()1 Ch
0 O O
I I I

._r
O O
I I

 

Molar Absorptivity (M'lcm‘l)
N 15 A

 

 

O

 

 

22 20 18 l6 14 12
Energy (x 103 cm'l)

Figure 3-12. The four lowest energy transitions of Cu(phacac)2
determined from a TD-DF T calculation plotted against the ground state
absorption spectrum of Cu(phacac)2 in CHzClz (black). The green and
red dotted lines represent the G2- and G3-based transitions,
respectively. The calculated oscillator strengths for all four transitions
are zero (Table 3-6); the intensities of the calculated absorptions have
therefore been arbitrarily scaled to facilitate comparison with the
experimental spectrum.

GaussView renderings of the orbitals involved in the four calculated
excited states are shown in Figure 3-13. Excited States 1 (18,831 cm'l), 2
(18,703 cm'l), and 4 (14,778 cm'l) are dominated by dz2 —> dxy (960 —>
1080), dyz —> dxy (970 —> 10813), and dxz —> dxy (10713 —> 1086) ligand-
field transitions, respectively. Excited State 3 (16,280 nm) has a small
contribution from the 9013 —> 10813 (dx2_y2 —> dxy) transition, but is
dominated by the 10013 —» 108B (73%) component that contains
contributions from both charge-transfer and metal-centered excited states. A

closer look at 1000 reveals a large metal-ligand bonding component to the

wavefunction, along with density located solely on the Cu[1 metal center

166

buried within this bonding contribution. The metal-centered density suggests
a significant fiaction of d-d character exists in the 10013 —+ 10813 transition,
which would explain the lack of oscillator strength associated with Excited
State 3. TD-DFT calculations were also performed on Cu(phacac)2 in frozen
Dzh symmetry in a CHzClz environment in order to compare with the results
from the minimized solution phase geometry. Results of the two TD-DFT
analyses exhibited close correspondence in both energy ordering and orbital
compositions for the four low-energy transitions, suggesting that the
structure of Cu(phacac)2 doesn’t vary significantly between the solution

phase and idealized geometries.

167

"'" i" " a?
;%.%w «nu-a baa-0a
«a. .~ . ’1»
9613 ——> 10813
Exited State 1

A. a 4 ’

'3 u 14““
#031310 we» “as; Q 3133—”;

a. 9. .10 9).

J J

  
  
   

 

97B —+ 10813

Excited State 2

J ‘ ’ ‘\ ‘
+0} 04 +0} WW:
I
a J a J
—_)

10813

10013

Excited State 3

JG; 40': T
10’ «.300: “at”!
0". I" .

10713 —» 10813

 

Excited State 4

Figure 3-13. Drawings of the orbitals involved in the lowest-energy
spin allowed transitions of Cu(phacac)2 based on a TD-DFT
calculation. The coordinate system used is that shown in Figure 3-5
with the CuO4 coordination environment drawn in the xy plane. See

Table 3-6 for further details.

168

The analysis of the energy transfer dynamics of compounds 1 through
5 indicated a significant difference in the rate of energy transfer for coupling
to the two absorption bands of the Cu11 absorption profile, a difference that
appears to be linked to the dipole orientation factor K2. At first glance this
would seem to be an unlikely scenario given the metal-centered nature of
ligand-field absorptions; as mentioned in the Introduction, d-d acceptor
states represent a nearly ideal manifestation of the point-dipole
approximation inherent to Forster theory. Fleming and co-workers have
demonstrated how one can use density matrix formulations to obtain detailed
representations of transition dipoles for energy transfer processes involving
carotenoids.123 The transition metal-based nature of the excited states
involved in the present study makes an analogous approach difficult,
however, visual inspection of the ground- and excited-state orbital
configurations reveals two interesting features. A comparison of the
calculated transition energies from TD-DFT with the Cu(phacac); ground
state absorption spectrum shows the higher energy G2 band is dominated by
the 9613 —+ 10813 (dz2 ——> dxy) and 9713 —+ 10813 (dyz -—> dxy) transitions,
whereas the lower energy G3 band is comprised mainly of the 10013 —-* 10813

and the 10713 ——> 10813 (dxz —-> dxy) transitions. One striking difference

between these transitions is the lack of an x-axis component in the

169

wavefunctions corresponding to the occupied orbitals of the two highest-
energy excited states. Ligand-field transitions — at least those arising from
either (i1 or d9 configurations where a one-electron orbital picture can be
invoked — can be thought of in terms of a transfer of charge between the
different planes defined by the orbital’s azithumal quantum number (m.). In

this regard, the transitions contributing to the G2 feature (dz2 —> dxy and
dyz —> dxy) will be distinct in a geometric sense from the dxz —> dxy

transition that characterizes the lowest-energy feature in G3. Absent more
detailed information it’s difficult to quantify whether this should give rise to
an increase or decrease in dipolar coupling to the Rel-based excited state, but
it seems reasonable to expect a difference.

A second distinction between the G2 and G3 bands that could be
playing a role in the observed dynamics is the nature of Excited State 3.
Despite its low oscillator strength and all indications from experimental data
of its ligand-field nature,‘03 Figure 3-13 illustrates that the 10013 —> 10813
transition is qualitatively different from the other three excited states
comprising the visible absorption spectrum of Cu(phacac)2. Based on the
spatial distribution of orbital 10013 there appears to be some charge-transfer
character to this transition, although clearly not sufficient to impact the

oscillator strength for the absorption. Nevertheless, the larger change in

170

charge distribution associated with Excited State 3 could be having a subtle
influence on the magnitude of the transition dipole, which in turn would
enhance the rate of energy transfer when coupling to this state. It is
interesting to note the fact that the G3 feature is composed of distinct excited
states explains at least qualitatively why the value of K2 calculated from eq
3-5a for compounds 4 and 5 was found to exceed the theoretical maximum
of 4 (Table 3-5). Given the above analysis, each of these excited states
should be viewed as an independent acceptor: coupling to both of these
states as part of the overlap factor for G3 would therefore lead to an
“effective” value of K2 (e.g., a sum of contributions) as opposed to the value
one would calculate assuming a single donor-acceptor interaction. A more
detailed theoretical treatment of this system would clearly be required in

order to explore these ideas fiirther.

3.5 Conclusions

The synthesis, structures, and photophysical properties of a series of
covalently linked assemblies containing Rel-bipyridyl donors and a Cull—acac
acceptor have been described. Steady-state and time-resolved emission
spectroscopies indicated that the strongly emissive Rel-based 3MLCT

excited-state was significantly quenched in the presence of the Cull center

171

relative to electronically benign BelI analogues. Favorable overlap between
the donor emission and the visible absorption spectrum of the acceptor,
coupled with a ca. 10 A donor-acceptor separation and unfavorable driving
forces for electron transfer, allowed for an assignment of Forster energy
transfer as the dominant mechanism for excited-state reactivity in this
system. A detailed examination of the rates of energy transfer across the
series revealed that the degree of dipolar coupling between the donor and
acceptor was not constant across the absorption envelope of the Cu11
acceptor, but instead exhibited preferential coupling to the lower-energy
portion of the ligand-field manifold. Fitting the absorption spectrum of the
Cull chromophore with a series of Gaussians allowed for a differential
analysis of the spectral overlap that quantified differences in the dipole
orientation factor K2 as the emission profile was tuned across the absorption
spectrum of the acceptor. A TD-DFT analysis of the central Cull species
revealed the composition of the Cull-based ligand-field acceptor states and
permitted the identification of the specific molecular orbitals responsible for
the dipolar energy transfer quenching as well as the likely origins of their
distinct roles in energy transfer. This study, in addition to providing a unique

example of orbitally-specific energy transfer, also illustrates the importance

172

of considering the substructure of absorption bands in terms of its potential

impact on dipolar energy transfer dynamics.

173

3.6 References and Notes

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(106) It should be noted that the presence of this emissive impurity is
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respectively, where E00 is the energy gap between the ground and
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182

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183

Chapter 4. Confirming Forster Energy Transfer Involving Ligand—Field

Acceptor States: CrRe3 Complexes.

4.1 Introduction
The theoretical formalism of dipole-dipole energy transfer is
described by the well-known Forster equation (equation 4-1), that elegantly

relates experimental observables to the rate constant for energy transfer

(kEnT): 1,2

szrJ
kEnT = C R6 (4-1)

 

where C is a collection of constants, and K2, k,, J, and R are as defined
previously (eq. 1-1). The spectral overlap integral (J) describes the
resonance condition for energy transfer and is by far the most essential

variable in the Forster equation, and is given in equation 4-2.

We}: 510’) (4—2)

 

The spectral overlap integral (J) quantifies the energetic correspondence
between the donor emission and the acceptor absorption transition moment
dipoles, and is described in detail in Chapter 1. The degree of spectral

overlap is directly proportional to the magnitude of the Forster rate constant

184

(kEnr), and must be non-zero in order for dipole-dipole energy transfer to be
considered as a possible excited-state quenching pathway.

Results from the CuRez assemblies presented in Chapter 3 showed that
ligand-field acceptor states of Cu(pyacac)2 (pyacac = 3-(4-pyridy1)-2,4-
pentanedione) can quench the 3MLCT (metal-to-ligand charge transfer)
excited states of Rel-polypyridyl donor complexes. The spectral overlap '
values between the ReI donor emission spectra with the ligand-field
absorption profile of Cu11 did not manifest a direct correlation with the
magnitude of the energy transfer rate constants within the structurally
homologous series. The ground state absorption spectrum of the d9 CuII core
exhibits two broad absorption bands that are attributed to four spin-allowed
ligand-field transitions,3 which did not allow for the spectral overlap
integrals of each individual d-d transition with the ReI emission to be
determined. The ground state absorption profile of Cu(phacac); (phacac =
3-phenyl-2,4-pentanedione) was fit with a series of Gaussians in order to
evaluate the spectral overlap of the individual Cull—based ligand-field
acceptor states with the emission spectra of the Rel-polypyridyl donor
moieties. The trend in spectral overlap values with each of the Gaussians,
along with computational efforts on Cu(phacac)2 revealed that the through-

space orientation (1(2) of the 3d orbitals relative to the 3MLCT donor

185

emission dipoles also play a role in the Forster energy transfer dynamics. In
contrast to the four distinct orbital based transitions corresponding to the
ligand-field bands of Cu“, the 4A2 —» “T2 ligand-field absorption of Cr‘" is
characterized by a multielectronic wavefunction deriving from a single

configuration (Figure 4-1).4

.,,_1_l_1__"11

Figure 4-1. One electron-orbital description of the 4A2 —> 4T2 electronic
transition in Crm. This diagram depicts the electronic configurations
from which the multielectronic wavefunction corresponding to the 4A2
and 4T2 are derived.

The focus of the work contained in this chapter is two fold, 1) utilize the
nature of the Cr"l ligand-field transition to examine if the rate of 3MLCT
emission quenching can directly correlate with the spectral overlap between
ligand-field-type transitions and Rel-based emission spectra and 2) utilize
the emissive properties of CrIII ligand-field excited states to provide
unequivocal proof that the Rel-based donor complexes undergo energy
transfer dynamics with ligand-field acceptor states.

The synthesis, structure, and photophysical properties of a series of

isostructurally related molecules, with the general formula

186

[Cr(Py3036)3(Re(tmb)(C0)3)3](0T03 (1), [Cr(PyacaC)3(Re(bPY)(C0)3)3]-
(OTf)3 (2), and [Cr(pyacac)3(Re(deeb)(CO)3)3](OTf); (3) (where pyacac = 3-
(4-pyridyl)-acetylacetonate, tmb = 4,4’-5,5’-tetramethyl-2,2’-bipyridine, bpy
= 2,2’-bipyridine, deeb = 4,4’-diethylester-2,2’-bipyridine, and OTf =
CF3SO3') are described. The Crm metal centers are covalently attached to
three fac-Re(bpy’)(CO)3 (bpy’ = tmb, bpy, and deeb) moieties through three
pyridyl-acetylacetonate bridging ligands (Figure 4-2). It was observed that
the Rel-based 3MLCT excited-states for complexes 1, 2, and 3 are

"1 metal center relative to

significantly quenched in the presence of the Cr
structurally analogous AlRe3 analogues (complexes 4, 5, and 6 from Chapter
2). Varying the bpy’ attached to the ReI metal center permitted a systematic
spectral overlap analysis with the Crm ligand-field absorption profile.
Emission and excitation spectra for complex 2 and Cr(phacac)3 were also
acquired in frozen 4:1 EtOH/MeOH, the results of which serve to confirm

quenching of the Rel-based 3MLCT excited state by an energy transfer

mechanism.

187

 

 

73+

cl

/// \\

O
12R1=R2=CH3
22R1=R2=H

3: R1 = COzEt, R2 = H

 

Figure 4-2. Structure of the [M(pyacac)3(Re(bpy’)(CO)3)3](OTf)3
assemblies (where M = Cr'll or All" and bpy’ = tmb, bpy, or deeb).

4.2 Experimental Section

4.2.1 Synthesis and Characterization

General. All solvents used were purified and dried according to previously
reported methods.5 Spectroscopic grade CHzClz was used for all
photophysical measurements; the solvent was dried under CaHz reflux until

no water was detected by lH NMR and degassed using freeze-pump-thaw

188

techniques. 3-(4-pyridyl)-2,4-pentanedione,6 Cr[N(SiMe3)2]2(TI-IF)2,7
Al(pyacac)3,6 Re(tmb)(CO)3(OTf),8 Re(bpy)(CO)3(OTf),8 Re(deeb)(CO)3-
(OTf),8 and fac-[Re(bpy)(CO)3(4-Etpy)](PF6) (4-Etpy = 4-ethylpyridine)9
were prepared following literature procedures. The synthetic procedures for
[A1(PyacaC)3(Re(tmb)(C0)3)3](0T03 (4), [A1(PyacaC)3(Re(bPY)(C0)3)3l-
(OTD3 (5), and [A1(pyacac)3(Re(deeb)(CO)3)3](OTO3 (6) are reported in
Chapter 2. 3-phenyl-2,4-pentanedione was purchased from TCI America.
Elemental analyses and F T-IR data were obtained through the analytical
facilities at Michigan State University. Mass spectra were obtained through
the analytical facilities at The University of South Carolina.
[Cr(pyacac)3(Re(tmb)(CO)3)3](OTf)3 (1). Preparation of [Re(tmb)-
(CO)3(pyacac)](OTf) precursor was prepared as follows: 193 mg (0.306
mmol) of Re(tmb)(CO)3(OTf) and 185 mg (1.04 mmol) of pyacac were
dissolved in 40 mL of THF. The reaction solution was flushed with argon
for 20 min and then refluxed for 6 hrs under argon. The volume of the
reaction mixture was then reduced to 10 mL and hexanes were added to
precipitate a yellow solid. The solid was then washed with ~ 100 mL of
diethyl ether to remove excess pyacac, dried under vacuum overnight, and
then used as is for the synthesis of compound 1. The synthesis of compound

1 is given as follows: 78 mg (0.151 mmol) of Cr[N(SiMe3)2]2(TI-IF)2 was

189

dissolved in 5 mL of THF and cooled to —78°C . in a Nz-filled drybox. 368
mg (0.455 mmol) of [Re(tmb)(CO)3(pyacac)](OTf) in 20 mL of THF was
added dropwise to the cold solution over 5 min. A light green precipitate
was observed immediately upon addition, and the mixture was allowed to
stir for 20 min. The reaction mixture was removed from the drybox and
additional green solid was precipitated by addition of hexanes. The green
precipitate was dissolved in ACN, filtered through celite, and the solvent
removed under vacuum. The product was recrystallized several times from
acetonitrile/ether (1:1 v/v). Yield: 166 mg (44%). Anal. Calcd for
C34H73N9F9024S3CrRe3'2HzO: C, 40.17; H, 3.29; N, 5.02. Found: C, 39.99;
H, 3.56; N, 5.02. IR (KBr, v(CO) (cm")): 2030 s, 1912 s, 1565 s, 1031 s,
638 m. MS: [ESL m/z (rel. int.)]: 676 (100) {[Cr(pyacac)3(Re(tmb)-
(C0)3)3]}3+, 1088-6 (33) {[Cr(py3030)3(Re(tmb)(CO)3)3](OTf)}2+, 2326-3 (1)
1[Cr(pyacac)3(Re(tmb)<C0)3)3](0m}"i
[Cr(pyacac)3(Re(bpy)(CO)3)3](OTf)3 (2). Preparation of the [Re-
(bpy)(CO)3(pyacac)](OTf) precursor was prepared as follows: 365 mg
(0.634 mmol) of Re(bpy)(CO)3(OTf) and 372 mg (2.10 mmol) of pyacac
were dissolved in 40 mL of THF. The reaction solution was flushed with
argon for 20 min and then refluxed for 4 hrs under argon. The volume of the

reaction mixture was then reduced to 10 mL and hexanes were added to

190

precipitate a yellow solid. The solid was then washed with ~ 100 mL of
diethyl ether to remove excess pyacac, dried under vacuum overnight, and
then used as is for the synthesis of compound 2. The synthesis of compound
2 is given as follows: 67 mg (0.130 mmol) of Cr[N(SiMe3)2]2(TI-{F)2 was
dissolved in 5 mL of THF and cooled to -78°C in a Nz-filled drybox. 294
mg (0.390 mmol) of [Re(bpy)(CO)3(pyacac)](OTf) in 10 mL of THF was
added dropwise to the cold solution over 5 min. A light green precipitate
was observed immediately upon addition, and the reaction mixture was
allowed to stir for 20 min. The mixture was removed from the drybox and
additional green solid was precipitated by addition of hexanes. The green
precipitate was dissolved in ACN, filtered through celite, and the solvent
removed under vacuum. The product was recrystallized several times from
acetonitrile/ether (1:1 v/v). Yield: 36 mg (12%). Anal. Calcd for C72H54N9—
F902433CrRe3: C, 37.49; H, 2.36; N, 5.46. Found: C, 37.41; H, 2.50; N, 5.45.
IR (KBr, v(CO) (cm")): 2032 s, 1916 s, 1568 m, 1031 m, 638 m. MS: [E81,
m/z (rel. int.)]: 619.9 (63) {[Cr(pyacac)3(Re(bpy)(CO)3)3]}3+, 1004.4 (14)
{[Cr(pyacaC)3(R6(bpy)(C0)3)3](0T0}2+, 2158 (1) {[Cr(Pya080h(Re(bPY)-
(CO)3)31(0T02}‘*.

[Cr(pyacac)3(Re(deeb)(CO)3)3](OTf)3 (3). Preparation of the [Re-

(deeb)(CO)3(pyacac)](OTf) precursor was prepared as follows: 165 mg

191

(0.230 mmol) of Re(deeb)(CO)3(OTf) and 51 mg (0.288 mmol) of pyacac
were dissolved in 40 mL of THF. The reaction solution was flushed with
argon for 20 min and then refluxed for 12 hrs under argon. The volume of
the reaction mixture was then reduced to 10 mL and hexanes were added to
precipitate a yellow solid. The solid was then washed with ~ 100 mL of
diethyl ether to remove excess pyacac, dried under vacuum overnight, and
then used as is for the synthesis of compound 3. The synthesis of compound
3 is given as follows: 48 mg (0.092 mmol) of Cr[N(SiMe3)2]2(TI-IF)2 was
dissolved in 5 mL of THF and cooled to —78°C in a Nz-filled drybox. 248
mg (0.276 mmol) of [Re(deeb)(CO)3(pyacac)](OTf) in 10 mL of THF was
added dropwise to the cold solution over 5 min. A light green precipitate
was observed immediately upon addition, and the reaction mixture was
allowed to stir for 20 min. The mixture was removed from the drybox and
additional green solid was precipitated by addition of hexanes. The green
precipitate was dissolved in ACN, filtered through celite, and the solvent
removed under vacuum. The product was recrystallized several times from
acetonitrile/ether (1:1 v/v). Yield: 76 mg (30%). Anal. Calcd for
C90H73N9F9036S3CrRe3-1.5HzO C, 39.08; H, 2.95; N, 4.56. Found: C, 38.69;
H, 3.18; N, 4.86. IR (KBr, v(CO) (cm")): 2036 s, 1925 s, 1577 m, 1031m,

638 m. MS: [ESI, m/z (rel. int.)]: 764.1 (16) {[Cr(pyacac)3(Re(deeb)-

192

(c0)3)3]}3+, 1220.6 (2) {[Cr(pyacac)3(Re(deeb)(CO)3)3](OTf)}2+, 2590.3
(05) {[Cr(PyacaC)3(Re(deeb)(C0)3)3](OTflz}1+-

Tris(3-phenyl-acetylacetonato)chromium(III), Cr(pghacac)3. This
compound has been previously synthesized by a different method.10 269 mg
(0.519 mmol) of Cr[N(SiMe3)2]2(THF)2 was dissolved in 15 mL of THF and
cooled to -78°C in a Nz-filled drybox. 275 mg (1.56 mmol) of pyacac 10
mL of THF was added dropwise to the cold solution over 5 min. A deep
green colored solution was observed immediately upon addition, which was
allowed to stir for 20 min. The reaction mixture was removed from the
drybox and a dark blue solid was precipitated by addition of hexanes. The
blue precipitate was dissolved in CHzClz, filtered through celite, and the
solvent removed under vacuum. The product was recrystallized from
CHzClz/hexanes (l :1 v/v) and yielded purple crystals. Yield: 150 mg (50%).
Anal. Calcd for C33H33O6Cr: C, 68.62; H, 5.76. Found: C, 68.41; H, 5.71.
4.2.2 Physical Measurements

Electronic Absorption and Steady-State Emission Spectroscopies.
Extinction coefficients for all compounds were acquired using a Varian Cary
50 UV-Visible spectrophotometer. All spectra were acquired in room-
temperature CHzClz solution. Steady-state emission spectra were acquired

using a Spex Fluoromax fluorimeter and corrected for instrumental response

193

using a NIST standard of spectral irradiance (Optronic Laboratories, Inc.,
OL220M tungsten quartz lamp)” Spectra were acquired on samples
dissolved in thoroughly degassed CH2C12 under optically dilute conditions
(o.d. ~ 0.1) and sealed under an argon atmosphere in 1 cm path length quartz
cuvettes.

Radiative quantum yields ((1),) were determined relative to fac-
[Re(bpy)(CO)3(4-Etpy)](PF6) ((1)r = 0.18 in CHzClz).9 Quantum yields were

calculated according to equation 4-2,

(Iunk /Aunk) [flunk )2

unk : s d ..
t (Istd /Astd) ”std (4 3)

where (Dunk and (Dad are the radiative quantum yields of the sample and the
standard, respectively, Iunk and [std represent the areas of the corrected
emission profiles for the sample and the standard, Aunk and Astd are the
absorbance values of the sample and the standard at the excitation
wavelength, and nunk and 115m correspond to the indices of refraction of the
sample and standard solutions (taken to be equal to the neat solvents).
Excitation wavelengths were 355 nm for the bpy and tmb analogs and 400
nm for the deeb analogs. The corrected excitation spectrum of fac-
[Re(bpy)(CO)3(4-Etpy)](PF6) in CHzClz compared favorably with the

compound’s absorption spectrum over the range of wavelengths examined

194

(355 - 400 nm), implying that the radiative quantum yield for fac-
[Re(bpy)(CO)3(4-Etpy)](PF6) does not vary significantly over this spectral
window. The reported value of (I)r = 0.18 was therefore used for determining
the radiative quantum yields at both T.,, = 355 nm and 400 nm.

Values for the zero-point energy gap (E00) of the Rel-based 3MLCT
excited states were determined by fitting the emission profiles of complexes
4-6 based on the approach described by Claude and Meyer.12 Wavelength
data were converted to energy units following the correction of Parker and
Rees;l3 the best fit was determined by visual inspection of the results of a
least-squares minimization routine.

Time-Resolved Emission Spectroscopy. Nanosecond time-resolved
emission data for the AlRe3 model complexes 4-6 were collected using a
NszAG-based laser spectrometer that has been described previously.ll
Data were acquired at room temperature in thoroughly degassed CHzClz
solutions having absorbances of ~0.1 at the excitation wavelengths.
Samples were sealed under an argon atmosphere in 1 cm path length quartz
cuvettes. The decay traces correspond to an average of 250 shots of the
signal probed at the emission maximum of each compound.

Picosecond time-resolved emission data for the CrRe3 complexes 1-3

were collected using a time-correlated single photon counting (TCSPC)

195

apparatus that has been described previously.14 Data were acquired in
thoroughly degassed CHzClz solutions having absorbances of ~0.1 at the
excitation wavelength. Samples were sealed under an argon atmosphere in 1
cm path length quartz cuvettes. Each reported decay trace corresponds to a
signal average of three data sets, with each data set resulting from ca. 30
minutes of data acquisition time. Data were fit using the OriginPro 7.5
software package. '5

Low-Temperature Spectroscopy. Low-temperature steady—state
spectroscopic data were acquired using a Janis SVT-lOO optical dewar. The

experimental protocols that were employed have been described elsewhere.16

4.3. Results and Discussion

4.3.1 Synthesis and Characterization. Although the structural and redox
properties of the Rel-based multinuclear assemblies presented in this
document strongly support energy transfer, the arguments have been based
on exclusion of other possibilities as opposed to direct evidence.
Observation of emission from an acceptor moiety involved in excited-state
quenching dynamics is fairly bulletproof evidence of an operative energy
transfer mechanism. None of the acceptor metal centers discussed thus far

(Fem and Cu") exhibit emission characteristics, and therefore no

1. 96

photophysical signatures have been observed to unequivocally prove energy
transfer. Cr(acac)3-type complexes show emission out of the 2E ligand-field
excited state at low-temperature,4 which inspired the incorporation of Cr111 as
an acceptor metal center in the Rel-based donor-acceptor structural motifs.

The formation of the tetranuclear CrRe3 assemblies, as with the FeRe3
complexes reported in Chapter 2, was facilitated by the low steric crowding
afforded by the 120° separation of the three pyacac ligands attached to the
Cr"I center. ESI-MS data for complexes 1-3 in acetonitrile solution are
consistent with the formation of the desired CrRe3 assemblies. For example,
complex 2 shows peaks corresponding to [Cr(pyacac)3(Re(bpy)(CO)3)3]3+,
[Cr1pyacac)3(Re(bpy>(c0m(our: and [Crtpyacac)3(Re(bpy)(CO)3)31-
(OTf)2H. The general robustness of the CrRe3 complexes is also evidenced
by the relative invariance of the steady-state emission intensity of complex 2
in CHzClz solution over the course of several hours (Vida infi'a), in stark
contrast to what was observed for the FeRe3 complexes described in Chapter
2.

The v(CO) stretching bands of the Re(bpy’)(CO)3 moieties reported in
the Synthesis and Characterization section (4.2.1) were assigned based on

previously reported data for fac-[Re(4,4’-X2-bpy)(CO)3(4-Etpy)](PF6)

complexes.17 The characteristic spectral profile consists of two very intense

197

peaks. The broad band at lower energy corresponds to two overlapping
transitions assigned to the A'(2) and A" modes (Cs symmetry), whereas the
sharper, higher energy band is assigned as A'(l).18 The similarity of the
carbonyl frequencies observed for each CrRe3/AlRe3 pair is indicative of

"1 metal

minimal direct electronic communication between the Re1 and Cr
centers in the ground states of the CrRe3 assemblies.19

4.3.2 Electronic Absorption Spectroscopy. The electronic absorption
spectra of complexes 1-6 were acquired in room-temperature CH2C12
solutions; spectra for all 6 complexes are shown in Figure 4-3. The
observed features were assigned based on previously reported analyses of
Re1 polypyridyl absorption spectra. ReI polypyridyl complexes exhibit a 1A.
-—> lMLCT (t2g —> n1 (bpy’)) transition occurring from approximately 330 to

2021
d.

430 nm depending on the substituents of the bpy’ ligan The absorption

profiles of the AlRe3 compounds permitted analysis of the donor-based
IMLCT transitions without overlapping transitions associated with the Crm-
containing systems (1-3). The absorption maximum for the lMLCT excited-
state reflects the electron donating/withdrawing ability of the bpy’
substituents, with Max for complexes 4, 5, and 6 at occurring at 344 (a =

18,500 M’lcm'l), 364 (a = 11,500 M'lcm'l), and 394 (a = 14,300 M‘cm“)

nm, respectively. As the substituents become progressively more electron

198

donating (e.g., H for bpy and (CH3)4 for tmb) this feature systematically
shifts to the blue and begins to overlap with the ligand-based absorptions in

the ultraviolet (Figure 4-3).

199

 

75000 W A
60000 4

45000

T

 

30000-

15000-

 

 

 

 

 

75000
62500
50000-
37500-
25000
12500b

 

 

 

 

 

75000-
62500~
50000-
37500-
25000-
12500-

 

Molar Absorptivity (M'lcm'l) Molar Absorptivity (M'lcm’l) Molar Absorptivity (M‘lcm')

 

 

 

300 350 400 450 500 550 600
Wavelength (nm)

Figure 4-3. Electronic absorption spectra of [M(pyacac)3(Re(bpy’)-
(CO)3)3](OTf)3 assemblies, where M = CrIII (red traces) and All" (blue
traces). All spectra were acquired in room-temperature CHzClz
solution. A. [Cr(pyacac)3(Re(tmb)(CO)3)3](OTf)3 (1) and [Al(pyacac)3-
(Re(tmb)(CO)3)3](OTf)3 (4)- R [Cr(pyacac)3(Re(bpy)(CO)3)3](OTf)3
(2) and [A1(Pyaca0)3(Re(bpy)(C0)3)3](OTf): (5)- C- [Cr(pyaca0)3(Re-
(deeb)(C0)3)3l(0Tf)3 (3) and [A108/3030):;(Re(deeb)(C0)3)3](OTfls (6)-

200

l
I
7
v
U" 51’ ‘ .

 

The presence of Crlll in complexes 1-3 gives rise to a new, broad
absorption feature on the high-energy side of the Rel-based charge-transfer
band. Figure 4-4 shows the absorption spectrum of Cr(phacac)3 in room—
temperature CHzClz which exhibits a strong transition centered at 390 nm
assigned to a 4A2 ——> 4LMCT (71: (acac) -—> tzg) transition. The 4A2 —-> 4LMCT
absorption in complexes 1-3 is somewhat obscured by the 1A1 -—> lMLCT
band of the Re1 chromophore, but a small shoulder is apparent in the three
CrRe3 absorption spectra (Figure 4-3). Figure 4-4 also shows a broad peak
at 570 nm (e = 113 M'lcm'l) which is assigned to the 4A2 —-> 4T2 ligand-field

III

transition of the Cr metal center. These assignments are based on the well-

known ground state absorption properties of Cr(acac)3 derivatives.”24

 

700
600
500_
400»
300-
200~
100~

Molar Absorptivity (a)

 

 

 

400 500 600 700 800
Wavelength (nm)

Figure 4-4. Electronic absorption spectrum of Cr(phacac)3 acquired in
room-temperature CH2C12 solution.

201

4.3.3 Steady-State and Time-Resolved Emission. Emission spectra for the
CrRe3 and AlRe3 assemblies were obtained in room-temperature
deoxygenated CH2C12 solution. As discussed in Chapter 2, the AlRe3
complexes represent an ideal structural model for the dynamics associated
with the ReI based 3MLCT emission due to the inability of the AlIII ion to
engage in either electron or energy transfer quenching. The emission
profiles for the CrRe3 and AlRe3 complexes are given in Figure 4-5. The
spectral profiles for both the Crm- and Alm-containing complexes
correspond to previously reported photophysical studies of Re1 polypyridyl
systems, with the emission originating from the 3MLCT —> 1A.
phosphorescence.25 The emission maximum for the tmb (526 nm), bpy (566
nm), and deeb (624 nm) derivatives reflect the electron donating and
withdrawing behavior of the polypyridyl ligands. The radiative quantum
yields ((1),) for complexes 1-6 were determined relative to [(bpy)Re(CO)3(4-
Etpy)](PF6) (<1)r = 0.18 in CHzClz) and are given in Table 4-1. The (I)r values
for the AlRe3 model complexes, which were originally reported and
discussed in Chapter 2, are comparable to the reported values for the
corresponding mononuclear ReI polypyridyl derivatives.8 As can be seen in
Figure 4-5, the emission intensity for all three CrRe3 complexes are

significantly attenuated compared to the AlRe3 model systems indicating

202

 

extensive quenching of the Rel-based 3MLCT excited-state. In addition,
smaller radiative quantum yields ((1),) calculated for the CrRe3 complexes (1-
3) compared to the All" model complexes (Table 4-1) indicated significant
quenching 3MLCT excited-state as well.

It should also be noted that (I), values for previously reported
chromophore-quencher complexes of FeRe3 (Chapter 2) and CuRez (Chapter
3) were analytically unreliable due to small amounts of H20 contained in the
CHzClz that caused slight dissociation of the complexes over time, which
produced Re(bpy’)(CO)3(pyacac) species as emissive impurities. The
emission intensity of complex 2 remained constant when monitored over the
course of 4 hrs in CHzClz solution (Figure 4-6), which attests to the
robustness of the CrRe3 complexes. Therefore, quantitative radiative

quantum yield values for complexes 1-3 could be reported (Table 4-1).

203

 

 

Emission Intensity
Emlss10n Intensny

 

 

 

 

 

 

 

 

500 600 700 800 500 600 700 800
Wavelength (nm) Wavelength (nm)

 

1F (:

Emission Intensity

 

 

 

 

500 600 700 800
Wavelength (nm)

Figure 4-5. Corrected steady-state emission spectra for [M(pyacac)3_
(Re(bpy’)(CO)3)3](OTf)3 assemblies, where M = All" (blue traces) and
Cr‘” (red traces). A. [Al(pyacac)3Re(tmb)(CO)3])3](OTt)3 (4) and
[Cr(pyacac)3(Re(tmb)(CO)3)3](OTD3 (1). B. [Al(pyacac)3Re(bpy)-
(C0)3])3](0Tf)3 (5) and [Cr(pyacaC)3(Re(bPY)(C0)3)3](0T03 (2)- C-
[Al(pyacac)3(Re(deeb)(CO)3)3](OTD3 (6) and [Cr(pyacac)3(Re(deeb)-
(CO)3)3](OTf)3 (3). All six spectra were acquired in deoxygenated
room-temperature CH2C12 solutions following excitation at 355 nm
(complexes 1, 2, 4, and 5) and 400 nm (complexes 3 and 6). For each
plot, the emission profiles have been normalized with respect to the
absorbance of each sample at its excitation wavelength. The relative
magnitudes of the signals are therefore accurate representations of their
relative intensities.

204

 

 

300000 -
250000 -
200000 -
150000-
100000 - N
50000 - Whig»
0

 

Intensity (cps)

<1,
*4“

2

 

 

 

500 550 600 650 700
Wavelength (nm)

Figure 4-6. Corrected steady-state emission spectra for [Cr(pyacac)3-
(Re(bpy)(CO)3)3](OTt)3 (2) monitored over the course of 4 hrs in room-
temperature CHzClz solution. The various emission traces are
represented as follows: the initial spectrum (black trace), after 1 hr (red
trace), after 2 hrs (blue trace), after 3 hrs (green trace), and after 4 hrs
(orange trace).

Additional details concerning the excited states of the AlRe3 and
CrRe3 complexes were obtained through nanosecond emission lifetime and
time-correlated single photon counting (TCSPC) measurements. The
nanosecond emission lifetime data in room-temperature CH2C12 solution for
complexes 4, 5, and 6 were initially reported in Chapter 2, with rob, = 2260 i
100, 560 i 30, and 235 i 20 ns, respectively. The corresponding radiative
and non-radiative decay rate constants for 4, 5, and 6 are given in Table 4-1.
As with the quantum yields, the observed excited-state lifetimes and rate
8.26

constants are all consistent with Rel-based 3MLCT —+ 'Al emission,

which were discussed in Chapter 2.

205

 

The lack of discemable signal observed for the CrRe3 systems during
nanosecond lifetime experiments prompted the employment of time-
correlated single photon counting (TCSPC) methods. Plots of the TCSPC
data obtained in room-temperature CHzClz solution for the CrRe3 complexes
are shown in Figure 4-7. Complexes 1 and 2 could be fit with single-
exponential models with rob, = 6.8 :1: 0.1 ns and 4.8 i 0.1 ns, respectively
(Table 4-1). Complex 3 exhibited bi-exponential kinetics with lifetimes of
1'1 = 7.0 i 0.3 ns and 12 = 190 :t 15 ns (Table 4-1). Due to the similar values
of T2 and the AlRe3 model complex (deeb, 6), the slower component is
assigned to residual amounts of [Re(deeb)(CO)3(pyacac)](OTf) starting
material remaining after purification of complex 3. The signal-to-noise ratio
for the three TCSPC data sets (1-3) are relatively poor owing to a
combination of virtually complete quenching of the Rel-based 3MLCT states
coupled with radiative rate constants that are on the order of 105 s". As can
be seen from the 3MLCT lifetimes in the A1Re3 and CrRe3 compounds
mentioned above, 1:01,, for the three Alm assemblies are significantly longer

111

than what was observed for the Cr complexes, indicating that 3MLCT

relaxation in complexes 1-3 is dominated by reaction with the Cr"I core.

206

 

 

 

60
E 1,? 15~
e, 40 3
b as 10
i E
8 20 .3 5-
E. E

 

 

 

0 5 10 15 20 25 30

Time (ns)

 

 

 

1400:
1200
1000

800

Intensity (cps)

600

 

 

 

400

 

0 215 510 715 100 125 150 115
Time (ns)

Figure 4-7. Time correlated single-photon counting (TCSPC) data for
the CrRe3 complexes: 1 (rob, = 6.8 i 0.1 ns), 2 (robS = 4.8 i 0.2 ns), and
3 (11: 7.0 i 0.3 ns) and 1:2 = 190 i 15 ns). All data were collected in
deoxygenated room-temperature CH2C12 solution. The red solid lines
correspond to fits to single-exponential decay models for 1 and 2, and a
bi-exponential decay model for 3 due to the presence of [Re(deeb)-
(CO)3(pyacac)](OTf) as an emissive impurity. See text for further
details.

207

Table 4-1. Photophysical Data of Complexes 1-6.

 

E00 kobs (SJ) kt knr
complexes km, (cm'l)a (I)r (><108 s") (X 106 s")b (X 106 s'l)c
CI'R63
tmb (1) 526 19,700 0.024 1.5 :t 0.1 3.6 i 0.2 150 i: 10

 

bpy(2) 566 18,600 0.009 2.1401 19401 210410

deeb(3) 624 17,000 0.003 1,440.1d 04840.02 140410

AlRe3
tmb(4)c 526 19,900 0.51 0.0044 0.234001 02240.01

bpy (5)6 566 18,700 0.16 0.018 0.29 3: 0.01 1.5 3: 0.1

deeb (6)6 624 16,800 0.07 0.043 0.30 i 0.01 4.0 d: 0.2

 

aZero-point energy difference between 3MLCT excited state and ground
state based on spectral fitting analysis. bkr = kobs*(1),. ck“, = kob, - k,.
dValue reported is the faster component (1'1) of the bi-exponential
behavior observed in the TCSPC data for complex 3. See text for
further details. eThese data were reproduced from Chapter 2 for
comparison purposes.

4.4 Mechanistic Considerations

4.4.1 Electron versus Energy Transfer Quenching. Both electron and
energy transfer processes can be envisioned to occur out of the Rel-based
3MLCT excited state. Based on the presence of Cr"1 in the ground states of
the CrRe3 complexes, electron transfer would proceed as either an oxidative

quenching reaction to produce (bpy’)Re"—CrII or a reductive quenching

208

 

reaction to produce (bpy")Re"—Crw. Based on previously reported
electrochemical data by Murray and Hiller on Cr(acac)3,27 a CrIV redox-
product that would be produced in the reductive quenching mechanism
would be highly unlikely and will not be discussed further. In order to
determine if the oxidative quenching process is a thermodynamically viable

1" ——> CrII reduction potential of

mechanism in the CrRe3 systems, the Cr
Cr(acac)3,27 the Re1 —> RelI oxidation potentials for the AlRe3 model
complexes (4-6), and the zero-point energy gaps of the 3MLCT states (E00)
determined from fits of the emission spectra of the corresponding CrRe3
analogues (Table 4-1), were used to calculate the thermodynamic driving
force for photoinduced electron transfer (AGET) in complexes 16.2839 The
calculated AGET values are all significantly uphill with values of +1.18 V
(tmb, 1), +1.56 V (bpy, 2), and +1.87 V (deeb, 3). The large endothermic
AGET values effectively rule out electron transfer as a viable 3MLCT
quenching mechanism in the CrRe3 series. It should also be noted that
cyclic voltammetry on Cr(phacac)3 in CH2C12 solution was performed with

m . .
—> CrII reductlon peaks observed 1n

neither the Cr-III —» Crlv oxidation or Cr
the solvent window, agreeing with the very large AGET values calculated

from the Cr(acac)3 literature values.

209

 

Direct experimental evidence for energy transfer in the CrRe3 series
was obtained from low-temperature emission spectroscopy. The lowest
energy excited state of Cr(acac)3-type complexes (the 2E ligand-field state)
does not emit at room-temperature but exhibits a sharp emission spectrum
with km, from approximately 750 to 850 nm in frozen solution.22 The
narrowness of the band, which is assigned as the 2E —> 4A2 emission, results
from the minimal nuclear displacement associated with the
intraconfigurational nature of the excited state.”33 Figure 4-8 shows the
emission spectrum of Cr(phacac)3 in 4:1 EtOH/MeOH at 82 K that exhibits
the sharp 2E —> 4A2 emission. It should also be noted that this low-
temperature 2E —> 4A2 emission of Cr(phacac)3 represents the expected result
of Forster energy transfer quenching of the 3MLCT donor state by the Crm-
based 4A2 —-* 4T2 ligand-field transition in the CrRe3 series, and not by the
4A2 —-> 2E state. The extremely small spectral overlap calculation between
_ Rel-based emission and the spin-forbidden 4A2 —> 2E transition negates the

possibility of dipolar energy transfer coupling between the two states.

210

 

 

 

Emission Intensity (au)

 

 

 

 

Wavelength (nm)

Figure 4-8. Steady-state emission spectrum of Cr(phacac)3 in 4:1
EtOH/MeOH at 82 K following excitation at 375 nm.

, . 4 . . 111
As dlscussed above, observatlon of 2E —> A2 em1ss10n fi'om the Cr

core (Figure 4-8) in the CrRe3 assemblies would be direct experimental
evidence for energy transfer quenching. Steady-state emission data for
complex 2 in 4:1 EtOH/MeOH at 82 K is given in Figure 4-9. Based on the
Cr(phacac)3 emission spectrum shown in Figure 4-8, the resulting emission
characteristics for complex 2 have been assigned to the Crm-based 2E —> 4A2
transition. In order to distinguish between direct excitation of the Crm-based
4A2 —+ 4T2 absorption giving rise to the observed 2E —> 4A2 emission, and 2E-
based emission signal resulting from energy transfer dynamics in complex 2,
the extinction coefficient values at Mun“, = 375 nm in CH2C12 were compared
between complex 2 and Cr(phacac)3. The comparison showed that ~ 9% of
the 375 nm photons absorbed by complex 2 directly excite the Cr(acac)3

core, while the rest are absorbed by the IA] —> 1MLCT transition of the Rel

211

 

donor. The red trace in Figure 4-9 represents the emission spectrum of
complex 2 if only absorption by the Cr"I acceptor (9% contribution) was
responsible for producing the 2E ——> 4A2 emissive photons (ie. no energy
transfer). The emission spectrum of complex 2 (black trace) clearly shows a
much larger intensity than would be expected from simple Cr(acac)3
excitation, which implicates energy transfer fi'om the Rel-based 3MLCT

lll

excited state into the Cr core as being responsible for the increase in 2E —>

4A2 emission intensity.

 

~
00
l

 

6

Emission Intensity (x 10 cps)

 

 

 

Wavelength (nm)

Figure 4-9. The black trace is the corrected steady-state emission
spectra of [Cr(pyacac)3(Re(bpy)(CO)3)3](OTf)3 (2) in 4:1 EtOH/MeOH
at 82 K following excitation at 375 nm. The red trace corresponds to
the intensity of the emission spectrum scaled by the relative
absorbances of the Rel-bpy and Cr'"(pyacac)3 chromophores. See text
for further details.

Additional evidence for excited state energy transfer was obtained by
measuring the excitation spectrum of complex 2 at an energy corresponding

111

to emission solely from the Cr fluorophores. Figure 4-10 shows the

212

 

absorption spectrum of complex 2 at 298 K along with the excitation
spectrum of complex 2 at 82 K resulting from monitoring the Crm-based 2E
-—> 4A2 emission maximum (km, = 795 nm). As can be seen in Figure 4-10,
the overall shape of the absorption and excitation spectra are conserved,
which demonstrates that emission produced from the 2E —> 4A2 transition of
the Cr"I center originates from both the Rel-based lMLCT and CrIH-based
absorptions. These spectra provide additional evidence that the 2E —+ 4A;
emission results from energy transfer quenching of the Rel-based 3MLCT
excited state. A noticeable deviation in the two spectra are noted in the near
UV region of Figure 4-10, and results from poor photon-to-current
efficiency for the photo-diode employed to adjust for lamp fluctuations in

excitation spectrum.

 

p.-
1
p—l

 

 

Absorbance (normalized)
(pazrleuuou) Kirsuaml

O
0

4L

350 400 450 500
Wavelength (nm)

 

Figure 4-10. Ground-state absorption spectrum of [Cr(pyacac)3(Re-
(bpy)(CO)3)3](OTf)3 (2) in 4:1 EtOH/MeOH solution at 298 K (black
trace), and the excitation spectrum of 2 in 4:1 EtOH/MeOH at 82 K
monitoring at item = 795 nm (red trace).

213

4.4.2 Dexter vs Fiirster Energy Transfer. In light of the prohibitively large
electron transfer driving forces and the low-temperature emission and
excitation spectra for complex 2, the 3MLCT quenching in all three of the
CrRe3 assemblies has been assigned as excited-state energy transfer. The
two most important mechanisms for energy transfer are electron
superexchange (Dexter)34 and through-space dipole-dipole coupling
(Fiirster).35 Dexter energy transfer is subject to a distance dependence that
falls off as e'2R due to its reliance on orbital overlap. As such, it is usually
relegated to covalently linked systems in which the donor and acceptor are
in close proximity (S 5 A)”37 Due to the structural and electronic
similarities between the CrRe3 series and the FeRe3 assemblies (Chapter 2),
Dexter energy transfer has been ruled out as a possible quenching
mechanism in the CrRe3 complexes. See the detailed comparison between
the two energy transfer mechanisms given in Chapter 2 (2.4.2) for further
details.

In addition to the similarities between the CrRe3 and FeRe3 assemblies
mentioned above, the applicability of the Ffirster mechanism in the CrRe3
series is supported by the moderate degree of spectral overlap that exists

between the Rel-based 3MLCT emission and the Crm-based 4A2 —-) 4T2

absorption: this is depicted graphically in Figure 4-11. The overlap between

214

 

the 3MLCT —> [A] emission of the Alm-containing tmb (4) (blue trace), bpy
(5) (green trace), and deeb (6) (red trace) analogues with the 4A2 —> 4T2
absorption of Cr(phacac)3 (black trace) is shown, with the bpy derivative
possessing the greatest amount of spectral overlap. The area ascertained
from the product of the normalized emission profile of an AlRe3 analogue
with the extinction coefficient spectrum of Cr(phacac)3 was used to calculate
the reported spectral overlap values. Forster theory predicts that as the
spectral overlap (J) increases across a structurally homologous series, the
observed rate constant for energy transfer should increase by that same
factor. The magnitude of spectral overlap (J) shows excellent agreement
with the trend in k,.,, values of the CrRe3, with J values of 6.92 x 10'16 (k,,, =
1.5 x 108 s"), 8.07 x 10'16 (kos. = 2.1 x 108 s"), and 6.28 x 10''6 (ks.s = 1.4 x
108 s") M"cm3 for the tmb (1), bpy (2), and deeb (3) analogs, respectively.
It is quite clear that the overlap requirement in these systems is satisfied,
which strongly implicates Férster transfer as the dominant quenching

mechanism in the CrRe3 complexes.

215

 

 

 

Absorbance (normalized)

 

 

(pazneuuou) Ansuarul uoyssgwg

22 20 l8 16 14
Energy (x 103 cm'l)

Figure 4-11. Overlay of the emission spectra of [Al(pyacac)3(Re(tmb)-
(C0)3)3](0Tf)3 (4. blue), [A1(Pyacac)3(Re(bPY)(C0)3)3](OTfls (5,

green), and [Al(pyacac)3(Re(deeb)(CO)3)3](OTf)3 (6, red) with the
electronic absorption spectrum of Cr(phacac)3. Data were acquired in
room-temperature CH2C12 solution.

4.4.3 Comparison with the CuRez Assemblies. A plot of the spectral
overlap between the Rel-based 3MLCT emission and the Cull-based ligand-
field absorptions for the CuRez assemblies was reported and discussed in
Chapter 3 (Figure 3-10). The ground state absorption spectrum of
Cu(phacac)2 in CHzClz solution shows a two-band structure, with these
bands representing four discrete d-d transitions. Table 4-2 gives the spectral
overlap integrals, energy transfer rate constants (kEnT), and kEnT/k, values for
the CuRez and the CrRe; families. As discussed in Chapter 3, kEnT/k, values
are used as the energy transfer rate constants due to variations in kr for the

Rel donor moieties. Dividing kEhr by kr essentially normalizes out the

variation in the radiative component and allows for sole evaluation of the

216

 

energy transfer coupling. As can be seen from Table 4-2, the differences in
spectral overlap (J) among the CrRe3 series directly corresponds to the
observed trend in the kEnT/k, values, but an analogous correlation for the
CuRez series is not observed. In addition to spectral overlap considerations
in the CuRez series, variations in through-space coupling (1(2) of the 3MLCT
transition moment dipoles with the four separate ligand-field absorption
dipoles of the CulI acceptor had to be taken into account to interpret the
dipolar energy transfer dynamics (Chapter 3). It was found that the two
CuRez complexes possessing the lowest energy 3MLCT excited states (dclb
and deeb) possess very favorable energetic (J) and through-space
interactions (K2) with the lowest energy Cuu d-d transition, which in turn
yielded the fastest observed energy transfer rate constants for the two
derivatives (Table 4-2). The direct dependence of the trend in kEnT/k, with
the amount of spectral overlap seen for the CrRe3 complexes essentially
validates the analysis of the Cull-containing complexes presented in Chapter

3. This validation comes from the fact that the 4A2 —) 4T2 ligand-field

acceptor transition derives from a single configuration of the Cr"I ion, which
results in identical through-space interactions with the three 3MLCT

emission dipoles within the series and produces a direct spectral overlap

dependence of the energy transfer rate constants.

217

 

Table 4-2. Calculated Overlap Integral Values (J) and Energy Transfer
Rate Constants for the CrRe3 and CuRez Complexes.

 

 

kEnr
compound J a (X 108 s' 1)b anT/kr
[Cr(pyacac)3(Re(tmb)(CO)3)3](OT1)3 (1) 6.92 1.50 650
[Cr(pyacac)3(Re(bpy)(CO)3)3](OTf)3 (2) 8.07 1.70 718
[Cr(pyacac)3(Re(deeb)(CO)3)3](OTf)3 (3) 6.28 1.40 452
[Cu(pyacac)2(Re(tmb)(CO)3)2](OT1)2c 4.29 0.67 208
[Cu(pyacac)2(Re(dmb)(CO)3)2](OTf); 5.14 1.18 295
[Cu(pyacac)2(Re(bpy)(CO)_~,)2](OT1)2c 5.52 1 . 18 347
[Cu(pyacac)2(Re(dclb)(CO)3)2](OTf);c 6.68 1.71 658
[Cu(pyacac)2(Re(deeb)(CO)3)2](OTf);c 7.09 1 .96 700

 

alReported in units of 10'16 M'lcm3. bkEnT = kob,(CrRe3) — kob, (AlRe3).
°These data were reproduced from Chapter 3 for comparison
purposes.

4.5 Conclusions
The synthesis and photophysical properties of a series of donor-
acceptor complexes based on Rel-bpyridine donors and Crm-acac acceptors

have been described. Steady-state and time-resolved emission

spectroscopies indicated that the strongly emissive Rel-based 3MLCT

excited state was significantly attenuated when compared to model

Ill 1111

complexes in which the Cr center had been replaced by A . Observation

111

of enhanced 2E —) 4A2 emission of the Cr acceptor in complex 2 in frozen

218

 

4:1 EtOH/MeOH unequivocally identified energy transfer as the 3MLCT
quenching mechanism. The favorable overlap between the 3MLCT donor
emission and the 4A2 —> 4T2 acceptor absorption profile, coupled with a ca.
10 A donor-acceptor separation, allowed for an assignment of F6rster
(dipolar) energy transfer as the dominant excited-state reaction mechanism.

Lastly, enhancement of the Cr"I

acceptor emission observed in complex 2
gives direct evidence for energy transfer as the 3MLCT excited state

quenching mechanism assigned in all of the Rel-based polynuclear donor-

acceptor assemblies discussed throughout this dissertation.

219

 

4.6 References and Notes

(1)
(2)

(3)
(4)
(5)

(6)

(7)
(8)

(9)
(10)
(11)

(12)
(13)
(14)

(15)
(16)

Ftirster, T. Discuss. Faraday Soc. 1959, 27, 7.

Van Der Meer, B. W.; Coker, G. 1.; Chen, S.-Y. Resonance Energy
Transfer, Theory and Data; VCH Publishers: New York, 1994.

Belford, R. L.; Carmichael, J. W., Jr. J. Chem. Phys. 1967, 46, 4515.
Juban, E. A.; McCusker, J. K. J. Am. Chem. Soc. 2005, 127, 6857.

Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals; 3rd ed.; Pergamon Press: Oxford, U. K., 1988.

Mackay, L. G.; Anderson, H. L.; Sanders, J. K. M. J. Chem. Soc.,
Perkin Trans. 1. 1995, 18, 2269.

Harada, Y.; Girolami, G. Polyhedron 2007, 26, 1758.

Hino, J. K.; Della Ciana, L.; Dressick, W. J .; Sullivan, B. P. Inorg.
Chem. 1992, 31, 1072.

Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952.
Murakami, Y.; Nakamura, K. Bull. Chem. Soc. Jpn. 1966, 39, 901.

Damrauer, N. H.; Boussie, T. R.; Devenney, M.; McCusker, J. K. J.
Am. Chem. Soc. 1997, 119, 8253.

Claude, J. P.; Meyer, T. J. J. Phys. Chem. 1995, 99, 51.
Parker, C. A.; Rees, W. T. Analyst (London) 1960, 85, 587.

DeWitt, L.; Blanchard, G. J.; LeGoff, E.; Benz, M. E.; Liao, J. H.;
Kanatzidis, M. G. J. Am. Chem. Soc. 1993, 115, 12158.

Origin; 7.5 ed.; OriginLab Corp.: Northhampton, MA, 1991 - 2004.

Damrauer, N. H.; McCusker, J. K. Inorg. Chem. 1999, 38, 4268.

220

(17)

(18)

(19)

(20)

(7-1)

(22)
(23)

(24)

(25)

(26)

(27)
(28)

(29)

(30)

Dattelbaum, D. M.; Omberg, K. M.; Schoonover, J. R.; Martin, R. L.;
Meyer, T. J. Inorg. Chem. 2002, 41, 6071.

Dattelbaum, D. M.; Martin, R. L.; Schoonover, J. R.; Meyer, T. J. J.
Phys. Chem. A. 2004, 108, 3518.

Argazzi, R.; Bertolasi, E.; Chiorboli, C.; Bignozzi, C. A.; Itokazu, M.
K.; Murakami Iha, N. Y. Inorg. Chem. 2001, 40, 6885.

Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J .; Demas, J. N.;
DeGraff, B. A. Inorg. Chem. 1990, 29, 4335.

Worl, L. A.; Duesing, R.; Chen, P.; Della Ciana, L.; Meyer, T. J. J.
Chem. Soc., Dalton Trans. 1991, 849.

Forster, L. S. Chem. Rev. 1990, 90, 331.
Kirk, A. D. Chem. Rev. 1999, 99, 1607.

Endicott, J. F.; Ramasami, T.; Tamilarasan, R.; Lessard, R. B.; Ryu,
C. K.; Brubaker, G. R. Coord. Chem. Rev. 1987, 77, 1.

Striplin, D. R.; Crosby, G. A. Chem. Phys. Lett. 1994, 221, 426.

Kestell, J. D.; Williams, Z. L.; Stultz, L. K.; Claude, J. P. J. Phys.
Chem. A. 2002, 106, 5768.

Murray, R. W.; Hiller, L. K. Anal. Chem. 1967, 39, 1221.
Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259.

The form of the Rehm-Weller equation employed to characterize the
oxidative quenching process is AGETox = Eox (Rel/H) - Bred (Crmm) - E00,
where E00 is the energy gap between the ground and lowest energy
3MLCT excited state of the donor. The small work term typically
employed for bimolecular systems was not included in the analysis.

Krause, R. A.; Trabjerg, I.; Ballhausen, C. A. Acta. Chem. Scan. 1970,
24, 593.

221

(31)
(32)
(33)
(34)
(35)

(36)

(37)

Chen, S.; Porter, G. B. J. Am. Chem. Soc. 1970, 92, 2189.
Atanasov, M. A.; Schonherr, T. Inorg. Chem. 1990, 29, 4545.
Yardley, J. T.; Beattie, J. K. J. Am. Chem. Soc. 1972, 94, 8925.
Dexter, D. L. J. Chem. Phys. 1953, 21 , 836.

F ’o'rster, T. Annalen der Physik 1948, 2, 55.

Brown, W. R.; O'Boyle, N. M.; McGarvey, J. J .; Vos, J. G. Chem.
Soc. Rev. 2005, 34, 641.

Goeb, S.; De Nicola, A.; Ziessel, R.; Sabatini, C.; Barbieri, A.;
Barigelletti, F. Inorg. Chem. 2006, 45, 1173.

222

 

Chapter 5. Spin Dependent Energy Transfer: CoRe3 and NiRez

Complexes.

5.1 Introduction

The area of focus contained in this chapter is on understanding the
role of electronic spin in determining the rate of excited-state energy transfer
in polynuclear transition metal-based donor-acceptor complexes. In addition
to possessing favorable thermodynamics, excited-state energy transfer
reactions, just as in simple bimolecular reactions, must conserve the total
spin angular momentum of the reactants and products throughout the course

of the reaction. From vector physics, the possible total spin angular

—

momentum (lsrl) values for a combination of two complexes with spins

fi fl

1811 and 1 21 are

“fl

S1 +S2

 

 

1345—1

 

,..., 151 - Sz

 

. For example,

if 1 11 2 and 1821 = 1 then the ISTI values are 3, 2, and 1. A generic

representation of an excited-state energy transfer reaction between an

_ —

*

SD

 

 

excited donor with and a ground state acceptor with 1SA| is given in

equation 5-1:

223

 

T “—T kn _— T
1801+ s, ——ET—>1s,,1+ s,

 

 

(5-1)

 

 

In addition to favorable thermodynamics, the reaction defined by equation 5-

fl

1 must possess a common IST1 between the reactants and products. A

classic example of spin conservation involved in energy transfer reactions is
the quenching of 3MLCT (metal-to-ligand charge transfer) excited-states of
low-spin d6 polypyridyl complexes.l Figure 5-1 illustrates total spin angular

momentum conservation in an excited-state energy transfer reaction between

#

:1:

SD

 

 

a d6 polypyridyl-based 3MLCT state ( = 1), which describes the excited

state properties of the Re1 donors utilized throughout this dissertation, and an

fl
fl

0; molecule that possesses a triplet ground state (1 AI = l). A ISTI value

of 1 is conserved between the reactant complexes and product complexes
involved in the energy transfer process, indicated by the connecting arrow,
and therefore the excited state reaction is spin-allowed. Investigating this
fundamental property of excited-state reactivity utilizing transition metal-

based donor-acceptor systems, will be the focus of this chapter.

224

 

 

 

* Energy Transfer *
D ——-A > D A

80:1 SA=1 80:0 SA=1

ST=2,1,0 ST:

Figure 5-1. Illustration of total spin angular momentum conservation in
the energy transfer quenching of a 3MLCT excited-state by Oz.

The first section of this chapter will focus on the synthesis and
photophysical properties of a tetranuclear complex with the general formula
[Co(pyacac)3(Re(bpy)(CO)3)3](OTf)3. The CoIII metal center is covalently
attached to three fac-Re(bpy)(CO)3 moieties through three pyridyl-
acetylacetonate bridging ligands (Figure 5-2). It was observed that the
relaxation dynamics of the Rel-based 3MLCT excited-state in the CoRe3
assembly is similar to a structurally analogous AlRe3 model complex. Based
on spectral overlap determinations, the 1A1 ——> 'T1 ligand-field state of the
Co"1 acceptor shows very favorable energetic matching with the Rel-based
3MLCT donor emission, which indicates the Col" ligand-field state should
serve as an efficient quenching pathway out of the ReI excited-state. These
results contrast with the excited-state dynamics described in the previous

chapter for the isostructural CrRe3 complex - [Cr(pyacac)3(Re(bpy)(CO)3)3]-

225

 

(OTt)3. The difference in reactivity can be explained by considering the role
of spin conservation in the energy transfer reaction: whereas the 4A2 ——> 4T2
ligand-field transition of the Crlll center in the CrRe3 assembly yields a spin-

allowed energy transfer pathway out of the Rel-based 3MLCT excited-state.

fl

Evaluation of IST1 for excited-state energy transfer in the CoRe3 system

shows that coupling to the Com-based 'Al -—+ 1T1 ligand-field transition
represents a spin-forbidden relaxation pathway. This represents the first
example of an energy transfer reaction in which spin polarization is dictating

reactivity.

226

 

 

Figure 5-2. Structure of the CoRe3 assembly.

The second section of this chapter will focus on the structural and
photophysical properties of a chromophore-quencher molecule with the
general formula [Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2](OTDZ (where pyacac
= 3-(4-pyridyl)—acetylacetonate, THF = tetrahydrofuran, bpy = 2,2’-
bipyridine, and OTf = CF3SO3'). The NiII metal center is covalently attached
to two fac-Re(bpy)(CO)3 moieties through two pyridyl-acetylacetonate
bridging ligands (Figure 5-3). The X-ray structural data for the NiRez

system shows two THF solvent molecules axially coordinated to the NilI

227

 

metal center that produces a pseudo-octahedral NiO6 core (3A2). It was
observed that the Rel-based 3MLCT excited-state for the NiRez assembly is
significantly quenched when compared to an electronically benign BeRez
model complex. Based on spectral overlap considerations, the 3A2 —> 3T2
ligand-field state of NiIl was assigned as the thermodynamically viable
quenching pathway for the Rel-based 3MLCT excited state. Measuring the
3MLCT emission properties of the NiRez compound in CHzClz and in THF

solutions revealed an axial ligand dissociation mechanism. The NiRez

 

assembly, and particularly chemistry associated with the Ni" ion, represents
an excellent example of the utility of basic coordination chemistry in

controlling excited-state reactivity.

 

Figure 5-3. Structure of the NiRez assembly.

228

5.2 Experimental Section
5.2.1 Synthesis and Characterization

General. All solvents used were purified and dried according to
previously reported methods.2 Spectroscopic grade CHzClz was used for all
photophysical measurements; the solvent was dried under CaHz reflux until
no water was detected by 1H NMR and the solvents used in the emission
experiments were degassed using freeze-pump-thaw techniques. 3-(4-
pyridyl)-2,4-pentanedione,3 Ni(phacac)2,4 and fac-[Re(bpy)(CO)3(4-
Etpy)](PF(,)5 (4-Etpy = 4—ethylpyridine) were prepared following literature
procedures. The synthetic procedure for [Re(bpy)(CO)3(pyacac)](OTf) is
presented in Chapter 4. The synthetic procedure for [Al(pyacac)3(Re(bpy)-
(CO)3)3](OT1)3 is presented in Chapter 2 and the synthetic procedure for
[Be(pyacac)2(Re(bpy)(CO)3)2](OTf)2 is presented in Chapter 3. 3-phenyl—
2,4-pentanedione was purchased from TCI America and Ni(1,5-
cyclooctadiene); was purchased from Sigma-Aldrich. Elemental analyses
and FT-IR data were obtained through the analytical facilities at Michigan
State University. Mass spectra were obtained through the analytical
facilities at The University of South Carolina.

[Co(pyacac)3(Re(bpy)(CO)3)3](OT03. Amounts of 11 mg (0.04

mmol) of Co(acetate)2-4HzO and 98 mg (0.13 mmol) of [Re(bpy)(CO)3-

229

'. -3 4‘? 1

 

(pyacac)](OTf) were dissolved in 10 mL of ethanol and heated at 100°C for
20 min in air. The reaction solution turned a dark green color and a
yellowish precipitate formed in the solution. The green filtrate was
collected, the solvent removed under vacuum, and the resulting green solid
placed under vacuum overnight. The product was recrystallized several
times from acetonitrile/ether (l :1 v/v). Yield: 25 mg (25%). Anal. Calcd for
C72H54N9F9024S3C0Re3-2H20: C, 36.80; H, 2.49; N, 5.36. Found: C, 36.40;
H, 2.47; N, 4.98. MS: [ESL m/z (rel. int.)]: 622.3 (5)
1[CotpyacacMRetbpyXCOh)3113*, 1007.9 (2) {1Co(pyacac)s(Re(bpy)-

(CO>3)31(0T0}2*.

[Ni(pyacac)2(Re(bpy)(CO)3)2](OTf)2. In 30 mL of THF were
dissolved 285 mg (0.379 mmol) of [Re(bpy)(CO)3(pyacac)](OTf) in a N2-
filled drybox. 52 mg (0.189 mmol) of Ni(1,5-cyclooctadiene)2 were then
added and the solution turned a deep red color, along with the formation of a
green precipitate. The reaction mixture was allowed to stir for 24 hrs in the
drybox, after which time the reaction mixture was removed from the drybox
and the green precipitate collected. The green solid was washed with THF,
placed under vacuum overnight, and then recrystallized several times from
acetontrile/ether (1:1 v/v) to yield the product. X-ray quality crystals were

obtained by slow diffusion of THF into an acetonitrile solution of the

230

 

01:“

compound. Yield: 41 mg (14%). Anal. Calcd for C43H36N6F6016S2NiRe2: C,
36.91; H, 2.32; N, 5.38. Found: C, 35.86; H, 2.61; N, 4.99. IR (KBr, v(CO)
cm“): 2033 s, 1922 s. MS: [ESL m/z (rel. int.)]: 632.0 (35)
{[Ni(pyacac)2(Re(bpy)(CO)s)2]}2‘1 1413.0 (1) {[Ni(pyacac)2(Re(bpy)-

(COthOTDVi

Tris(3-phenyl-acetylacetonato)cobalt(III), Co(phacac)3. The
synthesis of this compound has been previously reported by a different
method.6 Amounts of 85 mg (0.534 mmol) of Co(acetate)2-4HZO and 205
mg (1.16 mmol) of phacac were dissolved in 10 mL of H20 and heated at
100°C for 20 min in air. A dark green precipitate that formed in the reaction
solution was collected, washed with water, and recrystallized from hot
ethanol. Yield: 100 mg (50%). Anal. Calcd for C33H33O6Co-0.3EtOH: C,
67.45; H, 5.86. Found: C, 67.05; H, 5.45.
5.2.2 Physical Measurements

X-ray Structure Determinations. Single-crystal X-ray diffraction
data for [Ni(pyacac)2(TI-IF)2(Re(bpy)(CO)3)2](OTf)2 were acquired at the X-
ray facility of Michigan State University. Diffraction data were collected on

a Siemens SMART diffractometer with graphite-monochromatic Mo K01
radiation (A. = 0.71073A). Data were collected at -100 °C by using an Oxford

Cryosystems low temperature device. Crystallographic data are summarized

231

in Table 5-1; selected bond distances and angles are listed in Table 5-2.
Lattice parameters were obtained fiom least-squares analyses and data were
integrated with the program SAINT.7 The integration method employed a
three dimensional profiling algorithm and all data were corrected for Lorentz
and polarization factors, as well as for crystal decay effects. The absorption
correction program SADABS8 was employed to correct the data for
absorption effects. The structures were solved by direct methods and
expanded using Fourier techniques. All structure calculations were
performed with the SHELXTL 6.12 software package.9 Anisotropic thermal
parameters were refined for all non-hydrogen atoms. Hydrogen atoms were
localized in their calculation positions and refined by using the riding model.
Further details concerning the structure determinations may be found in
Supporting Information.

Electronic Absorption and Steady-State Emission Spectroscopies.
Extinction coefficients for all compounds were acquired in CHzClz solutions
using a Varian Cary 50 UV-Visible spectrophotometer. Steady-state
emission spectra were acquired using a Spex F luoromax fluorimeter and
corrected for instrumental response using a NIST standard of spectral
irradiance (Optronic Laboratories, Inc., OL220M tungsten quartz lamp).10

Spectra were acquired on samples dissolved in thoroughly degassed CHzClz

232

 

under optically dilute conditions (o.d. ~ 0.1) and sealed under an argon
atmosphere in 1 cm path length quartz cuvettes.

Radiative quantum yields ((1),) were determined relative to fac-
[Re(bpy)(CO)3(4-Etpy)](PF6) (ch, = 0.18 in Cinch).5 Quantum yields were

calculated according to equation 5-2,

2
(I) (I) (Iunk/Aunk){nunk]

unk Std (Istd /Astd) ”std (SI-2)

where (Dunk and (1),“, are the radiative quantum yields of the sample and the
standard, respectively, Iunk and Istd represent the areas of the corrected
emission profiles for the sample and the standard, Aunk and Astd are the
absorbance values of the sample and the standard at the excitation
wavelength, and nunk and 1],“! correspond to the indices of refraction of the
sample and standard solutions (taken to be equal to the neat solvents). An
excitation wavelength of 355 nm was used for all the compounds studied in
this chapter.

Time-Resolved Emission Spectroscopy. Nanosecond time-resolved
emission data for [Co(pyacac)3(Re(bpy)(CO)3)3](OTf)3, [Al(pyacac)3(Re-
(bPY)(C0)3)3](0Tf)3. and [Be(PyacaC)2(Re(bp)')(C0)3)2](OTf)2 were
collected using a NszAG-based laser spectrometer that has been described

previously.10 Data were acquired at room temperature in thoroughly

233

 

degassed CH2C12 solutions having absorbances of ~0.1 at A.,, = 355 nm.
Samples were sealed under an argon atmosphere in 1 cm path length quartz
cuvettes. The decay traces correspond to an average of 250 shots of the
signal probed at the emission maximum of each compound.

Picosecond time-resolved emission data for [Ni(pyacac)2(THF)2(Re-
(bpy)(CO)3)2](OTf)2 were collected using a time-correlated single photon
counting (TCSPC) apparatus that has been described previously.ll Data were
acquired in thoroughly degassed CHzClz solutions having absorbances of
~0.1 at A1,, = 370 nm. The sample was sealed under an argon atmosphere in
1 cm path length quartz cuvette. The reported decay trace corresponds to a
signal average of three data sets, with each data set resulting from ca. 30
minutes of data acquisition time. Data were fit using the OriginPro 7.5

software package. '2

5.3 Results and Discussion for [Co(pyacac)3(Re(bpy)(CO)3)3](OTf)3

5.3.1 Synthesis and Characterization. The interest in the CoRe3 system
was to synthesize a donor-acceptor complex with similar overall charge and
structural characteristics to the CrRe3 compounds presented in Chapter 4.
The 4A2 —+ 4T2 ligand-field acceptor state of Crm(acac)3 is replaced by a 1A,

-—9 'Tl transition in the case of the CoRe3 system, allowing the comparison

234

of energy transfer rates simply on the differences in spin-state associated
with the acceptor.

As with the F eRe3 (Chapter 2) and CrRe3 (Chapter 4) complexes, the
formation of the tetranuclear CoRe3 assembly was facilitated by the low
steric crowding afforded by the 1200 separation of the three pyacac ligands

111

attached to the Co center. ESI-MS data for [Co(pyacac)3(Re(bpy)-

(CO)3)3](OTf)3 in acetonitrile solution are consistent with the formation of
the desired complex, with m/z peaks observed for [Co(pyacac)3(Re(bpy)-
(count and lCotpyacachtRetbpyXCOhhl2".

5.3.2 Electronic Absorption Spectroscopy. The electronic absorption
spectra of [Co(pyacac)3(Re(bpy)(CO)3)3](OTf)3 and [Al(pyacac)3(Re(bpy)-
(CO)3)3](OTf)3 were acquired in room-temperature CHzClz solutions and are
shown in Figure 5-4. Transitions associated with the AlIII complex were
assigned in Chapter 2, with the shoulder centered at 360 nm assigned to the
Rel-based 1A. -—> 1MLCT (t2g —> 11:. (bpy’)) transition (Figure 5-4). The
AlRe3 complex allows for analysis of the ground-state absorption behavior

of the ReI donor moieties without overlapping with the Com-based charge-

transfer transitions associated with the CoRe3 assembly.

235

 

 

Absorbance (au)

 

 

 

 

 

300 400 500 600
Wavelength (nm)

Figure 5—4. Electronic absorption spectra of [Co(pyacac)3(Re(bpy)-
(CO)3)31(OTD3 (red trace) and [Al(pyacac)3(Re(bpy)(CO)3)3](OTf)3
(blue trace) acquired in room-temperature CHzClz solution.

The presence of Co"1

in the CoRe3 complex gives rise to a new, broad
absorption feature occurring at approximately 340 nm and overlapping with
the Rel-based charge transfer band (Figure 5-4). Figure 5-5 (Top) shows the
absorption spectrum of Co(phacac)3 that exhibits the 340 nm transition
which is assigned to a 1A] —+ lLMCT (7t (acac) —> eg*) transition.
Co(phacac)3 also possesses a transition at km, = 605 nm (Figure 5-5,
Bottom) with a measured extinction coefficient of 193 M'lcm'l. This band is
easily assigned to the 1A, —> 1T] transition, the lowest-energy spin-allowed
d-d band of a pseudo-octahedral CoIn species.“3 It should also be noted that
the IA. —> IT] transition occurs at approximately the same energy as the 4A2

-—> 4T2 transition of Cr(phacac)3 reported in Chapter 4, and should have

similar spectral overlap with 3MLCT emission of the Rel-bpy fluorophore.

236

 

 

 

Comparing the effect on the rate of 3MLCT relaxation incurred by the Co"l

and Cr"! ligand-field acceptor states will be the focus of the discussion

section.

 

35000-
30000-
25000-
20000-
15000.
10000-
5000-
0

 

 

 

 

 

Molar Absorptivity (M'lcm'l)

250 300 350 400 450 500 550
Wavelength (nm)

Figure 5-5. Top. Electronic absorption spectrum of Co(phacac)3
showing the higher energy charge transfer and organic-based
transitions. Bottom. Electronic absorption spectrum of a concentrated
solution of Co(phacac)3 showing the mid-visible 1A1 —> 1T1 ligand-field
transition. Both spectra were acquired in room-temperature CHzClz
solution.

237

 

Figure 5-5 (cont’d)

 

200

l60~\

120-\

4:.
O
//

 

 

Molar Absorptivity (M'lcm'l)
00
O
C

500 600 700 800 900 1000
Wavelength (nm)

O
r

 

5.3.3 Time-Resolved Emission. Insight into the Rel-based 3MLCT excited
state relaxation behavior of the CoRe3 and AlRe3 complexes were obtained
through nanosecond emission lifetime measurements. Nanosecond time-
resolved emission data in room-temperature CHzClz solution for
[A1(pyacac)3(Re(bpy)(CO)3)3](0T03 and [C0(pyacac)s(Re(bpy)(CO)3)3l-
(OTf)3 are given in Figure 5-6. As discussed in Chapter 2, emission from
the AlRe3 complex could be fit to a single-exponential model with rob, = 530
i 20 ns, with corresponding radiative (k,) and non-radiative (km) decay rate
constants of 3.0 i 0.1 x 105 s'1 and 1.6 d: 0.1 x 106 s", respectively,
corresponding to ground-state recovery for the Rel-based 3MLCT excited
state. The time-resolved emission data for [Co(pyacac)3(Re(bpy)(CO)3)3]-
(OTf); could likewise be fit to a single-exponential model with rob, = 595 i

20 ns. The 3MLCT emission lifetime in the CoRe3 assembly is ~60 ms

238

 

longer than the electronically benign AlRe3 model compound. The slightly
longer observed time constant is not completely understood, but the data do
illustrate the absence of 3MLCT quenching in the CoRe3 system. Additional
CoRe3 material needs to be synthesized and the time constants re-measured

before the longer emission lifetime in the CoRe3 complex can be scrutinized.

 

 

 

 

 

 

 

 

 

 

 

 

50- Al 50- Co
A 40» A 40-
> >
E 30— E 30-
32‘ .E‘
E 20 - a 20 -
8 2
t: t:
"‘ 10 - "‘ 10 -
0 J l l I l 0 I l I I
0 1 2 3 4 0 l 2 3 4
Time (x103 ns) Time (x103 ns)

Figure 5-6. Lefi‘. Nanosecond time-resolved emission data for [Al-
(pyacac)3(Re(bpy)(CO)3)3](OTD3 (robS = 530 i 20 ns). The data were
originally presented in Chapter 2 and are reproduced here for
comparison purposes. Right. Nanosecond time-resolved emission data
for [Co(pyacac)3(Re(bpy)(CO)3)3](OTD3 (robs = 595 i 20 ns). Both
traces (Al and Co) were acquired in deoxygenated room-temperature
CH2C12 solution monitoring at the emission maximum (565 nm). The
solid red lines correspond to fits to mono-exponential decay models for
both complexes.

5.3.4 Spin-Dependent Dipolar Energy Transfer. As mentioned in the
Introduction, in addition to possessing a downhill thermodynamic driving
force (i.e. spectral overlap), the total spin angular momentum must be

conserved during an energy transfer event. In Chapter 4, significant

239

quenching of the Rel-based 3MLCT excited state was observed for a

3 MLCT
structurally similar CrRe3 complex ( 1'0133 = 4.8 :t 0.1 ns) that possesses

a similar ligand-field absorption profile and maximum as the CoRe3
assembly, and therefore should produce similar spectral overlap with the
3MLCT emission profile and manifest corresponding rates of 3MLCT
relaxation. The similarity in emission lifetimes between the CoRe3 and

AlRe3 complexes indicates that the CoIll

center is not engaging in excited-
state reactivity with the Rel-based 3MLCT state, even though F firster energy
transfer quenching is thermodynamically favorable due to a significant
amount of spectral overlap observed between the 1A1 —> IT, ligand-field
transition of Co(phacac)3 and the Rel-based 3MLCT donor emission
spectrum (Figure 5-7). The only differing characteristic that could be giving
rise to the large variance observed for the 3MLCT relaxation kinetics

between the CoRe3 and CrRe3 assemblies is the spin-state of the Com ('Al)

and CrIII (4A2) metal centers.

240

 

 

 

.—-
—n

 

 

 

O I l I I I 0
500 600 700 800 900 1000
Wavelength (nm)

 

Molar Absorptivity (normalized)
(pozueuuou) Ansuarul uogssrurg

Figure 5-7. Overlay of the emission spectrum of [Al(pyacac)3(Re(bpy)-
(CO)3)3](OTf)3 (blue trace) with the electronic absorption spectrum of
Co(phacac)3 (red trace).

Figure 5-8 shows spin conservation diagrams between the Rel-based
3MLCT excited state with the Cr"I (Top) and Com (Bottom) acceptor cores in

111

the CrRe3 and CoRe3, respectively. The Cr ground state in the CrRe3

complex is 4A2 (18A1= 3/2), and when coupled with the 3MLCT excited

_.

SIS

 

 

state of the Re1 donor ( = 1) generates total spin angular momentum

(18T1) values of 5/2, 3/2, and 1/2. As an energy transfer (EnT) reaction

occurs out of the] Rel-based 3MLCT excited state to produce the Crm-based

4A2 —> 4T21igand-field transition, an 1ST1 = 3/2 is generated on the products

side of the diagram from the coupling of the resulting ReI ground state

241

 

#

 

A = 3/2). As can been

 

(lSDl = 0) with the Crm-based 4T2 excited state (

fl

seen in Figure 5-8, an 1311 value of 3/2 between the reagents and the

products is conserved, which provides the spin-allowed energy transfer
pathway.
Conversely, Figure 5-8 (Bottom) shows the corresponding process for

the CoRe3 assembly. The C0111 ground state in the CoRe3 complex is IA]

(lSAl = O), and when coupled with the 3MLCT excited state of the ReI

—'
an:

SD

#

 

 

donor ( = 1) generates a total spin angular momentum (ISTj ) value of

III

1 on the reagents side. The acceptor state of the Co ion is the 1A1 —> 1T1

fl

32.

 

 

ligand-field state ( = 0), and when coupled with the ReI ground state

— _——'

(ISDI = 0) an 18T1 = 0 is produced on the products side. As can'be seen in

— #

Figure 5-8 (Bottom), a mutual IST1 value between the reagents (ISTI = 1)

—’

and the products (IST1 = O) is not conserved, therefore the energy transfer

reaction is spin-forbidden. The spin statistics outlined in Figure 5-8

illustrates the theoretical framework behind the shorter 3MLCT emission

242

 

lifetime observed in the CrRe3 complex compared to the CoRe3 assembly.
The results discussed throughout this section highlight the control one can

have on excited-state reactivity by simply changing the spin state of a single

 

reagent.
EnT
- II III I III *
(bpy )Re mCr spimauowed (bpy)Re~’W(Cr )
80:1 SA=3/2 80:0 SA=3/2
S1- = 5I2, 3/2, 1I2 S1- = 3I2
EnT
b - R "MC ||| .................. , b Re'wv Col" *
( py) e o spin-forbidden ( py) ( )
80:1 SA=O 30:0 SA=O
ST = 1 ST = 0

Figure 5-8. Top. Spin conservation diagram illustrating the spin-
allowed energy transfer pathway between the 3MLCT excited-state of
the ReI donor and the CrlII acceptor. Bottom. Spin conservation diagram
illustrating the spin-forbidden pathway between the 3MLCT excited-
state of the ReI donor and the CoIII acceptor.

243

 

 

5.4 Results and Discussion for [Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2]-
(OTfh
5.4.1 Synthesis and Characterization. The interest in the NiRe2 complex
was to explore the effect solvent coordination can play on Forster energy
transfer dynamics in covalently attached donor-acceptor complexes, and
whether the rate of energy transfer can be controlled by simply varying the
coordination strength of the solvent. Unlike the square-planar Cu"(acac)2
center utilized in the CuRez series (Chapter 3), square-planar Ni"(acac)2
possesses a large propensity to bind molecules in the axial positions.”'7
These solvent interactions change the ligand-field environment from 4- to 6-
coordinate, and more importantly to the work in this chapter, the spin-state
of the Ni" center.‘8

The formation of the trinuclear NiRez assembly was facilitated by the
low steric crowding afforded by the 180° separation of the two pyacac
ligands attached to the Ni11 center. ESI-MS data for [Ni(pyacac)2(THF)2(Re-
(bpy)(CO)3)2](OTf)2 in acetonitrile solution are consistent with the formation
of the desired complex, with m/z peaks observed for [Ni(pyacac)2(Re-
(bpy)(c0)3)212* and [NitpyacachtRe<bpy><COhhl(0T0"1 corresponding to
the loss of the two axial THF ligands during the course of the measurement.

X-ray quality crystals were generated by diffusion of THF into an

244

 

 

acetonitrile solution of the compound over the course of approximately one
week, which provides evidence for the general robustness of the complex in
solution. In addition, the steady-state emission intensity of the NiRez system
in THF solution remained constant when monitored over the course of 5 hrs,
which proves the compound remains intact during the duration of the various
photophysical measurements that will be presented.

The v(CO) stretching bands of the Re(bpy)(CO)3 moiety reported in
Section 5.2.1 were assigned based on previously reported data for the fac-
[Re(bpy)(CO)3(4-Etpy)](PF6) complex.19 The characteristic spectral profile
consists of two very intense peaks. The broad band at lower energy
corresponds to two overlapping transitions assigned to the A'(2) and A"
modes (C, symmetry), whereas the sharper, higher energy band is assigned
as A'(l).20 The similarity of the carbonyl fi'equencies observed in the
NiRez/BeRez pair is indicative of minimal direct electronic communication
between the Re1 and Ni]1 metal centers in the ground states of the NiRez
assemblies.21
5.4.2 Single-Crystal X-ray Structures. X-ray quality crystals were
obtained for [Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2](OTf);. The NiRe2
assembly crystallizes in the triclinic space group P-l. Crystallographic

details are given in Table 5-1 with selected bond distances and angles for the

245

two complexes listed in Table 5-2. A square-planar coordination
environment about the central metal ion (Figure 5-9) is formed from the four
oxygen atoms of the acac ligands lying in the equatorial plane. The Ni—O
bond distances of 1.971 :t 0.005 and 1.981 i 0.005 A and a 0.0 A mean-
plane deviation of the NiII ion from the O4 plane are both consistent with
square-planar NiII and compare favorably with other structurally
characterized examples of Nin-acac systems.4’22 The structure of the NiRe2
assembly also shows Ni-O (THF) distances of 2.155 :1: 0.006 A
corresponding to axial bonding of THF molecules to the Ni11 core. The axial
Ni-O distances are similar to Ni(acac)2(HzO)2,23 which indicates the two
axial interactions represent true covalent metal-ligand interactions. The
internal geometries of the Re(bpy)(CO)3 moieties exhibit the classic pseudo-
C3v coordination environment common to ReI complexes in this class.24'26
Lastly, the Rel-"Ni" distances (9.868 A) are similar to the Rel-"M distances
reported for the FeRe3 (Chapter 2) and CuRez (Chapter 3) complexes, which
will manifest similar through-space donor-acceptor interactions in the NiRez

assembly as has been discussed for the previous MRex (x = 2 or 3)

complexes.

246

Table 5-1. Crystallographic Data for [Ni(pyacac)2(THF)2(Re(bpy)-

 

(C0)3)2](0Tf)2.
formula C56H52N6F601382N1R62
Mw 1706.26
cryst syst triclinic
space group P-l
T/K 173(2)
a/A ”11.220(2)
b/A 11.819(2)
c/A 15.737(2)
01/° 80.188(9)
13/° 72.111(9)
y/° 68.904(8)
V/A3 1848.7(5)
Z 1
Dc/g cm'l 1.800
20max 57
reflns measured 9762
independent reflns 6125
observed reflns [I > 20(1)] 4075
u(Mo K01)/cm" 3.738
Rim 0.0775
R1a 0.0509
wR2b 0.1351
GOF 0.995

 

3R1 = sum. chll/ZIFOI. waz = [2w(F02 — Fc2)2/2w(F02)2]'/2, w
= l/[O' 2(13,") + (aP)2 + bP], where P = [F.3- + 21:31/3.

247

 

Table 5-2. Selected Bond Distances (A) and Angles (deg) for
[Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2](OTDZ.

 

Bond Distances (A)
Ni( 1 )—O( 1) 1 .971(5)
Ni(1)—O(2) 1.981(5)
Ni(1)—O(1A) 1.971(5)
Ni(1)—O(2A) 1.981(5)
Re( 1 )—N( 1) 2.23 8(5)
Re( 1 )—N(2) 2.152(6)
Re(l)—N(3) 2.112(10)
Re(1)—C(12) 1.903(12)
Re(1)—C(13) 1.901(10)
Re(1)—C(14) 1.912(9)
Ni(1)--- O(6) 2.155(6)
Ni(1)--- O(6A) 2.155(6)
Ni(1)°"Re(1) 9.868
Ni(1)°°°Re(1A) 9.868
Bond Angles (deg)
O(1)—Ni(1)—O(2) 89.6(2)
O(1)—Ni(l)—O(1A) 179.999(2)
N(1)—Re(l)—N(2) 88.0(2)
C(12)—Re(l)—N(1) 92.1(4)
C(14)—Re(l)—N(1) 178.2(4)
aplane Implane 2 87.539
bplane 1mplane 2 87.539

 

aPlane 1 is defined by atoms 0(1), 0(2), C(7), C(8), C(8), C(10), C(11);
plane 2 is defined by atoms N(1), C(2), C(3), C(4), C(5), C(6). bGiven
by the atoms that define the two planes in the adjacent pyacac ligand.

248

 

 

Figure 5-9. Drawing of the cation [Ni(pyacac)2(THF)2(Re(bpy)—
(CO)3)2](OT1)2 obtained from single-crystal X-ray structure
determination. Atoms are represented as 50% probability thermal
ellipsoids.

5.4.3 Electronic Absorption Spectroscopy. The electronic absorption
spectra of [Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2](OTDZ and [Be(pyacac)2(Re-
(bpy)(CO)3)2](OTf)2 were acquired in room-temperature CH3CN solutions
and are shown in Figure 5-10. Transitions associated with these complexes
were assigned based on previously reported analyses of ReI polypyridyl
absorption spectra and”28 the ground state absorption data for the
structurally similar CuRez complexes previously discussed (Chapter 3). The
lowest energy absorption feature for BeRez is centered at 360 nm (a = 6,800
M'lcm") and is assigned as the Re[(bpy)-based 1A. —> lMLCT (t2g —-> rt*
(bpy’)) transition. The BeRez complex allows for analysis of the ground—

state absorption behavior of the Re1 donor moieties without overlapping with

249

 

 

the Niu-based charge-transfer absorptions associated with the NiRez
analogue. These charge transfer transitions are very apparent in the NiRez
absorption spectrum due to the sizable increase in the molar absorptivity

values observed in the ~ 340 to 400 nm range.

 

50000
40000 '
30000 ~
20000 -

10000 -

 

 

 

Molar Absorptivity (M'lcm'l)

 

 

250 300 350 400 450 500 550
Wavelength (nm)

Figure 5-10. Electronic absorption spectra of [Ni(pyacac)2(THF)2(Re-

(bPY)(C0)3)2](0Tf)2 (red trace) and [36(pyacaC)2(Re(bPY)(C0)3)21-
(OTf); (blue trace) acquired in room-temperature CH3CN solution.

In addition to the Rel-based charge transfer transition, the NiRez
complex also possesses ligand-field transitions associated with the Nill core.
The electronic absorption spectrum of Ni(phacac)2 in CHzClz solution,
which serves as a model for the ligand-field transitions associated with the
Ni11 center of the NiRez assembly is shown in Figure 5-11. The shape of
these low energy Nin-based transitions with Max = 640 nm, as well as the
observed amax = 17 M'lcm'l has been observed previously by Fackler and

Cotton.29

250

 

 

 

 

N N
O LI.
1 r

t—l
LII
T

U!
r

 

 

Molar Absorptivity (M'lcm'l)
8

O

 

500 600 700 800 900
Wavelength (nm)

Figure 5-11. Electronic absorption spectra of Ni(phacac)2 acquired in
room-temperature CHzClz solution.

5.4.4 Steady-State and Time-Resolved Emission. Emission spectra for
[Ni(pyacac)2(THF)2(Re(bpy)(CO);)2](OT02 and [Be(pyacac)2(Re(bpy)-
(CO)3)2](OT1)2 were obtained in deoxygenated room-temperature CH2C12
solution. The BeRe2 analogue represents an ideal structural model for the
dynamics associated with the Re1 based 3MLCT emission, due to the
inability of engaging in quenching dynamics incurred by the NiII core in the
NiRez system. The emission profiles are given in Figure 5-12, and show the
emission intensity for the NiRez is significantly quenched relative to the
Ben-containing system. The spectral profile for the BeRe2 compound was
first observed in Chapter 3, with the emission originating from the 3MLCT
—-> 1A] phosphorescence.30 The emission maximum for both the Ni"- and
Ben-containing derivatives is 565 nm suggesting similar emission

characteristics for the NiRez complex. A radiative quantum yield ((1),) of

251

 

 

0.18 was calculated for the BeRe2 derivative (Chapter 3) when determined
relative to [(bpy)Re(CO)3(4-Etpy)](PF6) ((1)r = 0.18 in CHzClz). The (I)r
value exactly corresponds with the reported value of the mononuclear
Rel(bpy) standard indicating the 3MLCT relaxation in the BeRe2 model
complex can be solely assigned to the ReI donor moiety. A (I), value for the
NiRez complex is analytically unreliable due to dissociation of the pseudo-
octahedral NiO6 coordination environment in CHzClz solution. Figure 5-13A
shows the emission spectrum of NiRez in CHzClz solution over a period of
27 hours, which shows a steady increase in the 3MLCT -—> 1A1 emission
intensity occurring as the NiRez complex remains in the CHzClz solution
over the course of the experiment. The increase in intensity is attributed not
to dissociation of the Re(bpy)(CO)3(pyacac) donor moiety from the NiII
metal center, as was seen for the F eRe3 (Chapter 2) and CuRez complexes
(Chapter 3) due to residual amounts of H20 contained in the CH2C12, but
loss of the axial THF ligands attached to the NiII center to generate a N106-x
(x = 1 or 2) environment. This is confirmed by Figure 5-13B, which shows
the emission intensity of the NiRez complex in THF solution remains
constant when monitored over the course of 5 hours. If the NiRez complex
was dissociating to generate Re(bpy)(CO)3(pyacac) species in solution, the

dissociation process would be occurring to varying degrees regardless of

252

 

solvent. The possible NiO6.x structures formed in solution are discussed in

the next section.

 

 

 

 

Emission Intensity (normalized)

 

Wavelength (nm)

Figure 5-12. Corrected steady-state emission spectra for [Ni(pyacac)2-
(THF)2(Re(bPY)(C0)3)2](OTf)2 (red tr8109) and [Be(PyacaC)2(Re(bPY)-
(CO)3)2](OT1)2 (blue trace). Acquired in deoxygenated room-
temperature CH2C12 solution following excitation at 355 nm. The
emission profiles have been normalized with respect to the absorbance
at 355 nm, and therefore the relative magnitudes of the signals are
accurate representations of their relative intensities.

253

 

 

 

 

 

4000000-
3500000-
3000000-
2500000-
2000000-
1500000—
1000000-
500000-
0

Intensity (cps)

 

 

 

 

 

450 500 550 600 650 700 750 800
Wavelength (nm)

 

100000-

80000r

60000»

40000-

lntensity (cps)

20000—

 

O

 

 

 

 

500 550 600 650 700
Wavelength (nm)

Figure 5-13. A. Corrected emission spectra of [Ni(pyacac)2(THF)2(Re-
(bpy)(CO)3)2](OTf)2 in CH2C12 solution over a period of 27 hours (hex =
355 nm) exhibiting a steady increase in emission intensity. See text for
further details. B. Corrected emission spectra of [Ni(pyacac)2(THF)2-
(Re(bpy)(CO)3)2](OTf)2 in THF solution over a 5 hour time period (3.“
= 355 nm) showing a constant emission intensity over the course of
data collection.

Additional details concerning the excited-state states of the BeRe2 and
NiRez complexes were obtained through nanosecond emission lifetime and
time-correlated single photon counting (TCSPC) measurements. The

nanosecond emission decay lifetime (robs) in room-temperature CHzClz

254

 

 

solution for [Be(pyacac)2(Re(bpy)(CO)3)2](OTf)2, which was originally
reported and analyzed in Chapter 3, is shown on the'lefi side of Figure 5-14
and could be fit to a single-exponential model with rob, = 530 :1: 30 ns. The
corresponding radiative (k,) and non-radiative (km) decay rate constants are
3.4 i 0.2 x 105 s'1 and 1.6 i 0.1 x 106 s", respectively, which were all
consistent with the assignment of 3MLCT —-> 'A1 emission given in Chapter
3. A plot of the TCSPC data obtained in room-temperature CH2C12 solution
for [Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2](OTf); is shown on the right side of
Figure 5-14. The decay trace could be fit with a bi-exponential model with
1'] = 5.0 i 0.4 ns and ӣ2 = 25 i 4 ns, with the source for the bi-exponential
behavior observed in the NiRe2 system proposed in the next section.
Regardless of the bi-exponential behavior, the observed time constants for
excited-state decay in the Nin-containing system are significantly larger than
the corresponding BeRe2 model complex, indicating the presence of a very
efficient quenching process stemming from a reaction between the Rel-based

3MLCT excited state and the Ni" core.

255

 

 

 

 

 

 

 

 

 

 

 

 

40- Be 1400~\ Ni
12001
9 30', ”g 1000—1‘11
é ' 3’ 800-
5 20-1 3? 1
g 1 2 600—1
+2 10_l E 400-.
T 1 200:1
0 1 0 . . . . . . . . .
0.0 05 1:0 1:5 2:0 2:5 3:0 3:5 0 10 20 3o 40 50 60 7o 80
Time (ns) x 103 Time (ns)

Figure 5-14. Lefi. Nanosecond time-resolved emission data for
[Be(pyacac)2(Re(bpy)(CO)3)2](OTDZ (“cobs = 530 i 30 ns). Right.
TCSPC data for [Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2](OTf)2 (1:1= 5.0 :t
0.4 ns and 12 = 25 i 3 ns). Both traces (Be and Ni) were acquired in
deoxygenated room-temperature CHzClz solution monitoring at the
emission maximum (565 nm). The solid red lines correspond to fitting
to a mono-exponential decay model for BeRe2 and to a bi-exponential
decay model for NiReg. See text for discussion of the bi-exponential
kinetics for NiRez.

5.4.5 Mechanistic Considerations: Dexter vs Ftirster Energy Transfer.
Dexter energy transfer,“ which is described as a simultaneous double-
electron exchange mechanism, requires orbital overlap between the donor
and acceptor involved in the energy transfer.32 The X-ray structure data for
[Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2](OTf)2 (Figure 5—9) shows a Rel-"Ni"
separation of nearly 10 A, a value that lies at the maximum of what is
typically considered for an exchange-based process due to the exponential

dependence (e'ZR) (R = donor-acceptor separation) of orbital overlap.33 The

10 A metal-metal distance, coupled with observed similarities in the v(CO)

256

frequencies of the NiRez and BeRe2 complexes (Section 5.2.1), largely
negate the possibility of significant electronic coupling between the Re1 and
Ni" subunits. This point is amplified by the computational results of Meyer
and coworkers on similar ReI mononuclear complexes that show the 3MLCT
wavefilnction would not exist on the bridging pyacac ligand,20 which
drastically negates the possibility of the pyacac bridging ligands imparting
significant electronic coupling between the ReI and NiII metal centers. This
combination of the structural and electronic characteristics of the NiRez
complex makes it unlikely that Dexter-type exchange is playing a dominant
role in the energy transfer dynamics of these systems.

Work discussed in Chapter 3 on the structurally similar [Cu(pyacac)2-
(Re(bpy)(CO)3)2](OTt)2 assembly found Férster energy transfer to be the
dominant excited-state quenching mechanism. The CuRez compound
contains the same ReI donor species (ie. equivalent redox potentials and
emission characteristics), pyacac bridging ligand, and similar donor-acceptor
separations as the NiRez system. Although the results of the CuRez
derivative can’t unequivocally assign the quenching mechanism for the
NiRe2 complex, the findings do point to analogous excited-state reactivity
out of the 3MLCT state of the Re1 donor. In addition, the spectral overlap

between the Rel-based 3MLCT emission with the Nin ligand-field states also

257

 

implicates Féirster-type reactivity as the dominant excited-state quenching
mechanism in the NiRez system. A qualitative picture of the spectral
overlap of the Re(bpy) donor moiety with the NiII ligand-field manifold of
Ni(phacac)2 is shown in Figure 5-15, which corresponds to a calculated
spectral overlap value of J = 2.14 x 10'16 M'lcm3. The spectral overlap value
(J = 5.52 x 10'16 M 1cm3 ) between the Rel-based 3MLCT emission and the
CuIl ligand-field states in the CuRez complex (Chapter 3) is similar to the

[Ni(pyacac)2(TI-IF)2(Re(bpy)(CO)3)2](OTf)2, which manifests as similar

kCURez

observed kinetics between the two complexes ( obs = 1.2 d: 0.1 x 108
kNiRez

and obs = 2.0 i 0.2 x 108). The similar spectral overlap and energy

transfer quenching dynamics observed between the CuRez and NiRez

complexes provides additional proof of Forster-type reactivity occurring in

[Ni(PyacaC)2(THF)2(Re(bPY)(C0)3)2](OTf)2-

258

 

 

 

Molar Absorptivity (normalized)
(pozgeuuou) Ansuaiul uogssrulg

 

 

Energy (x 103 cm'l)

Figure 5-15. Overlay of the emission spectrum of [Be(pyacac)2(Re-
(bpy)(CO)3)2](OTf)2 (blue trace) with the electronic absorption
spectrum of Ni(phacac)2 (red trace).

5.4.6 Spin Dependent Dipolar Energy Transfer. As mentioned in the
Introduction, in addition to possessing a downhill thermodynamic driving
force, the total spin angular momentum - an abiding property of reactants
and products - must be conserved during the course of a chemical reaction.
Conservation of total spin angular momentum during an excited state energy
transfer event must also be retained. From the structure of the NiRez
complex given in Figure 5-9, two THF solvent molecules are coordinated in
the axial positions of the Ni" metal center to generate a Ni(acac)2(THF)2
complex (i.e. Ni06 coordination sphere) in the core of the donor-acceptor
assembly. The single-crystal X-ray structure data of the NiRez assembly
(Table 5-2) shows the Ni—O (THF) distances are ~ 0.2 A longer than the

Ni—O (acac) distances, suggesting a weaker metal-ligand covalent

259

interaction between Ni" and the oxygens of the THF ligands. The
dissociation of both THF ligands from NiRe2 in solution would yield a ratio
of donor-acceptor complexes with a pseudo-octahedral NiO6 and pseudo-D2},
NiO4 cores in solution depending on the propensity for NiRez to lose THF
ligands. Figure 5-16 shows spin conservation diagrams between the Rel-
based 3MLCT excited state with a pseudo-octahedral NiO6 (Top) and

pseudo-D21| NiO4 (Bottom) acceptor cores. The NiII ground state in the NiO6

fl

core is 3A2 (ISAI = 1), and when coupled with the 3MLCT excited state of

fl

SB

—

 

 

the ReI donor ( = 1) generates total spin angular momentum (ISTI)

values of 2, 1, and 0. Based on the overlap integral plot given in Figure 5-
15, the acceptor state of the NiO(, core is the 3A2 —> 3T2 ligand-field.

transition. As energy transfer (EnT) quenching occurs out of the 3MLCT

_

excited state to produce the NiH-based 3A2 —> 3T2 transition, an ISTI of 1 is

generated on the products side of the diagram from the coupling of the

_p

resulting ReI ground state (1801 = 0) with the Nin-based 3T2 excited state

fl #

A = 1). As can been seen in Figure 5-16, an 1311 = 1 between the

(

 

 

260

 

reagents and the products is conserved, which provides the spin-allowed
energy transfer pathway.
Conversely, Figure 5-16 (Bottom) shows coupling between the ReI

excited state and the ligand-field states of the MO, core. The Nill ground

fl

state in MO. is 'A. (1S A1 = 0), and when coupled with the 3MLCT excited

—.§

Sh

 

 

state of the Re1 donor ( = l) generates a total spin angular momentum

(ISTI ) value of 1 on the reagents side. On the products side, the acceptor

states of the MO, core are S = O —> S = 0 ligand-field transitions (ISAI = O)

—

and the ground state of the ReI produced after the energy event is ISDI = 0

fl #

A mutual ST value between the reagents (ISTI = 1) and the products (IST1

= 0) is not conserved, therefore energy transfer is spin-forbidden.

261

 

 

EnT

 

- II ' I ' *
(bpy )Re m NIOo spimauowed (hm/)Re M(N'06)
80:1 SA=1 SD=O SA=1
ST = 2,1,0 S1- = 1
EnT
b - R "mN'o ------------------ > b Re'JW NiO *
( py ) e I 4 spin-forbidden ( DY) ( 4)
80:1 SA=O 80:0 SA=0
s1- = 1 s1- = 0

Figure 5-16. Top. Spin conservation diagram illustrating the spin-
allowed energy transfer pathway between the 3MLCT excited-state of
the Re1 donor and the pseudo-octahedral NiO6 acceptor. Bottom. Spin
conservation diagram illustrating the spin-forbidden pathway between
the 3MLCT excited-state of the ReI donor and the pseudo-D2}, NiO4
acceptor.

—

It is proposed that the differences in total spin angular momentum (ISTI)

conservation in the energy transfer reaction between the Rel-based 3MLCT
state and the ligand-field states of NiO6 and MO, cores, illustrated in Figure
5-16 (Top), give rise to the different behavior observed in the steady-state
emission spectra of Ni(pyacac)2(TI-IF)2(Re(bpy)(CO)3)2](OTf); in CHzClz
and THF solutions (Figure 5-13). This proposed axial ligand dissociation
mechanism is manifested in the steady-state emission spectrum of the NiRez
assembly in CH2C12 solution (Figure 5-13A), in which the emission signal is

seen to steadily increase over the course of many hours in solution indicating

262

 

 

 

lesser quenching of the 3MLCT excited states over time. The THF
dissociation mechanism has been attributed to a simple ligand exchange
process between the NiRez complex and the very large concentration of
CHzClz solvent molecules. This projected route is confirmed by the steady-
state emission data for the NiRez complex in THF solution (Figure 5-13B),
in which no change in the emission intensity was observed over the course
of 5 hrs. The unvarying emission intensity of the NiRez system in THF
solution is assigned to suppression of the axial ligand dissociation
mechanism occurring in the presence of a large excess of THF molecules.
The results of the steady-state emission data for the NiRez assembly in the
two solvent systems reveal that dipolar energy transfer dynamics can be
manipulated by simply varying the spin of the acceptor metal center, and
furthermore, can be controlled by simply varying the coordination
environment around a single metal center.

Due to the above discussion on the steady-state emission data for
Ni(pyacac)2(THF)2(Re(bpy)(CO)3)2](OTf)2 in CHzClz, the time-correlated
single photon counting (TCSPC) data for the NiRez assembly (Figure 5-14,
Right) requires further discussion. The two relaxation processes (rl= 5.0 ns
and 12 = 25 ns) observed are both significantly faster than the rob, = 530 ns

measured for the BeRe2 model complex (Figure 5-14, Left), which suggests

263

 

 

 

 

there are two competing quenching processes out the Rel-based 3MLCT
excited state. If the NiO6 and NiO4 acceptor cores (discussed above) were
the only two NiII coordination environments produced in the CH2C12
solution, a time-constant resulting from the spin-allowed pathway (13] = 5.0
ns) and a baseline offset representing the spin-forbidden coupling

Nio4 ~ BeRe2 . . . .
obs ~ obs ) would be the only observed klnetrc processes wrthrn the

( 13
time frame shown in the TCSPC data (Figure 5-14, Left). Another possible
coordination environment for the NiII core in the NiRez complex is NiOs,
which would result from dissociation of a single THF solvent molecule to
form Ni(acac)2(THF). Based on the work of Raymond and coworkers in the
1960’s, the NiO5 structure could exist either in square-pyramidal or trigonal
bipyramidal geometries due to the soft potential that exists between the two
geometries.34 Existence of either the square-pyramidal (pseudo-C4,) or
trigonal bipyramidal (pseudo-D3),) NiOs structures in solution could be
responsible for the r; = 25 ns observed in the TCSPC data for the NiRez
assembly (Figure 5-14). Further work, including photophysical and solution

phase magnetic susceptibility data on the NiRez and Ni(acac)2(TI-IF)x

complexes, will need to be performed to unmask the 25 ns component.

264

 

 

 

5.5 Conclusions

The synthesis, structures, and photophysical properties of a series of
donor-acceptor complexes based on Rel-bipyridine donors with a Com-acac
acceptor (CoRe3; Section 5.3) and a Nin-acac acceptor (NiRez; Section 5.4)
have been described. Steady-state and time-resolved emission
spectroscopies indicated that the Rel-based 3MLCT excited state relaxation
kinetics is significantly affected by the spin-state of the acceptor metal
center. In the CoRe3 complex, the Com-based ligand-field transition (1A1 —)
1T1) represents a thermodynamically viable relaxation pathway out the
3MLCT donor state, but the similarity in 3MLCT emission lifetimes between

"1 ligand-

the CoRe3 assembly and an AlRe3 model complex ruled out the Co
field state as a possible reaction pathway. The reason for the similar kinetics
between the complexes was shown to derive from lack of total spin angular
momentum conservation in the excited-state energy transfer reaction of
CoRe3. In the NiRez system, the Nin-based ligand-field transition (3A2 —>
3T2) provided a spin-allowed quenching pathway out of the Rel-based
3MLCT excited-state when compared to a Ben-containing model complex,
and a much faster 3MLCT relaxation was observed in the NiRez assembly.

Favorable spectral overlap between the 3MLCT donor emission and acceptor

ligand-field states coupled with a ca. 10 A donor-acceptor distance, allowed

265

 

 

 

 

for an assignment of Ftirster (dipolar) energy transfer as the dominant
excited-state quenching mechanism in the Nin-containing complexes. These
complexes represent excellent examples of the ability to manipulate excited-

state reactivity by simply varying the spin-state of a single reagent involved.

 

 

266

5.5 References

(1)

(2)

(3)

(4)

(5)
(6)
(7)
(8)
(9)
(10)

(11)

(12)
(13)
(14)
(15)
(16)

Mulazzani, Q. G.; Sun, H.; Hoffman, M. 2.; Ford, W. E.; Rodgers, M.
A. J. J. Phys. Chem. 1994, 98, 1145.

Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals; 3rd ed.; Pergamon Press: Oxford, U. K., 1988.

Mackay, L. G.; Anderson, H. L.; Sanders, J. K. M. J. Chem. Soc,
Perkin Trans. 1. 1995, I8, 2269.

 

r
Doehring, A.; Goddard, R.; Jolly, P. W.; Krueger, C.; Polyakov, V. R.
Inorg. Chem. 1997, 36, 177.
Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952. ,1

i
Murakami, Y.; Nakamura, K. Bull. Chem. Soc. Jpn. 1966, 39, 901. l9:

SAINT; ver 6.02a ed.; Bruker AXS, Inc.: Madison, WI, 2000.
Sheldrick, G. M.; ver 2.03 ed.; Bruker AXS, Inc.: Madison, WI, 2000.
Sheldrick, G. M.; ver 6.12 ed.; Bruker AXS, Inc.: Madison, WI, 2001.

Damrauer, N. H.; Boussie, T. R.; Devenney, M.; McCusker, J. K. J.
Am. Chem. Soc. 1997, 119, 8253.

DeWitt, L.; Blanchard, G. J.; LeGoff, E.; Benz, M. E.; Liao, J. H.;
Kanatzidis, M. G. J. Am. Chem. Soc. 1993, 115, 12158.

Origin; 7.5 ed.; OriginLab Corp.: Northhampton, MA, 1991 - 2004.
Barnum, D. W. J. Inorg. Nucl. Chem. 1961, 21, 221.

Graddon, D. P. Nature 1956, 183, 1610.

Hashagen, J. T.; Fackler, J. P. J. Am. Chem. Soc. 1965, 87, 2821.

Holm, R. H.; Cotton, F. A. J. Phys. Chem. 1961, 65, 321.

267

(17)

(18)
(19)

(20)

(21)

(22)
(23)

(24)

(25)

(26)

(27)

(28)

(29)
(30)

(31)
(32)

Tsukahara, Y.; Kamatani, T.; lino, A.; Suzuki, T.; Kaizaki, S. Inorg.
Chem. 2002, 41 , 4363.

Fackler, J. P.; Cotton, F. A. J. Am. Chem. Soc. 1960, 82, 5005.

Dattelbaum, D. M.; Omberg, K. M.; Schoonover, J. R.; Martin, R. L.;
Meyer, T. J. Inorg. Chem. 2002, 41 , 6071.

Dattelbaum, D. M.; Martin, R. L.; Schoonover, J. R.; Meyer, T. J. J.
Phys. Chem. A. 2004, 108, 3518.

Argazzi, R.; Bertolasi, E.; Chiorboli, C.; Bignozzi, C. A.; Itokazu, M.
K.; Murakami Iha, N. Y. Inorg. Chem. 2001, 40, 6885.

Cotton, F. A.; Wise, J. J. Inorg. Chem. 1966, 5, 1200.
Montgomery, H.; Lingefelter, E. C. Acta. Cryst. 1964, 17, 1481.

Busby, M.; Liard, D. J.; Motevalli, M.; Toms, H.; Vlcek, A. Inorg.
Chim. Acta. 2004, 35 7, 167.

Chen, P.; Curry, M.; Meyer, T. J. Inorg. Chem. 1989, 28, 2271.

Lucia, L. A.; Abboud, K.; Schanze, K. S. Inorg. Chem. 1997, 36,
6224.

Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.;
DeGraff, B. A. Inorg. Chem. 1990, 29, 4335.

Worl, L. A.; Duesing, R.; Chen, P.; Della Ciana, L.; Meyer, T. J. J.
Chem. Soc., Dalton Trans. 1991, 849.

Fackler, J. P.; Cotton, F. A. J. Am. Chem. Soc. 1961, 83, 3775.
Striplin, D. R.; Crosby, G. A. Chem. Phys. Lett. 1994, 221, 426.
Dexter, D. L. J. Chem. Phys. 1953, 21, 836.

Brown, W. R.; O'Boyle, N. M.; McGarvey, J. J.; Vos, J. G. Chem.
Soc. Rev. 2005, 34, 641.

268

 

(33) Soler, M.; McCusker, J. K. J. Am. Chem. Soc. 2008, 130, 4708.

(34) Aristides, T.; Raymond, K. N.; Spiro, T. G. Inorg. Chem. 1970, 9
2415.

 

269

Chapter 6. Future Work

6.1 Overall Goals

Chapter 6 focuses on continuing the fundamental investigation of
Féirster energy transfer dynamics in Rel-based donor-acceptor assemblies
discussed throughout this dissertation. The majority of the discussion in this
chapter will center on proposing various spectroscopic tools and additional
donor-acceptor complexes that will be needed to address two remaining
goals of the research effort: 1) the complete characterization of the
dynamical behavior (i.e. vibrational) of the Re(bpy’)(CO)3 donor moieties
undergoing 3MLCT energy transfer quenching processes and 2) the synthesis
of additional CoRe3 derivatives to further understand the spin-dependent
energy transfer processes proposed for the CoRe3 and CrRe3 complexes

outlined in Chapter 5.

6.2 Goal 1: Monitoring 3’MLCT Relaxation in the Re(bpy’)(CO)3 Donors
6.2.1 Background. The 3MLCT excited state of the ReI fluorophores can be
thought of as Re"(bpy')(CO)3, in which the Re1 metal has been formally
oxidized and the polypyridyl ligand formally reduced to generate

Re(tzg5 )—bpy(1r*'). A brief description of the electronic structure and

270

 

 

photophysical properties of d6 polypyridyl complexes is given in the
Introduction. As one can imagine, the ground state vibrational properties of
the Re(bpy)(CO)3 moiety will be significantly perturbed in the charge
transfer excited state. The ground-state vibrational signatures for the
polypyridyl-based C-C and C-N stretching modes and the CO stretching
frequencies are the two particular areas of the Re(bpy)(CO)3 donor moiety
that will be significant altered in the excited-state. The bond order of the C-
C and the C-N bonds of the bpy will be decreased due to population of a
bpy-based 71:1 orbital, and the bond order of the CO ligands will be increased
due to less backbonding contribution from the oxidized Re metal center.1
Time-resolved infrared spectroscopy (TRIR) has been shown to be an
excellent technique for probing the relaxation dynamics associated with
transition metal-based excited states.2’3 Due to the high oscillator strengths
and backbonding interactions associated with the CO ligands, TRIR has
been used to monitor the excited-state CO stretching frequencies to
characterize the 3MLCT excited state relaxation in [Re(Xz-bpy)(CO)3(4-
Etpy)](PF6) (where X = CH3, H, or COzEt and 4-Etpy = 4-ethylpyridine).4’5
The v(CO) bands associated with [Re(Xz-bpy)(CO)3(4-Etpy)](PF6) were
observed to shift to higher energy in response to an increase in the triple-

bond character of the CO ligands due to less Re-CO backbonding. In

271

 

addition, the technique has been shown to monitor and assign the
mechanism of excited-state reactivity out of the 3MLCT excited state of Rel-
based donor-acceptor assemblies by monitoring the extent to which the
v(CO) bands shift in energy.6'8

Based on the TRIR data reported for the mono- and poly-nuclear Re]-
based complexes discussed above, application of TRIR spectroscopy to the
MRex (x = 2 or 3) assemblies presented throughout in dissertation would be
a nice compliment to the steady-state and time-resolved emission data. All
of the MRex complexes possess Re(bpy’)(CO)3 donor moieties that possess
v(CO) bands that could serve as spectroscopic tags to monitor relaxation out
of the 3MLCT excited state. Additionally, even though the donor-acceptor
distances, electrochemical properties, and spectral overlap considerations in
the MRex complexes prove dipolar energy transfer as the dominant 3MLCT
quenching process, the energies for the v(CO) bands in the 3MLCT excited
state derived fiom TRIR would be different depending on whether energy or
electron transfer quenching was operative (Vida infra).
6.2.2 Dicyano Derivatives of CuRez and BeRe2. With an eye toward
developing a system that could be used for assigning the excited-state
quenching mechanisms in the MRex complexes Via TRIR, I have synthesized

and characterized [Cu(pyacac)2(Re(dcnb)(CO)3)2](OTf)2 and [Be(pyacac)2-

272

 

 

(Re(dcnb)(CO)3)2](OTf)2 (where dcnb = 4,4’-dicyano-2.2’-bipyridine). Just
as with the complexes reported in Chapter 3, the Cu11 and Bell metal centers
are covalently attached to two fac-Re(bpy’)(CO)3 moieties through two
pyridyl-acetylacetonate bridging ligands (Figure 6-1). The BeRe2 complex
was synthesized to investigate the v(CO) bands associated with the 3MLCT
excited state of the Re(dcnb)(CO)3 moiety in the absence of emission

quenching reactivity in the CuRez assembly.

CN‘I 2+

 

Figure 6-1. Structures of [Cu(pyacac)2(Re(dcnb)(CO)3)2](OTf); and
[Be(pyacac)2(Re(dcnb)(CO)3)2](OTf)2

273

 

 

6.2.3 Synthesis and Characterization

The synthesis procedure for [Cu(pyacac)2(Re(dcnb)(CO)3)2](OTDZ is
same as reported for the CuRez series in Chapter 3. Amounts of 75 mg (0.18
mmol) of Cu(pyacac); and 225 mg (0.360 mmol) of Re(dcnb)(CO)3(OTf)
were used in the preparation of the CuRez compound. Yield: 83 mg (28%).
Anal Calcd for C52H32N10F6OI6S2CuRe2 °4HzOz C, 35.92; H, 2.32; N, 8.05.
Found: C, 35.74; H, 2.10; N, 7.84. MS: [ESI+, m/z (rel. int.)]: 684.4 (35)
{[Cu(pyacac)2(Re(dcnb)(CO)3)2]}2+, 1517.9 (1) {[Cu(pyacac)2(Re(dcnb)-
(COthorori

The synthesis procedure for [Be(pyacac)2(Re(dcnb)(CO)3)2](OTf)2 is
same as reported for the BeRez series in Chapter 3. Amounts of 67 mg (0.19
mmol) of Be(pyacac)2 and 233 mg (0.372 mmol) of Re(dcnb)(CO)3(OTf)
were used in the preparation of the BeRe2 compound. Yield: 141 mg (47%).
Anal Calcd for C52H32N10F6016S2BeRez °4HzOz C, 37.08; H, 2.39; N, 8.31.
Found: C, 37.22; H, 2.18; N, 8.04. MS: [ESI+, m/z (rel. int.)]: 657.1 (41)
{[Be(pyacac)2(Re(dcnb)(CO)3)2]}2+, 1463.3 (1) {[Be(pyacac)2(Re(dcnb)-
(COthOToti
6.2.4. Electronic Absorption Spectroscopy. The electronic absorption
spectra of [Cu(pyacac)2(Re(dcnb)(CO);)2](OTf); and [Be(pyacac)2(Re-

(dcnb)(CO)3)2](OTf)2 were acquired in room-temperature CH3CN solutions

274

 

 

and are shown in Figure 6-2. Transitions associated with these complexes
were assigned based on previously reported analyses of CuRez/BeRez
complexes reported in Chapter 3. The Rel-based 1A. —> lMLCT (t2g ——> 71*
(bpy’)) transition occurs at Am, = 395 nm for both complexes, with the
absorption profile of [Be(pyacac)2(Re(dcnb)(CO)3)2](OTf); solely exhibiting
these lMLCT based characteristics. The BeRe2 complex allows for analysis
of the ground-state absorption behavior of the Re1 donor moieties without
overlapping with the Cull-based charge-transfer transitions associated with
the CuRez assembly. In addition, the similarity in the shape and energy of
the 1MLCT absorption bands in the two complexes indicates minimal
electronic communication between the Re1 and Cu'I metal centers in the

ground state.

 

60000 1-
50000 -
40000
30000 -
20000 -
10000

 

 

 

Molar Absorptivity (M'cm'l)

 

 

300 400 500 600
Wavelength (nm)
Figure 6-2. Electronic absorption spectra of [Cu(pyacac)2(Re(dcnb)-

(CO)3)2](OTf)2 (red trace) and [Be(pyacac)2(Re(dcnb)(CO);)2](OTf)2
(blue trace) acquired in room-temperature CH3CN solution.

275

 

 

6.2.5 Time-Resolved Emission. Quantitative information concerning
emission quenching by the Cu" center was obtained via time-resolved
emission spectroscopy. Data for the BeRe2 model complex could be fit to a
single-exponential decay kinetic model (robS = 110 d: 10 ns); emission trace
for the observed decay rate in the BeRe2 complex is given on the left side of
Figure 6-3. Due to a lack of signal observed with the nanosecond emission
apparatus for the CuRez complex, time-correlated single-photon counting
(TCSPC) was employed to measure the excited-state lifetime of the Cu"-
containing complexes. A plot of the TCSPC data obtained for the CuRez
complex in deoxygenated CHzClz solution is shown on the right side of
Figure 6-3; the observed time constant (4.1 i 0.4 ns) for excited-state decay
in the CuRez complex is significantly shorter than the BeRe2 model
complex, and shows the Rel-based 3MLCT excited state is significantly
quenched in the presence of CuII relative to Be". The analysis of the time-
resolved emission data for [Cu(pyacac)2(Re(dcnb)(CO)3)2](OTf)2 and
[Be(pyacac)2(Re-(dcnb)(CO)3)2](OTDZ were performed analogous to the
CuRez complexes reported in Chapter 3 and the reader is directed there for

further discussion.

276

 

 

250 - ' Cu

200- ‘

I—i

Ur

O
I

O
O

 

Intensity (mV)

 

Intensity (cps)

soul

0 200 400 600 800 0 10 20 30 40

Time (ns) Time (ns)

 

 

 

 

 

 

 

 

Figure 6-3. Left. Nanosecond time-resolved emission data for
[Be(pyacac)2(Re(dcnb)(CO)3)2](OTf)2 (robS = 110 d: 10 ns) acquired in
room-temperature deoxygenated CHzClz solution (kpump = 400 nm;
31mm, = 625 nm). Right. Time correlated single-photon counting
(TCSPC) emission data for [Cu(pyacac)2(Re(dcnb)(CO)3)2](OTf)2
acquired in room-temperature deoxygenated CH2C12 solution ()tpump =
400 nm; Ambe = 625 nm). The solid red line corresponds to a fit to a bi-
exponential decay model with t, =4] :1: 0.4 ns and 1:, = 110 ns (fixed);

the latter corresponds to emission from Re(dcnb)(CO)3(pyacac) present
in solution as a trace impurity.

6.2.6 Proposed TRIR Experiments. The Re(dcnb)(CO)3 donor moieties of
[Cu(pyacac)2(Re(dcnb)(CO)3)2](OTf)2 and [Be(pyacac)2(Re(dcnb)(C0);)2]-
(OTf); represent ideal candidates for probing quenching dynamics of charge
transfer-based excited states using TRIR. In addition to monitoring the
dynamical shift in the CO ligands that occur in response to the RelI (d5)
metal center in the 3MLCT excited state, the high oscillator strength of the
CN substituents on the bipyridyl ligand should serve as ideal spectroscopic

tags for measuring the excited state relaxation associated with the reduced

277

polypyridyl ligand. Measuring the energies of the v(CO) and v(CN) bands
associated with the 3MLCT excited state in the BeRe2 complexes will be
critical to the mechanistic analysis in the CuRez analogue. If excited-state
energy transfer is the dominant mechanism in the CuRez complex, then the
shifts (v(GS) to v(ES)) observed for the v(CO) and v(CN) bands in the Be"
and Cull-containing systems will correspond. Due to the shorter excited-
state lifetime measured for the CuRez complex (Figure 6-3), the only
difference between the two experiments will be the shorter time window that
will be needed to collect the CuRez data. The energy transfer process in the
CuRez system is in essence, a faster pathway for the 3MLCT state to relax
and therefore will exhibit similar v(CO) and v(CN) bands as seen for the
charge transfer excited state in the BeRe2 complex. If electron transfer is the
dominant quenching process in the CuRez assembly, the shifts in the v(CO)
bands of the Ben- and Cull-containing systems will not correlate due to the
“complete” oxidation of the ReI metal center that would occur during an
electron transfer event in CuRez, compared with partial oxidation (~70%)
occurring during a charge transfer excitation. In addition, the v(CN)
observed in the BeRe2 complex should disappear due to electron transfer
quenching removing the excited electron from the all level of the dcnb

ligand.

278

 

 

 

6.3 Goal 2: Additional Derivatives for the CoRe3 Series

6.3.1 Background. In Chapter 5, a CoRe3 donor-acceptor complex -
[Co(pyacac)3(Re(bpy)(CO)3)3](OTf); - was synthesized in order to explore
the spin-state dependence of excited state energy transfer reactions. Based
on the observation of significant spectral overlap between the 3MLCT
emission profile of the Re(bpy) donor moiety and the 1A1 —) lT1 ligand-field
absorption band in the Co111 acceptor, dipolar energy transfer quenching of
the 3MLCT state is thermodynamically favorable and predicted to occur.
Nanosecond emission lifetime measurements on the CoRe; complex in
CHzClz solution exhibited similar 3MLCT relaxation kinetics as an
electronically benign AlRe; model system. Determination of the total spin
orbital angular momentum (ST) for the energy transfer quenching reaction in
CoRe; revealed a spin-forbidden coupling mechanism between the photo-
induced reagents and the energy transfer products. Furthermore, comparison
of the 3MLCT emission lifetimes of CoRe; and a structurally identical CrRe;
complex that possesses similar spectral overlap characteristics and a spin-
allowed coupling mechanism between the reagents and products showed a
ca. 100 fold decrease in the lifetime of CrRe3, and confirmed that dipolar
energy transfer can be turned on or off depending on the spin-state of a

single ion.

279

The trend in observed energy transfer rate constants for the series of
CrRe; complexes presented in Chapter 4 showed a direct dependence on the
magnitude of spectral overlap between the Rel-based 3MLCT emission
spectra and the Crm—based 4A2—> 4T2 ligand-field acceptor transition. Figure
6-4 reproduces the spectral overlap plot for the CrRe; series originally
reported in Chapter 4, with the bpy derivative (green trace) exhibiting the
highest spectral overlap value and the largest energy transfer rate constant.
To adequately compare the CrRe; (spin-allowed) and the CoRe; (spin-
forbidden) 3MLCT relaxation processes, additional Re(bpy’) derivatives of
the CoRe; complex will need to be prepared to generate an analogous

spectral overlap plot for the CoRe; series.

 

 

 

Absorbance (normalized)

 

 

(pazgeuuou) Knsuarul uogssrurg

22 20 18 l6 14
Energy (x 103 cm")

Figure 6-4. Overlay of the emission spectra of [Al(pyacac)3(Re(tmb)-
(C0)3)3](0Tf)3 (b11112), [A1(pyacaC)3(Re(bPY)(CO)3)3l(OTf)3 (green),
and [Al(pyacac)3(Re(deeb)(CO)3)3](OTf)3 (red) with the electronic
absorption spectrum of Cr(phacac)3. Plot is reproduced from Figure 4-
12 (Chapter 4). The largest calculated spectral overlap (bpy; green)
resulted in the fastest energy transfer rate constant within the series.

280

6.3.2 Proposed CoRe3 Derivatives. Based on the spectral overlap plot
between the [Al(pyacac)3(Re(bpy)(CO)3)3](OTf); emission profile (Xmax =
565 nm) and the Co(phacac)3 1A; —> 1T; ligand-field manifold (Amax = 605
nm) reported in Chapter 5, additional CoRe; derivatives possessing lower
energy Re(bpy’) emitters need to be synthesized (preferably one with an
emission maximum of ~ 600 nm) to generate an analogous spectral overlap
plot created for the CrRe; series. One obvious choice is to synthesize the
deeb derivative (deeb = 4,4’-diethylester-2,2’-bipyridine), due mainly to
already possessing [Al(pyacac)3(Re(deeb)(CO)3)3](OTf); as a structural
model and secondly that the Amax for emission is 625 nm which lies just on
the low energy side of the Co(phacac)3 'A; -—> 1T1 ligand-field band but still
retains descent spectral overlap.

Based on literature reports that show an emission maximum of ~ 600
nm in CH2C12 solution, the next CoRe; analogue proposed is the dclb
derivative (dclb = 4,4’-dichloro-2,2’-bipyridine). The synthesis and
photophysical characterization of the [Al(pyacac)3(Re(dclb)(CO)3)3](OTf);
structural model have already been performed based on experimental
protocols used for similar AlRe3 complexes reported in Chapter 2. The

steady-state and nanosecond time-resolved emission data for [Al(pyacac)3-

(Re(dclb)(CO)3)3](OTf)3 is shown in Figure 6-5. The photophysical

281

 

properties of [Al(pyacac)3(Re(dclb)(CO)3)3](OTf)3 correspond with the other
AlRe; derivatives reported in Chapter 2, and the observed emission

properties are assigned to the Rel-based 3MLCT —r 1A] transition.

 

 

 

 

 

 

 

 

 

 

 

40 -
A? A 1
E E .
:: §‘ 20 - 1
O m
'8 E
"8 E 10 -‘
LL)
I I I A O I l l 1 I
500 600 700 800 0 200 400 600 800
Wavelength (nm) Time (ns)

Figure 6-5. Left. Steady-state emission spectrum of [Al(pyacac)3(Re(-
dclb)(CO)3)3](OTf)3 in room-temperature CH2C12 solution O‘rump = 375
nm); Am“ = 595 nm for 3MLCT —> 1A1 emission. Right. Nanosecond
time-resolved emission data for [Al(pyacac)3(Re(dclb)(CO)3)3](OTf)3
(rob, = 120 d: 10 ns) acquired in deoxygenated room-temperature
CH2C12 solution (kpump = 415 nm; Apmbe = 595 nm).

Figure 6-6 shows the spectral overlap plot between the Alm-
containing bpy, dclb, and deeb 3MLCT emission profiles with the IA] —> 1T1
ligand-field absorption band of Co(phacac)3. As can be seen from Figure 6-
6, ideal spectral overlap between the Re(dclb) moiety (green trace) and the
CoIII 1A] —> 1T1 absorption band (black trace) is observed, and most

importantly a spectral overlap plot analogous to the one in the CrRe; series

has been generated. In other words, a spin-allowed spectral overlap plot and

282

a spin-forbidden spectral overlap plot have now been generated to compare
two structurally identical donor-acceptor complexes that differ simply by the

acceptor spin-state.

 

 

 

 

(pozneuuou) Ausualul uogssrula

 

Molar Absorptivity (normalized)

500 600 700 800
Wavelength (nm)

Figure 6-6. Overlay of the emission spectra of [Al(pyacac)3(Re(bpy)-
(C0)3)31(0Tf)3 (blue), [A1(PyacaC)3(Re(dclb)(C0)3)3](OTfla (green),
and [Al(pyacac)3(Re(deeb)(C0););](OTf); (red) with the electronic
absorption spectrum of Co(phacac)3.

In closing, the final two compounds required to complete the CoRe;
series — [Co(pyacac)3(Re(dclb)(CO)3)3](OTf); and [Co(pyacac)3(Re(deeb)-
(CO)3)3](OTf)3 - need to be synthesized and characterized. The synthesis
procedure for these two remaining complexes should follow the same

procedure used for [C0(pyacac)3(Re(bpy)(CO)3)3](OTf); reported in Chapter

5.

6.4 References

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

Glyn, P.; George, M. W.; Hodges, P. M.; Turner, J. J. J. Chem. Soc.,
Chem. Commun. 1989, 1655.

Vlcek, A.; Farrell, I. R.; Liard, D. J.; Matousek, P.; Towrie, M.;
Parker, A. W.; Grills, D. C.; George, M. W. J. Chem. Soc., Dalton
Trans. 2002, 701.

Schoonover, J. R.; Strouse, G. F. Chem. Rev. 1998, 98, 1335.

Dattelbaum, D. M.; Martin, R. L.; Schoonover, J. R.; Meyer, T. J. J.
Phys. Chem. A. 2004, 108, 3518.

Dattelbaum, D. M.; Omberg, K. M.; Schoonover, J. R.; Martin, R. L.;
Meyer, T. J. Inorg. Chem. 2002, 41, 6071.

Chen, P.; Palmer, R. A.; Meyer, T. J. J. Phys. Chem. A 1998, 102,
3042.

Claude, J. P. ,Omberg, K. M.; Williams, D. S.; Meyer, T. J. J. Phys.
Chem. A 2002,106,7795.

Alsindi, W. Z.; Easun, T. L.; Sun, X.-Z.; Ronayne, K. L.; Towrie, M.;

Herrera, J.; George, M. W.; Ward, M. D. Inorg. Chem. 2007, 46,
3696.

284

 

 

     
 

         

 

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