THESIS

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This is to certify that the
dissertation entitled
PROTON COUPLED ELECTRON TRANSFER MEDIATED BY A SALT

BRIDGE

presented by

James A. Roberts

has been accepted towards fulfillment
of the requirements for

 

 

 

Ph.D. degreein Chemistry
Major professor
9-15—97

Date

 

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DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

me chlRCanDm.pGS—p.14

 

PROTON COUPLED ELECTRON TRANSFER MEDIATED BY A SALT
BRIDGE

By

James A. Roberts

A DISSERTATION

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

DOCTOR OF PHILOSOPHY

Department of Chemistry
1997

 

ABSTRACT
PROTON COUPLED ELECTRON TRANSFER MEDIATED BY A SALT BRIDGE

By

James A. Roberts

Many proteins and enzymes derive their function through the mediation of
electron transfer processes by proton transfer. The coupling of proton motion to charge
separation is a basic mechanism of biological energy conversion systems. However, the
inherent complexity of biological systems does not allow for the direct measurement of
coupled proton and electron kinetics. To this end the design of electron transfer
complexes incorporating design features of fixed distance electron transfer in
combination with photoinduced proton transfer have been prepared.

Tris—bipyridyl ruthenium(II) complexes functionalized with hydrogen-bonding
salt-bridges permit a photoinduced excited state electron transfer to be established. Initial
investigations based on the excited state electron transfer donor chemistry of
[(bpy)2Ru(Me-bpy)COOH(PF6)2]2*, [(bpy)2Ru(Me-bpy)C00' (PFe)]* and [(bpy)2Ru(Me-
bpy)AmH+ (PF6)3]3+ (bpy = 2,2'—bipyridine, Me-bpyCOOH = 4-methyl,4'-carboxylic
acid-2,2'-bipyridine, Me-bpyAmI-I+ = 4-methyl,4'-amidinium—2,2'-bipyridine) complexed
with the complimentary salt-bridge functionalized 3,5-dinitrobenzene show rates for
electron transfer dependent upon the orientation of the salt-bridge interface. When the
arrangement is donor-amidinium-carboxylate-acceptor (AMH",COO") the observed
intramolecular electron transfer rate constant is a factor of 2 less than for the same donor-

acceptor system bridged by a (COOH); interface. The rate for the (COO‘,AMH+)

interface orientation indicates a rate enhancement of 4 relative to that observed for the
switched analog.

Energetics of bipyridyl systems are not convenient to a clear electron transfer
measurement as the energy of the interface functionalized and non—functionalized
bipyridyl ligands are similar. Thus systems designed with alkylated bipyridyl ligands
[(tmb)2Ru(Me—bpy)COOH(PF6)2]2+, [(tmb)2Ru(Me-bpy)COO'(PF5)]“and [(tmb)2Ru(Me—
bpy)AmH+(PF6)3]2+ (tmb = 4,4'5,5'-tetramethyl-2,2'-bipyridine Me-bpyCOO' = 4-methyl
,4'-carboxylate 2,2'-bipyridine) and ester modified bipyridyl ligands [(decb)2Ru(Me-
bpy)COOH (PF5)2]2+ and [(decb)Ru(Me-bpy)AmH+ (PF5)3]2+ (decb = 4,4'diethylcarboxy—
2,2'-bipyridine) were used. For the (tmb) systems, the observed intramolecular electron
transfer rate is 36 times greater for the (COO',AmH+) interface system relative to the
(An1H+,COO') system. Reductive electron transfer, with the (decb) functionalized Ru(lI)
complexes, toward the protonated residue of the metal complex exhibits a fast
intramolecular rate constant. However, a comparative study of switched interface systems

is hampered by large changes in the electron transfer donor potential.

 

To my family.

 

ACKNOWLEDGMENTS

My advisor, Professor Dan Nocera, has provided me the freedom to explore
science in an environment that has always been exceptional and invigorating. My
understanding and awareness of the big picture and machinery of science have changed
as a result of the five years spent in his research group. I would also like to thank my
committee members Professors Babcock, Cukier and Dunbar and also Professor
Blanchard who served on my committee on extremely short notice.

Special thanks to Yongshi Deng and Eric Saari who spent the last eventful weeks
like me, deeply entrenched in the sometimes overwhelming process of finishing. Thanks
also to Dr. A] Barney who helped to maintain my focus during the final days and to Linda
Krause who always kept things on a lighter note, without her this thesis would assuredly
have been much more of a burden. Finally I would like to thank Wanda Hartmann for her
diligent proofreading when things were not running smoothly.

The Nocera group, most directly JP and Deng for a continuous supply of
compounds, and the rest of the group Adrian, Al, Jude, Jean, Mark Mortellaro, Niels,
Ann, Carolyn, Claudia, Doug, Dan, Dimitris, Eric, Janice, Jeff, Mark Torgerson, Sara,
and Zoe, have provided a sound environment in which to develop as a scientist. Most
importantly I would like to thank Wanda, who has for the last 2 1/2 years been a source
of inspiration and support during the trying times of graduate school.

And Finally, I would like to thank Al and Kathy Barney and also Dan, Amy and
Ariel Engebretson for their hospitality during my last homeless days at Michigan State

University.

TABLE OF CONTENTS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TABLE OF CONTENTS VI
LIST OF FIGURES IX
LIST OF TABLES XIII
CHAPTER 1 1
INTRODUCTION 1
ELECTRON TRANSFER 1
Fixed Distance Systems 3
Model systems 5
Rigid donor-acceptor svctpmc 10
NON-COVALENT INTERACTIONS 13
PROTON TRANSFER 17
Proton transfer in L" ' o ' nl svcrpmc l7
NON-COVALENT SYSTEMS IN ELECTRON TRANSFER I 8
PROTON COUPLED-ELECTRON TRANSFER 24
Incorporation of Ruthenium Systems into the study of PCE T ?7
ORGANIZATION OF THE THESIS ’39
CHAPTER 2 41
EXPERIMENTAL METHODS 41
MATFRIAI s 41
ELECTROCHEMISTRY 4 1
OPTICAL SPECTROSCOPY 47
PHOTOPHYSICAL MEASUREMENTS so
Infrared spectroscopy 51
Nana-Second Lifetime Measurements 5 7
Pica-second Lifetime measurements 5 ?
SYNTHESIS OF RUTHENIUM COMPLEXES 55

 

vi

CHAPTER 3 <7

PHOTOINDUCED ELECTRON TRANSFER WI I HIN AN ELECTRON DONOR~
ACCEPTOR ASSENflSLY JUXTAPOSED BY A HYDROGEN -BONDING

 

 

 

 

 

 

 

INTERFACE 57
INTRODUCTION 57
RESULTS 61
Electronic Absorption and Luminescence spectroscopy 6 I
Luminescence and Lifetime Measurements 65'
DISCUSSION 72
CHAPTER 4 78

 

PHOTINDUCED ELECTRON TRANSFER WI I HIN A SYNTHETICALLY
MODIFIED RUTI-IENIUM DONOR ACCEPTOR PAIR2THE EFFECT OF

 

 

 

 

 

 

 

 

SWITCHING THE SALT-BRIDGE INTERFACE 78
INTRODUC'IION 78
RESULTS R?
Electrochemistry 8?
Electronic Absorption and Luminescence photophysical characterization ............... 83
Association Constants 87
Determination of Ru(II)polypyridyl complex energetics 94
Luminescence and Lifetime Measurements 94
DISCUSSION 101
CHAPTER 5 I 09

 

PHOTOINDUCED REDUCT IVE ELECTRON TRANSFER WI I HIN AN
ELECTRON DONOR-ACCEPTOR PAIR:ASSESSMENT OF THE REVERSAL OF

 

 

 

 

 

 

 

THE ELECTRON TRANSFER DIRECTION 109
INTRODUCTION 109
RESULTS 1 13
Electronic Absorption / Emission Spectroscopy 113
Luminescence Spectra I I 7
Lifetime Quenching Electron Transfer Measurements [24
DISCUSSION 1 33
LIST OF REFERENCES 138

 

APPENDIX A I 49

 

vii

DETERMINATION OF THE ACID—BASE PROPERTIES OF A SERIES OF
ELECTRON TRANSFER DONOR-ACEPTOR PAIRS: ASSESSMENT OF THE pKa*

AND ApKa IN AQUEOUS SOLUTION ...................................................... 149
INTRODUCTION .................................................................................... 149
RESULTS ............................................................................................. I 53
Absorption/Emission Ground State and Excited State Acid-Base Reactions of the
Ruthenium Complexes ....................................................................... 153
Ground State Acid-Base Properties of the Free Ligands ............................... 164
Excited State Acid-Base Properties of the Ruthenium Complexes; Lifetime
Measurements ................................................................................. I65
Ground State Acid-Base Properties of Salt-Bridge F unctionalized 3, 5-
Dinitrobenzene Electron Transfer Quenchers ........................................... 170
DISCUSSION ........................................................................................ I7 1

viii

 

LIST OF FIGURES

Figure 1. Phtotsynthetic reaction center mimics, coupled porphyrin model systems design
of (a) Osuka et al. and (b) a tetrad system of Gust and Moore

 

Figure 2. Model system for the assessent of electric field effects in photosynthetic
reaction centers

 

Figure 3. Electron transfer model systems (a) Biphenyl bridged model system of
McClendon (b) Peptide linked chromophore-quencher system of Meyer and (c) rigid
covalent norbomadiene system of Paddon-Row. 9

 

Figure 4. Non-covalent electron transfer systems implementing guanine-cytosine base-
pair recognition for donor-acceptor separation. Conformationally flexible system (a).
conformationally rigid system (b) two point (c) and four point (d) hydrogen bonding
of quinoncs by functionalized porphyrins.

 

Figure 5. Non-covalent systems based on asymmetric hydrogen—bonding (a) and by metal
ligation (b) 71

 

Figure 6. Non—covalent cyclodextrin porphyrin assembly for the investigation of electron
transfer to quinone. 7’)

 

Figure 7 Zn(II)— -porphyrin- -3 ,-4 dicarboxylic acid donor- -acceptor symmetric interface
system. (a) Zn(H)- Fe(III) diprophyrin symmetric interface system with pheny71
spacer (h)

 

Figure 8. Pholtophysical scheme for tris-bipyridyl ruthenium(II) complexes. lMLCT,
3MLCT, l1t1t*,and 3d- d refer to the singlet and triplet charge transfer, the singlet
intraligand and the triplet metal centered states respectively. k and km represent
decay processes 37

Figure 9 Bipyridyl ligated systems. Symmetric dicarboxylic acid (a) asymmetric
amidinium-carboxylate (b) and asymmetric carboxylate- -amidinium (c) salt- -bridge
interfaces for electron transfer studies.

 

Figure 10. Biexponential fit of lifetime decay indicating concentration-dependent and
concentration-independent components of a representative lifetime decay for
[(bpy)2Ru2+(Me- bpy)AmH+] (1. 0 x 10‘1 M) with complimentary 3, 5- dinitrobenzoate
[1.5 x 10 M] 68

 

 

Figure 11. Concentration-independent component ( O ) and concentration dependent
component ( I ) for the lifetime titration of (bpy)2Ru2“(Me-bpy)AmH+ [1.0 x 104
M] with 3,5-dinitrobenzoate in CH2C12 at 22°C. 69

FigureIZ. Energetic scheme for the excitation of the Ru(II)polypyridyl con;plexes
[(bpy)2Ru2+(Me-bpy)AmH+]3+, [(bpthuz’rMe-bpy)COOHI2+ [(bpthu *(Me-
bpy)2Ru2+(Me-bpy)COO-]+ in CH3CN at 22°C. 74

 

Figure 13. Tetramethyl bipyridyl Ru(H) complexes for electron transfer through the salt-
bridge interface formed by the association of the complimentary quenchers 3,5-
dinitrobenzoate (a), 3,5-dinitrobenzoic acid (b) and 3,5-dinitrobenzamidinium (c). 81

Figure 14 Absorption and Absolute emission of the three Ru(H) electron transfer
complexes: [(tmb)2Ru2+(Me-bpy)AmH+]3+ (line), [(tmb)2Ru2"(Me-bpy)COO—]+
(dashed line), and [(tmb)2Ru2+(Me-bpy)COOH]2+ (dotted line). ............................. 84

Figure 15 Association constant determination from Absorption titration of
[(tmb)2Ru2+(Me-bpy)AmH+]3+ (3.0 x 10'5 M) with 3,5-dinitrobenzoate in CH2CIz.
Inset plot shows the change in absorbance at 495 nm through the titration .............. 90

Figure 16. Association constant determination from Absorption titration of
[(tmb)2Ru2+(Me-bpy)AmH+]3+ (3.0 x 10‘5 M) with benzoate in CHZClz. Inset plot
shows the change in absorbance at 470 run through the titration. ............................ 91

Figure I 7. Association constant from the ratio of biexponential fit function prefactors (A1
and A2) of the lifetime decay of [(tmb)2Ru2+(Me-bpy)COO']+ (0.10 mM) titrated
with 3,5-dinitr- L ’J' ' 92

 

Figure 18 Concentration-dependent ( I ) and concentration-independent ( O ) observed
rate constants for the quenching of [(tmb)2Ru2+(Me-bpy)COO-]+ (0.10 mM) by 3,5-
dinitrobenzamidinium in CHZClz. 95

 

Figure 19. Top panel. Lifetime decay data for the quenching reaction between
[(tmb)2Ru2“(Me-bpy)COO-]+ and 3,5-dinitrobenzamidinium. Biexponential fitting
of the decay data is indicated with the solid line. Bottom panel. Relative percentages
of the biexponential fitting of the decay data from Top panel. ................................ 96

Figure 20. Concentration-dependent ( I ) and concentration-independent ( O ) observed
rate constants for the quenching of [(tmb)2Ru2+(Me-bpy)COOH]2+ (0.06mM) by 3,5-
dinitrobenzoic acid in CHZClz. 97

 

Figure 21. Concentration-dependent ( I ) and concentration-independent ( O ) observed
rate constants for the quenching of [(tmb)2Ru2+(Me-bpy)AmH*] (0.064mM) by 3,5-
dinitrobenzoate in CH2C12. 98

 

Figure 22. Ru(mpolypyridyl complexes for reductive electron transfer quenching (a)
[(bpy)2Ru2*(Me-bpy)AmH*I3* (b) [(bpy)2Ru2*(Me-bpy)coo-I*. (c)
[(decb)2Ru2+(Me—bpy)Aml-I+]3+ and (d) [(decb)2Ru2+(Me-bpy)COOH]2+' Figures

 

shown are the salt-bridged complexes formed from the 1:1 association with N,N‘-
dimethylaminobenzoate and N,N' Jinnah} ' L 'J' ' 112

 

Figure 23. Absorption and emission spectra for amidinium functionalized electron
transfer donor complexes [(bpy)2Ru2+(Me-bpy)AmI~l+]3+ and [(decb)2Ru2+(Me-
bpy)AmH+]3+ in aerated CH3CN solution. 114

Figure 24. Absorption and emission spectra of carboxylate functionalized electron
transfer donor complexes [(bpy)2Ru2“(Me-bpy)COO-]+ and [(decb)2Ru2+(Me-
bpy)COOH]2+in aerated CH3CN solvent. 115

Figure 25. Absorption titration of [(bpy)2Ru2+(Me—bpy)AmH+] (2.0 x.10'5 M) by N,N'-
dimethylaminobenzaote in CH3CN 118

 

Figure 26. Absorption titration of [(decb)2Ru2+(Me-bpy)AmI-I+]3+ (2.0 x 10‘5 M) with
N,N'di...e:l._,1nmin---L . in CH3CN 119

 

 

Figure 27. Absorption titration of [(bpy)2Ru2+(Me-bpy)AmH"]3+ with N,N'-
dimethylfimin- L 'J' ' in CH3CN. 120

 

Figure 28. Absorption titration of [(decb)2Ru2+(Me-bpy)COO-]+ with N,N'-
diurcthylflmin- L 'J' I in CH3CN. 121

 

Figure 29.Concentration-independent ( O ) and concentration-dependent ( I ) observed
rate constants for the quenching of [(bpy)2Ru2+(Me-bpy)AmH“]3+ by N,N'-
dimethylaminobenzoate in CH3CN 125

 

Figure 30.Ratio of pro-exponential factors Al ( I ) and A2 ( O ) from biexponential
fitting of the quenching data for [(bpy)2Ru2+(Me—bpy)AmH+]3+ with N,N'-
dimethylamino-L me 126

Figure 31. Concentration-independent ( O ) and concentration-dependent ( I ) observed
rate data for the quenching of [(decb)2Ru2‘”(Me-bpy)AmH"]3+ by N,N'-
dimethylaminobenzaote in CH3CN 127

 

Figure 32. Marcus fit of bimolecular electron transfer rates constants for the kinetic
determination of redox potentials of the reductive quenchers N,N'-
dimethylaminobenzoate and N,N'—dimethylamin- L 'J' ' in CH3CN u = 0.1
22°C 130

Figure 33. Ru(IDpolypyridyl complexes used in the aqueous acid-base property
determinations. (a),(b) [(x—bpy)2Ru2“(Me-bpy)COOH]2+ {(a) x = H and (b) x =
(war. and (c).(d) [(x-be)2Ru2+(Me-bpy)AmH+]3* «O x = H and (d) x =
(CH3)4} ......................................................................................... 152

Figure 34. Absorption pH titration of [(bpy)2Ru2+(Me-bpy)COOH][(PF5)]2. The spectrum
for the intraligand 1r—1t* region is shown at the left and the spectrum in the dn—It*
MLCI‘ region at the right. The inset of each of the plots shows the change of the
absorption (1» = 286 and 456 nm) over the pH range
investigated .................................................................................... 154

 

 

xi

 

Figure 35. Absorption pH titration of [(tmb)2Ru2+(Me-bpy)COOH][(PF5)]2. The
spectrum for the intraligand 1t—‘tt* region is shown at the left and the spectrum of the
dn—rt* MLCT region is shown at the right. The inset of each of the plots shows the
absorption (71. = 290 and 447 nm) over the pH range
investigated ..................................................................................... 155

Figure 36. Absorption pH titration of [(bpy)2Ru2+(Me-bpy)AmH+][(PF6)]3. The spectrum
for the intaligand 1t—1t* region is shown at the left and the spectrum of the d1t—11: *
MLCT region is shown at the right. The inset of each of the plots shows the
absorption (I. = 286 and 457 run) over the pH range
investigated .................................................................................... 157

Figure 37. Absorption pH titration of [(tmb)2Ru2+(Me-bpy)AmH+][(PF5)]3. The spectrum
for the intraligand 1t—1r* region is shown at the left and the spectrum of the dit—ir*
MLCT region is shown at the right. The inset of each of the plots shows the
absorption (7» = 289 and 440 nm) over the pH range
investigated .................................................................................... 158

Figure 38. Emission pH titration for [(bpy)2Ru2+(Me-bpy)COOH][(PF6)]2. The inset of
the plot indicates the changes of the emission intensity at 630 nm over the pH range
investigated .................................................................................... 160

Figure 39. Emission pH titration for [(tmb)2Ru2+(Me-bpy)COOH][(PF6)]2. The inset of
the plot indicates the changes of the emission intensity at 630 nm over the pH range
investigated .................................................................................... 161

Figure 40. pH dependence of the lifetime measurements. Top trace ( O )is for the
complex [(bpy)2Ru2+(me-bpy)COOH][(PF6)]2, bottom trace ( I ) is for the complex
[(tmb)2Ru2+(Mebpy)COOH][(PR5)]; ....................................................... 167

Figure 41. pH dependence of the lifetime measurements. Top trace ( O )is for the
complex [(bpy)2Ru2+(Me-bpy)AmH+][(PF6)]3, bottom trace ( I )is for the complex
[(tmb)2Ru2+(Mebpy)AmH+][(PR5)]; ....................................................... 168

xii

 

LIST OF TABLES

Table 1. Electrochemical data for the three (bpy)2Ru(H) ‘ m

I

 

Table 2. Emission Characterization of (bpy)2Ru(II) complexes 67

 

Table 3. Observed electron transfer rates for concentration— —dependent and concentration-
independent electron transfer of bipyridine based donor-acceptor complexes in
CHZCIZ at 22°C

 

Table 4. Electrochemical data for (tmb)2Ru(H) complexes In CH3CN at 22°C. Em (V) vs.
SCE.

 

Table 5. Ru(H) polypyridyl emission characterization data in CH2C12 22°C. Excitation by

 

 

Xe/Hg lamp at 435.8 nm 88
Table 6. Rates for concentration-dependent and concentration-independent quenching of
(tmb)2Ru(H) electron transfer complexes. 99
Table 7. Absorption data for (decb)2Ru(II) and (bpy)2Ru(H) , ' 116

 

Table 8. Emission Data for (decb)2Ru(II) and (bpy)2Ru(II) complexes in CH3CN. ....... 123

Table 9. Photophysical Characterization for (decb)2Ru(II) and (bpy)2Ru(H) complexes in
CH3CN. 123

Table 10. Redox potentials and rate constants for the bimolecular quenching reactions
between Ru(bpy);2 and the reductive electron transfer I L 131

 

Table I 1. Rates for concentration-dependent and concentration-independent quenching of
(decb)2Ru(II) and (bpy)2Ru(H) salt-bridge electron transfer complexes. ............... 132

xiii

 

CHAPTER 1

INTRODUCTION

Electron Transfer

Theoretical descriptions of electron transfer have been presented in the past at a
level of detail and sophistication that was not experimentally accessible. However, with
the advent of fast laser techniques, and with the design of fixed distance donor-acceptor
systems, the study of electron transfer reactions have become accessible at a level
befitting of the theoretical framework. Of particular significance, is the ability to probe
fast vibrational events occurring as a result of, or promoting, an electron transfer event

[1].

' A

2

Traditionally, the study of photoinduced electron transfer has been dominated by
the determination of distance, conformational, and stereoelectronic relationships to the
observed electron transfer rate. The myriad of systems designed to incorporate these
issues have occupied the literature significantly for the past twenty years since Marcus'
original description of activated electron transfer in the 1950's [2]

Subsequent to Marcus' theory, photoinduced electron transfer reactions were
composed of bimolecular interactions between donor and acceptor in homogeneous
solution. Deactivation of an electronically excited donor by either oxidative or reductive
quenching species is shown schematically in Eq (1.1) Typical of bimolecular reactions is
the oxidative and reductive quenching of Ru(bpy)32+ with viologen, aromatic nitro,
aromatic amine and inorganic complex quenchers [3]. In these bimolecular electron

transfer reactions, organization of donor and acceptor in a specific distance or

 

D+A D*:A —> D‘+A’

 

D+A =_.. D*:A —> D’+A’
(1.1)
conformational arrangement is inherently difficult and the electron transfer information
gleaned from such experiments is either rate limited by the diffusional encounter of the
complexes in solution or by the nuclear activation necessary to convert reactants to
products. However, In 1981 Sutin [4] introduced ion-pairing to bimolecular electron
transfer reactions with the system composed of Ru(bpy)32+ and Fe(CN)63', effectively

organizing donor and acceptor in solution. This addition of electrostatic complimentarity

‘

3

to the bimolecular system, characterized by moderate association (~ 350 M'l) eliminated

the diffusional dependence of previous non-specific bimolecular interactions.

Fixed Distance Systems

Bimolecular reactions are still of great interest to the study of electron transfer
reactions [5] but more generally, the determination of specific contributions to electron
transfer reactions over the past 15 years has developed around the incorporation of
spacers tethering two electron transfer centers in a specific distance, stereochemical, or
conformational arrangement.

The elegant flash quenching experiments performed by Gray et al. [6] spurred the
development of fixed distance electron transfer systems. Formation of a metalloprotein
electron transfer complex resulting from reaction of (NH3)5Ru(III)(H20) with horse heart
cytochrome c (PFem) produced the covalently attached (NH3)5Ru(III) (histadine-33),
creating a electron transfer complex (PFem—Rum) with donor and acceptor fixed in
distance by the protein. Bimolecular reduction of the surface bound (NH3)5(histadine-
33)Ru(III) by Ru(bpy)32+ in the presence of EDTA generated the (PFem-Ru") complex.
Subsequently, intramolecular electron transfer across a distance of approximately 11.8 A
occurred with a rate constant of 30 :1: 3 5". Further studies utilizing the cytochrome c
protein in which the driving force of the reaction was enhanced by the substitution of the
Fe(IH) with Zn(II) were developed. Coupled with the variation of the Ru(HI) complex
position at the surface of the protein, temperature dependence studies of the electron
transfer and electrochemical measurements were performed to assess the AH, AS and AG

of the reaction, providing a determination of the electronic coupling (Hab) and

A

 

4

reorganizational energy (A) for the system. Large reorganizational energy and significant
decay of the electronic coupling through the protein separating the electron donor and
acceptor were revealed. The attenuation of the electronic coupling (Hub) found from these
measurements (2.0 A") was attributable to poor donor-acceptor overlap through the
medium of the protein environment. [7]

Prompted by these initial findings, the assessment of electron transfer rates
through protein matrices of many small protein complexes, modified by attachment of
various ruthenium complexes have been undertaken. Blue copper proteins, azurin,
plastocyanin, and histidine derivatives of Fe-cyt c and myoglobin were developed [8].
The surprising finding of these protein mediated electron transfer reactions was the large
reorganizational energies (~1.0 V) fairly independent of the protein environment and the
donor—acceptor electronic coupling, found to be significantly variable, with the
parameters not well accounted for by semi-classical electron transfer descriptions. In
stark contrast to these findings, the observations of Dutton [9], surveying a wide range of
electron transfer reactions in proteins having variable distances between donor and
acceptor were found to follow a "universal" model for the electron donor-acceptor
coupling (Hab) and distance (d) separating the donor and acceptor. This one-dimensional
square barrier (131053) model proposed that a general relationship exists for the
exponential decay of the maximum electron transfer rate constant km (s") =
10'3exp‘1‘4w'3‘6) versus d (A), where B = 1.4 A'1 satisfactorily explaining all of the distant
dependent rate data. However, the works of Onuchic and Beratan have suggested an
alternative tunneling pathway model for the decay of the electron transfer within the

protein environment [10]. This model accounts for dominant pathways for electron

‘

5

transfer contributions from bonding interactions existent within the protein environment.
Covalent, hydrogen bonding interactions and through-space jumps, are separately
assigned decay components, which additively provide contributions to the electron
transfer pathway. This model effectively predicts exponential decay dependent on the
tunnel length and composition rather than the direct distance between donor and acceptor.
A recent analysis comparing both of these electronic coupling models (Dutton and
Onuchic-Beratan) applied to electron transfer in 23 protein systems has shown a clear
distinction between the photosynthetic reaction center proteins and other electron transfer
proteins [1 1]. However, the method of the of the analysis used in this investigation
generalizes the Onuchic-Beratan tunneling pathway model incorporating only through-
space interactions including the protein as a continuous medium with characteristic

electronic bandwidth.

Model systems

The observation of long range electron transfer in both the photosynthetic reaction
center and other protein electron transfer systems have prompted model system studies
aimed at assessing the controlling factors. Simulation of the protein environment with
small molecule model systems possessing design features representative of the
secondary, structure of proteins has been a recent focus in the work of Isied, Wishart, and
Ogawa [12]. Recent examples from Ogawa [13] have incorporated the use of B-sheets
and B-tums separating donor and acceptor complexes. Preliminary results from this group
have substantiated Gray et al. [14] in the prediction of different electronic coupling decay

constants for Ot-helices (1.26 AI) and B-strands (1.00 Al).

A

6

Previously, Meyer [15] has incorporated the design feature of peptide spacers in
ruthenium based inorganic chromophore-quencher assemblies. These model system data
are not treated by either of the distant dependent decay models, due primarily to the short
range of the eletron transfer, but rather are analyzed by conventional Marcus
relationships.

Although no account has been made in these model systems as to the dipole
moment associated with the amino acid residues oriented parallel to the a—helical axis,
the incorporation of electrostatic potential effects generated from the dipole induced field
on electron transfer in systems such as these is an issue of significant current interest
[16]. Electrostatic field effects have been incorporated into both theoretical and
experimental studies of the photosynthetic reaction center in relation to possible
mechanistic implications governing the rapidity of electron transfer over long distances
[17].

Many model system designs encompassing the orientation and distance
relationships of chromophores in electron transfer patterned after the photosynthetic
reaction center have been developed [18]. Figure (la), shows the design of Osuka [19]
incorporating three porphyrin macrocycles; two in a close conformation and the third
appended with a pyromellitimide acceptor representative of the arrangement of the
chromophores found in the bacterial reaction center. The distinct narrow anion radical
spectra of the pyromellitimide quencher in these systems provides a great advantage in
the observation of a charge separated state owing to the electron transfer reactions

relative to other similar quinone based reaction center mimics. Gust and Moore (1b) have

 

Figure I. Phtotsynthetic reaction center mimics, coupled porphyrin model systems design
of (a) Osuka et al. and (b) a tetrad system of Gust and Moore.

 

Figure 2. Model system for the assessent of electric field effects in photosynthetic
reaction centers

 

(b)

(C)

Figure 3. Electron transfer model systems (a) Biphenyl bridged model system of
McClendon (b) Peptide linked chromophore-quencher system of Meyer and (c) rigid
covalent norbomadiene system of Paddon-Row.

taken a different approach to mimicking the photosynthetic reaction center with their
design of triad, tetrad and pentad systems [20]. These systems produce quantum yields of
ion-pair formation of 0.83 and long lived (55 us) charge separated states; however, the
conformational flexibility and the participation of multiple electronic excited states make
these systems intractable for the determination of specific effects controlling electron
transfer reactions. Yet other studies by Wasielewski (2) have shown model
photosynthetic reaction center complexes inCOrporating induced electrostatic fields
generated as a result of the charge separation event [21]. The design considerations of
these model systems are primarily concerned with the effects of orientation and distance.
The aromatic spacer systems of McClendon (1c) [22] offer a contrast to these studies by
suggesting that the nature of the spacer and the relative orientation dependence of the
donor and acceptor porphyrins play a key role in the electronic coupling between the

donor and acceptor.

Rigid donor-acceptor systems

Not all donor/acceptor systems have been relegated to photosynthetic reaction
center models. The series of systems incorporating dimethoxynapthalene covalently
attached to dicyanoethylene through the hydrocarbon bridge (3c) of Paddon-Row [23]
have detailed the effects of distance, conformation, and conjugation within this one
versatile system.

Perhaps the most important model system designed to address these issues are the

rigid steroid spacer between donor and acceptor groups (4) investigated by Closs and

11

Miller [24]. The attachment of donor and acceptor groups at the 3 and 16 positions
respectively of the 5-01-Androstane spacer allowed the creation of a fixed 10.3 A distance
separating the donor and acceptor. Tuning the free energy for the reaction over 2.5 V was
accomplished through the introduction of a series of aromatic, and substituted quinone
groups. The free energy dependence of the electron transfer rate results in the observation

of a non—parabolic fall-off in the observed electron transfer for these systems providing

° 0

(4)
the first unambiguous verification of the Marcus inverted region predicted in

Eqs. (1.1 - 1.2). Fitting of this data to Marcus theory however, required the incorporation

 

of
—AG*
k — Aexp|: kBT ] (1.1)
2
AG[(A_GM;>] (1,2,

quantum mechanical considerations derived by Marcus [25], Levich and Dogonadze [26]
based on the Fermi Golden Rule for the quantum mechanical probability for the transition

of an electron between two states as given in Eq. 1.3. The assumption may be made from

12

2
km : 7"]vab|21=c (1.3)

Eq.l.3 for non-adiabatic electron transfer that the Franck—Condom factor may be given in
the form FC = (47thT)"’2 exp-((AG+7L)2/ 4kka), yielding the semi-classical expression
for electron transfer as given in Eq. (1.4). In this form the electron transfer rate constant

maintains a dependence upon the free energy for reaction (AG), the temperature (T), and

211'. 2 1/2 (AG + A)2
k =—V 4kkT ex —— 1-4
..,1..1<..r [ w t)
the reorganizational energy of the solvent (1.5). Incorporation of bond length and bond
angle changes accompanying an electron transfer reaction, especially for highly
exothermic reactions, may be represented by a single vibrational mode having energy hv

and additional reorganizational energy for this mode

211 2 °° -5 Sv) (AG + 71.5 + mhv)2
k = — V — — —— 1.5
e‘ h I “bl "ale v! ”p 4ka1 ( )
A
s = —V 1.6
hv ( )

M ( S = M/hv ). This expression originally derived by Closs and Miller [24] successfully
accounts for the observation of the non-parabolic fall-off of the rate in the inverted region
according to Eq. (1.5).

Despite advances made by the virtue of fixed distance electron transfer systems,

non-covalent interactions and bond-making/bond-breaking reactions so important in

13

many biological transformations have yet to be explored. The prevalence of these factors

in biological systems provides new frontiers for electron transfer research.

Non-covalent interactions

The most common non—covalent interaction found in redox active systems is
established by the hydrogen bond. The premier example of hydrogen bonding in
biological systems is exemplified by the non-covalent, base-pair, self-association
interactions found in the complementary strands of the DNA double helix. However,
hydrogen-bonding interactions in biological systems are by no means restricted to this
particular system. Hydrogen-bonding interactions have been implicated in mechanisms of
catalytic enzyme reactions [27], , the secondary and tertiary structure of proteins [28] and
the stabilization of bound oxygen to hemoglobin and myoglobin [29]. Recent site
directed mutagenesis studies have implicated hydrogen-bonding interactions in tuning the
redox potentials of critical components of the photosynthetic reaction center [30].

Classifications of hydrogen-bonding interactions have been advanced based on
the strength and internuclear separation of the interaction [31,32]. Measurements of
hydrogen-bonding interactions typically involve NMR chemical shift of the hydrogen-
bonded nucleus, and the determination of the internuclear distance between the heavy
atoms by x-ray crystallography. Strong hydrogen-bonding interactions are exemplified by
the interaction of F-H-F, having an interfluorine distance of 2.26 A [32]. Typical energies
for the hydrogen-bonding in these interactions may be > 24 kcal/mol. An intermediate
range, of hydrogen-bonding interactions, is Low-Barrier-Hydrogen-Bonding, typically

yielding bonding energies 12-18 kcal/mol. This classification of hydrogen bonding,

 

14

although established in solution, solid state and gas phase, remains a controversial issue
[33] regarding the catalytic stabilization afforded by hydrogen-bonding interactions in
enzymatic reactions. The final categorical class of hydrogen-bonding interactions are
Traditional Hydrogen Bonds (THB), also called weak hydrogen-bonds, typically
exhibiting bonding energies of < 12 kcal / mol. This thesis will focus on systems
exhibiting the latter types of hydrogen bonds

Important aspects in the description of any hydrogen-bonding interaction are van
der Waals, electrostatic and pKa effects [34]. Within symmetric or asymmetric hydrogen-
bonding systems van der Waals interactions of the electron deficient proton with an
neighboring electron rich anion influences a directional and close association of
substituents. The association of two hydrogen bonding species in symmetric hydrogen
bonding arrangements has been most commonly investigated. However, the investigation
of non-symmetric hydrogen-bonding interactions via infrared measurements [35] has
shown that the incorporation of asymmetry into the hydrogen-bonding interaction may
substantially influence the thermodynamic association inherent to the system. Bolstered
by the increased specificity of the directional interaction, pairing of the donor and
acceptor are enhanced. Furthermore, addition of complimentary charges into an
asymmetric hydrogen-bonding interaction may further augment the already enhanced
association. Charges implemented within a hydrogen-bonding system yield

complimentary electrostatic interactions, which may be described by a simple

 

EH = 9192 (1.8)

electrostatic model Eq. (1.8). In this expression, changes in the hydrogen-bond energy

(E") are described by changes in Coulombic interactions between the partial effective

 

15
charges of the hydrogen bond donor and the hydrogen bond acceptor (q. and qz). The

presence of the distance (r) and the solvent dielectric (e) modify the strength of the
interaction between the charges as a function of distance and solvent dielectric. As shown
in Eq (1.8) the strength of the hydrogen bonding is inversely proportional to the dielectric
constant of the solvent and the distance between the charges. Additional electrostatic
considerations enhancing the strength of the hydrogen bonding interactions have been
shown by Jorgenson [36]. The Secondary Electrostatic Interaction (SEI) results in the

development of enhanced Coloumbic attraction within hydrogen bonding interactions

/H
FN o -------- +1 N X +
”\dN --------- H—N/ \ - +
— N
N—H -------- o +
(5)

exhibiting complimentary charges in a diagonal orientation in hydrogen-bonding
substituents. For the case of guanine and cytosine (5) in the Watson-Crick hydrogen-
bonding arrangement, there are both positive and negative secondary electrostatic
interactions between the partial positive and negative charges of the nitrogen and oxygen
atoms composing the hydrogen bond. Positive contributions to the hydrogen bond
strength are found between the carbonyl oxygen and the imino nitrogen functionalities of
the guanine with the cytosine amino and protonated imino functionalities represented
schematically at the right of Figure (5). Furthermore, negative contributions to the

hydrogen bond strength are found for the guanine amino and the cytosine protonated

16

imino functionalities. Description of hydrogen-bonding in terms of acid and base or
ionizable residue concepts (pKa), using the electrostatic argument shown in Eq. (1.8)
predicts that the hydrogen bonding strength will increase linearly with increasing acidity
(positive charge) of the hydrogen bonding donor or with increasing bascity of the
hydrogen bonding acceptor (negative charge) reaching a maximum at ApKa = 0 [31].
Description of the hydrogen bonding interactions based on the pKa of the participating
substituents is a thermodynamic argument and a first approximation for a proton transfer
process. While there is an expected definite relationship between the hydrogen bonding
strength and the pKa it has been shown in model compounds that there is no special
stabilization associated with the equalization of the pKa of hydrogen bond donor and
hydrogen bond acceptor [37]. However, there are important consequences of short and
strong hydrogen bonding arrangements, as this may give rise to potential energy
descriptions for the system involving two wells separated by a low barrier or possibly to a
single well with no barrier to proton transfer. The greater the distance or the weaker the
interaction, the greater will be the barrier to, and the less likely will be, proton transfer.
The derivation and interpretation of potential energy surfaces describing the internuclear
proton lability or transfer between the two heavy atoms is difficult to desribe solely from
these primarily thermodynamic observations [38]. Theoretical approaches and
implications of the nature and type of hydrogen-bonding interaction are diverse and both
classical and quantum mechanical descriptions may be necessary to fully describe proton

transfer reactions occurring in solution. [39]

Proton Transfer

The transfer of a proton between two chemical sites is the most fundamental of
chemical reactions and may occur as a result of either thermal or electronic excitation.
Photoinduced proton transfer reactions have been studied in a variety of intermolecular
and intramolecular hydrogen bonding chemical systems for many years. Accompanying
photoexcitation of an intramolecularly hydrogen bonded complex, a fast reorganization
of molecular structure generally occurs resulting from the shift or transfer of the proton
within the hydrogen bonded complex. Frequently, a signature of the photoinduced proton
transfer within a hydrogen bonded complex is the observation of dramatically
(4000 - 10000 cm") Stokes-shifted emission resulting from the electronic and/or
conformational change occurring in the production of the proton transfer tautomer [40].

Upon photoexcitation, the electronic composition of the chromophore is altered

such that barring steric or strain considerations, the proton transfer reaction will proceed

A A.‘ Ij
r r

with a driving force for ‘inn determined by the change in the
acidity or basicity of the chromophore. Complete proton transfer within hydrogen-bonded
complexes results when a large change in the acidity or basicity of chromophore occurs
in the excited state. The observation of an incomplete transfer or proton shift within the

hydrogen-bonded complex may result in complexes in which there is a relatively small

change in the excited state pKa of the chromophore relative to the ground state
Proton transfer in biological systems

Perhaps nowhere does the relevance of proton transfer have such a prominent role

as in biological systems. The coupling of proton transfer to charge separation is a

‘

18

fundamental component of biological energy conversion mechanisms. These coupled
events continue to emerge as fundamental components of structure/function relationships
in a variety of proteins and enzymes including nitrogenase [41], non-heme iron
containing proteins [42,43], multicopper oxidases [44] and reductases [45]. However, the
significance of the effect of the coupling of proton motion to electron transfer is best
demonstrated in the active site of cytochrome c oxidase. Despite similar separation of the
two heme sites from the binuclear CuA center, electron transfer to heme a is 102—104
faster than to heme a 3 [46]. Proton transfer accompanying heme reduction is believed to
be the origin of the slow electron transfer to heme a3 and accordingly the controlling
factor for directional electron transport via heme a [47]. In photosynthesis and respiration
the photoinduced redox processes creates a proton potential across the mitochondrial
membrane which is utilized for the production of ATP. Yet the inherent complexity of
these systems does not allow the direct observation of the electron/proton transfer steps.
Incorporating the design feature of non-covalent interactions specifically those containing
hydrogen-bonding interactions between the electron transfer donor and acceptor may

provide the means of differentiating these inherently coupled processes

Non-covalent systems in electron transfer

The incorporation of hydrogen-bonding interactions in electron transfer systems
are relatively few. Mataga, [48] has shown the effects of hydrogen-bonding interactions
for a variety of organic based electron transfer systems incorporating single point
acid/base complementary interactions of amino and alcohol functionalized compounds.

Additionally, complementary hydrogen-bonding interactions utilizing DNA base-pair

A

19

recognition (guanine-cytosine) for the binding of electron transfer donor and acceptor
moieties held at a fixed orientation and distance have been shown in the work of Sessler
[49]. The electron transfer ensembles of Sessler (4a, 4b) represent some of the first
preparations of non-covalent electron transfer model systems. A primary issue in the use
of non-covalent electron transfer systems is the strength of the interaction, and therefore
the association constant maintaining the two interacting components in contact in
solution. In (43) the characterized by a heteroassociation constant of 3100 :i: 470 M'| in
CDzClz. Despite the association of the complimentary cytosine and guanine components
of the electron transfer complex are significant association of the two components in low
dielectric solvent, the orientation is such that the electronically excited zinc
metalloporphyrin donor and 2-cytidinyl benzoquinone acceptor are capable of through
space interactions. Thus the conformational flexibility in this complex may lead to net
electron transfer quenching by collisional interaction in addition to that through the
hydrogen-bonding interface. To alleviate these concerns, further synthetic modifications
of this system have led to the development of (4b). The association constant in this
system is significantly greater (8900 :I: 600 M'l) than that of (4a). Determination of
intramolecular electron transfer in each of these systems was conducted using time-
resolved fluorescence measurements yielding unimolecular electron transfer rates of 4.2
10.7 x 103 s‘1 and 8 x 108 s'1 for (4a) and (4b) respectively. While charge-separated
intermediates have not been observed, these fluorescence data are consistent with
electron transfer occurring through hydrogen-bonding interfaces. Control experiments are

also consistent with electron transfer. Addition of ethanol to the solutions results in loss

20

a 1
%§ 0 {k Fwd : t

(a)

 

(C) ((1)

Figure 4. Non-covalent electron transfer systems implementing guanine-cytosine base-
pair recognition for donor-acceptor separation. Conformationally flexible system (a).
conformationally rigid system (b) two point (e) and four point ((1) hydrogen bonding of
quinoncs by functionalized porphyrins.

O

\ / N ------ -H—N 8 N—(CH2)30TBDMS

N—H- ------ o o
o:§
O
(a)
o
"'C6H13\N
o N
o
/

 

Figure 5. Non-covalent systems based on asymmetric hydrogen-bonding (a) and by metal
ligation (b)

22

 

Figure 6. Non-covalent cyclodextrin porphyrin assembly for the investigation of electron
transfer to quinone.

23

of the biphasic decay corresponding to emission from the formation of bound and
unbound donor in the return of the monoexponential decay of the uncomplexed zinc
metalloporphyrin lifetime 1.4 ns (7.1 x 108 s'I ).

A beautiful set of experiments investigating the effects of hydrogen bonding in
electron transfer systems has recently been undertaken by Mataga [50]. Efficient
quenching of the porphyrin singlet excited state by l,8:4,5—napthalcne-tetracarboxamide
in benzene solution is observed. Quenched emission for the complex was not resolved
despite an instrument response of 3-5 ps, revealing only emission due to unbound donor.
Static quenching of this complex, however, indicates an efficient quenching, when
coupled with the appearance within 10 ps of transient spectral features associated with
the formation of a charge separated complex. The electron transfer quenching is
established as occurring at rates faster than 3-5 ps (2.0—3.3 x 10118"). An experimental
advantage of this complex is the appreciable association (1.6 i 104 M") in CDC13 and
(1.3 i- 105 M'l) in C6D6. Further systems that have been developed for non-covalent
hydrogen-bonding electron transfer reactions are those of (4c) and (4d) developed by
Aoyama, Ogoshi [51], and co-workers and Hayashi, Ogoshi and co-workers [52],
respectively. These systems take advantage of two point and four point hydrogen-
bonding interactions between the para quinone and the porphyrin complex in systems
reflective of the covalent complexes of Lindsey and Mauerzall [53]. Interestingly, for
these complexes quenching of the fluorescence of the porphyrin is found to depend upon
the binding of the quinone and not on the redox properties, suggesting that the close
spatial relationship gives rise to large electronic coupling dominating the Franck-Condon

factor.

24

The use of non-covalent interactions between electron transfer donor and acceptor
based on metal complex ligation by pyridyl containing acceptors have been performed by
Sanders and co-workers [54] (5a). In this system a dramatic example of the effects of the
solvent reorganizational energy are evidenced by comparison of complex (53) to the
additionally synthesized meso tetra-phenyl porphyrin analog. The intramolecular electron
transfer rates observed in these two complexes are found to be 213 x 108 s'1 and 53.3 x
108 s‘l respectively. Further systems developed for the investigation of non-covalent
interactions in electron transfer have been demonstrated in the design of (6) [55] two-
cyclodextrin molecules covalently linked to a central porphyrin. In these systems the
inclusion of a quinone or substituted quinone into the cyclodextrin cavity provides the

acceptor for the electron transfer reaction.

Proton Coupled-Electron Transfer

The coupling of proton transfer to electron transfer in model systems have been
addressed with Oxo bridged di-Manganese OEC model complexes. These model systems
have shown proton and electron transfer events to be coupled processes [56]. An earlier
example showing the coupling of proton and electron transfer events involved the use of
ruthenium oxo derivatives, [57]. While each of these systems incorporate the proton and
electron transfer events the ultimate mechanism of the coupling of the proton to the
electron transfer event is not resolved.

The work of Mataga employing electron transfer within single point hydrogen-
bonding of 13 and 7H dibenzocarbazole and l-aminopyrene with pyridine have shown

the coupling of proton shift within the hydrogen-bonded pair to facilitate fast electron

25

transfer reactions [58]. The basic tenets for coupling of proton transfer to electron transfer
were given in these studies, where it was assumed that the ionization potential of the
donor would be reduced, and the electron affinity of the acceptor would be enhanced due
to the presence of the hydrogen-bonding interaction. Furthermore, the hydrogen-bonding
in the excited state was expected to give rise to greater delocalization of charge causing
the proton to shift toward the acceptor further lowering the ionization potential of the
donor and increasing the electron affinity of the acceptor. These factors were expected to
facilitate a rapid electron transfer between donor and acceptor in contrast to expectation,
based upon the assumption that the electronic coupling through a hydrogen-bonding
interface would be reduced relative to a system composed of a direct attachment of the
donor and acceptor. Observed intramolecular electron transfer rates for the aminopyrene-
pyridine-x (where x = H, 2-methyl and 4-methyl ) are found to be ~l.0 x lO'os'l. The
basic shortcoming of this system is the single-point hydrogen bonding yielding meager
(< 10 M") association constants even in non-polar hexane.

To circumvent this issue, complementary hydrogen-bonding interactions of
dicarboxylic acids in aprotic low dielectric solvents may be implemented giving rise to a
more strongly associated hydrogen-bonding proton transfer interface. Wenograd [59] and
Pitzer [60]. First described this interface in solvents with low dielectric constant. Many
examples of the use of this interface in proton and electron transfer [61] systems have
appeared since making this interface an exceptional experimental model.

Previous studies in the Nocera group [62] have elaborated the details of the
symmetric dicarboxylic acid (COOH); interfaces (7a, 7b) as a spacer between the

electron transfer donor and acceptor. Electronic excitation of the zinc metalloporphyrin to

26

the singlet excited state when associated through the (COOH)2 interface to the electron
transfer acceptor 3,4—dinitrobenzoic acid leads to the observation of fast intramolecular
electron transfer (kcs = 1 x10ms’l and kcr = 5 x10‘°s‘1). Definitive evidence supporting the
determination of electron transfer through the (COOH); interface is corroborated by
addition of ethyl-3,4-dinitrobcnzoate. In the presence of this non hydrogen-bonding
complex only the electronically excited zinc metalloporphyrin complex is observed and
there is no evidence of quenching. Furthermore, conclusive evidence of the coupling of
the proton to the electron transfer occurring through the (COOH); interface is established
in isotope exchange studies, whereby, the deuteration of the hydrogen-bonding interface
protons results in the observation of a deuterium isotope effect of kH/kp = 1.7 on charge
separation and kH/kD = 1.8 on the charge recombination reaction.

Further studies of the effect of symmetric dicarboxylic acid (COOH)2 interfaces
were found in system (7b) where the symmetric (COOH); interface is isolated from the
redox centers by the incorporation of a phenyl spacer [63]. In this system results of
kinetic data in the observation of quenching of zinc metalloporphyrin emission as a
function of added complimentary benzoic acid functionalized iron metalloporphyrin. It is
proposed in this system that the incorporation of the phenyl spacer results in decreased
effective electronic coupling between the redox centers, and firrthermore proton transfer
within the interface is decoupled from the electron transfer yielding an observed electron
transfer rate of 7.6 x 108 s", significantly attenuated with respect to (7a) where only a
methylene group is present between the porphyrin and the dicarboxylic acid interface.
The electron transfer quenching results of (7b) are obscured by an energy transfer

component.

27

 

Figure 7 Zn(II)-porphyrin-3,4-dicarboxylic acid donor-acceptor symmetric interface
system. (a) Zn(II)-Fe(IH) diprophyrin symmetric interface system with phenyl spacer (b).

‘

28

determination of electron transfer through the (COOH); interface is corroborated by
addition of ethyl—3,4-dinitrobenzoate. In the presence of this non hydrogen—bonding
complex only the electronically excited zinc metalloporphyrin complex is observed and
there is no evidence of quenching. Furthermore, conclusive evidence of the coupling of
the proton to the electron transfer occurring through the (COOH); interface is established
in isotope exchange studies, whereby, the deuteration of the hydrogen-bonding interface
protons results in the observation of a deuterium isotope effect of kit/k1) = 1.7 on charge
separation and kH/kD = 1.8 on the charge recombination reaction.

Further studies of the effect of symmetric dicarboxylic acid (COOH)2 interfaces
were found in system (7b) where the symmetric (COOH); interface is isolated from the
redox centers by the incorporation of a phenyl spacer [64]. In this system results of
kinetic data in the observation of quenching of zinc metalloporphyrin emission as a
function of added complimentary benzoic acid functionalized iron metalloporphyrin. It is
proposed in this system that the incorporation of the phenyl spacer results in decreased
effective electronic coupling between the redox centers, and furthermore proton transfer
within the interface is decoupled from the electron transfer yielding an observed electron
transfer rate of 7.6 x 108 s], significantly attenuated with respect to (7a) where only a
methylene group is present between the porphyrin and the dicarboxylic acid interface.
The electron transfer quenching results of (7b) are obscured by an energy transfer

component.

Incorporation of the non-covalent dicarboxylic acid interface in electron transfer
systems presents a unique problem in electron transfer research. The interface resulting

from this dicarboxylic acid (COOH)2 configuration is symmetric, and proton

‘

29

displacement from one side of the interface is compensated by the displacement of a
proton on the opposite side. As a result, protons are not pumped by an electron transfer
event and proton motion does not lead to a net charge build-up within the interface.
Subsequently, proton motion may influence an observed electron transfer rate through
modification of the electron coupling matrix element between the electron donor and

acceptor states.

Recently the incorporation of the dicarboxylic acid interface organizing electron
transfer donor and acceptor complexes composed of benzoic acid functionalized
trimethyoxy phenyl (Zn" and Fem) porphyrin complexes have appeared [65].The results
analyzed by the additive contribution of specific types of bonding interactions of the
tunneling pathway. However, these analyses of the results are partially misinformative as
it is merely assumes that the primary contribution to the electron transfer rate in each
system is the coupling of the donor and acceptor over the nearly constant distance
separating the donor and acceptor in each system. This particular study serves to further
validate the use of non—covalent hydrogen—bonding interfaces as viable spacer systems for
the observation of electron transfer reactions.

The two carboxylic acid functionalized systems (7a) and (7b) provide the
benchmark for the investigation of a photoinduced proton transfer within a fixed distance
electron transfer system. The theoretical assessment of the results of system (7a) show
that the effect of the proton modulation of the observed electron transfer rate stems from
the dependence of the electronic coupling matrix element (Vab), responsible for the
transfer of the electron, upon the configuration of the interface protons [66]. The

electronic coupling is found to reach a maximum at a configuration in which the protons

30

are located equidistant within the interface. This configuration represents a maximum for
the charge delocalization, facilitating the observation of fast electron transfer. Because
the symmetric interface does not invlove a net charge rearrangement upon proton transfer

only a weak coupling of the protons with the solvent is expected.

42 """" ”2%— a: """" “new“ """" in

OH ------- O----H~---O ----- H-0
(6)

The use of the symmetric dicarboxylic acid interface has presented the initial
framework for the determination of the involvement of the proton in the modulation of an
observed electron transfer rate. However, the use of the symmeric dicarboxylic acid
interface has several experimental limitations. The association of the carboxylic acids
presents a small association (500 M") constant even in solvents of low dielectric
constant, and furthermore the homo-association of donor and acceptor complexes reduces
the solution concentration of the desired electron transfer assembly. To alleviate these
concerns and to potentially reveal more information concerning the modulation of the
electron transfer rate by the proton interface, the incorporation of an asymmetric salt-
bridging interface has been included in the PCET system design. This hydrogen-bonding
arrangement presents several advantages over the symmetric analog. The directional and
specific interaction of donor and acceptor by charge complimentarity affords a strong
interaction of donor and acceptor in solution and furthermore eliminates the potential

formation of the homoassociated species. The most important aspect of the charged

31

asymmetric salt-bridge interface is the significantly enhanced association constant.

H H\
\
—H o H ----- 9
—<" + — :: VE ,l\‘ + —’>_
/N—H lN—H ----- O
H H
(7)

Additionally, the charge within the asymmetric interface offers the opportunity for

greater coupling of the charged proton to the solvent polarization.

H\ H\
N H --------- o N ---------
D" _G) 9 ,i'}—A —> D‘ \ \> A
N—H -------- N --------- H—
/ /
H H
/H /H
o --------- H N o --------- N
o-4< o ®>7A —. o—<:' e ®>7A
O -------- o -------- H N
\ \
H H
(8)

For electron transfer reactions described by Figure 8 the change in the localization

of the proton within the interface is described by Eq (1.11).
21: 2 1/2 . 2 (E +AG+hosv)2
kc, = TM (Eska) VEO|(oi|vf)| exp— [W (1.11)

S!

[(01 | «)12 = [e‘s 3] (1.12)

32
In either representation of the interface, shown in (8) the displacement of the proton is
expected to influence the observed rate for the electron transfer reaction. Describing the
electron transfer according to Eq. (1.11) accounts for this dependence by the
incorporation of the proton coordinate v having characteristic oscillator frequency co.
Accordingly, the reorganizational energy and the exothermicity due to the proton transfer
shown in Eq. (1.12) are given as the summation over the proton vibrational frequencies in
Eq. (1.11).This additional contribution, Franck-Condom factors for the proton states, may
allow a clear distinction to be made between the participation of the proton transfer in the
electron transfer system composed of the donor amidinium-carboxylate acceptor system

relative to the case where the interface is reversed [67].

Incorporation of Ruthenium Systems into the study of PCET

This thesis focuses on Ru(H) polypyridyl complexes as electron transfer donors
through amidinium-carboxylate salt bridge interfaces. Thus a brief review of these
systems with regard to electron transfer will now be described

Synthetic modifications of Iris—bipyridyl ruthenium complexes synthesized to
obtain information concerning the nature and the systematic details of electron transfer
are numerous [68]. All systems take advantage of the well-documented photophysical
properties of this class of compounds. Particularly advantageous aspects of these
molecules are the well-characterized radiative and non-radiative decay properties and the
means by which these may be manipulated and controlled in an orderly manner to create

tailor-made electron transfer complexes.

33
Absorption Spectroscopy of Ruthenium Bipyridyl Complexes

The ground and excited states of ruthenium tris-bipyridyl molecules have been
well characterized over the past 30 years by a vast assortment of spectroscopic techniques
[69]. Features present in the electronic absorption spectra consist of transitions found in
ultra-violet region, characterized by intraligand 1t-rt* absorption in the 270 - 300 nm
region. Typically the transitions have molar absorptivites 8 ~ 4 - 10 x 104 M" cm’1 and
reflect the absorption cross sections observed for the free bipyridyl ligands. The 11-1c*
transitions gradually red shifting upon the introduction of more strongly electron
withdrawing substituents at the periphery of the bipyridyl rings.

The spectral features of Ru(bpy);t2+ complexes, however, are dominated by the
split non-gaussian, metal—to-ligand charge transfer (MLCT) band in the 400 - 500 nm
region. General consensus is that the bands originate from 1E (1t*)(-—3 IA] ((111)
transitions (D3 point group symmetry), involving the promotion of an electron from the
metal-centered drt manifold to the ligand antibonding 1t*. Typically, the molar
absorptivities for these transitions are (e~ 6 - 15 x 103 M'lcm"). This relatively large
range in the molar absorptivity values arises largely from the introduction of asymmetry
in the complexes via the incorporation of ligands having different electronic (0' donating
and 11: bonding) composition. Reduction of the symmetry of the complex due to the ligand
environment may be achieved in the ground state through the synthetic manipulation of
the ligand set surrounding the central metal. Upon charge transfer excitation, promotion
of an electron to the ligand based 11* orbital results in an inequivalence of one of the three

ligands, effectively increasing the transition dipole and hence the molar absorptivity.

34

In a systematic study of the absorption of Ru(bpy)32+ complexes in 19 solvents
spanning a range from nitromethane to water, slight shifts (the largest observed
difference was found to be 310 cm") of the absorption maxima in the d1r-1t* (MLCT)
transition region have been observed. This solvent dependant shifting of the MLCT band
has been given as evidence of the localization of the excited state electron at one
bipyridyl ligand upon photoexcitation [70]. The consequence of solvent changes of the
localization due to lower dielectric constant creating a configuration in which strong
coupling of the hole at the metal and the electron at the ligand would be preferred for
solvents having a higher dielectric constant, the opposite would be expected for a low
dielectric thus a dependence upon the solvent is expected. The basis for these conclusions
is drawn from the derivation of an expression for the change in the total polar spectral
transition energy (AET) associated with a particular transition. However, caution has been
advised in the over interpretation of this solvent dependant absorption data due primarily
to the small observed shift found when a comparison is made to the solvent dependent

shifts observed in charge transfer absorption spectra for organic complexes [71].

Luminescence Spectroscopy of Ruthenium Bipyridyl Complexes
Paris and Brandt first reported the luminescence of Ru(bpy);2+ in 1959 [72]. The
observed luminescence was attributed the to 1t*-d charge transfer. Initially, skepticism of

this assignment arose from Porter [73] and from Crosby [74] who attributed this

luminescence as d*-d emission or as d*-d phosphorescence influenced by spin-orbit

35
coupling considerations. However, the original contention of the 1t*-d assignment
prevailed, after considerable investigation during the 1960's [75].

More detailed studies have since shown that photoexcitation of the MLCT of
Ru(bpy)32+ complexes results in the initial population of a singlet 1MLCT excited state.
Due to strong spin-orbital effects intersystem crossing occurs from this state with unity
efficiency producing the emissive 3MLCT excited state of the complex. Recent studies
have shown that the appearance of the triplet excited state of the Ru(bpy)32+ occurs with a
time constant of approximately 500fs [76]. The emissive 3MLCT is known to contain a
manifold of three closely lying (100 cm") states. Deactivation of the excited state occurs
by three major pathways: radiative decay, non-radiative decay to the ground state of the
complex and thermally activated cross over to metal centered d-d excited states. The
energy gap between the emissive the 3MLCT and the non-emissive MC states may be
determined by temperature dependence studies but is dependent upon the ligand
composition surrounding the metal center. For Ru(bpy)32+ the metal centered states are
located approximately 3600 cm'1 above the 3MLCT. Population of the MC states gives
rise to extremely fast radiationless decay including decomposition of the complex by
ligand substitution processes, While luminescence from the 3MLCT of tris-bipyridyl
complexes of ruthenium are found to be significantly red shifted approximately
6000 cm'l from the absorption of the MLCT transition maxima.

The bandshape of luminescence spectra are generally nondescript at room
temperature consisting of broad, fairly gaussian-shaped bands. Upon cooling, vibrational
structure begins to appear, the spacing of which reflects dominant (medium frequency)

vibrational modes of the bipyridyl ligands and to a lesser extent metal-nitrogen bond

36

vibrations. The quantum yield of emission of Ru(bpy)32+ complexes are typically found
to be approximately 10% ((1)em s 0.10 ) [68] upon excitation into the MLCT transition
and has been found to be significantly independent of excitation wavelength.

k
I z =-—-—'——=kt 1.7
e ¢em kernr r0 ( )

Large variations in the band position and quantum yield of the emission occur
with ligand substitution where increasing delocalization of excited state electron density
and electron withdrawing substituents causes the emission to progress to lower energy
and decreased quantum yield. These effects are adequately understood through the
application of energy gap law considerations as applied to ruthenium, and osmium
complexes by Meyer [77].

Ligand variation and substitution of electron withdrawing and donating
substituents may affect the decay of the 3MLCI‘ state of Ru(bpy)32+ complexes.
Generally, it is found that a decrease in the energy gap between the ground and excited
states decreases the radiative rate, consistent with the Energy Gap Law. Two mechanisms
by which the excited state decay may be influenced by the energy gap of the complex are
evident. The overlap of the initial and final states of the acceptor modes resulting from a
change in the equilibrium displacement (AQe) or frequency (v = 01/21:) between initial
and final electronic states promotes deactivation of the complex by non-radiative decay.
Additionally, the difference of the excited state energy relative to the ground state may
increase or decrease km through the efficiency of ovelap of vibrational wavefunctions.

Thus an increase in the vibrational overlap between ground and excited states through a

37

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1
M 01;; 3d-d
LCT
G.S
lung:
11
"MLCT 3
- d-d
klsc
<1>~1 3MLCT
E"
(D
C
LIJ
hv kr knr knr
(31.8.
i v i

 

 

 

 

 

Figure 8. Photophysical scheme for tris-bipyridyl ruthenium(II) complexes. lMLCT,
3MLCT, 11111’“, and 3d—d refer to the singlet and triplet charge transfer, the singlet
intraligand and the triplet metal centered states respectively. kr and km represent decay
processes.

38

change in the equilibrium displacement (AQe) or by a decrease in the energy gap (AE)
results in the effective increase in the non-radiative decay km of the complex.

As with the absorption profile the position of the emission band observed is
largely dependent upon the nature of the ligand electronic composition and the nature of
the solvent environment. With the introduction of electronically withdrawing or donating
ligands emission maxima is observed to red or blue shift accordingly. The sensitivity of
the emission to the solvent environment is attributable to the charge transfer nature of the
transition, producing a significant dipole that may interact with the solvent dipoles. As
expected, the more strongly the solvent interacts with the dipole of the complex, the more
dramatically red shifted will be the emission maxima.

Non-radiative decay processes dominate the emissive properties of ruthenium
complexes, and lifetimes of these complexes typically vary with ligand substitution.
However, measurements of the lifetime of these complexes commonly yield decay
constants of 1 us in polar aprotic solution.

The loss of the ligands in the excited state is particularly problematic in solvents
of low dielectric in the presence of complexing counterions such as Cl‘, C104“ and CNS“.
A mechanism for the photoanation reaction has been published, however, the direct
experimental evidence supporting the mechanism remains largely speculation. Yet, recent
evidence for imidazolyl ligands suggests that the de-ligation proceeds through the
sequential removal of nitrogens from the metal center [78].

The complications of deligation are known and understood such that synthetic

strategies for the circumvention of these processes may be incorporated into complex

39

design [68]. The placement of ligands surrounding the metal center having different 0

and 1: characteristics allows the manipulation of the energetic levels of the complex such
that the energy gap between the emissive MLCT state and the photochemical d-d MC
state are separated to an extent that de-ligation (decomposition) processes are not

competitive with deactivation of the complex via radiative and nonradiative channels.

Organization of the Thesis

The incorporation of each of hydrogen-bonding proton transfer in simple electron
transfer systems composed of asymmetric salt-bridge hydrogen bonding interactions, as a
potential means of providing insights to these two fundamental reactions is the subject of
this thesis.

Chapter Two elaborates the experimental procedures used for the determination
and characterization of ruthenium and porphyrin complexes. Electrochemical and
photochemical determination of the free energy driving forces (AG) for each of the
electron transfer reactions are given. This chapter additionally includes general details for
the characterization of each of the chromophores in the solvents in which the electron
transfer reactions were studied.

Chapter Three develops the PCET of synthetically functionalized tris—bipyridyl
ruthenium complexes incorporating dicarboxylic acid and amidinium-carboxylate
interfaces. Considerations of the excited state energetics on the PCET rate is presented.

Chapter Four is concerned with the introduction of the tetrarnethylated bipyridyl
ruthenium complexes. This designed feature enforces localization of the MLCT transition

toward at the interface, clearing up considerations of equilibrium distribution among

40

ancillary ligands. A consideration of the effects of the nuclear (proton) motion in the
interface as it pertains to the observed electron transfer reaction is discussed. The effects
of the coulombic field generated in the charged interface are assessed in relation to the
potential role in the electron transfer reactions.

In Chapter Five a Ru(H) acceptor-salt bridge-donor system is detailed. This
section includes a description of the electrochemical determination of the energetics of

the various ligands composing the complex.

CHAPTER 2

EXPERIMENTAL METHODS

Materials

All Ru(H) polypyridine complexes were prepared in the laboratory of Professor
Nocera at Michigan State University [79].
Electrochemistry

Electrochemical measurements were performed with the use of a workstation
comprising an EG&G PAR 173 potentiostat/galvanostat a PAR 175 universal
programmer, and a PAR 179 digital coulometer. The output of the digital coulometer was
fed directly into a Houston Instrument Model 2000 X-Y recorder. Cyclic voltammograms
were measured at room temperature by using a Pt-disk working electrode (A = 0.08 cmz),
Pt-wire auxiliary electrode, and an Ag wire pseudo-reference potential electrode in a

standard H-cell configuration. Redox couples for the ruthenium(II)polypyridyl

41

42

complexes, typically (2-5 mM) were measured in CH3CN or CH2C12 solution containing
0.1 M tetrabutylammonium hexafluorophosphate (TBAH) as supporting electrolyte.
Typical scan rates were 50 to 200 mV/sec. Oxidation-reduction couples were referenced
to SCE via the use of an internal reference ferrocenium/ferrocene having a potential of
0.307 V vs. SCE [80]. The reversibility of each redox couple was determined through the
measurement of the anodic to cathodic peak separation AEp of the oxidative and reductive
cycles for each complex and also from the in,l lip,c ratios. For each of the redox couples,
the typical measured anodic to cathodic peak separation ABp varied from the theoretical
59 mV by 40mV. This deviation can be attributed to minimal uncompensated cell
resistance and fluctuations due to slight thermal effects.

Cyclic voltammetry measurements in water were undertaken to assess pH effects.
Experiments were conducted in H20 at constant ionic strength maintained at 0.1 M with
the use of LiCl and buffered for each pH range used as described previously [81]. The pH
was adjusted with the addition of stock solutions of HCl and NaOH. The NaOH solution
used in these measurements was prepared via standard techniques; the HCl solution was
titrated against the NaOH standard to determine the solution normality. Serial dilutions of
these stock solutions were performed to obtain the correct concentration for volumetric
additions. During these measurements it was found that the use of a platinum electrode
gave very irreversible and inconsistent results, having AEp's significantly greater than 100
mV. Thus a glassy carbon disk electrode was used for these experiments. For the best
reversibility and most reproducible results, activation of the surface of a glassy carbon
electrode was necessary. The glassy carbon activation procedure of Meyer [81] was

altered slightly as follows: the potential of the electrode was held at 1.8 V for an initial

43

period of 25 minutes in an 0.1 M H2SO4 solution followed by cycling the electrode 5-10
times between a potential of 1.8 V for a period of 30 seconds followed by a potential of —
0.2 V for 15 seconds. This procedure was repeated for each point of the titration as the
activation of the surface results in the formation of a fragile oxide layer that flakes off
from the surface of the electrode upon extended immersion in solution.

Reduction potentials of the oxidative electron transfer quenchers were determined
in CH2C12 solution containing TBAH ( u = 0.1 ) as the supporting electrolyte. Quencher
concentrations were typically 2-5 mM. The reduction potentials of the protonated
quenchers 3,5—dinitrobenzamidinium (DNBAmH+) and 3,5-dinitrobenzoic acid (DNBA)
were performed using a glassy carbon electrode as the working electrode because the
platinum electrode resulted in a quasi-reversible redox couple from which the
determination of reduction potentials could not be determined with sufficient accuracy or
reliability.

The effect of protonation on the redox potential of each of the quenchers in
CH2C12 solvent was determined with DBU (1,8 Diazabicyclo[5.4.0]undec-7-ene) solution
2 x 10'3 M or through the direct addition of the DBU base in concentrated form by micro-
pipet. For the direct DBU additions, it was found that the addition of base in excess of
200 ill resulted in excessive asymmetry of the current response and a dramatic reduction
of the effective potential range over which the electrode response in CH2C12 could be
measured. The use of DBU in these experiments thus required a thorough cleaning and
polishing procedure between each measurement during repeated use. The cleaning
procedure consisted of a brief 5-10 minute soaking in concentrated nitric acid, followed

by thorough rinsing with house distilled H20, polishing of the electrode surface with

44

(BAS 100 um polishing alumina) and brief sonication of the electrode in a small 25 ml
beaker containing distilled/deionized H20. Cleanliness of the electrodes in these
measurements was assumed upon the return of reversibility of the measurement of the
ferrocenium/ferrocene redox couple.

Standard chronocoulometric techniques [80] were applied to the measurement of
the diffusion coefficients for each of 13 nitro-aromatic oxidative quenchers [Ru(bpy)3
(PF(5)2]2+ and also measurements were made for the 3,5 dinitroaromatic salt-bridge
functionalized oxidative quenchers. For each of the nitroaromatics the same potential
(—l.2 V) was set for the platinum electrode. For the determination of the diffusion
coefficient of Ru(bpy)32+ (PF6)2 the potential was set at both (1.4 V and —l.4 V) and
values found were nearly identical (3.98 x 10'5 cm2/sec at +1.4 V and 9.6 x 10’5 cm2/sec at
-l.4V) to previous literature determinations (4.2 x 10‘5 cmzlsec) for this compound. The
current time response of for each of the nitroaromatics was converted to l/t"2 vs. i ( i is
the current in amperes and t is the time in seconds) plotted in Kaleidagraph. A least
squares linear regression was performed with fitting parameters of the slope yielding the
diffusion coefficients D(cm2/sec) from known concentration C0* (mollcm3) for each of
the nitroaromatics from the Cottrell equation Eq. (2.1) [80].

. . nFAD “2c *
l“) : 111(1) = 1/(2t1/2 0
1r

 

(2.1)

In this equation n is the number of equivalents, F is Faraday's constant (9.6485 x 104
C/equivalent), and A ( 0.0314 cmz) is the area of the electrode. The supporting electrolyte

for each of these determinations was tetrabutylammonium hexafluorophosphate (TBAH

45
p. = 0.1M). Before each measurement of the diffusion coefficient of the nitroaromatic was
determined, the measurement of the solution (solvent with supporting electrolyte)
contribution to the observed diffusion coefficient was determined and analyzed in the
exact same manner as mentioned above. The current time response of the solution was
minimal and subtraction produces no noticable change in the diffusion coefficient
measurements for the nitroaromatics nor for the Ru(bpy)32+(PF6)2. For the diffusionally

controlled regime of electron transfer the bimolecular steady-state rate is given by the

Debye-Smoluchowski equation Eq. (2.2) [82],where D = Dd + D a and a' = the effective

k D = 4713133. (2.2)

encounter distance. As it has been determined [83] that for bimolecular quenching
reactions involving nitroaromatics the quenching occurs through the nitro functionality,
the value of a' was assumed to be identical for each of the nitroaromatics used here.

Solution concentration of the Ru(bpy)32+(PF6)2 was 3 x 10'5 M with u = 0.2 M in
tetrabutylammonium perchlorate (T BAP). The concentration of each of the respective
quenchers were 0.0015- 0.2 mM as necessary for the reduction of the natural lifetime (130)
of the Ru(bpy)32+(PF6)2 to (Io/2) as has been previously determined for reliable
determination of the linear Stem-Volmer bimolecular quenching rate constants kq [84,85].
The lifetime data was transformed into quenching rate kq using the relation of Meyer
given in Eq. (2.3).

_ = _ + — (2.3)
kq- kD

46

Where kq is the measured quenching rate constant and K; is the rate constant of activated
quenching and finally kD is the determined diffusion limited rate constant in the medium
used. The data obtained from the flourescence quenching experiments were plotted as Itq
vs. concentration of the quencher and a least squares linear regression analysis was
performed using Kaleidagraph and Origin fitting routines based on the linear Stem-

Volmer Eq. (2.4).

L0 = 1+ kq[Q]‘r (2.4)
1'

Furthermore, the data of the redox potentials measured for the complexes previously
determined by Meyer in CH3CN solvent were converted to AG and the quenching rates as
RTlnka and fitted to Marcus Theory using two different expressions developed both by
Meyer [3] and by Schuster [86].

Bulk electrolysis measurements were performed on solutions of the ruthenium
complexes ~3.0 x 10'5 M, contained in a previously constructed cell, modified to fit into
the optical path of a CARY 17 absorption instrument. This cell has the same general
configuration as the standard electrochemical H-cell, with the exception of a reduced
volume compartment for the reference electrode. The initial determination of the
potential at which the compound was reduced/oxidized was determined from cyclic
voltammetry measurements. Bulk electrolysis was carried out 0.2 V past the halfwave
potential. Stirring was maintained throughout the experiment by a micro magnetic stir

bar.

47
Optical Spectroscopy
Absorption Spectroscopy

Absorption measurements were performed using a Cary 17 spectrometer
retrofitted with the hardware and software design modifications of On-Line Systems Inc.
for each of the ruthenium(H) complexes. All absorption measurements were collected
using 1cm Suprasil or matched quartz emission cells in aerated or degassed solutions by
Ar(g) or freeze/pump/thaw cycles. Absorption spectra were collected throughout the full
spectral region encompassing the d1t-1t* (MLCT) and intraligand 11—1t* regions for each
of the ruthenium complexes and throughout the Q-band and Soret regions for the
porphyrin complexes.

Molar absorptivites for each [(bpy)2Ru(Me-bpy)COOH][(PF6)2]2+,
[(be)2Ru2+(Me-bpy)COO-l[(PFo)]+. [(bPY)2RU(Mc-bpy)AmH+l[(PFe)3l3+. and the
analogous tetramethylated ruthenium complexes were measured on solutions ~2.0 x 10'5
M in CH2C12 ( bpy = 2,2'-bipyridine, Me-bpyCOOH = 4-methyl4'-carboxylic acid-2,2'-
bipyridine, (Me-bpy-AmH+ = 4-methyl-4'-amidinium—2,2'bipyridine). The
[(decb)2Ru(Me-bpy)COOH][(PF6')2]2+ and [(decb)2Ru2+(Me-bpy)AmH+][(PF6')3]3* (decb
= 4,4'-diethylcarboxy-2,2'-bipyridine) complexes in which molar absorptivites data were
collected in CH3CN solvent. These molar absorptivites reported for the (bpy) and (decb)
complexes in Chapter 5 were determined from Beer-Lambert plots [87] of six points
covering an absorption range 4.0-40x10"5 M.

Absorption titration for the ruthenium complexes ~2.0 x 10'5 M were collected
throughout the quencher concentration range ~0.5-15 equivalents relative to the Ru(II)

polypyridyl concentration. Addition of 111 quantities of a stock solution 2.0 x 10'3 M

48

quencher was used for each of the respective salt-bridge and hydrogen-bonding
quenchers. Solution volume was maintained at a constant 5.0 i 0.15m] solution volume
throughout the titration range.

Absorption data, collected for each of the ruthenium complexes as a function of
DBU in Cinch, were performed on samples 2.0 x 10‘5M. Addition of DBU
encompassing a range of 0.2 - 10 equivalents of the ruthenium complex concentration
was accomplished by the addition of a stock 2.0 x 10'3 M solution of DBU by micropipet.
Solution volume was maintained at a constant 5.0 :1: 0.15 ml throughout the titration

range.

Emission Spectroscopy

Emission measurements were performed using an instrument previously designed
and constructed at Michigan State University [88]. This instrument was recently modified
to include photon-counting detection, supported by associated hardware and software
[89]. Excitation of all samples was accomplished using either of two sources, 150 W Xe
or 200 W Xe/Hg lamps. Isotropic emission from the samples was detected using a
standard 90° orientation by a dry-ice cooled Hamamatsu R1104 or R943-02 gated
photon-counting photomultiplier tube mounted to a single monochromator (2.5 mm / 2.5
mm). All emission measurements were performed at room temperature unless stated
otherwise. All samples were contained in 1cm suprasil or matched quartz cells with
aerated solutions or that included a high vacuum adaptation design constructed at

Michigan State University.

49

Emission titration measurements for the [(tmb)2Ru(Me-bpy)COOH (PF6)2]2+,
[(tmb)2Ru(Me-bpy)COO- (PF(5)]+ and [(tmb)2Ru(Me-bpy)AmH+ (PF6')3]3+ (tmb =
3,3'4,4'-tetramethyl-2,2'-bipyridine) complexes were performed using DBU,
complimentary salt-bridged benzene and 3,5-dinitrobenzene complexes. For these
measurements, concentrations of the respective quencher or base were measured in
CH2C12 solvent. The addition of the 3,5-dinitrobenzamidinum quencher covered a range
of from 0.5-15 equivalents of the ruthenium complex. For each of the [(tmb)2Ru2+(Me-
bpy)COOH (PF,)2]2+ and [(tmb)2Ru2+(Me-bpy)AmH* (PF6)3]3+ complexes, the
concentration range of the complementary salt-bridging 3,5 dinitrobenzene was
abbreviated relative to that of the [(tmb)2Ru2+(Me-bpy)COO- experiment. For the
[(tmb)2Ru2+(Me-bpy)COOH]2+ the concentration range of the 3,5-dinitrobenzene
complex encompassed the range from 0.5-5 equivalents. For the (tmb)2Ru2+(Me-
bpy)AmH+ the 3,5-dinitrobenzoate was varied over the concentration range of 0.5 - 10
equivalents relative to the ruthenium complex. Similarly, the concentration range used in
the titration with the salt-bridge and hydrogen-bonding functionalized benzene
complexes encompassed an identical concentration range to that of the 3,5-
dinitrobenzene analogue. However, for the addition of DBU, the concentration range was
identical for each of the [(tmb)2Ru2+(Me-bpy)COOH (l>1=.,)2]2+ and [(tmb)2Ru2+(Me-
bpy)AmH+ (PF6)3]3+ comprising a range of 0.2 to 10 equivalents.

In the emission titration measurements of the reductive electron transfer systems,
the concentration of each of the ruthenium complexes was maintained at ~2.0 x 10'5 M.
The quencher titration range comprised a 0.2 - 2 M equivalent concentration range

relative to the concentration of the ruthenium complex. For this series of complexes, the

50

quencher additions included the addition of DBU, salt-bridge functionalized
dimethylaminobenzene complexes, salt-bridge functionalized benzene complexes, and
dimethyl-p-toluidine. Each of the emission titration measurements was performed open to

air and after freeze/pump/thaw degassing cycles.

Photophysical Measurements

Quantum yield measurements for each of the ruthenium complexes were
conducted on the above described emission instrument. The excitation wavelength for
these determinations was the 435.8nm Hg line of a 200W Xe/Hg lamp. All measurements
were performed at room temperature in either Ar(g) degassed or freeze/pump/thaw
degassed solutions. Absolute quantum yields were determined by referencing emitted
light to the emission of a standard Ru(bpy)32+(PF6‘)2 solution. ((bem = 0.029, 0.062, for
CHzClz and CH3CN [68] respectively). Appropriate corrections for differences between
sample and quantum yield standard were applied according to Eq. (2.1). As and Ac are the

absorbance of the standard (3) and the Ru(IDpolypyridyl (c) complex at the excitation

2
_ A; Is He
Will...)

wavelength, Is and Ic are the integrated emission spectra corrected for photomultiplier
characteristics and 11c and 11, are the refractive indices of the solvents of PCET complex

and standard, respectively.

51

Infrared spectroscopy

Infrared measurements were performed with the use of the departmental Nicolet
FT -IR instrument. The FT -IR spectra of the CHzClz solvent and of the CHzClz solvent
with the addition of the DBU were collected for baseline corrections. Collection of the
FT-IR data for each of the samples required the use of high sample concentrations
(~1 x 10'3 M) and the collection of at least 64 (usually 256) scans at 4 cm" resolution
were required for the observation of well resolved spectra for each of the ruthenium
complexes.

Association constants were determined for the ruthenium complexes in CH2C12
following a similar procedure. Stock solutions of ruthenium complex were 9.2 x 104 M
and 1.1 x 10'3 M and that of the stock solution of complexing salt-bridge functionalized
complexes 1.3 x 10'3 M, 9.1 x 104 M, 1.15 x 10'3 M and 8.6 x10‘1 M for benzoate,
benzarnidinium, 3,5-dinitrobenzoate, and 3,5-dinitrobenzamidinium respectively. The
concentrations of the ruthenium complexes were constant throughout the titration
procedure. Aliquots of the complexing salt-bridge acceptor complexes were taken
volumetrically by micropipet. Each aliquot of the salt-bridge complex was added to a
sample vial and the CH2C12 solvent was removed by evaporation. To this evaporated
residue was added the constant volume of ruthenium complex solution. Subsequent to
mixing a 100 u] aliquot of the resultant solution was added to the 200nm CsI infrared

sample cell by syringe.

52

N ano-Second Lifetime Measurements

Emission decay lifetimes were collected in the LASER Laboratory at Michigan
State University. The 504 nm excitation wavelength used for excitation of the drt—1t*
(MLCT) transitions of salt-bridge functionalized tris—bipyridyl ruthenium(H) complexes
was provided by the third harmonic output (355 nm) of a DCR-2 Nd:YAG laser Operating
at 10 Hz, pumping a 25-50 psi H2(g) filled Raman shifting tube. The isolation of the
second Stokes-shifted 504 nm light was accomplished with the use of a Pellin-Broca
prism placed after the Raman shifting tube to disperse the Stokes and anti-Stokes shifted
H2(g) lines. After dispersion, the 504 nm light was further isolated by passing the beam
through an iris diaphragm before being turned by a glass prism. The excitation light was
collimated using a telescope and turned once again at a glass prism before reaching the
sample. Excitation powers measured at the sample were typically 1-5 mJ/pulse (10 ns
FWHM). Reduction of scattered light in the observed emission decay data was
accomplished by passing the emission through an appropriate combination of long and
short wavelength pass filters placed before the focusing lens to the slit of a SPEX 1702
single pass monochromator. Lifetime decays, monitored at the ruthenium complex
emission maxima (unless otherwise stated), were collected with a Hamamatsu R928 PMT
operating at 600V. The output of the PMT was signal averaged with a TEK 602A
Digitizing Signal Analyzer storage oscilloscope averaging 512 decay traces/lifetime data
file. Decay data were stored as two column ASCII files to be manipulated via

Kaleidagraph or Origin software packages.

Low Temperature lifetimes

53

Low temperature (77K) lifetime measurements were made on EPA glass samples
contained in 9 mm o.d. NMR tubes immersed in a liquid N2 Dewar with the windows of
the Dewar kept free of condensation by a stream of nitrogen gas. For each ruthenium

sample the data set was composed of the average of 512 decay traces per lifetime file.

Temperature Dependent Lifetimes

Temperature dependent lifetimes (-20 — 60°C) were collected with the use of a
temperature-controlled cell housing obtained from Hitachi. Cooling or heating of the cell
housing was provided by the flow of a solvent mixture of water and ethylene glycol
(50:50) in a programmable Neslab chiller. At temperatures below 0°C, condensation on
the surface of the cell was prevented with a flow of N2(g). All samples were

freeze/pump/thaw degassed prior to measurement.

Pico-second Lifetime measurements

Lifetimes collected at the picosecond timescale were accomplished with the use
of a recently modified TCSPC instrument previously designed and constructed at
Michigan State University [90]. The design modifications made were, the inclusion of a
Hamamatsu 6 pm R3809U (MCP-PMT) mounted to a CV] 112 0.8 m scanning
subtractive double monochromator. The output of the MCP detector was fed to an EG&G
9806 pre-amplifier prior to reaching an EG&G 9307 pico-Timing Discriminator. These
modifications were made to reduce transit time spread (TTS) of the monochromator and
risetime of the detector and consequently the overall instrument response function for the

instrument. Other modifications to the instrument include the introduction of an Ealing

54

microscope objective, directed through a CVI DPL polarization scrambler after the
sample to collimate the luminescence light onto the entrance slit of the monochromator.
These modifications give rise to instrument response functions typically 40-50 ps at the
selected wavelength ranges used.

Excitation is provided by the output of a synchronously pumped cavity-dumped
Coherent 702 dye laser typically operating at a frequency of 3.8 MHz (FWHM 6-8 ps,
13 nJ/pulse) The Coherent Antares 76-s Nd:YAG laser second harmonic (532 nm, 100 ps
FWHM, 3W) output provides the pump source for the dye laser. Tunability of the
excitation wavelength provided by the 702 dye laser is accomplished with one of the
three dye circulators running pyrromethene 567, R6G, or LDS 698 laser dyes for access
to wavelength ranges through 560-750nm. Doubling, accomplished with an external
crystal placed in the beam path, increases the useful range of the 702 dye laser by
providing additional excitation wavelengths of 280-375nm. The intermediate wavelength
range is adequately covered with the use of the blue-dedicated 702 dye laser providing
excitation wavelengths from 420 - 470 nm.

The fast time response necessary for the collection of fluorescence data below the
nano-second time-scale requires special electronic design provided by reversed timing
Single-photon-counting techniques. For reversed timing, the sample luminescence
resulting from the excitation pulse provides the start signal at the time to amplitude
converter (T AC). Splitting of the excitation beam equally (1" glass cube 50 l 50 beam
splitter) prior to sample excitation provides both the excitation and the reference signal
which is optically delayed through a 1.0 111 external fiber optic cable coupled to a variable

length internal fiber optic delay cable. This configuration allows the spacing of the

55

fluorescence signal and reference laser pulses up to 180 ns. The addition of either of two
external 50 m or 100 m fiber optic spools provides the further spacing of the signal and
reference laser pulses to a total of 680 ns. Additionally, for the collection of fast events in
long fluorescent lifetime samples the excitation frequency may be set continuously from
38 MHz to 1.5 kHz through the 702 dye laser cavity dumper driver. With an external
trigger source constructed at Michigan State University the frequency may be further
tuned to 1 kHz.

Other modifications of the instrument include the addition of a Tennelec TC 864
replacing the older TC 862, changing the range of the smallest temporal window from 25
to 5 ns. Data collection was accomplished with new 4096 channel Viking Instruments
MCA card and data was displayed and stored with the use of Edinburgh FLA-900A
software followed by further analyses with Kaleidagraph or Origin software packages

For the [(tmb)2Ru(Me-bpy)COO’][(PF6)] complex titrated with 3,5-
dinitrobenzamidinium and the [(decb)2Ru(Me-bpy)AmH"][(PF15)3]3+ titrated with N ,N'-
dimethylaminobenzoate lifetime data collection was accomplished with excitation at 575
nm and emission collected at the emission maximum for each point of the lifetime

titration.

Synthesis of Ruthenium Complexes

Synthesis of [Rn(Me-bpy-COOII),]2+ and [Ru(Me-bpy-AmH*)3]5*. Complexes
were prepared using standard literature procedures A 95% ethanol (10 ml) solution
containing 4-methyl-4'carboxylic acid-2,2'-bipyridine (1.0g, 4.1 mmol) or 4-methyl—

4'amidinium-bipyridine (4.0g 20 mmol) and RuC13°(H20)3 (0.5g 1.9 mmol) were allowed

56

to reflux for 10 days. The solvent for each complex was evaporated under reduced
pressure followed by dissolution in a minimal amount of water and filtered. The deep red
PF6 salt was obtained by the addition of a saturated aqueous solution of NH4PF6. Further
purification of the [Ru(Me-bpy-COOH)3]2+ complex was accomplished by charging a
neutral alumina column with the complex and eluted with CH2C12, (CH3)2CO, and MeOH
followed evaporation under reduced pressure and washing with anhydrous ethyl ether on
a medium frit. For the [Ru(Me-bpy-AmH“)3]5+ complex the purification consisted of
charging a silica gel column with the complex and eluting, followed by evaporation under

reduced pressure and washing with anhydrous ethyl ether.

Deuteration of samples for lifetime measurements

Deuteration of the dissociable protic substituents of the salt-bridge and hydrogen-
bonding interface of [(bpy)2Ru2+(Me-bpy)COOH][(PFG')]2, [(bpy)2Ru2+(Me-bpY)AmH+]
[(PFe')13. [(tmthuzxMe-bpylAmHulure->13 and [(tmthuzxMe-bpnCOOHI
[(PF5‘)]2 complexes was accomplished by the addition of each complex and NaOCH3 to a
50 ml round bottom flask. The contents of the flask were rigorously dried under vacuum
and gentle heating. Upon cooling of the flask to ambient a dried degassed solution of
MeOD was transferred under vacuum to each of the flasks containing the ruthenium
complexes. Subsequent to the addition of ~10 ml of MeOD to each flask the contents
were warmed to room temperature and thoroughly mixed for a period of 10 minutes and
the excess solvent was removed under vacuum. This procedure was repeated 3-5 times
before complete. The confirmation of deuteration was assessed by FAB-MS, IR, and

NMR spectroscopy.

CHAPTER 3

PHOTOINDUCED ELECTRON TRANSFER WITHIN AN ELECTRON DONOR-
ACCEPTOR ASSEMBLY JUXTAPOSED BY A HYDROGEN-BONDING
INTERFACE

Introduction

The ubiquity of electron transfer processes in chemical reactions have led to the
investigation of a great variety of model systems designed to address specific questions
regarding the factors controlling electron transfer processes [91]. Development of model
systems incorporating long range electron transfer reactions has been a central focus in
these developments [18].

Pioneered by the observation of long range electron transfer reactions in
biological systems, incorporation of structural components found in these systems have
been implemented in a variety of different electron transfer models [91]. To better

understand the relationship of the biological framework, developments of electron

57

58

transfer reactions occurring in proteins, protein complexes and in systems containing
synthetic polypeptides, or-helices, B-sheets and B-tums as spacers have been undertaken
[12,13,15].

A fundamental design consideration of these photoinduced electron transfer
model system investigations is the incorporation of photoexcitable donor molecules
capable of either oxidative or reductive electron transfer reactions in the excited state. To
this end the photophysical behavior of Ru(II)polypyridine complexes have been the
subject of numerous investigations of photoinduced excited state electron transfer
reactions. Development of this class of complexes has been extensively investigated in
regard to both bimolecular electron transfer reactions and more recently to the study of
intramolecular electron transfer reactions[68,92]. Recent examples using these
photoexcitable donor molecules may been seen in the chromophore-quencher complexes
of Meyer [3,15]. These systems contain both an appended reductive quenching
substituent and oxidative substituent covalently tethered by peptide residues to the
bipyridyl ligands composing the metal complex. The relative synthetic ease with which
bipyridyl ligands may be functionalized make these complexes particularly amenable to
the development of electron transfer assemblies. Other recent examples of the mimicry of
biological frameworks toward the understanding biological function are found in the
work of Ogawa [13]. Similarly, the incorporation of design features representative of
reactions occurring in the biological medium have been developed in model systems [93].

Coupling of proton motion to charge separation is a fundamental mechanism in
biological energy conversion systems. The energy storing processes of small molecule

activation and the translocation of protons across membranes in the protein and enzymes

59

of photosynthesis and respiration are predicated upon proton coupled electron transfer
(PCET). One approach to investigating the mechanism of PCET is to photoinduce
electron transfer within an donor/acceptor pair that is juxtaposed by a proton transfer
interface such as those formed from carboxylic acid dimers [62] or guanine-cytosine
base-pairs [49] Electron transfer systems based on the dicarboxylic acid design have
previously been described where it was found that electron transfer reactions occur with
rates comparable to systems in which the electron transfer donor and acceptor are
tethered by a covalent pathway form the standpoint of distance and driving force [62].
The incorporation of an symmetric interface into a system in which the electron

2+ complex may be used,

donor composed of a carboxylic acid derivatized Ru(bpy)3
requires the modification of one of the bipyridyl ligands with carboxylic acid
functionality. These molecules have been previously synthesized in other laboratories
[94] and have subsequently been reproduced in the Nocera group [95] in which the
carboxylic acid functionality is placed at the 4' position of a bipyridyl ligand. In order to
utilize this complex and create an electron transfer reaction, an electron transfer quencher
is necessary which can recognize and bind with the complimentary carboxylic acid
functionality of the donor complex. One well-known organic oxidant, 3,5-dinitrobenzoic
acid, provides this complimentary portion for the creation of a symmetric interface
system. The electron transfer donor/acceptor complex generated in solution between
these molecules is shown in Figure (9a) below.

The present system based on the photoinduced excited state of a modified tris-

bipyridyl ruthenium (II) chromophore in CHzClz solvent. The results of the findings are

compared to systems composed of the introduction of a charged asymmetric amidinium-

60

carboxylate salt-bridge hydrogen bonding interaction as shown in Figure (3.1), for
modified Ru(II) polypyridyl electron transfer donor complexes. Introduction of
asymmetry to both the donor and acceptor moieties of the proton transfer interface allows
an experimentally reversible configuration such that the direction of the electron transfer
reaction may be analyzed in relation to the direction of proton transfer within the salt—

bridging interface. The advantages garnered through the incorporation of the charged

H H
\_ o \N__ ......
—<\ + \>— ——- e>—
.__..O N— ......
/ /
H H
(9)

asymmetric interface are, foremost, the selective and directional hydrogen-bonding
interactions effectively abolishing the competitive self-associations prevalent in the
symmetric dicarboxylic acid (COOHz) system. Furthermore, the charge complimentarity
of the interface enhances the production of a stable and strong hydrogen-bonding
configuration even in solvents having high dielectric constant [95].

The comparison of observed electron transfer rates in these asymmetric systems
provides the potential to determine the effects of proton motion within the interface. In
this chapter the measurement of the time-resolved quenching of emission intensity by
photoexcitation is discussed with specific attention to the kinetic isotope effects observed
through selectively introducing deuterated functionalities to the salt-bridge interface

components.

61

Results
Electronic Absorption and Luminescence spectroscopy

Absorption and emission spectra for the three hydrogen-bonding and salt-bridge
modified ruthenium(H) electron transfer donors are typical of modified
ruthenium(H)polypyridine complexes [68]. The 400 - 500 nm region shows typical,
slightly split d1t—1t* (MLCT) absorption bands (km = ~ 450 for (1), (2) and (3)), and
single intraligand 1t—1t* absorption bands in the region 260 - 300 nm (11m, = ~285 nm for
(1), (2) and (3))-

The emission spectra for each of the three ruthenium complexes are broad and
structureless at room temperature and show vibrational progressions of 1000 - 1500 cm"
upon cooling to N2(l) temperatures (77K). Emission maxima of the spectra are found to
change position slightly in response to changes in the electronic composition of the
ligand set surrounding the metal center. (km = 646 for (9a), 639 for (9b) and 629 for
(9c)). The photophysical characterization data for absorption and emission is summarized
in Table 2.

Free energy driving force (AG) determinations for each of the electron transfer
systems (9a-c) was accomplished by two methods. First, the determination of the free
energy of the excited state relative to the ground state by the measurement of the FWHM
of the emission band required the estimation made through the use of a relationship
previously derived by Meyer [15] with corrections for reorganizational energy of medium
frequency vibrational modes of the bipyridyl ligands, using Eq's. (3.1) and (3.2). The

operating principle of these Eq's. (3.1 and 3.2) is generalized from a Franck- Condon

62

\
{.1 /
(bpy). Hu'
\ 0...,
/ "H
o
\H"~.~_ / N02
N02
(a)

/
/ (bpy) Ru"
(hm/12m" \ 2 \

NO2

 

(b) (C)

Figure 9 Bipyridyl ligated systems. Symmetric dicarboxylic acid (a) asymmetric
amidinium-carboxylate (b) and asymmetric carboxylate-amidinium (c) salt-bridge
interfaces for electron transfer studies.

63

Table I. Electrochemical data for the three (bpy)2Ru(II) complexes

 

 

Ru (11) compound E3”?+ AEp (mV) 1,, / ipc
(bpy)2Ru2+(Mebpy)COOH 1.27 100 1.1
(bpy)2Ru2+(Me-bpy)AmH+ 1.26 65 1.00

(bpy)2Ru2*(Me-bpy)Coo— 1.29 60 1.00

 

64

150,0 = AGes ‘1' X (3-1)
Av

X E ( 0.1/2)2 (32)
16111 ZKfiT

emission spectral analysis [96] of tris-bipyridyl ruthenium(H) complexes derived from
the Energy Gap Law considerations of Jortner [97]. The determination of the E00 is
derived from the energy of the emission spectra taken at the maximum. The additional
factor x is a result of the displacement of the excited state potential energy surface
relative to the ground state surface. The extent of the displacement and its contribution to
the luminescence is accounted for in Eq (3.2) by the incorporation by the measurement of
the full width at half maximum (FWHM) of the emission spectrum, as the primary
contribution to the spectral bandshape are determined to be due to one vibrations of the
bipyridyl ligands (1200 - 1500 cm") and to a lesser extent to the metal-nitrogen bond
changes (200 - 500 cm") resulting from photoexcitation and thermal factors. Entropic
considerations are accounted for in the term x (x = xi + Xo) and are assumed to be small
[98].

Measurement of the reduction potentials for the 3,5-dinitrobenzoic acid (DNBA)
and the 3,5-dinitrobenzamidinium (DNBAmH+) results in irreversible electrochemical
data by standard cyclic voltammetry techniques. Thus the free energy necessary for the
reduction of the oxidative quenchers were determined by the derivation of a linear free
energy relationship between the known reduction potential and the bimolecular
quenching rate constant of Ru(bpy)32+ (CH3CN u = 0.1) by a series of nitroaromatic
complexes (0, m, and p-dinitrobenzene). The reaction free energy for the bimolecular

quenching of these complexes encompasses a range of (—AG = 0.3V). Collection of this

65
data allows an estimation of the thermodynamic free energy (—AG) for the electron
transfer reactions of the proton coupled-electron transfer quenchers determined as shown
in the following relation Eq. (3.3).

AG... = Ru” — 15.,2 Rub"2+ — Q°" (3.3)

Luminescence and Lifetime Measurements

Each of the three ruthenium complexes (bpy)2Ru2+(Me-bpy)COOH, (9a)
(bpy)2Ru2+(Me-bpy)AmH+ (9b) and (bpy)2Ru2+(Me-bpy)COO— (9c) exhibit long lived
and monoexponential luminescence in room temperature CH2C12 solution throughout 4‘:
of decay data (I = 1100 us, 1030 ns and 1300 ns). In these lifetime decays there is no
evidence of change in the decay lifetime upon altering the emission detection
wavelength. Similarly, the use of excitation wavelength 436, 504 or 532 nm causes no
appreciable change in the observed lifetime decay. That these monitored decay lifetimes
do not change with alteration of excitation wavelength is consistent with the contention
[99] that the emissive state from which the electron transfer occurs in these complexes
represents a thermodynamically equilibrated complex on the nano-second time-scale.
Additionally, the risetime to the emissive state giving rise to the lifetime decays are not
observed to vary.

For each of the synthetically modified Ruthenium complexes the d1t—1t* (MLCT)
excited state is well poised for the efficient reduction of the complimentary carboxylic
acid, amidinium or carboxylate aliphatic or aromatic electron transfer acceptors. The
addition of complimentary hydrogen-bonding or salt-bridge functionalized non-electron

transfer active compounds, the emission and the decay lifetimes are altered from the

66

either the uncomplexed species or the electron transfer complexes. Upon the addition of
the hydrogen-bonding or salt-bridge functionalized 3,5-dinitro aromatic quenchers, to the
complimentary ruthenium(H) complexes (9a-c) there is an observable dramatic
attenuation of the luminescence attributable to quenching by an electron transfer
mechanism consistent with the observation of the electron transfer quenching of
electronically excited tris-polypyridyl complexes of ruthenium(H) by nitroaromatics [3].
The lifetime decay data for each of these three systems exhibit biphasic decay with one
component dependent upon the concentration of the quencher and one component
remaining independent of the concentration of the quencher throughout the quencher
concentration range investigated Figures (10) and (11). The concentration dependent
component is assigned to bimolecular quenching of the excited state lifetime of unbound
ruthenium complex. Bimolecular electron transfer rates for this component of the electron
transfer reactions for each of the complexes obeys typical linear Stem-Volmer
bimolecular quenching kinetics. Relative quenching determined from the bimolecular
component of the lifetime decays for (bpy)2Ru2+(Me-bpy)COOH (9a), (bpy)2Ru2+(Me-
bpy)AmH+ (9b) and (bpy)2Ru2+(Me-bpy)COO' (9c) complexes are 80% , 64% and 49%
respectively. Fitting this lifetime decay data versus quencher concentration yields the
bimolecular electron transfer rate constants (kg. = 2.4 x 109 M'1 s'1 for (9a), 1.4 x 109 M"
s'1 for (9b) and 1.0 x 109 M'1 s" for (9c))

The concentration independent component observed in the lifetime quenching
experiments for each of the ruthenium complexes exhibit rate constants of 8.0 x 10° 3",

12.0 x 106 s" and 4.3 x 10° 8" for (bpy)2Ru2+(Me-bpy)COOH, (bpy)2Ru2+

67

Table 2. Emission Characterization of (bpy)2Ru(II) complexes

 

 

Ru (II) compound E0 (cm’l) Avo, 1,2(cm'l) AGes(V)
(bpy)2Ru2+(Mebpy)COOH 15480 2297 2.20
(bpy)2Ru2+(Mebpy)AmH+ 15649 2586 2.26
(bpy)2Ru2+(Mebpy)Coo— 15800 1760 2.18

 

68

 

 
 
  
 

13 1= 205 (38%)

/

Arb. Intensity

‘C 2: 656 (62%)

  

 

 

 

0 1 2 3 4

time/ 106 seconds

Figure 10. Biexponential fit of lifetime decay indicating concentration-dependent and
concentration-independent components of a representative lifetime decay for
[(bpy)2Ru2+(Me-bpy)AmH+] (1.0 x 104 M) with complimentary 3,5-dinitrobenzoate
[1.5 x 10“ M].

69

 

 

 

 

1 2
7O
1 0
v- 7?
.0) '1]
mo 50 3
I 8 0,,
x 30 "
6
1 0 _. L 1 l l I I 4

 

0.5 1.5 2.5 3.5
[3,5-DNBCOOH 1/10‘4 M

Figure 11. Concentration-independent component ( O ) and concentration dependent
component ( I ) for the lifetime titration of (bpy)2Ru2+(Me-bpy)AmH+ [1.0 x 104 M]
with 3,5-dinitrobenzoate in CH2C12 at 22°C.

Table 3. Observed electron transfer rates for concentration-dependent and concentration-
independent electron transfer of bipyridine based donor-acceptor complexes in CH2C12 at

 

 

      

 

 

22°C.

Ru(H) salt bridge complex AGET / V kET/ M’1 8" km s"

\

1‘ °2

1 N-Hm 'O
(hm/>29" \N (e e} -021 2.4 x 109 4.3 x 106

N—H----o
H’ No2
(9a)
\ /
N02
o----H—O
(bpylanu'\N/ \ / / -014 1.4 x109 8.0 x 106
_ _H___
N02
(9b)

\
b R
( ”)2 ” \ -0.05 1.0 x 109 12.0 x10°

-0.15 1.9 x109 —

 

 

71

(Me-bpy)COO‘ and (bpy)2Ru2+(Me-bpy)AmH+ respectively. This component of the
lifetime decay data is attributed to proton coupled-electron transfer (PCET) through the
symmetric hydrogen bonding or salt-bridge functionalized interfaces. The data for each
of the three systems is summarized ( -AG, ka, and km ) in Table 3.

The assessment of these kinetics data is suggestive of electron transfer occurring
through the symmetric dicarboxylic acid and the asymmetric salt-bridge hydrogen-
bonding interfaces. Validation of the association between electron donor and acceptor
reactants in DMSO solution have been shown in NMR titration experiments for
(bpy)2Ru2+(Me-bpy)AmH+ [95] In DMSO the association constants measured for system
(9b) is found to be dependent upon the dielectric constant of the solvent (Ka = 1197 :t 93
M" in DMSO and Ka = 2432 in CH3CN). Measurement of the association of complex
(9a) reveals an association constant of 702 M'1 in CHzClz solvent.

Control experiments for the lifetime measurement using the non-complexing
ethyl-3,5-dinitrobenzoate yield monoexponential decays throughout a concentration
range comprising that of the 3,5-dinitrobenzene salt-bridge and carboxylic acid
quenchers. These experiments reveal no evidence of biphasic decay processes throughout
the concentration range of the titration. Supporting the contention that the observed
concentration independent component of the lifetime decay data represent an electron
transfer reaction occurring as a result of the association of the symmetric hydrogen-

bonding or asymmetric salt-bridge complexes.

72

Discussion

The subtle differences in the concentration independent rate as a function of the
change in the direction of the interface suggests that the direction of the interface plays a
role in determination the observed electron transfer rate. But by the substitution of the
dissociable protons within the interface of electron transfer complexes (9a) and (9b) the
participation of the proton in the electron transfer rates are more clearly shown. The
lifetime measurements incorporating the deuterated complex (9a) reveals a kH/kD rate
ratio of 1.5 while the observed kinetic isotope effect observed for the electron transfer
complex (9b) gives rise to a rate ratio kit/kn = l.34(3)

Referring to Table 3 summarizing the electron transfer kinetics for each of the
three complexes it may be seen that the relative energetic difference in the driving force
between the complexes are small. In fact, the difference between the asymmetric system
(bpy)2Ru2+(Me-bpy)AmH+, 3,5-dinitrobenzoate (AG = —0.21V) and the (bpy)2Ru2+
(Me-bpy)COOH, 3,5-dinitrobenzoic acid (AG = —0. 14 V) is merely —0.07 V. Yet, despite
the smaller driving force for the complex composed of the symmetric (COOH); interface
the observed intramolecular electron transfer rate is nearly a factor of 2 greater than that
observed for the asymmetric (AmH",COO’) interface system. Comparison of the two
asymmetric interface systems (AmH’ZCOO’) vs. (COO’,AmH+) shows a difference in
driving force of 0.15V and a difference in observed intramolecular electron transfer rates
of only 3.

For each of these systems the energetics of the bipyridyl ligands surrounding the

ruthenium play a role in the determination of the electron transfer reaction. Both the

73

carboxylic acid and amidinium functionalized bipyridyls should have 1t* levels lying
lower than that of the respective non-functionalized bipyridyl ligand despite the presence
of the methyl substituent located at the 4 position. For the carboxylate functionalized
bipyridyl ruthenium complex, however, the energy of the bipyridyl 1t* level is expected
to lie higher than that of the bipyridyl due to the presence of the carboxylate at the 4'
position and also due to the additional presence of the methyl at the 4 position. Thus the
energetics of the ligands are expected to be Me-bpy-AmH+ < Me—bpy-COOH < bpy
< Me-bpy-COO- Respectively. Electrochemical measurements of the reduction potentials
Em (Ru2+/l+) and Em (Ru'+/°) of each of the complexes allow an assessment of these
energetics and show a qualitative agreement to the expected trend. The relationship of the
electrochemical data to the energetic splitting of the MLCT states may be expected since
the charge transfer transition results in the placement of the electron in the 1t* of the
bipyridyl ligand and similarly the electrochemical measurrement does likewise. However,
the difference being that the charge transfer transition results in the change of the charge
of the metal from Ru2+ to Ru”. Yet the irreversibility of the redox couples associated
with the COOH and AmH+ bipyridyl ligands obscure the direct observation and disallow
the placement of the relative potentials of the ligands in each complex.

The electrochemical data do, however, allow an assessment of the magnitude of
the participation of electron transfer contributions from the ancillary ligands. In a manner
similar to that previously noted by Cooley et al. [100] the incorporation of the remote
ligands in the observed electron transfer rates may be achieved through the use of Eq's.

(3.3) and (3.4) accounting for the effects of the equilibrium established in charge transfer

74

9% a 23.5 E .ToooEeozvaamfiaeezv.8333: aEoooaaeezvaomaaaé
FmanAEniozvaacmen: 85388 Kathi—0255M 05 we 5:885 05 .5.“ 080:8 ouoweoem .2 «Cami

a
+m

O-§oe-o§__=ma§§

 

10.04

 

 

- c-r;aa-ae__aa§ae . I..|

see

 

=

 

 

Esmeuozv=és§§
#045. .0 <

o.§ae-ae__=a§e$amz

 

 

 

75

 

K
eq
0. = (3.3)
1+Keq
EAR
ch = [EA-Je‘i RT l (3.4)
'1 R

to ligands 11A and mi (adjacent and remote bipyridyls) having energetics of similar
magnitude yielding the partitioning factor or. The magnitude of or in each of complexes
derived from the measurement of the reduction potentials for adjacent bipyridyl ligands
(EA) and remote bipyridyl (ER) (9a-c) is quite small. And more importantly for these
complexes (9a-c) is the comparison of this energetic difference (EAR) relative to the tris-
bipyridyl ruthenium complex in which the value of EAR is found to range from 180 - 200
mV in comparison to that observed for the Me-bpyCOO- (220 mV), and Me-bpyAmH+
(210 mV). As a result the energy difference between the reduction potentials of the
adjacent and remote bipyridyl ligands for each of the bipyridyls are no more than 50 mV
which would yield or = 0.066. This value for or is relatively insignificant amounting to 10-
15% changes in the observed electron transfer rates. Thus the use of this estimation does
not lie far outside of the error expected in collection of the lifetime decay measurements.
Alternatively, the evaluation of the participation of the remote ligands in the
observed electron transfer may be determined through the assessment of the free energy
of the excited state relative to the ground state for each of the salt-bridge functionalized
bipyridyl complexes. Using the relationship given in Eq. (3.1) and (3.2) in combination

with the emission spectra for each of the complexes supports these trends for the

76

changing electronic composition of the bipyridyls, reflected in the relative position of the
emission maxima and calculated emission energetics summarized in Table 2. From these
estimations it is seen that the energetic difference between the three ligands is quite small
and that the excited electron is essentially delocalized over the three bipyridyls of the
ruthenium complex.

This distribution of the charge transfer electron density is expected to participate
in the observation of the electron transfer rate constant through the hydrogen-bonding
interface. The similarity of the charge transfer energetics for transitions to each of the
ligands thus obscure a measurement of the direct electron transfer rate constant
corresponding to electron transfer through the hydrogen-bonding or salt-bridge interface.
As a result these data suggest as has been previously shown for the symmetric (COOH);
interface that the electron transfer rate exhibits a dependence on the proton interface.
However, the data represented by the two asymmetric interface systems (AmH+,COO')
and (COO',AmH+) suggest that the proton motion within the interfaces in an electron
transfer reaction may be manifested in ways other than the electronic coupling as the
suggested explanation of the observed electron transfer rates in the symmetric
dicarboxylic acid interface [66]. In that case the proton displacement from one side of the
interface is directly compensated by displacement of the proton at the other side. This
symmetric configuration of protons allows a maximization of charge delocalization and
the electronic coupling between the donor and acceptor facilitating the electron transfer.
Yet, in this interface as a result of the neutral symmetric configuration little dependence
upon coupling of the proton to the solvent is expected. However, in the asymmetric

interfaces significant charge rearrangement upon proton motion in the interface is

77

expected [38]. A direct consequence of this charge rearrangement is the coupling of the
solvent dipoles to the charge redistribution occurring upon the electron transfer. For each
of the asymmetric interfaces the effects of the charge redistribution will be significantly
different. In the case of the complex (9b) (AmH+,COO") the electron transfer may
influence the resulting proton transfer while for (9c) (COO‘,AmH+) the proton is located
such that the transfer of the proton does not occur. Thus for these two asymmetric
systems effects of the proton (charge) within the salt—bridge interface are expected to
yield significant coupling to the solvent potentially being manifested as differences in
additional Franck-Condon factors for the intramolecular electron transfer event.
Furthermore, the charge within the interface establishes a coulombic field over the salt-
bridge, which is expected to enhance or attenuate the driving force for the electron
transfer reaction. However, for these systems since the relative energetics of the bipyridyl
ligands are such that the functionalized and non-functionalized ligands are energetically
similar, observation of substantial differences in the electron transfer rate are not evident.
The results of tris-bipyridyl ruthenium(H) systems in which the energetics of the
ancillary ligands are synthetically altered such that these issues may be further addressed

is the subject of the Chapter 4.

CHAPTER 4

PHOTINDUCED ELECTRON TRANSFER WITHIN A SYNTHETICALLY
MODIFIED RUTHENIUM DONOR ACCEPTOR PAIR:THE EFFECT OF
SWITCHING THE SALT-BRIDGE INTERFACE

Introduction

The definition of pathways in electron transfer studies has an established
significance in the determination of the factors controlling the overall observed rate
constant for an electron transfer event [10]. Specifically, in protein matrices, definition of
the pathway for a particular electron transfer system, is principally predetermined by the
protein structure. Secondary structure of the protein, composed of B-sheets, B-tums, and
or-helices sets the overall tertiary structure of the protein. The shortest distance through
this medium by which the electron may reach the acceptor from the donor will be the
principle rate determining path. Detailed studies of the relative contributions of each type

of protein sequence in relation to the observed electron transfer rates have been assessed

78

79

by Gray et al. [14]. More recently, the attachment of electron transfer donor and acceptor
moieties to the ends of a-helical oligopeptide monomers [16] have begun to elaborate not
only on the pathway through the peptide backbone, but also the direction of electron
transfer within the pathway by the attachment of electron transfer donor and acceptor
relative to the N-terminus and the carboxy—terminus of the a-helix. By these means
effects of the electrostatic field, calculated by vacuum electrostatics to be as large as
109 V/m, generated between the ends of the oligopeptide are established. Wasielewski
has prepared systems representative of the photosynthetic reaction center implementing
the generation of an electrostatic field, by photoinduced electron transfer in a donor-
acceptor pair ((1 = 8.4 A). In these systems the assessment of the effects of an electrostatic
field generated as a result of photoinduced electron transfer are monitored by the spectral
features of a carotenoid probe molecule. The suggestion that is inferred in this system is
the electrostatic field generated as a result of long range charge separation systems may
perturb electron transfer in nearby donor acceptor systems, effectively tuning the redox
potentials of these species and subsequently modify the electron transfer rate constants.
The relative energetics of the dn—7t* (MLCT) charge transfer transitions in Ru(II)
polypyridyl salt-bridge electron transfer complexes are determined by the ligand set. The
implementation of ligands of different energetics ultimately may be responsible for
influencing the overall pathway traversed by an electron through the intervening medium
of an electron transfer donor-acceptor pair. Relative energetic splitting of the MLCT
transitions to each of the ligands of a Ru(H) polypyridine complex may be determined
through a comparison of electrochemical, absorption and emission data [68]. Although a

recent study [71] suggests that an exact correlation does not exist between the

80

spectroscopic properties and the electrochemical data. Nonetheless, the ground state
electrochemical data yields an assessment of the extent of the o donating and 1t accepting
capacities afforded by each of the ligands complexed to the central metal. Manipulation
of the o donating ability of the ligands via placement of electron donating groups at the
ligand destabilizes metal centered (in: states making a poorer n-accepting ligand.
Similarly, the addition of an electron withdrawing functionalities result in the net
lowering of the 1t* energy of the coordinated ligand resulting in decreases in the energy
of the MLCT transition. For tris—heteroleptic Ru(II)bipyridyl complexes in which the
different ligand substituents vary electronically, the dipole of the d1t—1t* MLCT charge
transfer transition will be predominantly oriented toward the ligand lying at lower energy
[71].

The determination of the localization of the excited electron in Ru(H) polypyridyl
complexes has been thoroughly investigated by many different researchers [101].
Consistant with the electrochemical determinations, EPR experiments of chemically
reduced Ru(bpy)32+ have shown that the signal observed is bipyridine based and is
localized. For these complexes photoexcitation leading to the transfer of charge localized
at the ligands has been verified by transient absorption and resonance Raman techniques.
These investigations suggest that the ligand most easily reduced is the resulting location
for the charge transfer. Findings of these studies suggest that the initial population of the
electron resides at one ligand at timescales < 10 ns. Recent ultrafast spectroscopic studies
have suggested that the localization of the excited state of ruthenium complexes is

obtained in the first 500 fs of excitation [76].

81

/
(tmbpy)2Ru"

~02

 

(a)
\
,1 /
(tmbpthtu"
/ l
\
/ ‘H\
O
\H. ..... ‘0/ N02
N02
(b)
(imbthFiU'I

N02

 

(C)

Figure 13. Tetramethyl bipyridyl Ru(H) complexes for electron transfer through the salt-
bridge interface formed by the association of the complimentary quenchers 3,5-
dinitrobenzoate (a), 3,5-dinitrobenzoic acid (b) and 3,5-dinitrobenzamidinium (c).

82

In addition to the splitting of the energetic levels for each of the respective charge
transfer transitions, separation of the MLCT energetics with ligand variation has broad
consequences in the overall symmetry of the complex. Thus, in addition to the energetic
splitting, the different ligand electronic composition may impose a different point group
symmetry to the complex, giving rise to electronic transitions having different basic
photophysical details [69]. In the simplest sense, a tris-bipyridyl ruthenium complex may
be considered as having a D3 point group in the ground state. However, in the localized
model [69,] charge transfer transitions make one of the ligands inequivalent, reducing the
symmetry of the complex to a C2 point group. Substitution of a ligand into a tris-
bipyridyl ruthenium(H) complex having electronic composition different from the
remaining ligands may impose reduced symmetry in the ground state of the complex
resulting in an observable changes in the the charge transfer transitions [102]. Thus, the
electron transfer donor/acceptor pathway for complexes formed via salt-bridge hydrogen
bonding is critical for the determination and observation of an electron transfer rate
through the interface.

Three tris-bipyridyl ruthenium(II) complexes used for the formation of the
hydrogen-bonding and salt-bridge intramolecular electron transfer complexes
[(tmb)2Ru2+(Me-bpy)AmH+]3+ [(tmb)2Ru2+(Me-bpy)COOH]2+ and [(tmb)2Ru2+(Me-
bpy)COO—]+ (tmb = 3,3',4,4'-tetramethyl-2,2'-bipyridine, Me-bpyAmH+ = 4-methyl-4'-
amidinium-2,2'bipyridine, Me-bpyCOO— = 4-methyl-4'-carboxylate-2,2'-bipyridine)
incorporated these design features. Association of these complexes with the respective

complimentary electron transfer quenchers 3,5-dinitrobenzoate (DNBCOO—), 3,5-

83

dinitrobenzoic acid (DNBCA) and 3,5-dinitrobenzamidinium (DNBAmHi’) respectively

are shown in Figure (13).

Results
Electrochemistry

Results of the electrochemical measurements indicate that the three hydrogen-
bonding salt-bridge functionalized complexes (l3a-c) are typical for ruthenium(H)
polypyridyl complexes in CH3CN [68,103]. Since the reduction of the two protonated
3,5-dinitrobenzene quenchers 3,5-dinitrobenzamidinium+ (DNBAmH+) and 3,5-
dinitrobenzoic acid (DNBA) are irreversible by by standard electrochemical techniques
the determination of the potentials for these compounds was obtained by measurement of
the deprotonated quenchers these potentials for each of the quenchers are shown in Table
4. These measurements suggest that the effective change in the potential upon

protonation/deprotonation is O. 1 V.

Electronic Absorption and Luminescence photophysical characterization

For the [(tmb)2Ru2+(Me-bpy)COOH]2+, [(tmb)2Ru2+(Me-bpy)AmH+]3+ and
[(tmb)2Ru2“(Me-bpy)COO-]+ complexes addition of the (tmb) ligands slightly alter the
appearance and position of the absorption bands relative to those observed for complexes
composed of 2,2'—bipyridyl ligands as seen in the absorption spectra for the three
ruthenium complexes shown Figure (14).

For the [(tmb)2Ru2+(Me-bpy)AmH+][(PF6)3] complex (13a) the d1t—1t* (MLCT)

absorption band indicates a distinct splitting with two maxima (itmax = 427 and 490 nm).

84

 

 

 

 

 

 

 

 

 

2.5
I"
I: :t ‘1 - 2
:' 5'1.
:' :" l i — 1.5 Ene-
5 i, * 3
:' .‘ ‘ —1 37
a: “ a;
l. : X5“\ ‘3. V
I '1': .t‘ ‘3‘ H 0.5
O I .—’l x}; " a: I ‘\‘~hi:- O
300 400 500 600 700 800

Wavelength / nm

Figure 14 Absorption and Absolute emission of the three Ru(H) electron transfer
complexes: [(tmb)2Ru2+(Me-bpy)AmH+]3+ (line), [(tmb)2Ru2+(Me-bpy)COO-]+ (dashed

line), and [(tmb)2Ru2+(Me-bpy)COOI-I]2+ (dotted line).

85

The intraligand 1t—7t* absorption band has a single maxima (Mm = 292 nm), while the
[(tmb)2Ru2+(Me-bpy)COOH][(PF6)2] (13b) absorption spectra resembles that for the
amidinium modified complex. However, the splitting of the dit—1t* (MLCT) region is not
nearly as distinct, still the observation of two maxima (km = 433 and 478 nm) are
observed. The intraligand 1t—1t* absorption spectra for this complex lies isoenergetically
with that of the amidinium n—n* absorption band (Km, = 292 nm). Finally, the
[(tmb)2Ru2+(Me-bpy)COO-][(PF6)] (13c) complex exhibits an absorption spectra very
similar to that observed for the tris- tetramethyl bipyridyl complex [Ru(tmb)3]2+[(PF6)2]
having drt-n“ (MLCT) absorption band maxima (Am = 456 nm) and a less pronounced
shoulder (km = 426 nm). The intraligand 1t—1t* absorption for this complex is also found
to be isoenergetic with the amidinium and carboxylic acid intraligand absorption bands
(km = 292 nm). Absorption spectra for each of the protonated complexes (13a) and
(13b) are typical for Ru(II)polypyridyl complexes having one or more ligands of
distinctly different electronic composition [102]. The electronic absorption and emission
spectral parameters for these three complexes as well as for three independently
synthesized tris-homoleptic complexes formed from the respective ligands composing the
electron transfer donor complexes are found in Table 5.

Emission spectra from 500 - 800 nm for each of the three Ru(II)polypyridyl
complexes used in the intramolecular electron transfer studies are shown in Figure (14).
Each of the emission spectra show broad gaussian shaped emission bands. For the
[(tmb)2Ru2+(Me-bpy)AmH+][(PF6)3] the maxima of the ernisson is found (km = 678nm).

For the complexes [(tmb)2Ru2+(Me-bpy)COOH][(PF6)2] and [(tmb)2Ru2+(Me—bpy)COO']

86

Table 4. Electrochemical data for (tmb)2Ru(II) complexes in CH3CN at 22°C. E1 ,2 (V) vs.
SCE.

 

 

Ru(II)polypyridyl E1/2(Ru3+’2+) E 1,2(Ru2+n+) Eln(Rur+/°) E 1n(Ru°/')
(tmb)2RuI+AmH+ 1.02 -1.17* -1.62 -l.82
(tmb)2Ru(II)COOH 1.04 -1.06* -1.65 -191
(tmb)2Ru(II)COO- 1.02 -1.14* — —
(tmb)3Ru(II) 1.01 -1.63 -1.82 -2.07

 

*irreversible redox couples values are estimated from the midpoint of the cathodic half of
the cyclic voltammetric wave
Supporting electrolyte (0.1 M TBAH)

87

[(PF6)] the emission maxima are found (7km = 658 and 618 nm) respectively. Quantum
yield measurements for each of these complexes determined relative to a standard
[Ru(bpy)32+] [(PF5)2] in CH2C12 yield values greatly reduced relative to CH3CN as has
been previously found for similar Ru(II)polypyridyl complexes [68]. Summary of the
electronic absorption and emission data for each of the three electron transfer donor
complexes (13a-c) and those for the three tris-heteroleptic complexes are given in
TableS. Upon cooling to 77K each of the complexes give rise to distinct vibrational
progressions (Avav = 1480, 1850 and 1800 cm!) for [(tmb)2Ru2+(Me-bpy)AmH+][(PF6)3],
[(tmb)2Ru2+(Me-bpy)COOH][(PF6)2] and [(tmb)2Ru2+(Me-bpy)COO-][(PF6)]
respectively, typical of tris-bipyridyl ruthenium(H)complexes. Furthermore, lifetimes of
low temperature glass samples are (t = 4.38, 6.0 and 4.9 us) respectively, reflective of

lifetimes for similar complexes [68].

Association Constants

The Ru(II)polypyridyl complexes appended with the amidinium and carboxylate
functionalitites readily form the salt-bridge with their complimentary acceptors in
solution. Solution association of the respective components for each of the electron
transfer complexes have been thoroughly characterized by NMR methods in DMSO-d6
[79]. Determinations of the stoichiometry and strength of the interaction show 1:1
association from Job's plots and yield binding constants of 386 M'1 and 2297 M'l for the

[(tmb)2Ru2+(Me-bpy)AmH+]3+ and [(tmb)2Ru2+(Me-bpy)COO—]+ with the 3,5

88

Table 5. Ru(H) polypyridyl emission characterization data in CH2C12 22°C. Excitation by
Xe/Hg lamp at 435.8 nm

 

 

Rum) polypyridyl km 0 (AV0,1/2)2 AGes(V)
(x102) (em-1)
(tmb)2Ru2+AmH+(PF6)3 678 0.22 2278 2.23
(tmb)2Ru2+COOH(PF6)2 658 2.6 2496 2.22
(tmb)2Ru2+COO‘(PF6) 618 2.1 2470 2.31
Ru2+(COOH)3(PF6) 626 — 2105 2.22

Ru2+(AmH+)3(PF6)5 673 — 2476 2.17

 

89

dinitrobenzoate (DNBCOO—) and 3,5dinitrobenzamidinium (DNBAmH+) acceptors
respectively.

The electrostatic interaction of the complimentary charges of the electron transfer
donor and acceptor components comprising the salt-bridge are expected to yield
significant association constants in CH2C12 solvent in which the solvent dielectric is
(e = 8.9). Previously, it has been found for the association of the respective salt-bridge
complexes that lowering of the solvent dielectric from DMSO-d6 (e = 48) to CD3CN
(8:36) results in significant increases in the observed association constant [104].
However, the determination of the association constants in CH2C12 by NMR methods are
not easily measured. For the [(tmb)2Ru2+(Me-bpy)AmH+]3+-3,5-dinitrobenzoate complex
absorption titrations reveal significant changes in the d7t—1t* (MLCT) absorption region
concomitant with increased concentrations of the complimentary 3,5 dinitrobenzoate in
solution. Specifically, the split MLCT coalesces upon salt-bridge formation maintaining
an isosbestic point at 487 nm as seen in Figure (15). Fitting of this data by the method of
Wilcox [105] for the changes in the absorption spectra at 495 nm reveals association
constants of 5.6 x 105 M". When the titration is instead performed with the benzoate
complex similar changes in the absorption spectra are observed, revealing a slightly
enhanced association constant of 8.5 x 105 M". Greater association for this complex is
expected, as there are no electron withdrawing nitro groups present reducing the negative
charge at the salt-bridge interface. For the complex formed by the association of

[(tmb)2Ru2+(Me-bpy)COOH]2+ with 3,5-dinitrobenzoic acid the changes observed in the

90

 

 

 

 

 

 

 

 

 

3 0.12
1..
2
0 2 4 6 8
[3,5 DNBCOO—1/105 M
1 x 2

 

 

 

300 400 500 600

Figure 15 Association constant determination from Absorption titration of
[(tmb)2Ru2+(Me-bpy)AmH+]3* (3.0 x 10'5 M) with 3,5-dinitrobenzoate in CH2C12. Inset
plot shows the change in absorbance at 495 nm through the titration.

91

 

 

 

 

 

 

 

 

 

 

 

3
(no
2
2 _ (D
a, 2
O
C
(6
e
8 0 1 1 1 J
5 0 1 2
1 [Benzoate1/1O'4 M
O l l
250 350 450 550

Wavelength

Figure I 6. Association constant determination from Absorption titration of
[(tmb)2Ru2+(Me-bpy)AmH+]3* (3.0 x 10'5 M) with benzoate in CH2C12. Inset plot shows
the change in absorbance at 470 nm through the titration.

92

 

A1/A2

 

 

 

 

[ 3,5 DNBAmH+ ]/ 10‘5 M

Figure I 7. Association constant from the ratio of biexponential fit function prefactors (A1
and A2) of the lifetime decay of [(tmb)2Ru2+(Me-bpy)COO’]+ (0.10 mM) titrated with
3,5-dinitrobenzamidinium.

93

absorption titration are not and this complex gives rise to association constants
Ka = 51000 M", similar to previous determinations of the association constant of
[(bpy)2Ru2+(Me-bpy)COOH]—3,5-DNBA complex found to be 702 M". Titration of
[(tmb)2Ru2+(Me-bpy)COO‘]+ complex with the complimentary 3,5-dinitrobenzamidinium
does not reveal absorption changes lending accurate assessment of the association
constant by absorption methods. However, infrared measurements performed with
benzamidinium monitoring the change of the C=N stretching region of the
benzamidinium complex reveal Ka = 105 M'l. Assessment of the association constant for
this complex from the pre-exponential factors A. and A2 of biexponential fitting of the
lifetime data yield association constants for the complex > 3.0 x 104 M'l is shown in

Figure (17).

The design of the excited state of the Ru(II)polypyridyl complex is crucial to a
proper kinetics study of the electron transfer reactions of these complexes. Thus
tetramethylating the ancillary bipyridyl ligands is necessary, as upon photoexcitation into
the d7t—7t* (MLCT) state of the complex the oxidative electron transfer reaction may
proceed by two potential different pathways.

For a complex in which the lowest energy MLCT excited state is localized on the
salt-bridge modified Me-bpy ligand, photoexcitation will place the transferring electron
directly into the PCET pathway. Alternatively, a lowest energy MLCT excited state
involving the ancillary bipyridyl ligand will remove the excited electron from the PCET

pathway. Consistent with this expectation as shown in Table 6 the energetic splitting of

94

the tetramethylated bipyridyl is removed from the PCET reaction pathway and thus the

photoexcitation produces the desired orientation of the charge transfer transition.

Determination of Ru(II)polypyridyl complex energetics

The data in Table 5 allow the evaluation of the free energy of the excited state of
each of the complexes. Analysis of the emission profiles of the three homoleptic
complexes [Ru2+(tmb)3]2+, [Ru2+(Me-bpyCOOH)3]2+ and [Ru2+(Me-bpyAmH+)3]5+ allows
the determination of the energy of the excited state for charge transfer to the salt-bridge
functionalized ligands for each of the complexes. Accordingly, the evaluation of the
excited state energy for the complexes may be determined according to the procedure
developed by Meyer [2] and by Schuster [86] as described in Chapter 3. The data

determined by this method are given in Table 5.

Luminescence and Lifetime Measurements

In the absence of electron transfer quenchers for each of the complexes exhibit
long lived and monoexponential luminescence over 41: of the decay data having lifetimes
‘C = 860, 770 and 1030 ns for [(tmb)2Ru2+(Me-bpy)AmH+]3+, [(tmb)2Ru2+(Me-
bpy)c00H]2+ and [(tmb)2Ru2+(Me-bpy)COO-]+ respectively.

Upon the addition of the complimentary benzoate, benzamidinium or benzoic acid
complexes, energetically incapable of electron transfer reaction, the luminescence for
each of the complexes is slightly altered. For the [(tmb)2Ru2+(Me-bpy)AmH+]3+ complex
the emission is found to shift 10 nm to the blue and undergoes a concomitant 10%

relative increase in the emission intensity. For the [(tmb)2Ru2“(Me-bpy)COOH]2+ very

400

300

200

6 -1
km?r /10 s

100

95

 

2.2

 

 

 

 

l I

 

   

2.0 4.0 6.0 8.0 10.0 12.0

[3,5 DNBAmH+] / 10'5M

Figure 18 Concentration-dependent ( I ) and concentration-independent ( O ) observed
rate constants for the quenching of [(tmb)2Ru2”(Me-bpy)COO-]+ (0.10 mM) by 3,5-
dinitrobenzamidinium in CH2C12 ,

96

 

            

  
  

         

   

 

 

 

_ 600 .
d) .
c .
C
“I
.r: v1: . ‘ '
:2 400 (52.111111? .arttlge-
g ....-,. .......,1...... M...“
Q _
l'I-ln.
m
g 200 —
O
U ._

0

5 15 25
Time/10‘9s

80
N 70 1 o
P 60 — III .
'U
E 50 - I

40 — 0
P.— .0 I

30 .- '

20 ' ' I '

 

 

2.0 4.0 6.0 8.0 10.0 12.0
[3,5 DNBAmH+ ]/ 10 '5 M

Figure 19. Top panel. Lifetime decay data for the quenching reaction between
[(tmb)2Ru2*(Me-bpy)COO-]+ and 3,5-dinitrobenzamidinium. Biexponential fitting of the
decay data is indicated with the solid line. Bottom panel. Relative percentages of the
biexponential fitting of the decay data from Top panel.

97

 

 

 

 

 

 

4
I‘D ~\
:00 .5
1— om
l: 2 i
8 (n
x l.
o l L l l I l 0
0 4 8 12
[3,5 DNBCOOH 1/10‘4 M

Figure 20. Concentration-dependent ( I ) and concentration-independent ( O ) observed
rate constants for the quenching of [(tmb)2Ru2+(Me-bpy)COOH]2+ (0.06mM) by 3,5
dinitrobenzoic acid in CH2C12.

98

 

 

 

 

 

[3,5 DNBCOO'] / 10 '4

Figure 21. Concentration—dependent (I) and concentration-independent ( O ) observed
rate constants for the quenching of [(tmb)2Ru2+(Me-bpy)AmH+] (0.064mM) by 3,5-
dinitrobenzoate in CHZClz.

99

Table 6. Rates for concentration-dependent and concentration-independent quenching of
(tmb)2Ru(II) electron transfer complexes.

 

Salt Bridge Complex AG 0 / eV ker kPCE'r
(x 109 M‘s") (x 106 s")

 

-O.l4 1.2 8.4
\—/
N02
(tmbpy)2"RL / \ 9mm
_ _W_ / -0.34 3.3 43
no2

 
  

no2

1H!
G)" -023 3.2 310
\1 02

(tmbpyh ' ' BL / \

 

100

little difference is observed in emission spectra upon addition of benzoic acid showing
only a slight 5 nm red shift observed concomitant with little 5% relative change in the
emission intensity. For the [(tmb)2Ru2+(Me-bpy)COO-]+ the addition of benzamidinium
results in observation of a 10 nm red shift of the emission maximum and a 10% increase
in the emission intensity. Lifetimes obtained for each of the complexes bound to
benzoate, benzamidinium, and benzoic acid are long lived having excited state lifetimes
of ‘C = 1200, 470, and 895 us for [(tmb)2Ru2+(Me-bpy)COO-]+, [(tmb)2Ru2+(Me-
bpy)AmH+]3+ and [(tmb)2Ru2+(Me-bpy)COOH]2+ respectively. Upon the addition of the
complimentary 3,5-dinitrobenzene quenchers the luminescence is dramatically quenched.

Excited state lifetimes of each of the tetramethylated Ru(H) complexes in the
presence of the quenching 3,5 dinitroaromatic complexes are biphasic. One component of
the emission decay is clearly dependent upon the concentration of the quencher whereas
the other component remains concentration-independent. Concentration-dependent
lifetimes for these complexes are attributed to bimolecular electron transfer quenching of
the unbound Ru(II)polypyridyl complexes. For this component, the quenching obeys
typical linear Stem-Volmer quenching kinetics yielding electron transfer rates given in
Table 6. The observed bimolecular electron transfer rates for each of these three
complexes are found in Table 7. Similar observations are found for equi-exergonic
reactions of Ru(II)polypyridyl complexes with nitroaromatic quenchers [3] consistent
with the determination of bimolecular electron transfer reactions in these complexes. Of
greater relevance to the investigation of PCET are the attendant concentration-
independent rate constants, which are attributed to the unimolecular electron transfer of

each complex associated by the hydrogen bonding salt- bridge interactions. For the two

101
salt-bridging complexes [(tmb)2Ru2+(Me-bpy)COO-]+ and [(tmb)2Ru2+(Me-
bpy)AmH+]3+ the observed differences in the unimolecular rates are seen to depend
significantly on the salt-bridge and its orientation with respect to the electron transfer

pathway as is seen in Figures (19-22)

Discussion

The observation of luminescence lifetime quenching in these complexes is
indicative of electron transfer occurring through the salt-bridge interface. For each
complex the bimolecular component of the lifetime corresponds well with the static
quenching results. For the [(tmb)2Ru2+(Me-bpy)COO‘]+ associated with 3,5
dinitrobenzamidinium 83 % relative quenching in the emission experiments is observed
over a range identical to that used in the lifetime quenching experiments. In the
determination of the bimolecular contribution to the observed lifetime quenching the
relative change in the free complex quenching component is found to be 67 % quenched.
Finally, for this complex the measurement of the fast PCET component of the quenching
allows the additional assessment to be made for the relative quenching of the free Ru(II)
complex by the analysis of the pre-exponential factors of the biexponential curve fitting.
In Figure (18) contributions from both concentration-independent PCET component A]
and the concentration-dependent component A2 reveal 80% quenching of the free
complex lifetime through the concentration range. This is strong support that the
quenching occurrs by electron transfer and furthermore, that the mechanism of the
quenching is the same. In the bimolecular quenching observed in static quenching

experiments for the [(tmb)2Ru2+(Me-bpy)AmH“]3+ complex formed with the

102

complimentary 3,5-dinitrobenzoate quencher similar observations are made revealing a
relative reduction of the emission intensity by 37% and quenching of the free complex
lifetime measurements a relative change in lifetime of 67 % are observed.

Several issues of the electron transfer relevant to the association constant
determinations in the two salt-bridge complexes (13a) and (13c) are important for the
establishment of the intracomplex electron transfer rate constant. The interaction between
the electron transfer donor and acceptor in a non-covalent system is predicated upon the
establishment of an equilibrium between bound and unbound quencher during the long
excited state lifetime ~1p.s (kr = 1.0 x 1065") of the ruthenium complexes. A

manifestation of this equilibrium process is the competition of the rate of the electron

D*-NH+--- 'OA
ko/ kPCEt
koft
D*-NH+ + 'OA D+-NH+--- 'OA'
(4.1)

transfer relative to the dissociation rate of the quencher complex. If the rate of the
interaction of the two complimentary species in solution may be considered as limited
only by diffusion, then the maximal rate at which the two species may encounter one
another is approximately kg for CH2C12 = 4 x 1010 M" s" in CH2C12 solvent and not
substantially different in CH3CN, in; being established for nitro aromatic oxidative
electron transfer quenchers as 2 x 1010 M’1 s'1 [3]. For the [(tmb)2Ru2+(Me-bpy)COO']+

complex the association constant between the two species is found from the A1/A2 ratio

103

of the biexponential fitting of the lifetime decay to be 3.2 x 104 M". The maximal off rate
for this complex (13c) will effectively be 1.25 x 106 s". The observed PCET rate for this
complex is 310 x 106 s", yielding a kPCET/koff ratio of 250. For the remaining two
systems, the observed PCET rate constant is found to be 43 x 10‘5 s'1 for [(tmb)2Ru2+(Me-
hpy)C00H]2+ (13b) and 8.4 x 106 s" for [(tmb)2Ru2+(Me-bpy)AmH+]3+ (13a). In each
case the maximal expected off rate constant will be 4 x 107 s'1 (kPCET/koff = ~ 1 0) and 7.1
x 104 s'1 (km/keg: 120), respectively. Thus only in the (COOH)2 system will the effect
of the excited state equilibrium be significant, as the observed PCET rate constant is of
the same order of magnitude as the predicted off rate. However, the equilibrium
established in the electron transfer measurements is with the electronically excited Ru(II)
complex and not the ground state complex, as is measured by the absorption titrations.
Yet the typical ApKa for Ru(II)polypyridyl complexes between ground and excited state
is ~3; for instance, Ru(II) bipyridyls directly appended with carboxylic acids exhibit
ApKa of 2.5 [106,68]. Hence the observed intramolecular electron transfer decay rates
well exceed the equilibration rates, and intramolecular electron transfer occurs for a static
complex. This is not the case for the symmetric dicarboxylic acid omplex (13b) in which
it is expected that the PCET rate is complicated by a dynamic equilibrium between the
donor and the acceptor.

In addition to the charges at the interface significantly enhancing the association
constants for each of the salt-bridge functionalized complexes as is reflected in the
association constant determinations. The charge at the interface is expected to be directly
affected by the presence of the nitro groups of the quenchers. For the amidinium this is

expected to increase the relative positive charge, and conversely for the carboxylate the

104

electron withdrawing nitro groups are expected to stabilize the negative charge at the
interface thus reducing the strength of the salt-bridging interaction. A direct relationship
may be expected between the association constant and the electronic coupling as the
efficiency of the electronic coupling may be manifested in the differences in the strengths
of the salt-bridging interaction [107]. The stronger interaction of the salt-bridge will be
expected to enhance the mixing of the donor and acceptor states relative to the symmetric
dicarboxylic acid (COOH)2 interface in which it has been previously assessed that the
electronic coupling is ~10cm'l [66]. Infrared data for the two salt-bridge complexes (13a)
and (13c) suggest that the shifts of the amidinium C=N stretch change by 20 cm‘1 in the

complex formed from [(bpy)2Ru2‘“(Me-bpy)AmH”]3+ with 3,5-dinitrobenzoate C=N

\
N—H --------------- o
N—H -------------- o
/
H
H
/
Q --------------- H—N
law—<9 <——l- @}>—DNB
o -------------- H—h
\
H

(4.2)

105
stretches changing from while for [(bpy)2Ru2‘”(Me-bpy)COO-]+ with benzamidinium a
25 cm" shift is found. These shifts are consistent with shifts found for C=N in schiff base
complexes [108].

More significantly, is the effect of charge in the salt-bridge on the observed,
differences in the intramolecular electron transfer rates. In complex (13a) the permanent
dipole (5+8) of the (AmH+,COO—) salt bridge lies in the direction of electron transfer
whereas in (13c) the electron transfer opposes the (COO-,AmH+) salt—bridge dipole as
schematically represented in Figure (4.2). Additionally, internal electrostatic fields
created as a result of the charged salt-bridge interface affecting the rates of electron
transfer by altering the driving force of the reaction in the complex relative to that for the
isolated constituents [109]. Using typical bond distances for Ru(bpy)33+’2+, and

dinitrobenzene” and the amidinium-carboxylate salt-bridge, a determination of the field

effect for these interfaces from the Eq. (4.2) yields 0.37 V. The magnitude of the internal

Ec = ‘1qu (4.2)

 

field effect is expected to be will be favorable for complex (13a) and thus unfavorable for
complex (13c) effectively augmenting the relative driving force for intramolecular
electron transfer in complex (13a). The thermodynamic attenuation of the electron
transfer rate constant for (13¢) may be further augmented by a greater reorganizational
energy associated with the salt-bridge. In (13a) accompanying proton transfer (from the
Ru(II) amidinium donor to the carboxylate acceptor) can stabilize the charge of the
electron as it develops on the acceptor. In this case, since the proton charge is strongly

coupled to the solvent dipoles, as with the electron, charge shift within the salt bridge

106

may be accompanied by significant solvent polarization thereby giving rise to additional
Franck-Condon factors for the electron transfer reaction. This is not the case for (13¢).
Here, the proton is already residing on the acceptor, and hence it is likely to remain upon
the arrival of the electron.

The assessment of the electron transfer rates found for each of the asymmetric
salt-bridge interfaces may be analyzed in terms of semi-calssical electron transfer theory
as given in Eq. (4.3). Under the assumption that the electronic coupling (Vab) of the two

interfaces is the same and equal to 20cm", and furthermore, that the outer-sphere

4}»ka

(AGHt )2 ]

ket = z—th-|Vab|2(47t.kBT)—“ 2 exp—[ (4.3)

contribution (2.0) to the reorganizational energy (it) of each system may be found from a
two-sphere dielectric continuum mode model [2,110] (2.: L, + 714) according to Eq. (4.4).

Substituting into Eq. (4.4) the diameter of the Ru(II) polypyridine ((1 =14.5 A,

x_e21+1_1 l___1_ (44)
0 41(80 21D 21'A RDA 80p 83 .

r0 = 7.25 A ) and for the dinitrobenzene complexes ((1 = 7.1 A, rA =3.55 A ) estimated

 

 

from molecular modeling (Spartan) and that the inner sphere reorganizational energy for
each interface is the same and equal to a maximum expected [11]] value it, = 0.25V

yielding it = 0.75V. The 60p and as for CH2C12 are the optical dielectric constant (the

107

square of the refractive index) 2.03 and the static dielectric constant 9.08 respectively e2
the charge for and electron and 41teo the vacuum premittivity, yield the expected electron
transfer rates based upon the free energy of reaction [AmH+,COO— (AG = -0.14V)] and
[COO—,AmH+ (AG = —0.34 V)] for each interface system [km (AmH+,COO—)
= 214 x 106 5"] and that for the switched analogue [k2, (COO—,AmH+) = 3240 x 106 s" 1.
Thus each of these predicted electron transfer rates are found to be effectively 25 and 10
times greater for each of the (AmH+,COO—) and (COO—,AmH+) interfaces respectively.
Further modifying the semi-classical electron transfer equation to bring the theoretical
rates to the observed values again assuming that the electronic coupling for each interface
is the same requires increasing the reorganizational energy it for each (AmH+,COO—)
and (COO—,AmH+) interfaces to 1.2 and 1.15V respectively or ~0.5V greater than the
predicted reorganizational energy found from the simple spherical reactant continuum
dielectric models.

Recently, Cukier [67] has shown that a representation for observed electron
transfer rates in asymmetric proton coupled-electron transfer systems may be given

according to Eq's. (4.5) and (4.6). Here the additional reorganizational energy associated

 

2n 2 1/2 . 2 (E +AG+hov)2
ket =-h-|V| (EsktsTl EDWIN» exv-[ S 4EskBT ] (4.5)

P
|<0i|vf>|2 =[e’S-SSTJ (4.6)

with the coupling of the proton to the solvent is given as a summation over available
proton vibrational levels, Eq. (4.6), initial (i) and final (1) associated with the localization

of the proton during the electron transfer event.

108

These results show that intervening salt bridges can significantly affect the rates
of intramolecular electron transfer reactions and may provide an insight into the coupled
proton/electron transfer events found in photosynthetic [112] and respiratory [113]
biological systems and the more recent example given by Brzezinski [47] in cytochrome

c oxidase.

CHAPTER 5

PHOTOINDUCED REDUCTIVE ELECTRON TRANSFER WITHIN AN
ELECTRON DONOR-ACCEPTOR PAIRzASSESSMENT OF THE REVERSAL
OF THE ELECTRON TRANSFER DIRECTION

Introduction

The directionality of electron transfer processes is crucial to the effective function
of many biological organisms. This necessity for directional electron transfer reactions is
of fundamental importance to the storage of solar energy for conversion to necessary
metabolites. Necessarily this scheme requires maintainance of a stable charge separated
state which can undergo useful chemical reactions in competition with the deleterious
back electron transfer reaction. A means by which this may be accomplished is to control
the direction of the electron transfer reaction. A premier example of the directional
electron transfer reactions coupled to chemical reactions is found in photosynthetic [112]
and respiratory [113] biological systems. Prevention of the energy wasting back electron

transfer effectively accomplishes the the task. A particular objective recently has been the

109

110

mechanistic and theoretical experiments detialing directional electron transfer in the
photosynthetic reaction center. Absorption of light in and subsequent energy transfer to
the special pair leads to the selective and directional electron transfer through the L
subunit rather than the structurally identical M subunit. Recent investigations have
applied a variety of different spectroscopic techniques aimed at unraveling the details of
this directionality [17]. Many suggestions are centered on the dielectric asymmetry of the
RC environment, the Coulombic (electric field) asymmetry has been shown in the work
of Honig as a potential explanation, and also the incorporation of hydrogen-bonding
interactions of particluar residues of the peptide backbone encompassing the reaction
center have all been suggested as playing potential roles in the asymmetric eletron
transfer in this system.

Yet another recent example indicating directional and specific electron transfer
has been shown in cytochrome c oxidase in which the rate of electron transfer along the
heme a3 is found to be 104 times greater than that observed along the nearly identical
(structurally and distance) heme b pathway. Furthermore in this system the potential
control of the observed electron transfer rate has implicated the coupling of proton
transfer. Yet the nature and complexity of each of these systems, photosynthetic reaction
centers and the cytochrome c oxidase do not allow the thorough investigation of each of
the speculated controling interactions.

The design of model systems based upon the electron transfer reactions of
Ru(II)polypyridyl systems functionalized with salt-bridging substituents allow the control
of the direction of electron transfer reactions. To this end a systematic series of tris-

bipyridyl ruthenium(II) complexes was designed to explore reductive electron transfer

11]

reactions. This orientation of electron transfer donor and acceptor maintain the previously
developed directionality of the asymmetric hydrogen-bonding salt-bridge interface while
reversing the direction of the electron transfer. Thus, use of the oxidation capacity of the
excited state of the Ru(H)polypyridyl complexes allows an electron transfer to be elicited
from an organic donor associated through the salt-bridge interface. Electron transfer
reactions for this orientation of donor and acceptor may be regarded as a hole transfer
where the excitation of the charge transfer state of the ruthenium complex creates a metal
centered Ru3+ hole upon formation of which an electron is transferred from the acceptor
through the salt-bridge functionalized bipyridine.

Crucial to the unambiguous observation of electron transfer in these
Ru(II)polypryidyl systems is the energetic splitting of the MLCT states between the
ancillary and salt-bridge functionalized bipyridine ligands. For bipyridine based systems
charge transfer to the ligand having the lowest lying reduction potential results in the
localization of the excited state electron at the amidinium functionalized bipyridine. For
the carboxylate functionalized complex, due to the higher energy MLCT transition of
carboxylate, electron localization is maintained at the ancillary bipyridine ligands. To
further facilitate the resultant excited state charge transfer localization onto ancillary
ligands, electron withdrawing substituents have been incorporated, specifically, two 4,4'-
diethylcarboxy-2,2'—bipyridine (decb) ligands. Four Ru(H) systems Figure (23a-d) have
been synthesized composed of the electron transfer acceptor complexes [(bpy)2Ru2+(Me-
bpylAmHu’t [(bpy)2Ru2*(Me-bpy)co0‘lt [(decb)2Ru2+(Me-bpy)AmH*]3* and
[(decb)2Ru2+(Me-bpy)COO']+ (bpy = 2.2-bipyridine, decb = 4,4'-diethylcarboxy-2,2'-

bipyridine, Me-bpyAmH+ = 4-methyl-4'-amidinium—2,2'-bipyridine, and Me-bpy-COO- =

112

 

(bPY)2RU"\ (decbhriuf

  

(bpy)2Ru" (decb)2Ru(
\

(b) (d)

Figure 22. Ru(II)polypyridyl complexes for reductive electron transfer quenching (a)
ltbpy>2Ru2*(Me-bpy)AmH*l3* (b) [(bpy)2Ru2*<Me-bp )COO-r. (c) [(decb)2Ru2*(Me-
bpy)AmH+]3+ and (d) [(decb)2Ru2+(Me-bpy)COOH] +‘ Figures shown are the salt-
bridged complexes formed from the 1:1 association with N,N'-dimethylaminobenzoate
and N,N'-dimethylaminobenzamidinium

113

4-methyl-4'-carboxylate-2,2'—bipyridine) upon formation of the salt-bridge with the

organic donors N,N'-dimethylaminobenzoate and N,N'-dimethylaminobenzamidinium.

Results
Electronic Absorption I Emission Spectroscopy

Absorption profiles for the two bipyridine complexes are typical for Ru(II)bpy
complexes [68] having a high energy 7t—1t* intraligand absorption band and a low energy
d7t—7t* MLCT absorption band. These spectral features are maintained for the amidinium
and carboxylate modified complexes. Absorption and emission spectra for each of the
ruthenium complexes are shown in Figures (24) and (25). The distinguishing differences
evident in the absorption spectra for the two (decb) modified complexes of moderate
intensity observed at the low energy tail of the intense intraligand band and high energy
tail of the MLCT are typical of complexes with alkyl and ester modified bpy ligands. For
instance, Ru(II)tris(decb) exhibits transitions at 355 and 407 nm [103a], which are
energetically coincident with the additional transitions appearing in the absorption spectra

of the amidinium modified (Amax = 356 and 405 nm) and carboxylate modified (Amax =

366 and 400 nm) (decb) complexes. Similarly, Ru(II) tris(dimethylbipyridine) shows
absorption features at 328 and 360 nm [114]. These transitions of the alkyl and ester
modified bpy ligands are believed to be MLCT in character. Summary of the electronic
absorption spectra for each of the Ru(H) complexes are given in Table 7. Figures (26 -

29) display the changes for each of the Ru(II) bpy complexes (23a-d) occurring upon the

114

 

 

 

 

1
5 104
Z
2
a
a
5 s
g 0.6 m
8 3 104 g.
‘8 ca.
5 8
o —
E 2%“
3
l’.
q
1 104 0'2

 

 

 

 

 

 

300 500 700
Wavelength (nm)

Figure 23. Absorption and emission spectra for amidinium functionalized electron
transfer donor complexes [(bpy)2Ru2“(Me-bpy)Ami-F]3+ and [(decb)2Ru2+(Me-
bpy)AmI-I‘”]3+ in aerated CH3CN solution.

115

 

 

 

 

 

 

 

5104
0.8 E
,2 a
.2 fi'
1;: a
0 I11
<0 4
g 3 10 ('35
(.3 92.
.5 0.4 8
2 5'
5'
3
se.
2
1 104
0

 

 

 

 

300 500 700
Wavelength (n m)

Figure 24. Absorption and emission spectra of carboxylate functionalized electron
transfer donor complexes [(bpy)2Ru2"(Me-bpy)COO-]Jr and [(decb)2Ru2+(Me-
bpy)c00H]2+in aerated CH3CN solvent.

116

Table 7. Absorption data for (decb)2Ru(II) and (bpy)2Ru(II) complexes

 

 

Ru(II) polypyridyl complex Mom") 8 (it—7t*) Mcm") e (dit—7r*)
(dech)2RtF*(me-hpymmH+ 32680 44668 21 186 14978
(decb)2Ru2+(me—bpy)COOH 32468 62685 20534 18527
(bpy)2Ru2+(me-bpy)AmH+ 34722 58301 21930 1 1894

(bpy)2Ru2+(me-bpy)COO' 34965 55996 22026 12017

 

117

addition of the complimentary N,N'-dimethylaminobenzoate (DMABCOO-) and N,N'-
dimethylaminobenzamidinium (DMABAmH+) quenchers. Pronounced shifts in the
absorption profile for each of the complexes are observed. Whereas the MLCT transitions
remain relatively unperturbed, the intensity of the intraligand transition increases, as does
the intensity of the weak transition on the low energy tail. These spectral changes are
attributable to the formation of the electron transfer complexes Figure (23 a-d). As
observed from the insets of Figures (26-29), the inverse of the change in the absorption
cross-section scales linearly with the inverse of the quencher concentration. Benesei-
Hildebrand treatment [115] of these data yields the association constants given as inserts

to Figures (26-29).

Luminescence Spectra

Emission spectra for each of the (decb) and (bpy) amidinium and carboxylate
complexes are shown in Figure (24) and (25) respectively. Corresponding energy
maxima, quantum yields, and excited state lifetimes are listed in Tables 8 and Table 9.
The appreciable red shift in the emission maxima observed for each of the decb
complexes is in accordance with the stabilization of the MLCT excited state resulting
from the greater electron withdrawing ability of the bpy modified with the 4,4'-dicarboxy
substituents. The energy of the MLCT transitions to each of the (bpy) and (decb)
complexes may be determined using the procedure of ’Meyer Eq's. (3.1) and (3.2) in
Chapter 3. For the (decb) complexes, as indicated from the red-shifted emission in

Figures (24) and (25) the (lit —) 7t* (decb) MLCT excited state is considerably stabilized

118

 

 

    

 

 

 

 

2 ' 7_

E -

35-

(U

I ..

is-
m P
g K,,=32eoom-1
to 1 l 1 1 l 1
‘g _ 0 10 20 30
g 1 1/[DMABCOO-l/1O'3

 

 

 

I I I I I J I I I I I ' _ -
250 350 450 550 650
Wavelength / nm

Figure 25. Absorption titration of [(bpy)2Ru2+(Me-bpy)AmH+] (2.0 x 10'5 M) by N,N’-
dimethylaminobenzaote in CH3CN.

119

 

 

         
  

 

 

   

 

 

 

 

 

7
E 5’
1.5 I _
g a-
g 7- - Ka=28000M-1
to 1 I 1 l L 1
'g 1 0 10 20 30
(I) 1/[DMABCOO-‘l /10-3
2
0.5
0 I J
250 350 450 550

Wavelength / nm

Figure 26. Absorption titration of [(decb)2Ru2*(Me-bpy)AmH+]3+ (2.0 x 10-5 M) with
N ,N'-dimethylaminobenzoate in CH3CN

120

 

 

  

   

 

 

 

 

 

 

2.5
6 .—
2 " g 4 ..
I L-
a) g 2 -
o .—
g 1'5 _ - Ka=7800M'1
e 0 1 l l l l
8 0 10 20 30
g 1/[DMABAmH+] I10 -3
1 .—
0.5
O , I I I I I I I I I I

250 350 450 550 650
Wavelength / nm

Figure 27. Absorption titration of [(bpy)2Ru2+(Me-bpy)AmH+]3+ with N,N'-
dimethylaminobenzamidinium in CH3CN.

121

1.5

 

.0
N

 

 
   
  
   
 

m
l

:3
I

  

1/ ([ast-[abs] 0)
I

 

&=%MM4
l l l l I l I
o 10 20 30 40

1/ [DMABAmH+] / 10 '3

 

 

 

 

 

Absorbance

 

 

 

250 350 450 550
Wavelength / nm

Figure 28. Absorption titration of [(decb)2Ru2+(Me-bpy)COO-]+ with N,N'-
dimethylaminobenzamidinium in CH3CN.

122

with respect to that of (bpy). Estimates of the Ru(decb)32+ MLCI‘ from electrochemical
data [103,116] gives AGES 2 2.05 eV as compared to AG... (Ru(bpy)32+*) = 2.15 eV [15].

Moreover, the drc —) 7t* (decb) MLCT is energetically stabilized relative to the MLCT
excited states of Mebpy-amH+ and Mebpy-COO‘. Previous analysis of the emission

spectra of the homoleptic Mebpy-amH+ and Mebpy-COO' Ru(II) complexes with Eqs.
3.1 and 3.2 places the MLCT excited states for these ligands at 2.09 and 2.22 eV,
respectively. Consistant with the stability fo the charge transfer transition of the (decb)
complexes are the long excited state lifetimes. Lifetimes for each of the four
Ru(II)poylpyridyl complexes (23a-d) are summarized in Table 9.

Eqs. (3.1) and (3.2) are also necessary for the estimation of excited state redox
potential, which provide the relevant driving forces for the reductive quenching reactions
of 23a-d. In the simplest treatment, the excited state reduction potential of the Run metal

complex can be ascertained from the simple thermodynamic relation: E1/2(*Ru"”) =

AG 0 — E1 /2(Ru"”) where E1 /2(Ru"”) and E1 /2(*Ru"") are the ground state and excited
es

state Run” reduction potentials, respectively. The redox potentials for the various Ru(H)
polypyridyl complexes are summarized in Table 10. The Ru(II) complexes undergo

reversible one-electron oxidation as indicated by ip,a/ip,c ratios of 1.10 :1: 0.10. Anodic
to cathodic peak separations (AEp) were greater than 59 mV, but were comparable to that
measured for ferrocene, thereby establishing that deviations of AEp from the theoretical

limit are primarily due to uncompensated cell resistance.

123

Table 8. Emission Data for (decb)2Ru(H) and (bpy)2Ru(II) complexes in CH3CN.

 

 

Ruthenium Complex item(cm") Ava-[(2 x(cm") AGm(eV)
(decb)2Ru2+(me-bpy)AmH+ 14950 (2370) 1885 2.09
(decb)2Ru2+(me-bpy)COOH 14800 1900 1588 2.03
(bpy)2Ru2+(me-bpy)AmH+ 15750 2470 2684 2.28
(bpy)2Ru2+(me-bpy)COO— 15850 2000 1760 2.18

 

Table 9. Photophysical Characterization for (decb)2Ru(H) and (bpy)2Ru(II) complexes in
CH3CN.

 

 

Ruthenium Complex ¢(em) T/us

(decb)2Ru2*(me-bpy)AmH* 0.1053 1.30 (5.20)“)
(decb)2Ru2+(me—bpy)COOH 0.0960 1.50 (5.50)”
(bpy)2Ru2+(me-bpy)AmH+ 0.0835 1.06 (5.45)“)
(bpy)2Ru2+(me-bpy)coo- 0.0738 1.18 (5.30)“)

 

a) lifetimes for each of the Ru(II)polypyridine complexes measured at 77K in
EtOHzMeOH glass (4:1 v/v).

124

Lifetime Quenching Electron Transfer Measurements
The long-lived luminescence from the Ru(H) polypyridyl complexes is preserved

upon formation of the salt-bridge in the absence of an electron donating group. Addition

of benzoate to acetonitrile solutions of [(bpy)2Ru2+(Mebpy-AmH+)]3+ (23a) and

[(decb)2Ru2"(Mebpy-amH+)]3+ (23c) lead to a modest attenuation in the observed

excited state lifetimes (t = 975 and 1330 ns, respectively), which remain relatively

independent of benzoate concentrations. Similarly, long-lived excited state lifetimes of

900 and 1130 ns are observed for [(bpy)2Ru2+(Mebpy-COOU]+ (23b) and

[(decb) Ru2+(Mebpy—COO")]+ (23d) associated to benzamidinium. The Ru(II) excited
2

state luminescence of (23a) and (23c) is dramatically quenched when phenyl of benzoate
is substituted with the dimethylaniline. This result is consistent with an electron transfer
quenching mechanism, which is a well-established reaction between electronically
excited Ru(II) tris(polypyridyl) complexes and dialkylamine aromatic electron donors
[3]. Biexponential decay kinetics are observed for each of the amidinium functionalized

complexes where one lifetime component of the emission decay is dependent on the

concentration of donor and the other is not over a DMABCOO‘ concentration range of

0.062 to 1.24 mM ([(bpy)2Ru2+(Mebpy-AmH+)]3+ = 0.051 mM) Figure (30). The

concentration-dependent lifetimes obey typical linear Stern-Volmer quenching kinetics

and the bimolecular rate constants of 6.4x 108 M'ls‘1 for (23a) and of 3.5 x108 M"1s-l

125

 

 

 

 

 

8
,_ x
'0) '31
mo 3
I Ca:
1; 4 3
x8 (”-3
0

 

[DMABCOO-] / 10'5

Figure 29.Concentration-independent ( I ) and concentration-dependent ( I ) observed
rate constants for the quenching of [(bpy)2Ru2+(Me-bpy)AmH+]3+ by N,N'-
dimethylaminobenzoate in CH3CN .

126

 

 

 

 

100 . +
80 —
O
.i“ 60 —
'0 _ O
8 I ' '
1: 40 -
20 L- '
O __!1 I I_ I I I 1 I I I 1 I1

[ DMABCOO' ]/ 10'4 M

Figure 30.Ratio of pre-exponential factors A1 (I) and A2 ( I ) from biexponential fitting
of the quenching data for [(bpy)2Ru2“(Me-bpy)AmHi]3+ with N ,N'—dimethylamino-
benzoate.

127

4.5

 

9 -1
kPCEr/10 s

0)

b1

1"
01

.L3
l15,111 901/ >1

 

 

 

 

0.0 I I l l l l l 1 .5
0 20 4o 60 80
[ DMABCOO-1/ 10‘5 M

Figure 31. Concentration-independent ( O ) and concentration-dependent ( I ) observed
rate data for the quenching of [(decb)2Ru2‘“(Me-bpy)AmH”]3+ by N,N'-
dimethylarninobenzaote in CH3CN.

128

for (23c) accord well with the kinetics of equi-exergonic bimolecular reactions between

Ru(bpy)32+ and dialkylanilines [3]. Analyses of the concentration- independent lifetime

decay curves yield intramolecular rate constants of 1.9x107 s—1 and 1.7x 109 s"1 for
(23a) Figure (30) and (23c) Figure (32), respectively. The independence of the rate
constant on quencher concentration is consistent with an electron transfer quenching
reaction occurring within the associated pair of (23a) and (23c). When the interface is
carboxylate and amidinium moieties are interchanged on the Ru(II) acceptor and DMAB
donor, no electron transfer quenching of the Ru(II) luminescence is observed. The
lifetime of (23b) and (23d) is invariant with the addition of DMABAmH+ over a

5 M. The significant differences in the

quencher concentration range of 1.0 - 140 x 10—
electron transfer reactivity between the (23a,23c) and (23b,23d) systems are reflected in

the electron transfer driving forces. Table 10 lists the estimated driving forces for the

reaction of the various systems as determined from the following relation: AGo =
E1 /2(Q+’°) - E1 /2(*Rum) where E1 /2(*Ru"”) was determined as described above and
E1 /2(Q+’°) is determined from redox potential measurements. For N,N’-

dimethylaminobenzamidinium, a reversible half-wave potential at 1.10 V vs. SCE was

observed in the cyclic voltammogram. Conversely, N,N’-dimethylaminobenzoate (TBA+
salt) was electrochemically irreversible as evidenced by the presence of only an anodic
wave in the cyclic voltammetric scan. Accordingly, the potential for this compound was
estimated from a Marcus curve by using methods previously described by Schuster and

co-workers in their estimation of redox potentials for borate quenchers [86]. A linear free

129

energy relationship for the electron transfer quenching of *Ru(bpy)32+ by aromatic
amines has previously been defined by Meyer [3]. The Marcus curve for the quenching
reaction was verified by re-determining the rateconstants from emission lifetime decay
curves of selected quenchers (dimethyl—p-toluidine, diethylaniline, and tetramethyl-p—
phenylenediamine) under identical experimental conditions. The measured bimolecular
rate constants were corrected for diffusional effects and found to be 110% the values

previously measured by Meyer et. al. The corrected observed rate constant for N,N’-

dimethylaminobenzoate was 3.1><108 M'] s‘l, falling within the linear range of the

Marcus free relationship for aromatic amines (0.20 < AGO < —0.25). Correlation to the

Marcus plot gives a redox potential of 0.70 V vs. SCE. It must be emphasized that the
Marcus curve in Meyer’s study is constructed for neutral quenchers whereas for the
correlation here, the quencher is anionic. Thus the observed rate constant from which the
redox potential is determined is compromised to the extent that work term corrections
have not been applied. At the high ionic strengths [.1 = 0.1 M at which the quenching
reaction was performed, however, the error associated with work term contributions is
expected to be small (< 0.1 V) according to standard Debye-Hiickel formalisms. To
ensure that work terms were indeed insignificant in our analysis, we determined the

reduction potential of N,N’-dimethylaminobenzamidinium from the same Marcus curve.

A corrected, observed rate constant of 1.1x105 M.1 s“1 yielded a E1/2 of 1.09 V vs.

SCE, which is excellent agreement to the electrochemically measured Q'"O reduction

potential of 1.10 V vs. SCE, Table 11.

130

 

0.7

0.6

0.5

0.4

RTlnkq'

0.3

 

 

 

 

0.2
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

Figure 32. Marcus fit of bimolecular electron transfer rates constants for the kinetic
determination of redox potentials of the reductive quenchers N,N'-
dimethylaminobenzoate and N,N'-dimethylaminobenzamidinium in CH3CN u = 0.1 22°C

131

Table 10. Redox potentials and rate constants for the bimolecular quenching reactions

 

 

between Ru(bpy)3 and the reductive electron transfer quenchers.
7
Reductive quencher 15:45:11)) Em (V) AGet(V)
0:13 /CH3
IN—HN\ 1450 0.12 -0.78
CH3 CH3
(TMPD)
3
\CH3 945 0.71 -0. 19
(DM-p-T)
N/Cl-13
\CHg 4.7 0.76 -014
(DEA)
Q /CH3
9 1+©~
o \
°“3 34 0.70:) -0.08
(DMABCOO-)
H
H-N’ /CH3
H-N‘ \CH3
H 0.105 1.09 0.15
(DMABAmH+)

 

Tetramethyl-p-phenylenediamine ( TMPD ), dimethyl-p-toluidine ( DM-p—T ),
diethylaniline ( DEA ), N,N'-dimethylaminobenzoate ( DMABCOO" ) and N,N'-
dimethylaminobenzamidinium ( DMABAmH+ ).

a) Redox potential determined from Marcus fit of quenching data.

132

Table 11. Rates for concentration—dependent and concentration-independent quenching of
(decb)2Ru(H) and (bpy)2Ru(II) salt-bridge electron transfer complexes.

 

Ru(II) salt bridge complex AGET / V kE-r/ M”1 s'1 km s'1

 

 

-039 3.5x108 1.7x109
\ /
I -0.52 6.4 108 1.9 107
(bpbeu'K / \ 3g” of?) O ”PW x x
'_ )V-H-n-O CH3
H
(238)
0.17 8x104 —

0.35 — —

 

 

133

Discussion

Amidinium—carboxylate salt bridge complexes (23a) and (23b) readily form in
CH3CN solutions. The exceptionally high association of the salt bridge agrees well with
Jorgensen’s classification for two favorable secondary interactions composing the
hydrogen bonded interface [36], bolstered by the stabilization imparted by the molecular
recognition of the negatively charged carboxylate by the positively charged amidinium.

The significantly higher Kassoc of (23a) is consistent with an increased basicity of the

carboxylate group conferred by the electron donating dimethylamino group, resulting in a
stronger hydrogen bonding interaction than that observed for switched interface system
(23c and 23d).

Consistent with the equilibrium measurements, the Ru(II) excited state undergoes

electron transfer with unbound and bound donor when the reaction is exergonic. The

luminescence from [(bpy)2Ru2+(Mebpy-amH"')]3+ and [(decb)2Ru2“(Mebpy-amH"')]3+ is

efficiently quenched by DMABCOO‘. That the bimolecular rate for the reaction of the
bpy complex, (23a), is greater than that of the decb complex, (23c), is sensible in light of
the greater driving force of the bimolecular electron transfer reaction of the former

(AAG = 0.13 V). Along these lines, the inversion of the relative ordering of the rate

constants, and their large difference (23a is ~102 less than that of its decb congener 23b),
for the salt-bridge assembled acceptor-donor pair appears peculiar. We believe that the
origins of this behavior arise from disparate MLCT energetics and consequently

electronic structure of the Ru(II) polypyridyl acceptor complex.

134

The electron transfer reaction of (23a) is established by photogenerating a hole at
the Ru(II) center by MLCT excitation. Figure (34) shows the relative energy levels of the
different ligand systems as established from the spectral fits of the emission profiles of

the homoleptic complexes and data from the literature. The energy ordering decb

~ Mebpy-amH+ << bpy follows a simple trend where the energy of the MLCT excited
state increases as the electron withdrawing ability of the substituents on the polypyridyl

rings decreases. In the case of (23a), simple Boltzmann considerations suggest that _
MLCT excitation places an electron on the Mebpy—amH+ and thus the electron transfer

pathway contains a photoreduced Mebpy ligand. For (23c), the decb and Mebpy-amH+
MLCT excited states are nearly equi-energetic and therefore the photoexcited electron is
partly removed from the electron transfer pathway. In an electronic sense, the transferring
electron traverses a less obstructed pathway from DMAB to the photogenerated hole on
the metal. Similar observations ct for the oxidative quenching reaction of amidinium- and
carboxylate-modified Ru(II) polypyridyls by dinitrobenzene were shown in Chapter 4,
but the trend is opposite to that observed here. In this case, the photoexcited electron
moves away from the metal to an acceptor and the electron transfer pathway is properly
established with MLCT of an electron onto the Mebpy ligand to which the salt bridge is
attached. This situation is achieved by tetramethylating the ancillary bpy ligand, which
raises its MLCT excited state energy well above that of the Mebpy ligand. The
photoinduced electron transfer event is cleanly established by MLCT excitation to the

Mebpy ligand, from where the electron can smoothly advance to the dinitrobenzoic acid

135

acceptor. Conversely, the photoexcited electron is removed from the electron transfer
pathway when the ancillary bpy is unmodified (owing to the lower energy of the bpy
MLCT excited state) and electron transfer proceeds from the remote ancillary ligand.
Similar to the magnitude of the effect described here for (23a) and (23c), the rate of the
properly established electron transfer for the tmbpy complex is 40 times greater than that
observed for the more convoluted electron transfer pathway of the bpy complex.

The electronics of (23b) and (23d) are straightforward with regard to

photoinitiating the electron transfer reaction. The energetics for the

d7: —) 7t* (polypyridine) MLCT excited state are decb < bpy < Mebpy-COO”.
Photoexcitation of (23a) and (23c) cleanly establishes a hole at the metal ion without any
contribution of a photoreduced ligand along the electron transfer pathway. Nevertheless,
electron transfer is not observed for either salt-bridge complex (23b) or (23d). Moreover,
the corresponding bimolecular electron transfer reaction of (23b) is absent and that of
(23d) is negligible. Inspection of the free energies listed in Table x reveal that the
electron transfer reactions for these systems are endergonic.

The reactivity difference between the Ru(H) polypyridyl amidinium systems (23a
and 23c) and carboxylate congeners (23b and 23d) is primarily a result in a shift of the

DMAB donor potential upon the presence of amidinium at the para position of
dimethylaniline. DMAB-AmH+ is 0.40 V more difficult to oxidize than DMAB-COO’,

which is a slightly better reductant than DMA (E1/2 = 0.81 V vs. SCE). The increased

difficulty in oxidizing DMAB-AmH‘" is sufficiently destabilizing to drive the electron

transfer reactions of (23a) and (23c) energetically uphill. Within the 2/4 series, the further

136

II” redox

decrease in driving force of (23d23d) finds its origins in the unfavorable Ru
potential, which is largely ligand—centered in Ru(H) polypyridyl complexes. As observed
from the energy of the MLCT excited states, which typically track the Ruml reduction
potential [100], the reduction potentials of (decb) and (Mebpy-AmH‘”) are similar and
much less than (bpy) and (Mebpy-COO"). For this reason, as described above, the overall
driving forces among these systems reflect for the most part, the difference in the donor
potentials. The reduction potential of (23a), however, is expected to be centered on the

[III

bpy ligand, which is born out by the similarity of this complexes Ru half-wave

potential to that of Ru(bpy)32+ in CH3CN (E1/2(*Ru"”) = 1.32 v vs. SSCE [103]).

Consequently, the large decrease in the electron transfer driving force for (23c) and (23d)
with regard to (23a) and (23b) reflects the difference in reducing (bpy) as opposed to the
more oxidizing (decb) ligand.

The overall electron transfer kinetics of (23a) follows the emerging trend that
electron transport through salt bridges is fast for the D—(carboxylate-amidinium)-A
complexes. In this orientation, electron transfer is in the direction of the permanent dipole
of the salt bridge and the internal electric field contributes favorably to the driving force
of reaction relative to the isolated constituents (for which the redox potentials in Table 10
are measured) [16,17]. The thermodynamics may be further augmented by favorable
kinetics associated with a D—(carboxylate-amidinium)—A orientation. Charge
redistribution within the salt-bridge may be strongly coupled to the solvent dipoles [67],
thereby giving rise to Franck-Condon factors in excess of that expected for simple
electron transfer. In the case of the D—(carboxylate-amidinium)-A complex, the proton

already resides on the acceptor and hence it is likely to remain upon the arrival of the

137

electron. Franck-Condon factors arising from proton motion within the salt bridge are
therefore minimized. On the basis of the thermodynamic and kinetics considerations, we
can account for the differences in the oxidative and reductive quenching pathways. For
the oxidative quenching reaction, electron transfer should be fast out of the electronically
excited Ru(H) complex. Conversely, fast electron transfer into the metal center should be
observed for a reductive quenching pathway. In both cases, the orientation of the salt
bridge with regard to the direction of electron transfer is the same and favorable with
regard to electron transfer. Unfortunately, as was provided in Chapter 4 by a comparative
kinetics studies of switched interface systems for the oxidative quenching pathway, a
more explicit analysis of the effect of the salt bridge on electron transfer pathway is

obviated by the large difference in the electron transfer driving forces of (23c) and (23d).

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APPENDIX A

APPENDIX A

DETERMINATION OF THE ACID-BASE PROPERTIES OF A SERIES OF
ELECTRON TRANSFER DONOR-ACEPTOR PAIRS: ASSESSMENT OF THE

pKa* AND ApKa IN AQUEOUS SOLUTION

Introduction

Excited state proton transfer reactions of terminal oxo complexes Of Re(I),
Mn(IV), and Ru(H) have been investigated extensively [57,117]. Still, other proton
transfer reactions utilizing the excited states of functionally substituted bipyridyls Of tris-
bipyridyl ruthenium complexes are used in the development Of effective solar energy
conversion systems [118] and proton transfer reactions have been implicated as playing a
role in investigations into the development Of photovoltaics [119].

The study of inorganic complexes for solar energy storage applications have
provided many examples based upon the tris—bipyridyl ruthenium class Of complexes
[120] The coordination Of bipyridyl ligand to metal atoms make the complexes

particularly suitable for pKa studies. The addition Of protonatable substituents at the

149

150

periphery Of the ligands is synthetically straight-forward and the compounds are easily
synthesized. The excited state properties Of these complexes have many desireable
characteristics, the lifetimes are long compared to the time constant Of the proton transfer
reaction under study, the luminescence Of the excited state is strong and in most cases
very dependant upon the degree Of protonation.

Protonation/deprotonation investigations utilizing bipyridyl ruthenium complexes
have been investigated for more than 25 years [121]. The first example Of excited state
protonation/deprotonation processes in which both protonated and deprotonated forms of
[(bpy)2Ru2+(bpy-(COOH)2][(PF6)] in a buffered aqueous solvent was studied by
Wrighton and coworkers [122]. These studies provided a preliminary insight into the
photophysical properties exhibited under changing pH conditions. Since that time the
development of Ru(H) complexes utilizing a wide range of protonatable bipyridyl
[57,123] and imidazolyl and benzimidazolyl [124] ligands have been reported. Excited
state properties Of Ru(bpy)32+ complexes and synthetic variations, allow convenient
control Of the energy gap (AE between the emissive 3MLCI‘ state and the deactivating
MC state) [67] allowing the design of complex having desireable photophysical

characteristics. The depth Of the studies Of Ru(bpy)32+

complexes has allowed their
incorporation into both electrochemical and photophysical sensors for the determination
Of pH in solution [125]. Design of an O2(g) sensor based on the
protonation/deprotonation properties of Ru(H)polypyridyl complexes has also been
reported [126]. Furthermore, these complexes have been implemented as luminescent

tags [127] for the study of protein folding/denaturing in aqueous solution as a function of

solution pH. More fundamental approaches, have been concerned with implementation of

151

protonation/deprotonation processes in the activation Of substrates and energy conversion
schemes [120,128]. Work oriented toward the investigation of protonation/deprotonation
processes in the tuning of redox potentials in electron transfer schemes have aslo
appeared [129].

The work presented in this Appendix is comprised Of the examination of acid—
base properties of these two sets of Ru(II)polypyridyl complexes designed for the study
Of excited state photoinduced electron transfer reactions; the data is supplemented with
that collected the very capable summer undergraduate research student, Sam Wall. In
Chapters 3 and 4 the salt-bridge complexes, Figure 33, presented in non-aqueous aprotic
organic solvents, CH3CN and CH2C12 have been used in protonation/deprotonation
investigations in buffered aqueous solvent. The tendency of a proton tO change position
within the salt-bridge interface in response to a change in the electronic composition of
reactants initiated by the absorption Of a photon in electron transfer reactions is an issue
of the acidity/basicity of each component of the salt-bridge complex. TO this end
synthesis of the ruthenium compounds Of Figure 33, having different energetics for the
ancillary ligands bpy (2,2'-bipyridine) and tmb (4,4' 5,5' tetramethyl-2,2'-bipyridine) were
synthesized as a necessary components of the electron transfer investigations (see
Chapter 4). The remaining bipyridine ligand, Me-bpyCOOH or Me-bpyAmH+ (Me-
bpyCOOH = 4'-methyl-4'-carboxy-2,2'—bipyridine and Me-bpyAmH+ = 4-methyl-4'-
amidinium-2,2'—bipyridine) serve as one half Of the salt-bridge electron transfer complex.
The investigation Of these complexes to gain insight into the propensity of the proton Of

the salt-bridge interface to change position in response to a change in the electronic

152

(x-bpylafitJ"

 

(e).(d)

Figure 33. Ru(II)polypyridyl complexes used in the aqueous acid-base property
determinations. (a),(b) [(x-bpy)2Ru2“(Me-bpy)COOH]2+ {(a) x = H and (b) x = (CH3)4,
and (c),(d) [(x—bpy)2Ru2"(Me-bpy)AmH-1-]+ {(c) x = H and (d) x = (cram.

153

composition Of reactants resulting from the absorption of a photon by the donor, or as a
result Of the electron transfer reaction in the formation of the charge separated state
within the bound complex, and assess this information with regard
to the electron transfer reactions Of the non-aqueous aprotic solvent systems is the

primary aim Of this Appendix.

Results
Absorption / Emission Ground and Excited State Acid-Base Reactions of the
Ruthenium Complexes

In Figure 34. is shown the absorption titration for each of the carboxylic acid
derivatized ruthenium complexes (33a) and (33b). The insets show the change in the
absorption at wavelengths in the respective MLCT region (hm, = 457 and 440 nm) and
(1mm = 287 and 290 nm) for the intraligand 7c—7t* region. Observation of Figure 34 shows
that for complex (33a) there is evidence for two isosbestic points throughout the pH
titration at the red and blue edges of the MLCI‘ region located at 400 and 467 nm, while
in the bipyridyl 7t—7t* region there is evidence for two additional distinct isosbestic points
at 274 and 297 nm. The inset titration associated with each plot shows one inflection
point located at a pH of aproximately 2.0. For the tetramethylated complex (33b) the
more clear evidence of two clean isosbestic points located at each side of the MLCT band
located at 418 and 477 nm between the pH range 1.85 and 12 are Observed. However, at
pH below 1.85 the isosbestic point shifts approximately 2nm to the red and drops slightly

in intensity. While it is unlikely that this change in the isosbestic point below pH Of 1.85

154

 

 

 

 

 

 

 

 

 

 

 

 

 

0.2
0.80
0.15
(D
‘5?
e - 0.1
g 0.40
2
0.05
0.00 o
260 400 500 600

 

Wavelength

Figure 34. Absorption pH titration of [(bpy)2Ru2+(Me-bpy)COOH][(PF6)2]. The spectrum
for the intraligand 7t—7t* region is shown at the left and the spectrum in the d1t—7t*MLCT
region at the right. The inset of each Of the plots shows the change of the absorption ( A»:
286 and 456 nm) over the pH range Of the experiment.

155

 

0.60 0.12

 

 

0.40 0.08

 

 

 

 

 

 

Absorbance

0.20 0.04

 

 

 

 

0.00
260

 

 

Wavelength

Figure 35. Absorption pH titration of [(tmb)2Ru2*(Me-bpy)COOH][(PF6)]2. The
spectrum for the intraligand 7t—1t* region is shown at the left and the spectrum Of the
d7t—7t* region is shown at the right. The inset of each Of the plots shows the absorption (2.
= 290 and 447 nm) over the pH range investigated.

156

indicates a second protonation step as has been found in ruthenium complexes having
two independent titratable carboxylic acid residues [106] the reversibility leads to the
interpretation of the data as due to solution/environmentaleffects. With reference to
Figures 34 and 35 it is seen that for the titration range examined for each Of the
ruthenium complexes (33a) and (33b) there is a decreased intensity and broadening of the
MLCT band with dereasing pH from 12 to 2.0. In complex (33a) the broadening is less
substantial than is Observed for the tetramethylated complex (33b) and little or no
evidence Of the splitting of the MLCT band as can be readily Observed for complex (33b)
showing evidence Of two disernable peak maxima located at 431 and 482 nm.

In Figures 36 and 37 is shown the absorption titration for each of the amidinium
derivatized ruthenium complexes (33c) and (33d). The inset for each region Of the spectra
both bipyridyl and MLCT show the change in the absorption at wavelengths 457 nm and
440 nm Of the MLCT transition for each Of these ruthenium complexes. For complex
(33c) one clean isosbestic point located at 470 run between the pH range 5 and 12 while
at pH below 5 to the lowest pH in the range studied 1.8 a new isosbestic point is Observed
shifted approximately 3 nm to the red. For complex (33d) the same behavior is Observed,
as found for complex (33c) except that for this complex the isosbestic point is located at
479 nm and occurs for the pH range 9.8 to 13.5. At pH below 9.8 through to pH 1.03 the
new isosbestic point shifted ~ 2 nm to the red is Observed. For each Of these complexes
the inset absorption change plots mirror the changes in absorption Observed throughout
the pH range examined. Once again, the shifting isosbestic points are indicative of the

establishment of a new equilibria yet for these complexes there are no additional

157

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1.20 0.3
a) 0.80 - 0.2
o
5
g 1'2
a)
9 0.40 - 0.1
000 I I L I I I 1 n o

 

 

260 300 340 380 400 500 600

Wavelength

Figure 36.Absorption pH titration Of [(bpy)2Ru2+(Me-bpy)AmH+][(PF5)]2. The spectrum
for the intraligand 7t—7t* region is shown at the left and the spectrum Of the d7t—1t* region
is shown at the right. The inset of each of the plots shows the absorption (9. =286 and 457
nm) over the pH range investigated.

158

 

 

 

 

0'80 0.12

 

 

 

 

 

p
4:.
O

 

Absorbance

 

 

 

 

 

0.00
260 300 340 380 400 500 600

 

 

Wavelength

Figure 37. Absorption pH titration of [(tmb)2Ru2+(Me-bpy)AmH+][(PF5)]3. The spectrum
for the intraligand 7t—7t* region is shown at the left and the spectrum of the dtt-tt* region
is shown at the right. The inset of each of the plots shows the absorption (7. =289 and 440
nm) over the pH range investigated.

159

dissociable substituents. Alternatively, the changes in the isosbestic points may indicate
decomposition, although the changes are reversible.

Further Observation Of the data presented in Figure 36 and 37 reveals a
pronounced broadening, and most particularly for the tetramethylated Ru(II) complex, a
distinct splitting of the absorption in the MLCT region at low pH. The splitting gives rise
to two distinct maxima located at 427 and 491 nm of the MLCT band. Upon increasing
the pH the MLCI' region shows a coalescence of the absorption band and a shifting of the
red edge absorption maxima to higher energy with little or no variation at the higher
energy blue edge Of the absorption.

In Figures 38 and 39 are shown the emission data and titration profiles Of the acid
base reactions for the complexes (33a) and (33b). Observation of the emission data
presented in Figures 38 and 39, the emission spectra are seen to be broad structureless
gaussian shaped profiles typical Of Ru(bpy)32+ compounds. Insets for each of the figures
show the titration profiles as a function Of pH over the range of pH 2-10 for (33a) and pH
2-12 for (33b). For these two complexes the position of the emission maxima throughout
the titration are found to have Opposite trends in regard to the direction Of the shift
relative to the addition Of base. For complex (33a) the protonated emission maximum is
found to be located at ~ 650 nm wheras, the deprotonated emission maximum is located
at ~ 630 nm. For complex (33b) the protonated emission maximum position is similar tO
the protonated form Of (33a) at 650 nm, yet the deprotonated emission maximum is
located approximately 10 nm to the red at 660nm. For each of the complexes the same

trend is Observed for the change in the emission intensity. For complex (33b) the relative

160

0.30

 

 

0.20

 

 

 

0.10

 

 

 

 

Wavelength

Figure 38. Emission pH titration for [(bpy)2Ru2+(Me-bpy)COOH][PF5]2. The inset Of the
plot indicates the changes of the emission intensity at 630 nm over the pH range
investigated.

161

 

 

(140

 

 

 

(120

 

 

 

 

550 650 750 850

Figure 39. Emission pH titration for [(tmb)2Ru2+(Me-bpy)COOH][PF6]2. The inset Of the
plot indicates the changes Of the emission intensity at 630 nm over the pH range
investigated.

162

change in the emission intensity is 10 times more intense at pH of 12 than at pH 2. The
10 nm change of the emission maximum represents a change Of approximately 0.028 V
or 226 cm" in the energy Of the emissive excited state. That the change is so small is
somewhat suprising considering the change in both the extent Of delocalization/electronic
coupling of the ligand orbitals with the metal centered orbitals throughout the
protonation/deprotonation process [130]. Furthermore, the direction Of the shift is
somewhat contrary to what is expected considering that the deprotonated complex
carboxylic acid ligand on becoming a carboxylate is electronically similar to a methyl
group, rendering the ligand energetically near to that for a tetramethyl. The inset titration
profile for complex (33b) shows data plotted for the emission maxima Of both the fully
deprotonated complex ~ 670 nm. In this plot Of the data indicates a single inflection at pH
4.0-4.5. For complex (33a) there is a more distinct change in the position Of the emission
maxima. The fully protonated complex at pH 2 has maxima located at 670 nm while for
the fully deprotonated complex pH 12 the maxima is located at 630 nm. Throughout this
pH range the concomitant change in the intensity of the emission is relativly 10 times
more intense at pH 10 than at pH 2. With the change in the emission maxima Of 40 nm
the change in the energy Of the emissive excited state is 0.118 V (948 cm"). This change
in the energy/wavelength Of the emission maxima Of the excited state concomittant with
the protonation/deprotonation process energetically, is consistant with expectation. The
loss of a proton at the carboxylic. acid Of the salt-bridge derivatized ligand results in a
dramatic change in the energetics Of the complex. Initially the energy of the carboxylic

acid functionalized bipyridine ligand lies slightly below that of the 2,2'—bipyridine

163

ancillary ligands. Yet upon deprotonation the ligand becomes significantly decreased in 71:
accepting ability relative to the ancillary ligands resulting in a reversal Of the directon of
the charge transfer transition, and the Observed emission represents emission
predominantly from a charge transfer state localized at the ancillary bipyridyl ligands.
The resulting complex resembles energetically the tris-bipyridine complex and not
suprisingly the emission maximum is nearly the same as for Ru(bpy)32+ complex in H2O
solution. The inset Of Figure 38 for complex (33a) shows the titration profile through the
pH range 2-10. The presentation of the data in the titration profile for the emission
maximum of the fully protonated and fully deprotonated complex (data not shown) show
similar behavior throughout the titration with inflection point located at pH 4.5 for both

plots.

For complex (33c) the data (data not shown) show that at pH below 5 the
emission maximum is located at 690 nm while for pH above 7 the emission maximum is
located at 670 nm The relative change in the emission intensity through the titration is 4
times greater at pH 12 than at pH below 5. The change in the emission maximum
represents a change Of 0.054V (432 cm"). Furthermore the data for complex (33d)
indicate that there is a substantial change in the emission intensity between pH 9 and pH
12 and a plot of the data shows that the inflection point for the titration also lies in this
region at pH Of ~11. The data for complex (33d) indicates that for the titration through
pH 2 to pH 12 similar behavior to that for complex (33c) is observed. The emission
maximum shifts to the red throughout the titration beginning at 615 for pH 2 and
eventually ending at 665 for pH 12. Throughout the titration it can also be seen that the

emission intensity changes by a factor Of 5; being greater for the fully deprotonated

164

complex than for the fully protonated complex. The change of the emission maximum
represents a change of 0.152 V (1223 cm") difference in the energy Of the excited state
emission. For this complex there is a dramatic change in the emission intensity between
pH 8 and pH 12 and a plot of the titration profile shows the inflection point for the

titration occurs in this region at pH 10.0.

Ground State Acid Base Properties of the Free Ligands

For the Ruthenium complexes (33a), (33b), (33c) and (33d) the there are four
different free ligands Of each complex 2,2'-bipyridine, (4,4' 5,5' tetramethyl) 2,2'
bipyridine, (4-methyl-4'carboxylic acid) 2,2' bipyridine, and (4-methyl-4'amidinium) 2,2'
bipyridine. The acid-base properties of the free ligands provide a direct measure Of the

interplay of the electronic 6 donor strength and 7t bonding properties affecting the

relative energy of the d7t—7t* MLCT transitions. The pKa values for a large variety of
coordinating bipyridine ligands have been determined [123j]. Additionally the pKa of the
(4-methyl-4'carboxylic acid)-2,2' bipyridine has also been previously determined to be
2.2 for the carboxylic acid substituent, with very little change of the two coordinating
nitrogen atom pKa values from the 2,2' bipyridine ligand having values Of 0.52 and 4.45
[131]. Assessment fo the pKa Of the free ligand 4-methyl,4'amidinium-2,2'-bipyridine has
not been determined with sufficient accuracy to merit a clear estimate of the pKa,

however absorption titration experiments place the pKa between 6-8.5.

165

Excited State Acid-Base Properties of the Ruthenium Complexes; Lifetime
Measurements

The detectable emission Of the four ruthenium complexes (33a), (33b), (33c) and

(33d) is a benefit that is not universal for ruthenium bipyridyl complexes. This additional

piece of information is enormously useful for the determination of the excited state pKa

(pKa*) for each of the complexes. Typically the excited state (pKa*) for many molecules

may be approximated from emission as a function Of pH data using the thermodynamic

Fbrster cycle [132], a knowledge Of the energy Of E0,o and the changes of the absorption

(0.625)

A1
T(VB " VH3) ( )

 

pKa* = pKa +

and emission maxima resultant from the protonation/deprotonation process. This relation

has been particularly useful for the determination of the pKa* values of many organic
compounds. But the spin forbidden charge transfer emission 3MLCT -> IA] 6.5. for
ruthenium bipyridyl complexes makes the use Of this approximation tenuous as the
rigorous use of the FOrster cycle relationship requires changes shown by the Stokes shift

of the emission relative to the absorption having the same spin multiplicity. Herein lies
the problem, the absorption spectra of Ru(II)polypyridyl complexes with bipyridine
ligands is dominated by the singlet transitions of the MLCT while the emission is
predominantly triplet in character, making the approximation Of the E0,0 for the triplet
excited state very uncertain. Thus additional means are necessary for the determination of

the pKa*. One such method incorporating corrections for the lifetime of both the

 

166

protonated and deprotonated complexes may be derived from these relationships based
on the ground state absorption and the excited state emission. To determine pKa* more
rigorously the changes in the ground and excited state equilibria Of the acid-base

reactions need to be entered into the equation. The form of the

pKa* = pKa, 409313- (A2)

Te
expression is closely related to the Henderson-Hasselbach equation for volumetric acid-
base titrations. In this expression THE is the lifetime Of the protonated complex, 13 is that
for the deprotonated complex and pKi is the inflection point of the luminescence titration
profile. The primary drawback for the application Of this equation stems from the
necessity Of observable liminescence having a lifetime in non-aqueous aprotic solvent
longer than the diffusional rate for proton transfer, as typical diffusional rates in solvents
such as CH3CN are approximately 2 x 1010 M'ls" and substantially less in many other
solvents. In H2O solvent this drawback is far less severe as it has been determined that
the diffusional rate Of the proton is of the order of >1011 M'1 s'1 and has been suggested
as being as great as 1013 M"s" in the Grotthuis mechanism [133]. For the Ruthenium
complexes (33a,33b) and (33c,33d) there is a significant reduction Of the emission
intensity and quantum yield concomittant with the reduction Of the pH (see above),
however, even in the lowest pH solution 0.54 for [(tmb)2Ru2“(Me-bpy)COOH]2+
observable luminescence and measureable lifetimes are found I", > 20 ns (kn. > 5 x 107
s"). The data of all four complexes is summarized and plotted in Figures 40 and 41.

Inspection of the lifetime profiles of complexes (33a) and (33b) in Figure 40 show the

167

 

lifetime / s

 

 

 

 

Figure 40. pH dependence of lifetime measurements. Top trace ( O )is for the complex
[(bpy)2Ru2+(Me-bpy)COOH][PF6]2, bottom trace ( I )is for the complex [(tmb)2Ru2*(Me-

bPY)C00Hl[PF6]2.

 

168

 

6 10'7 I

410-7

 

lifetime / s

2 10"7

 

 

 

 

Figure 41. pH dependence of lifetime measurements. Top trace ( O )is for the complex
[(bpy)2Ru2+(Me-bpy)AmH+][PF5]3, bottom trace ( I )is for the complex [(tmb)2Ru2+(Me—

bpy)AmH+][PF6]3.

169

pH vs. lifetime titration data collected for each Of the complexes at the emission maxima.
For complex (33a) it is seen that the protonated complex lifetime is 125 ns (kg,. = 8.0 x
106 s") and that for the deprotonated complex 560 ns ( log, = 1.79 x 106 s") yielding a Kal
Of 4.47 with an apparent inflection point at pH = 5.2. Application Of equation A2 to the
emission titration data and the lifetime data yields an assessment Of the excited state pKa
Of 4.4. For complex (33b) the lifetime titration profile shows the protonated lifetime to be
50 ns (kH, =2.0 x 107 s") and that for the deprotonated complex to be approximately 250
ns (kg+ = 4 x 10‘5 s") yielding a K, of 5.0 with an apparent inflection point at pH = 4.8.
Application of equation A2 tO these emission and lifetime data yield the assessment Of
the excited state pKa = 4.6.

For the complexes (33c) and (33d) the lifetime data is less well behaved in that
for each Of the complexes there are two apparent steps in the protonation sequence as the
lifetime profiles show two distinct breaks. For complex (33c) the protonated lifetime is
found to be approximately 45 ns (kg+ = 2.22 x 107 s") the lifetime at the intermediate step
is found to be 275 ns (k = 3.64 x 106 s") and the lifetime Of the complex at pH between
10.5 and 12 is 575 ns ( k = 1.74 x 106 s"). The K, for the first and second processes is 6.1
with an apparent inflection point a pH 7.8 and that for the second and third processes is
2.1 with an inflection point at pH 10.5. The K,, for the sum Of the two processes is thus
8.2. For complex (33d) the protonated lifetime is found to be 25ns ( k“, = 4.0 x 107 s")
that for the intermediate step is 120 ns ( k = 8.33 x 106 s"). For the final step a limiting
value is not reached , however, using the average Of the last two points of the titration a

value for the lifetime is found tO be 225 ns ( k = 4.44 x 10‘5 s"). These data then yield

170

values Of 4.8 for the first Ka and 1.9 for the Ka of the last step giving K,, for the overall
process Of 6.7. The inflection points for these two processes are located at pH 7.2 and

11.0.

Ground State Acid-Base Properties of Salt-Bridge Functionalized 3,5-
dinitrobenzene Electron Transfer Quenchers

The acid-base properties Of each of the electron transfer quenchers determined by
standard volumetric titration procedures [134] were found to be 2910.2 and 8.5i0.3.
The value for the acid functionalized complex is consistant with the pKa for similar
compounds: p-nitrobenzoic acid pKa = 3.4 and benzoic acid pKa = 4.2. However, the
pKa value found for the amidinium functionalized compound is substantially less than
expectation. The pKa reported for benzamidinium as the chloride salt is 11.6 [135].
Addition of the nitro groups would be expected to shift the pKa to lower values but
assessment of the pKa utilizing linear free energy relationships for both inductive and
resonance effects [136] which provide assessment Of pKa values for a great variety of
organic complexes in very good agreement with measured values, yield a value Of ~ 10;
substantially higher than this experimental value. It is possible that the influence Of the
tetraphenyl borate counter ion lends some charge transfer character to the complex
possibly lowering the observed pKa but no assessment can be made to further rationalize

this result.

171

Discussion

The utility of the pKa data derived from the measurement of the photophysical
properties of these complexes allows an assessment Of the relative tendency Of the proton
tO transfer in response tO altered acidity-basicity of the electron transfer donors and
acceptors due to photoexcitation Of the ruthenium chromophore, but is limited as a means
to a simple determination Of the intrinsic barrier to proton transfer in non-aqueaou
solvent. The limitations Of the measurements of proton transfer rate constants for each Of
these ruthenium donors (33a,33b) and (33c,33d) stems from the equilibrium established
in the excited state. Since both protonated and deprotonated forms of the complexes are
emissive, and furthermore that the lifetime Of the protonated form does not satisfy the
condition lltHB >> ILH+* the deactivation of the excited state protonated complex tO

ground state protonated complex is in competition with back reaction to give the

 

 

 

 

 

 

2 k H”: 2
Ru ”B" + H+ = h Ru +B-H*
11 k -H+* 1r
11V 1 ITB ITV '1 lTHB
I k H+ y
Ru2+B + H+ -= '= Ru2+B-H
k -H+

172

deprotonated complex according to the scheme shown in Figure A1.The magnitudes Of
the KA* for each Of the ruthenium complexes of ~5 is found for three Of the four
complexes complexes throughout the pH titration range 2-12 the only exception being
that Observed for complex (33c) showing an apparent value of 6.1. In the limit that the
excited state equilibrium is fast with respect to the escited state deactivation the above

expression for the determination of the excited state pKa* should be modified to account

+
[HE ] : THBQCB " Tobsd) (A3)
1131 18(Tobsd - tr113)

 

pKa-tr = pKai _ log 1H3 (1:3 — tobsd) (A4)
t151(Tobsd - 11118)

for the excited state concentration distribution according to equations A3 and A4. That
the lifetimes measured in these experiments show only monoexponential decay over the
modest range Of wavelengths surrounding the emission maxima Observed, is an indication
that the above assessment is correct and that the fast equilibrium interconverting the
protonated and deprotonated forms gives rise to emission contributions from each
species. The addition of large excess Of acid or base through the pH titration influences
the Observed emission and lifetime, driving the equilibrium further toward one side or the
other and the lifetime and / or emission of the authentic fully protonated or deprotonated

complex is not obtained.

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