. . -4! ,
:11 1.. 1. .
31-...sfibnoxxju . b
l ..u....d.$r.c. ...I
... I-‘

..
N
is?

Us. .
‘ Ill
Ix: {I J: V

. S"

v.
_ on!

 

9...--. .
(.513? . . .
, . , ,
‘ ‘ . . Kl}

. . ql .lAv
57.: l
:OUufl. . 1...! H.
AIL

z: .
.Duhwbyl'fi. HM...

. A... .
.sl. .Xshn 3N3.

4r.'
M!-

.‘\r‘
.a .0.’.

7a

_‘ 3,.
i! 2

Z
1

33:3
.lnit‘r.

. . H...
Skull:
, It.

'd
Mu

V. ‘1‘!
.(u‘IvhflovJ
In? In! 5‘:
‘0! o.wluvtfli...~.

‘vtd‘vhflMml
{.3 .ul‘v-

 

uh {darn
\fibfi‘du» .wmv...\o. tan;

*h".uv.5.tmfitflwn~.m.fiwulv .e 3 4.5.5.0} ..
H: b I , ”.51.. ‘ Nahum? . in. .9?!

. 2. ‘1‘qung . . .II I WWWVH

'3'.
an

I
v
o
I
..I

.H. .0! :1
. .9‘ n. . 1--. .. ‘
K nub“ ! 9 .t‘ I. l I.’ 0.»
19.133‘. -Wvli . thlhiiurfio‘adv.‘ . 3.. ({ qul

1‘
.
‘.
v
.,

      

‘ \ ‘
:2
'in

   
 

 

 

 

‘ ’0..=1u.\. r.» .11. I: 1: ... ,l i.._.... 5.... , .Cr .: r..7...

.v. I
(‘1.‘..{ J...

l ‘ - bin-II , . . s .. ._. . . ..
X 16-min“. Iv.= < ".1 y .‘ u q u ”pr! LEE-n... urn-p:

.V\
, :v-li I! n u

v --.vlv

 

 

 

 

2 mulllii‘mmuJnumnwmnmum

23 01563 9168

 

 

 

 

 

 

 

 

This is to certify that the

dissertation entitled

Luminescent Cyclodextrin Supramolecular Recognition

Sites for Optical Chemosensing
presented by

Wanda K. Hartmann

has been accepted towards fulfillment
of the requirements for

Ph.D. degree in Chemistryfi

 

 

home;

Major professor U

Date IL? 3-5:; 1%,]

MS U i: an Affirmatiw Action/Eq ual Opportunity Institution 0- ‘l 2771

 

 

 

 

 

 

 

 

 

 

LIBRARY

Michigan State
University

 

 

 

PLACE ll RETURN BOX to romovo this chookout from your rooord.
TO AVOID FINES rotum on or More duo duo.

DATE DUE DATE DUE DATE DUE

IL;— :I

 

 

 

 

 

 

 

 

 

 

 

 

 

 

_. :l—
feel

MSU to An Affirmative Adlai/Equal Opportunlty Inotltwon

 

 

 

 

 

LUMINESCENT CYCLODEXTRIN SUPRAMOLCULAR RECOGNITION SITES
FOR OPTICAL CHEMOSENSING

By

Wanda K. Hartmann

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

LUMINESCENT CYCLODEXTRIN SUPRAMOLCULAR RECOGNITION SITES
FOR OPTICAL CHEMOSEN SING

By

Wanda K. Hartmann

Novel physical and chemical processes can be studied with supramolecular
assemblies. Molecular recognition allows one to design an active site in a supramolecular
assembly to target an analyte for chemical sensing applications. Supramolecular
assemblies can be designed for chemical sensing that ranges from monitoring of
environmental contaminants such as benzene to medically relevant analytes such as
glucose, pH, and blood gases. One class of molecule that is useful in designing a
supramolecular assembly is a cyclodextrin (CD); CDs have a hydrophobic cavity that
binds molecules that are insoluble in water. We have used the hydrophobic pocket to bind
chromophores and lumophores that are used for triggering a luminescent optical signal in
the presence of analytes. One supramolecule designed in our laboratory triggers
' phosphorescence of l-bromonaphthalene (l-Ber) upon formation of a ternary complex
with hydrogen-bonding substrates (alcohols) and cyclodextrin (Chapters 2-4). A second
active site that is suitable for an optical sensor for sensing of aromatic hydrocarbons
(benzene, toluene, xylene, biphenyl, naphthalene) is also discussed in detail (Chapters 5

and 6).

To my brother and parents.

iii

ACKNOWLEDGMENTS

My two advisors, Professor Dan Nocera and Professor George Leroi, have helped
me to think and communicate scientifically. I am a more confident scientist as a result of
their guidance. They also impressed upon me the value of communication skills in the
modern scientific world that surrounds us. I would also like to thank my committee
members Professor Ned Jackson and Professor Marcos Dantus. Thanks as well to my
dissertation proofreaders: Jude Rademacher, James Roberts and my father.

I thank Tom Carter, who was always willing to help me and very patient. His
technical expertise with lasers and computers is very thorough. Our department is very
fortunate to have him. I would also like to thank Professors Blanchard, Cuckier and
Wagner. Professor Gary Blanchard gave me endless advice on time—correlated single
photon counting and other laser experiments. Professors Cuckier and Wagner also spent
several hours discussing my research project with me. Finally, I extend a personal thanks
to Professor Leslie Kuhn in Biochemistry, who has supported me throughout the past
years as a graduate student and pottery artist.

The staff in the Chemistry Department have been extremely helpful to me. I
would especially like to thank everyone in the Electronics shop, the Machine shop, Glass
shop and the Business office. I would also like to thank Ann Kirchmeier, Phil Krausshar
and Ruby Ghosh in the Center for Sensor Materials. I would like to extend a million

thanks to Linda Krause for being very efficient, helpful and always cheerful.

iv

The Nocera and Leroi group, Adrian, Al Barney, Ann, Claudia, Dan, Dimitris,
Eric, Janice, Jean, Jeff, Jerry, JP, Jude, Mark Mortellaro, Mark Torgerson, Niels, Sara,
Tom, Zoe and, most of all, Jim have been a very supportive research group. First and
foremost I would like to thank Adrian Ponce, who taught me the tools of the trade. Mark
and Jude were also especially helpful in teaching me organic chemistry and cyclodextrin
synthesis. I am especially indebted to them. Thanks to my other coworkers Professor
Peter Wong (Andrews University), Jeong-Uk Kim, and undergraduates, Mike Gray and
John Flenner. Last and most importantly, Jim has helped me through the most difficult
times in graduate school and helped me to believe in myself.

I would like to give a personal thanks to my cousin Theresa, my godmother Maria
Weiner and all of my aunts (especially Cathy, Margie, Marie and Betty) and especially
my uncles Richard and Larry. I would like to thank my friends Lisa, Natsuko, Marie,
Melissa and Tara and all the friends I made at M.S.U. My father and brother have always
encouraged me to do my best. I am fortunate to have a wonderful family.

Lastly, I would like to acknowledge all those family members and friends who
have passed away, especially Chris Serafin and my mother, both of whom died at the
very young age of 35. I will always remember how hard my mother fought in her battle

against lung cancer and the courage that she inspired in me.

TABLE OF CONTENTS

LIST OF FIGURES ........................................................................................................ VIII
LIST OF TABLES ........................................................................................................... XII
CHAPTER 1
INTRODUCTION ............................................................................................................... l
OPTICAL SENSING: TURN ON vs. QUENCHING DETECTION OF LIGHT FROM EXCITED
STATES ............................................................................................................................. 4
INTERSYSTEM CROSSING OF EXCITED STATES .................................................................. 8
SUPRAMOLECULAR ASSEMBLIES .................................................................................... 1 1
BUILDING A SUPRAMOLECULE FROM A CYCLODEXTRIN BUCKET ................................... 16
CHAPTER 2
OPTICAL CHEMOSEN SING OF ALCOHOLS BY AN ABSORPTION
INTERSYSTEM CROSSING EMISSION PROCESS .................................................... 24
INTRODUCTION ............................................................................................................... 24
EXPERIMENTAL ............................................................................................................... 31
Materials ............................................................................................................................................... 3l
Electronic Absorption Spectroscopy and Binding Constants ............................................................... 32
Steady State Luminescence Spectroscopy, Quantum Yield and Titrations .......................................... 34
Stern-Volmer Quenching ...................................................................................................................... 36
Nanosecond Time-Resolved Spectroscopy .......................................................................................... 37
Molecular Modeling ............................................................................................................................. 38
RESULTS ......................................................................................................................... 38
Quantum Yields and Binding Constants ............................................................................................... 38
Stern-Volmer Quenching ...................................................................................................................... 47
Other Hydrogen-Bonded Substrates ..................................................................................................... 48
Molecular Modeling ............................................................................................................................. 52
CONCLUSIONS ................................................................................................................. 55
CHAPTER 3
REGIOISOMERIC EFFECTS ON THE EXCITED STATE PROCESSES OF A
CYCLODEXTRIN MODIFIED WITH A LUMOPHORE .............................................. 58
INTRODUCTION ............................................................................................................... 58
EXPERIMENTAL ............................................................................................................... 59
RESULTS ......................................................................................................................... 59
CONCLUSIONS ................................................................................................................. 66

vi

CHAPTER 4
ELECTRONIC STRUCTURE EFFECTS OF BROMONAPHTHALENE-CD

COVALENT AND TERNARY COMPLEXES ............................................................... 67
INTRODUCTION ............................................................................................................... 67
EXPERIMENTAL ............................................................................................................... 68
RESULTS ......................................................................................................................... 68
MOLECULAR MODELING ................................................................................................. 75
CONCLUSIONS ................................................................................................................. 82

CHAPTER 5

ENERGY TRANSFER THROUGH COVALENT BONDS: .......................................... 85

LANT HAN IDE CHELATES ........................................................................................... 85
INTRODUCTION ............................................................................................................... 85
FORSTER COULOMBIC THEORY ....................................................................................... 87

Dexter Exchange Theory ...................................................................................................................... 89
Lanthanide Spectroscopy and Electronic Structure .............................................................................. 92
N ON-RADIATTVE TRANSITIONS ....................................................................................... 94
EXPERIMENTAL ............................................................................................................. 101
RESULTS ....................................................................................................................... 102
CONCLUSIONS ............................................................................................................... 1 12

CHAPTER 6

PAH AND BTX OPTICAL CHEMOSENSING BY AN ABSORPTION-ENERGY

TRANSFER- EMISSION PROCESS ............................................................................. 113
INTRODUCTION ............................................................................................................. 113

Design Strategy for Chemosensing by a Triggered Luminescent Response ...................................... 116
EXPERIMENTAL ............................................................................................................. 121
Reversed Timing and TCPC Instrumental Modifications ................................................................... 124
RESULTS ....................................................................................................................... 126
Binding Constants .............................................................................................................................. 126
Titrations and Steady State Luminescence ......................................................................................... 127
Overlap Integral and Critical FOrSter Energy Transfer Distance ........................................................ 133
Time-Resolved Luminescence and Stern-Volmer Quenching ............................................................ 136
DISCUSSION .................................................................................................................. 145
SUMMARY AND FUTURE WORK .................................................................................... 150
LIST OF REFERENCES ............................................................................................. 172

vii

LIST OF FIGURES

Figure 1. Electronic state diagram. M, 'M* and 3M"‘ and denote the ground, singlet
excited state and triplet excited state of a molecule, respectively. The photochemical
product is P. .................................................................................................................. 5

Figure 2. Supramolecules: (a) carcerand with a guest molecule in the cavity shown in
gray (b) molecular cleft with guest molecule (c) molecular gondola ((1) molecular
hinge with host (6) molecular octopus (f) pagodane (Conzo) (g) scorpiand with guest
(h) molecular tweezer with guest. .............................................................................. 12

Figure 3. Supramolecular encapsulation of a tetrahedral ammonium ion, N 3— , sodium

ion and multiple ions in cryptands (a-d). Methylamine guests in an amphiphilic
Speleand and nitrobenzene cyclointercaland in a cyclophane (e-f). ........................... 14

Figure 4. Cyclodextrins with a hydrophobic cavity, hydrophilic rims and three sizes of
cavities. Shown in the middle is the glucose unit with the carbon atoms labeled with
the standard notation. C2 and C3 are primary carbons, and C6 is a secondary carbon.
.................................................................................................................................... 18

Figure 5. Three component multiprocess supramolecular assemblies for optical sensing
(a) Absorption-Intersystem Crossing-Emission (AICE), where RX is a hydrogen-
bonding substrate. (b) Absorption-Energy Transfer-Emission (AETE) ..................... 21

Figure 6. Ternary complex of l-bromonaphthalene, glucosyl-fl-CD and hydrogen
bonding substrate. The alkyl group (R) is a bulky t-butyl or cyclohexyl and the
hydrogen bonding group (X) is a hydroxyl, carboxylic acid, amine, aldehyde or ester.
.................................................................................................................................... 39

Figure 7. Luminescence spectra of l-Ber from aerated aqueous solution of Gfl-CD (10'
3 M) in the presence of ~ 3% v/v of (a) NpOH (b) t-BuOH (c) DBM (d) CycMeOH
(e) CchH (0 CchtOH. Spectra were normalized for l-Ber absorbance and were
collected in the asymptotic limit of the titration profiles shown in Figure 8 .............. 41

Figure 8. Dependence of relative phosphorescence intensity of l-Ber at 530 nm from
aerated solutions containing Gfl-CD (10'3 M) concentration of t-BuOH (O) NpOH
(O) and DBM (G), and CchH (El), CycMeOH (I), and CchtOl—l (131). As a
reference, the plateau region for NpOH is 2.1 x 105 times greater than the
phosphorescence intensity in absence of alcohol. ...................................................... 42

viii

Figure 9. Job plot of the relative phosphorescence intensity from a l-BerOG CDOt-
BuOH complex monitored at 530 nm. The l-Ber concentration was 1 x 10' M, and
[GB-CD] + [t-BuOH] = 2.5 x 10’3 M .......................................................................... 45

Figure 10. Oxygen quenching rate constant as a function of quantum yield for the 1-
Ber-Gfl-CDrsubstrate complexes listed in Tables 1 and 2. ..................................... 49

Figure 11. Molecular model of ternary complexes of l-Ber (light gray), fl-cyclodextrin
(Shaded) and alcohol substrates (black): t-BuOH (top), NpOH (middle) and DBM
(bottom). ..................................................................................................................... 54

Figure 12. Synthesis of regioisomers of l-bromonaphthalene covalently tethered to the
primary ( l) and secondary (2) hydroxyl rims of ,B-CD [100]. ................................... 60

Figure 13. Relative emission intensity of (a) 2 and (b) 1 (each 1.0 x 10“1 M) in air-
saturated aqueous solution (excitation wavelength Aex = 290 nm). ........................... 61

Figure 14. Phosphorescence intensity at 520 nm from 2 in aerated aqueous solution
titrated with DTAB. .................................................................................................... 63

Figure 15. Absorption spectra of (a) l-Br—2—NpA, (b) l-Ber, (c) l-Br-2-MeNp (d) l-Br-
4-MeNp, (e) 5-BrAch in methanol. ......................................................................... 69

Figure 16. Absorption spectra of (a) 2-Br—6-MeONp, (b) 2-Br-6-NpOH, (c) l-Br-2-
NpOH (d) l-Br-2-MeONp, (e) l-Ber in methanol. ................................................. 70

Figure 17. Dependence of relative phosphorescence intensity of l-Ber (O) l-Br-4-
MeNp (O) 1-Br-2-MeNp (I), 5-BrAch (El), l-Br-2-NpOH (V) and 2-Br-6-
MeONp (A) at 530 nm from aerated solutions containing Gfl-CD (10'3 M)
concentration and increasing concentration of t-BuOH. ............................................ 73

Figure 18. Luminescence spectra of (a) l-Ber (b) l-Br-4-MeNp (c) l-Br-2-MeNp (d) 5-
BrAch (e) l-Br-2-NpOH (gray) (f) 2-Br-6-NpOH (dotted line) from aerated
aqueous solution of Gfl-CD (10'3 M) in the presence of ~ 3% v/v of t-BuOH. Spectra
were collected in the asymptotic limit of the titration profiles shown in Figure 17... 74

Figure 19. Fluorescence (a,c) and phosphorescence (b,d) of 2-bromo-6-alkoxy-fl-
cyclodextrin in water for degassed (a,b) and aerated (c,d) solutions. ........................ 76

Figure 20. Equatorial and axial conformations of naphthalene in a ,B-cyclodextrin cavity.
.................................................................................................................................... 77

Figure 21. (a) Energy minimizations of bromonaphthalene derivatives covalently bound
to Gfl—CD. From left to right the molecules are: (first row) l-bromo-7-methyl-Gfl-
CD, l-bromo-S-methyl-Gfl-CD, l-bromo-3-methyl-Gfl-CD, (second row) l-bromo-
6-methyl-Gfl-CD, l-bromo-4-methyl-Gfl-CD, l-bromo-2-methyl-Gfl—CD. (b) Energy
minimizations of binary complexes of bromonaphthalene derivatives. l-bromo-7-
methylnaphthalenewith Gfl-CD (left), l-bromo-6-methylnaphthalenewith Gfl-CD
(middle) and 1-bromo-5-methylnaphthalenewith Gfl-CD (right). ............................. 78

ix

Figure 22. Energy minimization of (a) l-bromonaphthalene with Gfl-CD (b) 2-
bromonaphthalene with G-fl-CD. The cyclodextrin is shown as a gray stick,
bromonaphthalene is a black stick and bromine is a solid ball. ................................. 80

Figure 23. Energy minimization of (a) 1-bromo-2-methylnaphthalene with Gfl-CD (b) 1-
bromo-4-methylnaphthalene with Gfl-CD (c) 1-bromo-2-naphthoate with Gfl-CD. .81

Figure 24. Two possible mechanisms of energy transfer and the rate expressions [17] D.
and A are the excited electronic states of the donor and acceptor. D and A are the
ground States. RDA is the intermolecular distance. Other variables are described in the
text. ............................................................................................................................. 86

Figure 25. Energy level diagram for Eu3+ and Tb3+ in the free ion and crystal field limit
of the chemical environment. The configuration is split into terms by electrostatic
interactions, levels by spin—orbit coupling and sublevels by crystal field splittings
[137]. .......................................................................................................................... 93

Figure 26. Three mechanisms for intramolecular energy transfer in lanthanide chelates:
energy transfer through the triplet, singlet and multiple levels [148]. ....................... 98

Figure 27. Energy level diagram of Gd“, Eu3+ and Tb“ with a typical aromatic
hydrocarbon, showing the range of energy of the triplet and Singlet states. The

emissive transitions of europium and terbium are shown. ....................................... 100
Figure 28. Luminescence Spectrum of Gd(DBM)3 A6,, = 365.7 nm. .............................. 103
Figure 29. Luminescence Spectrum of Gd(dNDBM)3 Ac, = 435 nm .............................. 104

Figure 30. Excitation (300-410 nm) and emission spectra (500-650 nm) of Eu(DBM)3,
showing excitation of the dibenzoyl ligand and emission from Eu3+ at 548 and 616
nm. ............................................................................................................................ 106

Figure 31. Excitation (300-410 nm) and emission spectra (425-600 nm) of Tb(DBM)3,
showing excitation of the dibenzoyl ligand and emission from Tb3+ at 488 and 546
nm. ............................................................................................................................ 107

Figure 32. Excitation (300-475 nm) and emission spectra (400-650 nm) of Eu(dNDBM)3
showing excitation of the dinitrodibenzoyl ligand and emission from dNDBM at 500
nm and from Eu3+ at 616 nm. ................................................................................... 108

Figure 33. Energy flow from DBM (left) and dNDBM (right) triplet excited state to Eu3+
and Tb3+ excited states. The arrow from the Singlet to triplet excited state of the
ligands Shows the intersystem crossing. The rates labeled (a) and (b) were measured
in our laboratory and by Speiser and coworkers, respectively [140]. ...................... 111

Figure 34. (a) Swing aza crown-,B-cyclodextrin coordinated with Eu“. (b) Cradle aza
crown-fl-cyclodextrin coordinated with Eu“. (c) Diethylenetriaminepentaacetic acid-
,B-cyclodextrin coordinated with Tb“. ..................................................................... 119

Figure 35. Emission Spectra (Ac, = 266 nm) for a 2.5 x 104 M aqueous solution of
TbDTPACD with increasing concentrations of Bph (data courtesy of M.A.
Mortellaro [178]). ..................................................................................................... 130

Figure 36. Excitation Spectra (item: 544 nm) for a 2.5 x 10“1 M aqueous solution of
TbDTPACD with increasing concentrations of Bph (data courtesy of M.A.
Mortellaro [178]). ..................................................................................................... 131

Figure 37. Relative Emission intensity from an aqueous solution of TbDTPACD ( 2.5 x
104 M ) as a function of Bph (O) and Np (Cl) concentrations. The luminescence of
the Tb3+ ion was detected at 544 nm with excitation wavelengths of 268 and 275 nm,

respectively (data courtesy of M.A. Mortellaro [178]). ........................................... 132
Figure 38. Overlap Integral (c) of Bph emission (a) and Tb(NO3)3°5H20 absorption (b).

.................................................................................................................................. 135
Figure 39. TDCI3'6H20 absorption spectrum (0.05 M in methanol). ............................. 137

Figure 40. Time-resolved luminescence of Tb(NO3)3°6HzO (open circles) and TbDTPA-
fl-CD (squares) at 546 nm. ....................................................................................... 138

Figure 41. Time-resolved luminescence of TDCI3’6H20 (open circles) and TbDTPA-fl-
CD (squares) at 546 nm. ........................................................................................... 139

Figure 42. Bph fluorescence quenching by Tb“. Fluorescence (a) decreases upon
addition of terbium(III). A concomitant increase in terbium luminescence (b,c) is
observed at 488 and 544 nm. .................................................................................... 143

Figure 43. Stern-Volmer plot of loll and Tolt of Bph in DTPACD quenched with Tb“.
.................................................................................................................................. 144

Figure 44. Molecular modeling showing twisted Bph in the cavity of TbDTPA-fl-
cyclodextrin. The lanthanide ion is the shaded ball, the DTPA-fl-cyclodextrin is
shown as a stick and Bph in CPK rendering. The distance from the center of the
closest Bph aryl ring to the center of the lanthanide is 6-7 A. ................................. 149

Figure 45. Three supramolecular antenna lanthanide complexes (a) bipyridine cryptand
(b) hexakis(4-carboxyphenoxy)cyclotriphosphazine (c) [anbpy.bpy.bpy.]3+. ..... 154

Figure 46. (a) Covalent Bph-CD-lanthanide AETE system. (b) Potential new scheme for
sensing benzene by an antenna amplification of the signal. (c) Multichromorphic
cyclodextrins (see reference [131]) .......................................................................... 156

xi

LIST OF TABLES

Table 1. Quantum yields, equilibrium constants and oxygen quenching rate constants for
alcohol substrates ........................................................................................................ 43

Table 2. Quantum yields, equilibrium constants and oxygen quenching rate constants for
other hydrogen bonding substrates. ............................................................................ 50

Table 3. Rate constants (kc,) for the phosphorescence quenching by Co(NH3)(,3+ in
systems containing l-bromonaphthalene and/or flcyclodextrin, in deaerated aqueous
solutions at 25 °C. Emission lifetimes were measured at 520 nm; excitation
wavelength 309 nm ..................................................................................................... 65

Table 4. Quantum yields, equilibrium constants and oxygen quenching rate constants for
bromonaphthalene phosphor derivatives with GB-CD (10'3 M) and t-BuOH (in the
asymptotic limit) ......................................................................................................... 72

Table 5. Lifetimes and Risetimes for Eu(DBM)3. Eu(dNDBM)3, Tb(DBM)3, Gd(DBM) 3,
and Gd(dNDBM)3. ................................................................................................... 110

Table 6. Molar absorptivities of aromatic hydrocarbons, binding constants of cyclodextrin
complexes and lanthanide luminescence quantum yields for aza swing, aza cradle and
DTPA cyclodextrins and aromatic hydrocarbons [175,177]. ................................... 128

Table 7a. Overlap Integral from 28703—30012 cm'1 and Critical Distances for Singlet (S)
and Triplet (T) States of Bph and Np Donor and Tb(NO3)3-5HZO Acceptor. ......... 134

Table 8. Quantum yields of fluorescence, triplet state, fluorescence lifetime and triplet
lifetime for mono- and bicyclic aromatic hydrocarbons. Data taken from reference
[182]. ........................................................................................................................ 146

Table 9. Energy levels of substituted Bphs for a Dexter Marcus curve study. ............... 153

xii

CHAPTER 1

INTRODUCTION

The modern world has seen an explosion of the chemical industry as it responds
to an increased global population. With industrialization, humankind has put an
enormous environmental demand on the global ecosystem, resulting in significant levels
of air, water and soil pollution [1]. In the past few decades, significant efforts have been
devoted to seeking out and remediating geographical areas polluted by gaseous
emissions, organic waste, heavy metals and pesticides from manufacturing and other
sources. The Comprehensive Environmental Response, Compensation and Liability Act
or "Superfund" of 1980 was signed into law to address the issue of cleaning up inactive
hazardous waste sites [2].

Of the contaminants found in land pollution, some of the largest contributions are
from industrial solvents and wastes including polycyclic aromatic hydrocarbons (PAHS),

polychlorinated biphenyls (PCBs), trichloroethylene (TCE), benzene, toluene, xylene

2

(BTX), and halogenated hydrocarbons [3]. The common properties of these molecules
are their hydrophobicity and the presence, generally, of a halogen heavy atom (F, Cl). A
goal of this dissertation is to exploit these properties to selectively design active
recognition sites to detect these environmental contaminants by optical transduction
means.

Chemosensing extends beyond the realm of environmental applications. Another
potential application for optical chemical sensors is for the control of advanced systems
[4]. Industrial processes require monitoring of manufacturing, processing and operation
of large plants. A wide variety of sensors can be implemented to monitor these processes.
For instance, a modern automobile requires a great deal of technology that is related to
sensors. There are over 24 sensing functions that measure operations of only the engine
and the transmission of an automobile [5]. In addition to physical sensors found in the
engine, some of the chemicals of interest in sensing applications are alcohols such as
ethanol and methanol used in fuel blends; also concentrations of exhaust gas oxygen
(EGO) must be monitored. Although there are currently sensors available for oxygen
based on zirconia [6] and titania [7], these sensors are based on electrical Signals (voltage
is proportional to a logarithmic function of oxygen partial pressures between exhaust gas
and ambient air) and indicate only the transition from oxygen rich to oxygen lean fuel
burning; i.e., they do not give an indication of the concentration Of oxygen present in the
exhaust gas. Ideally, one would like to be able to determine the exact oxygen to fuel ratio.
Given the electrically noisy environment of an automobile, it is also preferable to use an

electrically quiet transduction mechanism.

3

A chemical sensing active site (sometimes called chemosensor) can be classified
by the property used to detect the target molecule. The word sensor is derived from the
human senses of sight, taste, touch and smell. More specifically, a sensor is referred to as
a transducer, “a device which provides a usable output in response to a specified
measurand [8].” The output is typically an electrical quantity and the measurand is a
physical quantity, property or condition. Chemical sensing can be achieved by various
transduction methods involving conductivites, potentials, capacities, heats and masses of
a sensor when an analyte is present. However, with the advent of advanced fiber optic
technologies, spurred by advances in the communications industry, potentially the most
powerful transduction mechanism for chemical sensors is optical [9-10].

Molecule—based optical sensing schemes can report on nanometer—length scales
with nanosecond time responses and continuously monitor analytes and their influences
in real-time. They possess an inherently large bandwidth (and hence information
capacity), feature intrinsic selectivity owing to choices of wavelength and polarization,
and can be married to a variety of imaging technologies, including optical wave guides,
the most important of which are optical fibers. The latter issue has been the primary
driving force behind the emergence of optical sensing schemes in recent years [1 1-13].
Fiber optic sensors (sometimes called optodes) permit "wireless" communication
between the detection element and the analyte, making them particularly attractive for in
situ remote sensing applications. Moreover, because signals of many different
wavelengths can propagate in either direction within a optical fiber, sensor arrays with

multiple sensing capabilities may be constructed.

4

Optical Sensing: Turn On vs. Quenching Detection of Light from Excited States

Optical sensing is based on physical and chemical properties of excited states.
Molecules normally reside in dark or ground states and enter excited states upon
absorption of energy from the environment. Electronic excited states result typically from
absorption of ultraviolet or visible light. They are generally very reactive and will
eventually return to the original ground state or react photochemically, resulting in a
different product ground state molecule (P) (in Figure 1). The departure of the molecule
from the excited state may occur in the absence of a reacting partner molecule
(intramolecular decay) or in the presence of a second molecule (intermolecular decay).
Because emitted light is more easily detected than changes in transmitted light,
luminescence detection schemes are generally implemented with greater facility than
absorption schemes, especially when the signal is proportional to the intensity of the light
[14,15].

Figure 1 shows the intramolecular decay processes for an isolated excited state
molecule (Ml). Competing thermal emission and photon emission pathways for the
overall excited state decay are described by the non-radiative and radiative rate constants,
km and k,, respectively. The radiative rate constant is an intrinsic property of the molecule
itself and is related to the absorption cross-section [16]. All decay pathways that do not
lead to photon emission are included in the non-radiative rate constant. Typically, non-
radiative decay entails the conversion of the electronic energy of the excited state into
high energy vibrations of the ground state molecule. Relaxation to the vibrationally
equilibrated ground state molecule is accompanied by the concomitant release of heat

(km); alternatively, the excess vibrational energy may be manifested in the creation of a

 

I kisc

 

    

km(P) hv kr(f|uor) knr(M) kr(phos)

 

 

 

Figure 1. Electronic state diagram. M, 'M* and 3M* and denote the ground, singlet
excited state and triplet excited state of a molecule, respectively. The photochemlcal
product is P.

different ground state molecule (km(P)). The overall non—radiative decay rate constant,
km, is simply the sum of kn,(M) and km(P).

The radiative and non-radiative rate constants are important because they define
the fundamental properties of molecules in excited states [16]. The intensity of
luminescence, 1., is directly dependent on the luminescence efficiency, usually called the
emission quantum yield, ale, which is the ratio of the number of photons emitted per
photons absorbed. The emission quantum yield is directly related to k, and k,., by the
following expression,

kr

I": =——
e 4%: kr+knr

=k,r0 (1)

where 1'0 is the observed lifetime of the electronic excited state. Usually, the non-radiative
relaxation pathways dominate, k,,, >> k,, and as can be seen from Eq. (1) molecules
remain dark upon excitation. However, when thermal emission is inefficient with respect
to photon emission, excited states will literally light up or luminescence. The excited
state molecule is termed a lumophore, which is a general term for an emissive molecule.

In the presence of a reacting partner, the emission intensity of the excited state of
the indicator molecule, 1,, is related to the quantum yield, gig, by the following equation
[17, 18]:

= k‘ (2)
k, + km + kq[Q]

 

1e We

where k, is the radiative rate constant, k,,, the non-radiative rate constant, kq the quenching
rate constant of the excited state molecule and [Q] is the concentration of the quencher

molecule. Many Chemosensing schemes detect the presence of an analyte by quenching

7

the luminescence of an active site. Detection occurs when the last term in the
denominator (kq[Q]) increases in the presence of an analyte. This leads to an overall
attenuation of visible light signal in the presence of increasing concentrations of analyte
[19]. Consequently, schemes whose function is derived from quenching mechanisms are
hampered by the necessity to detect small differences in luminescence intensity relative
to a high background. In addition, selectivity is compromised because excited states are
highly energetic and thus may be quenched by a variety of analytes.

A goal of our research in recent years has been to generalize the design of
Chemosensing schemes based on a triggered luminescence response so that the approach
can be used to detect a variety of neutral and charged analytes. Our strategy centers on
synthesizing supramolecules containing multiple sites of complementary function. The
analyte is detecting an increase in visible phosphorescence or luminescence from a dark
background [20]. At one Site is a photoactive center capable of emitting visible light; at
the other is a docking Site for the analyte. Our work is based on perturbing the rate
processes of an excited State center by an analyte. The fundamental parameters governing
the luminescence intensity of the photoactive center are manipulated such that the
molecular recognition of the analyte at the docking site causes k, >> k,,, and k ; this
condition is met by increasing the effective k,, decreasing k,,, or decreasing kq upon the
molecular recognition of analyte by the supramolecule. In this manner, (15,, approaches its
theoretical limiting value of unity, and thus bright luminescence from the photoactive

center heralds the presence of an analyte in the supramolecule’s environment.

Intersystem Crossing of Excited States

The photoactive molecules used in our optical sensing schemes are organic
aromatic systems. The first excited state of most organic molecules is a singlet state,
where there are no unpaired electron spins. The radiative emission from a Singlet excited
state, fluorescence, is very short lived (nanoseconds) and typically occurs in the
ultraviolet (higher energy) region of the electromagnetic Spectrum. For these two reasons,
optical sensing by detection of fluorescence is not desirable. Our approach uses longer
lived and visible (500 — 700 nm) phosphorescence from supramolecular assemblies to
detect analytes. However, to reach triplet excited states (electronic states with two
electrons possessing parallel spins) and higher order spin states (luminescent states) an
excited state molecule typically passes through the singlet State, and one or more
electrons must undergo a spin flip. This non-radiative processes of an excited state
molecule traveling between electronic excited states of different spin multiplicities is
termed intersystem crossing.

The rates of intersystem crossing must be faster than fluorescence in order to
promote population of excited states that will lead to phosphorescence or luminescence.
However, aromatic hydrocarbons such as PAHs and BTXS, have comparatively slow
intersystem crossing rates (kg = 108 - 10‘5 s"), which are considerably smaller in
magnitude than the values for molecules with a "heavy atom" (kST(bromonaphthalene) =
109 s") or n,1r* excited states (k5T(benzophenone) = 10H 3") [21]. The three factors that
contribute to the magnitude of the intersystem crossing rate are the electronic

configuration of the two states, the energy gap between the two states (8; and T1)

9

involved in the non-radiative transition, and the Spin-orbit coupling between the two
states.

Coulombic and exchange interactions are responsible for the energy gap between
the triplet and singlet states. The energy gap is inversely related to the magnitude of
intersystem crossing rates, as is true for many radiationless processes. The energies of the
ground state, first excited singlet and first excited triplet of a generic aromatic

hydrocarbon with rt,1t' states are given below [17,21]:

E(SO)=0 (3)
E(Sl)= E(7r,7r*) +J(7r,rr*) + K067") (4)
H Ti): E057”) +1057") - KHAN) (5)

where J is the coulombic repulsion and K is the electron exchange repulsion. Therefore,
AE(St-Tl) = 2K(1t,1t’) >0. The smaller splittings for ma" states compared to It,1t' states is
one explanation for faster intersystem crossing in aromatic hydrocarbons containing a
ketone group such acetylacetonates and carboxylates. The singlet-triplet excited state
energy gap decreases and the rate of intersystem crossing increases along the series:
benzene (AE(S.-T.) = 40 kcal/mol), naphthalene (35 kcal/mol) and anthracene (30
kcal/mol) [17].

In addition to the effects of the energy gap between the singlet and triplet states,
spin-orbit coupling of internal and external heavy atoms will increase intersystem

crossing. Spin-orbit coupling arises when a magnetic moment is created from the intrinsic

10

spin of an electron. The Hamiltonian for spin-orbit coupling in a many-electron field is

given by the sum of the terms for the individual electrons [21],
Ze2 l
H = ———1s- (6)
SO zi34m2c2 ri3 ' I

where Z is the atomic number, e is the charge of an electron, m is the electronic mass, c is
the velocity of light, r is the distance between the heavy atom and the electron being
perturbed, and l and S are the electron’s orbital and spin quantum numbers. The
dependence on Z is the heavy atom effect and is dependent on the Coulomb nature of the
field created.

The rates of intersystem crossing via spin-orbit coupling of aromatic
hydrocarbons (BTXS, PAHS, phosphor active sites) can be affected by an internal heavy
atom (e.g. bromine or iodine) or external heavy atom (e.g. lanthanide ions). Heavy
halogens (Cl, Br, I) and lanthanide ions such as Eu“, Gd3+ and Tb3+ promote intersystem
crossing through a spin-orbit coupling process. In general, the heavy atom effect will act
to decrease 4);: (fluorescence quantum yield) and increase ¢isc if the rate of intersystem
crossing is comparable to the rate of fluorescence. However, the increase or decrease of
¢p (phosphorescence quantum yield) and (1)1-5 (triplet to Singlet quantum yield) will depend
on how kp and kTs (rate of triplet conversion to ground state) are influenced by the heavy
atom. The maximum heavy atom effect that can be induced by bromine substitution is kisc
= 108 - 109 s'1 [17]. We will use the heavy atom bromine to induce intersystem crossing

in our phosphor supramolecular assembly for alcohol detection.

ll

Supramolecular Assemblies

Our approach to designing optical sensors is to build a supramolecular assembly
where different molecular structural sites have an complimentary optical functions. Jean-
Marie Lehn, one of the founders of the field of supramolecular chemistry, defines the
field as encompassing the study of "chemical species of greater complexity than
molecules themselves, that are held together and organized by means of intermolecular
(non-covalent) binding interactions [22]." Because a supramolecule is held together by
intermolecular bonds this imparts selective binding and molecular recognition of
substrates by a molecular receptor. The receptors are designed to posses steric and
electronic features that complement the target substrate molecule, balancing enough
flexibility for the function to be performed. Molecular recognition implies binding and
selection of a substrate by a receptor, which in most cases leads to a specific function.
Some of the functions are changes in the electronic, ionic, optical or conformational
properties, thereby generating a signal at the molecular level.

There are many sizes and shapes of receptor molecules ranging from spherical to
tetrahedral, linear to branched, charged to neutral, organic to inorganic to biological.
Chemists have learned to sculpt molecules into shapes ranging from molecular bracelets
to tiny pagodas. Some of the exotic molecules are (Figure 2): boxes [23], butterflies [24],
calixes [25], carcerands [26], cavitands [27], clefts [28], crowns [29], cryptands [30],
cyclophanes [31], footballene [32], gondolas [33], hinges [34], ladders [35], octopus [36],
pagodanes [37], scorpiands [38], sepulchrands [39], speleands [40], Spherands

I41],torands [42], tweezers [43-44], vessels [27] and wires [45].

12

 

 

Figure 2. Supramolecules: (a) carcerand with a guest molecule in the cavity shown in
gray (b) molecular cleft with guest molecule (c) molecular gondola ((1) molecular hinge
with host (e) molecular octopus (f) pagodane (Conzo) (g) scorpiand with guest (h)

molecular tweezer with guest.

13

Whereas some of these molecules are merely attractive (such as the pagodane), others can
act as receptors for guests by relying on an elaborate network of intermolecular bonds.
For instance, the carcerand is a hollow cylindrically shaped host molecule that possesses
an interior cavity that may trap simple organic molecules, gases and inorganic ions
(Figure 2a). This cavity provides an interesting environment with unique physical and
chemical properties.

Various types of substrates can be recognized by deliberate design of the
supramolecular assembly. The assemblies are held together by hydrogen bonding,
hydrophobic and electrostatic interactions. Tetrahedral, anionic, cationic, linear and
multiple recognition sites can be fused into the supramolecular backbone. Several
examples are given in Figure 3. The top four molecules of this figure are derivatives of
cryptands. The smaller cryptands can host small cations such as the tetrahedral

ammonium ion or spherical sodium cation, or they can host ellipsoidal negative ions
N3_. In some cases cryptands have enough space and coordination sites for two metal

cation guests (Figure 3d). The Speleand shown in Figure 3e optimizes two Sites, an
aromatic portion which supplies a hydrophobic interaction and an aza crown ether
providing a site for binding a positive charge. These two recognition sites are ideal for
binding molecules such as methyl amine. Lastly, the cyclophane derivative shown in
Figure 3f combines binding by intercalation and electrostatic charge [22].

Other supramolecules converge to provide a platform for guest molecules. The
molecular cleft (Figure 2b) can interact via its carboxylic acid groups thereby to bind

molecules by hydrogen bonding interactions. Metal ions and neutral substrates will also

14

a b
r\ /T\
W: ‘17.: (1”.@ 1O
[N V0;_/H\/ U03 (@2V0ufi’J>

OCH3 O O OCH3 OCH 0
5.... f @112"
CE)
/_I\
—N., 0
<0— 'H’N\ 2 0

Figure 3. Supramolecular encapsulation of a tetrahedral ammonium ion, N 3‘ , sodium

ion and multiple ions in cryptands (a—d). Methylamine guests in an amphiphilic Speleand
and nitrobenzene cyclointercaland in a cyclophane (e-f).

15

bind to the cleft. The Size of the cleft can be made larger or smaller by substituting the
naphthyl ring with a benzyl or anthryl ring to impart Size selectivity. Molecular hinges
and tweezers (Figure 2 d,h) also recognize substrates by hydrogen bonding as well as 1:-
stacking intercalating interactions. Nucleic acid guests, such as l-butyl-thymine and
adenine, bind to the molecular hinges and clefts. Molecules such as the gondola, octopus
scorpiand and (Figure 2 c,e,g) are closely related to the cyclophane, cryptand and
Speleand supramolecules shown in Figure 3 and can also be used to capture guest
molecules.

Supramolecular assemblies can be used to make molecular devices based on the
manipulation of photons, electrons and ions, incorporate photosensitive groups for light
conversion and charge separation, be employed as catalysts, use redox active moieties to
make molecular wires and ion channels, form helices and other higher order structures
based on self-assembly, and finally supramolecules can be used for recognition reaction,
translocation, transformation and regulation [22c]. Supramolecular species can transport
amino acids, metal cations, anions across membranes, transport electrons in a redox
gradient and protons in pH gradients and drive light transport. Our interest centers on
controlling and manipulating light transport to design optical based chemical sensors,
where the substrate is the analyte of interest and the receptor is the active site of the
sensor. The structure of the chemical sensor is critical in determining the outcome of the
function, because it triggers the visible optical signal.

The most prolific work on chemical sensor design, which incorporates highly
Specific membranes and selective receptors, comes from the laboratories of GOpel and

coworkers [46]. Some examples of such sensors are those that operate by chemically

l6

sensitive field effect transistors (CHEMFET) [47] and light-addressable potentiometry
(LAP) [48]. Sensor arrays and neural networks are also being developed that will target a
collection of substrates, with a specific pattern for each compound [46c-d]. Self-
assembled monolayers of cavitand receptors on gold surfaces have been examined that
are selective for perchloroethylene (C2C14) [22a]. Many of the supramolecular
chemosensors operate by transduction of electrical and mass Signals, which are subject to

more noise interferences than the optical methods being investigated in our laboratory.

Building a Supramolecule from a Cyclodextrin Bucket

Supramolecular chemosensors must be built around the receptacle that will
capture the substrate. This container must be invisible to light, to avoid an interfering
signal emanating from this component of the supramolecule. Because we are interested
in Chemosensing of hydrophobic molecules (PAHS and BTXs) as well as those involving
hydrogen-bonding substrates (alcohols), we require a molecule whose structure will bind
the analyte with hydrophobic and/or hydrogen-bonding interactions. There are many
molecules that have a hydrophobic cavity (calixarenes, cavitands, hemicarcerands,
speleands, and cyclodextrins). However, there are few that meet our specification of
optical transparency. One such supramolecular template is a cyclodextrin (CD).

There are many reviews of cyclodextrins and 7000 publications between 1979 and
1994 [49-52] as well as two texts [53,54]. Cyclodextrins are simple and inexpensive to
produce by the fermentation of starch and are soluble in methanol, ethanol, dimethyl
sulfoxide, dimethylformarnide, N,N-dimethylacetamide, pyridine and, most importantly,

in water. They are versatile molecules because they are chemically stable, non-reducing,

l7

non-toxic, biodegradable and chiral. Cyclodextrins are active catalysts, used to study
weak interactions and are used to encapsulate drugs, perfumes and flavorings.

A cyclodextrin is one of the few molecules that has, in addition to the
hydrophobic pocket, hydrophilic rims available for hydrogen-bonding and water
solublization, and can be modified regioselectively on the primary or secondary side (see
Figure 4). There are three Sizes of cyclodextrins, a—, ,6—, and y—CD, which have 6-, 7-,
and 8-glucose units. We have chosen the mid-sized 6CD for designing a chemosensor
site for an optical sensor, as it is an ideal size for mono- and bicyclic aromatic
hydrocarbons. The narrow primary hydroxyl rim has seven hydroxyl groups (on carbon
number 6, C6, of the glucose group) available for functionalization, whereas the wider
secondary side has 14 hydroxyl groups attached to secondary carbons (C2 and C3). If we
desire further size selectivity, we can use the a—CD for monocyclic aromatic
hydrocarbons (BTXS) and the y—CD for larger PAHS. In the first study described in this
dissertation, glucosyl-fl-CD was used as a cyclodextrin host. ,B-CD derivatized with a
glucosyl (Gfi-CD) attached to the primary hydroxyl rim via a 1-6 linkage is a better host
structure than underivatized ,B-CD because of its increased (40-fold) water solubility [55].
The binding of guests to the glucosyl derivatized CD cup is only marginally attenuated
when compared with underivatized ,B-CD.

A cyclodextrin is transparent to excitation in the ultraviolet and visible regions of

the electromagnetic spectrum, as it is replete with saturated carbon-carbon bonds.

 

 

 

 

 

 

 

 

 

 

 

 

 

Qt 0 01‘1on
O o
w o
0 l “
O H H o
H O 9
O . H H
O
. 0 H 906’
0
"0e
’9 H o
0 H
D
o H o
0 H
:l:
. 0 H H H O “(’3
' I
L/Oe . H . . O
O
'b‘?‘
‘29
f—13-7A—>, 7—15.3A —->. .4—16.9A_..
i *SJA-‘r ‘ i <—7,3A-> g «951‘... g
l g 3 1 1 Secondary
‘- l .4 "t 1 i
‘1] ‘3‘ it: 3‘: / s de
I] 1, / ‘1, l
"i ‘1. f a". Primary
side
a—CD B-CD y-CD
n=6 n=7 [i=8

Figure 4. Cyclodextrins with a hydrophobic cavity, hydrophilic rims and three Sizes of

cavities. Shown in the middle is the glucose unit with the carbon atoms labeled with the
standard notation.

19

This property is advantageous in our studies of photophysical properties of
supramolecular assemblies that employ components that will absorb and emit light in
ultraviolet and visible wavelengths. Yet another advantageous property of a cyclodextrin
supramolecular system is the distance constraint imposed when several chemical species
are brought together by intermolecular forces. In solution, the distance over which these
interactions can dominate is too long to be efficient. We plan to use the distance
restrictions of a cyclodextrin to enforce properties that are distance dependent, such as
spin-orbit coupling from external heavy atoms and electronic energy transfer.

Guest molecules form inclusion complexes with cyclodextrins upon binding to
the hydrophobic cavity. The driving force for binding is the expulsion of energetic water
molecules from the CD cavity and replacement by a guest molecule. Although they bind
neutral molecules, cyclodextrins are polar molecules, where ,B-CD has a dipole moment
two-thirds the magnitude of a—CD (12-13 D) [56-57]. Cyclodextrins are electrophilic
molecules, and nucleophilic bases bind to their cavities. The strongest bases form the
most stable complexes with CDs. However, apolar molecules will also bind to the cavity,
as inclusion will induce a dipole moment in the guest molecule. Polarizability is a crucial
factor in determining complex formation. Inclusion complexes of photoactive molecules
such as methyl orange [58], crystal violet [59], methylene blue [60], dansyl [61] and 1-
anilinonaphthalene-8-sulfonate (ANS) [62] have been examined with cyclodextrin. The

binding of dye molecules causes a change in the absorption spectrum as well as change in

the pH.

20

Two Optical Strategies for Chemical Sensing

Schematic diagrams of two optical strategies we have designed for use as active
Sites in optical chemosensors are shown in Figure 5. At the top of the figure is the
template for an AICE (Absorption-Intersystem Crossing-Emission) process used for
optical sensing of alcohols. The supramolecular chemosensor is based on self-assembly
of a ternary system. The foundation of the supramolecule is a water soluble glucosyl-fl-
CD with a cavity size that is ideal for inclusion of a phosphor molecule. The glucosyl
attachment imparts water solubility to an otherwise less soluble fl-CD. A hydrophobic
interaction holds the phosphor in the water repelling cavity. The phosphor is directly
excited with ultraviolet light to create a singlet excited state. Although the phosphor has
an inherently large intersystem crossing rate between the singlet and triplet states, oxygen
gas present in aerated aqueous solutions is able to quench the vulnerable phosphor
excited state at primary and secondary openings of the cyclodextrin basket. Migrating at
the rate of diffusion (kD = 109 5"), oxygen non-radiatively quenches or steals the
phospor’s excited state energy before it arrives at the ground state by radiative decay.
Phosphorescence (emission from the first triplet state to the ground singlet State) is
typically a slow process, as the electronic transition is dipole forbidden. Thus, the
phosphor, although included in the cyclodextrin cavity, is unable to emit light due to the
rapidity of oxygen quenching. We have exploited the shortcomings of the phosphor
cyclodextrin complex by adding a third molecule that inhibits diffusion of oxygen to the
phosphor. The third molecule is an alcohol substrate, which is recognized by the
cyclodextrin rims by an intermolecular hydrogen bond. The structure of the alcohol

all-ows bitopic or two-site recognition by the cyclodextrin receptor.

21

Components Processes
hv
Binding Properties

  
 
 
  

Binding Site for

Phosphor Topology of Complex
Cyclodextrin
Quantum Yield of Phosphor
Binding Site for Alcoh.

or other substrate
Oxygen Quenching Rate Constant

302 102
Components Processes
h" Binding Properties
Binding Site for
Aromatic Hydrocarbon
\ Absorption of aromatic donor

Cyclodextrin Energy Transfer

Binding Site for

Lanthanide ion Photophysics of Ln3+ acceptor

hv'

F igure 5. Three component multiprocess supramolecular assemblies for optical sensing
(a) Absorption—Intersystem Crossing-Emission (AICE), where RX is a hydrogen-bonding
substrate. (b) Absorption-Energy Transfer-Emission (AETE).

22

The unique structure of this molecular basket results in bright, glowing emission from the
phosphor, which can efficiently radiate energy from the triplet state, and results in a
triggered photonic signal. The lock and key structure of the ternary complex permits us to
effect an optical functionality. These results will be described in Chapters 2-4.

Chapters 5 and 6 describe another molecular architecture that has been designed
for optical Chemosensing derived from a fl-cyclodextrin (Figure 5b). The specific
cyclodextrin receptor used for sensing of BTXs and PAHS also has a bitopic structure that
allows recognition of the analyte substrate and of a lumophore. The hydrophobic cavity
in this case does not hold the lumophore, but instead acts to capture the substrate, a light
absorber. On the narrow end of the cyclodextrin bucket, a macrocyclic ligand containing
chelating nitrogens and oxygens has been tethered. This ligand acts as a binding Site for
metal lumophore ions. The macrocycle also prevents water from entering the
coordination sphere of the ion, thereby decreasing km of the lumophore. The metal ion
coordinated to the cyclodextrin macrocycle has an intrinsically low absorption cross-
Section, and therefore, when it is directly excited with ultraviolet light, the excited state
luminesces weakly. Molecular recognition of a BTX or PAH by the cyclodextrin’s cavity
greatly enhances the population of the upper state of the lumophore by energy transfer,
and intense triggered excited state emission is observed. This process of absorption by the
BTX, energy transfer across the interior cavity of the cyclodextrin, and emission by the
lumophore ion is termed AETE (Absorption-Energy Transfer-Emission). The shortened
distance between the aromatic hydrocarbons and the lumophore imposed by the
cyclodextrin cavity allows ultraviolet light energy absorbed by the aromatic analyte to be

transferred non-radiatively to the lumophore. Therefore, the analyte of interest must have

23

a sufficient absorption cross-section and the lumophore should have a large quantum
yield of emission. We have chosen the lanthanide ions, terbium (III) and europium(III) as

the lumophores in our cyclodextrin AETE system.

CHAPTER 2

OPTICAL CHEMOSENSING OF ALCOHOLS BY AN ABSORPTION
INTERSYSTEM CROSSING EMISSION PROCESS

Introduction

Alcohol (ethanol) is one of the most prevalent molecules in our society. We
consume it in alcoholic beverages, use it as a fuel additive or as an industrial solvent. AS
an alternative fuel additive with the promise for reduced level of emissions produced by
combustion, ethanol and methanol need to be detected in hydrocarbon backgrounds [5].
To enable automobiles to burn mixed fuel efficiently, the engine must have information
about the relative percentages of gasoline and alcohol. Chemosensors for simple alcohols,
ethanol and methanol, are therefore highly desirable [63]. Ethanol sensing is useful for
many biotechnological processes as well [64,65]. Techniques that have been employed to
determine ethanol concentration include electronic absorption, infrared spectroscopy, fuel
cells, refractive index changes, piezoelectrics, electrochemical techniques,

semiconductors and enzymatic reactions. The latter has been mostly successful, yet these

24

25

measurements indirectly monitor ethanol by oxygen consumption by alcohol oxidase
[66,67] or NADH production by alcohol dehydrogenase [68-72]. However, there is a
need for more selective optical sensors for alcohol sensing, especially for fuel
composition measurements, in order to improve vehicle performance and public
acceptance of alternative fuels. Our laboratory is interested in developing a direct ethanol
sensor based on triggered luminescence for monitoring fuel composition.

Optical methods for alcohol detection are prominent for selective applications
such as measuring blood alcohol concentrations. Determining if a driver is under the
influence of alcohol has been a challenge for law enforcement for many years [73]. The
Breathalyzer, a well-known ethanol sensor, is based on absorption spectroscopy. The
concentration of ethanol is determined by reduction of potassium dichromate by ethanol
and a subsequent change in the absorption spectrum as orange chromium(V) is reduced to

green Cr(III). The reaction is given schematically below:

primary orange
alcohol H
RCHZOH + Cr2072' ——> R—C\ + of“
o pale green
02072-
O H
+ Cr3+

/
R—C\O

pale green

26

Blood alcohol level concentrations can be measured from 0 to 0.40 % weight by volume
by measuring the absorbance difference between the test sample and a reference solution
of potassium dichromate. A catalyst must be added to ensure the complete oxidation of
ethanol to acetic acid. Due to the poor reliability of the test, new modern methods have
been developed for testing blood alcohol levels. Infrared spectroscopy (IR) has been the
primary test of ethanol Since the 19805 [74]. One advantage of IR spectroscopy iS the
real-time measurement of ethanol concentrations compared to the Breathalyzer, which
requires a reaction time of 1.5 minutes. Although there are advantages to IR detection,
the disadvantages are the high cost of achieving specificity, inaccuracy at low alcohol
concentrations and the nonlinear output of the detector. A third alternative for blood
alcohol sensors are fuel cells, where a breath sample passes through a layer of platinum
black into an acidic electrolyte solution. An electrochemical Signal heralds the presence
of ethanol, which is converted to acetic acid. Fuel cell technology, in contrast to IR
spectroscopy is very specific to ethanol [75].

Other optical tests for atmospheric ethanol based on absorption spectroscopy are
based on films of a vanadium-oxinate-polymer [76]. Vanadium oxinate is insoluble and
non-reactive in water and has been used to determine levels of ethanol in beer, wine,
blood and other solvents [77,78]. Ethanol concentrations are determined by UV-VIS
spectrometry, where a change in color from the gray-green vanadium oxinate complex to
a red color, the complex with alcohol, is observed. Two obvious disadvantages of this
method is that the detection is based on absorption (and thus a reference 10 is needed),
and the active compound chemically reacts with the analyte, making the sensor

irreversible.

27

\ /N O N\ / C2H5OH \_/N 0
/ O—U—O \ / O_l|}_o

OH—(CZHSOH)X

 

grey-g reen red

Alcohol sensors based in fluorescence have recently appeared. An Optode (optical
electrode) used to monitor the content of ethanol in alcoholic beverages [79] has been
developed with an active material composed of n—acetyl-N-dodecyl-4-
trifluoroacetylaniline (TFAn) coated onto polyvinylchloride (PVC) polymer. A change in
absorption Spectrum is transduced upon contact of ethanol with TFAn from the formation
of hydrates and hemi-acetals. The mechanism of sensing occurs because of the highly
electrophilic nature of the carbonyl carbon atom of TFAn, which is magnified further by

the highly electron withdrawing trifluoromethyl group.

C12H25

0 /
N Base
F30 )1 _... Catalyzed 0| : /C12H25
O i ‘ F30 N

 

 

R'OH

Interaction with ethanol results in a loss of electron delocalization and a hypsochromic
shift of the absorption band. Selectivity towards primary alcohols such as methanol,

ethanol, l-propanol, l-n-butanol is imparted by the nucleophilic character of the analyte,

28

and secondary alcohols such as tert-butanol and isopropanol are rejected by a factor of
10. A dynamic range of ethanol concentrations from 0.5 to 35 % (v/v) can be determined
by this optical method based on absorption.

A similar optical sensor is also available for use in determining ethanol
concentrations in alcoholic beverages [80]. This optode is fabricated with a chitosan and
poly(vinyl alcohol) cladding. The chitosan membrane separates ethanol from water as
swelling and shrinking of the cladding induces a change in refractive index and
consequently the intensity of transmitted light. This sensor operates in the ethanol
concentration range of 0-60 % (v/v).

In addition to optical sensors based on absorption and refractive index changes,
there are also optodes that rely on changes in fluorescence to herald the presence of
alcohol. The active receptor site of the two sensors are 1,3-oxazole or thiazole [81] and
fluorescein octadecyl ester (FODE) [82]. The oxazole/thiazole fiber optic sensor is based
on the effect of twisted intramolecular charge transfer (TICT), which causes a decrease
and a red Shift of the fluorescence of the dye molecule upon exposure of these dye
molecules to polar ethanol or methanol. Since the most common application of optical
sensing of simple alcohols is for measuring their levels in fuel blends, the fiber optics can
be immersed in non-polar octane solutions. Concentrations of polar alcohols between 0.5
and 6% will trigger a change in the fluorescence of the dye emission. The fluorescence
spectrum of fluorescein, also a dye molecule, is sensitive to hydrogen bonding
interactions with alcohols, resulting in an increase in the relative fluorescence intensity
upon addition of alcohol. This sensor also triggers fluorescence in the presence of

carboxylic acids such as formic, acetic, propionic and benzoic acid. Both of these sensors

29

use visible light to excite the laser dyes and measure increases or decreases in
fluorescence intensity. However, the selectivity of the sensors is compromised by the
variety of polar and hydrogen bonding substrates that can also trigger a luminescent
signal.

Thus there is a diverse scope of ethanol sensors, based mainly on optical
(infrared, UV-VIS absorption and refractive index) and electrical techniques (fuel cells).
However, they posses several disadvantages. For an automobile application, electrical
transduction techniques (fuel cells) are undesirable in an environment already crowded
with electrical signals. The optode sensors are based on absorption Signals, which involve
measuring small changes in transmitted light. Finally, many of the devices and
mechanisms described are not Specific to ethanol and methanol.

Optodes based on luminescence spectroscopy have the advantages of low cost,
real-time measurements, Simple design and ease of miniaturization. They do not require a
reference signal and are not subject to electrical interferences, [83]. The challenge of
sensing ethanol and other alcohols by luminescence spectroscopy is that the analytes are
neutral molecules that do not absorb or emit light. Although ethanol can effect a change
in absorption of vanadium oxinate and trifluoroacetylaniline, perturbations in
phosphorescence and fluorescence are more difficult to achieve and many times non-
specific.

How can substrates such as alcohols be detected? They do not coordinate to metal
ions, absorb light or have any characteristics that would easily lend themselves toward
optical detection by triggered luminescence. Our interest in elaborating a method for

alcohol detection by a luminescence response led us to become interested in the

30

properties of lumophores included in cyclodextrin cups. It is well-known that the
nonradiative decay processes of lumophores are significantly attenuated, and hence
luminescence intensity increased, in the protected microenvironment of a CD cup [84-
87]. Many investigations have focused on the fluorescence of CD-included polyaromatic
hydrocarbons or azo dyes, and the effect of alcohol on fluorescence [88]. The
perturbation by the alcohol on the fluorescence properties has been attributed to the
alcohol increasing the association of the fluorophore within the CD cup [89-91],
modifying CD-fluorophore interactions [88,92,93], and/or physically shielding
fluor0phore from quenchers present in bulk solution [94]. We became interested in
integrating the concepts of room-temperature phosphorescence of lumophores in CD cups
[95-98] with that of ternary complex formation among aromatic hydrocarbons, CD5, and
alcohols to design a scheme for the luminescent detection of alcohols. Specifically, we
were intrigued by Turro’s investigation of host-guest inclusion complexes between CDs
and bromonapthalene appended to detergent chains [96b,99]. Although the cationic
surfactant phosphorescent probes are quenched efficiently by Co(NH3)r,3+ in aqueous
solution, their inclusion within the CD cup preserves their intense phoshorescence. The
bromonaphthyl moiety is protected from quencher by the coiled hydrocarbon chain
within the CD cavity. We wondered if the protective function of the long detergent chain
could be assumed by alcohols in the formation of a complex among bromonaphthalene,
,B-CD, and alcohol. In this case, the possibility exists for alcohols to trigger the intense,
green, room-temperature phosphorescence of bromonaphthalene, which is Otherwise

efficiently quenched by oxygen.

31

We have shown that simple alcohols can assume the role of long detergent chains
in ternary complexes [100]. The alcohol acts as a lid for the CD cup, shielding l-Ber
from oxygen and thereby preserving l-Ber’s intense green phosphorescence.
Phosphorescence enhancements can be very large, depending on the fit of the alcohol lid
on top of the CD cup. We investigate here the topological features of the alcohol that lead
to the best shielding. In addition, the recognition of other hydrogen bonding substrates by
the CDOI-Ber is defined and the relationship between the structure of the CD°1-Ber-S
(S = substrate) and its photophysical properties leading to a triggered luminescence

response is described.

Experimental
Materials

Mono glucosyl-flcyclodextrin (G-fl-CD) was purchased from CTD, Inc. and used
without further purification. Tert-butanol (t-BuOH), pivalic acid (PA), and
neopentylamine (NpNHg) were purchased from Fisher, MCD and ICN Biomedical,
respectively and used as received. All other substrates, neopentanol (NpOH), 3,3-
dimethyl-l-butanol (DMB), cyclohexanol (CchH), cyclohexylmethanol, (CycMeOH),
2-cyclohexylethanol (CchtOH), cis-1,2-cyclohexanediol (1,2-Cyc-diOH), 1,3-
cycloheanediol (1,3-Cyc—diOH), 1,4-cyclohexandiol (1,4-Cyc-diOH) tert-butylacetic acid
(t-BuAA), tert-butylamine (I-BUNHz), trimethylacetaldehyde (TMA) and methyl
trimethylacetate (MTMA) were purchased from Aldrich and used without purification.
The diols 1,3-Cyc-diOH and 1,4-Cyc-diOH were used as mixtures of cis and trans
isomers. 1-Bromonaphthalene (l-Ber) was purchased from Aldrich and vacuum

distilled before use.

32
Electronic Absorption Spectroscopy and Binding Constants

All experiments were performed using room temperature aerated aqueous
solutions unless otherwise noted, where the concentration of oxygen in water is
2.4 x 10’4 M. Absorption Spectra were collected on double beam Hitachi U200] or OLIS
retrofitted Cary 17 spectrophotometers. Changes in the absorption Spectra upon binding
were used to determine association constants. In all cases the hydrophobic guest
molecules were insoluble in water. We therefore devised a novel way to measure the
binding of cyclodextrins in saturated aqueous solutions with a large excess concentration
of the aromatic hydrocarbon of interest [100-101].

Before the association constant can be measured, two parameters must be
accounted for: the solubility concentration of aromatic hydrocarbon in water and the
changes in index of refraction. The first quantity is gleaned by collecting an absorption
Spectrum of aromatic hydrocarbon saturated in water versus a water reference. Using
Beer's Law we obtain the concentration as [Ar] = Aolcb, where [Ar] is the concentration
of aromatic in water, A0 is the absorption of the solution, a is the molar absorptivity of
the aromatic at the maximum and b is the path length. In all subsequent equations the
path length will be set to 1 cm and therefore appears to be excluded. Secondly, the
refractive index must be corrected for, as addition of cyclodextrin creates a new
equilibrium between the aromatic solute and water solvent, which results in solubilization
of the aromatic hydrocarbon. Therefore, a baseline is scanned prior to titration, with a
saturated solution of aromatic in both the sample and reference chambers.

AS concentrated cyclodextrin solution (25 mM CD) is added to the sample

compartment, the absorption increase results from‘the complex of cyclodextrin with

33

aromatic hydrocarbon, [CD-Ar], where CD = cyclodextrin and Ar = aromatic
hydrocarbon. A plot of [A-Ao]/A0 versus [CD].m-[CD°AI] yields K, the binding constant
as the slope. The derivation of the equation is given below, where [CD] is the
concentration of unbound cyclodextrin, CD0 is the initial concentration of titrated CD
(before complexation with Ar), [CDOAr] is the amount of complexed CD, A0 is the
absorbance of an aromatic saturated in water, ACD is the absorbance of an aromatic

hydrocarbon with added CD in water, a is the molar absorptivity at the wavelength where

the A0 and ACD is collected (in this case A = 284 nm),

K
[Ar] + [CD] V'— [Ar°CD] (7)
ACD — A0
[CD] = CD0-[ CD °Ar] and [CD 0 Ar] = l:——————] (8)
e
[CDO Ar]
= — 9
[calm] ‘)
[CD . Ar] = K[CDIAr] (10)
l—ACD‘A°l=KlCDo-——-“CD“‘°ll-"11
a s e
[ACD_A0]:K[CDO_ACD-AO] (12)
A0 8

After separating the independent ([CD0]) and dependent (A) variables, the equation is,

KA08

m[CDO]+AO (I3)

ACD =

34

Therefore the binding constant is obtained from the slope, molar absorptivity and the
absorption of saturated aromatic in water and the y-intercept is related to the
concentration of aromatic in water.

The following assumptions are made for this experiment. First, extinction
coefficient of l-bromonapthalene does not change significantly upon complexation in the
cyclodextrin cavity. Secondly, we do not see a shift in the absorption maximum of 1-
Ber upon complexation. Lastly, the solution remains saturated with aromatic
hydrocarbon throughout the titration and never reaches the asymptotic limit that is typical
for most binding curves. This linear equation applies because the titration occurs in the

linear regime of binding, where [CD0] << [l-Ber].

Steady State Luminescence Spectroscopy, Quantum Yield and Titrations

Quantum yields were determined on a high resolution emission instrument built at
Michigan State University [102] that employed an R943 or R1104 Hamamatsu PMT as
the detector. The instrument was modified from its original lock-in amplifier
configuration (EG&G) to photon counting detection (SRS 400 Photon Counter) [103].
LABVIEW is used to computer control the photon counter, lock-in amplifier and two
SPEX monochromators, a 0.22 M double 168 and 0.5 M Single 18708, for selecting
excitation and emission wavelengths respectively. The 313 nm line of a XeHg lamp was
used to excite l-Ber Light and dark counts are collected when a 13 or 338 Hz chopper
is open and closed, and the Signal is calculated as the difference between light counts and
dark counts. Two chopper frequencies allow us to control the time interval for collecting

fluorescence or phosphorescence quantum yields. A single blade chopper (Terahertz

35

Technologies) with speeds ranging from 3-4000 Hz was added to allow for more
consistent positioning, more flexibility in the optical path and also for computer control
by LABVIEW.

The phosphorescence intensity of l-Ber (saturated) in Gfl-CD (2.5 mM) was
observed to increase to an asymptotic limit with increasing amounts of alcohol or
hydrogen-bonding substrates. Substrate additions were between 0 and 300 pl, so that the
total volume change for the solution (10 ml) was not Significant. All measurements were
performed at 22 i 2°C in aerated aqueous solutions. The stoichiometry of the ternary
complex was determined from a Job plot by the method of continuous variation [104].
The concentrations of Gfl-CD and tert-butanol were varied so that their total
concentration was equal to 2.5 X 10'3 M as the concentration of l-Ber was held constant
at 1 x 10'5 M, the solubility limit in water. The phosphorescence intensity was measured
at 530 nm. Quantum yields were measured using solutions saturated in l-Ber, 2.5 mM
Gfl-CD and in the asymptotic limit of added substrate. Absorbances measured at 313 nm
were ~ 0.1. Quantum yields were derived by integrating the phosphorescence of l-Ber
from 450 to 700 nm and correcting for the refractive index differences between hexane
and water. The quantum yield was evaluated relative to 2,3-butanedione (biacetyl; (ppm, =
0.05 in hexane at room temperature) [105]. Biacetyl was freeze-pump-thawed 3-5 times
before the phosphorescence was observed, and contributions due to fluorescence were
subtracted by measuring fluorescence from aerated solutions. The phosphorescence
quantum yields of all bromonapthalene compounds in chapters 3 and 4 are measured

relative to biacetyl.

36

Stern-Volmer Quenching

The extent of static and dynamic quenching can be determined from the Stem-
Volmer method of luminescence lifetimes [106]. If quenching is purely dynamical then
plots of intensity and lifetime as a function of quencher concentration will be linear and
the slope proportional to the Stern-Volmer quenching rate constant, which is the product

of the lifetime in the absence of quencher and the bimolecular quenching rate constant.

10

— :31 =1+KSV[Q]I (14)
I I"

stszTO (15)

If static quenching is also present, then there is binding in the excited state as well, and
the intensity plot has a positive deviation from the lifetime plot, which is given by the

following equation.

1% = (1+ stlQDQ + Keq[Q]) (16)

Therefore if the Stern-Volmer quenching rate constant is known from the lifetime plot,
the equilibrium constant can be found by the quadratic equation.

Stem-Volmer quenching was used to establish the extent of dynamic and static
quenching [107]. In the case of l-Ber phosphorescence in a ternary complex, the
dynamic quenching of triplet oxygen was obtained by measuring the lifetime for three
oxygen concentrations in degassed solutions, aerated solutions at atmospheric pressure
and in saturated oxygen solutions (0 M, 2 x 10“1 M, 12 x 10“1 M, respectively) on a

nanosecond-time resolved laser instrument. Oxygen concentrations were determined

37

according to Henry’s law with methods that we have previously discussed [100]. For the
covalent compounds of l-Ber and cyclodextrin, a cobalt hexammine chloride complex,
[CO(NH3)6]C13 ,was used as a quencher to define the dynamic quenching kinetics, since
for the case of compound 2 (bromonaphthalene on the secondary side) oxygen was not an
efficient quencher. Monoexponential decays of l-bromonapthalene were fit using

Kaleidagraph or Origin Software packages.

Nanosecond Time-Resolved Spectroscopy

All laser systems used were in the LASER laboratory shared facility in the
Department of Chemistry at Michigan State University. To determine the lifetimes and
risetimes of phosphorescence, nanosecond pulsed lasers were used. Lifetimes longer than
the pulsewidth of a 10 ns can be measured on the nanosecond Quanta Ray DCR2
Nd:YAG systems. In the normal timing mode, the laser pulse directly or the laser pulse
scattered onto a photodiode acts as the trigger signal for data collection. A TEK DSA
602A fast oscilloscope was employed to collect time-resolved luminescence and lifetimes
of l-Ber phosphorescence. Phosphorescence of l-Ber at 530 nm was collected at
right angles to the excitation beam and measured by a R928 Hamamatsu PMT (700-800
V) after being selected by a SPEX 1681 single monochromator. The entrance slit was
kept as small as possible for stray light rejection purposes. AS phosphorescence is
typically long lived (milliseconds), it was necessary to open the exit slit to an average of
4 mm.

The 532 nm output of a nanosecond Quanta Ray DCR2 Nd:YAG laser was used

to pump a variety of laser dyes (rhodamine, DCM, kiton red) in a side pumped

38

configuration. The wavelengths selected by the dye were doubled by a WEX, where the
angle of the doubling crystal could be tuned if the laser was scanned or a fixed
wavelength could be selected by a Pellin—Broca prism. The power of ultraviolet light
emanating from the WEX was typically 10-40 mW (10 -15 % conversion efficiency).
Alternatively, a nanosecond Quanta Ray DCR2 Nd:YAG laser could be used as a source
when it is configured with a H2 Raman Shifter (10-20 psi) to obtain 309 nm anti-Stokes
shifted 355 nm light. This wavelength (309 nm) was used to measure phosphorescence

decay of l-Ber.

Molecular Modeling

Energy minimized conformations of the l-Ber0Gfl-CD03ubstrate ternary
complex were determined from an energy minimization run on a Silicon Graphics IRIS
Elan x2 computer using Biosym Insight Discover with the assistance of Mike H.B. Gray.
The method of steepest descent and conjugate gradient algorithms of a consistent valence
force field (CVFF) were employed. The Gfl—CD was minimized first, and then successive

minimizations were performed as the l-Ber and substrate molecules were added.

Results
Quantum Yields and Binding Constants

When excited with ultraviolet light, an aerated aqueous binary system of l-
Ber°CD emits only blue fluorescence at 320 nm typical of aromatic hydrocarbons,
although inclusion of l-Ber does cause slight changes in the vibrational fine structures

(Figure 6)[108]. Green phosphorescence of l-Ber at 530 nm appears only when

39

 

 

 

CH3

R= +CH3 N
CH3

x-_— OH, NH2, COOH, CHO, coon

Figure 6. Ternary complex of l-bromonaphthalene, glucosyl-,B-CD and hydrogen
bonding substrate. The alkyl group (R) is a bulky t—butyl or cyclohexyl and the hydrogen
bonding group (X) is a hydroxyl, carboxylic acid, amine, aldehyde or ester.

40

alcohols are added to the binary system of l-Ber0CD to form a ternary complex. Figure
7 Shows the Similar phosphorescence profiles for l-Ber in ,B-CD in ternary complexes
with tert-butyl and cyclohexyl alcohols. Although the shape of the phosphorescence
profile is unchanged, the intensity varies markedly as a function of the nature and
concentration of the alcohol.

Titration profiles of l-Ber at item = 530 nm (Aex = 313 nm) as a function of
substrate concentration are shown in Figure 8. For all substrates shown, the emission
intensity increases monotonically with increasing alcohol concentration to a different
asymptotic limit for each substrate. Within the tert-butyl and cyclohexyl series the
greatest response is observed for neopentanol (NpOH) and cyclohexylmethanol
(CycMeOH), respectively, where the bulky groups are removed from the alcohol group
by one methylene spacer.

The quantification of the phosphorescence response is achieved by measurement
of the quantum yields. Table 1 provides a list of the phosphorescence quantum yields of
l-Ber in ,B—CD in the presence of various tert-butyl and cyclohexyl alcohols at
concentrations range near the asymptotic limit provided by the titration profiles. The
largest quantum yields are observed for alcohols in which the aliphatic bulky group is
separated from the hydrogen—bonding hydroxyl group by one methylene Spacer, followed
by alcohols with no spacer. The quantum yields decrease when -(CH2)n- reaches n > 2.
The method of continuous variation (Job plot) was used to determine the stoichiometry of
the l-Ber'Gfl-CD-substrate complex for t-BuOH [109]. The concentration of l-Ber

was held constant while the molar ratio of Gfl-CD and t-BuOH were varied, keeping the

total concentration ([Gfl-CD] + [t-BuOH] = 2.5 x 10'3 M) the same throughout the course

41

 

(a)

b
( ) (C)

phos

 

 

 

 

,.’/ F
l l l l l l 144 l J l r l l l l l l L J L l l l l
450 500 550 600 650 700

A/nm

Figure 7. Luminescence spectra of l-Ber from aerated aqueous solution of Gfl-CD
(10'3 M) in the presence of ~ 3% v/v of (a) NpOH (b) t-BuOH (c) DBM (d) CycMeOH
(e) CchH (1) CchtOH. Spectra were normalized for l-Ber absorbance and were
collected in the asymptotic limit of the titration profiles shown in Figure 8.

42

 

 

O OO
0CD ‘3 D
If. .0. .
O
IO .9
D 0000
O .
O
.1." U o.
a .0
v 81 .
(D
2
QCEJ '
'ESE.
S
O
IE.
O
O
.
lllllJllJLLllllllllIllll

 

 

 

0 0.1 0.2 0.3 0.4 0.5
[ROH]

Figure 8. Dependence of relative phosphorescence intensity of l-Ber at 530 nm from
aerated solutions containing Gfl-CD (10'3 M) concentration of t-BuOH (O) NpOH (O)
and DBM (G), and CchH (El), CycMeOH (I), and CchtOH (1S1). As a reference, the
plateau region for NpOH is 2.1 x 105 times greater than the phosphorescence intensity in
absence of alcohol.

43

Table 1. Quantum yields, equilibrium constants and oxygen quenching rate constants for
alcohol substrates.

 

 

Substrate Symbol limos/104 K4/M" kq/ 105 M'ls'T
tert-Butanol t-BuOH 340 1344 8.9
Neopentanol NpOH 1 149 1264 0.3
3,3-Dimethyl- l -butanol DMB 789 2162 1.6
Cyclohexanol CchH 350 1972 1.2
Cyclohexylmethanol CycMeOH 77 l 1 107 0.2
Cyclohexylethanol CchtOH 364 1022 0.8
cis-1,2-Cyclohexanediol 1,2-Cyc-diOH 883 763 2.0
1,3-Cyclohexanediol 1,3-Cyc-diOH 51 653 32

1,4-Cyclohexanediol 1,4-Cyc-diOH <102 <10 >104

 

44

of the experiment. Figure 9 Shows the Job plot for l-BerOGfl-CDOIert-butanol as the
molar ratio of tert—butanol and Gfl-CD is varied. As the binary complex of l-BerOGfl-
CD does not phosphoresce, the phosphorescence intensity at 530 nm directly reflects the
stoichiometry of the ternary complex. A maximum value of the intensity occurs between
0.4 and 0.5, thus providing evidence for a nearly equimolar ratio of CD and alcohol.
Since naphthalene and its derivatives have been proven to form 1:1 complexes with
cyclodextrins [110-112], we can conclude with confidence that the data are consistent
with the formation of a 1:1:1 ternary complex of l-BerzGfl-Cth-BuOH.

Many equilibria can be expressed for the l-Ber0Gfl-CD05ubstrate system. Either
l-Ber or substrate will bind to CD to form binary complexes (K1, K3 respectively). The
ternary complex l-Ber0Gfl-CD03ubStrate can be formed by either binary complex (1-
Ber°Gfl-CD or GflCD°substrate) associating with the third molecule (substrate and
alcohol respectively) given by K2 and K4. The equilibria are described as follows, where

S equals the substrate,

Kl
l-Ber + Gfl-CD Ber'Gfl-CD

 

 

 

 

 

K2
l-Ber-Gfl-CD + S l-Ber-Gfl-CD-S
K3
GflCD + S GflCD-S
K4

 

Gfl-CD°S + l-Ber l-Ber0Gfl-CD'S

The equilibrium constants are related by 1(le = K3K4, as verified by previous

measurements [100]. K4 was shown to be the dominant equilibrium by those studies. The

45

 

 

 

 

30
O
25 r O
o
O
8
Va, 20 r O
*2
_ O
15 _
O
O
10? l l l l
0 0.2 0.4 0.6 0.8 ‘l
[CD]/[CD] + [ROH]

Figure 9. Job plot of the relative phosphorescence intensity from a 1-Ber0Gfl-CD0t-
BuOH complex monitored at 530 nm. The l-Ber concentration was 1 x 10'5 M, and
[GB-CD] + [t-BuOH] = 2.5 x 10'3 M.

46

concentrations of free Gfl—CD and l-BerOGflCD are negligible in the limit of large
excess concentrations of alcohol with respect to Gfl-CD. Consequently, measurement of
the binding constant of l-Ber to GflCD in an alcohol/water mixture is an effective
measure of K4. The binding is determined experimentally as described previously by
measuring an optical change in the absorbance of l—Ber at 284 nm upon the addition of
Gfl—CD alcohol/water solutions. K; for each ternary complex was determined from the
slope of the linear plot of absorbance vs. [Gfl-CD]; the values are summarized in Table l.
The association constants were generally large (> 1000 M"), but did not correlate well
with the quantum yield. For example, NpOH has the highest quantum yield of
phosphorescence of l-Ber, whereas the binding to CD iS smaller than t-BuOH or DBM.
In the cyclohexyl series, the quantum yield of CycMeOH is the largest, but the magnitude
of the value for K; is intermediate. Accordingly, a comparison of the binding and
photophysical data (quantum yield) reveal that factors other than the association of the
ternary complex govern the radiative processes of the triplet State of l-bromonaphtalene
from a l-Ber-Gfl-CD0alcohol complex.

The photophysics of l-Ber phosphorescence included in Gfl-CD dissolved in

water/alcohol mixtures is described by the following scheme:

   

K4
*1-Ber + coco-s —._._-_ *1-BerGflCD-S
1-Ber-Gfl-CD-S 1-Ber-Gfl-CD-S
+ +

102 102

47

where kg and k] are the excited state decay rate constants of the phosphorescence of l-
Ber in water and within the Gfl-CD and kqo and kq. are the bimolecular oxygen
quenching rate constants of l-Ber in water and within the Gfl-CD, respectively. Gfl-CD
and l-Ber°G,B-CD were excluded since their concentrations are negligible as mentioned
previously. The above scheme is based on the dominance of the K4 equilibrium and on
the assumption that the binding constant of the ground and excited states of l-Ber to
GflCD are comparable. Ground and excited state binding differ when excitation
produces a large dipole change in the excited State, as is the case for molecules that may
contain ketone groups [113]. A dipole moment change that causes the exit rate from the
CD to increase in the excited state relative to the ground state will result in a smaller
excited state formation constant. In the case of l-Ber, a significant change in charge
redistribution is not expected. Further, dipole moments can be predicted by changes in
the Stokes Shift in solvents of varying dielectric constants [114]. Solutions of l-Ber in
acetonitrile, toluene, ethanol and ethyl acetate did not show an appreciable Stokes shift.

Consequently, similar K43 are expected for l-Ber and l-Ber*.

Stern-Volmer Quenching

The long-lived green phosphorescence of l-Ber (To = 0.64 ms) is quenched
efficiently by oxygen in aerated solutions of methanol and water (Ill). The quenching
rate constant of l-Ber determined by linear Stern-Volmer plots of the lifetime as a
function of oxygen concentration is 5.0 x 108 M"s'l [100]. Addition of alcohols
diminishes the rate of oxygen quenching dramatically in solutions with Gfl-CD to 103 -

104 M"s". In fact, the quenching rate constants and quantum yields directly correlate

48

with one another, as shown in Figure 10, which plots k,,(02) and the quantum yield of
phosphorescence of l-Ber, 0pm,, for all the ternary complexes in Tables 1 and 2. The
quantum yield increases with decreasing quenching rate constant and generally follows
equation 1 in Chapter 1.

Addition of a second hydroxyl group also significantly alters the phosphorescence
of l-Ber in the ternary complex. Table 1 lists the quantum yields, binding constants and
quenching rate constants for cyclohexyldiol substrates. The quantum yield of l-Ber
phosphorescence with a 1,2-Cyc-diOH is greater than that observed for CchH capping
l-Ber°G,8-CD. However as the two hydroxyl groups are spaced further apart around the
cyclohexyl ring, an attenuation of l-Ber emission is observed. Although the formation
constants of 1,2-Cyc-diOH and 1,3-Cyc-diOH are similar, 1,3-Cyc-diOH has a smaller
quantum yield and larger quenching rate constants by a factor of 15. When the two
hydroxyl groups occupy the 1,4- position of the cyclohexyl ring the phosphorescence is
barely detectable and the oxygen quenching rate constants are at the detection limit.
Hence, it appears the 1,4-Cyc-diOH is very weakly associated with l-Ber0G-fl-CD and

inefficiently protects the triplet state of l—Ber.

Other Hydrogen-Bonded Substrates

Alcohols are not the only substrates that are recognized by the l-BersGfl-CD
complex. Table 2 presents the data collected for a variety of other hydrogen-bonding
substrates that trigger l-Ber phosphorescence from a ternary complex at room
temperature. The t-butyl moiety is maintained across the amine, aldehyde, carboxylic

acid, ester series for purposes of comparison. The overall quantum yield of l-Ber

49

35

 

0'15—

k / M“S"1

10_—

01

O

O
O 0% 1 $2 I n 1 Q
0.00 0.05 0.10 0.15 0.20
Quantum Yield

 

 

 

Figure 10. Oxygen quenching rate constant as a function of quantum yield for the 1-
Ber°G,B-CD°substrate complexes listed in Tables 1 and 2.

50

Table 2. Quantum yields, equilibrium constants and oxygen quenching rate constants for
other hydrogen bonding substrates.

 

 

Substrate Symbol limos/10'4 K4/M'I kq/IO5 M'IS-I
Pivalic acid PA

pH 2.9 1697 2340 0.21

pH 9.0 27 3771 3.9
tert-Butylacetic acid t-BuAA

pH 2.9 765 1881 0.99

pH 9.0 292 3934 1.9
tert-Butylamine t-BuNHg 210 1330 34
Neopentylamine NpNHg 1024 359 3.0
trimethylacetaldehyde TMA 299 330 3.7

Methyl trimethylacetate MTMA 17 l 1817 14

 

51

phosphorescence increases along the series ester (MTMA) < aldehyde (TMA) < amine (t-
BuNHz) < alcohol (t-BuOH) < acid (PA). In the case of the amines, t-BuNHz and NpNHz,
additional quenching pathways involving charge transfer quenching of the excited State
are possible. Secondary and tertiary amines are known to quench triplet states of aromatic
hydrocarbons by single electron transfer, owing to their low ionization potentials [115].
In deaerated methanol, t-BuNHz does quench l-Ber phosphorescence, with a Stem-
Volmer quenching rate constant of 0.84 X 105 M"s". Notwithstanding, l—Ber included
in a cyclodextrin cup is not quenched. An invariant phosphorescence lifetime for l-Ber
in Gfl-CD provides evidence that addition of the amines, t-BuNHz and NpNHz, have little
effect on the triplet state radiative decay. On that account, the rate constants listed in
Table 2 reflect exclusively the quenching of l-Ber triplet by oxygen.

Similar trends appear for amine substrates as were observed for alcohol
substrates. In particular, the most intense phosphorescence was noted when the t-butyl
group was separated from the hydrogen-bonding group by one methylene spacer.
Although the overall quantum yields are smaller than the t-butyl alcohols, the quantum
yield measured for NpNHz was substantially larger (5 times) than I-BUNHz. Consistent
with the data for alcohols, the quenching rate constant for the amines also decreased by
an order of magnitude upon addition of the methylene Spacer. A comparison to
carboxylic acids, PA and t-BuAA, also yields similar results, except the carbon spacer
that was previously a methylene is replaced with a carbonyl group.

The carboxylic acid complexes, PA and t-BuAA, also have a significant
dependence on pH, as is shown by the quantum yield at 2.9 and 9.0. The

phosphorescence of l-Ber decreases with increasing pH and shows the largest changes

52

at the pKa of PA (5.24) and t-BuAA (5.05). The association constant of PA and t-BuAA
is larger for the carboxylate form than for the acid form; however, the oxygen quenching
rate constant is larger as well. Oddly enough, although the carboxylate forms have larger
binding, their ability to shield photoexcited l-Ber is decreased. The increase in the
formation constant is most likely due to the fact that an ionic species has a stronger

interaction with the hydroxyl rim of the cyclodextrin than does a neutral molecule.

Molecular Modeling

Energy minimizations on the Biosym program Discover have given us some
insight on the topology of the ternary complexes and allowed us to visualize their
geometrical arrangements as a result of changes to the substrate. In many instances the
visualizations helped make sense of the experimental results and gave us explanations for
the unpredictable binding behavior.

The minimizations were performed by successive additions of each of the three
molecules in the ternary complex. The binary inclusion complex of l-Ber and Gfl-CD
was first minimized. The results demonstrated that the bromine of bromonaphthalene has
a preferred orientation on the secondary (wider) side of the cyclodextrin, most likely due
to the fact that the glucose moiety is located on the primary side. It was thought that
bitopic recognition of bulky hydrogen-bonding substrates occurred as the hydrogen-
bonding group associated with the hydroxyl rim of the CD and the bulky portion
protected the hydrophobic interior. We then added substrates to the primary side of the
cyclodextrin (narrower side); however, it is possible that substrates could interact with

the secondary side as well. The secondary Side provides 14 hydroxyls that a substrate

53

could interact with, whereas the primary side has 7 (3 more when a glucosyl is attached).
It remains to be seen which side of the cyclodextrin and therefore which part of the l-
Ber is most susceptible to attack by oxygen; the next section will describe results for
covalent systems that may explain the mechanism of oxygen quenching.

Molecular models of t-BuOH, NpOH and DBM complexes reveal that the
methylene spacer better positions the t—butyl group over the unprotected l-Ber, as
compared to no spacer (t-BuOH) and an ethylene spacer (DBM) (Figure 11). In the case
of t-BuOH, it is pulled Slightly inside the cyclodextrin, pushing the bromonaphthalene
out of the secondary side. The addition of a methylene spacer, however, acts as a hinge,
and this substrate sits snugly atop the cyclodextrin. When the ethylene spacer of DBM is
added however, it appears that the aliphatic group has an affinity for the cup and pushes
l-Ber even further outside than t-BuOH.

The disparate properties of carboxylate and carboxylic acids are also explained
very well when one looks at the molecular modeling minimizations. The models show
that ion-dipole interactions of the carboxylate at the hydroxyl rim lead to a pronounced
distortion of the cup. Since there is a negative charge present, more hydroxyl groups must
surround the carboxylate than in the case of a neutral carboxylic acid, resulting in the
distortion. The ability of the substrate to shield l-Ber is compromised, as the
cyclodextrin conformation is drastically changed and as a result l-Ber is exposed to

oxygen.

54

 

Figure 11. Molecular model of ternary complexes of l-Ber (light gray), fl—cyclodextrin
(shaded) and alcohol substrates (black): t-BuOH (t0p), NpOH (middle) and DBM
(bottom).

55

Conclusions

The first study illustrated that substrates such as alcohols trigger phosphorescence
from l-bromonapthalene included in the cavity of ,B-CD. The substrate is bitopically
recognized by the hydroxyl rim and hydrophobic interactions with the cyclodextrin. The
bulky tert-butyl groups and cyclohexyl groups were maintained across the series of
substrates because the steric bulk of the aliphatic groups does not permit them to
coinclude with guests in the fl-CD. The position of the t-butyl or cyclohexyl group over
the CD cup is one important factor in shielding 1-Ber from oxygen. For all the series
investigated, the presence of a single carbon spacer produces the highest emission
quantum yields. Molecular models shed some qualitative insight into the role of the
spacer (Figure 11). Of the three alcohols in the t-butyl series, the methylene spacer of
NpOH better positions the t-butyl group over the CD as compared to t-BuOH with no
spacer and DMB, which has an ethylene Spacer between the t-butyl and hydroxyl groups.
The positioning of the alcohol over the primary side of the cyclodextrin causes the l-
Ber to protrude from the secondary side. Interestingly, NpOH sits on top of the CD,
while minimially perturbing the orientation of the l-Ber within the cup. Conversely,
both the t-BuOH and DMB tend to get pulled inside the cavity as the hydrogen bond
develops, pushing the l-Ber slightly outside the CD cup on the secondary Side. For t-
BuOH, the hydrogen bond directs the t-butyl group into the CD cup. With the addition of
a flexible methylene spacer, the t-butyl group is better able to Sit on top of the cup.
However, as the Spacer increases in length, the t-butyl group tends to Show a higher
affinity for the CD cup and therefore sterically crowds the l-Ber, pushing deeper into

the CD. The cyclohexyl alcohol series shows Similar results with the cyclohexyl group

56

crowding its way into the CD cavity to partially expel the l-Ber; this steric crowding is
minimized for CycMeOH.

The strength of the hydrogen bond appears to be an additional contributing factor
to determining the photophysics of the complex. The luminescence intensity increases
along the series ester < aldehyde < amine < alcohol < acid and generally follows the
propensity Of the substrates to hydrogen bond to the rim of the CD cup. The better
hydrogen bonding ability of the substrate is reflected in smaller oxygen quenching rate
constants of l-Ber phosphorescence, presumably resulting from the substrates ability to
be a more effective cap of the l-Ber-Gfl-CD complex.

Because hydrogen-bonding is important, we suspected that the introduction of
additional hydroxyl groups on cyclohexanol might significantly enhance the intensity of
the observed phosphorescence of the complex. This appears to be the case but only if the
hydroxyl are judiciously placed about the cyclohexyl ring. For hydroxyls at the 1,2
positions of the ring, the diol appears to be a particularly excellent cap of the 1-Ber0Gfl-
CD complex and we Observe the smallest quenching rate constant for any substrate.
Examination of molecular models of the diol and CD cup shows the possibility for two-
point hydrogen bonding, which is the Situation for 1,3- and 1,4-diols. However, the 1,3-
diol exhibits only intermediate behavior for oxygen inhibition and the 1,4-diol Shows
negligible inhibition of the oxygen quenching reactivity. While racemic mixtures of the
cis and trans diols were employed, as opposed to the cis form of the 1,2-diol, the excess
concentration of diols was large enough to ensure that the trans and cis conformers were
each in the concentration independent regime of the luminescence response. As the

hydrogen bonding groups move around the ring, the dispersion interaction between the

57

aliphatic cyclohexyl group and the hydrophobic interior of the CD is reduced, leading to
a decrease in the stability of the complex owing to stronger dipolar interactions of the
diol with water. Consequently, the substrate is less able to shield the l-Ber from
oxygen. Decreased stability of CD complexes in aqueous solution upon increases in
substrate polarity are well documented and also occurs when the basicity of the substrate
increases [116]. Better hydrogen bonding interactions available in the more acidic

environment of water are preferred to those offered by the CD hydroxyls.

CHAPTER 3

REGIOISOMERIC EFFECTS ON THE EXCITED STATE PROCESSES OF A
CYCLODEXTRIN MODIFIED WITH A LUMOPHORE

Introduction

Supramolecular topologies based on cyclodextrin molecules may be exploited to
engender novel physical and chemical processes. The hydroxyl rims, as shown in the last
chapter, can be used as points of intermolecular attachment in a ternary supramolecular
complex. In this chapter, the covalent attachment of lumophore to the primary and
secondary rims of CDs will be discussed as receptor sites for bulky alcohol
chemosensors. A covalent lumophore-CD complex is advantageous for marrying to
optical fibers. Typically, the primary hydroxyls provide the most convenient points of
attachment to the CD, because the reactivity of the secondary hydroxyl groups is
diminished. However, when CDS are derivatized on the primary and secondary OH
groups with the same functional group [117-119], substantial differences in thermal
reactivity have been observed.

In recent years, the use of CD5 to control molecular processes has been extended
to reaction pathways proceeding through excited states. As a complement to the

macromolecular fluorescent schemes designed for ion sensing [120], lumophores have

58

59

been appended [20,121] onto CDs to afford supramolecules that produce a luminescent
signal upon association of neutral substrates. An important and outstanding issue in the
design of photoactive CD supramolecules is whether regioisomeric effects on reactivity,
so prominent in ground state processes, are manifested in excited state processes as well.
The synthesis of photoactive CD regioisomers is rare [122], and to date no significant
differences have been observed in the photophysics or photochemistry of these
congeners. We now report an unusual example of differences in the excited state
properties of regioisomers and in the chemistry of a bromonaphthalene lumophore

appended to the primary (1) and secondary (2) OH groups of flcyclodextrin.

Experimental

The reaction of dry ,B—CD with NaH in dry DMF, followed by addition of l-
bromo-2-bromomethylnaphthalene, yielded two regioisomers of the covalently bound
phosphor-CD (Figure 12). The l-bromonaphthalene is covalently tethered either to the
primary or the secondary side of ,B-CD. These syntheses were developed by Dr. Mark
Mortellaro and have been described in detail previously [101]. The compounds will be
referred to as l-Ber-fl-CDI or 1 and as l-Ber-fl-CDZ or 2, for the primary and
secondary side complexes, respectively. Emission spectra were collected on a Hitachi

F4500 and lifetimes were collected as described in Chapter 2.

Results
The emission spectra of the two compounds showed dramatic differences in the

photophysical properties of the triplet state (Figure 13). Whereas both molecules

60

 

 

 

 

 

 

 

A :9 NaH,DMF C A ”'0
Ar
——>
Br Br +
OH Br 0
1 2

Figure 12. Synthesis of regioisomers of 1-bromonaphthalene covalently tethered to the
primary (1) and secondary (2) hydroxyl rims of ,B-CD [101].

61

 

 

  

 

 

 

 

40
35 _.
a
a ( )
“if: 30 __
C
2
C
E 25 __
.9
(I)
.‘L’ 20 _.
E
LIJ
g 15 _
E
a 10 e
5 _
(b)
O l .40000‘4d
350 400 450 500 550
Mnm)

Figure 13. Relative emission intensity of (a) 2 and (b) 1 (each 1.0 x 10'4 M) in air-
saturated aqueous solution (excitation wavelength Aex = 290 nm).

62

fluoresce at 350 nm, with nearly equivalent quantum yields, the phosphorescence

quantum yield of l-Ber-fl—CDZ at 530 nm is more than 102 times more intense than that

observed for equimolar solutions of l-Ber-fl—CDI (¢53O(1) S 104; (15:30 (2) = 0.05 in

;.
aerated H20; ¢Z30(1 and 2)z0.06 in the absence of oxygen). Deoxygenation by freeze
pump thaw or nitrogen purging results in a dramatic increase in the phosphorescence
intensity of l-Ber-fl-CDI, but only a slight increase for l-Ber-fl—CDZ, suggesting that
the bromonaphthalene moiety of 1 is unprotected, whereas 2 is not. The fluorescence
intensity is not affected significantly by deaeration.

The dissimilar properties of l and 2 may be attributed to the latter compound
forming an intramolecular complex in which the bromonaphthalene moiety is included in
the CD cavity. To prove this hypothesis, we looked for competitive binders that would
displace the bromonaphthalene moiety of the cavity, thereby quenching the
phosphorescence. Dodecyltrimethylammonium bromide (DTAB), a water soluble
surfactant, was chosen as it is known to form inclusion complexes with fl-CD [123].
Titration of 2 with DTAB results in a decrease of phosphorescence intensity of the
bromonaphthyl moiety at 520 (Figure 14). The phosphorescence intensity approaches
zero as DTAB concentration increases (to below the critical micelle concentration 5-15
mM). A fit of the data to a least squares routine [124], yields a Ka of 16001- 160 M'l for
the binding of the bromonaphthyl moiety of 2 with the ,B-CD cavity, which is larger than
the association of DTAB to ,B-CD [123]. Other cyclodextrins modified on the secondary
face Show a greater tendency to bind aromatic residues owing to increased hydrophobic

interactions and solvent shielding [125]. Therefore the increased stability of 2 leads to an

63

 

350
<5
300..
O
.4?
g 250 __O
E e
E» 200_
.8 O
LIEJ 150-— o
(D
E, O
{—3 100_. 0
CC
0
50._ O
O
00
| l 10010 lo

 

 

 

|
00 0.5 1 1.5 2 2.5 3 3.5
[DTAB] / 10'3 mol dm"3

Figure 14. Phosphorescence intensity at 520 nm from 2 in aerated aqueous solution
titrated with DTAB.

64

attenuation in the binding of DTAB. The results of the titration of DTAB can be ascribed
to the quenching of l-Ber phosphorescence when the phosphor is excluded from the
CD cup by a competitive guest.

To better define the quenching kinetics of the bromonaphthalene moiety of 1 and
2, a water-soluble inorganic complex, Co(NH3)63+, was procured as a quencher of the
excited state in deoxygenated solutions [126]. A list of the phosphorescence quenching
rate constants in the absence of oxygen for 1, 2, 2 with DTAB, the binary complex of l-

Ber-G-fl-CD and free l-Ber in a methanol/water solution is given in Table 3. Stem-

Volmer plots of the phosphorescence lifetime of l-Ber (To/‘1') versus concentration of
Co3+ were linear with a y-intercept of unity. Several conclusions can be drawn from the
data in Table 3. First, the large quenching rate constants of 1 and free 1-Ber are similar
in magnitude, indicating that the bromonaphthalene moiety of l is exposed to bulk
solvent. Since l-bromonaphthalene included in GflCD shows nearly two orders of
magnitude lower quenching, the data are consistent with in inability of l to form an
intramolecular inclusion complex at a low concentration (intermolecular complexation is
not observed). Table 3 shows that the quenching rate of l-Ber-fl-CDZ is more than 104
slower than 1. However, when 2 is saturated with DTAB, the bromonaphthalene moiety
is quenched by Co“ at a high rate, 1.7 x 108 M'ls", coinciding with the rates for 1 and
free l-Ber. We can therefore conclude with certainty that compound 2 contains an
intramolecularly included bromonaphthalene and that addition of DTAB displaces Ber,
thus exposing the excited state to quenching in solution by Co”. CPK modeling provided

a more accurate description Of the topology of the covalent complexes of Ber and ,6-

CD, 1 and 2. The secondary side of ,B-CD is wide enough for inclusion of Ber;

65

Table 3. Rate constants (kq) for the phosphorescence quenching by CO(NH3)63+ in
systems containing l-bromonaphthalene and/or fl—cyclodextrin, in deaerated aqueous

solutions at 25 °C. Emission lifetimes were measured at 520 nm; excitation wavelength
309 nm.

 

 

Compound kq/M"5'l
l-Ber-eCDz 1.5 x 103
l-Ber-G-fl-CD“ 1.9 x 106
l-Ber-fl-CD] 8.2 x 107

l-Ber-eCDzoDTAB 1.7 x 108
18erb 1.8 x 108

 

a binary complex formed from G-fl-CD
and l-Ber [Ref. 100-101]
b l-Ber in methanol/water (1/1).

66

however, conversely, the primary side is too narrow for the unobstructed intramolecular
binding of Ber. We hope to use these novel results for future design of CD-based
supramolecular assemblies and in determining how energy flows, based on the topology
of a phosphor appended CD. The next section discusses model ternary systems for

designing linkages that will maximize the geometry restraints of bromonaphthalene in a

,B-cyclodextrin cavity.

Conclusions

Electronic structure and conformational geometry both contribute to the overall
phosphorescence response of bromonaphthalene emission in regioisomeric covalent
complexes of cyclodextrin and bromonaphthalene and in ternary complexes of CDs,
alcohols and bromonaphthalene derivatives. From the study of regioisomeric compounds,
only the derivative 2 formed an intramolecular complex in which the appended
bromonaphthalene is inside the cyclodextrin cavity, giving rise to novel room
temperature phosphorescence in an oxygenated and aqueous environment. These results
Show that regioisomeric effects can control reactions of the excited state of photoactive
CDs. Such issues will play an especially prominent role in the design of CD-based
supramolecules for optical sensing, especially those that derive their function from

equilibria of appended lumophores inside and outside the CD cup.

CHAPTER 4

ELECTRONIC STRUCTURE EFFECTS OF BROMONAPHTHALENE-CD
COVALENT AND TERNARY COMPLEXES

Introduction

The synthesis of regioisomeric compounds of l-Ber and fl—CD, 1 and 2,
revealed that the structure of an intramolecular inclusion complex can Significantly affect
the photophysical properites. Covalent attachment of l-Ber to ,B-CD at the 2-position of
the naphthyl ring with one methyl spacer did not provide us with a phosphorescence
triggered optical Signal in the presence of bulky alcohols. We presumed that the close
proximity of the bromine to the CD linking position was not the optimal topology for the
intramolecular complex. Therefore, an investigation of substituted bromonaphthalenes in
ternary complexes with G-flZD and t-BuOH was initiated in an attempt to predict the
photophysical properties of a covalent lumophore-CD complex. Our efforts to study
various substituted 1- and 2-Ber and to predict the behavior of bromonaphthalene

appended cyclodextrin complexes prior to synthesis are now described.

67

68

Experimental

The synthesis of 2-bromo-6-alkoxynaphthyl-fl-CD was achieved by Jude
Rademacher [127] using methods similar to the synthesis of the regioisomeric
cyclodextrins prepared in Chapter 3. Mono glucosyl-fl-cyclodextrin (Gfl-CD) was
purchased from CTD, Inc. and used without further purification. Tert-butanol (t-BuOH)
was purchased from Fisher and used as received. The following substituted naphthalenes
were obtained from Aldrich: 5-bromoacenaphthalene (5-BrAch), 1-bromo-2-
methylnaphthalene (1-Br—2-MeNp), 1-bromo-4—methylnaphthalene (1-Br-4-MeNp), l-
bromo-2-naphoic acid (l-Br-2-NpA), 2-bromo-6-naphoic acid (2-Br-6-NpOH), 2-bromo-
6-methoxynaphthalene (2-Br-6-MeONp). Other compounds employed in this study were
1-bromo-2-naphthoic acid (1-Br-2-NpA, Sigma) and 1-bromo-2-methoxynaphthalene (1-
Br-2-MeONp, Crescent Chemicals). Absorption spectra were collected on the OLIS Cary
17, and emission spectra and quantum yields were collected on the high-resolution

research instrument as described in Chapter 2.

Results

The absorption Spectra of bromonaphthalene derivatives, l-Ber, 1-Br-2-NpA, l-
Br-2-MeNp, 1—Br-4-MeNp, 1-Br-2-NpOH, 2-Br-6-NpOH, 1—Br—2-MeONp and 2-Br-6-
MeONp have different absorption maxima (Figure 15 and Figure 16). The maxima for l-
Ber, l-Br-2-NpA and l-Br-2-MeNp coincide at 285 nm, whereas l-Br—4-MeNp is red-
shifted (Am, = 290 nm) and all other derivatives are blue shifted, 1-Br-2-MeONp (Amax =
283 nm), l-Br-2-NpOH (Am. = 279nm), 2-Br-6-NpOH (7tmax = 276nm) and 2-Br-6-

MeONp (Amx = 272nm). Additionally, the hydroxyl and methoxy substituted

69

 

(a)
(b)

l

(C)

Absorbance

(d)
(e)

 

 

l l l l

240 260 280 300 320 340
Wavelength (nm)

—

 

Figure 15. Absorption spectra of (a) l-Br-2-NpA, (b) l-Ber, (c) l-Br-2-MeNp (d) l-Br-
4-MeNp, (e) 5-BrAch in methanol.

7O

 

Absorbance

BE:
”A“

 

 

 

240 260 280 300 320 340 360
Wavelength (nm)

Figure 16. Absorption spectra of (a) 2-Br-6-MeONp, (b) 2-Br-6—NpOH, (c) l-Br-2-
NpOH (d) 1-Br-2-MeONp, (e) l-Ber in methanol.

71

bromonaphthalenes show the appearance of a second absorption band in the 320-430 nm
range. Therefore, the excited state energy is unchanged when substitutents in the 2-
position are methyl or carboxylic acids, is stabilized upon substitution in the 4- position
and is destabilized when hydroxyl or methoxy groups are substituents. Table 4 lists the
absorption maxima and the molar absorptivities measured in methanol solutions. The
molar absorbtivites were derived from Beer's law with a dilute solution (1 x 10‘4 M). The
molar absorptivies of l-Ber, 1-Br-2-MeNp, 1-Br-4-MeNp and 5-BrAch are all around
8000 M"cm", whereas the compounds containing hydroxyl or carbonyl groups have
significantly smaller molar absorptivies around 5000-6000 M'lcm’l.

The titration profiles of tert-butanol with l-Ber, 1-Br-4-MeNp, l—Br-Z-MeNp,
1-Br-2—NpOH and 2—Br-6-MeONp and 2.5 mM Gfl-CD are shown in Figure 17. The
methyl substituted bromonaphthalenes show the largest phosphorescence response with
added alcohol, followed by the bromonaphthols. 2-Br-6-MeONp shows no increase in
bromonaphthalene phosphorescence. The phosphorescence emission spectra of
bromonaphthalene derivatives are Shown in Figure 18. With the exception of 2-Br-6-
MeONp the phosphorescence profiles for 1-Br-4-MeNp, l-Br-2-MeNp, 1-Br-2-NpOH
and S-BrAch have identical phosphorescence profiles. The maxima of 1-Br-4-MeNp is
noticeably red shifted relative to 1-Ber and to the other derivatives.

The results for the bromonaphthalene derivatives are summarized in Table 4.The
quantum yields Show that bromonaphthalenes with only methyl substituents have a larger
quantum yields than 1-Br-2-NpOH, 2-Br-6-NpOH, 1-Br-2-NpA, l-Br-2-MeONp and 2-
Br-6-MeONp. The oxygen quenching rate constant follows the trend of the quantum

yields; roughly, the quenching rate constant decreases with increasing quantum yield.

72

Table 4. Quantum yields, equilibrium constants and oxygen quenching rate constants for
bromonaphthalene phosphor derivatives with GB—CD (10'3 M) and t-BuOH (in the
asymptotic limit).

 

 

Phosphor Am“ /nm abs (1),,th 10'4 e/M'l cm’l kq/ lOSM'IS'I
l-Ber 285 340 8670 8.87
l-Br-2-NpA 285 0 5162 >103
l-Br-2-MeNp 285 124 8006 5.89
1-Br-4-MeNp 290 269 7837 4.28
l-Br-2-NpOH 279 37 5027 33.0
S-BrAch 299 134 8315 11.0
2-Br-6-NpOH 276 0 5054 >103
l-Br-2-MeONp 283 0 6210 >103

2-Br-6-MeONp 272 0 5787 >103

 

73

 

Rel. Intensity at 530 nm
0
O
O

 

 

 

 

0.5““ 1' 1.5
[t-BuOH]

Figure 17. Dependence of relative phosphorescence intensity of l-Ber (C) l-Br-4-
MeNp (O) l-Br-2-MeNp (I), 5-BrAch (El), 1-Br-2-NpOH (V) and 2-Br-6—MeONp
(A) at 530 nm from aerated solutions containing Gfl-CD (10'3 M) concentration and
increasing concentration of t-BuOH.

74

 

 

Rel. Intensity

 

 

£1) (C) \\

 

 

I
it . (
I
J..- ---,eflfl,.w
, .. l ~~--j--_— " n. __ _

 

450 500 550 600 650 700
Wavelength (nm)

Figure 18. Luminescence spectra of (a) l-Ber (b) l-Br-4-MeNp (c) 1-Br-2-MeNp (d) 5-
BrAch (e) l-Br-2-NpOH (gray) (f) 2-Br-6-NpOH (dotted line) from aerated aqueous
solution of Gfl-CD (10'3 M) in the presence of ~ 3% v/v of t-BuOH. Spectra were
collected in the asymptotic limit of the titration profiles shown in Figure 17.

75

We were interested in studying the photophysical properties of 2-Br-6-MeONp
covalently linked to ,B-CD because of its lack of phosphorescence in the presence of
added alcohol. Figure 19 shows the fluorescence and phosphorescence of the covalent
lumophore-CD complex, 2-bromo-6-alkoxy-naphthyl-fl-CD (2-Br-6-NpCD), in aerated
and degassed solutions. Addition of t-butanol and cyclohexanol to 2-Br-6—NpCD did not
result in triggered phosphorescence. The explanation for this behavior may be related to
the conformational and electronic differences between 1- and 2-substituted
bromonaphthalenes in a cyclodextrin cavity. It has been shown by circular dichroism that
l- and 2-substituted naphthalenes have different orientations in a ,B-cyclodextrin [128].
The two modes of inclusion of naphthyl-moities in a CD cavity are axial and equatorial,
as shown in Figure 20. Naphthalenes substituted in the 1 position on the ring can orient in
either axial or equatorial positions, whereas 2-substituted naphthalenes prefer an axial
conformation. Since 2-Ber and its derivatives are axially oriented in the cyclodextrin
cavity, this topology could result in a triplet excited state that is readily quenched by
oxygen. The equatorial conformation that l-Ber can assume in a CD cavity appears to
be optimal for Shielding the triplet state. Therefore complexes that favor the equatorial

conformation are ideal for the design of covalent Ber-CD complexes.

Molecular Modeling
Energy minimizations on Discover were used for visualizing the geometrical
constraints of covalent and binary complexes of l-bromo-X-methyl naphthalene (where

X is the position on the naphthyl ring) and Gfl-CD. The results are shown in Figure 21.

Rel. Intensity

76

 

 

L

L

(d)

 

 

1

  

L

 

 

300

400

500
Mnm

600

700

Figure 19. Fluorescence (a,c) and phosphorescence (b,d) of 2-bromo-6-alkoxy-fl-

cyclodextrin

in

water

for

degassed

(ab)

and

aerated

(c,d)

solutions.

77

 

 

 

 

O
Equatorial Axial

Figure 20. Equatorial and axial conformations of naphthalene in a ,B-cyclodextrin cavity.

78

(a)

 

Figure 21. (a) Energy minimizations of bromonaphthalene derivatives covalently bound
to Gfl-CD. From left to right the molecules are: (first row) l-bromO-7-methyl-Gfl-CD, l-
bromo-S-methyl-Gfl-CD, l-bromo-3-methyl-Gfl-CD, (second row) l-bromo—6-methyl-
Gfl-CD, l-bromo—4-methyl-Gfl-CD, 1-bromo-2-methyl-Gfl-CD. (b) Energy
minimizations of binary complexes of bromonaphthalene derivatives. 1-bromo-7-
methylnaphthalenewith Gfl-CD (left), 1—bromo-6-methylnaphthalenewith Gfl-CD
(middle) and 1-bromo-5-methylnaphthalenewith Gfl-CD (right).

79

The covalent complexes of l-bromo-5-methylnaphthalene and l-bromo-4-methyl
naphthalene have geometries where the naphthyl ring is close to the hydrophobic cavity.
The naphthalene moieties are attached to the secondary side and the glucosyl group is on
the primary side. The bromonaphthyl substituted covalent cyclodextrins (X = 2, 3, 6, and
7) give conformations where the phosphor moiety is far away from the hydrophobic
pocket. In all cases of the covalent complexes it appears that the bromonaphthalene is

unprotected.

The lower half of the figure shows minimizations for binary complexes of l-
bromO-7-methylnaphthalene, 1 -bromo-6-methylnapthalene and 1 -bromo-5-
methylnapthalene with Gfl—CD. In all cases the included molecule prefers an orientation
where the bromine atom extends away from the secondary side of the cyclodextrin. A
comparison of the binary complexes of l-Br-7-MeNp, l-Br-6-MeNp, 1-Br-5-MeNp and
Gfl-CD with the covalent complexes shows that none of these complexes have similar
conformations once they are covalently linked to the cyclodextrin. This indicates that a

covalent bond Significantly hinders the movement of the bromonaphthyl moiety.

Figure 22 and Figure 23 reveal the conformations for binary complexes of l-
Ber, 2-Ber, 1-Br-2-MeNp, 1-Br-2-MeNp and l-bromo-Z-naphthoate with Gfl-CD.
The structures on the left are minimized with the bromine portion of bromonaphthalene
away from the cavity and the right side Shows minimizations for binary complexes where
bromine was pointed towards the cyclodextrin cavity. From these models we observe that
bromine protrudes from the either primary and secondary side of the cyclodextrin for
both isomers depending on the orientation prior to minimization. Another observation is

that l-bromonapthalene orients itself at a 45° angle between an axial and equatorial

80

(a)

 

Figure 22. Energy minimization of (a) l-bromonaphthalene with Gfl-CD (b) 2-
bromonaphthalene with G-fl-CD. The cyclodextrin is shown as a gray stick,
bromonaphthalene is a black stick and bromine is a solid ball.

81

(a)

 

Figure 23. Energy minimization of (a) 1-bromo-2-methylnaphthalene with Gfl-CD (b) 1-
bromo-4-methylnaphthalene with Gfl-CD (c) 1-bromo—2-naphthoate with Gfl-CD.

82

orientation, whereas 2-bromonapthalene prefers an axial position. These molecular
models are consistent with the circular dichroism data. Additionally, they may explain
why we do not see phosphorescence for 2-bromonapthyl derivatives. Turro et al. have
also observed weaker emission from 2-bromonaphthalene in comparision with 1-
bromonaphthalene [96a]. The dipole moment of bromonapthalene is also related to the
orientation in the cyclodextrin cavity [128]. However, although there is a slight difference
in the ground state dipole moment between l-Ber and 2-Ber, orientations of l-Ber
and 2-Ber in the cyclodextrin cavity iS dramatic.

The orientation of binary complexes of l-Br-2-MeNp and 1-Br-4-MeNp in G6-
CD is Shown in Figure 23. The Similarity between the orientation of the two methyl
substituted naphthalenes and the l-bromonapthalene is striking. In contrast, the
minimization of l-bromo-Z-naphthoate, reveals that the bulkiness of the group hinders

the molecule from entering the hydrophobic cavity.

Conclusions

Synthesis of another appended bromonaphthalene-6CD, 2-bromo-6-alkoxy-fl-
cyclodextrin, also reveals that the orientation of bromonaphthalene in the cyclodextrin
has profound effects on the photophysical properties of the excited state. Although
bromonaphthalene was attached at the secondary face of the cyclodextrin, room
temperature phosphorescence was not Observed under aerated conditions, and only
appeared when the solutions were degassed. Addition Of tert-butanol and cyclohexanol
also failed to trigger a luminescent response. It appears that 2-Br-6-alkoxy—fl-CD does

not form an intramolecular complex with the cavity.

83

As a result of the unpredictable results of covalent complexes of Gfl-CD and
bromonaphthalene, investigations of the photophysics of other bromonaphthalene
compounds were performed. In general, bromonaphthalene derivatives containing methyl
groups only slightly perturb bromonaphthalene phosphorescence from ternary complexes
of l-bromo-X-naphthalene°G,B-CDOt-BuOH, whereas hydroxy and methoxy groups
greatly decrease the phosphorescence intensity. It has been proven that the orientation of
the bromonaphthalene in cyclodextrin cavities is related to the dipole moment of the
guest [128]. Therefore a measurement of the dipole moment may allow us to predict the
orientation of bromonaphthalene in a CD cavity. Circular dichroism studies may assist in
determining the orientation and therefore allow us to determine which orientations lead to
bromonaphthalene phosphorescence. It appears that an equatorial conformation of Ber
in the cyclodextrin cavity is necessary for phosphorescence. Molecular modeling, through
the use of Biosym Insight Discover, has proven insightful in explaining many of the
properties of binary and tertiary cyclodextrin complexes with bromonaphthalene guests
and alcohol substrates. However, it does not provide an accurate description of the
topology of the covalent compounds of bromonaphthalene attached at the primary and
secondary rims (1 and 2). The difficulty in visualizing these structures by computer
modeling is that the structures shown are only one of many possible local minima. CPK
modeling is more accurate in predicting the structure and orientational restrictions of
covalent complexes, as the models are more rigid.

In conclusion, we have demonstrated that we can synthesize supramolecular
architectures with a phosphor and host structure that trigger very bright luminescence.

The enhancements in phosphorescence approached 105. From the study in this chapter it

84

appears that the next logical choice of bromonaphthalene starting material for the
synthesis of covalent phosphor materials is l-Br-4-MeNp with ,B-CD. Secondly,
lengthening the linking group between the bromonaphthlalene and cyclodextrin may also
allow for more flexibility. Lastly, studies of other water soluble methylated ,6-
cyclodextrins may be another alternative to Gfl-CDS for synthesizing photoactive sites for
optical chemosensin g of alcohols. The challenge that remains is balancing the structure of

the complex to compliment the desired photophysics. We are in the final steps of

incorporating optical transductions schemes into a prototype sensing device.

CHAPTER 5

ENERGY TRANSFER THROUGH COVALENT BONDS:
LANTHANIDE CHELATES

Introduction

Energy transfer processes for the design of BTX and PAH optical sensors will be
developed with (1) an understanding of Simple model complexes where energy transfer
donors and acceptors are held at fixed distances; and (2) authentic systems where the
PAH or BTX analyte is juxtaposed to the lanthanide ion and energy transfer does not
occur through covalent bonds. Electronic energy transfer is the transfer of energy from an
excited state donor molecule to a ground state acceptor. The functional form of the
distance dependence is determined by the type of interaction between the donor and
acceptor. Dipole-dipole interactions (FOrster) engender a transfer rate proportional to 1/R6
(Figure 24a) [17]. For this mechanism, energy transfer may occur over distances as long
as 80-100 angstroms (A), as is the case for photosynthetic reaction centers [129].

Conversely, exchange (Dexter) (Figure 24) interactions vary as a function of e'R

85

86

 

FOrster Dexter
(Coulombic) (Exchange)
A
—O — —— —O— —O— — — —O—
l l /\
o— m —00 —O— o— 00- 00 —0—
MB” “”6 “”0 “”6
_ _1_ R0 6 _ 31 , “ZRDA
ken _ to (RDA) ken - h KJ exp< L )

9000(In 10)K2(DDJ

 

128115114NAV

Figure 24. Two possible mechanisms of energy transfer and the rate expressions [17] D‘
and A are the excited electronic states of the donor and acceptor. D and A are the ground
states. RDA is the intermolecular distance. Other variables are described in the text.

87

and require that the donor and acceptor orbitals overlap at van der Waals contact.

Consequently, this type of energy transfer dominates at short distances.

F Orster Coulombic Theory

FOrster described the theory of dipole-dipole (coulombic, resonance) interactions
where a fluorescent molecule loses its excitation energy non-radiatively and
noncollisionally [130]. Since light, a beam of photons, is an electromagnetic wave
propagating through space, there are electric and magnetic vectors both oriented
perpendicular to the direction of propagation. The probability that light is absorbed is
dependent on the angle between the electric field vector of the incident light and the
absorption axis of the molecule. When a donor molecule is excited by this
electromagnetic wave there is a transition dipole moment created for the absorption and
emission of the light [131]. The transition moments are related to the absorption and
emission spectra of the molecule. For dipole-dipole energy transfer, only the emission
dipole of the donor and absorption dipole of the acceptor are involved. When these two
dipoles interact across a distance of 1/R6, the donor transition dipole induces a dipole
oscillation in the acceptor and causes non-radiative energy transfer. The equation that
describes dipole-dipole energy transfer is,

k = ilflié (17)
ID R
where T0 is the donor lifetime, R0 is the critical energy transfer distance or F'Orster

distance and R is the actual distance between the donor and acceptor. The equation for R0

88

measured in angstroms (A) is

I
R0 = 9. 7811103 {QDrzn‘U}? (18)
where J, the overlap integral, is measured in units of M'lcm3 units. QD is the donor
quantum yield, K is the orientation factor (2/3 for an isotropic distribution), and n is the
index of refraction, which appears because of the two point dipole interaction in an
isotropic medium.

The overlap integral is measured by taking the emission spectrum of the donor,
the absorption spectrum of the acceptor, and evaluating the following integral over
wavenumber, 17 .

J :12 fD(v~):/1 (9)6117
0 V (19)
It is important to note that this overlap integral includes the normalized donor emission

spectrum, f 0(17) , and the molar absorptivity, 54W), which is not normalized to unity.

The factor of ( V )4 originates from the classical harmonic oscillator approximation and

oscillator strengths. The overlap integral, J, is directly related to the critical distance, R0,
and can vary from 10"4 to 10'20 M" cm3.

If one can experimentally determine the rate of the energy transfer and therefore
determine the distance R, it is possible to determine the efficiency of energy transfer by
inspection of the calculated values of R0 derived from J. When R0 = R the efficiency of
energy transfer is 50%. Half of the energy goes to the acceptor, and the other half is
dissipated by heat and other processes. The smaller R becomes relative to R0, the more

efficient the process; the larger R becomes relative to R0, the less efficient the process.

89

From R0 and R, we can evaluate the efficiency of the process by the following equation:

R0 + R
There are numerous examples of dipole-dipole energy transfer in the literature,
among the most prominent are photosynthetic chlorophyll [129] and artificial light-
harvesting centers [132]. Resonance energy transfer (another term for the dipole-dipole
mechanism) is invoked in many biological systems to obtain donor-acceptor distances
[131]. One additional example of FOrster energy transfer where a solid state photon

antenna device with efficiencies approaching unity is being developed is the work of

Daschak and Mallouk [133].

Dexter Exchange Theory
Energy transfer events occur at van der Waals distances if exchange interactions
dominate. The theory of exchange energy transfer was derived by D.L. Dexter [134]. The

rate constant for this type of interaction is,

k = K] exp[-_—:£) (21)

where K is a constant related to specific orbital interactions, J is the spectral overlap
integral of the normalized emission of the donor and normalized absorption of the
acceptor, R is the distance from the donor to the acceptor relative to L, the sum of the van
der Waals radii [17].

There are several differences between FOrster and Dexter energy transfer.
Electron exchange interactions are purely quantum mechanical and, unlike FOrster

dipole-dipole interactions, have no classical description. The efficiency cannot be related

90

to an experimental quantity. Exchange interactions are strongest at shorter distances and
dipole-dipole interactions dominate at longer distances, depending on the oscillator
strengths of the acceptor and the overlap integral. Photosynthetic systems, where the
oscillator strengths of chlorophyll are large, have efficient transfers at 80-100 A because
the values for the overlap integral J are larger (1 x 10’'4 M" cm3). The rate of dipole-
dipole energy transfer depends on the oscillator strength, whereas for exchange
interactions there is no dependence on oscillator strength as the acceptor's molar
absorptivity is normalized.

Closs and coworkers, have defined the most renowned study of triplet-triplet
energy transfer, an exchange interaction involving an electron transfer and a hole transfer
[135]. AS shown in Figure 24 the rate for triplet-triplet energy transfer can actually be
derived from the rates for electron transfer (km) and hole transfer (km):

kn(St,N)= CkET(St,N)kHT(St,N) (22)
where St represents the steric factor, N is the number of 0 bonds between the donor and
acceptor, and C is a constant containing the ratio of the electronic coupling matrix
element and the Frank Condon weighted density of states. Distance and driving force
were varied in the Closs triplet-triplet energy transfer systems.

Another example of a triplet-triplet exchange energy transfer iS the non-covalent
system of biacetyl donor and hemicarcerand-aromatic acceptor complexes examined by
Farran and Deshayes [136]. Bimolecular energy transfer between biacetyl and aromatic
hydrocarbons has a diffusion controlled energy transfer rate. However when trapped
inside a hemicarcerand cage, the rate of the energy transfer from biacetyl to various

aromatic acceptors follows a Marcus curve and is well below the diffusion limited value,

91

so that energy transfer rates in the Marcus inverted region are observed. In this case, the
hemicarcerand walls prevent direct contact between donor and acceptor and retard the
rates of energy transfer. Structural models revealed that the distance between the biacetyl
donor and the acceptor on the outside of the hemicarcerand complex is 7 A. The decrease
in rate when the hemicarcerand is used as a spacer is concomitant with a decrease in
electronic coupling. Another equation for exchange energy transfer relates the rate to the
driving force (AG) of the reaction. The driving force is proportional to the energy gap

between the donor and acceptor excited states:

 

2
(71,+AG+2,) ] (23)

k = Aex —
pi 42,1937

where A is the preexponential factor, A, is the solvent reorganization energy, 71,. is the
vibrational reorganization energy and AG is the thermodynamic driving force. The
theoretical curve was fit with values of 0.20 eV for both 21, and 71,.

A third example of a Dexter mechanism that has been studied extensively in the
literature is energy transfer in photosynthetic systems where the donors are carotenoids
(polyenes) and the acceptors are chlorophyll [129]. Originally it was thought that energy
transfer was due to a dipole-dipole interaction; however upon closer inspection some
groups are beginning to find evidence that both singlet-singlet and triplet-triplet
mechanisms actually follow Dexter exchange theory more closely [129, 137]. The reason
given for a Dexter mechanism was a small spectral overlap between carotenoid emission
and Qy transition of chlorophyll, a small transition dipole moment and forbidden optical

and spin transitions.

92
Lanthanide Spectroscopy and Electronic Structure

Lanthanide ions are ideal for optical Chemosensing of BTXS and PAHS. When
directly excited very weak emission is observed, owing to the low molar absorptivites of
lanthanide ions. However, once excited indirectly these ions luminesce efficiently in the
visible region of the electromagnetic spectrum with long radiative lifetimes.

The electronic structure of lanthanide ions is unique as a result of the properties of
f-orbitals and spin-orbit coupling (Figure 25). Because the f—Orbitals are buried in the
atom core, their bonding involves a much weaker overlap when compared to d-orbitals.
The Spectroscopy of d-block elements is therefore dominated by the coordination
environment of the metal ion. The crystal field splittings due to the presence of the
ligands is large in d—block elements, whereas the spin-orbit coupling is small. However,
lanthanide spectroscopy is dominated by spin-orbit coupling; therefore transitions
between “spin forbidden” states is no longer an issue. The spherical symmetry of a
lanthanide ion is destroyed when it is surrounded by ligands in a crystal, glass or solution
[138]. Figure 25 shows the splitting into terms due to interelectronic repulsions (~ 20000
from spin-orbit coupling, and into sublevels from ligand crystal field splittings.

The selection rules for lanthanide electronic transitions are derived from Judd-
Ofelt theory of forced electronic dipole transitions, since transitions within the f—orbital
manifold are electronically and spin forbidden [139]. The forced electronic dipole
selection rules for lanthanides state that neither the AS = 0 nor the AL = 0 rules are
obeyed. A description of the lanthanide selection rules can be found in Btinzli’s chapter

in Lanthanide Probes in Life, Chemical and Earth Sciences [138]. As a result of the

 

 

 

 

 

415 5d 5 5
11 L D4
503
502 ..... =
EU3+ SD 5014::"———
l -- __
500
2x104 cm'1
415 7,:
.. J 6
5
4
7F 3 }103 cm'1
2 -- 1
1..-::... —. 102cm
0- ..... —
417 5d

 

 

 

 

 

 

 

 

 

3+
Tb
0
1
2 6x103 cm"
3
4
§_ ---'_—_—__——
5N.“ }21(102cm'1
Configuration Elemtis t t' Levels Sublevels
ec ros a to Spin-Orbit Crystal
Interactions Coulping Field
Splitting

Figure 25. Energy level diagram for Eu3+ and Tb3+ in the free ion and crystal field limit
of the chemical environment. The configuration is split into terms by electrostatic
interactions, levels by spin-orbit coupling and sublevels by crystal field splittings [138].

94

forbidden nature of the intra f-electronic transitions, the oscillator strengths and

consequently the molar absorptivites are small (<1 M'l cm").

Non-Radiative Transitions

Lanthanide ions can be easily deactivated by vibrational oscillations in the
surrounding environment. Multiphonon (vibrational) deactivation leads to energy loss
from electronically excited lanthanide ions. Non-radiative deactivation is dependent on
the energy gap between manifolds, the photon energy and the photon-electron
Hamiltonian overlap integral [140]. The larger the energy gap between the two lanthanide
states, the slower the rate. The non-radiative deactivation has been measured in some
cases [141]. Our aim is to try to avoid populating the higher excited state levels of the
lanthanide ion by judiciously choosing the donor molecules.

One potential quencher of lanthanide luminescence is water, whose O-H
vibrations non-radiatively deactivate the excited states of lanthanides. To avoid
quenching by water, the nine coordination sites need to be occupied by ligands that will
cage the lanthanide. Typical ligands that have been employed for this purpose are
cryptands, which have seven nitrogen and oxygen coordination sites. The remaining two
sites are occupied by water molecules. However, shielding by the macrocycle is still
effective. Horrocks et al. developed the following equation for determining the number of
molecules of water located in the coordination sphere of a lanthanide [142] by measuring
the lifetimes of various lanthanide complexes in water and deuterium oxide solutions.

anO=C[ 1 _ I ] (24)

T1120 tD20

 

95

where c is a constant (c = 4.2 for Tb3+ and 1.05 for EU“ ) and THZQ and Togo are the

lifetimes in solutions of water and deuterium oxide, respectively.

To shield the deleterious effect of O-H oscillators, lanthanide ions can be
encapsulated in molecular cages such as cryptands. Two nitrogens and five oxygens
comprising the three straps of 2.2.1 cryptand ligand occupy seven of the nine
coordination Sites of the Ln3+ ion. The two remaining coordination sites of the Ln3+
cryptand provides the opportunity for light-harvesting substrates to enter the coordination
sphere of the lanthanide and be detected by the energy transfer process. In this manner
the optical excitation is efficiently pumped to the Ln3+ excited state via the light-
harvester. We have been able to increase the emission efficiency of Tb3+ and Eu3+c 2.2.1
complexes by upwards of three orders of magnitude upon coordination of light-
harvesting substrates such as ,B-diketonates. Compelling evidence that the increase in
emission intensity is due to energy transfer comes from the excitation spectrum, which is
not that of the native Eu3+c2.2.1 as is the case when no light harvester is present, but
rather corresponds to the absorption spectrum of the light harvester.

In our laboratories, we have studied the energy transfer rates of acetylacetonate
(acac) ligands to a cryptand—encapsulated lanthanide ion (Tb3+c2.2.1) in real-time at
room temperature [143]. The luminescence decay of acac at 435 nm is monoexponential
with a lifetime of 1.7 i- 0.4 ns. This Shortened acac emission is attributable to oxygen
quenching of the 37t7t* radiative decay at room temperature [144]. A monoexponential
decay of 1.4 i 0.2 ns is also observed for Gd(acac)3 at 435 nm. AS the lowest energy

level of Gd3+ (32000 cm") is energetically far removed for energy transfer relative to the

96

acac triplet (25600 cm"). The Shorter triplet lifetimes for the chelate Gd(acac)3 is
consistent with the presence of a heavy atom enduced intersystem crossing rate.

In the presence of Tb“: 2.2.1, a biexponential lifetime is observed, indicative of
more complicated kinetics. The two components of the decay are 103 :1: 23 ps and 1.73 :1:
0.07 ns; the Short component arises from triplet decay of acac associated with
Tb3+c 2.2.1 and the long component originates from unbound acac. The relative weights
of the short and long component are 11% and 89% respectively, and the short component
increases with increasing concentration of TD“: 2.2. 1. Associated with the biexponential
decay is the appearance of Tb“ luminescence at 546 nm (5D4—)7F5). The risetime of the
Tb3+ luminescence is 85 :t 40 ps, consistent with the timescale of decay of acac emission
(103 ps). The lifetime of the 5D4 State Tb3+ state (546 nm) in the acac-Tb“: 2.2.1
complex is 1.7 ms. The length of the decay is typical of a lanthanide excited state. The
correspondence of acac phosphorescence decay and Tb3+ luminescence rise components
supports the hypothesis of direct energy transfer from the acac triplet (25600 cm") to the
metal luminescent excited state (20500 cm"). Fast energy transfer with a rate of 9.7 x 109
s" is observed with this system of acac bound to Tb3+c 2.2.1. Excluding studies done in
our laboratory there have been no real-time measurements of the rates of through bond
energy transfer in lanthanide chelate systems.

The acac-Tb3 +c2.2.1 adduct represents energy transfer in a system under
dynamic equilibrium. 1H NMR Spectra reveal that acac undergoes an equilibrium with
Tb3+c2.2.1 (15 % for 1:1 binding). It is therefore difficult to determine the distance of

the energy transfer between the acac and Tb“. Energy transfer rates and mechanisms of

few aromatic systems to lanthanide acceptors have been investigated. Lanthanide chelates

97

[145] and triphenylene doped into lanthanides in rigid glasses at 77K [146] exhibit
energy transfer by a Dexter mechanism. The energy transfer mechanisms to two other
well studied systems, aromatics (naphthalene and Bph) in micelles with Tb3+ and Eu3+
[147] and lanthanide calixarenes [148] have not yet been determined. We therefore
decided to investigate the energy transfer process lanthanide acetylacetonate chelate
complexes where the acac is directly coordinated to the lanthanide ion.

Three mechanisms have been proposed for through-bond energy transfer of
lanthanide chelates: (1) fast intersystem crossing (S. to T1) and transfer from the ligand
triplet to the lower levels of the lanthanide excited state; (2) transfer from the ligand
singlet state to the rare earth excited state; and lastly (3) a cascading of energy back and
forth from the ligand to the metal to the lowest emissive state of the lanthanide (Figure
26) [149]. Energy transfer from the triplet requires a fast intersystem crossing rate that is
competitive with singlet radiative emission. Intersystem crossing rates of chelates were
examined by Tanaka [150] and El Sayed [151]. The rates were estimated to be 10H 3" by
Bhaumik and El Sayed and measured by Tanaka and coworkers at 7.5 x 108 s" for
lanthanide ions complexed with methyl salicylate. This chapter describes measurements
in our laboratory of the emission, excitation and time-resolved luminescence for
lanthanide chelates and evidence of a triplet state mechanism (Mechanism 1).

First, gadolinium acac chelate complexes are used to determine the ligand
phosphorescence. Phosphorescence of the ligand from Gd(acac)3 is observed for two
reasons, spin-orbit coupling of the donor molecule is enhanced by the heavy atom effect,

and the first excited state of gadolinium is energetically indisposed relative to the donor

 

 

 

 

98

 

 

 

 

 

Mechanism I Mechanism ll
3111':— 2 31 1r—~. 2
T1—+“‘ T1q— ‘ \‘

‘__ 1 ‘__ 1
hv hv
Ligand Lanthanide Ligand Lanthanide
Mechanism Ill
81 11"" “““ f. 2
T1 -—<:"'
‘__ 1
hv
Ligand Lanthanide

Figure 26. Three mechanisms for intramolecular energy transfer in lanthanide chelates:

energy transfer through the

triplet,

singlet and multiple levels [149].

99

excited state (Figure 27). Secondly, europium and terbium are substituted for gadolinium
order to select a Specific lanthanide excited state. Europium(III) has low lying excited
states at 17000 and 19000 cm", whereas terbium (HI) has states at 20000 and 26000
(Figure 27). The two acac based ligands that have been chosen are dibenzoylmethide
(DBM) and dinitrodibenzoylmethide(dNDBM). The triplet state of DBM is located at an
energy that will populate both excited states of Eu3+ but only the lowest level emissive
excited state of Tb“. Located lower in energy than DBM, dNDBM populates only the
emissive state of Eu3+ and no energy transfer occurs to Tb“.

We have chosen lanthanide chelate systems for many reasons, the most prominent
being the precedence in the literature [149]. Lanthanide chelates were examined in the
605 and 703 as potential candidates for lasing materials. In a 1966 review of rare earth
complexes, Crosby [145] gives an estimate of the energy transfer rate of 109 to 1013 s"
for ,B-diketonate complexes and an exchange mechanism is postulated, implying that the
lanthanide and diketonate orbitals are physically overlapped. Tanaka [150] and Crosby
[145] performed experiments on lanthanide fl—diketonate complexes, indicating that a
short distance and a paramagnetic lanthanide are important for enhancement of spin-orbit
coupling. The heavy atom effect must therefore operate over the distance of a bond in
order to have a pronounced effect on the intersystem crossing rates of aromatics.
Covalent bonds in lanthanide ,B-diketonate complexes impose a Short energy transfer
distance, and therefore it is reasonable that the energy transfer rates are fast.

Energy transfer has been examined for a non-covalent system comprised of an
aromatic (Bph and naphthalene) imbedded in the core of a micelle with lanthanide ions

on the polar surface. Energy transfer involves diffusion of the aromatic donor to the

100

 

6pm 32200

 

 

 

 

 

 

 

 

 

 

l y 51
_— 26000 503
4 —-—- SD 21519 ~ .
f 1 2 .._.J J “1:“;
l ”-1-: 5‘1“). 59.28
(5) 500 17277
a no energy
a; transfer
c
LLl
$1733 r, lit; 11”?)
616 nm :4.-. ll 1']
j A .1...
857/2 1 S "E: 7 7
0 F0 F6
/ 2“

’ 3., , /"l in";
3,.Gd31'} Aromatic Tb3 ,3.
“-4.22" Hydrocarbon 1

Figure 27. Energy level diagram of Gd“, En3+ and Tb3+ with a typical aromatic

hydrocarbon, showing the range of energy of the triplet and Singlet states. The emissive
transitions of europium and terbium are shown.

101

lanthanide acceptor. Almgren and Thomas [147] and Wagner and Schott [152] have
independently shown that the second order rates are slow, 105 M" s". From a measure of
the binding (K = 500), they were able to determine that the rate due to bound complexes

is 6.2 x 102 s‘1 and 1.4 x 103 s" for l-bromonapthalene and Bph respectively.

Experimental

Dibenzoylmethide (DBM) ligand, GdCl3-6HzO, EuCl3-6HzO and TbCl3-6H20
were obtained from Aldrich and used without further purification. Tris chelate
(Ln(DBM)3) complexes were synthesized by refluxing a mixture of 1.3 g (6 mmol) of
dibenzoylmethide ligand, 48 ml of acetone and 12 ml of 0.5 M KOH to which a 2 mmol
of solution of LnCl3-6H20 (Ln = Gd, Eu, Tb) in 10 ml of water was added. The solution
was filtered hot to remove all impurities and upon cooling yielded yellow needles in 60
% yield. Characterization was done by infrared spectroscopy and melting point analysis.

Dinitrodibenzoylmethide (dNDBM) ligand was synthesized by Adrian Ponce by
reacting 54 g of anhydrous Ale and 70 g of p-nitrobenzoylchloride in 100 g of
tetrachloroethane with 17g of vinyl acetate. The product of this reaction was worked up
with HCl and followed by distillation to remove tetrachloroethane. dNDBM was
recrystallized in glacial acetic acid in 30% yield. Characterization of dNDBM was done
by NMR, IR and GC mass spectrometry. The chelate complexes Ln(dNDBM)3 were
synthesized by the same method as Ln(DBM)3.

Emission spectra from 1x10'5 M or 1x106 M lanthanide chelates in EPA glasses
(ethanol/diethyl ether/isopentane 2:5:5) were collected on the high resolution emission

instrument at Michigan State University. A 13 Hz chopper was used to collect long-lived

102

luminescence and phosphorescence. Emission spectra were collected by exciting DBM
chelates at 365.7 nm, and scanning the emission from 400 nm to 600 nm. dNDBM
chelates were excited at 435 nm and the emission monitored from 450-650 nm.
Nanosecond lifetimes and risetimes of Gd, Eu and Tb complexes of Ln(DBM)3 and
Ln(dNDBM)3 were collected with a TEK 602a oscilloscope, following excitation of the
ligand at Aex = 355 run, one of the harmonics of the Nd:YAG DCR 2 nanosecond laser.
The ligand phosphorescence lifetimes were measured for Gd(DBM)3 at 494 nm and for
Gd(dNDBM)3 at 550 nm. Europium and terbium luminescence lifetimes were collected at

616 and 546 nm, respectively.

Results

Energy transfer does not occur in gadolinium chelate complexes because the
lanthanide ion excited state is too high in energy (32200 cm"); therefore ligand
luminescence, is only observed. Figure 28 and Figure 29 show the luminescence spectra
of Gd(DBM); and Gd(dNDBM)3. Ligand fluorescence (400—460 nm) and
phosphorescence (470-600 nm) is observed from Gd(DBM)3 upon excitation at 365.7
nm. The phosphorescence band shows a vibrational progression at 488, 518, 552 nm,
which gives an energy spacing of 1186 and 1189 cm". This progression can be attributed
to the vibrational modes of the phenyl ring. The lifetime of Gd(DBM)3 is 1.8 us at 494
nm and < 10 ns between 400-460 nm.

The emission of Gd(dNDBM)3 shows four bands at 492, 550, 590 and 640 nm.
The first band at 492 nm corresponds to fluorescence of the ligand and has a lifetime of

<10 ns. The other three peaks correspond to vibrational structure in the phosphorescence.

103

 

 

Intensity

 

 

 

l 1 1

400 450 500 550 600
Wavelength (nm)

 

Figure 28. Luminescence Spectrum of Gd(DBM)3 A“ = 365.7 nm.

104

 

Intensity

 

 

pa 1 l l l

400 450 500 550 600 650

 

Wavelength (nm)

Figure 29. Luminescence Spectrum of Gd(dNDBM)3 kg. = 435 nm.

105

(separations of 1266 and 1324 cm"). The progression is lower in energy due to the
electron withdrawing nitro groups on the phenyl ring. The lifetime of phosphorescence at
550 is 1.2 as.

The excitation and emission spectra of Eu(DBM)3 are shown in Figure 30. The
excitation spectrum Shows a broad band with a maximum at 375 nm (26666 cm") and
vibrational structure, which is assigned to the first excited singlet State of the DBM
ligand. The emission Spectrum shows two narrow emission lines at 548 nm (magnified
100 times) and 616 run that are assigned to the SDI—VF] and 5D0—-)7F2 electronic
transitions, respectively. The former transition is very weak whereas the latter transition
is intense; the intensities are related to the symmetry of the coordination environment of
the lanthanide ion. The 5D0—>7Fp_ is a hypersensitive transition; if the ion lies on a center
of inversion this transition is absent from the Spectrum. The tris chelates (C3 point group)
do not have a center of inversion and therefore the 616 nm line is very intense. The
excitation spectrum for Tb(DBM)3 (Figure 31) shows a similar excitation profile to that
of Eu(DBM)3. The emission spectrum, however, shows narrow emission lines that are
typical for Tb3+ at 488 (5D4—>7F6) and 546 nm (5D4—>7F5). Therefore we observe ligand
absorption in the excitation Spectrum and lanthanide emission in the emission spectrum,
providing evidence of an absorption-energy transfer-emission process.

The excitation spectrum of Eu(dNDBM)3 shows a broad band centered at 425 nm,
23529 cm" (Figure 32). The emission spectrum shows some residual singlet emission
from the dNDBM ligand and, in contrast to Eu(DBM)3, there is only one narrow Eu3+

emission line at 616 nm. The presence of only one luminescence transition, 5D0—>7F2, is

106

 

 

 

 

 

Eu3+

DBM
.é‘
(D
C
.93
E

Eu3+
1 l 1 - l_ ...1 11.1111 hi

 

300 350 400 450 500 550 600 650
Wavelength (nm)

Figure 30. Excitation (300-410 nm) and emission spectra (500-650 nm) of Eu(DBM)3,
showing excitation of the dibenzoyl ligand and emission from Eu3+ at 548 and 616 nm.

107

 

DBM

Tb3+

Intensity

 

 

 

 

 

l l 1 Julius ll 1 J

300 350 400 450 500 550 600 650

Wavelength (nm)

Figure 31. Excitation (300-410 nm) and emission spectra (425-600 nm) of Tb(DBM)3,
showing excitation of the dibenzoyl ligand and emission from Tb3+ at 488 and 546 nm.

108

 

Eu3+

dNDBM

Intensity

dNDBM

. . .11

300 350 400 450 500 550 600 650
Wavelength (nm)

 

 

 

 

 

Figure 32. Excitation (300-475 nm) and emission Spectra (400-650 nm) of Eu(dNDBM)3
showing excitation of the dinitrodibenzoyl ligand and emission from dNDBM at 500 nm
and from Eu3+ at 616 nm.

109

consistent with the lower energy singlet and triplet states of the dNDBM ligand.
Apparently, the higher energy 5D] state is not populated by energy transfer and no
emission from that state is observed. These data sustain the hypothesis that light absorbed
by dNDBM is transferred only to the lower state of EU“, affirming the mechanism I
(Figure 26), energy transfer from the ligand triplet state to the lanthanide manifold of
states.

The luminescence and phosphorescence lifetimes of all complexes of DBM and
dNDBM are listed in Table 5. The lifetimes of lanthanide luminescence of the 5D0—-)7F2
and 5D4—>7F5 states of Eu(DBM)3, Eu(dNDBM)3 Tb(DBM)3 respectively are very
Similar at 400, 338 and 555 us. The risetimes of the lanthanide states, 5Do for
Eu(dNDBM)3 and 5D4 for Tb(DBM)3, are both too fast to be measured by a nanosecond
laser pulse (< 10 ns). However the risetime of the 5Do state of Eu(DBM)3 is slow (1.8 us).
This measurement of the buildup of the 5Do state includes three processes: intersystem
crossing of the ligand, energy transfer from the ligand to the metal and internal
conversion (metal) from 5D1—) 5D0. The non-radiative transition from 5D] —+ 5Do has been
measured previously to be 1.8 us and is slower than expected upon comparison to organic
systems [141]. This slow intersystem crossing is typical for non-radiative transitions
between states in lanthanide ions. Inspection of the energy diagram in Figure 33 shows
the timescale for non-radiative transitions. The 1.8 us process that is measured for the
risetime of the 5D0 of Eu3+ is the sum of three processes: singlet to triplet intersystem
crossing, energy transfer and internal conversion between the lanthanide states. As
intersystem crossing for ketones is inherently fast (~l x 108 s") we can assume that for

our case this value is the lower bound of kisc and that the presence of the heavy atom will

Table 5. Lifetimes and Risetimes for Eu(DBM)3, Eu(dNDBM)3, Tb(DBM)3, Gd(DBM) 3,

110

 

 

and Gd(dNDBM)3.
Chelate Electronic Transition Lifetime Risetime Wavelength/nm
Eu(DBM)3 5D0—57F2 400 us 1.8 us 616

5D.—+71=l 1.8 118 <10ns 548
Eu(dNDBM)3 5D0—>7F2 338 Its <10 ns 616
Tb(DBM)3 5D4—)7F5 555 pg <10 ns 546
Gd(DBM) 3 T1—-) So 1.8 ps - 494
Gd(dNDBM)3 81—) So < 8 ns - 490

T1 —> 30 1.2 us - 550

 

111

s1 27248

 

 
 
   

kisc therefore a is fast

—— S, 22946

 

 

 

 

T, 20242 a 5D, 19028 kisc
<10ns 413%— T, 18181
50017277
5 . ' . sz
>5
9
(D
C
[U
s‘ 27248 _ 5D3 26000
k.
'Sc 3, 22946
< 10ns
T, 20242185,;
D4 20500
T, 18181
C)2” NO
Th“ 0 a 2
o O f-\

Figure 33. Energy flow from DBM (left) and dNDBM (right) triplet excited State to Eu3+
and Tb3+ excited states. The arrow from the singlet to triplet excited state of the ligands
shows the intersystem crossing. The rates labeled (a) and (b) were measured in our
laboratory and by Speiser and coworkers, respectively [141].

112

increase the rate by an order of magnitude [150]. The internal conversion of the
lanthanide, as mentioned previously, is slow (1.8 us) and by deduction we can
approximate the rate of energy transfer to be very fast! The fast rate is consistent with
the risetime of Tb“, which occurs on a timescale within the pulsewidth of the

nanosecond laser (< 10 ns).

Conclusions

Data from steady state and time—resolved luminescence of chelate complexes of
Ln(DBM)3 and Ln(dNDBM)3 where Ln = Gd“, Eu3+ and Tb3+ point to one conclusion:
there is absorption by antennae acac ligands, rapid intersystem crossing, rapid energy
transfer, and long lived lanthanide luminescence. The timescale of energy transfer is
faster than diffusion, with a lower limit of 1 x 10 8 SI (10 ns). This rate is consistent with
two other measurements in the literature by Nocera et al. for acac-Tb3+c2.2.l cryptate
[143] and Lehn et al. for Tb3+cbpy.bpy.bpy cryptate (bpy = bipyrdine) [153], where the
rates were derived to be 9.7 x 109 s" and 0.5 x 108 s", respectively. With the addition of
the data for the lanthanide chelates there are three examples where energy transfer
through a bond, whether it be an adduct held together by an equilbrium or a metal ligand
bond, is faster than diffusion. In the next chapter energy transfer through space will be
discussed as it relates to the active site mechanism for chemosensing of PAHS and

BTEXS.

CHAPTER 6

PAH AND BTX OPTICAL CHEMOSENSING BY AN ABSORPTION-ENERGY
TRANSFER- EMISSION PROCESS

Introduction

Aromatic hydrocarbons such as PAHS (polycyclic aromatic hydrocarbons) and
BTEXS (Benzene, Toluene, Ethylbenzene, Xylene) are environmental pollutants that are
powerful mutagens and carcinogens [154-157]. The Urban Air Toxic Program (UATP)
sponsored by the EPA has measured 397 benzene compounds in 14 cities, and the
National Ambient Volatile Compound (NAVOC) program found 564 benzene
compounds in 31 cities. These chemicals are therefore a great health risk to humans and
animals, demanding that water, air and soil be monitored for these contaminants.

Many of the techniques involved in detecting and monitoring levels of PAHS and
BTEXS involve time-consuming and meticulous laboratory analyses that must be
performed in an off-line or indirect manner. Some of the techniques employ

micoextraction-GC-FID [158], FTIR spectroscopy [159], capillary electrophoresis [160],

113

114

synchronous fluorometry [161], synchronous phosphorimetry [162], or synchronous
luminescence spectroscopy [163]. One recently developed method is based on fiber optic
sensors [164-167], which can be applied directly (on-line, on-Site or in-situ). The
challenge in building fiber optic sensors for aromatic hydrocarbons, especially BTEXS, is
that they absorb and emit light in the ultraviolet region of the electromagnetic spectrum
where it is difficult to obtain inexpensive powerful light sources, sensitive detectors and
quartz fiber optics.

One fiber optic probe of BTES (excluding xylene) based on infrared excitation
uses Raman spectroscopy, with a near infrared (NIR) diode laser and a CCD detector
[164]. These components and fiber optics are less expensive than those required for
ultraviolet light. This fiber optic is used in quality and process control for industrial
applications in petroleum fuel production [168,169]. Typical analysis of fuel products
involves gas chromatography, which is time consuming and labor intensive [170].
Currently, NIR reflectance/absorbance spectroscopy is used to remotely monitor
petroleum chemical composition. However, the disadvantages of NIR absorption are that
the peaks are few, broad and overlapping [171-173]. Raman spectroscopy provides many
peaks that are well resolved and that can yield information about the analyte.
Concentrations of BTES ranging from 1 to 16 % w/w can be measured by Raman
spectroscopy, at an instrumentation cost of < $20,000.

A near-ultraviolet fiber optic absorption sensor for BTEXS has been developed for
soil remediation at the Lawrence Livermore Dynamic Underground Stripping Site [166].
Both off-line laboratory analysis and on-line optical sensors were used to gain a better

understanding of the dynamic stripping process, whereby resistive heating or steam

115

injection raises the ground temperature, causing volatilization of organic contaminants
trapped in the arid soil. Direct detection of BTEXS is obtained from the absorption
spectrum (230-300 nm) from moisture-saturated vapor steam via fiber optic on-line
probes. Near UV absorption has advantages over infrared spectroscopy as there are no
interferences from water. A flow-through cell is used to determine the absorption of the
vapor, which contained aromatic hydrocarbons. All optics are made of fused silica and
quartz, and a UV photodiode with a current-to-voltage amplifier was used as the detector.
The optical source for the sensor is a 30 Watt deuterium lamp whose wavelengths are
selected by a monochromator and modulated with a chopper. The sensor was used at
temperatures ranging from 12 to 43 °C for five weeks to monitor benzene concentrations
at the absorption maximum (243 nm) with a detection limit of 13 ppm [166].

A third optical sensor based on time-resolved fluorescence spectroscopy has been
developed for in situ detection of PAHS in water. The sensor device employs a fiber
length of 50 meters and a detection limit in the nanograrn per liter range [167]. The
apparatus consists of a 337 nm pulsed nitrogen laser light source, which is coupled to the
50 m quartz fiber optic (core diameter of 600 um) connected to a PMT and boxcar.
Thirteen environmentally relevant analytes have been detected with this instrumentation,
including fluoranthene, benzo(b)fluoranthene, benzo(e)pyrene, chrysene,
benzo(flfluoranthene, indeno(1,2,3-CD)pyrene. The lifetimes of these analytes range
from 5 to 33 ns, with varying emission spectra from 395 to 540 nm. This sensor is
compact enough to fit into a briefcase and is very sensitive to PAHS [167].

Other portable and on-line means of measuring PAHS and BTEXS have been

developed, in addition to fiber optic sensors. Compact analytical instrumentation for

116

benzene has also been designed with gas chromatography and mass spectrometry (GC-
MS) [174,175]. These instruments are designed for operation on an electrically powered
vehicle for monitoring exposure to benzene. The accuracy of measuring benzene is
achieved by enrichment with deuterated benzene. Close to real-time monitoring is
obtained with 4 minute inter-analysis time. The limit of detection is approximately 1

jig/m3 for benzene and 2 [lg/m3 for toluene.

Design Strategy for Chemosensing by a Triggered Luminescent Response

Although there are many optical sensors for PAHS and BTEXS, all of the
absorption and emission techniques rely on detection of ultraviolet light, which can be
subject to many interferences. For our strategy of optical sensing by triggered visible
luminescence, a means for converting ultraviolet light to visible light is necessary. The
source of visible emission that we have selected is lanthanide ions, commonly found in
phosphors in television screens. Lanthanide ions such as europium (Eu3+) and terbium
(Tb3+) are intrinsically bright lumophores, but they Show little or no luminescence under
direct irradiation owing to the low absorbance (8 <1 M" cm") of the 5D] emitting state
manifold. With regard to eq (1) in Chapter 1, the kr of the emitting state of Ln3+ ions is
small and hence the luminescence produced by direct irradiation is weak.
Nothwithstanding, the emitting state can be indirectly excited with energy from a
sensitizer. As popularized by Balzani and Lehn [153], an absorption-energy transfer-
emission (AETE) process produces an emitting excited state in very high yields as long
as the rate of energy transfer from the sensitizer to the photoluminescent center is
efficient. We recognized that this AETE process could be useful for chemosensing if the

sensitizer was an analyte possessing a large absorption cross—section with respect to that

117

of the active site. However, one additional problem must be solved in order to implement
lanthanide-based chemosensing schemes. Even if Ln3+ ions are efficiently excited, their
luminescence is quenched efficiently by water because the O-H bonds act as good
thermal receptors of the energy of lanthanide-ion excited states [142]. This deleterious
effect of water can be overcome by encapsulating the Ln3+ ions in supramolecular cages
that exclude water.

In order for an AETE process to be effective, the distance between the sensitizer
analyte and lanthanide lumophore must be short. In normal aqueous solutions, the
average distance from an aromatic hydrocarbon to a lanthanide is too far for efficient
energy transfer to occur. For example, using the Perrin formulation, the quenching sphere
of a donor molecule in a dilute solutions (10'2 M) places the acceptor at a distance (R)
greater than 30 A (R = 6.5[A]”3 where [A] is the concentration of the acceptor) [17]. This
distance challenge was addressed by using a cyclodextrin cup (CD) as the molecular
recognition center for the aromatic hydrocarbon. Different sizes of cyclodextrins can be
used to control Size selectivity for BTEXS and PAHS. Alone, a cyclodextrin is an
inadequate chemosensing active site because there is no photoactive center. However, a
photoactive center can be incorporated into a cyclodextrin by adding a binding site for a
lumophore. Macrocyclic ligands, such as aza crown ethers or
diethylentriaminepentaacetic acid (DTPA) can be tethered to the cyclodextrin at the
primary Side, leaving the secondary side of the cyclodextrin accessible to binding of the
hydrophobic pocket by mono- and bicyclic aromatic hydrocarbons.

In our efforts to design supramolecular assemblies capable of optically detecting

aromatics and polycyclic aromatics, several derivatized cyclodextrins were synthesized in

118

our group, specifically the aza crown swing and cradle cyclodextrins [176,177] and
diethylenetriaminepentaacetic acid (DTPA) (Figure 34) [178]. In the absence of aromatic
hydrocarbons, weak emission is observed when all three complexes, Eu3+ aza crown
swing cyclodextrin, Eu“ cradle cyclodextrin and a Tb3+ triacid cyclodextrin complex, are
directly excited with ultraviolet light. When aromatic hydrocarbons are present, however,
enhanced luminescence is observed. Since aromatic hydrocarbons bind with high
affinities to the cyclodextrin cavity, they are held close to the ligand, at a distance where
energy transfer can occur to a lanthanide coordinated to the macrocyclic ligand. We
attribute the enhancement in luminescence to an absorption-energy transfer-emission
process from the aromatic that resides in the cavity to a lanthanide ion coordinated to the
aza crown and DTPA macrocycles.

We are particularly interested in sensing benzene, toluene and xylene (BTX) by
energy transfer. Tethered at only one nitrogen, the aza crown prefers a conformation that
is swung away from the hydTOphobic cup; therefore the triggered luminescence response
is weak, owing to the long distance for energy transfer. Studies with the Eu3+ aza crown
swing cyclodextrin and benzene analyte indicated that the small binding constant
(117 M") and small increase in emission intensity of Eu3+ could be ascribed to the long
energy transfer distance [176]. Because a shorter distance is imposed when the lanthanide
ion binding site is cradled under the CD, we suspected that the aza crown tethered to the
CD cup via its two nitrogens would show a significantly larger triggered luminescence
response. To address this issue, the distance was shortened by tethering the aza ligand at

both nitrogens [177]. However, although the distance from benzene to the lanthanide

119

(b)

 

Figure 34. (a) Swing aza crown-,B-cyclodextrin coordinated with Eu“. (b) Cradle aza
crown-flcyclodextrin coordinated with Eu“. (c) Diethylenetriaminepentaacetic acid-,6-
cyclodextrin coordinated with Tb”.

120

(Eu3+) was closer, instead of observing an increase in lanthanide luminescence, there was

little improvement. Indeed, the binding constant was smaller (< 10 M"). Apparently the
3+ charge of the appended europium cradle at the bottom of the cup makes the interior of
the CD less hydrophobic and hence decreases the association of benzene in the cup. It
was predicted that the hydrophobicity of the cyclodextrin cavity would remain unaltered
if the overall charge of the CD supramolecular assembly could be made neutral.
Neutralization of the charge is achieved by linking a tricarboxylic acid macrocyclic
ligand synthesized from diethylenetriaminepentaacetic acid to the cyclodextrin. The
synthesis of a CD supramolecule appended with a diethylenetriaminepentaacetic acid
(DTPA) binding site that neutralizes the charge of the lanthanide ion in a cradle geometry
has been reported by Mortellaro and Nocera [178]. The neutral charge of the Tb3+ triacid
CD complex was the key to binding neutral substrates; it showed sensitivity to BTXS, and
to bicyclics such as Bph [179].

The ultimate aim of the investigation reported in this chapter is to determine the
mechanism of the energy transfer process leading to detection of the BTXS (aromatic
hydrocarbon) to the acceptor (lanthanide ion). The rate of energy transfer, distance
dependence, FOrster energy transfer mechanism, and overlap integral of aromatic donor
and lanthanide acceptor will be presented in this chapter. Predicting the distance of
energy transfer is possible by measuring the energy transfer rate, the overlap integral of
the donor emission and absorption of the acceptor, and the donor lifetime and quantum
yield. These measurements and their implications on the theory of energy transfer

formalisms will be discussed.

121

Experimental

Biphenyl (Bph) was obtained from Fischer and recrystallized from methanol.
Scintillation grade naphthalene (Np) was purchased from Aldrich and used as received.
Tb(NO3)3°5H20 and EU(NO3)3°5H20 were obtained from Aldrich and used as received.
The synthesis of the aza crown swing, cradle and diethylenetriaminepentaacetic acid
(DTPA-fl-CD) cyclodextrin molecules was conducted by Dr. Zoe Pikramenou [176], Dr.
Mark A. Mortellaro [178] and Dr. Jude Rademacher [127]. The compounds were
characterized by 300 and 500 MHz NMR, thin layer chromatography and mass
spectrometry. With the assistance of Dr. Mark Mortellaro, the synthesis of the swing
cyclodextrin was repeated although the compound was not fully purified by gel
chromatography. Mark Mortellaro attempted the reaction under more vigorous conditions
with pyridine as the solvent, at higher temperatures. These conditions did not improve the
yield of the reaction and in some cases caused decomposition. Jude Rademacher has
repeated the syntheses of swing and cradle cyclodextrins and improved the purification
methods. He has observed that the aza crown swing CD can exhibit a pale yellow color
instead of the typical white powders observed for cyclodextrin compounds.

All experiments were performed using room temperature aerated aqueous
solutions unless otherwise noted. Absorption spectra were collected on double beam
Hitachi U200] or OLIS retrofitted Cary 17 spectrophotometers. To obtain data for
overlap integral estimations, lanthanide absorption spectra were collected for highly
concentrated aqueous 0.05 M solutions of Tb(NO3)3'5H20 (8 ~ 1 M" cm") or
TbCl3°5H20 in methanol. The data were scanned at 0.2 nm intervals to match the interval

of the emission spectra of aromatic hydrocarbons. The spectral overlap integral of donor

122

emission (Bph) and absorption of the terbium (III) acceptor was calculated using a
spreadsheet format by utilizing Kaleidagraph software.

Binding constants of aromatic hydrocarbons to DTPA-,B-CD coordinated with
Tb3+ (TbDTPA-fl-CD) were determined by the same technique used to determine binding
of l-Ber to Gfl-CD described in Chapter 2. The absorption spectra of two cells
saturated with aromatic hydrocarbon were measured to give a relative baseline of zero.
Concentrated DTPA-6CD solution (25 mM) was added to the sample compartment,
resulting in an increase in absorption that is proportional to the concentration of complex
[CD-Ar], where CD = cyclodextrin and Ar = aromatic hydrocarbon. A plot of [A-Ao]/Ao
versus [CD],o.-[CD-Ar] yields K, the binding constant. To verify the binding constants, a
measurement was also obtained from titrations of the emission profiles described in the
next section and fit with a Standard equation [124].

Excitation and emission spectra were collected under the same conditions as
described in Chapter 2. Titration profiles were collected for emission at 544 nm of
TbDTPA-fl-CD as a function of added aromatic hydrocarbon at an excitation wavelength
specific for each analyte (A6,, = 266 nm for Bph and 275 nm for Np). A 2.5 x 10'4 M
solution of TbDTPA-fl-CD was made by dissolving equimolar amounts of
Tb(NO3)3°5HzO and DTPA-6CD in doubly deionized water and assuming 100%
complexation. Excitation and emission spectra were collected as aliquots of 1-100 ul of a
0.02 M solution of Bph or 0.1 M Np in ethanol were titrated into a 2 ml solution of 2.5 x
10" M TbDTPA-fl—CD. Quantum yields of Tb3+ in TbDTPA-fl-CD in the presence of
Bph and Np were determined by integrating the 544 nm line in the asymptotic

concentration limit of Bph and Np, using a standard of degassed biacetyl in hexane [105].

123

Other quantum yield standards (quinine bisulfate and Ru(bpy)32+) gave irreproducible
results.

Luminescence lifetimes of Tb3+ in TbDTPACD complexed with Bph (TbDTPA-
fl-CD-Bph) were collected on a pulsed ns laser system. The 532 nm output of a
nanosecond Quanta Ray DCR2 Nd:YAG was doubled by a WEX to obtain 266 nm light
used to excite the aromatic hydrocarbons (Np and Bph). The power of ultraviolet light
out of the WEX was typically 10-40 mW (10—15 % conversion). The luminescence
risetime and lifetime of Tb3+ were collected at 544 nm, the luminescence line with the
largest intensity. The slits were set at the same widths as described for the l-Ber
lifetimes described previously.

The Stern-Volmer method of luminescence lifetimes was also used to determine
the extent of static and dynamic quenching of Bph fluorescence by Tb3+ quencher [107].
Quenching of Bph and Np singlet excited states in DTPA by Tb3+ was determined by
picosecond lifetime and steady state luminescence experiments. A concentrated solution
of Tb(NO3)°5HzO (20 mM) was titrated into a solution of 3 x 10" M Bph and 2.5 x 10’4
M DTPA-6CD. The emission intensity and lifetime of Bph fluorescence were measured
at 320 nm with the excitation set at A6,, = 266 nm. The fluorescence lifetime decays of the
singlet state of Bph in the absence and presence of Tb3+ were fit using an instrument
response function between 30-80 ps with software provided by Edinburgh Instruments
(FLA 900) in a GEM platform.

Energy minimizations were run on a Silicon Graphics IRIS Elan x2 computer
using Biosym Insight Discover. Approximations were made of the distance of energy

transfer from the lanthanide acceptor at the bottom of the cyclodextrin to the donor

124

aromatic in the CD cavity. As lanthanide forcefields are not available on Discover, the
lanthanide was excluded from the energy minimization calculation of swing and cradle
CD. An ammonium ion was used to approximate the charge interactions of a cation (to
replace Tb3+ and Eu3+ ) with the triacid group of the DTPACD. The results of the
minimization were plausible as the three negative charges formed a triangular orientation
about the ammonium cation. The average distance from the center of one ring of Bph to

the center of mass of the lanthanide was around 6-7 A.

Reversed Timing and TCPC Instrumental Modifications

When lifetimes and risetimes become Shorter than the pulsewidth of a nanosecond
laser, it is necessary to use a picosecond laser as the excitation source, where the pulses
are typically 6-8 ps FWHM. The repetition rate of the laser can be varied with a cavity
dumper and external trigger from 76 MHz to l KHz, corresponding to 13 118 to 1 ms
intervals between pulses, respectively. At a repetition rate of 76 MHz, the light pulses are
arriving too quickly for the electronic detection system to respond. AS a result, time-
correlated single photon timing (TCPC) was developed [180]. As the name implies, only
one photon can be counted at a time. Therefore, one is able to avoid excessive dead time
on the detector where fluorescent photons arrive too quickly and cannot be counted.
TCPC is a reverse timing mode where the Start (fluorescence) light pulse is detected prior
to the stop (reference) pulse. In this case the trigger or start signal is initiated by an
emitted fluorescent photon at the MCP-PMT detector. The stop signal is the photon from
the laser pulse that is optically and electronically delayed. Previously in our laboratory
Jeonga Yu et al. [143] was able to measure fast risetimes with subsequent slow

luminescent decays of Tb3+c2.2.1 with acac when the cavity dumper was externally

125

triggered at a repetition rate of 1 kHz. Unfortunately, this led data collection times of ~
80 hours to collect 1000 counts. We are still investigating an alternative method to data
collection.

The picosecond laser system that was used as the excitation source is a Coherent
702 dye laser. For amplification of the 702 pulse (typically ~12 nJ/pulse, 6-8 ps) a dye
laser a Continuum Regenerative amplification system is used. The second harmonic of a
Coherent Antares 76S (100 ps FWHM, 3W of 532 nm) was used to pump the laser dyes
LDS 698, Rhodarnine 6G and Pyrromethene 567 in either ethylene glycol or propylene
glycol solutions. The output of the 702, which varied between 100-400 mW depending
on the dye and wavelength, was doubled with a KDP crystal. The doubling efficiency
was typically 10-15 %. The light that was not doubled was filtered out with a Corning 7-
54 filter. Bph was excited to the first singlet state with 280 nm obtained from 560 nm
(Pyrromethene 567). Stern-Volmer dynamic quenching of Bph fluorescence lifetime in
DTPACD by Tb3+ was collected on this laser system by TCPC.

The TCPC instrument described by Bowman et al. [180] was rebuilt in order to
decrease instrument response function (IRF) by eliminating the transit time spread (TTS)
of the monochromator and decreasing the risetime of the detector [181]. The SPEX 1681
single grating monochromator was replaced with a subtractive double monochromator
from CVI (CM112 and DK1200 controller), thereby eliminating the use of a 10 mm mask
previously placed on the 1681 grating. A subtractive double monochromator eliminates
the TTS between blue and red photons that the 10 mm mask previously provided.
Therefore, the throughput of photons was larger. A TAC biased amplifier (Oxford,

Instruments Model TC 864) replaced the TC 862, changing the range of the smallest

126

temporal window from 25 ns to 5 ns. The 220 ps risetime of the detector was improved to
156 pS by replacing the 12 11M R1564U/11 MCP—PMT (Hamamatsu) with a 6 11M
R3809U MCP-PMT (Hamamatsu). The shortened risetime of the 6 1.1M detector
necessitated a shortening of the 9 cm internal delay loop of the fluorescence channel of
the CFD (Tennelec CFD 454). However, after an exhaustive variation of the internal
delay lengths longer and shorter than 9 cm (electronics design by John Rugis), the IRF
still did not improve. Therefore a picotiming discriminator (9606) and fast preamp (9607)
were purchased from EG&G ORTEC. The picotiming discriminator replaces the CFD for
the fluorescence channel, and the Tennelec CFD is still used for the reference channel
from the laser pulse. The fast preamp was necessary for the MCP-PMT pulse shaping. A
picotiming analyzer (pTA 9608) is also available from EG&G ORTEC and would replace
the need for a TAC and allow longer lifetimes to be measured. Another advantage of the
9608 is that the software runs in a Windows platform. A demo model of this instrument
was investigated, but due to limited funds was not purchased. AS there are a finite
number of bins for data, short lifetimes can be measured with high resolution; however,
as the lifetimes lengthen, the resolution is poorer. Hence, for measuring short risetimes on

a long exponential decay, this instrumentation is not beneficial.

Results
Binding Constants

Binding constants of mono- and bicyclic aromatic hydrocarbons to TbDTPA-fl-
CD were measured by absorption and independently verified by emission spectroscopy

by Mark Mortellaro. The binding constant of Np to TbDTPA-fl-CD is previously

127

reported at 18000 M" [176,178]. The binding constant measured for Bph is also 18000
M'l [182]. Bicyclic aromatic hydrocarbons bind more tightly to TbDTPA-fl-CD than
monocyclic aromatics such as durene (4000 M") [176,178]. The binding constants of
other monocyclic aromatic hydrocarbons investigated in our studies: benzene, toluene
and xylene, are too small to be measured by changes in absorption because the molar
absorptivies (< 200 M"cm") and the changes in the terbium luminescence upon
complexation are too small.

A comparision of DTPA-,B-CD to previously studied aza crown swing and cradle
as seen in Table 6. reveals that a Tb3+ cyclodextrin complex with an overall neutral
charge, TbDTPA-fl-CD, binds neutral complexes more strongly than a cyclodextrin that
does not neutralize the +3 charge of the lanthanide (aza crown swing and aza crown
cradle). The swing cyclodextrin results demonstrate that charged carboxylate analytes
have the strongest binding of all analytes examined. A more detailed study of the aza
crown swing and aza crown cradle CD binding properties with neutral aromatic

hydrocarbons (Bph and Np) is necessary for a direct comparison.

Titrations and Steady State Luminescence

Steady state luminescence spectra of TbDTPA-fl-CD, when titrated with aromatic
hydrocarbons, provide evidence of an energy transfer process. When TbDTPA-fl-CD
complexed with Bph is excited at 266 nm, emission from Tb3+ at four wavelengths, 488,
544, 585 and 620 nm, increases with increasing amounts of Bph (Figure 35). The four
transitions of Tb3+ are from the lowest J level of the excited state J = 4, 504—)7F6 (488

nm), 5D4—>7F5 (544 nm), 5D,—>7F,, (585nm), 5D,—>7F3 (620 nm).

128

Table 6. Molar absorptivities of aromatic hydrocarbons, binding constants of cyclodextrin
complexes and lanthanide luminescence quantum yields for aza swing, aza cradle and
DTPA cyclodextrins and aromatic hydrocarbons [176,178].

 

 

Cyclodextrin Aromatic Molar Binding Quantum
Analyte Absorptivity/ Constant/ M" Yield
M'lcm'l
azaCrown Swing fl-CD benzene 196 117 -
azaCrown Swing ,B-CD benzoic acid 546 36000 -
azaCrown Swing 6CD naphthoic 1370 43000 -
acid
azaCrown Cradle ,B-CD benzene 196 <10 -
TbDTPA-fl-CD biphenyl 6000 18000 0. 103a
TbDTPA-fl-CD naphthalene 5500 18000 0.0293
TbDTPA-flCD durene 692 4000 -

 

3 Measured for the most intense transition 5D4—)7F5 relative to biacteyl in hexane (<1)p =
0.05).

129

Bph emission from the singlet state is also present in the emission spectrum, at 320 nm,
but is not shown in Figure 35.

Excitation spectra were collected by scanning the excitation monochromator
while detecting at the 544 nm Tb3+ emission line. Figure 36 shows the excitation
Spectrum of the TbDTPA-flCD complex with increasing amounts of Bph. Two
excitation bands are observed, one at 220 nm, a Tb3+ f-d transition, and another at 266
nm, the first excited state of Bph (8,). The Bph singlet state grows in with increasing
amounts of Bph and is structureless due to the non-planar character of Bph in the ground
state. The emission spectrum of Bph is slightly more structured, indicating that the
excited state of Bph is more planar in character [106].

The titration profiles of Bph and Np with 2.5 x 10'4 M TbDTPACD are shown in
Figure 37. For these measurements A6,, = 275 nm for Np, A6,, = 266 nm for Bph and item =
544 nm for Tb3+ emission. For each case Tb3+ emission increases to a plateau by 1.5 x
10'4 M. The relative emission intensities in the asymptotic limit are 35 for Bph and 5 for
Np. The factor of 7 difference can be explained in part by differences in the molar
absorptivity; Bph is 6600 M'lcm" (266 nm), compared to 5000 M'lcm" for Np (275 nm).
However, the ratio of the emission intensities is much larger than ratio of the molar
absorptivites.

As the binding constants of Np and Bph are identical, differences in the plateau
intensity must be due to another factor in addition to the molar absorptivites.
Measurement of the quantum yields of emission for TbDTPACD-Bph and

TbDTPACD-Nap, relative to biacetyl, yields a quantum yield for emission of 0.103 and

130

 

 

   

 

 

 

 

 

 

 

70

60 p
E:
8 50
(D
L5
g 40
a 1%
.22 1‘:
LE 30
T5 5 ‘

10

0 fi—

450 500 550 600 650

A/nm

Figure 35. Emission Spectra (A,x = 266 nm) for a 2.5 x 10" M aqueous solution of
TbDTPACD with increasing concentrations of Bph (data courtesy of M.A. Mortellaro
[179]).

131

 

120

 

 

 

 

100 '
a? l
(é) l
,9 80
E
C i
60 “
E
Lu
33' 40
m
20
0 l | ~
200 220 240 260 280 300
A/nm

Figure 36. Excitation Spectra (item: 544 nm) for a 2.5 x 10’4 M aqueous solution of
TbDTPACD with increasing concentrations of Bph (data courtesy of M.A. Mortellaro
[179]).

132

 

4O

30 °
25
20
15

10

Relative Emission Intensity ()l’l)

 

 

0 I1] I l
0 0.0001 0.0002 0.0003

[Biphenyl or NaphthalenellM

Figure 37. Relative Emission intensity from an aqueous solution of TbDTPACD ( 2.5 x
10" M ) as a function of Bph (O) and Np (Cl) concentrations. The luminescence of the
Tb3+ ion was detected at 544 nm with excitation wavelengths of 268 and 275 nm,
respectively (data courtesy of M.A. Mortellaro [179]).

133

0.029 respectively as given in Table 6 a factor of 3.55 difference. The quantum yield
reported is only for the most intense transition of Tb3+ the transition 5D4—)7F5. This
transition accounts for only 50% of the intensity of the four emission bands. Therefore if
we consider all four bands the overall quantum yield should be 0.206 and 0.058 for

TbDTPA-fl-CD-Bph and TbDTPA-fl-CD-Nap complexes respectively.

Overlap Integral and Critical Flirster Energy Transfer Distance

The overlap integral is essential in ascertaining if a transition is allowed by a
dipole-dipole mechanism. The overlap integral is proportional to the critical energy
transfer distance by the relationship given in equation 19 and can vary from 1 x 10"4 to l
x 10'20 M" cm3 for a strongly and weakly overlapped donor-acceptor pair, respectively. A
large value of J leads to efficient transfer over long distances (80—100 A). A small J limits
energy transfer to shorter distances. The overlap integral of Bph emission and Tb3+
absorption was calculated using a Kaleidagraph spreadsheet format and integrating over
the product of normalized Bph emission and Tb3+ absorption divided by the wavenumber
taken to the fourth power. The values for the overlap integral of Bph and Np with Tb3+
are given in Table 7. for two integration intervals, 17241 to 28703 cm" and 17241-30012
cm". The spectral overlap as a function of wavenumber of the Bph emission and Tb3+
(Tb(NO3)3-5HZO) absorption is Shown as a bold line in Figure 38. The emission spectrum
of Bph (a) shows the tail of the singlet state transition, which Shows a maximum at 266
nm (37594 cm"). There is little evidence of emission from the triplet state of Bph under

these conditions, which should appear at around 22900 cm'l [183].

134

Table 7a. Overlap Integral from 17241-28703cm" and Critical Distances for Singlet (S)
and Triplet (T) States of Bph and Np Donor and Tb(NO3)3°5H20 Acceptor.

 

Aromatic J/M"ch Ro/A R/A Efficiencyc ROIA R/A Efficiency°

 

Hydrocarbon (S)a (S)b (T)a (T)b
Bph 2.86 x 10'19 4.17 8.57 1.3 % 4.62 3.12 91 %
Np 3.99 x 10‘19 4.59 5.77

 

Table 7b. Overlap Integral from 17241-30012 cm" .

 

 

Aromatic J/M'lcm3 Rn/A R/A Efficiencyc Ro/A R/A Efficiencyc
Hydrocarbon (S)a (S)b (T)a (T)b

Bph 5.24x10‘19 4.62 14 0.13% 6.15 4.16 91%

Np 7.35 x 10''9 5.08 6.39

 

a Evaluated with Ro=9.78 x 103 (Qchzn'UW6 with quantum yields given by the previous
table, K = 2/3 and n = 1.332 for water. Equation from Reference [131].

1’ Fit with kg, = 8.3 x 10“ s" and k = l/TD(Ro/R)6

c Efficiency: l/1+(R/Ro)6

135

 

 

 

“

"a, (a)
a
e i.
0 '=.
Z 1
\. '1
3 5.;
<1: i '-
2 't
\
g t
t y

(b)
.3, . w vawmfl’ ”‘WW
(0)
l l l l

 

28000 26000 24000 22000 20000

Wavenumber (cm '1 )

Figure 38. Overlap Integral (c) of Bph emission (a) and Tb(NO3)3-5H20 absorption (b).

136

The absorption spectrum of Tb(NO3)3 (Figure 38b) shows 5 narrow lines at 28500
UFO—>504), 27800 (7196-5505), 27100 (7F,—>5L,0), 26500 (Ga—>503, 506 ) and 20500
cm" UFO—>504) with the transitions given in parenthesis [184]. The transition at 20500
represents direct population of the emissive excited state of Tb3+ at 544 nm (504—)7F 5).
A very large absorption band from nitrate counter ions is observed above 29000 cm" and
was not included in the calculation of the overlap integral.

TbCl3'6HzO can also be used as a source for terbium. The absorption spectrum of
TbCl3°6HZO (Figure 39) shows 9 narrow lines from 25000 to 36000 cm", but lacks the
large absorption band observed for Tb(NO3)3°5HzO around 30000 cm". The absence of
the nitrate band should decrease the overlap integral. The value of J for TbCl3-6HZO
absorption and Bph emission yields a value of 3.15 X 10"9 M" cm3 (28703 to 30012
cm"). Therefore, the difference between the overlap integral of Tb(NO3)3'5HzO and

TbC13°6H20 is not significant.

Time-Resolved Luminescence and Stern-Volmer Quenching

The risetime and lifetime of Bph with TbDTPACD were measured at room
temperature in aerated solutions with a Nd:YAG nanosecond laser. Lanthanides are
notorious for their slow non-radiative deactivation in the excited state [141], and
therefore it was necessary to measure the risetime when the lanthanide excited state is
directly populated. The risetimes of Tb(NO3)3°5H20 and TbCl3-6H20 (SD, 546 nm) are <
10 ns when excited directly (7Lex = 266 nm) in the absence of any added Bph light
harvester, and the quantum yield of terbium luminescence is small (Figure 40 and Figure

41). This implies that internal conversions from the higher level 5D3 excited State is very

137

 

—L
l l

Molar Absorptivity/ M'1 crn'1
C
'CD

 

; I l l 1

36000 34000 32000 30000 28000 26000

Wavenumber (cm")

Figure 39. TbCl3°6HzO absorption spectrum (0.05 M in methanol).

 

138

 

 

 

lntensi

 

 

 

 

Time/ms

Figure 40. Time-resolved luminescence of Tb(NO3)3-6H20 (open circles) and TbDTPA-
,6»CD (squares) at 546 nm.

139

 

 

 

 

 

 

 

Emcee.

0.4 0.6 0.8

Time/ms

0.2

Figure 41. Time-resolved luminescence of TbCl3°6H20 (open circles) and TbDTPA-fl-

CD (squares) at 546 nm.

140

fast leading to a fast population of the luminescent excited state 5D4. Therefore Slow
internal conversion of the excited states of Tb(NO3)3°5HzO and TbCl3-6HZO is not
apparent at room temperature.

When 2 pl of 0.02 M Bph in ethanol is titrated in to the solution there is a
dramatic change in the risetime (Figure 40 and Figure 41), which is actually very slow in
the presence of Bph. Concomitant with the longer risetime is a large increase in the
quantum yield of Tb3+ emission. A fit of the risetime of terbium luminescence at 546 nm
results in a 12 us time constant, which is consistent over the entire titration range as
shown in (Figure 40). The risetime is attributable to the population buildup of the 5D4
state at 20500 cm'l when light harvested by Bph is transferred to the terbium excited
state. This risetime is equivalent to a rate for energy transfer of 8.3 x 104 8", assuming the
reaction is first order.

Time-resolved luminescence of the excited state of Tb“, 5D4, was fit to a standard
kinetics equation involving an exponential rise, representing the buildup of population by
energy transfer in the Tb?” excited state and the exponential decay of the state,
representing the radiative decay. The kinetics scheme of Bph exicted state donor and

TbDTPA-fl-CD is given below, where DTPA-,B-CD is abbreviated CD and TbDTPA-fl-

CD is represented by TbCD
"' Bph . TbCD —“'—> Bph . TbCD + hv (25)
* Bph-TbCDL; Bph+TbCD (26)

*Bph+TbCD—k3—>*Bph-TbCD (27)

141

*Bph——k4—+Bph+hv (28)
* Bph . TbCD ——"5—) Bpho’TbCD + hv (29)
Bpho“ TbCD —“—6——> Bph . TbCD + hv (30)

This scheme was adapted from Thomas et al. [147]. Step 1 is the phosphorescence
(fluorescence) of Bph when encapsulated in the cyclodextrin cavity. Step 2 is the
dissociation of excited state Bph from the CD cavity. The rate of excited state Bph
association with a cyclodextrin cavity is given by the third step. In the fourth step excited
state Bph radiatively decays in bulk solution. The most important steps (5) and (6) Show
the rate of energy transfer and the rate of terbium luminescence. The last two steps define
the time evolution of the excited state of terbium(III). This luminescence time profile is
given by [147]:

—t

k5[Bph oTbCD]i e—

1*Tbl= , ‘P —e"‘6‘ (31)

 

kO—Ip

where 1,, is the phosphorescence lifetime of Bph and t is dependant variable, time. The
data was fit to the equation given above where the decay of Bph (1,) is incorporated into
the risetime of Tb3+ luminescence and k6 represents the decay of luminescence of Thy.
According to this equation the energy transfer rate is given in the preexponential factor
k5. However, for our purposes we will use the risetime as the energy transfer rate. The
slowness of the risetime is indicative of a slow population of the Tb3+ 5D4 excited state by
the triplet state of Bph. If the singlet state is involved in energy transfer then one would

expect a much faster energy transfer rate with a nanosecond risetime (or picosecond) for

142

Tb3+ luminescence. Therefore the delayed buildup of Tb“ verifies that the mechanism of
transfer is through the triplet state of biphenyl.

The description of Bph energy transfer to TbDTPA-fl-CD as a unimolecular
process is tenuous. Stem—Volmer quenching of ‘Bph by Tb3+ will discriminate between a
unimolecular and bimolecular process. Since we do not observe Bph phosphorescence in
aerated solutions, we cannot measure quenching of Bph triplet. We therefore investigated
Stern-Volmer static and dynamic quenching of Bph fluorescence by measuring the

intensity and lifetime of Bph and DTPA-fl-CD with increasing concentrations of

Tb(NO3)3°5H20. The lifetime of Bph in DTPA—,B-CD was measured by TCPC to be 11 ns
in the absence of Tb”. This lifetime is shorter than the literature value of 16 ns for polar
solvents [183]. As the concentration of terbium increases, the lifetime and intensity of
Bph emission at 320 nm (fluorescence) decreases. The Stern-Volmer plot of 1011 and To/‘t
of Bph fluorescence in DTPA-fl-CD as a function of Tb3+ concentration is shown in
Figure 43. The intensity and fluorescence lifetime of Np was also investigated and similar
results were observed. The dynamic (bimolecular) quenching derived from the slope of
the lifetime plot of both Np and Bph are roughly equivalent at 1.66 x 1010 and 2.10 x
10'0 s", respectively. These bimolecular rates are within the limit of diffusional processes.

In an attempt to obtain the static rate constant (binding constant) the loll
quenching data was fit with a quadratic equation. The fit failed to give a meaningful
value for static quenching rate constant. However, we can conclude that the extent of

static quenching is significant, given the large differences between the plots of M and

143

 

Rel. Emission Intensity

(b) (A)

1- 1 g

320 360 400 440 480 520 560
Wavelength (nm)

 

 

 

 

 

 

Figure 42. Bph fluorescence quenching by Tb“. Fluorescence (a) decreases upon
addition of terbium(III). A concomitant increase in terbium luminescence (b,c) is
observed at 488 and 544 nm.

144

 

 

 

 

1.6
1.5 -- 1:1
:1
1.4 L— '0/1
\l”
P 1.3 _
L Cl
0 CI
1.2 —
:0 1:1 :1
1.1 —- TO/T
O O
1 :1—3': 0 o 0 9
0.9 14.1.L...I......,,,| .........
0 0.0001 3 0.0002
lTb+l

0.0003

Figure 43. Stern-Volmer plot of loll and wk of Bph in DTPACD quenched with Tb“.

145

rolt. Therefore the value of 8.3 x 104 s" for the risetime is truly a reflection of the energy

transfer rate of a bound complex of Bph in the hydrophobic cavity of TbDTPACD.

Discussion

The very slow rate of energy transfer from light harvesting aromatic hydrocarbons
to luminescent lanthanide ions we observe in supramolecular cyclodextrin assemblies is
precedented. Thomas and coworkers observed very slow rates of energy transfer from Np
and Bph encapsulated in a micelle to lanthanide ions on the surface of the micelle [147].
In this case risetimes of ~ 20 us were observed. Another example of a Slow and
inefficient rate of energy transfer was observed in a protein system studied by Horrocks
where a tryptophan residue to a terbium ion residing in a calcium binding site transfers
energy over a distance of 8-10 A [185]. The very slow transfer can be accounted for by
many factors. The primary factor is the exceedingly small value for the overlap integral
that leads to an inefficient transfer of energy. Involvement of the triplet state will also
slow down the transfer, as the decay is long-lived and the FOrster rate is proportional to
the inverse of the donor lifetime.

It seems evident from the magnitude of the energy transfer rate that the triplet
state of biphenyl is involved. However, perhaps more proof is needed. The presence of
the heavy lanthanide in close proximity to the light harvesting aromatic donor may
enhance intersystem crossing to the triplet manifold of the donor. At this point we have
no data to support or refute this hypothesis. Inspection of Table 8 shows that the triplet

yield of Bph and Np are already very large, 0.84 and 0.75. Addition of a heavy lanthanide

146

Table 8. Quantum yields of fluorescence, triplet state, fluorescence lifetime and triplet
lifetime for mono- and bicyclic aromatic hydrocarbons. Data taken from reference [183].

 

 

Aromatic Hydrocarbon (1)" (in tfl/ns TT/us
Bph 0.15 0.84 16 130
Np 0.19 0.75 96 175
l,2,3,4-tetramethyl benzene 0. 14 - 36.2 -
Benzene 0.06 0.25 34 -
Toluene 0. 14 0.53 34 -

p-Xylene 0.22 0.63 30 -

 

147

atom would increase the intersystem crossing by approximately an order of magnitude if
the lanthanide were within bonding distance of the aromatic hydrocarbon [150].

Amazingly enough, there is a lack of intensity of Bph triplet observed in the
overlap integral (Figure 38), which is typically observed at 22900 cm" in the emission
spectra. It appears that the intensity and lifetime of the triplet state of Bph is quenched by
oxygen in aerated solutions as noted by the absence of emission at 22900 cm". An
attempt was made at collecting emission spectra of degassed solutions at room
temperature. This yielded inconsistent results. Gadolinium was used in place of terbium
as a control experiment to test for phosphorescence of Bph at exceedingly high
concentrations [186]. Some evidence of Bph triplet emission was observed when
precipitation occurred. The same results were observed for solutions of Gd3+ and Bph in
the absence of the cyclodextrin.

The conformation of Bph may play some role in determining whether the triplet
or singlet excited states are involved in energy transfer. Many studies of the conformation
of aromatic hydrocarbons, particularly Bph, have been published [187]. The general
consensus is that more planar, rigid molecules have smaller intersystem crossing rate
constants. In one study the quantum yield, phosphorescence quantum yield and
phosphorescence lifetime of Bph in viscous propylene glycol were measured as a
function of temperature and excitation wavelength. The intersystem crossing rate and
radiative phosphorescence rate are both faster in twisted Bph in comparison to planar
Bph. It is difficult to determine the extent of planarity of Bph in a cyclodextrin cavity,

however molecular modeling minimizations show a twisted conformation for Bph

148

(Figure 44). Circular dichroism measurements may shed light on the conformation of
Bph in the cyclodextrin cavity.

Approximations of the distance of energy transfer from the lanthanide acceptor at
the bottom of the cyclodextrin to the aromatic donor in the CD cavity were also modeled
on the Silicon Graphics system using Insight Discover Biosym. As lanthanide forcefields
are not available on Discover, the lanthanide was excluded from the energy minimization
calculation of swing and cradle CD. An ammonium ion was used to approximate the
charge interactions of a cation (to replace Tb3+ and Eu“) with the triacid group of the
DTPACD. The results of the minimization were realistic, as the three negative charges
formed a triangular orientation about the ammonium cation. The average distance was
around 6-7 A.

Table 7 lists the values of the critical energy transfer distance (R0) and the actual
distance calculated with the experimental rate and lifetime of the donor. Two different
values of the overlap integral give similar results. An accurate measurement of the
overlap integral is pivotal to determining the validity of the dipole-dipole mechanism for
this type of energy transfer. The values of R were evaluated for using equation 20, with
the values for R0 from measurement of the overlap integral and k from the risetime. The
singlet state of Bph gives a distance of 8-14 A between the Bph and Tb3+ ion and a very
low efficiency of 1% or less. Distances of 3-4 A are approximated for the triplet state
with larger efficiencies of 91%. The slow rate along and the measurements of the
efficiency are a convincing argument for a triplet state mechanism. The small efficiency

of energy transfer from the singlet state coupled with the longer distances for energy

149

 

Figure 44. Molecular modeling showing twisted Bph in the cavity of TbDTPA-fl-
cyclodextrin. The lanthanide ion is the shaded ball, the DTPA-fl-cyclodextrin is shown as
a stick and Bph in CPK rendering. The distance from the center of the closest Bph aryl
ring to the center of the lanthanide is 6—7 A.

150

transfer make it unlikely that energy will flow directly through this state to the lanthanide

excited state.

Summary and Future Work

The rate of absorption of Bph and energy transfer to lanthanide ions in a non-
covalent supramolecular cyclodextrin assembly is slower than diffusion, (8.3 x 104 8").
Two issues that remain unresolved are whether the triplet or singlet state of Bph is
involved in energy transfer and if a significant contributions from exchange interactions
are also possible. To address the first point, it seems very unlikely that buildup of terbium
population is arriving by population from Bph singlet state (1' = 11 ns). If this were true
one should observe a risetime that is orders of magnitude faster than what has been
measured. Inspection of the values of R0 and R for energy transfer from the singlet state
(Table 7) the efficiency is 1%. Since all experiments were performed in aerated solutions
at room temperature, the quantum yield and lifetime of Bph triplet state will be very
small. Despite the apparent absence of Bph triplet state emission and extremely small
overlap of Bph emission and terbium absorption, an energy transfer efficiency of 91% is
estimated.

Balzani and coworkers have measured the triplet-triplet transient absorption of
[anbpy.bpy.bpy]3+ cryptand where Ln = Gd,Tb and Eu. They observed a very large
triplet state absorption for the bipyridine ligands having a AOD of 0.4 and a non-
exponential decay in the us time scale. The transient triplet signal was quenched by

oxygen with a rate of 5 x 108 M'l s". In our laboratory, attempts at measuring the

transient absorption of Bph, GdDTPACD with Bph and TbDTPACD with Bph yielded

151

inconclusive results. The pump pulse was set at 266 nm and the monochromator scanned
from 280-400 nm. Degassing of a solution of Bph in ethanol also failed to produce a
transient signal at moderate slit widths. A possible false signal was observed when the
slits were opened to nearly 8 m. If the proper set of optics coated for 266 nm were
employed excitation power of the laser pulse and overlap with the probe white light
could be greatly improved.

Preliminary energy transfer studies of the risetime of Tb3+ in TbDTPACD in the
presence of 4-bromobiphenyl and 3-bromobiphenyl donor show that the rate of buildup is
also very slow (20-50 us) adding further evidence that the triplet state of the aromatic is
involved [62]. Although intersystem crossing is enhanced by attachment of bromine,
energy transfer is still limited by the decay of the triplet state. Small changes in the
conformation of Bph by using various 2-, 3-, 4- substituted derivatives may prove
essential in determining whether the route of energy transfer flows through the singlet or
triplet state by providing an assessment of how the extent of twisting increases
intersystem crossing rates.

The second issue concerning the proof or disproof of an exchange mechanism is
difficult to undertake until a way to systematically varying the distance is found. Also, it
is not possible to completely eliminate the possibility of contributions from Dexter

energy transfer. A general rule of thumb for the rate dependence of Dexter energy

—2R
transfer as a function of distance is, k=k0exp 2-3 where k0, the limiting rate at

collisional distance (R = 0) is 10'3 s'1 [17]. This crude model gives an estimate of the rate

to be ~104 s'1 when R = 10 A. One control experiment would be a measurement of the

152

rate of a covalent system of Tb3+ and biphenyl where the distance is set between 6-7 A.
This would give a value for a through bond rate.

Although the exchange mechanism is typically viewed as a through bond
mechanism, there is no reason not to believe that a through space mechanism is
impossible. Farran and Deshayes have demonstrated that exchange energy transfer can
occur bimolecularly on the extremities of a hemicarcerand to a biacetyl molecule trapped
in the cavity over a distance of around 7 A [136]. One possible solution is to choose a
series of donors where the triplet/singlet state is varied over a region where a Marcus
curve can be measured. One is limited by sizes of donor aromatic hydrocarbons that will
fit in the cavity of the cyclodextrin and by the energy of the triplet state. Experiments
with pyrene and anthracene donors show that no energy transfer occurs due to the low
energy of the triplet state [188]. Possible molecules in the a Marcus curve series are
substituted biphenyls and naphthalenes. Table 9 lists substituted Bphs whose triplet states
vary from 2.5-2.79 eV. Other potential donors that have triplet energies above the Tb3+
5D; (20500 cm") are fluorene (Emplep 23000 cm'l), triphenylene (Em-pm~ 23400 cm’l),
benzophenone (Etriplet~ 24200 cm‘l), xanthone (Emma~ 25905 cm") and xanthene (Emplm~
27700 cm'l), to name a few. Xanthene has previously been used as a light harvester for
terbium complexes [189].

In conclusion, we have succeeded in designing a sensor that is optimized for
neutral aromatic hydrocarbons. Other supramolecular AETE systems exist (Figure 45),
however, in these systems the chromophore is built into the backbone of the molecule.

The bipyridine calixarene complexes with Tb3+ synthesized by Sabbatini, Ungaro and

153

Table 9. Energy levels of substituted Bphs for a Dexter Marcus curve study.

 

 

Bph Donor Energy of
Triplet State/eV

4,4'-dinitro 2.50
4,4'-dibenzoyl 2.50
2-nitro 2.57
2-iodo 2.73
4,4-dichloro 2.73
4,4'-dihydroxy 2.73
4,4'-dimethoxy 2.74
4-chloro 2.75
3-chloro 2.76
4,4'-difluoro 2.76
4-hydroxy 2-78
3,3'-dichloro 2.79

4,4'dimethyl 2.79

 

154

   

 

Figure 45. Three supramolecular antenna lanthanide complexes (a) bipyridine cryptand
(b) hexakis(4-carboxyphenoxy)cyclotriphosphazine (c) [Ln c bpy.bpy.bpy.]3+.

155

coworkers has a quantum yield of 0.12 [190]. Matusumoto has created a complex with a
quantum yield of 0.30 [191]. Lanthanide bipyridine cryptates (anbpy.bpy.bpy) have
large lanthanide quantum yields from between 0.01 to 0.10 in H20 and D20 [153]. These
values are comparable to the quantum yields of 0.10 and 0.03 obtained for
Bph°TbDTPA-,B—CD and Np‘TbDTPA-fl-CD. The rate of energy transfer from the
bipyridine triplet state through the bonds of the cryptate cage to the 5D4 of Tb in
Tbcbpy.bpy.bpy is derived from a kinetics scheme involving decay of the transient
absorption triplet state and of the 5D4 state [153]. The solution to a best fitting procedure
gives an energy transfer rate of 0.5 x 108 s".

In our systems the chromophore is bound to the supramolecule by an equibrium
process, allowing us to turn bright visible lanthanide luminescence on and off. There are
very few cases of lanthanide luminescence used in optical sensing by this type of
technique. The only other example is the work by Shinkai [192]. In this AETE sensor
binding of saccharides to a boronic acid inhibits quenching by a tertiary amine and bright
luminescence of a lanthanide ion is observed. The challenge that we face with our system
is fine tuning the properties of the AETE supramolecule to enhance the luminescence
response for benzene and other weakly absorbing monocyclic aromatic hydrocarbons. As
we can see from Figure 42 the luminescence intensity of the donor molecule is still very
large relative to terbium luminescence.

There are many potential future directions for this research project. Several
possibilities are outlined in Figure 46. One control experiment (Figure 46a) is to make a
covalent AETE complex with Bph, Np and benzene covalently bound to the secondary

side of the primary side linked DTPA-,B-CD. This experiment will define a measurable

156

(b)

 

 

Figure 46. (a) Covalent Bph-CD-lanthanide AETE system. (b) Potential new scheme for

sensing benzene by an antenna amplification of the signal. (c) Multichromorphic
cyclodextrins (see reference [132])

157

distance and reveal if energy can transfer from the upper rim of the CD through the bonds
to the lanthanide ion on the secondary side.

A second possibility is to make a DTPACD with multichromophore antenna
molecules on the secondary side (Figure 46b). The light harvesting backbone on this
cyclodextrin may significantly magnify the luminescent signal from benzene by
cascading the energy down a light collecting channel of two short distance steps (kc... and
km) to the lanthanide ion, akin to the way that energy is funneled to the reaction center in
photosynthetic units. We may be able to convert high energy light absorbed by benzene
to lower energy by using smaller energy gaps to transfer the energy from benzene to the
lanthanide ion. Addition of through bond step may yield more efficient transfer and faster

rate S .

LIST OF REFERENCES

(a) Silkworth, J.B.; Lipinskas, T.; Stoner, C.R. Toxicology, 1995, 105, 375. (b)
Johnson, B.L. Chemosphere 1995, 31, 2415. (c) Ahlfeld, D.P.; Page, R.H.; Pinder,
G.F. Groundwater 1995, 33, 58. (d) Pascoe, G.A.; Dalsoglio, J .A. Environmental
Toxicology and Chemistry, 1994, 13, 1943. (e) Karnrin, M.A.; Fischer, L.J.; Suk,
W.A.; Fouts, J .R.; Pellizzari, E.; Thornton, K. Environmental Health Perspectives
1994, 102, 221. (f) Royer, M.D.; Selvakumar, A.; Gaire, R. Journal of the Air &
Waste Management Association 1992, 42, 970.

Reisch, M.; Johnson, S.L.; Bearden, D.M. Congression Research Service Report
for Congress, Superfund Fact Book, Environment and Natural Resources Policy
Devision, 1994, 94-464 ENR.

Jones, K. in Sensors, A Comprehensive Survey; Gopel, W.; Hesse, J .; Zemel, J .N.
Eds.; VCH: Weinheim, 1990; Vol. 8, Ch 16.

Soloman, S. Sensors and Control Systems in Manufacturing, McGraw-Hill: New
York, 1994.

Westbrook, M.H.; Turner, J .D. Sensors Series: Automotive Sensors Jones, B.E.
(ed.) Institute of Physics Publishing: Philadelphia, 1994.

Shulman, M.A.; Hamburg, D.R. Society of Automotive Engineers Paper 1980,
800018,.

158

10.

11.

12.

13.

14.

15.

16.

17.

18.

Pfeifer, J.L; Wertheimer, H.P. Proc. 5’” Int. Automotive Electronics Conf.
(ImechE Conf. Publication 1985-12), 1985, 357.

Norton, H.N. Sensor and Analyzer Handbook; Englewood Cliffs, NJ: Prentice
Hall, 1982.

Gopel, W.; Hesse, J.; Zemel, J.N. (eds.) Sensors: A Comprehensive Survey.
Fundamentals and General Aspects, VCH: New York, 1988, Vol. 1.

J anata, J. Principles of Chemical Sensors, Plenum Press, New York 1988, Ch. 5.

Wolfbeis, 08. (ed.) F iber-Optic Chemical Sensors and Biosensors, CRC Press:
Boca Raton, FL, 1991, Vol. 1.

Wagner, E.; Dandliker, R.; Spenner, K. (eds.) Sensors: A Comprehensive Survey.
Optical Sensors, VCH: New York, 1991, Vol. 6.

Rogers, A]. in W. Gopel, J. Hesse and J.N. Zemel (eds.), Sensors: A
Comprehensive Survey. Fundamentals and General Aspects, Optical Sensors
Plenum Press: New York, 1992, Vol. 6, Ch. 15.

Goldberg, M.C. (ed.) Luminescence Applications in Biological, Chemical,
Environmental and Hydrological Scineces, ACS Symposium Series 383,
Amcerican Chemical Society, Wasington, D.C.

Wolfbeis, O.S. (ed.) Fluorescence Spectroscopy, New Methods and Applications,
Springer-Verlag, Berlin.

(a) Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds,
Academic Press: New York, 1970. (b) Wayne, R.P. Principles and Applications of
Photochemistry, Oxford University Press: New York, 1988.

Turro, NJ. Modern Molecular Photochemistry, University Science Books: Mill
Valley, 1991.

Ferraudi, GJ. Elements of Inorganic Photochemistry, Wiley-Interscience: New
York , 1988, Ch. 1.

159

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

Balzani, V.; Moggi, L.; Manfrin, M.F.; Bolletta, F.; Coord. Chem. Rev. 1975, 15,
321.

Mortellaro, M.A.; Nocera, D.G. ChemTech, 1996, 26, 17. (b) Nocera, D.G. New
Scientist, 1996, 149, 24. (c) Pikramenou, 2.; Yu, J.-a; Lessard, R.B.; Ponce, A.;
Wong, P.A.; Nocera, D.G. Coord. Chem. Rev. 1994, 132, 181.

McGlynn, S.P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the Triplet
State; Prentice-Hall: Englewood Cliffs, 1969; Chapter 5 and Appendix 1.

(a) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives; VCH:
New York, 1995. (b) Lehn, J .-M. in Frontiers in Supramolecular Organic
Chemistry and Photochemistry Schneider, H.-J.; Diirr, H. (eds.) VCH: New York,
1991, Ch. 1. (c) Lehn, J.-M. Angew. Chem. Int. Ed. Engl. 1988, 27, 89.

Odell, B.; Reddington, M.V.; Slawin, A.M.Z.; Spencer, N.; Stoddart, J.F.;
Williams, D.J. Angew. Chem. Int. Ed. Engl. 1988, 27, 1547.

El Khalifa, M.; Pétillon, F.Y.; Saillard, J .Y. and Talarmin, J. Inorg. Chem. 1989,
28, 3849.

Gutsche, C.D. Calixarenes Royal Society of Chemistry: Cambridge, 1989.

Cram, D.J.; Karbach, S.; Kim, Y.H.; Baczynskyj, L. Marti, K.; Sampson, R.M.;
Kalleymeyn, G.W. J. Am. Chem. Soc. 1988, 110, 2554.

Cram, D.J.; Karbach, S.; Kim, Y.E.; Knobler, C.B.; Maverick, E.F.; Ericson, J .L.;
Helgeson, R.C. J. Am. Chem. Soc. 1988, 110, 2229.

Rebek, J ., Jr. Science, 1987, 235, 1478.

Pedersen, C.J. Science, 1988, 241, 536.

Dietrich, B.; Lehn, J .M. Sauvage, J .P Tetrahed. Lett. 1968, 2285.
Dietrich, B. Angew. Chem. Int. Ed. Engl. 1988, 27, 362

Kroto, H.W. Nature, 1987, 329, 529.

160

33.

34.

35.

36.

37.

38.

39.

40.

41.

42.

43.

45.

46.

Vinod, T.K.; Hart, H. J. Am. Chem. Soc. 1988, 110, 6574.
Hamilton, A.D.; Van Engen, D. J. Am. Chem. Soc. 1987, 109, 5035.

Barr, D.; Clegg, W.; Hodgson, S.M.; Lamming, G.R.; Mulvey, R.E.; Scott, A.J.;
Snaith, R.; Wright, D.S. Angew. Chem. Int. Ed. Engl. 1989, 28, 1241.

Hyatt, J.A. J. Org. Chem. 1978,43, 1808.

Fessner, W.D.; Sedelmeier, G.; Spurr, P.R.; Rihs, G.; Prinzbach, H. J. Am. Chem.
Soc. 1987, 109, 4626.

Pallavicini, P.S.; Perotti, A.; Poggi, A.; Seghi, B.; Fabbrizzi, L. J. Am. Chem Soc.
187, 109, 5139.

Sargeson, A.M. Chem. Brit. 1979, 15, 23.

Canceill, J.; Collet, A.; Gabard, J .; Kotzyba-Hilbert, F.; Lehn, J.M. Helv. Chim.
Acta, 1982, 65, 1894.

Cram, D.J. Angew. Chem. Int. Ed. Engl. 1986, 25, 1039.

Bell, T.W.; Firestone, A.; Ludwig, R.; J. Chem. Soc., Chem. Commun. 1989,
1902.

Leppkes, R.; Vogtle, F. Angew. Chem. Int. Ed. Engl. 1981, 20, 396.
Zimmerman, S.C.; Wu,W. J. Am. Chem. Soc. 1989, 111, 8054.

Arrhenius, T.S.; Blanchard-Desce, M.; Dvolaitzky, M.; Lehn, J.M. Malthete, J.
Proc. Natl. Acad. Sci., 1986, 83, 5355.

(a) Schierbaum, K.D.; Weiss, T.; Thoden van Velzen, E.U.; Engbersen J.F.J;
Reinhoudt, D.N; Gopel, W. Science, 1994, 265, 1413. (b) Gopel, W. Sens.
Actuators B., 1994, 18-19, 1. (c)Weimar, U.; Vaihinger, S.; Schierbaum, K.D.;
Gopel, W. Chemical Sensor Technology, Kodansha: Tokyo, 1991, 3, 51. (c)
Hierlemann, A.; Weimar, U. Kraus, G.; Gauglitz, G.; Gopel, W. Sensors and
Materials, 1995.

161

47.

48.

49.

50.

51.

52.

53.

54.

55.

56.

57.

58.

59.

60.

61.

(a) Reinhoudt, D.N.; Sudholter. E. J.R. Adv. Mater. 1990, 2, 23. (b) Cobben,
P.L.H.M.; Egberink, R.J.M.; Bomer, J .G.; Bergveld, P. Verboom, W.; Reinhoudt,
D.N. J. Am. Chem. Soc. 1992, 114, 10573.

Hafeman, G.; Parce, J .W. McConnell, H.M. Science, 1988, 240, 1182.
Wenz, G. Angew. Chem. Int. Ed. Engl. 1994, 33, 803.

(a) Saenger, W. Angew. Chem. Int. Ed. Engl. 1980, 19, 344. (b) Saenger, W.
Inclusion Compounds 1984, 2, 231.

Breslow, R. Acc. Chem. Res. 1995, 28, 146.
Tabushi, 1. Acc. Chem. Res. 1982, I5, 66.

Szejtli, J. Cyclodextrins and Their Inclusion Compounds, Aladémiai Kiadc’):
Budapest, 1982.

Bender, M.L.; Korniyama, M. Cyclodextrin Chemistry Springer Verlag: New
York 1978.

Okada, Y.; Kubota, Y.; Koizumi, K.; Hizukuri, S.; Ohfuji, T.; Ogata, K. Chem.
Pharm. Bull. 1988, 36, 2176.

Broser, W. Z. Naturforsch. 1953, 8b, 722.

Sakurai, M.; Kitagawa, M.; Hoshi, H. Chem. Lett. 1988, 895.
Cramer, F. Chem. Ber. 1951, 84, 851.

Broser, W.; Lautsch, W. Naturwiss., 1953, 40, 220.

Cramer, F. Chem. Ber., 1953, 86, 1582.

Kinoshita, F.; Iinuma, F.;Tsuji, A. Biochem. Biophys. Res. Commun. 1973, 51,
666.

162

62.

63.

65.

66.

67.

68.

69.

70.

71.

72.

73.

74.

75.

76.

77.

Cramer, F.; Hettler, H. Naturwiss. 1967, 54, 625.
Labianco, D.A. J. Chem. Ed. 1990, 67, 259.
Wisemann, A. Trends Anal. Chem. 1988, 7, 5.

Ruz, J.; Fernandez, A.; Luque de Castro, M.D.; Valcarcel, M.J. Pharm. Biomed.
Anal. 1986, 4, 559.

Volkl, K.-P.; Opitz, N.; Liibbers, D.W. F resenius' Z. Anal. Chem. 1980, 301, 162.
Wolfbeis, O.S.; Posch, H.B. F resenius' Z. Anal. Chem. 1988, 332, 255.
Schirmer, R.E.; Zetter, M. Adv. lnstrum. 1987, 42, 173.

Maeder, G.; Veuthey, J .-L.; Pelletier, M.; Haerdi, W. Anal. Chim. Acta 1990, 231,
115.

Walters, B.S.; Nielson, T.J.; Arnold, M.A. Talanta 1988, 35, 151.
Scheper, T.; Buckmann, A.F Biosens. Bioelectron., 1990, 5, 125.
Gautier, S.M.; Blum, L.J.; Coulet, P.R. Biolum. Chemilumin. 1990, 5, 57.

(a) Borkenstein, R.F. US. Patent 2 824 789, 1958. (b) Borkenstein, R.F.; Smith,
H.W. Med. Sci. Law 1961, 2, 13.

Intoximeters Inc. 1994-1995 http://www.intox.com

Stafford, D.T. "Investigation of the Response of Fuel Cell Based Alcohol Breath
Test Instruments to Substances Other than Ethanolz" 1993, Tennessee: University
of Tennessee Toxicology Laboratory.

Hayes, E.T.; Galal, A.; Mark, H.B, Jr. Talanta, 1995, 42, 873.

Stiller, M. Anal. Chim. Acta 1961, 25, 85.

163

78.

79.

80.

81.

82.

83.

84.

85.

86.

87.

88.

89.

90.

91.

Tanaka, M. Talanta 1960, 5, 162.
Seiler, K.; Wang, K.; Kuratli, M.; Simon, W. Anal. Chim. Acta 1991, 244, 151.
Kurauchi, Y.; Yanai, T. Ohga, K. Chem. Lett. 1991, 1411.

Orellana, G.; Gomez-Cameros, A.M.; de Dios, C.; Garcia-Martinez, A.A.;
Moreno-Bondi, M.C. Anal. Chem. 1995, 67, 2231.

Zeng, H.-H.; Wang, K.-M.; Dong, L.; Yu, R.-Q. Talanta, 1994, 41, 969.
Woflbeis, 0.8. F resenius' Z. Anal. Chem. 1986, 325, 387.

Ueno, A. In Photochemistry in Organized and Contrained Media; Ramamurthy,
V., Ed. ; VCH: New York, 1991.

Balzani, V.; Scanola, F. Supramolecular Photochemistry; Ellis Horwood: West
Sussex, England, 1991; Chapter 10.

Kalyanasundaram, K. Photochemistry in Microheterogenous Systems; Academic:
Orlando, FL, 1987.

Eaton, D.F. Tetrahedron 1987, 43, 1551.

Schuette, J .M.; Ndou, T.T.; Mufioz de la Pefia, A.; Mukundan, S., Jr.; Warner,
I.M. J. Am. Chem. Soc. 1993, 115, 292 and references therein.

(a) Patonay, G.; Fowler, K. Shapira, A.; Nelson, G.; Warner, I.M. J. Inclusion
Phenom. 1987, 5, 717. (b) Nelson, G.; Patonay, G.; Warner, I.M. Anal. Chem.
1988, 60, 274. (c) Zung, J .B.; Munoz de la Pena, A.; Ndou, T.T.; Warner, I.M. J.
Phys. Chem. 1991, 195, 6701. (d) Munoz de la Pefia, A.; Ndou, T.T.; Zung, J.B.;
Greene, K.L.; Live, D.H.; Warner, I.M. J. Am. Chem. Soc. 1991, 113, 1572.

Nakajim, A. Bull. Chem. Soc. Jpn. 1984, 57, 1143.

Ueno, A.; Takahashi, K.; Hino, Y.; Osa, T J. Chem. Soc., Chem. Commun. 1981,
194.

164

92.

93.

94.

95.

96.

97.

98.

99.

100.

101.

102.

103.

104.

105.

Hamai, S. J. Am. Chem. Soc. 1989, 111, 3954. (b) Hamai, S. J. Phys. Chem.
1989, 93, 2074.

Bergmark, W.R.; Davis, A.; York, C.; Macintosh, A.; Jones, G., 11 J. Phys. Chem.
1990, 94, 5020.

(a) Nelson, G.; Warner, I.M. J. Phys. Chem. 1990, 94, 576. (b) Nelson, G.; Neal,
S.L.; Warner, I.M. Spectroscopy 1988, 3, 24.

(a)Scypinski, 8.; Love, L.J.C. Anal. Chem. 1984, 56, 322. (b) Femia, F.A.; Love,
L.J.C. J. Phys. Chem. 1985, 89, 1897.

(a) Turro, N.J.; Bolt, J .D.; Kuroda, Y.; Tabushi, I. Photochem. Photobiol., 1982,
35, 69 (b) . Turro, N .J .; Cox, G.S.; Li, X. Photochem. Photobiol., 1983, 37, 149.

Alak, A.M.; Vo-Dinh, T. Anal. Chem. 1988, 60, 596.

Richmond, M.D.; Hurtubise, R.J. Anal. Chem. 1989, 61, 2643.

Turro, N.J.; Okubo, T.; Chung, C.-J. J. Am. Chem. Soc. 1982, 104, 1789.

Ponce, A.; Way, J .J .; Wong, P.A.; Nocera, D.G J. Phys. Chem. 1994, 97, 11137.

Mortellaro, M.A.; Hartmann, W.K.; Nocera, D.G. Angew. Chem. Int. Ed. Engl.
1996, 35, 1945.

Mussel, R.D.; Nocera, D.G. J. Am. Chem. Soc. 1988, 110, 2864.
Saari, E. Ph.D. Thesis, M.S.U.

(a)Muiioz de la Pefia, A.; Ndou, T.T; Zung, J.B.; Greene, K.L; Live, D.H.;
Warner, I.M. J. Am. Chem. Soc. 1991, 113, 1572. (b)Hamai, S. J. Phys. Chem.
1988, 92, 6140. (c)Hamai, S. J. Phys. Chem. 1989, 93, 2047.

Eaton, D.F. in Handbook of Photochemistry; Scaiano, J .C. Ed.; CRC: Boca Raton,
1989; Vol 1, Chapter 4.

165

106.

107.

108.

109.

110.

111.

112.

113.

114.

115.

116.

Lakowicz, J.R. Principles of Fluorescence Spectroscopy; Plenum Press: New
York, 1983.

Jackson, J .A.; Turro, D.; Newsham, M.D.; Nocera, D.G. J. Phys. Chem. 1990, 94,
4500.

Mufioz de la Pefia, A.; Ndou, T.T.; Zung, J .B.; Warner, I.M. J. Phys. Chem. 1991,
95, 3330.

(a) Mufioz de la Pefia, A.; Ndou, T.T.; Zung, J.B.; Greene, K.L.; Live, D.H.;
Warner, I.M. J. Am. Chem. Soc. 1991, 113, 1572. (b) Hamai, S. J. Phys. Chem.
1988, 92, 6140 (c) Hamai, S. J. Phys. Chem. 1989, 93, 2074.

Turro, N.J.; Bolt, J .D.; Kuroda, Y.; Tabushi, I. Photochem. Photobiol. 1982, 35,
69.

Hamai, S. J. Am. Chem. Soc. 1989, 111, 3954.

(a)Harata, K.; Uedaira, H. Bull. Chem. Soc. Jpn. 1975, 48, 375. (b)Hamai, S. Bull.
Chem. Soc. Jpn. 1982, 55, 2721. (c)Yorozu, T.; Hoshino, M. Imamura, M.;
Shizuka, H. J. Phys. Chem. 1982, 86, 4422. (d)Hashimoto, S.; Thomas, J.K. J.
Am. Chem. Soc. 1985, 107, 4655. (e)Nelson, G.; Warner, I.M. J. Phys. Chem.
1990, 94, 546. (DPitchumani, K. Vellayappan, M. J. Incl. Phenom. 1992, 14, 157.

Liao, Y.; Frank, J.; Holzwarth, J .F; Bohne, C.J. J. Chem. Soc. Chem. Commun.
1995, 199.

(a) Ravi, M.; Soujanya, A.; Samanta, A.; Radhakrishnan, T.P. J. Chem. Soc.
Faraday Trans, 1995, 91, 2739. (b) Ravi, M.; Smanata, A.; Radhakrishnan J.
Phys. Chem. 1994, 98, 9133. (c) Abe, T. Bull. Chem. Soc. Jpn. 1991, 64, 3224.

(a)Bissel, R.A.; de Silva, A.P. J. Chem. Soc. Chem. Commun. 1991, 223.
(b)Beecroft, R.A.; Davidson, R.S.; Goddwin, Pratt, J .E.; Luo, X]. J. Chem. Soc.,
Faraday Trans. 2, 1986, 82, 2393. (c)Yip, R.W.; Loutfy, R.O.; Chow, Y.L.;
Magdzinski, L. Can. J. Chem. 1972, 50, 3426.

Park, J .H.; Nah, T.H. J. Chem. Soc. Perkin Trans. 2 1994, 1359.

166

117.

118.

119.

120.

121.

122.

123.

124.

125.

126.

127.

128.

129.

130.

(a)Breslow, R.; Czamik, A.W. J. Am. Chem. Soc. 1983, 105, 1390 (b) Breslow,
R.; Hammond, M.; Lauer, M. ibid. 1980, 102, 421.

Martin, K.A.; Mortellaro, M.A.; Sweger, R.W.; Fikes, L.E.; Winn, D.T.; Clary,
S.C.; Johnson, MP; Czamik, A.W. J. Am. Chem. Soc. 1995, 117, 10443.

Rao, K.R.; Srinivasan, T.N.; Bhanumathi, N.; Sathur, P.B. J. Chem. Soc. Chem.
Commun. 1990, 10 (b) Cramer, F.; Mackensen, G. Angew Chem. Int. Ed. Engl.
1966, 5, 601.

Fabbrizzi, L.; Poggi, A. Chem. Soc. Rev. 1995, 197 (b) Czamik, A.W. Acc.
Chem. Res. 1994, 27, 302. (c) Fluorescent Chemosensors for Ion and Molecular
Recognition (ACS Symp. Ser. 1993, 538).

Pikramenou, Z; Yu, J.A.; Lessard, R.B.; Ponce, A.; Wong, P.A.; Nocera, D.G.
Coord. Chem. Rev. 1994, 132, 181.

Ueno, A.; Suzuki, 1.; Osa, T. J. Am. Chem. Soc. 1989, 111, 6391.
Park, J .W.; Park, K.H. J. Inclusion Phenom. Mol. Recog. Chem. 1994, 17, 277.
Connors, K.A. Binding Constants; Wiley: New York, 1987.

van Dienst, E.; Snellink, B.H. M.; von Piekartz, I.; Engbersen, J F] .; Reinhoudt,
D.N. J. Chem. Soc. Chem. Commun. 1995, 1151.

Turro, N.J.; Okube, T.; Chung, C.-J. J. Am. Chem. Soc. 1982, 104, 1789.
Rademacher, J .T.; Nocera, D.G., unpublished results.
Harata, K.; Uedaira, H. Bull. Chem. Soc. Jpn. 1975, 48, 375.

van Grondelle, R; Dekker, J .P.; Gilbro, T.; Sundstrom, V. Biochem. et. Biophys.
Acta 1994, 1187, 1.

(a)Forster, Th. Annalen der Physik 1948, 2, 55; English translation in Biological
Physics Mielczarek, E.V., Greenbaum, E., and Knox, R. S., Eds.; American

167

131.

132.

133.

134.

135.

136.

137.

138.

139.

140.

Institue of Physics: New York, 1993; p.148-60. (b)Forster, Th. Discuss. Faraday
Soc. 1959, 2 7, 7. (c)F6rster, Th. Z. Naturforsch. 1949, 4a, 321.

Van Der Meer, W.B.; Coker, G.; Chen, S.S.-Y. Resonance Energy Transfer
Theory and Data; VCH: New York, 1994.

(a)Berberan-Santos, M.N.; Canceill, J.; Brochon, J.C.; Jullien, L.; Lehn, J.-M.;
Pouget, J .; Tauc, P.; Valeur, B. J. Am. Chem. Soc. 1992, 114, 6427. (b)Berberan-
Santos, M.N.; Pouget, M.; Valeur, B.; Canceill, J.; Jullien, L.; Lehn, J.M. J. Phys.
Chem. 1993, 97, 11376. (c)Berberan-Santos, M.N.; Canceill, J.; Gratton, E.;
Jullien, L.; Lehn, J.-M.; So, P.; Sutin, J.; Valeur J. Phys. Chem. 1996, 100, 15.
(d)Jullien, L.; Canceill, J.; Valeur, B.; Bardez, E.; Lefévre, J-P.; Lehn, J.-M.;
Marchi-Artzner, V.; Pansu, R. J. Am. Chem. Soc. 1996, 118, 5432.

Daschak, D.M.; Mallouk, T.E. J. Am. Chem. Soc. 1996, 118, 4222.
Dexter, D.L. J. Chem. Phys. 1953, 21, 836.

(a)Closs, G.L.; Piotrowiak, P.; MacInnis, J .M.; Fleming, G.R. J. Am. Chem. Soc.
1988, 110, 2652. (b)Closs, G.L; Johnson, M.D.; Miller, J.D.; Piotrowiak, P. J.
Am. Chem. Soc. 1989, 111, 3751. (c) Sigman, M.E.; Closs, G.L. J. Phys. Chem.
1991, 95, 5012.

Farran, A.; Deshayes, K.D. J. Phys. Chem. 1996, 100, 3305.

(a)Farhoosh, R.; Chynwat, V.; Gebhard, R.; Lugtenburg, J.; Frank, H.A.
Photosyn. Res. 1994, 42, 157. (b)Wasielewski, M.R.; Liddell, P.A.; Barrett, D.;
Moore, T.A; Gust, D. Nature, 1986, 322, 570.

Bunzli, J .-C.G. in Lanthanide Probes in Life, Chemical and Earth Sciences; J.-
C.G. Bunzli and GR. ChOppin Eds; Elsevier: Amsterdam, 1988; Ch. 7, p. 219.

(a)Judd, B.R. Phys. Rev. 1962, 127, 750. (b)Ofelt, G.S. J. Chem. Phys. 1962, 73,
511. (C)Ofelt, G.S. J. Chem. Phys. 1963, 38, 2171.

(a)Fong, F.K.; Naberhuis, S.L.; Miller, M.M. J. Chem. Phys. 1972, 56, 4020.
(b)Miller, M.M. Chem. Phys. Lett. 1971, 10, 408. (c)Ta11ant, D.R.; Miller, M.R.;
Wright, J.C. J. Chem. Phys. 1976, 65, 510.

168

141.

142.

143.

144.

145.

146.

147.

148.

149.

150.

151.

152.

153.

Richter-Lusting, H.; Ron, A.; Speiser, S. Chem. Phys. Lett. 1982, 85, 576.

(a)Horrocks, W.D.; Albin, M. in Prog. Inorg. Chem. S.J. Lippard Ed.; Wiley:
New York, 1984; Vol. 31, p. 1. (b)Horrocks, W.D.; Sudnick, D.R. J. Am. Chem.
Soc. 1979, 101, 335.

Yu, J .-a.; Lesarard, R.B.; Bowman, L.E.; Nocera, D.G. Chem. Phys. Lett. 1991,
I87, 263.

(a) Crosby, G.A.; Watts, R.J.; Westlake, S.J.; J. Chem. Phys. 1971, 55, 4663. (b)
Sahbari, J.J.; Bomben, K.D; Tinti, D.S. J. Am. Chem. Soc. 1983, 105, 5577. (c)
Marciniak, B.; Buono-Core, G.E. J. Photochem. Photobiol. A 1990, 52, 1.

(a)Crosby, G.A. Molecular Crystals 1966, I, 37. (b)Freeman, J .J .; Crosby, G.A. J.
Phys. Chem. 1963, 67, 2717. (c)Whan, R.E.; Crosby, G.A. J. Mol. Spectry. 1962,
8, 315. (d)Crosby, G.A.; Whan, R.E.; Alire, R.M. J. Chem. Phys. 1961, 34, 743.

Brown, A.; Wilkinson, F. J. Chem. Soc. Faraday Trans. 2 1979, 75, 880.
Almgren, M.; Thomas, J .K. J. Am. Chem. Soc. 1979, 101, 2021.

(a)Rudkevich, D.M.; Verboom, W.; van der Tol, E.; van Staveren, C.J.;
Kaspersen, F.M.; Verhoeven, J .W. Reinhoudt, D.N. J. Chem. Soc. Perkin Trans.
2, 1995, 131. (b)Steemers, F.J.; Verboom, W.; Reinhoudt, D.N.; van Der Tol,
E.B.; Verhoeven, J .W. J. Am. Chem. Soc., 1995, 117, 9408.

Buono-Core, G.E.; Li, H.; Marciniak, B.; Coord. Chem. Rev. 1990, 99, 55.
Tobita, S.; Arakawa, M.; Tanaka, I. J. Phys. Chem. 1985, 89, 5649.
Bhaumik, M.L.; El Sayed, M.A. Appl. Opt. Supp. 1965, 2, 214.

Wagner, P.J.; Schott, H.N. J. Phys. Chem. 1968, 72, 3702.

Alpha, 3.; Ballardini, R.; Balzani, V.; Lehn, J .-M.; Perathoner, S.; Sabbatini, N.
Photochem. Photobiol. 1990, 52, 299.

169

154.

155.

156.

157.

158.

159.

160.

161.

162.

163.

164.

165.

166.

167.

Onuska, FL in B.K. Afghan and A.S.Y. Chau (Eds) Analysis of Trace Organics
in the Aquatic Environment, CRC: Boca Raton, F. 1990, 205.

Neff, J.M. Polycyclic Aromatic Hydrocarbons in the Aquatic Environment,
Applied Science: Barking, 1979.

Vo-Dinh, T. in Vo-Dinh (Ed.) Chemical Analysis of Polycyclic Aromatic
Hydrocarbons, Wiley: New York, 1990, 1.

Futoma, D.J.; Smith, S.R.; Smith T.E.; Tanaka, J. Polycyclic Aromatic
Hydrocarbons in Water Systems, CRC: Boca Raton, FL, 1981.

Thomas, S.P.; Ranjan, R.; Webster, G.R.B.; Sama, L.P. Environmental Science
and Technology 1996, 30, 1521.

CE. Anson, C.S. Creaser, G.R. Stephenson, Spectrochimica Acta A 1996, 52,
1183.

Brown, R.S.; Luong, J.H.T.; Szolar, O.G.J.; Halasz, A.; Hawari, J. Anal. Chem.
1996, 68, 287.

Ariese, F.; Kok, S.J.; Verkaik, M.; Gooijer, C.; Velthorst, N.H.; Hofstraat, J .W.
Aquatic Toxicology, 1993, 275.

Carretero, A.S.; Blanco, C.C.; Gutierrez, A.F. Anal. Chim. Acta 1996, 329, 165.

Alarie, J.P.; Vo-Dinh, T.; Miller, G.; Ericson, M.N.; Eastwood, D.; Lidberg R.;
Dominguez, M. Rev. Sci. Instrum. 1993, 64, 2541.

Fletcher, P.E.; Cooper, J .B.; Vess, T.M.; Welch, W.T. Spectrochimica Acta Part
A 1996, 52, 1235.

M.T. Tena, M.D. Luque de Castro, M. Valcarcel, Anal. Chem. 1996, 68, 2386.
Barber, T.E.; Fisher, W.G.; Wachter, E.A. Environ. Sci. Technol. 1995, 29, 1576.

Niessner, R.; Panne, U.; Schroder, H. Anal. Chim. Acta 1991, 255, 231.

170

168.

169.

170.

171.

172.

173.

174.

175.

176.

177.

178.

179.

180.

181.

182.

Weiss, M. Today ’s Chemist at Work 1995, 4, 25.

Owen, H. Proc SPIE—Int. Soc. Opt. Eng. (Opt. Based Methods Process. Anal. ),
1992, 1681, 128

Pauls, R.E.; Weight, F.J.; Munowitz, P.S. J. Chromatogr. Sci, 1992, 30, 32.
Schirmer, R.E.; Fopulk, S. Adv. Instrum. Control, 1990, 45, 1529.
Maggard, S.M. Am. Chem. Soc. Div. Fuel Chem, 1990, 35, 95.

Parisi, A.F.; Nogueiras, H.P. Anal. Chim. Acta, 1990, 283, 95.

Davoli, E.; Cappellini, L.; Moggi, M.; Ferrari, S.; Fanelli, R. Internal Archives of
Occupation and Environmental Health 1996, 68, 262.

Riedel, K.; Ruppert, T.; Conze, C.; Scherer, G.; Adlkofer, F. J. of
Chromatography A 1996, 719, 383.

Pikramenou, Z.; Nocera, D.G. Inorg. Chem. 1992, 31 , 532.
Pikramenou, 2.; Johnson, K.M.; Nocera, D.G. Tetrahedron Lett. 1993, 34, 3531.
Mortellaro, M.A.; Nocera, D.G. J. Am. Chem. Soc. 1996, 118, 7414.

Hartmann, W.K.; Mortellaro, M.A.; Nocera, D.G.; Pikramenou, Z. Chemosensors
of Ion and Molecule Recognition, NATO ASI series; Kluwer Academic:
Dordrecht, 1997, in press.

(a)Bowman, L.E.; Berglund, K.A.; Nocera, D.G. Rev. Sci. Instr. 1993, 64, 338.
(b)Bowman, L.E. M.S.U. Ph.D. Thesis, 1991.

Kim, J.; Hartman, W.K.; Schock, H.J.; Golding, B.; Nocera, D.G. Chem Phys.
Lett. 1997, 267, 323.

Mortellaro, M.A.; Nocera, D.G., unpublished results.

171

183.

184.

185.

186.

187.

188.

189.

190.

191.

192.

Murov, S.L; Cannichael, 1.; Hug, G.L. Handbook of Photochemistry; Marcel
Dekker: New York, 1993.

Carnall, W.T.; Fields, P.R.; Rajnak, K. J. Chem. Phys. 1968, 49, 4447.
Horrocks, W.D. ACS Meeting, San Francisco, 1997
Rudzinski, C.K.; Nocera, D.G., unpublished results.

(a)Fujii, T..; Suzuki, A.; Komatsu, S. Chem. Phys. Lett. 1978, 57, 175.
(b)Nijegorodov, N.I.; Downey, W.S. J. Phys. Chem. 1994, 98, 5639.

Hartmann, W.K., Nocera, D.G., unpublished results.

Suzuki, Y.; Morozumi, T.; Kakizawa, Y.; Bartsch, R.A.; Hayashita, T.;
Nakamura, H.; Chem. Lett. 1996, 547.

Casnati, A.; Fischer, G; Guardigli, M.; Isemia, A.; Manet, 1.; Sabbatini, N.;
Ungaro, R. J. Chem. Soc. Perkins Trans. 2 1995, 395.

Itaya, T.; Sasaki, Y.; Tanigaki, T.; Matsumoto, A.; Inoue, K. Chem. Lett. 1996,
305.

Matsumoto, H.; Ori, A.; Inokuchi, F.; Shinkai, S. Chem. Lett., 1996, 301.

172