EXCITED-STATE PHENOMENA ASSOCIATED WITH

SOLVATION SITE HETEROGENEITY

By

Khader Ahmad Al—Hassan

A DISSERTATION

Submitted to
Michigan State University

in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1981

To My Family

ii

ABSTRACT

EXCITED—STATE PHENOMENA ASSOCIATED WITH

SOLVATION SITE HETEROGENEITY

By

Khader Ahmad Al—Hassan

Organic molecules that undergo a large change in
dipole moment upon excitation may exhibit large shifts
in absorption or emission spectra. Interesting excited-
state phenomena involving solvent—solute interactions and
solvent-cage relaxation may occur when such polar molecules
are excited in a polar matrix. These phenomena include:
temperature—dependent inhomogeneous spectral broadening,
solvent-assisted electronic energy transfer, time—dependent
spectral shift, lack of fluorescence depolarization, and
excitation-wavelength dependent red shift of fluorescence
and phosphorescence in rigid media (Red—Edge Effect).
These phenomena can be explained in terms of the statisti-
cal interaction between the polar solute molecules and
their immediate polar environment. Thus, even in a homo-

geneous condensed phase the polar solute molecules are

Khader Ahmad Al-Hassan

expected to occupy a variety of solvation sites at any given
time giving rise to different absorption energies correspond-
ing to the same electronic transitions. Such variation in
solute-solvent local interactions introduces a significant
source of broadening of both the absorption and emission
spectra.

We have examined the role of microenvironmental hetero-
geneity in electronic spectra by studying the effects of
medium polarity, rigidity, temperature, concentration and
excitation-wavelength dependence of the spectra of pyridine
merocyanine dye, 2-amino-7-nitrofluorene (ANF), A,A'-
aminonitrodiphenyl (AND) and methyl- and tertbutyl-esters
of 9-anthroic acid (9MA and 9TBA respectively). Pyridine
merocyanine dye represents a class of organic molecules
that undergo a decrease in dipole moment upon electronic
excitation while ANF and AND represent a class of organic
molecules that undergo a substantial increase in di—
pole moments upon electronic excitation. 9MA and 9TBA
represent a class of organic molecules that are flexible
and undergo geometrical changes upon electronic excitations.

The absence of excitation wavelength dependence of the
fluorescence at room temperature in fluid polar media is
explained in terms of orientational and translational relaxa-
tions of various "solute-solvent conformations" that occur
prior to fluorescence. The fluctuations in the interaction

of solutes with different solvation sitesare~dynamic in fluid

Khader Ahmad Al-Hassan

media. Once the solution is made rigid (glass, polymer
matrix, etc.) the dynamic character is lost. One may think
of a viscosity—dependent barrier between these sites, the
height of which depends on the rigidity of the medium.
Under these circumstances, the lifetimes of various "solute-
solvent conformations" are larger than the excited-state
lifetime, and hence excitation wavelength dependent fluores-
cence will occur (Red—Edge Effect).

The presence of BBB in flexible 9MA and 9TBA in nonpolar
matrix (3MP) is explained in terms of different conformers

that have slightly different absorption energies.

ACKNOWLEDGMENTS

I would like to express my deep appreciation to my
advisor, Dr. M. Ashraf El-Bayoumi, for his guidance, en-
couragement and friendship during the course of this study.

I would like to express my gratitude to the members of
my Committee, Dr. Kathy Hunt, Dr. James Harrison, Dr.
William ReuschaniDr. Andrew Timnick.

Many thanks go to Mr. Ron Hass for his expert assistance
in electronics and to Ms. Bev Adams for patiently drawing
all the figures in the dissertation until they possessed
unmatched perfection.

To my laboratory colleagues, especially to Kamal Ismail
and Nahid Shabestary, I thank them for their continuous
support and friendship.

Special thanks are extended to Yarmouk University in
Irbid-Jordan for their financial and moral support during

the course of this study.

iii

Chapter

LIST OF
LIST OF

CHAPTER
CHAPTER

II.

III.

IV.

TABLE OF CONTENTS

TABLES.
FIGURES
I - INTRODUCTION.

II - SOLVENT EFFECT ON ELECTRONIC
SPECTRA.

Solvent Effect on Absorption
Spectra . . . . . .

Effect of Hydrogen Bonding
Solvents. .

Theories of Solvent Spectral
Shift . . .

Solvent Shifts as an Aid in

Characterizing Electronic
States. . . . . . . .

l. n + n* vs n + n* Transitions.

2. Locally—Excited States vs.
Charge-Transfer States.

3. Electron—Transfer Transitions
A. Singlet-Triplet Transition.

Solvent Effect on Emission
Spectra . . . . . .

1. Spectral Shifts
2. Viscous-flow Barriers

3. Solute—Solvent Relaxation in

the Nano and Pico—second Range.

A. Excited State Level Inversion

iv

Page

vii

viii

1U

17

29
29

31
3A
36

38
38

Al

A5
52

Chapter

CHAPTER

CHAPTER

I.

II.

III.

CHAPTER

I.

II.

III - THE RED EDGE EFFECT AND A
RELATED PHENOMENON.

Shpol'skii Effect
IV - MOLECULAR SYSTEMS.
Molecular Systems that Undergo

a Large Decrease in Dipole
Moment as a Result of Excitation

1. Merocyanine Dyes.
2. Alkyl Pyridinium Iodides.

Molecular Systems that Undergo a
Large Increase in Dipole
Moment as a Result of Excitation.

l. 2-Amino-7-Nitro Fluorene.
2. A,A' Amino Nitro Diphenyl

Flexible Molecules that May Undergo
a Change in Dipole'

Geometric Configuration Upon
Excitation. . . . . . .

l. 9-Tertbutylanthroate: (tertbutyl-
ester of 9-anthroic acid) .

2. 9- -Methylanthroate (methylester
of 9- anthroic acid) . .

V - RESULTS AND DISCUSSION.
Molecular Systems that Undergo a

Large Decrease in Dipole
Moment as a Result of Excitation.

A. Pyridine-Merocyanine Dye.
B. Benzothiazole Merocyanine
Molecular Systems That Undergo a

Large Increase in Dipole
Moment as a Result of Electronic

Excitation.

Page

57
67
73

7A
7A
75

79
79

8A

8A

8A

88

88
88

116

119

Chapter

A. 2—Amino-7-nitrof1uorene (ANF)
B. A,A'-Amino Nitro Diphenyl (AND)
III. Flexible Molecules that May Undergo
a Change in Dipole
Geometric Configuration Upon
Excitation. .
CHAPTER VI - EXPERIMENTAL
A. Materials . . . . . . . . . .

I. Purification of Solvents.

II. Preparation and Purification
of Chemical Compounds

III. Preparation of Polymer Films.
B. Spectral Measurements . . . .
1. Absorption Spectra.
2. Emission Spectra.
3. Temperature Variation System.
CHAPTER VII — CONCLUSION AND FUTURE WORK.
I. Pyridine Merocyanine.
II. ANF and AND . .
III. 9MA and 9TBA. . . . . . . . . . . . . . .

REFERENCES. . . . . . . . . . . . . .

vi

Page

119

133

1A0
151
151

151

152
15A
155
155
155
156
157
158
159
160

161

Table

II

LIST OF TABLES

Page
Optical Properties of Pyridine
Merocyanine Dye in Various
Solvents. . . . . . . . . . . . . . . . 77
Fluorescence Properties of
ANF (2—Amino-7-Nitrofluorene)
in Various Solvents . . . . . . . . . . 83

vii

Figure

3a

3b

5a

LIST OF FIGURES

Solvent shifts due to hydrogen
bonding. A) Hydrogen bonding is
stronger in the ground state.

B) Hydrogen bonding is stronger

in the excited state. . . . .

Room temperature absorption spectra
Of p-nitroaniline in different sol—
vents . .

Room temperature absorption spectra
of halogen ions in aqueous solution
(D20) . . . . . .

Room temperature absorption spectra
of sodium iodide in acetonitrile,
water and ethanol . . .

Mechanical viscous-flow-barrier
cage proposed by Dellinger and
Kasha . . . . . . . . .
Steady-state fluorescence spectra
for ANS in n—propyl alcohol at the

indicated temperatures.

viii

Page

15

33

35

35

U2

A8

Figure

5b

5c

7a

7b

8a

Page

Time-dependent fluorescence spectra
for ANS in n-propyl alcohol at -90°C.
A, O nsec; B, 2.5 nsec; C, 12.5 nsec;
D, 21.5 nsec; E, 31.5 nsec. . . . . . . . U8
Time-dependent fluorescence spectra
for ANS in n-propyl alcohol at -150°C.
A, 2 nsec; B, 11 nsec; C, 68 nsec . . . . A8
Tor versus solution viscosity for
rhodamine 6G in various solvents. . . . . 51
Effect of temperature on the
fluorescence spectra of 5 x 10-5 M
DEAB (p-diethylaminobenzonitri1e)
in butylchloride-methylcyclohexane-
is0pentane mixture (12:3:1 in volume).

293°K, ------ 23A0K, ————— 173°K,
----~-- 1A8°K . . . . . . . . . . . . . . 55
Excited state level inversion caused
by mutual interaction between solute
in excited state and polar solvent. . . . . 55
Diagram showing orientation of
parallel and perpendicularly polarized
emission with respect to plane of
polarization of exciting radiation

in the laboratory coordinate system . . . 59

ix

Figure

8b

9a

9b

lOa

Excitation polarization spectra

of indole in propylene glycol at

-70°C.

.I, 0.01m; A, 0.05 M;

X, 0.1 911; 0.0.2 1’1; 0, 0-14 M.

Fluorescence and 0-0 region

phosphorescence spectra of 10-3 M

indole in 1:1 ethylene glycol-water
media at 202 and 77 K, respectively,
demonstrating the shift between the

emission excited at 280 and 295 nm.

Plot of exciting wavelength de-

pendence for the 10-3 M fluores-

cence and phosphorescence spectra

versus the temperature of the A:l

glycerol-water medium. Fluores-

cence and phosphorescence shifts

were measured as the average of

the red edge and blue edge dif-

ferences between the spectra at

280 and 295 nm excitation

Fluorescence spectra of mlO-l5 M

TP (p—terphenyl) in ethanol at 77 K

at different excitation wavelengths.

280 nm; ------ 310 nm;

Page

59

62

62

6A

Figure

10b

11a

11b

12

Page

Fluorescence spectra of «.10.5 M

QP (quaterphenyl) in ethanol at

77 K at different excitation wave-

lengths. —————— 320 nm; ------ 330 nm;

°°°°°° 335 nm . . . . . . . . . . . . . . 6A
Fluorescence spectrum of coronene

in a frozen n—heptane matrix. The

sharp lines arise because of the

unique conformation of the coronene

in the matrix . . . . . . . . . . . . . . 68
Variation in the appearance of the

A03 nm band system in the fluores-

cence spectrum of benzOEaprrene in

heptane matrices at 15 K under dye

laser excitation as a function of

excitation wavelength: (a) 385.5 nm,

(b) 385.7 nm, (c) 385.9 nm, (d) 386.1

nm. . . . . . . . . . . . . . . . . . . . 68
The limiting cases of homogeneous

and inhomogeneous broadening. In

the homogeneous case (a) the spectrum

summed over all molecules in the sample
corresponds identically to that for

any one of the individual molecules.

In the inhomogeneous case, (b), the

xi

Figure

13

1A

l5

16

17

Page

spectrum summed over all molecules in

the sample differs from the spectrum

that would be seen for any single

molecule. . . . . . . . . . . . . . . . . 71
Room temperature absorption spectra

of pyridine merocyanine dye in dif-

ferent solvents. DMSO = Dimethyl

sulfoxide and EtOH = Ethanol. . . . . . . 76
Room temperature fluorescence

spectra of pyridine merocyanine

dye in different polar solvents . . . . . 78
Room temperature absorption spectra

of ANF in different solvents: (1) 3MP,

(2) Paraffin Oi1,(3) Polystyrene film,

(A) Ethanol, (5) Polyvinyl alcohol

film. . . . . . . . . . . . . . . . . . . 81
Room temperature absorption spectra

of AND in different solvents: (l) 3MP,

(2) Paraffin Oi1,(3) Polystyrene film,

(A) Ethanol, (5) Polyvinyl alcohol

film. . . . . . . . . . . . . . . . . . . 82
Room temperature absorption spectrum

of (7 x 10'5 M) 9TBA and its

fluorescence spectra at room tem-

perature ( oooooo ) and at 77 K in

3MP . . . . . . . . . . . . . . . . . . . 86

xii

Figure

l8

19

20a

20b

21

22

Page

Room temperature absorption spectrum

of (7 x 10—5 M) 9MA and its

fluorescence Spectra at room

temperature ( ------ ) and at 77 K

( ------ ) in 3MP . . . . . . . . . . . . . 87
Room temperature absorption spectra

of aqueous solutions of pyridine

merocyanine dye (2 x 10—5 M) at

different pH. . . . . . . . . . . . . . . 89
Resonance structure of pyridine

merocyanine dye in its neutral and

acidic form . . . . . . . . . . . . . . . 9O
Photochemical cycle of pyridine

merocyanine dye and its cis—trans
isomerization . . . . . . . . . . . . . . 9O
Fluorescence spectra of a dilute

solution (<lO—5 M) of pyridine

merocyanine dye in ethanol at 77 K

as a function of excitation wave-

lengths . . . . . . . . . . . . . . . . . 96
Fluorescence spectra of a dilute

solution of pyridine merocyanine

dye in 1-propanol at 77 K as a

function of excitation wavelengths. . . . 97

xiii

Figure

23

2A

25

26

27

28

Fluorescence spectra of a dilute
solution of pyridine merocyanine

dye in l-hexanol at 77 K as a func-
tion of excitation wavelengths.
Fluorescence spectra of pyridine
merocyanine dye in PVA (polyvinyl
alcohol) thin film at 77 K as a
function of excitation wave-

lengths . . . . . . . . . .
Temperature dependence of fluores-
cence edge excitation red shift of
pyridine merocyanine dye in ethanol
Fluorescence spectra of a dilute
solution of pyridine merocyanine

dye in ethanol at —135°C as a func-
tion of excitation wavelengths.
Temperature variation of REE (Red
Edge Effect) for pyridine merocyanine
dye in two different solvents
Fluorescence spectra of pyridine
merocyanine dye of different concen-
trations in ethanol at 77 K at fixed

excitation wavelength (the absorption

maximum at 77 K WA7O nm).

xiv

Page

98

99

100

102

103

10A

Figure

29

30

31

32

33

Fluorescence spectra of pyridine
merocyanine dye in PVA (polyvinyl
alcohol) thin film at room tem-
perature as a function of excita-
tion wavelengths. . . . . . . .
Fluorescence spectra of (<10'5 M)
pyridine merocyanine dye in ethanol
at different temperatures. (The
excitation wavelength was the ab-
sorption maximum at each tempera-
ture.).

Fluorescence spectra of a dilute
solution of (<10.5 M) pyridine
merocyanine dye in glycerol at dif-
ferent temperatures. (The excita-
tion wavelength was the absorption
maximum at each temperature.)
Absorption spectra of pyridine
merocyanine dye in ethanol

(“’10-5 M) as a function of
temperatures. The temperature

in °C is indicated on each
spectrum. . . . . . . . . .
Absorption spectra of pyridine

merocyanine dye in PVA (polyvinyl

XV

Page

106

107

108

109

Figure

3A

35

36

37

38

Page

alcohol) thin film as a function

of temperature. . . . . . . . . . . . . . 110
Ground and excited state potential

energy curves for a molecular system

in which the dipole moment decreases

as a result of excitation. Solvation

sites a, b, c and d differ in the

orientation of the solvent molecules,

6, but are assumed to have the same
intermolecular separation R for

simplicity. Solvation site a repre-

sents the most stable orientation . . . . 112
Fluorescence spectra of a dilute

solution of ANF in ethanol at 77 K as

a function of excitation wavelengths. . . 123
Fluorescence spectra of ANF in PVA

thin film at 77 K as a function of

excitation wavelengths. . . . . . . . . . 12A
Fluorescence spectra of ANF in

polystyrene thin film at 77 K as

a function of excitation wave-

lengths . . . . . . . . . . . . . . . . . 125
Fluorescence spectra of ANF in

PVA thin film at room tempera-

ture as a function of excitation

wavelengths . . . . . . . . . . . . . . 126

xvi

Figure

39

A0

A1

A2

Page

Fluorescence spectra of ANF in

polystyrene thin film at room

temperature as a function of ex—

citation wavelengths. . . . . . . . . . 127
Absorption spectra of ANF in

ethanol (3 x 10‘5

M) as a func-

tion of temperature: (1) 2A°C,

(2) 0°C, (3) -36°C, (A) —78°C,

(5) —10A°c, (6) —115°c. . . . . . . . . 128
Ground and excited state po-

tential energy curves for a

molecular system in which the

dipole moment increases as a

result of excitation. Solvation

sites, a, b, c and d differ in

the orientation of the solvent

molecules, 0, but are assumed to

have the same intermolecular

separation R for simplicity.

Solvation site a represents the

most stable orientation . . . . . . . . 130
Fluorescence spectra of a dilute

solution of AND in ethanol at 77 K

as a function of excitation wave-

lengths . . . . . . . . . . . . . . . . . 135

xvii

Figure Page

A3 Fluorescence spectra of a dilute

solution of AND in l-propanol at

77 K as a function of excitation

wavelengths . . . . . . . . . . . . . . . 136
AA Fluorescence spectra of AND in

PVA thin film at 77 K as a func-

tion of excitation wavelengths. . . . . . 137
A5 Fluorescence spectra of AND in

polystyrene thin film at 77 K as

a function of excitation wave-

lengths . . . . . . . . . . . . . . . . . 138
A6 Fluorescence spectra of a dilute

solution of AND in ethanol at

-l26°C as a function of excitation

wavelengths . . . . . . . . . . . . . . . 139
A7 Fluorescence spectra of AND in

PVA thin film at room temperature

as a function of excitation wave—

lengths . . . . . . . . . . . . . . . . . 1A1
A8 Fluorescence spectra of AND in

polystyrene thin film at room

temperature as a function of ex-

citation wavelengths. . . . . . . . . . . 1A2
A9 Absorption Spectra of AND in

ethanol (W10_u M) as a function

xviii

Figure

50

51

52

of temperature.

(2) -23°C,

Fluorescence spectra of (7 x 10_5 M)

(1) 2uoc,

(3) -86°C, (A) -l20°C.

9TBA in 3MP at 77 K as a function

of excitation wavelengths.

(Spectrum #1 corresponds to

365 nm excitation and #A cor-

respond to 397 nm excitation.).

Fluorescence spectra of (7 x 10-5 M)

9MA in 3MP at 77 K as a function

of excitation wavelengths

Variation of fluorescence in-

tensity and resolution of vibronic

band of (7 x 10'5 M) 9TBA in 3MP

at 77 K at different excitation

wavelengths which are:

(2) 355 nm,
(5) 370 mm:
(8) 386 nm.

(3) 360 nm,
(6) 375 nm,

XIX“

(l) 350 nm,
(A) 365 nm,
(7) 383 nm,

Page

1A3

1A6

1A7

150

CHAPTER I

INTRODUCTION

This study deals with solvent effects on absorption
and luminescence properties of molecules that undergo
large change in dipole moment upon electronic excita-
tion. Changes in quantum yield (¢F) and energy of fluores—
cence (AF) in pure solvents of different polarities and
mixed solvents were sought. Our objective is to examine
the effects of solvent-cage relaxation on emission spectra
of certain molecular systems. From the beginning we
suspected that different solvation cages, hence different
excitation energies may occur and hence we looked for
excitation wavelength dependence of luminescence energy
and spectral resolution due to edge excitation (excitation
at the red edge of the first absorption band) at low
temperatures.

Three types of molecular systems were examined; the
first molecular system undergoes a substantial decrease ig
dipole moment upon electronic excitation. As an example
of this molecular system we have studied the optical and
luminescence properties of pyridine merocyaninecrnawhich

undergoes 5.5D decrease in its dipole moment upon

3 _ \ Oo’

Pyridine merocyanine dye

excitation. The second molecular system undergoes an ig-
crease lg dipole moment upon excitation: as an example of
this system we have studied the optical and luminescence
properties of 2-amino-7-nitrofluorene and A,A' amino nitro—

diphenyl (abbreviated as ANF and AND, respectively).

AN F

AND

 

These molecules undergo an increase in their dipole moments
by 18D and 12D respectively upon electronic excitation.

The third molecular system represents flexible molecules
that undergo geometric relaxation in the excited state

upon electronic excitation. As an example of this system
we have studied 9-methylanthroate (9MA) and 9-tertbutyl

anthroate (9TBA).

 

9-Methyl Anthroate (9MA) 9-Tertbutyl Anthroate (9TBA)

For merocyanimedye we have found a large blue shift
in absorption, a small blue shift in emission in more
polar solvents. Also the emission undergoes a blue shift
upon lowering the temperature of the solution, and the
absorption spectra exhibit an "apparent" blue shift upon
lowering the temperature of the solution. While no excita-
tion wavelength dependence of the fluorescence (red-edge
effect) was observed in fluid media, a large red-edge
effect was measured in rigid glass at low temperature and
in polymer matrices at room temperature. The red-edge
effect observed in rigid glass at low temperature gradually
decreases when the concentration of the solute increases.
These observations were interpreted in terms of a potential
energy diagram built upon the assumption of a statistical
distribution of solute polar molecules in different solva-
tion sites. Thus even in a homogeneous condensed phase,
solute molecules are expected to occupy a variety of sol-

vation sites at any given time giving rise to different

absorption energies corresponding to the same electronic
transitions. Such variation in solute-solvent local inter-
actions which is orientation dependent is responsible for
the observed REE and the inhomogeneous broadening of the
absorption spectra. The absence of REE at higher concen-
trations is explained in terms of energy transfer among
solute molecules in different solvation sites. ANF and
AND offered the converse example of the pyridine mero-
cyanine dye. ANF and AND absorption spectra undergo a

red shift in more polar solvents. Their low temperature
absorption undergoes a red shift instead of the blue shift
observed in the case of merocyanine dyes at low tempera—
tures. The REE observed for ANF and AND in different
solvents at 77 K and in polymer matrices at room tempera—
ture were explained also in terms of a potential energy
diagram that considers the statistical distribution of

the solute among different solvation sites. The main
difference here with respect to merocyanine is that the
most stable solvation site lies at longer wavelength

(red edge side) of ANF and AND'absorption spectrum and not
at shorter wavelength of the spectrum as in the case of
pyridine merocyanine dye. This is of course due to the
fact that ANF and AND undergo an increase in dipole moment
upon excitation and in the case of pyridine merocyanine
dye a decrease in dipole moment occurs.

For 9MA and 9TBA, the excitation wavelength

dependence of the fluorescence in 3MP at 77 K vwns studied.
The variation of emission intensities of different vibronic
bands of 9TBA at different excitation wavelength at 77 K
were studied. The results are interpreted in terms of
Shpol'skii effect.

The second chapter gives a detailed review of solvent
effects on electronic absorption and emission spectra.
The third chapter is a review of the red edge effect
phenomenon (REE): different observations and interpreta-
tions are given by different authors. Two kinds of REE
are distinguished. A related effect,name1y the Shpol'skii
effect, is also briefly discussed. Chapter IV describes
the molecular systems we have studied with their corres-
ponding absorption and emission spectra. The detailed
study of pyridine merocyanine dye, ANF, AND, 9MA and 9TBA
is presented in Chapter V together with our interpreta-
tion. The experimental part that describes the preparation,
purifications and spectral measurements 1&5 summarized
in Chapter VI. In Chapter VII I have discussed the
significance of our study together with some suggestions

for future work.

CHAPTER II

SOLVENT EFFECT ON ELECTRONIC SPECTRA

I. Solvent Effect on Absorption Spectra:

 

In the vapor phase at reduced pressure the observed
absorption is due to electronic transitions in isolated
molecules and high-resolution spectra showing vibrational
and rotational structure are observed. Such resolution
will decrease under conditions of high pressure due to
intermolecular interactions in a molecular collision and
this is called pressure broadening. This is similar to the
spectrum of a dilute solution where the solute and solvent
molecules are in close contact, all rotational structure
is eliminated even in inert solvents, e.g., hydrocarbon
solvents that are transparent in the region of solute ab-
sorption. Intermolecular collisions perturb the vibrational
levels causing a blurring of the vibrational structure of
the spectrum. Besides such a blurring effect, one may also
observe an intensity change as well as a shift in the
wavelength of the absorption bands depending on the kind
of transition that the solute undergoesznuiits environ-
ment. Thus in the case of polar solutes in polar solvents

one usually observes large changes in the position, intensity

and shape of absorption bands.

The interpretation of solvent effect is made difficult
because they are sometimes small and thus not easy to
measure precisely, and also because they are often the
results of several effects which sometimes reinforce one
another and sometimes cancel out. There is also some
difficulty in the fact that the most easily measured and
most often recorded solvent effect is the displacement or
shift of the absorption maximum whereas theoretical con—
siderations of electronic energy states refer to the posi-
tion of (0,0) band, which is not necessarily affected in
the same way as the maximum. Since it is practically im—
possible to locate the (0,0) band in diffuse or structure-
less solution spectra, the spectral shifts are referred
to absorption maxima.

Solvent effects are qualitatively interpreted1 in
terms of dipole polarization and hydrogen-bonding forces
between solute and solvent molecules. The following im—
portant factors must be considered: a) the momentary transi-
tion dipole during the optical absorption; b) Franck—Con-
don effects; c) the difference in permanent dipole moment
between the ground and excited state involved in the
electronic transition of the solute; d) properties of the
solvent such as its dipole moment, its dielectric constant,
its donicity and its ability to interact specifically with

solute molecules, i.e., via a hydrogen bond; and d) a

factor to account for the statistical nature of inter-
molecular forces.2
Different intermolecular interactions such as hydrogen
bonding, dipole—dipole, dipole-induced dipole and dispersion
forces which occur in solutions may cause different shifts*
in the electronic spectra depending on the nature of
solvent-solute interaction and the electronic transition
involved.
When optical absorption occurs, a transition dipole of
the solute induces a momentary polarization in the sol-
vent causing a stabilization of the excited electronic
state relative to its energy in the vapor phase giving
rise to a red shift (few hundred wave numbers, cm-l) in
the absorption spectrum. This polarization red shift is
a result of dispersion forces and is operative in all
solutions, whether the solute and solvent molecules are
polar or not. It was found3 that the magnitude of the

polarization red shift increases with the refractive index

of the solvent.

 

*
(Red shift): A shift in absorption and emission to
lower energy (i.e., to longer wavelength).

*(Blue shift): A shift in absorption and emission to
higher energy (i.e., to shorter wavelength).

In the case of polar solute molecules that undergo a
large change in dipole moment (either an increase or a de-
crease) upon excitation, there will be different interaction
energies between the polar solute and the solvent molecules
particularly if the latteriisalso polar. In the case of a
non-polar solvent, dipole-induced dipole forces dominate
while in the case of polar solvent dipole-dipole interactions
play the major role. Solvent molecules will interact with
the solute molecules in the ground and excited state to
different extents depending on the change in dipole moment
of the solute upon excitation.

The Franck-Condon principle plays a major role in
solvent effects on absorption and emission spectra. A
solute molecule in solution is in equilibrium with the sur-
rounding solvent molecules. The energy of the equilibrium
ground state depend on two factors: (a) a packing factor
which depends on the geometry of the solute and solvent
molecules, and (b) a factor which depends on the degree
of mutual orientation interaction if the solute and solvent
are polar or if there is hydrogen bonding between them.

The geometry, charge distribution, and dipole moment of
the solute may be different in the excited state from the
ground state; therefore, the equilibrium configuration of
the solvent cage would also be different in the excited
state from that in the ground state. The Franck-Condon

principle states that an optical transition occurs in a

10

time that is short compared with the period of nuclear
motions, therefore at the instant cfi‘ the transition, the
solvent configuration around the excited-state solute mole-
cule is not the equilibrium configuration, but a conforma-
tion geometrically identical to the solvated ground state,
i.e., a Franck-Condon state configuration. The energy

of this configuration is higher than that of the equilibrium
configuration in the excited state, which is reached upon
relaxation of the system. A time of at least several mol-

—13

ecular vibrations ( 10 sec) is required for a geometri-

cal rearrangement of the solute molecule and a time of the

-11

order of 10 sec is required for the solvent reorienta-

tion. Since the lifetime of an excited singlet state is
of the order of 10'9 - 10'8 sec, there is ample time for
excited-state equilibrium to be established before de-
activation occurs. In the same manner, the ground-state
configuration of the system after fluorescence is not an
equilibrium configuration, but a Franck-Condon state whose
energy is higher than that of the ground-state equilibrium
configuration; the system then relaxes to its equilibrium
ground—state. Franck-Condon factors may contribute sig—
nificantly to the observed shifts in absorption and emission
spectra. One may classify the solution spectra into four

different cases and consider solvent shifts observed in

each case.

11

Case I: Nonipolar Solute in Non-polar Solvent

 

The transition dipole induces a dipole in the neighbor—
ing solvent molecules and the solution spectrum is shifted
to the red owing to the polarization effect and the shift
depends on the solvent refractive index. The solution

spectrum will tend to retain its vibrational structure.

Case II: Non—polar Solute in Polar Solvent:

 

This is similar to Case I, except there might be a
slight packing strain since dipolar and particularly hydro—
gen bonding forces between the solvent molecules themselves
will tend to increase the solvent-cage relaxation time after

the transition”. A red shift that depends on the solvent

 

refractive index will be observed together with an increase

in the blurring of vibrational spectra relative to Case I.

Case III: Polar Solute in Nonfipolar Solvent:

 

Depending on whether the excited state dipole moment
is smaller or larger than that of the ground state one has

two cases:

a. Solute Dipole Moment Decreases Upon Excitation — Since
the solvent is non-polar, there will be no orientation

strain. Because of dipole-polarization (polarization of

the solvent molecules by solute dipoles), the excited state

12

solvation energy will be less than that of the ground
state. This results in a blue shift of the absorption
spectrum, the amount of which will be dependent on the re-
fractive index of the solvent. The polarization red shift
still occurs and the observed shift will depend on the

relative magnitude of the two effects.

b. Solute Dipole Moment Increases Upon Excitation -
The increased solute dipole moment in the excited state
makes its solvation energy greater than that of the ground
state. This results in a red shift in the absorption
spectra depending on the solvent refractive index.
The red shift adds to the polarization red shift

and gives rise to the observed shift in the spectrum.

 

Case IV: Polar—Solute in Polar Solvents:

In this case, dispersion, dipole-polarization and di-
pole-dipole forces are operative. In the case of hydrogen-
bonding solvents the hydrogen-bonding forces must also be

considered.

a. Solute Dipole Moment Increases_gpon Excitation -
In this case the dipole-dipole forces between the solute
and solvent are greater in the excited state than in the
ground state and a red shift is expected. Orientation
strain in this case is small because the solvent molecules

are already partly oriented.

13

b. Dipole Moment Decreases Upon Excitation - If the
dipole moment decreases as a result of excitation the solva—
tion energy is larger in the ground state than in the ex-
cited state. The excited solute molecule finds itself in
a cage of dipoles whose orientation is that appropriate
to the equilibrium ground state. In this case the blue
shift resulting from dipole-dipole interaction is due to
the greater solvation energy of the ground state and to
the orientation strain which contributes a term equal to the
energy required to orient solvent dipoles around the excited
solute molecule.

The magnitude of the blue shift will depend on several
factors, including the magnitude of the change in dipole
moment during the transition, the value of the solvent di-
pole moment, and the sizes of solute and solvent molecules.
If the solvent molecules are small, for example, more of
them can get close to the solute dipole, and greater inter-
action will result. The superimposed polarization red
shift will usually be dominated by this dipole blue Shift.
If the dipole moment changes its direction during excita-
tion, one should expect a large orientation strain and
hence a large blue Shift due to dipole-dipole forces.

The last factor (e) dealing with the statistical nature
of intermolecular forces and the fluctuation of all physi-
cal characteristics of the liquid resulting from it, ac-

counts for the Spectral inhomogeneity inherent in

1A

experimentally observed absorption and emission spectra
and represents the superposition of "elementary" spectra
corresponding to molecules with different "molecular en-

vironment" potentials.

II. Effect of Hydrogen Bonding Solvents

 

Bayliss and McRael considered hydrogen bonding inter—
actions to be a special case of dipole-dipole interactions.
However, Pimentel5 pointed out that dipole-induced dipole
and dipole-dipole interactions produce small solvent shifts
compared with those due to hydrogen bond. He discussed
the influence of hydrogen bonding formation on electronic
transitions in terms of the Franck-Condon principle. Sol-
vent shifts due to hydrogen bonding can be expressed as fol-

lows (See Figure l):

Ava = va-vo = Wg-We + we . . . . . . l

-We—w.......2

g

where:

Ava is the energy Shift in absorption maximum,

Avf is the energy shift in emission (fluorescence)
maximum,

Va is the energy of absorption maximum,

vf is the energy of emission (fluorescence) maximum,

v0 is the energy of absorption maximum in the gas phase,

l5

 

 

ENERGY

 

 

 

 

 

 

 

 

 

 

 

R(A..B)

Solvent shifts due to hydrogen bonding. A) Hydro-

gen bonding is stronger in the ground state.

B) Hydrogen bonding is stronger in the excited
state.

16

WE is the energy in the ground state which results

from H—bonding interaction of the solvent with the

solute,

W is the energy in the excited state which results
from hydrogen bonding interaction of the solvent with
solute upon excitation.

(we and WE) are the energies implied by the Franck-

Condon principle and always positive.

Since we and WE are always positive, the shift in ab-
sorption and emission will depend on whether the value of

Wg is greater or less than We, when

Wg > We: i.e., the hydrogen bODGijsstronger in the ground
state than in the excited state as shown in Figure 1A.

A blue shift in an absorption spectrum which exceeds we
by a value Of Wg-We will result. In emission, the shift

is less than W -We by w so that a blue or red shift may

g g’

be observed depending on Wg-we and w However, the Shift

g'
is small compared to wg.

If We > W i.e., the hydrogen bond is stronger in the

g)
excited state than in the ground state as shown in Figure
1B, a red shift in the absorption spectrum which is less

than Wg-We by we is expected. In emission, the red shift

exceeds Wg—We by wg.

Well characterized hydrogen bonds have energies in the

range of 1-7 Kcal/mole (350-2500 cm’l). According to

17

the above discussion, a blue shift in absorption may

exceed the ground state hydrogen bonding energy, hence, the
expected blue shift occurs in the range of 350-2500 cm-1
or larger than 2500 cm-1. But a red shift in absorption

, should not be larger than

should never exceed Wg, i.e.

2500 cm-1. For W* + n transition in hydrogen bonding media,
a blue shift in absorption is usually observed.6’7 This
is due to the decrease in charge density (M1 the lone pair
atom as a result of lone pair promotion. Thus, the hydrogen
bond is always stronger in the ground state.

A red shift in absorption Spectrum indicates that hy-
drogen bonding is stronger in the excited state. This
may indicate an increase in the acidity or basicity depend-
ing on the functional group at the chromophore involved in

hydrogen bonding.

III. Theories of Solvent Spectral Shift:

 

The general theory which relates Spectral shifts to
various interactions between the solute and the solvent
molecule is incomplete. These Spectral shifts are at-
tributed to (a) physical interactions between the solute
and solvent molecules and (b) to some important specific
effects like: hydrogen bond formation; proton or
charge transfer between solvent and solute; and solvent-
dependent aggregation; ionization or dissociation and iso-

merization equilibria.

18

Ooshika8 presented a theory of solvent Shifts for
polar and non-polar solvents. McRae9 published a similar
10

theory and applied it to dye molecules He used the

11 in his theory of the dielec-

Same model used by Onsager
tric constant of polar liquids. By Onsager's theory one cal»
culates the polarization due to one molecule by representing
it as a polarizable point dipole in a cavity electric field.
All other molecules give rise to a homogeneous dielectric
with a dielectric constant equal to the bulk liquid. Mc-

Rae divided the polarization of the dielectric into two

parts: one due to orientation and the other due to elec-

tronic polarization of the solvent molecules. The inter-

action between the polar solute and dielectric was cal-
culated to second order in perturbation theory. Liptayl2
presented a theory of solvent sensitivity and discussed

it qualitatively in terms of hydrogen bonding properties
of the solvent. Kirkwood-Frohlich theory13'15 represents
a modification of Onsager's theory that takes into account
short-range order in the solvent.

16’ Ahen

Several other theories by Marcus and Weigang

and Wild18

have also been presented.

The starting point of most theories of the general sol-
vent spectral shift is the presumably known set of wave
functions for the unperturbed electronic states of iso-

lated solute and solvent molecules. Second-order per-

turbation theory was used to develop expressions for the

19

change of the stationary electronic energy levels and of
energy differences or transition energies of a single
solute molecule which arise from electric fields due to a
particular (but unspecified) configuration of an individual
solvent molecule. The resulting Shifts were then averaged
over all appropriate configurations of the solvent mole—
cules of the solution.

According to the scheme derived by Nicollg, the approp-
riate solute—solvent pair distribution function was ex-
pressed in terms of the wave functions 1K,i> = |K>Ii> for
the K—th electronic state of an isolated solvent molecule
and the unperturbed i-th electronic state of an isolated
solute—chromophore. (Solvent index will always precede
the solute index and the 0 designates the ground state of
either molecule.)

The Hamiltonian H = H + H' for this solvent-solute

0

pair was defined in terms of HO the sum of the separate,

a
unperturbed Hamiltonians for the isolated solvent and solute
molecules, and H', the interaction operator. |K>, |i>

and Ho were presumed to be known from vapor phase spectra

H' was defined in terms of an electric field Em that the
solvent medium produces at the site of the solute molecule.
EKi was taken to be the sum of the unperturbed energies of
the solvent molecule in state K and the solute in state i

while EKi representstflmacorresponding perturbed energy

including terms to second order in the perturbation.

20

If one deals with one particular transition of the
solute from state 0 to state 1 then E(00-01) is the un—

perturbed solute transition energy and equal to EOl — EOO

while 3(00-01) is the corresponding energy for the per-
turbed (solvated) solute and equal to €00 - 801, The

Shift obtained for this particular configuration was:

Au(cm-l) (hc)-1[AE(00-01)1

(hc)-l[€(OO-01) - E(OO-Ol)] (3)

This has been averaged over the configurations of the
solvent molecules to obtain expressions for the general
solvent spectral Shifts.

The perturbed energies of the solute chromophore-
solvent pair were written in terms of quantities just

defined:

l<ooIH'|Ko>l2

 

500 = E00 + <00IH'I00> + z

 

 

KfiO EOO - EKO
2 . 2
+ z 1<ooig}101>1 + z I<ooJH'1K1>I (u)
l#0 E00 - E01 13K#O E00 - EKI

21

 

2
| 01 H' K1 |
801 = E01 + <01|H'|01> + Z < I l >

K50 E01 ’ EKl

 

 

I . 2 2
+ £1 [<211H j01>1, + 1&1 |<01|H'|Ki>l (5)
i - E D E - E o
01 01 K#0 01 K1

The first term of the right hand side of equations (A)
and (5) is the zero-order unperturbed energy for the iso-
lated chromophore. The second term in both equations is
the first-order term and represent the interaction of the
permanent dipole moments of solvent and solute molecules.
This contributes to the shift only if both molecules are
polar. The three remaining terms in both equations arise
in second order and represent respectively:

(a) the third term is the interaction of the permanent
dipole moment of the solute with the dipole moment induced
in the solvent by the solute dipole. The contribution of
this term is zero if the solute is non-polar.

(b) the fourth term is the interaction of the permanent
dipole moment of the solvent with the dipole moment induced
in the solute by the solvent dipole. This term vanishes
for a non-polar solvent.

(c) the fifth term is the interaction of the mutually
induced dipoles of the solute and solvent. This term is

non-zero in all cases.

The average shift of the transition energy due to

22

solute-solvent interaction, was obtained by subtracting
Equation (A) from Equation (5) and averaging the resulting
expression over the appropriate two particle distribution
functions for the separation and mutual orientation of a
solvent molecule relative to the solute. This was repre-

sented19 as follows:

<AE(00—01)>AV. <e(OO-Ol)>AV. _ <E(00‘Ol)>Av.

[(<01|H'|01>)AV. _ (<00|H'|00>)AV 1

 

 

 

 

 

 

(|<01|H'|K1>|2) (|<00|H'|K0>|2)
+ [ z { AV' - AV} 1
K50 E01 ’ EKl Eoo ‘ EKo
(I<01|H'I0i>l2)AV (|<00|H'10i>|2)AV
+ [ z - E l
lfii E01 - E0i 1%0 E00 — E0i
. 2 . 2
(|<01|H'|Ki>l ) (|<00|H'|Ki>l )
+ I ; Av _ 2 Av (6)
K o E — E . K¢o E — E .
1%1 01 K1 i#0 00 K1

The interaction operator was approximated as follows:

' .- -> O ->
H " TEm Usolute (7)

+

where Em is an electric field that the solvent medium pro—

duces at the site of the solute molecule and Psolute is

23

the dipole moment operator on the solute coordinates. Wave—

functions for the system that are Simple products of iso-
lated molecule wavefunctions were introduced so that each

matrix element of the perturbation in Equation (6) is ex-

pressed as a product of three terms:

a + + o
<01IH'IKj> = <OiI—Em - “soiutelKJ>

= _ <O|Em|K><l|u j>COSO (8)

solutel

where

<0|Em|K> represents the electric field at the Site of

the solute produced by the u K th permanent or

0

transition dipole moment of the solvent molecules.

<i|usolutelj> represents the appropriate permanent or

transition dipolenmmmnm;of the solute; and

cose represents the effect of mutual orientation of

the solvent and solute.

Each of the four terms in Equation (6) was decomposed and

related to macroscopic properties: Only the final forms

will be given without their mathematical details.

A. Interaction between permanent dipoles of the solute

and the solvent is represented by the first term of Equation

(6) and is written as

2A

2

 

[<01IH'|0|>]AV - [<oolH'|00>]AV = D[§}% _ “2‘:] (9)
n+
where
D = —t<2a-3>(ugolute><pgolute cosB — ugolute>1 (10)

e, n, represent the dielectric constant and the refractive

l 0

index of the solution, respectively. “solute’ “solute

are
the magnitudescfi‘the permanent dipole moment of the excited
and ground states, respectively, and B is the angle between
them. d is the radius of a solute molecule obtained by ap-

proximating it as a spherical particle.

B. Interaction between the permanent solute dipole and
induced solvent dipole is represented by the second term

of Equation (6) and thus can be written as

 

 

 

2 2
(I<01IH'IK1>I )AV (|<00|H'|K0>| )Av _ n2-1
E - E ' E E 7 CE 2 J

K#O 01 K1 00 - K0 2n +1

(11)
where
_ -3 l 2 _ o 2

C " '[d (“solute) (“solute) J (12)

25

C. Interaction between induced solute dipole and per—
manent solvent dipole is represented by the third term of

Equation (6) and has the following final form:

 

 

 

 

. 2 . 2
(|<01|H'|Ol>| )AV _ (|<00|H'|01>| )AV _
1%1 EOl - EOi 1%0 EOO - EOi
2 2
l - 2 +
e (n+2)
where
2
1 = _ 108 An (R/d) l 0
B E R3 kT (asolute - OLsolute)] (1A)
h 0 d 1 ' b‘lit' f
ere “solute an asolute represent the polariza 1 1es o

the initial (ground) and final (excited) states of the
solute respectively. R represents the radius of the sol-
vent shell taken from the center of the solute. k, T are

the Boltzmann constant and temperature in Kelvin scale.

D. Interaction between mutually induced dipoles of

solute and solvent if; represented by the fourth term of

Equation (6) has the following final form:

(|<01|H'|Ki>|2)AV] _ (IOOIH'IKi>'2)Av]

 

 

z [ z =
K#O K O
131 E01 ' EKi 1:0 Eoo ’ EKi
2
n -l
A[-—§——1 (15)

2n +1

26

where

11

solute Oi )2)1 (16)

2
) )-( Z A0i(usolute

1
A = -[—-1[( Z A .(u
d3 11 ifio

i#l
where Ali and AOieuwaweighting factors9 depending upon the
transition frequencies of both the solute and solvent.

The final form of Equation (6) would be then:

n2—1
2 1
2n +1

 

<AE(OO-Ol)>AV = (A + C) [

l [(e-n2)(2e+n2)]

 

+B

 

e(n+2)2
2
6‘1 n -1

The parameters A, B1, C and D may be viewed either as

phenomenological constants to be evaluated from experimental
results or as parameters to be calculated from Equations
(l6, l3, l2 and 10) respectively. Equation (17) is still
approximate and similar to Equation (18) found by other

authors.9’20’21

 

 

n2—l e—l n2-1
Av = dispersion term + B( ) + C(——— - ) + Stark

2n +1 6+2 n2+2 effect term

(18)

27

Either equation is still approximate and describes
only Shifts resulting from interactions between the
solute and solvent molecules. They do not take care of
any specific interactions like, hydrogen-bonding, proton-
transfer, etc.

The ability of different terms in Equation (18) to
explain solvent shifts in different types of solvent system
has been examined for both n-n* and n-n* transitions of

2220321 n-n* transition

organic molecules, including dyes.
energies of C=O and C=S groups in different solvents are
found to vary linearly with the stretching frequencies in
the same solvents, indicating the importance of ground-
state stabilization by solvents.2o’21
The London dispersion term in Equation (18) (causing
red shifts with respect to the gas phase) also involves the
function (n2-l)/(2n2+l). A plot of spectral shifts of non-
polar solutes like aromatic hydrocarbons against (n2-l)/
(2n2+1) is found to be linear. The linearity is strictly
expected for non-polar solvents (aliphatic hydrocarbons and
so on). It is not surprising, therefore, that different
linear plots are found for different families of solvents,
particularly when some of them, like ketones, are quite

18’19 It is noteworthy that solvent Shifts of

polar.
polar solutes in non-polar solvents are also accounted for
by the (n2-l)/(2n2+l) term, although the observed solvent

shift would be due to the combined effect of the first

28

two terms in Equation (18). One could, in principle,
rationalize Spectral shift data in different types of
solvent by incorporating a dielectric constant or Stark
effect term. This has been done by Nicol19 who derived
Equation (17) and found a linear plot of Shifts in the
absorption maxima of aromatic hydrocarbons in a variety
of solvents. This equation also Shows that the dispersion
effect on the refractive index terms of the solvent alone
cannot account for spectral shifts in polar solvents.
Equation (18) can be simplified to study the effect of
the dielectric constant term (third term) alone by measur—
ing band shifts in two polar solvents of nearly the same
refractive index but different dielectric constant.20’21
From such a study, one can obtain estimates of the excited-
state dipole moments of solute molecules. Diethyl ether
(n = 1.356 and s = A.3) and acetonitrile (n = 1.3AA and
e = 37.5) seem to make a good pair of such solvents.
Basu22 has given a detailed quantum mechanical treatment
of frequency Shifts in solutions by considering Onsager's
reaction field model. Basu evaluated the stabilization
energy of electronic states due to solute-solvent inter-

nd order perturbation theory.

action in terms of 2
Finally, I would like to mention that Kosower23 has
given a solvent polarity scale based on the effects of

solvents on the intramolecular ChargeeTransfer band of

pyridinium iodide. This scale is defined in terms of Z

29

values of solvents given by the energies in Kcal/mole of the
absorption maxima of A—methoxycarbonyl-l-ethylpyridinium
iodide. The solvents listed in this scale vary from non-
polar to highly polar and hydrogen bonding solvents. The
change in charge-transfer absorption spectra are so large
and their measurement so readily made the Z values have

been preferred by physical organic chemists over the Y
valueszu obtained from solvolysis kinetics of t-butyl-
chloride. Charge transfer to solvent Spectra of I' and

other systems have been correlated with Z values.25’26
While such empirical parameters may be useful for correla-
tions, they do not provide the exact mechanism of solvent

effects, considering the wide variation in the nature of

solvents included in obtaining the scale.

IV. Solvent Shifts as an Aid in Characterizing Electronic

 

States

Measurement of the effect of different solvents on
absorption bands is one convenient way of characterizing
the electronic states involved. As an example, solvent
effect on different types of electronic absorption transi-

tions can be classified as follow:

1. n + n* vs n-+n* Transitions

 

In an n + n* transition in a carbonyl group the oxygen

atom has less electron density in the excited state, and

30

the carbonyl group, as well as the molecule as a whole,
becomes less polar. In a hydroxylic solvent such as ethanol,
hydrogen bonding between the carbonyl group and the solvent
is stronger in the ground state than in the excited state.7
The net result is that n-n* absorption bands are blue-
shifted in hydroxylic solvents relative to non-polar, hydro-
carbon solvents. A typical shift may be on the order of
Moo—800 cm’l.
A more extreme case occurs when a molecule with a non-'
bonding orbital is dissolved in acid solution. Under these
conditions, the atom with the n electrons is protonated,
and the energy difference between the ground and excited
states becomes much greater than in alcoholic solutions;
the solvent shift to shorter wavelengths may be so large
that the n-w* transition appears to vanish completely.
Transitions in ketones that are classified as n-n*
generally show a slight increase in polarizability in the
excited state; the result is a small red shift (usually
less than 600 cm-1) of a w-n* transition in a polar solvent
relative to a nonpolar solvent.27 Transitions in aromatic
hydrocarbons which contain no heteroatoms are considerably
28

less affected by polar solvents. Intramolecular charge

transfer statesixlaromatic ketones, by contrast, become

much more polar relative to the ground state. Red shifts

1

of 2000 cm' or more in polar solvents are common,27 for

intramolecular charge transfer bands in aromatic ketones.

31

The general rule for solvent Shifts is that if the
excited state is more polarizable than the ground state
or has an increased permanent dipole, the Spectrum is red-
shifted in the more polar solvent; if the reverse applies,
the Spectrum is blue—shifted.

29 applied the criteria of solvent shifts in

Kuboyama
assigning the lowest—energy, singlet-singlet transition in
fluorenone, which had long been considered to be an n + n*

transition, as a n + n* transition. His results were later

substantiated by Yoshihara and Kearns.3O

2. Locally—Excited States vs Charge-Transfer States

 

One may classify the electronic states of substituted
molecules as being locally excited (L.E.) or charge-trans-
fer (C.T.) states in the zeroth order approximations.

A charge-transfer (C.T.) state involves the transfer of an
electron between the hydrocarbon and the substituent. Com-
paring the spectra of a substituted benzene molecule taken
in hydrocarbon solvents with those taken in polar solvents
will offer a means to identify "C.T." bands. For substi-
tuted benzenes,especially those containing a strong donor
and a strong acceptor substituent, the dipole moment
increases in the same direction in the CT state and thus

a red shift is observed whose magnitude depends greatly

on the polarity of the solvent and the change in dipole

moment during excitation. One Should also mention that

32

the CT bands of chloro, bromo and iodobenzenes are ex-
pected to shift to the blue since their dipole moments de—
crease or may change direction as a result of excitation

to CT states.31’32 Relatively large red shifts are usually
characteristic of intramolecular C.T. transitions of sub-
stituted benzenes.

Room temperature absorption spectra of p-amino-nitro-
aniline in methylcyclohexane MCH, acetonitrile CH3CN and
ethylalcohol EtOH are Shown in Figure (2).33 It is clear
that the first absorption band is more sensitive than the
2nd band, the first band (CT band) undergoes a red shift

1 in going from hydrocarbon solvent to ethanol.

of NA000 cm-

As a result of charge—transfer migration from the amino
to the nitro group, the dipole moment of p-aminoaniline is
expected to increase in the CT state but remain in the
same direction as that of the ground state. Larger solva-
tion energies are therefore expected in the CT state com—
pared to the ground state due to the increase in the dipole
moment. Also larger hydrogen-bond energies are expected
due to the increase of the acidity of the amino group
hydrogen atoms and the increase of the nitro group basicity.
This explains the red shift observed in polar solvents,
particularly hydrogen bonding solvents.

The spectrum of p-nitroaniline in alcohol (Figure 2)

1

shows a shoulder at ~29A and 305 nm corresponding to the B211

Locally Excited (LE) benzene transition. Its frequency is

33

 

 

I ' I I j .r A I r 1 I
’\
.0 .
a." no
r- [I ‘- '
__ —:nm ALcoroL
.’ A. --------- Miran cvcmrcum:
ze+ f 3/ \\ .__m____ '
1 : }
\. '

‘ l \ . ~£rncn

 

 

 

\
/' // \ \ I
as” ./}/ ,‘ \. \
L__._L._.J_- 1’ J -e; 1-- 11°C; I. -_ "i ------ _~._-\_-;— in»
2000 2400 2.00 3200 3600 0000 4400 4.00
‘ IAVE LENGTH IN ANGSYROHS
Figure 2.

Room temperature absorption spectra of p-
nitroaniline in different solvents.33

3A

not much affected by change of solvents.

3. Electron-Transfer Transitions

 

Halogen anions, 1’, Br’ and Cl’ show strong absorption

373' Their Spectra are shown

bands in the ultraviolet region.
in Figure (3a);fl”)where one can see that the I' and Br"
exhibit two absorption bands. The spectra of halogen anions
were ascribed to electron transfer absorption,35’36 where

an electron was transferred from the halide anion to the
solvent.

Strickler and Kasha37 studied the Spectra of halogen
anions in acetonitrile, water and ethanol. The Spectra of
iodine in these solventsenwzshown in Figure (3b).37 They38
have studied the electronic transitions in N03 and N05
ions in solvents of different polarities and differentiated
between electron transfer and other internal transitions
(n + W*, n + v* . . ., etc) in these ions.

9,A0 studied the electronic

Kosower and co-workers3
spectra of N-alkyl-pyridinium iodide in various solvents
and found that the spectra are very sensitive to solvent
polarity and a large blue shift occurs with increasing sol-
vent polarity. Each absorption band was assigned to the
charge transfer band in which an electron is promoted from
the iodide ion to the pyridinium ring and therefore the CT

state is characterized by a small dipole moment which lies

in the plane of the ring.

35

 

    
  

I'M!)

 

 

 

 

79 1.0 - I ‘
a 'l - SOLVENT
U o " 0-0
' a
'1
as» t -
(puma: ‘
E
E
‘0
x
1 ‘."~_;L._ \4 1
“> 55 a: 45 40 ,5

D (KKJ

Figure 3a. Room temperature absorption spectra of halogen
ions in aqueous solution (D20).3Ab

 

 

 

 

 

ANGST ROMS

Figure 3b.. Room temperature absorption spectra of sodium
iodide in acetonitrile, water and ethanol.37

36

u = 13.1D p = 8.6D
This is a case where the solvation energy of the ground
state is larger than that of the excited state. Moreover,
the dipole moment changes its direction as a result of excita-
tion and therefore a large Franck-Condon orientation strain
is expected. Both effects contribute to the large ob-

served blue shift in more polar media.

A. Sihglet-Triplet Transition

 

The probability Of Tl + SO transition in organic com-
pounds is a highly sensitive function of the presence of
heavy atoms which enhance spin-orbit interactions and in-
creases the intensity of the transition. The Spin-orbit
interaction is treated quantum mechanically by introducing
into the Hamiltonian operator, a term HSO for each electron

of the form

H80 = K a (L - s) (19)

37
where L is the orbital angular momentum operator, S is

the spin angular momentum operator and g is a factor depend-
ing on the nuclear field. g and therefore HSO is propor-

A because of the reciprocal rela-

tional to Z/r3, 343;, to Z
tion between Z and r. (Z is the nuclear charge and r is
the distance between the electron and the nucleus. Per-
turbation theory shows that if mg and w% are the wave-

functions of "pure" singlet and triplet states, respec-
tively, then the triplet state produced under spin—orbit

coupling can be written!41 in the form of the following

equation

< 0 [H W°>
SK SO T
wT = w% + z - ng (20)

K ET ’ ESK

 

A Similar expression can be written for the singlet state.
Thus the effect of Spin—orbit coupling is to mix a small
amount of singlet character into the triplet states and
vice-versa, so that "pure" Singlet and triplet states no
longer exist. The probability of S + T transition increases
rapidly with the atomic number and thus solvents with a heavy
atom such as iodine will enhance Singlet-triplet absorption

bands; this will help in their identification.

38

V. Solvent Effect on Emission Spectra

1. Spectral Shifts

 

A solute molecule in its ground state is surrounded by
solvent molecules in equilibrium in solution. The geom-
etry, charge density and dipole moment of the solute mole-
cule may be different in the excited state; therefore, the
equilibrium excited-state configuration of the solvent cage
will also be different. The solvent configuration around
the excited solute molecule immediately after the electronic
transition does not correspond to the equilibrium-excited
configuration, but to a configuration geometrically identi-
cal to the solvated ground state, i.e., a Franck-Condon
state configuration. The relaxation of this configuration
occurs to the excited state equilibrium configuration.

The time required for solvent reorientation is around 10—11
sec in fluid media. Since the life time of an excited
singlet state is of the order of 10'8 sec, there is enough
time for excited state equilibrium to be reached before
deactivation occurs if the solvent is not viscous. Also
the ground state configuration after fluorescence is not
the equilibrium ground state configuration but a state

of strain whose energy is higher than that of the ground

state equilibrium configuration.

If the dipole moment of the solute changes (in magnitude

39

and/or direction) upon excitation and the solvent is polar,
reorientation of solvent molecules occurs before emission.
However, if the solvent is rigid, relaxation times are
several order of magnitude larger than the excited state
life time, and emission occurs before solvent rearrangement
takes place. If however, the polar solvent is fluid, relaxa-
tion is much more rapid and emission may occur from the
equilibrium excited state where dipole reorientation is
completed. Therefore, absorption will occur to the meta-
stable Franck-Condon state (Va = absorption frequency in
wave numbers cm-l) and emission will occur from the equilib-
rium state (;a = fluorescence emission frequency in wave

-1). The quantitative expression for A? in

numbers cm
absorption is different from that in emission and the 0-0

band will not coincide. The difference (Stokes' shift) is

 

2
2 2 D—l n -1 2
= (u _u ) [— _ _—:l + T
a3hc e g D+2 n2+2 a ho

n2-l
n2+2

 

[<ae-ag><3u§ - 5n: + 2ugue>1£%}% - 1 (21)

where a is an effective cavity radius appropriate for the

solvent, pg, “e, D, n, “e and a have their usual meanings.

g
The second term originates from the dipole-induced

A0

dipole interaction, in many cases can be considered as a
second order interaction term and makes a negligible con-

tribution to the shift and the equation is Simplified as

follows:
2
_ = I " — 2 2 (D—l) M
A v v - v - ———— (u —u ) [T___I - l (22)

The dipole moment difference in ground and excited state
can be obtained directly from the Stokes' Shift. An esti-
mate of the excited state dipole moment can be made from
experimental absorption and emission shift data and the
known ground state dipole moment using Equation (22).

In highly viscous solutions this solvent relaxation
is slow and fluorescence occurs, but from a non-equilibrium,
Franck—Condon state. Since the Franck-Condon state is
always higher in energy than the equilibrium excited state,
fluorescence in highly viscous solutions is blue Shifted
with respect to that in solutions of low viscosity. Another
consequence of increasing the viscosity of the solution is
the increase in the intensity of emission. In fluid media
radiationless transitions are very fast in these relaxing
systems.“2

One method of increasing viscosity is to use solvents

which form rigid glasses at liquid nitrogen temperature;

A1
a blue Shift of the low—temperature fluorescence Spectra
compared with those at room temperature in the same sol-

vent occurs.

2. Viscous-flow Barriers“3

 

Molecules in condensed media always are surrounded by
a solvent cage. The cage may be a liquid solvent, a
macromolecular enclosure (enzyme site), a lipid membrane,
a multi-layer lamellar system, or may be a crystal or
surface-adsorption cage. Although photochemical and other
kinetic studies have long taken cognizance of the effect
of solvent cages on recombination rates, Spectroscopic
studies of the mechanism of solvent cage action have been
neglected in comparison.

The mechanical Viscous- Flow Barrier solvent cage has
been analyzed on a quantum—mechanical basis by Dellinger
and Kasha.uu’145 Figure (A) indicates schematically the
basis of their model. If a molecule upon excitation
requires a rather large distortional motion for relaxa—
tion, then a large volume of solvent in the cage
around the solute must be displaced. For example, in
trans-stilbene the torsional relaxation about the double-
bond requires that the phenyl groups sweep out a large
volume of solvent to reach an equilibrium excited state

configurationus (upper part, Figure A). On the other hand,

A2

ISOMERIZATIONAL EXCITATION

 

O O Q 0 0
0C? 000%”) Q 000 g°©o°3o
0m ' ‘oW 9
03,00 s00“? 0098898%39

00 Q00Q§00
DISSOCIATIVE EHCITATIPN .
0 00 0 A 07230 ‘ 0 00 00 o
o€§8§fiég on 0693978937100 90
@009 690$?) U&@0%9%;08%
'00 0’ 0 0Q O -
SOLUTE GROUND STATE . SOLUTE EXCITED STATE

Figure A. Mechanical viscous-flow-barrier cage proposed
by Dellinger and Kasha.A3

A3

a hydrogen-bonded molecular pair, such as 9,10—diazaphen-

anthrene complex with t-perfluorobutyl alcohol:

0

N-N
.. .. CF3
H\ /
O-IC-CF3
CF3

would require displacing a large volume of solvent mole-
cules in the photo-dissociative excitation of this
molecular complex.”5
According to the Dellinger—Kasha model, the solvent
cage imposes a barrier to the molecular motion, which ap-
pears as a Gaussian-flow barrier added to the potential
function where the latter become horizontal, igeg, in the
dissociative case, at the dissociative limit. The phenom-
enological reality of these viscous-flow barriers has been
tested by low-temperature spectroscopic studies. Thus,
luminescence phenomena reveal that the molecule is trapped
in a ground state configuration in rigid glass solvents.
Recently, Mohammadi and Henry”6 have verified the

viscous—flow barrier to molecular motion by an entirely

AA

different route. They studied the infrared overtone an-
harmonicity of C-H vibrations in methylbutanes, and were
able to show that the decrease in anharmonicity predicted
by the Dellinger-Kasha model was experimentally verified
and could be calculated by the Lennard-Jones potential as
a quantitative perturbation of Morse potential.“6
An interesting case of excited state proton transfer

spectroscopy was discovered recently in flavones by Sengupta

and Kasha.u7 In 3—hydroxy flavone

 

it was discovered that in hydrocarbon solution at room
temperature a green fluorescence was observed, unrelated

to the ultraviolet absorption of the molecule. It was
deduced that an intramolecular proton-transfer had occurred
within the internal H-bond to the carbonyl group, and that
a pyrilium-like excited state tautomer yielded the green
emission. Upon freezing in a rigid glass solvent, the
normal violet-ultraviolet fluorescence could be inhibited
by the solvent cage,u7 so that the viscous-flow barrier

prevented tautomerization.

A5

3. Solute—Solvent Relaxation in the Nano and Pico—

second Range

 

Molecular relaxation occurring in the "molecule-solvate
Shell" system after optical excitation is governed by the
correlation between radiation lifetime If and environment
relaxation time TR. The main cases possible here are as

follows:2

1) Tf >> TR - all molecular relaxations are completed
within the time Tf, 143;, the molecule at the moment
of deexcitation is in thermodynamic equilibrium with
all modes of the "molecule-solvate Shell" system;

2) 1f m TR - molecular relaxations are realized only
in part with the result that at the moment of
emission the system is not in thermodynamic equili-
brium;

3) If << TR - the case of extreme nonequilibrium,
for the relaxations (orientational and transla—

tional in particular) do not proceed at all.

Picosecond and nanosecond time resolved spectra are
used to measure these relaxation processes.

For a solute that undergoes a large change in dipole
moment upon excitation, the study of time dependent spectral
Shifts will reveal information regarding solvent relaxation.

Time dependent emission spectra taken at different delay

A6

times after the excitation pulse are measured.

An example of a time-dependent spectral Shift given
by the polar molecule ANS (1-Anilino-8-naphthalene sul-
fonate) in a polar solvent, n-propylalcohol, is shown in

Figure (5).”8

Figure (5a) shows the steady-state fluores-
cence Spectra of ANS at room temperature, intermediate tem-
perature and liquid nitrogen temperature. A blue shift of
A1 nm is observed. Large time-dependent spectral shifts
were observed only in the temperature range from -700 to
—l70°C in n-propyl alcohol. At higher or lower tempera-
tures the emission spectra are essentially time independent
over a time range of 70 nsec. (the lifetime of ANS of ap-
proximately 20 nsec places a limit on the length of time
available for time—resolved Spectral measurements). Fig-
ures (5b) and (SC) Show the time-resolved spectra in n-
propyl alcohol at -90° and —l50°C. The indicated delay-
time for each spectrum is given relative to the initial
rise of the nanosecond flash lamp. It is observed from
the figures that the time required to approach the relaxed
spectrum (corresponding to the spectrum at room tempera-
ture) increases with a decrease in temperature or an in-
crease in viscosity of the solvent.

Wareu8'50 interpreted the phenomenon of time-dependent
spectral Shift as being due to solvent reorientation about
the excited molecule that is required to accommodate the

change in dipole moment and moment direction that occurs

A7

Figure 5. (a) Steady—state fluorescence spectra for
ANS in n—propyl alcohol at the indicated
temperatures.

(b) Time-dependent fluorescence spectra for
ANS in n—propyl alcohol at -90°C. A, 0
nsec; B, 2.5 nsec; C, 12.5 nsec; D, 21.5
nsec; E, 31.5 nsec.

(c) Time-dependent fluorescence spectra for
ANS in n-propyl alcohol at -l50°§. A,
2 nsec; B, 11 nsec; C, 68 nsec.

FLUORESCENCE INTEWTY

A8

 

 

 

 

 

 

 

 

a
)-
p.
Q
E
Z
A
8 496 450'
(D .
Id
§ 42:» ~29
d
" v. 550 3&1
wwaimnnmm)
c
' V
‘(
).
c
U)
2
E
E
LIJ
O
2
U
U
, 3
c §
; d
A
Kb 130 its 400

 

 

 

 

WAVELENGTH («rd

1

 

Figure 5

l _
450 500

WAVELENGTH (nm)

550

 

A9

upon excitation. The extent to which time-dependent spec—
tral shifts are observed is a measure of the degree of

relaxation of the excited molecule towards its equilibrium

 

configuration at a given temperature.

Azumi51 related the time—dependent Spectral shift to
the edge excitation red Shift and interpreted the two
phenomena as being due to the same mechanism, which was given
by Ware. Naturally the Azumi model based on the above
mechanism cannot explain the lack of time-dependent spectral
Shift and the appearance of REE at 77 K. We believe that the

two phenomena have different mechanisms. The time-dependent

spectral shift represents spectra corresponding to dif-
ferent stages of relaxation of the bulk continuum at a par-
ticular temperature, while the red edge effect represents

the spectra corresponding to different solvation sites
(different local environments) when excited monochromatically
at the red edge of the bulk continuum.

The picosecond light pulse method was used to measure
the molecular orientational relaxation times, and time
dependencecfi‘solute rotation. The principle idea of this
laser technique is to induce an anisotropy in the orienta-
tional distribution of the solute molecule with a pico-
second excitation polarized pulse, and to monitor the return
of the system to isotropic distribution with an attenuated

52

picosecond pulse. Due to the induced anisotropy, the

absorption of the probe pulse is polarization dependent.

50

The decay of this dichroism with time, due to thermal
molecular motions is determined by measuring the relative
transmitted intensities III/Ii of the probe light; I||(t)
and 11(t) are the components, respectively, of the probe
light polarized parallel and perpendicular to the excitation
light at the time t after the excitation pulse. For rota—
tional motion describable by the rotational diffusion equa—

52

tion, Eisenthal et al. obtained a relation

I
2n Till .. expE-(6D + mm (23)

where D is the rotational diffusion constant and T is the
excited state lifetime. The orientational relaxation time
is given by (6D)'l.

Eisenthal et al.52 reported the orientational relaxa-
tion of rhodamine 6G in a series of normal alcohols, ethylene-
glycol, chloroform and formamide together with the effect
of hydrogen bonding and the structure of liquids on the
molecular rotational motion using picosecond light pulses.

It was found that even though methanol forms a stronger
hydrogen bond with rhodamine 6G than chloroform, the
relaxation times are found to be equal. The insensitivity
of the orientational relaxation of rhodamine 6G to its
formation of hydrogen bonded complexes with the molecules
of the solvents was explained in terms of the orientational

freedom of the hydrogen bond and the fact that the complex

51

 

3o 1 v 1 I I T I T ' '
28 P I l—Undecanol 'l
26 t ‘
24 ~ ‘
I l—Decanol
22 t I ‘
Eth lane
20 - Glycol
E 18r 7
O
.0 16 __ I—Octanol .1
ha 14 e "
12 _ ‘
lO - ‘
Formam'd
8 ' l—Pentanol .1
6 e l—Butanol ‘
4 _ l—Propanol -
2 +- Ethanol
Methanol q
Chloroform 1 1 1 L 1 1 1 1

 

 

 

0 2 4 6 8 10 12 14 16 18 20 21

nlcentipoises)

Figure 6. T versus solution viscosity for rhodamine 6G

or 52
in various solvents.

52

is dynamic in that the solute-solvent hydrogen bonds are
breaking and reforming. The same results was found by

53.514

Levshin et al., for the same solute in formamide and

pentanol solvents.

Those observations are clear in Figure (6)52 and show
that the relaxation times of rhodamine 6G in the liquids
through octanol vary linearly with the solution viscosity.
The departure from linearity in case of decanol and un-
decanol has been interpreted in terms of greater linear
dimension of solvent relative to solute. The deviation of

rhodamine 6G in ethyleneglycol may be due to extensive

solvent—solvent aggregation by hydrogen bonding interactions.

A. Excited State Level Inversion

 

If two different excited energy states S1 and 82
which have quite different electronic structure lie in
close proximity, there is a possibility that the solvent
effect will bring about the inversion of two different
energy states because the solvent effect on these two ex-
cited states may be considerably different. When this
kind of inversion occurs in some solvents, a consider-
able change of fluorescence Spectra, intensity, polarization
and lifetime is invariably observed, though absorption
spectra will remain almost unchanged. The dual fluores-
cence character of p-cyano-N,N-dimethylaniline (DMAB)

in different solvents first pointed out by Lippert

53

et al.55,56 is Shown in Figure (7a).55 Such dual fluores—

cence caused by environmental perturbations has been ob-

61 62

served also in indole,57'6O 1-naphthylamine and l-naphthol.
Suzuki et al.63 have observed fluorescent level inversion
of dual fluorescence in alcoholic solutions of l—naphthol
by a change in temperature only.

These results can be interpreted in the following way.

These molecules have two electronic states lLb and l

1

L o
all’l

L

the region of their lowest absorption band. The a

state lies above lLb in free molecules, but these states

do lie close to each other. In fluid solutions at room
temperature, the solvent molecules around the excited solute
molecule will generally have time to reorient themselves
before light emission occurs and hence they relax to their
preferred equilibrium configuration which is of lower energy.
In non p018? solvents, the 1L states of these molecules

a

still lie above 1Lb even in the preferred equilibrium con-

1L

b

state. In polar solvents, the interaction between dipole

figuration and hence fluorescence occurs from the

moments of such excited molecules and solvent molecules low-
ers the energy of the 1La state below that of lLb in their
equilibrium configuration and hence fluorescence occurs

from the 1La state. The above sequence is shown in Figure
(7b).6u At sufficiently low temperatures and high vis-
cosities the relaxation processes which lead to an equil—

ibrium configuration will not occur to any appreciable

5A

(a) Effect of temperature on the fluorescence
spectra of 5 x 10-5 M DEAB (p-diethylaminobenzo-
nitrile) in butylchloride-methylcyclohexane-
isopentane mixture (12:3:1 in volume). ——————293°K,

______ 23A°K, -._._ 1730K, -.._..— 1A8°K.55

Figure 7.

(b) Excited state level inversion caused by
mutual interaction between solute in excited

state and polar solvent.

55

IF

 

 

J 1 J J 1

20.3 22.9 24.0 25.6 27.4 29.9

7°10”[cm"] —_.

IL increasing lorlly
o ——————E:

 

 

 

 

 

 

 

 

 

 

 

F‘.‘
§ ‘Lb . 5 3 1.1—1—
3. Obi floor. lluor.
obsorpllon fluorosconoo
in in
nonpolar polar-solvonlo
solvonl

Figure 7

56

extent during the lifetime of the excited state and there-
fore emission will take place from an unrelaxed configura-
tion.

Mataga measured the time-resolved fluorescence spectra
and fluorescence decay curves of the 1L and lLb fluores_

a
cence of DMAB and demonstrated clearly the relaxation
process forming the lLa—solvated state. The 1Lb fluores—
cence approaches its intensity maximum after A nseconds and

1La fluorescence after about 7 nseconds after the end of a

short excitation pulse.

CHAPTER III
THE RED EDGE EFFECT AND A RELATED PHENOMENON

Fluorescence and phosphorescence of some organic mole-
cules exhibit a red Shift when excited at the long wave-
length edge of the first absorption band. This phenomenon
has been termed the red—edge effect65 (REE), or edge—excita-
tion red shift66 (EERS) which actually is the difference
in wave numbers (cm-l) between the emission maxima ob-
tained on shorter wavelength excitation and excitation at
the red edge of the first absorption band.

Narrow band excitation is one of the necessary condi-
tions to be able to observe the REE. This can be accom-
plished by using a good quality excitation monochromator
or recently by using tunable dye lasers. Another condi-
tion to observe this phenomenon is the rigidity of the medium
such that the life time of the excited state would be in
the range of solvation site life time. This condition can
be satisfied by lowering the temperature of the solution
or by dissolving the material in a polymer matrix (e.g.,
poly (vinyl alcohol) or polystyrene) at room temperature.

A review of observations of the phenomenon made by dif-

ferent authors during the last two decades is now given.

57

58

In 1960 G. Weber reported67 that concentration depolariza—
tion in rigid solutions, which results from singlet—singlet
intermolecular energy transfer, failed to occur upon excita-
tion into the red edge of the chromophore absorption band.
The emission polarization experiment is represented dia-
gramatically in Figure (8a). The polarization P is defined

by the following equation:

Ill - II

P=I||+Il (2“)
where III is the number of photons per unit time (intensity)
emitted with their electric vector parallel to the electric
vector of the exciting radiation in the laboratory frame
of reference and Ii is the number of photons per unit time
(intensity) emitted with their electric vector perpen-
dicular to the direction of the electric vector of the
exciting radiation. Polarization reflects anisotropic
character and hence lack of energy transfer while de—
polarization reflects isotropic character and hence an
energy transfer. Figure (8b)67 shows Weber's study of the
concentration depolarization of indole in thin propylene
glycol films at 220°K. Figure (8b) shows that excitation
at the red edge of the absorption band (above 290 nm)
resulted in enhanced polarization which indicates that

little or no energy transfer occurred under these conditions

 

 

59

 

5 F-—‘ Perpendicular
w tP/larized (IL)

5" /l
’1‘--. . Parallel Polarized
Sample Cell Emission (In)

 

 

 

 

 

    

Polarized
Excitation

Figure 8a. Diagram showing orientation of parallel and
perpendicularly polarized emission with
respect to plane of polarization of exciting
radiation in the laboratory coordinate system.

 

 

01111111LJ

230 240 250 160 170 280 190 300 3‘0
limp)

Figure 8b. Excitation polarization spectra of indole in
propylene glycol at -70°C. I, 0.0l M; A, 0.05 M;
x, 0.1 M; 0.0.2 131; o, 0.14 131.67

60

even in concentrated solutions. Other studies68 by the
same author have indicated that this is a general phenomenon
in aromatic molecules.

Galley and Purkey69 measured the excitation wave-
length dependence for emission spectra of indole in rigid
media. Figure (9a)69 displays the fluorescence and 0-0
region of the phosphorescence spectra of indole in a 1:1
ethyleneglycol-water glass excited near the center (280 nm)
and the red edge (295 nm) of the absorption band. Clearly
the emission spectraenwanot independent of the exciting
wavelength as is generally assumed, for the Spectra gen-
erated at the longer exciting wavelength are red shifted
by about 900 cm.1 in fluorescence and 90 cm"1 in phos-
phorescence. The emission Shift decreases with increasing
concentration, at 0.5 M the shift in the fluorescence of
indole is 600 cm’1 for 280 and 295 nm excitation. An
exciting wavelength dependence was not observed for the
fluorescence spectra at room temperature. This depen—
dence of the phenomenon on solution "rigidity" is. depicted
in Figure (9b).69 The differences in emission wavelength
for both the phosphorescence and fluorescence spectra of
indole excited at 280 and 295 nm in A:l glycerolzwater
were plotted as a function of temperature. The exciting—
wavelength dependence is lost in either case, but the tem-
perature at which the transition occurs for the phosphor-

escence is much lower than that for fluorescence.

Figure 9.

(a)

(b)

61

Fluorescence and 0-0 region phosphorescence
spectra of 10‘3 M indole in 1:1 ethylene
glycol-water media at 202 and 77 K,
respectively, demonstrating the shift
between the emission excited at 280 and

295 nm.

Plot of exciting wavelength dependence for
the 10'3 M fluorescence and phosphorescence
spectra versus the temperature of the

A:l glycerol-water medium. Fluorescence
and phosphorescence shifts were measured

as the average of the red edge and blue
edge differences between the Spectra at

280 and 295 nm excitation.59

62

wavenumhets ism")
3

 

relative emission Intensity

 

313 311 2]? fig: 25 151.5 251.0 241.5 x10
lNlllllE nunnescsuc: mum: Pnnspunns’scenca

exclling, beam

 

 

 

 

 

 

 

 

 

 

 

 

 

 

l l l l l l l l
300 325 350 375 ' 395 400 405 410
wavelength lnml
(a)
0007— “I" — ’ 3m
0 r:-—_.I—:’. L ......... 0
non-- - .’ ‘ - Ill
2 I, -0
g sun — . l.’ - Ell
é "
400,. llumescence ': phosphmescenca _. 40
i \ ,1 /
E200— /. I" - 20
' ’o/ I ’I
’5 n 1. o -—-o -..-o' o —1 o
.L_ l i _1 i i
l) -4ll -80 -I20 - 160 —200

tempevatme (“Bl

(b)
Figure 9

63

K. Nagvi et al.70 showed that the shape of the fluores-
cence spectrum is sensitive to the exciting wavelength in
rigid media. Figures (10a and 10b)70 shows better resolu-
tion in the spectra of P-terphenyl (TP) and P-quaterphenyl
(QP) in rigid ethanol when excited at the red edge of the
absorption band.

Similar observations to the above were found by us for
different systems and are discussed in detail in Chapter V.
The phenomenon has been confirmed experimentally by many
other authors.71-79

Earlier interpretations of the phenomenon will now be
given. Weber7l’72 interpreted the phenomenon present in
naphthyl amine derivatives as due to the existence of out-
of—plane transition moments in absorption and emission,
normal to the plane of the aromatic rings,which he thought
arose as a result of coupling between out-of—plane nuclear
vibrations and in—plane transition moment. His interpre-
tation was based on the measurement of polarization at
different wavelengths of excitation and calculation
of the rates of in-plane and out-of-plane rotations
in aromatic compounds.71

Galley69 interpreted the REE in terms of heterogeneity
of solvation sites and emphasized the composite nature of
both absorption and emission in rigid solution and pro-
vided a simple rationalization for the red edge failure of

concentration depolarization obtained by Weber. Excitation

6A

 

INTENSITY

 

l J l l

 

 

 

L 1 L
320‘ 3‘0 300

WAVELENGTH (hm)

Figure 10a. Fluorescence spectra of «.10'5 M TP (p-ter—
phenyl) in ethanol at 77 K at different excita-

tion wavelengths. ——————-280 nm; ----- 310 nm;
..... 315 nm.

 

INTENSITY

 

 

 

 

340 300 ~ 300 400 ‘30 4‘0

WAVELENGTH (nm)

Figure 10b. Fluorescence spectra of "’10"5 M QP (quater-
phenyl) in ethanol at 77 K at different excita-
tion wavelengths. —————— 320 nm; ------ 330 nm;
...... 335 nm.70

65

into the red edge of the chromophore absorption band selects
a certain subpopulation of the solute molecules of small
absolute concentration, which by virtue of their par-
ticular local environments are characterized by relatively
low 0—0 singlet electronic transition energy. Because the
bulk of the chromophore molecules in the solution possess
0-0 transitions of higher energy than the subclass of
molecules at the red edge, members of the latter subclass
have, in general, only neighboring molecules with higher
energies. Singlet-singlet energy transfer and, as a result
fluorescence depolarization, thus cannot occur for those
molecules selectively excited at the red edge of the chromo—
phore absorption band. On the other hand, at Shorter
exciting wavelengths, molecules of generally higher transi-
tion energies are excited and readily undergo energy trans-
fer and fluorescence depolarization. This model of dif-
ferent local environment given by Galley explained the
excitation wavelength dependence of the fluorescence in
Figure (9a).69 The results depicted in Figure (9b)69

were explained by Galley as follows: at -110°C the mobility
of the glassy solution is sufficient to randomize the
chromophore solvation sites within the triplet—state life—
time of indole (”7 seconds), whereas a temperature of -53°C
is necessary to render the solution fluid enough to average
the chromophore local environments within the nanosecond

excited singlet state lifetime.

66

Nagvi7O interpreted the REE observed in P-terphenyl
and P-quaterphenyl as a result of emission from different
conformers. Chen73 interpreted the observation in quinene
and 6—methoxyquinoline as a result of emission from more

7A

than one electronic states. Tomin and Pavlovich75

gave an interpretation similar to Galley's model in terms
of inhomogeneous spectral broadening. Azumi et a1.51
noted that the mechanism of the REE is similar to the one
proposed by Ware et al.79’SQ in interpreting the time—
dependent spectral shift.

It may be convenient to distinguish two cases where
the solute may assume different conformations: An example
of this has been observed in the case of P-terphenyl and
P-quaterphenyl in ethanol at 77°K7O and in the case of the
esters of 9-anthroic acid in 3MP at 77°K, Chapter V,
observed by us. The other case involves polar solute mole-
cules interacting with various solvation sites in a polar
solvent, iLQL, a distribution of different solvation sites;
an example of this is the case of indole in ethylene glycol-
water media at low temperatures69. Another example is our

recent observation of a large REE in the case of mero—

cyanine dye in polar solvents.79

67

 

A large number of polyaromatic hydrocarbons exhibit

vibronic spectra consisting of a large number of qgasilines

 

(i.e., a series of narrow bands which in most cases can be
called lines) when present in a frozen n-alkane matrices at
77°K and lower temperatures. This phenomenon has been
called "SHPOL'SKII EFFECT" after the Russian physicist

E. V. Shpol'skii who discovered it in 1952.80 The crystal-
linity and the transparency of the matrix (Shpolskii
matrix) is one of the essential conditions to obtain the

80-8A

quasi-linear spectra. Temperature as well as the
rate of cooling of the matrix are other significant factors
in observing Shpolisktieffect. This effect has become a
general phenomenon in molecular spectroscopy.85:86

A typical example of the Shpolwfldi.effect is represented
by the fluorescence spectrum of coronene in a frozen n-
heptane matrix shown in Figure (11).86 The excitation wave—
length dependence of the fluorescence Spectrum of benzoEaJ-
pyrene in n-heptane matrix is also shown. Certain ali-
phatic alcohols can also be used as solvents for the develop-
ment of this effect.

To explain the line-like structure one presumes that
the aromatic hydrocarbon replaces an aliphatic hydrocarbon
at a lattice site and thereby exists in a state that can

be described as an oriented gas molecule. Thus the spectra

are of unperturbed systems corresponding to the free

Figure 11a.

Figure 11b.

68

 

 

 

a (nun)

Fluorescence Spectrum of coronene in a frozen
n-heptane matrix. The sharp lines arise be-

cause of the unique conformation of the coronene
in the matrix.

 

 

 

,1

l
"V— f -—
CO! ‘00 CO! 0.. 000

‘0'

Variation in the appearance of the A03 nm band
system in the fluorescence spectrum of benzo[a]
pyrene in heptane matrices at 15 K under dye
laser excitation as a function of excitation
wavelength: (a) 385.5 nm, (b) 385.7 nm, (c)
385.9 nm, (d) 386.1 nm.86

69

molecule. The ideal situation results when the dimensions
of the solute and solvent molecules are comparable so that
the solute molecule can occupy a definite position in the
host lattice with minimum deformation; for example, the
emission spectrum of naphthalene in pentane shows quasi-
linear structure, but is more diffuse in hexane and heptane.
If the solvent molecule is too small, a similar situation
arises. The broader, more diffuse Spectra that can result
tend to be similar to those obtained in clear, rigid glass
solvent systems. These can be a variation in the number

of lines and their intensity depending on the solvent, ex—
citation wavelength and temperature. The variations in
part appear to originate from the fact that there are local
differences in the crystal field surrounding different
molecules. Thus, the multiplet structure appears in Fig-
ure (11)86 is explained in terms of a number of distinct
sites in the host crystal, in which the guest molecule can
reside. Each Site will perturb the energy levels to a
different extent and so a number of component Spectra,

each with its own electronic origin, will be produced. Also,
the rate of freezing of the mixture is important. Slow
freezing results in the loss of some short wavelength com-
ponents and weakening of some long wavelength components.
This probably results from formation of rotational solvent
isomers.82 Under slow freezing it is more likely that a

greater proportion of the solvent molecules will be in the

70

most stable conformation and the spectrum will be simple.
Such fine-line spectra can be used to determine structural
details of the emitting molecule.82
In an amorphous medium (2;8;a solution), guest mole-
cules occupy many different microenvironments or "sites"
in a low-temperature matrix. Thus, the purely electronic
energy levels of different molecules of the same solute
are no longer the same and will be shifted to different

87-91

extent. Alternatively, it can be said that guest
molecules reside in different "sites" in their "0-0"
(purely electronic) absorption frequencies are different.87
The band width over which these different 0-0 absorption
frequencies are found is a measure of the extent of "in-
homogeneous broadening" which may contrast with the intrinsic
or homogeneous band width of a single molecule. This
contrast is clear from Figure (12).87 Upon monochromatic
excitation near the 0-0 absorption band (where the density
of vibronic state is small), only those molecules whose
absorption bands overlap the laser line will absorb and
consequently fluoresce. The resulting emission will
therefore exhibit much narrower bandwidths than if a con-
ventional broad-band lamp excitation source is used. This
effect is called "site selection", "energy selection", or
"fluorescence band narrowing".87"91

One of the experimental observations of site selection

spectroscopy is shown in Figure (11).86 It shows the

\

Figure 12.

71

0)

AA
A

MOLECULES ENSEMBLE
bl
MOLECULES ENSEMBLE

The limiting cases of homogeneous and inhomo-
geneous broadening. In the homogeneous case
(a) the spectrum summed over all molecules in
the sample corresponds identically to that for
any one of the individual molecules. In the
inhomogeneous case, (b), the Spectrum summed
over all molecules in the sample differs from
the Spectrum that would be seen for any Single
molecule. 7

72

excitation wavelength dependence of the benzo[a1pyrene
emission band in a solid matrix at low temperature. Other
examples in the literature Show the same concept as being
useful in proving either Shpol'skii spectra or the Red

Edge Effect discussed in the previous pages.

CHAPTER IV

MOLECULAR SYSTEMS

The goal of our research is to study solvent effects
on the absorption and luminescence properties of mole-
cules that undergo large changes in their dipole moment
(p) due to electronic excitations. One can study the change
in fluorescence yield and energies as the polarity of the
solvent is changed. Such changes can be examined in terms
of specific interaction and bulk medium effects. One can
also study the effect of solvent cage relaxation on the emis—
sion spectra using nanosecond and picosecond time resolved
Spectrosc0py, in fluids as well as in viscous media.
Since different solvation cages may exist, different
excitation energies may occur in dilute solutions of polar
molecules. Thus, it was interesting to study the depen-
dence of luminescence energy, lifetime and spectral resolu-
tion on excitation wavelength at low temperatures. The
molecular systems which we have studied are also potentially
useful as fluorescence probes of biological systems.

Three categories of molecular systems can be defined:

73

7A

I. Molecular Systems that Undergo a Large Decrease

 

in Dipole Moment as a Result of Excitation -

 

i.e., pe < u

 

g

where Hg is the dipole moment of the ground state and He

is the dipole moment of the excited state.

Examples:

1. Merocyanine Dyes

 

+ /
CTfli-' \\ I
_ \ 6

(l-methyl-A—hydroxystyryl)pyridinium betaine or 1-methyl-

A-((oxocyclohexadienylidene)-ethylidene-l,A—dihydropyridine.

3"”

3-methyl—2-((A-oxocyclohexadienylidene)-ethylidene-

benzothiazol.

75

2. Alkyl pyridinium iodides

:[ III

 

methyl pyridinium iodide.

For Dye I the static dipole moment decreases by 5.5D
upon excitation to the first excited singlet state.92393
The room temperature absorption spectra of pyridine mero-
cyanine (Dye I) in solvents of different polarities are
shown in Figure (13). The absorption maximum in chloro-
form occurs at 620 nm while in water the maximum occurs at
l

AAO nm, a blue Shift of 6500 cm- The half-band width of

 

the absorption band increases with the polarity of the
solvent. Table (I) shows the absorption maxima, Stokes'
Shift A5 (cm-l) with respect to chloroform and the half
band width (r l/2 cm-l) in some solvents. The fluores-
cence spectrum of this dye undergoes a much smaller Rige-
shlit as the solvent polarity is increased as shown in
Figure (1A). These observations are consistent with a
dipole moment decrease in the excited state.

Methyl pyridinium iodide is another example which

76‘

.Hocmcpm u mowm one oofixomHSm szpoefim u omza .mpco>H0m pcoeom
IMHU CH who mcficmmoonoe ocfiofinma mo mnpoodm coflpanmnm onsummoQEou Eoom .ma oesmflm

IIIAES £o§o>o>>nlll
00» com com own com 09 00? Con
_ q _

_ _ q _

 

l

A

.203

1

10m

 

8:5 1

 

<— aauquosqv

6333.5

 

 

 

77

Table 1. Optical Properties of Pyridine Merocyanine Dye
in Various Solvents.

 

 

 

Solvent rl/2 cm'l Amax’ nm AC, cm"1
Water 1900 AAO 6600
Ethanol 1500 508 3500
Dimethyl
sulfoxide 1000 570 1AOO
Chloroform A00 620 __--

 

 

Tl/2 is the width of the absorption band at half absorbance.
A; (cm-l) is the shift of the absorption maximum with

chloroform taken as reference.

78

 

 

 

3

'7.

C

Q)

.‘s' 2 g

m E m 8
L)

5 a
L)

A

2

LI.

l
500 550 600 650

——.Wovelength (nm)—->

Figure 1A. Room temperature fluorescence spectra of pyri-
dine merocyanine dye in different polar sole
vents.

 

79

undergoes a large blue shift ( 20000 cm-l) in polar sol-
vents.9u The first absorption band corresponds to a charge
transfer transition where 1’ acts as the electron donor and
the pyridinium acts as the electron acceptor. In the ex-
cited state, the intermolecular distance between the neutral
component is increased. Dramatic relaxations involving the
solvent cage as well as the two components solute will
occur.

In this category (“e < pg), we have studied in detail

the pyridine merocyanine (Dye I).

II. Molecular Systems that Undergo a Large Increase

in Dipole Moment as a Result of Excitation -

 

 

i.e., he > pg

Examples

1. 2-Amino-7-Nitro Fluorene

HN .

 

N0

ANF

80

2. A,A' Amino Nitro Diphenyl

H N N02

AND

95

The static dipole moment increases upon excitation to

the first excited Singlet state by 18D for the case of ANF
and by 12D for the case of AND.

The room temperature absorption spectra of ANF and AND
in solvents of different polarities are shown in Figures
(15) and (16), respectively. The absorption maximum for
ANF in 3MP (3-methyl pentane) occurs at 365 nm while in
ethanol the maximum occurs at 385 nm, a red Shift of
1

1A00 cm- The red shift in the AND spectra is about

1

2300 cm- in going from 3MP to ethanol. The fluorescence

spectra of these molecules undergo a larger red Shift as

 

the solvent polarity increases as shown in Table 11.96

These observations are consistent with a dipole moment

increase in the excited state. Both of these molecular

systems were studied in detail.

81

.Hocoeom ASL
”muco>Hom econommfio CH mz< mo mnuoodm soapanomnm oeSpmnoQEop Eoom .mH osswfim

0000mm

 

.sHHe Hocooeo asee>seom Amy
.sHec oeoasomsaom AmV.Heo eeeeoaom Amv .m2m AHV

 

AIIAEE 505.903
00m one 00v 0mm 0mm 0mm
. _ —

<

 

l

l

l

l

1

l

0.0

<—aouoq105qv—

0.0

 

 

82

.saae Honooao esca>seom Amy
.Hoeoaom Any .saee oeoasomsaom Amv.aeo anemones ANS .azm AHV
”mp:o>aom psoLoMMHp :H 02¢ mo mhpooam coapanompm onSmeoQEou Boom .mH onswfim

 

IAES 505.925 \
80 0mm com one 00v 0mm 00» 0mm com

._fi...—...-_uq14_.1q—....A.._._...ad.un_4

e . .

 

aauoqmsqv

1,

 

 

 

 

83

Table II. Fluorescence Properties of ANF (2—Amino—7-
Nitrofluorene) in Various Solvents.9

 

 

 

Amax’ nm
Solvent Absorption A0 (cm-l) Fluorescence A§(cm’l)
Benzene 388 —-— 515 _-_-
Diethylether 39A A00 532 620
l-Propanol A00 800 687 A800

Methanol 397 600 752 6100

 

 

8A

III. Flexible Molecules that May Undergo a Change in

 

Equilibrium Geometric Configuration Upon Excitation -

 

Examples:

1. 9—tertbutylanthroate: (tertbutylester of 9-

anthroic acid)

 

9-Tertbutyl Anthroate (9TBA)

2. 9-methylanthroate (methylester of 9—anthroic acid)

 

9-Methyl Anthroate (9MA)

85

These molecular systems undergo geometric relaxation

to different equilibrium positions in the excited state
depending on the size of the ester group, viscosity and
polarity of the solvent matrix. The room temperature ab-
sorption and fluorescence at room temperature and at 77°K
of 9TBA and 9MA in 3MP are shown in Figures (17) and (18).
respectively. The excitation wavelength dependence of the
fluorescence Spectra in rigid media has been studied for

97

both anthroate esters.

86

.EZm cs
AIIIIV x um um Mam A....V magpwpoqup Eoos pm mppooom oocoomonosam
mufi pew <mem A: mica x NV mo Esnpowdm COfipQLomnm pneumnodsop Eoom

E5 508.903

 

 

 

 

 

Aigsuaiul aouaosalonld

 

 

 

l 0.0

 

 

.efi magmas

V

O.

S

0

m
.lmnvnu

U

3

a

87

.mzm CH
AIIIIV m um um pCm A....V oCCmeoQEop Eoon pm mnuomqm ooCoomoeosHm

 

mpH UCm <20 Ha mIoH x NV Co ECCpooCm CoHquoma oCCCmCoCEou Eoom .wH oCCme
ES 592995
000 00¢ 09» 0mm own 00m
— q u q _ q — q, — H
V
O.
L. m ,
. J
Ino m
U
«4
8

Ausuaiul aoueosalonl :1

 

 

 

 

CHAPTER V
RESULTS AND DISCUSSION

I. Molecular Systems that Undergo a Large Decrease in

Dipole Moment as a Result of Excitation
A. Eyridine-Merocyanine Dye

As an example of these molecular systems we have
studied in detail the absorption, emission and excitation
wavelength dependence of the fluorescence of pyridine
merocyanine I (Au : -5.5D upon excitation)98 in solvents

of different polarities at different temperatures as well

as at different viscosities.

The electronic absorption spectrum of this merocyanine
dye is pH dependent as shown in Figure (19).99 The
protonated form of the dye undergoes cis-trans isomeriza-

tion as shown in Figure (20),99 however, the trans form

88

89

 

 

‘0—
‘0
g‘ 35— .
e
g 30—
'1
5 25-
a
0
;§ 20-
3
0
° 15— _
C
.9
E 10-
‘; .
C
3 5’ /\
3 ,
E I
1 1 1 1 1
200 300 £00 . 500 600

Wavelength lnml

Figure 19. Room temperature absorption spectra of aqueous
solutions of pyridine merocyanine dye (2 x 10‘5 M)

at different pH.99

90

H,c-N©=/=‘.:° H H,c3©—¢_.—§r .14

lb

Fifi-N? . .OH H ”c2943 ¢ © OH AM:
no 7 no I ‘

Figure 20a. Resonance structure of pyridine merocyanine
dye in its neutral and acidic form.99

Eat
Init£;.“=====t hath

hunk} bu AIkLh)

+ 3.1-3.1
M H I l'fl TI! METER.

Figure 20b. Photochemical cycle of pyridine merocyanine dye
‘ and its ciS-trans isomerization.99 .

91

is photochemically stable in basic media. All our Spectro-
scopic measurements were made in a basic media by adding
either piperidine or potassium hydroxide to the solu-
tions.

The room temperature absorption and emission spectra
in solvents of different polarities are demonstrated in
Figures (13) and (1A), respectively. The absorption
Spectrum of this dye exhibitsealarge blue shift in media
of increasing polarity. The absorption maximum in chloro-
form occurs at 620 nm while in water the maximum occurs at

1 as shown in Table I.

uuo nm, a blue Shift of e65oo cm—
The half band width of the absorption increases with the
polarity of the solvent. The room temperature fluores-
cence Spectra in Figure (1A) shows a much smaller blue
Shift as the solvent polarity is increased. These obser-
vations are consistent with a dipole moment decrease in
the excited state.

Earlier investigations of merocyanine dyes were limited
to their absorption spectra in solvents of different pol-
arities and in solvent mixtures. This particular dye I

was synthesized by Brooker and co-workers100 who proposed

it as an indicator of solvent polarity. They explainedloo’101

the results similar to those obtained in Figure (13) in
terms of a large ground state dipole moment which is sig-

92,102

nificantly reduced upon excitation The work of

Balyiss and McRae103 has established that the solvent

92

shifts of the merocyanine dyeseumeonly substantial in the
case of specific solvent interaction with both the oxygen
and nitrogen atoms; in other words the bulk dielectric

effect of the solvent has a small effect on the Spectrum.

10A

Benson and Murrel used an SCF n—electron theory with

bond length optimization to calculate the effect of solvent
polarity on the structure and spectroscopic properties of
the pyridine-merocyanine dye. McRae applied his theory

9 10 He considers the

of solvent shift to merocyanines.
dye to be a polarizable dipole in a cavity in a homogeneous
dielectric. The magnitude of the electric reaction field

from the cavity due to orientational polarization is given

by

R = EH [5'1 “2'1] (2 )
he??? 5

where R is the magnitude of the orientational cation field,
u the dipole moment of the dye in the ground state, e the
dielectric constant of the solvent, n the refractive index
of the solvent and a the cavity radius. The maximum ab-

sorption energy of the dye, Vmax’ is expected on the basis
of McRae's theory to vary regularly with the term in
parentheses of the above equation. This term is referred

to as F by McRae. Hydrogen bonding solvents Show much

93

larger shifts of 0 than do non-hydrogen bonding solvents

max
105 considers the

for comparable values of F.10 Campas
interaction between the dye and the solvent as a dipole-
dipole interaction between the dye molecule and a small

number of solvent molecules near the dye. The total di-
pole moment of the solvent molecules is calculated using

the theory of polar liquids developed by Kirkwoodl3’lu

15 In Onsager's theory of the

and modified by Frolich.
dielectric constants of liquids, one considers a Single
molecule contained in a cavity in a homogeneous dielectric.
In the Kirkwood-Frohlich theory one assumes there are M
molecules in the cavity and the limit is taken as M be-
comes large. A statistical mechanical average over the
orientations of the M molecules is performed in two steps;
first one molecule is considered fixed and an average is
performed over all orientations of the other M-l molecules.
Then an average is taken over all orientations of the one
remaining molecule. The total dipole moment of the M
molecules with one held fixed tends to a limit as M becomes
large enough to encompass the short range order around the
molecule which is held fixed. This limiting dipole moment
E is different from the dipole moment 0 of the fixed mole-
cule. Kirkwood expressed u - E = gu2 and estimated g from
X-ray diffraction data on coordination numbers in liquids.
From those g values he calculated dielectric constants of

water and several alcohols in reasonable agreement with

11

9A

13

Campas used Frohlich's modification of Kirk-
111

experiment.
wood's theory. Frohlich considered the M molecules as
M non—polarizable point dipoles imbedded in a Sphere with
dielectric constant he, the index of refraction squared.
The dipole moment of the molecule in the liquid, U, is
increased by a factor of (n2+2)/3 over the dipole moment
of the molecule in the gas phase, “0' Frohlich obtained

the result

2 = 9KT(e-n2)(2e+n2)
O unNE<H2+2)

 

(26)

where g is the Kirkwood correlation factor, “0 the dipole
moment of the molecule in the gas phase, K the Boltzmann
constant, T the absolute temperature, 5, n, defined as
before and N the number of molecules per unit volume.
Equation (26) reduces to Onsager's equation for g = l.
The problem of the undetermined Kirkwood correlation factor
was handled98 by assuming that the short-range order around
the merocyanine is the same as the short-range order in the
bulk liquid. Since the above investigations were limited
to absorption studies, we decided to study the fluorescence
Spectra under various conditions.

The room temperature fluorescence of fluid dilute

solutions (<10.5 M) of the dye is excitation wavelength

95

independent. Once these solutions are made rigid, excita-
tion wavelength dependence, lLSL’ a Red Edge Effect (REE)
is observed. We have observed a large REE for this dye in
solvents of different polarities (ethanol, l-propanol,
l—butanol, l-hexanol, glycerol, glycerol-water mixture,
dimethyl sulfoxide, pyridine and polyvinyl alcohol polymer
matrix) at 77 K and in rigid polymer matrices at room tem—
perature. The fluorescence spectra of dilute solutions
(e10-6M) of the dye in ethanol, l-propanol, l-hexanol and
polyvinyl alcohol (PVA) film at 77 K as a function of the
excitation wavelengthzmneshown in Figures (21-2A),
respectively. A large red Shift of e=l200 cm.1 in ethanol,
l-propanol and PVA and 3000 cm"1 in 1-hexanol has been
observed as the excitation wavelength is increased from

the absorption maximum (the number at top left of Figures
cited) at 77 K to the red edge of the absorption band (the
number at top right of the Figures cited). The magnitude
of this red edge effect is viscosity dependent. For ex-
ample, when the temperature in ethanol is higher than -62°C
an insignificant REE is observed. However, below this temp-
erature the viscosity of ethanol solution becomes large, and
the REE gradually increases reaching its maximum value at
77 K. Figure (25) shows such observations in ethanol

where the REE increases from 2200 cm-1 at -ll9°C to

z1200 cm.1 at 77 K. The Shape of fluorescence spectra as

a function of excitation wavelength at -l35°C is Shown in

96

l00 —

m
o
I

Fluorescence Intensity
.5
o
l

N
C)
I

 

() l, 1 l 1 I 1 I 1 J 1 I 17
480 500 520 540 550 580 600

Wovelengih (nm)

 

 

Figure 21. Fluorescence Spectra of a dilute solution
(<10"5 M) of pyridine merocyanine dye in ethanol
at 77 K as a function of excitation wavelengths.

97

8
.l
j

 

90- E
630- Si §§§s§

03V
00
I I
O
,H—‘T’.,

U"
Q
I

E?

Fluorescence Intensity
JS
0
I

—I\)
00
l I

O
O

 

 

L l l l 1 l 1 l

l 1 l 1 l 1 1
48050052O 540 560580 00 62
-—-Wovelength (nm)—>

 

‘

Figure 22. FluoresCence Spectra of a dilute solution of
pyridine merocyanine dye in l-propanol at 77 K
as a function of excitation wavelengths.

98

 

P 069
089

0&9
099

099

099

_ 029
009

(N9
009
0th
099

_W

 

J

 

 

 

 

l
O
03

Mysuaiul aauaosaiom :1

550 600 650 700
1———>. max. (nm)—->

500

Fluorescence spectra of a dilute solution of pyridine merocyanine dye

in l-hexanol at 77 K as a function of excitation wavelengths.

Figure 23.

99

 

 

 

 

90— 5.
L §§§§§§

e P \

er

9

E .

Q)

U

E; -

3

22 -

C)

2

“7 .

oo-
111111111111111111111L]

450 500 550 600 650

—>\ mox(nm)—->

Figure 2A. Fluorescence spectra of pyridine merocyanine dye
in PVA (polyvinyl alcohol) thin film at 77 K as
a function of excitation wavelengths.

100

 

 

 

 

 

 

 

l 5607

E -

v 540-

S -

In} T

'5 520-

3‘: _

“5

.c 500” 3+

‘2‘ .

2

9 480- := 3? N

s . t m

I ll 1 I 1 l 1 I 1 I 1
460 500 520 540 560 580 600

—Wovelengih of emission (nm)—->

Figure 25. Temperature dependence of fluorescence edge_ex¥
citation red Shift of pyridine merocyanine dye
in ethanol.

101

Figure 26. Another example of viscosity dependence, the
magnitude of REE in l-propanol at -108°C and in glycerol

1 in both solvents) proving the

at 0°C are equal (2A00 cm-
Significant role of viscosity in this phenomenon as Shown
in Figure (27).

Careful inspection of Figures (21—23) and (26) shows
that the Shape of the fluorescence spectrum is highly
dependent on the excitation wavelength. As the excitation
wavelength is changed toward the red edge of the absorption
band a shoulder is observed. The viscosity of the
medium as well as the length of the alcohol chain affect
the Shape of the spectrum. In a longer aliphatic chain
alcohol solvent a more defined shoulder appears in the
spectrum.

We have also studied the REE as a function of the con—
centration of the dye. In Figure (28) the emission of 10-5,

A, 10‘3 and 2 x 10'3 M of the dye in ethanol at 77 K,

10'
is shown; all solutions are excited at the same wavelength,
namely the absorption maximum A60 nm. A red Shift of

21000 cm'1 is observed when the concentration is increased

from 10'5 M to 2x10"3 M. lfimalOOO cm.l Shift represents the
decrease in the magnitude of the REE as the concentration

is increased, since no REE is observed for the concentrated
solution (2 x 10—3 M) and its emission is identical to less

concentrated solutions excited at the red edge.

Besidesifluaobservation of a REE at low temperatures, we

102

 

Xexc. (om)

Fluorescence Intensity
l

 

0.0-

 

 

1J1114111111111Ll

500 - 550 600 650
——>\ max (nm)-->

 

Figure 26. Fluorescence spectra of a dilute solution of
pyridine merocyanine dye in ethanol at -135°C
as a function of excitation wavelengths.

103

 

g

i

°$°

 

5.3”

glycerol

  

i - Proponol
I

—'-— AREE Cm"——>
A
E?

i

 

 

 

l l.
lOO 150 200 250 300
——1"K——>

Figure 27. Temperature variation of REE (Red Edge Effect)

for pyridine merocyanine dye in two different
solvents.

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105

have also observed a REE in rigid media at room tempera—
ture. The results for the pyridine-merocyanine dye in PVA
polymer matrix is shown in Figure (29). The magnitude of

the REE at room temperature was about :A50 cm"l

compared to
.1200 om’1 at 77 K.

The fluorescence spectrum of the dye undergoes a blue
Shift of 2100 cm’1 as the temperature is lowered to 77 K.
This observation is demonstrated in Figures (30, 31) for
ethanol and glycerol solutions, respectively; all solu-
tions were excited at the absorption maximum.

The absorption spectra of the dye in ethanol (10'5 M)

as a function of temperature are Shown in Figure (32).
A blue shift of z2100 cm—1 in going from room temperature
to 77 K is observed. Only a small (2500 om’l) blue Shift
in absorption is observed for the dye in PVA thin film as
shown in Figure (33).

So far we have made the following experimental obser-
vations: figst, the absence of a REE in fluid media; second,
a large REE as the medium becomes rigid; tMlpg, the ap-
pearance of Shoulders in the emission Spectrum when the
media become more rigid or when working with long chain
alcohol solvents; fourth, the appearance of REE at room
temperature in polymer films; gigth, the gradual decrease of
the REE as the concentration is increased; slgpg, the blue
Shift of the emission spectrum as the temperature is low-

ered, and seventh, the blue Shift of the absorption spectrum

106

 

 

 

 

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11l1111|1111|1111l11111
450 500 55a 600 650

—Wavelength (nm)—>

Figure 29. Fluorescence Spectra of pyridine merocyanine dye
in PVA (polyvinyl alcohol) thin film at room
temperature as a function of excitation wave—

lengths.

107

 

 

 

 

 

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Figure 30. Fluorescence spectra of (<10'5 M) pyridine mero—

cyanine dye in ethanol at different tempera-
tures. (The excitation wavelength was the ab—

sorption maximum at each temperature.)

108

 

 

 

 

 

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—Wovelength (nm)—>

Fluorescence spectra of a dilute solution of
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at different temperatures. (The excitation
wavelength was the absorption maximum at each

temperature.)

109

 

 

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Wavelength (nm)

Figure 32. Absorption spectra of pyridine merocyanine dye
in ethanol («:10-5 M) as a function of tempera-
tures. The temperature in °C is indicated on
each spectrum.

110

 

Absorbonce

 

0.0 -

 

 

1 11 1 11 11 1 11 1] 1.11 1J 1e11 1

450 500 550 600
--Wovelength (nm)—-

 

Figure 33. Absorption spectra of pyridine merocyanine dye
in PVA (polyvinyl alcohol) thin film as a func-
tion of temperature.

\

111

as the temperature is lowered.

In order to interpret most of these observations we79
assume a statistical distribution of the solute among dif-
ferent solvation sites. Thus, even in a homogeneous con-
densed phase the polar solute molecules are expected to
occupy a variety of solvation sites at any given time giving
rise to different absorption energies corresponding to the
same electronic transitions. A qualitative potential energy
diagram representing the interaction of the dye molecule in
its ground and excited states with a particular solvation
site a is shown in Figure (3h). The interaction energy is
assumed to depend on two coordinates, one representing the
orientation of the polar solvent molecules with respect to
the solute dipole symbolized by e. The other is a transla-
tional coordinate, R, representing the intermolecular
separation between the solute and the solvent molecules.

The dependence of the interaction energy with respect to

6 and R if; drawn in two perpendicular planes. In the
excited state, the magnitude of the interaction energy of
the equilibrium configuration is smaller than that of the
ground state reflecting a decrease in the dipole moment of
the solute upon excitation. Excitation results in a sudden
decrease of the solute dipole moment giving rise to a
strained Franck-Condon state which will relax along the
translational coordinate, R, to the equilibrium excited-

state configuration followed by emission. Emission leads

112

 

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/
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;/3}.'5 c
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Figure 3“. Ground and excited state potential energy
curves for a molecular system in which the
dipole moment decreases as a result of excita-
tion. Solvation sites, a, b, c and d differ
in the orientation of the solvent molecules,
6, but are assumed to have the same inter-
molecular separation R for simplicity. Solva-
tion site a represents the most stable orien-
tation.

113

to a sudden increase in the dipole moment of the solute
giving rise to a strained Franck-Condon ground state which
will relax along R to the equilibrium ground state. The
solute may occupy different solvation sites differing in
the orientation of the solvent molecules, 0, as well as the
intermolecular separation R. In Figure (3“), we have repre—
sented different solvation sites a, b, c and d having dif-
ferent 6's and the same R for simplicity. Solvation site a
represents the most favorable orientation with the largest
interaction energy while site d represents the least favor-
able orientation. In a fluid medium the lifetime of the
various solvation sites is short compared to the excited-
state lifetime and a dynamic equilibrium exist. One may
think of viscosity-dependent barriers between these sites“3
the heights of which depend on the rigidity of the medium.
Excitation of the solute dye molecule in site d will lead
to a fast relaxation along the R coordinate as well as the
a coordinate and emission originates from the equilibrium
excited-state configuration corresponding to the most
stable solvation site, i;e;, site a. Emission will occur
at the same wavelength independent of the excitation wave-
length. This accounts for the lack of REE in fluid media
represented by the first observation where a dynamic
equilibrium exists among the various solvation sites.

If the medium is made rigid so that the relaxation rate

constant along 6 and R is smaller than the fluorescence

11“

rate constant, emission will occur from a Franck-Condon
excited state corresponding to the solvation site specific-
ally excited. In rigid media the dynamic equilibrium
between different solvation sites is lost, and excitation
of the solute in a given solvation site will lead to an
emission of the solute in that solvation site. In this
case, an excitation wavelength dependence of the fluores-
cence energy results, i.e., a REE is observed and is ex-
perimentally observed in Figures (21-2u).

The observation of shoulders in the emission spectra
as shown in Figures (21-23 and 25) as the medium becomes
rigid and as the alcohol chain becomes longer is explained
in terms of inhomogeneous broadening and is similar to the
Shpol'skii effect.80 The inhomogeneous broadening results
from the statistical distribution of solute molecules among
the different solvation sites that become static in rigid
media. The presence of different local environments gives
rise to the excitation wavelength dependence of the emis-
sion and the appearance of shoulders in the emission
spectra particularly in long—chain alcohols.

The appearance of REE at room temperature in PVA
polymer film, Figure (29) is explained in terms of a
matrix that is rigid enough at room temperature to pre—
vent complete equilibrium between different solvation

sites. However the REE will be less than that observed

at 77 K (Figure 24).

115

The decrease in the emission dependence upon excitation
wavelength at higher concentration as shown in Figure (28)
is anticipated as a consequence of singlet-singlet non-
radiative transfer from molecular subclasses of higher
transition energies to those of lower energy emissions.

As a result, emission obtained by excitation into the middle
of the chromophore absorption band becomes red—shifted at
higher concentrations, which decreases the observed excit-
ing wavelength dependence. In the high concentration limit
it is expected that only long wavelength emission (cor—
responding to solvated dye molecules with lowest transition
energies) would be observed at all exciting wavelengths.

The blue shift of the emission spectrum upon lowering
the temperature (Figures 30 and 31) is explained in terms
of the relaxation rates of the solvent. In fluid media,
solvent-shell relaxation around solute molecules occurs
fast enough before chromophore emission, lifiia emission
will occur from an equilibrium excited state configuration.
In a rigid media, solute molecules emit radiation long
before solvent cage relaxation, i;g;,from a non-equilibrium
excited state configuration, the strained Franck-Condon
state. Hence emission will be blue shifted relative to the
room temperature emission. The lifetime of the excited
solute and the relaxation time of the solvent cage become
comparable at intermediate temperatures, iggg, at inter-

mediate viscosities. Hence, emission will occur from

116

intermediate states which will be red shifted relative to
emission in rigid media, or blue shifted relative to room
temperature (fluid) emission. The blue shift of the ab-
sorption at low temperatures (Figures 32 and 33) is explained
in terms of temperature—dependent inhomogeneous spectral
broadening. Lowering the temperature will change the
statistical distribution in favor of solute molecules
occupying the most stable solvation sites which absorb

at shorter wavelengths. This leads to an apparent blue—

shift of the absorption as one lowers the temperature.

B. Benzothiazole Merocyanine

S
4.
\ o“
I
CH3
This system undergoes a decrease in dipole moment upon
excitation. We have prepared this particular merocyanine
dye and have measured its absorption and emission in sol-
vents of different polarities. Its absorption spectra
was already measured by McRae.10
Like that of pyridine merocyanine, its absorption spec-

trum is pH dependent and it probably undergoes cis—trans iso-

merization. The longest wavelength absorption band of the

117

dye in aqueous solution occurs at u95rm1in basic media. The
acidic form of the dye absorb at “10 nm in H2O. Both forms
are present at intermediate pH values. We have observed
the disappearance of the color of the dye solutions after
leaving them overnight, and another absorption at 335 nm
was obtained. Adding acid generates the “10 nm band absorp-
tion which gives the “95 nm absorption when excess base
was added.

Our experiments show that the 335 nm absorption band
is a.result of chemical reaction of the dye with the base.
Private communication with Professor Reusch106 helped in
the elucidation of the basic hydrolysis of the dye at the

C-S bond:

Scheme I in water

 

Acid Base

0" 1410 nn

 

118

Scheme II

0H

acid 8 £2.)

 

u\ "—

CH

3 335 nm

The photochemical reactions that occur in this system
made it difficult to study its emission and optical properties
Since many authors have worked with this dye, it seems im-

portant to report our findings especially because similar

dyes are used in membrane probe studies in aqueous media.

119

II. Molecular Systems That Undergo a Large Increase
in Dipolg Moment as a Result of Electronic Excita-

tion

As an example of these molecular systems we have studied
the absorption, emission and excitation wavelength de-
pendence of the fluorescence of 2-amino-7-nitrofluorene
(ANF) and A,A'-amino-nitrodiphenyl (AND). These molecules
undergo a dipole moment increase92 by 18D and 12D, res-

pectively upon electronic excitation.

A. 2-Amino—7-nitrof1uorene (ANF)

ANF

The room temperature absorption spectra and emission
maxima of ANF in solvents of different polarities
are shown in Figure (15) and Table (II).92’96’107
The absorption spectrum of ANF exhibits a red shift in
media of increasing polarity. The absorption maximum
in 3MP (3-methylpentane) occurs at 365 nm while in
ethanol the maximum occurs at 398 nm, a red shift of
22300 cm-1. The room temperature fluorescence spectra

)92,96,l07

in Table (II show a much larger red shift as

120
the solvent polarity is increased. It undergoes =6000 cm-l
red shift in going from benzene to methanol. These observa-
tions are consistent with a dipole moment increase in the
excited state.

Earlier investigationscfl7ANF include the work of

92,107-109 who has made observations of the

Lippert, et al.
fluorescence shift as a function of temperature and sol-
vent mixture. Their conclusions were that in viscous

media (or alcoholic solvents at low temperatures) the
fluorescence spectrum undergoes a blue-shift because the
rate constant of dielectric relaxation becomes comparable

to the fluorescence decay time. Thus, the fluorescence
originates from progressively less relaxed states as molecu-
lar motion is inhibited. The work of Topp, et al.96’110’lll
indicatestflufi:the shape of the spectra and the fluores-
cence quantum yields also depend on the solvent indicating
the presence of some dynamic processes which are in the
picosecond range. Thus, they used special techniques to
obtain the fluorescence time profiles and emission spectra
of the molecule in different states of solvation. This

is similar to the nanosecond time resolved spectroscopy

technique developed by Ware, et al.“8 for ANS and has been

d96’llo’lll that the

discussed in Chapter II. They showe
Stokes'shift of ANF can be time—resolved in a picosecond
laser experiment andgfives information about the rate of

solvent polarization by the excited state dipole. They

121

have shown that this rate is faster in both 2-propanol
and 0—dichlorobenzene than the calculated T2 relaxation
time measured by microwave absorption. They also measured
the time profile at different places in the fluorescence
spectrum of ANF by varying the relative concentration of
benzene and 2-propanol in mixed solutions. The delay pro-
files measured at U90 and 680 nm were found to be exponen-
tial. What they found also is that in hydrogen bonded
solvents such as 2-propanol, the rate of dielectric polariza-
tion is fast enough that at room temperature they could
not resolve the shift from their pulse duration. However,
when 2-propanol was diluted by benzene the rate of the
shift decreased proportionately and the extent of the shift
could no longertmeexplained by thetnflUcdielectric properties
of the solvent. In conclusion, they showed that the en-
vironmental Franck-Condon relaxation is complete before
relaxation decay occurs, the differential in rates being
about one order of magnitude. They proved that the actual
position of the fluorescence spectrum observed by time-
integrated measurements does not result from a competition
between the two processes as suggested by Lippert.l

The room temperature fluorescence of fluid dilute
solutions of ANF in different polar solvents is hardly
fluorescent. Our excitation wavelength dependence of the
fluorescence was studied in rigid media at 77 K and in

polymer matrices at 77 K and room temperature. Under

122

these circumstances we have observed a large red edge
effect (REE) for ANF. The fluorescence spectra of dilute
solutions of ANF in ethanol, PVA (poly(vinyl)alcohol)

film and polystyrene film at 77 K as a function of excita-
tion wavelength are shown in Figures (35—37), respectively.

A large red shift of 2900 cm-1 in ethanol, 21300 cm'1 in

1 in polystyrene film was observed

PVA film and 21600 cm-
as the excitation wavelength was increased from the ab—
sorption maximum at 77 K (the number at the top left of
figures cited) to the red edge of the absorption band (the
number at the top right of figures cited). Such observa-
tions were accompanied by the appearance of shoulders in
the emission spectra as the excitation wavelength was moved
to the red. As in the case of pyridine merocyanine dye,
the magnitude of this REE is viscosity dependent. It is

decreased by warming up the solutions and it disappears

in fluid media at room temperature. The magnitude of the

l at room temperature compared

REE in PVA film is z600 cm-
to z1300 cm.1 at 77 K. Similarly the REE in polystyrene
film at room temperature is 2850 cm_1 compared to 21600
cm-1 at 77 K. These observations are illustrated in
Figures (38 and 39) respectively.

The absorption spectra of ANF in different polar sol-
vents undergo red shifts when the temperature is reduced.

Figure (A0) shows the absorption spectra of dilute solu-

tion of ANF in ethanol at different temperatures. A

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90— E
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Fluorescence Intensity

0.0 -

 

 

 

 

Wavelength (nm)—>

Figure 39. Fluorescence spectra of ANF in polystyrene thin
film at room temperature as a function of ex-
citation wavelengths.

128

 

09

Absorbance

 

 

 

 

l I l I l
350 400 450 500 550
—Wavelenglh (nm)—->

Figure H0. Absorption spectra of ANF in ethanol (3 x 10-5 M)
as a function of temperature: (1) 24°C, (2) 0°C,
(3) -36°C, (A) -78°C, (5) —1ou°c, (6) -115°c.

129

red shift of 21500 cm—1 was observed upon lowering the
temperature to z-115°C.

So far we have made the following experimental ob-
servations: the absence of a REE in fluid media, a large
REE in rigid solvents at 77 K and in polymer matrices at
room temperature and 77 K, the appearance of shoulders in
the emission spectra when exciting at the red edge, and
the red shift of the absorption spectrum as the temperature
is lowered.

In comparing these observations with those in the case
of pyridine merocyanine dye, one should recall that for ANF
the dipole moment is increased in the excited state. A
statistical distribution of the solute among different sol-
vation sites is assumed. Thus even in a homogeneous con-
densed phase, the polar ANF molecules are expected to
occupy a variety of solvation sites at any given time,
giving rise to different absorption energies corresponding
to the same electronic transition. A qualitative potential
energy diagram representing the interaction of the ANF
molecule in its ground and excited states with a particu-
lar solvation site a is shown in Figure (Al).

In the excited state, the magnitude of the interaction
energy of the equilibrium configuration is greater than
that of the ground state,reflecting an increase in the
dipole moment of the solute upon excitation. Excitation

results in a sudden increase of the solute dipole moment,

130

 

 

 

 

Figure Al.

 

 

 

 

 

 

 

 

Ground and excited state potential energy curves
for a molecular system in which the dipole
moment increases as a result of excitation.
Salvation sites, a, b, c and d differ in the
orientation of the solvent molecules, 6, but

are assumed to have the same intermolecular
separation R for simplicity. Salvation site

a represents the most stable orientation.

131

giving rise to a strained Franck-Condon state which will
relax along the translational coordinate, R, to the equilib-
rium excited state configuration followed by emission.
Emission leads to a sudden decrease in the dipole moment

of the solute giving rise to a Strained Franck-Condon
ground state which will relax along R to the equilibrium
ground state. The solute may occupy different salvation
sites differing in the orientation of the solvent mole-
cules, 6, as well as the intermolecular separation R.

In Figure (“1) we represented different salvation sites

a, b, c, and d having different 6's and the same R for
simplicity. Solvation site a represents the most favorable
orientation with the largest interaction energy and the
least transition energy while site d represents the least
favorable orientation with the highest transition energy.
In fluid medium the lifetime of the various solvation sites
is short compared to the excited-state lifetime and a
dynamic equilibrium exists. One may think of a viscosity-
dependent barrieruu’u5 between these sites, the height of
which depends on the rigidity of the medium. Excitation of
the solute molecule in site d will lead to a fast relaxation
along the R coordinates as well as the e coordinate and
emission originatingiflwmithe equilibrium excited—state
configuration corresponding to the most stable solvation
site, 143;, site a. Emission will occur at the same wave-

length independent of the excitation wavelength, i.e., no

132

REE is observed in fluid media. If the medium is made rigid
such that the relaxation rate constant along 6 and R is
smaller than the fluorescence rate constant, emission will
occur from a Franck-Condon excited state corresponding to
the salvation site specifically excited. In rigid media

the dynamic equilibrium between different salvation sites is
lost, and excitation of the solute in a given salvation site
will lead to an emission of the solute in that salvation
site. In this case, an excitation wavelength dependence

of the fluorescence energy results, ELEL’ a REE is expected
and is experimentally observed, of. Figures (35,37).

The observation of shoulders in the emission spectra
as shown in Figure (35) as the medium becomes rigid and
as the wavelength of excitation move to the red are ex-
plained in terms of inhomogeneous spectral broadening which
results from the statistical distribution of solute mole-
cules among the different solvation sites that become static
in rigid media.

The appearance of REE at room temperature in PVA poly-
mer film and polystyrene film of Figures (38 and 39) is
explained in terms of matrices that are rigid enough at
room temperature to prevent complete equilibrium between
different salvation sites. However, the REE is less than
that observed at 77 K (Figures 36,37).

The red shift of the absorption at low temperatures

(Figure U0) is explained in terms of temperature—dependent

133

inhomogeneous spectral broadening. Lowering the temperature
will change the statistical distribution in favor of solute
molecules occupying the most stable solvation site which ab-
sorb at longer wavelengths. This leads to an apparent red-

shift of the absorption as one lowers the temperature.

B. MLA'-Amino Nitro Diphenyl (AND)

 

"2" N02

The room temperature absorption spectra of AND in differ-
ent solvents are shown in Figure (16). Its absorption ex-
hibits a red shift as the case of ANF in media of increasing
polarity. The absorption maximum in 3MP (3-methyl—pentane)
occurs at 339 nm while in ethanol the maximum occurs at
377 nm, a red shift of 33000 cm-1. The absorption spectra of
AND in different solvents are blue shifted relative to the
ANF absorption spectra in the same solvents as shown in
Figures (16 and 15), respectively. Such observations are ex-
pected from ground state geometries of these molecules. Con-
jugation and hence more W-interaction is expected in the
ground state of ANF relative to AND because of the carbon
bridge in the former making it more planar. Thus ANF ab-
sorbs at lower energy than AND in different solvents.

In the excited state the two phenyl groups of AND will
be at a smaller angle relative to each other and the Stokes

shift of the emission is expected to be large even in

13M

hydrocarbon solvents. However, the excited state dipole
moment is increased and a large red shift in the emission
is expected to occur as the polarity of the solvent is
increased. Inspection of Figures (15 and 16) reveals

the absorption spectrum of ANF is more resolved than

that of AND. This is consistent with a planar rigid ANF
and a flexible AND molecule.

We have observed a large REE for AND in different polar
solvents at 77 K as well as in polymer matrices at room
temperature and at 77 K.

The fluorescence spectra of dilute solutions of AND
in ethanol, l-propanol, PVA polymer film and polystyrene
polymer film at 77 K as a function of excitation wave-
length are shown in Figures (A2-A5), respectively. A
large red shift of 21200 cm.1 in ethanol, 2750 cm-1 in l-
propanol, zlllOO cm'1 in PVA film and 21300 cm“1 in poly-
styrene film were observed as the excitation wavelengths
were increased from the absorption maximum at 77 K (the
number at top left of figures cited) to the red edge of the
absorption band (the number at the top right of figures
cited). Such observations are accompanied by the appear-
ance of shoulders in the emission spectra in ethanol as the
excitation wavelengths were moved to the red or by changing
the solvent to l-propanol. As is the case of ANF,the magni—
tude of these REE are viscosity dependent. At -l26°C in

ethanol, the REE is z200 cm—1 as shown in Figure (U6).

135

 

 

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1U0

The valuescfi‘AND REE's in PVA film and polystyrene film
at room temperature are :500 cm"1 and 21400 cm'l, respec-
tively. Their corresponding spectra are shown in Figures
(A7 and U8).

The absorption spectra of AND in different polar solvents
undergo red shifts when the temperature is reduced. Figure
(A9) shows the absorption spectra of a dilute solution of
AND in ethanol at different temperatures. A red shift of
21500 cm.1 was observed upon lowering the temperature to
-l20°C.

Since AND undergoes an increase in dipole moment upon
excitation and also has the same functional groups as ANF,
one can explain the above experimental observations as in

the case of ANF.

III. Flexible Molecules That May Undergo a Change in

 

Equilibrium Geometric Configuration Upon

Excitation

 

This kind of molecular system is exemplified by the
methyl and tertbutylesters of 9-anthroic acid I and II
whose ground and excited state geometries were studied

earlier.97

Their room temperature absorption and emis-
sion spectra together with their emission spectra at 77 K
in 3MP (3-methylpentane),are shown in Figures (17 and 18)

respectively. It was concluded that the ground state

1A1

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Fluorescence Intensity
I

 

Acne. (um) d

\400

470
490

\

 

I 1 1 1 1 .I 1 1 L41 I 1 1i 1 L I LL 1 1 1 I

 

Figure M8.

450 500 550 600 650
—fWavelength (nm)—->

Fluorescence spectra of AND in polystyrene thin
film at room temperature as a function of ex-
citation wavelengths.

1A3

 

-—Absorbance —-—-

 

 

 

 

0'0 - I J I J I
250 300 350 400 450 500 550
-——Wavelength (nm)—->

 

-A
Figure “9. Absorption spectra of AND in ethanol (~10 M)

as a function of temperature. (1) 2A°C, —
(2) —23°C, (3) -86°C, (A) -120°c.

iuu

geometries of these molecules are similar to each other

 

9-Methyl Anthroate (9MA) 9-Tertbuty1 Anthroate (9TBA)

with the carboxyl group out of plane, perhaps of 90°

with respect to the anthracene ring, indicating no inter-
action between the two moieties.112 A greater interaction
between the carboxyl group and the ring in the excited
state is expected. Such interactions depend on the size
of the ester group, viscosity, structure and polarity of
the solvent matrix. While a near-coplanar configuration
between the carboxyl group and the ring in the excited
state of 9MA is expected, a less planar configuration

is expected in the excited state of 9TBA. It was shown97
that 9MA relaxes to its equilibrium excited state con-
figuration even in a rigid glass hydrocarbon (3MP), while

9TBA does not relax under the same conditions. The

1A5

branched tertbutyl group is interlocked in the rigid sol—
vent matrix while the methyl ester group can undergo some
relaxation in the same solvent matrix. It should be noted
that rigid parafin oil at -80°C does not prevent excited
state relaxation of 9-anthroic esters. The structure of the
solvent matrix around the anthroic ester molecule is such
that there is enough free volume for the ester group to
relax during the excited state lifetime.

An excitation wavelength dependence of the fluorescence
was carried out for 9MA and 9TBA solutions in a 3MP matrix
element at 77 K. Figure (50) shows a red shift of $600

cm"1 when the excitation wavelength was increased from

385 nm to the far red edge (2397 nm) of the first absorp-
tion band of 9TBA in 3MP. Exciting at the far red edge

is accompanied with a highly resolved fluorescence spec-
trum. The shifts observed in the case of 9MA in 3MP matrix
were very small. As Figure (51) shows, a red shift of only

2200 cm’1

was observed when exciting at the far edge of
the first absorption band of 9MA. Such shift was not ac-
companied by more resolution as in the case of 9TBA.

The above experimental observations were interpreted
in terms of a red edge effect. The flexibility of these
molecules allow the presence of different conformers which
are unstable in fluid media. Upon freezing the solution,

some of these conformers are trapped in certain configura-

tions. Their lifetimes in the excited state will be

146

 

 

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1 i 1 1 1 1 I 1 4 1 1 I 1 1 111 J 1 1 1 1 l

400 450 500 550 600
—Wavelength (nm)—>

Figure 50. Fluorescence spectra of (7 x 10"5 M) 9TBA in
3MP at 77 K as a function of excitation wave-
lengths. (Spectrum # l correspondstx>365 nm
excitation and #u correspondstx>397 nm excita-
tion.)

 

1H7

 

I l I I I
90- E
S on
- zxz§

E
m -
if»
S I-
d.)
8 -
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U
(D
g -
E _

0.0-

1 I 1 1 1 1 1 1 I 1 1 1 1 1 1 I 1

 

 

 

1 l 1 1 I 1
400 450 500 550 600
+Wawlength (nm)—>

Figure 51. Fluorescence spectra of (7 x 10"5 M) 9MA in 3MP
at 77K as functions of excitation wavelength.

1&8

dependent on the degree of the medium rigidity as well as
the size of the ester group.

Different conformers of 9TBA are being trapped in
certain configurations which can't relax in their excited
state, lLQL, they have a long lifetime compared to the
fluorescence life time. The large tertbutyl ester group
does not have enough free volume to rotate in the solid

“3 "if a

matrix. As proposed by Dellinger and Kasha,
molecule upon excitation involves a rather large distor-
tional motion for equilibrium relaxation, then a large
volume of solvent in the cage around the solute must be
displaced". 0ur interpretation is similar to the "Shpol'skii
Effect"80 where heterogeneity in the local environment of
rigid matrices gives rise to solute molecules absorbing at
slightly different energies.

Irradiation at the red edge of the first absorption
band excites a subclass of molecules (conformers) which
are more planar and hence absorb at lower energy than other
solute molecules. The emission from this subclass will
be red shifted relative to the bulk emission. Selective
excitation produces a highly resolved emission spectrum
result. However, the extent of resolution of the emis-
sion spectrum of 9TBA in 3MP at 77 K did not change

regularly with excitation wavelength. Careful inspec-

tion of Figure (50) shows that the emission spectrum

1A9

excited at 390 nm was less resolved than the emission
spectrum excited at 385 nm. More detailed results
shown in Figure (52) confirm this observation.

One must point out that it is only in the case of
red-edge excitation that we select certain conformers that
absorb at that energy. At shorter wavelengths one will
excite a mixture of conformers, and the proportion in the

mixture varies irregularly with excitation wavelength.

150

 

 

 

 

 

 

1 IA14J 1 II 1

l 1 I 1 I l 1 l
360 380 4CD 420 440 460 480 500
—Wavelenglh (nm)—>

 

Figure 52. Variation of fluorescence intensity and resolu-'

tion of vibronic band of (7 x 10"5 M) 9TBA in
3MP at 77 K at different excitation wavelengths
which are: (l) 350 nm, (2) 355 nm, (3) 360 nm,

(A) 365 nm, (5) 370 nm, (6) 375 nm, (7) 383 nm,
(8) 386 nm.

CHAPTER VI

EXPERIMENTAL

A. Materials

 

I. Purification of Solvents

 

l. Ethanol:

200 proof ethanol was fractionally distilled at a rate
of five drops per minute. Portions of about 50 ml were
collected and the absorption spectrum taken in a 10 cm cell
to check for benzene. Distillation was continued until the
characteristic benzene UV absorption was no longer ap-

parent. Ethanol was then distilled and used as needed.

2. 3—methyl Pentane (3MP)

 

The Philips pure grade 3MP was mixed with nitric acid
and sulfuric acid, then stirred for two days. Then it was
separated and washed with base and water. After drying
with sodium sulfate for one night, the solution was refluxed
over sodium wires for two days and distilled. The vapor
passed through a four foot vacuum jacketed column and

condensed at a speed of two drops per minute. The purity

151

152

was checked by obtaining the absorption spectrum.

3. Water was doubly distilled in our laboratory.

A. l-Propanol, 1-butanol,gglycerin and DMSO

(Dimethyl Sulfoxide) - from Fisher Scientific Company

and chloroform from Mallinckrodt were used without further

purification.

5. Paraffin Oil from Mc/B, Piperidine and Polystyrene
from Aldrich Chemical Company, l-hexanol from Eastman and
Poly(vinylalcohol) [PVA] from Polyscience, Inc. (Cat. No.
2815, 99-99.8 Mole %) were also used without further puri-

fication.

II. Preparation and Purification of Chemical Compounds
1. Merocyanine Dyes:

l—Methyl-U—hydroxystyryl)peridinium betaine whose struc-

ture is

153

was prepared using the general procedure described by

lOO and others.lou’ll3 The merocyanine was puri-

Brooker
fied by repeated recrystallization in methanol and dried
under vacuum. The benzothiazol merocyanine dye was syn-
thesized in the same general procedure used by BrookerlOO

and described in detail by Kampas.98

2. ANF(2-Amino—7-Nitro Fluorene)

 

ANF was obtained from K & K Laboratories, Inc. and was

purified by repeated recrystallization in ethanol.

3. AND (A—Amino-U'-Nitro Diphegyl)

 

AND was obtained from K & K Laboratories, Inc. and was

purified by repeated recrystallization in ethanol.

A. 9MA and 9TBA (Methyl and Tertbutyl Esters of 9-

 

Anthroic-Acid)

9MA and 9TBA were prepared from the purified 9-anthroic

11A

acid according to the method of Parish and Stock and

purified by repeated recrystallization from ethanol.

15“

III. Preparation of Polymer Films

 

1. Poly(vinylalcohol) PVA Films

 

PVA (molecular weight l33,000-99% mole hydrolyzed
Polyscience Cat. #2815) was combined with water in a ratio
of 0.125 gram/ml. The mixture was heated over a steam
bath and was stirred occasionally (to prevent dehydration
at the surface for 20 minutes. The solution was then per-
mitted to cool to room temperature in a moist environment
(a desiccator with water in it works well). The dye solu-

tion (1 ml of 10'“

M in ethanol for each 1.5 gm PVA) was
added to the cool PVA water solution, mixed very well, and
put back in a moistened environment for about one hour.
The dye solution in PVA was then transferred into a small
flexible centrifuge tube and centrifuged at the speed of
15,000 round per minute for about 10-15 minutes (this will
remove any dehydrated polymer). The dye solution was
then poured into clean, dry molds (molds were prepared

by pouring melted paraffin in glass dishes to a depth of

8 mm. After solidification, A cm x A cm squares were cut
out with a razor blade to form the molds). The films were
then dried by directing air at them at a very slow rate

for about 70 hours at room temperature. Then the films

were removed from the molds and the needed measurements

made.

155

2. Polystyrene Films

A few billets of polystyrene were dissolved in the chloro-
form solution of the dye. The contents were then poured
into a flat dish and let to dry slowly in a clean atmos-
phere. These films were very thin and had an absorbance

of about 0.1.

B. Spectral Measurements:
1. Absorption Spectra
All reported absorption spectra were run on Cary 15

and Cary 17 spectrophotometers.

2. Emission Spectra

 

Fluorescence spectra were recorded on a multicomponent
system used in this laboratory consisting of a 500 W Xenon
light source, 500 mm Bausch & Lomb excitation monochromator
(which provides a narrow excitation band width), McPherson
Model 235 Emission Monochromator and EMI Model 9558QB
Phototube. Noise reduction and amplification of the PMT
signal is achieved by using a Princeton Applied Research
Model HR-8-LOCK IN Amplifier and appropriate chopping ap-
paratus. Some of the room temperature emission spectra
were recorded by using Aminco—Keirsard.Aminco-Bowman

spectrofluorometers. The phosphorescence spectra were

156

obtained with these instruments equipped with a rotating

can phosphoroscope.

3. Temperature Variation System

 

A quartz dewar with flat quartz excitation and emission
windows and a narrow glass tube for the samples were used.
(A 0.1 cm absorption cell connected to a long pyrex tube
was used for low temperature absorption). The temperature
of the sample was controlled by boiling liquid nitrogen
using a power resistor and allowing the N2 gas to flow into
the sample dewar. The temperature of the sample was moni-
tored through a thermocouple (copper, constantan) dipped
inside the solution in the sample tube immediately above
the point of excitation or absorption. The lowest stable
temperature which can be reached by such a system is around
-l60°C. Low temperature absorption or emission studies
with polymer films were performed by hanging the films
vertically in the dewar through a wire and a glass tube.
For 77 K measurements the films were dipped directly in

the liquid nitrogen.

CHAPTER VII

CONCLUSION AND FUTURE WORK

One of the interesting applications of merocyanine
dyes in particular and dyes in general is their extensive
use in biology and photography. In biology, merocyanines have
been found to be useful in measuring membrane potentials.115
Membrane potential sensitive dyes have been useful for
studying rapid electrical activity in single nerve and
muscle cells, in collections of individual cell bodies and in
single cells in tissue culture. Dyes have also been used
to determine changes in the membrane potential level of
cells, organelles and vesicles in suspension and of single
cells in conjunction with fluorescence activated cell sort-

116 used with lipid

ers. Merocyanines have been recently
bilayer membranes in which the trans-membrane potential can
be rapidly and accurately controlled. Potential-dependent
changes in the fluorescence and absorption spectra of these
dyes were obtained along with the polarization dependence

117,118

of these properties. In photography a stable posi-

tive image which is not thermally erasable below 70-80°C

119

was produced from merocyanines when irradiation of a

recording paper took place. It leads to the reprography

157

158

of transparent documents and also produces continuous tone
printing. This process requires no subsequent development,
is dry, is an autoprocessor and can be realized with a
relatively simple technology.

Besides their importance in measuring membrane poten-
tials and this application in photography, these dyes
(merocyanines, ANF and AND) could be used as probes to
study the dynamic behavior of liquids and could be po-
tentially used in photography storage.

Since these molecular systems (Chapter IV) are sensi-
tive to polarity and structure of the solvent found from
their absorption, emission and the significant role of REE,
they could be used to further investigate the following
important phenomena in the previous chapter: Shpol'skii
effect, site selection spectroscopy, inhomogeneous spectral
broadening, time dependent emission spectroscopy and viscous
flow barriers.

Some of the studies that could be suggested are now

summarized.

I. Pyridine Merocyanine:

 

l. The room temperature emission Spectra of pyridine
merocyanine undergoes a gradual red shift upon increasing
its concentration in ethanol. Such observations in other

120-123

systems were interpreted in terms of energy migra—

tion between molecules whose energy levels differ in position

159

as a result of inhomogeneous spectral broadening. Careful

investigation of the concentration effect in the pyridine

merocyanine dye is needed.

2. The REE phenomenon we observed requires further
study. Thus polarization experiments at different excita-
tion wavelengths at different temperatures in different
solvents may reveal polarization enhancement at red edge
excitation. Lifetime measurements in the picosecond range
could be performed at room temperatures in different sol-
vents and at different excitation wavelengths at low tem-
peratures should be made. Excitation wavelength dependence
could be performed using a tunable dye laser which will
enable us to select solute molecules with the same sol-

vent environment.

II. ANF and AND

 

l. The suggested experimental investigations of

pyridine merocyanine should be extended to these dyes.

2. We have observed a large REE in the phosphores-
cence spectra of these molecules in paraffin oil, poly-
styrene and polyvinyl alcohol at low temperatures. De-
tailed steady state and time-resolved study of the solva-
tion of the solute in its triplet state would be very

interesting.

160

III. 9MA and 9TBA

 

It would be very interesting to study the lifetimes
of 9MA and 9TBA in hydrocarbon at 77 K using different

excitation wavelengths.

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