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This is to certify that the

dissertation entitled

Ultrafast Stimulated£Spectroscoby Studies of
Vibrational Relaxation and Short Range
Solvent Organization in Organic Solutions

presented by

Ying Jiang

has been accepted towards fulfillment
of the requirements for

Ph . D degree in Chemis t ry

 

 

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ULTRAFAST STMULATED SPECTROSCOPY STUDIES OF VIBRATIONAL
RELAXATION AND SHORT RANGE SOLVENT ORGANIZATION IN ORGANIC
' SOLUTIONS

by

YING JIANG

A DISSERTATION

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

DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMISTRY

1995

ABSTRACT

ULTRAFAST STIMULATED SPECTROSCOPY STUDIES OF VIBRATIONAL
RELAXATION AND SHORT RANGE SOLVENT ORGANIZATION IN ORGANIC
SOLUTIONS '

By

YING JIANG

Understanding solute-solvent molecular interactions and local solvent organization are of
great importance for chemical reactions because the solution phase is the most widely
used medium for chemical reactions. In solution, however, molecules are not isolated
from one another like in the gas phase and are not spatially fixed like in the solid phase.
Chemically important events take place on the femtosecond and picosecond time scale,
and these processes depend sensitively on intermolecular interactions. Organization and
intermolecular interactions are, at present, not well understood in the liquids. This thesis
focuses on understanding solute-solvent interactions and local solvent organization
through studies of the vibrational energy relaxation and rotational diffusion dynamics of
fluorescent probe molecules. Subsequent to excitation and any optical emission, virtually
all of the excess of energy in the system is dissipated as heat or vibrational energy. The

7‘!

redistribution of this excess vibrational energy into surrounding molecules depends

crucially on the environment surrounding the probe molecule. A novel laser technique has
been developed to study vibrational energy relaxation dynamics in dilute solutions. A
pump—probe strategy is used to monitor the stimulated emission response of probe
molecule in dilute solutions. The measurement scheme can be modeled as a coupled three
level system, where the levels are the vibrationless electronic ground state, the vibrational
state of interest in electronic ground state and the vibrationless electronic excited state.
The pump laser is operated at the frequency of 0-0 transition, and the probe laser is
operated at a frequency corresponding to the difference between the pump laser and the
vibrational state of interest. The stimulated response is S(t) = - a exp(-t/T1) + b exp(-
t/Tclcc). The vibrational population relaxation times, T1, of four perylene vibrational modes
were measured in both polar and nonpolar solvents using this technique. We found that
T; times range from <10 ps to a few hundred picoseconds and are strongly mode- and
solvent-dependent. Measuring perylene T1 times as a fimction of aliphatic chain length in
a series of normal alkanes revealed the presence of solvent organization on a few A length
scale. Comparison of the vibrational energy relaxation and rotational diffusion dynamics
of perylene and l-methylperylene provides information on the persistent length of the local
solvent organization. For perylene in the n-alkanes, the exchange of vibrational energy
proceeds through in quadrupole-quadrupole interactions. For perylene, T1 relaxation and
rotational diffusion measurements do not correlate in an obvious way. For 1-
methylperylene, the solute-solvent vibrational energy exchange is through dipole-
quadrupole interactions, which operate over a longer range (cc r“) than quadrupole-
quadrupole interactions (at r’7). For l-methylperylene, there is a direct correlation

between T1 and rotational diffusion dynamics.

To China,
where I was born and grew up,
where my parents, sister, brother
and their families are living.

iv

ACKNOWLEDGMENTS

I am immensely grateful to my advisor, Dr. Gary Blanchard, for providing me with an intellectual
and pleasant environment, for the financial support and enlightening advice and inestimable
training. Perhaps the most valuable thing I learned from him is how enjoyable science can be.

I also want to thank my committee members, Dr. Bob Cukier, Dr. Greg Baker and Dr. Jeff Ledford
for their stimulating and helpful discussion through the course of years. My thanks also go to Dr.
Tom Carter, MSU laser lab manager, who helped me with the lasers and the spectroscopy without
reservation. Without the cooperative help of Ron. Tom and Scott in the Electronic shop this thesis
may not have been possible.

It is impossible to separate my thesis work from my colleagues in the group. Selezion Hambir
taught me how to take care of my worries and gave me a lot of help and suggestions for my
research work. It will be a precious memory for me to think of Lee Dewitt, Pat McCarthy, Dave
Karpovich, Jeff Rasimas, Jen Horn and their willingness to share their knowledge, expertise and
life experience (family stories, traveling, wild life in the woods and hunting...) with me.

I am specially indebted to Geurt for his continuous encouragement and support during the
composition of this thesis.

Finally I am grateful for the grant from National Science Foundation that made thesis possible.

TABLE OF CONTENTS

ABSTRACT
LIST OF TABLES
LIST OF FIGURES
Page

CHAPTER 1. INTRODUCTION ............................ 1
CHAPTER 2. EXPERIMENTAL ........................... 11
CHAPTER 3. ULTRAFAST STIMULATED EMISSION SPECTROSCOPY 0F

PERYLENE IN DILUTE SOLUTION - MEASUREMENT OF

GROUND STATE VIBRATIONAL POPULATION

RELAXATION ............................. 18

Summary
3.1. Introduction
3.2. Experimental
3.3. Results and Discussion
3.4. Conclusion
3.5. Literature cited

CHAPTER 4. VIBRATIONAL POPULATION RELAXATION OF PERYLENE IN
ITS GROUND AND EXCITED ELECTRONIC STATES ....... 49

Summary
4.1. Introduction
4.2. Theory
4.3. Experimental
4.4. Results and Discussion
4.5. Conclusion
4.6. Literature cited

CHAPTER 5. VIBRATIONAL POPULATION RELAXATION OF PERYLENE IN n-
ALKANES - THE ROLE OF THE LOCAL SOLVENT ORGANIZATION
IN LONG RANGE VIBRATIONAL ENERGY TRANSFER ...... 68

Summary
5.1. Introduction
5.2. Experimental
5.3. Results and Discussion
5.4. Conclusion
5.5. Literature cited

CHAPTER 6. ROTATIONAL DIFFUSION DYNAMICS OF PERYLENE IN n -
ALKANES - OBSERVATION OF SOLVENT LENGTH DEPENDENT
CHANGE OF BOUNDARY CONDITION ............... 92

Summary
6.1. Introduction
6.2. Background
6.3. Experimental
6.4. Results and Discussion
6.5. Conclusion
6.6. Literature cited

CHAPTER 7. VIBRATIONAL POPULATION AND ORIENTATIONAL
RELAXATION DYNAMICS OF l-METHYLPERYLENE IN n-
ALKANES - THE EFFECTIVE RANGE OF DIPOLAR ENERGY
RELAXATION IN SOLUTION .................... 114

Summary
7.1. Introduction
7.2. Experimental
7.3. Results and Discussion
7.4. Conclusion
7.5. Literature cited

CHAPTER 8. SUMMARY AND FUTURE WORK .................. 147

vii

2.1.

3.1.

3.2.

4.1.

4.2.

5.1.

6.1.

7.1.

7.2.

7.3.

7.4.

LIST OF TABLES

Information for each dye laser: including making of the dye solutions,
operating wavelength range and the mirror sets. ......... ‘ ........ 13

q

The pump, probe wavelengths and probed vibronic region for each solvent ..... 2.:
Build-up and decay times determined from the experimental data. All times

are given in ps. The vibrational frequencies refer to the final state in the

stimulated transition. The spectral origin for each solvent was estimated

from the static spectroscopic data ......................... 39

The pump, probe wavelengths and dyes for each solvent .............. 55

Vibrational population relaxation times for the ground state and excited
state v7 mode of perylene in n-alkane solvents ................... 61

T1 times for the V7 and (v7 + V”) modes of perylene in n-alkanes .......... 78
Experimental zero time anisotropies and reorientation times for perylene
in several n-alkanes. The asterisksindicate an excited electronic state

measurement. ..... q ................. . ............ 99

Spectral origin of l-methylperylene in the solvents used in this work,
determined from the linear optical response .................... 122

T1 relaxation times for the l-methylperylene 1370 cm'1 mode in the
n-alkane solvents .................................. 126

Experimental rotational diffusion time constants and zero-time
anisotropies. All uncertainties are reported as standard deviations (i 10). . ‘. . . 138

Cartesian components of the rotational diffusion constant extracted from
experimental data using Equations 5 ........................ 141

viii

2.1.

3.1.

3.2.

3.4.

3.6.

3.7.

LIST OF FIGURES

Schematic of the stimulated emission pump-probe spectrometer .........

Absorption and emission spectra for perylene in n-octane. The boxed
emission feature indicates the Spectral region over which time resolved

.12

stimulated emission spectra were recorded ..................... 25

Time resolved stimulated emission spectra of perylene in n-octane
over the spectral region indicated in Figure l. (o) = 10 ps delay, (0) = 100 ps,
(V) = 200 ps, (inverted filled triangle) = 300 ps, (Cl)= 400 ps, (I) = 500

ps, (A) = 600 ps, (filled triangle) = 700 ps, (0) = 800 ps, (9) = 900 ps ....... 26

. Time resolved stimulated emission spectra of perylene in l-butanol.

(o) '= 10 ps delay, (0) = 100 ps, (V) = 200 ps, (inverted filled triangle) =
300 ps, (El)= 400 ps, (I) = 500 ps, (A) = 600 ps, (filled triangle) = 700 ps,

(0) = 800 ps, (9) = 900 ps ............................

Time resolved stimulated emission spectra of perylene in l-octanol. (o) =
10 ps delay, (0) = 100 ps, (V) = 200 ps, (inverted filled triangle) = 300 ps,
(El)= 400 ps, (I) = 500 ps, (A) = 600 ps, (filled triangle) = 700 ps, (0) =

.27

800 ps, (0) = 900 ps ................................. 28

. Time resolved stimulated emission Spectra of perylene in DMSO. (0) = 10

ps delay, (0) = 100 ps, (V) = 200 ps, (inverted filled triangle) = 300 ps,
(Cl)= 400 ps, (I) = 500 ps, (A) = 600 ps, (filled triangle) = 700 ps, (0) =

800 ps, (9) = 900 ps ................................ 29

Time resolved stimulated emission spectra of perylene in toluene. (o) = 10
ps delay, (0) = 100 ps, (V) = 200 ps, (inverted filled triangle) = 300 ps,
(D): 400 ps, (I) = 500 ps, (A) = 600 ps, (filled triangle) = 700 ps, (0) =

800 ps, (9) = 900 ps ................................ 3O

Schematic of coupled three level system used to model the experimental

data. The terms k are described in the text ....................

ix

33

3.8.

3.9.

4.1.

4.2.

4.3.

4.4.

4.5.

5.1.

5.2.

LIST OF FIGURES (Continued)

Time scans for perylene in n-octane at probe wavelengths corresponding
to distinct vibronic resonances, where the ground state vibrational level

is indicated Build-up and decay times are given for these data in Table 3.2. . . .

Time scans for different IMW ratio ......................

(a) Schematic of the coupled three-level system used for interpretation of 0-0
excitation experiments. (b) Schematic of the coupled four-level system used for

.38

41

interpretation of excited state T1 measurements .................. 52

Linear optical response of perylene in n-hexane. The absorption and emission -
spectra have been normalized. Arrow “a” and “b” indicate the excitation

wavelengths used for the 0-0 and v7* experiments, respectively .......... 57

Schematic of the exaggerated atomic displacements for the perylene v7 mode.
The directions of the displacements were estimated from semiempirical

calculation results .................................

Experimental stimulated response and laser cross-correlation for measurement
of the ground state v7 mode of perylene in n-hexane. For this experiment,

AP.” = 432 nm and AW” = 462 nm. ........................

(3) Experimental stimulated response for the 0-0 and v7* excitation of perylene.
For these scans the ground state v5 mode of perylene in n-hexane is probed;
A,” = 432 nm for 0-0 excitation and 409 nm for v7* excitation and Ami” = 466
nm for both excitation conditions. See Table 3.1 for the best fit results.

(b) Difference signal, S(t), for the two scans shown in (a), with the best fit

58

6O

fiInction shown as a solid line through the data ................. 62

Absorption and emission Spectra for perylene in n-octane .............. 73

Exaggerated atomic displacements associated with the v7 and v1, normal modes

of perylene ..................................... 74

5.3.

5.4.

5.5.

6.1.

6.2.

6.3.

6.4.

LIST OF FIGURES (Continued)

Stimulated response of the (v-, + V”) mode of perylene in n-octane, presented
with the cross correlation response function .................... 76

T1 relaxation times for v7 (0) and “(v7 + v15) (Cl) modes in perylene as a function
of solvent chain length ............................... 77

Calculated probability, <<P>>, for long-range energy transfer for exact donor-
acceptor resonance, 0) = 0 (Equation 5) and co = 300 cm'l, as a function of
donor-acceptor separation, d ................ ' .......... y . . 85

Normalized absorption and spontaneous emission spectra of perylene
in n-octane ..................................... 98

(a) Time scans for ground state recovery response of perylene in n-octane. The

laser cross-correlation is presented with the time scans for pump and probe
electric-fields polarized parallel (larger AT/T at early time) and perpendicular to

one another. (b) R(t) signal produced from the experimental data shown in (a)

using Equation 3. These data fit best to a single exponential decay functionality,

as indicated by the dashed line .......................... 100

(a) Time scans for ground state recovery response of perylene in n-hexadecane.
The laser cross-correlation is presented with the time scans for pump and probe
electric fields polarized parallel (larger AT/T at early time) and perpendicular
to one another. (b) R(t) signal produced from the experimental data shown in
(a) using Equation 3. These data fit best to a single exponential decay

'fiinctionality, as indicated by the dashed line .................... 101

Ground state and excited state reorientation times for perylene as a function of
n-alkane solvent viscosity. For all measurements the ground state and excited

state reorientation times are the same to within the experimental uncertainty.

See text for a discussion of these data ........................ 103

6.5.

6.6.

7.1.

7.2.

7.3.

7.4.

7.5.

7.6.

7.7.

7.8.

LIST OF FIGURES (Continued)

Illustration of prolate and oblate ellipsoid rotor shape and the axial ratio, p. . . . 105
Dimensions and Cartesian axis assignments for perylene .............. 108
Steady-state absorption and emission spectra of l-methylperylene in n-hexadecane.
The arrow indicates the 0-0 transition energy, and the box the range over which

the stimulated response, shown in Figure 7.3, was taken. . .. . .. . . . . . . . . .121
(a) Infrared and (b) Raman spectra of l-methylperylene. The asterisks indicate

the vibrational resonance for which we determined T1 times ............ 123
The time-resolved stimulated spectra of 1-methylperylene in n-hexadecane, from
21978 cm’1 to 21231 cm“. This range corresponds to vibrational frequencies
between 1170 cm" and 1920 cm"1 ......................... 125
The stimulated response of 1-methylperylene (1370 cm‘1 mode) in eight alkanes.

The instrumental response function indicates zero delay time ........... 127
Solvent chain length dependence of the l-methylperylene 1370 cm'1 ’1‘, relaxation
time ......................................... 129
(a) Tail-matched parallel and perpendicular intensity data and (b) anisotropy decay,
R(t), of 1-methylperylene in n-pentane ....................... 136
(a) Tail-matched parallel and perpendicular intensity data and (b) anisotropy decay,
R(t), of l-methylperylene in n-hexadecane ..................... 137
Orientational relaxation time, 10,, as a function of solvent viscosity. For the long

chain alkanes (n-nonane to n-hexadecane), two exponential decays are found in
R(t) ......................................... 139

CHAPTER 1. INTRODUCTION

Dissimilar molecules interact with one another in the liquid phase according to their
chemical compositions and conformations. Gaining a detailed understanding of these
interactions has attracted significant research attention. Liquids are by far the most
commonly used medium for performing chemical syntheses and analyses, and gaining
predictive control over molecular interactions in liquids would be of great value to a large
portion of the chemical community. Unfortunately, the moderately strong intermolecular
interactions that give rise to the existence of the liquid phase are difficult to probe
experimentally because of the lack of either long range or long time organization in this
medium. For a given molecule in solution, there exist many energetically similar solvent
“cage” configurations, and exchange between these configurations occurs at a rate that is
fast compared to almost all experimental measurement schemes. Despite this inherent
structural complexity, there is a great deal of steady state spectroscopies and solubility
data that demonstrates the existence of highly Specific interactions between dissimilar
molecules. Gaining an accurate picture of local organization in liquids and relating this
information to macroscopic properties, such as solubility or reactivity, continues to be an

area open to investigation and debate.

Themost common experimental approach to the detection and characrerization of local
organization in solution is to interrogate, in some manner, the spectroscopic response of a
probe molecule dissolved in the liquid of interest. Because the probe molecule is different
from the liquid, its presence necessarily disrupts any local organization of the solvent, and
it is not possible to interrogate intrinsic solvent intermolecular interactions directly with
probe molecules. The systems of interest to us are solutions of either chemically reactive
or spectroscopically active molecules, where the interactions between dissimilar molecules
define the important properties of the system. There are a variety of measurement
schemes that have used spectroscopically active molecules as probes of solvent
organization, and each of these approaches senses a different component of the probe
molecule local environment. One can divide these measurements into two broad

categories; molecular motion and energy relaxation.

Measurements of probe molecule rotational diffusion in solution have shown that, when
the probe molecule is large compared to individual solvent molecules, there is little need to
account for specific intermolecular interactions, and the interaction between solvent and
solute can be treated as largely friCIional. This limit. described by the Debye-Stokes—

”"91 and with

Einstein equation,” has been shown to be valid for many polar systems.
appropriate treatment of the friction coefficient.'2"'22' for nonpolar systems as well.'23'25]
For solvent-solute systems where the molecular volume of the solvent and solute moieties
are similar, the molecular nature of solvation processes must be taken into account. In this

thesis the work on the orientational relaxation dynamics of perylene in n-alkanes shows

that, as the hydrodynamic volumes of the solvent and the solute become Similar, the

b)

solvent cage formed around the solute can alter the ability of the solute to rotate about
specific axes. Such data do not necessarily point to any specific structure within the
solvent cage, but do demonstrate that the motional freedom of the solute can be predicted
by relatively intuitive models. Under selected circumstances, such as those included in this
thesis, one can gain insight into the shape of the volume swept out by the reorienting
molecule. For this comparatively special case, we can gain some insight into the average

organization of solvent molecules around the solute.

- - - 6-30 -
Energy relaxation measurements, such as excrtation transport,‘2 ' transrent spectral

4.50 - - -
H l provrde information on energy

shiftm'm and vibrational population relaxation,
dissipation both within the probe molecule and between the probe molecule and the
surrounding medium. Excitation transport measurements have been used to determine
whether or not diffusive behavior dominates at short times or low concentrations in
solution and, as such, have placed limits on the ability to treat energy relaxation events

[27.29]

statistically. One of the more popular techniques for measuring “solvation” times in

liquids has been detection of the dynamic spectral shifts exhibited by modified

35.36.314.40 . . . .
l ' In these measurements, the evolution Oftlie coumarin emlSSlOl'l band

coumarins.
is monitored after excitation with a short light pulse. The timescale of this spectral
relaxation has been correlated with the bulk dielectric relaxation time(s) of the solvents
examined. Recent experimental and computational work has shown that the spectral
relaxation behavior of the coumarins is dominated by intramolecular relaxation between

53'

. . 51.- . . .
several overlapped electronic manifolds' The experimental Signature of this

intramolecular relaxation is a pronounced excitation energy dependence of the coumarin

emission band dynamics. Despite the experimental difficulties associated with using
coumarins as probes of solvation, much valuable information on solvent relaxation has
been gained from these experiments. The physical “picture” of solvent dynamics
developed to explain these data is appealing and will likely be proven correct. if a probe

molecule with a sufiiciently simple spectroscopic response can be found.

In addition to using probe molecule electronic relaxation dynamics to interrogate local
solvent organization, vibrational population relaxation has found its use for this purpose as
welll‘w'w' Using vibrational relaxation dynamics to interrogate local solvent organization
is explored in this thesis. The motivation for using vibrational states instead of electronic
states stems from the comparatively short length scale over which vibrational energy

transfer processes operate and intrinsic directionality of molecular vibrational motions.

Numerous studies have shown that there is indeed Short range order in liquids.'5‘”’”' For
instance, in liquids composed Of molecules with an anisotropic shape. such as the long-
chain n-.alkanes, there are thermodynamic effects associated with the presence of a short-
range molecular order in which the more extended conformations are stabilized by a
cooperative effect. There is some spectroscopic evidence that gauche conformations are

[61

relatively scarce ' and depolarized Rayleigh scattering in liquids shows that orientational

(12
' ' The pressure dependence Of

order is higher in normal alkanes than in branched alkanes,
the excess enthalpy, dHE/dP has been used to illustrate order destruction and order
creation in liquids by E. Aicart eta/.156] The discrepancy between measured dHE/dP values

and calculated values is due to the presence of short-range orientational order in the higher

n-alkane liquids which makes dH/dP more negative and which, upon mixing, is destroyed,

producing a positive contribution to dH/dP not accounted for by theory. Snyder‘s” found
that, in the case of n-alkanes, the observed C-H stretching frequencies tend to fall in
clusters that are regularly spaced with an average separation of about 145:1 cm". The
clustering occurs because the isolated C-H stretching frequencies are determined by the

'58] and

structure of the n-alkanes in the immediate vicinity of the C-H bond. Ohtaki
Marcusm’ made an attempt to paramerize the “structuredness” of a solvent from the
viewpoint of intermolecular interactions using the structuredness parameter Sp. Stengle er
aim” used the NMR chemical shift of Xe(l) to probe liquid structure. The Xe nucleus has
a spin 1 = 3/2', it has an electric quadrupole moment which causes short relaxation times

and leads to broad NMR lines. The relaxation rate is sensitive to the environment in a way

that differs from the chemical shift.

In this thesis, a novel pump-probe measurement scheme to detect the vibrational
population relaxation dynamics of dilute fiuorophores in solution is developed. We have
chosen the chemical system carefully so that the vibrational energy relaxation rate reflects
the local solvent organization, and is not dominated by intramolecular processes. Chapter
2 describes the pump-probe laser experimental set-up we use to measure vibrational
relaxation of probe molecules in dilute solution, In Chapter 3, the stimulated emission
measurement scheme is discussed extensively and the vibrational relaxation of four modes
of perylene in various solvents is presented. Chapter 4 demonstrates the capability of

measuring T. in both electronic ground and excited states using this technique. In

Chapters 5, 6 and 7 the focus is on the studies of vibrational energy relaxation and

rotational diffusion dynamics of perylene and l-methylperylene in a series of normal
alkanes. Information on the solvent local organization and the nature of intermolecular

vibrational energy exchange is extracted from these data.

Literature Cited

1. P. Debye; Polar Solvents, Chemical Catalog CO., New York, p. 84 (1929).

2. M. J. Sanders; M. J.Wirth; Chem. Phys. Lett, 101 361 (1983).

b)

. E. F. Gudgin-Templeton; E. L. Quitevis; G. A. Kenney-Wallace; J. Phys. Chem, _8_9_
3238 (1985).

4. A. Von Jena; H. E. Lessing; Chem. Phys, 40 245 (1979).

1J\

. AVon Jena; H. E. Lessing; Ber. Bunsen-Gm: Phys. Chem, Q 181 (1979)
6. A. Von Jena; H. E. Lessing; Chem. Phys. Let/., E 187 (1981).

7. K. B. Eisentha1;Acc. Chem. Res., 8 118 (1975).

8. G. R. Fleming; J. M. Morris; G. W. Robinson; Chem. Phys., 1_7 91 (1976).
9, C. V. Shank; E. P. lppen; Appl. Phys. Lett, E 62 (1975)

10. D. P. Millar; R. Shah; A. H. Zewail, ('hem. Plots. l.etl., 99 435 (1979).
11. E. F. Gudgin-Templeton; G. A. Kenney-Wallace. .1. Phys. Chem, fl) 2896 (1986).
12. G. J. Blanchard; M. J. Wirth, ./. Phil's. Chem, 9_ 2521 (1986).

13. G. J. Blanchard, J. Chem. Phil-5., §_7 6802 (1987).

14. G. J. Blanchard; C. A. Cihal; .l. Phys. ('hem., 22 5950 (1988).

15. G. J. Blanchard; J. Phys. Chem, 9;, 6303 (1988).

16. G. J. Blanchard; J. Phys. Chem, fl 4315 (1989).

17. G. J. Blanchard;; Anal. Chem, _6_1_ 2394 (1989).

18. D. S. Alavi; R. S. Hartman; D. H. Waldeck; J. Phys. Chem, fi 6770 (1991).
19. R. S. Hartman; D. S. Alavi; D. H. Aldeck; J. Phys. Chem, 5 7872 (1991).
20. C. M. Hu; R. Zwanzig;°J. Chem. Phys, @ 4354 (1974).

21. G. K. Youngren; A. Acrivos; J. Chem. Phys, 6_3 3846 (1975).

22. R. Zwanzig; A. K. Harrison; J. Chem. Phys, 88 5861 (1985).

23. D. Ben-Amotz; T. W. Scott, J. Chem. Phys, 8_7 3739 (1987).

24. D. Ben-Amotz; J. M. Drake; J. Chem. Phys, E 1019 ( I988).

25. Y. Jiang; G. J. Blanchard; J. Phys. Chem, 98 6436 (1994).

26. Th. Forster; Ann. Phys. Lie/121g, 2_55 (1948).

27. C. H. Gochanour; H. C. Andersen; M. D. Fayer. .l. Chem. thx, m 4254 (1979).
28. S. G. Fedorenko; A. I. Burshtein; Chem. Phi/.81, % 341 (1985).

29. P. A. Anfinrud; w. s. Struve; J. Chem. Phys, E 4256 (1987). ’

30 T. P. Causgrove; S. Yang; W. S. Struve; .1. Phys. Chem, 98 6844 (1989).

31. A. Mokhtari; J. Chesnoy; A Laubereau, Chem. Phys. l.eu.. 155 593 (1989).

 

32 A. Wagener; R. Richert. Chem. l’lll‘x. l.e/I.. 1_76 329 (1991)

33. A. Declemy; C. Rulliere; Ph. Kottis; Chem. Phys. l.eII., 133 448 ( I987).

 

34. E. W. Castner; M. Maroncelli; G. R. Fleming, J. Chem Plum, & 1090 (1987)

35. V. Nagarajan; A. M. Brearly; T. J. Kang; P. F. Barbara; .1. Chem. Phys, _8_6 3183
(1987)

9

36. M. Maroncelli; G. R. Fleming; J. Chem. Phys, & 6221 (1987).

37. J. D. Simon; Acc. Chem. Res, _2_1_ 128 (1988).

38. P. F. Barbara; W. Jarzeba; Adv. Photochem, 1_5 l (1990).

39. M. Maroncelli; R. S. Fee; C. F. Chapman; G. R. Fleming; J. Phys. Chem, fi 1012
(1991)

40. W. Jarzeba; G. C. Walker; A. E. Johnson; P. F. Barbara; Chem. Phys, 1-2 57 (1991).

41. T. Elsaesser; W. Kaiser; Annu. Rev. Phys. Chem, g 83 (1991).

42. R. Lingle, Jr; X. Xu; S. C. Yu; H. Zhu; J. B. Hopkins; J. Chem. Phys, 23_ 5667
(1990)

43. P. A. Anfinrud; C. Han; T. Lian, R. M. Hochstrasser; J. Phys. Chem, 9_4 1 180 (1990).

44. E. J. Heilweil; M. P. Casassa, R. R. Cavanagh; J. C. Stephenson, Ann. Rev. Phys.

Chem, Q 143 (1989).
45. E. J. Heilweil; R. R. Cavanagh; J. C. Stephenson; Chem. ths. Lett, 134 181 (1987).

 

46. E. J. Heilweil; R. R. Cavanagh; J. C. Stephenson; J. Chem. Phys, 8_ 230 (1989).

47 E. J. Heilweil; M. P. Casassa. R. R Cavanagh; J. C. Stephenson, .l. Chem. Phys, E
5004(1986)

48. S. A. Hambir, Y. Jiang; G. l. Blanchard; .1. Chem. Phys, 28 6075 ( 1993)

49. Y. Jiang; G. J. Blanchard; J. Phys. Chem, 28 941 1 (1994).

50. Y. Jiang; G. J. Blanchard; J. Phys. Chem, 28 9417 (1994).

51 N. Agmon; J. Phys. Chem, 24_ 2959 (1990).

52. P. K. McCarthy, G. J. Blanchard; .l. Phys. Chem., 9_7 12505 (1993)

53. Y. Jiang;; McCarthy, P. K; G. J. Blanchard;; Chem. Phys, 183, 249 (1994).

 

54. J. J. Moura Ramos; J. Sol. Chem; _18 957, (1989)

it)
55. P. Padila; S. Toxvaerd; J. Chem. Phys, 28 509, (1991).
56. E. Aicart; G. Tardajos; M. Costas; J. Sol. Chem, Q 369, (1989).
57. R. G. Snyder; A. L. Aljibury; H. L. Strauss; J. Chem. Phys, 81 53 52, (1984).
58. H. Ohtaki; J. Sol. Chem; 2_1 39, (1992).
59. Y. Marcus; J. 501. Chem, 2_1 1217, (1992).
60. T. R. Stengle; S. M. Hosseini; K. L. Williamson; J. Sol. Chem, 1_5 777, (1986).
61. (a).R. G. Snyder; J. Chem. Phys, 47 1316, (1967). (b). R. G. Snyder; J. 'H.

Schachtschneider; Spectroch/micaAela1_9 85, ( I963 ).
62. S. N. Battacharyya; D. Patterson; J. Sol. Chem, 2 753, (1980).

11

CHAPTER 2. EXPERIMENTAL

A mode-locked CW Nd:YAG laser (Coherent Antares 76-8) is used to produce 30 W
average power at 1.064 um (IR) with 100 ps pulses at 76 MHz repetition rate (See
schematic of the spectrometer in Figure 2.1). The output of this laser is frequency
doubled using a Type I temperature tuned LBO SHG crystal (7 mm) to produce 3 W of
average power at 532 nm (green), with the same pulse characteristics as for the
fundamental. Both the collinear green and residual IR light are combined in an angle
tuned Type I BBO SHG crystal to produce 1 W of average power at 355 nm (UV), again
with the same pulse width and repetition rate as the fundamental. The 355 nm light is
divided using a 56/44 (reflectance/transmittance) beam splitter and used to pump
synchronously two cavity dumped dye lasers (Coherent 701-3). Both dye lasers are
Operated with three plate birefriengent filters as the wavelength tuning element and no
saturable absorber is used. The dye circulating in each laser is cooled to ~ 2 °C to
increase the viscosity of the dye solutions and to reduce the rate of thermal degradation of
the dyes. Stilbene 1, Stilbene 3 (Stilbene 420, Exciton) and Coumarin 1 (Coumarin 460,
Exciton) dyes, as well as different sets of laser cavity mirrors were used, depending on the

wavelength requirements of the experiments.

12

 

 

' .—> AT

 

 

 

 

 

 

r— mvi #1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

—>T
/L_l probeDL ._ lg
’ I ‘ delay stage S(t) = AT/T
1064nm(IR)
V\“_ me : SHG CWImde-lockede'YAG
355nm(UV) 532nm(Green) 1064m0R)

P - Polarization rotator DL - Dye laser PD - Photod/oale detector

Figure 2.1. Schematic of the stimulated emission pump-probe Spectrometer.

The dye solutions and the corresponding wavelength range as well as the optics set used

are summarized in Table 2.1.

Table 2.1. Information for each dye laser: including making of the dye solution, operating
wavelength range and the mirror sets.

 

 

Dye Preparation Operating Mirror set
wavelength range (coherent coating
rage)
dissolve 1 g Stilbene 1 directly 405 - 450 nm 03
Stilbenel in 1.2 L warm ( ~ 100 °C)
ethylene glycol.
dissolve 2 g Stilbene 3 in 150
Stilbene 3 ml benzyl alcohol, then dilute 425 - 470 nm 03
(Stilbene 420) into 1.2 L warm ethylene
glycol.
dissolve 2 g coumarin 460 in
Coumarin l 150 ml benzyl alcohol, then 460 - 490 nm 04
(Coumarin 460) dilute into 1.2 L warm
ethylene glycol.

 

 

Both dye lasers were cavity dumped at 7.6 MHz. This repetition rate was found to be
optimum for the comparatively low gain blue laser dyes. The laser pulses are characterized
by background-free non-collinear second order autocorrelation (Spectra-Physics model
409). The average autocorrelation trace was found to be ~ 7 - 10 ps depending on
wavelength and dye. The pulses can be modeled with the noise-burst model,'” and the

'2' The instrumental response

frequency resolution was found to be ~ 4 cm'l for each laser
function, determined by cross correlation ofthe pump and the probe laser pulse trains, is

typically 10 ps FWHM and the cross correlation is taken to establish the zero time ofthe

experiment.

14

The pump laser is used for instantaneous excitation and the probe laser is used to initiate
the stimulate emission. The vibration of interest is excited at the difference frequency
between pump and probe lasers. The pump and probe beams are focused on to the sample
and the transmitted probe beam is directed to a monochromator and photodiode detector
while the pump beam is stOpped before the monochromator. The signal we detect is the
transient gain or loss of the probe laser intensity, which is usually very small. For our
experimental condition, changes in probe laser intensity are on the order of 10" to 10'5 of
the probe laser intensity, and the lifetime of the signal is significantly less than the inverse
of the laser repetition rate, so no cumulative gain or loss can be used to advantage.
Because the low frequency fluctuations of the probe laser can be as high as several percent
of the average output intensity, a detection scheme is required that can separate the
transient response from background noise. Based on the noise power spectrum ( N at f'
l), the shot noise limit can be achieved by shifting the detection frequency to a few MHz.
The magnitude of the shot noise is between ~ 10"’ to 10") of the laser beam intensity. We
use a radio and audio frequency triple modulation shot noise limited detection scheme '3'“
to encode the signal. Both dye lasers are modulated using electro-optic modulators. Each
modulator contains KTP crystals ( four of ~ 2.5 cm each) and a Glan-Thompson polarizer.
A Sinusoidal electric field applied across the crystal causes a voltage dependent
birefi'iengce in the crystal, leading to a rotation of the polarization of light passing through.
The polarizer lets the vertically polarized component through to the mechanical delay line

and the sample. The Sinusoidal electric fields used to drive electro-optic modulators have

a maximum amplitude of~ 300 V The pump laser is amplitude modulated at 3.01 1 MHz

15

(con), and the probe laser at 2.110 MHz ((99). The pump laser is further modulated by a
mechanical chopper at ~ 100 Hz ((0,). The signal detected is of the form AT/T, where AT
is the difi‘erence in the probe laser transmitted intensity for the pump beam on and off.
The form of the signal implies that the signal of interest interacts with both incident lasers,
and in so doing acts as a molecular mixture for the modulations applied to the two lasers.
In effect the sample multiplies the two modulation frequencies, where the efficiency of this

modulation is a measure of the Signal magnitude.

1 1
coswa o coswfl = Ecos(a)a +wfl)+§cos(a)a — (05) [1]

and the signal of interest is detected at a modulation frequency ((1),, + (Op), which is preset ‘

against a shot noise limited background.

In time-resolved experiments most often the radiation applied is linearly polarized, so there
is selectivity for the excitation and the collection of the response relating to the orientation
of the molecules that are investigated. It is frequently of interest to measure experimental
signals uncontaminated by orientational relaxation information. This can be done by
collecting response at so-called Magic angle, 6..., which is the angle between polarization
of the pump and probe in this thesis work. The induced anisotropy associated with the

transition dipole y is defined by Equation 1.

N t, —N t,
r (1,7): //( 7) .L( 7’) [2]
N//(t,7)+2Ni(l.r)

 

16

NI/(t) and N1(t) are the population with their orientation parallel and perpendicular-to the
polarization of the pump laser respectively. In the case where only one initial state is
prepared, r(t) is given by the expression involving the second Legendre polynomial (P;) of

the correlation of the transition moment direction at time zero with that at time t.

2 .
rm) = ng (cos6)<P2<p<01- 7(0)} 131

where 11(0) is the transition dipole associated with the preparation of the excited state (e. g.
absorption) and y(t) is the transition dipole associated with the probing event (e. g.

fluorescence or absorption. At zero time,

23(cosc5)2 —1
5 2

 

r(0)= %P2(cosé‘)= [4]

The vanishing of r(O) requires 5 to be 547° that is the Magic angle 5....

All the stimulated emission responses for T1 time measurements were taken at Magic
angle. For the experiments of rotational diffusion dynamics, the decay of the orientational
anisotropy that is induced by excitation of a polarized light is measured to obtain
information about how fast probe molecules reorient in solution. For vibrational
population relaxation measurements, where the time dependent population dynamics are
of interest, the stimulated response is collected for pump and probe electric fields oriented

at 54.7°1with respect to one another.

N

DJ

17

Literature Cited

. H. A. Pike; M. Hercher; J. Appl. Phys, 31 4562 (1970).
. Y. Jiang; S. A. Hambir; G. J. Blanchard; Opt. C ommnn., 29 216 (1993).
. P. BadO; S. B. Wilson; K. R Wilson; Rev. Sci. Instrum, _5_3 706 (1982).

. L. Andor; A. Lorincz; J. Siemion; D. D. Smith; Rice, S. A; Rev. Sci. Instrum, g 64
(1984)

. G. I. Blanchard; M. J. Wirth; Anal. Chem, 3 532 (1986).

CHAPTER 3. ULTRAFAST STIMULATED EMISSION SPECTROSCOPY OF
PERYLENE IN DILUTE SOLUTION - MEASUREMENT OF GROUND STATE
VIBRATIONAL POPULATION RELAXATION

Summary

Ultrafast stimulated emission measurements of perylene in a series of polar and nonpolar
solvents are reported. In all solvents the perylene stimulated emission spectra evolve in
time. Individual features corresponding to distinct vibronic resonances in the stimulated
emission spectra were Observed. The intensities of these features increase subsequent to
excitation and persist for hundreds of picoseconds. The fast build-up seen at short delay

times is related directly to the vibrational population relaxation time, T1, of the ground

vibrational state that is the lower energy state of the stimulated transition. The measured

T1 times for perylene vary with both ground vibrational state and solvent. The slow decay

rates for these data, the sum of the stimulated and spontaneous decay rates for the
particular transition, depend critically on the particular transition that is resonant with the

probe laser electric field.

18

l9

3. 1. Introduction

All optical spectroscopies require that the molecular system being examined is excited in
some way. Following this excitation, the excess energy lefi in the system ends up on the
vibrational states of the excited molecules. The redistribution of this excess vibrational
energy into the immediate environment of the excited molecule can affect the rate of a
chemical reaction or potentially a product branching ratio. Vibrational relaxation is a
dissipative process that damps the nuclear motion. There are two time scales associated

with the relaxation of vibrational resonances; the phase relaxation or dephasing time, T2,

and the population relaxation time, T1, and there are a variety of means used to measure

eachw’] The utility of vibrational relaxation data lie in the different physical origins Ole
and T2 and therefore the different types of interactions they represent. T2 is a measure of

the time required to disrupt or dephase a group of coherently excited vibrations, and this
time is determined by the frequency of elastic collisions experienced by the vibrationally

excited molecule. The population relaxation time, T1, is determined by both

intramolecular and intermolecular processes. The intramolecular decay channels available
to a particular vibrational mode are related to its anharmonicity and the density of solvent
states in close energetic proximity. Collisional depopulation and resonant energy transfer

are the dominant intermolecular mechanisms for T1 relaxation in dilute solution. The

efficiency of the collisional process depends on the density of states of the solvent system

(bath modes) near the same absolute energy as the populated vibrational mode and the

20

frequency of inelastic collisions between the solvent and solute. Resonant energy transfer
between the solvent and the solute will depend strongly on vibrational spectral overlap
between the solvent and solute. Both the energetic and physical proximity of the solvent

and solute thus determine the T1 time measured experimentally. Actual measurement of

vibrational relaxation can be complicated. For a homogeneously broadened vibrational
transition, frequency domain measurements can be used. The vibrational linewidth for a

given (homogeneous) transition is related directly to the relaxation time for that mode,

Av- _—
2rrT
[1]
1 1 1
— — __+.—__.
T 72 2T1

The time T is the observed relaxation time, with the dephasing time being given by the

quantity T2 and the population relaxation time by T]. Only one value is Obtained in a

homogeneous linewidth measurement. and it is not possible to separate the relative
contribution of T1 and T2 to the experimental data. In addition. most vibrational
resonances Observed for room temperature condensed phase systems are inhomogeneously
broadened, thereby invalidating the relationship between Av and T.. For these reasons
there have been a number of experimental schemes devised to measure selectively either

T1 or T2, each with their own advantages and limitations. Heilweil's group has developed
an especially useful infrared pump-probe technique that can be used to measure T1
directly for the IR-active C=O stretching resonances of metal carbonyl compounds in

. 15.61 17.8] , [9-13)
solution. In addition, the Hochstrasser and Hopkins groups have developed

transient vibrational spectroscopic techniques to study T] relaxation. The Kaiser group

21

has developed anti-Stokes Raman measurement scheme to study Tim. Results from all of
these groups indicate that vibrational population relaxation depends critically on the

specific chemical system under examination.

An experimental study has been undertaken in this work to investigate the nature of
solvent-solute interactions in polar and nonpolar systems using time resolved stimulated
emission spectroscopy. For nonpolar probe molecules that do not exhibit a significant
change in dipole moment on excitation, the state-dependent change in solvation is
expected to be more modest than it is for polar probe molecules like the coumarins. A
more subtle experimental signature of solvation dynamics is expected in these systems. In
this chapter ultrafast stimulated emission evolution measurements of perylene in several
solvents of varying polarity is presented. It is found that, indeed, the time evolution of
stimulated emission does contain information relevant to probing solvation phenomena for
nonpolar systems. This technique offers a new and comparatively general means to

determine T1 times for probe molecules in dilute solutions using visible lasers. The

method developed in this work is not sensitive to unwanted overtone contributions to the
14-61 . . . . . .

response, and is less affected by variations in the instrumental response function than

11°31 .. . . . .
Raman based methods. In addition, stimulated emisSIOn measurements provrde data on
the excited electronic state population dynamics, and this information can be separated

from the vibrational relaxation contribution. It is found that the T1 vibrational population

relaxation times for perylene depend strongly on both the specific vibrational mode and the

22

solvent. These findings imply that T1 measurements can be used to probe the intrinsic

anisotropy in the solvation environment experienced by a fluorescent solute.

3 .2. Experimental

Ultrafast stimulated emission spectroscopy. The details of the spectrometer are described
in Chapter 2. For this experiment, the pump dye laser is operated near 435 run, its exact
wavelength adjusted to be coincident with the spectroscopic origin of perylene in each
solvent (see Table 3.1), using Stilbene 420 laser dye (Exciton Chemical Co.) The output
of this laser is ~ 60 mW average power at 7.6 MHz repetition rate, ~ 5 ps FWHM
autocorrelation. The probe laser is operated in the range of 460 nm to 486 nm using
Coumarin 1 laser dye (Aldrich Chemical Co.) (See Table 3.1). Output from this laser is ~
20 mW average power, 7.6 MHz repetition rate with a ~ 5 ps FWIHVI autocorrelation
trace. For all experiments the probe laser polarization is set to 547° with respect to the

pump laser polarization to eliminate rotational diffusion contributions to the data.

23

Table 3.1. The pump, probe wavelengths and probed vibronic region for each solvent.

 

 

solvent 2pm,, (nm) 2pm, (nm) probed vibronic
(O-O frequency (cm")) region (6112")
n-octane 437 (22883) 460 - 480 l 144 - 2050
l-butanol‘ 435 (22988) 460 - 480 1249 - 2155
l-Octanol 435 (22988) 460 - 480 1249 -2155
DMSO 435 (22988) 466 - 486 1529 - 2412
toluene 440 (22727) 466 - 486 1268 - 2151

 

 

Steady state spectroscopies. The static absorption spectra of all perylene solutions were
measured using a Beckman DU-64 spectrophotometer, with ~ 1 nm resolution.
Fluorescence spectra were _recorded using a Perkin-Elmer model LS-S fluorescence
spectrophotometer, with ~ 1 nm resolution. Raw data from these instruments were
digitized and input into a computer using a digitizing tablet and associated software
(Jandel Scientific). These data were used to estimate the spectral origin of perylene in

each solvent.

Chemicals and sample handling. Perylene, 1-butanol and dimethyl sulfoxide (DMSO)
were purchased from Aldrich Chemical Company and used as received. The solvents
octane. l-Octanol and toluene were purchased from J. T. Baker Inc. and used without
further purification. The solutions used for the time resolved stimulated emission

measurements were ~15 11M and were flowed through a 1 mm pathlength flow cell to

24

minimize thermal lensing contributions to the signal. The sample was temperature

controlled at 300 i 0.1 K using a thennostatted bath.

3.3. Results and Discussion

The absorption and emission spectra of perylene in n-octane are presented in Figure 3.1.
The absorption and emission spectra Of perylene in the other solvents that were studied
appear very Similar, the only variation being in small solvent-dependent shifts in the
bands. The absorption and emission peaks shown in Figure 3.1 are themselves composed
of several vibronic transitions each. Attention has been focused on stimulated emission
measurements of the most intense Spontaneous emission band, indicated by the box in
Figure 3.1. The time-resolved stimulated emission spectra of perylene in the solvents
studied here are presented in Figures 3.2-6. These data are time slices taken at 10 ps after
excitation and 100 ps to 900 ps delay, at 100 ps intervals. The spectra were reconstructed
from time scans taken at fixed pump and probe wavelengthsand were normalized using
the measured stimulated emission spectrum taken at the fixed delay time of 800 ps. The
stimulatedemission response at 800 pS delay time is the same as the spontaneous emission
profile to within our experimental uncertainty. There are several important features in

these data. and not all of them are immediately apparent from inspection of Figures 3.2-6.

origin

1.0 - ‘ ©

 

 

 

 

 

 

 

 

3
E
'g 0.5 — A
3
0.0 5 J 4 i : . .
350 400 450 500 550

wavelength (nm)

Figure 3.1 Absorption and emission spectra for perylene in n-octane. The boxed
emission feature indicates the spectral region over which time resolved
stimulated emission spectra were recorded.

Intensity (a.u.)

Figure 3.2.

26

 

 

 

 

 

r 1934 1734 1578 1375
1.5 —
0.0 F 1 4 l 1 I A 1

 

 

20800 21 200 21 600 22000

frequency (cm'l)

Time resolved stimulated emission spectra of perylene in n-octane over
the spectral region indicated in Figure 3.1. (o) = 10 ps delay, (0) =
100 ps, (V) = 200 ps, (inverted filled triangle) = 300 ps, (Cl)= 400 ps,
(I) = 500 ps, (A) = 600 ps, (filled triangle) = 700 ps, (0) = 800 ps,

( O) = 900 ps.

27

1934 1734 1578 1375

j

 

 

 

 

C
I

Intensity (a.u.)

 

 

O. O J I 1 A J 4 #1
20800 21200 21600 22000

frequency (cm")

Figure 3.3. Time resolved stimulated emission Spectra of perylene in l-butanol.
(o) = 10 ps delay, (0) = 100 ps, (V) = 200 ps, (inverted filled triangle)
= 300 ps, (13)= 400 ps, (I) = 500 ps, (A) = 600 ps, (filled triangle) =
700 ps, (0) = 800 ps, (0) = 900 ps.

28

1934 1734 1578 1375

 

 

 

 

 

 

 

15 -
. eN/fl
33;. '
I“\i A
A'\A/.—>3
1.0 - "‘ ’0
f0” /.
A ”0".
=1
3 L
.3
£9.
.E
0f5'—
00 i 1 I I i 1 I
20800 21200 21600 22000

frequency (cm")

Figure 3.4. Time resolved stimulated emission spectra of perylene in l-octanol. (o)
= 10 ps delay, (0) = 100 ps, (V) = 200 ps, (inverted filled triangle) = 300 ps,
(D)= 400 ps, (I) = 500 ps, (A) = 600 ps, (filled triangle) = 700 ps, 0) =
800 ps, (0) = 900 ps.

29

1934 1734 1578 I375

 

 

 

 

 

 

1.5 —
e C
- /:\./s
:1 V v
I
/ /2
. A" \A
10 - /‘\543
’5 ‘ °
=1 / 0“.
3 . /
3 ‘O
:E) ’ 0 \v
.V/
:1 If \
>‘A
I .
a . \
<>/ 0
0.5 — ’
J l J l 1 1
20400 20800 21200 21600

frequency (cm")

Figure 3.5. Time resolved stimulated emission spectra of perylene in DMSO. (0) = 10 ps
delay, (0) = 100 ps, (V) = 200 ps, (inverted filled triangle) = 300 ps, (CI)= 400
ps. (I) = 500 ps, (A) = 600 ps, (filled triangle) = 700 ps, (0) = 800 ps, (0) =
900 ps.

30

1934 1734 1578 1375

 

 

 

 

 

 

 

1.5 F
1.0 '-
3
3
.3“
§
0.5 —
.
1 1 1 1 1' 1
20400 20800 2 1200 2 1600

frequency (cm")

Figure 3.6 Time resolved stimulated emission spectra of perylene in toluene. (O) =
10 ps delay, (0) = 100 ps, (V) = 200 ps, (inverted filled triangle) = 300 ps,
(Cl)= 400 ps, (I) = 500 ps, (A) = 600 ps, (filled triangle) = 700 ps. (0) =
800 ps. (0) = 900 ps.

31

The most obvious feature in these stimulated emission profiles is the presence of solvent-_
dependent sub-structure within the band, and the gradual evolution of the stimulated
emission profile to correspond with the spontaneous emission spectrum. The physical

origin of these features and the chemical information that is contained within them will be

addressed below.

The individual features seen in the time resolved spectra are red-shified from the perylene
spectroscopic origin by vibrational resonance frequencies. These features correspond to
individual vibronic transitions within the spontaneous emission envelope that was studied.
The spectral width of the individual bands is dominated by the thermal width of the

vibrationless excited state. Over the wavelength range that was examined, four Raman
active vibrational modes at 1375 cm'l, 1578 cm'l, 1734 cm'1 and 1934 cm‘1 dominate
the spectrum. The modes at 1375 cm'1 and 1578 cm'1 have been assigned previously as

the v7 and v5 ag modesmm The 1734 cm'1 and 1934 cm'1 modes are combination
modes; v1734 z v7 + 358 cm'l, and v1934 2: v7 + 550 cm'1 or v5 + 358 cm'lml Both
the v15 mode at 358 cm’1 and the v14 mode at 550 cm'1 are prominent Raman active ag

. 16 . . . . . . . . .
modes in perylene.l ] It 15 not possrble to distinguish between these possrbilities for v1934

based on the frequency domain spectra. The temporal responses of these modes give an

indication of their composition.

32

The single most unusual characteristic of these data is the persistence time of the
individual vibronic features. The instrumental response time of the laser system is 10 ps,
and thus a signal that persists for hundreds of ps can not be attributed to a coherent
Raman response. These vibronic features do, however, dwindle to a level that is difficult

to detect before the population of S] is exhausted. If this were a simple excited state
absorption response the features would persist for the duration of the 51 population, and

such a process would likely produce a spectral Signature very different from the one that
was Observed. Coherent hyper-Raman scattering from excited state vibrational .modes
with a resonant intermediate state could be invoked to explain the observed energy
dependence in the data, but such a signal would persist only for the duration of the
instrument response function. Modeling these data within the framework of pump pulse
absorption followed by simple Stimulated emission from the excited state of perylene does
not provide satisfactory agreement with the experimental data either, because such a

process should nOt expose vibronic sub-structure.

To understand these results we must consider a more complete model of the spectroscopic
processes possible for our experimental conditions. For simplicity we model these data as
a linear superposition of coupled three level systems. This simplification ultimately leads
to small differences between the experimental data and the model, but the qualitative
features and chemical information Contained in the data are represented well by this model.

The three states in each coupled three level system are the electronic ground State (So).

the electronic excited state (S 1) and a vibrational level in the 50 manifold, as depicted in

33

Figure 2.7, where the S] state is assigned the label A, the ground vibrational state B and

the vibrationless ground state C. It is assumed that the excitation laser pulsewidth is Short
compared to the processes we detect. Experimentally the pump wavelength is chosen to
be v0.0, and to probe the vibrational state vvib the probe wavelength is chosen to be v0.0 -

v..,.,. This is how two laser radiations forces a molecule to vibrate at the difference

 

 

 

 

frequency.
‘ A
1 1.,
k ”probe :( (”O-0’ (0v)
(11 0-0 = (”pump 2
I B ( m. )

 

 

Figure 3.7. Schematic of coupled three level system used to model the experimental data.
The terms k are described in the text.

In 1917 Einstein proposed a model for transitions between two states that relates
absorption, spontaneous emission and stimulated emission'm For a two level (AB = hv)
an incident electric field at frequency v will serve to diminish the population difference
between two states. This happens because the population in the higher energy state will
be transfered to the lower energy state and the population in the lower energy State

absorbs the radiation to make a transition to the higher energy state (absorption). This

34

pOpulation exchange will not proceed indefinitely due to the finite lifetime of the.upper

state (A) and vibrational state B. In this model k1 is the rate constant for emission from
level A to level B. k1 = klstimulated + klspontaneous' The rate constant k2 is for
absorption from level B to level A. The rate constant k3 represents the vibrational

population relaxation from state B to state C. Because this is a spontaneous process, the
mechanism of depopulation from B to C are not well determined. It is assumed that the B
—> C transition is irreversible since the vibrational levels are at energies high enough that
Boltzmann population of state B is negligible. The complete model is shown in Figure
3.7. The Einstein coefficients 8,: and 82) for a two level system are equal save for

differences in the degeneracy of the two states involved (k) z k2).

By virtue of the way the transient response is detected, the observed Al/I signal is the sum
of the population changes for states A and B: S(t) = A(t) + B(t). This assertion appears
outwardly to be counter-intuitive because stimulated emission produces a gain in the
number of photons in the probe laser electric field while absorption involves the loss of
photons from the same electric field. The response we sense is the net gain on the probe
laser electric field, Le. (Stimulated emission - absorption). ”The reason that the
experimental signal we detect is sensitive to the sum, and not the difference, in populations
lies in the manner in which the experimental Signal is encoded and decoded. Although the
details of the experimental laser beam modulation scheme are more complex, as discussed
in Chapter 2, one can consider conceptually that the pump electric field imposes a

sinusoidal amplitude modulation, (Dmod (“'5 MHz), on the excited state (A) population. For

35

a DC probe laser electric field applied at a time t after excitation, the stimulated emission
signal will appear as a sinusoidal gain at comod, and the absorption component of the signal
will appear as a sinusoidal loss, also at mm), but because of the population exchange
between the emitting and absorbing states, the phase of the gain modulation is shifted by It
from the phase of the loss modulation. In other words, When stimulated emission from A
to B is at a maximum because the population of A is at its maximum level, absorption
from B to A will be at a minimum because the population of B will be at its lowest level
during the modulation cycle, and vice versa.

Gam(a)mod.t) = A(t)sin(wm0d ~t) Loss(rumod.t) = B(t)srn(a)mod -t + 12')

S(wnde) = (Jam((um0d.t) — Loss(wm0d .t) [l]
S((umodJ) = A(t)sin(wm0d 0!) — B(t)sin(a)mod -t + II) = (A(t) + B(t))s1n(w,,,0d -t)

By synchronous demodulation detection. we obtain S(t) = A(t) + B(t). The'coupled
differential equations that describe the time-dependent population changes for the three

level system are [2]

"1 "3
A<:>B—>C
"2

11114414192181

dt

337?] = I‘ll/11— (*2 + @1113}
{[5] = "3l’31
dt

36

. . [18.19] .
The solution to these equations has been reported by several authors, and the time
dependence of each species can be found by making a series of substitutions. Following

[191
the treatment and nomenclature of Szabo,

a = [Al/[Ale fl = [Bl/[Ale 7 = [Cl/[Ala

T=k11 K1=k2/kl K2=k3/k1
d

——=-a+K

dr 1'8

gig: —K1fl—K7,B+a

dr "

d7

—=K

dr 2'6

The solutions for or and B are given by

 

K2-l3 K‘z-Az
at = exp —}mkt — exp -/1 kt
) 41302-13) ( J 1) 1202-43) ( 2 1)

1
12-13

 

[exp(—/l3k1t)- exp(-/12k1t)]

A(t) =

where

I 1
2.2 = %{1+K‘] +K‘2 +[(1+K1+K2)2 -4K2] '}

l

l.

7 ./
13 = Elz—{l-i-tq +K‘2 —[(1+KI+K2)" —4K2]' 2}

S(t) = or(t) + B(t). S(t) has the form of two exponential decays,

37

-__’_‘_2__ L. _ __1_. ._
S(t)— (’12 -,l3){,l3 exp( A3k1t) ’12 exp( 121(10}

The exponential decay terms in S(t) containing 71.2 and A3 are related in a straightforward
manner to the transition rate constants k} and k3 when the vibrational population

relaxation rate constant, k3, is substantially larger than the overall rate constant k1 for the

A -—> B transition. l/klspontaneous z 6.4 ns for perylenem] and, while there is no T1 data

available for perylene in solution, Zinth et al. have reported a T1 for the v2 mode of

anthracene to be ~240 psm The condition k3 >> k1 thus appears to be satisfied. For large
values of k3/k1, 3.3 approaches unity. The exponential decay term in S(t) containing A3
will therefore be determined by k1 when vibrational relaxation from B is fast. Comparison
Of A2 and k3/k1 reveals that, except near k3 ~ k), the term Azkl ~ k3. The decay term in
S(t) containing 712 will therefore be related directly to k3. A build-up with a time constant

T) = l/k3 and a decay with a time constant Telec = l/k] (te hereafter) are expected in our

experimental time scans. In Figure 3.8 the time scans for perylene in n-octane at four
probe wavelengths corresponding to the vibronic resonances are shown, where the
relevant ground vibrational state is indicated. Data for times < 20 ps delay have been
truncated so that there is no contribution to these decays from the instrumental response
function. For all cases a fast build-up followed by a slow decay was observed. Both of

these time constants vary as a fiinction of perylene vibrational mode and solvent, and are

38

1.5 l" r.
‘L

AITT(att)
U.
\l
m

 

 

l.) 2110

delay finie (ps)

Figure 3.8. Time scanes for perylene in n-octane at probe wavelengths corresponding to
distinct vibronic resonances, where the ground state vibrational level is
indicated Build-up and decay times are given for these data in Table 3.1.

reported in Table 3.2. Analysis of these data show that the approximations of A3 ~ 1 and

Azkl ~ k3 are good to within <5°/o, i.e. better than the uncertainty in the values of T. and

Te.

Table 3.2. Build-up and decay times determined from the experimental data. All times are
given in ps. The vibrational frequencies refer to the final state in the stimulated transition.
The spectral origin for each solvent was estimated from the static spectroscopic data.

 

vibrational mode

 

 

500W" [3 75 cm'1 1578 cm'1 1734 cm'1 1934 cm'1
n-octane T1=25i3 T1=51i6 T1=55i4 T1=362i87
te=2163:10 te=2858i27 te=1828i13 te=872i15
l-OCIanOl T1=40i3 T1=100i1 l T1=54i5 T1:75i9
te=2521i20 te=21 1 1:42 te=2654i42 te=2165i41
l-butanol T1=357i86 T1—79i6 T1=3 541-40 Tl=188i25
te=1928i285 Te 1833:32 te=1575i58 te=2236il 17
DMSO T.=66:9 T.=32i12 T1=80i7 T1=26i4
te=3682i 147 te=256 1:47 te=2006i3 5 18:1526i13
toluene T.=364:133 T1=- --- T1: ---- Tl=55i5
te=l962i288 te=2805i20 te=3874i78 te=1697:15

 

 

There are several points regarding these data that require further attention. First, it is not
immediately apparent that the B —> A transition needs to be considered to explain these
data. The probe pulse stimulates emission from state A and, we assert, also stimulates
absorption from state B.

If only stimulated emission occurs then the effective rate

constant for this process should depend on the probe laser intensity, and we should

40

measure a probe laser intensity dependent decay constant. If absorption from state B also
contributes to the observed response one should observe no probe laser intensity
dependence because the effective absorption rate constant depends on probe laser intensity

in exactly the same manner as stimulated emission. Two time scans for perylene in octane

where Iprobe E Ipump/ 1000 and Iprobe _—“: Ipump were measured (Figure 3.9). The slow

decay constant is the same within experimental uncertainty for the two scans, establishing
the contribution of absorption from B —> A during the probe pulse. Over the duration of

the probe laser pulse, a population equilibrium is established between states A and B with
the equilibrium constant given by klstimulated/kz. An intensity dependence in the signal

was observed only when the laser pulses are overlapped in time, and this effect is
attributed to stimulated Raman gain, a response that is expected to scale with the intensity

Of both the pump and probe lasers and persist only for the duration of the instrument

. 121]
response function.

Another noteworthy feature of these data is that the measured rate constant for the A —> B

transition varies with probe laser wavelength. For a given transition, the stimulated

. . . . . . 1221
em1s510n rate constant 15 related directly to the spontaneous emnssron rate constant, and

both of these rate constants depend on the oscillator strength of the particular transition
. 1231 . . . . . .
be1ng probed. Because of the comparatively narrow detection bandwrdth 1ntnnsrc to

. . . . [241 . .
our stimulated emissron experiments, different total rate constants for each specrfic

transition accessed were observed.

41

stimulated Raman gain
f IPump ~ Iprobe
‘ 'l ‘1
. til H 1'. 1 l
6- Ii i Ill )1"

 

 

 

 

 

 

’K. i,“
3. 4 '— r 1 Hi 1 i
g .1 .' 1 l l i .
). 1
1 1 ~ .
E ‘ i .’ I10.1I ‘
Q 2 1- 1
Iprobe ~ 0.001 ipump
0
_2 n . 1 1 l 1 I 1 I A 1 l I A L A 1 l I 1 _1
0 1 00 200 300 400 500 600 700 800 900 1 000

scan time

Figure 3.9. Time scans for two different pump/probe intensity ratios.

42

The detail chemical meaning of the T, values reported in Table 3.2 is discussed as
followed. The vs (1578 cm-l), V7 (1375 em-l), v14 (550 em-l) and v15 (358 cm-1)
modes in perylene are all ag in-plane ring distortion modes. It is not surprising that the

different modes relax at. different rates. The solvent "cage" surrounding perylene is almost
certainly anisotropic, and each solute vibrational mode couples difierently to its immediate
surroundings for both geometric and energetic reasons. Comparison of the relaxation

times T1 for v7 and v1734 (V1734 = v7 + v15) reveals that for all solvents except toluene,

the v7 mode couples more efficiently to the available solvent bath modes than the v15

mode. For toluene, however, T11375 = 364 i 133 ps, and T11734 < 10 ps, indicating
efficient relaxation mediated by v15 at 358 cm'l. The v15 mode in perylene is essentially
a stretching mode between the two naphthalene moieties and represents a significant
concerted motion of different portions of the perylene It systemml The data indicate
Strong coupling of this mode to the surrounding toluene solvent, suggesting at least partial

alignment of the solvent and solute 1: systems. The first step inthis process appears to be

intramolecular relaxation into v15 followed by energy transfer between the perylene

modes and 'toluene. The infrared spectrum of toluene Shows a band at ~350 cm‘hml

implicating resonant energy transfer as the dominant intermolecular relaxation mechanism.

The perylene v5 mode, a symmetric breathing mode of the individual naphthalene
moieties, also appears to be coupled strongly to the toluene solvent bath modes, and there

are strong Raman and infrared active toluene vibrations between 1550 cm‘1 and 1650 cm”

43

1261 . . . ..
1. This spectral correlation also suggests the dominance of non-collis10nal energy

transfer between perylene and toluene.

The operative coupling mechanism for the alcohols appears to be fundamentally different.
There are-substantial differences in T1 for perylene in l-butanol and l-octanol. The Raman
and infrared spectra of l-butanol and l-octanol are similar, suggesting that if non-
collisional coupling of perylene to the solvent bath is operative then we should observe a
correlation in the measured T) times in the two alcohols. The non-correspondence we
observe suggests that inelastic collisions may be an important relaxation mechanism in the
alcohols. It is also possible that non-collisional relaxation efficiency depends critically on
local organization in the solvents. This possibility is suggested by the data present in
Chapter 5. The generally more efficient relaxation of perylene in the longer chain alcohol
suggests that solute inelastic collisions with the solvent aliphatic moiety are more efficient
than interactions with the polar hydroxyl group.

The v1934 mode can be described as either v7 + v14 or v5 + v15. If v1934 = v5 + v15
then its T) values should correspond with either those of v5 or v1734. No such
correlation was noted in the data. Conversely, if V1934 = v7 + v14 then one expects a
correlation between T. values for this mode and v7. Again no such correlation was
observed, implying, by process of elimination the relaxation of this mode is dominated by
v14 at 550 cm'l. It is likely that the lack of correspondence with other modes that have

been examined is due to the participation of all of these modes in this spectral region,

where v14 provides the dominant relaxation pathway.

44

If the stimulated emission response of perylene is modeled as a linear superposition of
coupled three level systems, the vibrational features in the data Should not disappear

completely until population of the 51 state is exhausted. Experimentally, however, these
vibrational features do not persist at a detectable level for the lifetime of the 51 state

(Figures 3.2-6). We believe that the failure of our model to account for this observation
lies in two factors, both of which must be present. First, the vibrational modes must
exhibit some anharmonicity for us to observe combination modes. This anharmonicity can
also allow coupling to other (anharmonic) perylene modes, and this possibility was not
considered. Second, it is implicitly assumed that once the vibrational energy is dissipated

from state B. it is lost irreversibly. For high frequency modes this is a good
approximation. but for low frequency modes it is not. The v15 mode at 358 cm'1 is < 2

kT at 300 K. Boltzmann population will be significant for low frequency components of
combination modes. It is believed that the slight discrepancies between the experimental
data and the model that was used to understand it arise from the shortcomings of the

simplistic model.

Finally, in comparison with other methods like 1R pump/ IR probe and IR pump/ UV,VlS
probe, this technique uses visible lasers and has high spectroscopic selectivity. A wide
range of vibrational resonances can be accessed with our pump-probe scheme. A
significant advantage of this technique, which are touch upon in later chapters, is the

ability to excite probe molecule vibrational resonances that are degenerated with solvent

45

vibrations. Using IR lasers to excite such a vibration is not possible owing to the size of

the solvent background absorption.

46

3.5. Conclusion

A new technique for the measurement of T1 ground state vibrational population relaxation

times using time resolved stimulated emission spectroscopy was developed. The
measured population relaxation times were found to vary dramatically with the identity of
both the solute vibrational mode and the solvent. Both intermolecular and intramolecular

relaxation pathways depend on the identity of the solute vibrational mode. The T]

variations from solvent to solvent for a particular mode serve to underscore the
importance of intermolecular relaxation pathways in the vibrational relaxation process.
The experimental data is modeled qualitatively by assuming that the spectrosc0pic
response of perylene can be treated as a linear superposition of coupled three level
systems. Differences between the experimental data and the model are likely due to
anharmonic coupling between vibrational modes in perylene that is not accounted for. It is
believed that other pOchyclic aromatic hydrocarbons will exhibit these effects due to the
comparatively modest anharmonicities they exhibit. Polar dye molecules may not yield an
analogous stimulated emission response because of the greater extent of anharmonic
coupling in these systems. Nonetheless. the limitations of the model do not detract from
the physical significance of the build-up and decay times seen in our experimental data.

This means of measuring T1 is anticipated to prove useful in elucidating the molecular

nature of solvent-solute interactions.

47

3 .6. Literature Cited

1. W. Zinth; C. Kolmeder; B. Benna; A. Irgens-Defregger; S. F. Fischer; W. Kaiser; J.
Chem. Phys, 18 3916 (1983).

N

.N H. Gottfried; W. Kaiser, Chem. Phys. Lett, __1331 (1983)

. A. Seilmeier; P. O. J. Scherer; W. Kaiser; Chem. Phys. Lett, 121140 (1984).

b)

4. E. J. Heilweil; M. P. Casassa; R. R. Cavanagh and J. C. Stephenson; J. Chem. Phys, Q
5004(1986)

LII

..E J. Heilweil; R R Cavanagh andJ. C. Stephenson; Chem. Phys. Lett, ___4 181
(1987)

6. E. J. Heilweil; R R. Cavanagh andJ C. Stephenson, J. Chem. Phys, _982 30(1988).

\I

. P. Anfinrud; C. Han; P. A. Hansen; J. N. Moore and R. M. Hochstrasser; Ultrafast
Phenomena VI, T. Yajima; K. Yoshihara; G. B. Harns; and S. Shionoya; Eds. Springer,
Berlin, p. 442-446, (1988).

8. P. A. Anfinrud; C. Han; T. Lian and R. M. Hochstrasser; J. Phys. Chem, % 1180
(1990)

9. R. Lingle Jr.; X. Xu; S.-C. Yu; H. Zhu and J. B. Hopkins; J. Chem. Phys, 9_3 5667
(19.90).

10 R Lingle Jr. X Xu; S -.C Yu; Y J Chang and] B. HOpkins; J. Chem. Phys, 92
4628(1990)

11.S-.C Yu; X Xu; R Lingle Jr and]. B. Hopkins; J Am. Chem. Soc, 12 3668
(1990).

12. X. Xu; R. Lingle, Jr.; S.-C. Yu; Y. I. Chang and J. B. Hopkins; J. Chem. Phys, 22
2106(1990)

13. J. B. Hopkins andP M Rentzepis; Chem. Phys. Lett, __2503 ( 1987)

14. N. H. Gottfried; W. Kaiser; Chem. Phys Lett, 1(1983)

48

15. S. J. Cyvin; B. N. Cyvin and P. Klaeboe; Spec. Lett, _l_§ 239 (1983).

16. S. Matsunuma; N. Akamatsu; T. Kamisuki; Y. Adachi; S. Maeda and C. Hirose; J.
Chem. Phys, 88 2956 (1988).

17. A. Einstein; Zur Quantentheort’e der Strahlung, Phys. 2., 18 121 (1917).

18. D. H. McDaniel and C. R. Smoot; J. Chem. Phys, 60 966 (1956).

19. Z. G. Szabo in Comprehensive Chemical Kinetics, Vol. 2, ed by C. H. Bamford and C.
F. H. Tipper; Elsevier, 24-26 (1969). '

20. I. B. Berlman; Handbook of Fluorescence Spectra of Aromatic Molecules, Academic
Press, p. 399, (1971).

21. B. F. Levine; C. V. Shank and J. P. Heritage; IEEE J. Quant. Electron, QE-lS 1418
(1979)

22. J. T. Verdeyen; Laser Electronics, Prentice Hall, p. 139, (1981).

23. G. Herzberg; Molecular Spectra and Molecular Structure, Vol. 111, Van Nostrand, p.
418,(1966)

24. G. J. Blanchard;J. Chem. Phys, 95 6317 (1991).

25. We have calculated the normal mode atomic displacement vectors for these perylene
modes using the Spartan v.2.0 software package.

26. H. A. Szymanski; Correlation of Infrared and Raman Spectra of Organic
Compounds, Hertillon Press, Cambridbge Springs, PA, (1969).

CHAPTER 4. VIBRATIONAL POPULATION RELAXATION OF PERYLENE IN
ITS GROUND AND EXCITED ELECTRONIC STATES

Summary

A novel scheme for the measurement of molecular vibrational population relaxation in
both ground and excited electronic states using ultrafast Stimulated spectroscopy is
proposed. This technique was demonstrated using 10'5 M perylene in n-pentane, n-hexane
and n-heptane. The vibrational population relaxation times (T.) of the perylene V7 mode
are 304:44 ps in the ground state and 140:1 1 ps in the first excited singlet state, and both
T. times are solvent independent to within the experimental uncertainty. In contrast, the

T. relaxation time of the perylene ground state v5 mode exhibits a measurable solvent

dependence, ranging from 160 i 37 ps in n-pentane to 308 i 41 ps in n-heptane.

49

50

4.1. Introduction

Despite the high density of states intrinsic to comparatively large molecules, certain
chemical systems exhibit slow vibrational population relaxation. Perylene, for example.
exhibits T. times that can vary from less than 10 ps to hundreds of ps, depending on the
specific vibrational mode examined and the chemical identity of the surrounding
medium.“I Perylene is non-polar, and the nature of its interactions with its surroundings
are, for the most part, shorter range than for polar systems, where electrostatic and dipolar
processes dominate. Perylene is a USCfUl probe molecule for vibrational population
relaxation experiments because it exhibits only modest anharmonicity and Strongly mode-
dependent relaxation rates in liquids. Perylene has been chosen as a well-characterized
probe molecule for the experiments reported in this thesis. It is found that the vibrational
population relaxation time, T., is mode sensitive and solvent sensitive. The purpose of
this work is two-fold. First. the ability to measure vibrational population relaxation rates
in excited electronic states of perylene in dilute solution will be demonstrated, and second,
the state-dependent vibrational population relaxation times for the perylene v7 mode will
be presented. T. for both 5.. and S. perylene for the v7 and v). modes in three n-alkanes
were measured, and it was found that relaxation in the S. state proceeds a factor of two
more rapidly than in the Sn State. These findings will be discussed in the context of inter-
and intramolecular relaxation pathways that are expected to exhibit an electronic state-

dependence.

51

4.2. Theory

As discussed in the previous chpater, the measurement of ground state vibrational
population relaxation times using transient stimulated spectroscopy can be understood in
the context of a coupled three level system (Figure 4.1a). To recap briefly, the

experimental signal is of the form,

B
S(t) = B(t) + ((1): HOT. {(12 + k3)exp(—klt) - (k2 + k1)exp(-k3t)} [1]

For the measurement of vibrational population relaxation within an excited electronic
state, one needs to consider the population dynamics for four states (Figure 4.1b). In this
four level system. the states B, C and D are the same as those Shown for the three level
system. State A is the vibrational mode ofinterest in the excited electronic state. The rate
constants k., k; and k. are the same for the four level system as for the three level system.
The primary differences between the observed relaxation dynamics for these two systems
lie in the initial populations of states A and B. For the excited state vibrational population
relaxation measurement, the initial population in state B is zero, where for the three level
system. state B is populated directly by the exciting laser pulse The transient population
relaxation dynamics for the four level system are described by the four coupled differential

equations shown in Equation 2

 

 

 

 

 

 

B a
1 A
k1
k C”probe
(0 2
pump
V\ C
k
3 D
b
A

 

 

0’ pump

 

 

 

 

Figure 4.1. (a) Schematic of the coupled three-level system used for interpretation
of 0-0 excitation experiments. (b) Schematic of the coupled four-level
system used for interpretation of excited state T. measurements.

53

A—->B<:>C—>D

dA

—=-k A

d1 4
Eg=k4A—le+k2C
‘” In
dC

——=kB— k~+k C
cl] 1 (.1 2)
ink3C

dt

For the three level system, it was implicitly assumed that k; >> k. E k2 we apply this same
assumption plus one other, k. ~ k., for the solution of the four level system. Integration of

these equations yields

(k2 + A3)
("3 ‘k1)("4 "'11
+ (k1 +k2 +k3 —k4)
l Mt-kuw4-kn

("3 ‘k1)(’*'3 ‘ "41

 

 

1
exp(—klt) + exp(—k3t)

S(t) = B(l)+ ("(1) = AOL-4

 

cxp(—k4t)

The Signal from our experiments contains three exponential terms. In principle, one can fit
the experimental decay curve using Equation 3, but with five variable parameters and a
finite signal-to-noise ratio, it is difficult to obtain fits to the data where the fitted
parameters possess the requisite mutual independence. This is, however, not an
insurmountable problem. Using the information obtained from experiments where
excitation was at the origin, one can determine k. with acceptable certainty. With k. and
k; obtained from the 0-0 excitation experiments, either the raw data from the blue
excitation experiments can be fitted directly, or the difference between blue and red

excitation data can be taken and fitted with the requisite certainty. It is possible for a

54

ground state vibrational mode and an excited State vibrational mode to decay with the
same time constant, i.e. k3. = k... If this condition were to occur, it would lead to
undefined pre-exponential terms in Equation 3. Such a condition does not pose a problem
experimentally because the ground state and excited state vibrational modes used in the
measurements can be different, as long as the same ground state vibrational mode is used
for both excitation frequencies in the determination of the excited state T. time. In fact,
this is a strategy which was employed in the acquisition of T. times in the first excited

singlet electronic state of perylene, as detailed below.

4.3. Experimental

Spectrometer: The spectrometer used for these measurements has been described in detail
in Chapter 2. Stilbene l and Stilbene 420 dyes (Exciton) were used for the pump dye laser
and Stilbene 420 dye was used for the probe dye laser. The pump laser is operated at 434
nm for 0-0 excitation of perylene in n-pentane and n-hexane, and at 437 nm for tt—heptane.
For excitation of the v7 mode in the first excited Singlet electronic state, the pump dye
laser iS operated at 409 nm for n-pentane and n-hexane. and at 412 nm for n-heptane. For
red (0-0) excitation experiments measuring v. T. times, the probe dye laser is operated at
462 nm for n-pentane and n—hexane and at 466 nm for n-heptane (See Table 4.1). For the
measurement of v7‘ we used probe laser wavelengths of 466 nm for perylene in n-pentane
and n-hexane, and 470 nm for perylene in n-heptane.

Table 4.1 . The pump, probe wavelengths and dyes for each solvent.

55

 

 

0-0 excitation 1_J-* excitation
solvent pump probe dyes pump .probe dyes
(nm um) (nm 1 nm)

 

n-pentane 434 /462 Stilbene 420/ Stilbene 420 409 / 466 Stilbene l / Stilbene 420
n-hexane 434 /462 Stilbene 420/ Stilbene 420 409 / 466 Stilbene 1 / Stilbene 420

n-heptane 437 I466 Stilbene 420/ Stilbene 420 412 / 470 Stilbene 1 / Stilbene 420

 

 

Steady state .spectroscopt‘es: The steady absorption spectra of the perylene solutions were
measured using a Beckman DU-64 spectrophotometer, with ~ 1 nm resolution.
Fluorescence spectra were recorded using a Perkin-Elmer model LS-S fluorescence
spectrophotometer, with ~ 1 nm resolution. These data were used to estimate the spectral

origin of perylene in each solvent.

Chemicals and sample handling: Perylene and the n-alkanes were purchased from Aldrich
Chemical Co. and were used as received. The solutions used for the time resolved
Stimulated measurements were ~ 10 ul_vl_ and were flowed through a 1 mm pathlength flow
cell to minimize thermal lensing contributions to the signal. The sample was temperature

controlled at 300:0.1 K using a thermostatted bath

4.4. Results and Discussion

56

The static absorption and emission spectra of perylene in n-hexane are presented in Figure
4.2. The corresponding spectra of perylene in n—pentane and n-heptane are virtually
identical. For excitation of the perylene 0-0 transition, the pump laser is set to the
wavelength indicated by the arrow labeled “a” in Figure 4.2, and for excitation of the v7.
mode, the pump laser wavelength is at ~1400 cm'1 above the origin, indicated by arrow
“b” in Figure 4.2. While the ground state v7 mode has a characteristic frequency of 1372
cm", in the first excited singlet state this mode shifts to ~ 1393 cm".m The v7 mode in

perylene is an in-plane Raman-active ring distortion mode of (1,. symmetry?“ and an

exaggerated schematic of its atomic displacements is shown in Figure 4.3.

The difference in resonance frequency between the ground and excited electronic states
suggests that the potential well for this mode is slightly different in the electronic states,
and it is therefore not surprising that the vibrational population relaxation dynamics for

this mode will be unique to each state.

Before the results for v7 and v7. population relaxation times are discussed, the method of
acquisition and analysis of the raw data is described briefly. As noted in the experimental
section, two groups of experiments were performed; the first aimed at measurement of the
V7 model and the second group designed for measurement of the V7. mode. In the first

group of experiments, the V7 mode (state C in Figure 4.1a) was accessed directly. For the

57

1.0 -

08 .—

t. . .
Absorp ton Emrssmn

0.4 r-

linear response (a.u.)

 

 

350 400 450 500

wavelengh (nm)

Figure 4.2. Linear optical response of perylene in n-hexane. The absorption and emission
spectra have been normalized. Arrow “a” and “b” indicate the excitation
wavelengths used for the 0-0 and v7* experiments, respectively.

58

 

 

V7

Figure 4.3. Schematic of the exaggerated atomic displacements for the perylene v7
mode. The directions of the displacements were estimated from semi-
empirical calculation results.

59

second group of experiments, the ground state v5 mode (state C in Figure 4. lb) instead of
the v7 mode was used. As discussed in the Theory section, it is possible that, for a given,
vibrational mode, T. = T.‘ (k3=k1), and such an experimental condition could lead to an ill-
defined condition in the interpretation of our data. The use of different ground and
excited state modes serves to minimize, but not eliminate, the possibility that k3=k... In
fact. for perylene in n-pentane, k;(v5) ~ k..(v7'°), but because the agreement is not exact,
we are able to extract information on k. for v7. from the experimental data. A second
reason for using v5 instead of vv in the determination of v7. is that the highest possible
signal to noise ratio in the data is desired, and the stimulated transition cross section for
perylene is larger for a probe laser energy of(v0-.. - 1578 cm'l) than for (V...) - 1372 cm'l).
For the 0—0 excitation experiments on both the v7 and v5 modes, the time resolved
stimulated signal S(t) was fitted with a double exponential function, and this process has
been detailed previously. A time resolved scan used for the determination of T. for v7 is
shown in Figure 4.4. For the determination of V). the k. and k;(v5) information obtained
from the 0-0 excitation experiments are used to fit the data from the v7. excitation
experiments In principle. one can either fit the data directly, or take the difference
between the two experimental signals, normalizing for intensities at long delay times,
where spontaneous and stimulated emission from state B are the dominant relaxation
processes in both data sets. Both of these methods yield identical results, save for
arbitrary pre-exponential factors, and the difference signal approach was chosen because it

presents a clearer view of the difference in responses for the two excitation conditions.

60

 

 

 

 

 

l
1.0 *-
411.]
> ' - ’ "-41“ ‘ ,‘
08 1— ' ." ""r'"'1".-..
. 'Vli. l'hl‘."
_ T111 ‘
.i‘.',"L_ .‘
t
E—' 04 1-
G
11.2 —
l .
‘1
0.0 I L
l I l M 1 I l I J I
0 200 400 600 800

delay time (ps)

Figure 4.4. Experimental stimulated response and laser cross-correlation for
measurement of the ground state v7 mode of perylene in n-hexane. For
this experiment, Am... = 432 nm and two... = 462 nm. The line through
the data is the fitted result See Table 4.1 for best fit results.

61

The difference signal, AS(t), (V7. excitation response - 0-0 excitation response) is of the

form (Equation 3 - Equation 1),
112 4.1% k4A0 ] [k7 4111.1 k4A0 ]
AS! = 3 —B ex ——lrt + " —B ex -lrt
() (k3-/‘1 [Qt-kl 0 p( I) k3-k1 k3-k4 0 p( 3)

[kl/1”“! + k2 + k3 - k4 )] exP("k4l) ‘
(k4 _ k1 )(k-l ‘ k3)

 

 

[4]

ln Figure 4.5a both the 0-0 and V7. excitation time scans for probing the V5 mode in n-
hexane and the difference signal, AS(t), in Figure 4.50 The T. (=k3") and If (=k4")
population relaxation times for V5, V7 and V7. in the three n-alkanes are presented in the

Table 4.2.

Table 4.2. Vibrational population relaxation times for the ground state and excited state
V7 mode of perylene in n-alkane solvents.

 

 

solvent 'l',( v5) 711/ V') TIY V5.)
(1)3) ‘ (ps) (1)5)
n-pentane 160:3 7 276:46 141:2
n-hexane 300: l 00 281:177 150:17
n-heptane 308:4] 355:100 129:71

 

 

These data Show several interesting features. We note that v5 exhibits a solvent-
dependent T. time where V7 does not. In our previous work we have found that several
different perylene vibrational modes exhibit unique solvent-dependent relaxation

properties. and the data we report here on V5 and V7 are consistent with this trendm

12

10

intensity (a. u)

dillerence signal (an)
O

Figure 4.5.

62

 

 

 

 

_ 0.," excitation a
'_ /
* ”Wm
' ' “We
)-
/ 01-1111
_. 1 0-0 excitation 'Vltf'kii...}1.u
l-
1
Q1 1 1 I 1 1 1 3 1
’ b

—
F—'
M‘
_J
‘_—
I
q
.—
—
'—
l

 

 

 

 

 

200 400 600 800 1000
delay time (ps)

(a) Experimental stimulated response for the 0-0 and V7* excitation of
perylene. For these scans the ground state V5 mode of perylene in n-hexane
is probed; Mp = 432 nm for 0-0 excitation and 409 nm for v7* excitation
and Am... = 466 nm for both excitation conditions. See Table 4.1 for the best
fit results. (b) Difference signal, S(t), for the two scans shown in (a), with the
best fit function shown as a solid line through the data.

63

It. is important to place the data we present here in context with the data we have reported
in the following chapter. The T. values we present for the perylene V7 mode in n-pentane,
n-hexane and n-heptane are significantly longer, by a factor of ~10, than the T. time we
have reported for the V7 mode in tt-octane.["5' As discussed in Chpater 5, the perylene v7
mode relaxes anomalously fast in n-octane, compared to its relaxation in the other 11-
alkanes, because of efficient V-V resonance coupling to the n-octane terminal CH3
rocking mode (1378 cm"). The perylene V7 mode relaxes with a time constant of 298 :
102 ps in tt-CgD.g, demonstrating that the dominant coupling mechanism for the
anomalously fast T. time seen in n-CgH... is resonant V-V energy transfer. The solvent
vibrational receptor mode is significantly localized at the termini of the alkyl chains and
‘ resonant V-V coupling is extremely sensitive to the spatial proximity of the “donor” and
“acceptor” modes, i.e., the spatial relationship between the alkane terminal CH3 groups
and the perylene v7 vibrational coordinate determine sensitively the efficiency of T.
relaxation for this systemm The T. relaxation times of the perylene V7 mode in the n-
alkanes indicate the presence of short range order in solution, and we have discussed this
point in detail in Chapter 5. Rotational diffusion measurements of perylene in the n-
alkanes Show that solvent organization exists on a length scale much less than 10 A, the

“length” of the perylene molecule.‘6|

The state-dependence of the relaxation times measured for the perylene V7 mode is
discussed below. T. for V7 is the same for all three n-alkanes to within the experimental

uncertainty and that T.’ is the same for the three solvents, but T.‘ < T. for V7. It is

64

important in and of itself that the T. times for both the V7 and V7. modes are solvent-
independent for these three n-alkanes, but perhaps of more importance is that the
relaxation times for V7 and V7. are different. There are two possible reasons for the
difference between T. and T.‘ for these modes. The first is that the state-dependence
arises from intramolecular changes in the coupling between the vibrational modes of
interest and lower energy modes. A difference in anharmonic coupling between modes in
the two electronic states is possible, as indicated by the ~21 cm'l blue shift of V7 on

121

excitation. Indeed, there are several modes, both IR and Raman active, in Close
energetic proximity to V7 and V7., and the state-dependent frequency shifts seen for these
modes are not, in general, the same as those seen for V7 and V7.12“ Any coupling between
these modes will necessarily vary with the anharmonicity of each mode and their frequency
differences. Because the measurements were performed in a room temperature liquid and
the spectral resolution of the system is ~15 cm", one cannot separate cleanly anharmonic
coupling effects from the non-selective simultaneous excitation of several nearly
degenerate modes. The second possible reason for the difference in T. and T.° is that the
intermolecular relaxation pathways available to V7 and V7. are different. If such a state-
dependent intermolecular process is dominant, then it must be due to a V-V resonant, i.e.
non-collisional. interaction between the perylene modes and the n-alkane solvent. A
prerequisite for the existence of this mechanism is. of course. that there is a solvent
vibrational resonance in the vicinity of the perylene 1375 cm'l mode. The n-alkane
solvents possess a vibrational resonance at 1378 cm'l corresponding to a rocking motion

of the terminal the CH; groups. Other recent data on ground State T. times for perylene in

a broader series of n-alkanes indicate that this V-V coupling can be strong under certain

65

conditions, and may contribute to the state-dependent relaxation that is seen in this
work”! V-V processes are believed to dominate the intermolecular contribution to the,
observed relaxation because a state-dependent change in the inelastic collisional rate
would require substantial local heating on excitation. For these experimental conditions
the transient temperature rise for the perylene molecule should not exceed several K at
most. It is likely that neither the intermolecular nor the intramolecular processes by

themselves account completely for the measured difference in T. and T.‘ for V7, but,

rather, that both factors combine to produce the observed result.

66

4.5. Conclusions

The feasibility of measuring vibrational population relaxation times of complex organic
molecules in dilute solution for both their ground and excited electronic states has been
demonstrated. Specifically, we have examined the state dependent T. relaxation time of
the perylene V7 mode in three n-alkanes and find that for a given electronic state the T.
times are solvent independent. The excited state (V7.) mode relaxes approximately twice
as rapidly as the ground state (V7) mode. This difference in relaxation times are attributed
to either state-dependent changes in the anharmonic coupling Ofthis mode to other modes
of equal or lower energy or to changes in the efficiency of intermolecular coupling to the
solvent bath modes. If the latter mechanism is operative, then the intermolecular coupling
must be predominantly through a near-resonant V-V channel. Further experimental work

is in progress to elucidate the dominant relaxation pathway for this mode.

67

46. Literature cited

1. S. A. Hambir; Y. Jiang; G. J. Blanchard; J. Chem. Phys. , 28 6075 (1993).

2 S. Matsunuma; N. Akamatsu, T. Kamisuki; Y. Adachi; S. Maeda. C. Hirose, .l. Chem
Phys, 88 2955 (1988).

3. M. A. Kovner, A. A. Terekhov; L. M. Babkov; Opt. Spectrosc. Lett. 28 (1971)
4. S. J. Cyvin; B. N. Cyvin; P. Klaeboe; Spectrosc. Lett, E 239 (1983).

5. Y. Jiang; G. J. Blanchard; J. Phys. Chem, 28 9411 (1994).

6. Y. Jiang; G. J. Blanchard; J. Phys. Chem, 28 6436 (1994).

CHAPTER 5. VIBRATIONAL POPULATION RELAXATION OF PERYLENE IN
n-ALKAN ES - THE ROLE OF THE LOCAL SOLVENT ORGANIZATION IN

LONG RANGE VIBRATIONAL ENERGY TRANSFER

Summary

The vibrational population relaxation times of the Raman active v7 mode (1375 cm") and
(v7 - v.5) combination mode (1733 cm") of perylene in eight liquid n-alkanes were
measured using ultrafast Stimulated emission spectroscopy. The vibrational population
relaxation time of the perylene V7 mode ranges from ~300 ps to < 10 ps depending on the
n-alkane solvent chain length, but there is no Simple correspondence between alkane
length and T. for V7. Energy transfer from the perylene V7 vibrational mode to a specific
"-alkane solvent vibrational mode is dominated by long range resonance coupling. The
Perylene (v7 + V. 5) combination mode exhibits additional efficient relaxation pathways for
difI‘erent length n-alkanes. These data point collectively to short range order in the n-

a”(fine solvent surrounding perylene molecule.

68

69
5. 1. Introduction

In low pressure gas phase experiments, where an excited molecule is comparatively
isolated from its neighbors, vibrational energy will dissipate slowly within the molecule to
lower energy modes according to the extent of anharmonic coupling between the
vibrationalmodesln For probe molecules in liquids, intramolecular relaxation is often less
important than direct intermolecular relaxation because the number of collisional
interactions with the surrounding medium is large and individual molecules are in closer
spatial proximity to one anotherml In solution, excess vibrational energy within a
molecule can be dissipated directly into the surroundings, sometimes very efficiently.
Intermolecular vibrational population relaxation processes include energy transfer from
solute vibrational modes into the translational, vibrational and rotational degrees of
freedom of the solvent. The transfer of energy into these degrees of freedom can proceed
by short range inelastic collisionsli's' or, for vibrational to vibrational (v-v) processes, can
also proceed by long range polar coupling between excited solute Vibrational modes and
the Vibrational states of solvent molecules.'6'9' Long range v-v processes occur because of
dipole - dipole, dipole - quadrupole, dipole - induced dipole or other polar interactions.
FOF collisional (short range) v-v energy transfer, the acceptor and donor molecules must
necessarily be separated by fractions of an angstrom during the collision, and for long
range processes, the efficiency of energy transfer will scale with r'". where it depends on
the nature of the coupling and varies according to the model used.Ill For any given
Vibrationally excited molecule, there is a non-zero probability that it will relax according to

any Of the aforementioned processes. The interests of this work is in determining

7O

experimentally which process(es) dominate relaxation in room temperature liquids. There
is very little information available in the literature on understanding in detail the pathways
of vibrational population relaxation in complex organic systems, and one must necessarily
assume, as a starting point, that the physics of the problem are essentially the same as
those of gas phase vibrational relaxation processes. The vibrational population relaxation
rate constant is proportional to the probability of the transfer process. The probability of
collisionally mediated, short range v-v energy transfer ranges up to ~10'3 depending on the
difference in vibrational resonance frequencies of the donor and acceptor species and the
frequency of collisionsm The probability of long range polar V-v process ranges up to ~l,
depending on the difference between the donor and acceptor vibrational frequencies as
well as the nature of the polar interaction responsible for the energy transfer. An
important and distinctive feature of long range polar v-v processes is that the probability
of a relaxation event depends on the distance between the donor and the acceptor and the
alignment of the species. The probabilities of collisional v-t,r processes (up to ~10'5) are
Wpically much smaller than either of the v-v processes.'” Separating the contributions of
each of these processes in a given chemical system requires the examination of several
relaxation pathways available to a given vibrational excitation, as well as the ability to vary

eXperimental conditions in a controlled manner.

To gain predictive control over vibrational energy flow in complex chemical systems, one
needs first to determine the relative efficiency of each vibrational relaxation mechanism in
a comparatively simple organic system and identify the chemical reasons for these relative

effiCiencies. Much work has been done on small and medium size molecules in the gas

71

phase.“"” where the density of species available to accept vibrational energy is low, and in
low temperature solid matrices,“°'”] where the spatial relationships between donor and
acceptor are well defined and fixed on a time scale that is long compared to the dissipation
of energy. Significantly less work has been focused on understanding vibrational
relaxation of medium size molecules in solution at room temperature.[2'3'”’ It has been
demonstrated previously that the efficiency of each relaxation pathway available to the
excited molecule is determined by the specific chemical system and vibrational mode

excited, and it is the purpose of this work to understand the dominant processes

responsible for vibrational pOpulation relaxation of the v7 and v.5 modes of perylene in a

series of n-alkane solvents.

5 .2. Experimental

Ultrafast stimulated spectroscopy: The spectrometer used for Stimulated measurements
Of perylene has been described in detail in Chapter 2““ Stilbene 420 dye (Exciton) is
used for both dye lasers. The pump laser is operated at 434 nm, corresponding to the 0-0
transition for perylene in n-pentane and n-hexane or at 437 nm to pump the 0-0 transition
OF Perylene in the longer n-alkane solvents. The probe dye laser is operated in the range
of 460 to 480 nm. with the exact wavelengths depending on the mode accessed and the 0-
0 transition energy.
Steady state spectroscopies: The steady state absorption spectra of all perylene solutions
Were measured using a Beckman DU-64 spectrophotometer, with ~l nm resolution.

Fluorescence spectra were recorded using a Perkin-Elmer model LS-5 fluorescence

72

spectrophotometer, with ~1 nm resolution. These data were used to estimate the

spectroscopic origin of perylene in each solvent.

Chemicals and sample handling: Perylene and all the n-alkane solvents were purchased

from Aldrich Chemical Company and used as received. The concentrations of the

perylene solutions used for the time resolved stimulated measurements were ~10 .184. To
minimize thermal lensing contributions to the Signal, sample solutions were flowed
through a 1 mm pathlength flow cell. The sample temperature was controlled at 30.0 : 0.1
K using a thermostatted bath. For experiments using n—octane-dm. Stimulated
measurements were performed in a 1 cm pathlength cuvette equipped with a magnetic
stirrer As a check, perylene relaxation in n-octane-hm was measured in the same cuvette

and results were identical to those obtained using the flow cell.

5.3. Results and Discussion

The absorption and emission spectra of perylene in n-octane are presented in Figure 5.1.
The absorption and emission spectra of perylene in other n-alkane solvents are similar,
with only minor solvent -dependent band shifts. The excitation wavelength is indicated by

Ihe arrow and the vibrational modes accessed are contained within the emission feature

73

origin

_ ©©
1.0 - ©©©

 

 

 

 

 

 

lnlensrty(a u )
0
L11
1
>
r11

 

 

0.0 t 4 ‘ t - : .L c .
350 400 450 500 550

wavelength (nm)

Figure 5.1. Steady-state absorption and emission spectra of perylene in n-octane.

l \
x ‘ ’
y I
‘ A
V
\ V V I
1 A A \
A
V V
A A
, V ‘ U
x ’ 7
p <

 

i
3“ re 5 .2. Exaggerated atomic displacements associated with the v7 and V15 normal modes
of perylene.

75

boxed in Figure 5.1. While there are a variety of Raman active vibrational modes

accessible to this measurement scheme,“7°’9' the focus has been on the relaxation dynamics

of two particular vibrational modes in the ground electronic state of perylene; the V7
fimdamental mode at 1375 cm’1 and a combination mode composed of the V7 and the v.5
(358 cm") modes, at 1733 cm'1 (Figure 2). The V7 mode is an in-plane ring deformation
mode, and the v.5 mode is the dominated by the in-plane stretching motion of the center
ring between the two naphthalene moieties. Both of these modes are of ag symmetry and
are Raman active. The (V7 + V .5) combination mode was used instead of accessing the v..
mode directly because spectral overlap of the Steady state absorption and emission bands
prevents separation of ground and excited electronic state responses in the region of the
(v01. - v.5) resonance. The relaxation times measured for both the V7 and (V7+V15) modes
depend sensitively on the identity of the n-alkane solvent, and this aliphatic chain length-

dependence provides direct evidence for solvent structure around the perylene molecule.

The origins of the solvent dependent response will be discussed in below.

In Figure 5.3 an example of the experimental stimulated signal, S(t), for perylene in n-
octane excited at 437 nm, and probed at 472 nm to access the (V7 + V..) mode is shown.

The spectroscopic selectivity is sufficient to distinguish between individual perylene

-1 [16]

Vibrational modes in liquidsm The frequency width of the probe laser pulse is ~4 cm

While the vibrational linewidths of the perylene resonances are significantly broader at 300

K- The time resolved stimulated Signals were fitted using Equation 3. The vibrational

AT/T (a.u.)

Figure 6.3.

0.8

0.6

0.4

0.2

0.0

76

T1 = 60:14 p5

   

7, = 2180214 ps

\

 
 

 

 

 

0 200 400 600 800 1000

delay time (ps)

Stimulated response of the (V7 + v..) mode of perylene in n-octane,
presented with the cross correlation response function.

77

 

 

 

 

SUU —
400 — . +
300 - _L
0
Z _ 0 I375 om'l mode
s: 0 I733 an" mode
e— 200 — 1734 an“=1375cm"+359cm"
,. L T
IUH r- d I
. § §
1 §
1) O 1 l I l O
4 6 8 10 l2 I4 16

Number of C's in n-alkane

Figure 5.4. T. relaxation times for v7 (0) and (w + v”) (0) modes in perylene as a
function of solvent chain length.

78

relaxation times, T 1 = k;", in the eight n-alkane solvents we have studied, are presented in

Figure 5.4 and Table 5.1 for the v7 and (v7+V15) modes.

Table 5.1. T. times for the v7 and (v7 + V”) modes of perylene in n-alkanes.

 

 

solvent v- T, (v- - V15) T1
(ps) (ps)

n-Csle 276:46 286:.36
n-C6H14 281:177 < 10
n-Cva 355:100 316:9
n-CgHis 30:3 60:14
n-Cgqu 125:26 56:17
n-Cmsz 376:92 273:183
Il-Clezo 410:106 41:10
II-C16H34 < 10 < 10
II-Cngs 298:102 -----

 

 

Each of these data values can be reproduced at will and are determined by regression of

the average of at least 15 individual time scans. The large uncertainty in these values is

reflective of the small fractional contribution of the k; process to the overall transient gain
resDonse (Equation 3). For the v7 mode, T1 ranges from 30 : 3 ps in n-octane and <10 ps
i n N~hexadecane to ~3OO ps in the other n-alkanes. The majority ofthe T1 times are found
‘0 be much longer than the 10 ps instrument response time, and thus no attempt at
deconvolution was made to determine the exact T. values for the few conditions where
the relaxation proceeds in < IO ps, .The unexpected dependence of T. on solvent identity
provides an important indication of the mechanism by which vibrational energy relaxes

frOm perylene. For the (v7+vl5) combination mode, the measured T. times are

79

qualitatively similar to those for the v7 mode, but exhibit additional efficient relaxation

pathways, which must necessarily be associated with the V” mode.

There are at least two questions regarding these data which require an answer at some
level, and, indeed, the information contained in the T1 data provide insight into vibrational

population relaxation at a molecular scale. The first consideration lies in determining the

dominant relaxation mechanism, and the second question is centered on any local
molecular structural information the solvent chain length dependence provides. For the
dilute solutions that were investigated, intermolecular energy exchange between perylene
molecules will contribute negligibly to the observed response, and the focus is on the
interactions between perylene and the surrounding n-alkane solvents. While anharmonic
coupling in perylene is significant, as is apparent from the prominence of the (V7+V15)
combination mode, it is expected that intramolecular energy transfer to low energy modes
Will contribute little to solvent chain length dependence in the T1 times that were
"'1 easured, because the linear optical response of perylene appears to be virtually solvent-
independent. The intramolecular relaxation processes for perylene were approximated to
be solvent-independent. It should be noted that the separation of intermolecular and
intramolecular relaxation processes is more likely to be accurate for a rigid molecule such
as perylene than for a labile molecule, where photoisomerization or other large amplitude
molecular motions can influence the efficiency of intramolecular decay pathways.'2°‘2”
1 h principle, intermolecular energy relaxation from a solute molecule to a solvent molecule
can proceed by collision-mediated short range v-t,v,r, processes or by long range

resonance polar v-v coupling. Except in carefully chosen cases, the probability of

80

collisional v-t,r processes is much smaller than that for collisional v-v processes. For. most
liquid phase systems, it is expected that the dominant relaxation pathway will be through
v-v coupling, and the central mechanistic question is whether the coupling is short range,
collisionally mediated or long range, through-space in nature. For the chemical systems
we have chosen to examine, we believe that the dominant relaxation processes must be v-v
because, in n-alkanes, the terminal CH3 rocking mode resonance occurs at 1378 cm'1 and
this resonance exhibits negligible variation among the different n-alkanes. The perylene v7
mode is centered at 1375 cm", functionally degenerate with the solvent CH; rocking bath
IT) ode. Since there is no applicable theoretical treatment of different v-v processes in room

temperature liquids, gas phase treatments were used in attempting to distinguish between

long range polar and short range collisional relaxation processes.

The probability of a v-v energy transfer event for short range collisional processes

betheen the donor and acceptor species at exact resonance is given by

/ all/PH" 2

 

WhEre Uir is a vibrational matrix element for the collisional interaction, u is the reduced
mass of the system undergoing the collisional interaction, and L is the length scale over
which the collisional interaction can take place, Le. L is the effective intermolecular

di 31 ance of the colliding species when v-v energy transfer takes place. The value of L will

depend on the chemical identity of the colliding species, and is typically taken as L = 0.2 A
f0? both v-t and v-v gas phase collisional interactions. While the value of L may not be

precisely the same in liquids, it is not likely to be much different because the fundamental

 

81

nature of the interaction responsible for the energy transfer is the same in both the liquid
and the gas phases. For typical experimental conditions, a maximum value of <<P>> for
collisionally mediated short range energy transfer is ~10'3. It is important to note that, for
short range collisional processes, the probability of energy transfer does not vary smoothly
with the distance between molecules. For an energy transfer event to occur, the donor

and acceptor molecules must be in intimate contact. Long range energy transfer processes

are fimdamentally different than collisionally mediated energy transfer. Long range
interactions involve polar coupling between donor and acceptor species.
4yC3
-—-— [51

/ 1)‘:
\< >2 file/VT

There are a number of different formulations for long range v-v energy transfer,l("8' and all

yield qualitatively the same result. The term C contains information on the matrix

elements for the donor and acceptor vibrational transitions involved in the energy transfer

Process as well as geometric alignment terms. <<P>> for long range energy transfer

depends inversely on the distance, d, between the acceptor and donor species, with the
ex act distance dependence being determined by the type of interaction (i.e. dipole-dipole,
Clipole-quadrupole, dipole-induced dipole, etc). Thus the length scale over which this
er‘ergy transfer takes place depends on the chemical system, but, for exact resonance
COTlditions and in a condensed phase system, where d is small, <<P>> ~ 1‘” This latter

mechanism is significantly more efficient than short range, collision-mediated v-v energy

trE‘lnsfer for our experimental conditions. In addition to the predictions of the above

referenced models, the experimental data indicate the dominance of long range energy

tr'cansfer.

82

The data in Figure 5.4 show that T. for the v7 mode does not change smoothly with the
length of the aliphatic solvent. If collisional interactions dominate the relaxation process
sensed by the T. measurements, then we would expect a smooth progression of T. times
that is proportional to the frequency of collisions between perylene and the terminal
methyl groups of the alkane solvent molecules. The frequency of collisional interactions
between the solvent and the solute should vary with the solvent viscosity and density, both
of which are well-behaved functions of aliphatic chain length: If short range v-v energy
transfer were responsible for the data shown in Figure 5.4, then collisional interactions
between perylene and n-octane would have to be a factor of ~l0 more frequent than they
are for either n-hexane or n-decane, and this possibility is physically unreasonable.

The dominance of long range polar v-v energy transfer implies necessarily that the solvent

exhibits local structure about the perylene molecule. Equation 5 shows that the

P rObability of a long range energy transfer event is related to the separation distance of the
donor and acceptor, d, and the term C in Equation 5 also contains a geometric factor for
aligriment of the species. The solvent acceptor mode is significantly localized on the
t erI'hinal methyl groups on the alkane chains, and thus it is expected the efficiency of v-v
transfer to be proportional to the distance between perylene and the terminal methyl
grOUps of the solvent. The observed change in T. for the perylene v7 mode in n-octane
and n-hexadecane indicates that the terminal methyl groups of these solvents are, on

aVerage, in closer proximity to the perylene molecule than are the terminal methyl groups

0f the other n-alkane solvents.

-l

m-__~.__ awn...

‘M_~_

We expecte the response for the (v. + v.5) combination mode to be a superposition of the.
responses of each fundamental constituent mode. The same response for the (v. + v.5)
mode was observed as for the v7 mode, and in addition, efficient coupling to the solvents
n-hexane 'and n-dodecane, and to a lesser extent for n-nonane were observed. The
efficient coupling of the v.5 mode to the surrOUnding bath modes is likely due to v-v long
range resonance processes, but, in contrast to the v7 mode, the solvent and solute
resonances are not at exactly the same frequency and the length scale of the coupling may
be different than for the V7 mode because coupling may proceed from the perylene Raman
active mode to either Raman (AOL) or infrared (Au) active modes of the solvent. Despite
these possible differences in the nature of the coupling, the qualitative information content
ofthe data on the (v. + v.5) combination mode is expected to be the same as that for the
v7 mode. The enhanced coupling of the v., mode to the solvents n-hexane and n-
dodecane arises from arrangements of the solvent around the perylene molecule that are
sensitive to the motions ofthe v.5 mode. Because these motions are significantly different
than those for the V7 mode. a different solvent-dependence is expected for the relaxation
of this mode.

To establish the dominance of v-v processes in our measurements, especially for the cases
where efficient relaxation occurs, T. for the v7 mode in n-octane-d... (II-C3D...) was
measured. For this system T. = 298:102 ps was obtained, in contrast to 30:3 ps in n-

octane-h,,.. For n-octane-d... the terminal CD3 rocking mode resonance occurs at 1050

 

84

cm", Am = 325 cm'1 for v-v relaxation from the perylene v7 mode to this mode. The

detuning dependence of <<P>> is given by [71

 

27r2C 2150,11 pu'fl/
P = , exp —
<< >1 r-IthdsU [:7 2k?"
where [6]

. (ZdAaflch '1’3
u =
y
The experimental T. values, in conjunction with Equations 5 and 6 allow us to estimate
the average perylene-solvent methyl group spacing. For a frequency difference of ~300
cm", for long range resonance energy transfer, <<P>> was estimated to be ~ 0.07 for d=l
A and <<P >> ~ 0.0005 for d = 2 A. These values of <<P>> for perylene in n-octane-d...
yield T. ~ 430 ps for the perylene v7 mode in n-octane-d... for d=l A and T. ~ 4.2 ns for
d=2 A, based on the experimentally'observed time of T.=30 ps for perylene in n-octane-
In... The assumption is made that the deuteration of n-octane does not substantially alter
its solvation characteristics, and, if this assumption is valid, one can estimate the perylene-
methyl group spacing, d ~ 0.9 A in the n-octanes from the ratio of the experimental T.
times, i.e. <<P>> smucm-l ~ lO<<P>> x... 3......m-l (see Figure 5.5). From this estimate ofd
for the n-octanes and n-hexadecane, a T. of ~ 300 ps for the perylene v7 mode in other H-
alkanes suggests that the average perylene-methyl group separation for these solvents is
~l .7 A. There is also stoichiometric uncertainty involved in the interpretation of these

numbers. At this point it is important to note that the value of d we report correspond to

points of cloest contact, not intermolecular distances.

85

 

 

1.0 —
Aw=0
/ ..
0.8 [- 1
, Aw=300 crn'l L
/ .
/\ 0.6 -
A
a.
v
v
0.4 ~
\
\\
02 ~ “
\
r \\
0.0 K - l 4
1 .

d (K)

Figure 5.5. Calculated probability, <<P>>, for long-range energy transfer for exact
donor-acceptor resonance, 0) = O (eq.5) and a) = 300 cm", as a function
of donor-acceptor separation, d.

It is not clear to what extent the terminal CH3 rocking mode in n-alkanes behaves a
collective motion of both CH3 groups, as opposed to acting as a doubly degenerate but
spatially separated mode within an individual molecule. Despite this uncertainty, the
above estimates of d seem entirely plausible for a liquid phase system. Also for n-octane-
d... there is another vibrational mode, the CD2 scissors motion at 1080 cm'1 (Aw = 295 cm'
I) which can contribute to the measured relaxation time. It is likely that both of these
solvent modes act as acceptors for the perylene v7 mode. For a frequency difference of
Am ~ 300 cm", <<P>> for collisionally mediated transfer falls to ~ 105, indicating that, for
perylene in n-octane-d,,,, the dominant relaxation mechanism remains long range resonant
v-v coupling. For gas phase systems, the cross-over point between v-v long range and
collisional process dominance has been estimated to occur for A0) ~ 250 cm",‘81 Clearly the
density of the bath medium has a significant effect on vibrational relaxation.

[20.21] and 3

Mode- and solvent-specific intermolecular interactions have been seen before
central question in all such work is the nature of solvent organization around the solute.
The observation of mode-dependent .coupling to different n-alkane solvents invites
speculation on geometric arrangements of the solvent about the solute. Such a practice is,
of course, extremely speculative, and should be taken as such. In this context, we offer
only two observations. The dominant motion of the v7 mode is a distortion of the
individual naphthalene moieties, and this mode is found experimentally to couple
efficiently to n-octane and n-hexadecane. The “length” of n-octane, if it were in an all-

trans conformation, is quite close to that of the perylene long axis, which spans both of

the naphthalene moieties. In contrast, the dominant motion of the v.5 mode is an in-plane

87

moving together and apart of the individual naphthalene groups, and this mode is observed
to couple strongly to n-hexane and n-dodecane. Because the solvent resonances to which
the v.5 mode couples are not as localized as those to which the v7 mode couples, any
relevant geometric constraints are less well defined, but we note that the “length” of n-
hexane is close to that of naphthalene. For both modes the coupling efficiency exhibits
what appears to be a periodic effect, i.e. v7 couples to n-CgH... and n-C.6H3..; v.5 couples
to "-C.;H... and n-C.2H26. The origin(s) of this effect are, at present, unclear, but suggest a
regularity in the way aliphatic chains organize around a solute molecule. These
postulations are reminiscent in some sense of Shpol’skii‘s work on perylene in n-alkane
crystals at 77 Kim Shpol’skii used steady state emission and absorption measurements to
observe perylene spectral line narrowing in frozen n-alkane matrices. For such
measurements it is reasonable to expect that local structure will persist for the lifetime of
the emitting state (several ns). In liquids there is a significant body of information that

1220-231 (vide infra), albeit with a persistence time

points to short range solvent structure
much shorter than for solids. It is possible that the mode-specific short range transient

order detected in liquid n-alkanes is related to the structural effects detected by Shpol’skii,

but the connection between these bodies of data remains unclear at present.

The T. data for both the V7 and (V7 + v.5) modes indicate that local solvent structure is
important to vibrational energy relaxation. The length scale over which such structure
persists is not clear from the T. measurements alone, but other dynamical measurements
can place bounds on this persistence length. The rotational diffusion dynamics of perylene

in these same n-alkane solvents show that, while the boundary condition changes between

88

the solvent and the solute at ~n-octane, there is no discontinuous change in the viscosity

'2‘” A discontinuous response is expected only if

dependence of the reorientation time.
there is a substantial solvent chain length dependent change in solvent ordering in the
vicinity of the solute. The reorientation data are sensitive to changes in the relative
hydrodynamic volumes of the solute and the solvent, but show no evidence of
comparatively long range solvent structure. Thus the local structure sensed with the T.
measurements persists on a length scale much shorter than the perylene molecule (~10 A).

Finally, the need for a better model for vibrational energy transfer in liquids, particularly

for v-v long range resonance coupling vibrational energy transfer in liquids, is needed so

to aid the prediction and interpretation of these experimental results.

89

5.4. Conclusion

The aliphatic chain length dependence of vibrational population relaxation for the v7
fundamental and (v. + v.5) combination modes of perylene in dilute solution have been
measured. The measured T. times do not vary smoothly with solvent aliphatic chain
length. For certain solvent alkane chain lengths, vibrational energy in the perylene
molecule couples efficiently to the bath modes of the surrounding solvent. The dominant
mechanism for this vibrational population relaxation is long range resonance v-v energy
transfer. The observation of efiicient solvent-solute coupling for specific solvent aliphatic
chain lengths demonstrates the existence of persistent local structure in this chemical
system. Data from rotational diffusion measurements on perylene in the n-alkanes shows
that the local solvent ordering exists on a length scale significantly shorter than the length
of the perylene molecule (~10 A). The sensitivity of individual solute vibrational modes to
different components of local solvent structure offers the ability to interrogate selectively

the presence of structure in the solvent cage of a variety of condensed phase systems.

5.5. Literature cited

1. J. T. Yardley; Introduction to Molecular Energy Transfer, Academic Press, 1980.

to

. S. A. Hambir; Y. Jiang; G. J. Blanchard;J. Chem. Phys, 98 6075 (1993).

. Y. Jiang; G. J. Blanchard; J. Phys. Chem, 98 9417 ( 1994).

b)

4. J. T. Yardley; C. B. Moore; J. Chem. Phys, 4_6 4491 (1967).

5. C. B. Moore; Adv. Chem. Phys, Q 41 (1973).

6. B. H. Mahan; J. Chem. Phys, 4_6 98 (1967).

7. R. D. Sharma'. C. A. Brau; J. Chem. Phys, _SQ 924 (1969).

8. J. C. Stephenson, R. E. Wood; C. B. Moore; J. Chem. Phys, 48 4790 (1968).
9. J C. Stephenson, C. B. Moore, J. Chem. Phys, 52 2333 (1970).

10. T. C. Chang; D. D. Dlott‘, Chem. Phys. Lett, £2 18 (1988).

l l. J. R. Hill; D. D. Dlott', J. Chem. Phys, 8_9 830 (1988).

12. J. R. Hill; D. D. Dlott;J. Chem. Phys, 8_9 842 (1988).

13. T. C. Chang; D. D. Dlott,J. Chem. Phys, _9_O 3590 (1989).

14. H. Kim; D. D. Dlott,./. Chem. Phys, % 8203 (1991).

15. T. Elsaesser; W. Kaiser; Ann. Rev. Phys Chem, Q, 83 (1991).

16 Y. Jiang; S. A. Hambir, G. J Blanchard. Opt. Commun, 92 216 (1993)
17. E. V. Shpol‘skii; R. l. Personov; ()pt. .S'pectrosc., 8 172 (1960).

18. S. J. Cyvin, B. l\'. Cyvin, P. Klaeboe, .Syiec'tt'osc. Lett, 1_ 239 (1983).

19. S. Matsunuma; N. Akamatsu, T. Kamisuki; Y. Adachi; S. Maeda; C. Hirose, J. Chem.
Phys, E 2956 (1983).

20 W. L. Weaver; L. A. Huston; K. lwata; T. L. Gustafson; J. Phys. Chem, % 8956 (1992).

21 R. M. Butler; M. A. Lynn; T. L. Gustafson;J. Phys Chem, 91 2609 (1993).

91

. D. McMorrow; W. T. Lotshaw; J. Phys. Chem, fl 10395 (1991).
. D. McMorrow; W. T. Lotshaw; Chem. Phys. Lett., 29; 369 (1993).

. Y. Jiang; G. J. Blanchard; J. Phys. Chem, 28 6436 (1994).

CHAPTER 6. ROTATIONAL DIFFUSION DYNAMICS OF PERYLENE IN n-
ALKANE - OBSERVATION OF SOLVENT LENGTH DEPENDENCE CHANGE
OF BOUNDARY CONDITION

Summary

Orientational relaxation dynamics of perylene in both its ground electronic state and its
first excited singlet electronic state in the series of n-alkanes n-pentane through n-decane,
n-dodecane and n-hexadecane were investigated. A curvilinear relationship between
orientational relaxation time and solvent viscosity was observed, and these data were
interpreted in terms of a solvent length-dependent change in the nature of solvent-solute

interactions.

92

 

6. 1. Introduction

Understanding molecular-scale interactions between solvents and solutes has been a
subject of long standing interest because these interactions play a deterministic role in
many chemical processes and reactions. The central focus of many of these investigations
has been the nature of the local structure induced in the solvent by the presence of the
solute, and the mechanisms that are operative in the interactions between the two chemical
species. Indeed, these are questions that, for a given chemical system, have answers that
depend on the length and time scale of observation. One of the more widely used
experimental approaches aimed at understanding these complex interactions has been the
measurement of the rotational diffusion dynamics of a probe molecule dissolved in
selected solvents, where some property of the solvent such as bulk viscosity, dielectric
response, static dipole moment or molecular structure is varied in a regular mannern’m
The utility of rotational diffusion measurements stems from the fact that the length scale of
the measurement is relatively well defined by the size of the probe molecule, and that-
comparatively straightforward means exist for the theoretical treatment of the data. While
it is difficult to elucidate specific molecular interactions between solvent and solute using
rotational diffusion measurements except in special circumstances, such measurements
detect the solvent-solute interactions over a time scale where a large number of molecular

collisions occur. thereby providing insight into the average environment experienced by

the solute.

94

Many probe molecules in a wide range of solvent systems have been examined using
rotational diffusion measurements. There are essentially two classes of experiments; those
where polar probe molecules in polar solvents are used and those where non-polar probes
and solvents are used. The reorientation of the probe molecule in each type of system is
mediated by different types of intermolecular interactions. For polar systems, dielectric

“81 and even the formation of comparatively long-lived

frictionm’, dipolar interactions
solvent-solute complexes“9‘2°] have been shown to contribute to the observed response.
The multitude of comparatively long range interactions in polar systems yields
experimental results that are often ambiguous. Studies of molecular reorientation in non-
polar systems have demonstrated that, despite the absence of long range electrostatic
interactions, the data can contain contributions from a number of weaker interactions, with
at least a portion of the response being controlled by solvent “structure” on a length scale
comparable to the reorienting probe moleculem‘m In this chapter the rotational diffusion
results are presented for the non-polar probe molecule, perylene, in a series of n-alkanes,
where the size ofthe solvent molecules is varied in a regular manner. The data show that,
when the solvent molecules are significantly smaller than the solute, near stick-limit
hydrodynamic behavior is seen. and when the length of the solvent approaches that of the
probe molecule, a change to slip-limit behavior is seen. Further, as the solvent molecular

size is increased, perylene changes its effective rotor shape from essentially a spheroid to a

prolate ellipsoid.

5.2. Background

The rotational motion of a solute imbedded in a solvent has been treated theoretically by a~
number of workerslzmsl A common thread to these treatments is the approximation that
the solvent surrounding the probe molecule is essentially a continuous medium, 1‘. e. there
is no explicit consideration of the intrinsically molecular nature of the solvent surrounding
the probe molecule. A well established starting point for most treatments of rotational
[271

diffusion is the Debye-Stokes-Einstein (DSE) equation,

1 tyl'
_ _ l
TOR if] kl. [ l

where a... is the orientational relaxation time constant, n is the solvent bulk viscosity, V is
the hydrodynamic volume of the solute, and D is the rotational diffusion constant. The
assumptions implicit in Equation 1 are that the solute is spherical and that the solvent is a
continuous medium. Both of these assumptions are necessarily limited in their
applicability, but for cases where the solute is much larger than an individual solvent
molecule, the DSE model holds quantitatively. Both of the assumptions in the DSE
equation realize their limits in chemical systems where the solvent and solute have similar
hydrodynamic volumes. For polar solutes of V~300~500 A3, the DSE equation can
predict ton to within a factor of ~2,'°' despite the fact that this type of chemical system
possesses the largest number of intermolecular interactions likely to cause deviations from
the simplistic DSE picture. For non-polar systems, the difference between experimental
data and DSE predictions is substantial, often greater than a factor of two different, with
the experimental values of TOR being faster than the DSE prediction. To account for these

discrepancies, several groups have modified the DSE model to consider the boundary

96

condition for the solvent-solute interface. These works do not consider the molecular
nature of the solvent, but instead treat the solvent-solute interaction as purely frictional,
with a variable friction coefficient that depends on the solute shape. These corrections

enter multiplicatively into the DSE equation.

771' f
TOR =17; [

where f is a friction term to account for the solvent-solute interaction. The value off can
range from near zero in the slip limit to unity in the stick limit, depending on the shape of

1341

the efiective rotating ellipsoid. S is a shape factor, determined from Perrin’s

'281 which accounts explicitly for the non-spherical shape of the solute. The use

equations,
of these correction factors typically brings theory into significantly closer agreement with
experiment for many non-polar systems. While this model does not account for the

inherently molecular nature of rotational diffusion dynamics, it does provide a useful and

nearly quantitative basis for the interpretation of our experimental data.

5 . 3. Experimental

Spectrometer. The picosecond pump-probe laser spectrometer used to measure both the
ground state and excited state rotational diffusion dynamics of perylene in several n-
alkanes is same as the one used for vibrational relaxation T. measurements and has been
described in detail in Chapter 2 The excitation or pump dye laser was operated at 437 nm
using Stilbene 420 laser dye (Exciton). The output of this laser was ~ 60 mW average

power with a 5 ps FWHM autocorrelation trace at 8 MHz repetition rate. The probe dye

97

laser was also operated with Stilbene 420 laser dye, and was set to either 429 nm for
ground state recovery experiments to obtain the data for ground state reorientation
dynamics or 470 nm for stimulated emission experiments for excited state reorientation
dynamics. The probe laser wavelengths were chosen to coincide with the non-overlapped

regions of the perylene absorption and emission spectra (Figure 6.1).

Chemicals and sample handling. Perylene (99%) and all n-alkane solvents were
purchased as their highest purity grade available from Aldrich Chemical Co. and were used
as received. All perylene solutions were ~1x10°5 M and were flowed through a
temperature controlled flow cell to minimize thermal contributions to the experimental

signal. For all rotational diffusion measurements, the temperature of the sample was

maintained at 300 : 0.1 K.
6 4. Results and Discussion
Both the ground state and first excited singlet state rotational diffusion dynamics of

perylene in n-alkane solvents n-pentane through n-decane, n-dodecane and n-hexadecane

were measured. The data for R(O), ton. R°(O) and TOR. are presented in Table 6.1.

98

 

 

lntensity(a. u.)

 

 

 

0.0 ¢ + : T c . : fi
350 400 450 500 550

 

wavelength (nm)

Figure 6.]. Normalized absorption and spontaneous emission spectra of perylene
in n-octane.

99

Table 6.1. Experimental zero time anisotropies and reorientation times for perylene in
several n-alkanes. The asterisks indicate an excited electronic state measurement.

 

 

solvent 77 (cP)" 13(0) 1’10 10.. 1'10 R°(0) $10 {OR :16
(ps) (ps)
n-Can 0.24 0.18 : 0.08 8 : 1 0.12 : 0.06 9 : 2
n-CaH... 0.33 0.32 : 0.03 14 : 3 0.33 : 0.05 11 : 2
n.c.H.6 0.41 0.26 a: 0.08 15 : 3 0.22 : 0.06 14 : 2
n-CgH... 0.54 0.33 : 0.03 16 x 1 0.21 : 0.08 19 : 4
"£911... 0.71 0.21 : 0.05 19 : 3 0.24 : 0.06 21 : 5
"-00sz 0.92 0.19:0.05 21 :3 0.30:0.09 19:3
new... 135 0.27 : 0.03 28 : 3 0.25 : 0.06 32 : 7
”emit... 3.34 0.27 : 0.04 49 : 7 0.21 : 0.03 49 : 8

 

 

‘1 Data from CRC Handbook of Chemistry and Physics, 7lst Edition, D. R. Lide, Editor,
CRC Press. 1990.

The rotational diffusion time constants through individual time resolved scans were
determined, where the polarizations of the pump and probe laser pulses were set to be
either parallel or perpendicular to one another, and present representative data sets in
Figures 6.2a and 6.3a. The induced orientational anisotropy function was produced from

the individual time scans according to Equation 3 (Figures 6.2b and 6.3b).

[MU-II“)
l“(l)+2]i(l)

 

R(t):

The ground state and excited state reorientation times for perylene are found to be
identical for a given solvent, as is expected for a non-polar system. The number of
exponential decays contained in R(t) is determined by the effective rotor shape of the

reorienting species and the relative orientations of the pumped and probed transition

100

AT/I (a.u.)
N
o

 

 

 

 

 

 

 

 

10 ~
0
O 100 200 300 400
delay time (ps)
0 4i
. b
0.3 ~
0.2 P
E .
0.1 ~
0.0 ——“WW——
0 100 200 300 400

delay time (ps)

Figure 6.2. (a) Time scans for ground state recovery response of perylene
in n-octane. (b) R(t) signal produced from the experimental data
shown in (a) using Equation 3.

101

1.0 " I.(I)

AT/T (a u.)

 

 

 

 

 

O 100 200 300 400
dela time s)
0.4 ~ y (p
0.3 -
0.2 ~ ‘1

R(t)
95"”

0.1~ \

 

 

delay time (ps)

Figure 6.3. (a) Time scans for ground state recovery response of perylene in
n-hexadecane. (b) R(t) signal produced from the experimental

data shown in (a) using Equation 3.

0 0 WWW -
_ . . {4W

‘ I

102

dipole moments in the probe molecule.133‘ In principle, R(t) can contain up to five
exponential decays, but a single exponential decay is encountered for most systems. The

“"11” and in at least

rotational diffusion properties of perylene have been reported before,
two cases, a double exponential decay of R(t) was found.'“‘391 There are several
differences between those experiments and what are presented here. For the earlier
experimental works, a double exponential decay is an expected result. For these
experimental conditions, where the first excited electronic singlet state was accessed

spectroscopically, it is expected to observe a single exponential decay functionality for

R(t).

The viscosity dependence ofthe data presented in Table 6.1 is shown in Figure 6.4. These
data demonstrate a change in the effective boundary condition between the solvent and
solute as a function oftheir relative sizes. 1n the limits where the solvent is significantly
smaller or larger than the perylene probe molecule, a semi-quantitative agreement was
obtained with the modified DSE equation in the stick and slip limits, respectively,

(Equation 2).

The change over between these two limits apparently occurs at n-octane The agreement
of our data with the different limits of the hydrodynamic model is discussed below. The
focus ofthe discussion is on the slopes ofthe two clearly linear regions shown in Figure
6.4. The quantity TOR/1’] is related to the hydrodynamic boundary condition as described in

Equation 2. Because the same probe molecule for all of the measurements was used, the

103

 

 

DO _ excited state
40"- T .,.-'i’.’.”’
A O " N . ' I ’
a 3 .. , \
o , . ’ ground state
P 20 .. _I’ 7
IO — ‘11“
1:
O + J I l A: I
0 l 2 3

n (GP)

Figure 6.4. Ground state and excited state reorientation times for perylene as a
function of n-alkane solvent viscosity. For all measurements the ground
state and excited state reorientation times are the same to within the
experimental uncertainty.

104

quantity [’5 must necessarily be the solvent-dependent variable. From Pern'n’s
equationslm when modeling the probe as a prolate ellipsoid, the axial ratio of ellipsoid p =
a/b = (3 + 7.4) / 10 = 0.52 (< l), where b is the long molecular axis and a is the short

molecular rotational axis. (See diagrams in Figure 6.5)

 

= . ~ — 141

(2__p2) _:_£____h, __p2 1
“l-le p ,

where S is the shape factor in Equation 2.

 

 

 

For an oblate ellipsoid,

p=wb=uo+2h/3=29pr)

 

'7 _
-i- — ' p - 151
3

(2—p2) (p2 5) arctan «£02 — l) —p2

\“P'

l
s

 

 

.1

These equations gave S = 0 69 if perylene is modeled as a prolate ellipsoid and S = 0.70 if
perylene is modeled as an oblate ellipsoid. Therefore, because of the probe molecule
shape, the only quantity that can contain a measurable solvent-dependence isf Despite
the fact that S is virtually shape-independent for perylene, f contains information on

effective rotor shape, and we discuss this point below.

 

 

 

 

 

‘ g b - axial axis
Prolate ellrpsord a,c - other axes
K\ i ,\ p = [(a +c)/2]/b
f = [(3 + 7.4)/2]/IO
w k = 0.52 < l
Oblate ellipsoid

=[ (a +c)/2]/b
[(10 + 7 41/21/3

 

F1gure 6 5 Illustration of prolate and oblate ellipsoid rotor shape and
the axial ratio, p.

 

106

1t.should be noted at the outset of this discussion that any changes in the effective rotor
shape of perylene are manifested as changes in the relative values of the Cartesian
components of D, the rotational diffusion constant, and not in the actual shape of the

molecule.

For the four lowest viscosity solvents that were examined, n-pentane through n-octane,
the relationship between TOR and n is linear to within the experimental uncertainty, with a
slope of 40 : 3 ps/cP. The hydrodynamic volume of perylene is calculted to be 225 A3140]
For 5 = 0.7, T = 300 K, and f = 1 (stick limit), calculated TOR/T1 = 77 ps/cP. The
difference between experiment and the prediction of the model is slightly less than a factor
of two, but the data are not consistent with slip limit hydrodynamics (vide infra). For
perylene reorienting as an oblate rotor, slip hydrodynamics predicts TOR/T] = 5 ps/cP and
for a prolate rotor. the slip limit prediction is IOR/q = 16.5 ps/cP. Thus for n-alkanes C5
through C3 the observed reorientation behavior is intermediate between the slip and stick
limits. The weak point in any such analysis is knowledge of the effective hydrodynamic
volume V and thenon-spherical rotor shape correction, S. for the probe molecule. If
perylene is assumed to reorient as a sphere (S=l) then the stick limit DSE prediction is
IOR/n = 54 ps/cP. in excellent agreement with the experimental data. This nearly
quantitative agreement is viewed as fortuitous, but indicative that the effective rotor shape

of perylene in short chain alkanes is only weakly anisotropic.

For the four longest chain n-alkane solvents, n-nonane, n-decane, n-dodecane and n-

hexadecane. the slope of the best fit line for TOR/I] = 12.7 : 1.5 ps/cP, a factor of three

107

different from the value of IOR/n for the shorter n—alkanes. Clearly there is a fundamental

change in nature of the interactions between perylene and n-alkanes as a fimction of alkane,

length, and this change occurs for solvents longer than n-octane. The slope of TOR/T] for
the longer n-alkanes is in excellent agreement with slip hydrodynamic predictions for
perylene acting as a prolate rotor, i.e. reorientation predominantly along its x (long) axis
(Figure 6.6). The slip prediction for perylene reorienting as an oblate rotor is significantly
less than that observed experimentally. If we neglect the non-spherical shape correction
(S=1), the slip limit for a prolate rotor is predicted to be 11.6 ps/cP and the oblate rotor is
predicted to be 3.5 ps/cP. Thus, regardless of the extent of anisotropy in the shape of the
perylene molecule itself, the long chain aliphatic solvents constrain its motion to be

predominantly about its long axis.

The experimental TOR/T] data in the longer n-alkanes do not provide the only evidence for
perylene behaving as a prolate rotor. A supporting, but inconclusive, piece of evidence
lies in the observed fiinctionality of the experimental R(t) decay curves. The Chuang and
Eisenthal formulation for reorientation of an anisotropic probe molecule includes
treatments for conditions where the measured transition dipole moments lie along arbitrary

~~
lé-‘l

angles with respect to the molecular long and short axes.

For the purposes of this discussion the x axis is assigned as the perylene long axis in the
molecular plane, the y axis as the short in-plane axis and the z axis normal to the molecular

plane, as indicated in Figure 6.6.

“1‘

Ha-

108

7.421

 

 

10A 3A(z)

Figure 6.6. Dimensions and Cartesian axis assignments for perylene.

109

Because perylene is planar the full Chuang and Eisenthal expression simplifies to

R(t) = 0.3(,6’+ a)exp(—(6D + 2A)t) + 0.3(,6— a)exp(-(6D - 2A)t) [6]
where a and B are terms relating the values of the Cartesian components of the rotational
diffusion constant and the relative angles of the excited and observed transition dipole
moments with respect to the Cartesian axes. D is the average of the Cartesian
components of the rotational diffusion constant, and A is a term describing the anisotropy
in the Cartesian components of D. Equation 6 might be taken to suggest that R(O) can
exceed its theoretical maximum value of 0.4, but limits on the values of or and B preclude
this possibilitym' Equation 6 is a general expression for a planar molecule, and additional
restrictions on the orientations of the pumped and probed transition dipole moments and
the anisotropy in D allow the prediction of the number of exponential decays in the
experimental R(t) function. In principle, perylene can reorient either as an oblate rotor or
as a prolate rotor. For an oblate rotor, Dz > DX=Dy and for a prolate rotor, D. > Dy=Dz.
For a symmetric molecule such as perylene, the transition dipole moments will lie along
the Cartesian axes defined for the rotational diffusion constants. The S. <— S.. transition
accessed experimentally is polarized along the long (x) molecular axis. R(t) exhibits a
double exponential decay if perylene reorients as an oblate rotor and to decay as a single

exponential if perylene reorients as a prolate rotor.

oblate: R(t) = 0.3-exp(-(2/),.+4/)_.)1) + 0.1.exp(- 613,1)
[7]
prolate: R(t) : 0.4-exp(— 61):!)

A single exponential functionality was observed for R(t) providing support for, but not

proof of, our assertion that the reorientation dynamics of perylene in the longer n-alkanes

110

is consistent with those of an effective prolate rotor. The practical limit on the ability to
use the R(t) functionality to determine effective rotor shape lies in the finite signal to noise

ratio of the data and the unknown relative values of D., Dy and D...

The measured values for R(O) and R.(O) do not achieve the theoretical maximum of 0.4.
Previous work on a large number of polar systems show that the theoretical maximum
value for R(O) is difficult to obtain experimentally, with a typical maximum being ~ 0.33
for slowly reorienting molecules. The reasons for this experimental limitation are not
understood fully, but polarization scrambling by the flow cell face(s) or the finite
extinction ratio (~50: 1) of the pump and probe electric field polarizations could contribute
to the observed behavior. For very fast reorientation, it is expected that the instrumental
response function will serve to obscure the early time response and potentially reduce the
regressed R(O) value. Previous reports on perylene reorientation also report an inability to
achieve the theoretical maximum R(O) value."” In those reports, a double exponential
decay of R(t) was reported. For their experimental conditions, where the excited
transition was the S; (— So y-axis polarized transition and the So (— S. x-axis polarized
transition was monitored, a double exponential decay indicates that perylene behaved as an
effective oblate rotor. There is not discrepancy between earlier work on perylene
indicating an effective oblate rotor shape and our work indicating an effective prolate
rotor shape in longer n-alkanes because of the differences in the solvent systems examined.

It is entirely likely that perylene exhibits an environment-dependent effective rotor shape.

111

6.5. Conclusion

The rotational diffusion dynamics of perylene in a series of n-alkane solvents were
presented. The reorientation time does not depend linearly on the solvent viscosity, but,
rather exhibits two distinct linear regions. For shorter chain n-alkanes C5 through C3 the
solvent-solute boundary condition lies close to the stick hydrodynamic limit, and for
longer solvents C9, C..., C .2 and C... a Slip boundary condition applies. Further, the data
for the longer chain solvents suggest that perylene behaves as an effective prolate rotor.
The abrupt change in boundary condition occurs at n-octane. Solvents n-pentane, n-
hexane and n-heptane are shorter than the perylene long molecular axis, while solvents
longer than n-nonane are distinctly longer. n-Octane, the solvent at which the change in
boundary condition is seen is approximately the same length as the perylene long axis; for
this solvent-solute size ratio one of the basic assumptions of the DSE model is clearly
violated. While there is obviously not enough information contained in these
measurements to determine structural information about the solvent “cage” surrounding
perylene, we notethat for solvents of length greater than C.., the solvent cage can, in

principle, be comprised of molecules that span the entire solute length

112

66. Literature Cited

Ix)

b)

9.

10

. H. Labhart; E. R. Pantke; Chem. Phys. Lett., Q 482 (1973).

. G. R. Fleming; J. M. Morris; G. W. Robinson; Chem. Phys, 17 91 (1976).

. T. J. Chuang; K. B. Eisenthal; Chem. Phys. Lett., 11 368 (1976).

. P. E. Zinsli; Chem. Phys, 29 299 (1977).

. A. von Jena; H. E. Lessing; Chem. Phys, 40 245 (1979).
. D. P. Millar; R. Shah; A. H. Zewail; Chem. Phys. Lett., _6_6 435 (1979).
. U. K. A. Klein; H. P. Haar; Chem. Phys. Lett., Q 40 (1979).

. A. von Jena; H. E. Lessing; Chem. Phys, £187 (1981).

D. H. Waldeck; G. R. Fleming; J. Phys. Chem, Q 2614 (1981).

. E. F. Gudgin Templeton; G. A. Kenney-Wallace; J. Phys. Chem, fl 2896 (1986).
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. J. Blanchard; M. J. Wirth; J. Phys. Chem, 9_0 2521 (1986).

. Ben-Amotz; T. W. Scott; J. Chem. Phys, fl 3739 (1987).

. J. Blanchard; J. Chem. Phys, 8_7 6802 (1987).

. Ben-Amotz; J. M. Drake; J. Chem. Phys, _8_9 1019 (1988).

. J. Blanchard; C. A. Cihal; J. Phys. Chem, 2 5950 (1988)”.

. J. Blanchard; J. Phys. Chem, 22 6303 (1988).

. J. Blanchard; J. Phys. Chem, 9_3 4315 (1989).

C) C) C) O C C3 U C)

. J. Blanchard; Anal. Chem, 61 2394 (1989).

U

. S. Alavi; R. S. Hartman; D. H. Waldeck; J. Phys. Chem, 9_5 6770 (1991).

113

21. R. S. Hartman; D. S. Alavi; D. H. Waldeck; J. Phys. Chem, 2; 7872 (1991).

22. M. Lee; A. J. Bain; P. J. McCarthy; C. H. Han; J. N. Haseltine; A. B. Smith;III; R. M.
Hochstrasser; J. Chem. Phys, 85 4341 (1986).

10
L.)

. G. J. Blanchard; J. Phys. Chem, 9; 5293 (1991).

24. R. S. Hartman; D. H. Waldeck; J. Phys. Chem, 3 1386 (1994).

25. D. Kivelson; K. G. Spears; J. Phys. Chem, _8_9 1999 (1985).

26. R. S. Moog; D. L. Bankert; M. Maroncelli; J. Phys Chem, fl 1496 (1993).
27. P. Debye; Polar Molecular: Chemical Catalog Co: New York, p84 (1929).
28. F. Perrin; .1. Phys. Radium, Z l (1936).

29. L. D. favro; Phys. Rev, 119 Q (1960).

30. T. tao; Biopolymers, 8 609 (1969).

31. G. J. Weber;./. Chem. Phys, _5_5 2399 (1971).

32. G. G. Belford; R. L. Belford; G. Weber; Proc. Natl. Acad. Sci, U. S. A., Q2 1392
(1972).

33. T. J. Chuang; K. B. Eisenthal; J. Chem. Phys, 51 5094 (1972).
34. C.-M. Hu; R. Zwanzig; J. Chem. Phys, @ 4354 (1974).
35. G. K. Youngren; A. Acrivos; J. Chem. Phys, 63 3846 (1975).
36. R. Zwanzig; A. K. Harrison. J. Chem. Phys, 83 5861 (1985).
37. G. Weber; .1. Phys. Chem, 93 6069 (1989).
38. D. J. Kivelson; J. Chem. Phys, g 709 (1991).
39. J. T. Edward;J. Chem. Ed, Q 261 (1970).

40. D. W. Piston; T. Bilash; E. Gratton; J. Phys. Chem, 93 3963 (1989).

A}.-.

t.;._ ._-‘-*AI-g-A
1‘

CHAPTER 7. VIBRATIONAL POPULATION AND ORIENTATIONAL
RELAXATION DYNAMICS OF l-METHYLPERYLENE IN n-ALKAN ES - THE
EFFECTIVE RANGE OF DIPOLAR ENERGY RELAXATION IN SOLUTION

The vibrational population and orientational relaxation dynamics of l-methylperylene in
the series of normal alkanes n-pentane through n-decane, n-dodecane and n-hexadecane
were studied Both the vibrational population relaxation time constant. T., of the l-
methylperylene 1370 cm'1 mode and the orientational relaxation time constant(s), 10., were .
found to depend sensitively and nonlinearly on the aliphatic chain length of the n-alkane
solvent. The data show that the two relaxations are sensitive to solvent local organization
on approximately the same length scale, and stand in contrast to the relaxation dynamics
of perylene in the same n-alkane solvents, where the operative length scale of T. relaxation
was found to be substantially shorter than the length of the perylene molecule. These
differences were understood in the context ofthe different polar v-v coupling processes
utilized by perylene and l-methylperylene. The rotational diffusion data for l-
methylperylene indicate that the dominant reorientation axis of the chromophore changes

with solvent aliphatic chain length.

114

115

7.1. Introduction

Earlier investigations in this thesis have focused on the probe molecule perylene in both
polar and nonpolar solvents, and the ability to measure relaxation in both the ground and
excited electronic states of the “probe molecule using this technique has been
demonstrated.” The relaxation of the perylene Raman-active 1375 cm'1 mode in n-alkane
solvents has been used to explore the role of resonance energy transfer in non-polar
liquidsm Normal alkanes exhibit an infrared-active terminal methyl group rocking motion
at 1378 cm". The perylene/u-alkane systems were chosen to evaluate whether or not local
organization could be detected in nonpolar solutions. The data indicated that relaxation of
the perylene 1375 cm'1 mode in n-octane is ~10 times more efficient than in n-hexane or n-
decane. While these data indicated clearly that solvent local organization is important to
vibrational energy relaxation, the T. measurementsm did not correlate with orientational
relaxation measurementsm suggesting that the local “structure” in the n-octane
surrounding perylene persists, at most, over several Angstrdms. These data are
informative because they reveal the presence of local organization, but the length scale
which T. measurements probe remains unclear. In this chapter the investigation of both
the vibrational population (time constant T.) and orientational (time constant(s) To.)
relaxation dynamics of l-methylperylene in several n-alkanes are presented. It is found
that, in contrast to perylene, these two relaxation processes sense local solvent
organization over similar length scales for l-methylperylene. We understand the
differences between the perylene and l-methylperylene T. dynamical responses in terms of

the symmetry ofthe chromophores.

116

7.2. Experimental

Spectroscopy. The spectrometer used for stimulated measurements of l-methylperylene
has been described in detail in Chapter 2.“) Stilbene 420 dye (Exciton) is used for both
dye lasers. The pump laser is operated at 429 nm, corresponding to the 0-0 transition for
l-methylperylene in n-pentane and n-hexane, at 430 nm in n-heptane and n—octane, at 431
nm in n-nonane. u-decane and n-dodecane, and at 432 nm in n-hexadecane. The l-
methylperylene vibrational mode that was focused on occurs at ~1370 cm". The probe
wavelength is 456 nm for n-pentane and n-hexane, 457 nm for n-heptane and n-octane,
458 nm for n-nonane through n-dodecane, and 459 nm for n-hexadecane. For the T.
relaxation measurements the probe laser polarization was set to 547° with respect to the
pump laser polarization to ensure the absence of orientational relaxation effects in the
data. Rotational diffusion measurements were made separately and, for these
measurements, individual scans were taken for the probe laser polarization parallel and
perpendicular to the pump laser polarization. The form of the experimental signal we

obtain from these measurements has been presented in detail previously.

Steady-state spectroscopy. The steady-state absorption and emission spectra of all 1-
methylperylene solutions were measured with 1 nm resolution using a Spex Fluorolog2
model F1 1 lAT spectrometer. These data were used to estimate the spectroscopic origin

of l-methylperylene in each solvent.

CIT.

'-.‘A“-_.A__ l'_‘. _L

1
1
H 4‘

117

Iz'ibrational spectroscopy. The infrared spectrum of solid l-methylperylene on KBr was
recorded on an FTIR spectrometer at 4 cm'1 resolution (Mattson Galaxy 3000 ) using a
DTGS detector. The spontaneous (resonance) Raman scattering speCIrum of solid 1-
methylperylene in a capillary tube was obtained using 363.8 nm excitation from an Ar'
laser (Coherent Innova 200). The Raman detection equipment consisted of a SPEX 1877
Triplemate spectrometer equipped witha 1800 groove/mm grating, operating at ~ 6 cm'1
resolution. The CCD detector was an EG&G Princeton Applied Research Model

Spectrum One.

Chemicals and sample handling. l-methylperylene was synthesized by alkylation of
perylene with methyllithium. (see Scheme 7.1)‘51 This reaction was reported to methylate
perylene at the 1- position with >95% selectivity. Perylene, methyllithium and 10% Pd on
C catalyst were purchased from Aldrich and used as received. Following purification by
plate chromatography, the identity of l-methylperylene was confirmed by mass
spectrometry, 1H NMR, and UV-visible and infrared absorption measurements. All it-
alkane solvents were purchased from Aldrich in their highest purity grade and used
without further purification. The concentrations of the l-methylperylene solutions used
for the T. relaxation and rotational difiusion measurements were ~10 11M. Sample
solutions were flowed through a 1 mm path length quartz cell to minimize thermal lensing
contributions to the signal, and the sample temperature was controlled at 300 : 0.1 K

using a thermostatted bath.

118

©© H3C\ / CH3

@ . + CH3L1 + /NCHz—CH2N

oo ““3
H
80°C, 24 hrs CC H
o

 

+

ether 0‘
,. methyldihydroperylene

(I)

(I) Pd/C (5%) 4 @@© H3
©©

1 -methylperylene

 

Scheme 7. 1. Synthesis of l-methylperylene from perylene

119

7 .3. Results and Discussion

Both the motional and vibrational energy relaxation dynamics of l-methylperylene in the
alkanes n-pentane through n-decane, n-dodecane and n-hexadecane were measured, and
the solvent-dependence of these two dynamical responses are found to be correlated. This
finding is different from the case for perylene in the same n-alkanes, and we understand at
a qualitative level, the basis for this difference. First the vibrational population relaxation
dynamics of l-methylperylene and then its orientational relaxation dynamics in the n-
alkanes will be discussed. The comparison of these bodies of data to one another and to

12.31

our earlier reports on perylene provides an understanding of the intermolecular

interactions and local organization responsible for our observations.

l'ihranona/ population relaxation. The method for measuring vibrational population
relaxation time constants (T.) of fluorescent probe molecules in solution“'”’ has been
discussed in Chpater 3. In Chapter 5 on perylene in the n-alkanes, the perylene v. (1375
cm") vibrational mode was chosen to interrogate because of its degeneracy with the
terminal methyl group rocking mode of the n-alkanes. Because the dominant motion of
the n-alkane vibration is confined to the CH3 groups, this “acceptor” vibrational mode was
well suited to sensing local organization over short length scales in liquids. The data
indicated that, for this mode, the solvent n-octane organized in such a way as to place its
methyl groups in closer spatial proximity to the perylene vibrational coordinate than the
other alkanes, and that other vibrational modes revealed preferential organization of

different n-alkanes along their vibrational coordinates. While these results were important

120

because they demonstrated the existence of local organization within n-alkane solutions,
they were also perplexing because of their lack of correspondence with reorientation
dynamics measurements, where the molecular length scale of the measurement is
comparatively well defined. The investigation of l-methylperylene in the same n-alkanes
was undertaken because it is expected for the structural aspects of its interactions with the
n-alkanes to be qualitatively similar to those for perylene but with spectroscopic selection

rules that are relaxed significantly compared to those for perylene.

Before presenting the T. data for l-methylperylene, it is important to examine the
electronic and vibronic spectral response of this probe molecule. The absorption and
emission spectra of l-methylperylene in n-hexadecane are shown in Figure 7.1. Not
surprisingly. the linear optical response of I-methylperylene is very similar to that of
perylene, except that individual features for l-methylperylene are blue-shifted by ~5 nm
(~300 cm") compared to perylene in a given solvent, in agreement with previous
reportsn‘s' This blue shift can be understood in terms of the strain imposed on the
perylene ring structure by the presence of the methyl group at the 1- position. Semi-
empirical calculations of l-methylperylene indicate a dihedral angle in the range of 13° to

'8' The 0-0 transition energy of I-methylperylene in

23° between naphthalene moieties
each solvent is obtained from the linear response and these data are listed in Table 7. 1.

These data were used to determine the pump and probe laser wavelengths for the T.

relaxation measurements.

121

 

 

 

 

 

 

, 432 nm

1.0 -
A 0.8 -
L": .
3
“m’ 0.6 - Absorption Emission
g 1
Q.
Q .
a 0.4 l" /
Q)
.E
..1

0.2 P

0.0 . . A 1 - - 1 4

350 400 450 500

wavelength (nm)

Figure 7.1. Steady-state absorption and emission spectra of l-methylperylene in n-
hexadecane. The arrow indicates the 0-0 transition energy, and the box the
range over which the stimulated response, shown in Figure 7.3, was taken.

122

Table 7.1. Spectral origin of l-methylperylene in the solvents used in this work,
determined from the linear optical response.

 

 

Solvent 0-0 transition energy
‘ (001")
n—pentane 233 10
n-hexane ' 23310
n-heptane 23256
n-octane 23 256
n-nonane 23202
n-decane 23 202
n-dodecane 23 202
n-hexadecane 23 148

 

 

While the electronic spectra of l-methylperylene are similar to those of perylene, the
infrared and Raman spectra of l-methylperylene and perylene differ significantly. The
origin of the difference in the vibrational responses of these two structurally similar
chromophores lies in the reduction of symmetry by the addition of a methyl group.
Perylene is of D2. symmetry. a point group containing a center of inversion. Any Raman-

active vibrational mode is infrared inactive, and vice versa for a molecule belonging to a
point group containing an inversion center. The addition of the CH3. group to the perylene
molecule eliminates the center of inversion, and all l-methylperylene normal modes are
both Raman and infrared active. The infrared and Raman spectra of l-methylperylene

were present in Figure 7.2. The 1370 cm'l mode is present in both spectra. The 1370

100— a
9? 95~
51
90-
'_"=_'_ U
(L) h
E 85’

 

 

Intensity (cps)

 

 

 

Rarrnn shift (on!)

Figure 7.2. (a) Infrared and (b) Raman spectra of I-methylperylene. The asterisks
indicate the vibrational resonance for which we determined T. times.

124

cm'1 of l-methylperylene mode is believed to be derived from the perylene 1375 cm'1
mode based on the experimental energies of the resonances as well as semiempirical

calculations.

Figure 7.3 shows the time resolved stimulated emission spectrum of l-methylperylene in
n-hexadecane, where vpump was fixed at v0-.. and vpmb. was stepped over the range
corresponding to v... between ~117O cm'1 and ~1920 cm". At each pump-probe
frequency pair, a time scan was recorded, and the frequency-domain response(s) were
reconstructed by normalization of the time scans to the spontaneous emission spectrum at
long delay timelm The spectra are time slices of the reconstructed surface taken at delay
times of 10 ps, 30 ps , 50 ps and 100 to 700 ps delay, in 100 ps intervals. Multiple ground
state vibrational resonances were detected within one broad steady-state spontaneous
emission band (boxed feature in Figure 7.1), reminiscent of the earlier work of perylene in
polar solvents'“ Because the vibrational response of l-methylperylene has not been
assigned, and the Franck-Condon factors for the vibronic transitions accessed are
significantly different than those for the spontaneous Raman response shown in Figure 7.2,
To assign the individual resonances in the time-resolved stimulated spectra shown in

Figure 7.3. was not attempted. For perylene, several combination and overtone
resonances that are weak in the spontaneous Raman response are significant in the
stimulated spectrum. The similar resonances are expected to play a role in the 1-

methylperylene data as well.

Foo}

125

1.0 —-
—I—10 ps

—.—30
_ —A—50
—v—100
‘ 9/ ° —o—200

. V, g A —1:1—

\ / lA/.-\ _O_

/ -A-

ATfl (a.u.)

o..- .9375
g.

 

Il>0
\

OI

OID

 

l //OI ”a?

06 l 4 J 1 l 1 l I
21200 21400 21600 21800 22000

frequency (cm")

Figure 7.3. The time-resolved stimulated spectra of l-methylperylene in n-hexadecane,
from 21978 cm" to 21231 cm". This range corresponds to vibrational
frequencies between 1 170 cm’l and 1920 cm".

126

While the frequency-domain spectra contain a great deal of information, the quantitative
T. information of interest is contained in the time domain responses. Presented in Figure
7.4 are the individual time domain scans for the l-methylperylene 1370 cm'1 mode in each

n-alkane solvent where T. is observed as a build-up in intensity at early delay times. The

' mode are presented in Table 7.2.

time constants T. for the 1-methylperylene 1370 cm'
The uncertainties in these data are derived from at least five individual determinations for

each solvent. A typical single determination is itself the average of 10 to 15 data ,1

acquisition cycles (time scans). q3

Table 7.2. T. relaxation times for the l-methylperylene 1370 cm'l mode in the u-alkane
solvents.

 

 

solvent T. i l 0'
(ps)

n-pentane 14 : 3
n-hexane 14 : 3
n-heptane 18 : 2
n-octane 18 : 1
n-nonane 28 : 8
n-decane 70 : 8
n-dodecane 77 : 8
n-hexadecane 105 : 7

 

 

 

127

 

NWT (a.u.)

1 A J A

100 200 300 400 500

 

delay time (ps)

Figure 7.4. The stimulated response of l-methylperylene (1370 cm'1 mode) in eight
alkanes.

 

128

Shown in Figure 7.5 is the dependence of the measured l-methylperylene T. times on n-
alkane solvent identity. These data are significantly different than those reported for
perylenem Clearly, the addition of a methyl group to the perylene chromophore alters the
coupling between the solvent and the solute significantly. For short chain solvents, n-
pentane through n-octane, T. is fast (~ 20 ps) and changes little as the solvent aliphatic
chain length increases. For the solvents n~decane through n-hexadecane. T. is significantly
slower, and slightly more solvent-dependent. A transition in T. relaxation behavior occurs
between n-octane and n-decane. The T. times for l-methylperylene are, on aggregate.
faster than for perylene, indicating more efficient coupling to the solvent environment.
Perhaps more telling is that the dependence of T. on aliphatic chain length is very different

for the two molecules.

The magnitudes of the T. relaxation times measured are determined largely by the
mechanism of the relaxation. Any local molecular organization within the solvent
surrounding the chromophore is reflected in the modest solvent-dependent variations in T.
we detect experimentally. In designing these experiments, the l-methylperylene 1370 cm'1
mode and n-alkane solvents were deliberately chose to use because of the degeneracy of
this chromophore vibration with the solvent bath mode at 1378 cm’I (CH; rocking mode).
Because of this degeneracy, polar (non-collisional) v-v coupling processes dominate
vibrational energy transfer between the l-methylperylene and the solvent.“°'

lntramolecular relaxation processes can also occur, but their contribution will be the same

for all of the solvents we study here, and thus any solvent-dependence measured in T. will

120

100

80
E3:- 60
t:

40

20

 

129

 

E II
35 E
A l I J 1 l 1 l l 4]
6 8 10 12 14 16
Number ofC 's in alkane

Figure 7.5. Solvent chain length dependence of the l-methylperylene 1370 cm'1 T.
relaxation time.

arise from intermolecular relaxation processes. Aside from probability arguments relating
to the magnitude of T. that was presented in Chapter 6 and the solvent-dependence of the .
experimental T. data indicate the dominance of polar coupling over collisional relaxation
processes. If collisional energy transfer processes dominate, T. will vary continuously
with increasing solvent alkane chain length because of the direct relationship between
solvent-solute collision rate and solvent‘viscosity. Also, the decrease in fractional density
of solvent CH3 moieties with increasing solvent aliphatic chain length will contribute to a
smooth dependence of T. on solvent chain length. This trend was not observed

experimentally, implying the dominance of polar v-v coupling.

The T. times for the l-methylperylene 1370 cm'1 mode increase with solvent aliphatic
chain length, but there is an abrupt increase in T. starting with n-nonane. This solvent-
dependence in T. implies the existence of local solvent organization around 1-
methylperylene. 1n solvents shorter than the l-methylperylene normal mode coordinate
probed, both solvent CH; groups are likely in close spatial proximity to the probe
molecule, permitting efficient intermolecular energy transfer. For longer chain solvents,
where the length of a solvent molecule is similar to or greater than the maximum
dimension of the l-methylperylene vibrational coordinate, it is possible for the solvent
terminal CH; groups to interact with the probe molecule, but the average distance between
probe molecule and solvent terminal CH3. groups will be greater, on average than they are
for the shorter solvents. This argument is recognized to be qualitative, and in room

temperature liquids there is a broad distribution of n-alkane molecular conformations. this

 

131

interpretation is presented as a qualitative, empirical explanation consistent with the
experimental data. It is expected that the interactions between l-methylperylene and the

alkane solvents are similar to those between perylene and the alkanes, and that the
differences measured experimentally arise primarily from spectroscopic rather than
molecular geometry considerations. This point is discussed below. A recent paper by the
Topp group on the rotational coherence response of perylene/alkane complexes in a
supersonic jetl'” suggests that there are significant interactions between perylene and the
n-alkane(s) along the chromOphore long axis. The details of the connection between the
Topp group’s data and ours remains to be made, but structured perylene/n-alkane

complexes have been observed at low temperature.

The T. relaxation behavior of l-methylperylene in n-alkane solutions is different than that
for perylene/alkane solutions. The primary reason for this difference is believed to be the
removal of the chromophore center of inversion by the addition of the CH3. group. The
consequent change in vibrational selection rules is important because it allows access to
vibrational modes in l-methylperylene that are both infrared and Raman active. For
perylene, one can access and detect Raman active, infrared inactive vibrational modes.
Raman-active modes exhibit a change in polarizability on vibrational motion, while
infrared active modes exhibit a change in dipole moment on excitation. The Raman-active
perylene modes will exhibit modulations of their quadrupole moment (or higher multipole
moments) on vibrational excitation where, for l-methylperylene, without a center of

inversion, its vibrational modes exhibit a change in dipole moment on vibrational motion.

132

The dominant solvent bath mode is the infrared-active n-alkane terminal CH3. rocking
mode. For perylene the dominant relaxation is therefore via quadrupole-dipole coupling
(interaction energy 0: r'7)“2] while for l-methylperylene the most important relaxation

process is through dipole-dipole coupling (interaction energy cc r6)” Because these two
coupling processes operate over different length scales, we expecte the local environment
sensed by perylene T. relaxation measurements to be significantly more confined than that
for l-methylperylene. The difference in the T. data for perylene and l-methylperylene
indicates that, for perylene, anharmonic coupling between vibrational modes does not
contribute significantly to the measured T. response. If anharmonic coupling of the
perylene Raman-active modes to its infrared active modes was significant,

we would expect its solvent-dependent T. response to be similar to that of l-

methylperylene, and we do not observe this trend experimentally.

Rotational diffusion measurements. In the absence of a comprehensive theoretical
treatment of T. relaxation processes in liquids, and without a means to calibrate the length
scales over which these T. relaxations operate, it is necessary to compare these T. data to
a different dynamical response where the length scale of the dynamics is better
understood. Rotational diffusion is a technique used widely for understanding the
complex interactions between probe molecules and solvent molecules. Because rotational
diffusion measurements sense the motion of the entire electronic chromophore, it is
difficult to determine the existence of site-specific molecular interactions between the

solvent and solute except in cases where the molecules contain the appropriate polar or

reactive functionalitiesm'm' What is significant for the purposes of this work is that the
length scale of rotational diffusion measurements is comparatively well defined by the
hydrodynamic volume of the probe moleculelm This “benchmark” can provide insight

into the Operative length scale of polar v-v T. relaxation processes.

In a rotational diffusion experiment, excitation of an ensemble of probe molecules by a
polarized light pulse photoselects an anisotropic subset of the ensemble. This induced
orientational anisotropy relaxes to a random distribution with characteristic time
constant(s) and functionalities. The time course of the re-randomization contains
information on the shape of the volume swept out by the rotating probe molecule (its rotor

119-211

shape)‘”” and on the solvent-solute boundary condition. The induced orientational

anisotropy function is extracted from experimental data according to Equation 2

1..(t)—l.(t)

R(/) =
1"(l)+ 211(1)

 

where I..(t) and l,(t) are the signal intensity for pump and probe electric field polarizations
parallel and perpendicular to each other, respectively. In general, R(t) can contain up to
five exponential decaysml depending on the shape of the volume swept out by the
reorienting molecule and the orientation of the pumped and probed transition moments
with respect to the Cartesian diffusion constant axes. Under most circumstances. a single
exponential decay of R(t) is observed. and thus there can be significant ambiguity in the
interpretation of the experimental data. In cases where only limited information is
available about the probe molecule transition moment orientation(s) or where a single

exponential decay of R(t) is observed. the viscosity-dependence of the R(t) decay time

constant can be measured to extract information on the frictional interaction between
solvent and solutem The modified Debye-Stokes-Einstein (DSE) equation is used

frequently to relate the decay time of R(t) to the viscosity of the solvent and the volume of

the solute molecule,“9’2‘~23l
T — —l- : I]: . I. [
0" 6D H s

where T.,, is orientational relaxation time, n is the solvent bulk viscosity, V is the
hydrodynamic volume of the solute, (243 A3 for 1—methylperylene)“7] and D is the
rotational diffusion constant. The terms f and S are correction factors to account for the
solvent-solute boundary condition and the non-spherical shape of the reorienting species,
respectively. Before discussing the information content of the l-methylperylene
reorientation data. we want to make clear that we will focus our attention on the effective
rotor shape of the reorienting species, S, and not on the solvent-solute boundary
condition, f In Chapter 5 the focus was on the solvent-dependent change in the solvent-
solute frictional interactions. because the data were of a form amenable to this treatment”
The reorientation data presented here on l-methylperylene in the same n-alkanes are of a
significantly different functionality. This difference necessarily arises from the presence of
a single methyl group, producing an effect which is a combination of the transition dipole
moment orientation of l-methylperylene and subtle differences in the way this probe
molecule interacts with the solvents. A portion of this difference may also be the result of
the torsional strain introduced to the aromatic ring system by the addition of the CH3

group."" The rotational diffusion constant can be decomposed into its Cartesian

components and Chuang and Eisenthal have related the anisotropy decay determined

135

experimentally (Equation 2) to the relative directions of the pumped and probed transition
dipoles and the Cartesian components of the rotational diffusion constant (D) for a general
ellipsoidml For l-methylperylene the z axis is taken to be perpendicular to the molecular
1: system plane, with the transition dipole moment(s) along the x (long) axis of the
electronic chromophore. There are two general ellipsoidal forms used to describe the
volume swept out by a reorienting probe molecule. These are an oblate ellipsoid and a
prolate ellipsoid. For an oblate ellipsoid, the fastest reorientation occurs along the axis
perpendicular to the molecular plane (Dz > DS = D...) and‘for a prolate ellipsoid the
dominant reorientation axis lies within the molecular plane, usually along the longest in-
plane axis (D“ > D. = D.) For l-methylperylene, an experimental R(t) functionality that

depends on the effective rotor shape is expected?“

oblate: R(t) = (%..)exp(—(2Dx + 4D: )t)+(%o) exp(—6Dxt) [4]
prolate: R(t): (V...) exp(—6D_,t) [5]

The reorientation dynamics of l-methylperylene in the n-alkanes are such that one can
extract significant information on the effective rotor shape of the probe molecule. The
rotational diffusion data were presented in Figures 7.6 and 7.7. The data in Figures 7.6a
and 7.7a are the tail-matched 1(1) and 1(1) signals in n-pentane and n-hexadecane,
respectively. The data in Figures 7.6b and 7.7b are the anisotropy decays, R(t),

synthesized from the data in Figures 7.6a and 7.7a according to Equation 2.

136

1.0 —

 

1.10
0.5 ~

 

 

AT/T (a u.)

 

0.0

 

100 200 300 400 500

0.4 -

R(t)

 

 

 

delay time (ps)

Figure 7.6. (a) Tail-matched parallel and perpendicular intensity data and (b)
anisotropy decay, R(t), of l-methylperylene in n-pentane.

I37

 

1.0 - / mm a
:i
t l
l-
< .-

 

 

 

 

 

 

 

500
52’
J 1 L 1 L 1 I 1 I 1 1
0 100 200 300 400 500
delay time (ps)

Figure 7.7. (a) Tail-matched parallel and perpendicular intensity data and (b)
anisotropy decay, R(t), of l-methylperylene in n-hexadecane.

138

In n-pentane through n-octane, R(t) exhibits single exponential decay functionality,
whereas in n-nonane through n-hexadecane, a double exponential decay is required to
provide adequate agreement with the experimental data. The R(0) values as well as the
decay times, 10,, are presented in Table 7.3. The viscosity-dependence of these data is
shown in Figure 7.8.

Clearly there is a significant difference between the motional dynamics of 1-

methylperylene in short (5 C3) and long chain (2C9) alkane solvents.

Table 7.3. Experimental rotational diffusion time constants and zero-time anisotropies.
All uncertainties are reported as standard deviations (: lo).

 

 

solvent viscosity R .( 0) r0,(1) R2( 0) 5,0)
(cPf ' (as) (123)
n-pentane 0.24 0.22:0.02 12:1 -- --
n-hexane 0.33 0.25:0.02 13:2 -- --
n-heptane 0.41 0.24:0.04 15:1 -- --
n-octane 0.54 0.28:0.03 16:1 -- --
n-nonane 0.71 0.21:0.03 14:2 0.07:0.04 49:14
n-decane 0.92 0.24:0.03 17:3 0.04:0.03 70:26
n-dodecane 1.35 0.28:0.01 21:2 0.05:0.01 126:29
n-hexadecane 3.34 0.21:0 04 25:3 0.09:0.02 152:23

 

 

a. Data from CRC Handbook of Chemistry and Physics, 7lst ed.; Lide, D. R., CRC Press;
Boca Raton, FL, 1990

 

 

1

 

 

 

 

 

 

I39
175 — _—
130 r- —P T
12 0 '1’
D )—
C16
.00 .. C1o __
g . C12
‘6 75 — .
H C9 T
50 -
l.
- C.
25 — L is II E
0 1 J J I L l 1 I 1 1
00 0.5 10 l.' 20 2.5 30 35
0 (GP)

Figure 7.8. Orientational relaxation time, 10., as a function of solvent viscosity. For
the long chain alkanes (n-nonane to n-hexadecane), two exponential
decays are found in R(t).

E"

140

While there are several ways in which this solvent-dependent change in the functionality of
R(t) could be interpreted, these data were considered in the context of a solvent-
dependent change in the effective rotor shape of I-methylperylene. It is not possible to
extract information on the solvent dependence of the fictional boundary condition from
these data because of the different information content of the two forms of R(t). As the
solvent chain length increases the effective rotor shape changes from prolate to oblate.
Using the preexponential factors from the data in Table 7.3 and Equation 4, the 1-
methylperylene rotor shape for the longer chain solvents can be estimated. For short chain
solvents, where l-methylperylene behaves as a prolate rotor, only D. from the
experimental data can be extracted, and thus there is little or no “shape” information

available. For the longer chain solvents, however, where there are two exponential

decays, it is straightforward to extract Dx and D2, and thus the major-to-minor axial ratio 1

of the ellipsoid of rotation (Table 7.4). While the detailed information can not. provided
on the effective (ellipsoidal) rotor shape of l-methylperylene in n-pentane through it-
octane, it can be determined from the ratio (Dz ID.) that, for the longer chain solvents the
anisotropy of the rotational ellipsoid increases with solvent length to a limiting value in n-
dodecane. we interpret these results as representing a quasi-lamellar confinement of 1-

methylperylene in the long-chain solvents.

141

Table 7.4. Cartesian components of the rotational diffusion constant extracred from
experimental data using Equations 4 and 5.

 

 

solvent B. D,- D/Dx
(GHz) (GI-1:)

n-pentane --. 13.9 : 1.2 (< l)
n-hexane -- 12.8 : 2.3 (< 1)
n-heptane -- 11.1 : 0.8 (< 1)
n-octane -- 10.4 : 0.7 (< 1)
n-nonane 3.40 : 0.75 16.2 : 1.9 4.8 : 1.3
n-decane 2.38 : 0.64 13.5 :1.9 5.7 : 1.8
n-dodecane 1.32 : 0.24 11.2 : 0.9 8.5 : 1.9
n-hexadecane 1.10 : 0.15 9.5 : 1.2 8.6 : 1.9

 

 

The solvent-dependent change of rotational diffusion rotor shape and the onset of the
change in T. both occur between n-octane and n-nonane. The interpretation of the
reorientation data in terms of solvent local organization are consistent with the solvent
local organization implied by the T. solvent chain length dependence. In short n-alkanes,
the l-methylperylene rotor shape is prolate, i.e. the dominantirotational motion is about
the l-methylperylene long in-plane axis. The solvent molecules are small enough that both
terminal CH; groups are in close spatial proximity to the probe molecule, and fast T.
relaxation is expected. In the longer n-alkane solvents, I-methylperylene behaves as an
oblate rotor, where the dominant rotational m0tion is around the axis perpendicular to the
probe molecule molecular plane. We believe that this solvent-dependent change of rotor

shape for l-methylperylene is due to confinement of the probe molecule between solvent

.fl‘f‘:_;[, .1.
g . .

 

 

I42

“lamellae”. Alternatively, this confinement can be expressed in the context of the
individual solvent molecules being long enough to span the l-methylperylene long axis,
thereby significantly reducing the structural freedom of individual solvent molecules on the
probe molecule length scale. For such an environment, where the solvent terminal CH3
groups are, on average further away from the probe molecule than in the shorter n-
alkanes, one expects a longer T. relaxation time, consistent with the experimental findings.
The change in both the reorientation and T. data between n-octane and n-decane suggests
that the effective “length” of the l-methylperylene vibrational coordinate probed is in the
same range as the average length of the ensemble of these solvent molecules. These data
also indicate that dipolar v-v coupling processes responsible for T. relaxation occur over a
~10 A length scale. Recent work by the Topp group on the rotational coherence

spectroscopy of jet cooled perylene/n-alkane complexes suggests intermolecular

[111 In the isolated

interactions at least qualitatively in correspondence with these data.
perylene/n-alkane complex, the alkane chain lies parallel to the perylene long axis with a
3.6 A separation between the molecules, with the n-alkane located over the center of mass
of the perylene molecule. As the n-alkane chain length is increased (from n-octane), a
displacement from the parallel structure occurs. It is interesting that these data, for room

temperature solutions, and the low temperature perylene/n-alkane complex point to similar

intermolecular organization.

“'1?

1‘5... 1___~—.__L_h'__._x

 

143

6.4. Conclusions

The vibrational population and orientational relaxation responses of l-methylperylene in a
series of n-alkane solvents were measured using ultrafast stimulated laser spectroscopy.
The T. response for the l-methylperylene 1370 cm'1 mode is dominated by non-collisional
dipolar v-v coupling to the alkane solvent 1378 cm '1 CH3 rocking mode. Both the T. and
R(t) data for l-methylperylene in the n-alkanes differ significantly from the data of the
earlier work for perylene in the same solvents. The difference in the T. response is
understood for the two probe molecules. For perylene, the vibrational mode interrogated
is infrared inactive and therefore the dominant polar exchange process between solute and
solvent must be quadrupole-dipole coupling. For l-methylperylene the vibrational
resonance we access is both infrared and Raman active and thus the dominant exchange
mechanism is dipole-dipole coupling. These two coupling processes operate over different
length scales and it is therefore expected closer correlation between T. and R(t) data for
l-methylperylene than for perylene. For perylene in the n-alkanes, the R(t) decays
presented in Chapter 4 are single exponential in all cases, while for l-methylperylene, a
double exponential decay for longer alkane solvents was observed, This observation'is not
signal-limited. but rather represents a fundamental difference in the way the two molecules
reorient in the same solvent. More work is needed to understand the large differences in
dynamics that arise from the addition of a single methyl group to the chromophore, but
one possible basis for this difference is the torsional strain induced in the l-methylperylene
ring structure by the presence of the CH3 group. For the l-methylperylene data, the

correlation between the R(t) and T. dynamical responses can be understood in terms of

144

local solvent organization about the chromophore. Both of the dynamical responses point
to the close spatial proximity of the solvent terminal CH3 groups to the chromophore in
short alkanes and a greater average distance between these moieties in longer alkane
solvents. Both sets of data point to a change in solvent-solute interaction between it-
octane and n-decane. These data appear to be in excellent qualitative agreement with
recent low temperature examinations of perylene/n-alkane complexes in a jet expansion

using rotational coherence spectroscopy.

 

 

145

6. 5. Literature Cited

1. Y. Jiang; G. J. Blanchard; J. Phys. Chem, 98 9417 (1994).

Id

. Y. Jiang; G. J. Blanchard; J. Phys. Chem, 3 9411 (1994).

Lo)

. Y. Jiang; G. J. Blanchard; J. Phys. Chem, 28 6436 (1994).

. Y. Jiang; S. A. Hambir; G. J. Blanchard; Opt. Commun, 9_9_ 216 (1993).

J}.

5. H. E. Zieger; E. M. Laski; Tet. Lett., 32. 3801 (1966).

6. S. A. Hambir; Y. Jiang; G. J. Blanchard; J. Chem. Phys, 98 6075 (1993).

7. L. Lewitzka; H.-G. Lohmannsroben; M. Strauch; W. Luttke; J. Photochem. Photobiol.
A: Chem, 61 191 (1991).

8. S. Grimme; H.-G. Lohmannsroben; J. Phys. Chem, 96 7005 (1992).

9. Y. Jiang; S. A. Hambir; G. J. Blanchard; Chem. Phys, 183 249 (1994).

 

10. J. T. Yardley, Introduction to Molecular Energy Transfer, Academic, New York,
1980. .

l l. T. Troxler; J. R. Stratton; P. G. Smith; M. R. Topp; J. Chem. Phys, l___l 9219 (1994).

12. C. G. Gray; K. E. Gubbins; Theory of Molecular Fluids, Vol. I : Fundamentals
Oxford Science, pp. 91-100, 1984. '

13. G. J Blanchard; C. A Cihal; J. Phys. Chem, _9_2_ 5950 (1988).

14 G. J. Blanchard; J. Phys. Chem, 22 6303 (1988).

15. G. J. Blanchard; J. Phys. Chem, 93 4315 (1989).

16. G. J. Blanchard; Anal. Chem, _6_1 2394 (I989).

17. J. T. Edward; J. Chem. lid, 41 261 (1970).

146

1.8. F. Perrin; J. Phys. Radium, _7_ 1 (1936).

19. C. M. Hu.; R. Zwanzig; J. Chem. Phys, 60 4354 (I974).

20 G. K. Youngren; A. Acrivos; J. Chem. Phys, Q 3846 (1975).
21. R. Zwanzig; A. K. Harrison; J. Chem. Phys, 8_3 5861 (1985).
22. T. J. Chuang; K. B. Eisenthal; J. Chem. Phys, 57 5094 (1972).

23. P. Debye; Polar Solvents, Chemical Catalog Co., New York, p. 84, 1929.

 

 

CHAPTER 8. SUMMARY AND FUTURE WORK

This thesis work focuses on intermolecular interactions between solutes and solvents using
ultrafast laser spectroscopic method. A We have developed a novel pump-probe laser
spectroscopy scheme to study vibrational energy transfer between solute molecules and
surrounding solvent molecules. We are able to pump two blue picosecond dye lasers
synchronously with the third harmonic output of a Nd:YAG laser. Other vibrational
relaxation measurement schemes involve the use of more expensive and technically more
complicated picosecond infrared lasers. Our spectrometer, combined with our
measurement scheme, gives us time resolution of a few picoseconds and spectral
resolution of ~ 4 cm". Picosecond time resolution allows us to look at solution phase
dynamics such as vibrational relaxation and rotational diffusion. High spectral resolution
enables us to access the vibrational states of interest selectively. From our studies on
perylene and l-methylperylene we find that T. is vibrational mode and solvent sensitive as
well as state dependent for a given probe molecule. We are able to utilize the information
from rotational diffusion measurements to aid the interpretation of vibrational relaxation

data.

In general, vibrational relaxation can occur through intramolecular and intermolecular

energy transfer. For intramolecular energy relaxation, excess vibrational energy in high

147

148

frequency vibrational resonances transfers to low energy vibrational states within the
molecule because of anharmonic coupling between vibrations, typical of organic
chromophores. Intermolecular energy transfer includes vibrational energy exchange
between the solute molecules themselves and their surroundings. In solution, due to the
high density of the medium the energy exchange between solute and surrounding solvent
becomes dominant while intramolecular? vibrational relaxation is less important or at least
constant for a given mode. When there are solvent vibrational resonances at frequencies
close to solute modes, v-v long range resonance coupling allows the exchange to be very
efficient. It is difficult to separate the different channels for vibrational energy transfer in
solution. For studies of intramolecular energy transfer, the ideal system has a low
frequency of the collisions between solute molecule and the surrounding molecules and no
vibrational mode of the surroundings is close to energetic proximity of the solute modes.
For the systems we chose in this thesis work, we have focused on intermolecular
vibrational energy transfer between solute and solvent through v-v long range resonance

couphng.

For v-v long range resonance coupling vibrational energy transfer, the rate of transfer is a
function of the frequency difference between a solute mode (donor) and solvent mode
(acceptor) (A00), the distance between donor and acceptor (d) and the nature of the
interaction (u).

T.xf(Ac0,d,u) 11]
Ground state T. measurements of perylene in n-alkanes revealed the existence of local

solvent organization. For perylene in n-alkanes, A0), is ~ 0 for our conditions, and u is

149

related to d'7. For the perylene 1375 cm'1 vibrational mode (v..) in alkane solvents, the
CH3 end group rocking mode at 1378 cm" is the energy acceptor for v-v long range
resonance coupling. The variation of the perylene v.5 T. times in different alkanes reflects
different average distances between perylene v.5 vibrational coordinate and CH; groups.
These differences are caused by variation of the local solvent organization around the
solute. The dependence of T. on aliphatic chain length is not a smooth trend. There is a
transition occurring around octane between short and long chain solvents and the chain
length of all trans octane is close to perylene long axis length. In the future, T. times of
perylene in branched alkanes can be measured and the results can be compared to those in
normal alkanes. This information will provide insight into local solvent organization by

deliberate disruption of the solvent environment.

Among the three parameters, A0) and u, the latter two are relatively easy to determine, but
d is not possible to control in solution. one possible way to control, or vary (1 is to use
supercritical fluids instead of liquids. A supercritical fluid is a special phase between the
gas and liquid phases, where the intermolecular separation (d) can be controlled by varying

temperature and pressure.

The term u in Equation 1 is determined by the properties of both solute and solvent. We
observed a close correlation between energy transfer and rotational diffusion dynamics. for
l-methylperylene in n-alkanes. this correlation is not seen for perylene in same solvents.
We attribute this difference to variations in the nature of solute-solvent interaction for

energy transfer. For perylene, v-v energy exchange occurs between a solute Raman ag

mode and solvent IR mode. The intermolecular coupling is by quadropole-dipole
interactions, where the interaction energy is proportional to r'7. The solvent local
organization, reflected by the T. solvent chain length dependence persists on a shorter
length scale than the probe molecule itself so that no correlation between T. and To.
dynamics is found. For l-methylperylene, v-v long range resonance coupling occurs
between a solute IR mode and solvent IR mode. The coupling in this case is dipole-dipole
and the interaction energy is proportional to r‘. This length scale is similar to the size of
the probe molecule itself and thus a correlation between two dynamics is observed. To
continue exploring the dependence of T. on the interaction energy surface, experiments of
perylene in benzene and toluene, and l-methylperylene in benzene and toluene, need to be
performed. From these experiments the coupling between solute Raman mode with
solvent Raman or IR mode and solute IR mode with solvent Raman or IR mode can be

compared.

As we learn more and more about T. relaxation of organic chromophores in solutions
experimentally, there is an urgent need for developing a theory of v-v long range
vibrational energy transfer processes in solution. Future collaborations will be necessary
to relate the experimental data to a sound theoretical interpretation. The technique that we
have developed and the knowledge we have gained about intermolecular interactions can
be applied to more complicated systems such as proteins, where ultrafast relaxation
processes have been experimentally observed, but where the role of vibrational relaxation

remains only poorly understood.