Title Page
 Table of Contents
 List of Tables
 List of Figures
 Behavior of triplet benzene
 Duraldehyde in durene
 Biographical sketch

Title: Temperature dependence of molecular phosphorescence
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00097534/00001
 Material Information
Title: Temperature dependence of molecular phosphorescence
Physical Description: xii, 164 leaves : ill. ; 28cm.
Language: English
Creator: Moehle, William Emil, 1948-
Publication Date: 1975
Copyright Date: 1975
Subject: Phosphorescence   ( lcsh )
Temperature   ( lcsh )
Chemistry thesis Ph. D
Dissertations, Academic -- Chemistry -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: leaves 158-163.
Additional Physical Form: Also available on World Wide Web
General Note: Typescript.
General Note: Vita.
Statement of Responsibility: by William Emil Moehle.
 Record Information
Bibliographic ID: UF00097534
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000161489
oclc - 02676397
notis - AAS7830


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Table of Contents
    Title Page
        Page i
        Page i-a
        Page ii
        Page iii
    Table of Contents
        Page iv
        Page v
        Page vi
    List of Tables
        Page vii
    List of Figures
        Page viii
        Page ix
        Page x
        Page xi
        Page xii
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    Behavior of triplet benzene
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    Duraldehyde in durene
        Page 92
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    Biographical sketch
        Page 164
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Full Text






To my parents


I wish to express my appreciation to Dr. Martin T. Vala

for his advice and encouragement throughout this work. His

many suggestions and comments on this project were of in-

estimable value.

While there are many people who have helped and encour-

aged me in this work, special thanks and gratitude are due

Neil Weinstein, Arleen French, and Ann Kennedy for their

extraordinary efforts in helping me through the harder moments

of this project. Special thanks are also due the members of

Dr. Vala's research group, particularly Joseph Baiardo, Ralph

Spafford, and Joseph Wrobel, the Electronics Shop, who were

especially helpful on the technical side of my work, and Ken

Wagener and Dick and Barbara Galley, whose friendship has

been a particular source of strength.

Finally, I wish to mention again Ann Kennedy who per-

formed a major miracle in getting this manuscript finished

on time.



Background 1

The Phosphorescence Decay Problem 2

Decay from Thermally Equilibrated States 6

Decay with Thermal Activation
(Deactivation) 7

Difficulties in the Interpretation of
Phosphorescence Decays 8

Experimental Apparatus and Procedures 9

Phosphorescence Spectra 9

Phosphorescence Lifetimes 13


Introduction 18

Benzene Phosphorescence 18

Temperature-Dependent Work 19

Benzene Photochemistry 21

Benzene-Polyhaloalkane Complexes 22

Benzene Dissertation Work 23

Experimental Procedures 25

Experimental Results 31

Spectral Results 31

Lifetime Results 36

Discussion 56

Spectral Results 56

Lifetime Results 57

Theoretical Calculations on the Exciplex Model 65

Results of Open and Closed Shell INDO
Calculations 65

Discussion 77

Comments on Possible Exciplex Formation in
Other Phases 81


Introduction 92

Duraldehyde Phosphorescence 92

Duraldehyde Photochemistry 93

Durene Crystal Structure 96

Dissertation Work 97

Experimental Procedure 98

Experimental Results 100

Phosphorescence Spectra 101

Phosphorescence Lifetimes 103

Infrared Results 110

Discussion 117

Phenomenological Model 117

Lifetime Results 126

Kinetic Model 129

Infrared Results 150

Identification of Emitting Species 151

Comments on Other Aromatic Carbonyls 157




Table 1. Chemicals, Their Source and Purity 26

Table 2. Parameters Determined from Observed 51
Decay Rates for Benzene Complexes
(Forms I and II)

Table 3. Parameters Determined from Observed 52
Decay Rates for Benzene Complexes
(Forms III and IV)

Table 4. Sample Fit to C6H6-CDC13 Decay Rates 54
(Form I)

Table 5. Reported Parameters Determined from 66
Benzene Decay Rates in Different
Solvents (Form I)

Rable 6. Results of INDO Calculations on 71
H2-C6H6 Exciplex

Table 7. Results of INDO Calculations on 74
H2-C6H6 Ground State Complex

Table 8. Results of INDO Calculations on 78
HF-C6H6 Exciplex

Table 9. Results of INDO Calculations on 81
Stablest H2-C6H6 Exciplex as a
Function of Angle

Table 10. Parameters Determined from Observed 130
Decay Rates for Duraldehyde in Durene

Table 11. Sample Fit to Duraldehyde Decay Rates 131
(Form III)



Figure 1. Rate processes for a two level 5
triplet system

Figure 2. Experimental configuration for 11
recording spectra

Figure 3. Experimental configuration for 15
recording lifetimes

Figure 4. Sample holder 29

Figure 5. Phosphorescence spectra of 33
benzene and its complexes at

Figure 6. Phosphorescence spectra of 35
benzene and its complexes at

Figure 7. Plot In (I/I0) versus temper- 38
ature for completed and un-
complexed benzene

Figure 8. Plot In (I/I0) versus temper- 40
ature for slow and normal
cooled completed benzene

Figure 9. Plot of In (relative intensity) 42
versus time for a typical

Figure 10. Plot of lifetime versus temper- 44
ature for the benzene-chloro-
form complex

Figure 11. Plot of lifetime versus temper- 46
ature for the benzene-chloro-
form-d complex

Figure 12. Plot of decay rate versus inverse 48
temperature for the benzene-
chloroform complex


Figure 13. Plot of decay rate versus inverse 50
temperature for the benzene-
chloroform-d complex

Figure 14. Plot of relative intensity versus 59
wavelength for the 0,0 band of

Figure 15. Approach of hydrogen molecule to 70
triplet benzene

Figure 16. Plot of H2-C6H6(triplet) exciplex 73
energy versus distance for
different complex geometries

Figure 17. Plot of H2-C6H6(ground state) com- 76
plex energy versus intermolecular
distance for different complex

Figure 18. Plot of HF-C6H6(triplet) exciplex 80
energy versus intermolecular
distances for different complex

Figure 19. Plot of H2-CgH6(triplet) exciplex 83
energy versus relative inter-
molecular orientation

Figure 20. Molecular orbitals of benzene 86

Figure 21. Conformation of durene in its 96

Figure 22. Phosphorescence spectrum of single 102
crystal duraldehyde in durene
at 11K

Figure 23. Temperature dependence of the 105
duraldehyde in durene phosphor-
escence spectrum

Figure 24. Plot of ln (intensity) versus in- 107
verse temperature for two duralde-
hyde in durene bands

Figure 25. Plot of In (intensity) versus tem- 109
perature for two duraldehyde in
durene bands

Figure 26. Plot of lifetime versus temper- 112
ature for duraldehyde in

Figure 27. Plot of In (decay rate) versus 114
inverse temperature for
duraldehyde in durene

Figure 23. Temperature dependence of the 116
infrared spectrum of duralde-
hyde in durene for the 1650
1800 cm-1 region

Figure 29. Effect of irradiation on the 119
infrared spectrum (1650 -
1800 cm-1 region) of duralde-
hyde in durene

Figure 30. Phenomenological model of the 121
duraldehyde in durene system

Figure 31. Plot of In (12/11) versus in- 125
verse temperature

Figure 32. Kinetic model of the duraldehyde 135
in durene system

Figure 33. Theoretical fit to the variation 140
of the 411.2 nm band intensity
with temperature

Figure 34. Theoretical fit to the variation 143
of the 431.6 nm band intensity
with temperature

Figure 35. Theoretical fit to the variation 146
of the 408.0 nm band intensity
with temperature

Figure 36. Predicted behavior for a plot of 149
In (12) versus inverse temper-
Ii ature

Figure 37. Comprehensive model for the 153
duraldehyde in durene system

Abstract of Dissertation Presented to the
Graduate Council of the University of Florida
in Partial Fulfillment of the Requirements for
the Degree of Doctor of Philosophy



William Emil Moehle

December, 1975

Chairman: Martin T. Vala
Major Department: Chemistry

Phosphorescence emission has been used as a probe for

studying the triplet manifold of two molecular systems. For

the first, we have observed the phosphorescence decay times

of the benzene:chloroform:alkane system as a function of

temperature. We have fit the results to a number of differ-

ent rate equations and used the parameters thus obtained to

elucidate the radiationless processes in benzene. To account

for the data, we propose that an exciplex is formed between

triplet benzene, chloroform and solvent. This model pro-

vides a framework for understanding a wide variety of photo-

chemistry involving benzene in different phases. Calcula-

tions have been performed using open and closed shell INDO

semi-empirical theories that show the validity of our argu-

ments and shed additional light on the structure of the


For the second system, duraldehyde in durene, we have

studied in detail the temperature dependence of both the

phosphorescence lifetime and intensity. We have further

observed the temperature dependence of the infrared spectrum

in the critical carbonyl stretching region and have observed

the effects of ultraviolet photolysis on the intensity of

these bands. From these data we have formulated a coherent

kinetic model for the dissipation of triplet energy which is

consistent with previous results. Finally, we have proposed

a comprehensive model which describes all known photophysical

and photochemical processes occurring via the duraldehyde

triplet manifold.



It has been known for a long time that many molecules

exhibit long-lived emission (phosphorescence) after electronic

excitation. As early as 1935, Jablonski2 suggested a triplet

state as the metastable species involved. In 1944, Lewis

and Kasha3 reported the results of the first extensive study

of the emission from these metastable species and in the

following year Lewis and Calvin4 showed that this species

was paramagnetic. Actually, it wasn't until 1958 and the

photo-excited ESR experiments on naphthalene in durene that

Hutchinson and Magnum proved that this state was indeed a


Since that time there have been many studies undertaken

that have used phosphorescent emission as a probe to study

the triplet manifold of molecular systems.6 Benzene along

with its simple derivatives and aromatic carbonyls are two

classes of compounds that have received considerable atten-

tion. It is the intent of this dissertation to further our

understanding of both these classes of compounds. For the

first class, we have studied the phosphorescence character-

istics of a weak benzene complex, while for the second class

we have studied the phosphorescence characteristics and

ground state infrared of the system duraldehyde in durene.

While each of these studies is essentially independent of

the other, they do share a number of common features among

which is a general understanding of the problem of decay

from an excited state.

The Phosphorescence Decay Problem

If an excited level of a molecular ensemble has a

larger population than permitted by normal thermal excitation,

then that state will attempt to lose this excess population

by any means available. For the simplest situation involving

the triplet.manifold, this means that molecules in the lowest

triplet state (T1) will revert to the molecular ground state

(S0) by either (or both) radiative or nonradiative transitions.

Thus the decay of T is described by:
d = -(klor + klnr) T1] (1)

where t = time

[T1] = population of T1 at any time

r = radiative transition

nr = nonradiative transition

kl = rate constant for the T1 -So transition

The solution of this problem is:

[T1] = [T 10 e-kl0t (2)

where k0 = klonr + klor

[T1]0 = population of T1 at t = 0

and hence, we are dealing with a simple exponential decay.

The triplet manifold of a molecule is rarely this simple.

The triplet state consists of two unpaired electrons which

make it highly reactive, and the spin forbidden nature of

the transition to the ground state makes the triplet state

a long-lived species. Actually, all the familiar processes

of complex formation, chemical reaction, isomerization, etc.

may occur in the triplet manifold. It is frequently difficult

to decide where in the triplet manifold these processes may

be occurring, as only the lowest triplet state generally


If a second state, T2, exists that can also be "over-

populated" and have processes occurring from it (see Figure

1) then the decay problem becomes:

dt = -(k10 + k12 [T1] + k21[T2] (3)

t k= 2[T1] (k20 + k21) [T2] (4)

where the rates are defined in Figure 1.

These coupled differential equations may be solved using

LaPlace Transforms as by the method of DiBartolo. While

there is not much value in deriving the results, they are

worth examining especially in certain limits as they

illustrate what can be learned from a study of decay times.

Assuming processes linking levels 1 and 2 are faster

than the other processes leaving T1 and T2 (i.e., k10 and

k20), then the general solution to equations (3) and (4) are:

k + k]e- t (5)
k 12 20 k1 0 -pt k 12 1 0-k21[T2 0]e-qt
1 1 I k2 + k21 kl2 + k21

Figure 1. Rate processes for a two level triplet system

k k nr + k r
10 10 10

k = knr + klr
20 = k20 20

k20 = k20 + k2p

21 12 21

r20 ~kl20

o j~lo

so 0 --__

2 kl2 1[T +[T0 ]e-pt + [12[T20-k12[T1]0 qt (6)
k12 + k21 k12 + k21

where p = kl0k21 + k12k20
where p = (- 2,-
k12+ k21

and q = k12 + k21

Thus,the presence of a second level changes the decay of

T1 from exponential to nonexponential (where nonexponential

means a sum of exponentials). Additional information can be

gathered from these equations by considering two important

cases of the general equations (3) and (4): thermal equilib-

rium and thermal activation (or deactivation).

Decay from Thermally Equilibrated States

If thermal equilibrium exists between TI and T2 then:

[T20 12 e-E21/kT (7)
[T1]0 k21

where AE21 = energy separation between T, and T2

k = Boltzmann's constant

T = temperature in K

and the general solutions given by equations (5) and (6)


[T1] = (k1221 k 10 + T20)ePt (8)

[T2 = 12k k)(T10 + [T2]0)e-'t (9)
k12 2 k21

k10 + k20e-AE21/kT
where p' /IcT
h1 + e-AE21/kT

With >> 1, then equations (8) and (9) further reduce to:

[TI] = [T] e-Pt (10)

[T2] = [T2]e-"t (11)

where p" = k0 + k20e-AE21/kT

Thus,if a thermal equilibrium exists, the decay from either

state will be exponential and show the same decay rate.

Moreover, a study of the temperature dependence of the

measured decays will give 1) the rates for leaving T1 and T2

for all processes except those linking these states, and 2)

the energy separation between the levels. If more than two

states are in thermal equilibrium then k20eAE21/kT is

replaced by a sum of such terms as is the term e-21/kT

If there are n levels in equilibrium then there will be n-i

such terms.

Decay with Thermal Activation (Deactivation)

If we consider where level 2 is thermally populated

from level 1 but where k21 = 0, then the initial equations

(3) and (4) may be solved exactly to give:

[T1] = Tl0e-t (12)

[T2] = {[T1 kl2 10 e-k20t+ k12 e-at (13)
[ 210 -k10-k12+k20' k20-k10-k12

where 8 = kl0 + kl2

If k12 is an Arrhenius-type rate, i.e., kl2=kl e-AE21/kT

(where AE21 is not necessarily the difference in energy

between the two levels) then equations (12) and (13) remain

unchanged and B becomes
S= k0 + kloe-AE 21/kT
10 12 21

Thus a nonexponential decay from only the upper state

is predicted and at least one of the lifetimes from each

state should show a temperature dependent behavior. The

temperature dependent rate, which will be common to both

states, can yield information on the rate of depopulation

of TI, on the energy barrier between states T1 and T2, and

the intrinsic rate of conversion from T1 to T2. The other

decay will give information (if seen) on the rate of depop-

ulation of T2.

If k12 = 0 instead of k21 = 0, then the roles of T1 and

T2 are reversed (the actual equations may be obtained by

changing subscripts 1 to a 2 and 2 to a 1 in equations (12)

and (13)) and the decay from the lower state will be non-

exponential. If both k12 and k21 are zero, then the two

triplet levels are no longer coupled and the solutions are


[T1] = [T1]0ek10t (14)

[T2] = [T20e-k20t (15)

Difficulties in the Interpretation of Phosphorescence

There are many practical problems in the interpretation

of the decay results from a lowest triplet state. The

appearance of a nonexponential decay in itself does not

prove that an upper triplet level is feeding a lower one

since the existence of more than one emitting species

caused by, say, environmental effects, tautomerization, etc.,

will also produce a nonexponential decay. On the other

hand, a nonexponential decay caused by an upper triplet

level may appear as an exponential decay if the intensity

of one of the decays is sufficiently small or if one of the

decays is sufficiently fast to avoid detection in a given

experimental configuration.

Similarly, a change in the nonradiative decay rate as

a function of temperature may be induced by, say, an upper

reactive triplet state or by vibrationally induced inter-

system crossing from T1 to S Fortunately, processes such

as vibrationally activated intersystem crossing usually

have much smaller preexponential factors and activation

energies than such processes as reactions and hence can be

distinguished by temperature dependent decay studies.

Changes in steady state emission intensities may also

be used to determine the values for such parameters as the

energy separation between levels. Finally, it is important

to remember that changes in steady state emission inten-

sities may be effected by any process in the ground or first

excited singlet state that will effect the number of mole-

cules that intersystem cross to the triplet state.

Experimental Apparatus and Procedures

Phosphorescence Spectra

The apparatus for recording phosphorescence spectra

is shown in Figure 2. The lamp source was either a 1000

watt d.c. Xenon arc lamp or a 12 watt low pressure mercury












0 a C
g U

o X I-I
rl 5r

vapor lamp. The excitation wavelength was selected by

means of a 1/4 meter Scanning Monochromator (Heath Model

700) or an appropriate combination of filter solutions.

This output was focused by means of Suprasil quartz lenses

(Amersil Inc. ) onto a sample which was located at the

tip of a closed cycle refrigerator (Air Products and Chem-

icals, Inc. Model CSW 202A Displex), range 10 to 300K.

The sample holders were of homemade design and were

constructed from oxygen-free copper to insure high thermal

conductivity. Temperatures were monitored via a pair of

Chromel P vs. Gold,/.07 atomic % Iron thermocouples. One

was located in the tip of the Displex and used as input for

an Indicating Temperature Controller (Air Products and Chem-

icals, Inc. Model APD-IC 1) that had a + 1K temperature

control for a 48 hour period. The other thermocouple was

held in place by a set screw at the bottom of the sample

holder and used a liquid nitrogen bath as a reference. The

output of this thermocouple was monitored on a Digital

Multimeter (Keithley Model 160) whose readings were converted

to degrees K with an accuracy of 0.1K. The two thermo-

couples always agreed to within the reading errors of the

monitoring instruments but the second thermocouple because

of its smaller reading error and position was taken as more


Sample emission was collected at right angles to the

excitation beam and focused by means of Suprasil quartz

lenses onto the slits of a second 1/4 meter Scanning Mono-

chromator (Heath Model 700) which had a photomultiplier

tube (EMI 9558Q or EMI 9635 QB) with quartz window at its

exit slit. The signal from this tube was filtered and

amplified by a Picoammeter (Keithley Model 416) and recorded

on a Strip Chart Recorder (Heath Model EU-205-11) synchro-

nized to the second monochromator drive.

Phosphorescence Lifetimes

The apparatus for measuring the lifetimes is shown in

Figure 3. The same excitation lamps were used as before,

but since the intensity of exciting light was more important

than its monochromaticity for the lifetime work, optical

cells were used instead of the monochromator for isolating

the excitation wavelength. Actually, for the work involving

the low pressure mercury vapor lamp, even the optical cells

were omitted if there was no mercury line near the band whose

lifetime was being measured. Prior to impinging on the

sample the excitation light was chopped by means of a mechan-

ical chopper whose speed was controlled by a variable fre-

quency oscillator. The sample holders and temperature con-

trol systems were as previously described as were the

focusing optics, analyzing monochromator and photomultiplier


The output of the photomultiplier tube was directed to

the input of a Digital Signal Averager (Tractor Northern

Scientific, Inc. Model NS-570). The instrument's input

resistance is one megaohm; since its Analog-to-Digital Con-

verter requires only 100 millivolts for a full scale reading,














u '4

a 100 nanoamp current is required to provide this reading.

In practice, only 20-30 nanoamps are drawn from the photo-

multiplier tube, a value well below saturation current


The signal average contains 1024 channels. The decay

signals were allowed to build up until a good signal-to-

noise ratio was obtained. The capacitance of the system, as

determined by the photomultiplier tube, interconnecting cable

and a variable filter placed across the input resistor of the

signal average was adjusted such that the RC time con-

stant of the system was less than the sweep time for a

single channel.

The signal average was triggered off the chopper

system using a delayed triggering scheme that eliminates

any contribution to the decay signal from the mechanical

chopper. The amount of decay time required for any given

chopper frequency is determined by noting the effect of

excitation light on the signal average and simply delaying

the sweep until this light produces no effect in the signal


The output of the signal average is dumped via an

interface (Wang Model 705-1A Microface) into a Programmable

Calculator (Wang Model 700C) that analyzes the decay curve.

Exponential curves are most conveniently handled, but the

system has the capacity to handle dual exponential curves

also. The basic program used in the analysis was written by

Dennis Mahoney, although a few alterations were necessary

because of the use of a different signal average in our work.

The program deconvolutes nonexponential decays by first

calculating the lifetime result for the long-lived emission

starting from a time when the short-lived emission has

essentially ceased. It then subtracts off the contribution

of the long-lived emission from the actual emission and

calculates a lifetime for the short-lived emission. Choice

of the point where the short-lived emission ceases to in-

fluence the results is chosen by the operator from a plot of

ln Intensity (actually In of the number of counts) versus

time that is printed out on an Input/Output Writer (Wang

Model 711). For good input data, the lifetime results

should be accurate to about 10%, if the lifetimes are

different by at least a factor of two and the decays are

roughly of the same initial intensity. Lifetimes are cal-

culated using a weighted least squares procedure that weights

each point as the square of the signal intensity. Thus,

points where the signal-to-noise ratio is high are more

highly weighted.



Benzene Phosphorescence

Lewis and Kasha measured the phosphorescence spectrum

of benzene in EPA (5:5:2-ether, isopentane, alcohol by

volume) and noted its very high triplet energy (about 85 Kcal)

and its relatively broad though somewhat structured appear-

ance. Shull1 reinvestigated the same system and from a

detailed vibrational analysis concluded that the symmetry

of the emitting state was Blu and the geometry was hexagon-

ally symmetric planar. Later work by Albrechtl2 confirmed

this symmetry assignment and elucidated the pathways of

vibronic spin-orbit coupling. Nieman and Tinti3 and de
Groot and van der Waals4 further showed that Shull's deduc-

tion of hexagonal symmetry was in error and that in fact the

triplet state geometry is planar elongated (D2h).

Lewis and Kasha had not attempted to measure the ben-

zene phosphorescence lifetime, but later workers have done so

despite the early instrumental difficulties caused primarily

by the very weak emission intensity. In 1949, McClure5

reported that triplet benzene had a lifetime of 7.0 0.5

seconds in EPA at 77K. Coupled with later quantum yield

work, he deduced a value of 21 seconds for the radiative

lifetime of benzene. This value was later corrected by

Lim7 to 28 2 seconds.

Even with the advent of more sophisticated instrumen-

tation,differences in the values for the benzene phosphor-

escence lifetime were reported varying from solvent to

solvent and, in some instances, even in the same solvent.

Working at 4.2K, Wright, Frosch, and Robinson recorded

benzene lifetimes of 16 seconds (in methane and in argon)

and benzene-d6 lifetimes of 22 seconds (methane) and 26

seconds (argon). Russell and Albrechtl9 reported that

benzene has a triplet lifetime of 5.2 seconds in 3MP (3-

methylpentane) at 77K. Martin and Kalantar0 reported a

slight deviation from exponential decay for benzene in either

EPA or 3MP at 77K and state that the measured lifetime in

these solvents varies with the cooling time of the sample.

Benzene lifetimes in EPA varied from 7.80 to 8.45 seconds

over a five-hour period while in 3MP the lifetimes varied

from 4.70 to 5.75 seconds over a three hour period.

Temperature-Dependent Work

The temperature dependence of benzene's phosphorescence

lifetime was studied by three separate groups: Nieman,

Hatch, and Erlitz,21 Leubner and Hodgkins,22,23 and Kilmer

and Spangler.24 Their results showed a remarkable dependence

of benzene lifetime on both solvent and temperature. For any

given solvent the lifetimes were usually temperature inde-

pendent below 50 60K, but varied rapidly above this onset

temperature. This resolved the problem of the lifetime

variation in the same solvent (i.e., the temperatures were

not identical) and focused attention on the probable causes

of this effect.

Nieman et al. found that the temperature-dependent

results could be fit to an equation of the form:

kobs = kLT + CeE/T (16)

where kobs = observed decay rate

kLT = low temperature decay rate

C = constant

AE = activation energy

T = temperature in K

k = Boltzmann's constant

Furthermore, kLT, C and AE were found to vary in an apparently

arbitrary manner with solvent. C was found to vary between

10+3 and 10+9 with AE of the order of 500 2000 cm-1
At this time several investigators 9 reported

quantum theories for the radiationless transitions in large

molecules and two in particular looked specifically at the

temperature dependence of benzene phosphorescence. Fischer29

attributed the strong temperature dependence to the pseudo-

rotational motion of benzene in the lowered D2h symmetry of

the triplet state. This approach has been criticized31'32

on several grounds. First, this equation does not give a

good fit to the experimental results throughout the temper-

ature range investigated.31 Second,and more importantly,

pseudorotation and temperature-dependent lifetimes do not

always occur in the same temperature region. The phosphor-

escent lifetime work of Nieman et al.32 for mesitylene

in B-trimethylborazine show a temperature-dependent lifetime

above 150K while the ESR work of van der Waals33 indicates

that the mesitylene is freely interconverting among its

various forms by 77K.

Lin has derived a theoretical expression for the tem-

perature dependence which is identical in form to the empir-

ical one of Nieman, et al., but his estimates of the intra-

molecular radiationless decay rates (C) are orders of magni-

tude too small. Despite a great deal of discussion, there

appears to be no generally accepted explanation for the

strong temperature dependence of benzene's phosphorescence

and for its strong solvent dependence, although several have

been proposed. Since the AE values are of the same order of

magnitude as vibrational energies, vibrations of either the

benzene21'23 or of the solvent23 have been cited for the

effect. At high temperatures in electronically excited

benzene, different isomeric forms of benzene (that revert to

ordinary benzene in the ground state) have been found,34'35

and have been proposed as the cause of the effect. Currently,

there is no conclusive explanation of the observations.

Benzene Photochemistry

In 1953, Gibson, Blake, and Kalm36 discovered that benzene

when irradiated in IM (isopentane, methylcyclohexane) or EPA

at 77K produced a photoproduct that had a spectrum similar to

that of hexatriene. Subsequent independent work by Leach
37-42 43
and Migirdicyan37-42 and by Anderson, Chilton, and Porter43

showed that the photoproduct formed was a solvent substituted

hexatriene. Leach and Migirdicyan proposed that the re-
action went by a biradical mechanism, 2 while Porter
et al. proposed a four-center concerted mechanism.4 Sub-

sequent ESR work44 failed to provide any evidence for the

hexatriene biradical and thus a concerted mechanism is

assumed operative. Photoproduct formation is found to
depend linearly on exciting light intensity;4 of the two

possible precursor states (i.e., the first excited singlet

state or the triplet state), the longer lifetime of the

triplet is thought to make it the more likely precursor.

In 1962, Shelimov, Fok, and Voevodskii4 reported that

while irradiating benzene with 254 nm light in a solvent

glass at 77K, solvent radicals were produced. In 3MP, the

radical was formed by breakage of the C-H bond of the

tertiary carbon atom. It was later found that this process
required the absorption of two photons47 and as the excited

singlet state is too short-lived for this to occur, an

upper triplet is assumed responsible for the radical forma-

tion.4748 Using this fact and making certain assumptions

about the radical-forming process, they were able to obtain

a spectrum of the excited triplet level by following radical

production as a function of the wavelength of exciting light

from a second light source.49 The second triplet of benzene

lies approximately 1 eV above the first.

Benzene-Polyhaloalkane Complexes

In 1969, Simons and Perrins50 reported their results

for the system: benzene (10-3 M), chloroform (10-1 M), IM

(isopentane, methylclcohexane) solvent at liquid nitrogen

temperatures. Chloroform has long been known51 to possess

an affinity for benzene at room temperature and Simons et

al. were able to show that a 1:1 complex of chloroform with

(ground state) benzene was formed at lower temperatures

with a AHf of -1170 120 cal/Mole and a AG of -354 70

cal/Mole. The complex persisted in the first excited sin-

glet state of benzene with a AHf of -590 + 180 cal/Mole.

A 2:1 chloroform-benzene complex was also found at higher

relative chloroform concentrations.

Analyzing the phosphorescence spectrum in 3MP, Simons,

Perrins, and Smith52 discovered that there was a substantial

change in its appearance and that the spectrum was consistent

with a D2h symmetry for the benzene. It was noted that com-

pounds like C2HC15 had a similar effect to chloroform, but

that chlorocarbons like CCl4 showed no evidence of complex-

ation whatsoever.50 They further found that with the chloro-

form present the photoproduct changed from a solvent-substi-

tuted hexatriene to a chloroform-substituted hexatriene.50

A change from chloroform to chloroform-d as the completing

agent produced no change in the spectroscopic results for

the ground and first excited singlet levels, but resulted

in the solvent-substituted hexatriene being formed as the

photoproduct although this occurred at a slower rate than

in the absence of the chloroform-d. From phosphorescent

lifetime work at 77K, Simons and Smith53 established that

changes in the lifetime accompanied this change in photo-

product formation and they also noted a puzzling nonexpon-

ential decay from the benzene-chloroform complex while the

benzene-chloroform-d complex decayed exponentially.

Later work54 on hexatriene formation from benzene de-

rivatives (substituted as highly as hexaethylbenzene) led

them to conclude that steric factors were important in

hexatriene formation. They concluded that the chloroform

sits with the C-H bond of the chloroform on the six-fold

axis of the benzene and that a tilt of the chloroform must

occur to achieve hexatriene formation. Similar work on

benzene in either ethanol, ethanol-d, or ethanol-d655 led

them to conclude that an intermolecular vibronic coupling

is operative in these syst.eas and is instrumental in the

radiationless decay of the triplet state.

Benzene Dissertation Work

In the first part of this dissertation, the phosphor-

escence decay times of the benzene:chloroform:alkane system

have been investigated as a function of temperature. The

results have been fit to a rate equation of the form used

by Nieman and the parameters thus obtained used to elucidate

the radiationless processes in benzene. The nonexponential

decay exhibited by the chloroform-benzene system has also

been studied and related to data already available.

To account for the data we propose that an exciplex is

formed between triplet benzene, chloroform, and the solvent.

This model also provides a framework for understanding a

wide variety of photochemistry involving benzene in differ-

ent phases. Calculations have been performed using open

and closed shell Intermediate Neglect of Differential Overlap

(INDO) semi-empirical theories that show the validity of

our arguments and shed additional light on the structure

of the hexatriene precursor.

Experimental Procedures

Table I is a list of chemicals used, along with their

source and purity. Each solvent was irradiated with 254 nm

light in order to show that there was no impurity emission

that would interfere with the (0,0) emission (-338-340 nm)

of the benzene-chloroform complex. This is the spectral

region of importance in this experiment as benzene itself

has essentially no (0,0) emission and hence uncomplexed

benzene cannot interfere with either the spectral or life-

time results of the complex in this region.

Each solvent was passed through a gas chromatograph in

order to identify any significant impurity that could com-

plex with the benzene. Chloroform was found to contain some

traces of acetone, but acetone was not found to complex

with benzene. We also checked on the possibility of toluene

affecting the results, but found that the complex formed

with chloroform emits some 6 nm to the red of the benzene-

chloroform complex and hence could not have significantly

affected the results, even if it had been there in appreciable


Spectra were recorded using the low pressure mercury

Table I
Chemicals, Their Source and Purity

Chemical Source Grade or Purity

Benzene Matheson, Coleman & Bell Spectrograde
Norwood, Ohio

Benzene-d6 Aldrich Chemical Co., Inc. 99.5 atom %
Milwaukee, Wisconsin

Chloroform Malinckrodt Spectrograde
St. Louis, Missouri

Chloroform-d Aldrich Chemical Co., Inc. 99.8 atom %
Milwaukee, Wisconsin

3-Methylpentane Aldrich Chemical Co., Inc. 99+ %
Milwaukee, Wisconsin

vapor lamp since its output in the 254 nm region is more

than ten times that of the xenon arc lamp. Filter solu-

tions in the excitation pathway were used if a spectrum

of the entire emission region was required. In general,

only the (0,0) band was monitored; since the lamp had no

significant emission in this region, spectra were fre-

quently recorded without even the use of filter solutions.

The slit widths for these runs were generally on the order

of 150 microns (band pass: 0.3 nm).

For the lifetime work, the mercury vapor lamp with-

out cells was used. Slits were typically between 900 and

1200 microns for a band pass of 1.8 to 2.4 nm. Decay

curves were recorded over a period of four lifetimes. If

decay curves were nonexponential, then they were recorded

for four lifetimes of the longer lived species.

The design for the sample holder used in this work

is shown in Figure 4. Light could enter the cell through

the small window A and emission could be viewed at right

angles to the excitation source through window B. Alter-

natively, the cell could be placed at 450 to the excita-

tion beam and then both exciting and emitted light would

pass through window B. The former arrangement has the

advantage of effectively screening the analyzing equipment

from the excitation beam, while it suffers from the dis-

advantage of screening the analyzing equipment from

emission that occurs near the surface of window A. As

benzene is a strong absorber of 254 nm light, much of the

emission would be lost if the sample were excited through

Figure 4. Sample holder

A Window A for excitation light
B Window B for excitation and/or
emitting light
C Cover plate to hold windows
D Threaded screw for mounting on







window A and hence the latter configuration was used almost

exclusively. As liquids may readily lose 1/4 to 1/2 their

volume on cooling, a metal plate in the shape of a semi-

circle was frequently placed at the bottom of the cell holder

to increase the height of the glass layer that is close to

window B. Thus, the top half of the sample holder served

as a reservoir of solution for the glass forming process.

The O-ring seal in the sample holder prevented the liquid

from being exposed to vacuum during the cooling process.

In practice, very little, if any, solution was found to be

lost during the run. This design, however, prevented the

solutions from being degassed. Dry nitrogen was bubbled

through the solutions to remove oxygen which is frequently6

(although not usually for benzene)2456 a triplet quencher.

The sample cell was filled in a nitrogen atmosphere.

The solutions were prepared by placing 3.5 il of ben-

zene (from a 10 il syringe) and 320 Pi of chloroform (or

chloroform-d) into a 10 ml volumetric flask and filling to

volume with 3-methylpentane (3MP). This makes a solution

of 3.94 mM benzene and .399 M chloroform (.399 M chloro-

form-d). Solutions with one tenth of the above chloroform

concentration were also prepared in order to study the

effect of concentration on the spectrum.

Samples were cooled directly from room temperature to

operating conditions (10 to 90K) on the Displex. Stopping

the cooling process of the solution at the freezing point

of the glass and waiting 30 minutes was found to not

significantly affect the results. Two hours were allowed

for thermal equilibrium to be attained in the glass. Any

temperature changes required during the experiments were kept

to less than 10K and a period of 30 minutes was allowed for

thermal equilibrium to be reestablished. It is important

to observe that our method of sample preparation, while

corresponding to a slow cooling process (3K per minute)

probably results in a nonequilibrium ratio of free benzene

to 1:1 to 2:1 (chloroform to benzene) complex by freezing

some higher temperature configuration in the glass.

Experimental Results

Spectral Results

The spectra of benzene and its complexes with chloro-

form and chloroform-d at approximately 15 and 77K in 3MP are

shown in Figures 5 and 6. The spectra at 77K have been reported

previously952 and our spectra essentially agree with

these results. There is some change in the spectra upon

further cooling, with some peaks showing a doublet character

at the lower temperature. The benzene spectrum shows almost

no emission in the 338-340 nm region where its 0,0 band is
located, but does show 992 cm- ring breathing mode pro-

gressions based on false origins corresponding to one quanta

of the v. mode (1596 cm-1; symmetry e2g) and one quanta of

the v9 mode (1178 cm-1; symmetry e2g). The benzene-chloro-

form spectra show an intense 0,0 transition on which are

built progressions in the 992 cm-1 mode. Similar progressions






0 r
. H




















E n O
a u U
t a

0 IO ^ ^

O 30 30 30


o a) 2






* C)





built on the false origins corresponding to one quantum of

the vQ and v9 modes are also observed.

Figure 7 is a plot of the natural logarithm of the

intensity of the origin emission (0,0 band for the complexes

or false origin for the benzene) versus temperature and

show the strong dependence of emission intensity on temper-

ature. Figure 8 shows the effect of "slow" cooling (stopping

for 1/2 hour at the freezing point of the glass) on sample

emission intensity. In spite of an apparent effect on the

temperature dependence of the emission intensity, there was

no apparent difference in the emission spectra for the

complex formed by the different procedures.

Lifetime Results

Working solely in the 338-340 nm region, decay curves

for emission from the complexes were obtained. From a plot

of In (Intensity) versus time (see Figure 9) it is easily

seen that these curves are nonexponential. These curves

were deconvoluted by the method previously described. The

results for each component lifetime of each complex as a

function of temperature (13-90K) are shown in Figures 10

and 11. Figures 12 and 13 are plots of the decay rate versus

the inverse of the temperature for the same results. Above

80-90K, the intensity of the 0,0 band is too small to allow

the measurement of lifetimes on our equipment.

In Tables 2 and 3, the results for various fitting

schemes for the data are given. In each case a weighted

least squares program identified as Super, provided and

Figure 7. Plot In (I/I0) versus temperature for complex
and uncomplexed benzene
I = Intensity at temperature T
I0= Intensity at lowest temperature
0 = C6H6
A = CDC13-C6H6
0 = CHC13-C6H6

0.00oo n

-1.00 \




-5.00 .

-6.00 --- i
0.0 20.0 40.0 60.0 80.0

Figure 8. Plot In (I/I0) versus temperature for slow
and normal cooled completed benzene
I = Intensity at temperature T

10 = Intensity at lowest temperature
Q = CHC13-C6H6: normal cooling
A = CHC13-C6H6: slow cooling


\ \'


-3.00 \



0.0 20.0 40.0 60.0 80.0



















(ATsa4%uI aA!Te;TH) UT

Figure 10. Plot of lifetime versus temperature for the
benzene-chloroform complex
0 = Long-lived component
A = Short-lived component


20.0 40.0 60.0 80.0 100.0

6.0 -

1.00 4

0.00 I

Figure 11. Plot of lifetime versus temperature for the
benzene-chloroform-d complex
0 = Long-lived component
A = Short-lived component








20.0 40.0

T (K)

60.0 80.0

0 o0 o

Figure 12. Plot of decay rate versus inverse temperature
for the benzene-chloroform complex
0 = Long-lived component
A = Short-lived component




1.00 AA




12.0 24.0 36.0 48.0 60.0

1/T x 10+3 (K-1)

Figure 13. Plot of decay rate versus inverse temperature
for the benzene-chloroform-d complex
0 = Long-lived component
A = Short-lived component




1.40 -





42.0 54.0
1/T x 10+3(K-1)

66.0 76.0'

18.0 30.0

----I~i_ I _I_ I I I I _I


U 0 0 0 0
-l +1 +- +1 +1
N 0 0 0 0
C' ~N N
M in N N (N Cd
< N
(U 0
i i 0 0
A ) -I 0 0
B >, I 0 0 o nm m
o + r- r- +1 +
o U ) +1 +1 0 0
) 0 0 0 0 0
d N" m m N (N

C + 1 +1 +1 +N N
C ) 0
N m

E >l ->
I 0 0 0 0

0 --' +- +1 +1 +1
E4 0 N N 0 CO

H di dH
>HI 0 0 0 0 H 4.
Id N N in iLn .1 .1 I
NU' 3 -C +4 +1 +1 +1 Ni N I 4

+d N+ 0, 0 0 0- 1
di 11 14 U3 I0 N N N N N >1

H d 0 d di |i in 'di

0- U 4+ +4 -H +4 + + 14

0) 0 0 0 0
(i 0 N .i 0 0 Id
o ^ in in in in .0

') Od 0 0 0 0

-.4 C +4 i- +1 +4 0 0 0


a) U u u
0 H H

u U U U U

0 o o o o


o o
0- +1 +1 +I +1

V 0 +1 +1
X1 -01 a N 0

D i oo o o

0 C % o ,
0) C] NI O 0 -0 a)
0 r +1 +1 +1 +10
0 r 0 0 0

Q) .
H CO f

o 0 M +1 + +1 +1
0 0 PL -

0 01

0. 1 C 0 0 0
H- 411 If3I

Q1 -> 01

(I- H 0 4-O

C H> H H 4- 0P
N U O O 0 0 C C^ C 1N N

010 C L1 0 0 C C C C N P
Hl H -- H H 01 0 4 4 C 0
.0" H <1 010 0a 4)
011H 01 0 04 01
Hm> 4 N NCl 0 0.

01E K iH C CO C C > 01 0
m .0 I 0 0" 0 H 01
00 1 ^ +1 +1 +1 +1 o oC 01 3
o C N O C 0 rl H O

0 N U U 0 1

"0 H -0 0 n o

01 I 1 A AC U
*r4 m0 o o o o
S 1-1 o oC
01 H N N N N

1 H >
0 H H

4) 4) H n 4 4)
014 H H H H [1 &


0t en em en m

0 D0 10 10 10


modified by Joseph Baiardo, was used to fit the results.

The first form (I) is the one used originally by Nieman

(see equation 16) while form II is the theoretical expression

derived in the introduction (see equation 9). The third

form is a modification of the first in which the low tem-

perature decay rate, k10, first found by averaging the low

temperature results, is treated as a constant in the sub-

sequent determination of k20 and 2E 1. This form was ex-

plored because the data invariably showed a tendency to

depart from I in the region where the decay rates first start

to change. This deviation causes most fitting procedures to

give a somewhat high value for kl0 and correspondingly low

values of k20 and AE21. This general result is not as

pronounced here as in previous work because of the large

decay rate errors caused by the nonexponential deconvoluting

procedure. Form IV is the analogous modification of form II.

The solid line in Figures 10 through 13 is the calculated

fit to the data using form IV.

Tables 2 and 3 indicate that the accuracy of the kl0

results are better than the 10% previously estimated, but

this is felt to merely reflect a consistency in the choice

of break point in the deconvolution procedure that does not

necessarily denote accuracy. Table 4 gives the output of

Super for form I using the data of the benzene-chloroform-d

complex for both the slow and fast decay rates.

Table 4
Sample Fit to C6H6-CDCl3 Decay Rates (Form I)

Temperature (K) Rate (s-1)



Calculated Percentage
Rate (s-1) Deviation

Fast Decay

Slow Decay



Temperature (K)


Table 4
Rate (s-1)


- continued
Rate (s-1)





Spectral Results

Previous analysis of the uncomplexed benzene spectrum

has indicated on the basis of the absence of the 0,0

transition and presence of progressions based on false

origins that triplet benzene is only slightly distorted

from D6h symmetry.13,14 Simons has shown that the symmetry
of benzene in a complex is lowered to D2h. From this

symmetry assignment and the relative vibrational intensities

in the phosphorescence spectrum of the chloroform complex

it has been shown52'58 that the deviation from D6h symmetry

is at least three times that in the uncomplexed benzene.

Our phosphorescence spectral results show an apparent

discrepancy with the benzene complex spectra of Simons.

Relative to his results, our spectra show several peaks at

higher energies. Simons has attributed several shoulders

on the blue edge of his peaks to a 2:1 complex,52 and we

agree with this assignment. As previously mentioned, our

experimental conditions probably cause a nonequilibrium ratio

of 2:1 versus 1:1 chloroform-benzene complex pairs. With

a slower cooling procedure it is expected that greater re-

laxation can occur and a different proportion of the

chloroform-benzene complexes form. The slightly different

temperature dependence for the benzene-chloroform complex

phosphorescence intensities for the different cooling pro-

cedures (see Figure 8) areno doubt caused by this effect.

Strong support for the existence of two emitting species

comes from the shift in wavelength with temperature of the

0,0 band of the benzene-chloroform-d complex (see Figure 14).

This shift can best be understood in terms of different

intensity contributions from the two different complexes at

different temperatures. Reduction of the chloroform concen-

tration by a factor of ten also appeared to cause some shift

in the location of the 0,0 band but this result is compli-

cated by a lowering of the 0,0 band intensity and the

appearance of new peaks that indicate the presence of un-

complexed benzene.

Lifetime Results

The observation of nonexponential decays in all the

benzene complex systems studied is not surprising in view

of the presence of more than one emitting species. Indeed,

what is surprising is the observation by Simons and co-

workers of an exponential decay in his benzene-chloroform

system.55 They attribute this to the difference in inter-

molecular vibronic coupling and complex pair orientation
and fluctuation in the solvent cage. Although Simons

has identified two emitting species from his absorption and

phosphorescence spectra, he curiously ignores these as

possible causes of the nonexponentiality.

It appears possible that Simons and coworkers did not

observe any nonexponentiality in the benzene-chloroform-d

complex because of experimental conditions. Although chloro-

form and chloroform-d show the same completing strength to-

ward benzene,50 the emission intensity from the chloroform-

Figure 14. Plot of relative intensity versus wavelength
for the 0,0 band of C6H6-CDC13


H 0.50




Wavelength (nm)


d-benzene system is two to three times that of the chloroform-

benzene system.53 If the nonexponentiality were due to the

overlapping decays of the 2:1 and 1:1 chloroform-benzene

complexes and the enhanced intensity of the chlororform-d

complex versus the chloroform complex were predominantly due

to the 1:1 complex, one might expect to observe a nonexpon-

ential decay in the chloroform system and an exponential one

in the chloroform-d system. The fact that our cooling pro-

cedure produces both complexes about equally whereas Simons'

technique favors the 1:1 chloroform-benzene complex is con-

sistent with the above assumptions and conclusions.

The identification of the origin of the different com-

ponents of our nonexponential decays to specific complexes

is not easy, although Simons' previous work is again help-

ful. He has measured a phosphorescence lifetime of 1.7

seconds for the benzene-chloroform-d system in 3MP at 77K.53

This is in reasonably good agreement with our long-lived

component for the same system (2.2 seconds). The difference

is most probably due to a small temperature difference in the

two experiments. Figures 11 and 13 show that at ~77K both

components of our phosphorescence decay are very sensitive

to temperature. Because Simons was observing the decay of

the 1:1 complex, it is reasonable to conclude that our long-

lived component originates from the 1:1 chloroform complex

and the short-lived component from the 2:1 chloroform


From a comparison of the average lifetime and phosphor-

escence efficiency of the chloroform-benzene complex with

that of the chloroform-d complex, Simons has concluded that

the difference in lifetimes at 77K is due to a difference

in the nonradiative decay rates for the two complexes. This

was interpreted as evidence for an intermolecular vibronic

coupling between the C-H(D) motion in chloroformed) and the

benzene ring.53

From our results in Tables 2 and 3, it is possible to

explore further the reasons for the difference in the non-

radiative decay rates. The low temperature decay rate, kl0,

is seen to be the same, within experimental error, for both

the chloroform and chloroform-d complexes. Thus the com-

bined radiative and nonradiative decay rate from the lowest

triplet to the ground state is unaffected by the completing

partner, chloroform or chloroform-d. The large differences

observed for AE21 and k20 for the two complexes show, how-

ever, that it is some level above T1 (here labeled T2) in

equilibrium with TI, to which or through which radiationless

processes may occur at higher temperatures (T h 77K). We

propose that the T2 level is simply the potential barrier

over which the completed benzene must pass to form a sub-

stituted hexatriene. The hexatriene formation step is

envisaged as occurring via a concerted mechanism involving

the chloroform and/or solvent and the triplet benzene and

it is this hexatriene precursor that is deuterium sensitive.

There are a number of observations which support this

model as the major pathway of radiationless decay in benzene.

1) Simons and coworkers have observed that upon irradiation

of benzene in 3MP at 77K, solvent-substituted hexatriene is

formed. Similar experiments with chloroform-complexed

benzene yielded chloroform-substituted hexatriene while with

chloroform-d completed benzene, solvent-substituted hexa-

triene was formed, although at a slower rate than for un-

complexed benzene in 3MP.50'55 As Simons points out in

justifying his intermolecular vibronic coupling model, the

Franck-Condon overlap factor for the chloroform-d-benzene

complex is expected to be smaller than that for the chloro-

form complex.53 Thus, the probability of energy exchange

necessary for chloroform-substituted hexatriene formation

is decreased or prohibited in the chloroform-d-benzene com-

plex. 2) The increase observed in AE21 for either lifetime

component upon substitution of chloroform-d (see Table 2 or

3) is consistent with the contention that the CH(D) fragment

is the important energy acceptor since upon deuteration the

zero point energy of the complex is expected to decrease.

Simons and coworkers have also noted that the phosphorescence

intensity of the chloroform-d-benzene system is two to three

times more intense53 than the chloroform one. This again

is just a consequence of the larger activation energy (AE21

of the 1:1 complex) needed in the chloroform complex. 3)

Substituted hexatrienes are apparently formed in the triplet

manifold53 and are known to be formed via a one photon
process. They also show an external deuterium isotope

effect in their formation.50'5355 4) The large values of

k20 found previously for benzene phosphorescence decays in

many different solvents cannot be explained by current

theories (e.g., Lin's)9 of intramolecular radiationless

processes. 5) No other one-photon processes (including isomer-

ization)3 are known to occur in glasses containing benzene.

It has been implied in the above discussion that there

exists only one potential barrier, T2, to substituted hexa-

triene formation. There are several pieces of data which

indicate, however, that several such barriers (i.e., pro-

cesses) are possible. First is the previously discussed

observation of Simon's on the chloroform-substituted and

solvent-substituted hexatriene formation from irradiation of

chloroform and chloroform-d complex systems, respectively.

That two different types of photoproducts can be formed from

the 1:1 complex implies that the complex is more properly

viewed as a 1:1:1 chloroform-benzene-solvent complex. Ther-

mal activation of chloroform-benzene-solvent exciplex would

preferentially result in the chloroform-substituted hexa-

triene because of a larger Franck-Condon overlap between

benzene and the C-H molecular fragment of the chloroform

versus the solvent as evidenced by the larger 0,0 transition

in completed versus "uncomplexed" benzene. Deuterating the

chloroform causes the process leading to chloroform-substi-

tuted hexatriene to be more difficult because of the smaller

Franck-Condon factor in C-D versus C-H and actually results

in the reaction with the C-H molecular fragment of the

solvent becoming more efficient than reaction with the C-D

molecular fragment of the chloroform-d. Each exciplex

should then possess at least two barriers corresponding to

benzene reaction with either of its partners. The higher

energy associated with T2 in chloroform-d versus chloroform

(see Tables 2 and 3) may then reflect not just a lowering of

the zero point energy of the exciplex due to deuteration,

but also the larger barrier to solvent versus chloroform-

substituted hexatrienes. We will return to the question of

whether or not benzene can form an exciplex with the solvent


The second piece of evidence for additional processes

is more direct and comes from the temperature dependence of

the phosphorescence lifetimes of the "benzene-chloroform"

complex. The experimental lifetimes as shown in Figures

10 and 12 are substantially shorter than those predicted on

the basis of the fit to the data and indicate the need for a

second Arrhenius type term (i.e., k31e-AE 31/kT) to describe

the data. Unfortunately, the lifetimes may not be followed

to sufficiently high temperatures to allow the calculation

of the parameters k31 and AE31 but the need for this second

term is evident. It is significant that this T3 shows up

in the 1:1 "chloroform-benzene" complex as it is precisely

in this case that we would predict that it should be possible

to transfer energy to the C-H fragment of either the solvent

or the chloroform.

The proposal that benzene can form a triplet exciplex

with the solvent requires some justification. Not only does

such a proposal appear reasonable from the present results,

but it is consistent with other previous studies of "uncom-

plexed" benzene and its methyl derivatives in hydrocarbon

solvents. Simons55 has shown that benzene in ethanol (and

its various deuterated forms) exhibits an intermolecular

coupling which is specific to the methyl hydrogens rather

than the hydroxyl hydrogen. Nieman and coworkers have done

a great deal of research on temperature dependence of the

phosphorescence lifetimes of benzene and its methyl deriv-

atives in many different solvents. They have observed that

the kl0 values vary appreciably for different solvents, even

for the so-called inert alkane solvents (see Table 5).31,56

They have further noted that 1) methyl substitution in

deuterated benzene causes an essentially linear increase

in k10, 2) the relative decay rates (kl0[solute:solvent]/

kl0[C6D6:same solvent]) are solvent independent, and 3) the

relative decay rates (k10[solute:solvent]/kl0[same solute:

C7F141) are constant for any solute despite the fact that the

magnitude of the solvent effects is a function of methyl

substitution. These data which are solely concerned with the

decay rate from the lowest emissive triplet are all consis-

tent with the idea of a benzene-solvent exciplex from which

emission occurs.

Theoretical Calculations on the Exciplex Model

Results of Open and Closed Shell INDO Calculations

The computer program used for the INDO calculations

was supplied by the Quantum Chemistry Exchange Program

Table 5
Reported Parameters Determined from Benzene Decay Rates in
Different Solvents (Form I)


AE21(cm-1) k20

CgH12 (M) 450
C6H12 (M) 580
C6H12 (S) 570
C6H12 (S) 800
3MPb 960
Ethanol 1540
C7H14 unannealedd) 1160
C7H14 (crystal) 720
C7H14 (crystal) 480
C7H14 (crystal) 670
C7F14 unannealedd) 520
C7F14 unannealedd) 890
C7F14 unannealedd) 820
C7F14 unannealedd) 1060
C7F14 unannealedd) 730


asee reference 31

bsee reference 59








(QCPE #141) and originally written by P. Dobosh.6 It will

do open or closed shell INDO calculations for atoms H

through F and is a modified version of a program described

by Pople and Beveridge.61

We have investigated theoretically the potential of

triplet benzene to complex with an alkane solvent by doing

open shell, semi-empirical INDO calculations on this system.

As a model we chose to study the interaction of triplet

benzene with a hydrogen molecule. The bonding in a hydrogen

molecule can be likened to that of a C-H bond of an alkane

without the computational problems introduced by steric

factors caused by "inert" parts of the solvent molecule.

We have also carried out closed shell INDO calculations on

the interaction of ground state benzene with H2 in order

to compare the tendency of the S. and T1 states of benzene

to interact with the solvent. Further, we have done open

shell INDO calculations on the triplet benzene-hydrogen

fluoride system (fluorine approaching the aromatic ring)

to see if the apparent inertness of the fluorocarbons

toward triplet benzene is predicted by the computations.

In this way we could check the applicability of INDO calcu-

lations to this system, as well as defend our choice of

model system.

In considering the interaction of the hydrogen molecule

with benzene triplet, H2 was allowed to approach from above

the plane of the benzene ring as this would optimize the

H2 interaction with the pi electron cloud and is also

consistent with the probable geometry of the benzene-chloro-

form complex as determined by Simons.52,53 The line joining

the H-H bond was kept perpendicular to the plane of the

benzene ring since steric considerations in alkanes would

prohibit any other type of approach. There are then three

different ways in which H2 may approach: (Figure 15) from

above a particular carbon atom (approach A), from above the

center of a carbon-carbon bond (approach B), or from above

the middle of the ring along the C6 axis (approach C).

Table 6 is a compilation of the results of the INDO

calculation for these different lines of approach. r is

the distance of the first H atom above the ring (the second

H atom is kept at the equilibrium bond distance .74611 A

from the first). E is the total energy of the molecule in

Hartrees. The remaining three columns are the valence elec-

tron densities of the primary interacting atoms (i.e., the

two H atoms and the nearest interactive carbon atom) and

are given in order to illustrate how the electron density

of the complex changes as a function of distance. Figure

16 is a plot of r versus E for each of the different lines

of approach and graphically illustrates the interaction of

H2 and benzene triplet.

Table 7 is a compilation of the results for the inter-

action of ground state benzene with H2 for the approaches

that lead to the strongest and weakest interactions in the

triplet state. Figure 17 is a plot of r versus E for these

lines of approach. Similarily Table 8 is a compilation of

Figure 15. Approach of hydrogen molecule to triplet benzen'



-- H


H -H--

H N in in C i
N in i in N

n n n N N

i~n N in in
H H H 0 00

H H H H H Ni

in in N in m in
in N 0 H H

C 0 H H H H

0U N
iO LN t ) in

H *N
> in >1 i-i E
ILi i i-i I 9
0 K II 0 II 4 UiC I

W U MZ E-uj

i N in n n in n in io N

nmmmn NNNNm

n rH Nr n rN N in in N H
r-l I OO O 0 0 0 0 0

in in in N N N N Hl i N
O lrl 0 Hl H l o o

0 0 H H H H H H H H
O -- o

in 0 in 0 in 0 in 0 i 0 0 in 0 i 0 0
N 0 N in N 0 N 0 N in 0 N- 0 N in 0

Hl Hl H H N Hl Hl H N Hl Hl H N

Figure 16. Plot of H2-C6Hg(triplet) exciplex energy
versus intermolecular distance for
different complex geometries
O = Approach A
A = Approach B
a = Approach C




1.00 1.50 2.

r (A)

_,-C~c '

Hl 0 N 9~ N m 0 N N
Q 1 C Ce Ce Ce Ce Ce Ce Ce Ce
en en en e e e n emn e e

a l en c~ a o~ N en 0 en
en en N H- 0 en N N Hl C

-4 H 4 H4 H H H Hi

en en en 0 N L en H en w

0o 0- 0 0\ 0~ 0 0 0 0 0

U, C IC 0 0 0 eC C 0

4 H H N C-N


Figure 17. Plot of H2-C6H6(ground state) complex energy
versus intermolecular distance for different
complex geometries

0 = Approach A
O = Approach C




0 1.00
0 1.00



r (A)


the results for the interaction of triplet benzene with HF

(F approaching first) for the approach from above a carbon

atom. Figure 18 is a plot of r versus E for these different


One last set of calculations was performed in which

the most stable H2-benzene configuration was chosen; the

second H atom was moved such that the hydrogen molecule

was parallel to the aromatic ring. The second H atom was

then rotated about the first in order to see if there might

be steric considerations aside a more stable configura-

tion than the one chosen which could act as a driving force

to bring the H2 (or C-H for an alkane) into a position from

which concerted solvent-substituted hexatriene formation

might occur. The results are tabulated in Table 9; Figure

19 is a plot of E versus rotation angle for the various

configurations tested.


The results for the interaction of triplet benzene and

H2 clearly indicate that these species will complex with

each other (see Figure 6). The energy of interaction is

approximately 0.1 Hartree and is much larger than we would

qualitatively expect even for a model that essentially

ignores steric factors. It is significant that benzene in

its ground state shows no analogeous interaction (see

Figure 17) indicating the true exciplex character of the

complex. That the strongest exciplex is formed with H2

over a particular carbon atom rather than along the C6 axis

N N N \O W
P ~C N W O~ O


o w H 0n9
0 0 0 H H N


0 0 0 0 0 N
N H H 0 N N


C n C L
< C NM n N^ C

H- H- H- H- NM

. <

Figure 18. Plot of HF-C6H6(triplet) exciplex energy
versus intermolecular distance for
different complex geometries
O = Approach A






r (A)



Table 9
Results of INDO Calculations on Stablest H2-C6H6
Exciplex as a Function of Angle

Total Energy
Angle (O) (Hartrees)






Valence Electron Density

H1 H2 C (nearest)











Figure 19. Plot of H2-C6Hg(triplet) exciplex energy
versus relative intermolecular orientation


r -46.890




60.00 120.00

Angle (O)




of the molecule may be surprising at first sight but an
inspection of the molecular orbitals of benzene (see

Figure 20) provides a ready rationalization. For triplet

benzene there are two electrons in the alu molecular orbital

(m.o.), three in the two elg m.o.'s and one in the two e2u

m.o.'s. As Figure 20 clearly indicates, there is a node

along the axis of benzene for both the elg and e2u m.o.'s

and hence there can be no positive overlap of these orbitals

with any molecule approaching along this line. Approach

from above, say, carbon two, offers the potential for a

strong interaction with these orbitals, as does approach

from above a carbon-carbon bond. Only the alu m.o. offers

the possibility for positive overlap with the H2 molecule

and apparently this overlap, while sufficient to maintain

a relatively weak interaction (0.04 Hartrees) in the triplet

state, is not capable of maintaining a bond in the ground


Interestingly, the HF molecule showed about the same

tendency to complex with triplet benzene as did H2. The HF

complex is apparently weaker than the H2 complex as its

interaction energy is only about 0.07 Hartree for the

stablest configuration; however, the most significant result

of this calculation can be seen by studying the valence

electron densities. These two complexes are very different

in character. An examination of Table 8 reveals that the

fluorine atom in the complex is positive relative to what

it is in HF. While it is unclear whether F is actually

Figure 20. Molecular orbitals of benzene (the wave-
functions in the cross-hatched regions
are out of phase with those in the plain

b -l

e 2u+

e lg ~ 1


adapted from reference 62.

donating electron density to the benzene ring (F is slightly

negative at 1.25 A) this situation is in contrast

with H2 (see Table 6) in which it is the ring that clearly

donates electron density to the H2. This means that H2 must

accept electron density into an antibonding orbital of the

molecule and thus completing weakens the H-H bond. This is

an important result in explaining the photochemical activity

of the benzene triplet manifold in alkane solvents as it is

the C-H bond of the solvent that must be broken in order to

form the solvent-substituted hexatriene. (It is this same

C-H bond that is broken in the two photon process that gives

the solvent radical in glasses containing benzene.) Further,

benzene in fluorocarbons should show either less photochem-

ical activity as there is less tendency to populate an anti-

bonding orbital of HF, or a very different photochemical

activity if F donates electron density to the aromatic ring.

This latter alternative would entail the use of a filled

p or sp hybrid orbital that should have little effect on the

HF bond strength. While little is known of the photochem-

istry of benzene in fluorocarbons, these conclusions appear

to be consistent with current experimental results.31 Thus,

while our theoretical study of HF and benzene triplet shows

that complex formation is likely, this complex is expected

to behave very differently from the one involving H2 and is,

therefore, not inconsistent with experimental observations.

The idea of a benzene-fluorocarbon complex is an intriguing

one since the kl0 values for benzene do vary from one

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