Spectroscopic investigations of electronic transitions in certain acene quinones

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Spectroscopic investigations of electronic transitions in certain acene quinones
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xi, 124 leaves : ill. ; 28 cm.
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Capps, Rodger Neal, 1951-
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Quinone -- Spectra   ( lcsh )
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Thesis:
Thesis--University of Florida.
Bibliography:
Includes bibliographical references (leaves 117-123).
Statement of Responsibility:
by Rodger N. Capps.
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Typescript.
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Vita.

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University of Florida
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Full Text










SPECTROSCOPIC INVESTIGATIONS OF ELECTRONIC TRANSITIONS IN
CERTAIN ACENE QUINONES








By

RODGER N. CAPPS


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY












UNIVERSITY OF FLORIDA


1979























"Training is everything. The peach was once a
bitter almond; cauliflower is nothing but cabbage
with a college education."

-- Pudd'nhead Wilson's Calendar













ACKNOWLEDGEMENTS


The author is indebted to Professor Martin T. Vala for his initiation

and support of this work. His many suggestions and comments, as well as

his never-failing optimism that things would eventually work out, were

of incalculable value in the course of this research.

The expertise contributed by the electronics shop personnel was a

help in carrying out these experiments, as was the assistance in construc-

tion of equipment contributed by Mr. E.C. Whitehead.

Dr. Glenn Boutilier and Mr. Gary Walden are gratefully acknowledged

for the performance of lifetime measurements included in these experi-

ments. The author would also like to thank Professor J.D. Winefordner

for the loan of the Eimac lamp, and Dr. E.J. Gabbay for the daunorubicin.

Numerous people must be acknowledged for moral support. The author

would like to thank his fellow graduate students, Ed. Voigtman, Joe

Baiardo, Dave Powell, and Bob Brittain for their camaraderie. He would

also like to acknowledge his parents for support and encouragement.

J.M. Nicovich, Justine Strand, Bette Ackerman, and Dr. S.O. Colgate are

also gratefully acknowledged for their moral support.

Finally, the author would like to thank Adele Koehler for her able

assistance in preparing this manuscript in a finished form.













TABLE OF CONTENTS


ACKNOWLEDGEMENTS


Page

. . . iii


LIST OF TABLES . . .

LIST OF FIGURES . . .

ABSTRACT . . .

CHAPTER

I INTRODUCTION . . .

II EXPERIMENTAL DETAILS . .

Materials . . .
Monochromators and Light Sources . .
Detection and Recording System . .
Shpolskii Matrix Experiments . .
Lifetime Measurements . .
Polarized Excitation and Emission Spectra .

III THE ANTHRAQUINONE SYSTEM . .

Introduction . ..
Shpolskii Systems and Impurity-Lattice Interactions.
Experimental .. . .
Results and Discussion . .

Analysis of Phosphorescence Spectra .
Temperature-Dependent Emission .

IV QUINIZARIN AND DAUNORUBICIN . .

Introduction . . .
Theory of Photoselection . .
Experimental . . .. .
Results and Discussion . .

Infrared Spectra . .
Lifetimes . . .
Absorption and Luminescence Spectra .


Shpolskii Systems


Photoselection and Assignment of Transitions .


vi

vii

ix


S92


. . 97


. 105







Page
V SUMMARY AND CONCLUSIONS . . 115

REFERENCES.................. .. ....... .117

APPENDIX FREQUENCIES OF NORMAL MODES OF VIBRATION OF
9,10-ANTHRAQUINONE . . .. 122

BIOGRAPHICAL SKETCH ....................... 124












LIST OF TABLES

Table Page

I Experimental Equipment and Manufacturers (Lifetimes) 11

II Phosphorescence Vibrational Analysis for Anthraquinone
in Hexane at 10 K . . 32

III Phosphorescence Vibrational Analysis for Anthraquinone
in Heptane at 10 K . . 35

IV Representations of the Normal Modes of Vibration of
Quinizarin . . 91

V I.R.-Active Fundamental Modes of Vibration of Quinizarin 93

VI Fluorescence Vibrational Analysis for Quinizarin in
Octane at 14K . ..102














LIST OF FIGURES


Number

1. Block Diagram for Shpolskii Matrix Experiments .

2. Block Diagram for Lifetime Measurements .

3. Block Diagram of Experimental Set-Up for Polarization
Experiments . . .

4. Mechanism of the 3Blg 1A Phosphorescence in
Anthraquinone . . .

5. Phosphorescence of Anthraquinone in Hexane at 140K in
the Origin Region . .

6. Phosphorescence of Anthraquinone in Hexane at 540K in
the Origin Region . . .

7. Phosphorescence of Anthraquinone in Hexane at 740K in
the Origin Region . . .

8. Phosphorescence of Anthraquinone in Hexane at 940K in
the Origin Region . . .

9. Phosphorescence of Anthraquinone in Hexane at 1200K in
the Origin Region . . .

10. Phosphorescence of Anthraquinone in Hexane at 1590K in
the Origin Region . . .

11. Phosphorescence of Anthraquinone in Heptane at 14K
in the Origin Region . . .

12. Phosphorescence of Anthraquinone in Heptane at 74K
in the Origin Region . . .

13. Phosphorescence of Anthraquinone in Heptane at 1670K
in the Origin Region . . .

14. Phosphorescence of Anthraquinone in Octane at 14K in
the Origin Region . . .

15. Phosphorescence of Anthraquinone in Octane at 74K in
the Origin Region . . .


Page

S 8

S. 10


S. 17


41


S. 43


S. 45


S. 47


49


S. 51


S. 53


. 55


. 57


S. 59


61


. 63







Figure Pane

16. Phosphorescence of Anthraquinone in Pentane at 14K
in the Origin Region . . 55

17. Phosphorescence of Anthraquinone in Pentane at 74K
in the Origin Region . .... 67

18. Plot of Activation Energies of Temperature-Dependent
Emission of Anthraquinone in Hexane. . ... 69

19. Phosphorescence of Anthraquinone in Hexane at 140K .. 75

20. Phosphorescence of Anthraquinone in Hexane at 360K 76

21. Phosphorescence of Anthraquinone in Hexane at 760K 77

22. Structures and Axis System of Anthraquinone, Quinizarin,
and Daunorubicin . . 82

23. Illustration of Relative Polarization for an Electronic
Transition . . .. 87

24. Plot of A vs. C for Quinizarin in Hexane .... .. 95

25. Plot of A vs. C for Quinizarin in Methanol ... 96

26. Fluorescence of Quinizarin in Hexane at 140K ...... 98

27. Fluorescence of Quinizarin in Octane at 14K ...... 99

28. Fluorescence of Quinizarin in EPA at 770K. .. 106

29. Plots of Absorbance and Relative Polarization of
Quinizarin in EPA. .. . . 108

30. Plot of Polarization vs. Wavelength for Quinizarin
Fluorescence. . .. 110

31. Plot of Excitation Polarization of Anthraquinone as
Compared to Quinizarin . . 111

32. Plots of Absorbance and Relative Polarization of
Daunorubicin in Alcohol-Water-Glycerin ... 114


viii









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


SPECTROSCOPIC INVESTIGATIONS OF ELECTRONIC TRANSITIONS IN
CERTAIN ACENE QUIRONES

By

Rodger N. Capps

March 1979

Chairman: Martin T. Vala
Major Department: Chemistry

The positions and natures of the lowest excited emitting electronic

levels in 9,10-anthraquinone and two of its derivatives, quinizarin and

daunorubicin, have been characterized. In the first case, that of

anthraquinone, highly resolved emission spectra at low temperatures were

obtained in Shpolskii matrices of n-pentane, n-hexane, n-heptane, and

n-octane. In the latter three matrices, the origin of the purely

electronic transition is observed very weakly or not at all. In the

n-pentane matrix, a variety of both non-totally symmetric and totally

symmetric modes are present. In hexane, heptane, and octane, the phos-

phorescence of anthraquinone is shown to contain progressions built upon

vibrations of types blu, b2u, and b3u, with the most intense peaks being

those of type blu. Analyses of the phosphorescence emission of anthra-

quinone are presented which are shown to be consistent with the assign-

ment of the lowest triplet level of anthraquinone as 3Blg. The results

obtained in this work tend to corroborate those of Khalil and Goodman,

but include several additional spectral features not reported by them.

A temperature-dependent emission at energies higher than the origin of







the purely electronic transition is also obtained in n-hexane, heptane,

and octane, but not in n-pentane. Based upon the behavior of this

emission as a function of temperature and solvent, it is suggested here

that the emission is a manifestation of the anthraquinone-lattice inter-

action which allows the spin and parity forbidden electronic transition

to gain radiative properties.

The nature of the lowest excited emitting electronic level in quin-

izarin is characterized by lifetime measurements, photoselection measure-

ments, and low temperature Shpolskii matrix emission spectra. The

species involved in the absorption and emission is shown to be the

quinizarin molecule, rather than an aggregated form, by concentration

studies. An analysis of the absorption spectrum of quinizarin in the

infrared and far infrared regions is also presented.

Photoselection measurements were made upon both quinizarin and

anthraquinone. It was observed that the relative polarizations of

transitions in the two molecules were the same. Based upon the axis

system chosen for quinizarin, electronic transitions occurring in the

ultraviolet and visible absorption spectra of quinizarin have been

assigned.

Highly resolved, low temperature emission spectra were recorded for

quinizarin in n-hexane, n-heptane, and n-octane. It is suggested that

the emission in n-hexane and n-heptane arises from two sites, at least

one of which is severely distorted. An analysis of the vibrational pro-

gressions for quinizarin in n-octane is presented which is consistent

with the observed ground state I.R. frequencies.







Based upon the measured emission lifetime of 6.5 1 nsec, photo-

selection measurements, and analyses of low temperature Shpolskii matrix

spectra, the lowest excited emitting electronic level in quinizarin is

assigned as a ir,i* singlet arising from a charge-transfer interaction of

the hydroxy substituents with the benzene ring of the anthraquinone

skeleton.

Photoselection measurements upon daunorubicin revealed that the

polarization of major electronic transitions in this molecule are the

same as those of quinizarin. Based upon these results, it is concluded

that the polarizations of transitions in this molecule, as assigned

byGabbayand co-workers, are incorrect.












CHAPTER I

INTRODUCTION


The 9,10-anthraquinone molecule and its related compounds are of

chemical importance from both an applied and a theoretical point of view.

It is an excellent subject for studies of heteroatom effects upon ex-

tended r-electronic systems, of spin-orbit coupling mechanisms, and of

possible intramolecular exciton interactions. Its high degree of sym-

metry allows an excellent correlation of theory and experiment through

the methods of group theory. Anthraquinone is also the parent molecule

of a large number of compounds which find use in the dye industry.

Studies of its electronic structure and photochemical reactivity might

find use in understanding the photodegradation of dyes. It has also

been demonstrated that this compound and certain of its derivatives dis-

play a mutagenic activity upon intestinal bacteria found in humans (1),

while other compounds having the 9,10-anthraquinone molecule incorporated

into their skeletal structures show promise as anti-leukemia drugs

(2, 3).

The earlier investigations of the nature of electronic transitions

in 9,10-anthraquinone were concerned with visible and near ultraviolet

absorption spectra (4, 5). Labhart carried out investigations of the

excited states of substituted anthraquinones using polarized absorption

spectra (6, 7).. Using polarized single crystal spectra, Sidman established

the polarizations of the electronic transitions in anthraquinone (8).




-2-


Photoselection experiments (9, 10) and a determination of the T1 < S

absroption moment (11) showed that the phosphorescence of anthraquinone

was in-plane polarized, n-n* type.

In reference (11) it was concluded that the lowest triplet level of
3
anthraquinone was A Later studies (12, 13) indicated that this
u
assignment was incorrect. The earliest reported Shpolskii matrix study

(14) was inconclusive, since no ground state vibrational frequencies were

available at that time. A later study (15) at 77K indicated that the

lowest triplet was 3Blg. However, there is normally a great difference

in resolution of spectra run at 770K and those obtained at temperatures

approaching O'K. One of the objectives of this study was to try to

determine the nature of the lowest triplet state of anthraquinone from an

analysis of the vibrational structure of the phosphorescence spectrum.

Quinizarin, which is a 1,4-disubstituted hydroxyanthraquinone, has

not been as extensively studied. Absorption and fluorescence spectra

of quinizarin in the vapor phase and in ethanol solutions were recorded

by Borisevich and Gouzinskii (16). A later work (17) reported visible

absorption and fluorescence spectra in the vapor phase, along with a

partial analysis of the I.R. spectrum.

Shcheglova, Shigorin, and Dokunikhin (18) recorded the luminescence

spectrum photographically at 770K in n-octane, but no attempt at analysis

was made. An early work (5) on substituent effects upon the visible and

ultraviolet absorption spectrum of anthraquinone included a study of

quinizarin. ElEzaby et al. (19) studied the absorption spectra of

anthracene, anthraquinone, and several hydroxy-anthraquinones in dif-

ferent solvents in order to determine the nature of the electronic







transitions in these compounds. Platonova, Popov, and Smirnov (20)

concluded from dichroic ratios of absorption spectra of quinizarin in

polyvinyl alcohol films that the relative polarizations of electronic

transitions in quinizarin were the same as those of anthraquinone.

The above works indicated that the behavior of quinizarin was

peculiar in several respects. A discussion of the effect of substituting

OH groups upon anthraquinone by Abrahamson and Panik (21) indicated that

the n-electronic levels of the molecule should be shifted relative to

anthraquinone. Experimentally (17), a very large wavelength shift in

emission was observed. Also, the emission from the molecule was appar-

ently fluorescence, rather than phosphorescence (17). Normally, carbonyl

containing compounds will exhibit phosphorescence (22).

The present work was undertaken to resolve some of the ambiguities.

The daunorubicin was also included in this study to determine whether

the assignments of electronic transitions in an earlier work were cor-

rect (23). The results listed in this work indicated a polarization of

electronic transitions contrary to results obtained here for quinizarin,

suggesting that chromophores present in daunorubicin had a very large

effect upon the electronic energy levels of the anthraquinone skeleton.

This work, then, presents a study of electronic transitions in

anthraquinone and two of its derivatives, quinizarin and daunorubicin,

using room temperature luminescence and absorption spectra, Shpolskii

matrix emission spectra, I.R. spectra, and polarized excitation spectra.












CHAPTER II

EXPERIMENTAL DETAILS



Materials


The anthraquinone used in these experiments was synthesized by ring

closure of ortho-benzoyl benzoic acid with concentrated H2SO4 (24).

Technical grade ortho-benzoyl benzoic acid was extracted from an

ether solution with 3 M NaOH solution. The sodium salt solution of the

acid was then filtered, and the ortho-benzoyl benzoic acid was regenerated

with 3 M HC1 solution. This was followed by suction filtering and

washing with distilled water. The ortho-benzoyl benzoic acid was then

recrystallized twice from ethanol and water. The melting point was

127-1280C, in good agreement with the literature value of 1280C (25).

The ortho-benzoyl benzoic acid was placed in a large excess of 96%

H2SO4. The mixture was then heated to 150-1600C, and maintained in this

temperature range with vigorous stirring for approximately two hours.

The reaction mixture was then poured over ice and allowed to stand

overnight. It was then mixed thoroughly with saturated NaHCO3 solution,

suction filtered, and thoroughly washed with more NaHCO3 solution and

distilled water. Two recrystallizations from an acetone-chloroform

mixture, followed by four vacuum sublimations, were deemed adequate to

insure the purity of the anthraquinone. An I.R. spectrum was run to

insure that the compound formed was indeed anthraquinone. Since the







compound sublimes, rather than melts, purity was checked by thin-layer

chromatography. Only one band was observed.

The quinizarin used was received from Eastman Kodak, and was vacuum

sublimed twice before use. The melting point was 200-202C. The litera-

ture value is also 200-202C (25).

The daunorubicin was a gift from Dr. E.J. Gabbay, and was used as

received.

Aldrich Gold Label (purity > 99%) n-hexane, n-octane, and 2-methyl

butane (isopentane) were used as received. Fisher analytical grade

n-pentane and Mallincrockt analytical grade n-heptane were used as re-

ceived, along with absolute ethanol and anhydrous ether. All solvents

were checked for absorption and emission in the spectral regions of

interest, and found to be satisfactory.



Monochromators and Light Sources


Heath EU-700 monochromators were used in this research. These are

0.35 meter, single pass, modified Czerny-Turner type mount monochromators

with folding mirrors which provide a common optical axis for entrance

and exit beams. The excitation monochromator was supplied with a plane

grating ruled with 1180 lines/mm and blazed at 3500 A, while the

emission monochromator grating was blazed at 5000 A.

The aperture ratio of the monochromator is f/6.8 at 2000 A. Wave-

length accuracy, relative to a fixed reference line, is 1.0 A throughout

the wavelength range. Resettability of the monochromator is 0.1 A.

Slits are continuously manually variable from closed up to 2000 microns.

Reciprocal linear dispersion is approximately 20 A/mm at the exit slit.







An external control unit provides scanning rates from 0.05 to 20 A/sec,

and can be synchronized with the chart rate drive of the Heath 700-C

chart recorder module used.

An Eimac 300 watt xenon lamp (Eimac is a division of Varian Associ-

ates) with a regulated power supply was used as a continuum excitation

source for the polarized excitation and emission spectra. A Hanovia

12 watt low pressure mercury discharge lamp was ordinarily used as an

excitation source for the Shpolskii matrix experiments on anthraquinone.



Detection and Recording System


The photodetector used in the Shpolskii matrix experiments and the

photoselection experiments was an RCA 1P-28A photomultiplier tube with

an S-5 spectral response. It was enclosed behind a shuttered quartz

window in the.light-tight compartment of a Heath photomultiplier module.

The supply voltage normally used was -900 volts.

The current output of the photomultiplier tube was fed into a

Keithley 416-S picoammeter. The picoammeter amplified the signal, con-

verted it from a current to a voltage signal, and filtered it. The

output of the picoammeter was then fed into the potentiometric amplifier

section of the Heath 700-C chart recorder module.

The potentiometric amplifier will accept any signal from 1 millivolt

up to 50 volts, and convert it to 1 volt full scale output to the chart

recorder. The recorder module was also equipped with a D.C. offset

module which allowed the baseline to be shifted to compensate for any

D.C. component of the signal.







The recorder output was plotted as a record of signal intensity

versus wavelength. The recorder drive could be synchronized with the

scan drives of the monochromators, so that a predetermined scale of

A/inch of chart paper could be recorded.



Shpolskii Matrix Systems

A block diagram of the experimental arrangement is shown in Figure 1.

An Air Products Model CSW202 Displex helium closed cycle refrigerator was

used as the cooling source in these experiments. Samples were contained

in a copper cell sealed with an 0-ring and a quartz Suprasil window.

The cell was mounted on the cold tip of the Displex at a 135 degree

angle to the face of the mercury lamp. The temperature of the sample

was measured by two gold-Chromel thermocouples, the upper of which was

fixed to the cold tip of the Displex, and the other mounted directly to

the cell. Liquid nitrogen was used as a reference junction. The tem-

perature of the upper thermocouple could be read directly from the

temperature controller module, while that of the lower one was indicated

by the thermocouple emf as registered on a Keithley digital voltmeter.

In general, the two temperatures agreed to within 1K. Due to its

proximity to the sample, that of the lower thermocouple was taken as

being a more accurate indication of the sample temperature. If desired,

the sample temperature could be increased by using a resistance heater

on the cold tip of the Displex. Temperature control was normally con-

stant to within 0.50K after the system had equilibrated.

Samples of the compounds to be run in n-paraffin matrices were pre-

pared by the quick-freeze method. The cell was filled and mounted on the











































0

I-











w
Z L


4J-
i-jM
























F-- 0
-0
Lll







CC x






























o
L
0









S-






L
+o


















S-
cr
































1-1


crO
CO
00




=If
I- 0

-O







cold tip of the Displex. Good thermal contact was insured by the use of

an indium gasket between the cell and the cold tip. The heat shield of

the Displex was placed around the cell, and the cold tip was immersed

in liquid nitrogen. When the readout of the Displex temperature con-

troller indicated that the cell was at 770K, the outer shroud of the

Displex was put into place and the system put under vacuum by a two-stage

mechanical pump with a liquid nitrogen cold trap. After approximately

fifteen minutes, the compressor of Displex was started. Cool-down time

to 100K was approximately thirty mintues.

Light from the mercury discharge lamp was passed through a Corning

CS 7-54 filter and focused onto the sample cell by quartz optics.

Emission from the sample cell was viewed at right angles to the direction

of excitation. It was focused by quartz optics onto the entrance slit

of the monochromator. Radiation passed by the monochromator was detected

by the RCA 1P-28A photomultiplier tube. The photomultiplier current was

processed as described in the previous section.

Slit widths of the monochromator were in the range from 18 to 120

microns, for a spectral bandpass of approximately 0.4 to 2.4 A. Scan

speed was normally 0.5 A/sec, with 20 A/inch recorded on the chart re-

corder output.



Lifetime Measurements

A block diagram of the apparatus used to measure lifetimes of

various peaks in the phosphorescence spectrum of anthraquinone in hexane

at 770K is shown in Figure 2. The same apparatus was used in an effort

to determine whether or not any phosphorescence was present in the

emission spectrum of quinizarin. Table I lists the model numbers and





- I (-


w ()


0-


v,



E










E

-r
L
r-





4-





0











CD
0)
ro














r-^


LL
en


cu


0



(-
=)
Q-


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-11-


Table I. Experimental Equipment and Manufacturers (Lifetimes)

Itm Model Number
(Description) source

Laser


Nitrogen


Flashlamp


Sample Housing

Monochromator


Photomultiplier Tube


Photomultiplier Housing


High Voltage Power
Supply

Gated Amplifier

Signal Generator


Oscilloscope


Boxcar Integrator


Chart Recorder


Signal Averager


Tape Punch

Computer


C950


CMX-4




H-10


4837


180


EU-42A




110


122AR


CW-1


SRG


BIOMAC 1000


2

PDP 11/20


Avco Everett Research Labora-
tories, Everett, Mass.

Chromatix, Sunnyvale,
Calif. 94086

Laboratory Constructed

American ISA, Inc.,
Metuchen, N.J. 08840

RCA Corp., Lancaster, Pa.
17604

PAR Corp., Princeton,
N.J. 08540

Heath Co., Benton Harbor,
Mich. 49022

Laboratory Constructed

Wavetek, Inc., San Diego,
Calif. 92123

Hewlett-Packard, Palo Alto,
Calif. 94306

PAR Corp., Princeton, N.J.
08540

Sargent-Welch, Skokie,
Ill. 60076

Data Laboratories Ltd.,
Mitcham, Surrey, U.K.

Friden, Inc., Rochester, N.Y.

DEC, Maynard, Mass. 01754




-12-


manufacturers of the equipment. Both Figure 2 and Table I are reproduced

with permission from Dr. Glenn Boutilier (26). A complete discussion of

the design and operation of the system may be found in reference (26).

Samples were placed in a 30 cm length Suprasil tube of 2 mm I.D. and

4 mm O.D. This was fitted into a Teflon cylinder using Teflon tape and

slip-fitted into a NMR spinner assembly. The spinner assembly fitted into

the top of the cover of a sample compartment containing a nitrogen

immersion Dewar and was used to position the sample cell. Nitrogen was

flushed through the lower section of the compartment to prevent conden-

sation of moisture on the Dewar.

A nitrogen discharge laser with a pulse width of 7.7 nsec was used

as the excitation source. The 3371 A line was used. Emission lines from

the laser were focused into one plane by the beam steering mirrors. The

laser beam was passed through an interference filter with a peak trans-

mittance of 42% at 3400 A and a bandwidth of 100 A to block non-lasing

nitrogen emission lines at wavelengths longer than 3600 A. The beam was

then focused onto the sample tube by a 3 inch diameter quartz lens of

8 inch focal length.

Light from the sample was focused onto the entrance slit of a

monochromator by a 15 mm diameter 25 mm focal length quartz lens. The

monochromator was a 0.1 meter focal length monochromator with an aperture

ratio of f/3.5 and a reciprocal linear dispersion of 80 A/mm. Detection

of phosphorescence selected by the monochromator was by an RCA 4857

photomultiplier tube enclosed in a light-tight compartment.

The photomultiplier socket was especially wired so that voltage

supplied to the photomultiplier dynodes could supply sufficient current




-13-


to maintain a linear response for the large pulses encountered using a

pulsed nitrogen laser. A wiring diagram of the P.M. socket may be found

in reference (26).

Both the signal average and the boxcar integrator used required a

voltage input. The photomultiplier output current was passed through a

current-to-voltage amplifier which could be gated to avoid saturation

effects due to stray light or fluorescence. A discussion of the design

and operation of the amplifier is included in reference (26). The laser

pulse repetition rate was controlled by the Wavetek signal generator

shown in Figure 2.

In the case of lifetime measurements of anthraquinone, the slit

width of the emission monitored at 4974 A was 0.1 mm (giving a spectral

bandwidth of 8 A). In the case of the peaks at approximately 4503 and
0
4530 A, the slit width was increased to 0.5 mm for a spectral bandwidth

of 40 A. The same setting was also used in attempts to detect phos-

phorescence in the emission spectrum of quinizarin. Supply voltage to

the photomultiplier tube was -800 volts. The amplifier was gated off

for 200 usec in all phosphorescence experiments.

In measuring lifetimes, the filtered output of the amplifier was

connected to the signal average. Sweep time of the signal average

could be varied by factors of 2 from 5 msec to 81.92 sec. Delay before

the start of sweep could either be set to zero or varied in factors of

2 from 0.32 msec to 5.12 sec. The signal average acquired 1000 points

per sweep and summed the value at each point into memory. The proper

repetition rate, sweep time, and amplifier gain were selected from an

oscilloscope display of the signal. After the requisite number of sweeps

had been averaged, the contents of the signal average were output to




-14-


the paper tape punch in 16 bit words. A PDP 11/20 microcomputer was

later used to process the tapes. The programs used are also listed in

the appendices of reference (26).

The arrangement used to measure fluorescence lifetimes of quinizarin

was similar. A 1.5 nsec laser was used as the excitation source, and a

fast boxcar integrator was used to process the signal. Instead of being

digitized, the data were recorded on a chart recorder. The limiting

factor here was the response time of the photomultiplier (1.5 nsec).

What was actually measured was the fluorescence lifetime convoluted with

the rise and decay time of the photomultiplier. Lifetimes were estimated

by extrapolating the time for a decay curve of known width of time to

decay to l/e of its original value.



Polarized'Excitation and Emission Spectra


When measuring a polarized spectrum, the substance in question is

normally imbedded in a clear, strain-free glass, dissolved in a dilute,

highly viscous solution, or grown in a single crystal. A glass is

generally much more convenient to prepare than a single crystal and is

preferable to a solution, since rotational depolarization will be

minimized.

In the case of quinizarin, EPA was found to be the most satisfactory

substance for forming a glass (EPA is a 5:5:2 vol.-vol.-vol. mixture of

ether-isopentane-ethanol). Attempts were made to use 3-methylpentane

and isopentane-methyl cyclohexane mixtures, but solubility problems pre-

vented the use of these compounds. Daunorubicin was not soluble to any

appreciable extent in EPA or other organic solvents commonly used to form




-Ib-


glasses. A 55-25-20 vol.-vol.-vol. mixture of ethanol-methanol-water

was used in an attempt to form a glass, but was found to be unstable when

the daunorubicin was dissolved and the solution frozen. Alternatively,

the daunorubicin solution was added in a volume ratio of 1:6 to glycerin,

and the mixture cooled by the simple expedient of lowering the room

temperature with air conditioning to approximately 190C. This arrange-

ment gave satisfactory results for obtaining the polarized excitation

spectrum of daunorubicin.

A number of experimental methods for obtaining polarization ratios

are discussed by Parker (27). The method used was that of Azumi and

McGlynn (28). This procedure corrects for such factors as selective

transmission of the emission monochromator, selective reflection of the

cylindrical sample tube and cylindrical Dewar, and spectral shifts due

to rotation of the Glan-Thompson polarizers. It makes use of the fact

that emission viewed in a direction at a right angle to an incident

horizontally-polarized beam must be unpolarized. If the intensities of

the vertical and horizontal components of fluorescence with horizontally

polarized exciting light are denoted as IBE and IBB, then the ratio

IBE/IBB is a correction factor for the effects mentioned above, since
any difference between IBE and IBB will be due to instrumental factors.

If the corresponding intensities of fluorescence with vertically

polarized light are denoted by IEE and IEB, then the corrected polariza-

tion of emission is calculated from


EE E B (IBE/IBB)
SI + (I BEBI
EE EB BE BB




-16-


Two different experimental methods were used which gave essentially

the same results. Polarized excitation and emission spectra were first

run on an Aminco-Bowman spectrofluorimeter equipped with a liquid

nitrogen immersion Dewar and Glan-Thompson polarizing accessory. How-

ever, in obtaining the polarized excitation spectra there was some

second-order scattering of the exciting light at short wavelengths.

Spectra were recorded on a relatively small area of chart paper, which

made an accurate interpretation of intensity versus wavelength somewhat

difficult. In order to obtain more highly resolved spectra, the experi-

mental arrangement shown in Figure 3 was employed.

Quinizarin, dissolved in EPA, was placed in a 2 mm I.D. quartz

Suprasil tube. The tube was then placed in a quartz immersion Dewar,

which was clamped into place on a ring stand mounted on the optical table.

A stream of air was blown across the Dewar to prevent condensation.

Light from the Eimac lamp was passed through the excitation mono-

chromator. The exciting light was collimated and focused on the sample

by quartz Suprasil optics. Polarization of the excitation beam was

achieved by passing the focused light through a Glan-Thompson polarizer

placed directly in front of the immersion Dewar.

Emission was collected at right angles to the direction of excita-

tion. Polarization of the emitted light was obtained by using a

polaroid sheet mounted in front of the entrance slits of the emission

monochromator.

In obtaining the polarized excitation spectra, the excitation

monochromator had to be scanned from 2000 A up to past 5300 A. This,

coupled with two other factors, led to the necessity of scanning the

excitation spectra in two segments. The Dewar used held enough liquid




























w
cr
I LIj
a OO_
< Q00


LU
I-
,)
o




C) m
Ql-

<<.^_ r, 'I

A


-17-


of
CD
Q:
o


I
-o




-18-


nitrogen for approximately 36 minutes. In order for a set of spectra to

be internally self-consistent, a scan of all four possible orientations

of the polarizers had to be taken on a single glass sample without re-

filling the Dewar. Also, since the output of the Eimac lamp was much

greater in the visible than in the ultraviolet region, a readjustment of

monochromator slit widths had to be made before scanning into the visible

region to keep the chart recorder from going off-scale. Therefore, the

excitation spectra for quinizarin were usually scanned from 2000 A up

to 3800 A. A scan of the visible region was made on a fresh glass sample

from 3600 A up to past 5400 A. An average of polarization values from

the two different runs was taken in the area of overlap. In general,

the agreement was good between the two sets of data.

When a set of polarized excitation spectra was obtained, the emis-

sion monochromator was set on a wavelength corresponding to the maximum

of a vibronic band in the emission spectrum. The chart recorder was

synchronized to the scan drive of the excitation monochromator, and the

wavelength region of interest was scanned. Slits of the excitation

monochromator were maintained at 1800 microns or less, while those of

the emission monochromator were maintained at 1000 microns or less.

In the emission mode, the strip chart recorder was synchronized with

the scanning drive of the emission monochromator. The excitation mono-

chromator was maintained at a fixed wavelength, and the fluorescence

emission spectrum was obtained. Spectra were also obtained here for all

four possible orientations of the two polarizers.

The experimental arrangement for obtaining the polarized excitation

spectra of daunorubicin was similar, with the distinction that the quartz







immersion Dewar was not used. As mentioned before, it was necessary to

use the highly viscous glycerin-alcohol-water mixture to obtain polarized

excitation spectra, since no suitable glass could be found for daunoru-

bicin. The mixture was placed in a 1/2" O.D. quartz Suprasil tube and

mounted in the same position as the immersion Dewar had been, and spectra

were obtained as described in the case of quinizarin. Since the

daunorubicin exhibited broad, structureless fluorescence, no polarized

fluorescence emission spectra were recorded.












CHAPTER III

THE ANTHRAQUINONE SYSTEM



Introduction

The molecular structure of anthraquinone and its crystalline mor-

phology are well known (29, 30, 31). The 9,10-anthraquinone molecule is

planar and belongs to the D2h point group, while its crystalline space

group is C2h.

Group theoretical considerations of the electronic configurations of

anthraquinone show that emission from the triplet level could occur from

either the Au, B1g, B2g, or B3u states. It is generally accepted (13, 15)

that the lowest triplet energy levels of the 9,10-anthraquinone molecule

are the 3B and 3A states. These two states are degenerate if inter-

actions between the two carbonyl groups are neglected. On the basis

of polarized single crystal absorption measurements at 770K, Dearman

and co-workers (11) have assigned the lowest triplet state of 9,10-

anthraquinone as A This assignment was refuted in later polarized

Zeeman (13) and Stark-Zeeman (12) absorption measurements, where it was

concluded that the lowest triplet state of 9,10-anthraquinone is 3Bg
31g
and that it is separated from the A state by 410 cm- in the crystal

(13).
3 1
The transition Big A is forbidden on the grounds of spin and
3 1
parity conservation, while the transition A A is only spin for-
bidden.
bidden.


-20-




-LI-


It has been shown (9, 10) that the phosphorescence of anthraquinone

is n-n* type, in-plane polarized. Measurements of the polarized S-T

absorption spectra (11) showed that the dipole moment of the transition

lies in the plane of the molecule parallel to the C-O axis (the Z axis,

in Mulliken's notation). Therefore, the vibrational bands present in the

emission spectrum of anthraquinone may be viewed as primarily being

polarized along the Z axis. If the conclusions in (12, 13) are correct
3 3 1
and the 3Bg is the lower triplet level, then the transition 3B A

will occur primarily as a result of borrowing intensity from the nearby
lu (TT*) state.

It is well known (32) that, in the case of forbidden electronic

transitions where vibronic coupling plays an important part in allowing

the transition to occur, only odd quanta of the coupling vibrations will

appear. These vibrations will be antisymmetric with respect to any

symmetry operation to which the transition moment integral is antisym-

metric. In the case of 9,10-anthraquinone, this will be vibrations of

the type b If the carbonyl stretching frequency and vibrational pro-

gressions built upon it are present, as is normally the case for n-n*

phosphorescence from carbonyl-containing compounds (22), then the low

temperature Shpolskii matrix phosphorescence should reveal them. If the

lowest triplet state were indeed the 3B1g, then a close analysis of

vibrational progressions present in the phosphorescence spectrum should

confirm this.

Analyses of the spectra were handicapped by a lack of reliable Raman

data. As an adjunct to this portion of the research, it was decided to

obtain polarized single crystal Raman spectra of anthraquinone. However,

for reasons which are discussed in a later section of this paper, these




-22-


attempts were unsuccessful. Also, a highly resolved low temperature

Shpolskii matrix emission spectrum of anthraquinone was published (33)

before the analysis presented here was complete. Therefore, the present

results will only be reported to the extent that they extend those findings.

In comparing spectra obtained at 100K and 770K, it was noticed that

several small high energy peaks had appeared as the temperature was in-

creased. The separation between the location chosen as the 0-0 position

of the anthraquinone phosphorescence and the highest-energy temperature

dependent band was approximately 424 cm-1. This was very close to the

value of 410 cm-1 tentatively suggested as the triplet level separation

in the crystal (13). It was thought that perhaps the upper triplet level

might be thermally populated by the lower one. Such behavior has been

observed in other molecules (34, 35, 36). Therefore, a series of experi-

ments were undertaken in an effort to determine whether the higher-

energy peaks were emission from a thermally populated triplet level of

anthraquinone, or whether they had some other origin.

In summary, this portion of the research had three major objec-

tives: (1) to obtain and analyze highly resolved phosphorescence spectra

of anthraquinone in n-paraffin matrices at low temperatures; (2) to

obtain polarized single crystal Raman spectra in order to make definitive

assignments of Raman-active normal vibrations; (3) to determine the

exact nature of the temperature dependent higher energy emission bands

in the phosphorescence of anthraquinone.


Shpolskii Systems and Impurity-Lattice Interactions


The low temperature phosphorescence spectrum of anthraquinone in

n-hexane is an example of what is commonly referred to as the "Shpolskii




-23-


effect." The Shpolskii effect was discovered by E.V. Shpolskii (37)

as part of a systematic investigation of luminescence and absorption

spectra of large organic molecules in frozen crystalline n-paraffin

matrices. As in the case of substituted molecular crystals, i.e.,

naphthalene in durene, the effect is generally considered to arise from

substitution of "guest" molecules into the crystalline lattice of the

host. The guest molecules exist as discrete molecular substitutional

sites, rather than as crystalline aggregates (33, 38, 39). In inves-

tigations of electronic transitions, the Shpolskii matrix method

possesses the inherent advantage that the electronic energy levels of

the host are usually much greater than those of the guest, so that the

only electronic transitions which occur upon excitation are those of

the guest molecule.

In an experiment involving the Shpolskii method the substance

under investigation is dissolved in a suitable solvent, usually an

n-alkane, and the solution is then frozen. At very low temperatures the

guest molecules undergo electronic transitions in both absorption and

emission with a very high probability that either no change or small

changes in the phonon energy of the host will take place. This will

depend upon the nature of the electronic transition and upon the extent

of coupling of inpurity molecules to the host lattice.

Spectra produced by these transitions are quasi-linear in nature,

having bands which are very sharp and well-defined, with residual half-

widths of approximately 1-10 cm- at very low temperatures. They are

sometimes considered to be the optical analogue of the Mossbauer effect

(40). Since emission in a condensed medium normally occurs from the

ground state vibrational level of the electronic excited state, there is




-L'\-


usually a very good correlation between the frequencies of such peaks in

the emission spectrum and the ground state vibrational frequencies of

the guest molecules. These sharp peaks are often accompanied by more

diffuse, less intense bands which are usually assigned as "lattice" bands

in the analysis of a Shpolskii matrix spectrum. More will be said about

these later.

For molecules in which the electronic transition is an allowed one,

there is an overlap of the 0-0 transition in absorption and emission. A

temperature dependence upon the intensity of the spectrum is also ob-

served, with the intensity increasing with decreasing temperature. It is

also observed that considerable broadening and loss of fine structure

occurs as the temperature is raised. This is often accompanied by

thermal line shifts.

There also appears to be a critical dependence upon the size of the

solvent molecule. This is manifested experimentally in several different

ways. One of these is the fact that a spectrum can often be interpreted

in terms of several different spectra of varying intensities which are

displaced from each other. This effect is explained in terms of dif-

ferent sites in the host crystal in which the guest molecules reside. It

is also found that the solvent molecule which has a long axis dimension

comparable to that of the guest molecule gives the sharpest, most well-

resolved spectra.

As mentioned earlier, a common feature of electronic spectra in

Shpolskii matrix systems is the appearance of sidebands which are

associated with vibrational-electronic transitions. These sidebands are

usually taken -to be evidence of the electron-phonon interaction. The







exact nature of the impurity-host relationship is not well understood.

The first attempts at formulating a theory to describe the effect ob-

served in Shpolskii systems were made by Rebane (41, 42). The quasi-

linear bands were associated with a phononless optical transition and

the diffuse background was associated with phonon bands. Interest in the

exact nature of the impurity-lattice interaction has led to attempts to

refine the theory of interactions (43, 44, 45) and also to account for

thermal broadening and shifts of the so-called "zero-phonon" lines in

Shpolskii systems. Recently (46-53), experiments have been performed,

utilizing both absorption and emission spectra, upon various systems in

attempts to clarify the nature of the impurity-lattice interaction.

In any treatment of the theory of optical spectra of impurities in

solids (43-45, 54, 55), several assumptions are normally made. Both the

Born-Oppenheimer approximation and the Franck-Condon principle are taken

to be valid. It is generally assumed that the impurity concentration is

low, so that impurity-impurity interactions are taken to be negligible.

It is also generally assumed that electronic eigenstates of the impurity

are at different energies than the electronic bands or states of the

host crystal. The presence of the impurity will serve to destroy the

translational symmetry of the host crystal, so that the relevant sym-

metry will be the site symmetry of the impurity. It will also perturb

the lattice vibrations of the crystal by giving rise to localized

vibrations which do not exist in the vibrational spectrum of the unsub-

stituted host crystal. In general, the impurity will interact with both

band and localized vibrations (55).

Detailed.treatments of the theory of lattice-impurity interactions

in the optical spectra of impurities in solids are available (42, 43, 54,




-26-


55). In the general case, the optical spectrum of the impurity crystal

will consist of a narrow zero-phonon line corresponding to the purely

electronic transition in the impurity site, accompanied by broad phonon

sidebands caused by transitions with simultaneous excitation of phonons

(55). In the case where localized modes are present in the crystal,

vibrational replicas of the zero-phonon line will be produced at

frequencies + nw from the zero-phonon line, where w is the frequency of

the localized vibration, n is an integer, the plus sign corresponds to

absorption, and the minus sign to emission (55). Physically, the inter-

action of the electronic transition in the impurity site with the thermal

vibrations of the crystal will manifest itself in its influence upon the

intensity, linewidth, and position of the zero-phonon line.

The theory of the electron-phonon coupling predicts an exponential

dependence of the relative integrated intensity of the zero-phonon line

on temperature. The ratio of the intensity of the zero-phonon line to

the total spectrum intensity may be expressed as (56):


Io/I = exp[-. pi(2ni + 1)] = exp[-S(T)] (1)
1

In this expression, i is an index for acoustic and local vibrations,

pi is the dimensionless "Stokes loss" per crystalline oscillator i and
is equal to (mi mw Ag/2h), w. is the frequency of the ith oscillator,

mi is the effective mass, Ai is the shift in the equilibrium position

of the ith oscillator during an optical transition in the impurity site,

ni = [exp(fihi/kT) 1]-1 is the thermal average of the occupation number
of the ith oscillator, and S(T) is the Huang-Rhys factor which denotes

the strength df impurity-lattice coupling (56).




- L. I -


An examination of the relative integrated intensity of the zero-

phonon line as a function of temperature would serve to give information

about the strength of the lattice-impurity coupling, since the overall

integrated intensity of the zero-phonon line and its accompanying phonon

bands is independent of temperature in the Franck-Condon approximation

(55). Intensity lost by the zero-phonon line should be gained by the

phonon side band, since increased electron-phonon interaction would occur

as the temperature increased. The extent of the impurity-lattice

coupling would be characterized by the resulting value of S, since the

larger the value of S, the greater the impurity-lattice coupling is

taken to be.

The nature of the broadening of the zero-phonon line will depend

upon the structure of the electronic energy levels of the impurity (56).

For the case where the maximum phonon energy is greater or equal to the

difference in the energies of the electronic levels, the broadening may

be mainly determined by nonradiative thermal processes when absorption

or emission of a single resonance phonon occurs at the same time as the

electronic transition. In the opposite case, that where the difference

in the energies of the electronic levels is greater than the maximum

phonon energy, Raman scattering of phonons by the impurity center may

play an important part in the thermal broadening (53).

If the electronic transition which takes place is a phonon-assisted

one, certain additional features will manifest themselves. The vibronic

bands present due to a phonon-assisted transition will involve the

emission of photons and the creation or annihilation of phonons. De-

pending upon the energy levels involved, either or both of Stokes and




-28-


anti-Stokes emission of phonons will occur. Di Bartolo (57) has derived

the relevant matrix elements for such processes.

Since thermal vibrations are involved, the intensities of the

phonon bands on either side of a zero-phonon line will be temperature-

dependent. In the simplest case, that of a one-phonon transition, the

process will depend upon the number of phonons present. The temperature

dependence is contained in the expression for the number of phonons


np= [exp(h, p/kT) 1]-1 (2)


The value of n becomes very small as T is reduced. As shown in

(57), this will result in the almost total disappearance of emission of

anti-Stokes phonons at low temperatures, while the Stokes processes will

persist. It is also expected that as the temperature increases, an

increase of the multi-phonon background will occur. These processes will

tend to smooth out the peaks so that an almost continuous background

will result.



Experimental


Anthraquinone and n-paraffin solvents were obtained as described

in Chapter II. The techniques described there for obtaining Shpolskii

matrix spectra and measurements of phosphorescence lifetimes were em-

ployed. The I.R. spectra were obtained in KBr disks on a Perkin-Elmer

621 spectrometer.

As was mentioned in the introductory section of this chapter, un-

successful attempts were made to obtain polarized single crystal Raman

spectra of anthraquinone. There were several reasons for attempting




-29-


these experiments. There is some disagreement concerning the presence or

absence of certain lines in the powder spectrum of anthraquinone, as well

as the assignment of certain experimentally observed frequencies to

normal modes (58, 59). The most complete experimental study upon powder

samples is that of Rasanen and Stenman (59), with frequencies being

assigned on the basis of normal coordinate calculations of Strokach,

Gastilovich, and Shigorin (60, 61). In cases where the crystal struc-

ture of a compound is known, the method of polarized single crystal Raman

spectroscopy has proven to be very useful in the unambiguous assignment

of Raman active normal modes of vibration (62, 63).

Single crystals of anthraquinone were grown by the Bridgman tech-

nique. The design of the furnace was based upon those described by

Lipsett (64). Two weeks were normally required to pass an anthraquinone

ingot through the furnace.

The Raman spectrometer used was a Spex Ramalog 5, with a Spex 1401

double monochromator and an RCA 3140 cooled photomultiplier tube which

could be operated in the photon-counting mode. The excitation source

normally employed with this system was a Coherent Radiation Model CR-5

argon ion laser. Malfunctions of the argon ion laser and the photo-

multiplier tube resulted in a lengthy period of inactivity in this portion

of the research.

It was found that anthraquinone fluoresced strongly under excitation

by the argon ion laser lines, so that no Raman spectra could be obtained.

The exact nature of the fluorescence which often occurs in Raman spectra

is not clear. In this case, it was decided to employ the accessory

Coherent Radiation Model 490 dye laser as an excitation source to see if

going to longer wavelengths of excitation would decrease the fluorescence.






Attempts to use the dye laser proved to be only a little more

successful. The power output of the dye laser was very unstable. Since
-5
the Raman effect is ordinarily on the order of 10-5 times the accompany-

ing Rayleigh scattering, a very poor signal to noise ratio was obtained.

It was also discovered that no signal below 1000 cm-l of the exciting

line of the dye laser could be detected due to background in the anthra-

quinone powder spectrum. The reason for this was not immediately clear.

It was later discovered that the birefringent tuning element of this

particular model of dye laser transmits a structured fluorescence

emanating from the dye itself (65). In addition, background fluorescence

was observed in portions of the anthraquinone spectrum which could be

obtained.

It is possible that the fluorescence emanating from the dye itself

could have been removed by either an appropriate wavelength notch optical

filter or a tunable grating filter. It was also planned to use a KIM-1

microcomputer to employ the technique of digitized frequency-modulated

spectroscopy (66) to remove any fluorescence intrinsic to the anthra-

quinone itself. This technique requires an excitation source which is

constant in time. All attempts to correct the problem of the dye laser

power fluctuations, including those of a Coherent Radiation field service

representative, were unsuccessful.

The only spectra obtained were those of powder samples. The

accessory for mounting single crystals on the spectrometer was not

available. Attempts to fabricate holders resulted in unsuccessful

attempts to obtain single crystal spectra, so it is probable that the

problem was one of alignment. In any case, in view of all the diffi-

culties encountered, it was deemed expedient to terminate this phase




-JI-


of the research and rely upon the frequencies and assignments of Raisnen

and Stenman (59).



Results and Discussion


Analysis of Phosphorescence Spectra


Tabulations of the analyses of the phosphorescence spectra of

anthraquinone in Shpolskii matrices of n-hexane and n-heptane are shown

in Tables II and III, respectively. The analysis is complete in the case

of n-hexane, while only the major spectral features of the emission in

n-heptane are presented. The analysis is much more straightforward in

the case of n-hexane, since anthraquinone exhibits only one-site emission

in this solvent. Interpretation is much more complicated in the heptane

and octane matrices, particularly in the case of the weaker combination

bands, since two sites appear to be present. Two sites also appear in

the case of anthraquinone in n-pentane. In n-hexane, heptane, and

octane, the origin of the electronic transition is absent, while it is

observed in n-pentane. As was noted by Khalil and Goodman (33), this

may be attributed to the fact that at least the site symmetry of the

molecule is conserved in the fonner three matrices, while distortion

and subsequent lowering of symmetry occur in the n-pentane case.

Literature values (67) of the I.R. active fundamentals were checked

for accuracy. As explained earlier, attempts to obtain Raman spectra

were unsuccessful. The frequencies and assignments of Rasanen and

Stenman (59) were used instead. A tabulation of the fundamental modes

of vibration of 9,10-anthraquinone used in this work can be found in the

Appendix.







Table II. Phosphorescence Vibrational Analysis for Anthraquinone in
lexane at 10"K


Energy
(cm-1, vac)


AE
(cm-1, vac)


Assignment


4533.6
4555
4564
4570
4579
4582
4585.5
4589
4593
4596
4602
4619
4626
4639
4644
4674
4702
4719
4724
4729
4740
4743
4763
4768
4783
4797
4807
4823
4850
4854
4865
4874
4879
4889
4894
4919
4929
4932
4936
4949
4953
4961
4963
4968
4974
4980


22051
21948
21903
21874
21832
21816
21802
21783
21765
21752
21722
21645
21613
21549
21527
21390
21262
21184
21162
21138
21091
21077
20988
20967
20902
20840
20796
20716
20615
20595
20548
20512
20489
20448
20429
20324
20282
20269
20253
20202
20183
20152
20143
20122
20098
20076


Anti-Stokes
Anti-Stokes
Anti-Stokes
Anti-Stokes
Anti-Stokes
Anti-Stokes
Anti-Stokes







ttice


-268
-164
-119
-91
-48
-32
-18
0
18
31
61
138
170
234
256
393
521
599
621
646
692
706
795
816
881
943
987
1067
1168
1188
1235
1271
1294
1335
1354
1459
1201
1514
1530
1581
1600
1631
1640
1661
1685
1707


VVVW
VVW
VW
VW


W
not seen
VVW
VW
VW
VW
W
W
W
W
W

S
M
iw
W
W
S
W
VW
W
W


WM
W
VW
W
W
M
W


w
W



s
s
MS
MW
W
W



VS
S
MS
MW

VVS
S


+ lattice


+ 789 Raman
+ 789 Raman + lattice
+ 1146 Raman

+ lattice
+ 1146 Raman
+ 1146 Raman + lattice
+ 365 Raman

+ lattice


Wavelength
(A)


0-lattice,
0-lattice,
0-lattice,
0-lattice,
0-lattice,
0-lattice,
0-lattice,
0-0
0-lattice
0-lattice
0-lattice
0-lattice
0-v66
0-v32
0-v32 + la
0-v49
0-3 x v66
0-v32+ 36
0-v31
0-v31 + la
0-3 x V32
0-v63 I.-
0-v62 I.R.
0-v62 + la
0-v31 + 25
0-v47
0-v31 + 36!
0-v29
0-v28
0-v28 + la
0-v45


0-v27
0-v44
0-v44
0-v
0-V26
0-v63
0-v63
0-v49
0-v25

0-v64
^27
0-v27
0-v24
0-v24


ttice


5 Raman

ttice



ttice
8 Raman

5 Raman


__I~


-JL-




-JJ-


Table II. (Continued)

Wavelength Energy AE Assignment
(A) (an1, vac) (cm-, vac)

5031 19875 1908 MW 0-v24 + 239 Ranan
5039 19841 1942 W 0-v24 + 258 Raman
5044 19819 1964 W 0-v62 + 1146 Raman
5051 19972 1992 VW 0-v62 + 1146 Raman + lattice
5065 19737 2046 MS 0-v24 + 365 Raman
5071 19714 2069 MW 0-v24 + 365 Raman + lattice
5104 19589 2194 W 0-v46 + 1146 Raman
5121 19521 2262 VW
5124 19511 2272 VW
5130 19487 2296 MS 0-v31 + 1667 Raman
5136 19467 2316 VW 0-v31 + 1667 Raman + lattice
5148 19419 2364 VW 0-v24 + 688 Raman
5152 19404 2379 VVW 0-v24 + 688 Raman + lattice
5160 19373 2410 VVW
5176 19314 2469 MS 0-v24 + 789 Raman
5183 19290 2493 W 0-v24 + 789 Raman + lattice
5199 19228 2555 VVW 0-v61 + 1597 Raman
5228 19124 2659 VVW O-v24 + 978 Raman
5248 19051 2732 VW
5259 19009 2774 VVW
5276 18948 2835 M 0-v24 + 1146 Raman
5283 18922 2861 W 0-v24 + 1146 Raman + lattice
5306.5 18839 2994 VVW
5313 18816 2967 W 0-v27 + 1667 Raman
5329 18761 3022 VW 0-v25 + 1440 Raman
5360 18653 3130 VVW
5370 18616 3167 W 0-v24 + 1480 Raman
5379 18585 3198 VVW
5391 18544 3239 MS 0-v25 + 1667 Raman
5399 18518 3265 MW 0-v25 + 1667 Raman + lattice
5407 18489 3294 W 0-v24 + 1597 Raman
5411 18476 3307 W 0-43 + 789 Raman +1146 Raman
5422 18439 3344 VS 0-v2 + 1667 Raman
5428 18419 3364 M 0-v24 + 1667 Raman + lattice
5489 18213 3570 VW
5498 18183 3600 VW
5530 18087 3705 M 0-v24 + 1667 Raman+365 Raman
5537 18055 3728 W 0-v24+1667 Raman+365 Raman+lattice
5576 17928 3855 W
5608 17826 3957 W 0-v31 + 2 x 1667 Raman
5643 17715 4068 W 0-v25 + 1667 Raman+789 Ranan
5663 17654 4129 W
5748 17392 4391 VW O- 25+1597 Raman+1146 Raman
5756 17366 4417 VW 0-v24+1597 Raman+1146 Raman
5782 .17289 4494 W 0-v24+1667 Raman+1146 Raman
5791 17280 4509 W 0-v24+1597 Raman+1243 Raman
5820 17177 4606 W




-34-


Table II. (Continued)


Wavelength Energy AE Assignment
(A) (cm-1, vac) (cm-1, vac)

5893 16965 4818 VW 0-v24+1667 Raman+1480 Raman
5917 16896 4887 VW 0-v24 + 2 x 1597 Raman
5928 16864 4919 W 0-v25 + 2 x 1667 Raman
5946 16813 4970 W 0-v24 + 2 x 1667 Raman
5954 16791 4992 W 0-3 x v24




-35-


Table III.


Phosphorescence Vibrational Analysis for Anthraquinone in
Heptane at 10K


Low-Energy High-Energy
Site Site AE I Assignment
(cm-1) (cm-1) (cn-l)


VVVW 0-0, H.E. site
VVW 0-0, L.E. site


21761


21610
21527

21366



21230
21167
21140

20965

20780

20589
20471

20307


20117

20075
20059


19714
19699


19465
19446


19296
19279

18927

18799


21866

21705
21630


21467

21352
21271
21244



21071

20884

20694


20411

20266
20184

20160


19824
19804


19572
19556


19399
19382


19031

18906


Raman


Raman


0
0
161
236
151
234
399
395
514
595
622
521
594
621
795
796
982
981
1172
1172
1290
1455
1454
1600
1686
1594
1706
1686
1702
2042
2062
2047
2062
2294
2320
2296
2315
2466
2484
2465
2482
2835
2834
2960
2962


V

V
VV


V





V



M

VV


VV
V





V


V
M


M
M


0-v66
W 0-v32
0-v66
W 0-v32
W 0- 49
W 0-v49
'W 0-3 x v66
'W 0-v32 + 365
M 0-v31
W 0-3 x v66
W 0-v32 + 365
M O-v31
W 0-v6
M 0-v62
W O-v31 + 365
W 0-v31 + 365
rW 0-v28
M 0-v28
M 0-v27
W 0-v26
1W 0-v26
M O-v25
S 0-V24
S 0-v25
S 0-v2 + lat'
*S O-v24
'S 0-v24 + latl
M 0-24 + 365
W 0-24 + 265
S 0-24 + 365
1W O-v24 + 365
WI 0-v2 + 166
'W 0-v31 + 166
M 0-v31 + 166
W 0-v31 + 166
W 0-v24 + 789
'W 0-v24 + 789
W O-v24 + 789
1 0-v24 + 789
W 0-v24 + 114(
1W 0-v24 + 1141
1W 0-v27 + 166
W 0-v27 + 166


Raman
Raman


twice


twice
Raman
Raman + lattice
Raman
Raman + lattice
7 Raman
7 Raman + lattice
7 Raman
7 Raman + lattic
Raman
Raman + lattice
Raman
Raman + lattice
6 Raman
6 Raman
7 Raman
7 Raman


e

e




-36-


Table III. (Continued)

Low-Energy High-Energy
Site Site AE I Assignment
(an-1) (cm-1) (cm-1)

18524 3342 S 0-v + 1667 Raman
18508 3358 W 0-v24 + 1667 Ranan + lattice
18418 3343 VW 0-v24 + 1667 Raman
18401 3360 MW 0-v24 + 1667 Raman + lattice
18164 3702 W 0-v24 + 1667 Ranan + 365 Raman
18058 3703 MW 0-v24 + 1667 Raman + 365 Raman
17910 3956 VW 0-v31 + 2 x 1667 Raman
17803 3958 W 0-v31 + 2 x 1667 Raman
17787 4079 VW 0-v25 + 1667 Raman + 789 Raman
17700 4061 VW 0-v25 + 1667 Raman + 789 Raman
17372 4492 VW 0-v2 + 1667 Raman + 1146 Raman
17350 4516 VW -v24 + 1597 Raman + 1243 Raman
17269 4492 W 0-v24 + 1667 Raman + 1146 Raman
17245 4516 VW 0-v24 + 1597 Raman + 1243 Raman
16947 4919 VW 0-v + 2 x 1667 Raman
16874 4992 VW 0-3 x v24
16844 4917 VW 0-v25 + 2 x 1667 Raman
16771 4990 W 0-3 x v24




-JI-


We find 7 out of 11 possible blu vibrations to be present in the

phosphorescence spectrum of 9,10-anthraquinone in hexane. These, in

the notation of reference (67), are v 32' v31' V28' v27' V26' v25' and v24

This is in agreement with the results of Khalil and Goodman. One fea-

ture which was not included in reference (33)was the presence of a peak

at 1661 cm-1 from the origin. This peak is clearly visible, and is

assigned in the present study as a combination of v27 and the 365 cm-1

Raman line of anthraquinone.

The most intense peaks in the spectrum were those based upon the

non-totally symmetric C = 0 stretch, v24, as was expected. Both one and

three quanta of this vibration are reported in this work, although only

one quantum was reported in reference (33). Assignments of multiple

quanta of vibrations will be discussed later.

Khalil and Goodman reported combinations of v24 with Raman lines of

frequencies 240, 360, 790, 1149, 1595, 1666, 1666 + 360, and 1666 + 1149
-1
cm In the present study, peaks were found which were assigned as

combinations of v24 with Raman lines of frequencies 239, 258, 365, 688,

789, 978, 1146, 1480, 1597, 1667, 1667 + 365, 1597 + 1146, 1597 + 1243,

1667 + 1480, 2 x 1597, and 2 x 1667 cm1. The difference is probably

attributable to the much greater sensitivity in this region of the spec-

trum of the photomultiplier used here, as compared to that used by

Khalil and Goodman (33).

Khalil and Goodman (33) assigned lines in the phosphorescence

spectrum of anthraquinone of type b2u corresponding to v46' U44' and

v43. In addition, another b2u vibration, V47' appeared in combination
with the Raman 360 cm- line. In this study, 5 out of 11 possible

b2u vibrations were observed. These were v43' v44' v45' v47, and v49'







Combinations of v49 + 1146 cm-1 Raman, v46 + 1146 cm-1 Raman, and

v43 + 789 cm- + 1146 cm- Raman were observed.

At this point, some discussion concerning peaks corresponding to

"46 is necessary. There is some disagreement (67, 58, 68) concerning
the frequency of this fundamental mode of vibration, since it did not

appear in the single crystal spectra of anthraquinone and anthraquinone

d-8 (67). Normal coordinate calculations (60, 61) favor the assignment

of a frequency of 1034 cm- (68). Khalil and Goodman used a frequency
-l
of 1155 cm as the basis for an assignment of v46' In the present

work, no peak is observed at this position. Instead, a peak is observed

at 1067 cm1 from the origin which might be assigned to either v46 or

v29. Since other combination bands occur in the spectrum which agree

very well with a frequency of 1034 cm-1 for v46, this value is chosen

and assignments are made accordingly.

Out of six possible b3u fundamentals, three are observed in this

work and three are also reported by Khalil and Goodman. These are v66,

v63' and v62. Combinations of v64 with the 1146 cm-l Raman line are

found in the present work and in reference (33). Khalil and Goodman

assign a combination of v61 with a 360 cm- Raman line, while a com-

bination of v61 with the 1597 cm-l Raman line is found in this study.

A combination of v62 with the 1146 cml Raman line and of v63 with the

789 cm-l Raman line is found in both results.

As regards the search for odd quanta in the vibrational progressions

of the phosphorescence of anthraquinone, there is evidence for three

such peaks. The band located at 521 cm-1 from the origin may be assigned

to three quant-a of v66' a skeletal deformation, or a combination of v32

with the 305 cm- Raman line. This was assigned by Khalil and Goodman (33)






as 3 x v32 on the basis of deuteration experiments. The line which

occurs at 692 anc from the origin might either be assigned to the Raman
-l
685 cm line, to 3 x v32' or to a combination of the b2u mode vu4

(387 cm-l) with the Raman 305 cm-l line. On the basis of deuteration

experiments, Khalil and Goodman assigned this line to 3 x v32, so it has

likewise been assigned here.

The third peak in the spectrum of anthraquinone in hexane which

may be assigned as an odd quantum progression lies at 5954 A. This line

is close to the position of several mercury lines. However, it appeared

in spectra excited by the mercury lamp using a Corning CS 7-54 filter and

a WG-9 emission filter. The Corning 7-54 filter served to prevent

transmission of visible lines from the lamp, while the WG-9 emission

filter served to prevent the occurrence of second-order mercury lines

in the spectrum. The band in question, along with several other weak

peaks in the area, was observed in spectra excited by the Eimac xenon

lamp, so the conclusion that it belongs to the anthraquinone phosphor-

escence seems a valid one. This line, along with several others reported

in this work, was not reported by Khalil and Goodman, since the spec-

tral response of the photomultiplier to be used by them was very weak

in this region.

Results of analyses of Shpolskii matrix emission spectra of anthra-

quinone obtained in this study tend, in general, to confirm the results

of Khalil and Goodman. Crystal field effects of the host lattice do not

affect the location of the origin to a great extent, since the origin of

the electronic transition lies at similar positions in pentane, hexane,

heptane, and octane. In pentane, the origin is observed, with two site




-40-


emissions present. In hexane, heptane, and octane, it occurs weakly or

not at all. This indicates that at least the site symmetry (Ci) of the

anthraquinone molecule is preserved in the hexane, heptane, and octane

matrices. Non-totally symmetric I.R. active vibrations of types blu,

b2u, and b3u occur in the phosphorescence spectrum, indicating that the

electronic state from which the phosphorescence takes place is B g.
3 1
The BIg A transition is both spin and symmetry forbidden.

Apparently, the state from which it borrows intensity is a Blu

(IT-Tr* state). As discussed by Khalil and Goodman (33) and Strokach and Shi-

gorin (69) mixing of states to allow the transition to occur may happen

in three different ways. The 3Blg state may be viewed as mixing directly

in first order perturbation theory by means of the spin-electronic-

vibrational interaction operation H vso. The two states may also be

mixed in second order perturbation by intermediate states. These may
1 3
be either or both of a A 7,7,* state or a A n,,r* state. The possi-
g u
abilities are shown in Figure 4. Here, Hvso = Hso /Q where Hso is the

spin-orbit interaction operator with a component of Blg symmetry and Q

corresponds to a blu type normal mode of vibration. H is the

operator for electron-vibrational interaction, aH o/Qa, where Ho repre-

sents the Hamiltonian of the unperturbed system.



Temperature-Dependent Emission


The question of the temperature-dependent emission in anthraquinone

is now considered. Figures 5-17 illustrate the behavior of emission in

the region of the origin of the electronic transition of anthraquinone

in octane, heptane, hexane, and pentane as the temperature is increased.

The phenomenon of temperature-dependent emission is observed in octane,





-41-


,r
co


0co
r--'

,-


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t



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(,


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0

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=3
c,


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(A














































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S-C
0

Cd,





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cl)




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-113-


In 4-







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-65-


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-67-


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heptane, and hexane matrices, where the origin of the purely electronic

transition is absent, but not in the pentane case where the origin is

present.

The possibility that the peaks are an impurity emission can be

largely discounted by consideration of the purity of the chemicals used.

That the peaks might be due to a photoproduct being formed was eliminated

by demonstrating a reversible dependence of the emission upon temperature

and a lack of time dependence of intensity upon photopumping.

In cases where thermally activated emission processes between two

adjacent electronic levels occur, an Arrhenius plot of In I/I1 versus

T 1,will normally give a straight line whose slope agrees well with the

energy separation between the levels if no additional depletion processes

are present (34, 35). The results of such a plot for anthraquinone in

hexane are shown in Figure 18. There is an apparent exponential tem-

perature dependence, but the activation energies do not agree with the

energy level separations. If the highest-energy peak (- 4503 A) is taken

as the origin of a thermally populated 3A state, then it is separated

from the origin of the 3Bg state by approximately 420 cm-1. The value

of the activation energy obtained from the Arrhennius plot for this peak

is 220 cm- and the correlation coefficient of the plot is 0.981. There

is no obvious reason why the discrepancy of a factor of approximately

2 should occur if this peak is indeed the origin of emission from the

thermally populated 3A state. Likewise, the peak at approximately

4530 A gives an activation energy of 176 cm-. If it were a vibrational

progression in the emission from the 3A state, a separation of approxi-

mately 68 cm n- should be present. A careful study of the emission of

the origin region in hexane, as a function of temperature, as shown in




-69-


In I/I


A


4530
E = 176
a


4503 A
E. = 220 cm-1


-cm
cm


0.8 1.0 1.2 1.4 1.6 1.8
(1/kT x 103)


2.0


Figure 18.


Plot of Activation Energies
of Anthraquinone in Hexane


of Temperature-Dependent Emission


2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0


4554 A


A 1




-l')-


Figures 5-10, revealed that the peaks appeared sequentially as the

temperature was increased. The lifetime of the emission as monitored

at 4974 A (0-v24) was 3.21 + 0.32 msec, while those of the 4503 and

4530 A peaks were 3.77 + 0.38 and 3.71 + 0.33 msec, respectively. The

value of the lifetime of the emission monitored at 4974 A agrees well

with the literature values (70, 71). The close correlation of the life-

times of the three peaks indicates that they all originate from the same

electronic level. In view of all these factors, it was concluded that

the peaks were not emission from the second triplet level, but were

either vibrational hot bands or bands due to electron-phonon interaction.

This conclusion (that the peaks were not emission from the second triplet

level) was supported by a later paper (72) on the temperature dependence

of anthraquinone. The authors observed a strange effect upon adding

hexyl iodide to anthraquinone in heptane. No effect upon the emission
3
from the 31g level was observed, but a new set of weak bands in the

origin region were present which were assigned to the 3A level. These

peaks were separated from the 3B1g emission by approximately 60 cm-1

In this same paper, it was tentatively suggested that the temperature-

dependent emission was due to vibrational hot bands from the distorted

Blg level.

This does not seem to be a reasonable conclusion, since, in condensed

media, vibrational relaxation times are normally much faster (10-11 to

10-14 sec) than radiative lifetimes. Hot bands are therefore normally

found (73, 74) only in gaseous systems. In the rare cases (75, 76),

where it has been suggested that they were present in quasi-linear

systems, the electronic emission is spontaneous fluorescence where the

radiative lifetime is of a similar order of magnitude to the vibrational

relaxation times.




-11-


The fact that the temperature dependent emission is found in the

hexane, heptane, and octane matrices, but not in the pentane matrices,

lends support to the idea that the peaks are due to impurity-lattice

interactions rather than vibrational hot bands. That is, the transition

appears to be a phonon-assisted one in the cases of anthraquinone in

hexane, heptane, and octane, where the origin of the electron transition

is absent. Before proceeding further in this direction, a number of

points concerning the crystal structures and phonon spectra of the

n-paraffins C5-C7 will be discussed.

The crystalline structures of n-pentane, hexane, heptane, and octane

have been well characterized by X-ray methods (77-80). The n-pentane

structure is orthorhombic, with 4 molecules per unit cell and a volume
o3
of approximately 543 A Both n-hexane and n-octane are triclinic with

one molecule per unit cell and volumes of 165 and 208 A3, respectively.

N-heptane is triclinic, with 2 molecules per unit cell and a volume of

382 A3.

The phonon spectra of these crystals have been investigated using

Raman light scattering and neutron scattering techniques (81-83). A

remarkable similarity in the phonon spectra of n-hexane, heptane, and

octane is found, as might be expected in view of their similar crystal-

line structures.

The origin of the electronic transition in the phosphorescence of

anthraquinone occurs at similar locations in n-hexane, n-heptane, and

n-octane. Examination of Figures 5-15 shows that similar behavior

occurs in all three cases, with the temperature dependent emission

appearing at approximately the same position in all three matrices.

The discussion which follows is restricted to the case of anthraquinone







in n-hexane, since the situation is uncomplicated by multiple site

emission.

The temperature dependent emission is first noticeable at approxi-

mately 500K. Figures 5-10 illustrate the behavior of the origin region

of the phosphorescence of anthraquinone in n-hexane over the temperature

range 140K-159K. It can be seen that, as the temperature is increased,

the origin region begins to resemble a continuum, with maxima appearing

at positions separated by approximately 420, 290, and 180 cm-1 from the

origin. The word approximately is used here because the peaks are

slightly skewed and overlapped so that it is difficult to determine

their exact position. Also, an examination of the position of the most

intense component of the phosphorescence at 4974 A from 14-820K revealed

a shift on the order of 8-10 cm-1 toward the blue as the temperature

was increased. As Figure 10 shows, increasing the temperature to a

point just below the melting point of the n-hexane matrix results in an

almost complete loss of spectral definition in the origin region and an

extension of the emission to even shorter wavelengths. Although not

shown, similar behavior was observed in the heptane and octane cases.

In the case where a molecular impurity with intramolecular frequen-

cies of vibration is substituted into a crystal, two different situations

may arise (55). The molecular frequency wm may fall into a region

corresponding to frequencies also found in the perfect host crystal.

The resonance between the molecular and crystal frequencies will allow

energy to be transferred from the molecular impurity to the crystal.

The second case is that where the molecular frequency bm occurs in a

region which does not correspond to vibrations found in the perfect

crystal. If the perturbation produced by the impurity is strong enough,


-/f.-




-/J-


an anharmonic vibration may set in which does not correspond to a peak

in the phonon spectrum of the crystal (55). The relatively intense

phonon sidebands which accompany all the major vibrational progressions

in the phosphorescence of anthraquinone in hexane appear to be of this

type. For instance, in the emission corresponding to the vibrational

progression 0-v24, the accompanying phonon sideband occurs at a separa-

tion of 22 cm No such vibrational frequency occurs in the phonon

structure of n-hexane, while anthraquinone has a Raman active lattice

mode of 28 cm-1 (59). It is known (84) that such "lattice" modes will

appear in electronic spectra for either one of two reasons. The first

is that the excited molecule has an excited state equilibrium disposition

in the crystal lattice which differs from its normal ground state con-

formation. The second reason is that the lattice mode perturbs the

electronic transition in a way so as to assist the electronic motion to

gain radiative properties.

The temperature-dependent emission which occurs in the emission of

anthraquinone in hexane matrices has peaks separated from the origin of

the 3Bg level by approximately 180, 290, and 420 cm-l. Peaks in the

phonon structure of n-hexane (83) occur at 176 cm- 296 cm-, 372 cm-l

and 460 cm There is a relatively close correspondence between the

separation of the peaks occurring in the temperature dependent emission

and the peaks occurring in the phonon structure of n-hexane. Anthraquinone

itself has Raman active vibrations at frequencies of 301 cml, 419 cm-1
-l
and 150 cm so that it is more probable that the peaks correspond to

resonance modes set up between hexane vibrations and anthraquinone

normal modes.






As to the temperature-dependent emission, it is assigned here as

arising from a temperature-dependent anti-Stokes scattering of phonons

occurring simultaneously with the electronic transition. There are

several reasons for this conclusion. Figures 5-17 clearly reveal the

temperature dependent emission is present in the n-hexane, heptane, and

octane cases, but not in the n-pentane case. For each of the former

three cases, the origin of the purely electronic transition occurs with

negligible intensity, while in the latter case it is present. This in-

dicates that the transition is a phonon assisted one in the former three

cases. The occurrence of phonons in assisting transitions to take place,

while relatively rare, is not unknown. A most thoroughly documented

case (84) is that of the phosphorescence of molecular crystalline

pyrazine. Raman scattering of phonons has also been shown (50) to play

an important part in the thermal broadening observed in Shpolskii sys-

tems. Finally, the behavior of anthraquinone in n-paraffins C6-C8

resembles closely the temperature dependence of the emission of the

system V2+ in MgO, where anti-Stokes scattering of phonons was also

ascribed as the reason for the temperature-dependent emission observed

in this system (85).

A study of the impurity-lattice coupling in the anthraquinone-hexane

system might be helpful in corroborating the above conclusion. Judging

from the quasi-linear appearance of the spectra, it is probable that the

coupling is weak to intermediate. No attempt was made to evaluate the

coupling. This can be best understood with reference to Figures 19-21.

These indicate the decrease in intensity and broadening of the zero-

phonon line at 4974 A over the temperature range 14-780K. Studies of

impurity-lattice interactions (50) are usually performed on zero-phonon

































>)
















4925 4950 4975 5000 5025
Wavelength (4)


Figure 19. Phosphorescence of Anthraquinone in Hexane at 14K





-76-


4-,


















4925 4950 4975 5000 5025
Wavelength (A)


Figure 20. Phosphorescence of Anthraquinone in Hexane at 36K






































+--


'--
















I L I I I
4925 4950 4975 5000 5025

Wavelength (A)


Figure 21. Phosphorescence of Anthraquinone in Hexane at 760K




-/u-


lines which can be accurately separated from their phonon sidebands over

a wide range of temperature. The broadening in the anthraquinone case

makes this extremely difficult, and would introduce a large error into

calculations of the relative integrated intensity of the zero-phonon

line. Since the relative integrated intensity of the zero-phonon line

as a function of temperature characterizes the strength of the coupling,

this would cause a large error in the estimation of the strength of the

coupling.












CHAPTER IV

QUINIZARIN AND DAUNORUBICIN



Introduction

The geometrical structure of quinizarin is not known with certainty.

In all that follows, it will be assumed that the molecule is planar and

belongs to the point group C2v. There are several bases for this

assumption. Quinizarin is 1,4-dihydroxy anthraquinone. Free rotation

about the 0-H bonds could be expected to occur, so that on the average

the 0-H groups might lie either in or out of the plane of the molecule.

Experimentally (17, 86), it is found that the C=0 stretching frequency

in the infrared spectrum of this compound is 1631 cm-1, as opposed to
-l
1676 cm in anthraquinone. This indicates that quite strong hydrogen

bonding is present. Such a situation would occur if the 0-H groups lie

in the plane of the molecule. Quinizarin also displays a limited solu-

bility in polar solvents, which might be interpreted as additional

evidence that the 0-H groups are highly coordinated with the carbonyl

groups. Therefore, for purposes of analysis, it is assumed that 1,4-

dihydroxy anthraquinone (quinizarin) is a planar molecule in the ground

state and belongs to the point group C2v. The axis system in quinizarin

is chosen here to be the same as that used for anthraquinone in

reference (67).

The visible and ultra-violet spectra of quinizarin show several

quite strong absorption bands (5, 19). The longest wavelength absorption


-79-




-80-


band lies in the visible region, extending from approximately 4000 to

5300 A. The absorption appears to be due to one electronic transition

with vibrational fine structure. All of the UV and visible bands in the

quinizarin absorption spectrum have E values greater than 103, in-

dicating that the electronic transitions responsible for their occurrence

take place with a relatively high degree of probability. Under C2v

symmetry, elements of the electric dipole moment operator transform as

al, bl, and b2. Quinizarin has a closed shell electronic configuration,

so that its ground electronic state is A1 Transitions of the type

11 Al 1A1 B1 A1 B2 would be expected to occur with a high
degree of probability in absorption, while transitions of the type

1A1 1A2 would be forbidden by selection rules and would occur only

by "stealing" intensity through vibronic interactions with allowed

transitions.

Abrahamson and Panik (21) have discussed the effect of introducing

a basic group such as Cl, OH, NH2, etc.,into the rings of the anthra-

quinone system. If resonance interaction between orbitals on the basic

group and the 7 electronic system of anthraquinone occurs, then the

highest filled i orbital of the ground state will be raised in energy

and that of the basic group will be lowered. For highly electronegative

substituents, an inductive effect may be present which will interact

with n orbitals on the anthraquinone system to depress their energy

levels. Both of these effects will tend to decrease the energy level

separation of the n and n orbitals of the anthraquinone system and in

the extreme case might cause a reversal of the energy levels.

A study of the emission characteristics of the quinizarin molecule

can give information concerning the disposition of singlet energy levels.




-U1-


Compounds which have an n,w* singlet state as the lowest excited singlet

state do not normally exhibit fluorescence, since almost complete inter-

system crossing takes place from the lowest excited n,w* state to a

triplet state, from which phosphorescence occurs (22). Solvent effects

upon absorption spectra may also be useful, since n,r* transitions will

usually display a shift towards shorter wavelengths in going from a

non-polar to a polar solvent, while r,7* transitions display the opposite

behavior.

The structures of anthraquinone, quinizarin, and daunorubicin are

shown in Figure 22. It can be seen that the only symmetry element which

daunorubicin possesses is the identity element. It therefore belongs to

the point group C1. All electronic transitions will be of type A 1A,

with subsequent easing of symmetry constraints concerning the "allowed-

ness" or "forbiddenness" of transitions.

This portion of the research deals with investigations of electronic

transitions in quinizarin utilizing visible and ultraviolet absorption,

room temperature luminescence, lifetime measurements, photoselection,

and Shpolskii matrix emission. In addition, I.R. and far I.R. spectra

of quinizarin are reported, with an analysis of I.R. active normal modes

of vibration of quinizarin. Photoselection measurements upon daunoru-

bicin are also included, in an effort to determine whether the assign-

ments of electronic transitions of daunorubicin in reference (23) are

correct.


Theory of Photoselection

Numerous examples of the use of photoselection in the investigation

of electronic transitions may be found in the literature (28, 87-91). An




-82-


to















4-
cr O = =


















a oN
I -E



o ao c- \














OO t--
0C 3
5,







Of 50
--1 ~- C





4-).- M








tN
c--a
\r /- = f









Cr
\\, ,/ >0
\ '*- ^ / LTI
Uj --^- -- 10






extensive exposition of the theory of photoselection and discussions of

its application may be found in the article by Albrecht (92). The

treatment which follows is similar to Albrecht's.

In order to understand how photoselection might be used in the

assignment of electronic transitions, it is first appropriate to review

the concept of the polarization of an electronic transition. Since

electric dipole transitions are generally orders of magnitude stronger

than magnetic dipole or electric quadrupole transitions and are found

much more frequently in organic compounds, the example given is restricted

to the case of electric dipole transitions.

Consider, as the most general case, a transition between two

stationary electronic states qi and if brought about by incident radia-

tion of frequency v = (Ef Ei)/h. For simplicity, the states are

assumed to be non-degenerate and their eigenfunctions are assumed to be

real. The probability that the transition will occur is proportional

to the square of the transition moment integral


Mif = J i R 4 f dr f xi xf do (1)

where M = e ri', i is the spatial eigenfunction of the ith state and
1
xi is the corresponding spin function. For simplicity, it is assumed

that AS = 0. The electronic transition will be an allowed one if and

only if Mif is not equal to zero.

In the case where one or both states are degenerate, the transition

will be an allowed one provided that the direct product of the species

(r) of 0i with that of 1f belongs to the same species as one of the

components of.M.




-84-


The selection rule as discussed above applies only for fixed nuclei.

The nuclei are not actually fixed, and the dependence of the total eigen-

function upon nuclear motion must be considered. Neglecting rotational

motion (justified physically by the fact that during an actual experiment

the molecules undergoing a transition are normally fixed in a rigid

medium) the total eigenfunction may be expressed in the Born-Oppenheimer

approximation as


iik = ei(q,Q) ok (Q) (2)

where ,'ik is the vibronic state corresponding to the kth vibrational

level of the ith electronic state, q indicates all the electronic coor-

dinates, and Q stands for all the nuclear coordinates.

If the electric dipole is resolved into two parts, one due to

electrons and the other due to nuclei,


M = M + M (3)
e n

then the electric dipole transition moment becomes


Mik,fl f ik h1 dQ f e M of dq + I k mn Q1 dQ f o of dq (4)

Since the electronic wavefunctions for a given position of the

nuclei are muturally orthogonal, the second term in the above expression

vanishes.

If the dependence of 0. upon nuclear coordinates is taken into
1
account, we obtain


Mif = ei (q,Qo) M e f (q,Q ) dq (5)


(5a)


Mik,fl = Mif (Qo) f ak 1i dQ




-85-


where Q is the configuration of the nuclei near the equilibrium

position.
2
Thus, the transition probability, which is proportional to M2, has

been resolved into two components, one of which depends upon nuclear

motion, and the other which depends upon electronic motion.

Following Albrecht, this may be expressed in a slightly different

form by using the Q dependence of the electronic wave functions.


of (q,Q) = e + ~ fr (Q) oe (6)
r

The electronic wavefunction for state f at Q is expressed in terms

of the complete set of electronic wavefunctions evaluated at Q = 0,

corresponding to the ground electronic state. The sum in Eq. (6) goes

over all states but f, where the rth member of the set is The coef-
r
ficient Afr serves to mix the zeroth order wavefunction e6 with e through

a vibrational motion of the proper symmetry. Such mixing will be neg-

ligible for the ground state, so that 0. (q,Q) e?. The transition
1 1
moment may then be expressed as


Mikfl f = k 1 dQ + r M r fr dQ (7)

The physical interpretation of Mik,fl is that it represents the

polarization of the vibronic transition ik fl. Equation (7) may be

viewed as being made up of two parts, an "allowed" and a "forbidden"

part. It is the allowed part of the transition which will be the most

important part in determining the nature of the electronic transition.

This is the first term in Eq. (7), and represents the modification of

combining vibrational wave functions (Franck-Condon factor) upon an

electronic transition. The second term in Eq. (7) represents the forbidden







part of the transition. A given vibronic transition will be polarized

according to the vector sense of Mf if it is electronically allowed,

or according to the vector sense of the appropriate M? if the transition
ir
involves appropriate non-totally symmetric vibrations. This follows

from the fact that integrands in the integrals over nuclear space will

not contain totally symmetric components for both the allowed and for-

bidden components simultaneously for a non-totally symmetric Afr. There-

fore, vector addition of the two terms will not occur except when Xfr

contains a totally symmetric vibration.

In an actual experiment involving photoselection, there are essen-

tially two different methods which may be employed. In one case, there

is an active attempt to cause molecules to become oriented (as in the

case of a single crystal). The second method does not depend upon the

active orientation of molecules, but relies upon the anisotropic nature

of the exciting light. Ordinary light is isotropic. In order to render

it anisotropic, it is necessary to pass the beam through one of several

different types of polarizing media, so that the light which emerges is

a beam of plane polarized light.

Consider a rigid solution in which the fluorescent solute molecules

are randomly oriented. Assume, for the sake of simplicity, that the

transition moment for absorption coincides with that for emission. In

the absence of depolarizing factors this will correspond to an excitation

from the ground state to the first excited singlet state, and emission

from the first excited state to the ground state.

Following Parker (27), we may consider such a situation with

reference to Figure 23. The incident light is vertically polarized and




-87-


X -


Y <-

















Figure 23.


\ sR



V< E







Illustration of Relative Polarization for an Electronic
Transition




- Wu -


propagating along the X axis. Consider a molecule with a transition

moment directed along OR, inclined at an angle a with OZ and an azimuthal

angle y with OY. The R' is the projection of R on the plane XOY.

Emission is viewed along OY. The probability that such a molecule will

be excited by the vertically polarized light will vary as cos2a. For a

given number of excited molecules, the intensity of the vertical com-

ponent of light viewed along OY will also vary as cos a, while that of
2 2
the horizontal component will vary as sin asin y.

The degree of polarization is defined as


P = (I, I )/(lI + Ii) (8)

where In and LL are the intensities of the observed parallel and per-

pendicular components of the emission. In the case where the absorption

and emission moments coincide exactly, P is equal to 1/2 for vertically

polarized exciting light. Perrin (93) and Jablonski (94) have shown

that if the angle between absorption and emission oscillators is B, for

vertically polarized light P will be given by


P = (3cos28 l)/(cos2B + 3) (9)

For a = 0, P takes on the value of 1/2. For B = 90, when the

absorption and emission oscillators are at right angles, P will have the

value -1/3. Differences found in practice from these values are normally

attributed to such factors as overlap of two closely spaced transitions

or external factors such as strains in the rigid glass matrix.

The polarization of the emission as a function of excitation wave-

length can be.used to establish the relative orientation of an emission

transition dipole to a particular absorption band in the absorption




-89-


spectrum of the molecule in question. This is generally done by com-

paring the polarization curve to bands in the absorption spectrum. In

the ideal case, there will be a region of approximately constant polari-

zation for a given absorption band, followed by a region of changing

polarization where two absorption bands overlap, and then another region

of approximately constant polarization over most of the second absorption

band. The polarization may change sign or not depending upon whether

the two absorption bands have mutually perpendicular transition moments.



Experimental

The experimental procedures used to obtain Shpolskii matrix spectra,

photoselection measurements of quinizarin and daunorubicin, and lifetime

measurements for quinizarin are described in Chapter 2. Reagents and

equipment utilized in this set of experiments are also described in this

same chapter.

Infrared absorption spectra of quinizarin in the range 4000-250 cm-

were obtained in KBr disks on a Perkin-Elmer 621 spectrometer. Spectra

in Nujol mulls over the region 1800-400 cm-l and in the far I.R. were

obtained on a Digilab FTS-20C Fourier Transform I.R. spectrometer.

It has been suggested that dimerization takes place in the con-

densed phase in hydroxy anthraquinones (95). In order to determine

whether the emission and absorption spectra obtained were from quinizarin

molecules or from dimers, room temperature luminescence and absorption

spectra were run as a function of concentration in n-hexane and methanol

solvents. A Cary 17 spectrophotometer with a pair of matched 1 cm

path-length cells was used for absorption measurements, while a