STUDIES OF PHOSPHORESCENCE AND ENERGY TRANSFER BETWEEN TRIPLET STATES IN AROMATIC HYDROCARBONS

A thesis submitted for the Degree of Doctor of Philosophy of the University of London,

by Hannah Gay

Department of Chemistry September 1964. Imperial Collf;go. 1.

Abstract

Phosphorescence decay studios on aromatic hydro- carbons didsolved in rigid matrices have been carried out. Phosphorescence lifetimes of deuterated hydrocarbons are given. The effects of the presence of oxygen and of changes in temperature and solvent on the phosphorescence life- times and intensities have been investigated. It is shown that oxygen has a considerable effect in reducing the lifetimes and intensities, even when the hydrocarbons are dissolved in rigid matrices. The role of rigid media in allowing phosphorescence to be observed is discussed and it is concluded that the rigidity or degree of polymerisation does not affect first order phosphorescence decay; rather it is the permeability of the medium to oxygen and other impurities that is of importance. Glasses of low permeability have been investigated and shown to be efficient media for the study of phosphorescence. 2.

There is some evidence to show that phosphorescence is temperature dependent. Activation energies for internal conversion from the lowest triplet to the ground state have been determined; these arc found to be much smaller than those earlier reported. Energy transfer between triplet states has been studied. A slow transfer mechanism, not previously recorded, has bean demonstrated. The nature of this transfer is discussed and a mechanism proposed. 3.

Acknowledgement.

I should like to thank my supervisors Professor R. Mason and Dr. D.F. Evans for their guidance and encourage- ment over the past three years; Mr. J. Avery for his great assistance with energy transfer mechanisms and my husband Ian Gay for much useful discussion. I am also grateful to Mr. D. Alger for building the amplifier, to Professor D. Craig for a gift of deuterated phenanthrene and para-dideuterobenzene and to Mrs. S. MacGarry for typing this thesis. The award of a Morganite bursary is gratefully acknowledged. 4.

Contents

page

Abstract 1

Acknowledgement 3

Introduction 5

Experimental techniques 24

Lifetime studies 38

Energy transfer, experimental results 65

Energy transfer, mechanism and discussion 74

Appendix 87

References 90 5.

1. INTRODUCTION

1-1. Definition. The term phosphorescence has been used in a variety of contexts. Throughout this thesis it will be used to describe the long lived emission of radiation common among aromatic systems, which arises from transitions between the lowest triplet and ground states.

1-2. Phosphorescence and the triplet state. Phosphorescent compounds have been known for cen- turies although they were not seriously investigated until the end of the nineteenth century when Wiedemann (1) and Dewar (2) studied the phosphorescence of dyes in solid solutions and in crystals at low temperatures. Later Schmidt (3) introduced the use of glassy solvents and showed that pho*orescence bands were of lower fre quency than fluorescence bands. Prior to this the two phenomena were distinguished solely on the basis of 6. emission lifetime. The first apparatus used for accurate lifetime measurement was invented by Becquerel (4) and by using this and similar phosphoroscopes it was shown, principally by Vavilov (5) (6), that phosphorescence emission fol- lowed a first order decay law. It was further noted by Kautsky (7) that certain substances which phosphoresced when dissolved in or adsorbed on a solid matrix did not do so when in the crystalline state. His statement that molecules need to be "energetically isolated" in order to show phos- phorescence led Jablonski (8) to propose that emission occurred from a metastable state of lower energy than that from which fluorescence occurs. His scheme is essentially still in use although he did not recognise the triplet character of the metastable state. A Jablonski diagram using current nomenclature is given in Figure 1-1. Lewis, Lipkin and Magel (9), using fluorescein in boric acid, showed that the proposed metastable state had a characteristic absorption spectrum differing from that of the parent molecule. They claimed that either the state was a triplet or was due to tautomerism as was suggested by Franck and Livingston (10). Terenin (11) and Lewis and Kasha (12) argued in favour of the triplet state hypothesis. 7.

The existence of the triplet state was finally demonstrated by Lewis and Calvin (13) who observed photo- induced static susceptibility by using fluorescein in boric acid. Conclusive evidence was given by Hutchison and Mangum (14) who used paramagnetic resonance techniques to detect the triplet state of naphthalene which was dissolved in a durene crystal. Van der Waals and De Groot vastly improved the technique so that it was used to observe triplet state molecules dissolved in glassy matrices. (15)(16).

1-3. Decay mechanisms. Figure 1-1. shows the various processes possible subsequent to light absorption by a molecule. Phos- phorescence is seen to result from a radiative transfer from a triplet to a singlet state. Such transitions are electric dipole, electric quadrupole and magnetic dipole forbidden on account of the orthogonality of the spin wave functions of pure singlet and triplet states. Phosphorescence is obsc,rwed because spin orbit coupling brings about the mixing of singlet and triplet states. The theory of spin orbit coupling in molecules was developed by McClure (17), one of the tenets of the theory being that the spin orbit interaction operator involves the potential gradient. As this is largest in the vicinity

8. Figure 1-1 , Jablonski diagram

S2 o T 2

Si don. man, =NM

•

SO

Absorption and fluorescence pop Phosphorescence

MIL I... MP .10_ Intersystem crossing

Internal conversion

So) Si and S2 Ground, first and second excited, singlet states -q and T2 First and second excited triplet states 9•

of atomic nuclei, particularly those of high atomic number, McClure (17) was able to verify his theory by demonstrating that the spin intercombination involved in phosphorescence was enhanced when aromatic hydrocarbons were halogen substituted. Lifetimes, particularly for iodo and bromo substituted hydrocarbons, were considerably shortened. Apart from the direct effect of potential gradient, the heavy atom may facilitate spin orbit coupling by the mixing in of charge transfer states. Further experiments showing the effect of intermole- cular spin orbit perturbations were made by Wright, Frosch and Robinson (18) who studied the lifetime of the benzene triplet in inert gas matrices at 4.2°K. Shorter lifetimes were observed in the heavier gases. Figure (1-1.) also illustrates two types of radiation- less transition. Such transitions involve the exchange of energy between electronic and vibrational degrees of freedom. The probability of such transfer depends upon the nature of the environment and the potential energy surfaces of the different electronic states. Symmetry section rules, in this type of transition, are relatively unimportant for polyatomic molecules, because of the presence of antisymmetric vibrations. Kasha (19) reserves the term "internal conversioA" for radiationless transitions between states of like 10. multiplicity and "intersystem crossing" for such transi- tions involving a change in spin multiplicity. These definitions will be used throughout this thesis. With the exception of transitions between first excited singlet states and ground states, internal con- version mechanisms have rate constants of at least 1011sec-1. The radiative transition between the first excited singlet and the ground state, i.e. fluorescence, normally has a lifetime of the order 10-8sec. The life- times are related to the intensity of absorption from the ground to the excited stag. The intensity of an absorption band can be given in terms of the oscillator strength, f, as follows, (20). f = 4.319 x 10 -9)(- d7 (1-1.) where = molar absorption coefficient v = frequency in wave numbers (cm.-1 )

The lifetimes are given by the following expression, due in this form to Perrin, [cf. Kasha (19)].

2 1 V g.6 d7 (1-2.) 10 3.47 x 108 guj where To = natural mean lifetime of an excited. state in the absence of quenching processes. gz = ratio of multiplicity of lower state relative gu to that of upper state. 11.

By combining equations (1-1..) and (1-2.) the following expression is obtained.

f = 1.5 gu 1 (1-3.) ,217 0 From equation (1-3.) it can be seen that the greater the oscillator strength, the shorter will be the life- time of the excited state. The rate of internal conversion between the lowest excited singlet and the. ground state is not known although it must be less than 108sec-1. Phosphorescence life- times of aromatic hydrocarbons range in general from the order of seconds to that of hundredths of seconds. That there is a competitive non-radiative process from triplet to ground state was first made apparent by the quantum yield measurements of Gilmore et al. (21) (22). Further evidence was given by Hutchison and Mangum (23), Wright, Frosch and Robinson (18) and Van der Waals (24) who found a lengthening of the phosphorescence lifetimes on deuterium substitution. It was suggested (18) following the ideas expressed by Shull (25) and Craig (26) that this effect was due to the lower amplitude of the heavier atom vibration and hence a reduction in vibronic overlap between triplet and ground states. By using quantum yield measure- ments and by making the assumption that all non-radiative decay occurs via the triplet state [arguments in favour 12.

of this assumption are given by Robinson (27) and Ermolaelr (28)] it is possible to calculate intrinsic phosphorescence lifetimes. Such calculations have been made but owing to the difficulties in measuring quantum yields accurately the results are only approximate. For example in the case of naphthalene the observed phosphorescence lifetime is 2.6 seconds. The natural lifetime as calculated by Gilmore, Gibson and McClure (29) is 11 seconds, yet the observed lifetime of deuterated naphthalene is 17 seconds. For benzene the observed lifetime is 7 seconds, the natural lifetime as calculated by Robinson (30) and Lim (31) is 28 seconds and the observed lifetime of deuterated benzene is 12 seconds. The relatively high probability for the intersystem crossing S1 T1 compared to that for So „- ----T is anomalous. Explanations have been attempted by Fariser (32) and Pople (33). Although there has been much discussion (34) as to the role of solid media in the appearance of phosphorescence, it seems likely that they serve chiefly to inhibit second order quenching processes either between two triplet States or between a triplet and an impurity quencher such as oxygen. By using rigid matrices as solvents phosphorescence phenomena can be studied. 13.

1-4. Energy transfer. Energy can be transferred in ang of the following ways. (a) Emission of a photon by one atom or molecule and its reabsorption by a second. This is commonly referred to as the "trivial process"; its' prob- ability is governed by the Beer-Lambert law. (b) Collision. This is responsible for second order quenching of fluorescence and phosphorescence. (c) Transfer between well separated molecular or atomic electronic systems; this is a non-radiative process gnd is often termed resonance transfer. (d) Exciton migration. Such migration occurs in crystals or in regular molecular arrays because relatively strong coupling between molecules is necessary. The theoretical aspects were introduced by Frenkel (35) and developed by Davydov (36), Craig (37), Simpson (38) and others. The topic has been reviewed by McClure (39). Only case (c) will be dealt with further. The phenomenon was first demonstrated experimentally by Cario and Franck (40) in 1922. They irradiated a mixture of mercury and thallium vapour with light of frequency absorbed by mercury only and observed that the fluorescence spectrum showed emission from both types of atom. Transfer by the "trivial process" does not occur 14.

in this case, so that they postulated a non-radiative transfer although at the time they were unable to distin- guish between a collisional process and one involving distant atoms. They called the phenomenon sensitised fluorescence. Further examples of this were found in solution by J. Perrin and Choucroun (41) and to explain these results J. Perrin (42) proposed a theory of energy transfer on the basis of classical coupled oscillator theory. Quantum mechanical refinements of this theory were made by F. Perrin (43) and by Kallmann and London (44). It was further extended by Vavilov (45), F6rster (46) and Dexter (47). Figure 1-2. shows the energy levels of sensitiser molecule [S] and acceptor molecule [A]. The term resonance was used to describe this type of energy transfer because overlap of the emission spectrum of the sensitiser with the absorption spectrum of the acceptor allows the transitions to occur simultaneously. In general the interaction energy between the excited sensitiser SH and the acceptor A is of the type:-

u f( (x)eit(x')H(x,x1 )* (x)* (XI )dXdX1 s s AH

(1-4.) This represents an interaction between the two

15.

A

A 'A A $ A*

• V A II H

Radiative transition

- --► Transfer transition, ewsp. Internal conversion

Figure 1-2, Energy levels of sensitizer (s) and acceptor(A),(80). 16. configurations S A and SAS:

O are molecular electronic eigen- *4" * Mt VA, A SH functions. Superscript asterisk represents complex conjugate. x, x' represent all electronic and nuclear coordinates.

is the interaction operator, being the sum of the Coulomb interactions of all the charged particles.

i.e. H = 1. 1iS-7jA

1 j is the charge on particles i and j.

F -1. is the distance between the i'th iS jA charged particle of the sensitiser and the j'th charged particle of the acceptor.

The sum is expanded into a Taylor series in R, the inter- molecular distance, [Dexter (47)]. Hence u, the interaction energy, can be represented as a sum of multipole inter- actions between the various transition charge distributions of both molecules. The first non zero term of the expansion is a dipole-dipole interaction term and is the only term 17.

considered by F8rster in his treatment. For this case u is given by:-

al.a2 3(al.R)(d2.R) (1-5.) R3 R5 d1 ,d- are transition dipole moments between the ground and excited state.

Furthermore, in order to introduce optical properties, u may be written as:-

KMI2, u = (1-6.) n2R3

M is transition dipole moment.

n is refractive index of solvent.

K is a numerical factor introduced to account for Brownian rotational motion and is therefore only of concern when dealing with fluid media where rotation is fast compared with transfer. In this case for random orientation K = 2/3.

The rate of energy transfer is proportional to u2 hence from equations (1-5.) and (1-6.) :-

Transfer rate a 1— R6 18.

A critical transfer distance, Ro, at which the rate of transfer is equal to the rate of decay can therefore be defined by the equation:- R Rate of transfer, n3 — .\A = 1, 0\)6 (1-7.)

is the actual lifetime of the excited state. F8rster applied the theory just described and by introducing optical properties obtained the following expression for the rate constant of the resonance transfer process.

_ 9000 •e3a 10 K2 i 113E — - (1 6 4 • o 6 ) v e•A` v) dv4 -8.) 128 it n N ' SR •o

C(v) = molar decadic extinction coefficient of acceptor. 4v) = spectral distribution of fluorescence in sensitiser. v = wave number. N = Avogadro's number = natural lifetime of sensitiser, where

Is - -?-` • (1-9.) 0 RS = quantum yield of sensitiser fluorescence in the absence of transfer.

From equations (1-7.), (1-8.) and (1-9.) F8rster obtains:-

2 ° PP 9000 n 10 K 1 f (v) (v) dv Ro 6 4 i S A ---v 4 (1-10.) 128t n N

Equation (1-10.) predicts Ro values of 50-100A° if there 19.

is overlap of the donor emission with the acceptor absorp- tion spectrum. However dipole-dipole interaction may be weak because of forbidden optical transitions in either donor or acceptor molecule. Transitions may be forbidden either on account of symmetry or on account of spin intercombination. In the case where an optical transition is symmetry forbidden in the donor and allowed in the acceptor, with high is the slower transfer rate as cal- culated from equation (1-8.) is compensated by the longer lifetime of the sensitiser. Forbidden transitions in the acceptor, on the other hand, result in a low 4A(v) in equation (1-10.) so that only very short transfer dis- tances are predicted. The theory for this type of transfer in which symmetry forbidden transitions are involved has been developed by Dexter (47) who considered interaction of an electric dipole - electric quandrupole nature. The rate of transfer was found to be proportional

to -7i• • R' Dexter generalised the Perrin theory further to include spin intercombination in both donor and acceptors he concludes as do Fbrster and Terenin (48), that for triplet-triplet transfer the F8rster mechanism does not hold and an exchange mechanism has to be invoked. The operator H in equation (1-4.) includes an exchange integral, ignored in the F8rster treatment, which represents the 20. exchange interaction between the two charge clouds of donor and acceptor molecules. Since the function describing such clouds falls off exponentially with distance from the nucleus, the product of the functions will be small at distances much greater than the collision diameter. The selection rules for exchange interaction allow tran- sitions involving changes in spin multiplicity. Forster was able to verify his theory for singlet- singlet transfer experimentally (49). By making further calculations he obtained the following expression for the quantum yield of acceptor fluorescence, 11A. op, x2 / -t2 m . 2xe e dt 11A/71A ax j --DC

r-1 fit X0X2(14(X)]

bi is an error function, x = 2- -dC - • 0 3000 iCo s the critical transfer concentration 416-R3 corresponding to an average of one acceptor molecule in a sphere of radius Ro.

TIA max. is the maximum quantum yield of acceptor fluorescence obtained by direct excitation or by complete transfer.

x = acceptor concentration. 21.

Equation (1-11.) was confirmed by measurements of the quenching of the fluorescence of tryptaflavin by rhodamine B. The graph of equation (1-11.) is illustrated in Figure (1-3.) together with the experimental results. In his original experiments F8rster was unable to exclude the possibility of the trivial mechanism. This was shown to be absent in the work of Bowen et.al. (50) (51) who confirmed F8rster's theory in their study of the sensitisation of perylene by 1-chloranthracene. Further support for FBrster's theory lies in the phenomenon of the concentration depolarisation of fluorescence (52). The first experimental demonstration of sensitised phosphorescence was given by Terenin and Ermolaev (53). In their original experiments they used benzaldehyde and benzophenone as donors and naphthalene and a-methylnaph- thalene as acceptors. These compounds were chosen because the first excited singlet states of the donors were of lower energy than those of the acceptor molecules, while the triplet states were of higher energy. The donors were selectively excited and the appearance of acceptor phosphorescence noted. Transfer occurred rapidly, at a rate comparable with that of the decay of the donor triplet [in the Terenin case, of the order of milliseconds]. Ermolaev (54) found that in comparable conditions, triplet- 22. Figure 1-3. Fluorescence quenching of tryptophan in methanol by rhodamine B. Relative fluorescence yield as a function of rhodamine concentration,(46).

LN IQ

I • f 0 o n tio a tr en nc Co

O O 23. triplet transfer probabilities were approximately 0.007 compared to singlet-singlet probabilities of 0.4. Triplet- triplet transfer distances of up to 14A were found. The experiments of Terenin and Ermolaev have been confirmed by Farmer, Gardner and McDowall (55) who used the election spin resonance technique to observe the appearance of the acceptor triplet. This method has also been used by Smaller (56) who demonstrated triplet energy transfer from phenanthrene to naphthalene. He was unable to fit his results to any of the mechanisms discussed. 24.

2. 'EXPERIMENTAL TECHNIQUE The work to be described is concerned in part with the measurrcment of the phosphorescence lifetimes of aromatic hydrocarbons and with the ways in which these are affected by deuteration and by changes in solvent -or temperature. In addition experiments concerning energy transfer between molecules in the triplet state will be described. These also depend upon the analysis of phos- phorescence decay curves.

2-1. Apparatus. The apparatus used to measure phosphorescence decay is shown in Figure 2-1. A mercury discharge lamp [A.E.I. Mazda Me/D high pressure, 250 watt model enclosed in quartz envelope] was contained in a box fitted with a thick quartz window and used to irradiate the sample. The light was focussed by means of a quartz lens. The samples were sealed in glass or quartz tubes under vacuum and supported in a compartment with two shutters. These

•

V x a &T ear

-z - c - "

ddv v ua To amplifier and pen recorder L snq

L lens p quartz dewar flask M mercury lamp )( shutter

S sample F) photomultiplier V vent 26.

Figure 2-2., Circuit diagram 27.

were operated manually in order to expose the sample either to irradiation or to the photomultiplier [R.C.A. I.P.28]. The photomultiplier current was amplified by means of an electrometer in one arm of a Wheatstone bridge. The circuit shown in Figure 2-2. is an adaption of that described by Oster (57) known to have linear response over a wide sensitivity range. The amplifier has eleven sensitivity ranges, the eleventh having approximately six thousand times the sensitivity of the first. The signal was displayed on a pen recorder [Honeywell Brown, 0-1mV] with a response time of 1.3 seconds for full scale deflection and with a strip chart rate of 8 inches per minute. Linearity was checked and the sensitivity ranges calibrated as follows. The visible absorbance of a set of neutral density filters was measured at 5890A° on a Perkin Elmer 350 recording spectrophotometer. Sodium light from a stablised source was passed through an orange filter to remove lines of wavelength shorter than the 5890A°, 5896A°. doublet. The source was arranged so as to give full deflection on the pen recorder with the least sensitive range of the amplifier. By using the appropriate neutral density filters approximately full scale deflections were obtained on all the sensitivity ranges. The process was repeated for mid and low scale 28. deflections and the same sensitivity relationships between the ranges were found. The sensitivity of the apparatus is such that phosphorescence emission can be followed over long periods of time. In 10-3M benzene, for example, between ten and eleven halflives were followed.

2-2. Chemicals. Deuterium oxide Norsk-Hydro, isotopic purity 99.8°/o.

Benzene for molecular weight determinations was shaken repeatedly with sulphuric acid, washed, dried, fractionally distilled and fractionally frozen.

D-benzene from Koch Laboratories was used directly. The n.m.r. spectrum showed it to be > 990/0 deuterated.

Para dideuterobenzene was distilled before use.

Naphthalene. Two samples were used:- a) scintillation grade b) naphthalene for molecular weight determinations, recrystallised from alcohol.

D-naphthalene; zone purified sample from Merck, Canada. The n.m.r. spectrum showed it to be > 980/0 deuterated.

Toluene; sample was shaken with sulphuric acid, washed, dried and fractionally distilled. 29.

D-Toluene obtained from Koch Laboratories was used directly. The sample was > 98°/o deuterated.

Anthracene was crystallised from alcohol and passed through a column of acid washed grade III alumina, a mixture of benzene and petroleum ether, b.p. 60-80°C (1:1) was used as a solvent. The solvents were purified by distillation.

D-anthracene was prepared from this sample. [see below]

Phenanthrene was purified by Bachman's method (58) and then chromatographed in the same way as anthracene.

D-phenanthrene. Two samples were used. a) sample prepared from phenanthrene b) sample from Merck, Canada.

Triphenylene, 1.2 - benzanthracene, pyrene and 3.4 - benzpyrene were purified by chromatography. Their deuterated analogues were prepared from these samples.

2-3. Deuteration techniques. Three methods of deuteration were investigated. (a) Deuterium bromide method. Deuterium bromide was made in the following way. Deuterium oxide (8 ml.) was slowly added to phosphorus tribromide (10 ml.) which had been purified by distillation. Dry nitrogen was passed through the reaction flask and 30. the contents were mixed by means of a magnetic stirrer.

D20 and DBr were carried out in the nitrogen flow; the first was removed by condensation in a trap cooled to - 65°C with a solid carbon dioxide/methanol mixture. The remaining DBr was then condensed in a trap cooled with liquid nitrogen. This trap was then sealed from the other and the DBr distilled in vacuo from a carbon disulphide slush bath at - 110°C into a trap cooled with liquid nitrogen; the last traces of DBr were discarded. The DBr was then evaporated into a globe. Yield 7.3 g. The n.m.r. spectrum of the pure liquid indicated that < 1 0/o hydrogen bromide was present. General procedure for deuteration with DBr. The hydrocarbons (typical quantity 30 mg.) were weighed into thick walled pyrex tubes. The tubes were evacuated and DBr (ca. 0.7g.) was condensed onto the hydrocarbons. The tubes were sealed and left at room temperature for 6-12 hours, with occasional shaking. They were then cooled in liquid nitrogen and opened under an atmosphere of nitrogen. The tubes were warmed gradually and HBr and excess DBr evaporated. The procedure was repeated once or twice depending upon the degree of deuteration achieved. Aluminium tri- bromide was used as a catalyst for phenanthreno, 31.

triphenylene, 1.2 - benzanthracene and 3.4 - benzpyrene deuteration. Infra red spectra showed that for anthracene, pyrene, phenanthrene and 3.4 - benzpyrene ---98°/o deuteration was achieved when the procedure described was performed twice. Triphenylene was deuterated only 65-700/o and 1.2 - Benzanthracene was deuterated 70-75°/o at the third attempt; two having failed due to decomposition of the compound. The deuterated compounds were purified by chromato- graphy. u-V spectra showed that apart from deuteration the compounds had not altered. (b) Deuteration using deuterophosphoric acid boron trifluoride , D3PO4. BF3 Anhydrous phospharic acid was prepared by a method adapted from that described in ref. (59). Boron trifluoride gas was bubbled through the phosphoric acid, hydrogen fluoride having been removed by a sulphuric acid/boric oxide trap. D3PO4.BF3 is a colourless highly viscous liquid. Deuteration procedure. This method was used to deuterate only anthracene and pyrene. Anthracene (0.04g) was dissolved in dry carbon tetrachloride (3 ml.); D3PO4.BF3 (0.75g) was added and 32. the mixture shaken for six hours. Pyrene (0.04g) was dissolved in dry cyclohexane (3 ml.) and treated in the same way. Deuterium oxide (0.5 ml.) was then added to both mixtures and the two layers were separated. The hydro- carbons were recovered from the organic layer by evaporation and from the acid layer by filtration.- The procedure was repeated once. Infra red spectra showed ,_,50°/o deuteration in both cases.

(c) Deuteration using deutero-trifluoroacetic acid CF3CO2D. Deutero-trifluoroacetic acid was prepared as follows.

Trifluoroacetic anhydride (5 ml.) was added to dimethyl aniline (0.25 ml.) and then distilled in vacuo. Any trifluoroacetic acid remained behind as the salt. The anhydride was then distilled on to D20 (0.6 ml.) This method was used only to deuterate pyrene. Deutero-trifluoroacetic acid (1.5 ml.) was distilled in vacuo into a tube containing pyrene (0.03 g) and the tube was sealed. It was then heated at 11000 for 12 hours, cooled in liquid nitrogen and opened in an atmosphere of nitrogen. The acid was removed by distillation in vacuo and the sample purified by chromatography. An infra red spectrum showed the pyrene to be 750/0 deuterated. 33.

A second attempt at deuteration, this time at 130°C for 18 hours led to the formation of a yellow compound from which the pyrene could not be separated.

Technique (a) deuteration with DBr, although experimentally the most difficult, was used in general because of its greater efficiency.

2-4. Glassy solvents. (a) E.P.A. LEther/isopentane/ethanol (5:5:2 v/v),1 Ethanol and ether were purified by slow distillation through a 12 ins. fractionating column packed with glass helices. Isopentane was purified by distillation and by chromatography over silica gel. (b) Methylcyclohexane/isopentane (1:1 v/v) The solvents were purified by passing through a column of silica gel. (c) Perfluoromethylcyclohexane/perfluorokerosene (1:1) These solvents from Lights Chemicals were used directly without purification. The mixture gave a satisfactory non-proton glass at liquid nitrogen temperature. (d) D-Tolueno. This was also used without purification, a very ppor glass was obtained at liquid nitrogen temperature, even with rapid cooling. 34.

The above four solvents were used as follows. The hydrocarbons were weighed and placed in pyrex sample tubes; for benzene quartz sample tubes were used. The solvents were added and the tubes were frozen and pumped several times in order to eliminate air; they were then sealed under vacuum.

(e) Polymethylmethacrylata. Methylmethacrylate was purified from inhibitor, by shaking with a 1501/4 solution of podium hydroxide. It was then washed and dried over calcium sulphate. The hydrocarbons were dissolved in methyl- methacrylate and benzoyl peroxide (0.10/o w/V) was added. The solution was frozen and pumped several times and then sealed under vacuum. The tube was heated at 11500 for 12 hours and the methylmethacrylate polymerised giving a clear glass. (f) Ethylene glycol dimethylmethacrzlate. Ethylene glycol dimethylmethacrylate monomer was used in the same way as methylmethacrylate. Another polymer was made from a mixture (1:1) of the two com- pounds. The polymers, in both cases, were poor glasses as they were severely cracked. This was due to implosion resulting from considerable decrease in volume on poly- merisation. 35.

(g) Polymethylmethacrylate films. 'Perspex' was dissolved in chloroform. The hydro- carbons were weighed into tubes and the solution added. The solution was then slowly evaporated while the tubes were rotated. A thin film of hydrocarbon dissolved in perspex was thus formed on the walls of the tube. The films were then pumped. Of the three hydrocarbons that were used in making these films, D-naphthalene was pumped at room temperature, H-triphenylene and D-phenanthrene at 85°C, just below the melting point of perspex. The tubes were sealed after pumping for 4 hours. The pumping procedure proved too vigorous in the case of naphthalene which had largely disappeared from the glass. (h) Boric acid. Anhydrous boric acid was heated until one H3B03 molecule of water had been lost and its composition reduced to HBO,. The hydrocarbons were placed in pyrex tubes and mixed with the powder. The tubes were pumped and sealed in vacuum. They were then heated at 210°C in a furnace until the boric acid had melted. The molten solutions were then removed from the furnace while con- tinually rotating the tubes. A glass formed along the walls of the tube although a certain amount of cracking occurred on cooling. 36.

(i) Ethylene glycol citrate. The method used to make this polymer was an adapt- ation of that used by Sager (60). Citric acid (0.25 mole) and ethylene glycol (0.4 mole) were heated under nitrogen for one hour. at 180 - 18500. The product was clear and very viscous. A portion was dissolved in acetone and precipitated from ether to remove excess ethylene glycol. The polymer was then redissolved in acetone and phenanthrene and triphenylene added to two samples of this solution. The hydrocarbons were approximately 10-3M in ethylene glycol citrate. Acetone was evaporated and the tubes rotated so that a film of ethylene glycol citrate formed on the walls. The tubes were then pumped, sealed and heated so as to cross link the polymer.

Mannitol. The hydrocarbon was added to the mannitol which was then heated until it melted. The containing tube was rotated while the mannitol cooled.

(k) Polyvinyl alcohol. A concentrated solution of polyvinyl alcohol was made in water. The hydrocarbons, dissolved in acetone, were added to the solution. They dissolved on gentle heating. Water was slowly evaporated and the gelatinous solution poured onto a glass plate; this was heated at 37.

110°C until all the water had evaporated. Good clear films were obtained in this way.

(1) Durene. Durene and D-naphthalene were sublimed in vacuo several times, sealed, heated until molten and allowed to cool slowly. A polycrystalline mass was obtained.

2-5. Vacuum technique. In general a conventional vacuum line was used in the preparative work; vacuum was achieved by means of a mercury vapour pump backed by a rotary oil pump. Pressures -4 of ca. 10 mm. Hg were obtained in this way. For the higher vacuum work a conventional high vacuum system was employed, pressures were measured on a McCleod gauge. Oxygen was obtained by the decomposition of potassium permanganate; the gas evolved was passed through a trap cooled to 77°K to remove nitrogen dioxide and other impurities. Pure nitrogen was formed by the thermal decomposition of sodium azide and purified in the same way as oxygen. 38.

3. Lifetime Studies.

3-1. Definitions First order phosphoreseence decay as found in pure aromatic hydrocarbons at 77°K is expressed by the equation: - -kt I = Ioe (3-1.)

I = Phosphorescence intensity

Io= Intensity at time, t = 0. k = Decay constant. The term 'time constant' will be used for R1 and the terms 'half life' or 'lifetime' for n2 The symbol 7- will be used for lifetime.

3-2. Lifetimes at 77°K. The following half lives were measured using the apparatus described in section 2-1. Several of these lifetimes have been measured previously but as the literature values are not always in agreement results will be given for all the compounds studied.

39. •

E.P.A.Olass, 77°K. Literature' Hydrocarbon 0/0 Deuteration -r(secs.)+0.2 values (Ernes.). H-benzene 6.9 7.0+0.5 (17) H4.2 (16)

D-benzene > 99 11.6 13.1+1.0 (56) Para- dideutero- 8.4 benzene H-toluene 8.6 8.8+0.2 (17)

D-toluene > 99 11.3 H- naphthalene 2.4 H2.2 (24) 2.6+0.2 (17) 2.33 (63) D- naphthalene > 98 16.9 H16.9 (23) H18.0 (24) not H-anthracene detectable <0.1 (61) 0.03 (62) not D-anthracene > 98 detectable H- phenanthrene 3.8 3.3+0.2 (17) 3.8 (63) D- phenanthrene > 98 11.6 H- triphenylene 13.8 15.9+0.3 (17) ml3.3 (16) 40. Literature' Hydrocarbon 0/0 Deuteration -r(secs.) 0.2 values (secs.) D- triphenylene 65-70 18.0 H-1.2-benzan- thracene - 0.3 0.3+0.i (10(61) D-1.2-benzan- thracene 70-75 0.4 H-pyrene 0.4 0.7+0.2 (61) 0.2 (17) 0.33 (64) H-3.4-benz- not pyrene detectable <0.1 (61) D-3.4-benz- > 98 not pyrene detectable

E.P.A. solvent not used in these cases.

Lifetimes in other solvents at 77°K

Hydrocarbon Solvent -1(secs.)+0.2 D-naphthalene Methyl cyclohexane/ 17.0 isopentane (1:1 v/v) D-naphthalene Perfluoromethylcyclohexane/ 16.5 Perfluorokerosene (1:1 v/v) D-naphthalene D-toluene 16.4 D-naphthalene Polymethylmethacrylate 16.6 D-naphthalene Polyethyleneglycol 16.6 Dimethylacrylate D-naphthalene Boric Acid 16.5 41.

Hydrocarbon Solvent T(secs7)+0.2 D-naphthalene Polyvinyl alcohol 16.6 D-naphthalene Durene 17.0 D-phenanthrene Boric Acid 11.6 D-phenanthrene Perspex film 11.0 D-phenanthrene Polyvinyl alcohol 11.2 H-triphenylene Boric acid 1270 H-triphenylene Perspex film 12.8 H-triphenylene Polyvinyl alcohol 11.6

From these results it can be concluded that phos- phorescence lifetimes at 77°K appear to be virtually independent of the medium and reproducible. Smaller (56) states that the lifetime of D-naphthalene in C2 D5 OD• is 23.0 seconds and in E.P.A. made with C2D50D is 20.0 seconds. He invokes a solvent-solute complex to explain his results. However Hutchison and Mangum (23) found that for naphthalene in durene there was no change in lifetime on deuteration of the matrix. Similar results were noted by De Groot and Van der Waals (24). The lifetime of D-naphthalene in the fluorinated glass is 16.5 seconds; this does not support Smallers result because any vibronic interaction between solvent and solute should be lessened by fluorine substitution in the same way as by deuterium substitution.

42.

3-3. General decay mechanisms

Equation (3-1.) expresses the decay of triplet in rigid solutions at 77°K; the following is an expression for the decay of triplet in fluid solutions.

-d[T]/dt + k2[T]2 (3-2.)

where k1 and k2 are the first and second order decay con- stants respectively. [T] is concentration of triplet. The second order process is due to triplet - triplet interaction as follows:-

T1 + T1 S1 + So

S1 So + by resulting in delayed fluorescence (65). Reproducible values for k have been obtained only during the past 2 six years. The first order rate constants found in fluid media howevers have been the subject of much discussion and widely varying values have been given for supposedly identical systems. It is clear that both radiative decay

and internal conversion contribute to k1 but there is evidence that pseudo first order processes are also impor- tant. Early work on the determination of first order rate constants was in error because k values were not 2 well determined, however Porter and Wright (66) in 1959 with a good analysis of decay curves obtained accurate k2 43. values and discussed the first order rate constant. They found a very large viscosity dependence for kl and in solvents for which encounter is diffusion controlled they claimed the first order rate constant to be inversely proportional to viscosity. They also demonstrated pseudo first order quenching by paramagnetic impurities including oxygen. In discussion (34) following their paper, Longuet-Higgins proposed that in order to explain the viscosity dependence one could assume that the rate determining step in the internal conversion process was the loss of vibronic energy from the excited gound state to the medium. However Porter and Whiffen suggested that the rate determining step was the cross over from triplet to singlet state, partly because the lifetimes of triplets in the gas phase are shorter and because they are indepen- dent of inert gas pressure. This argument has since been shown to be incorrect as the lifetime of the triplet in the gas phase is limited by triplet-triplet quenching (73). Jackson, Livingston and Pugh (67) determined the rate constants for decay of the anthracene triplet in solution and showed that kI was reproducible only when the same sample of solvent was used and that the viscosity dependence was not as great as claimed by Porter and Wright. They showed that for hexane, viscosity, = 0.3 c.p. and for glycerol, = 1000 c.p., kl for anthracene only changed five fold. Further evidence given by Jackson and 44,

Livingston (68) also indicated that the ideas of Porter et al. on viscosity dependence were incorrect. In a later paper, Porter and Steif (69) still con- sidered the decay rate to be viscosity dependent although revising their earlier views. They claimed that kl varies only in an intermediate viscosity region and that the decay rata in very viscous solutions is reduced to the limit found in phosphorescence experiments. They propose that propylene glycol, 11 = 50 c.p. lies in the inter- mediate viscosity region-and assume a quenching rate donstant of ca. 1.3 x 108 1.mole-1 sec.- 1, a typical value found in other flash photolysis experiments. using the kT Stoke-Einstein equation, D = 73E71 D = diffusion coefficient, for calculating rate constants of diffusion controlled reactions, they showedthat a quencher concentration of -4 2 x 10 mole/1 would be necessary to produce the required quenching rate. However they found that the first order rate constant was independent of the solute in the region 2 x 10-4 to 2 x 10-6 mole/1 and independent of the solvent, six solvents having been used. From this they conclude oxygen quenching to be absent. Livingston and Ware (62) suggest that. Porters estima- tion of quenching concentration at 2 x 10-4 mole/1 is very high because it is made on the basis of the Stoke- Einstein relation, Ware (70) having indicated that this procedure gives unreliable results and that diffusion 45. rates so calculated, especially in viscous, H-bonded and polar media may be too small. An oxygen concentration much smaller than 2 x 10-4 mole/1 could have been present in Porter and Stief's experiments. Livingston and Ware, calculated Be given in the following expression:-

ki = k9ic1 exp(-LEH/RT)[Q]+k where k°1 = temperature independent constant k(;(), = pre-exponential constant related to quenches concentration [Q]. They claim that if the energy of activation. Be is a diffusional activation energy then it should be equal to the activation energy for viscosity change LE . However their experiments indicate Le to be invariably smaller than BE y particularly in viscous solvents. No explanation is given but the results appear to rule out a dominant viscosity effect. In a paper by Stevens and Walker (71) the authors obtain similar results to Porter and Stief. The variation 1 of In k1 with 7 exhibits characteristic behaviour in regions of high, medium and low viscosity. They state that the considerable dependence of k1 on viscosity in the intermediate region is kinetically consistent with reversible quenching of the triplet by a solute impurity. 46.

In the latest papers by Hilpern, Porter and Stief (73) and by Porter and West (74) they show that the vis- cosity dependence of the decay rate can be reduced by increasing the rate of internal conversion with heavy atom substitution or by using very pure solvents, the latter having previously been reported by Livingston and Ware (62) and by Linschitz, Steel and Bell (74). Hence they claim to have measured the true first order decay constants. However the effect of oxygen and other impuri- ties has not yet been estimated accurately. Porter et al. noted a considerable temperature dependence of and stated that this was due to viscosity change. Gas phase studies indicated no change in the rate constant between 80 and 140°C. A temperature effect has been proposed by Hadley, Rast and Keller (75) who measured the activation energy for the temperature dependent contribution to k 1, using H and D-naphthalene dissolved in single durene crystals. They obtained activation energies of 10.15 k.cals and 9.61 k.cals for H and D naphthalene respectively.

From the forgoing discussion it appears that there is doubt about the nature of the first order decay con- stant. The difficulties may be summarised as follows.

47. a) Is there a genuine viscosity dependence? b) How effective is oxygen in quenching the triplet? Is the viscosity dependence a reflection of diffusion_ of oxygen and other quenching impurities in the solvent? c) Is there a genuine temperature dependence? These problems have been investigated in the following experiments.

(3-4.) Effect of oxygen. Oxygen is known to be extremely efficient in enhancing singlet-triplet transitions, both radiative (76) and non- radiative (66). Lifetimes of triphenylene and D-phenanthrene in perspex films were measured for a series of oxygen pressures. Lifetimes were measured until constant values were attained, the time taken for equilibrium to be reached is given; the effect of oxygen was found to be reversible. Temperature 24 + 1 oC

Oxygen pressure D-Phenanthrene Triphenylene Approximate mm. H' -rsecs(+0:7 3secs(+0.2)_ time for equilibrium < 10-6, followed 9.6 8.5 3 hours by firing of cal- cium getter. • -6 < 10 9.4 8.3 30 mins. -4 3.2 x 10 7.5 7.2 1 hour -2 1.49 x 10 6.5 6.0 6 hours 1.19 no phosphorescence no phosphore- 3 days detectable scence detectable 48.

When the samples were pumped on a preparative vacuum line to the black vacuum stage the lifetimes obtained were, D-phenanthrene, S = 6.8 secs., triphenylene, T = 7.0 secs After exposing these samples to the atmosphere for thirty minutes no phosphorescence was detected. On re-pumping, the same lifetimes were obtained. The samples under a pressure of oxygen of 3.2 x 10-4 mm. were opened to the atmosphere while a decay measurement was being made. The resulting decay trace for D-phenanthrene is shown in Figure (3-1.). There is a striking and immediate drop in intensity and lifetime although the tail of the decay curve has a relatively long half life. These experiments indicate that oxygen can have a striking effect on phosphoreseence decay at room tem- perature even in a rigid matrix. The fact that reproducible results were obtained using the same pumping procedure [preparative vacuum line] is probably a reflection of the reproducibility of oxygen concentration under these con- ditions, [c.f. (62)]. It was further noted that when the samples were opened to the atmosphere the intensity fell proportionately much faster than the lifetime. This may be due to the non-uniform environment of the hydrocarbon molecules. Those to which oxygen can readily diffuse may be quenched immediately whereas others may continue to phosphoresce normally or be quenched by a slower diffusion controlled 49. Figures 3-1. (above) and 3-2. (below)

I

.80 Decay of D-phenanthrene in perspex film. Tube opened at 10 secs.

60

40

20

0 5 10 15 20 . secs.

Om. Phosphorescence not 10 detectable

—14 2 secs. = ca. 5 secs.

------T= ca. 7 secs.

•••• COY.

---______—_"1-= ca. 10 secs. 2

0 1 2 4 cm. 50. mechanism. This would account for non-exponential behaviour observed in the decay when the tubes were first opened to the atmosphere. The effect of oxygen was demonstrated further, as follows. A sample of D-naphthalene in polymethylmethacrylate, a clear solid glass, prepared in vacuum as described in section (2-4.) was opened to the atmosphere after the lifetime in vacuo had been measure. The lifetime .was measured immediately after opening the tube and was found to be unchanged at 10.2 secs. After 12 hours the lifetime was still unchanged although the intensity had dropped. After two days a gradient in lifetime was noted, this is illustrated in Figure (3-2.). The more rapid drop in lifetime and intensity observed in the perspex film as compared to the solid block of polymethylmethacrylate is probably a reflection of the shorter time necessary for equilibrium with oxygen to be achieved. The long lifetime of D-naphthalene observed in poly- methylmethacrylate, even though the glass was prepared under poor vacuum conditions may be due to the reaction of oxygen with free radicals, available during the poly- merisation. It is of intrest to examine other solid matrices and to attempt to correlate long phosphorescence lifetimes in 51. air at room temperature with glasses having low oxygen permeability. The following is a table giving phosphorescence life— times at room temperature in a variety of matrices. Temperature 23+1 °C

Pressure of air ca. 10-4 mm. Hg.

Hydrocarbon Matrix secs(+0.2) t 77°K E.P.A.

D—naphthalene Boric acid. 10.1 Polyvinyl alcohol 10.4 Block of poly— methylmethacry— late. 10.2 16.9 Po],y-othylene g]yc o1 dirdethy1acryLate 9.4 Dur ene 5.0 D—phenanthrene Boric acid 8.0 Ethylene glycol citrate 8.0 11.6 Polyvinyl alcohol 8.4 'Perspex film getter 9.6 Triphenylene Boric acid 9.0 Ethylene glycol 7.9 13.8 citrate. Polyvinyl alcohol 7.6 Mannitol (in air) 7.6

-8 pressure of air ca. 10 mm. Hg. 52.

The phosphorescence lifetimes of hydrocarbons in boric acid, polyvinyl alcohol and ethylene glycol citrate remained virtually unchanged when the samples were opened to the atmosphere. The lifetime of triphenylene in mannitol in air is the same as in polyvinyl alcbhol. Furthermore the lifetimes in air at room temperature in these particular glasses appear to be almost reproducible regardless of the matrix. In perspex and durene, however, phosphorescence is dependent on the absence of oxygen. It appears that triplet decay is governed not by the rigidity of.the glass but by the permeability of quenching impurities, especially oxygen, within it. Barrer (77) has collected data which shows that permeability is governed by the chemical nature of the matrix and he states that no connection can be observed between permeability and cry- stalline structure or degree of polymerisation. He gives data for the permeability of hydrogen, a non-polar molecule, and shows that it is very low in materials rich in hydroxyl groups. The permeability of oxygen may be assumed to behave likewise. It was in fact these considerations which led to the choice of polyvinyl alcohol and ethylene glycol citrate as glasses. Hence good matrices for studying phosphorescence at room temperatures in air would appear to be those rich in hydroxyl groups having low permeability to oxygen 53. rather than those of high rigidity. Of the glasses studied, polyvinyl alcohol is the easiest to make and gives reproducible results. Although the intensity of phosphorescence is high in boric acid this matrix is not entirely satisfactory because the lifetimes obtained are very sensitive to the amount of water removed from the boric acid while making the glass. This was demonstrated by using a mixture of D-phenanthrene in boric acid. When the mixture was heated until the boric acid had just melted the resulting glass gave a phosphorescence lifetime of 6.5 secs. Further heating to remove a little more water gave a glass having a lifetime of 7.8 secs. On further heating the lifetime was reduced to -,-6 secs. The optimum lifetime is obtained in a glass having the composition HB02 [c.f. (78)] but this is difficult to obtain regularly.

3-5. Temperature Effect. It has been noted that the lifetimes at room tem- perature aro virtually independent of the medium. The low lifetime obtained for naphthalene in durene may be due to the presence of oxygen because owing to the volatility of both durene and naphthalene pumping was restricted. The lifetime of 5.0 secs., however, is considerably greater 54. than ca. 1.5 secs. obtained by Hadley, Rast and Keller (75) at 298°K. Their durene sample was prepared under nitrogen so that an even greater concentration of oxygen may have been present. [For example, nitrogen containing only 1 p.p.m. oxygen is equivalent to an oxygen pressure of ca. 10-3 mm. Hg.] If Hadley, Rast and Keller were able, as they claim, to measure the true activation energy for internal conver- sion then this activation energy should be reproducible and independent of the medium. Accordingly the activation energy of D-naphthalene was determined in three other media. Some temperature studies have also been carried out for H-triphenylene and D-phenanthrene. The results are given below.

Polymethylmethacrylate / D-naphthalene T°K Tsecs.(+0.2 77 16.6 90 1646 172 16,2 195.3 15.0 20145 14.8 213,8 14,2 239.6 13.1 250.8 13.0 273,6 11,9 294.0 10.2 322.0 7.3 3/1/1 4.8 ) 361 1.6 non exponential 55. Ethylene glycol polydimethylacrylate / D-naphthalene o T K -rsecs. (+0.2) 77 16.6 90 16.6 155.5 16.2 176.6 15.2 193.3 15.0 216.0 15..0 233.6 13.6 256.3 12.0 273.0 10.8 309.6 8.8 325.0 8.3 3/111.5 7.4 360 4.5 ) 371 non exponential 385 >1.0 5

Boric Acid D-naphthalene

T°K ! secs.(+0.2) 77 16.5 go 16.5 155.5 16.0 178.6 15.8 194.4 15.8 56. Boric Acid / D-naphthalene T°K Tsecs.(+0.2) 212.8 14.8 236.8 14.2 256.8 10.9 273.0 10.2 308.7 10.1 324.0 9.8 342.0 9.4 360 8.6 376 7.6 396 7.1 413 6.8 425 3.1 ) 438 2.6 non-exponential 453 >1

Boric acid / Triphenylene

T°K izecs.(+0.2) 77 12.0 90 12.0 155.5 12.0 176.2 11.6 216.0 11.2 57. T°K 1secs.(+0.2) 241.1 11.0 273.0 9.8 312.6 8.3 323.0 8.2 3/0 7.9 358 7.5 393 5.5 405 5.1 418 3.1 ) 428 2.7 non exponential 448 2.5 ) 458 >1

Perspex film / Triphenylene T°K Tsecs.(+0.2) 77 12.8 90 12.8 156 1079 177.5 974 198.7 9.3 201.5 8.5 210.8 8.3 236.0 8.0 58. ' , T°K secs.(-0.2) 259.8 7.5 284.1 7.0 303.7 6.0 313.2 4.1 321.5 3.7 ) non exponential 3/14 1.8

Perspex film / D-phenanthrene

T°K Tsecs.(+0.2) 77 11.0 90 11.0 156, 11.0 176.0 10.2 208.5 9.4 213.0 9.2 238.0 8.8 259.5 8.6 273.0 7.3 294.5 6.4 303.7 6.0 312.0 5.6 323 2.8 ) non exponential 343 59• Figure 3-3., D-naphthalene 2 • 3 4 5 6 7 (1/T x 103 ) -2

A A Boric acid B 0 Polymethyl- methacrylate C • Polyethylene glycol dimethylacrylate 60. Figure 3-4., H-triralenylene

2 3 4 5 6 7 (1/T x 103) -2

0 Perspex film 0 Boric acid Figure 3-5., D-Ihenanthrene 61.

2 3 4 5 6 (1/T x 103 )

-2

-3

-5

62.

Graphs of In (k-ko) against ; are given in Figs. (3-3.), (3-4.) and (3-5.). k and ko are the rate constants for temperature dependent and independent decay respectively. The activation energies, found from slopes determined by least squares analysis, are as follows:-

Hydrocarbon Matrix Activation energy EA k.cal,/mole •. D-naphthalene Boric acid 2.2 Folymethyl- methacrylate 2.1 Polyethene glycol dimethylacrylate 2.1 H-triphenylene Boric acid 2.0 Perspex film 1.0 D-phenanthrene Perspex film 2.3

These activation energies are very low when compared to those obtained by Hadley, Rast and Keller (75). [D-naphthalene EA = 9.61k.cal H-naphthalene EA = 10.15k.cal. l• Their activation energies are comparable to the energies of activation for the diffusion of oxygen through organic material (77) and it is possible that this is what Hadley et al. in fact measured. The results given above show a fairly reproducible value for the activation energy of D-naphthalene of approximately 2.1 kcal. 735cm.-1). It is difficult to see with what mechanism this activation 63. energy is associated, although it is possible that excitation of a higher vibrational mode facilitates internal conversion. Since internal conversion occurs slowly at room temperature it is surprising that the activation energies are so low. [For a uninolocular reaction the rate constant, -E/RT k = ve whore v = frequency factor.] Generally 13 the frequency factor is very largo, 10 , but for reactions involving spin forbidden transitions it is greatly reduced. another factor may be the necessity of concentrating the energy in a particular vibration21 mode. As the same value of E was obtained for D-naphthalene in three different matrices a genuine temperature effect is indicated. However it remains possible that if all impurities including oxygen were absent, the lifetimes of the hydrocarbons would remain constant, at the value obtained at 77°K, independent of temperature. The discrepancy found for triphenylono between the two media could be due to impurity or to photochemical decomposition to which triphenyleno is prone. The temperature dependence of the internal conversion rate of D-phenanthrene was also investigated in the presence of 1.5 mm. oxygen pressure. The plot 1 of In k-ko against was non-linear, the slope became 64. steeper at higher temperatures. It was noted however that the activation energy associated with any one part of the curve was invariably greater than the 2.3 kcal. obtained for the evacuated sample. Non-linearly may be due to the combined factors of the oxygen diffusion rate and the rate at which the equilibrium solubility of oxygon in perspex is reached,

3-6. Effect of Nitrogen. Nitrogen was found to have no effect on the phos- phorescence intensity or lifetime or aromatic hydro- carbons. It would therefore appear that it is the oxygen in the air which is responsible for the effects described in section 3-4. 65.

4. Energy transfer; experimental results.

4-1. Introduction The purpose of the experiments to be described was to investigate transfer of energy between triplet state molecules in solid solution. As stated in section 1-4. resonance energy transfer between singlet states by the P8rster mechanism requires overlap of the emission spectrum of the donor with the absorption spectrum of the acceptor. Systems were chosen for which this condition holds well; it is likely to facilitate most transfer processes. Accordingly H and D isomers of the same hydrocarbon were used for investigation. The triplet levels of the deuterated isomers are of slightly higher energy their protonated analogues. Stern- licht, Nieman and. Robinson (79) give the differences in energy between the H and D triplet levels of benzene and of naphthalene as 200cm.-1 and 100cm.-1 respectively. Although it is likely that energy is transferred predominantly from the deuterated to the protonated isomer the energy 66.

difference is sufficiently small for the process to be reversible. It may be summarized as;-

TD + SH TH + SD

Benzene, naphthalene and phenanthrene dissolved in E.P.A. matrices at 77°K have been used in these experiments. Under these conditions second order quenching of the triplet is absent. This has been shown by the exponential nature of the triplet decay of single hydrocarbons over the range of concentrations studied. A mixture of the H and D isomers in E.P.A. at 77°K should, in the absence of transfer, exhibit the phos- phorescence decay of the two species independently so that the resultant emission is given by;-

Ti -k t + ID e D (4-1.) = Ia e o

kH and kD are the decay constants for the H and D isomers. D are the phosphorescence intensities of the Io and I H and D isomers at time, t = 0.

However, if there is interaction between the H and D triplets it is unlikely that the decay can be expressed as the sum of two exponentials in the way shown in equation (4-1.). Energy transfer was therefore investigated by analysis of the decay curves of mixtures of H and D isomers dissolved in E.P.A. 67.

4-2. H and D naphthalene For solutions of concentration > 5 x 10-5M con- taining 95°/o H isomer and 5VOisomer the decay of phos- phorescence did not fit equation (4-1:). In fact it was impossible to fit the decay to the sum of any two exponen- tials satisfactorily. In more dilute solutions the equation was obeyed within experimental error. [An expression of the form of equation (4-1.) was fitted to the decay curves by the method of least squares and deviations evaluated at all experimental points (average number 50).] In the more concentrated solutions graphs of lnI against t showed that after a certain period of time the decay could be expressed by a single exponential. The rate constants for the latter part of decay were larger than those found for the D-isomer alone, while in more dilute solutions the final decay exactly fitted that of the D-isomer. Sample plots of lnI against t are illustrated in Figure (4-1.). This shows the decay observed in the three situations:- -1 a) the most concentrated solution studied, 10 M. b) dilute solution in which slight deviation from equation(4-1.) was noted, 5 x 10-5M. c) very dilute solution in which equation(4-1.) is obeyed, 5 x 10-7M. Figure 4-1., Phosphorescence decay of H and D naphthalene mixtures dissolved in E.P.A.

B A 5 x 1077 M 1N(tail) = 16.9 0.2 sec. 13 5 x 16-5 rd 1(tail) = 16.8 0.2 sec. C 10-3- T(tail) = 13,9 0.2 sec.

rn 0 10 20 30 40 . 50 6o 70 8o 100 no (seconds) 69.

The lifetimes associated with the final, exponential region of the decay are given in the following table.

Solution of 95°/o H, '5°_/o D naphthalene in E.P.A. at 77° .

Concentration 17Secs. (+0.2)

H-naphthalene 2.4 D-naphthalene 16.9 10 1M 13.6 5 x 10-2M 14.2 10-2M 14.8 10-3M 15.1 5 x 15.8 10-4M 16.3 5 x 10-5M 16.8 10-5M 17.1 10-6M 17.0 5 x 10-7M 16.9

Graphs of lnI against t have not been given except in the three cases cited. as the plots for the other concentrations were all found to lie between the extremes given, in the expected order. The results given are mean values obtained from two runs taken on different samples of the same concentration) the deviation between 70. two such runs did not exceed +0.2 seconds. -1 The phosphorescence of a 10 M solution of H naphthalene could only be followed for approximately 16 halflives. Hence after about 40 seconds any direct contribution of the H isomer to the decay of a mixed solution can be ruled out. The results given indicate that some type of energy transfer may be occurring. However the question arises as to whether the results reflect some other phenomenon such as complexing or clustering of naphthalene molecules. This could give rise to Terenin-Ermolaev type exchange or quenching rather than to transfer between isolated molecules. Three facts appear to contradict the idea of complex- ing and clustering. a) Lifetimes of single hydrocarbons in E.P.A. are independent of the concentration. b) The same phosphorescence behaviour is observed regardless of whether the E.P.A. is cooled rapidly or slowly. c) A 1.26 x 10-2M solution of 95°/o H, 50/0 D naphthalene in polymethylmethacrylate, a polymer formed at a temperature (115°C) at which complexing is highly unlikely, had an exponential tail to its decay with a lifetime of 14.6 seconds, at 77°K. [c.f. Solution in E.P.A. of same molarity had Ttail = 14.6 secs. (extrapolated)]. 71.

4-3. H and D benzene. Similar experiments were performed with benzene using 75°/o H and 25°/o D isomer. Experimentally this is a more difficult system to follow because the difference in lifetimes between the two isomers is not so great as in the naphthalene case and a proportionately higher con- centration of H triplets is excited. Results analogous to those obtained for naphthalene were found but the exponential part of the decay could only be followed for approximately 40 seconds (c.f. Figure 4-1.). The following table shows the results obtained. Reproducibility of the results was not as good as in the naphthalene case and a correspondingly larger deviation is cited.

Solution of 75°/o HI 25°/o H benzene in E.P.A. at 77°K.

Concentration ecs.(+0.3)

H-benzene 6.9 (+0.2) D-benzene 11.6 (+0.2) 10 1M 9.0 10-2M 10.0 10-3 M 10.5 10-4M 11.6 72.

4-4. H and D phenanthrene. Similar results were obtained for phenanthrene using 800/0 H and 20°/o D isomer. Exponential decay for the latter part of phosphorescence was observed for about 70 seconds.

Solution of 800/0 H, 20°/o D phenanthrene in E.P.A. at 77°K.

Concentration nrsecs. (+0.2)

H-phenanthrene 3.8 D-phenanthrene 11.6 10- 1M 9.7 -2 10 M 10.2 -4 10 M 11.2 10-5M 11.6

4-5. Discussion. All the results show the latter part of the decay to be exponential. The lifetimes associated with this decay fall monotonically with concentration. It appears that transfer is occuring but that its' dependence on concentration is not very great. It is therefore unlikely that the results can be explained in terms of a Terenin- Ermolaev mechanism, which is in any case a fast process. (c.f. section 1-4.). 73•

The fact that deviation from equation (4-1.) was noted in solutions as dilute as 10-4M suggests that transfer may be occurring over long distances; however for this to be established it is necessary to consider the distribution of molecules in a rigid glass. This will be discussed in the following chapter in connection with the dependence of triplet-triplet transfer on inter- molecular distance. Mechanisms for the transfer will also be considered. 74.

5. Energy transfer - Mechanism and Discussion.

5-1. Introduction. The results given in the last section indicate that a slow energy transfer process is occurring between deuterated and protonated triplet state molecules. As mentioned previously this is inconsistent with the Terenin-Ermolaev mechanism nor can it be explained by the nrster theory, [c.f. section (1-4)]. It is unlikely that either clustering or complexing of the molecules is responsible for the transfer. As has been shown [c.f. section (4-2)] . complexing is not supported experimentally and furthermore it is likely that the decay kinetics would be different. The decay of non-complexed deuterated molecules would be expected to occur normally and this contribution would have been observed in the total decay. Smaller (56) however, supports the idea of a donor-acceptor complex. He studied triplet energy transfer from phenanthrene to naphthalene using E.S.R. techniques and observed that the decay of the donor phenanthrene remained exponential with

75.

a constant lifetime. Thus he concluded that rapid trans- fer occurred via a complex. However as shown in section (4-2) the lifetime of the donor D-naphthalene, the longest lived donor studied, was only reduced from 16.9 to 13.6 -1 seconds even in a 10 M solution. Since the lifetime measurements made with E.S.R. are not so accurate and in addition a forbidden E.S.R. transition was being observed it is possible that a small change in the life- time of phenanthrene ( ! = 3.3 secs.) would not be detected. However the possibility remains that transfer via a complex or by the Terenin-Ermolaev mechanism is occurring together with slow transfer. These processes would not have been detected under the experimental conditions.

5-2. Kinetic scheme. It was attempted to derive a kinetic expression relevant to the phosphorescence decay recorded in sections (4-2), (4-3) and (4-4). If the assumption is made that all the molecules are in the same environment then the following equations should hold: dN.R. (5-1) dt- (kH YH) NH 4- YDND

dND 737- = - (lc]) + YD) ND + yHNH (5-2) 76.

NH and ND are the number of protonated and deuterated triplet state molecules respectively. k/i and kD are the rate constants for the radiative (phosphorescence) decay of protonated and deuterated triplet state molecules respectively. yH is the rate constant for the process is the rate constant for the TH + SD SH + TD, andYD reverse process. t = time Equations (5-1) and (5-2) are solved simultaneously as follows.

Let A = kH + yH B = kD + yD Then, when D = dtd (D+A)NH - yDND = 0 (5-3) (D+B)ND - yHNH = 0 (5-4) (5-3) x (D+B) (D+A)(D+B)NH - (D+B) yDND = 0 (5-5) (5-4) x yD (D+B)yDND - yHyDNH = 0 (5-6) (5-5) + (5-6) [(D+A)(D+D) - yHyD] NH = 0 a. [D2+D(A+B)+AB-yHyD] NH = 0 (5-7)

The roots of equation (5-7), R1 and R2 are given by:-

77.

A+B) - 4(AB-yHyD) 2R1 = -(A+B) +

2R2 = -(A+B) - vi(A+B)2 4(AB-yHyD) Rlt R2t + K e (5-8) 1\-1H = 1K e 2 where K and K are constants. 1 2 Substituting (5-8) into equation (5-3) Rlt R,t (D+A)(Kle + K2e ) YDND =

R t R,t ND )e1 +K (A+R )e ] = --[KyD 1 (A+R 1 2 2 (5-9)

Hence the general solution of equations (5-1) and (5-2) is given by equations (5-8) and (5-9). HNp, where I and I Since ID = aDND and IH = a D H are the intenwities of the deuterated and protonated isomers, the total intensity, I = ID + IH, is seen to be the sum of two exponentials with exponential constants

Ri and R2. It should therefore be possible to find R1 and H2 from the decay curves and hence evaluate 7-H and y.D. These are given by the following expressions.

(R1-R2) YH = 4(kH-kn)

(Ri-R2)2-(Ri+R2+2kD)2 YD= 4(kp-kH)

78. y•H and YD are expressed in terms of the experimental parameters R1, R2, kD and However the analysis of the decay curves showed that the intensity did not follow the sum of two exponen- tials exactly. The reason for this deviation is probably that the original assumption of uniform environment for the molecules is incorrect. Distribution functions will be discussed in the appendix. As mentioned previously the latter part of the decay was found to be a very good single exponential. In order to determine the nature of this exponential the above decay scheme was simplified further by making an additional assumption that energy transfer occurred from the deuterated isomer only; i.e. only TD+SH >TH+SD. This assumption is more nearly correct in the final part of decay. Equations (5-1) and (5-2) are then modified to give:- dN D = -(k ) N (5-12) dt DD D

dNH dt HNH YDND (5-13)

-(kn+),D)t • • • ND = C1e -(k_+y dNH )t (5-14) • • - kHNH IDCle D

The general solution of equation (5-14) is the sum of the

79.

solution of the homogeneous equation (complementary function),

dNH dt + kHNH = 0

and a particular solution. The solution of the homogeneous equation i

-kHt NHh = C2e For a particular solution take:-

-(kD+yD)t NPH = C3 e

and substitute this into equation (5-14)

[-(kp+yD)+kii]C3 = ypCi

Hence the general solution is:-

(kt+YD)t ND = Cle

-k t C y • -(k +y )t N C e H 1 D D D H = 2 kH-(kD+yD) e

kHyD -(kD+yD)t = +IH = k C e +Ci[kn + 12, je D H 2 "H -("D'YD)

(5-15)

There is one situation for which the particular solution is degenerate, that is the case where YD = kH-kp and in this instance the relevant solution for equation (5-13) is:- 80. -k t C.F. = C e H 2 -k t P.I. = C te H 3 -k t kHt Hence I = H kH(kH-kD)Clte-

-k t + k C H (5-16) D le

This situation is of no practical importance. Equation (5.15) will not be a good representation of the decay when time, t, is small. However it is a fair approximation for the latter part of the decay and it was therefore used to calculate yp.

5-3. Comparison with the Forster theory. F5rster's theory for singlet-singlet transfer predicts a 1-7 dependence for the rate of transfer, where R' R is the intermolecular distance. Any mechanism with a 16 dependence, regardless of its origin, should there- fore follow equation (1-11) and a graph corresponding to that shown in Figure (1-3) should be obtained. In order to investigate whether al b mechanism is applicable to P the present results an expression equivalent to Ftirsters TIA/nA max of equation (1-11) has to be found. In the case of triplet-triplet transfer this is the phos- phorescence yield kp/kp-i-y-D. Hence kb/kfi-yD for naphthalene 81. was plotted against log concentration-) the result is shown in Figure (5-1). It can be seen that this only corresponds to the portion of the FBrster curve relevant to very dilute solutions, [c.f. Figure (1-3)]. The plot is almost linear except that a sharp drop in kD/kD+YD is noted when a solution of 10-1M concentration is reached. Forster also predicts a linear relationship between

TIA/T1A max and concentration, in dilute solutions. k A +y D D D was therefore plotted against concentration; the result is shown in Figure (5-2). The relationship is approximately linear. Hence although the results do not supply positive 16 evidence for a dependence they are not inconsistent R with a mechanism of this type. In order to obtain a value equivalent to Irorster's R , the distance at which o YD kD, studies in far more concentrated solutions would be necessary. This is practicably impossible for reasons of solubility; solutions of concentration greater than 10 1M cannot be studied satisfactorily in E.P.A. glasses at 77°K. It should be noted that several functions of R will give rise to linear plots as illustrated in Figure (5-1). It is likely that a more realistic distribu- tion function than that used by FBrster would result in an equation more sensitive to variation in the power of R. This is discusses briefly in the appendix. 82. Figure 5-1 ; plot of Ksjilpi-r against log concentration, naphthalene.

-4 -3 -2 log concentration 83. Figure 5-2 Plot of Kp/16.frm against concentration ; naphthalene

0 2 4 6 8 10 12 104 concentration moles/litre 84.

5-4. Transfer mechanism. Although the distance dependence of the rate of exchange has not been satisfactorily determined it is of interest to consider a mechanism to explain the slow triplet-triplet transfer demonstrated experimentally. Avery (18) has suggested that the mechanism involves spin-spin interaction; this would give rise to a dependence. The theory will be summarized briefly. The spin interaction term is given by

+1 2 . 0-=Q 3( GI- .711- )(-(1t-c ) H' = (17) ( ) (5-17) r3 5

are vectors having the three Pauli where(7-1 anc10-2 spin matrices as components, subscripts 1 and 2 denote action on the wave functions of electrons 1 and 2.

r =xl - x2 , where x1 and x, are the coordinates of electrons 1 and 2 respectively. The probability per unit time for a molecule (1) in the triplet state *t(1) to jump to the ground state kV0(1) while molecule (2) jumps from the ground *0(2) to the excited state *t(2) is given by:-

E1121 P 4,2(q2+4 I-, 2) (5-18) 85.

I dV2 4(1) ecE,(2)111 *0(1)1rt( ) where H12 = rdVi

(ile N 2 1 H12 '2mci R3

In using equation (5.18) the assumption is made that the spectral lines have a. Lorenzian shape of half- width n and that the emission line of the donor and the absorption line of the acceptor are centred respectively on frequencies w1 and w2. For the aromatic hydrocarbon under consideration spectral overlap between donor emission and acceptor absorption only occurs for the o - o bands.

Accordingly an approximate Ro value was calculated from equation (5-18). The position of the o - o band, H-naphthalene triplet = 21,200 cm-1 (76) (82)

According to Sternlicht et. al. (79) the D-naphthalene triplet is 100 cm-1 higher, i.e. 21,300 cm 1. 27cc Frequency of H naphthalene absorption band w1 = X 10 w1 = 67z x 10 x 21,200 15 = 4 x 10 c.p.s. Frequency of D-naphthalene emission band, 15 = 4.05 x 10 c.p.s. w2 and ri = 2.8 x 1013 c.p.s.

-11 me = Xo = 3.86 x 10 cm (Compton wavelength) 86.

-40 • 1.19 x 10 • • P jumps per second R6

R 1 6 P = ;( °.)6 9 where T.-- lifetime of donor

..' Ro 5. --- -, - 38A° .

value is certainly gfeater than that which This Ro would be found experimentally. However the calculation gives an order of magnitude estimate and demonstrates that slow transfer such as has been observed is possible. The actual distances involved in the transfer will be considered in the appendix. 87.

Appendix

FUrster used the following distribution function in deriving his expression.

w(R) dR = 4tR2dR where w(R) is the probability of finding one molecule at a distance R from another. It is likely that a more realistic distribution function imposed on equations (5-1) and (5-2) would result in an expression more sensitive to variation in the power of R than that of nrster. This was attempted without success for a random distribution and for a nearest neighbour distribution function. Both distributions were calculated by using Monte Carlo techniques (Ferranti mercury computer) but it was found too difficult to utilize them. In the simpler case where only transfer to nearest neighbours is considered, the exchange y is given by the equation, R y = X(-2)" where X is a constant. The number of molecules with a transfer rate between R y and y + dR dR is then X(if)" N(R) dR, where N(R) is the number of molecules at a distance between R and R+dR from their nearest neighbour. The concentration of deuterated molecules is then given by:- [c.f. equation (5-2)] • 88.

dND at (R,t) —Dcp + x(iprIND(R,t) and of protonated molecules [c.f. equation (5-1)]

dN R. (R' = -klINH(R,t)+X(112)n ND(R,t)

However although these equations are readily integrated with respect to time, it was found too difficult to integrate with respect to R. Furthermore a realistic distance of nearest approach has to be used in the calculation. Nevertheless the results of the calculations for the nearest neighbour distances are of interest and can be compared with the average distance between molecules. These distances are calculated on the assumption that the molecules are centred tn spheres which are close packed to occupy 74°/o of the total available volume.

Average distance, A° Molarity Nearest nei hbour distance Ao 15 10-1 13 18 5x10-2 15 30 10-2 25 62 10-3 34 -4 78 5x10 55 -4 136 10 168 5x10-5 120 286 10-5 89.

The nearest neighbour distance given is the peak of a distribution which was quite broad. About 550/0 of the molecules had nearest neighbours at the distance given but for the remaining 45°/o the distances ranged o + 30 A around the peak. This indicates that in a 5x10-5M solution (the most dilute solution in which transfer was observed) the energy may have been transferred over 100 A°. 90.

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