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Gay-H-1964-Phd-Thesis.Pdf 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.
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