Paramagnetic resonance in phosphorescent aromatic hydrocarbons

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Abstract
Electron resonance spectra have been recorded of the lowest triplet states of benzene, toluene, triptycene and tribenzotriptycene. The experiments were carried out by ultra-violet irradiation of these molecules in rigid glass solutions both at 77°k and 20°k. From an analysis of the line shape of the spectra, which prove to be strongly temperature dependent, the following conclusions could be drawn. Benzene in its first triplet state has a configuration of lowest energy which is not a regular hexagon. This finding, supported by the results for toluene, was substantiated by SCF-MO calculations; it is in agreement with a recent analysis by Liehr and Moffitt. Tunnelling between equivalent conformations of benzene is thought to occur at a rate of the order of 109–1010 sec-1. The spectra of the phosphorescent states of triptycene and tribenzotriptycene are dominated by intramolecular excitation transfer between the benzene or naphthalene sub-systems. In triptycene at 77°k transfer occurs at a rate ≫ 1010 sec-1, so that one observes the resonance signal of a triplet exciton. In tribenzotriptycene the transfer is somewhat slower. The rate of excitation transfer decreases with temperature and at 20°k most of the excitation in tribenzotriptycene is effectively trapped in the naphthalene sub-systems.

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