Paramagnetic resonance in phosphorescent aromatic hydrocarbons

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.