Photochemistry of aromatic thiones in their S2 and T1 states. Role of triplet self-quenching and excimer formation

Abstract

The quantum yields of photodecomposition of xanthione (XT), 4H- 1-benzopyran-4-thione (BPT) and 4H-pyran-4-thione (PT) subsequent to excitation to their S2 and S1 states have been measured as a function of thione concentration in inert perfluoroalkane solvents. The quantum yields of intramolecular photodecomposition of these thiones in S2 are immeasurably small. Intermolecular processes dominate the photochemical removal of the triplet states even at very low ( < 10–5 mol dm–3) thione concentrations. In inert perfluoroalkane solvents a photochemical channel in the diffusion-controlled triplet self-quenching process is responsible for the removal of the thione. A mechanism involving the intermediacy of triplet excimers is proposed.