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.