Triplet-state photophysics and transient photochemistry of cyclic enethiones. A laser flash photolysis study

The triplets of four cyclic enethiones, including thiocoumarin, have been investigated by nanosecond laser flash photolysis. Data are presented for transient spectra and kinetics associated with triplets, quantum yields of intersystem crossing and singlet oxygen photosensitization. The quenching of the thiocoumarin triplet (λTmax= 485 nm, εTmax= 8.8 × 103 dm3 mol–1 cm–1 in benzene) by several olefins, amines and hydrogen donors occurs with rate constants of 107–5 × 109 dm3 mol–1 s–1; the lower limits of quantum yields (ϕPC) for the related photoreactions, estimated from ground-state depletion, are generally small (0.0–0.11 in benzene, except for good hydrogen donors, namely, p-methoxythiophenol and tri-n-butylstannane). The radical anion of thiocoumarin (λmax= 405–435 nm) is formed in two stages upon triplet quenching by triethylamine in acetonitrile; the fast component is the result of direct electron transfer to the triplet and the slower component is assigned to secondary photoreduction of the thione ground state by the α-aminoalkyl radical derived from the triethylamine radical-cation.