Laser-flash-photolysis study of aliphatic thioketone triplets. Self-quenching and singlet-oxygen sensitization

Abstract

Seven aliphatic thioketones, several of them possessing bicyclo[2.2.1]heptane geometries, have been examined for triplet-state behaviour in benzene using laser excitation into their low-lying ¹(n, π*) band systems (485–490 and 532 nm). In the case of di-t-butylthioketone (DTBTK) a transient absorption (280–500 nm) was attributable to its triplet (ε^T≈ 1.5 × 10³ dm³ mol^–1 cm^–1 at 280 nm). The thioketone triplets were, however, probed in all cases by energy transfer to all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH). Results are presented for triplet lifetimes (0.1–0.6 µs at 1 × 10^–3 mol dm^–3 thioketone concentration), intersystem-crossing yields (0.85–1.0), self-quenching kinetics [(0.1–7)× 10⁹ dm³mol^–1 s^–1] and kinetics of energy transfer to DPH [(6–8)× 10⁹ dm³ mol^–1 s^–1]. The steric crowding at α positions severely hinders the self-quenching interaction in DTBTK. The efficiency of singlet-oxygen formation as a result of DTBTK triplet quenching by oxygen is close to unity (0.85 ± 0.17 in benzene).