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 (Îμ ^τ≈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 a 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).