Upon laser pulse excitation (λex = 532 nm) into the lowest-lying 1(n,π*) band system, pivalothiophenones in benzene solutions give rise to short-lived triplets (λmaxT = 325-335 nm, εmaxT = (11-15) × 103 M-1 cm-1) with quantitative intersystem crossing efficiencies. The triplet yields decrease slightly (by 10-30%) upon changing λex to 308 nm (i.e., upon excitation into S2). Kinetic data are presented for intrinsic triplet lifetimes, self-quenching, and quenching by oxygen, di-tert-butylnitroxy radical, and various reagents capable of interacting with the triplets via energy, electron, or hydrogen-atom transfer and by biradical formation (possibly leading to cycloaddition). The mechanisms of the quenching processes are discussed. Relative to rigid aromatic thiones, namely, xanthione and thiocoumarin, the interaction of pivalothiophenone triplets with most of the quenchers are kinetically inefficient. This is interpreted primarily as a manifestation of the steric crowding at positions α to the thiocarbonyl group.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry