Aromatic thioketone triplets and their quenching behaviour towards oxygen and di-t-butylnitroxy radical. A laser-flash-photolysis study

Abstract

Results obtained using nanosecond laser flash photolysis with various exitation wavelengths (337.1–600 nm) are reported for triplet-related properties of five aromatic thioketones, namely xanthione, thioxanthione, thiobenzophenone and p,p′-dimethoxy and p,p′-bis(N,N-dimethylamino) derivatives of thiobenzophenone. The triplet yields (ϕ_T) of these thioketones in benzene are high (0.5–1.0) and show a dependence on excitation wavelengths. The intersystem-crossing efficiency is less than unity (0.5–0.6) when laser excitation is carried out into the second excited singlet state, S₂(λ_ex= 355 and 425 nm), but approaches unity when the excitation leads to absorption into the lowest excited singlet, S₁(532 and 600 nm). The intrinsic triplet lifetimes are short (0.8–1.8 µs) and the self-quenching rate constants are in the range (2.6–7.1) 10⁹ dm³ mol^–1 s^–1. Quantitative data concerning triplet–triplet absorption spectra and triplet quenching by azulene, ferrocene, β-carotene and 2,5-dimethyl-2,4-hexadiene are presented. The oxygen quenching rate constants [(2.8–9.7)× 10⁹ dm³ mol^–1 s^–1] increase when electron-donating groups (methoxy and N,N-dimethyl-amino) are present in the thiobenzophenones, suggesting that charge-transfer interaction is important. The efficiency of singlet-oxygen generation in the course of the oxygen quenching of the p,p′-dimethoxythiobenzophenone triplet is unity. The stable free radical, di-t-butyl-nitroxide, quenches the thioketone triplets with unusually high rate constants [(1.4–3.3)× 10⁹ dm³ mol^–1 s^–1]; this behaviour appears to be a manifestation of electron-exchange-mediated intersystem crossing (S₀ [graphic omitted] T₁), enhanced by strong spin–orbit coupling or spin–spin interaction associated with the thiocarbonyl sulphur atom.