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