Quantitative description of the deactivation channels of the first excited singlet state of 2- and 4-thiosalicylic acids.

On the bases of picosecond and nanosecond laser flash photolysis with detection by emission and absorption spectroscopy, a quantitative description is given of all of the deactivation channels of the first excited singlet state of 2- and 4-thiosalicyclic acids (TS) such as fluorescence, intersystem crossing (ISC), chemical dissociation into radicals, and radiationless internal conversion (IC). For this purpose, the investigated compounds were studied in solvents of increasing polarity: 1-chlorobutane, acetonitrile, ethanol, and water. As an exception for thiols, and in contrast to recent findings on thiophenol and its methyl- and methoxy-substituted derivatives, the photoinduced first excited triplet state of the thiosalicyclic acids was directly observed by its T1-T(n) absorption spectra and characterized by sensitization experiments. According to the direct determination of the quantum yields of all deactivation channels of the first excited singlet state of 2- and 4-thiosalicylic acid isomers, it was found that the compounds studied have small fluorescence quantum yields (phi(F) = 0.003-0.009) and higher ones for triplet formation (phi(T) = 0.10-0.35) and S-H photodissociation (phi(D) = 0.05-0.15). As a difference to the total, the radiationless internal conversion (phi(IC) = 0.6-0.8) was found to be the dominating process.