Photochemistry of nitrobenzenethiol. Selective generation of the thio radical and thione triplet state as a function of solvent polarity

Transient absorption spectra of p-nitrobenzenethiol (NBSH) have been measured by the nanosecond laser flash photolysis method. In nonpolar and less polar solvents, nitrobenzenethio radical (NBS.) was predominantly formed by the homolytic fission of the S-H bond of NBSH. In protic polar solvents, on the other hand, formation of the triplet state of deprotonated nitrobenzenethione [(3)(NBT-)*], which was confirmed by triplet quenching experiments with O-2 and beta-carotene, was observed. Quantum yield of intersystem crossing (Phi(T)) and the lowest triplet energy (E-T1) Of (3)(NBT-)* were evaluated to be 0.36 and 58 kcal/mol, respectively, in ethanol. In aprotic polar solvents such as acetonitrile, both (3)(NBT-)* and NBS. were produced in the same time. Electron transfer occurs from donors to (3)(NBT-)* and to NBS.. For an electron acceptor, the electron-transfer reaction takes place from only (3)(NBT-)* and not from NBS.. Thus, NBSH can be used as a selective generator of the thione triplet and thio radical by changing the properties, such as polarity and protic/aprotic character, of the solvents used.