Photochemistry of Nitrobenzenethiol. Selective Generation of the Thio Radical and Thione Triplet State as a Function of Solvent Polarity

Abstract

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 O2 and β-carotene, was observed. Quantum yield of intersystem crossing (ΦT) and the lowest triplet energy (ET1) 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.