Formation of sigma- and pi-type dimer radical cations by the photochemicalone-electron oxidation of aromatic sulfides

The formation of dimer radical cations from aromatic sulfides has been stud ied by photochemical one-electron oxidation in acetonitrile. When dicyanona phthalene and thioanisole in acetonitrile were irradiated with nanosecond l aser flash (308 nm), two types of dimer radical cations were detected at 47 0 and 800 nm at the expense of the monomer radical cation (520 nm). The int ramolecular formation of similar radical ion complexes was observed for the cases of 1,n-bis(phenylthio)alkanes with n = 3 and 4, while bissulfides wi th n = 2, 6, and 8 showed radical cation spectra quite different from the a bove cases of n = 3 and 4. These facts indicate that dimer radical cations absorbing at around 460-500 nm are assigned as the sigma-type complex of th e sulfur-sulfur three-electron bond and that radical cations absorbing at a round 800 nm are of the pi-type complex associated with two phenylthio grou ps. For the case of p-methylthioanisole the formation of pi-type dimer was shown to be reduced owing to the steric hindrance of two methyl groups. No formation of dimer radical cations was observed for cases of p-methoxythioa nisole and diphenyl sulfide where the corresponding monomer radical cations are stabilized by the delocalization of positive charge on the sulfur atom . The density functional BLYP/6-31G* calculations on thioanisole predicted the existence of sigma- and pi-type dimer radical cations, in accordance wi th the experimental observation of approximately equal stability.