A REINVESTIGATION OF THE MECHANISM OF PHOTOREDUCTION OF BENZOPHENONES BY ALKYL SULFIDES

The photoreduction of benzophenone and 4-carboxybenzophenone by dimethyl sulfide was examined in aqueous, mixed water-acetonitrile (1:1 v/v) and acetonitrile solutions by the use of nanosecond laser photolysis. Bimolecular quenching rate constants were determined and were found to be in the range (1.5-4.6) X 10(9) M-1 s-1. Electron transfer from the sulfur atom to the triplet state of the benzophenones was found to be a primary photochemical step. This was established by the large values of quenching rate constants and by the observation of free radical ions, i. e. ketyl radical anions and (S therefore S)+ radical cations of dimethyl sulfide in aqueous and mixed water-acetonitrile solutions. The overall quantum yields of photoproducts (ketyl radicals and ketyl radical anions) are low (PHI(ketyl)total = 0.26 in aqueous solutions, are in the range 0.16-0.20 in mixed water-acetonitrile solutions, and decrease to less than or equal to 0.01 in pure acetonitrile), suggesting that back electron transfer within the charge-transfer complex to regenerate the reactants is the dominant process.