A REINVESTIGATION OF THE MECHANISM OF PHOTOREDUCTION OF BENZOPHENONESBY ALKYL SULFIDES

The photoreduction of benzophenone and 4-carboxybenzophenone by dimeth yl 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 t o be in the range (1.5-4.6) X 10(9) M-1 s-1. Electron transfer from th e sulfur atom to the triplet state of the benzophenones was found to b e a primary photochemical step. This was established by the large valu es 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), suggest ing that back electron transfer within the charge-transfer complex to regenerate the reactants is the dominant process.