A reinvestigation of the mechanism of photoreduction of benzophenones by alkyl sulfides

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The photoreduction of benzophenone and 4-carboxybenzophenone by dimethyl sulfide was examined in aqueous, mixed water—acetonitrile (1:1v/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)×109 M−1s−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∴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 (Φtotalketyl=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.