Triplet-state radical pairs benzophenone, photoreduction

Triplet-State Radical Pairs from the Photoreduction of Benzophenone by Hydrogen Donors... 

Aliphatic sulfides can be efficient co-initiators for the photoinduced polymerization induced by benzophenone [185, 186]. An exceptionally strong effect was observed for 2,4,6-trimethyl-1,3,5-trithiane (TMT). A model reaction for free-radical formation during photoreduction of an initiator triplet state by a sulfide is the photoreduction of benzophenone by dimethyl sulfide [171, 187-189]. In this process it was established that electron transfer from the sulfur atom to the triplet state of the benzophenone is a primary photochemical step. In this step, radical ions are formed. The overall quantum yields of photoproducts (ketyl radicals and radical anions) are low (Ed) 0.26) in aqueous solution, in the range 0.16-0.20 in mixed water-acetonitrile solution and less then 0.01 in pure acetonitrile. These results suggest that, in organic solvents, back electron transfer within the radical-ion pair to regenerate the reactants is the dominant process. 

The photoreduction of benzophenone (B) in solution is one of the most extensively investigated photochemical reactions, having been studied using a wide range of reductants, solvents, reactant concentrations, and irradiation conditions. In the presence of H-donating substrates, the triplet excited state of B is able to abstract an H-atom directly (Equation 13.17) or via a sequential electron/H transfer mechanism (Equation 13.18) to produce triplet radical pairs, BHR. 

A different type of photoreaction in ILs has been studied by Jones and co-workers the photoreduction of benzophenones by primary amines. Prior work by Cohen demonstrated that photolysis of benzophe-none in benzene in the presence of sec-butylamine afforded benzopinacol (and an imine). This reaction proceeds through a radical pair formed by hydrogen-atom abstraction by the triplet excited state ben-zophenone from the amine. In the much more polar environment of an IL, the radical pair may instead undergo single electron transfer to form an iminium cation and a hydroxyl-substituted carbanion. Proton transfer from the cation to the anion will yield benzhydrol and an imine. 

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