Photoreduction of benzophenone by amino acids, aminopolycarboxylic acids and their metal complexes. A laser-flash-photolysis study

Quantitative kinetic data have been obtained, by 337.1 nm laser flash photolysis, for the quenching of benzophenone triplet by several amino acids, aminopolycarboxylic acids and their FeII, CoII and NiII complexes. The efficiency of the resultant photoreduction has been measured in terms of the yield of ketyl radical or radical anion. The amino and aminopolycarboxylic acids (at neutral pH) were better quenchers when complexed with the metal ions. Under basic conditions, for many of the amino acids as well as for triethylamine, the formation of benzophenone radical anion was observed to be a relatively slow process with a time-scale longer than that associated with initial triplet quenching; this secondary reduction has been ascribed to electron transfer from aminoalkyl radicals to the ground state of the ketone. Among a series of metal complexes with a given acid, the triplet quenching rate constants (kq) and the yields (δ) of ketyl radical increased in the general order: NiII⩽ CoII < FeII; this is explained in terms of increasing exothermicity for charge-transfer interaction with the ketone triplet (for kq) and for back electron donation in the photogenerated radical pair (for δ).