Type-I and type-II photoprocesses in the system photosense–ascorbic acid

It was found significant difference in reactivity of undissociated ascorbic acid AH2 and its monoanion AH− towards singlet oxygen and in electron transfer to excited photosense. Kinetic analysis indicated that interaction of ascorbate with singlet oxygen in aqueous media results in quenching of the latter with rate constant (kq+kr)=(6.8±1)×108 mol−1 l s−1, where chemical quenching rate constant kr is 1.4×108 mol−1 l s−1. The value of kr for AH2 is a factor of about 20 lower. The similar trend was observed for ascorbic acid oxidation in electron transfer to excited photosense. The corresponding rate constants were established for ascorbate AH− and were found to be: for total quenching (kq+kr) is of order 107 mol−1 l s−1 and for radicals photogeneration kr=(2.5±0.5)×103 mol−1 l s−1. Analysis of the photosense–ascorbic acid system had revealed the low efficiency of type I pathway, initiated by electron transfer from ascorbate to excited photosense, in comparison to efficiency of the route, mediated by singlet oxygen.