A pulse-radiolytic and photochemical study of the oxidation of water by zinc porphyrin π-radical cations

Abstract

π-Radical cations of water-soluble zinc porphyrins have been formed by oxidation with Br^˙–₂ under pulse-radiolytic conditions. These π-radical cations decay with complex kinetics but the rate of decay was enhanced upon addition of colloidal RuO₂·2H₂O. For the π-radical cation derived from zinc tetrakis(4-sulphonatophenyl)porphine (ZnTSPP^4–) the rate constant for decay was a linear function of the concentration of RuO₂·2H₂O and the bimolecular rate constant for this interaction was dependent upon pH. At pH 12, interaction was diffusion controlled but the rate decreased with decreasing pH. The form of this pH dependence is consistent with the π-radical cation oxidising water to O₂ on the surface of the RuO₂·2H₂O colloid. Visible-light irradiation of ZnTSPP^4– in the presence of persulphate and RuO₂·2H₂O resulted in formation of O₂, and both the rate and total yield of O₂ formation were strongly dependent upon pH. Again, the optimum conditions for O₂ formation were found at pH 12 where the quantum efficiency was 50%. No O₂ was formed with zinc tetrakis (N-methyl-4-pyridyl) porphine, which has a much shorter-lived π-radical cation.