Zinc porphyrin π-radical cations in aqueous solution. Formation, spectra and decay kinetics

Abstract

Zinc porphyrins are oxidised readily to the π-radical cation by certain inorganic and organic radicals in aqueous solution under pulse-radiolytic conditions. Bimolecular rate constants for the oxidation process depend upon thermodynamic driving forces and Colulombic interactions between the reactants and lie within the range 10⁷–10¹⁰ dm³ mol^–1 s^–1. The resultant π-radical cations exist in aqueous solution in equilibrium with any anions or complexing agents (e.g. pyridine) present in the solution. This complexation affects both the absorption spectrum of the π-radical cation and its rate of decay. With zinc tetrakis(4-sulphonatophenyl)porphyrin the π-radical cation is quite persistent (t_1/2≈ 6 s) but its rate of decay increases when SCN^– ions are present. Positively charged zinc porphyrins form shorter-lived π-radical cations that decay on the microsecond timescale. Under some conditions this decay leads to the formation of a relatively long-lived intermediate which is either a π-radical dimer or an isoporphyrin derivative.