Photochemistry of manganese porphyrins. Part 8. Electrochemistry

Abstract

Water-soluble manganese(III) porphyrins undergo a well defined, one-electron reduction to the corresponding manganese(II) derivative for which the overall reversibility is very high. The intrinsic electrochemical rate constant (k⁰) was (1.2 ± 0.4)× 10^–3 cm s^–1 and it remained essentially independent of pH. The nature of the porphyrin ligand affected the magnitude of E_½ but caused no significant differences in electrode behaviour. One-electron oxidations were observed for which both E_½ and k⁰ depended markedly upon pH. At pH > 13, oxidation approached the fully reversible case but as the pH was decreased oxidation became progressively more difficult and less reversible. These differences are associated with acid–base equilibria of axially co-ordinated water molecules.