Formation and decay of zinc tetrakis (N-methyl-3-pyridyl)porphine π-radical cation in water

Abstract

Zinc meso-tetrakis(N-methyl-3-pyridyl)porphine [ZnTMPyP(3)⁴⁺] is readily oxidised to the π-radical cation under pulse radiolytic ([KBr]= 10^–2 mol dm^–3) or photochemical ([S₂O^2– ₈]= 10^–3 mol dm^–3) conditions. The π-radical cation undergoes bimolecular disproportionation to form a porphyrin π-dication which reacts with hydroxide ions to form an isoporphyrin (pH < 7) or a 5,6-dihydroxyporphyrin (pH > 7). This disproportionation reaction can be catalysed by colloidal RuO₂·2H₂O and Prussian Blue, but at pH > 7, the catalysed reaction leads to competitive formation of a ring-opened product. Photolysis (λ > 420 nm) of ZnTMPyP(3)⁴⁺ in water containing persulphate and catalyst does not result in generation of detectable amounts of O₂.