Resonance stabilisation of zinc porphyrin π-radical cations

Abstract

Pulse radiolytic oxidation of zinc tetrakis(hydroxyphenyl)porphyrins in neutral solution results in formation of the π-radical cations which decay via disproportionation over a few ms. In alkaline solution, the hydroxy groups are dissociated and for the ortho and meta isomers the rate of disproportionation is reduced relative to neutral solution due to Coulombic repulsion between the ions. In contrast, the para isomer shows a very long-lived π-radical cation (t_1/2≈ 5 s) in alkaline solution. This arises from resonance stabilisation associated with the ability of the substituent to push charge onto the porphyrin ring. The ortho isomer does not exhibit this effect owing to steric hindrance.