Photoredox properties of zinc porphyrin–viologen complexes

Abstract

Ground-state complexation occurs between zinc porphyrins and viologens in aqueous solution, and in cases where there is strong electrostatic attraction between the reactants the stability constants are high. This complexation ensures that static effects far outweigh dynamic effects in fluorescence quenching experiments, but direct excitation of the complexes does not result in the formation of redox-ion products. Both singlet and triplet excited states of the porphyrin are quenched when the viologen is covalently linked to the chromophore by a short, flexible chain. The quenching efficiencies depend upon chain length and solvent. In alcohol solvents, redox-ion products are observed by flash-photolysis techniques but there is no real evidence to show that the singlet excited state is capable of charge separation under such conditions.