Influence of electrostatic forces upon the efficiency of charge separation for the zinc porphyrin/methyl viologen system

Abstract

The bimolecular rate constant for quenching the triplet excited state of a metalloporphyrin by methyl viologen depends upon the electronic charge on the porphyrin ring. The rate constant increases with increased electrostatic attraction between the reactants and this is due to efficient formation of an encounter complex and a high thermodynamic driving force for formation of an ion-pair. However, dissociation of the ion-pair to separate ion products is favoured by strong electrostatic repulsion between the products due to electrostatic factors. In addition the yield of redox products depends upon the amount of electronic energy that must be dissipated in the form of heat, and the rate of internal conversion of the ion-pair decreases significantly with increased energy.