| Abstract: | Abstract— Twelve viologens (quaternary 4,4′-bipyridinium ions) are investigated as electron acceptors in vesicle suspensions containing chlorophyll-a (Chl) using laser flash photolysis. The structures of the viologens, which determine how they and their reduced radicals partition between the water and bilayer membrane phases, are systematically varied in order to probe the effects of acceptor solubility on the kinetics of photosensitized electron transfer reactions. The effectiveness of the viologens as quenchers of the Chl triplet excited state increases as they become less soluble in water, because the viologens are more likely to be incorporated into the membrane, but the efficiency of radical formation decreases and the rate of the reverse electron transfer reaction increases. The data obtained with a homologous series of di-alkyl viologens are analyzed quantitatively using a kinetic model which includes the partition coefficients of the viologens and their radicals. For the most part, the reactivities of the viologen species are proportional to their concentrations in the membrane phase, but the locations and mobilities of the viologens in the membrane phase also have to be considered. In order for the Chl and viologen radicals to separate efficiently. the viologen radical has to be able to diffuse from the membrane to the water phase at a rate that competes effectively with reverse electron transfer within the radical pair complex (> 105 s?1). |
