The surprisingly small effect of oxygen on photoelectron transfer in pigmented lipid bilayers is traced to a short lifetime of the excited states. Decreasing the oxygen concentration by greater than 100-fold decreases the half saturating concentration of acceptor by only threefold and has no effect on the maximum photovoltage observed at acceptor saturation. This holds true for both magnesium octaethylporphyrin and chlorophyll with both ferricyanide and methyl viologen as acceptors. Since oxygen quenches excited states at near the encounter limit, the lifetime of reactive state must be short, less than 100 ns. About 100-fold higher concentrations of acceptor are required to quench the fluorescence (in liposomes) than to saturate the photoeffect. Thus the reactive state is most likely the triplet. The short life of the excited state is caused by concentration quenching, i.e., their reaction with ground state molecules. The increase of photovoltage with increasing pigment concentration shows that this quenching in a condensed form of the pigment produces ions that lead to the observed photovoltage by interfacial reaction of the anion with acceptor.

The decay kinetics of the photovoltage formed on pulsed illumination of a chlorophyll a- (chl a-) containing lecithin-bilayer adjacent to a ferricyanide solution on one side show characteristics of a system with distributed rate constants, i.e., the decay approaches linearity in log of time. The kinetics can be explained by a distribution of the chl cation over a few angstroms depth in the interfacial region of the bilayer and a rate constant exponentially dependent on distance as expected from tunneling theory. Addition of the donor ferrocyanide both increases the average rate and sharpens the distribution. There is a competitive inhibition by ferricyanide of the reaction of pigment cation with ferrocyanide. Removal of oxygen increases the rate of decay when an acceptor, methyl viologen or anthraquinone-2-sulfonate, forms oxygen-sensitive radicals. The cation charge does not cross the bilayer on a time scale of less than 0.01 s. These data define a reaction localized precisely in the finite interfacial region of the lipid bilayer-water interface.