CHLOROPHYLL PHOTOCHEMISTRY IN CONDENSED MEDIA—II. TRIPLET STATE QUENCHING AND ELECTRON TRANSFER TO QUINONE IN LIPOSOMES*

Abstract— The fate of excitation energy and electron transfer to quinones within Chl‐a‐containing phosphatidyl choline liposomes has been investigated. The bilayer membrane of the liposome stabilizes the Chl triplet state, as evidenced by a three‐fold increase in the lifetime over that observed in ethanol solution. The relative triplet yield follows the relative fluorescence yield, indicative of quenching at the singlet level. Triplet state lifetimes are markedly shortened as the Chl concentration is increased, demonstrating that quenching occurs at the triplet level as well. This process is shown to be due to a collisional de‐excitation. In the presence of quinones, the Chl triplet reduces the quinone resulting in production of long‐lived electron transfer products. The percent conversion of Chl triplet to cation radical when benzoquinone is employed as acceptor is approximately 60 ± 10%, which is slightly less than in ethanol solution (70 ± 10%). The lifetime of the radical, however, can be as much as 1900 times longer. With respect to potentially useful photochemical energy conversion, the magnitude of this increased lifetime is far more significant than is the decreased radical yield.