Photoinduced Charge Separation and Recombination in Exciplex Systems in Lipid Bilayer Vesicles

Pulsed laser photolysis experiments are carried out in lipid bilayer vesicles in which the fluorescence of excited pyrene (P), pyrene‐ethanol (P‐CH₂OH) and pyrene‐monosulphonate (P‐SO⁻₃) are quenched by N.N‐diethylaniline (DEA). The observed photochemical patterns are basically different from those encountered in homogeneous solutions: No detectable exciplex emission is observed, and the yield of the separated ions P⁻ + DEA⁺ is small. Two alternative explanations are given for the complete lack of an exciplex fluorescence in the vesicles. One involves the requirement of a highly polar environment around the reacting species in the membrane interior. More plausibly, serious limitations imposed by the “liquid crystal” membrane microenvironment on the relative orientation of the ¹P* acceptor and the DEA donor, must be assumed. The quenching process leads to the formation of the P⁻ + DEA⁺ radical ion pair and of the pyrene triplet state ³P*. An analysis of the submicrosecond ions decay process, including its behaviour around the phase transition point of the lipids, indicates that the ions do not escape the vesicle and recombine by an intravesicle diffusion process. The relatively small yield of P⁻ and DEA⁺ is attributed to a microviscosity effect, inhibiting the primary separation of the solvent‐shared ion pair {P⁻ … DEA⁺}. An analysis of temperature (phase transition) and external magnetic field effects indicates that the pyrene triplet state, ³P*, is generated by geminate recombination in a geometrically correlated radical pair [P⁻ … DEA⁺]. The results bear on the general problem of photoinduced charge separation in nonisotropic media and on the structure of biomembranes.