Dynamics of Photoinduced Charge Separation on the Surface of Dihexydecylphosphate Vesicles

One-electron oxidation of triplet-photoexcited [5,10,15,20-tetrakis( N -methylpyridinium-4-yl)porphinato]zinc(II) ( 3 ZnTMPyP 4+ ) by N -alkyl-4-cyanopyridinium (C n CP + ) ions on an anionic dihexadecylphosphate (DHP) vesicle surface was studied by transient spectrophotometry. Surface rate constants for both oxidative quenching and charge recombination between the photoproducts, ZnTMPyP 5+ and C n CP 0, were insensitive to the alkyl chain length of the electron acceptor over the range n = 6-16 carbon atoms, suggesting that the rate was determined primarily by the lateral mobility of the metalloporphyrin. Interfacial reductive quenching of DHP vesicle-bound 3 ZnTMPyP 4+ to ZnTMPyP 3+ by dithiothreitol was 10 3 -fold slower than the corresponding reaction in homogeneous solution, an effect that was attributed to stabilization of the more highly charged tetracation by the anionic interface. The C n CP 0 radicals functioned as electron carriers to mediate photoinitiated transmembrane reduction of Co(bpy) 3 3+ by dithiothreitol; rate constants for transmembrane C n CP 0 diffusion, evaluated from the quantum yields for Co(bpy) 3 3+ reduction, were also insensitive to the alkyl chain length and had characteristic times of ∼3 × 10 -2 s. These values were nearly identical to those obtained for the same reaction measured in another photoreactive system where the C n CP 0 radicals were generated in the bulk aqueous phase, rather than on the membrane surface. The quantum efficiency for transmembrane redox was relatively low, primarily because charge recombination of the photoredox products on the vesicle surface was rapid relative to either transmembrane diffusion of C n CP 0 or interfacial reduction of ZnTMPyP 5+ by dithiothreitol.