Asymmetrically organized dihexadecylphosphate (DHP) vesicles containing inorganic photosensitizers bound to the outer surface and methyl viologen dication, an electron acceptor, bound to the inner surface were prepared by chromatography on strong-acid cation-exchange resins or dextran gels. The latter method proved superior because destabilizing medium conditions arising during manipulative procedures could be avoided. Vesicle integrity was confirmed by photon correlation spectroscopy, optical spectroscopy, and ultrafiltration measurements. Continuous illumination with visible light in the presence of external electron donors gave rise to formation of reduced viologen radical; the rate and extent of reduction were markedly enhanced by the presence of hydroxylic amine buffers in the aqueous medium. Examination of vesicle suspensions after illumination by addition of the DHP membrane-impermeable reductant, sodium dithionite, revealed that extensive diffusion of MV2+ from the inner to outer vesicle surface had occurred. Below 30 °C passive diffusion of MV2+ across the bilayer does not occur at an appreciable rate, but above 30 °C in the presence of the amine buffers, transmembrane diffusion is rapid. The mechanism for viologen reduction appears to involve photoinduced transmembrane diffusion of acceptor followed by reaction with photosensitizer bound to the same vesicle surface.
ASJC Scopus subject areas
- Colloid and Surface Chemistry