Photosensitised electron transport across phospholipid vesicle walls

Abstract

AS a means of solar energy conversion, we are interested in the photosensitised decomposition of water with pigmented lipid bilayer membranes that have O₂- and H₂-evolving catalysts at opposing membrane surfaces. Our present work toward this goal involves the investigation of light-induced electron transfer reactions between aqueous solutions of electron donors and acceptors that are on opposite sides of single-lamella liposomes (vesicles) with incorporated surfactant photosensitisers. Related experiments with planar bilayer membranes¹ or liposomes^2–4 using chlorophyll as the photosensitiser have been reported, but it is not clear that energy storage has been achieved in these systems. A possible exception is the photo-oxidation of water with vesicles that contain chlorophyll and trapped aqueous ferricyanide³, although attempts to duplicate this experiment have been unsuccessful⁵. We report here that we have observed the photosensitised reduction of methyl-viologen (1,1′-dimethyl-4,4′-bipyridinium²⁺, abbreviated to MV²⁺) dissolved in the exterior aqueous phase of a phospholipid vesicle dispersion containing a surfactant analogue of tris-(2,2′-bipyridine)ruthenium²⁺ when ethylenediamine-N,N,N′,N′-tetraacetate (EDTA) is dissolved in the enclosed aqueous phase of the vesicles.