Highly efficient quantum conversion at chlorophyll a–lecithin mixed monolayer coated electrodes

Abstract

THE design of solar conversion systems based on the photosynthetic primary reactions has attracted much attention recently. For instance, photoelectrolysis at a chlorophyll (Chl) a-water aggregate coated platinum electrode as a photocathode has been studied by Fong et al.^1–3, and simulation of the photoinduced electron transfer across the thylakoid membrane as well as of the biological ordered structure in a chloroplast has been attempted by Tien et al.^4,5 using a Chl a-containing bilayer lipid membrane (BLM). We have already attempted to combine these different approaches⁶ by using a Chl a monolayer coated SnO₂ transparent electrode as a photoanode, and a maximum photocurrent quantum efficiency of 12–16% was attained with Chl a-stearic acid mixed monolayer systems. In that case, however, a decrease of the quantum efficiency was observed at Chl a-stearic acid molar ratios <1.0; this might be due to the formation of pheophytin a and/or to a possible heterogeneity of the mixed monolayers. We describe here how we have overcome such problems by using a phospholipid instead of the fatty acid as a diluent for a Chl a monolayer. The hydrophobic interactions between Chl a and phospholipids have been well investigated both in vivo and in vitro, and the physical stability and photochemical activity of Chl a-synthetic lipid mixed layers were also confirmed in the BLM study⁷.