LIGHT-INDUCED HYDROGEN-PRODUCTION USING WASTE COMPOUNDS AS SACRIFICIAL ELECTRON-DONORS

Systems for the conversion and chemical storage of solar energy are us ually based on photo-induced electron transfer reactions from an excit ed sensitizer to an electron acceptor (i.e. an electron relay compound ). We have investigated the photo- and electrochemical properties of t wo novel electron relay compounds: 1,1'',1'''-trimethyl[4,2'; 4',4''; 6',4''']quaterpyridinium trichloride (1) and 1,1',1'',1'''-tetramethyl [4,2'; 4',4''; 6,4''] quaterpyridinium tetrachloride (2). When solutio ns containing Ru(bpy)3Cl2 or Zn-porphyrin (as photosensitizer), compou nd 1 or 2 and EDTA (as sacrificial electron donor) are irradiated by v isible light (lambda > 400 nm), only the formation of reduced relay co mpound 1 occurs. Addition of platinum catalyst to such solutions with compound 1 leads to the formation of hydrogen. Quantum yields are in t he range of 5%, calculated for absorbed light. Using other sacrificial electron donors such as alcohols or glucose (or waste compounds like 4-chlorophenol) did not result in hydrogen evolution. However, 1 and 2 are reduced by these sacrificial electron donors in the absence of an additional photosensitizer, when near UV irradiation light (lambda > 280 nm) is used. Quantum yields for hydrogen production with compound 1 are about 2% calculated for absorbed light out of GC-measurements fr om H2 in the gas phase above the irradiated solution. The photo- and e lectrochemical properties of compounds 1 and 2 are discussed.