Some photochemical and photophysical properties of a group of xanthene dyes have been studied in relation to their roles as sensitizers for the photoreduction of water. New spectroscopic and kinetic measurements have been carried out with these dyes. The triplet states of the dyes undergo energy-transfer and electron-transfer, but with rate constants differing by two orders of magnitude in favour of the former pathway. The very efficient transfer of triplet energy from a group of dyes to an acceptor molecule led to the design of a greatly improved system for the photochemical production of hydrogen.

Optical spectroscopy and cyclic voltammetry were used to study the monomer-dimer equilibrium of the one-electron reduction products of methyl viologen (MV2+), 1-methyl-1'-(3-sulfonatopropyl)-4,4'bipyridinium (MPVS+), and l,l/-bis(sulfonatopropyl)-4,4'-bipyridinium (SPV) in Nafion films deposited on SnOs electrodes. In Nafion the equilibrium constants are considerably smaller than in water and they are affected by the viologen loading.

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Energy transfer in monolayer assemblies has been studied with bis(2,2'-bipyridyl)(dioctadecyl 2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(~~) perchlorate (1) as acceptor and bis(2,2'-bipyridy1)-(4,4'-dieicosyl-2,2'-bipyridyl)ruthenium(~~) perchlorate (2), bis(2,2'-bipyridyl)(4-eicosyl-4'-methyl-2,2'-bipyridyl)ruthenium(~~) perchlorate (3), or bis(o-phenanthroline)(4,4'-dioctadecyl-2,2'-bipyridyl)ruthenium(11) (4) as donor. Forster theory was used to analyse transfer rate constants, obtained by measuring the reduction in intensity of the donor luminescence in the presence of acceptors. This luminescence quenching involves energy transfer and not electron transfer because of the sensitized luminescence intensity of the acceptor.The efficiency of energy transfer depends on the relative orientations of the alkyl chains. The observed rates of transfer greatly exceed the efficiency predicted from the Forster theory for the distance between the chromophores; it is suggested that the alkyl chains take part in the transfer of excitation energy. This effect can be largely suppressed by the insertion of a 'spacer' monolayer of cadmium arachidate between donor and acceptor layers.

The light-induced formation of hydrogen in the system H2O-Ru(bpy)32+-mv2+-edta-Pt (mv, inethylviologen) is accompanied by the hydrogenation of mv2+ to give the 1-methyl-4-(1'-methylpiperidin-4'-yl)pyridinium cation (1). The production of hydrogen is decreased by factors that favour the formation of this cation. The extent of hydrogenation depends on the pH of the mixture, the amount and type of catalyst present, and the partial pressure of hydrogen during irradiation.