Thiocarbamic acid metal complexes

The proven, and thus preferred, general structure for sensitizers is ML2(X>2, where M can be Ru or Os, L is 2,2 -bipyridyl-4,4 -dicarboxylic acid, and X represents a halide, cyanide, thiocyanate, acetyl acetonate, thiocarbamate, or water subsistent group [29]. The stfuctures of metal complexes used as sensitizers can also be mononuclear metal complexes (Figs. 38.2, 38.3, and 38.4a) [6,41, 18], binuclear metal complexes [Ru-Ru (Fig. 38.4b) [30], and Ru-Os (Fig. 38.4c)] complexes [30]. Polynuclear complexes have been employed in order to increase absorption coefficients. However, these bulky sensitizers require more space on the Ti02 surface and penetrate less easily in the small cavities of the nanocrystalline TiOj than the mononuclear complexes [34]. Hence, for polynuclear complexes, the increased absorption coefficients in solution do not necessarily lead to enhanced light absorption on the Ti02 electrode because of the reduced surface concentration of the bulkier sensitizer molecules on the nanoporous Ti02. 

An effective group of compounds are those which can decompose peroxides catalytically, so one molecule may destroy very many peroxide molecules. Typical examples are the thiodipropionate esters ROCOCH2CH2SCH2CH2COOR where R = C12H25, or CigH37, and the metal salts of thiocarbamic acids. The way in which these additives work is very complex indeed (Scott, 1983), involving oxidation of the sulfur by reaction with hydroperoxides. This in turn liberates sulfm aeids, powerful catalysts for decomposition of peroxides by ionic mechanisms. Overall, the effect is that each molecule of antioxidant destroys several molecules of peroxide, while itself being destroyed. 

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