Anchoring group and auxilary ligand effects on the binding of ruthenium complexes to nanocrystalline TiO₂ photoelectrodes

The thermodynamics and kinetics of binding to nanocrystalline TiO ₂ were investigated for four ruthenium complexes with different numbers of possible anchoring groups and auxiliary ligands. Infrared spectroscopic data indicated that the dyes bound in a bridging mode with the oxygen atoms bound to separate titanium atoms. Complexes with only one or two anchoring groups used all of their carboxy groups to bind. Complexes having four or six linkers, used two groups in binding. The dyes yielded similar maximum coverages on TiO₂, but different binding constants and desorption kinetics. The binding constant for the monocarboxy dye was lower than for the others. The adsorption rate constants were all similar, suggesting that formation of the first bond to TiO₂ was rate limiting. Binding of the dyes from a buffered ethanol solution yielded lower coverages than from neat ethanol, although binding constants increased up to 100 times, indicating competition for, and/or deactivation of, TiO₂ sites. For the monocarboxy dye, the integrated quantum yield was lowest, and electron transfer between TiO₂ and the redox couple was most facile.