Ruthenium Complexes of 2-(2-Pyridyl)benzimidazole as Photosensitisers for Dye-Sensitized Solar Cells

N-Alkylated carboxylic acid derivatives of 2-(2'-pyridyl)benzimidazole (pbimH) with different chain-lengths (pbim(CH)(n)(COH)-H-2 where n = 1-3) and their ruthenium complexes [Ru(bpy)(2)(pbim)](PF6)(2) have been synthesized and characterized. 2D COSY and NOESY NMR spectroscopy were used to aid the assignment of the pbim NMR spectrum. The effect of chain-length on the cyclic voltammetry (CV) was studied and the voltammetry of the parent pbimH complex was re-investigated. The ability of the carboxylic acid groups to bind to TiO2 coated electrodes was confirmed by the observation of a symmetrical, surface-confirmed Ru-III/II wave, while the specular reflectance IR revealed a band at 1620 cm(-1) due to the bound carboxylate (COO ... Ti) group. The efficiencies of solar cells using these sensitizers were rather low, due to the distance between the sensitizer and the surface and the inefficient coupling of the charge-separated excited state to the surface. A fall in the cell open-circuit voltage with chain length reflected this distance effect. Time-resolved luminescence spectroscopy indicated that rapid electron injection into the TiO2 conduction band was occurring (<30 ns), but this is not fast enough to compete effectively with alternative excited state processes.