Novel sensitisers for photovoltaic cells. Structural variations of Ru(II) complexes containing 2,6-bis(1-methylbenzimidazol-2-yl)pyridine

A series of new ruthenium(II) complexes was synthesised for the application in photoelectrochemical solar cells based on dye-sensitised nanocrystalline titanium dioxide. Design strategies for the development of new dyes were tested via structural variations of the prototype complex K[Ru(II) (bmipy) (4,4'-dcbpy)(NCS)] (7) with bmipy=2,6-bis(1-methyl-benzimidazol-2-yl)pyridine, and 4,4'-dcbpy = 2,2'-bipyridine-4,4'-dicarboxylate. A pi* level tuning was performed by replacing 4,4'-dcbpy with the low pi* level ligands dcbiq (2,2'-biquinoline-4,4'-dicarboxylate) and 5,5'-dcbpy. The resulting complexes showed decreased light-to-electric energy conversion efficiencies. K[Ru(II)(bmipy)(4-PO(3)bpy)(NCS)] (4-PO(3)H(2)bpy=2,2'-bipyridine-4-phosphonic acid) also sensitised TiO2 less efficiently than the model compound. The occurrence of two isomers was observed for the complexes containing 4-PO(3)bpy. In Na[Ru(II)(bmipy)(4,4'-dcbpy)X] with X = substituted phenylcyanamide (pcyd(-)) anions, the influence of substitution on the phenyl group was investigated. MLCT absorption maxima of the phenylcyanamide complexes at around 510 nm were shifted to lower energies in comparison with the model complex, however photoenergy conversion efficiencies were reduced. When introduced into the complex, phenylcyanamide was coordinated via the nitrile or the amido nitrogen. With prolonged reaction time, the amido-bound isomer was partially transformed into the thermodynamically more stable nitrilo-bound isomer. Linkage isomerism of coordinated NCS- and 4-Clpcyd- was studied with multinuclear NMR (H-1, C-13, P-31) spectroscopy and C-13-labelled ligands. Prospects for a substantial improvement of Ru(II) polypyridyl sensitisers for TiO2 are discussed.