Ruthenium(II) charge-transfer sensitizers containing 4,4'-dicarboxy-2,2'-bipyridine. Synthesis, properties, and bonding mode of coordinated thio- and selenocyanates

The synthesis and properties of several complexes of Ru(II) containing 4,4'-dicarboxy-2,2'-bipyridine (dcbpyH(2)), 2,6-bis(1-methylbenzimidazol-2-yl)pyridine (bmipy), or 2,6-bis(1-methylbenzimidazol-2-yl)-4-phenylpyridine (ph-bmipy), and monodentate ligands (X(-) = Cl-, I-, NCS-, NCSe-, CN-) are reported. The introduction of the ambident ligands X(-) = NCS-, NCSe-, and CN- into the coordination sphere of [Ru(bmipy)(dcbpy)I](-) and cis-Ru(dcbpyH(2))(2)Cl-2 has been studied in situ via H-1 and C-13 NMR spectroscopy using C-13-enriched ligands X(-). Introduction of thiocyanate and selenocyanate initially yields the two possible linkage isomers in comparable amounts; prolonged reaction time converts the S-bound isomer and the Se-bound isomer to the N-bound isomers. The isoselenocyanate complex decomposes rapidly, yielding the cyano complex under loss of Se. The N-bound isothiocyanato complex K[Ru(bmipy)(dcbpy)(NCS)] was found to be an efficient sensitizer for nanocrystalline TiO2; the incident monochromatic photon-to-current efficiency (IPCE) is nearly quantitative at 520 nm. Introduction of a phenyl group in the 4-position of the 2,6-bis(1-methylbenzimidazol-2-yl)pyridine ligand gives a red-shifted absorption maximum for the corresponding phenylated K[Ru(ph-bmipy)(dcbpy)(NCS)] complex with an increased molar absorption coefficient for the MLCT maximum at 508 nm. At longer wavelengths above 620 nm, phenyl substitution does not enhance the absorption coefficients of the complex. Compared to that of K[Ru(bmipy)(dcbpy)(NCS)], the performance of the phenylated complex is reduced in a solar cell due to lower IPCE values. The visible spectra of the halide complexes K[Ru(bmipy)(dcbpy)X] (X(-) = Cl-, I-) show enhanced red response, but the complexes exhibit strongly reduced overall IPCE values. A comparison of the complexes to cis-Ru(dcbpyH(2))(2)(NCS)(2) is presented. Possible strategies for the design of more efficient sensitizers are discussed.