Redox regulation in ruthenium(II) polypyridyl complexes and their application in solar energy conversion

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Abstract
Ruthenium(II) complexes of the type [Ru(dmbip)(Hdcbpy)X], [Ru(dmbip)(Hdcbiq)X] and [Ru(dhbip)(Hdcbpy)X], where dmbip = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine, dhbip = 2,6-bis(1-hexadecylbenzimidazol-2-yl)pyridine, H2dcbpy = 4,4′-dicarboxy-2,2′-bipyridine, H2dcbiq = 4,4′-dicarboxy-2,2′-biquinoline and X = Cl–, NCS–, CN– or H2O, have been synthesized and spectroscopically characterized. They act as efficient charge-transfer sensitizers, when anchored onto nanocrystalline TiO2 films. The lowest-energy metal-to-ligand charge-transfer transitions in these complexes could be tuned from 500 to 590 nm by choice of appropriate ligands and the highest occupied molecular orbital varied over 400 mV. Some of the complexes reported are emissive at room temperature. The ground- and excited-state pKa values of dcbpy complexes were measured by spectrophotometric and spectrofluorimetric titration. Resonance-Raman spectra show bands characteristic of the dmbip and dcbpy ligand for excitation at 468 nm, while excitation at 568 nm gave predominantly bands associated with the dcbpy ligand. The excited-state pKa values and the resonance-Raman data indicate that the lowest excited state is a metal to dcbpy or dcbiq ligand charge-transfer state.

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