Panchromatic Sensitization of Nanocrystalline TiO2 with cis-Bis(4-carboxy-2-[2‘-(4‘-carboxypyridyl)]quinoline)bis(thiocyanato-N)ruthenium(II)

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Abstract
We compared the spectral (IR and Raman), electrochemical, and photoelectrochemical properties of nanocrystalline TiO2 sensitized with the newly synthesized complex [NBu4]2[cis-Ru(Hdcpq)2(NCS)2] (1; [NBu4]+ = tetrabutylammonium cation; H2dcpq = 4-carboxy-2-[2‘-(4‘-carboxypyridyl)]quinoline) with those of TiO2 sensitized with [NBu4]2[cis-Ru(Hdcbpy)2(NCS)2] (2; H2dcbpy = 4,4‘-dicarboxy-2,2‘-bipyridine) and [NBu4]2[cis-Ru(Hdcbiq)2(NCS)2] (3; H2dcbiq = 4,4‘-dicarboxy-2,2‘-biquinoline). Complex 1 achieved efficient sensitization of nanocrystalline TiO2 films over a wide visible and near-IR region, generating a large short-circuit photocurrent. The absorbed photon-to-current conversion efficiency decreased in the order 2 > 1 > 3 with the decrease in the free energy change (−ΔGinj) of the electron injection from the ruthenium complex to TiO2. The open-circuit photovoltages (Voc's) of dye-sensitized solar cells decreased in the order 2 > 1 > 3 with the increase in the dark current resulting from reverse electron transfer from TiO2 to I3-. The sensitizer-dependent Voc value can be interpreted as a result of reverse electron transfer through the sensitizing dye molecules.