Panchromatic sensitization of nanocrystalline TiO2 with cis-bis(4-carboxy-2-[2′-(4-carboxypyridyl)]quinoline)bis(thiocyanato-N)ruthenium(II)

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; H(2)dcpq = 4-carboxy-2-[2'-(4'-carboxypyridyl)]quinoline) with those of TiO2 sensitized with [NBu4](2)[cis-Ru(HdcbPY)(2)(NCS)(2)] (2; H(2)dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) and [NBu4](2)[cis-Ru(Hdcbiq)(2)(NCS)(2)] (3; H(2)dcbiq = 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 (-DeltaG(inj)) of the electron injection from the ruthenium complex to TiO2. The open-circuit photovoltages (V-oc'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 I-3(-). The sensitizer-dependent V-oc value can be interpreted as a result of reverse electron transfer through the sensitizing dye molecules.