Synthesis and photophysical properties of ruthenium(II) charge transfer sensitizers containing 4,4′-dicarboxy-2,2′-biquinoline and 5,8-dicarboxy-6,7-dihydro-dibenzo[1,10]-phenanthroline

Ruthenium (II) complexes of the type cis-Ru(H(2)dcbiq)(2)X-2 and cis-Ru(H(2)dcdhph)(2)X-2, where H(2)dcbiq = 4,4'-dicarboxy-2,2'-biquinoline, H(2)dcdhph = 5,8-dicarboxy-6,7-dihydro-dibenzo[1,10]-phenanthroline, and X = Cl-, NCS- or CN-, have been synthesized and spectroscopically characterized. The resulting complexes show a broad and intense metal-to-ligand charge transfer (MLCT) band in the visible region with a peak between 580 and 700 nm and are emissive at room temperature. The ground-state first pK(a), value of cis-Ru(H(2)dcbiq)(2)(NCS)(2) (2) was determined to be 2.9 by the spectrophotometric method. Photoelectrochemical measurements show that all dyes, when anchored to a nanocrystalline TiO2 film electrode, present low light-harvesting efficiencies due to inefficient driving force for electron injection into the conduction band of TiO2 from their lower energy MLCT band. The photoelectrochemical performance of 2 was also investigated on a number of oxide semiconductor thin films such as Nb2O5, ZnO, SnO2 and In2O3. The results show that a high value of short-circuit photocurrent (J(sc)) is observed for the semiconductors having a low-energy conduction band potential (SnO2 and In2O3). In the dye 2-sensitized TiO2 film, the absorbed photon-to-current conversion efficiency (APCE) spectrum shows an absorption band selective electron injection yield, while a wavelength independent electron injection yield is observed when dye 2 is anchored to SnO2. These results indicate that the lowest excited MLCT state is energetically favorable for electron injection into the conduction band of SnO2 but not for TiO2. (C) 2001 Elsevier Science B.V. All rights reserved.