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)dcdh ph = 5,8-dicarboxy-6,7-dihydro-dibenzo[1,10]-phenanthroline, and X = Cl-, N CS- or CN-, have been synthesized and spectroscopically characterized. The resulting complexes show a broad and intense metal-to-ligand charge transfe r (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 ci s-Ru(H(2)dcbiq)(2)(NCS)(2) (2) was determined to be 2.9 by the spectrophoto metric method. Photoelectrochemical measurements show that all dyes, when a nchored to a nanocrystalline TiO2 film electrode, present low light-harvest ing efficiencies due to inefficient driving force for electron injection in to the conduction band of TiO2 from their lower energy MLCT band. The photo electrochemical performance of 2 was also investigated on a number of oxide semiconductor thin films such as Nb2O5, ZnO, SnO2 and In2O3. The results s how 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 el ectron injection yield, while a wavelength independent electron injection y ield is observed when dye 2 is anchored to SnO2. These results indicate tha t the lowest excited MLCT state is energetically favorable for electron inj ection into the conduction band of SnO2 but not for TiO2. (C) 2001 Elsevier Science B.V. All rights reserved.