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(H2dcbiq)2X2 and cis-Ru(H2dcdhph)2X2, where H2dcbiq=4,4′-dicarboxy-2,2′-biquinoline, H2dcdhph=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 pKa value of cis-Ru(H2dcbiq)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 (Jsc) 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.