Electrochemical and photoelectrochemical investigation of new carboxylatobipyridine (bis-bipyridine)ruthenium(II) complexes for dye-sensitized TiO2 electrodes

Methods for the preparation and purification of new carboxylated 2,2'-bipyridine ligands, two of which contain the new anchoring functionality malonate, and a reliable method for the synthesis of the corresponding [Ru(bpy)(2)(L)](PF6)(2) complexes are described. Their suitability for fundamental studies of the processes in wet solar cell applications has been investigated. All complexes show stable voltammograms in acetonitrile solution and although the complexes were obtained as lithium carboxylate salts, they were soluble in non-polar organic solvents such as dichloromethane, which allowed for good reproducibility in the dye-coating step. The electrochemical and photoelectrochemical properties of dye-TiO2 systems are discussed. The photoelectrochemical properties in monochromatic and white light are related to the attaching group that is varied in the series of complexes: A formally non-conjugated malonate group as anchoring group generally gives similar results to that of a conjugated carboxylic group, the second carboxylate compensating the absence of conjugation. Also, the complexes with malonate as attaching group are more efficient than a complex with a non-conjugated carboxylic group, especially in the red part of the action spectrum. The redox behavior of the adsorbed complexes is reversible at negative potentials and quasi-reversible at positive potentials. The latter feature is especially interesting since it is possible to electrochemically oxidize the surface-attached ruthenium complexes even at potentials in the mid-bandgap region of the metal oxide film. (C) 2000 Elsevier Science B.V. All rights reserved.