The series of cis-(Et2-dcbpy)2RuX2 (Et2-dcbpy = 2,2′-bipyridine-4,4′-diethoxydicarboxylic acid, X = Cl-, I-, NCS-, and CN-) sensitizer precursor complexes have been synthesized directly from the esterified ligand (Et2-dcbpy) rather than via the acid (H2-dcbpy) in order to obtain high yields. The RuII/RuIII oxidation process, which is utilized in photovoltaic cell reactions, has been studied in detail by voltammetric and spectroelectrochemical techniques. The [(Et2-dcbpy)2RuCl2]0/+ process represents an example of an ideal reversible one-electron oxidation process. The very high stability of the oxidized complex allowed [(Et2-dcbpy)2RuCl2]0/+ to be characterized by spectroscopic techniques. The ESR spectrum indicates deviation from axial symmetry, and electronic spectra show the disappearance of both MLCT bands and the appearance of one LMCT band as expected for a metalbased oxidation process. Oxidation processes for the other complexes are considerably more complicated. In the case of (Et2-dcbpy)2RuI2 an oxidatively induced ligand elimination process was observed to occur after formation of [(Et2-dcbpy)2RuI2]+ to yield [(Et2-dcbpy)2RuI(Solvent)]+ and [(Et2-dcbpy)2Ru(Solvent)2]2+ complexes in dimethylformamide and acetonitrile. The rate constants for these reactions were estimated from digital simulation of voltammetric data. When dichloromethane was used as the solvent, formation of the five-coordinate [(Et2-dcbpy)2RuI]+ complex was observed. The identity of these complexes formed after the initial one-electron oxidation process was confirmed by electrospray mass spectrometry. Oxidation of L2Ru(CN)2 is even more complicated than oxidation of L2RuI2. Both mono- and polynuclear ruthenium compounds are formed as a result of reactions that occur with the oxidized form of the ligand, cyanogen (CN)2, or its derivatives. Oxidation of L2Ru(NCS)2 leads to elimination of sulfur from the thiocyanate ligand and to formation of L2Ru(CN)2.
|Number of pages||11|
|Publication status||Published - 1 Dec 1999|