Electrochemical and spectroscopic studies on the reduction of the cis-(Et2-dcbpy)2RuX2 series of photovoltaic sensitizer precursor complexes (Et2-dcbpy = diethyl 2,2′-bipyridine-4,4′-dicarboxylate, X = Cl–, I–, NCS–, CN–)

Abstract

The series of (Et₂-dcbpy)₂RuX₂ complexes, where Et₂-dcbpy is diethyl 2,2′-bipyridine-4,4′-dicarboxylate and X = Cl^–, I^–, NCS^–, CN^–, and its reduced forms have been investigated voltammetrically and spectroscopically in dimethylformamide at 22 °C and at –58 °C. Four chemically reversible reduction processes were observed with short time scale voltammetric conditions for (Et₂-dcbpy)₂Ru(CN)₂. An interesting linear relationship was found between the reversible potentials and temperature. A simple conceptual model is provided that relates this potential–temperature dependence to “electron hopping” of spatially isolated electrons. Additional evidence for this model was found in UV/VIS/NIR spectra of [(Et₂-dcbpy)₂Ru(CN)₂]^–, generated in spectroelectrochemical experiments. A weak band at 3500 cm^–1 is also evidence of “ligand hopping”. As predicted, this band was absent for [(Et₂-dcbpy)₂Ru(CN)₂]^2–. The electronic spectra of other complexes are also discussed and bands tentatively assigned. The initial reduction products were not stable on longer time scales. Electrospray mass spectrometry was employed to identify decomposition products. In the case of (Et₂-dcbpy)₂Ru(CN)₂, the main reaction path was de-esterification, yielding the deprotonated acid. Additionally, reductively induced ligand elimination was the preferred decomposition pathway for the reduced complexes where X = Cl^– and I^–. The chemical reversibility of voltammetric reductions decreased in the order X = CN^– > NCS^– > Cl^– > I^–.