Dinuclear Ru-Cu Complexes: Electronic Characterisation and Application to Dye-Sensitised Solar Cells

We prepared the complexes [Ru{4,4′-(CO2R-bpy)2}{Cu(exoO2-cyclam)}][NO3]2 [R = Et (1), H (2)], which possess a CuII centre covalently linked to a (bipyridyl)RuII fragment. The complexes were characterised by cyclic voltammetry, UV/Vis spectroscopy, hybrid DFT and TD-DFT (time-dependent density-functional theory) calculations, EPR (electron paramagnetic resonance), emission spectroscopy and UV/Vis spectroelectrochemistry, with the latter showing reversible conversion to the mono- and di-oxidised and mono- and di-reduced species. The data suggest the first oxidation to be largely based on the Ru centre but with little energetic difference between the highest occupied orbitals based on Ru and based on Cu. There was also evidence of solvent coordination in both the electrochemistry and spectroelectrochemistry experiments. The redox potentials and the strongvisible absorptions of 2 (λmax = 562 nm, ϵmax = 22200 M–1 cm–1)make it appropriate for study as a sensitiser in a dye-sensitised solar cell. Charge-separated lifetime of photoexcited 2 on TiO2 and incident photon-to-current conversion efficiency (IPCE) as a function of wavelength were studied and are discussed alongside the formation of dye-sensitised solar cells with the best efficiency achieved of η = 2.55 %, Voc = 608 mV, Isc = 5.84 mA cm–2 and ff = 0.72. Limitations to the maximum efficiency obtained were attributed to a mismatch between the bipyridyl-based unoccupied orbital of the dye and the TiO2 conduction band edge.