Unusual properties of ruthenium(II) diphenylcyanamide complexes: chemistry and application as sensitizers of nanocrystalline TiO2

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Abstract
In this study, a series of ruthenium(II) polypyridyl complexes of the general formula Na4[RuII(dcbpy)2X2] (dcbpy is 2,2′-bipyridine-4,4′-dicarboxylate and X is a substituted phenylcyanamide anion) are investigated. When introduced into the ruthenium co-ordination sphere, the phenylcyanamide ligand showed linkage isomerism and was co-ordinated via the nitrile- or amide nitrogen, as shown by 1H NMR, 13C NMR spectroscopy and use of a 13C-labeled ligand. The title compounds exhibited unusually high first pKa values between 5 and 6, determined via spectrophotometric titrations. The first protonation occurred at the phenylcyanamide ligand, rendering the energy of the lowest energy 1MLCT absorption strongly pH-dependent. In photovoltaic devices, overall efficiencies were in the range of 3–4%, due to photon-to-current conversion efficiencies of 50–60% at 520 nm. The long-term stability of a representative complex under irradiation was investigated in solution and in a solar cell device.

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