Ruthenium(II) dichloro or dithiocyanato complexes with 4,4′: 2′, 2″:4″,4‴-quaterpyridinium ligands: Towards photosensitisers with enhanced low-energy absorption properties

Fourteen new complexes of the form cis-[Ru^IIX₂(R ₂qpy²⁺)₂]⁴⁺ (R₂qpy ²⁺ = a 4,4′:2′,2″:4″,4‴- quaterpyridinium ligand, X = Cl^- or NCS^-) have been prepared and isolated as their PF₆ ^- salts. Characterisation involved various techniques including ¹H NMR spectroscopy and +electrospray or MALDI mass spectrometry. The UV-Vis spectra display intense intraligand π → π* absorptions, and also metal-to-ligand charge-transfer (MLCT) bands with two resolved maxima in the visible region. Red-shifts in the MLCT bands occur as the electron-withdrawing strength of the pyridinium groups increases, while replacing Cl^- with NCS^- causes blue-shifts. Cyclic voltammograms show quasi-reversible or reversible Ru^III/II oxidation waves, and several ligand-based reductions that are irreversible. The variations in the redox potentials correlate with changes in the MLCT energies. A single-crystal X-ray structure has been obtained for a protonated form of a proligand salt, [(4-(CO ₂H)Ph)₂qpyH³⁺][HSO₄] ₃·3H₂O. Time-dependent density functional theory calculations give adequate correlations with the experimental UV-Vis spectra for the two carboxylic acid-functionalised complexes in DMSO. Despite their attractive electronic absorption spectra, these dyes are relatively inefficient photosensitisers on electrodes coated with TiO₂ or ZnO. These observations are attributed primarily to weak electronic coupling with the surfaces, since the DFT-derived LUMOs include no electron density near the carboxylic acid anchors.