Syntheses and properties of complexes with bis(2,2′-bipyridyl) ruthenium(II) moieties coordinated to 4,4′:2′,2′′: 4′′,4′′′-quaterpyridinium ligands

Eleven new complexes of the form cis-[RuII(bpy) 2(LA)]4+ (bpy = 2,2′-bipyridyl; L A = a pyridinium-substituted bpy derivative) have been prepared and isolated as their PF6- salts. Characterisation involved various techniques including 1H NMR spectroscopy and MALDI mass spectrometry. The UV-Vis spectra show intense intraligand π → π absorptions and metal-to-ligand charge-transfer (MLCT) bands with two distinct maxima in the visible region. Small shifts in the MLCT bands correlate with the electron-withdrawing strength of the ligand LA. Cyclic voltammograms show quasi-reversible or reversible RuIII/II oxidation waves, and two or more ligand-based reductions with varying degrees of reversibility. The variations in the redox potentials correlate with changes in the structure of LA, and also with the MLCT energies. Differential pulse voltammetry allows the first reduction process for two of the complex salts to be resolved into two peaks. Single-crystal X-ray structures have been solved for three of the new complex salts and also for a pro-ligand salt. Two carboxylate- functionalised compounds have been tested as photosensitizers on TiO 2-coated electrodes, but show only negligible efficiencies, in accord with expectations. © 2011 Elsevier Ltd. All rights reserved.