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

Abstract

Eleven new complexes of the form cis-[Ru II(bpy) 2(L A)] 4+ (bpy = 2,2′-bipyridyl; L A = a pyridinium-substituted bpy derivative) have been prepared and isolated as their PF 6 − 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 L A. Cyclic voltammograms show quasi-reversible or reversible Ru III/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 L A, 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.