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

Abstract

a b s t r a c tFourteen new complexes of the form cis-[Ru II X 2 (R 2 qpy 2+ ) 2 ] 4+ (R 2 qpy 2+ = a 4,4 0 :2 0 ,2 00 :4 00 ,4 000 -quaterpyridinium ligand, X = Cl À or NCS À ) have been prepared and isolated as their PF 6 À salts. Characterisation involved various techniques including 1 H NMR spectroscopy and +electrospray or MALDI mass spectrometry. The UV-Vis spectra display intense intraligand p ? p ⁄ 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 À with NCS À 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 2 H)Ph) 2 qpyH 3+ ][HSO 4 ] 3 Á3H 2 O. Time-dependent density functional theory calculations give adequate correlations with the experimental UV-Vis spectra for the two carboxylic acidfunctionalised complexes in DMSO. Despite their attractive electronic absorption spectra, these dyes are relatively inefficient photosensitisers on electrodes coated with TiO 2 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.

Quaterpyridines as Scaffolds for Functional Metallosupramolecular Materials

“…With 4,4′:2′,2″:4″,4‴-quaterpyridine ligands, the introduction of both redox-tuning and anchoring groups in one molecule is rather cumbersome. Such ligands function as bidentate species with pendent quaternized tails and are not expected to display high current efficiencies upon coordination with Ru­(II) ions because of the long distance between the metal center and the junction of the anchoring carboxylic groups as well as the high level of conformational freedom of pendent pyridyl arms. , Although four ruthenium dyes have been tested and found to show negligible DSSC efficiencies, the observation of a marked improvement in quaternized qtpy-based sensitizers upon a change of cathode substrate from TiO 2 to ZnO is of interest (Figure , Table ).…”

“…Figure 72. Schematic representation of quaternized Ru(II)−qtpy sensitizers [Ru(173) 2 ) 2 Cl 2 ](PF 6 ) 4 (H) and [Ru(173) 2 ) 2 (SCN) 2 ]-(PF 6 ) 4 (I) 334. …”

Allylic oxidation of cyclohexene over ruthenium-doped titanium-pillared clay

“…These values correspond to the UV spectrum of RuO 2 . 69,70 Fig. 5 presents representative IR spectra of all four samples, taken aer their exposure to pyridine, which interacts Pyridine coordinated to the Lewis sites (Lpy), absorbs near the bands at 1455, 1490, and 1610-1625 cm À1 ; the 1455 cm À1 band is typical for these sites.…”