Novel Ruthenium Sensitizers Containing Functionalized Hybrid Tetradentate Ligands:  Synthesis, Characterization, and INDO/S Analysis

Ruthenium sensitizers of the type trans-[Ru(L 1 )(X)2], trans-[Ru(L 2 )(X)2], trans-[Ru(L 3 )(X)2], and trans-[Ru(L 4 )(X)2] (where L 1 = 6,6‘-bis(1-H-benzimidazol-2-yl)-4,4‘-bis(methoxycarbonyl)-2,2‘-bipyridine, L 2 = 4,4‘ ‘‘-bis(tert-butyl)-4‘,4‘ ‘-bis[p-(methoxycarbonyl)phenyl]-2,2‘:6‘,2‘ ‘:6‘ ‘,2‘ ‘‘-quaterpyridine, L 3 = 4‘,4‘ ‘-bis[3,4-(dimethoxy)phenyl]-2,2‘:6‘,2‘ ‘:6‘ ‘,2‘ ‘‘-quaterpyridine, and L 4 = 4‘,4‘ ‘-diethoxycarbonyl-2,2‘:6‘,2‘ ‘:6‘ ‘,2‘ ‘‘-quaterpyridine; X = Cl-, NCS-) were synthesized and characterized by CV, NMR, and UV−vis absorption and emission spectroscopy. The trans-dichloro and dithiocyanate complexes show MLCT transitions in the entire visible and near-IR region. The lowest energy metal-to-ligand charge-transfer transition band of the trans-dichloro complexes is around 14 300 cm-1 in DMF solution, and these complexes show weak and broad emission signals with onset at above 10 500 cm-1. The absorption and emission maxima of the trans-dithiocyanate complexes are blue-shifted compared to those of its trans-dichloro analogues because of the strong π acceptor property of the NCS- compared to the Cl-. The electronic spectra of trans-[Ru(L)(X)2] complexes were calculated by INDO/S and compared with the experimental data. The extent of mixing between metal 4d and ligand π* orbitals is discussed. Extensive π-back-donation is observed. The panchromatic response of these novel complexes renders them as suitable sensitizers for solar energy conversion applications based on titanium dioxide mesoporous electrodes. Preliminary results using the trans-[Ru(L 4)(NCS)2] complex show 75% incident photon-to-current efficiencies (IPCE), yielding 18 mA/cm2 current density under standard AM 1.5 sunlight.