2,6-Bis(1-methylbenzimidazol-2-yl)pyridine: a new ancillary ligand for efficient thiocyanate-free ruthenium sensitizer in dye-sensitized solar cell applications.

We have designed and synthesized a new thiocyante-free ruthenium complex containing 2,6-bis(1-methylbenzimidazol-2-yl)pyridine, coded as SPS-G3, and it has been used as an efficient photosensitizer for dye-sensitized solar cells (DSSCs). Upon sensitization of SPS-G3 on nanocrystalline TiO2 film, the DSSC test cell yielded a large short-circuit photocurrent (16.15 mA cm(-2)), an open circuit voltage of 0.52 V, and a fill factor (FF) of 0.72, resulting in an overall power conversion efficiency (PCE) of 6.04% under simulated AM 1.5 solar irradiation (100 mW cm(-2)). DSSCs were prepared by adding various concentrations of multiwall carbon nanotubes (MWCNTs) (up to 0.5 wt %) into the TiO2 nanoparticles. Optimization of MWCNT concentration (0.3 wt %) lead to PCE values as high as 7.76%, while the test cells employing pure TiO2 photoanode obtained an efficiency of 6.04%. The results indicate that the PCE of MWCNTs/TiO2 composite DSSCs are dependent on the quantity of MWCNTs loading on the photoanodes. A small amount (0.3 wt %) clearly enhances the PCE of DSSC, while the excessive MWCNT loading lowers the photovoltaic performance of the DSSC. The increase in the PCE has been attributed to the decrease in charge-transport resistance, charge-transport time, and electron lifetime, which are estimated from electrochemical impedance spectra.

Tuning of redox potentials by introducing a cyclometalated bond to bis-tridentate ruthenium(II) complexes bearing bis(N-methylbenzimidazolyl)benzene or -pyridine ligands.

A series of asymmetrical bis-tridentate cyclometalated complexes including [Ru(Mebib)(Mebip)](+), [Ru(Mebip)(dpb)](+), [Ru(Mebip)(Medpb)](+), and [Ru(Mebib)(tpy)](+) and two bis-tridentate noncyclometalated complexes [Ru(Mebip)(2)](2+) and [Ru(Mebip)(tpy)](2+) were prepared and characterized, where Mebib is bis(N-methylbenzimidazolyl)benzene, Mebip is bis(N-methylbenzimidazolyl)pyridine, dpb is 1,3-di-2-pyridylbenzene, Medpb is 4,6-dimethyl-1,3-di-2-pyridylbenzene, and tpy is 2,2':6',2″-terpyridine. The solid-state structure of [Ru(Mebip)(Medpb)](+) is studied by X-ray crystallographic analysis. The electrochemical and spectroscopic properties of these ruthenium complexes were studied and compared with those of known complexes [Ru(tpy)(dpb)](+) and [Ru(tpy)(2)](2+). The change of the supporting ligands and coordination environment allows progressive modulation of the metal-associated redox potentials (Ru(II/III)) from +0.26 to +1.32 V vs Ag/AgCl. The introduction of a ruthenium cyclometalated bond in these complexes results in a significant negative potential shift. The Ru(II/III) potentials of these complexes were analyzed on the basis of Lever's electrochemical parameters (E(L)). Density functional theory (DFT) and time-dependent DFT calculations were carried out to elucidate the electronic structures and spectroscopic spectra of complexes with Mebib or Mebip ligands.