Electron Injection Efficiency in Ru-Dye Sensitized TiO2 in the Presence of Room Temperature Ionic Liquid Solvents Probed by Femtosecond Transient Absorption Spectroscopy: Effect of Varying Anions

We investigated the electron injection efficiency of a Ru-complex dye (N719) sensitized TiO2 system using femtosecond transient absorption in the presence of two different room temperature ionic liquids (ILs) varying their anionic counterparts (tetracyanoborate [TCB](-) and dicyanoamide [DCA](-)), keeping the cationic counterpart (ethylmethylimidazolium [EMIm](+)) invariant. We also investigated the effect of varying IL environments on the metal-ligand charge transfer band of the adsorbed N719 on TiO2 film using steady state absorption spectroscopy and the effect on cyano stretching frequency of ILs in the presence of TiO2 films using attenuated total reflectance-IR method. The downshift of conduction band (CB) edge energy of TiO2 nanoparticles in presence of [EMIm][TCB] IL environment has recently been reported (J. Phys. Chem. C 2011, 115, 816-822). Based on CB edge energy modulation and transient absorption spectroscopic data, we found that the electron injection efficiency is higher in the N719/TiO2 system in the presence of [EMIm][TCB] IL environment than in the presence of [EMIm][DCA] IL environment by similar to 20%.