Sub-picosecond injection of electrons from excited [Ru(2,2′-bipy-4,4′-dicarboxy)2(SCN)2] into TiO2 using transient mid-infrared spectroscopy

We have used femtosecond pump-probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO2 film electrodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4'-dicarboxy-2,2'-bipyridine)(2)(NCS)(2)] (N3). Pumping at one of the metal-to-ligand charge transfer absorption peaks and probing the absorption of electrons injected into the TiO2, conduction band at 1.52 mu m and in the range of 4.1 to 7.0 mu m, we have directly observed the arrival of the injected electrons. Our measurements indicate an instrument-limited similar to 50-fs upper limit on the electron injection time under ambient conditions in air. We have compared the infrared transient absorption for noninjecting (blank) systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al2O3 and ZrO2, and found no indication of electron injection at probe wavelengths in the mid-IR (4.1 to 7.0 mu m). At 1.52 mu m interferences exist in the observed transient absorption signal for the blanks.