Direct time-resolved infrared measurement of electron injection in dye-sensitized titanium dioxide films

Time-resolved infrared (IR) spectroscopy in the 6 mu m region was employed to study the excited-state properties of [Ru(4,4'-(COOCH2CH3)(2)-2,2'-bipyridine)(2,2'-bipyridine)(2)](+2) and [Ru(4,4'-(COOCH2CH3)(2)-2,2'-bipyridine)(4,4'-(C-H-3)(2)-2,2'-bipyridine)(2)](+2) in solution and anchored to nanostructured thin films of TiO2 and ZrO2. Excited-state spectra reveal a shift in the nu(C=O) of the ester groups for the free molecules in solution as well as attached to insulating ZrO2 substrates. For these molecules attached to TiO2 semiconductor films, a transient absorption appears that is attributed to electrons injected into TiO2. This absorption appears within the instrumental time resolution (ca. 30 ps) yielding an approximate 20 ps upper limit time constant for electron transfer from the sensitizer excited state to TiO2.