Ultrafast Photoinduced Processes in Alizarin-Sensitized Metal Oxide Mesoporous Films

The photoinduced dynamics in Al2O3, and TiO2 mesoporous films sensitized by the strongly coupled alizarin dye is investigated by femtosecond transient absorption spectroscopy in the spectral range from UV to mid-IR. Alizarin/Al2O3 acts as a nonreactive reference system, in which no electron transfer is observed. For comparison, the photoexcitation of the alizarin dye coupled to the surface of TiO2 films leads to ultrafast electron transfer from the dye to the TiO2 conduction band on the sub-100-fs timescale. We observe a fast relaxation of the alizarin excited state as well as a fast recombination of injected electrons with the alizarin cation on the picosecond timescale, which gives rise to very complex kinetics at short delay times. The infrared measurements clearly indicate that trapping of injected electrons is the main mechanism responsible for the observed long-lived charge separation in TiO2 mesoporous films. The experimental findings can be explained by a position of the dye excited state close to the conduction band edge.