Evidences of hot excited state electron injection from sensitizer molecules to TiO2 nanocrystalline thin films

Electron injection dynamics in dye sensitized TiO2 nanocrystalline thin fil ms are studied with femtosecond mid-infrared spectroscopy. Three classes of sensitizer molecules, Ru(dcbpy)(2)(X)(2) (X-2 = 2SCN, 2CN, and dcbpy), Fe( dcbpy)(2)(CN)(2), and ReCl(CO)(3)(dcbpy), are used to examine the dependenc e of injection rate and yield on the excited state redox potentials. We obs erved that electron injection occurred on the < 100 fs time-scale and injec tion quantum yield depended on the redox potential for the series of Ru dye s. These results suggest that electron injection to TiO2 competes with elec tronic and vibrational relaxation within the sensitizer excited states and the branching ratio between these two processes deter-mines the injection q uantum yield for sensitizer molecules with an excited state redox potential below the conduction band-edge.