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

Electron injection dynamics in dye sensitized TiO2 nanocrystalline thin films are studied with femtosecond mid-infrared spectroscopy. Three classes of sensitizer molecules, Ru(dcbpy)2(X)2 (X2 = 2SCN, 2CN, and dcbpy), Fe(dcbpy)2(CN)2, and ReCl(CO)3(dcbpy), are used to examine the dependence of injection rate and yield on the excited state redox potentials. We observed that electron injection occurred on the <100 fs time-scale and injection quantum yield depended on the redox potential for the series of Ru dyes. These results suggest that electron injection to TiO2 competes with electronic and vibrational relaxation within the sensitizer excited states and the branching ratio between these two processes determines the injection quantum yield for sensitizer molecules with an excited state redox potential below the conduction band-edge