Femtosecond Dynamics of Electron Injection from Ru-Based Dyes into Nanocrystalline TiO2 Electrodes

We are measuring the rate of electron injection from photoexcited Ru(4,4'-dicarboxy-2.2'- bipyridine)2(NCS)2 dye molecules adsorbed onto nanocrystalline TiO2 particles using femtosecond pump-probe spectroscopy. The Ru dye is pumped at 550 nm and the probe is at 1.52 mu-m. The probe is selected to monitor the time-dependent concentration of electrons in the TiO2 conduction band; the advantage ofthis approach is that we monitor the arrival time of the injected electrons rather than the lifetime of the excited dye molecules. Analysis of published experimental data for the absorption coefficient (at 1.5 mu-m) of the free electron absorption in TiO2 as a function of free electron density shows that we can detect a minimum electron density of 1 x 10 to the 16th cm-3 per pump pulse. Thephotoinduced infra-red free electron absorption profile shows a sub-picosecond rise time followed by a somewhat slower decay. Preliminary analysis of results from dye-TiO2 samples in air show an initial induced absorbance rate at 1.5 mu-m of 8x 10 to the 12th s-1, followed by a bleach rate of 2.5 x 10 to the 11th s-1/ Experiments are also being made with I present and with a complete cellconfiguration, as well as blank experiments with inert substrates like ZrO2 and Al2O3; we report the results of these experiments and our conclusion regarding electron injection rates from the excited dye molecule into the TiO2 nanoparticles.