Multiple-exponential electron injection in Ru(dcbpy)(2)(SCN)(2) sensitizedZnO nanocrystalline thin films

We have studied the electron injection dynamics of Ru(dcbpy)(2)(SCN)(2) sen sitized ZnO nanocrystalline thin films by femtosecond mid-IR absorption spe ctroscopy. Upon 400 nm excitation of the sensitizer, nonexponential electro n injection kinetics from the sensitizer MLCT excited state to ZnO were obs erved by probing the absorption of injected electrons in the 2000 to 1900 c m(-1) region on the < 1 ns time scale. A three-exponential fit to the data yields the following rise time constants and amplitudes (in parentheses): < 1ps (18%), 42ps (46%), and 450ps (36%). For films with increasing coverage of sensitizer molecules (optical density at 500 nm of 0.7, 2.3, 2.6), simi lar electron injection times were observed, while the injection yield decre ases. This result suggests that aggregation or multilayer formation of sens itizer molecules reduces the quantum yield of electron injection to ZnO on the < 1 ns time scale, and the observed multiexponential injection kinetics are attributed to the distribution of injection rates from the first monol ayer of sensitizers. Possible reasons for the nonexponential injection are discussed. The injection dynamics are also compared with the very different injection dynamics observed for the same sensitizer on TiO2.