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.