Interfacial electron transfer between Fe(II)(CN)(6)(4-) and TiO2 nanoparticles: Direct electron injection and nonexponential recombination

Photoinduced electron-transfer (ET) dynamics in Fe(II)(CN)(6)(4-) sensitize d TiO2 nanoparticles in D2O solution are studied by subpicosecond tunable l aser spectroscopy in the mid-infrared and visible region. The dynamics of t he injected electrons are monitored by the mid-IR absorption of electrons i n the semiconductor, and the corresponding dynamics of the adsorbate are mo nitored by the vibrational spectra of the CN stretching mode region and ele ctronic absorption in the visible. After 400 nm excitation, the forward ele ctron injection time from Fe(II)(CN)(6)(4-) to TiO2 occurs in <50 fs, indic ating a direct photoinduced charge-transfer process. The back ET from TiO2 to Fe(III)(CN)(6)(3-) in the <1 ns time scale is found to be a non-single-e xponential process. The best three-exponential fit to the data yields back ET time constants of 3 ps (35%), 40 ps (30%), and >1 ns (35%). Combining wi th previous measurements in the nanosecond to microsecond time scale (Lu et al. J. Am. Chern. Sec. 1993, 115, 4927 and Vrachnou et al. J. Chern. Sec., Chem. Commun. 1987, 868), the total back ET process is found to be highly nonexponential with time constants ranging from 3 ps to 3 mu s.