Femtosecond Electron Transfer from the Excited State of Chemically Anchored Chromophores into the Empty Conduction Band of Nanocrystalline Spong-like TiO₂ Films

Heterogeneous photo-induced electron transfer was measured with femtosecond transient absorption laser spectroscopy in ultra-high-vacuum. The electron acceptor was colloidal anatase TiO₂. The excited donor was a perylene Chromophore with covalently attached anchor and spacer groups and also a ruthenium dye. The former showed well-separated absorption spectra in its ground, excited, and cationic state. The rise of the perylene radical cation absorption as product state and the decay of the excited state absorption as reactant state gave an identical electron transfer time. The latter varied between 40 fs and 80 fs for seemingly identical preparations of the Ti₂ film and was found 30 fs and faster for a different anchor group. Radical cation absorption of the ruthenium dye was ambiguous but the rise time of intraband absorption of the injected electrons was measured faster than 25 fs in the case of the ruthenium dye. The ultrafast experimental electron transfer times were in agreement with the prediction of a straightforward adiabatic model.