Direct observation of ultrafast electron injection from coumarin 343 to TiO₂ nanoparticles by femtosecond infrared spectroscopy

Transient infrared (IR) absorption of injected electrons in colloidal TiO₂ nanoparticles in the 1900-2000 cm^-1 region are measured by femtosecond IR spectroscopy. The direct detection of electrons in the nanoparticles with subpicosecond time resolution provides a new approach to study ultrafast interfacial electron transfer between semiconductor nanoparticles and molecular adsorbates. The dynamics of electron injection from sensitizers to nanoparticles and the subsequent back-transfer and relaxation dynamics of the injected electrons correspond to the rise and decay of the transient IR signal of injected electrons. Using this technique, the injection time for coumarin 343 sensitized TiO₂ nanoparticles in D₂O is determined to be 125 ± 25 fs. The subsequent decay dynamics of the injected electrons in nanoparticles are found to be different from conduction band electrons in a bulk TiO₂ crystal.