Comparison of electron transfer dynamics in molecule-to-nanoparticle and intramolecular charge transfer complexes

Ultrafast electron-transfer dynamics of a catechol-TiO2 nanoparticle charge transfer (CT) complex and an intramolecular CT complex, [Ti(cat)(3)](2-) were compared. Both complexes show similar CT bands, suggesting similar donor (catechol) and acceptor(Ti) orbitals are involved in CT transition. However, the electron is localized on one Ti center in the [Ti(cat)(3)] Complex, but can delocalize into other Ti centers in TiO2 nanoparticles. The effect of charge delocalization on charge recombination dynamics was examined by comparing these complexes. In both cases, 400 nm excitation of the CT bands led to instantaneous promotion of an electron from a catechol ligand to a Ti(IV) center(s). In the molecular complex, the back electron-transfer dynamics from the Ti center to catechol ligand was observed on a 200 fs time scale. In the adsorbate-to-nanoparticle complex, back ET was much more complex, consisting of a 0.4 ps fast component and multiple slower components. The fast component was attributed to electrons trapped at or near the initial Ti centers, and slow components were attributed to trapped states farther away. Back ET kinetics from TiO2 to a series of catechol derivatives were also compared. Although all kinetics were highly nonexponential, the average back ET rate became slower at larger driving force, consistent with ET in the Marcus inverted region.