Dynamics of Interfacial Charge Transfer Emission in Small Molecule Sensitized TiO₂ Nanoparticles: Is It Localized or Delocalized?

Interfacial charge-transfer emission in molecule−TiO 2 semiconductor nanoparticles has been followed with femtosecond fluorescence upconversion in an attempt to characterize the localized/delocalized nature of the charge-transfer excitation. Ligand to metal charge-transfer absorption is observed in catechol, dopamine, acetyl acetonate, salicylate, and hydroxamic acid sensitized TiO 2 nanoparticles with significant absorption in the visible region. Emission from these charge-transfer states is revealed with femtosecond fluorescence upconversion and ascribed to interfacial charge-transfer emission. The emission lifetimes ranged from <100 fs to a few hundreds of femtoseconds for these systems depending on the electronic coupling of the molecule with TiO 2 . Fluorescence anisotropy measurements are carried out to understand the nature of charge transfer (CT) excitation. Anisotropy at very short time scales is close to what was expected from linear dipolar molecular systems for catechol, hydroxamate, and dopamine suggesting that the excitation is localized. In contrast, relatively lower anisotropy values are obtained for acetyl acetone and salicylate complexes which form comparatively weaker CT complexes with TiO 2 nanoparticles indicating the delocalized nature of excitation. Femtosecond transient absorption measurements on these molecule-sensitized TiO 2 nanoparticles have shown multiexponential charge recombination with lifetimes ranging from sub-picoseconds to a few tens of picoseconds indicating that the localized electrons diffuse and give rise to the charge-separated states. Results observed in the present investigation prove the presence of a localized charge-transfer exciton for the TiO 2 −catechol system with the charge transferred from ligand to localized Ti atom or the Ti atoms around it and not the entire nanoparticle. However, the slower components of emission and fluorescence anisotropy decay are observed for other small molecule sensitized systems which show the involvement of delocalized CT exciton thus confirming that both localized and delocalized charge-transfer excitations are indeed possible.