Ultrafast dissociation of excitons in CdSe quantum dots via electron transfer to adsorbed Re-bipyridyl complexes was demonstrated. The dissociation pathway was determined by the observation of reduced adsorbate using femtosecond IR spectroscopy. The rate of electron transfer was shown to increase at smaller QD sizes. Electron transfer time as fast as 2.3 ps was observed, faster than the exciton annihilation time in CdSe. The ultrafast charge separation in this quantum dot-adsorbate donor-acceptor complex provides a potential approach for separating multiple excitons in quantum dots.

The effects of anchoring groups on electron injection from adsorbate to nanocrystalline thin films were investigated by comparing injection kinetics through carboxylate versus phosphonate groups to TiO 2 and SnO 2 . In the first pair of molecules, Re(L A )(CO) 3 Cl (ReC1A) and Re(Lp)(CO)3Cl (ReC1P), [L A ) 2,2′-bipyridine-4,4′-bis-CH 2 -COOH, Lp) 2,2′-bipyridine-4,4′-bis-CH 2 -PO 3 H 2 ], the anchoring groups were insulated from the bipyridine ligand by a CH 2 group. In the second pair of molecules, Ru(dcbpyH 2 ) 2 (NCS) 2 (RuN3) and Ru(bpbpyH 2 ) 2 (NCS) 2 (RuN3P), [dcbpy ) 2,2′-bipyridine-4,4′-biscarboxylic acid, bpbpy ) 2,2′-bipyridine-4,4′-bisphosphonic acid], the anchoring groups were directly connected to the bipyridine ligands. The injection kinetics, as measured by subpicosecond IR absorption spectroscopy, showed that electron injection rates from ReC1P to both TiO 2 and SnO 2 were faster than those from ReC1A. The injection rates from RuN3 and RuN3P to SnO 2 films were similar. On TiO 2 , the injection kinetics from RuN3 and RuN3P were biphasic: carboxylate group enhances the rate of the <100 fs component, but reduces the rate of the slower components. To provide insight into the effect of the anchoring groups, the electronic structures of Re-bipyridyl-Ti model clusters containing carboxylate and phosphonate anchoring groups and with and without a CH 2 spacer were computed using density functional theory. With the CH 2 spacer, the phosphonate group led to a stronger electronic coupling between bpy and Ti center than the carboxylate group, which accounted for the faster injection from ReC1P than ReC1A. When the anchoring groups were directly connected to the bpy ligand without the CH 2 spacer, such as in RuN3 and RuN3P, their effects were 2-fold: the carboxylate group enhanced the electronic coupling of bpy π* with TiO 2 and lowered the energy of the bpy orbital. How these competing factors led to different effects on TiO 2 and SnO 2 and on different components of the biphasic injection kinetics were discussed.