Comparison of interfacial electron transfer through carboxylate and phosphonate anchoring groups

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 TiO2 and SnO2. In the first pair of molecules, Re(LA)(CO)3Cl (ReC1A) and Re(Lp)(CO)3Cl (ReC1P), [LA=2,2'-bipyridine-4,4'-bis-CH2-COOH, Lp=2,2'-bipyridine-4,4'-bis-CH2-PO3H2], the anchoring groups were insulated from the bipyridine ligand by a CH2 group. In the second pair of molecules, Ru(dcbpyH2)2(NCS)2 (RuN3) and Ru(bpbpyH2)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 TiO2 and SnO2 were faster than those from ReC1A. The injection rates from RuN3 and RuN3P to SnO2 films were similar. On TiO2, 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 CH2 spacer were computed using density functional theory. With the CH2 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 CH2 spacer, such as in RuN3 and RuN3P, their effects were 2-fold: the carboxylate group enhanced the electronic coupling of bpy pi* with TiO2 and lowered the energy of the bpy orbital. How these competing factors led to different effects on TiO2 and SnO2 and on different components of the biphasic injection kinetics were discussed.