Surface Ligand Composition of Polyoxovanadate-Alkoxides Influences Hole Transfer from CdSe Quantum Dots

In this study, we evaluate the impact of the modulation of the surface structure of polyoxovanadate-alkoxides (POV-alkoxides) on their hole transfer capabilities with cadmium selenide quantum dots (CdSe QDs). Initial studies probe the consequences of increasing the carbon chain length of bridging alkoxide ligands, through comparison of efficiencies of hole transfer to methoxide- and ethoxide-capped POV-alkoxides, [V6O7(OR)12]1- (R = CH3, CH2CH3). While [V6O7(OCH2CH3)12]1- is able to quench the photoluminescence of CdSe QDs, reduced efficiency is observed in comparison to its methoxide congener. This observation indicates that increasing steric bulk at the surface of the POV-alkoxide inhibits interactions of the hole acceptor with the surface of the nanocrystal. Subsequent studies investigate a POV-alkoxide capped with a tridentate ligand containing a pyridine substituent that we hypothesized would promote coordination of the cluster to the surface of the nanocrystal. Steady-state photoluminescence quenching experiments indicate improved hole transfer in comparison to structural controls, while time-correlated single-photon counting photoluminescence decay dynamics confirm an increase in the number of QD-cluster associations. However, slower hole transfer lifetimes per cluster are noted for the pyridine-substituted assembly. Collectively, our experiments provide insight into design criteria for molecular redox mediators that enhance the efficiency of hole transfer from CdSe QDs.