Effect of a redox-mediating ligand shell on photocatalysis by CdS quantum dots

Semiconductor quantum dots (QDs) are efficient photoredox catalysts due to their high extinction coefficients and easily tuneable band edge potentials. Despite the majority of the surface being covered by ligands, our understanding of the effect of the ligand shell on photocatalysis is limited to steric effects. We hypothesise that we can increase the activity of QD photocatalysts by designing a ligand shell with targeted electronic properties, namely redox-mediating ligands. Herein, we functionalise our QDs with hole-mediating ferrocene (Fc) derivative ligands and perform a reaction where the slow step is hole transfer. Surprisingly, we find that a hole-shuttling Fc inhibits catalysis, but that dynamically bound Fc ligands can promote catalysis by surface exchange and creation of a more permeable ligand shell. Despite poor catalytic behaviour, a hole-shuttling Fc confers much greater stability to the catalyst by preventing a build-up of destructive holes. Conversely, we find that trapping the electron on a ligand dramatically increases the rate of reaction. These results have major implications for understanding the rate-limiting processes in charge transfer from QDs and the role of the ligand shell in modulating it.