Site-Specific Ligand Exchange on Colloidal Semiconductor Nanocrystals: On-Edge Exchange versus On-Facet Position Swapping

Liquid-phase Fourier transform infrared (FTIR) is applied for in situ and quantitative studies on the most commonly investigated ligand exchange reaction, i.e., replacement of alkanoates by alkylthiolate ligands on CdSe nanocrystals (NCs). The forward reaction on CdSe NCs with controlled facet structures─zinc-blende (100) facets proceeds in two distinctive channels, with time constants of ∼5 and ∼10 6 s, respectively. The fast channel occurs between thiols in solution and alkanoate ligands on the edges of the facets, with a near-infinity equilibrium constant. FTIR and solid-state NMR measurements suggest that the remaining alkanoate ligands on a (100) facet are promoted from a chelating to a bridging motif by the incoming thiolate ligands. The slow channel combines the on-edge exchange and on-facet position swapping between a pair of adjacent thiolate and alkanoate ligands. Monte Carlo simulation reveals two on-facet position swapping pathways with time constants as ∼240 and ∼450 s. The reverse ligand exchange also possesses a large chemical equilibrium constant, but it can only occur on few unique bonding sites that are unsuited for the thiolate binding motif. Results here imply that the ligand chemistry of colloidal NCs can be advanced to a quantitative level with molecular resolution.