Directed Ligand Exchange on the Surface of PbS Nanocrystals: Implications for Incoherent Photon Conversion

Ligand-exchange procedures are ubiquitous in the functionalization of colloidal nanocrystals for applications in biological imaging, photocatalysis, and photonic/optoelectronic devices. However, the rich interactions between functional ligands and the nanocrystal surface offer a vast opportunity to achieve emergent self-assembled structures. Here, using H-1 NMR as a probe and L-type-promoted Z-type ligand displacement as a tool to study PbS nanocrystals, we demonstrate that 9-anthracene carboxylic acid (9-ACA) ligands strongly segregate to the highest-energy binding sites at the conclusion of X-for-X exchanges. These weaker sites are associated with nanocrystal facet-edges, and linewidth analysis corroborates that 9-ACA replaces the most conformationally dynamic native ligands. The templated assembly of this bulky model fluorophore at the nanocrystal surface is an opportunity to enhance energy transport and contrasts sharply with conventional aliphatic ligands, where we find that exchanges are isotropic. Our results indicate that ligand-ligand interactions and the spatial correlation of nanocrystal binding-site heterogeneity can be leveraged to produce functionalized particles with tailored, anisotropic ligand morphologies. This opportunity to promote clustering could influence the design of photoactive ligands for multiexcitation processes such as incoherent photon conversion.