Designing Janus Ligand Shells on PbS Quantum Dots using Ligand-Ligand Cooperativity.

We present a combined experimental and theoretical study of ligand-ligand cooperativity during X-type carboxylate-to-carboxylate ligand exchange reactions on PbS quantum dot surfaces. We find that the ligand dipole moment (varied through changing the substituents on the benzene ring of cinnamic acid derivatives) impacts the ligand-exchange isotherms; in particular, ligands with large electron withdrawing character result in a sharper transition from an oleate-dominated ligand shell to a cinnamate-dominated ligand shell. We developed a two-dimensional lattice model to simulate the ligand-exchange isotherms that accounts for the difference in ligand binding energy as well as ligand-ligand cooperativity. Our model shows that ligands with larger ligand-ligand coupling energy exhibit sharper isotherms indicating an order-disorder phase transition. Finally, we developed an anisotropic Janus ligand shell by taking advantage of the ligand-ligand cooperative ligand exchanges. We monitored the Janus ligand shell using 19F nuclear magnetic resonance, showing that when the ligand-ligand coupling energy falls within the order region of the phase diagram, Janus ligand shells can be constructed.