Designing Janus Ligand Shells on PbS Quantum Dots using Ligand–Ligand Cooperativity

Abstract

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.

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

“…40,41 The importance of interligand interactions has been recently highlighted by demonstrations of disorder-toorder transitions in the ligand shell, 39−42 and the exploitation of dipolar interactions between cinnamic acid derivatives to assemble Janus-type ligand coverage on PbS NCs. 40 These reports highlight the possibility that favorable aggregative properties could not only promote the clustering of surfacebound chromophores but guide their relative orientations, influencing the intermolecular couplings that govern phenomena such as singlet fission, energy transfer, and triplet fusion. 28,43−46 Thus, improved understanding of the ligand morphologies adopted by bulky PAH derivaties at the NC surface will identify opportunities to enhance and extend the performance of hybrid architectures for incoherent photon conversion.…”

Size-Dependent Janus-Ligand Shell Formation on PbS Quantum Dots

“…Functionally, the absorption enhancement provides an in situ observation of the exchange of oleate with RCA ligands. Here, we performed spectrophotometric titrations following previously reported methods ,, and generated ligand exchange isotherms as previously described . Briefly, the complete exchange of oleate is marked by the saturation of the absorption enhancement.…”

“…We model the isotherms presented in Figure (solid lines are the model) using a 2D lattice model described previously, which output nonlinear least-squares best-fit values of Δ G and Δ J , respectively with the equation Δ G AB = Δ G exc + θΔ G MF + 4Δ J (2 – ΔNN B ) . Here, θ is the fraction of the binding sites occupied by oleate ligands, NN B is the number of nearest neighbors of ligand B, Δ G exc is the binding energy difference of OA and RCA, Δ G MF is a mean-field energy difference (usually set to be a small number), and Δ J is the difference in nearest-neighbor ligand coupling energy between the oleate and cinnamate . We can qualitatively understand the impact of Δ G and Δ J on the behavior of the isotherms as the threshold concentration of RCA which the exchange begins to occur (Δ G ), and the slope of the rise (Δ J ), that is, a more negative Δ J corresponds to a sharper transition from all oleate to all cinnamate.…”

“…We have demonstrated some control over the ligand structure by exploiting favorable ligand–ligand cooperative interactions between surface bound polar ligands. In previous work, we exploited the fact that the linear absorption of colloidal PbS QD is enhanced when the ligand changes from the native oleate (OA) to various substituted cinnamic acids (RCA) in order to monitor the ligand exchange in situ , and construct ligand exchange isotherms . The exchange isotherms for these dipolar ligands exhibit cooperative behavior due to beneficial ligand–ligand coupling, that is, the ligand exchange becomes more favorable as the exchange reaction proceeds.…”

“…In previous work, we exploited the fact that the linear absorption of colloidal PbS QD is enhanced when the ligand changes from the native oleate (OA) to various substituted cinnamic acids (RCA) in order to monitor the ligand exchange in situ , and construct ligand exchange isotherms . The exchange isotherms for these dipolar ligands exhibit cooperative behavior due to beneficial ligand–ligand coupling, that is, the ligand exchange becomes more favorable as the exchange reaction proceeds. A two-dimensional (2D) lattice Ising-type model was developed to determine the ligand–ligand coupling energy (Δ J ) and the free energy (Δ G ) of the ligand exchange reactions.…”