The Studies On The Dynamics And Thermodynamics Of Ligand Exchange On CdSe Nanocrystals By NMR

Nanocrystals are an important class of functional materials,which are widely used in different fields,such as photoelectric conversion,heterogeneous catalysis and biological tracing.The surface ligands of nanocrystals play a crucial role in solution synthesis,stability maintenance,interface regulation and multi-level assembly processes.The strategy of ligand exchange is an important method of surface modification and performance regulation for nanocrystals.However,the understanding of the inherent physical and chemical mechanism of ligand exchange needs to be improved,especially in the precise measurement of exchange dynamics processes.Besides,further exploration is needed on the multiple influence factors of ligand exchange thermodynamics.Therefore,this article has used the exchange process of carboxylic acid ligands on the surface of Cd Se nanocrystals as the main research object s.Systematic studies on the dynamics and thermodynamics of ligand exchange were conducted,with exploring the effects of coordination and intermolecular interactions on ligand exchange rate constant and equilibrium constant.The NMR diffusion exchange sequence and the analytical model based on the two-component equilibrium constant have been developed to provide a universal measurement method for ligand exchange on the surface of nanocrystals.The first part of this article has investigated the dynamics of ligand exchange on nanocrystals in solution.In order to measure the millisecond order fast exchange rate constant in solution and avoid the limited resolution of ~1H NMR spectra,diffusion exchange pulse sequences were developed.This article has conducted systematic numerical simulations on various forms of the sequence,and optimized measurement conditions for ligand exchange measurement accurately.Based on the method,the exchange rate constant of different carboxylic acids and amine ligands in solution were obtained,and their underlying influencing mechanisms were disc ussed.This work has provided a universal method for studying the dynamics of ligand exchange on the surface of nanocrystals.The second part of this article has investigated the thermodynamics of ligand exchange on nanocrystals in solution.The two-component equilibrium constant of ligand exchange was found by combining nuclear magnetic resonance and gas chromatography.As a result,an analytical model for the equilibrium constant of two-component was proposed,and the generation mechanism of two-component was discussed.The effects of different Cd Se crystal morphology,crystal size,ligand density and chain length of ligands on the equilibrium constant were studied.Furthermore,the possible influence of solvation on ligand exchange was also discussed.This work has provided a more refined understanding of the thermodynamics of ligand exchange in nanocrystals.