Quantifying Ligand Exchange Reactions at CdSe Nanocrystal Surfaces

Abstract

The exchange reactions of X-type ligands at the surface of CdSe nanocrystals have been quantified via 1H NMR, absorbance, and emission spectroscopies. 1H NMR was used to quantify displacement reactions of oleate-capped CdSe nanocrystals with carboxylic-acid-, phosphonic-acid-, and thiol-terminated ligands that incorporate terminal alkene functionalities. The alkenyl protons of the native oleate ligand and the vinylic protons of the exchange ligand provide spectroscopic handles to quantify both free and surface-bound forms of these ligands. Undec-10-enoic acid was found to undergo an exchange equilibrium with oleate (Keq = 0.83), whereas phosphonic-acid- and thiol-terminated ligands irreversibly displace native oleate ligands. Absorption and emission experiments indicate that the carboxylic acid exchanges occur solely between surface-bound ligands, whereas exchange reactions with phosphonic acids and thiols alter the surface metal atoms of the nanocrystals, presumably through the displacement of Cd(oleate)...