title: Ligand exchange and the stoichiometry of metal chalcogenide nanocrystals: spectroscopic observation of facile metal-carboxylate displacement and binding.
abstract: We demonstrate that metal carboxylate complexes (L-M(O2CR)2, R = oleyl, tetradecyl, M = Cd, Pb) are readily displaced from carboxylate-terminated ME nanocrystals (ME = CdSe, CdS, PbSe, PbS) by various Lewis bases (L = tri-n-butylamine, tetrahydrofuran, tetradecanol, N,N-dimethyl-n-butylamine, tri-n-butylphosphine, N,N,N',N'-tetramethylbutylene-1,4-diamine, pyridine, N,N,N',N'-tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by (1)H NMR spectroscopy and depends most strongly on geometric factors such as sterics and chelation, although also on the hard/soft match with the cadmium ion. The results suggest that ligands displace L-M(O2CR)2 by cooperatively complexing the displaced metal ion as well as the nanocrystal. Removal of up to 90% of surface-bound Cd(O2CR)2 from CdSe and CdS nanocrystals decreases the Cd/Se ratio from 1.1 ± 0.06 to 1.0 ± 0.05, broadens the 1S(e)-2S(3/2h) absorption, and decreases the photoluminescence quantum yield (PLQY) from 10% to <1% (CdSe) and from 20% to <1% (CdS). These changes are partially reversed upon rebinding of M(O2CR)2 at room temperature (∼60%) and fully reversed at elevated temperature. A model is proposed in which electron-accepting M(O2CR)2 complexes (Z-type ligands) reversibly bind to nanocrystals, leading to a range of stoichiometries for a given core size. The results demonstrate that nanocrystals lack a single chemical formula, but are instead dynamic structures with concentration-dependent compositions. The importance of these findings to the synthesis and purification of nanocrystals as well as ligand exchange reactions is discussed.
authors.affiliation: Department of Chemistry, Columbia University , 3000 Broadway, MC 3121, New York, New York 10027, United States.
authors.forename: Nicholas C
authors.initials: NC
authors.lastname: Anderson
authors.forename: Mark P
authors.initials: MP
authors.lastname: Hendricks
authors.forename: Joshua J
authors.initials: JJ
authors.lastname: Choi
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Owen
mesh_terms: D001665:Binding Sites; D002264:Carboxylic Acids / Q000737:chemistry*; D018011:Chalcogens / Q000737:chemistry*; D008024:Ligands; D008670:Metals / Q000737:chemistry*; D053758:Nanoparticles; D011789:Quantum Theory; D013696:Temperature
chemical_list: D002264:Carboxylic Acids; D018011:Chalcogens; D008024:Ligands; D008670:Metals