Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal-Carboxylate Displacement and Binding

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• 作者：Nicholas C. Anderson ; Mark P. Hendricks ; Joshua J. Choi ; Jonathan S. Owen
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：December 11, 2013
• 年：2013
• 卷：135
• 期：49
• 页码：18536-18548
• 全文大小：545K
• 年卷期：v.135,no.49(December 11, 2013)
• ISSN：1520-5126

文摘

We demonstrate that metal carboxylate complexes (L鈥揗(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鈥?N鈥?tetramethylbutylene-1,4-diamine, pyridine, N,N,N鈥?N鈥?tetramethylethylene-1,2-diamine, n-octylamine). The relative displacement potency is measured by ¹H 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鈥揗(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 卤 0.06 to 1.0 卤 0.05, broadens the 1S_e鈥?S_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.