Thermodynamics of Ligand Exchange with Aromatic Ligands on the Surface of CdSe Quantum Dots

Abstract

Quantum dots (QDs) have great application prospects in optoelectronic devices because of their excellent optical properties. Nevertheless, the original ligands of the long carbon chain introduced in the synthesis process are not conducive to the application of CdSe QDs in optoelectronic devices due to their insulating nature. Therefore, short-chain and aromatic ligands are employed to replace the original ligands. However, the influences of conjugation structures on ligand exchange have rarely been systematically studied from a thermodynamic perspective. In this work, two types of aromatic ligands, namely, a series of benzoic acid (BA) ligands and cinnamic acid (CA) ligands that introduced electron-donating (methoxy-) and electron-withdrawing groups (fluoro-) at the meta and para positions of the benzene ring of aromatic ligands, were chosen to investigate the ligand exchanges on the surface of QDs from oleic acid (OA) to a series of aromatic ligands by 1H nuclear magnetic resonance (NMR) spectroscopy and isothermal titration calorimetry (ITC). 1H NMR showed that the average exchange ratio of CA ligands to OA was ∼1:1, which made it possible to obtain thermodynamic parameters through the determined ligand exchange model by ITC. Furthermore, thermodynamic results showed that ligand exchange with CA ligands was endothermic (ΔH > 0) and entropy-increasing (ΔS > 0), indicating that the process was entropy-driven. In addition, the equilibrium constants (K) of ligand exchange with CA ligands were larger than those with BA ligands. However, the K values of ligand exchange with BA ligands were ∼1, which resulted in ΔG ∼ 0 kJ/mol, demonstrating that ligand exchange with BA ligands was less favorable at 298.15 K. Finally, the introduction of substituents on meta and para positions of the aromatic ligand benzene ring had no significant effect on the ligand exchange process. This work not only elaborates on the process of exchange with aromatic ligands but also has fundamental significance for the application of aromatic ligand-modified QDs in optoelectronic devices.