Stereoelectronic Effects on the Binding of Neutral Lewis Bases to CdSe Nanocrystals

Using ³¹P nuclear magnetic resonance (NMR) spectroscopy, we monitor the competition between tri- n-butylphosphine (Bu₃P) and various amine and phosphine ligands for the surface of chloride terminated CdSe nanocrystals. Distinct ³¹P NMR signals for free and bound phosphine ligands allow the surface ligand coverage to be measured in phosphine solution. Ligands with a small steric profile achieve higher surface coverages (Bu₃P = 0.5 nm^-2, Me₂P- n-octyl = 2.0 nm^-2, NH₂Bu = >3 nm^-2) and have greater relative binding affinity for the nanocrystal (binding affinity: Me₃P > Me₂P- n-octyl ∼ Me₂P- n-octadecyl > Et₃P > Bu₃P). Among phosphines, only Bu₃P and Me₂P- n-octyl support a colloidal dispersion, allowing a relative surface binding affinity ( K_rel) to be estimated in that case ( K_rel = 3.1). The affinity of the amine ligands is measured by the extent to which they displace Bu₃P from the nanocrystals ( K_rel: H₂NBu ∼ N- n-butylimidazole > 4-ethylpyridine > Bu₃P ∼ HNBu₂ > Me₂NBu > Bu₃N). The affinity for the CdSe surface is greatest among soft, basic donors and depends on the number of each ligand that bind. Sterically unencumbered ligands such as imidazole, pyridine, and n-alkylamines can therefore outcompete stronger donors such as alkylphosphines. The influence of repulsive interactions between ligands on the binding affinity is a consequence of the high atom density of binary semiconductor surfaces. The observed behavior is distinct from the self-assembly of straight-chain surfactants on gold and silver where the ligands are commensurate with the underlying lattice and attractive interactions between aliphatic chains strengthen the binding.