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

Using 31 P nuclear magnetic resonance (NMR) spectroscopy, we monitor the competition between tri- n-butylphosphine (Bu3 P) and various amine and phosphine ligands for the surface of chloride terminated CdSe nanocrystals. Distinct 31 P 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 (Bu3 P = 0.5 nm-2 , Me2 P- n-octyl = 2.0 nm-2 , NH2 Bu = >3 nm-2 ) and have greater relative binding affinity for the nanocrystal (binding affinity: Me3 P > Me2 P- n-octyl ∼ Me2 P- n-octadecyl > Et3 P > Bu3 P). Among phosphines, only Bu3 P and Me2 P- n-octyl support a colloidal dispersion, allowing a relative surface binding affinity ( Krel ) to be estimated in that case ( Krel = 3.1). The affinity of the amine ligands is measured by the extent to which they displace Bu3 P from the nanocrystals ( Krel : H2 NBu ∼ N- n-butylimidazole > 4-ethylpyridine > Bu3 P ∼ HNBu2 > Me2 NBu > Bu3 N). 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.