Identification of Acidic Phosphorus-Containing Ligands Involved in the Surface Chemistry of CdSe Nanoparticles Prepared in Tri-N-octylphosphine Oxide Solvents

The surface ligand composition of CdSe nanoparticles prepared using technical grade tri-n-octylphosphine oxide (TOPO) was investigated using a nucleophilic ligand displacement methodology and (31)P {(1)H} NMR spectroscopy. 4-(N,N-Dimethylamino)pyridine (DMAP) and benzyltrimethylammonium propionate were added to tetrahydrofuran solutions of CdSe nanoparticles prepared in technical grade TOPO. DMAP was shown to be a sufficiently strong nucleophile to displace the more weakly coordinating ligands, TOPO, TOPSe, di-n-octylphosphinate, and n-octylphosphonate (OPA). Benzyltrimethylammonium propionate was shown to be a stronger nucleophile than DMAP in that it could displace all the aforementioned surface-bound ligands as well as a previously unidentified surface-bound phosphorus species. Independent synthesis and (31)P {(1)H} NMR spectral matching confirmed that the new species was P,P'-(di-n-octyl) dihydrogen pyrophosphonic acid (PPA). The PPA was shown to form during the nanoparticle synthesis via the dehydrative condensation of OPA. CdSe nanoparticle syntheses were performed using pure TOPO and added OPA, and subsequent displacement experiments showed that OPA and PPA were the predominant surface-bound ligands. CdSe nanoparticle syntheses were performed using pure TOPO and added PPA, and subsequent displacement experiments showed that PPA was the predominant surface-bound ligand. PPA was also shown to have the greatest affinity for the nanoparticle surface of all the ligands investigated. Thus, a model for the surface ligand composition could be developed for nanoparticles prepared using technical grade TOPO or other high-boiling solvents with added acidic phosphorus compounds.