Trioctylphosphine oxide

Trioctylphosphine oxide, frequently referred to simply as TOPO, is a chemical compound. TOPO, a tertiary alkylphosphines, can be used as an extraction or stabilizing agent. It is a stable white solid at room temperature.

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It can be synthesized from phosphorus trichloride, or from a phosphine.

It is now frequently used as a capping ligand for quantum dots (e.g. CdSe quantum dots). In these cases, it stabilizes the nanoparticles in organic solvents. TOPO-coated quantum dots are typically soluble in chloroform, toluene, and (to a lesser extent) hexane.

Tri-n-octylphosphine oxide (TOPO) contains three octyl chains resulting in different conformational structures. Density functional theory (DFT) calculations performed at different levels of theory suggest that the lowest energy conformation of TOPO has C3 symmetry. The next most stable conformation is very close to possessing a plane of symmetry. In addition, there are at least 14 other conformations having no symmetry (not including optical isomers) which lie within 1 kcal/mol of the C3 geometry. The energies of these gauche conformations do not vary in a systematic fashion as the kink (dihedral angle θ=60°) moves towards the phosphorus along the octyl chain.

TOPO has a large permanent dipole moment resulting from the phosphorus-oxygen bond. The calculated vibrational frequencies of P=O bond do not correlate with the electronic density at the bond critical point.