Collision-Induced Dissociation of II−VI Semiconductor Nanocrystal Precursors, Cd2+ and Zn2+ Complexes with Trioctylphosphine Oxide, Sulfide, and Selenide

The metal (M = Cd2+ and Zn2+) complexes with trioctylphosphine chalcogenide (TOPE, E = O, S, and Se) are prepared by electrospray ionization, and their relative stabilities and intramolecular reactions are studied by collision-induced dissociation (CID) with Xe under single collision conditions. These metal−TOPE complexes are considered as molecular precursors for the colloidal synthesis of II−VI compound semiconductor nanocrystals employing TOPO as a metal-coordinating solvent and TOPS or TOPSe as a chalcogen precursor. Of the various [M + nTOPE]2+ (n = 2−7) ions generated by ESI, the n = 2−4 complexes are characterized by CID as a function of collision energy. The collision energy at 50% dissociation (E 50%) is determined from the cracking curve and the relative stabilities of the complexes are established. Between the two metal ions, the zinc−TOPE complexes are more stable than the cadmium−TOPE complexes when n = 2−3, whereas their stabilities are reversed when n = 4. Of the TOPE, TOPO binds most strongly to the metal ion, while TOPSe does most weakly. Upon CID, loss of TOPE occurs exclusively from the tetra-TOPE complexes, while extensive fragmentation of TOPE takes place from the di-TOPE complexes, showing the signature of the metal chacogenide formation. The nucleation of nanocrystals appears to begin with cracking of [M + 2TOPE]2+ (E = S and Se).