CdSe Clusters: At the Interface of Small Molecules and Quantum Dots

We have prepared and isolated a variety of cadmium selenide cluster molecules by the reaction of Cd(O2C-R)(2)(H2NR')(2) (R = C4H9, 4-X-Ph; X = H, -OMe, -SMe, -Me, -F, and -CF3; R' = dodecyl, phenethyl) with tri-n-octylphosphine selenide (TOPSe) and diphenylphosphine selenide (DPPSe). Reactions conducted with 1.0 equiv of DPPSe lead to a cluster with a lowest energy electronic transition at 418 nm (d approximate to 1.5 nm, epsilon(418 nm) = 189 000 +/- 27 000 M-1 cm(-1)) and a broad (fwhm = 0.75 eV) and intense white light emission profile (Phi(PL) = 0.11(03)). Nuclear magnetic resonance spectroscopy, Rutherford backscattering spectrometry and solution molecular weight determinations support a molecular formula of [(CdSe)(4)(Cd(O2C-C6H5)(2))(H2NC12H25)(2)](7(+/- 1)). Mechanistic studies indicate that 2 equiv of DPPSe convert to [Se2PPh2](-)[H3NR'](+) and diphenyl phosphine upon mixing with H2NR' and prior to reaction with cadmium benzoate. Reactions with TOPSe result in a series of CdSe nanocrystals characterized by discrete steps in size with absorption features between 330 and 550 nm. The importance of precursor reactivity to the outcome of the cluster growth is discussed.