Oxo-Bridged Dinuclear Chromium(III) Complexes: Correlation between the Optical and Magnetic Properties and the Basicity of the Oxo Bridge

The synthesis and X-ray structure of a new member of the series of oxo-bridged, dinuclear chromium­(III) complexes, the methyl isocyanide complex [(CH₃NC)₅CrOCr­(CNCH₃)₅]­(PF₆)₄·2CH₃CN, is reported. This constitutes only the third oxo-bridged, dinuclear chromium­(III) complex with a homoleptic auxillay ligand sphere. Experimentally, the system shows unshifted narrow nuclear magnetic resonance (NMR) spectra that are consistent with calculations using broken symmetry density functional theory (DFT), which suggests it to be the strongest coupled, dinuclear chromium­(III) complex known. Furthermore, we report the crystal structure and computed magnetic properties for [(bpy)₂(SCN)­CrOCr­(NCS)­(bpy)₂]­(ClO₄)₂·2H₂O (bpy = 2,2′-bipyridine), which differs from other reported oxo-bridged species by featuring a bent CrOCr⁴⁺ core. We also interpret the spectacular 10-orders-of-magnitude variation in acid dissociation constant of the bridging hydroxo ligand in mono hydroxo-bridged dinuclear chromium­(III) complexes, in terms of a valence bond model parametrized by metal-to-metal charge transfer (MMCT) and ligand-to-metal charge transfer (LMCT) energies

Lanthanide Modification of CdSe/ZnS Core/Shell Quantum Dots

Lanthanide-modified CdSe quantum dots (CdSe­(Ln) QDs) have been prepared by heating a solution of Cd­(oleate)₂, SeO₂, and Ln­(bipy)­(S₂CNEt₂)₃ (bipy = 2,2′-bipyridine) to 180–190 °C for 10–15 min. The elemental compositions of the resulting CdSe­(Ln) cores and CdSe­(Ln)/ZnS core/shell QDs show this route to be highly reproducible. The optical absorption spectra of these composite materials are similar to those of the unmodified nanocrystals, but the QD-centered band edge photoluminescence (PL) is partially quenched. The time-gated emission and excitation spectra of the CdSe­(Ln) cores display sensitized lanthanide-centered PL upon higher energy excitation of the nanocrystal host but not upon excitation at the lowest energy QD absorption band. Growth of the ZnS shell led to the depletion of about 60% of the lanthanide ions present together with depletion of nearly all of the lanthanide-centered PL. On these bases, we conclude that the lanthanide-centered PL from the CdSe­(Ln) cores originates with Ln³⁺-related trap states associated with the QD surface