Superatom Fusion and the Nature of Quantum Confinement.

Quantum confinement endows colloidal semiconducting nanoparticles with many fascinating and useful properties, yet a critical limitation has been the lack of atomic precision in their size and shape. We demonstrate the emergence of quantum confined behavior for the first time in atomically defined CoSe(PEt) superatoms by dimerizing [CoSe] units through direct fusion. To accomplish this dimerization, we install a reactive carbene on the [CoSe] core to create a latent fusion site. Then we transform the reactive carbene intermediate into a material with an expanded core, [CoSe], that exhibits electronic and optical properties distinct from the parent monomer. The chemical transformation presented herein allows for precise synthetic control over the ligands and size of these clusters. We show by cyclic voltammetry, infrared spectroscopy, single crystal X-ray diffraction, and density functional theory calculations that the resulting fused [CoSe] material exhibits strong electronic coupling and electron delocalization. We observe a bandgap reduction upon expanding the cluster core, suggesting that we have isolated a new intermediate in route to extended solids. These results are further corroborated with electronic structure calculations of a monomer, fused dimer, trimer, and tetramer species. These reactions will allow for the synthesis of extended highly delocalized wires, sheets, and cages.