Core-Doped [(Cd1- xCox)10S4(SPh)16]4- Clusters from a Self-Assembly Route

Abstract

The incorporation of substitutional Co²⁺ impurities in [Cd₁₀S₄(SPh)₁₆]^4- (Cd₁₀) molecular clusters prepared by the self-assembly method where Na₂S is the sulfur precursor and a redox method where elemental S is the sulfur precursor is studied. The Co²⁺ ions provide unique spectroscopic and chemical handles to monitor dopant speciation during cluster formation and determine what role, if any, other cluster species play during Cd₁₀ cluster formation. In contrast to the redox method that produces exclusively surface-exchanged Co²⁺-doped Cd₁₀ (Co:Cd₁₀), the preparation of Cd₁₀ by the self-assembly method in the presence of Co²⁺ ions results in Co²⁺ incorporation at both the surface and core sites of the Cd₁₀ cluster. Electrospray ionization mass spectrometry (ESI-MS) analysis of the dopant distribution for the self-assembly synthesis of Co:Cd₁₀ is consistent with a near-Poissonian distribution for all nominal dopant concentrations albeit with reduced actual Co²⁺ incorporation. At a nominal Co²⁺ concentration of 50%, we observe incorporation of up to seven Co²⁺ ions within the Cd₁₀ self-assembled cluster compared to a maximum of only four Co²⁺ dopants in the Cd₁₀ redox clusters. The observation of up to seven Co²⁺ dopants must involve substitution of at least three core sites within the Cd₁₀ cluster. Electronic absorption spectra of the Co²⁺ ligand field transition in the heavily doped Co:Cd₁₀ clusters display clear deviation with the surface-doped Co²⁺-doped Cd₁₀ clusters prepared by the redox method. We hypothesize that the coordination of Co²⁺ and S^2- ions in solution prior to cluster formation, which is possible only with the self-assembly method, is critical to the doping of Co²⁺ ions within the Cd₁₀ cores.