Interplay of Charge State, Lability, and Magnetism in the Molecule-like Au₂₅(SR)₁₈ Cluster

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Abstract

Au25(SR)18 (R = -CH2-CH2-Ph) is a molecule-like nanocluster displaying distinct electrochemical and optical features. Although it is often taken as an example of a particularly well-understood cluster, very recent literature has provided a quite unclear or even a controversial description of its properties. We prepared monodisperse Au25(SR)18(0) and studied by cyclic voltammetry, under particularly controlled conditions, the kinetics of its reduction or oxidation to a series of charge states, -2, -1, +1, +2, and +3. For each electrode process, we determined the standard heterogeneous electron-transfer (ET) rate constants and the reorganization energies. The latter points to a relatively large inner reorganization. Reduction to form Au25(SR)18(2-) and oxidation to form Au25(SR)18(2+) and Au25(SR)18(3+) are chemically irreversible. The corresponding decay rate constants and lifetimes are incompatible with interpretations of very recent literature reports. The problem of how ET affects the Au25 magnetism was addressed by comparing the continuous-wave electron paramagnetic resonance (cw-EPR) behaviors of radical Au25(SR)18(0) and its oxidation product, Au25(SR)18(+). As opposed to recent experimental and computational results, our study provides compelling evidence that the latter is a diamagnetic species. The DFT-computed optical absorption spectra and density of states of the -1, 0, and +1 charge states nicely reproduced the experimentally estimated dependence of the HOMO-LUMO energy gap on the actual charge carried by the cluster. The conclusions about the magnetism of the 0 and +1 charge states were also reproduced, stressing that the three HOMOs are not virtually degenerate as routinely assumed: In particular, the splitting of the HOMO manifold in the cation species is severe, suggesting that the usefulness of the superatom interpretation is limited. The electrochemical, EPR, and computational results thus provide a self-consistent picture of the properties of Au25(SR)18 as a function of its charge state and may furnish a methodology blueprint for understanding the redox and magnetic behaviors of similar molecule-like gold nanoclusters.

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