Self-promoted solid-state covalent networking of Au25(SR)18 through reversible disulfide bonds. A critical effect of the nanocluster in oxidation processes

Abstract

Covalent crosslinking of ligand-protected gold nanoclusters offers interesting platforms to investigate the properties associated with the synergetic effects of multiple nanoclusters. In this paper, we report the synthesis of covalent networks of [Au₂₅(SR)₁₈]⁻ nanoclusters using reversible disulfide linkages, which was facilitated by the unique capabilities of the nanocluster to mediate oxidation processes. The conventional Au₂₅ synthesis using 1,6-hexanedithiol afforded a soluble oligodisulfide-appended [Au₂₅(SR)₁₈]⁻ monomer possessing uncoordinated anionic thiolate sites at the terminal ends. Upon exposure to O₂, the monomer spontaneously underwent intercluster crosslinking in the solid state to give free-standing transparent films, in which the nanoclusters were condensed with the retention of the original Au₂₅ framework. Through studies combined with model experiments, the Au₂₅ cluster was found to be involved in the O₂-mediated radical reactions, promoting the formation of intercluster disulfide linkages. The composition of the films implied the involvement of reversible exchange reactions between disulfide and thiyl radicals, from which it was suggested that solid-state crosslinking occurred in adaptive manners under the control of dynamic covalent chemistry. We also demonstrate that the nanocluster film can serve as a robust and efficient heterogeneous photosensitizer to mediate the generation of singlet oxygen. This work demonstrates a unique aspect of the Au₂₅(SR)₁₈-type nanocluster to mediate oxidation processes as well as the utility of the concept of dynamic covalent chemistry in the bottom-up construction of nanomaterials, which would widen the potential of ligand-protected nanoclusters.