N-Heterocyclic Carbene-Stabilized Gold-Copper Nanoclusters: Synthesis, Bonding and Mechanochromism

N-heterocyclic carbene (NHC) ligands have emerged as highly effective surface ligands for the protection and functionalization of metal nanoclusters (NCs). However, research on NHC-stabilized metal NCs, including their synthesis, structure, properties, and applications, is still in its early stages. In this study, we present the first gold-copper alloy cluster protected by both NHC and alkyne ligands, denoted as Au3Cu(iPrNHCiPr)(PA)4 (abbreviated as Au3Cu, where iPrNHCiPr is a bidentate N-heterocyclic carbene ligand with isopropyl as the N-substituent, and PA is a phenylethynyl group). The precise composition of Au3Cu was confirmed through the utilization of electrospray ionization mass spectrometry (ESI-MS), and its structure was determined via X-ray single-crystal diffraction. It is worth noting that although the Au3Cu clusters do not display substantial light emission when exposed to UV lamps, they are capable of emitting green fluorescence subsequent to undergoing mechanical milling (?em = 500 nm). Powder X-ray diffraction (PXRD) analysis reveals that this transition is attributed to a crystalline-amorphous transformation of the cluster crystals. These atomically precise alloy clusters are expected to serve as a model for further investigation into the principles of mechanical milling of metal clusters for discolouration.