Milling-Induced "Turn-off" Luminescence in Copper Nanoclusters

Atomically precise copper nanoclusters (NCs) attract research interest due to their intense photoluminescence, which enables their applications in photonics, optoelectronics, and sensing. Exploring these properties requires carefully designed clusters with atomic precision and a detailed understanding of their atom-specific luminescence properties. Here, we report two copper NCs, [Cu₄(MNA)₂(DPPE)₂] and [Cu₆(MNA-H)₆], shortly Cu₄ and Cu₆, protected by 2-mercaptonicotinic acid (MNA-H₂) and 1,2-bis(diphenylphosphino)ethane (DPPE), showing "turn-off" mechanoresponsive luminescence. Single-crystal X-ray diffraction reveals that in the Cu₄ cluster, two Cu₂ units are appended with two thiols, forming a flattened boat-shaped Cu₄S₂ kernel, while in the Cu₆ cluster, two Cu₃ units form an adamantane-like Cu₆S₆ kernel. High-resolution electrospray ionization mass spectrometry studies reveal the molecular nature of these clusters. Lifetime decay profiles of the two clusters show the average lifetimes of 0.84 and 1.64 μs, respectively. These thermally stable Cu NCs become nonluminescent upon mechanical milling but regain their emission upon exposure to solvent vapors. Spectroscopic data of the clusters match well with their computed electronic structures. This work expands the collection of thermally stable and mechanoresponsive luminescent coinage metal NCs, enriching the diversity and applications of such materials.