Solvent-Modulated Luminescent Spheroidal Assemblies of Cu₈ Nanocluster for Volatile Amine Sensing

Cluster-assembled luminescent microstructures built with metal nanoclusters (NCs) represent a promising class of nanomaterials with diverse applications in photonics and sensing. In this work, we have designed a strategy to make a photoluminescent material by assembling atom-precise NCs of [Cu₈(TFMPT)₄(DPPE)₄] (abbreviated as Cu₈), where TFMPT is 4-hydroxy-6-(trifluoromethyl) pyrimidine-2-thiolate and DPPE is 1,2-bis(diphenylphosphino)ethane. Single-crystal X-ray diffraction (SC-XRD) reveals a unique tetracapped tetrahedral Cu₈ core structure. Upon gradual addition of water (50-85 vol %) to the visibly nonluminescent dimethylformamide (DMF) solution of the clusters, a strong orange luminescence (emission at 625 nm under ultraviolet (UV) light) was observed. It is due to the formation of spheroidal assemblies of nanometer dimension. The cluster-assembled spheroids (CASs) are formed due to hydrophobic interactions among clusters as the concentration of water increases in the solution. Time-correlated single-photon counting reveals that the lifetime of luminescent aggregates is on a microsecond scale, which suggests phosphorescence. Such phosphorescent CASs show a fast response, high selectivity, and naked-eye detection of volatile amines (VAs). Spectroscopic studies and density functional theory (DFT) calculations provide an in-depth understanding of luminescence quenching of CASs and a mechanistic understanding of ammonia and trimethyl amine sensing. The limits of detection (LoD) of ammonia and trimethyl amine were measured to be 0.568 × 10^-7 M (0.001 ppm) and 0.362 0.568 × 10^-7 M (0.002 ppm), respectively. Overall, apart from enriching the family of copper clusters, this work additionally introduces a new photoluminescent material for volatile organic amine (VOA) compound sensing of environmental relevance.