Extensive studies of luminescent copper(I) complexes were conducted, revealing that some of them exhibit interesting chromic luminescence in response to external stimuli such as temperature, vapor, light, and mechanical force. In this review, recent progress in the field of luminescent chromic copper(I) complexes is discussed. Tetranuclear copper(I) clusters with a cubane-type {Cu4I4} core are the most prominent group, which shows thermochromic, vapochromic, and mechanochromic luminescence, and their responsiveness to external stimuli greatly depends on coordinating organic ligands. Coordination polymerization of copper(I) cluster cores using organic linkers provides new luminescent thermochromic and vapochromic materials with behavior strongly dependent on the flexibility of the framework. The diversity of the cluster core structure and the lability to the ligand exchange are also useful to derive unique behaviors of photochromic and vapochromic response. In addition to such stimuli-responsive luminescen... Luminescent copper(I) complexes have been extensively studied, and some of them exhibit interesting chromic luminescence in response to external stimuli such as temperature, vapor, light, and mechanical force. In this review, the recent progress in the field of luminescent chromic copper(I) complexes is introduced.

Simple mononuclear Cu(I)-halide complexes, [CuX(PPh₃)₂(4-Mepy)] (X = Cl⁻, Br⁻, I⁻; PPh₃ = triphenylphosphine; 4-Mepy = 4-methylpyridine), were prepared. They exhibit blue light emission, with extremely high photoluminescence quantum yields approaching 100% in the crystals. Emission lifetime analyses and density functional theory calculations revealed that the bright blue light emission at room temperature is mainly delayed fluorescence originating from the singlet metal-to-ligand charge transfer (MLCT) state combined with the halide-to-ligand charge transfer (XLCT) state, (¹(M + X)LCT), while that at 77 K is phosphorescence from the ³(M + X)LCT transition state, due to the small singlet–triplet energy differences (ΔE = 940–1170 cm⁻¹). The ternary ligand systems consisting of halide, bulky phosphine, and N-heteroaromatic ligands constitute inexpensive pure-blue-light-emitting materials, which can be fabricated by facile procedures such as simple manual grinding.