Bright Cyan‐Emissive Copper(I)‐Halide Single Crystals for Multi‐Functional Applications

Anti‐counterfeiting has become a serious issue around the global world; fluorescence‐based tags are praised for the low‐cost and facile decryption process. However, the high‐level anti‐counterfeiting application requires incorporating multifunctional luminescent materials with response to different excitation sources. Herein, a dimeric Cu(I)‐halide‐clusters‐assembled single‐crystal (AEP) 2 Cu 2 I 6 ·2I·2H 2 O (AEP = N ‐aminoethylpiperazinium) concerning a high down‐conversion photoluminescence quantum yield (PLQY) value of near unity is reported (λ ex = 305 nm), which can also be activated by the near‐infrared light and X‐ray. The ultrafast spectroscopy and density functional theory calculations reveal that the broad‐band emission originates from the recombination of self‐trapped excitons. Remarkably, the high PLQY and excellent attenuation capability for X‐ray confers an outstanding light yield of 55 650 photons MeV −1 to (AEP) 2 Cu 2 I 6 ·2I·2H 2 O, which is 5.25 times higher than the standard bismuth germaniumoxide scintillator. Such multi‐light‐stimulus response endows (AEP) 2 Cu 2 I 6 ·2I·2H 2 O with great application potential in anti‐counterfeiting, solid‐state lighting devices, and digital radiography.