Self‐Trapped Exciton Regulation in a Copper–Iodide Hybrid for Flexible X‐Ray Scintillation Imaging and Optical Encryption

ABSTRACT Controlling self‐trapped excitons (STEs) provides a promising strategy for achieving tunable photoluminescence (PL) in metal halides, but realizing multiple STEs in a single‐component system remains challenging. Herein, we report a Cu(I) halide hybrid scintillator, [(C 4 H 16 N 3 )·H 2 PO 2 ]CuI 3 , exhibiting excitation‐dependent dual emission, reversible thermochromic luminescence, and superior x‐ray scintillation performance. This emission behavior originates from two competing STEs whose relative populations are modulated by the excitation wavelength, temperature, and local lattice distortion. The high‐energy cyan and low‐energy yellow emissions originate from two distinct STE configurations associated with different local distortions and Cu···Cu interactions, enabling temperature‐dependent color tuning through STE population redistribution. The high photoluminescence quantum yields (PLQYs) also endow [(C 4 H 16 N 3 )·H 2 PO 2 ]CuI 3 with strong radioluminescence, an estimated light yield of 34 353 photons MeV −1 , and a low detection limit of 371 nGy air s −1 . Incorporating the crystals into a SEBS matrix produces a large‐area and highly flexible [(C 4 H 16 N 3 )·H 2 PO 2 ]CuI 3 @SEBS composite scintillation film for x‐ray imaging, achieving a spatial resolution of 14.25 lp·mm −1 . The abundant photophysical properties highlight the multiple optoelectronic applications of [(C 4 H 16 N 3 )·H 2 PO 2 ]CuI 3 in x‐ray imaging, anti‐counterfeiting, and secure communication.