Mechanochromism‐Free Cu ₄ I ₄ Complex‐Based Scintillators for Highly‐Efficient X‐Ray Excited Luminescence and Highly‐Resolution X‐Ray Tomography Imaging

Abstract Copper(I) halide cluster‐based complexes (especially Cu 4 X 4 ) are promising scintillators, but their practical application is limited by mechanochromism and nonradiative decay from cluster‐centered (3CC) transitions. Herein, novel octahedral or distorted‐octahedral Cu 4 X 4 (X = Cl, Br, I) clusters are reported, stabilized by a rigid diphenyl‐2‐pyridylphosphine ligand. These structures differ from classical cubane or stairstep geometries. The rigid ligand and intermolecular π – π interactions (π Ph –π Ph or π Py –π Ph ) effectively suppress 3 CC‐induced distortions and promote radiative 3 (M+X)LCT emission. A small singlet–triplet energy gap (ΔE ST ) enables dual emission pathways via thermally activated delayed fluorescence (TADF) and phosphorescence, leading to shortened lifetimes, high quantum yields (PLQY up to 87.77%), and a steady‐state X‐ray relative light yield 2.5‐fold higher than commercial scintillator Bi 4 Ge 3 O 12 . The corresponding flexible scintillator films exhibit excellent spatial resolution (>20 lp mm −1 ), and enable accurate 3D reconstruction of complex industrial components. These results highlight a robust strategy to enhance luminescence efficiency and stability of Cu 4 X 4 ‐based scintillators, offering great potential for high‐resolution panoramic X‐ray imaging and X‐ray tomography in medical diagnosis and industrial non‐destructive inspection.