Stimuli‐Responsive Triplet Emission and X‐Ray Scintillation via Reversible Structural Switching in Pyromellitic Diimide Cocrystals

ABSTRACT Smart stimuli‐responsive luminescent materials with programmable and reversible emission are highly desirable for intelligent imaging and information security. Stimuli‐responsive organic materials utilizing triplet excitons are particularly attractive owing to their pronounced sensitivity to solid‐state packing. However, crystallinity‐preserving reversible structural switching that enables simultaneous control over X‐ray‐excited luminescence (scintillation) remains largely unexplored. Herein, donor‐acceptor halogen‐bonded cocrystals were constructed using N , N ′‐bis(n‐butyl)pyromellitic diimide (Bu 2 PMDI) and 3,6‐dibromocarbazole (Br 2 Cz) or 3,6‐diiodocarbazole (I 2 Cz). The green cocrystal Bu 2 PMDI‐Br 2 Cz‐G exhibits room‐temperature phosphorescence (RTP) and bright radioluminescence, whereas its orange polymorph is nearly non‐emissive. In contrast, the I 2 Cz‐based cocrystal displays thermally activated delayed fluorescence (TADF), representing an alternative triplet exciton utilization pathway. Notably, mild organic‐vapor stimulation triggers reversible order‐to‐order polymorphic interconversion of Bu 2 PMDI‐Br 2 Cz, which reorganizes halogen‐bonding motifs and, thus achieves crystallinity‐retained “on/off” switching of radioluminescence. Benefiting from efficient triplet exciton utilization, Bu 2 PMDI‐Br 2 Cz‐G enables high‐resolution static X‐ray imaging (37 lp mm −1 ) and real‐time dynamic imaging (2 K, 60 fps) with negligible afterglow. Furthermore, the reversible solvent‐triggered luminescence switching facilitates rewritable multimodal information encryption by integrating photoluminescence and radioluminescence as orthogonal readout channels. This work demonstrates cocrystal engineering as a powerful strategy for reversibly programming triplet‐exciton emission and X‐ray scintillation via structural switching in organic solids.