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

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 (Bu2PMDI) and 3,6-dibromocarbazole (Br2Cz) or 3,6-diiodocarbazole (I2Cz). The green cocrystal Bu2PMDI-Br2Cz-G exhibits room-temperature phosphorescence (RTP) and bright radioluminescence, whereas its orange polymorph is nearly non-emissive. In contrast, the I2Cz-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 Bu2PMDI-Br2Cz, which reorganizes halogen-bonding motifs and, thus achieves crystallinity-retained "on/off" switching of radioluminescence. Benefiting from efficient triplet exciton utilization, Bu2PMDI-Br2Cz-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.