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 (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.

This work demonstrates that reversible structural switching in pyromellitic diimide cocrystals enables programmable triplet‐exciton emission, X‐ray scintillation, and high‐performance X‐ray imaging, showcasing stimuli‐responsive organic scintillators for rewritable multimodal information encryption.

1. Introduction

Smart stimuli‐responsive luminescent materials that enable high‐contrast and programmable emission switching are of growing interest for rewritable information security, adaptive sensing, and advanced imaging [1, 2, 3]. In this context, organic materials utilizing triplet‐state emission are particularly attractive: Their long‐lived states, not only provide intrinsic temporal contrast, but also render the luminescence highly sensitive to subtle changes in solid‐state packing and intermolecular interactions, as well as to external stimuli that perturb molecular organization [4, 5]. In addition, the fully organic composition of these materials offers advantages in flexibility, facile processability, and environmental compatibility, making them promising candidates for next‐generation smart luminescent platforms [6, 7].

Among the various strategies for developing stimuli‐responsive materials, cocrystal engineering has emerged as a powerful platform for modulating solid‐state photo physics through directional supramolecular interactions [8, 9]. By assembling donor and acceptor molecules into multicomponent crystals, cocrystals provide access to multiple nearby packing‐energy minima, enabling fine regulation of local rigidity, electronic coupling, and excited‐state dynamics without covalent modification [10]. For example, recent studies based on halogen‐bond cocrystals highlight the versatility of this approach, enhancing molecular packing and improving their optical properties [11, 12]. Despite these advancements, challenges remain in achieving stimulus‐responsive processes that rely on reversible or non‐destructive structural transformations, which limit the potential for reliable and clear structure–property correlations [13, 14, 15]. In materials systems based on triplet excitons, reversible and crystallinity‐preserving cocrystal‐to‐cocrystal transformations remain largely underexplored, making it difficult to achieve deterministic control over triplet‐state energetics, emission color, and lifetime. Therefore, developing a cocrystal system that couples reversible, order‐to‐order structural reorganization with predictable triplet‐state modulation remains a significant challenge.

This challenge becomes even more consequential under high‐energy excitation. Radioluminescence (RL) in organic systems depends, not only on X‐ray attenuation, which is enhanced by heavy atoms, but also on how supramolecular packing mediates intersystem crossing, triplet stabilization, and nonradiative decay pathways [16, 17]. To date, organic scintillators have been essentially static: Their RL output is fixed by the as‐grown packing motif and cannot be modulated by mild external stimuli [18]. If a cocrystal could be reversibly switched between two ordered polymorphs in which only one packing configuration efficiently harvests and emits triplets under X‐ray excitation, while also utilizing triplet excitons to achieve high radioluminescence, then stimuli‐gated X‐ray responsiveness—an RL “on/off” function—would become possible. Such orthogonal control over scintillation would provide a powerful foundation for multimodal information encoding, advanced anti‐counterfeiting, and adaptive X‐ray imaging.

To realize this concept, an organic cocrystal system capable of reversible cocrystal‐to‐cocrystal transformation and exhibiting stimuli‐gated radioluminescence is reported (Scheme 1). A donor–acceptor cocrystal system was designed, comprising a rigid imide acceptor, N, N'‐bis(n‐butyl)‐pyromellitic dimide (Bu₂PMDI), and halogenated carbazole donors, 3,6‐di(halogen)carbazole (X₂Cz, X = Br, I). One Br‐containing polymorph, Bu₂PMDI‐Br₂Cz‐G, exhibits strong RTP and bright X‐ray‐induced emission. In contrast, the alternative polymorph, Bu₂PMDI‐Br₂Cz‐O, is nearly non‐emissive under X‐ray irradiation. Interestingly, the I₂Cz‐based cocrystal demonstrates thermally activated delayed fluorescence (TADF) behavior, showcasing an alternative pathway for triplet exciton utilization. Moreover, a delicate balance between halogen bonding and π–π interactions results in multiple metastable packing configurations that can be interconverted by mild solvent‐vapor fumigation, all while preserving crystallinity. Based on this solvent‐triggered photoluminescence and radioluminescence switching, we further achieved rewritable multimodal information encryption. It establishes a supramolecular paradigm for programmable organic scintillators, where reversible solid‐state reorganization enables stimuli‐gated X‐ray functionality.

SCHEME 1.

Chemical structures of the cocrystals and a schematic illustration of the scintillation mechanism under X‐ray irradiation, including the distinct emission pathways in different cocrystals and the reversible polymorphic transformation.

2. Result and Discussion

Scheme 1 illustrates the radioluminescence mechanism of the host‐guest cocrystal system. Upon X‐ray irradiation, high‐energy photons interact with atoms in the cocrystal via the photoelectric effect and compton scattering, generating energetic secondary electrons. These electrons subsequently induce further ionization events, leading to the formation of electron–hole pairs that recombine to produce singlet (25%) and triplet (75%) excitons. The triplet excitons either relax radiatively to the ground state to yield phosphorescence or undergo reverse intersystem crossing (RISC) to regenerate singlet excitons, resulting in delayed fluorescence [19, 20].

Halogenated carbazole derivatives (Br₂Cz or I₂Cz), acting as planar electron donors, co‐assemble with Bu₂PMDI to form donor‐acceptor cocrystals. Building on the pioneering work of the George group, which established PMDI as promising triplet‐emissive scaffolds for tuning photo physics [21, 22], the introduction of external heavy atoms (Br or I) here, not only enhances X‐ray absorption, but also reinforces intermolecular interactions, thereby activating distinct triplet‐exciton emission pathways [23].

2.1. Cocrystal Structures and Reversible Switching

As shown in Figures S1–S3, Bu₂PMDI was synthesized and its chemical purity was confirmed by NMR and mass spectrometry. Subsequently, three cocrystals with distinct morphologies and polymorphic forms were obtained via a vapor diffusion method (Figure S4). Specifically, employing Br₂Cz as the guest afforded green needle‐like crystals (Bu₂PMDI‐Br₂Cz‐G) from dichloromethane (DCM) and orange cocrystals (Bu₂PMDI‐Br₂Cz‐O) from ethyl acetate (EA), whereas using I₂Cz yielded orange block‐shaped crystals (Bu₂PMDI‐I₂Cz) from EA. This pronounced solvent dependence and morphological diversity, not only highlight the inherent polymorphic adaptability of the cocrystal system, but also provide a robust foundation for achieving reversible structural regulation [11]. Besides, X‐ray photoelectron spectroscopy (XPS) confirmed the presence of C, N, O, and Br (or I) in the cocrystals (Figures S5–S8). Powder X‐ray diffraction (PXRD) further verified their phase purity, with experimental patterns in excellent agreement with those simulated from single‐crystal X‐ray diffraction data (CCDC 2417751, 2420157, and 2420076; Figures S9), confirming the high crystallinity and compositional integrity of the as‐prepared samples.

To elucidate the structural origins of the distinct photophysical behaviors and evaluate the feasibility of dynamic switching, the single‐crystal structures of the three cocrystals were systematically analyzed. As summarized in Table S1, Bu₂PMDI‐Br₂Cz‐G crystallizes in the monoclinic space group P2₁/c, whereas both Bu₂PMDI‐Br₂Cz‐O and Bu₂PMDI‐I₂Cz are triclinic lattices P‐1. The packing models show that, the guest molecules (Br₂Cz or I₂Cz) are arranged in both lateral and vertical directions relative to the Bu₂PMDI host (Figure S10). Given that Bu₂PMDI is a ditopic symmetric acceptor with two equivalent carbonyl acceptor sites [24], the intermolecular interactions were analyzed on the basis of representative host‐centered packing fragments containing both laterally and vertically associated guests. In Bu₂PMDI‐Br₂Cz‐G (Figure 1a), two lateral C═O···Br halogen bonds (3.48 Å) are formed on the two equivalent sides of the Bu₂PMDI core, accompanied by multiple lateral C═O···H─C contacts (3.36 and 3.53 Å), thus generating a symmetric interaction pattern. By contrast, Bu₂PMDI‐Br₂Cz‐O adopts an asymmetric lateral arrangement, in which one Br₂Cz engages in a C═O···Br halogen bond (3.66 Å), whereas the other forms a C═O···H─N hydrogen bond (2.82 Å), while retaining the C═O···H─C contacts. Both polymorphs preserve pronounced vertical π–π stacking interactions (Figure S11 and Table S2), which contribute to structural rigidification and lattice stabilization [25]. This cooperative coexistence pattern of halogen and hydrogen bonding is also consistent with previously reported PmDI‐based cocrystal systems [10, 22].

FIGURE 1.

(a) Schematic illustration of the reversible cocrystal transformation between Bu₂PMDI‐Br₂Cz‐G and Bu₂PMDI‐Br₂Cz‐O upon vapor fumigation, along with cocrystal structure and interaction analysis. The inset shows photographs of the corresponding powder samples under daylight. (b) PXRD patterns of Bu₂PMDI‐Br₂Cz‐O, transformed to Bu₂PMDI‐Br₂Cz‐G and subsequently restored, demonstrating the reversible cocrystal transformation. (c) Cocrystal structures of Bu₂PMDI‐I₂Cz and interaction analysis.

Motivated by the preferential crystallization of Bu₂PMDI‐Br₂Cz‐G and Bu₂PMDI‐Br₂Cz‐O from DCM and EA, respectively, solvent‐induced polymorphic interconversion was investigated. Upon DCM fumigation, the orange Bu₂PMDI‐Br₂Cz‐O powder transforms into a pale green powder, which reverts to its original state upon EA fumigation. This transformation was further verified by PXRD: The diffraction pattern of Bu₂PMDI‐Br₂Cz‐O is replaced by peaks characteristic of Bu₂PMDI‐Br₂Cz‐G after DCM treatment (Figure 1b), while subsequent EA fumigation restores the original Bu₂PMDI‐Br₂Cz‐O pattern, demonstrating a reversible, order‐to‐order cocrystal transformation [26]. Several diffraction peaks in the transformed powders are weaker than those of the corresponding solution‐grown crystals, likely owing to the reduced crystallinity associated with the solid‐state transformation process. In addition, a few weak residual peaks may arise from minor incompletely transformed domains [26]. Notably, despite retention of the global packing framework, subtle variations in the types of weak intermolecular interactions, together with conformational flexibility and sufficient free volume, enable this cocrystal transformation under mild vapor stimulation [27, 28, 29].

Optical microscopy was used to monitor the changes in interference color and UV emission during crystal fumigation (Figure S12). As shown in Video S2 and Figure S13, upon gradually increasing the DCM vapor concentration, the edges of the Bu₂PMDI‐Br₂Cz‐O crystals became progressively blurred, while newly formed microcrystals with distinct interference colors appeared, indicating the generation of the Bu₂PMDI‐Br₂Cz‐G phase through continuous dissolution and recrystallization. UV microscopy further supported this interpretation. The initial Bu₂PMDI‐Br₂Cz‐O crystals were essentially non‐emissive, whereas the newly formed microcrystals exhibited bright fluorescence (Video S3 and Figure S13). In contrast, when Bu₂PMDI‐Br₂Cz‐G was exposed to EA vapor, new needle‐like crystals gradually grew from the edges of the parent crystals and were accompanied by changes in interference color (Video S4 and Figure S14). At the same time, the initially emissive Bu₂PMDI‐Br₂Cz‐G crystals progressively lost fluorescence and transformed into the non‐emissive Bu₂PMDI‐Br₂Cz‐O phase (Video S5 and Figure S14). These observations indicate that, the crystal transformation proceeds through a dissolution‐recrystallization pathway in specific solvent environments [30].

In contrast, the Bu₂PMDI‐I₂Cz cocrystal exhibits a distinct interaction pattern. As shown in Figure 1c, the lateral interactions are dominated by two symmetry‐related C═O···H‐N hydrogen bonds (2.91 Å), while the vertical π–π stacking is accompanied by a C═O···I contact (3.87 Å). This altered balance between halogen bonding and hydrogen bonding generates a supramolecular environment fundamentally different from that of the Br‐based cocrystals. As complementary evidence, the Fourier transform infrared spectroscopy (FT‐IR) further corroborates these interaction differences. As shown in Figure S15, relative to pristine Bu₂PMDI (C═O stretching at 1694 cm⁻¹), all co‐crystals display a blue‐shifted C═O band near 1700 cm⁻¹, consistent with C═O···H─N and C═O···Br/I interactions [31]. Moreover, the N─H stretching bands of Bu₂PMDI‐Br₂Cz‐O and Bu₂PMDI‐I₂Cz shift to lower wavenumbers (3378 and 3324 cm⁻¹) compared with the amine N─H bands of the guests (Br₂Cz: 3410 cm⁻¹, I₂Cz: 3412 cm⁻¹), unambiguously confirming hydrogen‐bond formation [32].

2.2. Switchable Photophysical and Radioluminescence Properties

Differences in supramolecular interactions lead to pronounced disparities in the photophysical properties and multimodal excitation of these cocrystals. Compared with the individual components of Bu₂PMDI, Br₂Cz, I₂Cz, which show absorption bands in the 200–350 nm region arising from n–π^* and π–π^* transitions, Bu₂PMDI‐Br₂Cz‐O and Bu₂PMDI‐I₂Cz display pronounced broad absorption features in the 400–550 nm region, consistent with the formation of charge‐transfer (CT) states, whereas such CT character is much less evident in Bu₂PMDI‐Br₂Cz‐G (Figures 2a and S16). As shown in Figure 2b,c, temperature‐dependent measurements reveal that decreasing temperature from 280 to 77 K markedly prolongs the emission lifetimes and enhances emission intensities of Bu₂PMDI‐Br₂Cz‐O, Bu₂PMDI‐Br₂Cz‐G, and Bu₂PMDI‐I₂Cz cocrystals, consistent with suppressed non‐radiative relaxation of triplet excitons at low temperatures [33]. Besides, relative to pristine Bu₂PMDI with a maximum emission wavelength of 391 nm, all cocrystals display pronounced red‐shifted emissions, highlighting the critical role of intermolecular interactions in regulating excited‐state energetics (Figures S17 and 2d–f).

FIGURE 2.

(a) UV–vis absorption spectra, (b) Temperature‐dependent lifetime decay profiles and (c) temperature‐dependent steady‐state emission spectra of all cocrystals. Steady‐state photoluminescence spectra (solid lines) and delayed spectra (dashed lines) recorded at 280 K and 77 K for (d) Bu₂PMDI‐Br₂Cz‐G, (e) Bu₂PMDI‐Br₂Cz‐O, and (f) Bu₂PMDI‐I₂Cz. (g) RL spectra under X‐ray excitation at a dose rate of 278 µGy s⁻¹ (50 kV, 79 µA), with the inset showcasing photographs of Bu₂PMDI‐Br₂Cz‐G and Bu₂PMDI‐I₂Cz powders under X‐ray irradiation. (h) The dose rate dependence of RL intensities across the range of 4.58 to 278 µGy s⁻¹. (i) Enhanced RL of Bu₂PMDI‐Br₂Cz‐O after DCM fumigation, recorded under X‐ray excitation at a dose rate of 278 µGy s⁻¹. The inset shows the RL of Bu₂PMDI‐Br₂Cz‐O under identical conditions.

At 280 K, Bu₂PMDI‐Br₂Cz‐G exhibits an emission maximum at 493 nm with a long decay lifetime of 700 µs (Figure S18). The delayed spectrum overlaps with the steady‐state photoluminescence (PL), and no obvious nanosecond‐scale decay component is observed (Figure 2d), indicating that the emission mainly originates from triplet excitons [34]. Electron‐hole distribution analysis further confirms that, the triplet state is of locally excited (LE) character (Figure S19), and no obvious charge‐transfer absorption band is observed in the absorption spectrum. Therefore, the emission at 280 K can be assigned to ³LE phosphorescence [35]. Upon cooling to 77 K, the delayed and PL spectra remain largely overlapped, while the emission maximum red‐shifts to 517 nm. This red shift may reflect the stabilization of a lower‐energy triplet emissive state at low temperature [36], whereas the restricted molecular rotation and vibration further suppress nonradiative decay, making this emission more pronounced [37].

In contrast, the polymorph Bu₂PMDI‐Br₂Cz‐O shows red‐shifted delayed emission accompanied by mixed prompt fluorescence (2.93 ns) and phosphorescence (1.75 µs) at 280 K, indicative of coexisting ¹CT and ³LE emission (Figures 2e and S20–S22). Upon cooling, the emission of Bu₂PMDI‐Br₂Cz‐O becomes phosphorescence‐dominated and shows a red shift. Therefore, the temperature‐dependent change in the emission maximum more likely reflects a temperature‐dependent redistribution of the relative contributions of different emissive states [38].

Bu₂PMDI‐I₂Cz follows a distinct excited‐state relaxation pathway. When measured within a shorter time window, its decay lifetime decreases upon cooling (Figure S23), consistent with TADF [25]. Kinetic fitting of the temperature‐dependent lifetime data further affords a ΔE _ST value of 0.007 eV (Figure S24), which is sufficiently small to enable efficient RISC at room temperature and thus, supports its TADF character [39]. As shown in Figure 2f, Bu₂PMDI‐I₂Cz exhibits an emission maximum at 568 nm at 280 K, accompanied by a biexponential delayed decay (τ_avg = 0.91 µs; Figure S25) and a prompt fluorescence lifetime of 38 ns (Figure S26). The overlap of the prompt and delayed PL maxima indicates that the room‐temperature emission is mainly dominated by ¹CT fluorescence/TADF (Figure S27) [40]. Upon cooling to 77 K, thermally activated RISC is suppressed, and the PL spectrum splits into two bands at 539 and 568 nm. The higher‐energy band at 539 nm is assigned to ³LE phosphorescence, whereas the lower‐energy band at 568 nm is retained as residual ¹CT emission [41]. Furthermore, the emission efficiencies differ markedly across the samples: Bu₂PMDI‐Br₂Cz‐G shows a high PLQY of 35.3%, whereas Bu₂PMDI‐Br₂Cz‐O is nearly non‐emissive (PLQY = 0.5%). Bu₂PMDI‐I₂Cz exhibits a moderate PLQY of 4.3%, demonstrating that Bu₂PMDI‐Br₂Cz‐G is the most efficient emitter (Figure S28 and Table S3).

The above photophysical investigations confirm efficient guest‐activated triplet harvesting in the cocrystals, indicating their strong potential for X‐ray‐excited luminescence. To evaluate their multifunctionality under X‐ray excitation, the RL properties were subsequently investigated. According to the attenuation coefficients (μ) obtained from the photon cross‐section database (Figure S29), the halogenated guests Br₂Cz (Br, Z = 35) and I₂Cz (I, Z = 53) exhibit markedly stronger X‐ray absorption than the Bu₂PMDI host. Consistent with this expectation, both pristine host and guest molecules display negligible RL responses (Figure S30), indicating that efficient scintillation cannot be achieved without cooperative host–guest assembly. Upon co‐crystallization, the RL intensity is dramatically enhanced, with Bu₂PMDI‐Br₂Cz‐G and Bu₂PMDI‐I₂Cz exhibiting 31‐fold and 13‐fold increases with Bu₂PMDI, respectively (Figure 2g). Under identical conditions, the RL output of Bu₂PMDI‐Br₂Cz‐G surpasses 183% that of the commercial scintillator Bi₄Ge₃O₁₂ (BGO, 4.1 × 10⁴ a.u.). Using BGO (8000 photons MeV⁻¹) as a reference, the relative light yields are determined to be 18,667 and 3,107 photons MeV⁻¹ for Bu₂PMDI‐Br₂Cz‐G and Bu₂PMDI‐I₂Cz, respectively (Figures S31, S32) [42]. Moreover, Bu₂PMDI‐Br₂Cz‐G exhibits a low detection limit of 39 nGy s⁻¹ (Figures 2h and S33), corresponding to 1/25 of a typical medical diagnostic dose rate [20], together with excellent irradiation stability, retaining over 90% of its initial RL intensity after prolonged exposure (1800 s per day, Figure S34). Thermogravimetric analysis further reveals that, Bu₂PMDI‐Br₂Cz‐G possesses the highest thermal stability among the series, with a decomposition temperature of 250° C (Figure S35). These results establish Bu₂PMDI‐Br₂Cz‐G as a high‐performance organic scintillator combining high sensitivity and stability.

Building on the reversible cocrystal‐to‐cocrystal transformation between Bu₂PMDI‐Br₂Cz‐G and Bu₂PMDI‐Br₂Cz‐O, we further demonstrate stimuli‐gated modulation of both RL and PL. Owing to inefficient triplet utilization, Bu₂PMDI‐Br₂Cz‐O exhibits negligible scintillation. Upon mild fumigation with DCM vapor, this orange powder is converted into a pale‐green form, accompanied by the activation of intense RL emission (Figure 2i). This RL “turn‐on” behavior directly reflects the formation of the emissive Bu₂PMDI‐Br₂Cz‐G polymorph. Correspondingly, the PL emission maximum shifts from 583 to 493 nm after DCM treatment (Figure S36), closely matching the PL spectrum of Bu₂PMDI‐Br₂Cz‐G. Furthermore, when the resulting Bu₂PMDI‐Br₂Cz‐G is exposed to EA vapor, the emission peak shifts back to 580 nm, signifying the reformation of Bu₂PMDI‐Br₂Cz‐O, thus highlighting the reversibility of this transformation. This solvent‐triggered, crystallinity‐preserving structural switching enables reversible RL “on/off” gating, thereby providing a practical platform for multimodal, stimuli‐responsive luminescent systems.

2.3. Theoretical Simulation and Luminescence Mechanism

To elucidate the origin of the disparate luminescence properties in these cocrystals, model clusters were constructed directly from the crystal structures, comprising one central Bu₂PMDI molecule surrounded by four guest molecules. The intermolecular noncovalent interactions were then visualized and quantitatively evaluated by combining the independent gradient model based on Hirshfeld partition (IGMH) with Hirshfeld surface analysis. As shown in Figure 3a–c, the blue isosurfaces represent host‐guest noncovalent contacts. All three cocrystals exhibit well‐defined vertical π–π stacking, which rigidifies the local packing environment and thereby enhances the structural robustness and stability of the cocrystals [43]. Bu₂PMDI‐Br₂Cz‐G features dual C═O···Br halogen bonds, while C═O···H─C hydrogen bonds are also present on both sides, whereas Bu₂PMDI‐Br₂Cz‐O loses one halogen bond, which is replaced by C═O···H─N interaction. In contrast, Bu₂PMDI‐I₂Cz adopts a mixed interaction pattern, involving lateral C═O···H─N interaction and a vertical C═O···I halogen bond (Figure S37).

FIGURE 3.

Independent gradient model based on the Hirshfeld partition (IGMH) analysis of the corresponding fragments from (a) Bu₂PMDI‐Br₂Cz‐G, (b) Bu₂PMDI‐Br₂Cz‐O, and (c)Bu₂PMDI‐I₂Cz. Energy level diagrams and spin‐orbit coupling matrix elements (ξ) of (d) Bu₂PMDI‐Br₂Cz‐G, (e) Bu₂PMDI‐Br₂Cz‐O, and (f) Bu₂PMDI‐I₂Cz.

Hirshfeld surface analysis further enabled a quantitative comparison of the relative contributions (Figures S38–S41). The contribution of halogen‐related contacts follows the order: Bu₂PMDI‐Br₂Cz‐G (16.6%) > Bu₂PMDI‐Br₂Cz‐O (8.9%) > Bu₂PMDI‐I₂Cz (6.8%), whereas the contribution of hydrogen‐bonding interactions involving carbonyl oxygen follows the opposite trend: Bu₂PMDI‐I₂Cz (17.7%) > Bu₂PMDI‐Br₂Cz‐O (14.8%) > Bu₂PMDI‐Br₂Cz‐G (13.0%). These results indicate that, the emissive behaviors of these cocrystals are governed by a cooperative supramolecular environment defined by halogen bond, hydrogen bond, and π–π stacking. Hydrogen bond primarily directs the structural assembly and rigidifies the local environment [24], whereas halogen bond plays a complementary role by enhancing SOC through the external heavy‐atom effect [17, 44].

As shown in Figure 3d–f, Bu₂PMDI‐Br₂Cz‐G exhibits the highest fraction of halogen‐related contacts and, thus the largest SOC value (2.17 cm⁻¹). Meanwhile, the multiple lateral hydrogen‐bonding contacts further rigidify the local environment and suppress nonradiative relaxation. Consequently, Bu₂PMDI‐Br₂Cz‐G has a high PLQY (35.3%), giving rise to the most efficient PL and RL. In contrast, Bu₂PMDI‐Br₂Cz‐O contains only one halogen bond, and the reduced halogen‐contact contribution leads to a smaller SOC value (1.34 cm⁻¹). Although hydrogen‐bonding interactions are also present, their rigidifying effect is insufficient to compensate for the weakened halogen bond contribution. As a result, nonradiative decay remains dominant, leading to an extremely low PLQY (0.5%) and nearly quenched scintillation.

For Bu₂PMDI‐I₂Cz, although the higher atomic number of iodine would normally be expected to give rise to a larger SOC [45], both the fraction of halogen‐related contacts and the SOC value are the lowest (0.17 cm⁻¹). This indicates that effective SOC is determined not simply by elemental identity, but by the supramolecular geometry and electronic coupling between the heavy atom and the emissive excited states [17, 46]. In particular, the relatively long vertical C═O···I contact limits the contribution of the external heavy atom to the S₁→T₁ transition, resulting in inefficient ISC [47]. Nevertheless, the two lateral C═O···H─N hydrogen bonds help reinforce local rigidity and reduce nonradiative relaxation, such that its luminescence remains superior to that of Bu₂PMDI‐Br₂Cz‐O, although still lower than that of Bu₂PMDI‐Br₂Cz‐G.

In addition, the relatively large singlet‐triplet gaps of the two Br₂Cz‐based cocrystals (ΔE _ST ≈ 0.18‐0.19 eV), suppress RISC and thus favor RTP (Figure S42), whereas the much smaller ΔE _ST of Bu₂PMDI‐I₂Cz (0.007 eV) enables efficient RISC and supports its TADF character. Collectively, these results demonstrate that the scintillation performances of these cocrystals are dictated not merely by the presence of heavy atoms, but by the cooperative interplay of halogen bonding, hydrogen‐bond‐assisted rigidification, and supramolecularly controlled excited‐state coupling.

2.4. X‐Ray Imaging Based on Bu₂PMDI‐Br₂Cz‐G

Given the excellent scintillation performance of Bu₂PMDI‐Br₂Cz‐G, its applicability to X‐ray imaging was subsequently evaluated. The cocrystal powder was processed into microcrystals via mechanochemical ball milling in the presence of water (antisolvent) for 5 h, yielding particles with an average size of ∼ 620 nm (Figure S43). PXRD patterns recorded before and after milling were essentially identical (Figure S44), indicating that the long‐range cocrystal structure is well preserved during processing. Then, a uniform scintillator film is obtained by vacuum filtration of the microcrystal suspension onto polyester microporous membranes (Figure S45) [48]. Using a custom‐built X‐ray imaging system (Figure S46), we recorded images of a 20 lp mm⁻¹ line‐pair card under X‐ray irradiation (Figure 4a). After grayscale processing, pixel‐intensity profiles were extracted with ImageJ to obtain modulation transfer function (MTF) values of 0.52 at 16.6 lp mm⁻¹ and 0.47 at 20 lp mm⁻¹, indicating excellent spatial fidelity [49]. Further resolution analysis using the slanted‐edge method yields a spatial resolution of 37.0 lp mm⁻¹ (Figure 4b) [50], surpassing most reported organic phosphorescent scintillators (Table S4). Besides, the practical imaging performance was further demonstrated by visualizing biological and microelectronic specimens. As shown in Figure 4c–d, fine skeletal features of a small dried fish and the internal architecture of a microchip are clearly resolved, underscoring the applicability of this material to complex objects. Furthermore, this scintillator film exhibits good performance in dynamic X‐ray imaging, as demonstrated by video‐rate measurements (Video S1). At a temporal resolution of 16.67 ms, the internal spring structure of the capsule sample (Figure 4e) remains clearly resolved throughout motion, with no observable ghosting or motion blur, indicating negligible afterglow interference (Figure 4f). These results demonstrate the outstanding temporal response of Bu₂PMDI‐Br₂Cz‐G and its potential for dynamic X‐ray imaging applications.

FIGURE 4.

(a) Photographs of the line‐pair card under daylight and contrast images under X‐ray irradiation, accompanied by pixel‐intensity profiles along the indicated orange line. (b) X‐ray contrast image of a tungsten sheet (inset) and the corresponding MTF versus actual line‐pair spacing measured by the slanted‐edge method under X‐ray exposure. Photographs of (c) fish and (d) chip in daylight, alongside corresponding X‐ray static images showing the internal structure. (e) Photographs of spring inside an opaque in daylight. (f) Real‐time X‐ray dynamic images of a spring inside an opaque capsule, showing two consecutive frames with a 16.67 ms interval.

2.5. Multimodal Stimuli‐Responsive Information Encryption

Leveraging the solvent‐triggered reversible cocrystal‐to‐cocrystal transformation of Bu₂PMDI‐Br₂Cz, this system enables multimodal luminescence switching, thereby offering a platform for stimuli‐responsive information encryption. To begin with, Bu₂PMDI‐Br₂Cz‐O test paper was fabricated via vacuum filtration and used to evaluate environmental stability. As shown in Figure 5a, b, after immersion in aqueous solutions over a wide pH range (pH = 1–13), the test paper retained its bright sky‐blue emission with negligible intensity loss, demonstrating excellent acid‐base stability. In addition, alternating vapor fuming with EA and DCM induced highly reversible luminescence switching, with nearly unchanged emission maxima and only minor intensity fluctuations over multiple cycles (Figure 5c).

FIGURE 5.

(a) Photographs of Bu₂PMDI‐Br₂Cz‐G test papers under UV light at different pH values. (b) Photoluminescence intensity of Bu₂PMDI‐Br₂Cz‐G text paper under different pH. (c) Photoluminescence intensity of Bu₂PMDI‐Br₂Cz‐G text paper under cyclic EA/DCM vapor fumigation. (d) Schematic of the stimulus‐responsive multimodal information encryption. (e) Initial images under daylight and UV light. (f) X‐ray images after fumigation with DCM or EA.

Building upon the reversible phase transformation and excellent stability of Bu₂PMDI‐Br₂Cz, a multimodal stimulus‐responsive information encryption prototype was constructed. As shown in Figure 5d, a Morse code mask was placed on the test paper to encode information, where the genuine message was written using Bu₂PMDI‐Br₂Cz‐O doped with 2 wt% acridine orange (yellow regions in Figure 5d), while the remaining decoy patterns consisted of pure acridine orange (see chemical structure, Figure S47). As can be seen in Figure 5e, the Bu₂PMDI‐Br₂Cz‐O/acridine orange paper exhibits photoluminescence nearly identical to that of pure acridine orange under both daylight and UV excitation, causing the encoded information to misleadingly appear as “/72”. Notably, under X‐ray irradiation, neither material displays detectable radioluminescence, rendering the information completely invisible. Upon exposure to DCM vapor (Figure S48), Bu₂PMDI‐Br₂Cz‐O is transformed into the Bu₂PMDI‐Br₂Cz‐G polymorph. Under X‐ray excitation, the converted Bu₂PMDI‐Br₂Cz‐G/acridine orange paper exhibits intense radioluminescence, thereby revealing the genuine information “FZU”, whereas the pure acridine orange regions remain non‐emissive (Figure 5f). Subsequent treatment with EA vapor restores the initial state, enabling reversible ON–OFF switching of the encrypted information and re‐concealment of the genuine message. Therefore, through solvent‐triggered reversible phase transformation, this system achieves dynamic switching between UV‐ and X‐ray‐readable states, establishing a multimodal information encryption platform that integrates photoluminescence and radioluminescence channels. The combination of cocrystal structural transformation with X‐ray‐excited emission provides a versatile strategy for rewritable multilayer information encryption and next‐generation optical–radiation information systems.

3. Conclusion

In summary, we develop a donor‐acceptor organic cocrystal system that undergoes reversible, crystallinity‐preserving cocrystal‐to‐cocrystal transformations, enabling programmable control over triplet exciton utilization and X‐ray‐excited luminescence. Two polymorphic Bu₂PMDI‐Br₂Cz cocrystals with similar packing frameworks, but distinct halogen‐bonding motifs exhibit dramatically different emissive behaviors, ranging from efficient RTP to nearly quenched emission. Solvent vapor stimulation induces a reversible order‐to‐order structural switching that reorganizes supramolecular interactions without destroying crystallinity, thereby enabling reversible on/off switching of radioluminescence and photoluminescence. Mechanistic studies reveal that scintillation efficiency is governed by halogen‐bond‐mediated spin‐orbit coupling and excited‐state stabilization, rather than heavy‐atom identity alone. As a result, the optimized polymorph Bu₂PMDI‐Br₂Cz‐G enables high‐resolution static (37 lp mm⁻¹ @ MTF = 0.2) and real‐time dynamic X‐ray imaging with negligible afterglow. Furthermore, the solvent‐triggered cocrystal‐to‐cocrystal transformation between the two Bu₂PMDI‐Br₂Cz polymorphs, accompanied by pronounced differences in their emissive behaviors, further enables rewritable multimodal information encryption. This work establishes cocrystal engineering as an effective strategy for programmable organic scintillators.

Author Contributions

Yuxizi Guo: methodology, software, data curation, investigation, validation, and formal analysis. Hongyang Hong: data curation, software, and investigation. Yuanji Ye: methodology, data curation, and investigation. Qihao Xu: data curation, and investigation. Hongming Chen: conceptualization, project administration, funding acquisition, writing – original draft, and writing – review and editing. Mei‐Jin Lin: conceptualization, supervision, funding acquisition, visualization, project administration, resources, writing – original draft, and writing – review and editing.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: anie73136‐sup‐0001‐SuppMat.docx.

Supporting File 2: anie73136‐sup‐0002‐VideoS1.mp4.

Supporting File 3: anie73136‐sup‐0003‐VideoS2.mp4.

Supporting File 4: anie73136‐sup‐0004‐VideoS3.mp4.

Supporting File 5: anie73136‐sup‐0005‐VideoS4.mp4.

Supporting File 6: anie73136‐sup‐0006‐VideoS5.mp4.

Supporting File 7: anie73136‐sup‐0007‐cif.zip.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.