Guest-induced porous gating of a fluorescent nonporous adaptive crystal for efficient radioactive iodine sorption

Domain: chemistry
Published: 2026-02-21
Editorial outcome: minor_revision

Abstract
Nonporous adaptive crystals (NACs) are a new class of organic adsorbents that benefit from the ability of their crystalline structures to adaptively transform in response to specific guest molecules. Herein, a NAC with guest-induced porous gating property is demonstrated within a small organic molecule, BiPyBz. It produces two distinct crystals, the dense packing CryRod and the pore-gating CryQuad, in which the pores are opened by water. Remarkably, both CryRod and CryQuad can be used as adsorbents for gaseous iodine, with the latter demonstrating the highest uptake capacity of 3.1 g g−1 at 75 °C among all existing NACs constructed from small organic molecules. The superior iodine adsorption capacity is attributed to the synergistic effect of iodine loaded in gated pores and neighboring layers in the formed crystal. This study underscores the potential of small organic molecules as NACs for the effective adsorption and storage of guest molecules with promising real-world applications.

Introduction
Nuclear energy is regarded as a key pathway for achieving a low-carbon energy transition1. However, highly toxic and mobile radioactive iodine released from nuclear waste—such as the long-lived 129I and highly radioactive 131I—poses severe threats to ecosystems and human health2–4. Therefore, developing efficient and reliable radioactive iodine adsorbents is of critical significance for the sustainable utilization of nuclear energy5,6. To date, various porous amorphous or crystalline materials used as potential adsorbents for iodine capture have been developed and exhibited great application potential7–31. Among them, nonporous adaptive crystals (NACs) are emerging adsorbents and separation materials with excellent adsorption performance32–48. The feature of NACs lies in the negligible porosity compared with traditional porous materials with large surface areas, however, the guest molecules can be adsorbed into the crystalline structures with the adaptisorption, resulting in a crystalline structural change49,50, which has the potential for the long-term storage of radioactive iodine.

Most NACs are constructed by macrocyclic aromatic compounds containing intrinsically cavity (Fig. 1a), such as pillararenes and cages, owing to their intrinsically porous structure or cavity. For example, Huang group reported a nonporous EtP6 adsorbent for iodine capture38; Chi group reported NACs of a bipyridine cage for reversible iodine capture39. However, despite the impressive performance of these molecules, their relatively complex synthesis routes and modest yields may still pose challenges to broader development in this field.

NACs based on organic small molecules can overcome this limitation. Organic small molecules offer advantages such as straightforward synthesis, good crystallinity, and strong modifiability, allowing for the convenient introduction of functionally oriented groups. For instance, Li group reported the first small-molecule NAC, TPAPyH, with solvent dimension-dependent crystallinity property, achieving excellent iodine adsorption capacity of 1.27 g g−1 40. The challenge with NACs constructed by small organic molecules lies in their lower iodine uptake performance, which is due to the insufficient exposure of binding sites caused by dense intermolecular packing, as well as the absence of an intrinsically porous structure and flexibility within the crystal. Moreover, this packing structure hinders the diffusion of external guest molecules into the crystal interior (mass transfer process)51–53. Therefore, it is promising to consider that by creating a small amount of pores to expose the binding sites in the crystalline structures, facilitating the binding with active sites and increasing the mass transfer process in the crystal, then the adsorption capacity and efficiency of NACs of small organic molecules would be significantly enhanced54,55.

The intelligent phase transition of organic small-molecule crystals under external stimuli offers a novel approach for pore-gating process56–61. The rich intermolecular weak interactions (such as hydrogen bonding, π–π stacking, and van der Waals forces) within organic small-molecule crystals endow them with high structural tunability. This allows their crystal structures to undergo reversible and precise rearrangements in response to external guest molecules (e.g., solvents)57–63. In this process, external guest molecules can initially penetrate the crystal interior, disrupting the original packing arrangement to induce a phase transition and form a novel single-crystal structure, i.e. a gating process. Notably, guest molecules entering the crystal are not simply immobilized but exist as replaceable “pore units”. When guest molecules endowed with stronger interactions or higher affinity, they can readily replace the original molecules and occupy the active sites. This dynamically exchangeable phase transition mechanism not only enables guest molecules to interact more thoroughly with the active sites but also significantly reduces diffusion resistance and accelerates mass transfer processes within the crystal, thereby effectively enhancing adsorption performance.

Herein, an H2O-induced porous gating crystal is fabricated from a small organic molecule, BiPyBz, for superior nonporous adaptive iodine adsorption (Fig. 1b). The self-assembly of BiPyBz initially produces orange emitting CryRod, and then transforms into a CryQuad with green fluorescence emission upon the addition of a small amount of H2O. Single-crystal X-ray diffraction reveals an H2O-induced porous gating transformation mechanism. In this process, water molecules from the air are adsorbed, leading to the realignment of BiPyBz molecules. Due to the molecular-level porosity of CryQuad, it can serve as a solid adsorbent for iodine. When exposed to iodine vapor at 75 °C, it achieves an iodine uptake capacity of approximately 3.1 g g−1, the highest value among NACs formed by small organic molecules. The material also exhibits exceptional thermal stability, with minimal weight loss. Further characterizations show that iodine not only occupies the pores within the crystal structure but also expands the layers, much like a sponge swelling, contributing to its high adsorption capacity. This study highlights the potential of small organic molecules in the development of innovative organic sorbents for the capture of radioactive iodine, demonstrating both their feasibility and practicality.

Results
Preparation and characterizations of BiPyBz crystals
A molecule, BiPyBz (Fig. 1b), was designed and synthesized by a one-step coupling reaction, in which the pyridine motif was designed as a hydrogen or halogen bonding active site40, and the benzothiadiazole motif as an electron acceptor and chalcogen bonding site. The D-π-A-π-D molecular backbone ensures a high photoluminescence quantum yield. The synthesis details were listed in the Supplementary Information, and the obtained product was confirmed by 1H NMR, 13C NMR, and MALDI-TOF-MS (Supplementary Figs. 1–3). The UV-Vis spectrum shows two bands at 322 nm and 425 nm (Supplementary Fig. 4), the latter of which indicate the intramolecular electron transfer.

During the self-assembly process of BiPyBz in ethyl acetate, two species were found in the solution (Supplementary Fig. 5). One is the rod-like crystal with orange emission (termed CryRod, Fig. 2a), and the other is the quadrilateral crystal with green emission (termed CryQuad, Fig. 2a). After around 24 h, CryRod disappeared, leaving only CryQuad settled at the bottom of the bottle (Supplementary Fig. 5), which was stable for more than 1 month in solution. To reveal the supramolecular self-assembly pathway, in-situ monitoring of the crystal evolution process was conducted. As shown in Fig. 2b, the pre-formed CryRod was gradually dissolved into the solution over time, while the size of CryQuad simultaneously increased (Fig. 2c). Therefore, the self-assembly process of BiPyBz can be described as a competitive pathway (Fig. 2d), where CryRod represents a metastable aggregate in the off-pathway, while CryQuad serves as the thermodynamically stable aggregate in the on-pathway.

The physicochemical properties of the above two crystals were then characterized after manually collecting the bulk sample of individual crystal. In terms of emission properties, the maximum emission band blue-shifted from 570 nm for CryRod to 533 nm for CryQuad (Fig. 2e). Compared to the photoluminescent (PL) spectrum in toluene (1 µM, λ = 502 nm, Supplementary Fig. 6), CryRod displayed a larger redshift (68 nm) than CryQuad (31 nm) in the solid state, indicating that the intermolecular interaction had an important effect on the fluorescence emission energy. Moreover, the quantum yields of CryRod and CryQuad were measured to be 7.33% and 19.68% (Supplementary Figs. 7, 8) in the solid state, respectively. In addition, the fluorescence lifetime decreased from 3.74 ns to 1.50 ns (Supplementary Figs. 9, 10) during the crystalline structural transformation from CryRod to CryQuad (Fig. 2f). These data also imply the different types of intermolecular interactions in the two crystals. Furthermore, powder X-ray diffraction (PXRD) patterns proved their distinct molecular packing modes (Fig. 2g), as reflected from diverse diffraction patterns.

Besides, we also investigated the thermal stability of the above two crystals by observing their fluorescence images under in-situ heating. As shown in Supplementary Fig. 11, the emission band changed from 533 nm to 570 nm when CryQuad was heated from 30 to 180 °C, and the emission color of CryQuad gradually changed from green to orange, which is similar to the emission of CryRod. However, the emission band of CryRod was maintained at 570 nm under heating, only accompanied by a decrease in PL intensity (Supplementary Fig. 12). Besides, the PXRD pattern of heated CryQuad expressed the same pattern as CryRod (Supplementary Fig. 13). These data imply that CryQuad can be recovered to CryRod upon heating. To confirm this, thermogravimetric analysis (Supplementary Fig. 14) of CryQuad revealed a weight loss of 3% in the temperature window of 120–200 °C, which means the release of solvent. To precisely determine the phase transformation temperature, differential scanning calorimetry (DSC) measurements were performed on both CryQuad and CryRod crystals. As shown in Supplementary Fig. 15, CryQuad exhibit two endothermic peaks during the heating process at approximately 120 °C and 132 °C, corresponding to stepwise phase transitions or crystal rearrangements, but CryRod shows no obvious phase transition during heating. These results are consistent with the findings from in-situ fluorescence microscopy images.

Mechanism of the crystal evolution in solution
To reveal the evolution mechanism of BiPyBz in solution, the structure of CryQuad was analyzed using single-crystal X-ray diffraction (SCXRD). As shown in Fig. 3a, b and Supplementary Table 1, the crystal of CryQuad (CCDC 2369354) crystallizes in the triclinic P-1 space group, with unit cell parameters of a = 9.3297(4) Å, b = 9.8424(6) Å, c = 19.5238(8) Å, α = 79.940(4)°, β = 87.100(4)°, γ = 70.269(5)°, in which the asymmetric unit contains two host molecules and one water molecule. The experimental PXRD pattern and the simulated pattern from the SCXRD data coincided well (Fig. 2g), indicating the purity of respective phases. Surprisingly, the dimer formed by one H2O molecule bridging the two adjacent BiPyBz molecules through hydrogen bonds (-N···H-O-H···N-, 2.08 Å, Fig. 3b and Supplementary Table 2), which is the basic building block in the crystal structure. Then, the dimers linked to each other with π···π and C-H···π interactions, forming the crystal of CryQuad. The interlayer distance was measured to be 3.63 Å (Supplementary Fig. 16), which implies nontrivial π···π interactions between BiPyBz molecules. With this stacking mode, eight adjacent pyridine groups formed a pore with a size of 8.38 × 6.07 × 5.07 Å3 (Fig. 3c and Supplementary Fig. 17). It has to be noted that CryRod is a nonporous crystal, because of the low surface area of 3.37 m2 g−1 of CryRod from the BET measurement (Supplementary Fig. 18).

Despite the metastable nature of CryRod, the high-quality crystals were obtained successfully. The X-ray structure of CryRod (CCDC 2369352) was resolved (Fig. 3d and Supplementary Table 3). Unlike the CryQuad, CryRod crystallizes in the orthorhombic Pca21 space group, with unit cell parameters of a = 22.9015(9) Å, b = 3.95360(10) Å, c = 17.7923(8) Å, in which each unit cell contains four symmetry-dependent molecules. The structure of CryRod is shown in Fig. 3d. As we expected, no bound H2O molecules were found in the crystal structure. The BiPyBz molecules are stacked by multiple hydrogen bonds (meta-H of pyridine group and N of pyridine group, 2.67 Å and 3.81 Å) and chalcogen bonds (3.21 Å) between N···S (Fig. 3d), and the interlayer π···π interactions (Supplementary Fig. 19). The distance of interlayers was measured to be 3.39 Å (Supplementary Fig. 19), which indicates a more intense π···π interaction than that in CryQuad. Compared to the structure of CryQuad, CryRod has a more regular molecular packing mode and more intense intermolecular interactions, which result in a larger redshift of emission.

To quantitatively analyze the differences in the interactions between the two crystals, the weak interactions were evaluated by the Hirshfeld surface analysis64 (Fig. 3e, f, Supplementary Figs. 20, 21). The O-H···N hydrogen bonds exhibit a significant difference, changing from 19.6% for CryRod to 13.6% for CryQuad. Moreover, the binding energy of the three hydrogen bonds in CryRod and CryQuad was calculated by DFT calculation with the Gaussian 09 package (Supplementary Table 4-5)65. The binding energy of the hydrogen bond in CryQuad is stronger (−22.12 kJ mol−1) than that in CryRod (−10.51 kJ mol−1). These results suggest that the change of the hydrogen-bonding interactions from weak to strong is the driving force for the crystalline phase transformation from CryRod to CryQuad.

Further, the influence of molecular conformations of BiPyBz in CryQuad and CryRod crystals on their emissions was considered. Supplementary Fig. 22a illustrats two distinct conformations in the CryQuad, and one conformation in CryRod crystal. These molecular conformations are similar in that they are both planar, with only minor differences in dihedral angles. DFT calculations further analyzed their differences in HOMO and LUMO orbitals. In Supplementary Fig. 22b, they have similar HOMO and LUMO values; the main difference in HOMO-LUMO gap is approximately 0.05 eV, which is insufficient to account for the emission color change from orange to green. These results suggest that the differences in the intermolecular packing mode are the main reason for the emission color change.

Based on the above analysis, the evolution mechanism of the BiPyBz aggregates can be proposed. As shown in Fig. 2d, a competitive self-assembly process of BiPyBz, kinetically controlled off-pathway and thermodynamically controlled on-pathway, is involved. In the off-pathway, the monomer rapidly self-assembles into metastable nuclei, which then assemble into metastable crystals. Over time, H2O in the solvent interacts with BiPyBz, leading to the formation of dimers through hydrogen-bonding interactions. These dimers further assemble into new thermodynamically stable nuclei, which elongate into stable crystals via a self-catalyzed polymerization process. The thermodynamically stable nuclei, having a lower energy landscape, cause the disassembly of the higher-energy metastable crystals (Supplementary Fig. 23). This disassembly is the rate-determining step of the entire self-assembly process. Therefore, the second nucleation-elongation process represents the on-pathway.

H2O vapor-induced pore-gating
The crystal transformation from CryRod to CryQuad occurred not only in solution but also in air. During the experimental procedures, we found that CryRod changed its fluorescence from orange to green upon exposure to air (Fig. 4a). To elucidate the reason for this phenomenon, the fluorescence image of CryRod was monitored in-situ at ambient conditions. The orange emission of CryRod gradually changed into green emission over time when exposed to air (T = 25 °C, relative humidity RH = 60%). Interestingly, the transformation process begins from the two sides of the crystal and gradually moves towards the center of the crystal. Emission spectra (Fig. 4b) and PXRD patterns (Fig. 4c) prove that the green-emission aggregates are the same as the CryQuad phase. Then, we consider that if the fluorescence can serve as a built-in indicator for the phase transition. The in-situ fluorescence emission spectra of the phase transition from CryRod to CryQuad were measured. In Supplementary Figs. 24a–c, the in-situ emission monitoring of the phase transition process from CryRod to CryQuad exhibited the gradual blue-shift of the emission peak from 570 to 535 nm. Furthermore, the maximum emission peak exhibited a linear relationship with the transition time under vapor exposure, reaching equilibrium at 100 h. To further determine the correlation between mass ratio and transition time, the physically mixed samples with varying mass ratios of CryRod and CryQuad were prepared. The maximum emission peaks indicate a blue shift from 570 nm to 535 nm as the CryQuad ratio increased. A non-linear correlation is observed between the maximum emission wavelengths and the CryQuad mass ratios [CryQuad / (CryQuad + CryRod)] (Supplementary Figs. 24d, e). Based on this relationship, the correlation between the CryQuad mass ratio and the phase transition time can be obtained (Supplementary Figs. 24f). These results indicate that the position of the maximum emission peak value can reveal the mass ratios between CryRod and CryQuad. This provides a quantitative optical indicator for evaluating the extent of phase transition during the vapor-induced structural evolution process.

In addition, SEM images show the morphological changes from smooth rods to rough rods, which are composed of smaller bulk crystals (Fig. 4d, e). These results indicate the successful transformation from metastable crystals to thermodynamically stable crystals in air, suggesting the occurrence of the pore-gating process. Then, the effects of humidity and temperature on the crystalline phase transformation process were also studied. According to Fig. 4f and Supplementary Fig. 25, the crystalline phase transformation can occur in a wide humidity range from RH = 60% to 100%, in which the transformation time decreases with the rise of RH. Meanwhile, the transformation time accelerated as the temperature increased, likely due to the faster molecular motion and gas diffusion at higher temperatures (Supplementary Fig. 26). Additionally, other solvents, such as dichloromethane, tetrahydrofuran, methanol, and acetic acid, were also determined that they cannot trigger the phase transition from CryRod to CryQuad (Supplementary Fig. 27).

Subsequently, the crystal structural transformation process was further investigated via simulating the crystal morphology and indexing the major crystal faces to determine the evolution pathway of CryRod. In Fig. 4g and Supplementary Fig. 28, the simulated crystal morphologies are consistent with the experimental results. The macroscopically long axis direction of CryRod is the crystallographic b-axis of the unit cell, which is different from CryQuad. The molecular packing mode in the [101] facet of CryRod and CryQuad indicated the molecular transformation mode in the crystal phase transformation. This means the crystal evolution process started along the direction of the b-axis. On this basis, the evolution process can be established at the atomic level (Fig. 4h). In the first stage, H2O molecules are adsorbed on the surface of CryRod through hydrogen bonds between the nitrogen site in the pyridine group and H2O. In the second stage, H2O molecules distributed on the [101] facet further penetrate the crystal structure along the b-axis, altering the hydrogen bonds from BiPyBz···BiPyBz to BiPyBz···H2O. This transformation to higher-energy hydrogen bonds is identified as the driving force behind the crystal evolution. In the third stage, the molecular-level pores in CryQuad facilitate the transport of external H2O molecules into the crystal interior structure through pores on both sides, ultimately causing an emission transition of CryRod toward the center of the crystal on a macroscopic scale.

Gaseous iodine adsorption
To determine the adaptive adsorption properties of CryRod and CryQuad, they were used as adsorbents for iodine vapor adsorption due to their nitrogen-rich environments in the crystal structure. The as-prepared CryRod and CryQuad were exposed to iodine vapor at 75 °C and 25 °C to evaluate the adsorption performance, respectively. The mass of iodine uptake reached up to 3.1 g g−1 and 2.8 g g−1 for CryQuad at the temperatures of 75 °C and 25 °C, with the color of the crystal changed to dark black (Supplementary Fig. 29) respectively, corresponding to ~4.2 iodine atoms per BiPyBz molecule (Fig. 5a). Notably, the iodine adsorption speed was quite high, with the iodine uptake reaching 70% of the saturated adsorption capacity within the first 2 hours. The iodine adsorption performance of CryRod reached 2.5 g g−1 and 1.8 g g−1 at the temperatures of 75 °C and 25 °C, respectively. These results indicate that CryRod and CryQuad have high adaptive adsorption properties. The higher adsorption performance of CryQuad than CryRod suggests that the pores play a key role in the adsorption process. The iodine adsorption performance of CryQuad is among the highest reported for NACs constructed by small organic molecules, even better than many porous materials, like COFs, MOFs, and so on (Fig. 5b and Supplementary Table 6).

To investigate the adsorption kinetics, the adsorption behaviors of CryQuad and CryRod at 25 °C and 75 °C were quantitatively analyzed (Supplementary Fig. 30). The results show that CryQuad follows a pseudo-second-order kinetic model at both temperatures, indicating a chemisorption-dominated process. In contrast, CryRod exhibits temperature-dependent behavior, following the pseudo-second-order kinetic model at 75 °C. At 25 °C, however, the adsorption amount increases linearly with time, suggesting a diffusion-controlled process. This behavior of CryRod is likely due to limited exposure of active sites or pores at 25 °C, caused by dense molecular packing. To determine the effect of the phase transition for the iodine adsorption performance directly, the iodine adsorption experiments of both phases coexisting were incorporated, in which the samples were obtained at different phase transition times. In Supplementary Fig. 31, the iodine adsorption capacity exhibited an increase at both 75 °C and 25 °C in the presence of higher CryQuad ratios [CryQuad / (CryQuad + CryRod)]. Especially at 25 °C, there is an observed increase in the saturated iodine adsorption capacity, as well as a concomitant decrease in the adsorption time. These findings also suggest that the H2O-gated pores in CryQuad facilitate iodine adsorption by providing accessible channels for molecular diffusion and interaction.

To elucidate the reason for the high adsorption capacity, the iodine adsorption process of a single crystal of CryQuad was monitored in-situ by a fluorescence microscope at 25 °C. In Fig. 5c, with increasing adsorption time, the crystal size of CryQuad enlarged from 16.6 μm × 17.1 μm to 21.4 μm × 18.2 μm in length and width (Fig. 5c) at 120 min, and the emission intensity showed an exponential decay in 30 min without emission peak shift (Supplementary Fig. 32). Meanwhile, the crystal color of CryQuad turned from yellow to dark black, and the emission was gradually quenched by the adsorption of iodine. SEM images of CryQuad and I2@CryQuad demonstrated the structural expansion and cracking after the iodine adsorption (Fig. 5d, e). Notably, Energy-dispersive X-ray spectroscopy (EDS) mapping demonstrated that iodine is homogeneously distributed in the CryQuad (Fig. 5f). The significant crystal expansion and the structural changes indicated that iodine is not only absorbed via the pores in the crystal but also through the synergistic effect between layers, which is the underlying reason for the high iodine capacity.

The recycle adsorption performance of CryQuad was also evaluated. After four successive adsorption-desorption cycles with DMF washing methods, CryQuad retained >77% of the original iodine uptake capacity (Fig. 5g). The PXRD pattern of I2@CryQuad remains similar before and after desorption (Supplementary Fig. 33), which confirms iodine has a strong interaction with BiPyBz. 40 The decrease in the adsorption capacity during recycling is attributed to the iodine residue in CryQuad after desorption. Furthermore, the retention experiments were performed to verify the adsorption occurring within the porous rather than on the surface by exposing I2@CryQuad to air at room temperature and ambient pressure. The I2@CryQuad almost kept its weight at 99% for at least 7 days (Fig. 5h), suggesting the uptake originates from the strong interaction between CryQuad and iodine. The excellent iodine adsorption and high retention performance of CryQuad encouraged us to construct a device for iodine vapor adsorption at room temperature (Supplementary Fig. 34). A column filled with CryQuad powder was applied for real-time removal of iodine. Different concentrations of gaseous iodine were flowed through the adsorption column. Remarkably, the iodine concentration decreased from ppm to ppb levels (Fig. 5i and Supplementary Table 7), with an impressive removal rate of more than 99.9%.

Mechanism of iodine adsorption
The adsorption mechanism at the molecular level, including the iodine species in I2@CryQuad and intermolecular interactions between BiPyBz and iodine, is revealed by PXRD, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). PXRD patterns (Fig. 6a) show that the crystal structure changes after the saturated iodine adsorption of CryQuad, suggesting the formation of a new structure. Time-dependent PXRD patterns (Supplementary Fig. 35) exhibited the characteristic diffraction peaks of the pristine crystal phase gradually weakened and eventually disappeared as adsorption time increases. Meanwhile, a weak new set of weak diffraction peaks emerged. These observations suggest a structural transformation during iodine uptake, indicating that iodine molecules progressively penetrate the crystal lattice and lead to lattice rearrangement or the formation of a new iodine-coordinated phase. FTIR spectra (Fig. 6b) confirmed the intermolecular interactions between BiPyBz and iodine. The intensity of characteristic peak at 1660 cm−1, assigned to the C=N stretching vibration of pyridine groups, decreased after iodine adsorption26, which indicate the halogen bonding interactions between iodine and BiPyBz. Raman spectra (Fig. 6c) show two new bands appearing at around 114 cm−1 and 164 cm−1 of I2@CryQuad, respectively. The band located at 114 cm−1 is assigned to the symmetric stretching vibration of I3−. The band located at 164 cm−1 is associated with the asymmetric stretching vibration of polyiodide I3− and the symmetric stretching vibration of I5−. The complex of Py···I2 based on halogen bonding interaction ionizes into [Py···I]+ cation and I− anion66. Furthermore, the I− anion combines with one or two iodine molecules to form I3− and I5− anions (Supplementary Fig. 36). This is the reason iodine exists in the form of I3− and I5− in I2@CryQuad, and forms a charge-transfer complex with BiPyBz. Furthermore, XPS spectra were used to identify the form of iodine in I2@CryQuad. In Fig. 6d, signals located at 617.57 and 629.03 eV are assigned to I3−, and signals at 622.49 and 634.04 eV are assigned to I5−. Moreover, the N 1s orbital peak at 397.98 eV for the C=N bond in CryQuad shifted to 399.46 eV (Supplementary Fig. 37), indicating the formation of quaternary ammonium cations (N+). Therefore, the iodine adsorption mechanism of CryQuad is attributed to the charge transfer interaction between N+ and iodine anions (I3− and I5−)25.

To deeply understand the interactions between BiPyBz and I2, the crystal of BiPyBz-I2 (CCDC 2477064, Supplementary Table 8) was grown in a DMSO/H2O mixture solvent. It should be noted that the crystal structure of BiPyBz-I2 is not the same as the iodine-saturated I2@CryQuad. This is due to the differences in their PXRD patterns (Supplementary Fig. 38). It crystallizes into the triclinic P-1 space group, with unit cell parameters of a = 9.3353(4) Å, b = 10.2543(7) Å, c = 14.5105(8) Å, α = 72.809(5)°, β = 87.982(4)°, γ = 62.972(5)°. The structure of BiPyBz-I2 is shown in Fig. 6e, f. One BiPyBz molecule combined with two iodine molecules to form a supramolecule by the halogen bonding interaction. Then these supramolecules stacked in layers with a distance of 3.36 Å, indicating the π-π interactions. The distance between N of the pyridine group and the I atom was measured at about 2.39 Å, suggesting an intense halogen bonding interaction. The binding energy of the halogen bonding interaction was calculated at about −36.40 kJ mol−1 by DFT calculation (Fig. 6g, Supplementary Table 9)65. This is stronger than the binding energy of hydrogen bonding in BiPyBz-H2O (−22.12 kJ mol−1), suggesting that iodine can displace H2O in the CryQuad and fully occupy the pores.

The results of the Atomistic molecular dynamics (MD) simulation using the general AMBER force field (GAFF) confirmed the replacement of H2O. A molecular cluster containing 14 BiPyBz molecules, 2 H2O molecules, and 700 I2 molecules (the ratio of BiPyBz:I2 = 1:50) was filled in a box of 10 × 10 × 10 nm3. As shown in Fig. 6h, the I2 molecules gradually aggregated into the BiPyBz molecular cluster, and the size of the cluster expanded with the running time elongation. Notably, the H2O molecule was released during the iodine adsorption process. Thus, the adsorption mechanism is illustrated in Fig. 6i. The iodine molecules were adsorbed and filled into the pores through the electrostatic interaction and halogen bonding, replacing the bound H2O. Once the pores are filled, iodine induces an adaptive expansion of the crystal layers to accommodate more iodine.

Methods
Materials
4,7-dibromobenzo-2,1,3-thiadiazole, 4-vinylpyridine, palladium acetate (Pd(OAc)2), sodium acetate (NaOAc), tetrabutylammonium bromide (n-Bu4NBr), and iodine were purchased from Energy-chemical company. These compounds are used as received. The solvents of N,N-dimethylformamide (DMF), chloroform (CHCl3), acetonitrile, ethyl acetate (EA), tetrahydrofuran (THF), and dichloromethane (DCM) were bought from Sinopharm Chemical Reagent Co., Ltd., China.

Instruments and methods
NMR spectroscopic data were measured using the spectrometers Bruker AV 400. For 1H and 13C NMR spectra, chemical shifts were calibrated to the solvent lock signal (CDCl3). MALDI-TOF-MASS spectra were measured using a BRUKER microflex mass spectrometer. UV-vis absorption spectra were obtained using a Hitachi U-2910. PL spectra of monomers and aggregates were measured with a Hitachi F-4700. The samples of aggregates were prepared by spreading the aggregates on the glass substrates. Powder X-ray diffraction (PXRD) patterns were performed using a PAN alytical X’Pert PRO instrument (40 kV, 200 mA). Fluorescence quantum yields and Fluorescence lifetimes were measured with a FLS1000 photoluminescence spectrometer. SEM images were obtained on a Hitachi S-8010 field-emission microscope where the accelerating voltage and the current were set as 10 kV and 10 μA, respectively. Fluorescence microscope images were obtained on an inverted fluorescence microscope (Nikon Ti-2). The fluorescence microscope samples were prepared by spreading the aggregates onto the glass substrates. BET spectra were measured with ASAP 2460 Accelerated Surface Area and Porosimetry System. FTIR spectra were measured with a Bruker ALPHA II Compact FT-IR Spectrometer. Raman spectra were measured with LabRam HR Evolution. XPS spectra were measured with Thermo ESCALAB 250XI. ICP-MS data were measured with Agilent 7800. The samples that flowed out of the columns were collected with 0.3 M of sodium hydroxide solution. Single-crystal X-ray diffraction data were collected with a Bruker Apex Smart CCD diffractometer with graphite-monochromated Cu Kα (λ = 1.54184 Å) radiation at 170 or 180 K. All non-hydrogen atoms were refined with anisotropic displacement parameters, while the hydrogen atoms attached to carbons were placed in idealized positions with isotropic thermal parameters. Variable temperature fluorescence spectra were obtained using an Ocean Optics USB4000 fluorometer with a 385 nm LED lamp as the light source. The samples were prepared by spreading the aggregates on the glass substrates.

Synthesis of BiPyBz
4,7-bis((E)-2-(pyridin-4-yl)vinyl)benzo-2,1,3-thiadiazole (BiPyBz)67. A mixture of 4,7-dibromobenzo-2,1,3-thiadiazole (2.94 g, 10 mmol), 4-vinylpyridine (2.10 g, 20 mmol), Pd(OAc)2 (50 mg, 0.2 mmol), NaOAc (4.10 g, 50 mmol), n-Bu4NBr (0.65 g, 2 mmol), and DMF (50 mL) was heated at 100 °C with stirring. After reacting for 24 hours, deionized water was added to quench the reaction. The resulting precipitate was filtered and washed with deionized water to remove the DMF residue. The product was then purified by column chromatography on silica gel using ethyl acetate as the eluent, yielding BiPyBz (0.86 g, 2.5 mmol, 17% yield). 1H NMR (400 MHz, CDCl3) δ 8.65 (d, J = 5.9 Hz, 4H), 8.05 (d, J = 16.3 Hz, 2H), 7.84–7.74 (m, 4H), 7.52 (d, J = 5.8 Hz, 4H). 13C NMR (101 MHz, CDCl3) δ 153.74, 150.36, 150.32, 144.60, 131.42, 129.32, 128.68, 128.43, 121.11. Calcd. for BiPyBz: m/z [M-H]− = 341.4. Found: MALDI-TOF-MS: [M-H]− = 341.85.

Fabrication of BiPyBz aggregates
10 mg BiPyBz solid was added to 2 mL ethyl acetate solution, and heated to dissolve completely. CryRod was formed by cooling the solution to room temperature with orange emission. Then, it was transformed into CryQuad with green emission after 24 hours. The transformation cannot occur when the supra-dried ethyl acetate was used.

Iodine vapor uptake capacity
The procedure for static iodine vapor sorption was performed as follows: the crystal (CryQuad or CryRod, 10 mg) was placed in a vial (4 mL), then the vial was placed in a large wide-mouth bottle (20 mL) containing iodine. The bottle was sealed and kept at 25 °C or 75 °C. The weight of the small vial was measured periodically, until reaching the equilibrium.

In-situ iodine adsorption of CryQuad
The in-situ iodine adsorption of CryQuad was obtained by putting CryQuad and a small dish (1 cm in diameter) containing iodine into a culture dish (2 cm in diameter) with a glass bottom, sealed with sealing film, and observed under a fluorescence microscope.

Adsorption kinetics
The adsorption kinetics were quantified using Ho and McKay’s pseudo-second-order model, from which the apparent rate constant kobs can be calculated using the following equation:
t/qt = 1/(kobs × qe2) + t/qe (1)
where qt and qe are the adsorbate uptakes (g adsorbate per g adsorbent) at time t (h) and at equilibrium, respectively, and kobs is an apparent second-order rate constant (g g−1 h−1). The rate constant kobs can be calculated from the intercept and slope of the plot of t/qt against t.

Recycle adsorption
50 mg of I2@CryQuad solid was added to a 20 mL DMF solution and subjected to ultrasound for 1 h to induce iodine desorption. The resulting sample was filtered and washed with methanol and excess water before being dried. The sample was then used for iodine uptake again.

The adsorption performance of the column filled with CryQuad
CryQuad (50 mg) was filled into the column with a length of 8.5 cm, and the ends of the column were filled with cotton. Injecting 10 mL iodine vapor (concentrations of 3000 ppm, 1500 ppm, and 300 ppm, respectively) into the column at a rate of 3.3 mL/min, and the output gas was absorbed with 0.3 M sodium hydroxide solution (10 mL). The iodine concentration in the solution was detected by ICP-MS, and the iodine concentration (cvapor) of the output gas was calculated with the equation as follows:
cvapor = (csolution × Vsolution) / Vvapor (2)
where csolution is the iodine concentration in solution, measured by ICP-MS; Vsolution is the volume of the sodium hydroxide solution, 10 mL; Vvapor is the volume of iodine vapor, 10 mL.

Theoretical calculations
The density functional theory (DFT) calculation processes were carried out using the Gaussian 09 software package65. The imported BiPyBz molecular structure was derived from the single crystal X-ray diffraction analysis with further geometry optimizations for calculating the molecular orbitals. The gas-phase ground state calculations were performed at B3LYP/6-311g* for BiPyBz and H2O, and B3LYP/def2tzvp level for I2. The electrostatic potential (ESP) surfaces were mapped for all molecules using GaussView 5.0, with the isovalue of 0.0004. The H-bond interactions between BiPyBz and H2O were performed at m062x/def2tzvp with BSSE correction using the SCXRD geometry without further optimization. The halogen bonding interactions between BiPyBz and I2 were performed at m062x/def2tzvp with BSSE correction.

The molecular dynamics simulations were carried out using the Gromacs 2018.8 program with general Amber force fields (GAFF)68. All the molecules were put into a cubic box with a side length of 10 nm using packmol software69 (700 I2 molecules for 14 BiPyBz molecules). The cutoff for the neighbor list of the Verlet method and that for short-range interactions is 1.0 nm in all calculations with periodic boundary conditions in all three directions. After the energy minimization, the systems were run in the NVT ensemble for 1 ns at 298.15 K. The time step of each simulation was 1 fs.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors have investigated a nonporous adaptive crystal (NAC) derived from a small organic molecule of BiPyBz,\nrevealing its polymorphism: this includes the dense packing CryRod and porous CryQuad, which exhibit yellow and green\nemission, respectively. More notably, both CryRod and CryQuad function effectively as adsorbents for gaseous iodine, with\na maximum uptake capacity of 3.1 g g-1. These findings mark a significant advancement in the development of small organic\nmolecule-based NACs. The manuscript is well-structured, systematically elucidating the photophysical properties and iodine\nadsorption performance of BiPyBz. Given its scientific rigor and clear organization, the manuscript is recommended for\nacceptance following minor revisions.\n1. It is noteworthy that heating CryQuad from 30 to 180 ℃ induces a crystalline phase transition to CryRod. However, this\ntemperature range is excessively broad, and the critical transition temperature remains unspecified. It is therefore\nrecommended that differential scanning calorimetry (DSC) measurements be performed on both CryQuad and CryRod\ncrystals to systemically access their thermal properties and pinpoint the critical transition temperature.\n2. CryQuad and CryRod crystals exhibit distinct emission colors and molecular packing modes. To deepen the mechanistic\nunderstanding, the molecular conformations of BiPyBz in these two polymorphisms, as well as the underlying mechanism\naccounting for their differential emission behaviors, should be further elaborated.\n3. Water has been shown to trigger the crystalline phase transition from CryRod to CryQuad. A key question remains: can\nother solvents, including polar and nonpolar solvents, such as methanol, acetic acid, dichloromethane, tetrahydrofuran, also\ninduce this transition? Investigating this solvent-dependent behavior would significantly enhance the comprehensiveness of\nthis study.\n4. The iodine adsorption capacity of CryRod reaches 2.5 g g-1 and 1.8 g g-1 at the temperatures of 75 °C and 25 °C, with the\nproposed adsorption mechanism ascribed to the exposure of active sites or pores. To validate this mechanism, additional\nanalysis are required, for instance, XRD characterization should be conducted to confirm whether the crystalline structure\nvariations during iodine adsorption.", "reviewer_2": "This manuscript reports the design and iodine adsorption of nonporous adaptive crystals (NACs) based on BiPyBz, forming\ntwo phases: CryRod (guest-free) and CryQuad (H2O-containing). The authors emphasize guest-induced porous gating, N–I\ninteractions, and polyiodide formation (I3- / I5-), reporting a record iodine uptake of 3.1 g g-1 at 75 C. The coexistence of\nluminescence and phase transition is attractive. However, in its current form, the work does not reach the novelty and impact\nexpected for Nature Communications.\n(1) Introduction is generic and does not highlight the unique aspects of this system. The introduction focuses heavily on\nradioactive iodine capture but fails to emphasize the distinct features of this work, such as H2O-gated transition and\nluminescence. The text resembles many other iodine-capture reports, making originality difficult to perceive. Distinction from\nprevious NACs using macrocycles or cages is insufficient, and the comparison with COFs/MOFs is overstated.\n(2) Compound and crystal structures show limited novelty. BiPyBz itself is not new (Org. Biomol. Chem., 2019, 17, 9467), and CryQuad has been previously deposited in the CCDC1850341. CryRod is new, but the CIF lacks an FCF, preventing\nfull validation, and the CCDC-deposited data show Alert A inconsistencies, indicating possible refinement issues as flagged\nby checkCIF/PLATON. The authors provide a BiPyBz–I2 crystal structure as an intermediate, but the saturated iodine-\nloaded state corresponding to the record uptake is not structurally evidenced. This gap substantially weakens the reliability\nof the crystallographic claims, especially for a high-impact journal.\n(3) Reported iodine uptake values in Fig. 4a are inconsistent and raise doubts about quantification and mechanism. The\nuptake values are striking but not convincingly supported. The influence of water on the mechanism and the identification of\npolyiodide species are insufficient. Raman and XPS suggest I3- and I5-, but neutral I2 cannot be excluded, leaving the\nmechanism unclear. Furthermore, if I2-BiPyBz-I2 seen in the crystal structure is a stable intermediate, wouldn't the\ncorresponding 1.5 g g-1 be observed in adsorption?\nIn particular, SI-Fig.29 presents the pathway: R-Py -> R-Py-I2 -> R-Py-I...I -> R-Py-I...I3 -> R-Py-I...I5, with a total uptake of 3\ng g-1. However, based on molecular symmetry and the reported crystal structure, a more plausible sequence is: BiPyBz ->\nI2-BiPyBz-I2 (already crystallographically proven, 1.5 g g-1) -> I...I-BiPyBz-I...I -> I3...I-BiPyBz-I...I3 or I...I-BiPyBz-I...I5 (reach\nto 3 g g-1).\nThis scenario also allows prediction of I5...I-BiPyBz-I...I5, corresponding to 4.5 g g-1. If the uptake is truly quantitative, further\ncrystallographic evidence beyond the intermediate I2-BiPyBz-I2 is required.\n(4) The CryRod <-> CryQuad phase transition is not integrated into the iodine uptake study. The transformation between\nCryRod and CryQuad is potentially central to the uniqueness of the material, but its role in iodine capture is not properly\naddressed. For example, when both phases coexist (Fig. 3), what happens to I2 uptake? Without such discussion, the\nconcept of guest-induced phase transition remains underdeveloped.\nWhile the luminescent phase transition is quite interesting, the compound novelty is weak, the iodine uptake data are\ninconsistent and insufficiently supported, and the introduction does not distinguish the work from similar iodine-capture\nreports. Most importantly, only an intermediate iodine-loaded crystal (I2–BiPyBz–I2) is provided, while no structural evidence\nfor the high uptake state is given. Overall, the manuscript does not provide the originality or reliability required for Nature\nCommunications.", "reviewer_3": "In this paper, Zhang and coworkers report a fluorescent nonporous adaptive crystal (NAC) that undergoes guest-induced\nporous gating for efficient radioactive iodine sorption. The material, derived from a small organic molecule (BiPyBz), can\ntransform from a dense form (CryRod) to an open form (CryQuad) under water vapor, enabling exceptionally high iodine\nuptake of up to 3.1 g g⁻¹. Strong halogen bonding and formation of polyiodide species further enhance the capacity and\nretention. The concept of water-triggered structural gating in a small-molecule NAC is novel, and the iodine uptake\nsurpasses most reported NACs. The work is significant, well supported by crystallographic, spectroscopic, and adsorption\ndata, and suitable for publication after minor revision.\n1. The recyclability tests show ~23% capacity loss after four cycles, attributed to residual iodine. The authors may suggest\nstrategies to improve regeneration (e.g., extended heating or chemical washing) to further strengthen the case for practical\napplications.\n2. Since the title highlights fluorescence, it would be valuable to discuss the potential use of fluorescence changes (color\nshift and quenching) as a built-in indicator of phase transition or iodine loading.\n3. The comparison with closely related NAC systems (e.g., TPAPyH, ref. 53) could be made clearer in the introduction or\ndiscussion to emphasize how this work differs in molecular design and performance.\n4. Some language issues should be corrected (e.g., “adptive” → adaptive, “Different with” → Unlike, “replacment” →\nreplacement). Careful proofreading will improve clarity.\n5. For the iodine filtration column test, adding brief details such as flow rate or residence time in the main text or SI would\nenhance the understanding of scalability.\nOverall, this is an excellent contribution and I recommend acceptance after minor revision."}, "author_response": "We are very pleased and excited by the positive comments on the\nnovelty and significance of our study. We also thank the reviewer’s detailed\ntechnical comments/suggestions, which help us to improve the manuscript.\nWe have addressed them point-by-point below.\n1. It is noteworthy that heating Cry from 30 to 180 °C induces a crystalline\nQuad\nphase transition to Cry . However, this temperature range is excessively\nRod\nbroad, and the critical transition temperature remains unspecified. It is\ntherefore recommended that differential scanning calorimetry (DSC)\nmeasurements be performed on both Cry and Cry crystals to\nQuad Rod\nsystemically access their thermal properties and pinpoint the critical transition\ntemperature.\nResponse: Thanks for your constructive suggestion. To precisely determine the\nphase transition temperature, differential scanning calorimetry (DSC)\nmeasurements were performed on both Cry and Cry crystals. As shown\nQuad Rod\nin Fig. R1, Cry exhibits two endothermic peaks during the heating process\nQuad\nat approximately 120 °C and 132 °C, corresponding to stepwise phase\ntransitions or crystal rearrangements, but Cry shows no obvious phase\nRod\ntransition during heating. This result is consistent with the findings from in-\nsitu fluorescence microscopy images.\nThis discussion has been added to the revised manuscript.\nFig. R1. Differential scanning calorimetry (DSC) thermograms of Cry and\nQuad\nCry recorded during heating.\nRod\n2. Cry and Cry crystals exhibit distinct emission colors and molecular\nQuad Rod\npacking modes. To deepen the mechanistic understanding, the molecular\nconformations of BiPyBz in these two polymorphisms, as well as the\nunderlying mechanism accounting for their differential emission behaviors,\nshould be further elaborated.\nResponse: Thanks for your constructive suggestion. The molecular\nconformations of BiPyBz in Cry and Cry crystals were analyzed. Fig.\nQuad Rod\nR2a illustrate two distinct conformations in the Cry crystal, and one\nQuad\nconformation in Cry crystal. These molecular conformations are similar in\nRod\nthat they are both planar, with only minor differences in dihedral angles. DFT\ncalculations further analyzed their differences in HOMO and LUMO orbits. In\nFig. R2b, they have similar HOMO and LUMO values; the main difference in\nHOMO-LUMO gap is approximately 0.05 eV, which cannot cause the emission\ncolor change from orange to green. These results suggest that the differences in\nthe intermolecular packing mode are the main reason for the emission color\nchange.\nThis discussion has been added to the revised manuscript.\nFig. R2. Molecular conformations and electron cloud distributions of the\nHOMO and LUMO orbits in Cry and Cry crystals.\nQuad Rod\n3. Water has been shown to trigger the crystalline phase transition from Cry\nRod\nto Cry . A key question remains: can other solvents, including polar and\nQuad\nnonpolar solvents, such as methanol, acetic acid, dichloromethane,\ntetrahydrofuran, also induce this transition? Investigating this solvent-\ndependent behavior would significantly enhance the comprehensiveness of\nthis study.\nResponse: Thanks for your constructive suggestion. To determine the solvent-\ndependent emission change behavior from Cry to Cry , several organic\nRod Quad\nsolvents were used, including methanol, acetic acid, dichloromethane, and\ntetrahydrofuran. The experiments were performed in as same way as the water\nvapor induced phase transition. As Fig. R3-R5 demonstrate, the Cry crystals\nRod\nmaintained their structure and emission color in dichloromethane,\ntetrahydrofuran, and methanol vapors, which means these vapors cannot\ntrigger the phase transition from Cry to Cry . Contrary, in Fig. R6, the\nRod Quad\nemission of Cry red-shifted from 570 to 608 nm under the exposure of acetic\nRod\nacid vapor. With time elongation, the solid crystal was dissolved with the\nemission at 630 nm. These results may be caused by the hydrogen binding\ninteractions between carboxyl group of acetic acid and the pyridine group of\nBiPyBz.\nThis discussion has been added to the revised manuscript.\nFig. R3. Time-dependent fluorescence images and emission spectra of Cry\nRod\nexposed to dichloromethane vapor.\nFig. R4. Time-dependent fluorescence images and emission spectra of Cry\nRod\nexposed to tetrahydrofuran vapor.\nFig. R5. Time-dependent fluorescence images and emission spectra of Cry\nRod\nexposed to methanol vapor.\nFig. R6. Time-dependent fluorescence images and emission spectra of Cry\nRod\nexposed to acetic acid vapor.\n4. The iodine adsorption capacity of Cry reaches 2.5 g g-1 and 1.8 g g-1 at the\nRod\ntemperatures of 75 °C and 25 °C, with the proposed adsorption mechanism\nascribed to the exposure of active sites or pores. To validate this mechanism,\nadditional analysis are required, for instance, XRD characterization should be\nconducted to confirm whether the crystalline structure variations during iodine\nadsorption.\nResponse: Thanks for your constructive suggestion. Time-dependent PXRD\npatterns of Cry to Cry during the adsorption process were measured. In\nRod Quad\nFig. R7, these two crystals exhibit the same structural evolution. As adsorption\ntime increases, the characteristic diffraction peaks of the pristine crystal phase\ngradually weaken and eventually disappear. Meanwhile, a weak new set of\nweak diffraction peaks emerges. These observations suggest a structural\ntransformation during iodine uptake, indicating that iodine molecules\nprogressively penetrate the crystal lattice and lead to lattice rearrangement or\nthe formation of a new iodine-coordinated phase.\nThis discussion has been added to the revised manuscript.\nFig. R7. Time-dependent PXRD patterns of Cry and Cry during iodine\nRod Quad\nadsorption processes at 75 °C.\nResponse: We are very grateful for your meticulous and valuable comments to\nhelp us improve this manuscript. We fully understand the concern that, in its\ninitial form, the manuscript may not have sufficiently conveyed the novelty\nand potential impact of our work. In the revised version, we have therefore\nmade substantial efforts to highlight the novelty of this study more clearly.\nImportant new insights and scientific advances in this work are\nsummarized in a more brief fashion below:\n(1) A reversible phase transition trigger by H O\n2\nfrom a small organic molecule, BiPyBz. The self-assembly of BiPyBz initially\nproduced orange-emitting Cry , then it transformed into a Cry with\nRod Quad\ngreen fluorescence emission upon the addition of a small amount of H O. This\n2\nphase transition from Cry to Cry occurred not only in solution but also\nRod Quad\nin air with a wide humidity range from RH = 60 % to 100 %. SCXRD reveals\ntheir structures: the water-free, tightly packed Cry phase and the water-\nRod\ncontaining, pore-opening Cry phase.\nQuad\nImportantly, this phase transition is strongly coupled to a pronounced\nluminescence response, enabling a real-time optical tracking of the structural\nevolution. Such simultaneous phase transition and luminescence modulation\nin a simple organic molecular crystal is highly unusual and, to the best of our\nknowledge, has not been previously documented for NAC systems.\nThe coexistence of these two single-crystal forms enabled us to isolate the effect\nof pore opening on adsorption efficiency while keeping a single chemical\ncomposition rarely available in NAC research.\n(2) A new strategy to increase the iodine adsorption performance of NACs\nNACs have emerged as promising iodine sorbents because of their ability\nto undergo structural adaptation upon guest inclusion. However, the\nadsorption performance of NACs is limited due to the restricted exposure of\nactive binding sites and slow or incomplete diffusion of iodine molecules into\nthe dense crystal interior.\nIn this study, we demonstrated a new conceptual strategy that utilized a\nhumidity-gated phase transition to generate pores inside NACs before\nadsorption occurs. The phase transition from Cry to Cry introduced\nRod Quad\nwell-defined accessible pores, substantially increasd the exposure of the active\nsite of pyridyl and the diffusion pathways for iodine. This strategy successfully\nelevated iodine uptake to 3.1 g g⁻¹, the highest record among NACs formed by\nsmall organic molecules.\nThus, our work provides a generalizable strategy for enhancing NAC\nsorption performance by controlling crystal architecture through\nenvironmental factors.\n(1) Introduction is generic and does not highlight the unique aspects of this\nsystem. The introduction focuses heavily on radioactive iodine capture but fails\nto emphasize the distinct features of this work, such as H O-gated transition\n2\nand luminescence. The text resembles many other iodine-capture reports,\nmaking originality difficult to perceive. Distinction from previous NACs using\nmacrocycles or cages is insufficient, and the comparison with COFs/MOFs is\noverstated.\nResponse: Thanks for your constructive suggestion. According to the\nreviewer’s suggestion, we have rewritten the introduction part in the revised\nmanuscript. In the revised version, the background of radioactive iodine\ncapture is simplified, and the comparison with COFs/MOFs is omitted. Also,\nthe novelty of this work has emphasized: the H O-gated phase transition, the\n2\naccompanied luminescence transformation, and the advantages of the pore-\ngating structure for iodine adsorption. The difference between previous NACs\nusing macrocycles or cages and the NACs based on organic small molecules\nwas introduced with a more detailed style.\n(2) Compound and crystal structures show limited novelty. BiPyBz itself is not\nnew (Org. Biomol. Chem., 2019, 17, 9467), and Cry has been previously\nQuad\ndeposited in the CCDC1850341. Cry is new, but the CIF lacks an FCF,\nRod\npreventing full validation, and the CCDC-deposited data show Alert A\ninconsistencies, indicating possible refinement issues as flagged by\ncheckCIF/PLATON. The authors provide a BiPyBz–I crystal structure as an\n2\nintermediate, but the saturated iodine-loaded state corresponding to the record\nuptake is not structurally evidenced. This gap substantially weakens the\nreliability of the crystallographic claims, especially for a high-impact journal.\nResponse: Thanks for your constructive suggestions. For clarity, the\nexplanation to each point is separately listed below.\n(1) BiPyBz is an ideal molecular model for investigating the role of pores\nduring adsorption processes\nAt the molecular level, the pyridyl groups of BiPyBz act as active sites\ncapable of forming hydrogen and halogen bonds, while the D-π-A-π-D\nmolecular backbone endows the molecule with intrinsic luminescent properties.\nThese characteristics make BiPyBz a promising candidate for constructing\nfluorescence nonporous adaptive crystals (NACs).\nAs the reviewer pointed out, the Cry phase has been previously\nQuad\nreported. However, we present herein, for the first time, a kinetically stable\nCry polymorph of BiPyBz that is densely packed and completely water-free,\nRod\nwhich is distinctive from the H O-containing Cry structure. Specifically, we\nrevealed that both Cry and Cry exhibit in an unprecedented H O-gated\nQuad Rod 2\nreversible transition between two crystalline phases, accompanied by a\nremarkable luminescence switching behavior. The phenomenon of humidity-\nresponsive phase transition and photophysical coupling has not been\npreviously documented in BiPyBz-based systems or NAC analogous materials.\nFurthermore, the coexistence of these two distinct crystalline structures,\nwhich are obtained from the same molecule provides a unique opportunity to\nuse it as a model system to probe how structural differences govern adsorption\nbehaviors in NAC materials. The adsorption results also confirmed our\nhypothesis that the Cry exhibit higher iodine uptake than Cry , which\nQuad Rod\ncan be attributed to its porous-like structure facilitating iodine molecule\ndiffusion and binding.\nConsequently, while the building block BiPyBz has been identified, the\ndiscovery of its H O-gated reversible transition and structure-property\n2\nrelationship in iodine adsorption establishes a new direction beyond\nconventional static NAC frameworks.\n(2) Re-analysis of the single crystal structure of Cry\nRod\nFor the single crystal data of Cry , unfortunately our oversighting\nRod\ncaused an error. A re-analysis of the single crystal structure has been conducted\nand included in the revised manuscript, consequently eliminated the Alert A\ninconsistencies in the updated version. The new checkcif and cif files were\nattached.\n(3) Attempts to grow a saturated iodine-loaded single crystal (I -I-BiPyBz-I-I )\n2\ncrystal as an intermediate-state structure. The reviewer's comments have also\nbrought this issue to our attention. Therefore, the relevant discussion has\nremoved in the revised manuscript. The structure of BiPyBz-I crystal was\n2\npresented solely as a model to explore the halogen bond interactions between\nthe BiPyBz molecule and iodine molecules. In addition, following the\nreviewer's suggestion, we attempted to cultivate a saturated iodine-loaded\nsingle crystal (I -I-BiPyBz-I-I ). The following section delineates the attempts\n3 3\nthat were made.\n(I) Cultivating the crystal in solution\n(a) Solubility in different solvents\nA 2 mg of saturated adsorbed material was added to a 1.5 mL vial.\nSubsequently, 1.0 mL of organic solvent was added to facilitate the dissolution\nof the material, including n-hexane, toluene, chloroform, dichloromethane,\ntetrahydrofuran (THF), acetonitrile, methanol, Dimethyl Formamide (DMF),\ndimethylsulfoxide (DMSO), and water. As shown in Fig. R8, As shown in Fig.\nR8, the material dissolved notably in DMSO. It dissolved sparingly in THF,\nacetonitrile, methanol, and DMF, but remained nearly insoluble in the other\nsolvents.\nFig. R8. Pictures of dissolving saturated adsorbed material into different\nsolvents (from left to right, n-hexane, toluene, chloroform, dichloromethane,\nTHF, acetonitrile, methanol, DMF, DMSO, and water).\n(b) Cultivating the crystal in DMSO solution\nTable R1. The methods used for cultivating the crystal in DMSO solution.\nForming\nGood solvent Poor solvent Method\naggregate\nHeat-cool\nNo YES\nrecrystallization\nDMSO Water Vapor diffusion YES\nMethanol Vapor diffusion NO\nAcetonitrile Vapor diffusion NO\nWe attempted to cultivate single crystals from the iodine-loaded sample (I -\n3\nrecrystallization and poor-solvent vapor diffusion. The detailed procedures\nand outcomes are summarized below.\nHeating–cooling recrystallization. A volume of 1 mL of DMSO was added\ninto 15 mg of the saturated iodine-loaded solid, and the mixture was heated at\n80 °C for 1 h to obtain a clear solution. The solution was then slowly cooled to\nroom temperature. Ribbon-like crystals manifested (Fig. R9). PXRD analysis\ndemonstrated that the obtained crystals match exactly the pattern of the\nBiPyBz–I phase previously reported in our manuscript.\ncrystals grown in DMSO solution and simulated PXRD pattern of BiPyBz–I\n2\nphase.\nPoor-solvent vapor diffusion method. A 2 mg of the saturated iodine-\nloaded solid was dissolved in 1 mL of DMSO. Three identical solutions were\nplaced into separate 20 mL bottles, with each bottle containing 4 mL of a poor\nsolvent (water, methanol, or acetonitrile). The bottles were left undisturbed (Fig.\nR10). In the water system, a precipitate formed. As demonstrated in Fig. R10,\nthe SEM imaging revealed the cluster morphology. PXRD analysis further\nconfirmed that this precipitate corresponds to the BiPyBz–I phase (Fig. R10d).\nFig. R10. (a) Schematic diagram of the vapor diffusion method. (b-c) SEM\nimages of the aggregates formed in the DMSO/H O system. (d) PXRD pattern\n2\nof the aggregates formed in the DMSO/H O system, and the simulated PXRD\n2\npattern of BiPyBz–I .\n2\n(c) Cultivating the crystal in other solvent solutions\nTable R2. The methods used for cultivating the crystal in DMF, methanol,\nacetonitrile, and THF solutions.\nForming\nGood solvent Poor solvent Method\naggregate\nDMF water Vapor diffusion NO\nmethanol water Vapor diffusion NO\nAcetonitrile n-hexane Vapor diffusion NO\nTHF n-hexane Vapor diffusion YES\nFurther, we attempted to obtain single crystals from the iodine-saturated\nsamples using vapor diffusion with a wider set of solvents. Specifically, 2 mg\nof the saturated iodine-loaded solid was dissolved in 1 mL of DMF, methanol,\nacetonitrile, or THF in a 2 mL vial. Following complete dissolution, the\nsolutions were filtered through a 0.22 μm membrane to remove undissolved\nresidues, yielding clear homogeneous solutions. Each filtered solution was\nsubsequently transferred into a 20 mL bottle containing 4 mL of either water or\nn-hexane, which served as a poor solvent. The systems were left undisturbed\nfor several days.\nAmong all solvent combinations, precipitation occurred exclusively in the\nTHF/n-hexane system. SEM images of the obtained solid indicates crystalline\nmorphology (Fig. R11). PXRD analysis revealed that the precipitate\ncorresponds exactly to the BiPyBz-I phase reported in our manuscript.\nsystems, indicating that crystallization of polyiodide-rich states does not occur\nunder these conditions.\nFig. R11. (a) Schematic diagram of the vapor diffusion method. (b-c) SEM\nimages of aggregates formed in THF/n-hexane system. (d) PXRD pattern of the\naggregates formed in THF/n-hexane system, and the simulated PXRD pattern\nof BiPyBz–I .\n2\n(d) Cultivating the crystal by mixing BiPyBz and I ⁻\n3\nsingle-crystal growth experiments using a methanolic solution of BiPyBz and\nan aqueous I ⁻ solution prepared from KI and I . A trace amount of the I ⁻\n3 2 3\nsolution was added to the dilute BiPyBz methanol solution, and the mixture\nwas left undisturbed. After several days, aggregates formed at the bottom of\nthe vial (Fig. R12). PXRD analysis revealed that the obtained solid corresponds\nexclusively to the BiPyBz–I crystalline phase, which is identical to the\n2\nstructure previously obtained from iodine-loaded samples. However, no\ncrystalline phase containing I ⁻ was detected.\n3\nsolution, the system spontaneously reorganizes to form the thermodynamically\nfavored I -inclusion complex. This finding lends further support to the\n2\nconclusion that polyiodide-rich assemblies cannot be stabilized through\nsolution-based crystallization.\nFig. R12. (a) Schematic diagram of the cultivating method. (b-c) SEM images of\naggregates formed by mixing BiPyBz and I -.\n3\n(d) PXRD pattern of the\naggregates, and the simulated PXRD pattern of BiPyBz-I .\n2\nsolution.\nAfter conducting a thorough investigation, it was determined that the\nhypothetical I -I-BiPyBz-I-I species is likely to be unstable in solution.\n3 3\nThis phenomenon can be attributed to the reversible equilibrium of I ⁻ anion,\n3\nwhich readily decomposes into I and I- (Fig. R13). In solution, the neutral I\n2 2\nmolecules preferentially form strong halogen bonds with BiPyBz, which drive\nthe system to reorganize into the thermodynamically favored BiPyBz–I phase.\n3 3\nsolid state, where molecular mobility is restricted and the polyiodide framework\nis locked by the host lattice.\nTo test this hypothesis, Cry with large size were prepared and an\nQuad\nattempt was made to induce a solid-state crystal-to-crystal phase transition\nunder iodine vapor conditions.\n(II) Cultivating the crystal in solution by a solid-state crystal-to-crystal phase\ntransition\nIn order to avoid dissolution and to probe whether higher polyiodide\nspecies (e.g., I - or I -) can be stabilized exclusively in the solid state, large\n3 5\nCry crystals with typical dimensions of ~100 μm were grown (Fig. R14).\nQuad\nThese crystals were then exposed to iodine vapor at controlled and\nprogressively increasing concentrations (400 ppm, 200 ppm, 100 ppm). During\nthis process, the diffusion rate of iodine into the lattice was deliberately slowed\nin an attempt to preserve crystallinity.\nUpon exposure, a gradual change in the color of the crystals into brown\nwas observed, indicating successful iodine uptake (Fig. R14). However, in all\nexperiments, the single crystals developed internal cracks and eventually lost\nbirefringence when observed under a polarized optical microscope. This\nsuggests a transition from an ordered crystalline state to a disordered or\npartially amorphous state (Fig. R14). Consequently, no sample remained\nsuitable for SCXRD analysis.\nFig. R14. Brightfield and polarized light microscope images of the bulk Cry\nQuad\ncrystals exposed in different concentrations of iodine vapor: (a-b) 400 ppm, (c-\nd) 200 ppm, and (e-f) 100 ppm.\nTaken together, despite extensive efforts including vapor diffusion, slow\niodine ingress, and the use of large high-quality crystals, a crystalline phase\ncontaining I - or I - that is suitable for single-crystal X-ray diffraction could not\n3 5\nbe obtained. Literature surveys also indicate that, although I - or I - signatures\n3 5\nare frequently detected by XPS in iodine-loaded organic materials, the isolated\nsingle crystals reported in these systems invariably correspond to I -inclusion\n2\nstructures (J. Am. Chem. Soc. 2021, 143, 2325−2330; Adv. Mater. 2024, 36, 2402885;\nJ. Am. Chem. Soc. 2025, 147, 39, 35664–35674; J. Am. Chem. Soc. 2025, 147, 36,\n32352–32356, in Table R3 and Fig. R17), and to date, no single-crystal structure\ncontaining I ⁻ has been reported in charge-natural adsorbents. This collective\n3\nevidence strongly suggests that polyiodide species beyond I are inherently\n2\ndifficult to stabilize in an ordered crystalline lattice within the host frameworks.\n(3) Reported iodine uptake values in Fig. 4a are inconsistent and raise doubts\nabout quantification and mechanism. The uptake values are striking but not\nconvincingly supported. The influence of water on the mechanism and the\nidentification of polyiodide species are insufficient. Raman and XPS suggest I -\n3\nand I -, but neutral I cannot be excluded, leaving the mechanism unclear.\n5 2\nFurthermore, if I -BiPyBz-I seen in the crystal structure is a stable intermediate,\n2 2\nwouldn't the corresponding 1.5 g g-1 be observed in adsorption?\nIn particular, SI-Fig. 29 presents the pathway: R-Py -> R-Py-I -> R-Py-I...I -> R-\n3 5\nsymmetry and the reported crystal structure, a more plausible sequence is:\nBiPyBz -> I -BiPyBz-I (already crystallographically proven, 1.5 g g-1) -> I...I-\n2 2\nBiPyBz-I...I -> I ...I-BiPyBz-I...I or I...I-BiPyBz-I...I (reach to 3 g g-1).\n3 3 5\nThis scenario also allows prediction of I ...I-BiPyBz-I...I , corresponding to 4.5 g\n5 5\ng-1. If the uptake is truly quantitative, further crystallographic evidence beyond\nthe intermediate I -BiPyBz-I is required.\n2 2\nResponse: Thanks for your constructive suggestions. For clarity, the responses\nare listed item by item.\n(1) Determination of iodine adsorption capacity\nTo ascertain the adsorption capacity, the internationally accepted\nweighing method was employed (A review for Adsorption-based capture of\niodine and organic iodides, J. Mater. Chem. A, 2023, 11, 5460-5475). As\ndemonstrated in the Fig. R15, a small vial containing the adsorbent and a\nreference vial were placed within a larger vial.\nThe static iodine vapor sorption procedure was performed as follows: the\ncrystal (Cry or Cry , 10 mg) was placed in a vial (4 mL), followed by the\nQuad Rod\nvial being placed in a large, wide-mouth bottle (20 mL) containing iodine. The\nbottle was sealed and maintained at either 25 °C or 75 °C. The weight of the\nsmall vial was measured periodically until equilibrium was achieved.\nFig. R15. Schematic diagram of iodine adsorption devices.\n(2) Influence of water\nThe effect of water on iodine adsorption is achieved by providing pores at\nspecific locations. The presence of H O in the Cry has been demonstrated\nto induce the formation of a cavity, thereby enabling the entry and substitution\nof iodine molecules. DFT calculations reveal that the I---N halogen bond\npossesses a stronger binding energy (-36.40 kJ mol-1) than the N---H hydrogen\nbond (-22.12 kJ mol-1), thereby enabling it to replace the original N---H\nhydrogen bond. Moreover, molecular dynamics simulation results further\nsubstantiate this process, demonstrating that H O molecules are expelled from\n2\nthe molecular cluster during the iodine adsorption process.\n(3) Identification of polyiodide species\nThe X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy\nwere utilized to determine the forms of iodine in the saturated samples. In Fig.\nR16a-b, the wide-scan and high-resolution spectra are presented. The signals\nlocated at 617.57 and 629.03 eV are assigned to I -, and signals at 622.49 and\n3\n634.04 eV are assigned to I\n5\n- ( J. Am. Chem. Soc. 2022, 144, 1, 113–117). In Fig.\nR16c, Raman spectra show two new bands appearing at around 114 cm-1 and\n164 cm-1 of I @Cry , respectively. The band located at 114 cm-1 is coupled to\nthe symmetric stretching vibration of I -. The band located at 164 cm-1 is the\n3\ncharacteristic stretching mode of I - (J. Am. Chem. Soc. 2025, 147, 32352–32356;\nConsequently, there is an absence of discernible signals attributed to\nnatural I in the XPS and Raman spectra. The pyridine groups of BiPyBz have\n2\nbeen observed to donate electrons to I or form weak charge-transfer complexes\n2\nwith I . This process leads to the electron acceptance by I and its subsequent\n2 2\nreduction to I₋. Concurrently, the formation of polyiodide anions, such as I - and\nQuad 2 Quad\n(4) The utilization of BiPyBz-I2 as an intermediate is not a rational approach\nFollowing meticulous deliberation, it has been determined that the\nBiPyBz-I crystal should not be regarded as a stable intermediate structure. The\n2\nreviewer's comments have also brought this issue to our attention.\nConsequently, in the revised manuscript, the relevant discussion has been\nremoved. This structure is presented solely as a model for investigating the\nhalogen bond interactions between the BiPyBz molecule and iodine.\n(5) Response to the iodine transformation pathway\nThe iodine transformation pathway in SI-Fig. 29 (SI-Fig. 36 in the revised\nSI) indicated the origin of iodine anions (I - and I -) and the charge transfer\n3 5\ninteraction between N+ and iodine anions in the saturated sample. This\napproach enables the lucid elucidation of the XPS and Raman results.\nFurthermore, the iodine transformation pathway should be regarded as a\nconceptual representation of progressive polyiodide formation rather than a\nstrict stoichiometric process.\nSupplementary Fig. 36 | Formation and dissociation in the pyridine-iodine\nadduct.\n(6) Crystallographic characterization of I -I-BiPyBz-I-I is extremely\n3 3\nchallenging\nThe crystallographic characterization of I -I-BiPyBz-I-I or I -I-BiPyBz-I-\n3 3 5\nI is extremely challenging. In Comments 2, we have tried our best to cultivate\n5\nthese crystals, but failed. This phenomenon can be attributed to the fact that as\nthe iodine content increases, the crystalline order undergoes a collapse, thereby\nprecluding the formation of diffraction-quality crystals. This phenomenon is\ncommon in polyiodide–organic host systems. In most of the reported works,\nonly R-I crystals were obtained, such as the samples in Table R3 and Fig. R17.\nNo. Name of materials Reference\nnumber\nI\n2\n@AlOC-27-NC; J. Am. Chem. Soc. 2021,\n1 2042206; 2042207\nI @ AlOC-28-NC 143, 2325−2330\n2 β-I crystal 2119599\n2\n2402885\nJ. Am. Chem. Soc. 2025,\n3 c-1e 2375788\n147, 39, 35664–35674\nJ. Am. Chem. Soc. 2025,\n2\n147, 36, 32352–32356\nFig. R17. I -containing single crystals reported in the literature.\n2\n(4) The Cry <-> Cry phase transition is not integrated into the iodine\nRod Quad\nuptake study. The transformation between Cry and Cry is potentially\nRod Quad\ncentral to the uniqueness of the material, but its role in iodine capture is not\nproperly addressed. For example, when both phases coexist (Fig. 3), what\nhappens to I uptake? Without such discussion, the concept of guest-induced\n2\nphase transition remains underdeveloped.\nResponse: Thanks for your constructive suggestion.\nIn the revised manuscript, we incorporated the iodine adsorption\nexperiments of both phases coexisting, in which the samples were obtained at\ndifferent phase transition times. In Fig. R18, the iodine adsorption capacity\nexhibited an increase at both 75 °C and 25 °C in the presence of higher Cry\nQuad\nratios [Cry /(Cry +Cry )]. Especially at 25 ºC, there is an observed\nQuad Quad Rod\nincrease in the saturated iodine adsorption capacity, as well as a concomitant\ndecrease in the adsorption time.\nThis discussion has been added to the revised manuscript.\nFig. R18. Iodine adsorption capacity of two phases coexist samples with\ndifferent Cry mass ratios [Cry /(Cry +Cry )] at (a-b) 75 ºC and (c-\nQuad Quad Quad Rod\nd) 25 ºC.\nWhile the luminescent phase transition is quite interesting, the compound\nnovelty is weak, the iodine uptake data are inconsistent and insufficiently\nsupported, and the introduction does not distinguish the work from similar\niodine-capture reports. Most importantly, only an intermediate iodine-loaded\ncrystal (I –BiPyBz–I ) is provided, while no structural evidence for the high\n2 2\nuptake state is given. Overall, the manuscript does not provide the originality\nor reliability required for Nature Communications.\nResponse: Thanks for your constructive suggestion. In accordance with the\nreviewer's recommendations, the introduction of the revised manuscript has\nbeen modified to emphasize the novel aspects of this work, namely the\nluminescent phase transition and the significance of pores in the context of\niodine adsorption. In addition, an exhaustive array of methodologies has\nemployed to facilitate the growth of a single crystal from a saturated iodine\nadsorption sample. Our findings revealed that the acquisition of diffraction-\nquality crystals is unfeasible due to the collapses of crystalline order structure\nas the iodine content increases. Subsequently, the iodine adsorption\nperformance of both phases coexisting was examined, and it was revealed that\nthe pores can concurrently enhance the iodine adsorption rate and capacity.\nThus, the present work provides a new direction beyond conventional\nstatic NAC materials, where the sorption performance can be enhanced by the\ninner porous structures. It is our hope that these revisions and clarifications\nwill facilitate a more effective conveyance of the originality and robustness of\nour findings.\nWe are thankful and appreciate the positive comments on the\nnovelty and significance of our study. We believe that the detailed constructive\ncomments and suggestions have helped to improve the level of the manuscript.\nA point-by-point response to the comments is listed below.\n1. The recyclability tests show ~23% capacity loss after four cycles, attributed\nto residual iodine. The authors may suggest strategies to improve regeneration\n(e.g., extended heating or chemical washing) to further strengthen the case for\npractical applications.\nResponse: Thanks for your constructive suggestion. According to the\nreviewer’s suggestion, we employed extended heating and chemical washing\nmethods to regenerate the adsorption materials. The heating methods\nperformed using the devices are illustrated in Fig. R19, in which a vacuum\npump was used to promote iodine desorption. After four successive\nadsorption-desorption cycles, Cry retained >75 % of the initial iodine\nQuad\nuptake capacity (Fig. R20), which is analogous to the performance (>77 %) of\nthe DMF washing method. These results confirm iodine has a strong interaction\nwith BiPyBz.\nFig. R19. Schematic diagram of a heated gas desorption device.\nFig. R20. Recycle adsorption performance of Cry .\nQuad\n2. Since the title highlights fluorescence, it would be valuable to discuss the\npotential use of fluorescence changes (color shift and quenching) as a built-in\nindicator of phase transition or iodine loading.\nResponse: Thanks for your constructive suggestion. The in-situ fluorescence\nemission spectra of the phase transition from Cry to Cry and the iodine\nRod Quad\nloading process of Cry were measured.\nQuad\nIn Figs. R21a-c, the in-situ emission monitoring of the phase transition\nprocess from Cry to Cry exhibited the blue-shift of the emission peak\nRod Quad\nfrom 570 to 533 nm. Furthermore, the maximum emission peak exhibited a\nlinear relation with the transition time under vapor exposure, reaching\nequilibrium at 100 hours. To further determine the correlation between mass\nratio and transition time, we prepared the physical mixing samples with\nvarying mass ratios of Cry and Cry . The maximum emission peaks\nRod Quad\nindicate a blue shift from 570 nm to 533 nm as the Cry ratio increased. A\nQuad\nnon-linear correlation is observed between the maximum emission wavelength\nand the Cry mass ratios [Cry /(Cry +Cry )] (Figs. R21d-e). Based\nQuad Quad Quad Rod\non this relationship, the correlation between the Cry mass ratio and the\nQuad\nphase transition time can be obtained (Fig. R21f). These results indicated that\nthe position of the maximum emission peak value can reveal the mass ratios\nbetween Cry and Cry . This provides a quantitative optical indicator for\nRod Quad\nevaluating the extent of phase transition during the vapor-induced structural\nevolution process.\nThis discussion has been added to the revised manuscript.\nFig. R21. (a) Schematic diagram of the in-situ emission spectra monitoring\ndevice. (b) In-situ emission spectra of the phase transition from Cry to\nRod\nCry under exposure in a saturated vapor environment. (c) Linear\nQuad\ncorrelation between the maximum emission band and the transition time\nduring phase transition. (d) Emission spectra of the physical mixture of two\nphases with different mass ratios [Cry /(Cry +Cry )]. (e) Correlation\nQuad Quad Rod\nbetween the maximum emission band and Cry mass ratio by physical\nQuad\nmixing. (f) Correlation between Cry mass ratio [Cry /(Cry +Cry )]\nQuad Quad Quad Rod\nand the transition time.\nDuring the iodine adsorption of Cry at 25 ºC, the emission intensity\nQuad\nexhibited an exponential decay in 30 min without emission peak shift, which is\nconsistent with the results of in-situ fluorescence microscope monitoring. These\nresults indicate that the emission of BiPyBz is sensitive to the iodine vapor,\nsuggesting the potential for its future preparation as a fluorescence device for\nsensitive detection of iodine vapor.\nThis discussion has been added to the revised manuscript.\nFig. R22. (a) In-situ emission spectra of Cry during the iodine adsorption\nQuad\nprocess at 25 ºC. (b) PL intensity decay curve of Cry at 533 nm to exposure\nQuad\ntime in iodine vapor at 25 ºC.\n3. The comparison with closely related NAC systems (e.g., TPAPyH, ref. 53)\ncould be made clearer in the introduction or discussion to emphasize how this\nwork differs in molecular design and performance.\nResponse: Thanks for your constructive suggestion. A detailed comparison\nwith the representative NAC systems (including TPAPyH in Ref. 53) has been\nincorporated into both the Introduction and Discussion sections.\nIn introduction\n“Li group reported the first small-molecule NAC, TPAPyH, with solvent\ndimension-dependent crystallinity property, achieving excellent iodine\nadsorption capacity of 1.27 g g−1.“\nIn Discussion\n“A D-A molecule, BiPyBz (Scheme 1), was designed and synthesized by a one-\nstep coupling reaction, in which the pyridine motif was designed as a hydrogen\nor halogen bonding active site, [Adv. Mater. 2024, 36, 2402885.] and the\nbenzothiadiazole motif as an electron acceptor and chalcogen bonding site.“\n4. Some language issues should be corrected (e.g., “adptive” → adaptive,\n“Different with” → Unlike, “replacment” → replacement). Careful\nproofreading will improve clarity.\nResponse: Thanks for your constructive suggestion. We have carefully\nproofread the entire manuscript and corrected all spelling and grammatical\nerrors to improve clarity and readability.\n5. For the iodine filtration column test, adding brief details such as flow rate or\nresidence time in the main text or SI would enhance the understanding of\nscalability.\nResponse: Thanks for your constructive suggestion. In the revised manuscript,\nwe added experimental details.\nCry (50 mg) was filled into the column with a length of 8.5 cm, and the\nQuad\nends of the column were filled with cotton. Injecting 10 mL iodine vapor (gen\ninto the column at a rate of 3.3 mL/min, and the output gas was absorbed with\n0.3 M sodium hydroxide solution (10 mL). The iodine concentration in the\nsolution was detected by ICP-MS, and the iodine concentration (c ) of the\nvapor\noutput gas was calculated with the equation as follows:\n𝑐 ×𝑉\n𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛\n𝑐 =\n𝑣𝑎𝑝𝑜𝑟 𝑉\n𝑣𝑎𝑝𝑜𝑟\nwhere c is the iodine concentration in solution, measured by ICP-MS;\nsolution\nV is the volume of the sodium hydroxide solution, 10 mL; V is the\nsolution vapor\nvolume of iodine vapor, 10 mL.\nThis part has been added to the revised manuscript.\nOverall, this is an excellent contribution and I recommend acceptance after\nminor revision.\nResponse: We appreciate the constructive feedback and the recommendation\nfor acceptance following minor revisions. A thorough evaluation of all\nsuggestions has been conducted, leading to a substantial revision of the\nmanuscript. It is our earnest hope that the revisions will thoroughly address\nthe reviewer's concerns and meet their expectations."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I have reviewed the revised manuscript and the authors' responses to the concerns raised by me and Reviewer #1. The\nauthors have adequately addressed all the technical issues highlighted in the previous review round. The revisions made to\nthe manuscript are substantial and appropriate, effectively resolving the methodological concerns and strengthening the\noverall quality of the paper.\nTherefore, I recommend that this paper be accepted for publication in its current form.", "reviewer_3": "The authors have thoroughly revised the manuscript and addressed all my concerns. The revisions improve the clarity and\nquality of the work, and the manuscript is now suitable for publication."}, "author_response": ""}]

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