Reversible color switching of bright phosphorescence in purely organic materials for advanced data encryption

Domain: neuroscience
Published: 2026-04-01
Editorial outcome: major_revision

Abstract
Purely organic phosphors have emerged as promising materials for various optical applications. Herein, we report a single-component organic phosphorescent crystal, 1,1′-(2,5-dibromoterephthaloyl)bis(glutarimide) (BrGlu), which exhibits fully reversible, pseudopolymorph-dependent phosphorescence color switching. Under standard crystallization conditions, BrGlu forms green-emitting crystal (G-crystal), while crystallization in the presence of CHCl3 yields blue-emitting solvent-inclusion crystal (B-crystal). Notably, G-crystal converts into B-crystal upon exposure to CHCl3, whereas the blue crystals revert to green upon heating, demonstrating a reversible phosphorescence-to-phosphorescence switching mechanism. Through a combination of experimental analyses and quantum chemical calculations, we elucidate the underlying mechanism governing this triplet emission color transformation: syn–anti conformational reorientation of bromine–carbonyl substituents: upon solvent inclusion, hydrogen bonding stabilizes the syn-rotamer to yield bright-blue phosphorescence, whereas gentle heating reverts the system to the anti-rotamer for green emission. Exploiting this property, we develop an advanced data encryption platform featuring a dual-layer security system—phosphorescence emission combined with thermal- or solvent-induced stimuli-response—significantly enhancing security in secure data encryption and anti-counterfeiting.

Introduction
The study of room temperature phosphorescence (RTP) in purely organic compounds has a long history1–3, yet it remains a topic of interest due to its critical applications in OLEDs, photocatalysis, optical sensors, and security encryption, just name a few. Early efforts primarily explored the underlying mechanisms of RTP, with a particular focus on benzophenone4, which exhibits phosphorescence in degassed or frozen solution, as well as in the crystal state5,6. Since the early 2010s, RTPs by Kim, Tang, Fraser, and others have been observed in a diverse range of organic compounds beyond benzophenone7–15. As its underlying principles have been elucidated, the development of purely organic phosphors with varied structural designs has accelerated, along with extensive research into their broad applications, particularly in data encryption and anti-counterfeiting15–25. For these security-related applications, the defining feature of RTP materials is their intrinsically long-lived afterglow, driven by weak spin–orbit coupling (SOC), which slows the excited state deactivation associated with RTP15,17,26–29. This distinctive “time-gated” emission offers a key advantage over conventional fluorescent systems, whose luminescence fades immediately upon removal of the excitation source30–35.

However, its promise, relying solely on long phosphorescence lifetimes, is often inadequate for robust data protection, as a single security layer remains susceptible to counterfeiting or unauthorized duplication31–35. To address this, additional protective features—especially stimuli-responsive capabilities—are essential. These properties enable reversible modulation of emission in response to external stimuli such as heat, solvent exposure, or mechanical stress3,36–45. When integrated with the materials’ long-lasting emission, such reversible emission shifts introduce a second layer of complexity, heightening the challenge of counterfeiting.

Despite the clear value of reversible RTP for advanced encryption, achieving phosphorescence-color switching in purely organic materials under mild and practical conditions remains challenging. Most reported systems rely on multi-dopant matrices or require changes in excitation wavelength, complicating both the fabrication process and practical implementation (Supplementary Fig. 1a)31–33. Moreover, in single-component systems, external stimuli (e.g., temperature, solvent, pH, mechanical force) often lead to irreversible structural disruption or quenched emission (Supplementary Fig. 1b)46–57. For instance, Li et al. developed a stimulus-responsive RTP system by embedding phosphorescent and fluorescent dyes in a PVA matrix, enabling afterglow color modulation from blue to green to orange through triplet-to-singlet Förster resonance energy transfer31. However, exposure to water vapor compromised the rigid environment essential for phosphorescence, resulting in severe emission quenching. Similarly, Song et al. demonstrated a single-component phosphorescent material capable of shifting from blue to green under mechanical stress48. However, this transformation came at the cost of a significant reduction in phosphorescence intensity—from 68.4% (blue) to 7.6% (green)—as the looser packing after grinding failed to adequately suppress molecular motion. Recently, Ogawa and co-workers have reported a series of single-component organic crystals that exhibit RTP-to-RTP color changes in response to external stimuli via molecular conformational rearrangement58–62. However, these systems often suffer from limited reversibility, low emission efficiency in one of the states, or require complex external triggers, there by limiting their practicality and broader applicability.

Herein, we address these challenges by introducing an organic phosphorescent crystal, 1,1′-(2,5-dibromoterephthaloyl)bis(glutarimide) (BrGlu), which exhibits fully reversible, pseudopolymorph-dependent phosphorescence-to-phosphorescence color switching under mild external conditions (Fig. 1a)63–65. Under standard crystallization, BrGlu forms green-emitting crystals (G-crystal), while crystallization in the presence of CHCl3, yields blue-emitting solvent-inclusion crystals (B-crystal). This color change stems from syn–anti rotamer conformational rearrangements triggered by solvation. Syn/anti rotamers have commonly only a small impact on the emission properties66, but were shown to be relevant in the fluorescence process of the parent bromine-free TGlu series67. Structural analysis reveals that solvent-mediated hydrogen bonding flips the bromine–carbonyl orientation into a syn-rotamer, thereby increasing the triplet-state energy and resulting in a blue-shifted emission. Our system achieves reversible switching between G-crystal and B-crystal through two distinct pathways: dissolution–recrystallization in the presence of specific halogenated solvents and mild heating–driven solid-state desolvation. This transformation between well-defined conformational rotamers affords high photostability, high phosphorescence quantum yields (43% and 93% for B- and G-crystals, respectively), and complete reversibility. Unlike previously reported systems requiring complex fabrication or suffering emission quenching, BrGlu’s single-molecule design ensures reversible switching with high quantum yields (Supplementary Table 1). Quantum-chemical calculations further indicate a low energy barrier for this rotameric interconversion, allowing rapid and complete reversion to the original anti-rotamer upon mild heating. As desired for advanced data encryption, this color change is entirely reversible upon mild heating, facilitating repeated rapid switching between the two phosphorescent states.

On the basis of these findings, we have developed a proof-of-concept data encryption platform that takes full advantage of both BrGlu’s phosphorescence and its reversible emission switching (Fig. 1b). By intermittently applying solvent and mild heating, we achieve “advanced data encryption,” wherein information is encoded in both spatial patterns and emission lifetime in a manner that is not readily discernible through simple observation. Furthermore, the sub-millisecond phosphorescent lifetimes restrict visibility of the encoded signal to detection systems equipped for microsecond-scale readout, thereby adding another layer of security. This fully reversible, single-component RTP highlights the potential of stimulus-responsive organic phosphors for next-generation secure data storage and anti-counterfeiting applications.

Results
Synthesis
We designed and synthesized a purely organic phosphor, 1,1′-(2,5-dibromoterephthaloyl)bis(glutarimide) (BrGlu), incorporating two main design elements (Fig. 1c). Bromine atoms were here introduced to enhance SOC and promote intersystem crossing (ISC) from the lowest singlet state to triplet state manifold68. A sterically hindered acyl-glutarimide (Ac–Glu) moiety was introduced to suppress intramolecular motion and reduce π–π stacking in the solid state. This design aims to limit close intermolecular interactions that could facilitate nonradiative deactivation pathways, in particular exciton diffusion, towards defect sites; this was recently reviewed in the context of phosphorescent crystals3. Density functional theory (DFT) calculations of the optimized molecular geometry reveal that the glutarimide groups adopt an nearly orthogonal orientation relative to the dibromoterephthaloyl core (Fig. 1c, right), which minimizes orbital overlap between neighboring chromophores and supports efficient RTP.

Preparation and analyses of phosphorescent crystals
Crystals of BrGlu were grown from eight different solvents, resulting in diverse morphological and emissive properties (Supplementary Note 5 and Supplementary Fig. 2). Most solvents yielded green-emissive crystalline solids with varying forms, whereas CHCl3 produced a blue-emissive counterpart (Fig. 2a). Powder X-ray diffraction (PXRD) analysis supports these findings (Supplementary Fig. 3): crystals grown in CH2Cl2 (dichloromethane), ethyl acetate, tetrahydrofuran, and methanol exhibited nearly identical diffraction profiles, suggesting a common crystal structure. In contrast, the CHCl3-derived solid displayed a different PXRD pattern, indicating a distinct packing arrangement. These findings underscore the critical role of solvent selection in directing crystal structure, which in turn governs the resulting emission color—revealing a direct correlation between molecular packing and phosphorescent behavior.

To elucidate the relationship between emissive behavior and structural variations, we performed single-crystal (SC) XRD analysis (Fig. 2b–d, Supplementary Figs. 4 and 5, and Supplementary Table 2). The blue-emissive crystalline solid (B-crystal, solvate form) incorporates CHCl3 molecules and crystallizes in the I 2/a space group (a=16.9Å, b=10.3Å, c=15.1Å, β=120°, Fig. 2b, c). In contrast, the green-emissive crystal (G-crystal), grown from CH2Cl2, is solvent-free and adopts the P 21/c space group (a=12.1Å, b=6.0Å, c=13.3Å, β=106°, Fig. 2d). Notably, both crystals exhibit relatively large interlayer distances—7.58Å for B-crystal and 6.01Å for G-crystal (measured perpendicular to the c–a plane)—supporting the design principle that sterically bulky Glu units hinder close molecular packing. This structural feature is expected to reduce intermolecular excitonic coupling3, thereby limiting triplet exciton diffusion and effectively enhancing solid-state phosphorescence; this will be discussed in more detail further down.

Thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses reveal a clear distinction between the B- and G-crystals with respect to solvent incorporation and thermal stability (Fig. 2e, f). The B-crystal exhibits a mass loss of approximately 14.4 wt% upon heating to ~65°C, corresponding to the release of CHCl3 (Fig. 2e). This is consistent with a 1:1 BrGlu:CHCl3 solvate, supported by the calculated theoretical CHCl3 content of 18.8 wt% based on the molar masses of BrGlu (514.13 g/mol) and CHCl3 (119.38 g/mol). SCXRD confirms this stoichiometry, showing CHCl3 molecules occupying disordered but well-defined lattice sites, stabilized by C–H···O interactions.

In contrast, the G-crystal displays no observable mass loss and remains structurally stable until decomposition begins above 260°C (Fig. 2f and Supplementary Fig. 6). This exceptional thermal stability was further validated by repeated thermal cycling experiments. When subjected to five cycles of heating and cooling between 0 and 200°C, the G-crystal retained its structural integrity, as confirmed by PXRD analysis (Supplementary Fig. 7). These results demonstrate the BrGlu’s robustness under operational conditions relevant to reversible switching applications.

Raman spectroscopy (Fig. 2g) further underscores these structural differences. The C=O stretching mode shifts from 1749 cm−1 in the G-crystal to 1763 cm−1 in the B-crystal, correlating with a slight shortening of the average C=O bond (1.20Å vs. 1.19Å, Supplementary Fig. 8). This shift reflects bond reinforcement induced by CHCl3–lattice interactions, rather than classical hydrogen bonding. Additionally, a new peak at 1586 cm−1 appears exclusively in the B-crystal, which can be attributed to a change in the lattice leading to a local vibration of CHCl3–BrGlu interaction69. This peak reflects solvent-induced modulation of the lattice dynamics, not Br···O halogen bonding, which typically appears below 300 cm−1.

The B-crystal contains CHCl3 molecules that form hydrogen bonds with BrGlu (H to O distances: 2.280Å, 3.405Å, 4.085Å; Fig. 2c). These hydrogen bonds rearrange the local packing, affecting the orientation of the C=O and Br substituents (Fig. 3a)70. In the B-crystal, these groups largely point in the same direction (syn-rotamer; adjacent Br1 to O1 distance (d): 3.1Å; other Br2 to O1 distance (d): 6.3Å; carbonyl dihedral angle (θ): 36°). By contrast, in the G-crystal, an anti-rotamer of the C=O and Br substituents is found; d is 4.5Å, d is 5.2Å, and θ is 26°. While conformation-dependent RTP behavior is not widely reported, recent studies have shown that subtle conformational variations can affect triplet emission colors and rates60–65,70,71,72. In the BrGlu system, a single molecule exhibits a distinct conformational change between two emissive pseudopolymorphs, triggered by solvent inclusion. To explore this mechanism in detail, we performed further quantum calculations based on the crystal structures. Single point (SP) DFT calculations of the syn- and anti-rotamers in their x-ray crystal conformations reveal that the anti-form (i.e., the solvent-free G-crystal) is slightly favoured by about ΔG=5.0 kJ/mol (52 meV; see Fig. 3b and Supplementary Table 3); thus, the occurrence of the syn-form in the B-crystal is evidently stabilized by chromophore-solvent interactions. We note that both syn- and anti-conformations found in the crystals are not very different from the fully DFT-optimized ones in vacuum, see Fig. 3a and Table 1. For the latter, syn is slightly more stable than anti (5.6 kJ/mol (58 meV) in Fig. 3b).

In summary, SCXRD, Raman, and thermal analyses confirm that CHCl3-solvated BrGlu contain solvent molecules involved in hydrogen bonding. Comparing the B-crystal and G-crystal (see Fig. 3 and Table 1) reveal substantial differences in substituent orientation: in the B-crystal, the C=O groups are co-aligned with the bromine atoms (syn-rotamer), whereas in the G-crystal they are oriented in the opposite direction (anti-rotamer).

Phosphorescence color switching under mild external conditions
We first examined the optical properties of BrGlu in Ar-purged CHCl3 solution at room temperature (RT, 298 K) and low temperature (LT, 78 K), as shown in Fig. 4 and Supplementary Fig. 9. The absorption spectrum exhibits a main peak at 327 nm; according to our time-dependent (TD) DFT results (see Supplementary Fig. 10), this is ascribed to the electronic transition to the lowest excited singlet state (S0 → S1). The latter is essentially described by a one-electron excitation between the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO). According to the calculations, the lowest excited triplet state (T1), which is not visible in absorption, is located 0.74 eV below S1; the T1 state exhibits a complex electronic description, which thus requires natural transition orbitals, NTOs (Supplementary Fig. 10). At RT, the solution exhibits neither fluorescence nor phosphorescence under steady-state conditions; in contrast, at LT conditions pronounced phosphorescence occurs at 492 nm, whereas no fluorescence could be detected (Supplementary Fig. 9). The results evidence that nonradiative deactivation, being highly effective at RT, is efficiently blocked in the frozen solution through the rigid environment10. Moreover, the results show that ISC from S1 to the triplet manifold (Tn) is highly efficient in BrGlu, largely exceeding the fluorescence channel. This is in line with the TD DFT calculations, which indicate energetically accessible triplet states with considerable SOC elements (see Supplementary Fig. 10); this is a result of the participation of the free electron pairs of bromide in the respective NTOs, such providing a significant heavy atom effect15,65. Consequently, consistent with its behaviour in frozen solution, BrGlu is expected to exhibit phosphorescence also in its distinct crystalline forms, as indeed observed, see Fig. 4 and Table 2.

Building on these observations, we next explored how solvation-driven structural reorganization leads to distinctly different photophysical properties (Fig. 4). Steady-state PL measurements reveal emission maxima at 480 nm for the B-crystal and 510 nm for the G-crystal; low-temperature measurements are consistent with the experiments at ambient conditions, maintaining the bathochromic shift from B to G (Supplementary Fig. 12). As we will see below, this spectral shift reflects the different rotameric conformations, see Fig. 1a. Time-resolved PL spectra (Fig. 4a, delay: 0.2–1.0 ms) closely mirror the steady-state profiles, confirming triplet-state emission, with lifetimes in the microsecond regime (Fig. 4b). The phosphorescence quantum yields (ΦP) are 93% for the G-crystal and 43% for the B-crystal, with corresponding triplet lifetimes (τP) of 220 µs and 150 µs, respectively—demonstrating that both phases remain highly emissive despite the reduced efficiency of the B-crystal.

In order to rationalize the experimentally observed bathochromic shift of phosphorescence when going from B to G, we first conducted single-point TD-DFT calculations (i.e., giving the vertical transition energies Evert(S0 → T1)) of the molecules in those conformations, which were found in the crystal structures (see Fig. 4c and computational part for details). According to the calculations, Evert(S0 → T1) are somewhat higher in G-crystal compared to B-crystal (Supplementary Table 4), which seems to, at first glance, contradict the experiment. Similar results are found for Evert(S0 → T1) on the fully relaxed syn and anti conformations, see Supplementary Table 5. Nevertheless, we note that the figure of merit for the phosphorescence process is not the vertical energy for absorption, but that for emission, i.e., Evert(T1 → S0). Therefore, we optimized the T1 states, to calculate the latter; in fact, here the anti-conformation (G-phase) is found at 1.85 eV, i.e., 0.09 eV below the syn conformation (B-phase; at 1.94 eV), see Fig. 4c and Supplementary Table 6. This inversion is driven by the much larger structural relaxation in the T1 state of anti in comparison syn. The TD-DFT results on Evert(T1 → S0) are in reasonable agreement with the experimentally observed shift from 2.43 eV (510 nm) for G to 2.58 eV (480 nm) for B, that is 0.15 eV. The agreement is in particular good, as an additional small hypsochromic shift for the B-phase is expected to arise from the lower polarizability due to dilution by the solvent molecules in the B-crystal73, which is not accounted for in the TD-DFT calculations. Finally, we rule out excitonic contributions to the spectral shift, as this becomes only significant for high oscillator strength (f) of the lowest excited state and small intermolecular separation3,74; in the BrGlu crystal, however, f of the emitting state is self-evidently very small due to its triplet nature, and, at the same time, intermolecular separations are large, as evidenced from the crystal structure (vide supra).

To clarify the excited state deactivation processes, we derived the radiative and nonradiative rates from T1 via kP = ΦP / τP and knr = (1-ΦP) / τP (Table 2) by assuming that the ISC efficiency ηISC from S1 to the triplet manifold is about unity. The latter assumption is, on one hand, consistent with the absence of fluorescence at any temperature, while the nonradiative deactivation from S1 is expected to be very small due to the restricted access to the conical intersection for such emitters64. On the other hand, the spin-orbit coupling matrix elements are high for ISC from S1 to the triplet manifold Tn, due to the very significant participation of the free electron pairs of Br in S1 (see Fig. 4d for representative NTOs and Supplementary Figs. 10 and 13 for details); therefore, this heavy-atom contribution enables efficient ISC via effective SOC. Deriving kP, notably, the B-crystal and G-crystal show both high kP values (2.9×103 s−1 and 4.2×103 s−1, respectively), which arise from effective SOC through participation of the free electron pairs of Br also in the description of the T1 state, see Supplementary Fig. 13. This can be directly evidenced from quantum chemistry; for this, we calculated the oscillator strength f(T1) by TD-DFT under inclusion of SOC, and subsequently estimated kP via the Strickler-Berg equation3,75 (see Supplementary Note 10 for details). This yields ca. 1.0×103 s−1 for B and 3.4×103 s−1 for G, being in reasonable qualitative agreement with the experimental values; in particular, it confirms the larger value for kP in the G-crystal in comparison with B. The main factor for the enhancement of kP in G vs. B is essentially due to the higher f(T1) in G, indicating stronger SOC of T1 with S0 through the heavy atom effect of bromine.

The non-radiative rates knr from T1 are notably low (=3.8×103 s−1 for B-crystal, 0.32×103 s−1 for G-crystal). On one hand, phosphorescence quenching may be due to nonradiative recovery of the ground state S0 through inefficient reverse ISC3; the latter is commonly proceeding through a T1/S0 crossing point (CP). On the other hand, quenching may be due to trapping (in particular at interfaces/surfaces)63. These traps are reached by triplet exciton diffusion through long exciton lifetimes and effective exciton coupling via an exchange mechanism, which thus requires orbital overlap3. The latter is effectively suppressed in the BrGlu crystals due to the larger interlayer distances dIL of 7.58Å in B vs. 6.01Å in G as induced by the sterically bulky acyl-glutarimide groups. The larger knr for the B-crystal compared to G thus clearly evidences that trapping is not the predominant quenching mechanism, as the larger dIL in B vs. G should slowdown exciton coupling (and thus trapping) even more and not vice versa. Instead, the enhanced knr for B vs. G must be ascribed to more effective nonradiative ground state recovery via ISC0. In fact, although the densities of the B- and G-crystals hardly differ (see Supplementary Table 2), the presence of the solvent molecule in the B crystal may give BrGlu more structural flexibility in B vs. G, and thus facilitate opening the path to the T1/S0 CP. Additionally, the higher-lying T1 state in B compared to G should further promote the access to the CP, in analogy to the internal conversion (via a conical intersection) in the TGlu series64.

Importantly, the transition between the two phosphorescent phases is fully reversible. When G-crystal (100 mg) samples are suspended in 3 mL of CHCl3, they dissolve and recrystallize into the B-crystal over approximately 60 min (Fig. 4e, f and Supplementary Fig. 14). Photographic images in Fig. 4e under 365 nm irradiation and corresponding spectra show the transition from G-crystal to B-crystal states in CHCl3 while stirring. Subsequent heating to ~60°C removes the solvent, as confirmed by TGA and DSC, thereby restoring the original green emission. By contrast, exposure of the G-crystal to CHCl3 vapor for up to 2 days did not induce any phase transformation (Supplementary Fig. 15), indicating that solvent-assisted recrystallization from solution is necessary for pseudopolymorph interconversion. PXRD analyses of the post-heating sample further validate the transition between the B- and G-crystal states (Supplementary Fig. 7), with the final green-emissive material reverting to the same packing structure as the initial G-crystal. This complete reversibility is also illustrated by the emission switching in the CIE 1931 color space—from (0.216, 0.513) to (0.148, 0.257) (Fig. 4g)—which remains stable through at least ten solvation–desolvation cycles (Supplementary Fig. 16), with minimal variation in emission intensity (<5%) and λmax (<1 nm). In addition, both crystal forms exhibit excellent environmental stability under ambient conditions (25°C, 30% relative humidity). The G-crystal maintains a consistent emission profile over 7 days, indicating high structural robustness (Supplementary Fig. 17a). For the B-crystal, the emission remains largely unchanged during the initial 24 h, after which a gradual red-shift in λmax is observed, corresponding to a slow conversion to the G-phase as CHCl3 slowly evaporates (Supplementary Fig. 17b). These results confirm the high durability, reversibility, and practical reliability of the BrGlu system, supporting its potential for real-world applications in optical data storage and anti-counterfeiting technologies.

The facile conversion between the B- and G-phase is rationalized by DFT relaxed scans around the inner torsional angle θi (C1−C2−C4−N1), which converts the syn-rotamer (B-phase) into the anti-rotamer (G-phase), see Fig. 5; here, we additionally allowed for the torsion around the outer torsional angle θo (C1−C2−C4−N1) to release the steric constraints. A low-energetic path on the potential energy surface is evident from Fig. 5, which governs the transformation via a saddle point (X) at θi = 98° and θo = 109° with a barrier height of about 34 kJ/mol (0.35 eV), which explains why reversible interconversion of the two phases is rather easily accomplished in this material.

To further explore the solvent dependence of solvation-induced phosphorescence color switching, we investigated several halogenated solvents—including CDCl3, CHBr3, C2H2Cl4, C2H4Cl2, and CCl4; among them, besides CHCl3 (vide supra), only CDCl3, CHBr3, and C2H2Cl4 triggered the solvation–desolvation cycle (Fig. 6a–d). By contrast, solvents with negligible or no hydrogen-bond donation (e.g., C2H4Cl2 or CCl4) failed to induce a conformational reorganization of the crystal lattice, underscoring the pivotal role of hydrogen bonding in crystal dissolution, re-nucleation, and thus the phosphorescence color change. Interestingly, the solvation rates decrease in the following order, CDCl3 (10 min) > C2H2Cl4 (20 min) > CHCl3 (60 min) > CHBr3 (120 min), reflecting the interplay of isotope effects, solvent polarity, and molecular size. For instance, deuterated chloroform (CDCl3) accelerates solute–solvent interactions more effectively, whereas the increased number of chlorine atoms (electron-withdrawing groups) in C2H2Cl4 enhance its polarity, which partially compensates for its higher mass. Conversely, the larger van der Waals volume of CHBr3 slows down its diffusion into the G-crystal, thereby extending the time required for hydrogen-bonding interactions to form and resulting in the longest overall solvation time.

Furthermore, electrostatic potential (ESP) calculations (Supplementary Fig. 18) revealed that CDCl3, CHBr3, and C2H2Cl4 contain sufficiently “activated” hydrogen atoms capable of hydrogen bonding with BrGlu. In contrast, solvents such as C2H4Cl2 or CCl4 either possess minimal hydrogen-bonding capability or lack hydrogen entirely. Additional tests with polar protic and aprotic solvents—including methanol, ethanol, and acetonitrile—also failed to induce any phase transition, showing only minimal changes in PL spectra (Supplementary Fig. 19). These results suggest that the phase transition is governed not by polarity or hydrogen-bonding capability alone, but by a combination of factors, including ESP distribution, molecular size, and diffusivity, as summarized in Supplementary Table 7. PXRD and Raman spectra (Supplementary Figs. 20 and 21, respectively) confirm that each of these halogenated solvents with activated hydrogen induces a distinct solvated crystal form relative to the unsolvated G-crystal. Supporting this, TGA (Supplementary Fig. 22) and DSC (Supplementary Fig. 23) measurements on the solvated crystals with CDCl3, C2H2Cl4, and CHBr3 show mass losses in the range of 67–71°C, indicating that solvent acidity, polarity, and molecular size collaboratively influence the solvation/de-solvation behavior and associated emission characteristics.

Advanced data encryption
Based on BrGlu’s distinctive capability to toggle between two phosphorescent states, we engineered a multilayer encryption scheme built on two decryption keys (key 1 and key 2) for secure data access (Fig. 7a). In this configuration, B-crystal ink functions as the authentic cipher, whereas the FL dye ink adds a deceptive layer of visible camouflage. Both inks were formulated by dispersing the respective luminescent materials in mineral oil to form stable, solid-state dispersions suitable for precise patterning. The molds used for patterning were fabricated using a PDMS casting method, and hand-cut to produce features down to ~2×8 mm (see Supplementary Note 9 and Supplementary Fig. 24).

In Fig. 7b, we observe that under 365 nm UV light, the visible pattern appears as the spurious “8888” in blue (Supplementary Movie 1). Heating (~60°C, key 1) briefly suppresses BrGlu’s phosphorescence through increased molecular vibrations, distorting the display. Upon cooling, BrGlu transitions to its G-crystal phase, glowing blue and green mixed message. Turning off the UV lamp (key 2) maintains this green long-lasting phosphorescence emission, confirming the correct information, being “UM” (Supplementary Movie 2). Notably, this thermal cycle is fully reversible, enabling repeated thermal modulation, solvent-mediated color switching, and phosphorescent emission for secure data storage.

Additionally, a “3D” encryption strategy (see Fig. 7c, d, Supplementary Note 9 and Supplementary Movies 3–5) augments the security by harnessing the pseudopolymorpic kinetics of CCl4, CDCl3, and CHCl3 (key 1 for spatial color code), alongside controlled time intervals (key 2). A 1×1 cm grid was prepared using a silicone well plate to define pixel positions for selective solvent exposure. By segmenting G-crystal regions and exposing them to different solvents, TD phosphorescence color switching from green (0) to blue (1) was achieved, effectively generating a binary optical code.

At time zero, the entire grid remains green. After 10 min, only the CDCl3-exposed region converts to blue, corresponding to the ASCII-encoded message “GOBLUE.” Further exposure (e.g., 60 min) results in undesired color changes and scrambled output. A brief heating step effectively removes residual solvent, restoring all pixels to the original green-emitting G-crystal state, and enabling system reset for repeated use. This layered encryption framework underscores the synergy of thermal modulation, solvent-mediated color switching, and phosphorescent emission for secure data storage.

Collectively, these demonstrations establish BrGlu’s significant adaptability for complex data encryption protocols. By selecting specific solvents, timing, and thermal treatments, our platform delivers multiple security layers—from immediate visual decoys to time-gated or thermally controlled readouts—thereby showcasing the potential of reversible phosphorescence for sophisticated, stimuli-responsive encryption technologies.

Methods
Optical spectroscopy
To study the optical properties of BrGlu in solution, samples were prepared in spectroscopic-grade CHCl3 (Aldrich Chemical Co.) at a concentration of 1×10–5 M. To prevent phosphorescence quenching by O2, each solution was deaerated via three freeze–thaw cycles: the solution was frozen in liquid nitrogen, placed under vacuum for 10 min, then allowed to thaw. UV–visible absorption spectra were recorded with a Varian Cary 50 spectrophotometer. Photoluminescence (PL) emission spectra of both solutions and crystals were collected on a Photon Technologies International QuantaMaster spectrofluorometer (QM-400), equipped with a continuous 75 W Xe lamp. Emission was detected at a 90° angle using a Horiba PMT-928 photomultiplier tube; the spectra were corrected for the λ-dependence of the detector. For solid-state measurements, crystals were placed in Wilmad quartz NMR tubes (Aldrich Chemical Co.) on a quartz mount. Low-temperature measurements were carried out by immersing the quartz mount in liquid nitrogen. Except for low-temperature experiments, all other measurements of crystalline samples were conducted under ambient atmospheric conditions. For color-switching experiments (e.g., Fig. 4e, f), approximately 100 mg of G-crystals were placed in a Hellma fluorescence cuvette (QS grade) with 3 mL of anhydrous (non-deaerated) CHCl3 and stirred at room temperature. The reversible transition to B-crystal was monitored over time by PL spectroscopy under ambient air. Time-resolved PL measurements were performed on the same spectrofluorometer using a xenon flash lamp as the excitation source. Lifetimes were extracted by mono- or bi-exponential fitting (as appropriate) through deconvolution using the instrumental response function with the Felix GX software. PL quantum yields were determined using a calibrated integrating sphere attached to the spectrometer, with data measured against a blank sample. Each measurement was performed four times on freshly prepared samples to ensure consistency. All measurements showed high repeatability, and errors are reported as ±1 standard deviation. To verify measurement reliability, rhodamine 6G and coumarin 153 were studied as reference dyes following standard literature protocols76.

Crystal preparation
Crystals of BrGlu were grown by slow solvent evaporation (3 mg/mL) at ambient conditions using various organic solvents. CHCl3 and CH2Cl2 yielded single crystals within 48 h, referred to as B- and G-crystals, respectively. The B-crystals from CHCl3 exhibited blue phosphorescence, while G-crystals from CH2Cl2 showed green emission. Both formed well-defined crystals with uniform sizes ranging from 0.1 to 0.5 mm, as confirmed by optical microscopy (Supplementary Fig. 2). In contrast, ethyl acetate, THF, and acetone generated larger crystals—typically a few millimeters across—also emitting green phosphorescence. On the other hand, toluene, ethanol, and methanol produced thin, needle-shaped crystals with smaller dimensions, yet still maintained visible green phosphorescence under UV light (Supplementary Figs. 2 and 3).

Structural analysis
PXRD patterns were measured using a Rigaku MiniFlex 600 with CuKα radiation, operating at 40 kV and 15 mA. X-ray detection was performed using a D/TeX Ultra 2 one-dimensional silicon strip detector. PXRD data were collected over a 2θ range of 2–50° with a scan rate of 0.0100° per step. SCXRD data were collected using a Rigaku XtaLAB Synergy-S X-ray diffractometer with a kappa goniometer geometry configuration. The X-ray source is a PhotonJet-S microfocus Cu source (λ=1.54184Å) operated at 50 kV and 1 mA. X-ray intensities were measured with a HyPix-6000HE detector held 34 mm from the sample. The data were processed using CrysAlisPro v40.82 (Rigaku Oxford Diffraction) and were corrected for absorption. The structures were determined and refined using OLEX277 v1.5-ac5-024 with SHELXT78 and SHELXL79. All non-hydrogen atoms were refined anisotropically with hydrogen atoms located in idealized positions. Crystallographic data for BrGlu have been deposited in the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers 2431976 (B-crystal) and 2431977 (G-crystal). ORTEP-style illustrations with probability ellipsoids and detailed notes on A- and B-level alerts are provided in Supplementary Fig. 26 (B-crystal) and 27 (G-crystal). Raman spectra were measured using a Renishaw Qontor Raman spectrometer (1800 lines/mm grating) with a 100 mW 532 nm laser through a 50× Leica long working distance microscope objective. Static scans (2010–270 cm−1) were collected by focusing a single crystal of each sample with 10 s exposure time for 20 acquisitions using cosmic ray removal at 1% laser power. The collected data was smoothed, truncated (2000-900 cm-1), baseline-corrected, and normalized via WiRE 5.6. The single crystal of BrGlu solvated in CDCl3 was measured with static scans (2732~1164 cm−1) following the same procedure. The obtained spectra were truncated (2400~1700 cm−1), baseline-corrected, and normalized via WiRE 5.6.

Thermal Analysis
Thermogravimetric analysis (TGA) was performed on a TGA 5500 Discovery thermal analyzer (TRIOS 5.6). Approximately 1 mg of sample was placed in a platinum TGA pan and heated from 25°C to 600°C at a rate of 5°C/min. Differential scanning calorimetry (DSC) measurements were carried out on a Q20 DSC instrument. About 2 mg of sample was sealed in a Thermal Support aluminum hermetic pan with a lid, which was then pierced using a BD PrecisionGlide™ Needle (26G×5/8). The DSC instrument was calibrated with an indium standard using the same pan and pierced lid. For each run, the sample was first cooled to 0°C, then heated to 200°C at 5°C/min, and subsequently cooled back to 0°C at the same rate. This heating–cooling cycle was repeated five times.

Quantum-chemical calculations
Ground and excited state (S0, S1) optimization, MO and NTO analysis, and SP TD calculations were done at the DFT level in vacuum, using the B3LYP functional and 6–311G* basis set, as defined in the Gaussian 16 program package80. The SP TD-DFT calculations were either performed (i) directly on the molecules taking from the X-ray structures, (ii) under restricted optimization, where the torsional angles θ, θi,0 were fixed to those found in the X-ray structures, and (iii) on the fully optimized structures in syn- and anti-conformations, respectively. MO topologies were plotted with ChemCraft, using a contour value of 0.02. Additionally, SOC analyses, as well as the calculation of the oscillator strengths f(T1), were conducted in Orca 6.0.0, utilizing the B3LYP functional and 6–311G* basis set, in vacuum, based on the ground-state geometry previously optimized in Gaussian 1681. Specifically for the f(T1) calculation, the structure used was the crystal geometry (ground-state), which was subsequently optimized to the T1 state. Rate constants for phosphorescence kP were estimated from the Strickler-Berg equation (see Supplementary Note 10 for details)75, using an approximated form suitable for quantum-chemical output (utilizing f(T) and Evert for absorption and emission)3, under consideration of the state degeneracy, and inserting a refractive index of n≈2 to account for the crystal environment in the spectral region of emission73,82.

Fabrication of data encryption platform
To demonstrate the practical applicability of our system for data encryption, we fabricated luminescent patterns using two complementary approaches with distinct spatial and temporal encoding strategies.

For Fig. 7a, b, a PMMA mold was prepared using Silgard 184 silicone elastomer. The base and curing agent were mixed in a 10:1 weight ratio, degassed in a desiccator for 10 min to remove air bubbles, and then cured in an oven at 110°C. The mold was manually cut into the desired pattern (see Supplementary Fig. 24), with typical feature dimensions of approximately 2×8 mm. The encryption ink was formulated by dispersing the phosphorescent BrGlu material in mineral oil to increase viscosity and improve durability against solvent-induced degradation. The ink was applied using a micropipette to fill the mold cavities precisely.

For Fig. 7c, d, a 1×1 cm grid array was constructed using a silicone well plate sealed with a silicone mat. G-crystal suspensions were selectively deposited into individual wells, and time-resolved phosphorescence color switching was triggered by exposure to different solvents: CCl4 (no switching), CDCl3 (~10 min), and CHCl3 (~60 min) (see Supplementary Fig. 25). After solvent application, the plate was sealed and incubated to allow controlled solvent diffusion and rotameric conformation switching.

Afterglow images and Movies (Fig. 7b and Supplementary Movies 1–5) and dynamic pattern evolution were recorded using a high-frame-rate camera (1000 fps, ISO 12800, shutter speed 1/12800), enabling real-time visualization of emission changes.

These demonstrations confirm that the system can encode multistage, time-gated information with moderate spatial resolution (~mm scale) and maintain high durability through repeated solvent exposure. Further refinement using microfluidics, photopatterning, or inkjet printing techniques could enhance pixel resolution.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript entitled “Stimuli-Responsive Phosphorescence Switching in Purely Organic Materials for Advanced Data Encryption,” describes a technologically advanced investigation of a purely organic, room-temperature phosphorescent (RTP) system-BrGlu-with reversible, stimuli-responsive phosphorescence color switching. The authors have conducted an in-depth mechanistic investigation featuring crystallography, spectroscopy, thermodynamics, and quantum chemical modeling, and demonstrate its practical utility in multi-layered optical encryption. The novelty lies in achieving reversible solvent- and heat-driven switching between two crystalline polymorphs (G-crystal and B-crystal) of the same compound, having distinct phosphorescent emissions (green and blue) and high quantum yields. The structural switch between syn- and anti-rotamers driven by hydrogen bonding with guest solvent molecules is supported by experimental and computational data. This application in thermally and temporally gated anti-counterfeiting mechanisms renders this work scientifically and technologically relevant. While the manuscript is well-written, it can be enhanced by contextualization, quantitative contrast, and data presentation in certain parts. The manuscript can be accepted after addressing these major points:\n• The relevance of the variation of interlayer distance (7.58 Å vs. 6.01 Å) to triplet exciton delocalization may be elaborated, which can effectively link structure and emission. Also, discuss whether the disparity in space group symmetry between B- and G-crystal participates in RTP efficiency or molecular reorganization.\n• Authors may elaborate on the number of solvent molecules per BrGlu unit from TGA and relate it to crystal stoichiometry. Is 1586 cm-¹ peak is characteristic of some Br···O interactions or lattice distortion?\n• Clearly mention the solvent environment employed in the DFT (vacuum, PCM, etc.) since it has a direct influence on the stability prediction. It may be elaborated in context by referencing any analogous known rotamer-dependent photophysical properties.\n• Please clarify the hypsochromic shift (480 → 510 nm) more clearly, perhaps by including a simple schematic to show how conformational changes affect triplet energy levels. It would also be helpful to compare phosphorescence quantum yields and lifetimes directly in the text to emphasize the stability of performance.\n• Examine a quantitative plot of emission intensity or λmax versus cycle number to verify long-term cycling performance.\n• The explanation of solvent behavior (e.g., isotope effect for CDCl₃) would be useful with citations or calculated diffusion coefficients. A table summarizing solvation time, polarity, hydrogen bonding score, and ESP values for each solvent may be incorporated that would provide clarity.\n• Incorporate resolution or patterning capability for actual use. How small can encrypted pixels be? The durability of encryption materials is affected by multiple heat/solvent cycles, which may be briefly discussed.\nMinor Language/Editing Corrections\n1. Line 131: \"emission properties,\" which might emphasize a relationship between structure and properties.\n2. Line 163: \"anti-from\" should be \"anti-form.\".\n3. Line 229: \"arise from effective SOC through participation.\" Think about simplifying this sentence for ease of understanding.\n4. Line 243: \"remains robust through at least ten cycles\" can state whether intensity or wavelength differed across cycles.", "reviewer_2": "This manuscript by Kim, Gierschner, Kwon and co-workers reports new organic phosphorescent material, BrGlu, that exhibits reversible phosphorescence color switching between green and blue emission states upon exposure to specific halogenated solvents and mild heating. Through a combination of crystallographic analysis, thermal characterization, photophysical measurements, and density functional theory calculations, the authors attribute the switching mechanism to a solvent-induced syn–anti conformational reorientation of the bromine-carbonyl moieties, stabilized by hydrogen bonding interactions. The study is comprehensive, and the integration of this reversible switching behavior into advanced data encryption strategies is particularly interesting, offering practical relevance in information security applications. While the results are promising, several key aspects require further clarification and additional experimental support. Overall, I suggest that this important work can be published after some minor revision based on comments below:\n1. It would be helpful if the authors could clarify how many reversible phosphorescence switching cycles can be reliably realized without significant degradation.\n2. The authors may consider including a brief comparison between BrGlu and other recently reported stimuli-responsive RTP systems, highlighting differences in quantum yield, switching reversibility, and operational conditions. This would better contextualize the performance and advantages of the current system for readers.\n3. The authors emphasize that both the G-crystal and B-crystal phases maintain relatively high phosphorescence quantum yields. It would be interesting if the authors could elaborate on how the packing density or lattice rigidity in each crystal state contributes to the suppression of nonradiative decay. A comparison of packing efficiencies (e.g., unit cell volume per molecule) between the two crystal forms could offer additional mechanistic insights.\n4. The current study shows that mild heating (~60 °C) is sufficient to induce desolvation and restore the G-crystal state. What is the maximum temperature at which the G-crystal remains stable, and could thermal cycling (e.g., repeated heating and cooling) affect long-term structural integrity?\n5. How does the phosphorescence performance of BrGlu respond to prolonged exposure to ambient air and humidity? Are the G- and B-crystal emissions stable over time under real-world environmental conditions.\n6. Molecular RTP materials with stimuli-responsive switching and data encryption applications are hot topics in chemistry and materials. To arouse a broad interest from readership in this field, some strongly related works on the recent fabrications of molecular based RTP (Nat. Commun. 2024, 15, 9491; Angew. Chem. Int. Ed. 2025, 64, e202420139), and the wide stimuli-responsive properties as well as encryption applications (Sci. China Mater. 2024, 67, 3531; Angew. Chem. Int. Ed. 2025, 64, e202502782; Angew. Chem. Int. Ed. 2025, 64, e202505322) could be included as references.\n7. The manuscript demonstrates that only certain halogenated solvents induce the conformational switching. Could the authors discuss more about what solvent properties (such as hydrogen bond donating ability, polarity, molecular size) are critical for triggering the phase transition? Could experiments with additional solvents (e.g., acetonitrile, ethanol) clarify these criteria?", "reviewer_3": "The authors investigated room-temperature phosphorescence (RTP) properties of 2,5-dibromobenzene-1,4-bisamide bearing glutalimide substituents (BrGlu). While it does not exhibit RTP in solution, its crystal affords bright RTP with high quantum yields. Two differently emitting pseudopolymorphic crystals were obtained (G-crystal and B-crystal), where B-crystal contains CHCl3. The authors ascribed the origin of pseudopolymorph-dependent RTP colours to the molecular conformations in those crystals. Notably, interconversion between these two polymorphs is achieved by suspending them in CHCl3 and heating. Given the significance of developing highly emissive metal-free RTP materials and the relatively rare method (external stimulus) for changing the RTP colour, this work represents an interesting progress in the field of functional organic materials. However, the novelty of the work was not properly described, with several closely related works being uncited as specified below (comments #1-5). In addition, the interpretations of the experimental results are somewhat incomplete in some points. Therefore, the manuscript is potentially sound, but a major revision is needed to meet the very high standards of Nature Communications. Specific concerns are listed below.\n1. On page 3, lines 71–74, the authors mentioned that “Reported systems rely on multi-dopant matrices or require changes in excitation wavelength, complicating both the fabrication process and practical implementation.28–30,43–51 Additionally, in existing designs, external stimuli (e.g., temperature, solvent, pH, mechanical force) tend to quench phosphorescence rather than sustain or enhance it” and raised several precedents. However, a series of closely related works on stimulus-responsive organic RTP materials (J. Mater. Chem. C 2019, 7, 11926; Chem. Commun. 2020, 56, 6810; Front. Chem. 2022, 9, 812593; Chem. Sci. 2023, 14, 5302) is omitted. In those works, single-component molecular crystals respond to external stimuli (e.g., mechanical force, temperature, solvent vapour, and light) to exhibit RTP-to-RTP colour changes with enhanced efficiency or even RTP turn-on. Moreover, their response mechanism is based on the molecular conformation change, which is somewhat similar to the present case. Mentioning these works is indispensable to fairly evaluating the novelty of the present work.\n2. At the beginning of the conclusion part, the authors mentioned “This study presents a single-component organic phosphor, BrGlu, that exhibits fully reversible room temperature phosphorescence color switching between bright blue (B-crystal) and green (G-crystal) states by external stimuli such as solvent exposure and gentle heating.” Again, this has qualitatively been achieved in J. Mater. Chem. C 2019, 7, 11926. In my opinion, it would be better to clarify the uniqueness of the present work in a more specific manner. It may include excellent quantum yields for BrGlu crystals, RTP colour change based on pseudopolymorphs, and possible mechanism related to Viedma’s ripening (cf. comment #9).\n3. Related to comment #1, the authors mentioned “Although correlations between conformation and emission color are not commonly reported, they could affect triplet emission colors and rates,54 a possibility we investigate further through quantum calculations” on page 8, lines 160–162. This is also an important discussion closely related to the above-mentioned series of works. The conformation-dependent RTP colour change (green and yellow) is unambiguously demonstrated by X-ray crystal structure analyses in Chem. Sci. 2024, 15,10784.\n4. Structurally very similar compounds, 1,4-bis(aroyl)-2,5-dibromobenzenes, were reported to exhibit RTP in the crystal state (Eur. J. Org. Chem. 2016, 467–473), which would be better cited.\n5. Considering that the present work represents a rare example of pseudopolymorph-dependent RTP colour change, related work would be better cited (e.g., Chem. Asian J. 2024, 19, e202301114)\n6. In page 5, the authors mentioned the molecular design of minimising exciton coupling in the solid state. However, exciton coupling is typically very small in the triplet state due to small transition dipole moments for T1–S0 transition. Although the authors mentioned that “Previously, it was noted that this suppression of exciton trapping3 ultimately enhances intrinsic RTP efficiency”, the cited reference 3 actually did not note such things (they mentioned fluorescence). The authors argued the large S1-Tn SOC elements and assumed unity ISC efficiency, suggesting the fast ISC. Therefore, I recommend that the authors revise this introductory part as well as the discussion regarding crystal structures (page 7, lines 138–141) and decay behaviour (page 12, lines 231–233).\n7. On page 7, lines 148–151, the authors mentioned “hydrogen-bond-induced bond stiffening.” I could not catch the point here for two reasons. Firstly, in my understanding, hydrogen bond typically loosens the bond (unfortunately, I have no access to ref 52). Secondly, aroyl carbonyl moieties (whose bond lengths are compared) in both B- and G-crystal form hydrogen bonding with another BrGlu molecule, and the distance between the oxygen atom and H atom of CHCl3 is much longer (even longer than the sum of their van der Waals radii). The fact that the CHCl3 is disordered also indicates the weak interactions between BrGlu and CHCl3. What is actually the intention of the authors regarding this discussion?\n8. Preparation of crystal samples for optical measurements needs to be described in more detail. The authors only provided information about the solvent; however, I guess the shape and size of the crystals, as well as the homogeneity of those parameters, strongly depend on how the crystals were grown. Moreover, although the authors briefly mentioned “BrGlu is found to be phosphorescent also in powders and crystals” in line 194, I could not find information on how those two are distinguished.\n9. I wonder how stable the B-crystal is in air. Does it lose CHCl3 when kept open to air at room temperature at atmospheric pressure?\n10. The authors evaluated the rotamerisation pathway based on DFT calculations in Fig. 4. The result itself is interesting. However, I am not convinced that this is related to the reversible interconversion behaviour, because it is unclear whether the interconversion between G- and B-crystal occurs after the crystal is once dissolved. It seems to me that the phenomenon is quite similar to Viedma’s ripening. In relation to this, can the authors convert the G-crystal into B-crystal simply by exposing to CHCl3 vapor?\n11. The experiments summarised in Fig. 5 are interesting, but the interpretation of the results seems complicated. The difference in response may arise from the different solubility in those solvents, except CHCl3 and CDCl3. What is the clue for ascribing these phenomena to the difference in solvation, rather than in dissolution?\nMinor comments\n12. The atmospheric conditions for measurement of crystalline samples (in air or under N2 or Ar) should be provided. Also, whether the CHCl3 is deaerated or not for the measurement in Fig. 3e would be better noted.\n13. The authors mentioned “Lifetimes were extracted by mono-exponential fitting with the Felix GX software” on page 4 in SI. However, the decay curve for G-crystal apparently contains more than two components (Fig. 3b). It would be better to provide the fitting curves.\n14. In the caption of Fig. 1c, “Hydrogen atoms are omitted for clarity” should be for Fig. 1b and 1d.\n15. “da”, “do”, and “θc” at the bottom of Fig. 2a do not appear in the table.\n16. In the caption of Fig. 6, “(d,e)” should read “(c,d)”.\n17. On page 26 in SI, the predicted m/z value for BrGlu (513.93) is wrong. Also, the values for all compounds have only two digits, which should be four."}, "author_response": "Response Letter\nJournal: Nature Communications\nManuscript ID: NCOMMS-25-25620\nTitle: Stimuli-Responsive Phosphorescence Switching in Purely Organic Materials for Advanced Data\nEncryption\nResponse to the Reviewers’ Comments\nThe authors would like to express their sincere gratitude to the reviewers for their valuable feedback on our\nmanuscript. We have carefully reviewed each comment and revised the manuscript accordingly. To enhance the\nreadability, the reviewers’ comments and the authors’ responses have been color-coded in black and blue,\nrespectively. In the revised manuscript, revised parts are highlighted in yellow.\nReviewers’ Comments\nReviewer #1 (Remarks to the Author):\nThis manuscript entitled “Stimuli-Responsive Phosphorescence Switching in Purely Organic Materials for Advanced Data\nEncryption,” describes a technologically advanced investigation of a purely organic, room-\ntemperature phosphorescent (RTP) system-BrGlu-with reversible, stimuli-responsive phosphorescence color\nswitching. The authors have conducted an in-depth mechanistic investigation featuring crystallography,\nspectroscopy, thermodynamics, and quantum chemical modeling, and demonstrate its practical utility in multi-\nlayered optical encryption. The novelty lies in achieving reversible solvent- and heat-driven switching between\ntwo crystalline polymorphs (G-crystal and B-crystal) of the same compound, having distinct phosphorescent\nemissions (green and blue) and high quantum yields. The structural switch between syn- and anti-rotamers driven\nby hydrogen bonding with guest solvent molecules is supported by experimental and computational data. This\napplication in thermally and temporally gated anti-counterfeiting mechanisms renders this work scientifically and\ntechnologically relevant. While the manuscript is well-written, it can be enhanced by contextualization,\nquantitative contrast, and data presentation in certain parts. The manuscript can be accepted after addressing these\nmajor points:\n1. The relevance of the variation of interlayer distance (7.58 Å vs. 6.01 Å) to triplet exciton delocalization may\nbe elaborated, which can effectively link structure and emission. Also, discuss whether the disparity in space\ngroup symmetry between B- and G-crystal participates in RTP efficiency or molecular reorganization.\nResponse:\nWe thank the reviewer for this insightful suggestion, which is indeed very relevant for the understanding. We\nhave now added a detailed discussion in the manuscript as follows:\nRevised Manuscript Text:\nTo clarify the excited state deactivation processes, we have derived the radiative and nonradiative rates\nfrom T via k = Φ / τ and k = (1-Φ )/ τ (Table 1) by assuming that the ISC efficiency η from S to the triplet\nmanifold is about unity. The latter assumption is, on one hand, consistent with the absence of fluorescence at\nany temperature, while the nonradiative deactivation from S is expected to be very small due to the restricted\n1\naccess to the conical intersection for such emitters.63 On the other hand, the spin-orbit coupling matrix elements\n(SOCME) are high for ISC from S to the triplet manifold T , due to the very significant participation of the free\n1 n\nelectron pairs of Br in S (see Fig. 4d for representative NTOs and Supplementary Figs. 10, 13 for details);\n1\ntherefore, this heavy-atom contribution enables efficient ISC via effective SOC. Deriving k , notably, the B-crystal\np\nand G-crystal show both high k values (2.9 × 103 s–1 and 4.2 × 103 s–1, respectively), which arise from effective\nP\nSOC through participation of the free electron pairs of Br also in the description of the T state.\nnr 1\ncrystal). On one hand, phosphorescence quenching may be due to nonradiative recovery of the ground state S\n0\nthrough inefficient reverse ISC ;3 the latter is commonly proceeding through a T /S crossing point (CP). On the\n0 1 0\nother hand, quenching may be due to trapping (in particular at interfaces/surfaces).62 These traps are reached\nby triplet exciton diffusion through long exciton lifetimes and effective exciton coupling via an exchange\nmechanism, which thus requires orbital overlap.3 The latter is effectively suppressed in the BrGlu crystals due to\nthe larger interlayer distances d of 7.58 Å in B vs. 6.01 Å in G as induced by the sterically bulky acyl-glutarimide\nIL\ngroups. The larger k for the B-crystal compared to G thus clearly evidences that trapping is not the predominant\nnr\nquenching mechanism, as the larger d in B vs. G should slow down exciton coupling (and thus trapping) even\nIL\nmore and not vice versa. Instead, the enhanced k for B vs. G must be ascribed to more effective nonradiative\nnr\nground state recovery via ISC . In fact, although the densities of the B-and G-crystals hardly differ (see\nflexibility in B vs. G, and thus facilitate opening the path to the T /S CP. Additionally, the higher-lying T state in\n1 0 1\nB compared to G should further promote the access to the CP, in analogy to the internal conversion (via a conical\nintersection) in the TGlu series.63\n2. Authors may elaborate on the number of solvent molecules per BrGlu unit from TGA and relate it to crystal\nstoichiometry. Is 1586 cm-¹ peak is characteristic of some Br···O interactions or lattice distortion?\nResponse:\nWe have revised the manuscript to clarify the solvent stoichiometry and the origin of the 1586 cm⁻¹ Raman peak.\nOur responses are summarized as follows:\n1. Solvent Stoichiometry from TGA and Single-Crystal Structure: TGA shows a 14.4 wt% mass loss for\nthe B-crystal upon heating to ~65 °C (Fig. 1e), attributed to CHCl₃ release. Based on molar masses (BrGlu:\n514.13 g/mol; CHCl₃: 119.38 g/mol), a 1:1 BrGlu:CHCl₃ solvate yields a theoretical 18.8 wt%. The slightly\nlower observed mass loss is likely due to partial solvent evaporation during handling, but single-crystal X-ray\ndiffraction confirms a 1:1 stoichiometry, supporting our conclusion.\n2. 1586 cm–1 Raman Peak: This peak, exclusive to the B-crystal, can be attributed to a change in the lattice,\nleading to the local vibration of CHCl –BrGlu interactions, not Br···O halogen bonding (typically <300 cm–1).\n3\n3. Terminology Clarification: The phrase “hydrogen-bond-induced bond stiffening” was rephrased to “bond\nreinforcement due to solvent–lattice interactions” to clarify that the blue shift in C=O stretching (1749 to 1763\ncm–1) reflects increased local rigidity, not classical hydrogen bonding.\nThese clarifications have been reflected in the revised manuscript text provided below.\nResponse Letter\nRevised Manuscript Text:\nThermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses reveal a clear distinction\nbetween the B- and G-crystals with respect to solvent incorporation and thermal stability (Figs. 2e,f). The B-\ncrystal exhibits a mass loss of approximately 14.4 wt% upon heating to ~65 oC, corresponding to the release of\nCHCl (Fig. 2e). This is consistent with a 1:1 BrGlu:CHCl solvate, supported by the calculated theoretical CHCl\n3 3 3\ncontent of 18.8 wt% based on the molar masses of BrGlu (514.13 g/mol) and CHCl (119.38 g/mol). SCXRD\n3\nconfirms this stoichiometry, showing CHCl molecules occupying disordered but well-defined lattice sites,\n3\nstabilized by C–H···O interactions.\nIn contrast, the G-crystal displays no observable mass loss and remains structurally stable until\ndecomposition begins above 260 oC (Fig. 2f, Supplementary Fig. 6). This exceptional thermal stability was\nfurther validated by repeated thermal cycling experiments. When subjected to five cycles of heating and cooling\nbetween 0 and 200 oC, the G-crystal retained its structural integrity, as confirmed by PXRD analysis\n(Supplementary Fig. 7). These results demonstrate the BrGlu’s robustness under operational conditions\nrelevant to reversible switching applications.\nRaman spectroscopy (Fig. 1g) further underscores these structural differences. The C=O stretching\nmode shifts from 1749 cm–1 in the G-crystal to 1763 cm–1 in the B-crystal, correlating with a slight shortening of\nthe average C=O bond (1.20 Å vs. 1.19 Å, Supplementary Fig. 8). This shift reflects bond reinforcement induced\nby CHCl –lattice interactions, rather than classical hydrogen bonding. Additionally, a new peak at 1586 cm–1\n3\nappears exclusively in the B-crystal, which can be attributed to a change in the lattice leading to a local vibration\nof CHCl –BrGlu interactions.66 This peak reflects solvent-induced modulation of the lattice dynamics, not\n3\nindicating Br···O halogen bonding, which appears below 300 cm–1.\n3. Clearly mention the solvent environment employed in the DFT (vacuum, PCM, etc.) since it has a direct\ninfluence on the stability prediction. It may be elaborated in context by referencing any analogous known rotamer-\ndependent photophysical properties.\nResponse:\nWe thank the reviewer for the careful attention. In our experience, while solvent effects (commonly included via\nthe implicit PCM implementation) may have a significant impact on vertical transition energies, the effect on\ngeometries is usually negligible. Moreover, the description of the crystal environment via PCM appears somewhat\nrandom. Therefore, all calculations were done in vacuum. We have updated the manuscript (Quantum-Chemical\nCalculations) and Supplementary Information accordingly.\nFor the rotamers, we revised the relevant section of the introduction.\nThe following is added to the introduction:\nSyn/anti rotamers have commonly only a small impact on the emission properties,63 but were shown to\nbe relevant in the fluorescence process of the parent bromine-free TGlu series.64\n4. Please clarify the hypsochromic shift (480 → 510 nm) more clearly, perhaps by including a simple schematic\nto show how conformational changes affect triplet energy levels. It would also be helpful to compare\nphosphorescence quantum yields and lifetimes directly in the text to emphasize the stability of performance.\nResponse Letter\nResponse:\nWe have now extended the discussion and revised Fig. 1 in the text accordingly as follows:\nThe following sentence is added:\nAs we will see below, this spectral shift reflects the different rotameric conformations, see Fig. 1a.\nRevised Manuscript Text:\nIn order to rationalize the bathochromic shift of phosphorescence when going from B to G, we conducted\nfirst single point TD-DFT calculations of the molecules in those conformations which were found in the crystal\nstructures (see Fig. 4c and computational part for details). According to the calculations, the vertical transitions\nenergies for absorption to S and T are somewhat higher in G-crystal compared to B-crystal (Supplementary\n1 1\nTable 4), which seem to, at a first glance, contradict experiment. To gain further insight, in a second step, we\nperformed the single point TD calculations on the fully relaxed syn and anti conformations, which gave very\nsimilar results, see Supplementary Table 5. Nevertheless, we note that the figure of merit for the\nphosphorescence process is not the vertical energy for absorption, but that for emission, i.e. E (T S ). We\nvert 1 0\nthus optimized the T states, to calculate the latter; in fact, here the anti-conformation (G-phase) is found at 1.85\n1\neV, i.e. 0.09 eV below the syn conformation (B-phase; at 1.94 eV), see Fig. 4c and Supplementary Table 6.\nThis inversion is driven by the much larger structural relaxation in the T state of anti in comparison syn. The TD-\nvert 1 0\nnm) for G to 2.58 eV (480 nm) for B, that is 0.15 eV. The agreement is in particular good, as an additional small\nhypsochromic shift for the B-phase is expected to arise from the lower polarizability due to dilution by the solvent\nmolecules in the B-crystal,69 which is not accounted for in the TD-DFT calculations. Finally, we rule out excitonic\ncontributions to the spectral shift, as this becomes only significant for high oscillator strength (f) of the lowest\nexcited state and small intermolecular separation;3,70 in the BrGlu crystal, however, f of the emitting state is self-\nevidently very small due to its triplet nature, and, at the same time, intermolecular separations are large, as\nevidenced from the crystal structure (vide supra).\nFor the discussion of quantum yields and lifetimes, please see our response above (point 1).\n5. Examine a quantitative plot of emission intensity or λmax versus cycle number to verify long-term cycling\nperformance.\nResponse:\nTo address this, we have updated Supplementary Fig. 16 to include a quantitative plot of λ as a function of\nmax\ncycle number over ten solvation–desolvation cycles. The data confirm excellent stability, with variations\nremaining within <5% for intensity and <1 nm for λ across cycles. This new plot and analysis are now discussed\nmax\nin the revised manuscript and cited accordingly. These results highlight the robustness and reliability of the\nreversible switching mechanism, underscoring its suitability for encryption and sensing applications.\nRevised Manuscript Text:\nResponse Letter\nThis complete reversibility is also illustrated by the emission switching in the CIE 1931 color space—from\n(0.216, 0.513) to (0.148, 0.257) (Fig. 4g)—which remains stable through at least ten solvation–desolvation\ncycles (Supplementary Fig. 16), with minimal variation in emission intensity (<5%) and λ (<1 nm).\nmax\nSupplementary Fig. 16. Reversibility of the phosphorescence-to-phosphorescence transition upon\nsolvation and subsequent heating, corresponding to the blue and green phases, respectively. The\nprocess exhibits excellent reversibility, with negligible changes in emission intensity (<5%) and emission\nmaximum (λ < 1 nm).\nmax\n6. The explanation of solvent behavior (e.g., isotope effect for CDCl₃) would be useful with citations or calculated\ndiffusion coefficients. A table summarizing solvation time, polarity, hydrogen bonding score, and ESP values for\neach solvent may be incorporated that would provide clarity.\nResponse:\nWe have added Supplementary Table 7 summarizing key solvent parameters, including solvation times, dielectric\nconstants, diffusion coefficients, and ESP values for CHCl , CDCl , CHBr , C H Cl , C H Cl , and CCl . These\n3 3 3 2 2 4 2 4 2 4\nparameters are discussed in the revised manuscript to provide a mechanistic understanding of solvent-dependent\nswitching kinetics. To support our discussion, we reference dielectric constant data from Maryott and Smith (1951,\nTable of Dielectric Constants of Pure Liquids, Vol 514, NBS), and diffusion coefficients calculated using values\nfrom Ind. Eng. Chem. Res. 2012, 51, 2797–2803.\nAdditional tests with methanol, ethanol, and acetonitrile showed no phase transition. These findings are discussed\nin the manuscript also.\nRevised Manusciprt Text:\nFurthermore, electrostatic potential (ESP) calculations (Supplementary Fig. 18) revealed that CDCl ,\n3 2 2 4\ncontrast, solvents such as C H Cl or CCl either possess minimal hydrogen-bonding capability or lack hydrogen\n2 4 2 4\nentirely. Additional tests with polar protic and aprotic solvents—including methanol, ethanol, and acetonitrile—\nalso failed to induce any phase transition, showing only minimal changes in PL spectra (Supplementary Fig.\n19). These results suggest that the phase transition is governed not by polarity or hydrogen-bonding capability\nResponse Letter\nalone, but by a combination of factors, including ESP distribution, and diffusivity, as summarized in\nSupplementary Table 7.\nSupplementary Fig. 19. PL spectra of G-crystal before and after exposure to polar protic and aprotic\nsolvents. Normalized PL spectra of G-crystal before and after stirring for 2 days in acetonitrile, methanol, and\nethanol under ambient conditions. No significant spectral shift or intensity change was observed, indicating that\nthese polar protic and aprotic solvents do not induce a phase transition or affect the emissive properties of the\nG-crystal.\nSupplementary Table 7. Solvation times and solvent properties\n1. Dielectric constants are taken from: Ref 7.\n2. Diffusion coefficients were estimated using the genetic function approximation method reported in: Ref 8.\nRef 7. Maryott, A.A. & Smith, E.R. Table of Dielectric Constants of Pure Liquids, US Government Printing\nOffice 514 (1951).\nRef 8. Gharagheizi, F. Determination of diffusion coefficient of organic compounds in water using a simple\nmolecular-based method. Ind. Eng. Chem. Res. 51, 2797–2803 (2012).\nResponse Letter\n7. Incorporate resolution or patterning capability for actual use. How small can encrypted pixels be? The durability\nof encryption materials is affected by multiple heat/solvent cycles, which may be briefly discussed.\nResponse:\nWe have revised the Manuscript and Supplementary Note 9 describing the fabrication of patterned encryption\narrays, the achievable pixel dimensions, and the solvent-driven solvation rates across different halogenated\nsolvents.\nSpecifically, we demonstrated encryption patterns with feature sizes down to approximately 2 mm × 8 mm using\na hand-cut PMMA mold and micropipette deposition (Figs. 7a,b). Additionally, a 1 cm × 1 cm pixel-based time-\nresolved encryption format was constructed using a silicone grid and sequential solvent activation (Figs. 7c,d).\nAlthough current spatial resolution is limited by manual patterning and solvent diffusion, the system is compatible\nwith finer methods such as inkjet printing or soft lithography for future miniaturization.\nRegarding durability, the phosphorescent patterns exhibit stable performance over repeated solvent exposure\ncycles, as shown in Supplementary Fig. 16. Furthermore, ink formulations (see Supplmentary Note 9)\nincorporating mineral oil increase viscosity and suppress desorption or crystallite dissolution, contributing to\nimproved long-term stability.\nRevised Manuscript Text:\nBased on BrGlu’s unique capability to toggle between two phosphorescent states, we engineered a\nmultilayer encryption scheme built on two decryption keys (key 1 and key 2) for secure data access (Fig. 7a). In\nthis configuration, B‐crystal ink functions as the authentic cipher, whereas the FL dye ink adds a deceptive layer\nof visible camouflage. Both inks were formulated by dispersing the respective luminescent materials in mineral\noil to form stable, solid-state dispersions suitable for precise patterning. The molds used for patterning were\nfabricated using a PDMS casting method, and hand-cut to produce features down to ~2 mm × 8 mm (see Note\n9 and Supplementary Fig. 24).\nIn Fig. 7b, we observe that under 365 nm UV light, the visible pattern appears as the spurious “8888” in\nblue (Supplementary Video 1). Heating (~60 °C, key 1) briefly suppresses BrGlu’s phosphorescence through\nincreased molecular vibrations, distorting the display. Upon cooling, BrGlu transitions to its G‐crystal phase,\nglowing blue and green mixed message. Turning off the UV lamp (key 2) maintains this green long-lasting\nphosphorescence emission, confirming the correct information, being “UM” (Supplementary Video 2). Notably,\nthis thermal cycle is fully reversible, enabling repeated encrypt‐decrypt operations.\nAdditionally, a “3D” encryption strategy (see Figs. 7c,d, Supplementary Note 9 and Supplementary\nVideos 3–5) augments the security by harnessing the unique solvation kinetics of CCl , CDCl , and CHCl (key\n4 3 3\n1 for spatial color code), alongside controlled time intervals (key 2). A 1 cm × 1 cm grid was prepared using a\nsilicone well plate to define pixel positions for selective solvent exposure. By segmenting G‐crystal regions and\nexposing them to different solvents, time-dependent phosphorescence color switching from green (0) to blue (1)\nwas achieved, effectively generating a binary optical code.\nAt time zero, the entire grid remains green. After 10 minutes, only the CDCl -exposed region converts to\n3\nblue, corresponding to the ASCII-encoded message “GOBLUE.” Further exposure (e.g., 60 minutes) results in\nundesired color changes and scrambled output. A brief heating step effectively removes residual solvent,\nrestoring all pixels to the original green-emitting G‐crystal state, and enabling system reset for repeated use.\nThis layered encryption framework underscores the synergy of thermal modulation, solvent‐mediated color\nswitching, and phosphorescent emission for secure data storage.\nResponse Letter\nSupplementary Note 9. Patterning and Pixel Resolution for Data Encryption\nTo demonstrate the practical applicability of our system for data encryption, we fabricated luminescent\npatterns using two complementary approaches with distinct spatial and temporal encoding strategies.\nFor Figs. 7a,b, a PMMA mold was prepared using Silgard 184 silicone elastomer. The base and curing\nagent were mixed in a 10:1 weight ratio, degassed in a desiccator for 10 minutes to remove air bubbles, and\nthen cured in an oven at 110 oC. The mold was manually cut into the desired pattern, with typical feature\ndimensions of approximately 2 mm × 8 mm (see Supplementary Fig. 24). The encryption ink was formulated\nby dispersing the phosphorescent BrGlu material in mineral oil to increase viscosity and improve durability\nagainst solvent-induced degradation. The ink was applied using a micropipette to fill the mold cavities precisely.\nFor Figs. 7c,d, a 1 cm × 1 cm grid array was constructed using a silicone well plate sealed with a silicone\nmat (see Supplementary Fig. 25). G-crystal suspensions were selectively deposited into individual wells, and\ntime-resolved phosphorescence switching was triggered by exposure to different solvents: CCl (no switching),\n3 3\ncontrolled solvent diffusion and rotameric conformational switching.\nAfterglow images and videos (Fig. 7b and Supplementary Videos 1–5) and dynamic pattern evolution\nwere recorded using a high-frame-rate camera (1000 fps, ISO 12800, shutter speed 1/12800), enabling real-time\nvisualization of emission changes.\nThese demonstrations confirm that the system can encode multistage, time-gated information with\nmoderate spatial resolution (~mm scale) and maintain high durability through repeated solvent exposure. Further\nrefinement using microfluidics, photopatterning, or inkjet printing techniques could enhance pixel resolution.\nMinor Language/Editing Corrections\n1. Line 131: \"emission properties,\" which might emphasize a relationship between structure and properties.\nHave revised to “These findings underscore the critical role of solvent selection in directing crystal structure,\nwhich in turn governs the resulting emission color—revealing a direct correlation between molecular packing and\nphosphorescent behavior.” for clarity.\n2. Line 163: \"anti-from\" should be \"anti-form.\".\nHave corrected to “anti-form”.\n3. Line 229: \"arise from effective SOC through participation.\" Think about simplifying this sentence for ease of\nunderstanding.\nHave revised to “therefore, this heavy-atom contribution enables efficient ISC via effective SOC.”\n4. Line 243: \"remains robust through at least ten cycles\" can state whether intensity or wavelength differed across\ncycles.\nHave revised to “which remains stable through at least ten solvation–desolvation cycles (Supplementary Fig. 16),\nwith minimal variation in emission intensity (<5%) and λ (<1 nm).”\nmax\nResponse Letter\nReviewer #2 (Remarks to the Author):\nThis manuscript by Kim, Gierschner, Kwon and co-workers reports new organic phosphorescent material,\nBrGlu, that exhibits reversible phosphorescence color switching between green and blue emission states upon\nexposure to specific halogenated solvents and mild heating. Through a combination of crystallographic analysis,\nthermal characterization, photophysical measurements, and density functional theory calculations, the authors\nattribute the switching mechanism to a solvent-induced syn–anti conformational reorientation of the bromine-\ncarbonyl moieties, stabilized by hydrogen bonding interactions. The study is comprehensive, and the integration\nof this reversible switching behavior into advanced data encryption strategies is particularly interesting, offering\npractical relevance in information security applications. While the results are promising, several key aspects\nrequire further clarification and additional experimental support. Overall, I suggest that this important work can\nbe published after some minor revision based on comments below:\n1. It would be helpful if the authors could clarify how many reversible phosphorescence switching cycles can be\nreliably realized without significant degradation.\nResponse:\nTo address this, we have updated Supplementary Fig. 16 to include a quantitative plot of λ as a function of\nmax\ncycle number over ten solvation–desolvation cycles. The data confirm excellent stability, with variations\nremaining within <5% for intensity and <1 nm for λ across cycles. These results highlight the robustness and\nmax\nreliability of the reversible switching mechanism, underscoring its suitability for encryption and sensing\napplications.\nRevised Manuscript Text:\nThis complete reversibility is also illustrated by the emission switching in the CIE 1931 color space—from\n(0.216, 0.513) to (0.148, 0.257) (Fig. 4g)—which remains stable through at least ten solvation–desolvation\ncycles (Supplementary Fig. 16), with minimal variation in emission intensity (<5%) and λ (<1 nm).\nmax\nSupplementary Fig. 16. Reversibility of the phosphorescence-to-phosphorescence transition upon\nsolvation and subsequent heating, corresponding to the blue and green phases, respectively. The\nprocess exhibits excellent reversibility, with negligible changes in emission intensity (<5%) and emission\nmaximum (λ < 1 nm).\nmax\nResponse Letter\n2. The authors may consider including a brief comparison between BrGlu and other recently reported stimuli-\nresponsive RTP systems, highlighting differences in quantum yield, switching reversibility, and operational\nconditions. This would better contextualize the performance and advantages of the current system for readers.\nResponse:\nTo better contextualize the performance and advantages of our BrGlu system, we have revised the manuscript to\ninclude a comparative discussion with recently reported stimuli-responsive RTP materials. Specifically, we now\nhighlight key distinctions in design strategies (e.g., multi-component vs. single-component), switching conditions,\nreversibility, and photophysical performance such as quantum yield and stability.\nTo enhance reader comprehension, we have added a new schematic (Supplementary Fig. 1) that visually contrasts\ntypical multi-component and single-component RTP systems, emphasizing the simplicity, reversibility, and\nrobustness of our BrGlu platform under mild solvent/thermal stimuli.\nIn addition, we have included a new summary table (Supplementary Table 1) comparing BrGlu’s performance—\nincluding its single-component design, high quantum yields (93% for G-crystal, 43% for B-crystal), and reversible\nswitching—with representative systems from the literature (e.g., Li et al., Song et al.). Together, these additions\nclearly illustrate BrGlu’s advantages over conventional RTP systems.\nRevised Manuscript Text 1:\nDespite the clear value of reversible RTP for advanced encryption, achieving phosphorescence-color\nswitching in purely organic materials under mild and practical conditions remains challenging. Most reported\nsystems rely on multi-dopant matrices or require changes in excitation wavelength, complicating both the\nfabrication process and practical implementation (Supplementary Fig. 1a).31–33 Moreover, in single-component\nsystems, external stimuli (e.g., temperature, solvent, pH, mechanical force) often lead to irreversible structural\ndisruption or quenched emission (Supplementary Fig. 1b).46–57 For instance, Li et al. developed a stimulus-\nresponsive RTP system by embedding phosphorescent and fluorescent dyes in a PVA matrix, enabling afterglow\ncolor modulation from blue to green to orange through triplet-to-singlet Förster resonance energy transfer.31\nHowever, exposure to water vapor…..\nRevised Manuscript Text 2:\nUnlike previously reported systems requiring complex fabrication or suffering emission quenching,\nBrGlu’s single-molecule design ensures reversible switching with high quantum yields (Supplementary Table\n1).\nResponse Letter\nSupplementary Fig.1. A key limitation in previous stimuli‐responsive phosphorescence systems is the reliance\non distinct excitation wavelengths to achieve multicomponent emission (a). In addition, the loosely packed\nenvironments induced by external stimuli often promote non‐radiative decay from the triplet state (T ), resulting\n1\nin fluorescence‐dominated emission and reduced overall brightness (b).\nSupplementary Table 1. Summary of phosphorescence quantum yields, switching conditions, and reversibility\nreported in selected literature references\n3. The authors emphasize that both the G-crystal and B-crystal phases maintain relatively high phosphorescence\nquantum yields. It would be interesting if the authors could elaborate on how the packing density or lattice\nrigidity in each crystal state contributes to the suppression of nonradiative decay. A comparison of packing\nefficiencies (e.g., unit cell volume per molecule) between the two crystal forms could offer additional\nmechanistic insights.\nResponse:\nWe thank the reviewer for the suggestion. In the supporting information (Supplementary Table 2), we have\nprovided the detailed crystallographic analysis to assist the readers. This includes also the density; in the current\ncase the density may be a better number than the volume per unit cell as this number is not directly accessible in\nthe B-crystal due to the presence of the solvent molecule. The density is only very slightly higher in B (1.849\ng/cm–1) compared to G (1.834 g/cm–1); in our view, the more important factor is the presence of the solvent\nResponse Letter\nmolecule, which is expected to give more structural flexibility to the BrGlu molecule in the B-crystal and thus\nincreases the nonradiative decay in comparison with G. In response to this comment, we have extended the\ndiscussion in the text as follows:\nRevised Manuscript Text:\nTo clarify the excited state deactivation processes, we derived the radiative and nonradiative rates from\nT via k = Φ / τ and k = (1-Φ )/ τ (Table 1) by assuming that the ISC efficiency η from S to the triplet\nmanifold is about unity. The latter assumption is, on one hand, consistent with the absence of fluorescence at\nany temperature, while the nonradiative deactivation from S is expected to be very small due to the restricted\n1\naccess to the conical intersection for such emitters.63 On the other hand, the spin-orbit coupling matrix elements\n(SOCME) are high for ISC from S to the triplet manifold T , due to the very significant participation of the free\n1 n\nelectron pairs of Br in S (see Fig. 4d for representative NTOs and Supplementary Figs. 10, 13 for details);\n1\ntherefore, this heavy-atom contribution enables efficient ISC via effective SOC. Deriving k , notably, the B-crystal\np\nand G-crystal show both high k values (2.9 × 103 s–1 and 4.2 × 103 s–1, respectively), which arise from effective\nP\nSOC through participation of the free electron pairs of Br also in the description of the T state.\nnr 1\ncrystal). On one hand, phosphorescence quenching may be due to nonradiative recovery of the ground state S\n0\nthrough inefficient reverse ISC ;3 the latter is commonly proceeding through a T /S crossing point (CP). On the\n0 1 0\nother hand, quenching may be due to trapping (in particular at interfaces/surfaces).62 These traps are reached\nby triplet exciton diffusion through long exciton lifetimes and effective exciton coupling via an exchange\nmechanism, which thus requires orbital overlap.3 The latter is effectively suppressed in the BrGlu crystals due to\nthe larger interlayer distances d of 7.58 Å in B vs. 6.01 Å in G as induced by the sterically bulky acyl-glutarimide\nIL\ngroups. The larger k for the B-crystal compared to G thus clearly evidences that trapping is not the predominant\nnr\nquenching mechanism, as the larger d in B vs. G should slow down exciton coupling (and thus trapping) even\nIL\nmore and not vice versa. Instead, the enhanced k for B vs. G must be ascribed to more effective nonradiative\nnr\nground state recovery via ISC . In fact, although the densities of the B-and G-crystals hardly differ (see\nflexibility in B vs. G, and thus facilitate opening the path to the T /S CP. Additionally, the higher-lying T state in\n1 0 1\nB compared to G should further promote the access to the CP, in analogy to the internal conversion (via a conical\nintersection) in the TGlu series.63\nSupplementary Table 2. Summary of crystallographic data for B- and G-Crystal\nResponse Letter\n4. The current study shows that mild heating (~60 °C) is sufficient to induce desolvation and restore the G-\ncrystal state. What is the maximum temperature at which the G-crystal remains stable, and could thermal\ncycling (e.g., repeated heating and cooling) affect long-term structural integrity?\nResponse:\nWe have clarified in the revised manuscript that TGA and DSC data show the G-crystal remains stable up to\n260 °C, beyond which decomposition begins (Supplementary Fig. 6). Repeated thermal cycling (five cycles, 0–\n200 °C) showed no structural degradation, as confirmed by PXRD (Supplementary Fig. 7). This is now discussed\nin the manuscript, emphasizing the BrGlu’s thermal robustness for practical applications.\nRevised Manuscript Text:\nThermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses reveal a clear distinction\nbetween the B- and G-crystals with respect to solvent incorporation and thermal stability (Figs. 2e,f). The B-\ncrystal exhibits a mass loss of approximately 14.4 wt% upon heating to ~65 oC, corresponding to the release of\nCHCl (Fig. 2e). This is consistent with a 1:1 BrGlu:CHCl solvate, supported by the calculated theoretical CHCl\n3 3 3\ncontent of 18.8 wt% based on the molar masses of BrGlu (514.13 g/mol) and CHCl (119.38 g/mol). SCXRD\n3\nconfirms this stoichiometry, showing CHCl molecules occupying disordered but well-defined lattice sites,\n3\nstabilized by C–H···O interactions.\nIn contrast, the G-crystal displays no observable mass loss and remains structurally stable until\ndecomposition begins above 260 oC (Fig. 2f, Supplementary Fig. 6). This exceptional thermal stability was\nResponse Letter\nfurther validated by repeated thermal cycling experiments. When subjected to five cycles of heating and cooling\nbetween 0 and 200 oC, the G-crystal retained its structural integrity, as confirmed by PXRD analysis\n(Supplementary Fig. 7). These results demonstrate the BrGlu’s robustness under operational conditions\nrelevant to reversible switching applications.\n5. How does the phosphorescence performance of BrGlu respond to prolonged exposure to ambient air and\nhumidity? Are the G- and B-crystal emissions stable over time under real-world environmental conditions.\nResponse:\nWe have conducted additional environmental stability tests for both the B- and G-crystals under ambient\nconditions (25 °C, 30% relative humidity) over a period of 7 days.\nFor the G-crystal, which is solvent-free and structurally more rigid, showed no measurable change in its emission\nprofile throughout the 7-day test period. The phosphorescence intensity and spectral shape remained entirely\nstable, indicating excellent robustness under ambient conditions.\nIn contrast, the B-crystal, normalized PL spectra were recorded at 3-hour intervals during the first 24 hours and\nsubsequently every 24 hours up to 7 days. As shown in Supplementary Figure 17, the B-crystal exhibited minimal\nspectral changes during the first 24 hours. Over longer timescales, a gradual red-shift in the emission maximum\nwas observed. This shift is attributed to the slow evaporation of lattice-incorporated CHCl₃ solvent molecules,\nwhich slightly alters the local molecular packing environment and excited-state dynamics.\nThese results confirm that both crystal forms exhibit good stability in real-world environmental conditions, with\nthe G-crystal being particularly resilient due to its solvent-free, tightly packed structure.\nWe have included these results and the corresponding figure in the revised manuscript to clarify the environmental\nstability of both crystal forms.\nRevised Manuscript Text:\nIn addition, both crystal forms exhibit excellent environmental stability under ambient conditions (25 °C,\n30% relative humidity). The G-crystal maintains a consistent emission profile over 7 days, indicating high\nstructural robustness (Supplementary Fig. 17a). For the B-crystal, the emission remains largely unchanged\nduring the initial 24 hours, after which a gradual red-shift in λmax is observed, corresponding to a slow conversion\nto the G-phase as CHCl slowly evaporates (Supplementary Fig. 17b). These results confirm the outstanding\n3\ndurability, reversibility, and practical reliability of the BrGlu system, supporting its potential for real-world\napplications in optical data storage and anti-counterfeiting technologies.\nResponse Letter\nSupplementary Fig. 17. Environmental stability of G- and B-crystals under ambient conditions. (a) G-\ncrystal and (b) B-crystal were monitored under ambient conditions (25 0C, 30% relative humidity) to evaluate\nlong-term stability. The G-crystal exhibited no detectable change in emission profile over a 7-day period,\nconfirming its excellent environmental robustness. In contrast, the B-crystal showed minimal spectral change\nwithin the first 24 hours, followed by a gradual red-shift in the emission maximum over the course of one week,\nattributed to the slow evaporation of lattice-incorporated CHCl molecules. These results demonstrate that both\n3\ncrystal forms are stable, with the G-crystal remaining invariant and the B-crystal undergoing only modest spectral\nevolution due to solvent loss for a day.\n6. Molecular RTP materials with stimuli-responsive switching and data encryption applications are hot topics in\nchemistry and materials. To arouse a broad interest from readership in this field, some strongly related works on\nthe recent fabrications of molecular based RTP (Nat. Commun. 2024, 15, 9491; Angew. Chem. Int. Ed. 2025,\n64, e202420139), and the wide stimuli-responsive properties as well as encryption applications (Sci. China\nMater. 2024, 67, 3531; Angew. Chem. Int. Ed. 2025, 64, e202502782; Angew. Chem. Int. Ed. 2025, 64,\ne202505322) could be included as references.\nResponse:\nWe have carefully reviewed all the suggested references and incorporated them appropriately into the revised\nmanuscript to better contextualize our study within the broader field of molecular RTP materials. The citations\nhave been included in the Introduction and Discussion sections, where we discuss recent advances in stimuli-\nresponsive phosphorescent systems and their applications in information encryption. We believe this addition\nstrengthens the manuscript and broadens its relevance to the materials and chemistry communities.\n7. The manuscript demonstrates that only certain halogenated solvents induce the conformational switching.\nCould the authors discuss more about what solvent properties (such as hydrogen bond donating ability, polarity,\nmolecular size) are critical for triggering the phase transition? Could experiments with additional solvents (e.g.,\nacetonitrile, ethanol) clarify these criteria?\nResponse:\nWe have added Supplementary Table 7 summarizing key solvent parameters, including solvation times, dielectric\nconstants (as a measure of polarity), calculated diffusion coefficients, and electrostatic potential (ESP) values for\nCHCl , CDCl , CHBr , C H Cl , C H Cl , and CCl . These parameters are discussed in the revised manuscript to\n3 3 3 2 2 4 2 4 2 4\nprovide a mechanistic understanding of solvent-dependent switching kinetics. To support our discussion, we\nreference dielectric constant data from Maryott and Smith (1951, Table of Dielectric Constants of Pure Liquids,\nVol 514, NBS), and diffusion coefficients calculated using values from Ind. Eng. Chem. Res. 2012, 51, 2797–\n2803.\nTo further clarify the role of solvent properties, we have also tested non-halogenated polar solvents, including\nacetonitrile, ethanol, and methanol. These solvents did not induce a phase transition, and only negligible changes\nin PL spectra were observed. The results of these additional experiments are now included in Supplementary Fig.\n20. This contrast supports our hypothesis that halogenated solvents with appropriate dipolarity and molecular\ndimensions are critical for enabling the conformational switching of BrGlu.\nRevised Manuscript Text:\nResponse Letter\nFurthermore, Electrostatic potential (ESP) calculations (Supplementary Fig. 18) revealed that CDCl ,\n3 2 2 4\ncontrast, solvents such as C H Cl or CCl either possess minimal hydrogen-bonding capability or lack hydrogen\n2 4 2 4\nentirely. Additional tests with polar protic and aprotic solvents—including methanol, ethanol, and acetonitrile—\nalso failed to induce any phase transition, showing only minimal changes in PL spectra (Supplementary Fig.\n19). These results suggest that the phase transition is governed not by polarity or hydrogen-bonding capability\nalone, but by a combination of factors, including ESP distribution, molecular size, and diffusivity, as summarized\nin Supplementary Table 7.\nSupplementary Table 7. Solvation times and solvent properties\n1. Dielectric constants are taken from: Ref 7.\n2. Diffusion coefficients were estimated using the genetic function approximation method reported in: Ref 8.\nRef 7. Maryott, A.A. & Smith, E.R. Table of Dielectric Constants of Pure Liquids, US Government Printing\nOffice 514 (1951).\nRef 8. Gharagheizi, F. Determination of diffusion coefficient of organic compounds in water using a simple\nmolecular-based method. Ind. Eng. Chem. Res. 51, 2797–2803 (2012).\nSupplementary Fig. 19. PL spectra of G-crystal before and after exposure to polar protic and aprotic solvents.\nNormalized PL spectra of G-crystal before and after stirring for 2 days in acetonitrile, methanol, and ethanol\nunder ambient conditions. No significant spectral shift or intensity change was observed, indicating that these\npolar protic and aprotic solvents do not induce a phase transition or affect the emissive properties of the G-\ncrystal.\nResponse Letter\nReviewer #3 (Remarks to the Author):\nThe authors investigated room-temperature phosphorescence (RTP) properties of 2,5-dibromobenzene-1,4-\nbisamide bearing glutalimide substituents (BrGlu). While it does not exhibit RTP in solution, its crystal affords\nbright RTP with high quantum yields. Two differently emitting pseudopolymorphic crystals were obtained (G-\ncrystal and B-crystal), where B-crystal contains CHCl3. The authors ascribed the origin of pseudopolymorph-\ndependent RTP colours to the molecular conformations in those crystals. Notably, interconversion between these\ntwo polymorphs is achieved by suspending them in CHCl3 and heating. Given the significance of developing\nhighly emissive metal-free RTP materials and the relatively rare method (external stimulus) for changing the RTP\n the RTP\n RTP\ncolour, this work represents an interesting progress in the field of functional organic materials. However, the\nnovelty of the work was not properly described, with several closely related works being uncited as specified\nbelow (comments #1-5). In addition, the interpretations of the experimental results are somewhat incomplete in\nsome points. Therefore, the manuscript is potentially sound, but a major revision is needed to meet the very high\nstandards of Nature Communications. Specific concerns are listed below.\n1. On page 3, lines 71–74, the authors mentioned that “Reported systems rely on multi-dopant matrices or require\nchanges in excitation wavelength, complicating both the fabrication process and practical implementation.28–\n30,43–51 Additionally, in existing designs, external stimuli (e.g., temperature, solvent, pH, mechanical force)\ntend to quench phosphorescence rather than sustain or enhance it” and raised several precedents. However, a\nseries of closely related works on stimulus-responsive organic RTP materials (J. Mater. Chem. C 2019, 7, 11926;\nChem. Commun. 2020, 56, 6810; Front. Chem. 2022, 9, 812593; Chem. Sci. 2023, 14, 5302) is omitted. In those\nworks, single-component molecular crystals respond to external stimuli (e.g., mechanical force, temperature,\nsolvent vapour, and light) to exhibit RTP-to-RTP colour changes with enhanced efficiency or even RTP turn-on.\nMoreover, their response mechanism is based on the molecular conformation change, which is somewhat similar\nto the present case. Mentioning these works is indispensable to fairly evaluating the novelty of the present work.\nResponse:\nWe have carefully reviewed the suggested works and included them in the revised manuscript to better position\nour contribution within the existing literature. While these prior studies indeed report stimulus-responsive RTP\nwith single-component molecular crystals and conformation-dependent emission, our work distinguishes itself\nthrough the use of pseudopolymorph-specific switching enabled by solvent-mediated hydrogen bonding,\ndelivering exceptionally high quantum yields (93% and 43% for B- and G-crystals, respectively), a clear and\ntunable color change from blue (480 nm) to green (510 nm), and complete reversibility (> 10 cycles) without\nemission quenching. Unlike previous examples that often involve intensity loss, mechanical stress dependence,\nor partial conversion, our system operates under mild solvent/thermal stimuli with robust switching over multiple\ncycles. We have updated the Introduction to include and discuss these references in a balanced manner, thus\nensuring fair evaluation of the novelty and significance of our work.\nRevised Manuscript Text 1:\nDespite the clear value of reversible RTP for advanced encryption, achieving phosphorescence-color\nswitching in purely organic materials under mild and practical conditions remains challenging. Most reported\nsystems rely on multi-dopant matrices or require changes in excitation wavelength, complicating both the\nfabrication process and practical implementation (Supplementary Fig. 1a). Moreover, in single-component\nResponse Letter\nsystems, external stimuli (e.g., temperature, solvent, pH, mechanical force) often lead to irreversible structural\ndisruption or quenched emission (Supplementary Fig. 1b).\nSupplementary Fig. 1. A key limitation in previous stimuli‐responsive phosphorescence systems is the reliance\non distinct excitation wavelengths to achieve multicomponent emission (a). In addition, the loosely packed\nenvironments induced by external stimuli often promote non‐radiative decay from the triplet state (T ), resulting\n1\nin fluorescence‐dominated emission and reduced overall brightness (b).\nRevised Manuscript Text 2:\nRecently, Ogawa and co-workers have reported a series of single-component organic crystals that exhibit\nRTP-to-RTP color changes in response to external stimuli via molecular conformational rearrangement.1–5\nHowever, these systems often suffer from limited reversibility, low emission efficiency in one of the states, or\nrequire complex external triggers, thereby limiting their practicality and broader applicability.\nRevised Manuscript Text 3:\nOur system leverages a solvent-mediated pseudopolymorphic transformation between well-defined syn\nand anti rotamers, offering high photostability, high quantum yields (43% and 93% for B- and G-crystals,\nrespectively), and complete reversibility. Unlike previously reported systems requiring complex fabrication or\nsuffering emission quenching, BrGlu’s single-molecule design ensures reversible switching with high quantum\nyields (Supplementary Table 1).\nSupplementary Table 1. Summary of phosphorescence quantum yields, switching conditions, and reversibility\nreported in selected literature references\nResponse Letter\n2. At the beginning of the conclusion part, the authors mentioned “This study presents a single-component organic\nphosphor, BrGlu, that exhibits fully reversible room temperature phosphorescence color switching between bright\nblue (B-crystal) and green (G-crystal) states by external stimuli such as solvent exposure and gentle heating.”\nAgain, this has qualitatively been achieved in J. Mater. Chem. C 2019, 7, 11926. In my opinion, it would be better\nto clarify the uniqueness of the present work in a more specific manner. It may include excellent quantum yields\nfor BrGlu crystals, RTP colour change based on pseudopolymorphs, and possible mechanism related to Viedma’s\nripening (cf. comment #9).\nResponse:\nWe have revised the Conclusion to more clearly highlight the unique features of BrGlu that differentiate it from\nprevious systems, including: (1) high quantum yields (93% for G-crystal and 43% for B-crystal), which are rarely\nmaintained across both phases in RTP systems (see Supplementary Table 1); (2) a reversible pseudopolymorph-\ndependent RTP color switching mechanism driven by solvent inclusion, distinct from mechanically or\nphotochemically induced transitions; and (3) a low energy barrier (34 kJ/mol) for the syn–anti conformational\ninterconversion, which governs the transition between polymorphs without relying on chiral resolution as in\nViedma ripening (see response to Comment 10). These revisions aim to more explicitly and fairly contextualize\nthe novelty of our work in light of prior literature.\nRevised Manuscript Text:\nResponse Letter\nThis study presents a single-component organic phosphor, BrGlu, that exhibits fully reversible room\ntemperature phosphorescence color switching between bright blue (B-crystal) and green (G-crystal) states upon\nmild external stimuli, such as solvent exposure and gentle heating. Structural and photophysical analyses reveal\nthat this switching is driven by a solvent-induced pseudopolymorphic transformation involving syn–anti\nconformational rearrangements of bromine–carbonyl substituents, stabilized by hydrogen bonding. Structural\nand computational analyses reveal a low energy barrier (34 kJ/mol) for this interconversion, enabling rapid and\ncomplete reversibility under mild conditions. Notably, both crystalline states exhibit exceptionally high\nphosphorescence quantum yields—93% for the G-crystal and 43% for the B-crystal—without significant\nquenching or lifetime degradation. These results highlight the effectiveness of our molecular design, which\nintegrates heavy-atom-assisted intersystem crossing with a sterically rigid Ac–Glu framework to suppress\nnonradiative decay pathways\n3. Related to comment #1, the authors mentioned “Although correlations between conformation and emission\ncolor are not commonly reported, they could affect triplet emission colors and rates,54 a possibility we investigate\nfurther through quantum calculations” on page 8, lines 160–162. This is also an important discussion closely\nrelated to the above-mentioned series of works. The conformation-dependent RTP colour change (green and\nyellow) is unambiguously demonstrated by X-ray crystal structure analyses in Chem. Sci. 2024, 15,10784.\nResponse:\nWe have cited and discussed the Chem. Sci. 2024, 15, 10784 article in the revised manuscript. This study clearly\ndemonstrates that different conformations can lead to distinct RTP emission colors: the trans-planar conformation\nof compound 1a emits at 560 nm, while the skew conformation of compound 1b exhibits red-shifted emission at\n568 nm. Notably, however, these results were obtained from different molecules with distinct substituents (e.g.,\nhydrogen vs. triisopropylsilyl groups), which contribute to their respective crystal packing and conformational\npreferences.\nIn contrast, our system is based on a single molecular structure (BrGlu) that exhibits stimuli-responsive, reversible\nRTP color switching between two pseudopolymorphs (G- and B-crystals) through solvent-induced\nconformational interconversion (anti ⇌ syn rotamers). This transformation is driven by weak hydrogen bonding\nand proceeds with a low activation barrier (34 kJ/mol), enabling rapid and complete reversibility without chemical\nmodification or permanent lattice change. This distinction—conformational switching within a single molecule\nvia external triggers—enhances the practical significance and mechanistic novelty of our system. We have\nincorporated this discussion and the relevant reference into the revised manuscript.\nRevised Manuscript Text:\nWhile conformation-dependent RTP behavior is not widely reported, recent studies have shown that\nsubtle conformational variations can affect triplet emission colors and rates.3–7 In the BrGlu system, a single\nmolecule exhibits a distinct conformational change between two emissive pseudopolymorphs, triggered by\nsolvent inclusion. To explore this mechanism in detail, we performed further quantum calculations based on the\ncrystal structures.\n4. Structurally very similar compounds, 1,4-bis(aroyl)-2,5-dibromobenzenes, were reported to exhibit RTP in the\ncrystal state (Eur. J. Org. Chem. 2016, 467–473), which would be better cited.\nResponse Letter\nResponse:\nWe have now cited Eur. J. Org. Chem. 2016, 467–473 in the revised manuscript. This study supports the role of\nbromine atoms in enhancing spin–orbit coupling (SOC) and promoting intersystem crossing (ISC) from the\nsinglet to the triplet manifold—an important design principle also applied in our BrGlu system.\n5. Considering that the present work represents a rare example of pseudopolymorph-dependent RTP colour\nchange, related work would be better cited (e.g., Chem. Asian J. 2024, 19, e202301114)\nResponse:\nWe have included Chem. Asian J. 2024, 19, e202301114 in the revised manuscript, acknowledging prior\npseudopolymorph-dependent RTP systems. We clarify that BrGlu’s reversible switching via solvent and heat,\nwith high quantum yields, distinguishes it from these systems.\n6. In page 5, the authors mentioned the molecular design of minimising exciton coupling in the solid state.\nHowever, exciton coupling is typically very small in the triplet state due to small transition dipole moments for\nT1–S0 transition. Although the authors mentioned that “Previously, it was noted that this suppression of exciton\ntrapping3 ultimately enhances intrinsic RTP efficiency”, the cited reference 3 actually did not note such things\n(they mentioned fluorescence). The authors argued the large S1-Tn SOC elements and assumed unity ISC\nefficiency, suggesting the fast ISC. Therefore, I recommend that the authors revise this introductory part as well\nas the discussion regarding crystal structures (page 7, lines 138–141) and decay behaviour (page 12, lines 231–\n233).\nResponse:\nWe thank the reviewer for the critical comments. For Ref. 3, triplet exciton diffusion was actually discussed in a\ngeneral manner in Section 4.4.2. The relevance for phosophorescence quenching was indeed discussed in Section\n5.5 of Ref. 3. Furthermore, we agree that exciton coupling for T –S transitions is typically weak due to their low\n1 0\noscillator strength; however, this small exciton coupling for triplet states is over-compensated by the long triplet\nlifetimes so that exciton diffusion is very effective in these materials (as explicitly discussed in Ref. 3). However,\nwe agree that these issues were insufficiently discussed in the paper. Accordingly, we have extended the\ndiscussion in response to the reviewer's comments.\nRevised Manuscript Text 1:\nNotably, both crystals exhibit relatively large interlayer distances—7.58 Å for B-crystal and 6.01 Å for G-\ncrystal (measured perpendicular to the c–a plane)—supporting the design principle that sterically bulky Glu units\nhinder close molecular packing. This structural feature is expected to reduce intermolecular excitonic coupling,3\nthereby limiting triplet exciton diffusion and effectively enhancing solid-state phosphorescence; this will be\ndiscussed in more detail further down.\nRevised Manuscript Text 2:\nThe non-radiative rates k from T are remarkably low (= 3.8 × 103 s–1 for B-crystal, 0.32 × 103 s–1 for G-\nnr 1\ncrystal). On one hand, phosphorescence quenching may be due to nonradiative recovery of the ground state S\nthrough inefficient reverse ISC ;3 the latter is commonly proceeding through a T /S crossing point (CP). On the\n0 1 0\nother hand, quenching may be due to trapping (in particular at interfaces/surfaces).62 These traps are reached\nby triplet exciton diffusion through long exciton lifetimes and effective exciton coupling via an exchange\nmechanism, which thus requires orbital overlap.3 The latter is effectively suppressed in the BrGlu crystals due to\nthe larger interlayer distances d of 7.58 Å in B vs. 6.01 Å in G as induced by the sterically bulky acyl-glutarimide\nIL\ngroups. The larger k for the B-crystal compared to G thus clearly evidences that trapping is not the predominant\nnr\nquenching mechanism, as the larger d in B vs. G should slow down exciton coupling (and thus trapping) even\nIL\nmore and not vice versa. Instead, the enhanced k for B vs. G must be ascribed to more effective nonradiative\nnr\nground state recovery via ISC . In fact, although the densities of the B-and G-crystals hardly differ (see\nflexibility in B vs. G, and thus facilitate opening the path to the T /S CP. Additionally, the higher-lying T state in\n1 0 1\nB compared to G should further promote the access to the CP, in analogy to the internal conversion (via a conical\nintersection) in the TGlu series.63\n7. On page 7, lines 148–151, the authors mentioned “hydrogen-bond-induced bond stiffening.” I could not catch\nthe point here for two reasons. Firstly, in my understanding, hydrogen bond typically loosens the bond\n(unfortunately, I have no access to ref 52). Secondly, aroyl carbonyl moieties (whose bond lengths are compared)\nin both B- and G-crystal form hydrogen bonding with another BrGlu molecule, and the distance between the\noxygen atom and H atom of CHCl3 is much longer (even longer than the sum of their van der Waals radii). The\nfact that the CHCl3 is disordered also indicates the weak interactions between BrGlu and CHCl3. What is actually\nthe intention of the authors regarding this discussion?\nResponse:\nAs suggested, we have carefully revised the manuscript to clarify the solvent stoichiometry, the origin of the\n1586 cm⁻¹ Raman peak, and the use of the term “hydrogen-bond-induced bond stiffening.” Our responses are\nsummarized as follows:\n1. Clarification of “Hydrogen-Bond-Induced Bond Stiffening”\nWe appreciate the reviewer’s concern regarding the phrase “hydrogen-bond-induced bond stiffening.” We\nagree that classical hydrogen bonding often leads to bond weakening (i.e., red shifts). In our system, however,\nthe observed blue shift of the C=O stretching vibration (from 1749 to 1763 cm⁻¹) and the shortening of the C=O\nbond (1.20 Å → 1.19 Å; Supplementary Fig. 8) suggest increased rigidity in the local vibrational environment\nof the carbonyl group. While both B- and G-crystals involve BrGlu–BrGlu hydrogen bonding, the B-crystal\nuniquely incorporates CHCl₃, which perturbs the lattice through weak, non-directional interactions. Therefore,\nwe have rephrased the discussion in the manuscript to refer to “bond reinforcement due to solvent–lattice\ninteractions”, rather than classical hydrogen bonding, to avoid confusion.\n2. Solvent Stoichiometry from TGA and Single-Crystal Structure\nAs described in the revised manuscript, TGA of the B-crystal shows a mass loss of approximately 14.4 wt%\nupon heating to ~65 °C (Fig. 2e), attributed to the release of CHCl . Based on the molar masses of BrGlu\n3\n(514.13 g/mol) and CHCl (119.38 g/mol), a 1:1 BrGlu:CHCl solvate would yield a theoretical CHCl content\n3 3 3\nof 18.8 wt%. The observed slightly lower mass loss likely results from partial solvent loss during sample\nhandling. Importantly, single-crystal X-ray diffraction confirms the presence of one CHCl molecule per BrGlu\nunit in the B-crystal lattice. This consistent 1:1 stoichiometry justifies our conclusion regarding CHCl\n3\ninclusion.\n3. Origin of the 1586 cm–1 Raman Peak and Lattice Distortion\nThe 1586 cm–1 Raman band appears exclusively in the B-crystal and is absent in the G-crystal (Fig. 1g). This\nband does not originate from Br···O halogen bonding, which generally manifests at much lower frequencies\n(<300 cm⁻¹). Instead, we attribute this peak to a change in the lattice, leading to the local vibration of CHCl –\n3\nsum of van der Waals radii, such disordered solvent species can still modulate lattice dynamics through diffuse\nC–H···O contacts. These interactions introduce vibrational perturbations at the molecular level, contributing to\nthe emergence of new lattice modes such as the 1586 cm–1 peak.\nThese clarifications have been reflected in the revised manuscript text provided below.\nRevised Manuscript Text:\nThermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses reveal a clear distinction\nbetween the B- and G-crystals with respect to solvent incorporation and thermal stability (Figs. 2e,f). The B-\ncrystal exhibits a mass loss of approximately 14.4 wt% upon heating to ~65 oC, corresponding to the release of\nCHCl (Fig. 2e). This is consistent with a 1:1 BrGlu:CHCl solvate, supported by the calculated theoretical CHCl\n3 3 3\ncontent of 18.8 wt% based on the molar masses of BrGlu (514.13 g/mol) and CHCl (119.38 g/mol). SCXRD\n3\nconfirms this stoichiometry, showing CHCl molecules occupying disordered but well-defined lattice sites,\n3\nstabilized by C–H···O interactions.\nIn contrast, the G-crystal displays no observable mass loss and remains structurally stable until\ndecomposition begins above 260 oC (Fig. 2f, Supplementary Fig. 6). This exceptional thermal stability was\nfurther validated by repeated thermal cycling experiments. When subjected to five cycles of heating and cooling\nbetween 0 and 200 oC, the G-crystal retained its structural integrity, as confirmed by PXRD analysis\n(Supplementary Fig. 7). These results demonstrate the BrGlu’s robustness under operational conditions\nrelevant to reversible switching applications.\nRaman spectroscopy (Fig. 1g) further underscores these structural differences. The C=O stretching\nmode shifts from 1749 cm–1 in the G-crystal to 1763 cm–1 in the B-crystal, correlating with a slight shortening of\nthe average C=O bond (1.20 Å vs. 1.19 Å, Supplementary Fig. 8). This shift reflects bond reinforcement induced\nby CHCl –lattice interactions, rather than classical hydrogen bonding. Additionally, a new peak at 1586 cm–1\n3\nappears exclusively in the B-crystal, which can be attributed to a change in the lattice leading to a local vibration\nof CHCl –BrGlu interactions.66 This peak reflects solvent-induced modulation of the lattice dynamics, not\n3\nindicating Br···O halogen bonding, which appears below 300 cm–1.\n8. Preparation of crystal samples for optical measurements needs to be described in more detail. The authors only\nprovided information about the solvent; however, I guess the shape and size of the crystals, as well as the\nhomogeneity of those parameters, strongly depend on how the crystals were grown. Moreover, although the\nauthors briefly mentioned “BrGlu is found to be phosphorescent also in powders and crystals” in line 194, I could\nnot find information on how those two are distinguished.\nResponse:\nResponse Letter\nWe have added a detailed description of crystal preparation in the Supplementary Information, specifying that\ncrystals were grown via slow solvent evaporation (CHCl for B-crystal, CH Cl for G-crystal) over 48 hours,\n3 2 2\nyielding crystals of 0.1–0.5 mm with uniform morphology, as confirmed by optical microscopy (Supplementary\nFig. 2). These details are provided to address the reviewer’s concern regarding crystal shape, size, and\nhomogeneity.\nRegarding the mention of powders, we acknowledge the confusion and have removed the reference to powder\nsamples in the revised manuscript. Since this study is focused on the photophysical behavior of well-defined\nsingle crystals, we intentionally excluded powder data to avoid ambiguity and ensure a clear interpretation of\nstructure–property relationships.\nRevise Supplementary Note 5. Crystal Preparation\nCrystals of BrGlu were grown by slow solvent evaporation (3 mg/mL) at ambient conditions using various\norganic solvents. CHCl and CH Cl yielded single crystals within 48 hours, referred to as B- and G-crystals,\n3 2 2\nrespectively. The B-crystals from CHCl exhibited blue phosphorescence, while G-crystals from CH Cl showed\n3 2 2\ngreen emission. Both formed well-defined crystals with uniform sizes ranging from 0.1 to 0.5 mm, as confirmed\nby optical microscopy (Supplementary Fig. 2).\nIn contrast, ethyl acetate, THF, and acetone generated larger crystals—typically a few millimeters\nacross—also emitting green phosphorescence. On the other hand, toluene, ethanol, and methanol produced\nthin, needle-shaped crystals with smaller dimensions, yet still maintained visible green phosphorescence under\nUV light (Supplementary Fig. 2, 3).\n9. I wonder how stable the B-crystal is in air. Does it lose CHCl3 when kept open to air at room temperature at\natmospheric pressure?\nResponse:\nWe have conducted additional environmental stability tests for both the B- and G-crystals under ambient\nconditions (25 °C, 30% relative humidity) over a period of 7 days.\nFor the G-crystal, which is solvent-free and structurally more rigid, showed no measurable change in its emission\nprofile throughout the 7-day test period. The phosphorescence intensity and spectral shape remained entirely\nstable, indicating excellent robustness under ambient conditions.\nIn contrast, the B-crystal, normalized PL spectra were recorded at 3-hour intervals during the first 24 hours and\nsubsequently every 24 hours up to 7 days. As shown in Supplementary Figure 18, the B-crystal exhibited minimal\nspectral changes during the first 24 hours. Over longer timescales, a gradual red-shift in the emission maximum\nwas observed. This shift is attributed to the slow evaporation of lattice-incorporated CHCl₃ solvent molecules,\nwhich slightly alters the local molecular packing environment and excited-state dynamics.\nThese results confirm that both crystal forms exhibit good stability in real-world environmental conditions, with\nthe G-crystal being particularly resilient due to its solvent-free, tightly packed structure.\nWe have included these results and the corresponding figure in the revised manuscript to clarify the environmental\nstability of both crystal forms.\nRevised Manuscript Text:\nResponse Letter\nIn addition, both crystal forms exhibit excellent environmental stability under ambient conditions (25 °C,\n30% relative humidity). The G-crystal maintains a consistent emission profile over 7 days, indicating high\nstructural robustness (Supplementary Fig. 17a). For the B-crystal, the emission remains largely unchanged\nduring the initial 24 hours, after which a gradual red-shift in λmax is observed, corresponding to a slow conversion\nto the G-phase as CHCl3 slowly evaporates (Supplementary Fig. 17b). These results confirm the outstanding\ndurability, reversibility, and practical reliability of the BrGlu system, supporting its potential for real-world\napplications in optical data storage and anti-counterfeiting technologies.\nSupplementary Fig. 17. Environmental stability of G- and B-crystals under ambient conditions. (a) G-\ncrystal and (b) B-crystal were monitored under ambient conditions (25 oC, 30% relative humidity) to evaluate\nlong-term stability. The G-crystal exhibited no detectable change in emission profile over a 7-day period,\nconfirming its excellent environmental robustness. In contrast, the B-crystal showed minimal spectral change\nwithin the first 24 hours, followed by a gradual red-shift in the emission maximum over the course of one week,\nattributed to the slow evaporation of lattice-incorporated CHCl molecules. These results demonstrate that both\n3\ncrystal forms are stable, with the G-crystal remaining invariant and the B-crystal undergoing only modest spectral\nevolution due to solvent loss for a day.\n10. The authors evaluated the rotamerisation pathway based on DFT calculations in Fig. 4. The result itself is\ninteresting. However, I am not convinced that this is related to the reversible interconversion behaviour, because\nit is unclear whether the interconversion between G- and B-crystal occurs after the crystal is once dissolved. It\nseems to me that the phenomenon is quite similar to Viedma’s ripening. In relation to this, can the authors convert\nthe G-crystal into B-crystal simply by exposing to CHCl3 vapor?\nResponse:\nWe thank the reviewer for this thoughtful and constructive comment regarding the relationship between our DFT-\nbased rotamerization analysis and the experimentally observed reversible interconversion between the G- and B-\ncrystals. The reviewer also raises an important question about whether the G-crystal can be converted to the B-\ncrystal solely through CHCl vapor exposure, and whether the transformation process may be related to Viedma\n3\nripening.\nTo clarify this point, we have conducted additional control experiments in which G-crystals were exposed to\nCHCl 3 vapor for up to 48 hours under ambient conditions (see Supplementary Fig. 19). No visible phase\ntransition or change in phosphorescence color was observed, confirming that vapor exposure alone is insufficient\nto induce the transformation. This rules out a solid-state pathway and suggests that the phase conversion requires\ndissolution followed by recrystallization in CHCl solution.\nAccordingly, we conclude that the G-to-B transformation proceeds via a solution-mediated pseudopolymorphic\ntransition, rather than through a solid-state process or attrition-based mechanism like Viedma ripening.\nImportantly, our system does not involve mechanical stirring or racemization, further distinguishing it from\nViedma-type phenomena.\nWhile our DFT calculations were performed in the gas phase, they provide valuable mechanistic insight into the\nconformational switching that occurs during recrystallization. Specifically, the syn–anti rotamer interconversion\ninvolves a relatively low energy barrier (~0.25 eV), suggesting that such a transition is readily accessible in\nsolution, particularly when assisted by solvent–solute interactions. The calculations also show that solvent\ninclusion stabilizes the syn-rotamer through hydrogen bonding, rationalizing the selective formation of the B-\ncrystal in CHCl .\nBy contrast, exposure of the G-crystal to CHCl vapor for up to 2 days did not induce any phase\n3\ntransformation (Supplementary Fig. 15), indicating that solvent-assisted recrystallization from solution is\nnecessary for pseudopolymorph interconversion.\nSupplementary Fig. 15. PL spectra of G-crystal (black) and after exposure to CHCl vapor for 2 days (red). Only\n3\nminimal spectral changes are observed, indicating that CHCl vapor alone does not induce a transition to the B-\n3\ncrystal phase.\n11. The experiments summarised in Fig. 5 are interesting, but the interpretation of the results seems complicated.\nThe difference in response may arise from the different solubility in those solvents, except CHCl3 and CDCl3.\nWhat is the clue for ascribing these phenomena to the difference in solvation, rather than in dissolution?\nResponse:\nWhile differences in solubility may contribute to the observed switching behavior, our results indicate that\nsolubility alone does not govern the phase transition. We have added Table R3–2 summarizing relevant solvent\nparameters (dielectric constants, diffusion coefficients, ESP values, and switching times), which are now\ndiscussed in the revised manuscript.\nMoreover, we found that acetonitrile, despite its high solubility for BrGlu, does not trigger any crystal-to-crystal\ntransition (Fig. R3–4). Similarly, methanol and ethanol, which can form strong hydrogen bonds, also fail to induce\nResponse Letter\nany emission change. These observations further support the conclusion that the reversible pseudopolymorphic\ntransformation is not driven solely by solubility, but instead arises from a complex interplay of solvent inclusion,\npacking compatibility, and intermolecular interactions.\nWe hope that this expanded discussion better reflects the complexity of the underlying mechanism and resolves\nthe reviewer’s concern.\nRevised Manusciprt Text:\nFurthermore, Electrostatic potential (ESP) calculations (Supplementary Fig. 18) revealed that CDCl ,\n3 2 2 4\ncontrast, solvents such as C H Cl or CCl either possess minimal hydrogen-bonding capability or lack hydrogen\n2 4 2 4\nentirely. Additional tests with polar protic and aprotic solvents—including methanol, ethanol, and acetonitrile—\nalso failed to induce any phase transition, showing only minimal changes in PL spectra (Supplementary Fig.\n18). These results suggest that the phase transition is governed not by polarity or hydrogen-bonding capability\nalone, but by a combination of factors, including ESP distribution, molecular size, and diffusivity, as summarized\nin Supplementary Table 7.\nSupplementary Fig. 19. PL spectra of G-crystal before and after exposure to polar protic and aprotic solvents.\nNormalized PL spectra of G-crystal before and after stirring for 2 days in acetonitrile, methanol, and ethanol\nunder ambient conditions. No significant spectral shift or intensity change was observed, indicating that these\npolar protic and aprotic solvents do not induce a phase transition or affect the emissive properties of the G-\ncrystal.\nSupplementary Table 7. Solvation times and solvent properties\nResponse Letter\n1. Dielectric constants are taken from: Ref 8.\n2. Diffusion coefficients were estimated using the genetic function approximation method reported in: Ref 9.\nMinor comments\n12. The atmospheric conditions for measurement of crystalline samples (in air or under N2 or Ar) should be\nprovided. Also, whether the CHCl3 is deaerated or not for the measurement in Fig. 3e would be better noted.\nResponse:\nWe have clarified in the revised Supplementary Note 4 that, except for low-temperature measurements, all PL\nmeasurements of crystalline samples—including those shown in Fig. 3e—were performed under ambient\nconditions (room temperature, in air). Additionally, the CHCl used in the reversible switching experiment (Figs.\n3\n4e,f) was anhydrous but not deaerated, as the phase transition proceeds effectively even under aerobic conditions.\nThese clarifications are now included in the revised manuscript.\n13. The authors mentioned “Lifetimes were extracted by mono-exponential fitting with the Felix GX software”\non page 4 in SI. However, the decay curve for G-crystal apparently contains more than two components (Fig. 3b).\nIt would be better to provide the fitting curves.\nResponse:\nWe apologize for the oversight. As the reviewer correctly pointed out, the phosphorescence decay curve for the\nG-crystal exhibits multiple components. In our original analysis, we indeed applied a bi-exponential fitting model\nto the G-crystal data, but mistakenly described it as mono-exponential. We have now corrected the text in the\nrevised Manuscript to reflect this.\nAdditionally, we have included the corresponding decay fitting curves as Supplementary Figure 11, where the bi-\nexponential behavior is clearly shown and the fitted parameters are listed. We thank the reviewer for their careful\nobservation, which helped us improve the clarity and accuracy of the manuscript.\n14. In the caption of Fig. 1c, “Hydrogen atoms are omitted for clarity” should be for Fig. 1b and 1d.\nResponse:\nHave corrected in the revised Manuscript.\n15. “da”, “do”, and “θc” at the bottom of Fig. 2a do not appear in the table.\nResponse:\nHave corrected in the revised Manuscript.\n16. In the caption of Fig. 6, “(d,e)” should read “(c,d)”.\nResponse:\nHave corrected to “(c,d)” in the revised Manuscript.\n17. On page 26 in SI, the predicted m/z value for BrGlu (513.93) is wrong. Also, the values for all compounds\nhave only two digits, which should be four.\nResponse:\nHave corrected in the revised Supplementary Information.\nReferences\n1. Tani, Y., Terasaki, M., Komura, M. & Ogawa, T. Room-temperature phosphorescence-to-\nphosphorescence mechanochromism of a metal-free organic 1, 2-diketone. J. Mater. Chem. C 7, 11926\n(2019).\n2. Takewaki, Y. Ogawa, T. & Tani, Y. Modulating room-temperature phosphorescence-to-phosphorescence\nmechanochromism by halogen exchange. Front. Chem. 9, 812593 (2022).\n3. Tani, Y. Komura, M. & Ogawa, T. Mechanoresponsive turn-on phosphorescence by a desymmetrization\napproach. Chem. Commun. 56, 6810 (2020).\n4. Komura, M. Sotome, H. Miyasaka, H. Ogawa, T. & Tani, Y. Photoinduced crystal melting with\nluminescence evolution based on conformational isomerisation. Chem. Sci. 14, 5302 (2023).\n5. Tani, Y. et al. Fast, efficient, narrowband room-temperature phosphorescence from metal-free 1, 2-\ndiketones: rational design and the mechanism. Chem. Sci. 15,10784 (2024).\n6. Tsukiyama, Y. et al. Role of Halobenzene Guest molecules in modulating room temperature\nphosphorescence of benzophenone–naphthalene diimide inclusion crystals. Chem. Asian J. 19,\ne202301114 (2024).\n7. Ju, C.-W. et al. Evolution of organic phosphor through precision regulation of nonradiative decay. Proc.\nNatl. Acad. Sci. 120, e2310883120 (2023).\n8. Maryott, A.A. & Smith, E.R. Table of Dielectric Constants of Pure Liquids, US Government Printing\nOffice 514 (1951).\n9. Gharagheizi, F. Determination of diffusion coefficient of organic compounds in water using a simple\nmolecular-based method. Ind. Eng. Chem. Res. 51, 2797–2803 (2012).\nResponse Letter\nJournal: Nature Communications\nManuscript ID: NCOMMS-25-25620A\nTitle: Reversible Color Switching of Bright Phosphorescence in Purely Organic Materials for Advanced Data\nEncryption\nResponse to the Reviewers’ Comments\nThe authors appreciate the valuable comments and constructive suggestions on our manuscript. Reviewers #1 and\n#2 recommended publication of the manuscript's present version, while Reviewer #3 raised important points\nregarding the mechanistic context of the switching process and the usage of the term 'stimuli-responsive'. We\nhave carefully reviewed each comment and revised the manuscript accordingly to clarify these issues and to\nemphasize the pseudopolymorph-dependent RTP, high quantum yields, and the relevance to data-encryption\napplications. To enhance the readability, the reviewers’ comments and the authors’ responses have been color-\ncoded in black and blue, respectively. In the revised manuscript, revised parts are highlighted in yellow.\nReviewers’ Comments\nReviewer #3 (Remarks to the Author):\nThe authors have addressed most of the concerns raised by me and other reviewers. However, in their reply to\ncomment #10, the authors clarified that the present pseudopolymorph-switching is based on dissolution and\nrecrystallization. This fact has altered the context of the present work and raised several new concerns: the\ntransformation via the dissolution-recrystallization pathway is no longer regarded as a stimulus-responsive\ntransformation, but as a simple recrystallization from the solvent. Still, the pseudopolymorph-dependent RTP and\nhigh RTP quantum yield are outstanding properties, which may meet the standards of Nature Communications.\nSpecific concerns are listed below.\n1. In general, recrystallization of a (pseudo)polymorphic material from an appropriate solvent enables\ntransformation of one polymorph to another. This is commonly not regarded as stimulus response. Also, it is no\nsurprise that the properties are fully restored by the recrystallization, as it purifies the materials. Therefore, the\nemphasis of this manuscript should not be on the “reversible stimulus-response”, but rather on different aspects,\nsuch as pseudopolymorph-dependent RTP, high quantum yield, and the underlying mechanism behind these\nproperties.\nResponse:\nWe thank the reviewer for the thoughtful feedback and for recognizing the significance of pseudopolymorph-\ndependent RTP and the high phosphorescence quantum yields. Our work is motivated by data encryption, where\nreversible, externally triggered switching between emissive states enables reliable on–off information encoding\nby taking advantage of the robust photophysical properties of BrGlu.\nTo clarify the mechanism, we now state explicitly that the forward transformation from G-crystal (anti rotamer)\nto B-crystal (syn rotamer) proceeds via a solution-mediated (solvent-triggered) dissolution–recrystallization\npathway, whereas the reverse transformation from B-crystal back to G-crystal occurs through thermally driven\nsolid-state desolvation (stimuli-responsive preservation of the crystal shape, see Fig. R3–1). In the revised\nResponse Letter\nmanuscript, we therefore describe the phenomenon as phosphorescence color switching under mild external\nconditions, and we present the two pathways separately to avoid any confusion with a generic “stimulus” label.\nImportantly, we no longer use the expression reversible stimuli-responsive in the manuscript.\nIn practice, the switching is governed by two well-defined external inputs:\n1. Solvent exposure: selective dissolution–recrystallization converts G-crystal into B-crystal.\n2. Mild heating: solid-state desolvation regenerates G-crystal from B-crystal.\nBy distinguishing these pathways, we clarify that the observed switching is not a generic purification effect, but\na selective and reversible interconversion between two well-defined pseudopolymorphs, coupled with syn–anti\nconformational reorientation of bromine–carbonyl substituents stabilized by hydrogen bonding.\nIn line with the reviewer’s suggestion, the revised manuscript now emphasizes: (i) pseudopolymorph-dependent\nRTP, (ii) exceptionally high phosphorescence quantum yields (≈43% for B-crystal and ≈93% for G-crystal),\nand\n(iii) the mechanistic basis for efficiency and emission color differences. For the encryption demonstrations only,\nwe retain the term stimuli-responsive in an operational sense—denoting reproducible switching between emissive\nstates under solvent exposure and mild heating—as this property directly underpins the practical data encryption\nfunctionality.\nFig. R3–1. Optical microscope images of BrGlu crystals under external thermal stimulus, illustrating solid-state\nstability during the B-crystal → G-crystal desolvation process.\n2. It would be worth delving into the origin of the high quantum yields by theoretical calculation. For instance,\nthe authors mentioned on page 11 that “Deriving kp, notably, the B-crystal and G-crystal show both high kp\nvalues (2.9 × 10^3 s–1 and 4.2 × 10^3 s–1, respectively), which arise from effective SOC through participation\nof the free electron pairs of Br also in the description of the T1 state.” These kp values are indeed outstanding,\nand the origin is of significant interest. Interestingly, judged from the NTO (Supplementary Fig. 13), T1 state has\na strong (pi,pi*) character, which is typically unfavorable for large kp according to the perturbative intensity\nborrowing theory (Chem. Rev. 2017, 117, 6500-6537). Could the authors reproduce the experimental values by\ntheoretical calculation, or provide some discussion in more detail?\nResponse:\nWe thank the reviewer for highlighting the significance of the unusually high k values and the concerns about\nP\nthe pi–pi* character of the T state. Although the reviewer is right that the exclusive pi-pi* character of the\n1\ntransition does not favor SOC, we stress that in the current case we observe a strong participation of LCAO\ncoefficients of bromine's lone pairs, as shown in the NTO analysis (Supplementary Fig. 13). This induces a heavy\natom effect which promotes SOC and is expected to enhance the oscillator strength. Consequently, we now\nexplicitly calculated f(T ), which gave significant values (6.58×10−6 for B and 2.74×10−5 for G, see\nP\nequation (Strickler, S. J. & Berg, R. A. Relationship between Absorption Intensity and Fluorescence Lifetime of\nResponse Letter\nMolecules. J. Chem. Phys. 1962, 37, 814). The latter can be simplified for using the output of quantum-chemical\ncalculations, as reviewed earlier (J. Gierschner, J. Shi, D. Roca-Sanjuán, B. Milián-Medina, S. Varghese, S. Y.\nPark, Adv. Opt. Mater. 2021, 9, 2002251), taking into account the degeneracy of the states. For the refractive\nindex, we assumed n  2, being a typical value found for anisotropic molecular crystals in the spectral region of\nemission (H.-J. Egelhaaf, J. Gierschner, D. Oelkrug, Synth. Met. 2002, 127, 221; H.-J. Egelhaaf, J. Haiber, J.\nGierschner, D. Oelkrug, Opt. Mater. 1999, 12, 395-401). With this, we obtain k values of the same order of\nP\nmagnitude as experiment (1.0 x 103 s–1for B and 3.4 x 103 s–1 for G) and confirm the larger radiative rate in the\nG-crystal.\nAccordingly, the Discussion section of the main text has been revised as follows. This addition ensures that the\nexperimental findings are quantitatively corroborated by quantum chemistry, thereby strengthening our overall\ninterpretation.\n“This can be directly evidenced from quantum chemistry; for this, we calculated the oscillator strength f(T ) by\nP\nSupplementary Note 10). This yields ca. 1.0 × 103 s–1 for B and 3.4 × 103 s–1 for G, being in reasonable qualitative\nagreement with the experimental values; in particular, it confirms the larger value for k in the G-crystal in\nP\ncomparison with B. The main factor for the enhancement of k in G vs. B is essentially due to the higher f(T ) in\np 1\nG, indicating stronger SOC of T with S through the heavy atom effect of bromine.”\n1 0\n3. Regarding the pseudopolymorph-dependent RTP, the experimentally observed RTP is bluer for the B-crystal\n(syn-rotamer). However, DFT calculations indicate that the syn-rotamer provides RTP in longer wavelengths than\nthe anti-rotamer (Supplementary Table 4 and 5). In addition, the kp value for the B-crystal is smaller than that for\nthe G-crystal. Could this be reproduced by theoretical calculation? Evaluating these points (preferably in a\nquantitative manner) in more detail would strengthen the manuscript's quality, as it leads to a better understanding\nof the relationship between molecular conformation and RTP properties.\nResponse:\nWe appreciate the reviewer’s careful reading and the suggested theoretical reconciliation. For the question of k\nP\nwe refer to the comment above, where we provide the quantum-chemical estimation of k , which reproduces the\nP\ntrends observed in experiment. The question of the hypsochromic shift of phosporescence of B vs. G, was already\ndetailed step-by-step in the Discussion: we argued that the relevant metric for phosphorescence is not the vertical\nabsorption energy S T (given in Tables S4, S5) but the vertical emission energy, E (T S ). In fact, after\n0 1 vert 1 0\noptimizing the T states, we obtain 1.94 eV for the syn conformer (B-crystal) and 1.85 eV for the anti conformer\n1\n(G-crystal), i.e., the B-crystal T is 0.09 eV higher (Fig. 4c; Supplementary Table 6). This reproduces the\n1\nexperimentally observed blue shift of B-crystal relative to G-crystal and resolves the earlier appearance from\nvertical absorption values in Supplementary Tables 4–5.\nIn order to help the readers in this, we modified the text in the manuscript as follows:\n\" In order to rationalize the experimentally observed bathochromic shift of phosphorescence when going from B\nto G, we first conducted single-point TD-DFT calculations (i.e., giving the vertical transition energies\nE (S T )) of the molecules in those conformations, which were found in the crystal structures (see Fig. 4c\nvert 0 1\nand computational part for details). According to the calculations, E (S T ) are somewhat higher in G-crystal\nvert 0 1\ncompared to B-crystal (Supplementary Table 4), which seems to, at first glance, contradict the experiment.\nResponse Letter\nSimilar results are found for E (S T ) on the fully relaxed syn and anti conformations, see Supplementary\nvert 0 1\nTable 5. Nevertheless, we note that the figure of merit for the phosphorescence process is not the vertical energy\nfor absorption, but that for emission, i.e. E (T S ). Therefore, we optimized the T states, to calculate the\nvert 1 0 1\nlatter; in fact, here the anti-conformation (G-phase) is found at 1.85 eV, i.e. 0.09 eV below the syn conformation\n(B-phase; at 1.94 eV), see Fig. 4c and Supplementary Table 6. This inversion is driven by the much larger\nstructural relaxation in the T state of anti in comparison syn. The TD-DFT results on E (T S ) are in\n1 vert 1 0\nreasonable agreement with the experimentally observed shift from 2.43 eV (510 nm) for G to 2.58 eV (480 nm)\nfor B, that is 0.15 eV. The agreement is in particular good, as an additional small hypsochromic shift for the B-\nphase is expected to arise from the lower polarizability due to dilution by the solvent molecules in the B-crystal,73\nwhich is not accounted for in the TD-DFT calculations. Finally, we rule out excitonic contributions to the spectral\nshift, as this becomes only significant for high oscillator strength (f) of the lowest excited state and small\nintermolecular separation;3,74 in the BrGlu crystal, however, f of the emitting state is self-evidently very small due\nto its triplet nature, and, at the same time, intermolecular separations are large, as evidenced from the crystal\nstructure (vide supra).\"\n4. The authors used the term “solvent-mediated pseudopolymorphic transformation” in the 4th paragraph of the\nIntroduction. This might be misleading, as one might expect it to mean a solid-state pathway rather than the\ndissolution-recrystallization pathway. Consider clarifying this distinction.\nResponse:\nWe understand the concern the reviewer brought up. To resolve this, we have revised the manuscript to\nconsistently describe the forward process as a “solvent-triggered pseudopolymorphic transformation via\ndissolution–recrystallization”, which explicitly specifies the solution-mediated mechanism. In contrast, the\nreverse process is now described as a “thermally driven solid-state desolvation”, thereby clearly distinguishing\nsolution- and solid-state pathways. The corresponding revisions in the manuscript are provided below for clarity.\nRevised Introduction Section:\nOur system achieves reversible switching between G-crystal and B-crystal through two distinct pathways:\ndissolution–recrystallization in the presence of specific halogenated solvents and mild-heating–driven solid-state\ndesolvation. This transformation between well-defined conformational rotamers affords high photostability,\nhigh\nphosphorescence quantum yields (43% and 93% for B- and G-crystals, respectively), and complete reversibility.\nRevised Discussion Section:\nStructural and photophysical analyses show that the forward transformation from G-crystal to B-crystal occurs\nthrough a solvent-triggered dissolution–recrystallization, while the reverse transformation from B-crystal to G-\ncrystal proceeds by solid-state desolvation upon mild heating.\n5. To better contextualize the present work, a literature survey on polymorph-dependent RTP has become crucial.\nFor instance, a structurally very similar compound was reported to exhibit polymorph-dependent RTP and vapor-\nand heat-responsive interconversion (J. Mater. Chem. C, 2021,9, 2738-2743). Similarly, vapochromic RTP based\non conformation change has appeared on the ASAP of JACS (https://doi.org/10.1021/jacs.5c07339). Another\nexample may be Chem. Mater. 2023, 35, 22, 9799–9805, which reports polymorphism-mediated regulation of\nTADF and RTP.\nResponse Letter\nResponse:\nWe have added the recommended literature (now cited as Refs. 63–65) to the Introduction and Discussion to\nbetter contextualize the present work.\nMinor comments\n6. Regarding to previous comment #13, the sentence “Lifetimes were extracted by mono-exponential fitting with\nthe Felix GX software” still remains unrevised.\nResponse:\nWe apologize for the oversight. The text has been corrected as follows:\n“Lifetimes were extracted by mono- or bi-exponential fitting (as appropriate) through deconvolution using the\ninstrumental response function (IRF) with the Felix GX software.”\nResponse Letter\nJournal: Nature Communications\nManuscript ID: NCOMMS-25-25620B\nTitle: Reversible Color Switching of Bright Phosphorescence in Purely Organic Materials for Advanced Data\nEncryption\nResponse to the Reviewers’ Comments\nIn this round of revision, Reviewer #3 provided an additional comment concerning the explicit statement of which\nconformation was used for the calculation of the oscillator strengths f(T1). We have revised the manuscript\naccordingly to clarify this point. In the revised manuscript, revised parts are highlighted in yellow.\nReviewers’ Comments\nReviewer #3 (Remarks to the Author):\nThe authors have responded appropriately and sufficiently. It can be judged that the content and expression of the\npaper have been revised to more accurately reflect the experimental facts.\nIn the comparison of emission wavelengths among two pseudopolymorphs, it has been shown that the\nexperimental results are reproduced after structural optimization by DFT calculations. This could be considered\nto suggest that structural relaxation occurred even in the crystals, which is very intriguing. The large kp values\nhave also been estimated based on quantum chemical calculations and the Strickler-Berg equation, and it has been\ndemonstrated that they reflect the trends of the experimental results.\nOnly one point remains: it should be explicitly stated which conformation was used for the f(T1) calculation\n(whether it was the crystal structure or the conformation after structural optimization). If this point is addressed,\nthis paper would be worthy of acceptance without further reviewing.\nResponse:\nWe have addressed the remaining point raised by the reviewer. For the calculation of the oscillator strengths f(T1),\nthe crystal structure was employed and subsequently optimized at the T1 state. To clarify this explicitly, we have\nrevised the Quantum-Chemical calculations section as follows:\nRevised text in the Quantum-Chemical calculations section:\n… Additionally, spin–orbit coupling (SOC) analyses, as well as the calculation of the oscillator strengths f(T1),\nwere conducted in Orca 6.0.0, utilizing the B3LYP functional and 6-311G* basis set, in vacuum, based on the\nground-state geometry previously optimized in Gaussian 16.81 Specifically for the f(T1) calculation, the structure\nused was the crystal geometry (ground-state) which was subsequently optimized to the T1 state…."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Authors have addressed the comments, and it may be accepted for publications.", "reviewer_2": "In my view, the authors have answered all the questions and revised related points, and thus this revised work can be published as it is.", "reviewer_3": "The authors have addressed most of the concerns raised by me and other reviewers. However, in their reply to comment #10, the authors clarified that the present pseudopolymorph-switching is based on dissolution and recrystallization. This fact has altered the context of the present work and raised several new concerns: the transformation via the dissolution-recrystallization pathway is no longer regarded as a stimulus-responsive transformation, but as a simple recrystallization from the solvent. Still, the pseudopolymorph-dependent RTP and high RTP quantum yield are outstanding properties, which may meet the standards of Nature Communications. Specific concerns are listed below.\n1. In general, recrystallization of a (pseudo)polymorphic material from an appropriate solvent enables transformation of one polymorph to another. This is commonly not regarded as stimulus response. Also, it is no surprise that the properties are fully restored by the recrystallization, as it purifies the materials. Therefore, the emphasis of this manuscript should not be on the “reversible stimulus-response”, but rather on different aspects, such as pseudopolymorph-dependent RTP, high quantum yield, and the underlying mechanism behind these properties.\n2. It would be worth delving into the origin of the high quantum yields by theoretical calculation. For instance, the authors mentioned on page 11 that “Deriving kp, notably, the B-crystal and G-crystal show both high kp values (2.9 × 10^3 s–1 and 4.2 × 10^3 s–1, respectively), which arise from effective SOC through participation of the free electron pairs of Br also in the description of the T1 state.” These kp values are indeed outstanding, and the origin is of significant interest. Interestingly, judged from the NTO (Supplementary Fig. 13), T1 state has a strong (pi,pi*) character, which is typically unfavorable for large kp according to the perturbative intensity borrowing theory (Chem. Rev. 2017, 117, 6500-6537). Could the authors reproduce the experimental values by theoretical calculation, or provide some discussion in more detail?\n3. Regarding the pseudopolymorph-dependent RTP, the experimentally observed RTP is bluer for the B-crystal (syn-rotamer). However, DFT calculations indicate that the syn-rotamer provides RTP in longer wavelengths than the anti-rotamer (Supplementary Table 4 and 5). In addition, the kp value for the B-crystal is smaller than that for the G-crystal. Could this be reproduced by theoretical calculation? Evaluating these points (preferably in a quantitative manner) in more detail would strengthen the manuscript's quality, as it leads to a better understanding of the relationship between molecular conformation and RTP properties.\n4. The authors used the term “solvent-mediated pseudopolymorphic transformation” in the 4th paragraph of the Introduction. This might be misleading, as one might expect it to mean a solid-state pathway rather than the dissolution-recrystallization pathway. Consider clarifying this distinction.\n5. To better contextualize the present work, a literature survey on polymorph-dependent RTP has become crucial. For instance, a structurally very similar compound was reported to exhibit polymorph-dependent RTP and vapor- and heat-responsive interconversion (J. Mater. Chem. C, 2021,9, 2738-2743). Similarly, vapochromic RTP based on conformation change has appeared on the ASAP of JACS (https://doi.org/10.1021/jacs.5c07339). Another example may be Chem. Mater. 2023, 35, 22, 9799–9805, which reports polymorphism-mediated regulation of TADF and RTP.\nMinor comments\n6. Regarding to previous comment #13, the sentence “Lifetimes were extracted by mono-exponential fitting with the Felix GX software” still remains unrevised."}, "author_response": "Response Letter\nJournal: Nature Communications\nManuscript ID: NCOMMS-25-25620A\nTitle: Reversible Color Switching of Bright Phosphorescence in Purely Organic Materials for Advanced Data\nEncryption\nResponse to the Reviewers’ Comments\nThe authors appreciate the valuable comments and constructive suggestions on our manuscript. Reviewers #1 and\n#2 recommended publication of the manuscript's present version, while Reviewer #3 raised important points\nregarding the mechanistic context of the switching process and the usage of the term 'stimuli-responsive'. We\nhave carefully reviewed each comment and revised the manuscript accordingly to clarify these issues and to\nemphasize the pseudopolymorph-dependent RTP, high quantum yields, and the relevance to data-encryption\napplications. To enhance the readability, the reviewers’ comments and the authors’ responses have been color-\ncoded in black and blue, respectively. In the revised manuscript, revised parts are highlighted in yellow.\nReviewers’ Comments\nReviewer #3 (Remarks to the Author):\nThe authors have addressed most of the concerns raised by me and other reviewers. However, in their reply to\ncomment #10, the authors clarified that the present pseudopolymorph-switching is based on dissolution and\nrecrystallization. This fact has altered the context of the present work and raised several new concerns: the\ntransformation via the dissolution-recrystallization pathway is no longer regarded as a stimulus-responsive\ntransformation, but as a simple recrystallization from the solvent. Still, the pseudopolymorph-dependent RTP and\nhigh RTP quantum yield are outstanding properties, which may meet the standards of Nature Communications.\nSpecific concerns are listed below.\n1. In general, recrystallization of a (pseudo)polymorphic material from an appropriate solvent enables\ntransformation of one polymorph to another. This is commonly not regarded as stimulus response. Also, it is no\nsurprise that the properties are fully restored by the recrystallization, as it purifies the materials. Therefore, the\nemphasis of this manuscript should not be on the “reversible stimulus-response”, but rather on different aspects,\nsuch as pseudopolymorph-dependent RTP, high quantum yield, and the underlying mechanism behind these\nproperties.\nResponse:\nWe thank the reviewer for the thoughtful feedback and for recognizing the significance of pseudopolymorph-\ndependent RTP and the high phosphorescence quantum yields. Our work is motivated by data encryption, where\nreversible, externally triggered switching between emissive states enables reliable on–off information encoding\nby taking advantage of the robust photophysical properties of BrGlu.\nTo clarify the mechanism, we now state explicitly that the forward transformation from G-crystal (anti rotamer)\nto B-crystal (syn rotamer) proceeds via a solution-mediated (solvent-triggered) dissolution–recrystallization\npathway, whereas the reverse transformation from B-crystal back to G-crystal occurs through thermally driven\nsolid-state desolvation (stimuli-responsive preservation of the crystal shape, see Fig. R3–1). In the revised\nResponse Letter\nmanuscript, we therefore describe the phenomenon as phosphorescence color switching under mild external\nconditions, and we present the two pathways separately to avoid any confusion with a generic “stimulus” label.\nImportantly, we no longer use the expression reversible stimuli-responsive in the manuscript.\nIn practice, the switching is governed by two well-defined external inputs:\n1. Solvent exposure: selective dissolution–recrystallization converts G-crystal into B-crystal.\n2. Mild heating: solid-state desolvation regenerates G-crystal from B-crystal.\nBy distinguishing these pathways, we clarify that the observed switching is not a generic purification effect, but\na selective and reversible interconversion between two well-defined pseudopolymorphs, coupled with syn–anti\nconformational reorientation of bromine–carbonyl substituents stabilized by hydrogen bonding.\nIn line with the reviewer’s suggestion, the revised manuscript now emphasizes: (i) pseudopolymorph-dependent\nRTP, (ii) exceptionally high phosphorescence quantum yields (≈43% for B-crystal and ≈93% for G-crystal),\nand\n(iii) the mechanistic basis for efficiency and emission color differences. For the encryption demonstrations only,\nwe retain the term stimuli-responsive in an operational sense—denoting reproducible switching between emissive\nstates under solvent exposure and mild heating—as this property directly underpins the practical data encryption\nfunctionality.\nFig. R3–1. Optical microscope images of BrGlu crystals under external thermal stimulus, illustrating solid-state\nstability during the B-crystal → G-crystal desolvation process.\n2. It would be worth delving into the origin of the high quantum yields by theoretical calculation. For instance,\nthe authors mentioned on page 11 that “Deriving kp, notably, the B-crystal and G-crystal show both high kp\nvalues (2.9 × 10^3 s–1 and 4.2 × 10^3 s–1, respectively), which arise from effective SOC through participation\nof the free electron pairs of Br also in the description of the T1 state.” These kp values are indeed outstanding,\nand the origin is of significant interest. Interestingly, judged from the NTO (Supplementary Fig. 13), T1 state has\na strong (pi,pi*) character, which is typically unfavorable for large kp according to the perturbative intensity\nborrowing theory (Chem. Rev. 2017, 117, 6500-6537). Could the authors reproduce the experimental values by\ntheoretical calculation, or provide some discussion in more detail?\nResponse:\nWe thank the reviewer for highlighting the significance of the unusually high k values and the concerns about\nP\nthe pi–pi* character of the T state. Although the reviewer is right that the exclusive pi-pi* character of the\n1\ntransition does not favor SOC, we stress that in the current case we observe a strong participation of LCAO\ncoefficients of bromine's lone pairs, as shown in the NTO analysis (Supplementary Fig. 13). This induces a heavy\natom effect which promotes SOC and is expected to enhance the oscillator strength. Consequently, we now\nexplicitly calculated f(T ), which gave significant values (6.58×10−6 for B and 2.74×10−5 for G, see\nP\nequation (Strickler, S. J. & Berg, R. A. Relationship between Absorption Intensity and Fluorescence Lifetime of\nResponse Letter\nMolecules. J. Chem. Phys. 1962, 37, 814). The latter can be simplified for using the output of quantum-chemical\ncalculations, as reviewed earlier (J. Gierschner, J. Shi, D. Roca-Sanjuán, B. Milián-Medina, S. Varghese, S. Y.\nPark, Adv. Opt. Mater. 2021, 9, 2002251), taking into account the degeneracy of the states. For the refractive\nindex, we assumed n  2, being a typical value found for anisotropic molecular crystals in the spectral region of\nemission (H.-J. Egelhaaf, J. Gierschner, D. Oelkrug, Synth. Met. 2002, 127, 221; H.-J. Egelhaaf, J. Haiber, J.\nGierschner, D. Oelkrug, Opt. Mater. 1999, 12, 395-401). With this, we obtain k values of the same order of\nP\nmagnitude as experiment (1.0 x 103 s–1for B and 3.4 x 103 s–1 for G) and confirm the larger radiative rate in the\nG-crystal.\nAccordingly, the Discussion section of the main text has been revised as follows. This addition ensures that the\nexperimental findings are quantitatively corroborated by quantum chemistry, thereby strengthening our overall\ninterpretation.\n“This can be directly evidenced from quantum chemistry; for this, we calculated the oscillator strength f(T ) by\nP\nSupplementary Note 10). This yields ca. 1.0 × 103 s–1 for B and 3.4 × 103 s–1 for G, being in reasonable qualitative\nagreement with the experimental values; in particular, it confirms the larger value for k in the G-crystal in\nP\ncomparison with B. The main factor for the enhancement of k in G vs. B is essentially due to the higher f(T ) in\np 1\nG, indicating stronger SOC of T with S through the heavy atom effect of bromine.”\n1 0\n3. Regarding the pseudopolymorph-dependent RTP, the experimentally observed RTP is bluer for the B-crystal\n(syn-rotamer). However, DFT calculations indicate that the syn-rotamer provides RTP in longer wavelengths than\nthe anti-rotamer (Supplementary Table 4 and 5). In addition, the kp value for the B-crystal is smaller than that for\nthe G-crystal. Could this be reproduced by theoretical calculation? Evaluating these points (preferably in a\nquantitative manner) in more detail would strengthen the manuscript's quality, as it leads to a better understanding\nof the relationship between molecular conformation and RTP properties.\nResponse:\nWe appreciate the reviewer’s careful reading and the suggested theoretical reconciliation. For the question of k\nP\nwe refer to the comment above, where we provide the quantum-chemical estimation of k , which reproduces the\nP\ntrends observed in experiment. The question of the hypsochromic shift of phosporescence of B vs. G, was already\ndetailed step-by-step in the Discussion: we argued that the relevant metric for phosphorescence is not the vertical\nabsorption energy S T (given in Tables S4, S5) but the vertical emission energy, E (T S ). In fact, after\n0 1 vert 1 0\noptimizing the T states, we obtain 1.94 eV for the syn conformer (B-crystal) and 1.85 eV for the anti conformer\n1\n(G-crystal), i.e., the B-crystal T is 0.09 eV higher (Fig. 4c; Supplementary Table 6). This reproduces the\n1\nexperimentally observed blue shift of B-crystal relative to G-crystal and resolves the earlier appearance from\nvertical absorption values in Supplementary Tables 4–5.\nIn order to help the readers in this, we modified the text in the manuscript as follows:\n\" In order to rationalize the experimentally observed bathochromic shift of phosphorescence when going from B\nto G, we first conducted single-point TD-DFT calculations (i.e., giving the vertical transition energies\nE (S T )) of the molecules in those conformations, which were found in the crystal structures (see Fig. 4c\nvert 0 1\nand computational part for details). According to the calculations, E (S T ) are somewhat higher in G-crystal\nvert 0 1\ncompared to B-crystal (Supplementary Table 4), which seems to, at first glance, contradict the experiment.\nResponse Letter\nSimilar results are found for E (S T ) on the fully relaxed syn and anti conformations, see Supplementary\nvert 0 1\nTable 5. Nevertheless, we note that the figure of merit for the phosphorescence process is not the vertical energy\nfor absorption, but that for emission, i.e. E (T S ). Therefore, we optimized the T states, to calculate the\nvert 1 0 1\nlatter; in fact, here the anti-conformation (G-phase) is found at 1.85 eV, i.e. 0.09 eV below the syn conformation\n(B-phase; at 1.94 eV), see Fig. 4c and Supplementary Table 6. This inversion is driven by the much larger\nstructural relaxation in the T state of anti in comparison syn. The TD-DFT results on E (T S ) are in\n1 vert 1 0\nreasonable agreement with the experimentally observed shift from 2.43 eV (510 nm) for G to 2.58 eV (480 nm)\nfor B, that is 0.15 eV. The agreement is in particular good, as an additional small hypsochromic shift for the B-\nphase is expected to arise from the lower polarizability due to dilution by the solvent molecules in the B-crystal,73\nwhich is not accounted for in the TD-DFT calculations. Finally, we rule out excitonic contributions to the spectral\nshift, as this becomes only significant for high oscillator strength (f) of the lowest excited state and small\nintermolecular separation;3,74 in the BrGlu crystal, however, f of the emitting state is self-evidently very small due\nto its triplet nature, and, at the same time, intermolecular separations are large, as evidenced from the crystal\nstructure (vide supra).\"\n4. The authors used the term “solvent-mediated pseudopolymorphic transformation” in the 4th paragraph of the\nIntroduction. This might be misleading, as one might expect it to mean a solid-state pathway rather than the\ndissolution-recrystallization pathway. Consider clarifying this distinction.\nResponse:\nWe understand the concern the reviewer brought up. To resolve this, we have revised the manuscript to\nconsistently describe the forward process as a “solvent-triggered pseudopolymorphic transformation via\ndissolution–recrystallization”, which explicitly specifies the solution-mediated mechanism. In contrast, the\nreverse process is now described as a “thermally driven solid-state desolvation”, thereby clearly distinguishing\nsolution- and solid-state pathways. The corresponding revisions in the manuscript are provided below for clarity.\nRevised Introduction Section:\nOur system achieves reversible switching between G-crystal and B-crystal through two distinct pathways:\ndissolution–recrystallization in the presence of specific halogenated solvents and mild-heating–driven solid-state\ndesolvation. This transformation between well-defined conformational rotamers affords high photostability,\nhigh\nphosphorescence quantum yields (43% and 93% for B- and G-crystals, respectively), and complete reversibility.\nRevised Discussion Section:\nStructural and photophysical analyses show that the forward transformation from G-crystal to B-crystal occurs\nthrough a solvent-triggered dissolution–recrystallization, while the reverse transformation from B-crystal to G-\ncrystal proceeds by solid-state desolvation upon mild heating.\n5. To better contextualize the present work, a literature survey on polymorph-dependent RTP has become crucial.\nFor instance, a structurally very similar compound was reported to exhibit polymorph-dependent RTP and vapor-\nand heat-responsive interconversion (J. Mater. Chem. C, 2021,9, 2738-2743). Similarly, vapochromic RTP based\non conformation change has appeared on the ASAP of JACS (https://doi.org/10.1021/jacs.5c07339). Another\nexample may be Chem. Mater. 2023, 35, 22, 9799–9805, which reports polymorphism-mediated regulation of\nTADF and RTP.\nResponse Letter\nResponse:\nWe have added the recommended literature (now cited as Refs. 63–65) to the Introduction and Discussion to\nbetter contextualize the present work.\nMinor comments\n6. Regarding to previous comment #13, the sentence “Lifetimes were extracted by mono-exponential fitting with\nthe Felix GX software” still remains unrevised.\nResponse:\nWe apologize for the oversight. The text has been corrected as follows:\n“Lifetimes were extracted by mono- or bi-exponential fitting (as appropriate) through deconvolution using the\ninstrumental response function (IRF) with the Felix GX software.”\nResponse Letter\nJournal: Nature Communications\nManuscript ID: NCOMMS-25-25620B\nTitle: Reversible Color Switching of Bright Phosphorescence in Purely Organic Materials for Advanced Data\nEncryption\nResponse to the Reviewers’ Comments\nIn this round of revision, Reviewer #3 provided an additional comment concerning the explicit statement of which\nconformation was used for the calculation of the oscillator strengths f(T1). We have revised the manuscript\naccordingly to clarify this point. In the revised manuscript, revised parts are highlighted in yellow.\nReviewers’ Comments\nReviewer #3 (Remarks to the Author):\nThe authors have responded appropriately and sufficiently. It can be judged that the content and expression of the\npaper have been revised to more accurately reflect the experimental facts.\nIn the comparison of emission wavelengths among two pseudopolymorphs, it has been shown that the\nexperimental results are reproduced after structural optimization by DFT calculations. This could be considered\nto suggest that structural relaxation occurred even in the crystals, which is very intriguing. The large kp values\nhave also been estimated based on quantum chemical calculations and the Strickler-Berg equation, and it has been\ndemonstrated that they reflect the trends of the experimental results.\nOnly one point remains: it should be explicitly stated which conformation was used for the f(T1) calculation\n(whether it was the crystal structure or the conformation after structural optimization). If this point is addressed,\nthis paper would be worthy of acceptance without further reviewing.\nResponse:\nWe have addressed the remaining point raised by the reviewer. For the calculation of the oscillator strengths f(T1),\nthe crystal structure was employed and subsequently optimized at the T1 state. To clarify this explicitly, we have\nrevised the Quantum-Chemical calculations section as follows:\nRevised text in the Quantum-Chemical calculations section:\n… Additionally, spin–orbit coupling (SOC) analyses, as well as the calculation of the oscillator strengths f(T1),\nwere conducted in Orca 6.0.0, utilizing the B3LYP functional and 6-311G* basis set, in vacuum, based on the\nground-state geometry previously optimized in Gaussian 16.81 Specifically for the f(T1) calculation, the structure\nused was the crystal geometry (ground-state) which was subsequently optimized to the T1 state…."}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors have responded appropriately and sufficiently. It can be judged that the content and expression of the paper have been revised to more accurately reflect the experimental facts.\nIn the comparison of emission wavelengths among two pseudopolymorphs, it has been shown that the experimental results are reproduced after structural optimization by DFT calculations. This could be considered to suggest that structural relaxation occurred even in the crystals, which is very intriguing. The large kp values have also been estimated based on quantum chemical calculations and the Strickler-Berg equation, and it has been demonstrated that they reflect the trends of the experimental results.\nOnly one point remains: it should be explicitly stated which conformation was used for the f(T1) calculation (whether it was the crystal structure or the conformation after structural optimization). If this point is addressed, this paper would be worthy of acceptance without further reviewing."}, "author_response": "Response Letter\nJournal: Nature Communications\nManuscript ID: NCOMMS-25-25620B\nTitle: Reversible Color Switching of Bright Phosphorescence in Purely Organic Materials for Advanced Data\nEncryption\nResponse to the Reviewers’ Comments\nIn this round of revision, Reviewer #3 provided an additional comment concerning the explicit statement of which\nconformation was used for the calculation of the oscillator strengths f(T1). We have revised the manuscript\naccordingly to clarify this point. In the revised manuscript, revised parts are highlighted in yellow.\nReviewers’ Comments\nReviewer #3 (Remarks to the Author):\nThe authors have responded appropriately and sufficiently. It can be judged that the content and expression of the\npaper have been revised to more accurately reflect the experimental facts.\nIn the comparison of emission wavelengths among two pseudopolymorphs, it has been shown that the\nexperimental results are reproduced after structural optimization by DFT calculations. This could be considered\nto suggest that structural relaxation occurred even in the crystals, which is very intriguing. The large kp values\nhave also been estimated based on quantum chemical calculations and the Strickler-Berg equation, and it has been\ndemonstrated that they reflect the trends of the experimental results.\nOnly one point remains: it should be explicitly stated which conformation was used for the f(T1) calculation\n(whether it was the crystal structure or the conformation after structural optimization). If this point is addressed,\nthis paper would be worthy of acceptance without further reviewing.\nResponse:\nWe have addressed the remaining point raised by the reviewer. For the calculation of the oscillator strengths f(T1),\nthe crystal structure was employed and subsequently optimized at the T1 state. To clarify this explicitly, we have\nrevised the Quantum-Chemical calculations section as follows:\nRevised text in the Quantum-Chemical calculations section:\n… Additionally, spin–orbit coupling (SOC) analyses, as well as the calculation of the oscillator strengths f(T1),\nwere conducted in Orca 6.0.0, utilizing the B3LYP functional and 6-311G* basis set, in vacuum, based on the\nground-state geometry previously optimized in Gaussian 16.81 Specifically for the f(T1) calculation, the structure\nused was the crystal geometry (ground-state) which was subsequently optimized to the T1 state…."}]

Decision letter