Visualizing molecular diffusion direction and processes in the solid state via dichromatic fluorescent cocrystalization transformation

Domain: chemistry
Published: 2026-02-27
Editorial outcome: minor_revision

Abstract
Solid-state molecular motion (SSMM) plays a critical role in enriching material properties and functions, yet visualizing its detailed dynamics including direction, progression, and velocity to thoroughly elucidate its mechanism remains a great challenge. Herein, an intermolecular charge transfer (ICT)-mediated fluorescence strategy is developed to visualize and regulate SSMM in binary cocrystal systems, utilizing 6-methoxy-2-acetylnaphthalene (MA)/1,2,4,5-tetracyanobenzene (TCNB) as modeling platform. This approach leverages the reversible transformation between MA/TCNB = 1/1 (MT1, yellow emission) and MA/TCNB = 1/2 (MT2, green emission) crystalline phases, enabling real-time visualization of detailed SSMM information via localized dichromatic fluorescence signatures. It reveals an unexpected significant disparity in diffusion rates between electron donor MA and electron acceptor TCNB during the diffusion process at their interface, even exhibiting unidirectional diffusion from MA to TCNB within a restricted time. Applications demonstrated include pretreatment-free impurity analysis of MA (a key impurity in naproxen) down to 0.1% leveraging the fluorescence shift, and real-time monitoring of transesterification reactions based on the differing interaction capabilities of various naproxen esterification derivatives with TCNB. This study not only provides fundamental insights into SSMM and cocrystallization mechanisms but also demonstrates the potential of charge transfer cocrystallization for molecular sensing and dynamic reaction monitoring.

Introduction
Solid state molecular motion (SSMM) is the dynamic behavior of molecules in a solid substance, which contains various forms of photo- and thermally-induced continuously rotation1, 2, vibration3, and translation4-6, and so on. As SSMM plays important roles in affecting and regulating the properties and functions of materials through the influence of phase transition behaviors7, thermodynamic stability8, and optical characteristics9. Thus, a profound understanding of SSMM is paramount. The study of SSMM is more challenging compared to that of gaseous and liquid molecular motion due to the complexity of building theoretical models for SSMM and the difficulty in direct and real-time monitoring10. The precise elucidation and manipulation of SSMM mechanisms have thus emerged as central scientific challenges for optimizing material performance and designing novel functional materials4, 5, 8. Since visualizing and modulating SSMM holds substantial scientific and practical significance, some effective visualization methods based on optical signals from materials have been successfully applied to the study of processes in SSMM recently11, 12. Among them, fluorescence methods have been applied to the study of SSMM processes due to their high sensitivity and simplicity13, 14. However, the existing fluorescence methods are mainly confined to observing the occurrence of SSMSM based on monotonous signal changes. Therefore, the continuous visualization analysis of more detailed information about the SSMM, such as diffusion direction, velocity, and process is essential but challenging.

It's worth noting that organic cocrystals can leverage synergistic interactions, such as intermolecular charge transfer (ICT), hydrogen bonding, or - stacking15-17, between electron donor and acceptor molecules to engender novel optical18-20, electronic21, 22, and magnetic properties23, 24. This feature has enabled its varied applications in the construction of near-infrared imaging probes25, photothermal agents26, organic semiconductors27, analytical sensors28, 29, and medical treatment30. ICT interactions can lead to a newly formed composite state with a decreased energy gap, thereby exhibiting significant changes in optical signals31. Besides, it has been reported that the optical signals of ICT complexes formed between electron acceptors and donors of different structures may exhibit significant differences by varying donor-acceptor ratios32-34, controlled hierarchical self-assembly35, and the formation of excited charge-transfer states36. The diversified fluorescence signals in one cocrystallization system may enable the visualization of complex SSMM information28, such as velocity differences in bidirectional diffusion processes, and diffusion concentration gradients in different regions. So visualization methods based on these ICT cocrystals with more sophisticated optical signals are more favorable for visualizing complex SSMM information, and might supply an effective solution.

Our approach to visualize molecular diffusion direction and processes at the solid interface relies on the resulting dichromatic fluorescent signals via π-stacking of different ratios of electron donor/acceptor pairs. Experimental phenomena and computational data together indicate that the interconvertible color change of prepared dichroic cocrystals is attributed to their different molecular packing architecture. The different ratios between two components induce crystalline phase transformation and play a critical role in controlling the molecular arrangements in the resulting cocrystal structures. Herein, 6-methoxy-2-acetylnaphthalene (MA) and 1,2,4,5-tetracyanobenzene (TCNB) are molecular building blocks used to construct cocrystals. In situ fluorescence imaging revealed the dominant diffusion of MA toward TCNB by observing the variation of localized fluorescence, which is associated with the formation of MA/TCNB cocrystals with different ratios. The more detailed diffusion rate information in the motion of solid state molecules is monitored based on the dichromatic cocrystalization transformation (Figure 1).

Notably, MA is a key impurity of naproxen (S-NAP), a commonly used anti-inflammatory medicine. In contrast, S-NAP is unable to directly form ICT complexes with TCNB in the solid state. Based on this property, a fluorescence analytical method based on charge transfer cocrystallization was developed to detect trace amounts of MA in S-NAP. Besides, based on the easy formation of ICT complex between the propyl ester derivatives of naproxen and TCNB through SSMM, the transesterification monitoring between S-naproxen propyl ester (S-NAP-Pr) and S-naproxen methyl ester (S-NAP-Me) has been realized based on the yellow fluorescence of NAP-Pr/TCNB cocrystals.

Results
Study of the dichroism of MA/TCNB cocrystals. First, the interaction between the electron donor MA and the electron acceptor TCNB was investigated. The preparation method is shown in Figure 2a, MA and TCNB were co-crystalized in a 1:1 molar ratio in acetonitrile solution, yielding yellow MT1 crystals with yellow fluorescence peaked at 537 nm. In contrast, MT2 crystals formed under identical conditions from MA and TCNB at a 1:2 molar ratio emitted green fluorescence peaked at 505 nm. Both single crystals MT1 and MT2 exhibit fluorescence with a significant redshift relative to the monomer (Figure 2b-2e).

To gain insights into the dichroic properties of MA/TCNB cocrystals with different ratios, the molecular conformation and packing structures of MT1 and MT2 were next analyzed by X-ray crystallography (Tables S1, S2). As shown in Figure 2f and 2g, the two-component molecules MA and TCNB in two cocrystals are nearly planar and display the similar molecular stacking configurations. Specifically, MA and TCNB molecules arrange alternately into columnar stacks with donor-acceptor (D-A) distances of 3.428 and 3.375 Å for MT1, and 3.515 and 3.538 Å for MT2, respectively, suggesting the obvious D-A interactions in two cocrystals. Compared to MT2, the shorter D-A distances in MT1 indicate the stronger D-A interactions and may result in a redshifted emission, which is consistent with their emission spectra. Moreover, the columnar stacks of MT1 are further connected with each other by multiple intermolecular C−H···O (2.181-2.544 Å) and C−H···N (2.672-2.735 Å) interactions to form the three-dimensional architecture (Figure 2h; Figure S2a). Differently, the columnar stacks of MT2 are linked together through adjacent uncoupled TCNB molecules via C−H···O (2.239-2.709 Å), C−H···N (2.475-2.747 Å), and C−N··· (3.106-3.249 Å) interactions (Figure 2i; Figure S2b). The cooperation of the various interactions stabilizes two cocrystals.

To investigate whether ICT interactions can occur when MA and TCNB interact via alternative interaction approaches, we ground MA and TCNB together and examine the relationship between MA/TCNB=1/1 and MA/TCNB=1/2 (by grinding) with MT1 and MT2. When MA and TCNB are ground and mixed together, both MA/TCNB=1/1 and MA/TCNB=1/2 complexes emitted yellow fluorescent (543 nm and 544 nm), corresponding to the fluorescence emission of MT1 (542 nm) (Figure 3a; Figure S3). This indicates that MA and TCNB predominantly form complexes in the MT1 form when ground at ratios of 1:1 and 1:2. The complex formed by grinding MA and TCNB at a 1:1 ratio was further characterised. The quantum yield of MA/TCNB is 8.83%, which is obviously higher than that of MA (0.50%) and TCNB (2.81%). Consistently, the fluorescence decay curve indicated that the fluorescence lifetime of the complex formed by grinding MA and TCNB at a 1:1 ratio was 12.48 ns, which was obviously longer than MA (0.83 ns) and TCNB (6.23 ns) (Figure S4). The reason may be that the strong intermolecular interaction of the ICT cocrystals suppresses the rotational and vibrational non-relaxation of the ground state, thus increasing the quantum yield and extending the lifetime of cocrystals.

Further investigation of MA/TCNB confirms the occurrence of ICT interactions between MA and TCNB. Under visible light, the color of MA/TCNB deepened significantly and formed a yellow powder (Figure S3). UV-vis spectra analysis of the mixed powder revealed a broad absorption band from 330 to 600 nm, which was distinct from the spectra of either MA or TCNB (Figure 3b). Further Fourier transform infrared (FTIR) spectra and Raman spectra of the ground powders were performed. As shown in Figure 3c, the FTIR spectra bands of TCNB in MA/TCNB showed a slight shift (C−H str: from 3113 to 3111 cm-1; C−H str: from 3047 to 3041 cm-1; C≡N str: from 2245 to 2243 cm-1). An overall redshift observed in the spectral bands demonstrated augmented electron density of the benzene ring of the TCNB molecule. It is noteworthy that a similar shift also appeared in the FTIR spectral band of MT1 (C−H str: from 3113 to 3109 cm-1; C−H str: from 3047 to 3037 cm-1; C≡N str: from 22 45 to 2243 cm-1). From the Raman spectra, the ring bend of TCNB in both MA/TCNB and MT1 was blueshifted from the initial 723 to 724 cm-1, and the bend of cyan was redshifted from the initial 2241 to 2238 cm-1 (Figure 3d). On the other hand, shifts of TCNB bands in the MT2's FTIR and Raman spectra exhibit significant differences compared with MA/TCNB and MT1. Therefore, FTIR spectra data and the Raman characteristic results collectively indicate the presence of significant ICT interactions analogous to MT1 between MA and TCNB. In addition to the grinding-induced interactions, we also explored a solution-based route to generate ICT complexes by co-precipitation of MA and TCNB from organic solvents. Subsequent experimental investigations have revealed the formation of ICT complexes MA/TCNB (DCM) and MA/TCNB (ACN) through co-precipitation in dichloromethane (DCM) and acetonitrile (ACN), respectively. As demonstrated in Figure S5, both systems exhibit green fluorescence with maximum emission wavelengths at 512 nm and 510 nm, respectively. The spectral congruence between these solution-derived complexes and MT2 ( < 10 nm) provides compelling evidence for the preferential formation of MT2-type complexes during co-precipitation.

Next, we investigated the interaction between the MA structural analogue naproxen (S-NAP) and TCNB during grinding. Grind S-NAP and TCNB at a 1:1 molar ratio to obtain the S-NAP/TCNB complex. The S-NAP/TCNB=1/1 showed no significant change in fluorescence emission compared with the initial state, and its quantum yield of 2.98% exhibited no significant variation relative to S-NAP (2.05%) and TCNB (2.81%) (Figure S6). As shown in Figure S7-S9, S-NAP/TCNB complex displayed no significant change in absorption, whilst the FTIR and Raman spectra exhibited only minor peak shifts. Collectively, these findings indicated there was no significant interaction between S-NAP and TCNB. The above results demonstrated that even minor modifications to the donor substituent exert a significant influence on its interaction with TCNB.

To clarify the difference of MA and S-NAP in interacting with TCNB, we attempted to mix a small amount of S-NAP with an excess of TCNB and grind the mixture thoroughly. Under S-NAP/TCNB ratios of 1:50 and 1:100, a weak yellow–green fluorescence signal could be observed (Figure S10-S11). However, the phenomenon was not sufficiently distinct. Therefore, we evaluated the interaction between S-NAP and TCNB in triphenylphosphine (TPP) powder, which can not form charge transfer interaction with TCNB. The results indicated that the high concentration of S-NAP molecules makes them tend to form tightly self-aggregation in TPP powder, which inhibits the occurrence of ICT with TCNB. When the S-NAP concentration in the TPP is excessively low, the fluorescence intensity of the S-NAP/TCNB complex becomes too weak to be observed. Only a moderate concentration of S-NAP in the TPP could effectively form ICT with TCNB and thus emit a distinct yellow fluorescence (Figure S12-S13). This suggested that the donor’s strong intrinsic interactions of S-NAP prevent it from forming ICT interactions with TCNB. Consistent with this interpretation, subsequent experiments revealed that co-precipitation in DCM and ACN enables the formation of ICT complexes. Denoted as S-NAP/TCSNB (DCM) and S-NAP/TCNB (ACN), respectively. As shown in Figure S14, both precipitated complexes exhibit green fluorescence with emission maxima at 518 nm and 517 nm. This result suggests that S-NAP does not form a obvious ICT complex with TCNB via mechanical force. However, it can self-assemble into ICT complexes through a solution-mediated pathway. These results support a coherent mechanism: the strong self-aggregation of S-NAP dominates in the solid state and inhibits ICT formation with TCNB, whereas co-dissolution disrupts this S-NAP–S-NAP packing and promotes ICT complexation. The strengthened ICT interaction in the reassembled complexes consequently leads to a red-shifted emission.

Interconversion between MT1 and MT2. Subsequently, PXRD experiments were conducted to monitor the phase transitions of electron donors and acceptors after the grinding treatment. The PXRD patterns of S-NAP/TCNB show no additional bands or peak shifts, indicating that no ICT complex is formed. However, the PXRD of the MA/TCNB complex revealed distinct diffraction peaks that were absent in the individual components (MA or TCNB). The new diffraction peaks of the PXRD pattern of MA/TCNB cocrystals prepared by grinding (10.6°, 16.5°) were found to match well with that simulated from MT1 (10.6°, 16.7°) (Figure 4a). The new diffraction peaks became more obvious with the increased times of grinding during the preparation of MA/TCNB cocrystals (Figure S15). This indicated the intermolecular interactions between MA and TCNB, whether induced by grinding or solution-based assembly, yielded identical crystalline phases.

We further investigated the differences in interactions between MA and TCNB when mixed at different ratios. Interestingly, under UV irradiation, the complex of MA and TCNB made by grinding in different ratios exhibited a clear trend: the fluorescence changes from yellow to green with increasing TCNB content. MA/TCNB cocrystal exhibited yellow fluorescence when MA/TCNB > 1/5, and exhibited yellow-green fluorescence when 1/50 ＜ MA/TCNB ≤ 1/5. While it exhibited green fluorescence when MA/TCNB < 1/50 (Figure 4b).

The interactions between MA and TCNB in the formation of complexes of both configurations were further explored. Surprisingly, it was found that when the ratio of the complexes prepared by grinding was in the range of 1/50 ＜ MA/TCNB ≤ 1/5, the fluorescence color showed a blueshift from yellow to green over a period of 2 h, which was verified by the corresponding fluorescence spectra (Figure 4b; Figure S16-S22). This may be due to the partial conversion of the yellow MT1 cocrystals to the green MT2 cocrystals in the presence of excess TCNB. This conclusion was also verified by the PXRD data of the MA/TCNB=1/5 complex in Figure 4c.

To further investigate the relative thermodynamic stabilities of MT1 and MT2 single crystals, we immersed the MT1 in TCNB and immersed the MT2 in MA at 85oC and monitored their real-time changes in fluorescence spectra. The CT fluorescence peak of MT1 exhibited a clear blueshift over time (from 537 to 505 nm) upon exposure to TCNB, accompanied by a gradual color change from yellow to green within 6 h. In contrast, the CT fluorescence peak of MT2 exhibited inadequate redshift (from 505 to 525 nm) after immersion in MA for 6 h, only accompanied by uneven and insignificant green to yellow-green color change (Figure 5a-5b). The new diffraction peaks of the PXRD pattern of MT1 after immersion in TCNB (11.8o) matched with those simulated from MT2, indicating the existence of a structural transformation from MT1 to MT2. On the other hand, the new diffraction peaks with low intensity of the PXRD pattern of MT2 after immersion in MA (10.7o) matched with that simulated from MT1(10.6o), indicating a partial MT2 structural transformation to MT1(Figure 5c).

To investigate the thermodynamic preferences in the formation of different cocrystals from MA and TCNB, density functional theory (DFT) calculations were performed using the unit cells extracted from single-crystal structures as model systems. To preserve the crystallographic framework, only hydrogen atoms were allowed to relax during geometry optimization, while all non-hydrogen atoms were fixed. The unit cell of cocrystal MT1 consists of two MA molecules and two TCNB molecules, whereas that of cocrystal MT2 comprises one MA molecule and two TCNB molecules. Based on the optimized structures, the Gibbs free energy changes associated with the formation of MT1 and MT2 were calculated to be −360.36 kJ/mol and −410.95 kJ/mol, respectively (Figure 5d; Steps for Calculating Gibbs Free Energies in Supplementary Information). These results indicate that the formation of MT2 is thermodynamically more favorable, consistent with experimental observations that MT2 crystals are preferentially formed.

According to the above results, it can be concluded that the thermostability of MT2 is higher than that of MT1, and that the phases of MT1 and MT2 can form a dichromatic cocrystalization transformation system, in which sequential uptake of TCNB and MA under heating conditions triggers structural transformations between MT1 and MT2. At 1/50 ＜ MA/TCNB ≤ 1/5, the system preferentially exhibits kinetically stable yellow cocrystals with MT1 structure, which gradually transform into thermodynamically stable green cocrystals with MT2 structure within 2 h.

Visualizing molecular diffusion direction and processes. We further explored the visualization of SSMM in detailed. Since the donor and acceptor molecules could migrate toward each other in the solid state, the varied and dichromatic fluorescence signal could be used for the direct observation of this process. As shown in Figure S23-S24, MA was homogeneously mixed with TCNB, and a progressive emergence of intense yellow-green fluorescence was observed gradually with the passage of time, indicating the gradual self-assembly of MA/TCNB cocrystals without requiring grinding, in contrast, which was not observed for S-NAP. Comparatively, there was no fluorescence changes in the S-NAP/TCNB system under the same conditions. This aligned well with the structure and interaction studies based on CT cocrystals. As the investigation based on this bulk mixing can’t supply detailed diffusion information, a further visualized analysis of the interaction and subsequent diffusion process at the interface of two components in contact was designed.

As shown in Figure 6a, an equal amount of MA and S-NAP was loaded into adjacent compartments, and both of them contained a TCNB spot. It can be anticipated that as time passes, three possible molecular diffusion processes could lead to three different results. (Ⅰ) Bidirectional diffusion of TCNB and MA leads to ICT fluorescence of both the MA block and the TCNB spot; (Ⅱ) unidirectional diffusion from TCNB to MA leads to ICT fluorescence of the MA block; (Ⅲ) unidirectional diffusion from MA to TCNB leads to ICT fluorescence of the TCNB spot. After diffusion for 10 min, a faint yellow fluorescence was observed on the TCNB side close to the interface, indicating that MA moved toward TCNB at this stage (Figure 6b). With an extension of the time, the yellow fluorescence gradually diffused to the center of the TCNB part. After diffusion for six hours, starting from the interface to the center of the TCNB spot, a gradient emission color from yellow to green was observed, suggesting that MA constantly moved toward TCNB and formed a partial MT2 green cocrystal at the center of the TCNB spot. Finally, after diffusion for 1 day, the TCNB spot displayed a strong green emission in its entirety. The above phenomenon reveals an unexpected significant disparity in diffusion rates between MA and TCNB during the diffusion process at their interface, even exhibiting unidirectional diffusion from MA to TCNB within a restricted time.

It was noticed that the interface and center of the TCNB site showed yellow and green fluorescence signals, respectively, at the later stages of the diffusion process. The different fluorescence colors might be attributed to the uneven ratio of TCNB to MA. To confirm our assumption, the direction and process of molecular diffusion of MA and TCNB were further explored in the cuvette placed horizontally based on changes in fluorescence signals. As shown in Figure 6c, after eight hours of unidirectional diffusion of MA into TCNB, the TCNB layer showed yellow and green fluorescence signals near the interface and far away from the interface, respectively. Moreover, the TCNB layer presented strong green fluorescence when diffusion reached equilibrium. This tendency was almost the same as that in Figure 6b. The red/green ratio of fluorescence image from the distal and proximal parts of the TCNB layer was counted separately based on the RGB analysis with the software ColorPix. The distal 0.99 was significantly higher than the proximal 0.76, indicating a significant color difference between the fluorescence emitted from the distal and proximal ends of the TCNB layer (Table S3; Figure S25). After diffusion for 1 day, the TCNB section exhibited strong green fluorescence throughout, with the exception of the interface. Indicating that at this stage, MT2 green cocrystals should be the main body of the luminescent section. We have additionally extracted detailed RGB information from two fixed regions (proximal and distal) in the TCNB layer. The data shows the proximal region near the interface exhibits yellowing within the first 30 min, whereas after 8 h of diffusion, the distal region begins to show a clear trend toward green (Figure S26; Table S4). To further validate the diffusion-driven assembly mechanism in the cuvette, we added two additional diffusion experiments under modified conditions. First, the diffusion experiment was conducted at an elevated temperature of 75oC. Under this condition, a comparatively complete diffusion process could be achieved within 4 h, which is markedly faster than that at room temperature. This acceleration is attributed to the enhanced thermal motion of molecules at higher temperature, which substantially increases the diffusion rate (Figure S27). Second, our previous cuvette diffusion experiments were performed with the cuvette placed horizontally. Considering the potential contribution of gravity, we carried out an additional diffusion experiment at 25oC with the cuvette placed vertically (TCNB in the bottom layer and MA in the upper layer). The results showed no significant difference in diffusion rate compared with the horizontally placed cuvette, indicating that gravity does not appreciably influence the molecular diffusion behavior in this system (Figure S28).

Summarizing the above phenomenon, it is clear that in the diffusion process between MA and TCNB, due to the unidirectional diffusion of MA to TCNB, MT1 cocrystals will be formed in the TCNB layer close to the interface at the beginning, emitting yellow fluorescence. With the further unidirectional diffusion of MA, the TCNB layer far from the interface will interact with MA and tend to form MT2 cocrystals due to the presence of a large amount of TCNB, thus emitting green fluorescence. Thus, when MA and TCNB were in contact with each other without grinding, not only their motion direction in the solid state could be observed through the in situ fluorescence signal, but also further monitoring of the SSMM velocity and process could be realized based on the different emission color attributed to the binding ratio of MA/TCNB (Figure 6d).

Applications in impurity analysis and transesterification visualization. Notably, MA is a key impurity in the nonsteroidal anti-inflammatory medicine S-NAP according to the United States Pharmacopeia (USP), Chinese pharmacopoeia (Ch. P.), and European Pharmacopoeia (EP). Therefore, its limit detection is crucial for controlling the quality and safety of naproxen drugs37-39. The significant change of fluorescence due to the ICT interaction of the MA/TCNB, in contrast to S-NAP/TCNB, encouraged us to explore their applications in impurity analysis of trace MA in S-NAP. First, we ground TCNB with S-NAP containing 1% MA at different TCNB ratios to explore the appropriate TCNB ratio that can detect MA in the presence of a large amount of S-NAP. As shown in Figure 7a and Figure S29, when the TCNB ratio is set to a high level, MA can be detected even at trace concentrations. It was noteworthy that the dichromatic properties of MA/TCNB cocrystals could still be manifested by modulating the amount of TCNB in the presence of a large amount of S-NAP. The manifestation of dichroism in this analytical method ensured the strong specificity for MA detection. Specifically, by changing the proportion of TCNB, four representative regimes were observed. At 100:1:1, the emission does not show an ICT-induced red shift and remains blue; at 100:1:10, a MT1-dominated assembly forms and emits yellow fluorescence; at 100:1:50, the system transitions to a MT2-dominated assembly with green fluorescence; and at 100:1:500, MT2 remains the dominant species but the excess TCNB effectively dilutes the emissive complex, resulting in a noticeably weakened green fluorescence. The supplementary RGB component pie chart corroborates the systematic color shift from blue to yellow and subsequently to green intuitively as the TCNB proportion is increased (Figure S30). Furthermore, we also tested the detection limit of MA when the TCNB dosage was equivalent to that of S-NAP to evaluate the sensitivity of this method. The results showed that the MA content in S-NAP as low as 0.1% (impurity limit requirements in the pharmacopeias) could still emit green fluorescence through ICT interaction with TCNB, which suggested this method was promising for the impurity analysis of trace amounts of MA in S-NAP (Figure 7b; Figure S31-32).

To verify the specific response of TCNB toward MA, we determined the changes of the intensity at 540 nm in fluorescence spectra of representative coexisting molecules upon interaction with TCNB by grinding at a 1:1 molar ratio. These molecules include R-NAP, 6-methoxy-2-naphthoic acid (MNCA), 6-methoxy-2-naphthaleneacetic acid (MNAA), methyl 6-hydroxy-2-naphthoate (HN-Me), and 1-(6-methoxy-2-naphthyl) ethanol (MNE) (molecular structures are shown in Figure S33). As shown in Figure 7c, only when MA was added to the TCNB, the fluorescence intensity change obviously, implying that TCNB could be used for selective sensing of MA. Moreover, the anti-interference experiment results showed that the selective response for MA was not disturbed by the addition of other substances, further confirming that ICT strategy could act as an excellent fluorescence probe for MA detection based on the SSMM features (Figure 7d).

To further enable multilevel modulation of ICT based on SSMMS, an esterification derivatization strategy based on the modification of the NAP carboxyl group was employed. First, we successfully prepared five pairs of esterified derivatives of S-NAP, R-NAP, and rac-NAP in which the preparation process of R-NAP ester derivatives uses high-purity R-NAP, which is characterised by chiral HPLC in Figure S34-S35 and Tables S5-S6, namely S/R/rac- naproxen methyl ester (S/R/rac-NAP-Me), S/R/rac- naproxen ethyl ester (S/R/rac-NAP-Et), S/R/rac- naproxen propyl ester (S/R/rac-NAP-Pr), S/R/rac- naproxen butyl ester (S/R/rac-NAP-Bu), and S/R/rac- naproxen pentyl ester (S/R/rac-NAP-Pe) (Figure S1; Figure S36-S37) refer to previous reports28. The structure of the esterification derivative is shown in Figure S38. Interestingly, after interacting with the acceptor TCNB, the fluorescence emission peaks of both S/R-NAP-Pr/TCNB and all the rac-NAP ester/TCNB complexes showed an obvious redshift, exhibiting a classical ICT effect (Figure 8a-8c; Figure S39-S43).

To clarify the difference of S/R-NAP-Pr and other NAP esters in interacting with TCNB, we evaluated the interaction between S-NAP-Me and TCNB in TPP similarly. Similar to S-NAP, a high concentration of S-NAP-Me molecules makes them tend to form tightly self-aggregation in TPP powder, which inhibits the occurrence of ICT with TCNB. When the S-NAP-Me concentration in the TPP is excessively low, the fluorescence intensity of the S-NAP-Me/TCNB complex becomes too weak to be observed. Only a moderate concentration (1/100 and 0.5/100) of S-NAP-Me in the TPP could effectively form a weak but detectable ICT interaction (Figure S44-S45). This suggested that the donor’s strong intrinsic interactions of S-NAP-Me prevent it from forming ICT interactions with TCNB. Further research on the interaction between NAP esters and TCNB by Raman spectroscopy showed the ring bending of rac-NAP-Me in rac-NAP-Me/TCNB cocrystals was redshifted from initially 748 cm-1 to 744 cm-1, and the cyan bending was blueshifted from initially 2241 cm-1 to 2243 cm-1. The shift direction of each peak of NAP-Pr/TCNB is also basically the same as that of rac-NAP ester/TCNB, indicating that S/R NAP-Pr and rac-NAP esters can have ICT interactions with TCNB (Figure S46-S47).

Transesterification, the formation of two new esters from two esters catalyzed by acid or base, is a core process in the energy, chemical, and materials industries, etc40. The significant fluorescence differences between S-NAP-Me/TCNB and S-NAP-Pr/TCNB cocrystals prompted us to explore their application in monitoring the process of the transesterification reaction. As shown in Figure 8d, we chose propyl 4-hydroxybenzoate (PHBA-Pr) and S-NAP-Me as the raw materials for the transesterification reaction. Mixing these two molecules and grinding with TCNB did not lead to yellow fluorescence, indicating that the transesterification reaction does not occur at first (Figure 8e, the fluorescence spectrum exhibits high-intensity emission at 400-450 nm due to the presence of monomers). However, the complexes emitted significant yellow fluorescence after exposure to ammonia, which was a commonly used ester degradation catalyst, for 12 h (Figure 8f). Notably, with increasing ammonia fumigation time, the fluorescence intensity of the complexes gradually intensified, suggesting a time-dependent formation of the emissive ICT species during the ammonia-catalyzed transesterification process (Figure S48). Furthermore, mass spectrometry confirmed that the complexes exposed to ammonia contained S-NAP-Pr (Figure 8g-8h). Based on the above results, it can be concluded that PHBA-Pr can be transesterified with S-NAP-Me under alkaline environment to produce S-NAP-Pr and further form ICT complexes with TCNB to emit strong yellow fluorescence. This phenomenon suggests that the difference in SSMM regulated ICT fluorescence signals has potential for application in the monitoring of transesterification processes.

Methods
Materials and reagents
6-Methoxy-2-acetylnaphthalene (MA, ≥99.0%) was purchased from Aladdin Chemical Co., Ltd. 1,2,4,5-Tetracyanobenzene (TCNB, ≥98.0%) was obtained from TCI Chemicals. (S)-Naproxen (S-NAP, ≥99.5%), and (R)-naproxen (R-NAP, ≥99.0%) were purchased from Macklin Biochemical Co., Ltd. Triphenylphosphine (TPP, ≥99.0%), dichloromethane (DCM, HPLC grade), acetonitrile (ACN, HPLC grade), ethanol (≥99.7%), methanol (≥99.9%), n-propanol (≥99.5%), n-butanol (≥99.5%), and n-pentanol (≥99.5%) were obtained from Chuandong Chemical Reagent Co., Ltd. Propyl 4-hydroxybenzoate (PHBA-Pr, ≥98.0%) and aqueous ammonia solution (25–28 wt%) were purchased from Aladdin Chemical Co., Ltd.
All chemicals were used as received without further purification unless otherwise stated. Ultrapure water (18.2 MΩ·cm) was used in all experiments.

Instruments
Ultraviolet-visible (UV) absorption spectra were obtained by the UH4150 UV-Vis-NIR spectrophotometer combined with the diffuse reflection mode of the integrating sphere (Hitachi, Japan). Steady-state photoluminescence (PL) spectra were recorded on the F-7100 fluorescence spectrophotometer (Hitachi, Japan). PL lifetime was measured by the FLS1000 fluorescence spectrometer (Edinburgh Instruments, UK). Fourier transform infrared (FTIR) spectra were measured on the IRTracer-100 spectrophotometer (Shimadzu, Japan). Raman spectra were collected on the LabRAM HR800 spectrometer with a 633 nm laser excitation source (Horiba Jobin-Yvon, France). Powder X-ray diffraction (PXRD) measurements were made by the X'Pert3 Powder10300 (PANalytical, Holland). Enantiomer ratios were determined by HPLC (Chiralpak AS-H columns were purchased from Daicel Chemical Industries, LTD). All single-crystal X-ray diffraction data were collected on the XtaLAB Synergy, Dualflex, HyPix diffractometer with Cu Kα radiation (=1.54184 Å). With Olex2, all the structures were solved using the SHELXS structure solution program using the intrinsic phasing method, and all the structures were refined with the SHELXIL refinement package using least-squares minimization. Photographs were taken by an iPhone 13 camera.

Preparation of single crystals
Single crystals of different configurations were obtained by gas phase diffusion method. In a typical experiment, equimolar amounts of S-NAP (9.2 mg) and TCNB (7.1 mg) were dispersed in 4 mL of CH3CN using ultrasound to obtain a yellow solution, and the solution was passed through with a 0.45 μm syringe filter and transferred into a glass vial. The glass vial was placed in a sealed glass vial containing 8 mL of water under ambient conditions for several days, and high-quality single crystals were obtained by mutual diffusion of good and poor solvents.

Data availability
Crystallographic data for MT1 and MT2 have been deposited with the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers CCDC 2490358 (MT1) and CCDC 2490348 (MT2). These data can be obtained free of charge from the CCDC. The data supporting the findings of this study are available within the Article, Supplementary Information, or Source Data file. Source data are provided for Figures. 2b-e; 3a-d; 4a, c; 5a–c; 7b-d; 8b, c, e-h and Supplementary Figures. S4-S9, S11, S13-S26, S29, S32, S34-S37, S40-S43, S45-S48 in the associated source data file. Source data are provided with this paper. The Data is available from the corresponding authors on request.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The paper “Visualizing Molecular Diffusion Direction and Processes in the Solid State via Dichromatic Fluorescent Cocrystalization Transformation” by P. F. Gao et al. reports an intermolecular charge transfer-mediated fluorescence strategy to visualize and regulate solid state molecular motion (SSMM) in binary cocrystal systems. The study is important and presents a notable advance in the visualization of more detailed information about SSMM. Besides, the selective and unidirectional diffusion phenomenon in SSMM can provide valuable insights for the design of novel sensing platforms. Moreover, this work leverages the unique interaction characteristics between TCNB and MA to enable the impurity analysis of MA, a key impurity in naproxen, without the need for pretreatment. Furthermore, by exploiting the differential interaction capabilities between various naproxen esterification derivatives and TCNB, it enables the real-time monitoring of the transesterification reactions. The findings advance the field, meeting the high standards for urgency and novelty set by Nature Communications. I recommend acceptance after the minor revisions listed below.\n1) As illustrated in Scheme 1, MA and TCNB tend to form the MT2 complex via “co-precipitation from solution”. However, this process was not characterized and discussed in the subsequent main text. The corresponding experimental data for characterizing this process should be provided along with necessary discussions in manuscript.\n2) This study mainly investigates the unique interactions between TCNB and MA during grinding at the solid state and precipitation from solution. Regarding S-NAP, however, only the interactions induced by grinding are discussed. It remains to be determined whether the complex co-precipitated from the solution of S-NAP and TCNB is distinct from that formed by grinding.\n3) For better understanding the optical properties of MA/TCNB, the fluorescence lifetimes of MA, TCNB and MA/TCNB should be determined and discussed in the text.\n4) In Figure 5b and 5c, the authors observed unidirectional diffusion from MA to TCNB over time at 25°C. I wonder whether the SSMM between MA and TCNB would still exhibit unidirectional diffusion from MA to TCNB at higher temperatures.\n5) How is the cuvette placed in Figure 5c? Is it vertical or flat on the table during the observation experiment? I wonder whether the form of placement employed by the authors can eliminate the influence of gravity on the SSMM direction.\n6) I wonder how the luminescence of the complex would behave with further increases in the proportion of TCNB, as shown in Figure 6a. The authors are suggested to make more discussions about it. And what is the TCNB proportion in Figure 6b?", "reviewer_2": "The manuscript describes a fascinating approach to monitoring solid-state molecular diffusion via fluorescence signals based on intermolecular charge transfer (ICT). Using 6-methoxy-2-acetylnaphthalene (MA) and 1,2,4,5-tetracyanobenzene (TCNB) as a model platform, the authors reveal a significant disparity in diffusion rates between the electron donor (MA) and acceptor (TCNB) at their interface. Remarkably, the system exhibits unidirectional diffusion from MA to TCNB within a specific time frame. While the core concept is novel and promising, the presentation and mechanistic depth of the study can be significantly enhanced to better support the claims and improve readability. The following points are recommended for the authors' consideration:\n1. I think that Scheme 1 needs for molecular-level illustration. The current Scheme 1 appears to focus on macroscopic phenomena. To better align with the article's theme of \"monitoring solid-state molecular diffusion via dichroic fluorescence signals,\" it is highly recommended to redesign this scheme to illustrate the process at the molecular level. This should depict the interaction between MA and TCNB molecules, the resulting ICT effect, and how this ultimately leads to the observed macroscopic optical output.\n2. The inclusion of S-NAP as a control in Figure 2, while intended to show specificity, currently complicates the figure and dilutes the focus on the key finding (the MA-TCNB complex). It is suggested that Figure 2 be dedicated to a compelling series of data that conclusively demonstrates the formation of the MA-TCNB intermolecular complex and the ICT phenomenon. Furthermore, the authors claim that S-NAP fails to interact with TCNB due to self-aggregation. However, could the ICT phenomenon be achieved by modulating the S-NAP/TCNB ratio to disrupt this self-aggregation? A brief discussion or experimental data addressing this point would significantly strengthen the authors' claim regarding specificity.\n3. The FTIR and Raman spectra presented in Figure 2c and 2d show rather subtle peak shifts. The question remains whether this evidence is robust enough to firmly prove the interaction between MA and TCNB. To strengthen this claim, is it needed to include the results for the MT1 and MT2 cocrystals in these panels for a direct and convincing comparison?\n4. To enhance the visual impact and readability, it would be highly effective to include intrinsic fluorescence images of pure MA and TCNB powders in the main text. Presenting these alongside the fluorescence data of the cocrystals and diffusion composites would dramatically highlight the striking optical changes and underscore the \"interesting effect\" of the work.\n5. As shown in Figures 5 and S15-16, the spontaneous diffusion between unground MA and TCNB powders is a notable observation. Could the authors discuss the driving forces behind this phenomenon, and whether any special preprocessing was used? If not, what are the thermodynamic or kinetic driving factors of this diffusion process?\n6. Given the stated significance of monitoring diffusion processes and rates in solid state, Figure 5 presents a key opportunity for quantification. It is recommended that the authors add quantitative analysis, such as establishing a relationship between fluorescence color/intensity parameters and the amount of molecular diffusion. This could potentially form the basis for a general quantitative methodology for measuring solid-state diffusion kinetics.\n7. To enhance the readability and impact of the article, it is suggested that the authors consider combining Figures 6 and 7 to better showcase the potential applications of this study. Furthermore, could quantitative arguments be incorporated, such as providing a calibration curve for MA content detection or a kinetic analysis of the transesterification reaction process?\n8. The manuscript would be greatly improved by incorporating more schematic diagrams that elucidate the proposed mechanisms at the molecular level. Such illustrations would help readers quickly grasp the core concepts and highlights of the research.\n9. The annotations in Figures 7a and 7b appear inconsistent or incorrect. Furthermore, for the several molecules in Figure 7a that interact with TCNB, the \"obvious redshift\" described by the authors is not intuitively demonstrated. It is suggested that the authors provide a side-by-side comparison of the fluorescence images of these molecules before and after mixing with TCNB. This would allow readers to immediately comprehend the spectral shift.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "The authors exploited the fluorescence of molecular complexes to trace solid state reactions. Furthermore, they exploited the results to monitor both impurities and a transesterification reaction. Crystallographic analysis and spectroscopic data support the conclusions of the work and the methodology is in general clearly described. Some more details about Gibbs free energy calculations should be given.\nMinor notes:\n1) Some references on the crystallography of naproxen and TCNB co-crystal preparation are important to properly place the manuscript within the available literature. For instance:\nJang, Soeun Et al. effects of some polymeric additives on the cocrystallization of naproxen and nicotinamide via freeze drying Journal of Crystal GrowthArticle2024 DOI: 10.1016/j.jcrysgro.2023.127476\nErcicek, Fatma et al., Naproxen-bipyridine cocrystallization assisted by pressurized carbon dioxide Journal of Supercritical FluidsArticleOpen Access2023, DOI: 10.1016/j.supflu.2023.105976\nMirocki, Artur Et al., Exploring the molecular landscape of multicomponent crystals formed by naproxen drug and acridines CrystEngCommArticleOpen Access2022 DOI: 10.1039/d2ce00890d\nIn the last one, the authors highlighted that different preparation conditions allowed obtaining different polymorph. Several other work of the undersigned reviewer could be of interest for the authors concerning solid state reactions of both TCNB, Naproxen and parent compounds.\n2) Line186 “85oC” must be “85°C”\n3) Line 128 (and elsewhere) “cm-1” should be “cm-1”\n4) Line 159 “SNAP/TCNB was found to exhibit the superposition of the peaks of these two components.”: if his means “no reaction occurred, it should be written more clearly\n5) Figure 4b should be described in more details explaining what x and y are. Also the “CIE” acronym in the caption should be detailed.\nAfter such small changes are done, the manuscript could be accepted for publication"}, "author_response": "Point-by-point response to Reviewers' concerns and queries:\nReviewer 1\nThe paper “Visualizing Molecular Diffusion Direction and Processes in the Solid State via\nDichromatic Fluorescent Cocrystalization Transformation” by P. F. Gao et al. reports an\nintermolecular charge transfer-mediated fluorescence strategy to visualize and regulate solid\nstate molecular motion (SSMM) in binary cocrystal systems. The study is important and\npresents a notable advance in the visualization of more detailed information about SSMM.\nBesides, the selective and unidirectional diffusion phenomenon in SSMM can provide valuable\ninsights for the design of novel sensing platforms. Moreover, this work leverages the unique\ninteraction characteristics between TCNB and MA to enable the impurity analysis of MA, a key\nimpurity in naproxen, without the need for pretreatment. Furthermore, by exploiting the\ndifferential interaction capabilities between various naproxen esterification derivatives and\nTCNB, it enables the real-time monitoring of the transesterification reactions. The findings\nadvance the field, meeting the high standards for urgency and novelty set by Nature\nCommunications. I recommend acceptance after the minor revisions listed below.\nReply: We greatly appreciate the reviewer for his/her positive comments concluding that this\narticle deserves publication in Nature Communications, especially for “The study is important\nand presents a notable advance in the visualization of more detailed information about SSMM.\nBesides, the selective and unidirectional diffusion phenomenon in SSMM can provide valuable\ninsights for the design of novel sensing platforms”. We also greatly appreciate the reviewer for\nthe very careful review, and our manuscript has been improved according to the reviewer’s\nsuggestions.\nComment 1: As illustrated in Scheme 1, MA and TCNB tend to form the MT2 complex via “co-precipitation from solution”. However, this process was not characterized and discussed\nin the subsequent main text. The corresponding experimental data for characterizing this\nprocess should be provided along with necessary discussions in manuscript.\nReply: Thanks for the reviewer’s careful inspection and sorry for our lacking of data.\nSubsequent experimental investigations have revealed the formation of ICT complexes\nMA/TCNB (DCM) and MA/TCNB (ACN) through co-precipitation in dichloromethane (DCM)\nand acetonitrile (ACN), respectively. As demonstrated in the revised supplementary data\n(Figure S4), both systems exhibit green fluorescence with maximum emission wavelengths at\n512 nm and 510 nm, respectively. The spectral congruence between these solution-derived\ncomplexes and MT2 ( < 10 nm) provides compelling evidence for the preferential formation\nof MT2-type complexes during co-precipitation.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “In addition to the grinding-induced interactions, we also explored a\nsolution-based route to generate ICT complexes by co-precipitation of MA and TCNB from\norganic solvents. Subsequent experimental investigations have revealed the formation of ICT\ncomplexes MA/TCNB (DCM) and MA/TCNB (ACN) through co-precipitation in\ndichloromethane (DCM) and acetonitrile (ACN), respectively. As demonstrated in Figure S4,\nboth systems exhibit green fluorescence with maximum emission wavelengths at 512 nm and\n510 nm, respectively. The spectral congruence between these solution-derived complexes and\nMT2 ( < 10 nm) provides compelling evidence for the preferential formation of MT2-type\ncomplexes during co-precipitation.” In addition, we have added the corresponding Figure\n(Figure S4).\nFigure S4. Fluorescence spectra and corresponding photographs of MA/TCNB (DCM) and\nMA/TCNB (ACN) under UV light (365 nm).\nComment 2: This study mainly investigates the unique interactions between TCNB and MA\nduring grinding at the solid state and precipitation from solution. Regarding S-NAP, however,\nonly the interactions induced by grinding are discussed. It remains to be determined whether\nthe complex co-precipitated from the solution of S-NAP and TCNB is distinct from that formed\nby grinding.\nReply: Thanks very much for the reviewer’s professional and kind suggestion. Subsequent\nexperimental investigations have revealed the formation of ICT complexes S-NAP/TCNB\n(DCM) and S-NAP/TCNB (ACN) through co-precipitation in DCM and ACN, respectively. As\ndemonstrated in the revised supplementary data (Figure S13), both systems exhibit green\nfluorescence with maximum emission wavelengths at 518 nm and 517 nm. This indicates that\nalthough S-NAP cannot achieve ICT interaction with TCNB through mechanical force, the\ncomplex can self-assemble via co-precipitation following solution co-dissolution.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “Consistent with this interpretation, subsequent experiments revealed\nthat co-precipitation in DCM and ACN enables the formation of ICT complexes. Denoted as S-\nNAP/TCNB (DCM) and S-NAP/TCNB (ACN), respectively. As shown in Figure S13, both\nprecipitated complexes exhibit green fluorescence with emission maxima at 518 nm and 517\nnm. This result suggests that S-NAP does not form a obvious ICT complex with TCNB via\nmechanical force. However, it can self-assemble into ICT complexes through a solution-\nmediated pathway. These results support a coherent mechanism: the strong self-aggregation of\nS-NAP dominates in the solid state and inhibits ICT formation with TCNB, whereas co-\ndissolution disrupts this S-NAP–S-NAP packing and promotes ICT complexation. The\nstrengthened ICT interaction in the reassembled complexes consequently leads to a red-shifted\nemission.” In addition, we have added the corresponding Figure (Figure S13).\nFigure S13. Fluorescence spectra and corresponding photographs of S-NAP/TCNB (DCM)\nand S-NAP/TCNB (ACN) under UV light (365 nm).\nComment 3: For better understanding the optical properties of MA/TCNB, the fluorescence\nlifetimes of MA, TCNB and MA/TCNB should be determined and discussed in the text.\nReply: Thanks very much for your professional and kind suggestion. The fluorescence decay\ncurve indicated that the fluorescence lifetime of the complex formed by grinding MA and\nTCNB at a 1:1 ratio was 12.48 ns, which was obviously longer than MA (0.83 ns) and TCNB\n(6.23 ns) (Figure S3). This suggests the strong intermolecular interaction between MA and\nTCNB suppressed the rotational and vibrational non-relaxation of the ground state; thus\nextending the lifetime of MA/TCNB.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “Consistently, the fluorescence decay curve indicated that the\nfluorescence lifetime of the complex formed by grinding MA and TCNB at a 1:1 ratio was\n12.48 ns, which was obviously longer than MA (0.83 ns) and TCNB (6.23 ns) (Figure S3). The\nreason may be that the strong intermolecular interaction of the ICT cocrystals suppresses the\nrotational and vibrational non-relaxation of the ground state, thus increasing the quantum yield\nand extending the lifetime of cocrystals.” In addition, we have added the corresponding Figure\n(Figure S3).\nTCNB τ=6.23 ns QY=2.81%\nMA/TCNB τ=12.48 ns QY=8.83%\n0.1\n)\n.\nu\n.\na\n(\ny 0.01\nt\nsi\nn\ne\nt\nn\nI0.001\n0 50 100 150\nTime (ns)\nFigure S3. Fluorescence decay curves and quantum yields of MA, TCNB, and MA/TCNB.\nComment 4: In Figure 5b and 5c, the authors observed unidirectional diffusion from MA to\nTCNB over time at 25oC. I wonder whether the SSMM between MA and TCNB would still\nexhibit unidirectional diffusion from MA to TCNB at higher temperatures.\nReply: Thanks for the constructive comment from the reviewer. According to his/her kind\nsuggestion, we supplemented the unidirectional diffusion experiment of MA to TCNB at 75oC.\nAs shown in Figure S26, after 15 minutes of unidirectional diffusion of MA into TCNB, the\nTCNB layer showed yellow and green fluorescence signals near the interface and far away from\nthe interface, respectively. Moreover, the TCNB layer presented strong green fluorescence\nwhen diffusion reached equilibrium. This tendency was almost the same as the diffusion pattern\nat 25oC shown in Figure 5c. However, under identical experimental conditions, the diffusion\nprocess at 75oC is significantly accelerated compared to that at 25oC. This acceleration is\nattributed to the substantial enhancement of thermal motion of MA’s unidirectional diffusion\ninto TCNB.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “To further validate the diffusion-driven assembly mechanism in the\ncuvette, we added two additional diffusion experiments under modified conditions. First, the\ndiffusion experiment was conducted at an elevated temperature of 75°C. Under this condition,\na comparatively complete diffusion process could be achieved within 4 h, which is markedly\nfaster than that at room temperature. This acceleration is attributed to the enhanced thermal\nmotion of molecules at higher temperature, which substantially increases the diffusion rate\n(Figure S26).” In addition, we have added the corresponding Figure (Figure S26).\nFigure S26. Photographs of MA and TCNB interdiffusion experiments in cuvettes with a\nstraight interface (75oC).\nComment 5: How is the cuvette placed in Figure 5c? Is it vertical or flat on the table during\nthe observation experiment? I wonder whether the form of placement employed by the authors\ncan eliminate the influence of gravity on the SSMM direction.\nReply: Thanks for the constructive comment from the reviewer. To minimize gravitational\neffects, the cuvette in Figure 5c was flat on the table during the experiment. We have revised\nthe manuscript to prevent misunderstandings. Furthermore, we have supplemented the\nexperimental details for vertical cuvette placement in response to the reviewer's comments. As\ndemonstrated in Figure S27, MA successfully diffuses unidirectionally toward TCNB even\nwhen located in the lower layer under vertical orientation, overcoming gravitational constraints.\nCritically, the diffusion rate of MA in the vertical configuration is statistically indistinguishable\nfrom flat, confirming that gravity exerts a negligible influence on the unidirectional diffusion\nmechanism.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “Second, our previous cuvette diffusion experiments were performed\nwith the cuvette placed horizontally. Considering the potential contribution of gravity, we\ncarried out an additional diffusion experiment at 25°C with the cuvette placed vertically (TCNB\nin the bottom layer and MA in the upper layer). The results showed no significant difference in\ndiffusion rate compared with the horizontally placed cuvette, indicating that gravity does not\nappreciably influence the molecular diffusion behavior in this system (Figure S27).” In addition,\nwe have added the corresponding Figure (Figure S27).\nFigure S27. Photographs of MA and TCNB interdiffusion experiments in vertical cuvettes\nwith a straight interface (25oC).\nComment 6: I wonder how the luminescence of the complex would behave with further\nincreases in the proportion of TCNB, as shown in Figure 6a. The authors are suggested to make\nmore discussions about it. And what is the TCNB proportion in Figure 6b?\nReply: We are grateful for the reviewer's comments. In Figure 6b, the TCNB dosage was\nequivalent to that of S-NAP, and this detail has been incorporated into the manuscript. Besides,\nwe have incorporated additional experimental data into Figure 6a, illustrating a continuous\nincrease in TCNB proportion. The results demonstrate that further elevation of TCNB\nproportion does not alter the fluorescence color but reduces the fluorescence intensity.\nFurthermore, the supplementary RGB component pie chart (Figure S29) corroborates the\nsystematic color shift from blue to yellow and subsequently to green intuitively as the TCNB\nproportion is increased. These findings provide robust validation for the observed optical\nbehavior across varying TCNB concentrations.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “Specifically, by changing the proportion of TCNB, four representative\nregimes were observed. At 100:1:1, the emission does not show an ICT-induced red shift and\nremains blue; at 100:1:10, a MT1-dominated assembly forms and emits yellow fluorescence;\nat 100:1:50, the system transitions to a MT2-dominated assembly with green fluorescence; and\nat 100:1:500, MT2 remains the dominant species but the excess TCNB effectively dilutes the\nemissive complex, resulting in a noticeably weakened green fluorescence. The supplementary\nRGB component pie chart corroborates the systematic color shift from blue to yellow and\nsubsequently to green intuitively as the TCNB proportion is increased (Figure S29).” In\naddition, we have added and updated the corresponding Figures (Figure 6a; Figure S28-29).\nFigure 6. Applications of dichromatic cocrystals in impurity analysis. (a) Photographs and\nschematic diagrams illustrating the action mechanisms of S-NAP/MA/TCNB complexes with\ndifferent TCNB ratios under UV irradiation. (b) Fluorescence spectra of S-NAP/MA/TCNB\ncomplexes at 0%, 0.05%, 0.1%, 0.2%, 0.5% and 1% MA ratio. (c) Selectivity test against\nrepresentative coexisting molecules in 1 equivalent ratio with MA. (d) Anti-interference\ncapacity test by adding 1 equivalent coexisting molecule in MA group in (c).\n1.2\nS-NAP: MA: TCNB\ny\nt 100: 1: 1\nsi 1.0 100: 1: 5\nn 100: 1: 10\ne 100: 1: 20\nn t 0.8 100: 1: 50\nI 100: 1: 100\nL 100: 1: 150\nF 100: 1: 200\n0.6\nd 100: 1: 500\ne\nz\nali 0.4\nm\nr 0.2\no\nN\n0.0\n400 450 500 550 600 650 700\nWavelength ( nm )\nFigure S28. Fluorescence spectra of S-NAP/MA/TCNB complexes with different TCNB\nratios.\nFigure S29. RGB component pie chart of S-NAP/MA/TCNB complexes with different\nTCNB ratios under UV irradiation.\nReviewer 2\nThe manuscript describes a fascinating approach to monitoring solid-state molecular diffusion\nvia fluorescence signals based on intermolecular charge transfer (ICT). Using 6-methoxy-2-\nacetylnaphthalene (MA) and 1,2,4,5-tetracyanobenzene (TCNB) as a model platform,\nthe authors reveal a significant disparity in diffusion rates between the electron donor (MA)\nand acceptor (TCNB) at their interface. Remarkably, the system exhibits unidirectional diffusion\nfrom MA to TCNB within a specific time frame. While the core concept is novel and\npromising, the presentation and mechanistic depth of the study can be significantly enhanced\nto better support the claims and improve readability. The following points are recommended\nfor the authors' consideration:\nReply: Thank you for your thoughtful comments and positive recognition of the innovative\napproach in our manuscript. We sincerely appreciate your valuable suggestions to enhance the\npresentation and deepen the mechanistic analysis. In response to your recommendations, we\nhave comprehensively revised the manuscript to improve clarity, strengthen the experimental\ndescriptions, and provide a more rigorous discussion.\nComment 1: I think that Scheme 1 needs for molecular-level illustration. The current Scheme\n1 appears to focus on macroscopic phenomena. To better align with the article's theme of\n\"monitoring solid-state molecular diffusion via dichroic fluorescence signals,\" it is highly\nrecommended to redesign this scheme to illustrate the process at the molecular level. This\nshould depict the interaction between MA and TCNB molecules, the resulting ICT effect,\nand how this ultimately leads to the observed macroscopic optical output.\nReply: We sincerely appreciate the reviewer’s insightful suggestion to enhance Scheme 1. In\nresponse to the reviewer’s advice, we have carefully revised Scheme 1 to more explicitly\nillustrate the key molecular-scale interactions involved in our study. The updated schematic is\nintended to better reflect the article’s molecular-level theme, by clarifying how interactions\nbetween MA and TCNB ultimately give rise to the observed macroscopic optical signals.\nAccording to the reviewer’s suggestions, we have updated the Scheme (Scheme 1).\nScheme 1. Dichromatic cocrystalization transformation of MA/TCNB=1/1 and\nMA/TCNB=1/2.\nComment 2: The inclusion of S-NAP as a control in Figure 2, while intended to show\nspecificity, currently complicates the figure and dilutes the focus on the key finding (the MA-\nTCNB complex). It is suggested that Figure 2 be dedicated to a compelling series of data that\nconclusively demonstrates the formation of the MA-TCNB intermolecular complex and the\nICT phenomenon. Furthermore, the authors claim that S-NAP fails to interact with TCNB due\nto self-aggregation. However, could the ICT phenomenon be achieved by modulating the S-\nNAP/TCNB ratio to disrupt this self-aggregation? A brief discussion or experimental data\naddressing this point would significantly strengthen the authors' claim regarding specificity.\nReply: We sincerely thank the reviewer for this thoughtful and constructive suggestion. We\nfully agree that Figure 2 should maintain a clear focus on the formation of the MA–TCNB\nintermolecular complex and the associated ICT behavior. Based on the reviewer's suggestions,\nwe have revised Figure 2.\nRegarding the important question of whether the ICT phenomenon could be achieved for S-\nNAP by modulating the S-NAP/TCNB ratio to disrupt self-aggregation, we greatly appreciate\nthe reviewer highlighting this point. In response, we attempted to mix a small quantity of S-\nNAP with a large volume of TCNB and grind them thoroughly to mitigate the effects of S-\nNAP's self-aggregation. Results indicate that yellow-green fluorescent signals can be observed\nat S-NAP/TCNB ratios of 1:50 and 1:100 (Figure S9-S10). In addition, a similar trend was also\nobserved in our previous work. We evaluated the interaction between S-NAP and TCNB in\ntriphenylphosphine (TPP) powder, which can not form charge transfer interaction with TCNB.\nOnly a moderate concentration of S-NAP in the TPP could effectively form ICT with TCNB\nand thus emit a distinct yellow fluorescence (Figure S11-S12), which is consistent with the\ncurrent observations and further supports the conclusion that the donor’s strong intrinsic\ninteractions of S-NAP prevent it from forming ICT interactions with TCNB.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “To clarify the difference of MA and S-NAP in interacting with TCNB,\nwe attempted to mix a small amount of S-NAP with an excess of TCNB and grind the mixture\nthoroughly. Under S-NAP/TCNB ratios of 1:50 and 1:100, a weak yellow–green fluorescence\nsignal could be observed (Figure S9-S10). However, the phenomenon was not sufficiently\ndistinct.” In addition, we have added the corresponding Figures (Figure 2; Figure S9-S10).\nFigure S9. Photographs under UV irradiation of S-NAP/TCNB complexes with different\nTCNB ratios.\n1: 10\nu.) 16 1: 20\na. 1: 50\ny ( 12\n1\n1\n:\n:\n1\n5\n0\n0\n0\n0\nsit 1: 1000\nn\ne 8\nnt\nF L\nI\n4\n0\n400 450 500 550 600 650 700\nWavelength (nm)\nFigure S10. Fluorescence spectra of S-NAP/TCNB complexes with different TCNB ratios.\nComment 3: The FTIR and Raman spectra presented in Figure 2c and 2d show rather subtle\npeak shifts. The question remains whether this evidence is robust enough to firmly prove the\ninteraction between MA and TCNB. To strengthen this claim, is it needed to include the results\nfor the MT1 and MT2 cocrystals in these panels for a direct and convincing comparison?\nReply: Thanks for the constructive comment from the reviewer. We fully understand and\nsincerely appreciate the reviewer’s concern that the spectral changes observed in FTIR and\nRaman measurements can appear rather subtle. The evaluation of donor–acceptor complexes\nthrough peak shifts in FTIR and Raman spectra represents a well-established and rigorously\nvalidated approach in the field. At present, FTIR and Raman peak shifts are among the most\ncommonly adopted (and practically accessible) characterizations for probing ICT complex\nformation in the solid state. Based on our literature survey, these vibrational spectroscopic\nsignatures are typically modest in magnitude for many donor–acceptor systems, and thus the\nassociated band shifts are often inherently weak rather than dramatic. Subtle peak shift may\nalso be regarded as indicators of ICT complex formation. In this context, we believe that the\nobserved, consistent, and directionally meaningful shifts in our FTIR and Raman spectra still\nprovide supportive evidence for ICT formation, and they are fully consistent with our overall\nconclusion when considered together with other results. The following references 1 and 2 also\nemploy similar methodologies to demonstrate ICT complex formation. Moreover, our group\npreviously employed this approach when investigating the interaction between S-NAP and\nTCNB (reference 3).\n[1] Sun L, Zhu W, Wang W, et al. Intermolecular charge‐transfer interactions facilitate two‐\nphoton absorption in styrylpyridine–tetracyanobenzene cocrystals [J]. Angewandte Chemie\nInternational Edition, 2017, 56(27): 7831-7835. DOI: 10.1002/anie.201703439\n[2] Khan D, Li F, Zhang X, et al. Luminescent switching behavior achieved by adjusting\ncocrystal charge transfer excitons under acid stimulation [J]. Journal of Materials Chemistry C,\n2024, 12(29): 11060-11064. DOI: 10.1039/d4tc01741b\n[3] Wang W, Gong J, Zhao J, et al. Integration of Wallach's Rule into Intermolecular Charge\nTransfer: A Visual Strategy for Chiral Purification [J]. Advanced Science, 2024, 11(35):\n2403249. DOI: 10.1002/advs.202403249\nIn response to your suggestion, we have incorporated supplementary FTIR and Raman\nspectral data for MT1 and MT2 in the revised manuscript (Figure 2c and 2d), enabling a direct\nand convincing comparison to substantiate our conclusions. Further FTIR and Raman spectra\nof the ground powders were performed. As shown in Figure 2c, the FTIR spectral bands of\nTCNB in MA/TCNB showed a slight shift (C−H str: from 3113 to 3111 cm-1; C−H str: from\n3047 to 3041 cm-1; C≡N str: from 2245 to 2243 cm-1). An overall redshift observed in the\nspectral bands demonstrated augmented electron density of the benzene ring of the TCNB\nmolecule. It is noteworthy that a similar shift also appeared in the FTIR spectral band of MT1\n(C−H str: from 3113 to 3109 cm-1; C−H str: from 3047 to 3037 cm-1; C≡N str: from 2245 to\n2243 cm-1). From the Raman spectra, the ring bend of TCNB in both MA/TCNB and MT1 was\nblueshifted from the initial 723 to 724 cm-1, and the bend of cyan was redshifted from the initial\n2241 to 2238 cm-1 (Figure 2d). On the other hand, shifts of TCNB bands in the MT2's FTIR\nand Raman spectra exhibit significant differences compared with MA/TCNB and MT1.\nTherefore, FTIR spectra data and the Raman characteristic results collectively indicate the\npresence of significant ICT interactions analogous to MT1 between MA and TCNB.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “It is noteworthy that a similar shift also appeared in the FTIR spectral\nband of MT1 (C−H str: from 3113 to 3109 cm-1; C−H str: from 3047 to 3037 cm-1; C≡N str:\nfrom 2245 to 2243 cm-1). From the Raman spectra, the ring bend of TCNB in both MA/TCNB\nand MT1 was blueshifted from the initial 723 to 724 cm-1, and the bend of cyan was redshifted\nfrom the initial 2241 to 2238 cm-1 (Figure 2d). On the other hand, shifts of TCNB bands in the\nMT2's FTIR and Raman spectra exhibit significant differences compared with MA/TCNB and\nMT1.” In addition, we have updated the corresponding Figures (Figure 2c and 2d).\nFigure 2. Spectral characterisation of MA/TCNB (by grinding). (a) Fluorescence spectra\nof MT1, MT2, MA/TCNB=1/1, and MA/TCNB=1/2 (by grinding). (b) UV-vis spectra of\nTCNB, MA, and MA/TCNB=1/1. (c) FTIR spectra, and (d) Raman spectra of TCNB, MA,\nMT1, MT2, and MA/TCNB=1/1.\nComment 4: To enhance the visual impact and readability, it would be highly effective to\ninclude intrinsic fluorescence images of pure MA and TCNB powders in the main text.\nPresenting these alongside the fluorescence data of the cocrystals and diffusion composites\nwould dramatically highlight the striking optical changes and underscore the \"interesting effect\"\nof the work.\nReply: Thanks for the constructive comment from the reviewer. We have reformatted Figure 1\nin accordance with the reviewers' comments and incorporated the updated version into the\nmanuscript.\nAccording to the reviewer’s suggestions, we have updated the corresponding Figure (Figure\n1).\nFigure 1. Fluorescence spectra and molecular packing structures of MT1 and MT2. (a)\nSchematic diagram of the preparation process for MT1 and MT2. Fluorescence spectra of (b)\nMA, (c) TCNB, (d) MT1, and (e) MT2. Insert: the corresponding fluorescence photographs.\nIntermolecular - stacking of (f) MT1 and (g) MT2. Molecular packing structures of (h) MT1\nand (i) MT2 viewed along the crystallographic a-axis.\nComment 5: As shown in Figures 5 and S15-16, the spontaneous diffusion between unground\nMA and TCNB powders is a notable observation. Could the authors discuss the driving forces\nbehind this phenomenon, and whether any special preprocessing was used? If not, what are the\nthermodynamic or kinetic driving factors of this diffusion process?\nReply: Thank you for your insightful questions regarding the spontaneous diffusion\nphenomenon reported in our study. We confirm that no special pre‑treatment was applied to the\nsamples and all powders were used in their pristine state. The underlying driving forces for this\nphenomenon are rooted in fundamental molecular dynamics. Specifically, molecules in any\nsolid substance undergo continuous thermal motion. When two powder particles establish\nintimate contact, this thermal agitation induces minute relative displacements at their interface,\nenabling molecular migration from regions of higher concentration (i.e., unmixed domains) to\nthose of lower concentration (i.e., intermixed zones). In solid powder systems, such transport\noccurs via localized stochastic processes analogous to Brownian motion, operating at particle\ncontact points. Owing to surface roughness and lattice imperfections, molecules at these\ninterfaces can execute thermally activated “jumps” across short distances. Over extended time\nscales, the cumulative effect of these random events gives rise to observable macroscopic\ndiffusion. Moreover, the relatively weak intermolecular interactions among MA molecules\nlower the activation barrier for diffusion, thereby facilitating the unidirectional migration of\nMA into TCNB-rich domains. According to the reviewer's suggestion, we supplemented the\nunidirectional diffusion experiment of MA to TCNB at 75°C. As shown in Figure S26, the\ndiffusion process at 75°C is significantly accelerated compared to that at 25°C. This\nacceleration is attributed to the substantial enhancement of thermal motion of MA’s\nunidirectional diffusion into TCNB. This further demonstrates that the thermal motion of\nmolecules is the primary driving force of the spontaneous diffusion between unground MA and\nTCNB powders.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “To further validate the diffusion-driven assembly mechanism in the\ncuvette, we added two additional diffusion experiments under modified conditions. First, the\ndiffusion experiment was conducted at an elevated temperature of 75 °C. Under this condition,\na comparatively complete diffusion process could be achieved within 4 h, which is markedly\nfaster than that at room temperature. This acceleration is attributed to the enhanced thermal\nmotion of molecules at higher temperature, which substantially increases the diffusion rate\n(Figure S26).” In addition, we have added the corresponding Figure (Figure S26).\nFigure S26. Photographs of MA and TCNB interdiffusion experiments in cuvettes with a\nstraight interface (75oC).\nComment 6: Given the stated significance of monitoring diffusion processes and rates in solid\nstate, Figure 5 presents a key opportunity for quantification. It is recommended that the authors\nadd quantitative analysis, such as establishing a relationship between fluorescence\ncolor/intensity parameters and the amount of molecular diffusion. This could potentially form\nthe basis for a general quantitative methodology for measuring solid-state diffusion kinetics.\nReply: We are most grateful for the reviewer's comments, which enabled us to establish the\nrelationship between fluorescence colour and the diffusion process (Table S4; Figure S25). The\nproximal yellow fluorescence signal corresponds to the formation of the MT1 cocrystal, in\nwhich MA and TCNB molecules assemble in a 1:1 ratio. In contrast, the distal green\nfluorescence signal represents the formation of the MT2 cocrystal, where MA and TCNB\nmolecules co-crystallize in a 1:2 ratio. As the diffusion process progresses, the Red and Green\ncomponents constituting the yellow fluorescence signal at the proximal (yellow background)\nsignificantly increase at 30 minutes, indicating the gradual formation of MT1. After 8 hours,\nthe proportion of Green at the distal (green background) rises rapidly, reflecting the conversion\nof MA and TCNB molecules toward the MT2 phase. These findings provide stronger support\nfor the arguments presented in this article.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “We have additionally extracted detailed RGB information from two\nfixed regions (proximal and distal) in the TCNB layer. The data shows the proximal region near\nthe interface exhibits yellowing within the first 30 min, whereas after 8 h of diffusion, the distal\nregion begins to show a clear trend toward green (Table S4; Figure S25).” In addition, we have\nadded the corresponding Figure and Table (Table S4; Figure S25).\nFigure S25. RGB data of the proximal (yellow background) and distal (green background)\nregions at different diffusion times.\nTable S4. RGB data of the proximal and distal regions at different diffusion times.\nProximal (Distance=10) Distal (Distance=40)\nTime\nRed Green Blue Red Green Blue\n0 min 84 68 55 77 59 75\n10 min 112 101 73 96 62 89\n30 min 199 186 56 135 97 110\n1 h 173 173 43 101 82 75\n3 h 204 217 86 120 97 103\n8 h 244 253 48 161 192 133\n1 d 237 250 72 130 167 90\n3 d 240 255 83 156 179 127\nComment 7: To enhance the readability and impact of the article, it is suggested that the authors\nconsider combining Figures 6 and 7 to better showcase the potential applications of this study.\nFurthermore, could quantitative arguments be incorporated, such as providing a calibration\ncurve for MA content detection or a kinetic analysis of the transesterification reaction process?\nReply: We sincerely appreciate the reviewer's comments. In response to the reviewer's\nconstructive suggestions, we have incorporated a kinetic analysis of the transesterification\nprocess and the calibration curve for MA content detection (Figure S31; Figure S47). The\nresults demonstrate that the yellow fluorescence of the mixture progressively intensifies over\ntime. Such results are beneficial for applications in monitoring transesterification reactions. We\nalso attempted to merge Figures 6 and 7 as suggested. However, due to the substantial amount\nof information contained in each figure, a combined layout may become overly dense and\nwould compromise readability, resolution, and the logical flow of the results. For this reason,\nwe respectfully keep Figures 6 and 7 as separate figures, and we hope the reviewer will kindly\nunderstand our rationale.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “Notably, with increasing ammonia fumigation time, the fluorescence\nintensity of the complexes gradually intensified, suggesting a time-dependent formation of the\nemissive ICT species during the ammonia-catalyzed transesterification process.” In addition,\nwe have added the corresponding Figures (Figure S31; Figure S47).\n25\n20\n15\n10\n5\n0\n0 0.05 0.1 0.2 0.5 1\nFigure S31. I (fluorescence emission intensity of the mixture at 500 nm) /I (fluorescence\n1 0\nemission intensity of the mixture at 500 nm at 0 hours for the mixture reaction) of S-\nNAP/MA/TCNB complexes at 0%, 0.05%, 0.1%, 0.2%, 0.5% and 1% MA ratio.\nFigure S47. (a) Photographs under UV light, (b) fluorescence spectra, and (c) I (fluorescence\n1\nemission intensity of the mixture at 540 nm) /I (fluorescence emission intensity of the mixture\n0\nat 540 nm at 0 hours for the mixture reaction) of (PHBA-Pr+S-NAP-Me)/TCNB mixture at\ndifferent transesterification reaction times.\nComment 8: The manuscript would be greatly improved by incorporating more schematic\ndiagrams that elucidate the proposed mechanisms at the molecular level. Such illustrations\nwould help readers quickly grasp the core concepts and highlights of the research.\nReply: We thank the reviewer for this suggestion. According to the reviewers' suggestions, we\nhave incorporated additional schematic diagrams into the manuscript at the molecular level to\nelucidate the proposed mechanism (Figure 1a and 6a). Specifically, Figure 1a schematically\nillustrates that MA and TCNB can form two emissive ICT cocrystal phases depending on the\ndonor/acceptor stoichiometry under grinding. The 1:1 mixture preferentially yields MT1,\nexhibiting yellow fluorescence with an emission maximum at 537 nm, whereas the 1:2 mixture\nI/\nI\n0\nfavors MT2, displaying green fluorescence with an emission maximum at 505 nm. In Figure\n6a, we further summarize the four representative scenarios under varying TCNB fractions via\na stoichiometry-dependent schematic. At 100:1:1, the emission does not show an ICT-induced\nred shift and remains blue; at 100:1:10, a MT1-dominated assembly forms and emits yellow\nfluorescence; at 100:1:50, the system transitions to a MT2-dominated assembly with green\nfluorescence; and at 100:1:500, MT2 remains the dominant species but the excess TCNB\neffectively dilutes the emissive complex, resulting in a noticeably weakened green fluorescence.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “Specifically, four representative regimes were identified by varying the\nTCNB fraction: at 100:1:1, the emission does not exhibit an ICT-induced red shift and remains\nblue. At 100:1:1, the emission does not show an ICT-induced red shift and remains blue; at\n100:1:10, a MT1-dominated assembly forms and emits yellow fluorescence; at 100:1:50, the\nsystem transitions to a MT2-dominated assembly with green fluorescence; and at 100:1:500,\nMT2 remains the dominant species but the excess TCNB effectively dilutes the emissive\ncomplex, resulting in a noticeably weakened green fluorescence.” “The preparation method is\nshown in Figure 1a, MA and TCNB were co-crystalized in a 1:1 molar ratio in acetonitrile\nsolution, yielding yellow MT1 crystals with yellow fluorescence peaked at 537 nm. In contrast,\nMT2 crystals formed under identical conditions from MA and TCNB at a 1:2 molar ratio\nemitted green fluorescence peaked at 505 nm. Both single crystals MT1 and MT2 exhibit\nfluorescence with a significant redshift relative to the monomer (Figure 1b-1e).” In addition,\nwe have added the corresponding Figures (Figure 1a; Figure 6a).\nFigure 1. Fluorescence spectra and molecular packing structures of MT1 and MT2. (a)\nSchematic diagram of the preparation process for MT1 and MT2. Fluorescence spectra of (b)\nMA, (c) TCNB, (d) MT1, and (e) MT2. Insert: the corresponding fluorescence photographs.\nIntermolecular - stacking of (f) MT1 and (g) MT2. Molecular packing structures of (h) MT1\nand (i) MT2 viewed along the crystallographic a-axis.\nFigure 6. Applications of dichromatic cocrystals in impurity analysis. (a) Photographs and\nschematic diagrams illustrating the action mechanisms of S-NAP/MA/TCNB complexes with\ndifferent TCNB ratios under UV irradiation. (b) Fluorescence spectra of S-NAP/MA/TCNB\ncomplexes at 0%, 0.05%, 0.1%, 0.2%, 0.5% and 1% MA ratio. (c) Selectivity test against\nrepresentative coexisting molecules in 1 equivalent ratio with MA. (d) Anti-interference\ncapacity test by adding 1 equivalent coexisting molecule in MA group in (c).\nComment 9: The annotations in Figures 7a and 7b appear inconsistent or incorrect.\nFurthermore, for the several molecules in Figure 7a that interact with TCNB, the \"obvious\nredshift\" described by the authors is not intuitively demonstrated. It is suggested that the authors\nprovide a side-by-side comparison of the fluorescence images of these molecules before and\nafter mixing with TCNB. This would allow readers to immediately comprehend the spectral\nshift.\nReply: Thanks for the constructive comment from the reviewer. In response, we have added\nside-by-side fluorescence images of these molecules before and after mixing with TCNB in the\nrevised Figure 7a. The corresponding figure captions have been revised accordingly. This direct\ncomparison now clearly illustrates the fluorescence changes and allows readers to more readily\nappreciate the redshift behavior upon interaction with TCNB.\nAccording to the reviewer’s suggestions, we have updated the corresponding Figure (Figure\n7a).\nFigure 7. (a) Photographs of NAP-Me and NAP-Pr before and after interaction with TCNB.\nFluorescence spectra of (b) NAP-Me/TCNB and (c) NAP-Pr/TCNB (all complexes are formed\nby the interaction of monomers with a molar ratio of 1:1). (d) Schematic diagram of the\nmonitoring of transesterification between S-NAP-Me and PHBA-Pr. Fluorescence spectra of\nPHBA-Pr/TCNB, (PHBA-Pr+S-NAP-Me)/TCNB, and S-NAP-Me/TCNB (e) before and (f)\nafter ammonia fumigation. Mass spectrometry data of (g) S-NAP-Pr and (h) (S-NAP-\nMe+PHBA-Pr)/TCNB after ammonia fumigation.\nReviewer 3\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This\nis part of the Nature Communications initiative to facilitate training in peer review and to\nprovide appropriate recognition for Early Career Researchers who co-review manuscripts.\nReply: We sincerely thank the reviewer for the constructive comments and valuable\nsuggestions. Their careful evaluation and insightful feedback have been very helpful in\nimproving the quality and clarity of our manuscript. We have carefully considered all the\ncomments of the other reviewers and have revised the manuscript accordingly. We believe that\nthese revisions have strengthened the scientific rigor and presentation of our work.\nReviewer 4\nThe authors exploited the fluorescence of molecular complexes to trace solid state reactions.\nFurthermore, they exploited the results to monitor both impurities and a transesterification\nreaction. Crystallographic analysis and spectroscopic data support the conclusions of the work\nand the methodology is in general clearly described. Some more details about Gibbs free energy\ncalculations should be given.\nReply: We sincerely thank the reviewer for their positive assessment. We also greatly\nappreciate the reviewer for the very careful review, and our manuscript has been improved\naccording to the reviewer’s suggestion, supplementing the calculation conditions in the\nsupporting information. Test conditions were at a temperature of 298.15 K and a pressure of 1\nstandard atmosphere (1 atm).\nAccording to the reviewer’s suggestions, we have revised the Supporting Information by adding\nthe following description into the details about Gibbs free energy calculations: “At a\ntemperature of 298.15 K and a pressure of 1 standard atmosphere (1 atm).”\nComment 1: Some references on the crystallography of naproxen and TCNB co-crystal\npreparation are important to properly place the manuscript within the available literature. For\ninstance:\nJang, Soeun Et al. effects of some polymeric additives on the cocrystallization of naproxen and\nnicotinamide via freeze drying Journal of Crystal GrowthArticle2024 DOI:\n10.1016/j.jcrysgro.2023.127476\nErcicek, Fatma et al., Naproxen-bipyridine cocrystallization assisted by pressurized carbon\ndioxide Journal of Supercritical FluidsArticleOpen Access2023, DOI: 10.1016/j.supflu.2023.105976\nMirocki, Artur Et al., Exploring the molecular landscape of multicomponent crystals formed by\nnaproxen drug and acridines CrystEngCommArticleOpen Access2022 DOI: 10.1039/d2ce00890d\nIn the last one, the authors highlighted that different preparation conditions allowed obtaining\ndifferent polymorph. Several other work of the undersigned reviewer could be of interest for\nthe authors concerning solid state reactions of both TCNB, Naproxen and parent compounds.\nReply: We thank the reviewer for the careful evaluation and for recommending these specific\nreferences. The studies by Jang et al. (J. Cryst. GrowthArticle, 2024), Ercicek et al. (J. Supercrit.\nFluidsArticleOpen, 2023), and Mirocki et al. (CrystEngCommArticleOpen, 2022) provide\ndirectly relevant methodological guidance for our work. In particular, the approaches used to\ninvestigate naproxen cocrystallization—such as freeze-drying-assisted crystallization,\npressurized CO₂-assisted cocrystal formation, and structural characterization of\nmulticomponent crystal systems—offer concrete examples of how different crystallization\nconditions and analytical techniques can be applied to elucidate cocrystal formation\nmechanisms. These methodologies have been of considerable assistance in describing and\nanalysing crystallisation processes within our research. Following the reviewer’s suggestion,\nwe have carefully reviewed these publications and incorporated them as new references in the\nrevised manuscript.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing references:\n“[15] A. Mirocki, M. Lopresti, L. Palin, E. Conterosito, A. Sikorski, M. Milanesio,\nCrystEngComm 2022, 24, 6839-6853.\n[16] S. Jang, J. H. Park, I. W. Kim, J. Cryst. Growth 2023, 626, 127476.\n[17] F. Ercicek, C. Harscoat-Schiavo, P. Layrisse, M. Marchivie, Y. Cartigny, C. Brandel, T.\nTassaing, M. Conrad, S. Marre, P. Subra-Paternault, J. Supercrit. Fluids 2023, 200, 105976.”\nComment 2: Line186 “85oC” must be “85°C”\nReply: We sincerely thank the reviewer for identifying the error in Line186. The correction has\nbeen implemented in the revised manuscript.\nComment 3: Line 128 (and elsewhere) “cm-1” should be “cm-1”\nReply: We are grateful to the reviewer for highlighting the error in Line 128. The mistakes have\nbeen revised and the updated version is included in the revised manuscript.\nComment 4: Line 159 “SNAP/TCNB was found to exhibit the superposition of the peaks of\nthese two components.”: if his means “no reaction occurred”, it should be written more clearly\nReply: Thank you for the reviewer’s comment. We agree that the original wording is\nambiguous. We intended to indicate that no new phase/ICT complex was formed under this\ncondition and that the measured spectrum can be explained as a simple combination of the two\nindividual components.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “The PXRD patterns of S-NAP/TCNB show no additional bands or peak\nshifts, indicating that no ICT complex is formed.”\nComment 5: Figure 4b should be described in more details explaining what x and y are. Also\nthe “CIE” acronym in the caption should be detailed.\nReply: We thank the referee for catching this. The caption for Figure 4b has been supplemented.\nAccording to the reviewer’s suggestions, we have revised the manuscript by adding the\nfollowing description: “(b) CIE (Commission Internationale de l’Éclairage) chromaticity\ndiagram (x and y represent the CIE 1931 chromaticity coordinates (dimensionless). These\ncoordinates describe the perceived color of the luminescence and are calculated from the\nmeasured emission spectra using the standard CIE color-matching functions. After converting\nthe spectrum to tristimulus values X, Y, and Z, the chromaticity coordinates are obtained as x =\nX/(X+Y+Z) and y = Y/(X+Y+Z).”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "After the first round of revision, the authors have effectively resolved all the questions and suggestions put forward in the previous review, and I recommend accepting this manuscript.", "reviewer_2": "The manuscript titled \"Visualizing Molecular Diffusion Direction and Processes in the Solid State via Dichromatic Fluorescent Cocrystallization Transformation\" presents a creative approach to visualizing molecular motion in solid-state systems. The authors have thoroughly addressed the previous reviewers' concerns through additional experimental evidence. I believe this work will have a significant impact in the fields of molecular sensing and dynamic reaction monitoring. I recommend the paper for publication.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "The authors revised the manuscript according to reviewers' suggestions, and can be accepted for publication"}, "author_response": ""}]

Decision letter