Hierarchical single-crystal-to-single-crystal transformations of a monomer to a 1D-polymer and then to a 2D-polymer | Nature Communications

Nature Communications volume 15, Article number: 6638 (2024) Cite this article

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Designing and synthesizing flawless two-dimensional polymers (2D-Ps) via meticulous molecular preorganization presents an intriguing yet challenging frontier in research. We report here the single-crystal-to-single-crystal (SCSC) synthesis of a 2D-P via thermally induced topochemical azide-alkyne cycloaddition (TAAC) reaction. A designed monomer incorporating two azide and two alkyne units is synthesized. The azide and alkyne groups are preorganized in the monomer crystal in reactive geometries for polymerizations in two orthogonal directions. On heating, the polymerizations proceed in a hierarchical manner; at first, the monomer reacts regiospecifically in a SCSC fashion to form a 1,5-triazolyl-linked 1D polymer (1D-P), which upon further heating undergoes another SCSC polymerization to a 2D-P through a second regiospecific TAAC reaction forming 1,4-triazolyl-linkages. Two different linkages in orthogonal directions make this an architecturally attractive 2D-P, as determined, at atomic resolution, by single-crystal X-ray diffraction. The 2D-P reported here is thermally stable in view of the robust triazole-linkages and can be exfoliated as 2D-sheets.

Two-dimensional polymers (2D-Ps) are compelling class of materials, which are of great interest owing to their precise layered structure that confers interesting chemical, physical, and mechanical properties1,2,3,4,5,6,7. These interesting materials can find potential applications in optoelectronics, catalysis, molecular separation, desalination of water, sensing and energy storage and conversion8,9,10,11. While organic chemists have become adept at designing and synthesizing linear polymers of complex topologies12,13,14,15, guiding the desired polymerization in two orthogonal directions poses a significant challenge, the major obstacle being the entropy cost associated with the dimensionality increase16. On-surface/interfacial polymerizations present a possible route towards the synthesis of 2D-Ps17,18,19. Yet the key challenges are in the scalability, isolating the polymer sheets and structure elucidation20,21. It is noteworthy that the use of techniques such as scanning tunneling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS) for the structural analysis of monolayered 2D-Ps addresses some of these challenges17,22,23,24. Though there has been a surge of interest in covalent organic frameworks within the realm of organic 2D-materials25,26,27,28,29,30, their dynamic covalent bonds make them labile and often lead to topological imperfections31,32,33.

Recently, topochemical polymerization has been elegantly utilized to synthesize crystalline 2D-Ps, which holds advantages over other polymerization methods in producing flawlessly structured molecular sheets34,35,36,37,38,39,40,41. In topochemical polymerization, monomer molecules pre-organized with proximally placed reactive groups in crystals react to yield ordered polymers upon suitable stimuli42,43. In this approach, the transformation of monomer to polymer often occurs in a single-crystal-to-single-crystal (SCSC) fashion, which is advantageous for the accurate structure determination of the polymer at atomic resolution, by single-crystal X-ray diffraction44,45,46,47. King, Sakamoto and Schluter pioneered the topochemical synthesis of 2D polymers employing photo-induced cycloaddition reactions34,37. Later, King and Schluter achieved SCSC synthesis of 2D polymers, via photochemical [4 + 4] cycloaddition of anthracene-based monomers, whose structures were resolved by SCXRD analysis35,36. In addition, SCSC synthesis of three different 2D polymers by employing photo-induced [2 + 2] cycloaddition reaction have been reported38,39,40. Apart from the possible structural elucidation at atomic resolution, the SCSC nature of these polymerizations is helpful in proposing mechanisms for polymer growth48,49. However, all these reported topochemical synthesis of 2D-Ps involve the use of light-induced cycloaddition reactions and the growth of this field has been rather slow. The primary challenge here is the stringent requirement of ready-to-react monomer packing for photo-induced topochemical reactions. Additionally, the cycloadducts formed in these photo-induced reactions undergo retro-cycloaddition leading to depolymerization upon heating. While such thermal depolymerization is unarguably advantageous for recycling the polymer, it poses a potential hindrance in utilizing these polymers for high-temperature applications.

In order to increase the versatility of topochemical 2D-polymerization, it is essential to explore different topochemical reactions for the synthesis of 2D-Ps. Heat-induced topochemical azide-alkyne cycloaddition (TAAC) reaction has been widely used for 1D-polymer synthesis and in many cases, the geometrical criteria are not as stringent as in photochemical reactions in view of the thermal motion50,51. Here we report the synthesis of a thermally irreversible, robust and crystalline 2D-P employing the TAAC reaction, which involve sequential, hierarchical and regiospecific SCSC transformations of a monomer (M) to a 1D-polymer (1D-P) having 1,5-triazolyl-linkages and then to a 2D-polymer (2D-P) containing 1,5- and 1,4-triazolyl-linkages in orthogonal directions. Our results show that the conceivable reactions for topochemical 2D-polymerization are beyond light-induced cycloadditions and 2D-polymerizations need not adopt a simultaneous lateral growth always.

For topochemical 2D polymerization, a monomer is designed such that its molecules tile in two dimensions, in the crystal, positioning their reactive functional groups in close proximity, which allow polymerizations along these two dimensions when subjected to suitable stimuli. The 2D-Ps synthesized so far via topochemical means involved light-induced cycloaddition reactions and the resultant polymer undergoes thermal depolymerization when heated. Thus, one aspect of our design is to use a different chemistry that makes robust linkage that is stable even at high temperatures. TAAC reaction meets this criterion as this thermal cycloaddition reaction produces stable triazole linkages52. Depending upon the alignment of azide and alkyne groups in the crystal lattice the triazole linkage can be either 1,4-disubstituted or 1,5-disubstituted (Fig. 1a). Interestingly, TAAC reaction often proceeds in SCSC manner yielding products that can be characterized by single crystal XRD analysis53,54,55. Another feature we considered for the design is the architectural diversity. All the monomers employed so far for successful topochemical SCSC 2D polymerization are of tripodal geometry with C3 symmetric nodes which generate polymers having hexagonal cavities. In order to generate architecturally different 2D-Ps, we planned to use a structurally different monomer.

a Azide and alkyne groups oriented in antiparallel and parallel manner to form 1,4-disubstituted-1,2,3-triazole and 1,5-disubstituted-1,2,3-triazole respectively. b Chemical structure of monomer M. c Optical microscopy image of monomer M crystal. d Conformer A and conformer B depicting intramolecular hydrogen bonding interactions. Carbon atoms of different conformers are color coded for clarity.

Considering the above points, we designed and synthesized an X-shaped monomer M with a benzene core decorated with two azide groups and two alkyne groups (Fig. 1b, Supplementary Fig. 1). We anticipated that two thermally induced topochemical 1,3-dipolar cycloaddition of azide and alkyne units in orthogonal directions would result in a triazole-linked 2D-P. We obtained good quality long plate-like single crystals of the monomer from a saturated solution of methanol-isopropanol (10:1 v/v) or chloroform-isopropanol (2:1 v/v) via slow evaporation of the solvent at ambient conditions (Fig. 1c). We determined the crystal structure of the monomer using single-crystal X-ray diffraction (SCXRD). Monomer M crystallized in triclinic crystal system and adopted P−1 space group with two half molecules in the asymmetric unit (Supplementary Fig. 2). Intramolecular N–H…O and C–H…O hydrogen bonding lock the conformations of the four arms containing the reactive functional groups (Fig. 1d). The two conformers, A and B, of the monomer are alternately stacked via π…π interactions to form columns along a axis, which is additionally supported by two N–H…O and two C–H…O hydrogen bonding interactions (Fig. 2a, Supplementary Fig. 3). In each column, two azide units of each conformer A are parallelly and proximally arranged to one alkyne unit each of the two neighboring B-conformers (Fig. 2b). Based on Schmidt’s criteria56 for topochemical reactions, this constitutes an ideal arrangement for the TAAC polymerization to form 1D polymer having 1,5-triazolyl-linkage along a axis. The columns are arranged such that conformers A and B are self-sorted along b and c axes (Fig. 2c). Along c axis, the molecules translate by means of C–H…O interactions (Supplementary Fig. 4), however, none of the azide-alkyne pairs are in a reactive geometry. Along b axis, the molecules are connected by weak van der Waals contacts. The alkyne units of each conformer A and one azide group each of the two adjacent B-conformers are oriented in a perpendicular fashion with the end-to-end distances being 3.8 Å and 4.8 Å. Although the distance and geometry do not satisfy the Schmidt’s criteria for topochemical reactions, there are several examples of topochemical reactions via thermal molecular motions inside the crystal and the consequent transient attainment of distance and orientation suitable for reaction57. The crystal packing in monomer M suggests the possible rotation of the alkynyl group and the attainment of an antiparallel orientation and close proximity suitable for their TAAC reaction to form polymer having 1,4-triazolyl-linkage along b axis, at elevated temperatures. Thus, monomer M is expected to undergo polymerizations in two orthogonal directions; an easy reaction along a axis and a strenuous one along b axis.

a N–H…O hydrogen bonded column formed along crystallographic a axis. b Azide and alkyne groups arranged in a ready-to-react fashion along a axis. c Crystal packing of monomer M in the bc plane. d DSC thermograms of the monomer crystals, crystals heated till 160 °C and crystals heated till 210 °C (5 °C/min heating rate). e FT-IR spectrum of the monomer crystals, crystals heated till 160 °C and crystals heated till 210 °C.

The monomer crystals did not melt till 300 °C, but decomposed upon further heating. DSC thermogram of the crystals revealed three distinct exothermic peaks with onset temperatures around 110, 190, and 350 °C (Fig. 2d), wherein the last peak is due to the decomposition as confirmed from the thermogravimetric analysis (TGA, Supplementary Fig. 5a). The first two exothermic peaks could be due to the heat released during the TAAC reactions happening plausibly in a step-wise manner. To have a better understanding, we heated a few crystals till 160 °C, a temperature above the first exothermic event, and then cooled to room temperature and recorded its DSC profile again (Fig. 2d). Interestingly, the first exothermic peak has completely disappeared indicating the completion of the first reaction. FT-IR spectrum of this partially heated crystals showed a reduction in the intensity of azide stretching compared to the monomer, giving further evidence for the consumption of some of the azide groups during the first exothermic event (Fig. 2e). DSC profile of crystals that were pre-heated till 210 °C, a temperature above the second exothermic event, and cooled to room temperature showed complete disappearance of the first two exothermic peaks (Fig. 2d). Its FT-IR spectrum showed diminished azide stretching intensity to a further unchangeable level suggesting the reaction completion (Fig. 2e). The small residual azide peak seen in the FT-IR spectrum could be due to the presence of terminal azide groups at the edges of the 2D-polymer. Thus, DSC and FT-IR studies testified our hypothesis of step-wise TAAC polymerizations.

We followed the reactions at different heating rates via DSC analysis (Supplementary Fig. 6) and observed that the two thermal events are well resolved at a heating rate of 2 °C/min. Also, slow heating avoids rigorous reaction and thereby preserves the single crystal integrity. Hence, we heated the crystals of the monomer M at a heating rate of 2 °C/min. till 160 °C and cooled to room temperature. After the heating-cooling cycle, crystallinity was unaffected as evidenced from the birefringence pattern when viewed under a polarizing microscope (Fig. 3a) We have solved the crystal structure of this pre-heated and cooled crystal by SCXRD (Supplementary Fig. 7). The monomer M has undergone SCSC polymerization to a 1,5-triazolyl-linked 1D polymer (1D-P) along a axis as anticipated (Fig. 3b, c). The 1D-P crystal retained the P−1 space group. The polymerization resulted in the contraction of unit cell parameter a by 1.6% and expansion of the parameter c by 0.8%, while the parameter b remained more or less unchanged. The polymer chains adopted a zigzag ladder-like conformation. This secondary structure of the 1D-P is stabilized by intramolecular N–H…O hydrogen bonding within the polymer chain. Along b and c axes, the polymer chains are connected by weak van der Waals contacts (Fig. 3d). The unreacted azide and alkyne units are neither parallel nor proximal (centroid separation of 4.5 Å). However, void analysis revealed that large voids are present near the azide and alkyne groups, which can provide sufficient space for their rotation to attain a favorable geometry for the TAAC reaction (Supplementary Fig. 8). Rotation of azide and alkyne groups around C–N (162°) and C–C (96°) bonds respectively results in their antiparallel orientation with end-to-end distances of 3.8 Å and 3.9 Å (Fig. 3e), suitable for TAAC reaction to form 1,4-triazolyl-linkage. Thus, we anticipate that the 1D-P may undergo further polymerization along c axis to give a covalently linked 2D polymer that propagates in two orthogonal directions.

a Optical microscopy image of monomer M crystal heated till 160 °C (1D-P). b Propagation of the 1D polymer (1D-P) along a axis. N–H…O hydrogen bonding within the chain is shown. c View of a single column of 1D-P along a axis. d Crystal packing of 1D-P along bc plane. e Rotation of azide and alkyne groups in the crystal lattice to achieve a parallel orientation conducive to TAAC reaction.

DSC of the 1D-P showed an exothermic peak with onset temperature 190 °C, suggesting a second TAAC reaction. We heated a few 1D-P crystals till 210 °C at a heating rate of 2 °C/min. and then cooled to room temperature. The heated crystals maintained the birefringence indicating the preservation of crystallinity (Fig. 4a, Supplementary Fig. 9). SCXRD analysis of this heated-and-cooled crystal revealed a successful SCSC polymerization of the 1D-P to a 2D-P (Fig. 4b, Supplementary Fig. 10). TAAC reaction happened along c axis forming 1,4-triazolyl-linkage between the 1D chains, as expected from the 1D-P crystal packing. The 2D-P crystal also has retained the P−1 space group. The second polymerization results in a significant contraction of unit cell parameters c (11.8%) and b (10.7%) and expansion of the parameter a (8.6%). Fascinatingly, the polymer translates along a axis via 1,5-triazolyl-linkage, while along c axis the propagation is by means of 1,4-triazolyl-linkage. Thus, we achieved topochemical polymerization in two orthogonal directions via chemically different linkages. The polymer 2D sheets are connected only via weak van der Waals contacts along b axis, to form the 3D crystal (Fig. 4c). A single 2D-P sheet along ac plane is depicted in Fig. 4d.

a Microscopic image of 1D-P crystal heated till 210 °C. b Crystal packing of 2D-P along bc plane. c Crystal packing of 2D-P along ac plane depicting 2D sheets stacked along b axis. d Single 2D-P sheet along ac plane.

2D-P crystals can also be obtained by continuously heating the monomer crystals till 210 °C at a heating rate of 2 °C/min. The crystal structure of such 2D-Ps produced via continuous heating the monomer is indistinguishable from that of 2D-P obtained by heating the 1D-P crystals (Supplementary Table 1). In order to understand the temperature dependency of the polymerizations we followed the reactions at different temperatures using time-dependent DSC analysis (Supplementary Figs. 11 and 12). We found that at higher temperature, higher is the reaction rate. At 90 °C, the M to 1D-P polymerization completes in 5 h whereas at 100 °C, it takes only 2 h, as evidenced by the disappearance of the first exothermic peak. Similarly, 1D-P to 2D-P polymerization completes in 4 h at 140 °C and 2 h at 150 °C, as judged from the disappearance of the second exothermic peak. To get further insights about these polymerizations, we recorded time-dependent PXRD at 90 °C and 140 °C, respectively, for the first and second polymerizations (Supplementary Fig. 13). PXRD pattern of the monomer heated at 90 °C for 5 h was indistinguishable from the simulated PXRD pattern of 1D-P, suggesting the complete polymerization of M to 1D-P and in agreement with the time-dependent DSC experiment. Similarly, PXRD pattern of 1D-P heated at 140 °C for 4 h matched with the simulated PXRD pattern from 2D-P. Variable-temperature Raman spectroscopy of a single crystal of monomer M heated at a rate of 5 °C/min. from rt to 200 °C showed gradual reduction in the intensities of the characteristic alkyne stretching signal at 2130 cm−1, which completely vanished at 200 °C suggesting complete polymerization (Supplementary Fig. 14).

Mechanistically, 2D-polymerization in crystals is intriguing. Schluter et al. analyzed the plausible scenarios during the early stages of light-induced topochemical 2D-polymerization reactions and proposed three different mechanisms. When the reaction between two monomer molecules enhances the reactivity of the neighboring monomers, that would lead to the propagation of polymerization from one site to the surrounding areas of the crystal, analogous to the chain-growth mechanism in conventional polymerization; when the reaction inhibits the reactivity of neighboring monomers, that would follow a step-growth type mechanism; when the reacted monomers do not alter the reactivity of the neighboring monomers, it leads to a random polymerization58,59,60. In all these three cases, at the intermediate stages, the monomers, dimers, 1D-oligomers/polymers and 2D polymers co-exist and the growth happens simultaneously along two dimensions (2D plane). In this context, the thermal 2D-polymerization reported here is mechanistically different and interesting as it involves two discrete steps for the 2D polymerization, at no stage the monomer M co-exists with the 2D-P. Such hierarchical polymerization involving SCSC polymerization of a monomer to 1D-P and its subsequent SCSC polymerization to a 2D-P is intriguing. Clearly, the polymerization does not follow any of the three proposed mechanisms. As at both stages, polymerization proceeds only along a particular direction, the propagation has more resemblance with topochemical 1D-polymerizations, which mostly follow a cooperative mechanism61. However, the insolubility of the reaction mixture (oligomers/1D-P and 2D-P) prevented us from any kinetic studies.

Topochemical synthesis of 2D-Ps is advantageous as it offers almost error-free 2D-polymers and its structure can be determined at atomic resolution by SCXRD analysis. However, for most practical applications, such polymers are required as sheets than 3D crystals. Hence the topochemically synthesized 2D-P should offer the possibility of exfoliation. In the crystals, polymer sheets are formed parallel to ac plane and sheets are connected along crystallographic b axis through weak interactions suggesting that the sheets may be exfoliated easily. We matched the experimentally indexed crystal faces with the simulated BFDH morphology (Fig. 5a, Supplementary Fig. 15). We found that the largest rectangular faces of the crystal are (001/00−1) planes, which runs parallel to ab plane and the second largest face is (010/0−10) plane, which is parallel to the crystallographic ac plane containing the 2D-P sheet. Thus, it is expected that 2D sheets can be exfoliated from (010/0−10) face.

a Calculated BFDH morphology depicting the 2D-P sheet aligned along (0−10) face. b, c Microscopic images of exfoliating crystals. d, e SEM images of the exfoliated and exfoliating crystals. f AFM images and height profiles of exfoliated 2D-P crystals. g TEM images of exfoliating/exfoliated crystals.

While exfoliation of 2D layers from crystals can be done either through physical delamination using scotch-tape method or through chemical exfoliation using solvents. We have resorted to the solvent-mediated exfoliation as it is scalable. We attempted solvent-exfoliation using NMP (N-methylpyrrolidone) and TFA (Trifluoroacetic acid) which are common solvents known to induce delamination of 2D-layers62,63. We could not exfoliate the layers even after stirring the crystals in NMP at room temperature or at 50 °C for one month (Supplementary Fig. 16). Also, sonication (2 hours) of the crystals of 2D-P in NMP did not lead to exfoliation. We have also soaked 2D-P crystals in common solvents namely DMSO, DMF, chloroform, acetone, isopropyl alcohol, ethanol, and methanol for 12 h, but none of them could exfoliate the layers (see Supplementary Discussion 10) and their crystallinities were intact as confirmed by PXRD (Supplementary Fig. 17a). On the other hand, when the crystals were exposed to TFA, they underwent swelling within a few seconds as observed with naked eyes. Optical microscopy images revealed delamination of the 2D-P crystals to long thin 2D sheets (Fig. 5b, c, Supplementary Fig. 18). Zhao et al. had demonstrated acid-induced exfoliation of a 2D-P containing a triazine core39. We also expect a similar mechanism; strong acid such as TFA can protonate triazole nitrogen and repulsion between positively charged triazolium ions could help in the delamination of the sheets. To validate this, we have demonstrated delamination of the crystals using 1N HCl as well (Supplementary Fig. 19). As expected, the exfoliation occurred from the (010/0−10) plane. The delaminated thin layers maintain birefringence suggesting that the crystallinity is intact. SEM images of the drop-casted suspension showed very thin 2D sheets. The SEM images of the residues showed partially peeled or peeling crystals having thin layers. (Fig. 5d, e, Supplementary Fig. 20). Atomic Force microscopy (AFM) showed delaminated sheets of thickness ranging from ~1 nm to 4 nm (Fig. 5f, Supplementary Fig. 21). From the crystal structure analysis, it is clear that a single layer of the 2D-P possesses an average thickness of approximately 1 nm. This suggests that the exfoliation produces 2D-P sheets of single to a few layer thickness. TEM images also revealed exfoliated thin 2D sheets (Fig. 5g, Supplementary Fig. 22).

In conclusion, the topochemical 2D polymerizations known so far employ light-driven cycloaddition reactions that produce polymers that are unstable at high temperatures. We detailed here a thermally induced SCSC topochemical 2D polymerization that produces robust triazole-linkages that are stable at high temperature. A designed benzene-derived monomer anchored with two azide-alkyne pairs underwent two TAAC polymerizations in a step-wise and hierarchical manner. The monomer crystals on heating regiospecifically reacts first to form a 1D-P in SCSC manner, which on further heating progresses to yield a 2D-P, again through another regiospecific SCSC transformation. In view of the different regiospecificity in the two consecutive reactions, the repeating units in the 2D-P are connected by means of two different linkages in orthogonal directions; namely 1,5-triazolyl-linkage and 1,4-triazolyl-linkage. While the known 2D polymers have hexagonal pores in view of the tripodal repeating units, we show that the size and shape of the pore can be tuned by employing architecturally different monomers and different chemistry. TFA facilitates exfoliation of the 2D-P crystals into thin crystalline 2D sheets. Exploiting a hitherto unexplored linkage-chemistry and architecturally different monomers for the topochemical synthesis of 2D-Ps holds great potentials for tuning the topology, pore-size and properties. We hope this report will encourage researchers to venture into such areas.

Monomer crystals were placed on a glass slide and heated at a rate of 2 °C/min. from rt to 160 °C to obtain 1D-P. The crystals were held for additional 10 minutes at 160 °C to ensure reaction completion and then cooled to rt.

For obtaining the 2D-P, monomer crystals placed on a glass slide, were heated at a rate of 2 °C/min. from rt to 210 °C. The crystals were held for additional 10 minutes at 210 °C to ensure reaction completion and cooled to rt.

1D-P crystals placed on a glass slide, were heated at a rate of 2 °C/min. from rt to 210 °C. The crystals were held for additional 10 minutes at 210 °C to ensure reaction completion and cooled to rt.

Around 1 mg of the 2D-P crystals were suspended in 2 mL TFA and sonicated for 5 min. The sample was drop-casted on a silicon wafer and dried for SEM analysis. AFM analysis: 0.5 mg of the 2D-P crystals were suspended in 1 mL TFA and sonicated (bath sonication) for 1 h at rt. It was then centrifuged at 5000 rpm for 10 min and the supernatant was diluted with chloroform (1:9) and drop-casted on a silicon wafer, which was then dried under air for analysis. For TEM imaging, 4 μL of the same sample was drop-casted on a 400-mesh carbon-coated copper grid (42 μm hole size). The grid was allowed to dry well under air.

All other experimental methods are detailed in the Supplementary Information.

All data generated or analyzed in this work are available in the manuscript or in the supplementary materials. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers 2332792 (M), 2332791 (1D-P), and 2332793&2337070 (2D-P). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. All data are available from the corresponding authors upon request. Source data are provided with this paper.

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K.M.S. acknowledges support from SERB (CRG/568/2022) and J.C. Bose National Fellowship (JCB/2023/000039). H.B. thanks the Department of Science and Technology (DST), India for the INSPIRE fellowship. We thank Mr. Livin Paul for helping with Raman data acquisition.

School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Vithura, Thiruvananthapuram, 695551, India

Haripriya Balan & Kana M. Sureshan

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K.M.S. conceived and supervised the project. H.B. performed the experiments. H.B. and K.M.S. together analyzed the results and wrote the manuscript.

Correspondence to Kana M. Sureshan.

The authors declare no competing interests.

Nature Communications thanks Yingjie Zhao and the other, anonymous, reviewers for their contribution to the peer review of this work. A peer review file is available.

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Balan, H., Sureshan, K.M. Hierarchical single-crystal-to-single-crystal transformations of a monomer to a 1D-polymer and then to a 2D-polymer. Nat Commun 15, 6638 (2024). https://doi.org/10.1038/s41467-024-51051-z

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Received: 23 April 2024

Accepted: 26 July 2024

Published: 05 August 2024

DOI: https://doi.org/10.1038/s41467-024-51051-z

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