Fast self-healing in a layered molecular crystal mediated by stress-induced symmetry breaking

Domain: neuroscience
Published: 2026-02-09
Editorial outcome: minor_revision

Abstract
In recent years, symmetry-breaking has emerged as a powerful tool for significantly altering various physical properties in 2D layered materials. However, the breaking of symmetry by means of mechanical stress in organic crystals remains elusive. Here, we demonstrate a simple approach to engineer symmetry-breaking through mechanical stress fields in a layered molecular crystal, resulting in autonomous and fast self-healing under ambient temperature and pressure conditions. Fracture mechanics analysis reveals that the crystal adheres to an elasto-plastic model, with formation of a plastic zone at the crack tip, which prevents further crack propagation, facilitating the self-healing process. Spatially resolved Raman mapping reveals that the crack formation is accompanied by a distinct symmetry-breaking mechanism at the microstructural level. A six-fold increase in non-linear second harmonic (SH) activity, triggered by mechanical perturbation, further validates the local symmetry breaking in an otherwise centrosymmetric crystal. Furthermore, symmetry is restored following successful healing, as evidenced by the disappearance of the SH signal in the healed regions. This study not only broadens the scope of self-healing mechanisms viable in molecular materials but also offers key insights into the role of symmetry breaking and its potential for related technological applications.

Introduction
Symmetry in crystalline solids plays a critical role in determining their physical properties. Breaking of symmetry has been shown to impart exotic properties in inorganic materials, like increased ferroelectricity1, unconventional superconductivity2, and ultralow thermal conductivity3. Most symmetry-breaking events reported in literature are spontaneous, whereby an initially symmetric system becomes asymmetric during a spontaneous process like crystallization4 or a phase transition5. The other possibility, where the asymmetry is introduced explicitly through electric fields, optical excitations or mechanical strain, is more challenging to adapt6. Hence, to date, the potential implications of symmetry breaking, particularly in the context of molecular materials, remain poorly investigated.

Organic molecular crystals offer unique advantages like easy solution processing, long-range order, extensive functionalization possibilities and generally good biocompatibility, but their utility in devices is currently very limited as they are frequently prone to vertical stress-induced mechanical failure, cracks and abrupt reduction in performance at times7. Imparting self-healing ability can be a promising way to improve the mechanical robustness of molecular crystals, as this can help in repairing any eventual mechanical damage8. However, currently there is a lack of understanding of the underlying mechanisms and the factors that promote self-healing in crystalline materials, making it extremely challenging to design this property in building sustainable devices. Currently known self-healing approaches rely primarily on some form of external stimuli such as local heating9, aqueous media10, light11, compressive forces12 or combinations of them13 to trigger the healing process. This greatly limits their use, particularly in applications where external intervention is undesirable or impossible. To enable autonomous self-healing, typical strategies utilized in polymers, hydrogels and composites make use of either cross-linking14 or healing agents which are released in a controlled manner15,16 to repair the damaged areas. Such techniques prove inadequate for employing in crystalline materials, where reestablishing the complete long-range order at the molecular level is imperative for maintaining structural integrity. Most research on crystalline hard materials has been confined to the healing of minute cracks at the nanoscale17,18. A few molecular crystals relying on mechanical softness and dynamic bonding have also been reported in recent years to show self-repairing capability12,19–21. Certain non-centrosymmetric crystals possessing polar structures and inherent piezoelectricity are reported to self-heal via electrostatic surface charge-based actuation22,23. Given the immense potential of molecular materials in various miniaturized devices24, there is an urgent need to identify the inherent mechanisms that promote self-healing, so that this exotic phenomenon can be better understood and designed to a much broader set of crystals, not restricted by symmetry considerations.

Herein, we report a centrosymmetric crystal that exhibits reversible, mechanically induced symmetry-breaking, leading to ultrafast self-healing of large cracks (Supplementary Table 1 and Supplementary Note 1). The molecular crystal of 2-methyl-4-nitroimidazole (MNI), featuring a layered architecture analogous to 2D van der Waals materials1, develops macro-cracks parallel to its layers that undergo instantaneous autonomous self-healing (Supplementary Video 1). The self-healing crystals have an elastic modulus of 13.3 ± 0.5 GPa on their major face, overcoming the trade-off between healing ability and inherent stiffness (Supplementary Table 2). Successful self-repair is characterized by high-resolution scanning electron microscopy (SEM), atomic force microscopy (AFM) and single-crystal X-ray diffraction experiments, indicating an excellent efficiency with respect to crystallinity. Raman spectroscopic mapping and second-harmonic (SH) imaging techniques unambiguously confirm that mechanical stress can lead to local symmetry breaking, indicating its role in subsequent self-healing. In-depth analysis from the lens of fracture mechanics provides some fundamental understanding of the mechanism of crack propagation and its arrest, which is correlated with the underlying crystal structure. Our study elucidates a correlation between strain, structure and symmetry (Supplementary Fig. 1), demonstrating that the paths to self-healing are many and are yet to be discovered.

Results
Evaluation of fast self-healing under mechanical stress
Single crystals of MNI grown as large plates (~5 mm × ~2 mm × ~100 µm size; Supplementary Fig. 2) were investigated for their self-healing behavior by application of vertical as well as lateral loads and their real-time responses recorded under a stereomicroscope, integrated with a high-speed imaging capability (see Supplementary Videos 2 and 3 and Supplementary Fig. 3). A vertical mechanical stress on the (010) crystal face using a metal pin initiates a hairline fracture propagating from the point of stress, hereafter the defect point. From a defect point, the crack propagates in the form of a linear elliptical crack under external stress (Fig. 1a–c). When the applied force is removed, the crack growth halts and begins to close autonomously via reverse propagation towards the defect point, leading to a rapid self-healing event (Fig. 1c). No external compression is required to facilitate the healing process. For instance, a crack of length ≈336 µm could repair itself autonomously in an ultrafast timescale of four milliseconds (Supplementary Fig. 3), which is impressive considering that the crack spreads approximately 90% of the entire width of the crystal. The cracks generated under lateral loads on the (011) face of MNI crystals using a metal tweezer also heal in a similar fashion upon withdrawal of the external stress (Supplementary Fig. 4 and Supplementary Video 3).

The self-repair of micron-sized cracks is examined using cross-polarized optical microscopy (Supplementary Fig. 4), SEM (Fig. 1d and Supplementary Fig. 5) and AFM (Supplementary Fig. 6), which demonstrate only surface features devoid of any scars after healing. Such crack closure could be observed repeatably under cyclic loading at the same damaged area (Supplementary Video 4). Figure 1e shows seven consecutive cycles of crack formation and subsequent self-repair in a single MNI crystal, where each healing cycle takes ≈4 ms. Examination of a healed region using single-crystal X-Ray diffraction (SCXRD) produces sharp diffraction spots, confirming single-crystalline nature in the healed region (Supplementary Fig. 7). The diffraction profile remains similar to that of an undeformed (hereafter, pristine) crystal (see Supplementary Tables 3 and 4). We note that the value of mosaicity from the pristine crystal is 0.63 and that from the healed crystal is 0.85, which might be due to partial loss of intensity, indicating that the crystal healed with minimal structural degradation. Retaining a high level of crystallinity in healed regions is highly challenging, and our system with respect to these parameters is on par with the best self-healing crystals reported in literature17–23.

Excessive stress resulting in the propagation of cracks to either end of the crystal generally leads to permanent cracks (hereafter, fractured crystals). Such crystals with visible crack lines, show sharp but doubly split diffraction peaks (Supplementary Fig. 8). To further confirm the true nature of the cracks (gaps that extend deep into the bulk material), we experimentally examined them using AFM (Supplementary Fig. 9). The AFM topography profile of vertically strained crystals shows linearly oriented cracks that are approximately 1.5 µm in depth and 3 µm in width. Furthermore, the MNI crystals were examined using X-ray microtomography (XMT) imaging (see “Methods” and Supplementary Fig. 10). The reconstructed modeling of the three-dimensional spatial distribution of voids in the XMT image confirmed a deep visible fissure, whereas no such fissure lines can be seen in pristine and healed regions of the crystals (Supplementary Note 2).

To quantify the threshold stress for observing self-healing of cracks in our MNI crystals, we fixed a pristine sample to a support at one end and applied a uniaxial stress using a commercially available force sensor (see “Methods”). As shown in Supplementary Fig. 11, a crystal, having a thickness of 97 µm and a width of 2 mm, could self-heal from cracks formed by an external load of up to 45 mN. We also tested the crystals by mounting them on specially designed push-to-pull chips and applied a three-point bending load (see “Methods”) in a commercial indenter, Intraspect 90. Samples of thickness ranging from approximately 50 µm to 150 µm showed the ability to withstand a stress of 11.5 to 13.5 MPa before their first fracture event (see Supplementary Fig. 12). Subsequently, we quantified the self-healing efficiency (Fsh) from these mechanical tests by defining it as Fsh = F/F0 × 100, where F and F0 represent the fracture stress of the pristine crystal and that of the healed crystal, respectively. The fracture behavior of pristine and healed samples under controlled mechanical loading is summarized in Supplementary Fig. 13, demonstrating a healing efficiency of ≈40% for a sample of thickness approximately 100 µm. This is impressive considering that these crystals maintain excellent crystallinity at the fractured region after the self-healing process. It is to be noted that the healing efficiency observed here is with respect to the mechanical strength of the crystals. However, these self-healing crystals maintain a higher level of crystallinity than crystals of other classes following diffusion or twinning-based healing mechanisms (see Supplementary Note 3).

Healing mechanisms observed in structural materials often involve toughening to arrest the crack propagation25. Moreover, stress dissipation in such materials occurs via crack deflection and/or crack branching26 and sometimes, the healing of microcracks is accompanied by subsequent crack resumption along a different path17. This behavior contrasts sharply with that of our MNI system, where the induced cracks propagate along a well-defined direction, usually without any deflection or branching (Supplementary Video 5 and Supplementary Fig. 14). Furthermore, healing of a crack does not subsequently lead to new cracks in a different direction. When fresh block-shaped crystals are fragmented into pieces by applying a three-point bending stress, they show a mild recombination tendency via attraction behavior (Supplementary Video 6 and Supplementary Fig. 15). Understandably, the actuation in these crystals is not as prominent as in the case of non-centrosymmetric piezoelectric crystals22,23,27. Hence, we can’t directly adapt the mechanisms proposed for the piezoelectric crystals22 or those with mechanical softness8, reported recently.

Correlation of fracture mechanics with crystal symmetry
According to Griffith’s theory, a crack or hairline fracture is defined as “a localized breakdown of bonds in any material initiated by a stress concentration”28. Fracture and eventual failure of a material follow three basic stages: crack initiation, crack propagation and eventual rupture (see Supplementary Note 4). To prevent rupture and enable efficient self-healing, the crack propagation has to be terminated in Stage II itself25. To investigate the fracture mechanics of the MNI system, high-resolution SEM was employed (Fig. 2a–f). Herein, application of an external compressive force using a metal pin of diameter ≈10 µm creates a defect point on the surface (Fig. 2a), essentially a highly localized stressed point28. In Stage II, the local strain manifests as ductile micro-tearing events and provides opportunity for further propagation, while the crack tip bears the ultimate strain gradient26. In most known cases, the crack will propagate with increasing widths (Fig. 2b) to cause ultimate rupture of the crystal (Fig. 2c). Perfectly sharp cracks imply infinite stress concentration, meaning a minute crack formed by any chance in an ideal brittle material will propagate infinitely, causing total rupture28. However, perfectly sharp cracks are rarely formed due to some amount of plastic deformation at the crack tip, causing blunting25. As evident from Fig. 2d–f, local yielding (plastic deformation) and stress dissipation occur at the crack tip, which arrests crack propagation in this material. Once the external load is withdrawn, the self-repairing process begins. The presence of a combination of dispersive and hydrogen-bonding interaction regions in the crystal structure of MNI seems to provide a degree of plasticity, which allows for elasto-plastic deformation at the crack tips marked by crack arrest, which is further confirmed from AFM imaging of the crack tips (Supplementary Fig. 9).

To find the structural origin of the well-defined crack propagation followed by self-repair in this material, we examined the crystal structure obtained using SCXRD, which depicts a monoclinic space group P2/n (see “Methods”). The structure consists of polar imidazole molecules forming centrosymmetric dimers via C(sp2)–H···O bonds, which are further connected by N–H···N bonds to form an extended 2D sheet (Fig. 2g–i). These 2D sheets are connected by relatively weaker C(sp3)–H···O hydrogen bonds along the (10-1) plane of the crystal (Fig. 2j). The hydrogen bonding interactions of varying strengths constitute a 3D crystal with layered packing framework (Supplementary Fig. 16). The layers are stacked with inversion symmetry, similar to van der Waals materials like graphene or h-BN1. The layered organization of the structure is further highlighted from the total intermolecular interaction energies calculated by B3LYP DFT functional using CrystalExplorer (see “Methods” and Supplementary Fig. 17). Interestingly, the crack propagation direction in the single crystals is parallel to the 2D layers, as confirmed from crystal face indexing and Bravais–Friedel–Donnay–Harker (BFDH) morphology deduction (Supplementary Fig. 18).

To correlate the structure-property relationship of this material, we performed nanoindentation experiments on the (010) major face of MNI using a diamond probe of 150 nm radius (see “Methods” and Supplementary Fig. 19). Upon applying 3 mN load on the (010) face, microcracks appear from the edges of the indent impression (Supplementary Fig. 20a). Direction of these cracks is parallel to the 2D layers in the crystal, consistent with the cracks observed in qualitative tests. Furthermore, when the crystal is rotated by 90 degrees, the corresponding fissures and pile-up at the indent impressions also rotate by 90 degrees (Supplementary Fig. 20b). These observations confirm that the uniaxial stress generates cracks along a specific direction, i.e., parallel to the (10-1) cleavage planes, irrespective of the orientation of the tip or stress direction. The inversion center lies in between these obliquely aligned layers, forming the (10-1) plane as confirmed from crystal structure analysis (Fig. 2j). Hence, physical separation of the layers upon application of vertical stress causes interlayer bonds to be broken at the fracture interface. However, the subsurface layers immediately beneath the fracture surface will have “broken symmetry” with some distortions in the noncovalent interactions as compared to the pristine layers. Since the crack in this particular case follows an elliptical shape, there might be elastic strain too in the slightly curved surface, prompting the layers to spring back to normal, helping the self-healing. The newly created surfaces can raise the potential energy of the system, which can act as an additional driving force for crack closure, similar to the healing of enamel nanocrystals29.

Activation of new Raman modes by symmetry breaking
Theoretical Raman mode assignment reports Cs symmetry of the MNI molecule, consisting of 45 fundamental vibrations, active in both infrared (IR) and Raman spectroscopy30. Experimental Raman spectra were collected from pristine as well as fractured regions of an MNI crystal and compared to reveal distinct differences (see “Methods”). Pristine crystal has two prominent peaks centered at 820 cm-1 and 703 cm-1 (Fig. 3a). Traversing via a line-profile from pristine—crack—pristine area along [100] reveals emergence of two new modes at 826 cm-1 and 696 cm-1 adjacent to their corresponding parent peaks (Fig. 3b). The additional Raman modes observed from fractured zones are absent from the adjoining undeformed areas (full Raman spectra in Supplementary Fig. 21). To reveal the differences across strained vs unstrained regions on the crystal, we performed Raman spectral mapping on the tip of a vertically induced crack by selecting 120 x–y points in the form of an area map (Fig. 3c–g). Figure 3d, f shows the spatial variation of 696 cm-1 and 826 cm-1 modes, respectively, in and around the position of the crack tip. These additional modes are restricted only to the deformed regions, with the highest intensity being at the center of the crack. Crack tips, irrespective of the symmetry of the system, are known to bear the ultimate strain gradients that a material can withstand31. A similar Raman map over the same crack tip for the 703 cm-1 and 820 cm-1 peaks (Fig. 3e, g) reveals saturated intensities at all x–y positions, confirming the uniform distribution of the parent modes both in and around the fractured zones (Supplementary Figs. 22–25).

The new modes appearing in the fractured regions (Supplementary Fig. 26) are indicative of local strain arising out of vertical mechanical stress on the crystal lattice, which is absent in surrounding pristine regions32. Emergence of new Raman modes is widely reported to give direct evidence for the breaking of centrosymmetry in 2D layered structures like graphene33 and MoTe234, where the presence of an inversion center in the layer leads to in-plane shear modes being Raman inactive; however, when the crystal structure does not hold inversion symmetry, these modes become both Raman and IR active and begin to surface in Raman spectroscopy (Supplementary Note 5)34.

Role of symmetry breaking in self-healing
The symmetry-breaking effect and its role in the subsequent self-healing event were further probed by studying the non-linear susceptibilities of pristine, fractured, and healed crystals (see “Methods” and Supplementary Fig. 27 for experimental setup). Optical second harmonic (SH) generation is the final word on tracing any point-symmetry violation through micron-scale variations in SH intensity35–37. Figure 4a shows the schematic of the SH generation experiment, where an input femtosecond-pulsed laser centered at 800 nm is incident over a 250 × 250 μm area on the (010) crystal surface. SCXRD experiments undisputedly show the presence of a center of inversion in between the stacked layers in bulk MNI crystals (Fig. 4b). Under the influence of mechanical stress, local interlayer interactions predominantly disrupt, leading to dangling bonds and the inversion symmetry lying in between them gets broken (Fig. 4b). However, the intra-layer interactions, which are relatively stronger, remain intact, which prevents the overall crystal structure from collapsing. This is essential for allowing bonds to reform and for beginning the healing process.

To test our hypothesis, we first collected SH signal maps at 400 nm from pristine regions (Fig. 4c) and then from mechanically strained regions (Fig. 4d). The dramatic difference in non-linear optical response is further highlighted in Supplementary Figs. 28 and 29. The undeformed regions show a negligible SH signal, as expected, since this frequency-doubling phenomenon is not allowed in centrosymmetric media38–40. In contrast, bright oblique features are observed from a fractured region, demonstrating significant enhancement in SH intensity. One typical feature of these enhanced SH signals is that they come out in the form of an elliptical shape, which corroborates that the entire SH signal emanates only from the distortion of locally perturbed crystal layers.

Polarization-dependent SH generation (PSH) measurements were conducted by rotating the incident light polarization direction and collecting both parallel and perpendicular components, measured with respect to the laboratory axis (Fig. 4e). The polar plot of the PSH response in the fractured region exhibits a two-fold symmetry, while the pristine region shows a nearly circular polar plot (Fig. 4e). This difference arises due to different crystal symmetry of the interface layer of pristine area as compared to deformed area41. By repeating the experiments over multiple samples, PSH data over fractured regions are shown to exhibit almost 6 times higher intensity compared to pristine region (Fig. 4f). From the SH experiments, the highest intensity is observed at the center of the crack, consistent with the highest intensity of the new modes found in Raman spectroscopy (Supplementary Fig. 29). Figure 4g shows the SH efficiency which is linearly proportional to the intensity of the input beam, further verifying that the signal emanating from the perturbed region is a non-linear optical process. Furthermore, we recorded the SH signal after a successful self-healing event, demonstrating a clear loss in SH intensity comparable to that of a pristine surface (Supplementary Fig. 30 and Supplementary Note 6). These results reveal that the self-healing process is indeed associated with a symmetry-breaking pathway, indicating that the energy penalty associated with healing arises from this temporary loss of symmetry.

Methods
Sample preparation
The compound 2-methyl-5-nitroimidazole (MNI) was purchased from Sigma-Aldrich and used as-received without further purification. 50 mg of the compound was added to 5 mL of methanol in a clean conical flask with a flat base, warmed at 35 °C for 10 min and kept undisturbed to induce crystallization by slow evaporation. Macroscopically large plate-shaped crystals (a few millimeters in size) grew in one week’s time from the solution. Crystallization can also be performed from a water or ethanol solution. The single crystals of MNI typically exhibit spindle-shaped or open-book-shaped morphology, which is a result of growth twinning in the lattice, as shown in Supplementary Fig. 2.

Single-crystal X-ray diffraction
SCXRD data for the MNI crystal were collected at room temperature (R.T.) on a Bruker Quest D8 diffractometer using MoKα (λ = 0.71073) radiation having a 110 µm beam size. Using Olex2 (version 1.5)45, the structure was solved with the SHELXT structure solution program and refined with the SHELXL refinement package. Displacement parameters of all non-hydrogen atoms were refined anisotropically. All the crystal packing diagrams were prepared using Mercury (2022.3.0 version) and Chimera (1.16 version) software. Crystallographic refinement details can be found in Supplementary Note 7.

Characterization of self-healing
As-grown “pristine” crystals were subjected to vertical stresses under a Leica (M205 FCA) polarized optical microscope equipped with a Fastcam Mini high-speed camera. The magnification used was in the range of 4–8× while the videos were recorded at 250–1000 frames per second (fps) and a resolution of 1280 × 1024. Analysis of the videos was done using Fastcam software to visualize the healing in slow motion and real-time, and to quantify the time required for healing. On application of a vertical stress, typically, the crystals begin to crack from the point of stress application, termed as the defect point. Elliptically shaped cracks propagate on both sides of the defect point until the stress is applied. As soon as the force is withdrawn, the crack propagation stops, and the crack begins to heal. The efficiently healed crystals with no visible crack line when viewed under a polarized microscope are termed “healed” crystals. Whereas the crystals where the crack is unable to close perfectly show thin cracks termed as “fractured crystals”. Using excessive force, pristine crystals can be broken into pieces and separated apart, which are termed as “fragmented crystals”.

For characterizing self-healing using a scanning electron microscope (SEM), the images were obtained using a Zeiss DSM 950 and FEI QUANTA 200 3D microscope operating at 10 kV using a tungsten filament as the electron S4 source. Before the imaging, the healed samples were sputtered with gold (nano-sized film) using an SCD 040 Balzers Union sputterer to avoid charging during SEM analyses.

For characterizing self-healing using atomic force microscopy (AFM), a single MNI crystal was subjected to vertical stress under a microscope, allowed to heal, and the healed region was imaged under a commercial AFM Asylum Research Cypher ES using an AC240TS-R3 probe with a spring constant of ≈2 N/m and tip radius of ≈7 nm.

For characterizing self-healing using SCXRD, a healed single crystal of MNI at room temperature (RT) was carefully mounted on the diffractometer, and the incident X-ray beam was centered on the healed region of the crystal. The complete dataset obtained from a healed crystal was compared against that of a pristine crystal to characterize the self-healing (see Supplementary Tables 3 and 4). The structural models of both the pristine and neatly healed crystals exhibited comparable statistical refinement parameters, such as GooF and R1 (Supplementary Table 3). The hydrogen-bonding parameters (distances and angles) obtained for both pristine and healed crystals were consistent, confirming their identical crystalline phase before and after healing (Supplementary Table 4). The mosaicity value of the pristine crystal is 0.63, and that of the healed crystal is 0.85, which might be attributed to stress-induced local defects following the fracture and subsequent self-healing, leading to a reduction in the number of measurable reflections.

Computerized microtomography
Crystals were imaged using X-ray microtomography (Xradia 510 Versa X-ray Microscope, Zeiss X-ray Microscopy, Pleasanton, CA, USA) to obtain information from the bulk of any solid material via 3D reconstructed modeling. Crystals were loaded on the sample holder and kept between the X-ray source and the detector assembly. Detector assembly consisted of a scintillator, 20× optics and a CCD camera. The X-ray source was ramped up to 80 kV and 7 W. The tomographic image acquisitions were completed by acquiring 3201 projections over 360° of rotation with a pixel size of approximately 0.7 microns for a field of view of approximately 0.7 × 0.7 × 0.7 mm3. Each projection was recorded with a 2 seconds exposure time. The filtered back-projection algorithm was used for the reconstruction of the projections to generate two-dimensional (2D) virtual cross-sections of the specimens. Segmentation and further processing were performed using Dragonfly Pro software (Version 3.6, Object Research Systems, Montreal, Canada). 2D images were trimmed down to a sub-volume (approximately 300 × 600 × 700 microns) and filtered to remove noise. During the image segmentation process, the voxels pertaining to crystals and voids in the grayscale histogram were differentiated with the aid of an OTSU-mediated threshold selection program. The resultant 3D reconstructed model was used to visualize the spatial distribution of voids inside the crystals.

Energy frameworks calculations
Energy frameworks analysis was performed by CrystalExplorer using the B3LYP hybrid function with a 6–31G(d,p) basis set46. Energy frameworks calculations help to understand the energies among molecules at play in the lattice. The interaction energy among the layers of MNI was calculated by using energy frameworks analysis, where the thickness of blue bars represents the strength of interaction energies.

Nanoindentation
Nanoindentation experiments have been performed on MNI single crystals using TI Premier from Hysitron, Minneapolis, USA, equipped with an in-situ Scanning Probe Microscope (SPM). A Berkovich probe (three-sided pyramidal tip with a total included plane-edge angle of 142.3°) of radius ~150 nm was used to obtain the load–depth (P–h) curves, which were analyzed using the standard Oliver–Pharr method to determine the hardness (H) and elastic modulus (E) of the crystals47,48. The indent impressions were captured immediately after unloading using the Berkovich probe and were analyzed to draw structure-property correlations (Supplementary Figs. 19 and 20).

Quantitative analysis of healing events
To quantify the load responsible for self-healing, MNI crystals were mounted with a small amount of glue on a glass slide with the major portion of the crystal projected outwards. Then the stage having the crystals was translated towards the tip of a force sensor (accuracy of ±1%) with a minimum count of 1 mN, having a tip diameter of ~190 µm23. It is to be noted that the load values obtained from the instrument are directly reported here (Supplementary Fig. 11).

The in-situ fracture tests were performed with a PicoIndenter IntraSpect 90 coupled to a stereomicroscope integrated with a camera (M205 FCA). Crystals of different thicknesses were mounted on custom-designed push-to-pull chips in three-point bending geometry. A diamond flat punch tip of diameter 500 µm was driven in displacement mode with a rate of 4000 nm s-1. During the mechanical testing, load (F) and displacement (D) were recorded in real-time. The load versus displacement curves were obtained dynamically and converted to stress-strain curves using the following equations49.

Stress at any point (σ) = 3Fg / 2wd2 (1)

Strain at any point (ε) = 6d / g2 × D (2)

Where,
F is the load at a given point of time
g is the length of the crystal between supports
w is the thickness of the crystal
d is the width/breadth of the crystal
D is the loading arm displacement

Quantitative tests were performed on a pristine sample, a fractured sample and a healed sample. To quantify the healing efficiency, a pristine crystal and a healed crystal of two different thicknesses – one of 100 µm and another of 136 µm were chosen (see Supplementary Figs. 12 and 13).

Raman spectroscopy
Raman spectroscopy measurements were carried out using a Horiba-Jobin Yovn LABRAM HR spectrometer using 632 nm excitation and 100% laser power using 1800 grooves for an acquisition time of 5 s. Raman microscopy has been performed on the pristine and fractured zones of the same MNI crystal and used for further comparison. The full spectra were recorded from 200 cm-1 to 3300 cm-1. The typical approximate width of the crack, assessed from the 100× objective lens, is ~0.5 micrometer, which decreases as one approaches the crack tip. The appearance and disappearance of new modes can be ascertained as one moves across the crack (i.e. along the white line in the optical image, Fig. 3a) by looking at the spectra color coded as blue (pristine) and pink (fractured). Raman mapping was performed on the selected area in the green box (Supplementary Fig. 26). Raman mapping was recorded by selecting 120 x–y points in the form of an area map. The selected area, as shown in the optical image under a 100× objective lens, comprises both fractured and pristine regions of the crack tip. Mapping of the 696, 703, 820, and 826 cm-1 peaks has been done to obtain a 2-D Raman map and plotted by color contrast after normalization. Overall, the Raman maps confirm the appearance of new modes along the crack. For detailed experimental discussions, see Supplementary Note 5.

Second-harmonic generation microscopy
The polarization-dependent second-harmonic imaging of the organic crystal was performed using a standard non-linear optical microscopy imaging setup50. The system consists of a femtosecond pump laser source (Fidelity HP) at 1040 nm wavelength with a pulse width of 140 fs and repetition rate of 80 MHz, pumping an optical parametric oscillator (Levante-IR) tuned to generate a signal at 1600 nm wavelength with a pulse width of ~200 fs pulse width and repetition rate of 80 MHz. The output laser from the OPO is then fed into a Harmonic generator (APE HarmoniXX SHG) to generate a harmonic signal at 800 nm wavelength, which is used as an input laser source for the SH measurement. The organic crystal sample, glued on a glass slide, is then mounted over an XY motorized stage of the Olympus inverted microscope in an inverted position. The laser source with an average input power of 14.5 mW is focused on the sample using a 20x/0.75 NA Olympus objective. The same objective is used to collect the backward SH emission and then separated from the input laser using a dichroic beam-splitter. The SH signal is further spectrally filtered using a bandpass (400 ± 20 nm) and short-pass filters (550 nm cut-off) and detected using a photomultiplier tube (PMT). The set of bandpass and short-pass filters ensured effective rejection of the fundamental excitation, thus ensuring detection of only the SH signal. A pair of galvo-scanning mirrors is used to scan the incident laser beam over the sample to acquire two-dimensional SH images over a field of view of 150–200 µm mapped into a 256 × 256 pixels by mapping the PMT signal for each pixel. For the polarization-dependent SH studies, the input polarization was rotated from 0° to 180° in 5° steps using a half-wave plate to acquire SH generation images for each angle. An analyzer parallel to the laboratory horizontal axis is placed in front of the PMT to selectively detect the parallel-oriented SH emission from the sample with respect to the laboratory horizontal axis. For more experimental details, refer to Supplementary Note 6.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this article, the authors introduce a novel self-healing molecular crystal, 2-methyl-4-nitroimidazole (MNI), which can rapidly\nself-repair under mechanical stress. Research shows that this self-healing ability stems from the special layered structure\nand hydrogen bond interactions within the crystal, as well as the local symmetry breaking that occurs at the crack tip. In\naddition, the activity of the nonlinear second harmonic (SH) caused by mechanical disturbance surges six-fold, further\nconfirming the existence of local symmetry breaking in crystals that originally had a centrally symmetrical structure. This\nwork is certainly interesting from a scientific perspective. However, the originality and discussion of the results in this paper\nremain deficient. I will give further consideration after the authors address the following issues.\n1.While the paper verified local symmetry breaking via second harmonic generation, it did not fully explain its mechanism—\nspecifically, how it occurs and drives the self-repair process. The article's main innovation lies in the self-healing of\ncentrosymmetric crystals, which the author attributes to the breaking and rearrangement of interlayer molecular interactions.\nIs the driving force for this bond rearrangement consistent with the uneven charge distribution resulting from broken polar\nbonds in non-centrosymmetric crystals?\n2.The paper mentions a six-fold enhancement of nonlinearity at the fracture. Since centrosymmetric crystals theoretically\nlack SHG, how was this multiple derived? Moreover, since the second harmonic generation can be detected at the crack,\nwhether there is also a second harmonic generation signal at both ends of the crystal?\n3.Concerning MNI crystal's reported elastic modulus of 13.3±0.5 GPa: Is it anisotropic? If so, what are the values for other\ncrystal planes? How does this modulus compare to other self-healing molecular crystals, and is it linked to MNI's self-\nhealing capability?\n4.In the previous work (Nat. Commun. 2023, 14, 6589), the authors used SHG to verify the self-healing process of non-\ncentral symmetric crystals, and the SHG intensity at the crystal fracture was significantly reduced. Does this conflict with the\nconclusion of this paper?\n5.The text states that plastic regions prevent further crack propagation. How do these plastic regions relate to plastic\ncrystals? Given that elastic crystals can also exhibit self-healing (Proc. Natl. Acad. Sci. U.S.A., 2025, 122(7): e2417901122.),\nwhat is the flexibility of MNI crystals? Does the macroscopic elastic/plastic behavior of crystals influence their self-healing\nability?", "reviewer_2": "The authors demonstrate healing in 2-methyl-4-nitroimidazole (MNI) molecular crystal up on stress-induced failure. The\nstructural analyses of the crystal shows restoration of crystallinity post-healing and indicates the formation of plastic zone at\nthe crack tip. The images presented in the manuscript indicate a crack opening and a subsequent healing around a defect\npoint in under 4 ms, and the efficiency in mechanical strength on failure is limited to 40%. These results are interesting and\nprovide evidence for repeated cycling in molecular crystals (a topic that is of broad interest to the community). The authors,\nhowever, then claim that the fracture in the crystal followed by healing is attributed with the ``symmetry-breaking'' in crystals\nobserved in MNI. This latter reasoning/analyses is what we find to be tenuous and not well-supported based on the field's\nunderstanding of reversible symmetry-breaking. We identify some comments and questions for the authors below, and hope\nthat our review contributes to the strengthening of their work.\n(1) Symmetry-breaking in crystalline systems is often referred to the phenomena when the unit cells belonging to higher-\nsymmetry group deform to a lower-symmetry group. This symmetry-lowering or symmetry-breaking mechanism generates\nmultiple, crystallographically equivalent variants. A characteristic feature of this\nsymmetry-breaking phenomena is that each atom maintains the same nearest neighbors before and after transformation, and the\nbonds neither broken nor newly created in this process. Now, in the current work, the authors report a centrosymmetric unit cell in the ``pristine'' region of the\nmolecular crystal and a loss of inversion symmetry in the ``deformed'' region of the molecular crystal. This deformed region,\nhowever, is the region in the vicinity (perhaps also within) the crack i.e., region where bonds are broken and two new\nsurfaces are created. It's not clear what is the material system whose symmetry is being characterized or measured? And\nwhy do they call it symmetry-breaking and not a simple failure or separation of interfaces?\n(2) The authors correlate crystal symmetry with the healing process. But from what we see in their results it appears that the\nweak van der waals attraction when the newly formed surfaces are close enough contributes to the cross-linking of the\npolymer chains. Can the authors explain if and why the crystal symmetry is contributing to the self-healing behavior? How\ndoes its interplay with other factors such as molecular attraction, layered structure of the crystals, and other environmental\nfactors play a role? Would other layered molecular crystals also be expected to exhibit similar behavior?\n(3) The authors argue the origin of plasticity on page 6 – \"The presence of a combination of dispersive and hydrogen\nbonding interaction regions in the crystal structure of MNI seem to provide a degree of plasticity which allows for elasto-\nplastic deformation\". In supplementary note 4, the underlying mechanism of plastic deformation is described in terms of\ndislocation however, the formation and motion of dislocations is not described in the manuscript.\n(4) on page 10 it is also described that psd measurement differ due to different crystal symmetry of the interface layer of\npristine area as compared to deformed area—is it possible to determine the crystal symmetry of the fractured region? How is\nthe crystal or unit cell defined in this context?\n(5) While the experiments demonstrate nearly 100% recovery of crystallinity after self-healing, the recovery of mechanical\nstrength is limited to approximately 40%. The authors have discussed in Supplementary note 3 the trade-off between\ncrystallinity and mechanical strength for different healing mechanisms. However, a more detailed discussion of the factors\nlimiting mechanical recovery, despite full crystallinity restoration, would provide valuable insights for future work.\nAdditionally, the 40% efficiency is reported for the thickness of 100 μm; in reference to mechanical characterization provided\nin Supplementary Fig. 12, is the efficiency expected to be affected by crystal thickness.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts.", "reviewer_4": "Reviewer report:\n1. The manuscript reports an impressive demonstration of ultrafast autonomous self-healing in molecular crystals through\nstress-induced symmetry breaking. The combination of fracture mechanics, Raman mapping, and SHG studies makes the\nwork highly significant and suitable for a high-impact journal. The findings represent a clear advancement in the\nunderstanding of molecular crystal self-healing mechanisms.\n2. The study is supported by a strong set of experimental techniques including SCXRD, AFM, SEM, Raman, SHG,\nnanoindentation, and tomography. This multi-level approach strengthens the conclusions and ensures the results are robust.\n3.The correlation between symmetry breaking and healing is novel. However, the mechanism could be better articulated. I\nrecommend including schematic illustrations summarizing the process from crack initiation → symmetry breaking → self-\nhealing, which would improve mechanistic clarity for readers.\n4. Figures are comprehensive but occasionally dense. Simplifying figure labels and providing more comparative overlays\n(pristine vs healed crystals) would enhance clarity and accessibility.\n5. Authors should clarify points such as missing reflections, potential twinning (PLAT930/931), and residual outliers in either\nthe Supporting Information or Methods section.\n6. A noticeable difference exists between the number of reflections collected for pristine (1711) and healed crystals (1375).\nThis difference likely arises because the healed crystal may diffract more weakly due to altered mosaicity or partial loss of\nintensity, resulting in fewer usable reflections.\n7. Stress-induced disorder or local defects may also alter completeness. Authors should explicitly address these points in\nthe methods and discussion to prevent misinterpretation.\n8. The reported R-factors (R1 0.06, wR2 0.20) are slightly higher than ideal for small-molecule crystals. Authors should\nconfirm that refinement strategies (e.g., anisotropic displacement parameters, absorption correction, and twin refinement if\nnecessary) were fully optimized.\n9. The manuscript would benefit from additional discussion comparing this mechanism with other reported self-healing\ncrystals, such as piezoelectric, ferroelastic, and thermally activated systems. Seems very short explanation in all\nsections..Improve it. This contextualization would strengthen the broader impact of the study.\n10. The narrative is generally strong, but some sections are wordy. Streamlining the introduction and providing a clearer\nroadmap at the beginning would help readers follow the story more effectively."}, "author_response": "Response to Reviewers (Manuscript: NCOMMS-25-46023-T)\nWe thank all the reviewers for their insightful comments, critical queries, and constructive\nsuggestions. We have tried our very best to carefully address all the comments and\nquestions raised by the reviewers, which are supported by the additional experiments we\ncarried out as per the suggestions of the referees. We have added a discussion section\nto our revised manuscript for improving clarity of presentation and contextualization of our\nresults for the benefit of the readers.\nFor ease of readability:\n• The original reviewers’ comments are in italics.\n• Our responses are in blue.\n• Text with changes in the original manuscript or SI are shown in purple.\nReviewer #1 (Remarks to the Author):\nGeneral comment: In this article, the authors introduce a novel self-healing molecular\ncrystal, 2-methyl-4 nitroimidazole (MNI), which can rapidly self-repair under mechanical\nstress. Research shows that this self-healing ability stems from the special layered\nstructure and hydrogen bond interactions within the crystal, as well as the local symmetry\nbreaking that occurs at the crack tip. In addition, the activity of the nonlinear second\nharmonic (SH) caused by mechanical disturbance surges six-fold, further confirming the\nexistence of local symmetry breaking in crystals that originally had a centrally symmetrical\nstructure. This work is certainly interesting from a scientific perspective. However, the\noriginality and discussion of the results in this paper remain deficient. I will give further\nconsideration after the authors address the following issues.\nResponse: We thank the reviewer for appreciating the scientific depth of our work and\nfinding it interesting. We also thank the reviewer for correctly assessing its importance\nregarding self-healing mechanisms in molecular crystals. We wish to additionally highlight\nthe following distinctive and novel aspects of our work for the reviewer’s consideration.\n1. As the referee comments in the next point, our work mainly focuses on the novel\naspects of self-healing, a topic which is very promising but at its infancy. To the\nbest of our knowledge, although there are several reports on inversion symmetry\nbreaking (probed through Raman and SHG) in materials (mostly on inorganic or\nhybrid organic-inorganic crystals or 2D materials; but this is the first report in\norganic crystals under mechanical stress), its connections to self-healing\nmechanisms are not reported. The mechanistic insights from our centrosymmetric\n1\ncrystal will open new ways of understanding stress-induced behaviour in crystals.\nSince most organic crystals fall in the centrosymmetric space groups, the\nimplications of our work are wide reaching.\n2. The importance of our work is further articulated by an article in Nature Materials\n(https://doi.org/10.1038/s41563-025-02411-7) which appeared online while we\nwere completing our revision (i.e., a few days ago). This article talks about\n“diffusionless self-healing in a centrosymmetric crystal” (similar to ours) and the\nimportance of interactions between neighbouring layers in such crystals. In this\ncontext, our article provides deeper insights into the local structure at the cracked\nregion using multiple techniques, such as µ-Raman mapping, second harmonic\ngeneration (SHG) mapping, X-ray microtomography (XMT) imaging, SEM and\nAFM techniques. Hence, we feel that the referee will appreciate the novelty and\nimpact of our findings in this manuscript.\n3. The analysis of crack propagation and its mitigation in molecular crystals from the\nlens of fracture mechanics ̶ not documented in literature till date.\nWe feel that these multitude of aspects, connected for the first time, make our work highly\noriginal and impactful. We understand that these points may not have come out well in\nour manuscript and we have now addressed these shortcomings in the revised draft, page\n2 lines 31-41 and page 3 lines 1-5.\nComment 1: While the paper verified local symmetry breaking via second harmonic\ngeneration, it did not fully explain its mechanism—specifically, how it occurs and drives\nthe self-repair process. The article's main innovation lies in the self-healing of\ncentrosymmetric crystals, which the author attributes to the breaking and rearrangement\nof interlayer molecular interactions. Is the driving force for this bond rearrangement\nconsistent with the uneven charge distribution resulting from broken polar bonds in non-\ncentrosymmetric crystals?\nResponse: We thank the reviewer for raising this important question. In case of MNI\nsystem, the symmetry breaking occurs specifically under mechanical deformation,\nleading to local distortions in the centrosymmetric lattice and hence breaking of\ncentrosymmetry (confirmed by Raman & SHG studies). A similar mechano-induced\ninversion symmetry breaking is reported in a centrosymmetric perovskite (ref 32 of main\nmanuscript) and here we show, for the first time, the mechanical stress induced symmetry\nbreaking in an organic crystal.\nRegarding the connection between the self-healing mechanism and the symmetry\nbreaking, we tend to agree with the referee’s comment. Since what happens at a\n2\nmolecular level when the two shards rejoin is very difficult to capture using experimental\ntechniques, we tried not to overly present our views. We have got all the evidence for self-\nhealing, symmetry breaking and the attraction between the freshly broken shards\n(Supplementary Fig. 15 and Supplementary Video 6), and this certainly is a proof enough\nto present the following explanation, a part of which is now added to the draft page 11,\nlines 8-23.\nNew text: Application of vertical force on the (010) face of MNI probes the structure\nparallel to the pristine stacked layers, which are supported by weak van der Waals and\nC−HO interactions and related by inversion symmetry. The mechanical stress parallel\nto these layers leads to a crack formation, causing symmetry breaking in the subsurface\nlayers of the fracture surfaces. We hypothesize that when the crack is formed, the\ninteractions among the strained surface layers result in transient surface charges, thus\nleading to attraction between the fractured shards (SI, Video 6) and making them behave\nlike what we reported earlier in non-centrosymmetric crystals (refs 22-23 of main\nmanuscript). Since the symmetry of the pristine state is broken, the induced polarization\nhelps in reorganization of the interlayer interactions, enabling rapid self-healing. Like non-\ncentrosymmetric crystals where self-healing is shown to occur due to charge imbalance\nby broken polar bonds, herein a similar electrostatically driven mechanism is perhaps\nfollowed after the structure becomes locally non-centrosymmetric. Unlike the non-\ncentrosymmetric crystals, the opposite polarity of the broken surfaces could not be\nconfirmed by the KPFM measurements as the charges here are much smaller and short\nlived, but the evidence of instantaneous attraction between broken shards\n(Supplementary Video 6) supports our above hypothesis.\nComment 2: The paper mentions a six-fold enhancement of nonlinearity at the fracture.\nSince centrosymmetric crystals theoretically lack SHG, how was this multiple derived?\nMoreover, since the second harmonic generation can be detected at the crack,\nwhether there is also a second harmonic generation signal at both ends of the crystal?\nResponse: We thank the reviewer for this intriguing comment. We agree that\ntheoretically, second harmonic generation (SHG) signal is forbidden in the lattice of\ncentrosymmetric materials. However, due to the material discontinuity at the pristine\nsurface, we could detect very feeble SHG signals (refs 39-40 regarding surface SHG are\nadded in revised manuscript). Hence, the reported multiple is derived by taking the ratio\nof the SHG signal obtained from cracked region with this feeble SHG signal from the\npristine surface. We also measured the SHG signal at the pristine edge and found it to be\nmuch weaker in magnitude than that from the cracked region.\nanisotropic? If so, what are the values for other crystal planes? How does this modulus\ncompare to other self-healing molecular crystals, and is it linked to MNI's self-healing\ncapability?\nResponse: Molecular crystals are generally anisotropic due to their low-symmetry. In\ncase of MNI, most of the crystals, when viewed from the top, have a spindle-shaped\nmorphology, making it difficult to access the side faces for nanoindentation experiments.\nHowever, we made new attempts and could get a reasonably good number of indents on\n(10-1), which correspond to the ends of the crystal. The comparison of the mechanical\nproperties of the (010) face with (10-1) face show distinct mechanical behaviour, as\nexpected due to the crystal anisotropy. It can be seen from the scatter plot above that the\nelastic modulus of the (10-1) face is 5.71±0.398 GPa whereas that of the (010) face is\n13.3±0.5 GPa.\nFigure R1. Scatter plot of elastic modulus (E) vs hardness (H) of MNI on its two different\ncrystal faces; red corresponds to (10-1) while black corresponds to (010) face.\nRegarding the comparison of MNI’s modulus value with other self-healing molecular\ncrystals, it is worth noting that many studies do not report E and H values from indentation\nexperiments. We have included a table below (Supplementary Table 2) covering\nexamples that reported E and H values. However, currently, there are not enough\nexamples to comment on the correlation of self-healing with the mechanical properties of\nsingle crystals.\nmolecular crystals:\nMaterial system Elastic Instrumentation Reference\nmodulus\nMNI 13.3 GPa Nanoindentation Current work\nBipyrazole 8-15 GPa Nanoindentation [1]\nDibenzoate 7.1-7.5 GPa Nanoindentation [2]\nAnilinium bromide 4.19 MPa Tensile tester [10]\nDibenzothiophene 9.36 GPa Nanoindentation [11]\nMethylammoniumlead(II) 1.37 MPa Tensile tester [12]\niodide\nDisulfide 2 MPa Tensile tester [13]\nPenciclovir Not reported [14]\nSquaramide Not reported [15]\nComment 4: In the previous work (Nat. Commun. 2023, 14, 6589), the authors used SHG\nto verify the self-healing process of non-central symmetric crystals, and the SHG intensity\nat the crystal fracture was significantly reduced. Does this conflict with the conclusion of\nthis paper?\nResponse: We thank the reviewer for this insightful question and the opportunity to clarify\nthe apparent difference between the two studies. In our previous work (Nat. Commun.\n2023, 14, 6589; ref 23 of revised manuscript), we investigated the self-healing behaviour\nof a dibenzoate molecular crystal which was inherently non-centrosymmetric. Therein,\nthe strong SHG signal corresponds to its intrinsic bulk second-order nonlinear optical\nresponse (which is at its maximum due to the already existing asymmetry), and the\nreduced intensity at the fractured region was attributed to the increased surface\nroughness and the lower amount material due to a hairline gap.\nIn contrast, in this present work, the SHG signal in pristine centrosymmetric crystal is\nclose to nil while SHG intensity increases manifold at the mechanically stressed region.\nSimilar mechano-induced symmetry breaking in centrosymmetric perovskite lattices have\nbeen shown previously by Ruediger et al. (ref 32 of revised manuscript). The six-fold rise\nin SHG signal, despite the increased surface roughness and reduction in amount of\nmaterial at the crack junction, indeed supports our hypothesis of mechano-induced local\nsymmetry breaking. Hence, the results in both the papers are consistent with their\n5\nrespective underlying mechanisms as the origin of SHG in the two cases is fundamentally\ndifferent.\nComment 5: The text states that plastic regions prevent further crack propagation. How\ndo these plastic regions relate to plastic crystals?\nResponse: We thank the reviewer for this question. In case of MNI crystals shown in this\nwork, the presence of dispersive zones (methyl groups) in structure lends softness to the\ncrystal and gives rise to local plasticity at the crack-tip which prevents further crack\npropagation.\nIn contrast, plastic crystals (PCs) consist of molecules (generally globular or cylindrical\nshaped) having orientational freedom at their lattice positions, while maintaining long\nrange order in their plastic crystalline phase. The characteristic of a plastic crystal is a\nsolid-solid phase transition before the melting point, observable in DSC thermograms.\nThey also deform plastically and can be mold into different shapes like thin films, stretched\nrods, compressed rods, etc. There are other class of plastically bendable crystals which\ncan be bent on selected faces, but they cannot be usually casted into thin films or\nstretched/compressed rods. In both these cases, the deformation is observed at the\nmacroscopic as well as local level.\nIn case of MNI, we do not observe any macroscopic plasticity, consistent with its DSC\ndata (see figure below). Nonetheless, any crystal can experience elastic or plastic\ndeformation locally under mechanical stress. The plastic deformation discussed here\nrefers to such kind of local deformation (plastic zones) at the crack tip which arrests crack\npropagation (see Supplementary Note 4), and not the bulk or macroscopic plasticity.\nFigure R2. (a) Three-point bending of MNI shows absence of macroscopic plasticity. (b)\nDifferential Scanning Calorimetry (DSC) endotherms of MNI single crystals. There is no\ndiscernible phase transition peak before its melting point at ~253 °C.\n2025, 122(7): e2417901122.), what is the flexibility of MNI crystals? Does the\nmacroscopic elastic/plastic behavior of crystals influence their self-healing ability?\nWe thank the reviewer for bringing this interesting recent report to our attention. In our\nwork, we have not observed any noticeable or large mechanical flexibility in MNI crystals.\nThe MNI crystals are inherently brittle, and upon application of external stress they\nundergo crack-formation prompting symmetry breaking, which leads to self-healing. In\nthis sense, ours is a unique example and does not compare directly with other reported\ncases.\nThe report mentioned (Proc. Natl. Acad. Sci. U.S.A., 2025, 122(7): e2417901122) shows\nthe presence of elastic flexibility as well as self-healing behaviour in non-centrosymmetric\nBEzH crystals (space group P2 ) where the two phenomena are attributed to anisotropic\n1\nintermolecular interactions along different crystallographic axes. It is important to note\nthat elastic/plastic bending are macroscopic phenomena owing to the molecular\nmovements/ slippage of molecular layers under mechanical stress. However, we believe\nthat diffusionless self-healing in our MNI system is a symmetry-driven process where\ndangling bonds generated at cracked surfaces and the broken local symmetry among the\nlayers closer to the surface promote rejoining to return to previously symmetric state,\nresulting in self-healing. Since the crack in this particular case follows an elliptical shape,\nthere might be elastic strain in the slightly curved surface, prompting the layers to spring\nback to normal, helping the self-healing. Hence, intermolecular interactions and elastic\nenergy may play a role in both mechanical flexibility as well as self-healing. However, to\nconclude these suggestions, experimental and theoretical evidence through further\nstudies will be required. We feel that the self-healing in organic crystals is a very new\ntopic and further exploration to draw connections among various behaviours of crystals\nfrom the structure-property angle would be exciting.\nReviewer #2 (Remarks to the Author):\nGeneral comment: The authors demonstrate healing in 2-methyl-4-nitroimidazole (MNI)\nmolecular crystal up on stress-induced failure. The structural analyses of the crystal\nshows restoration of crystallinity post-healing and indicates the formation of plastic zone\nat the crack tip. The images presented in the manuscript indicate a crack opening and a\nsubsequent healing around a defect point in under 4 ms, and the efficiency in mechanical\nstrength on failure is limited to 40%. These results are interesting and provide evidence\nfor repeated cycling in molecular crystals (a topic that is of broad interest to the\ncommunity). The authors, however, then claim that the fracture in the crystal followed by\nhealing is attributed with the ``symmetry-breaking'' in crystals observed in MNI. This latter\nreasoning/analyses is what we find to be tenuous and not well supported based on the\n7\nfield's understanding of reversible symmetry-breaking. We identify some comments and\nquestions for the authors below, and hope that our review contributes to the strengthening\nof their work.\nResponse: We thank the reviewer for acknowledging the importance of this work in the\nfield of molecular crystals. We appreciate the comments/suggestions and hope that our\nrevised manuscript clarifies the context in which we described the symmetry breaking,\nand the connection of the same to self-healing mechanism in MNI.\nComment 1: Symmetry-breaking in crystalline systems is often referred to the\nphenomena when the unit cells belonging to higher-symmetry group deform to a lower-\nsymmetry group. This symmetry-lowering or symmetry-breaking mechanism generates\nmultiple, crystallographically equivalent variants. A characteristic feature of this\nsymmetry-breaking phenomena is that each atom maintains the same nearest neighbors\nbefore and after transformation, and the bonds neither broken nor newly created in this\nprocess.\nResponse: We thank the referee for giving us an opportunity to explain the context in\nwhich we used the term “symmetry breaking”. Symmetry breaking generally refers to the\ndisruption of an existing symmetry in a system. The comments made by the referee are\nparticularly true in the context of ferroelectric solid-to-solid phase transitions, wherein “the\nphenomenon in which some symmetry elements of the paraelectric (high-temperature,\nhigh symmetry) phase are absent in the ferroelectric (low-temperature, low-symmetry)\nphase is known as symmetry breaking.” (ref 5 of revised manuscript). In such cases, as\nboth the solid phases are highly crystalline (in different solid phases), each atom\nmaintains the same nearest neighbours through multiple, crystallographically equivalent\nvariants, which is either referred to as symmetry breaking (for e.g., going from space\ngroup P2 /c to P2 ) or symmetry lowering (for e.g., going space group P2 2 2 to P2 ). In\n1 1 1 1 1 1\nthe latter case, both the initial and final space groups are non-centrosymmetric.\nIn the present work, we refer to a symmetry breaking phenomenon by external\nmechanical stress, which causes the locally deformed areas to go from an initially\ncentrosymmetric state (high symmetry – no SHG signal) to a new non-centrosymmetric\nstate (lower symmetry – presence of SHG signal). In our case, the deformed regions,\nwhich emit SHG signal, are not same as what is observed in systems where solid-solid\nphase transitions occur. The breakage of non-covalent bonds involved in our case refers\nto the scenario at the “crack interface”, where the layers separate leading to fracture.\nSince here, the material layers are separated, the interlayer bonds have to be broken.\nHowever, the subsurface layers (those close to the fracture surface on either side) will\nhave “broken symmetry” with some distortions in the noncovalent interactions as\ncompared to the pristine or bulk layers (see Supplementary Figure 1). We have the proof\nfor symmetry breaking (SHG signal) at the crack junction, but proving the structure in an\n8\n“area of few microns” at the crack surface is very challenging and not in the scope of\ncurrent work as obtaining atomic resolution structure from such thin crystal regions\nrequires sophisticated techniques which are not easily available to us.\nNow, in the current work, the authors report a centrosymmetric unit cell in the “pristine”\nregion of the molecular crystal and a loss of inversion symmetry in the “deformed” region\nof the molecular crystal. This deformed region, however, is the region in the vicinity\n(perhaps also within) the crack i.e., region where bonds are broken and two new surfaces\nare created. It's not clear what is the material system whose symmetry is being\ncharacterized or measured? And why do they call it symmetry breaking and not a simple\nfailure or separation of interfaces?\nResponse: As the reviewer correctly points out, there are two components, (1) the freshly\ncreated surfaces through fracture where interlayer interactions are broken; (2) several\nsubsurface layers on either side of the crack which become distorted, resulting in\nsymmetry breaking, as evidenced by the SHG signal from the fractured surfaces. Such\nlocal symmetry breaking induced by mechanical stress has previously been reported in\ncentrosymmetric SrTiO , wherein the SHG activity shoots up on deformed crystal surface\n3\n(ref 32 of main manuscript).\nRegarding the referee’s comment on failure, according to fracture mechanics, failure is\ndefined as complete rupture of the crystal after crack propagation. Since our crystal is\nable to arrest the crack before complete rupture, we are not using the term “failure” for\nthese MNI crystals. It is to be noted, however, that under very high mechanical loading,\nindeed the crystals will fail, that is, will not self-heal and will lead to complete separation\nof interfaces.\nWe hope this makes our analysis clearer. We have added an additional paragraph (page\n8 lines 20-25) to include this important discussion in the main manuscript.\nNew text: Hence, physical separation of the layers upon application of vertical stress\ncauses interlayer bonds to be broken at the fracture interface. However, the subsurface\nlayers immediately beneath the fracture surface will have “broken symmetry” with some\ndistortions in the noncovalent interactions as compared to the pristine layers. Since the\ncrack in this particular case follows an elliptical shape, there might be elastic strain too in\nthe slightly curved surface, prompting the layers to spring back to normal, helping the self-\nhealing.”\nComment 2: The authors correlate crystal symmetry with the healing process. But from\nwhat we see in their results it appears that the weak van der waals attraction when the\nnewly formed surfaces are close enough contributes to the cross-linking of the\npolymer chains. Can the authors explain if and why the crystal symmetry is contributing to the self-\nhealing behavior? How does its interplay with other factors such as molecular attraction,\nlayered structure of the crystals, and other environmental factors play a role? Would other\nlayered molecular crystals also be expected to exhibit similar behavior?\nResponse: We thank the reviewer for this comment. We believe there may be a\nmisunderstanding, and we clarify below. We have not used the terms “polymer” or “cross-\nlinking” anywhere in describing our results as MNI is a “molecular crystal” and contains\nonly “molecules” (no polymerisation involved at any stage of our work).\nRegarding the correlation between crystal symmetry and self-healing, we request the\nreferee to refer to our previous reports (refs 22-23 of manuscript), where self-healing was\nrationalized based on the presence of non-centrosymmetry in crystals (polar space\ngroups). In case of such non-centrosymmetric crystals, application of stress creates\noppositely charged molecular dipoles on freshly created complimentary crystal faces.\nThese opposite charges at the fracture line led to attraction of the surfaces, eventually\nresulting in the diffusionless self-healing. Non-centrosymmetry in crystals is well known\nto cause charge polarization under external mechanical stress (piezoelectricity). In the\ncurrent work, we go a step further to show that in a centrosymmetric crystal having layered\nstructure, a similar kind of asymmetry can be locally induced by symmetry breaking under\nmechanical stress. In this manner, we can obtain autonomous diffusionless self-healing\neven in the centrosymmetric crystals such as MNI.\nRegarding the behaviour of other layered molecular crystals, we are carrying out a\nthorough follow up study on the role of intermolecular interactions, crystal symmetry and\nmechanical behaviour of other layered molecular crystals which will be published\nelsewhere. However, to answer the reviewer’s comment, we feel that not all layered\ncrystals will show self-healing as it is a very complex phenomenon involving many factors\nto enable the system to repair its damages.\nComment 3: The authors argue the origin of plasticity on page 6 – \"The presence of a\ncombination of dispersive and hydrogen bonding interaction regions in the crystal\nstructure of MNI seem to provide a degree of plasticity which allows for elasto-\nplastic deformation\". In supplementary note 4, the underlying mechanism of plastic deformation\nis described in terms of dislocation however, the formation and motion of dislocations is\nnot described in the manuscript.\nResponse: We thank the reviewer for allowing us to clarify this point. To address this, we\nhave modified the Supplementary note 4, which now read as follows.\nRevised supplementary note 4. Fracture mechanics: It is generally accepted that the\nprocess of crack growth under cyclic loading is divided into three phases: crack initiation\n(Stage I), crack propagation (Stage II), and structural fracture (Stage III).[16] Crack growth\n10\nis usually characterized by a crack-tip plastic zone, where plastic deformation occurs via\ndislocations. It is well-known that in case of molecular crystals, due to their low symmetry\nand high anisotropy, it is difficult to detect exact motion and direction of dislocations.\nHowever, it has been studied that plasticity tends to be anisotropic in low-symmetry\nmolecular crystals.\nIn a recent report by Collins et al, they have observed that the direction of dislocations in\nmolecular crystals “lie predominantly in planes containing weak intermolecular\ninteractions between layers of molecules without cutting across the molecular\nstructure”.[17] Pop-ins are also attributed to nucleation of dislocations under the indenter\ntip.[18] In case of MNI, upon nanoindentation on the (010) face, we observe several pop-\nins in the load-depth curves which indicates the formation of dislocations (see\nSupplementary Fig.19).\nComment 4: on page 10 it is also described that psd measurement differ due to different\ncrystal symmetry of the interface layer of pristine area as compared to deformed area—\nis it possible to determine the crystal symmetry of the fractured region? How is\nthe crystal or unit cell defined in this context?\nResponse: Given the symmetry breaking in the present study originates from local strain,\nthe notion of a well-defined unit cell becomes ill-defined in the fractured region. The\npresent PSHG (referee’s comment refers to this as “psd”, perhaps a typo) measurement\neffectively probes a second-order nonlinear response and the polarization dependence\nprovides only the in-plane components of the SHG tensor. Given that the point group of\nthe pristine unit cell is monoclinic 2/m, after symmetry breaking it can adopt three\npossibilities – 2, m or 1 (ref 41 in revised manuscript). Since these are very low symmetry\npoint groups having greater number of non-zero tensors, the information on the relative\nratios and absolute magnitudes of all the tensor elements is insufficient to unambiguously\ndetermine the crystal symmetry of the cracked region in MNI.\nComment 5: While the experiments demonstrate nearly 100% recovery of crystallinity\nafter self-healing, the recovery of mechanical strength is limited to approximately 40%.\nThe authors have discussed in Supplementary note 3 the trade-off between\ncrystallinity and mechanical strength for different healing mechanisms. However, a more detailed\ndiscussion of the factors limiting mechanical recovery, despite full crystallinity restoration,\nwould provide valuable insights for future work.\nResponse: We thank the referee for allowing us to clarify this point. To address this, we\nhave added a paragraph in Supplementary note 3 on the factors affecting mechanical\nrecovery and crystallinity restoration during self-healing in case of molecular crystals.\nmm), quantitative assessment of self-healing efficiency using regular tools that are\nemployed for studying macroscopic samples (mm to cm length) such as polymers and\nelastomers become inefficient. As such, the crystal habit and size greatly influence the\nfeasibility of conducting healing tests. Further, the amount of stress experienced by\ncrystals during fracture-healing tests is predominantly governed by its own morphology\nas well as inherent mechanical properties.\nIt is to be noted that the healing efficiency obtained via mechanical testing is that of a\nwhole crystal. In a previous report from our group,[1] we quantified the healing efficiency\nof a bipyrazole molecular crystal using Mueller Matrix Microscopy with a spatial resolution\nof 300 nm. The analysis of a neatly healed region showed that mechanical impact can\npermanently decrease the overall structural order by 10 to 15%, leading to introduction of\nsome permanent defects in the crystals.\nAdditionally, the 40% efficiency is reported for the thickness of 100 μm; in reference to\nmechanical characterization provided in Supplementary Fig. 12, is the efficiency expected\nto be affected by crystal thickness.\nResponse: We have observed that for pristine crystals of different thicknesses ranging\nfrom ~50 μm to ~150 um, the fracture stress (stress at the fracture point of pristine\ncrystals) falls in the same range (11-13 MPa). Hence it is expected that the healing stress\n(stress at break of healed crystals) will not vary significantly with the thickness of the\ncrystals, provided other parameters like crystallinity and environmental conditions are\nkept constant.\nHowever, we have performed an additional experiment to test the healing efficiency at\nanother crystal thickness of 136 μm, as shown in the plot below, wherein the healing\nthickness 136 µm, demonstrating a healing efficiency of ~ 45%.\nefficiency is ~ 45 %. From this additional data, we see that the healing efficiency falls in\nthe same range across different crystal thicknesses.\nReviewer #3 (Remarks to the Author):\nGeneral comment: I co-reviewed this manuscript with one of the reviewers who provided\nthe listed reports. This is part of the Nature Communications initiative to facilitate training\nin peer review and to provide appropriate recognition for Early Career Researchers who\nco-review manuscripts.\nResponse: We thank the reviewer for assessing this work.\nReviewer #4 (Remarks to the Author):\nComment 1: The manuscript reports an impressive demonstration of ultrafast\nautonomous self-healing in molecular crystals through stress-induced symmetry\nbreaking. The combination of fracture mechanics, Raman mapping, and SHG studies\nmakes the work highly significant and suitable for a high-impact journal. The findings\nrepresent a clear advancement in the understanding of molecular crystal self-healing\nmechanisms.\nResponse: We sincerely thank the reviewer for their positive and encouraging\nassessment of our work. We are glad that the reviewer recognizes the significance of\ncombining fracture mechanics, Raman mapping, and SHG analyses to elucidate the\nmechanism of stress-induced symmetry breaking and ultrafast self-healing in molecular\ncrystals. We appreciate the reviewer’s acknowledgment that our findings advance the\ncurrent understanding of self-healing mechanisms in molecular crystals.\nComment 2: The study is supported by a strong set of experimental techniques including\nSCXRD, AFM, SEM, Raman, SHG, nanoindentation, and tomography. This multi-level\napproach strengthens the conclusions and ensures the results are robust.\nResponse: We sincerely thank the reviewer for acknowledging the breadth of\nexperimental techniques employed in this study. We believe that the integration of\nSCXRD, AFM, SEM, Raman, SHG, nanoindentation, and tomography provided a\n13\ncomprehensive perspective, and we are pleased that the reviewer finds this multi-level\napproach supportive of the robustness of our conclusions.\nComment 3: The correlation between symmetry breaking and healing is novel. However,\nthe mechanism could be better articulated. I recommend including schematic illustrations\nsummarizing the process from crack initiation → symmetry breaking → self-healing,\nwhich would improve mechanistic clarity for readers.\nResponse: We thank the reviewer for highlighting the novelty of the correlation between\nsymmetry breaking and self-healing. Accordingly, we have included a figure (new\nSupplementary Fig.1) which illustrates the mechanism from crack initiation to symmetry\nbreaking to self-healing in the revised manuscript.\nSupplementary Figure 1 | Schematic illustration summarizing the process from\ncrack initiation → symmetry breaking in the subsurface layers → self-healing of the\nMNI crystals. Layers 1 and -1 constitute the fracture surfaces formed via breakage of\nnoncovalent interactions under the external stress, forming an elliptical crack with curved\nfracture surfaces; whereas the layers 2/-2 and 3/-3 represent the strained subsurface\nlayers lying beneath on either side of the crack line, leading to symmetry breaking. Upon\nwithdrawal of the external load, the elliptical crack closes, where the layers spring back\nto their original positions, facilitating the self-healing of crystal.\nComment 4: Figures are comprehensive but occasionally dense. Simplifying figure labels\nand providing more comparative overlays (pristine vs healed crystals) would enhance\nclarity and accessibility.\nResponse: We thank the reviewer for this valuable suggestion. While the reported figures\nwere designed to convey the full set of experimental observations, we have now simplified\nthem without altering the original figure layout. For instance, Fig. 3 of the main text has\nnow been modified as shown below:\nAdditionally, we have revised the figure captions and corresponding text (pages 7-8) to\nmore clearly emphasize the key comparisons between the pristine and healed crystals.\n(PLAT930/931), and residual outliers in either the Supporting Information or Methods\nsection.\nResponse: We thank the reviewer for this valuable observation. The additional details\nhave now been added as a new Supplementary Note in the Supporting Information file.\nRevised text: Supplementary note 7. (Crystallographic Refinement Details)\nCrystallographic structure refinements were performed using SHELXL within the Olex2\nsoftware. The alerts (Level-C and Level-G) from the checkCIF report (PLATON) were\ncarefully examined and are discussed below.\nPLAT911_ALERT_3_C Missing Reflections: A small number of reflections were omitted\nduring refinement due to poor agreement or being affected by detector artifacts. These\nreflections were identified through outlier analysis in SHELXL and excluded to ensure a\nstable refinement model. The final dataset retains a completeness of >99%, which is\nsufficient for reliable structure determination and validation.\nPLAT930/931_ALERT_2/5_G Potential twinning: A potential twinning was indicated.\nSome of the MNI crystals exhibit growth twinning along (001) which is known. On running\nPLATON TWINROTMAT, the twin component was found to be 7%. Since for organic\nmolecular crystals twinning less than 10% can be ignored hence the structure was not\nrefined as a twinned crystal.\nPLAT934_ALERT_3_C Number of (Iobs-Icalc)/Sigma(W) > 10 Outliers: The (231)\nreflection exhibited a large |Iobs – Icalc|/σ ratio (>10), likely arising from experimental\nnoise or an integration artifact. The reflection was retained in the final refinement, as its\ninclusion or omission did not significantly affect the R-factors or the quality of the electron\ndensity map. The structural conclusions remain unaffected.\nPLAT906_ALERT_3_C Large K Value in the Analysis of Variance: The relatively large K\nvalue (6.181) likely arises from anisotropic absorption effects inherent to the anisotropic\ncrystal habit, resulting in variation in reflection intensities. A multi-scan absorption\ncorrection (SADABS) was applied and adequately compensated for these effects.\nPLAT969_ALERT_5_G The ’Henn et al.’ R-Factor-gap value: For R gap 10-15, the\nstructure is usually fine and acceptable.\nComment 6: A noticeable difference exists between the number of reflections collected\nfor pristine (1711) and healed crystals (1375). This difference likely arises because the\nhealed crystal may diffract more weakly due to altered mosaicity or partial loss of intensity,\nresulting in fewer usable reflections.\nResponse: The number of reflections collected for the healed crystal (1375) was slightly\nlower than that for the pristine crystal (1711). As the reviewer points out, this difference\ncan be attributed to the marginally weaker diffraction from the healed region. We note that\nthe value of mosaicity from the pristine crystal is 0.63 and that from the healed crystal is\n0.85; which is in agreement with partial loss of intensity following the fracture and\n16\nsubsequent self-healing. Despite the reduced reflection count, the refined structure\nmaintains satisfactory data completeness and refinement quality, confirming that the\ncrystal healed with minimal structural degradation.\nComment 7: Stress-induced disorder or local defects may also alter completeness.\nAuthors should explicitly address these points in the methods and discussion to prevent\nmisinterpretation.\nResponse: We thank the reviewer for this valuable observation. We agree that stress-\ninduced disorder and the generation of local defects during the fracture and self-healing\nprocess could influence data completeness. We have now included an explicit statement\nin both the Methods as well as Discussion sections of the manuscript to clarify this point.\nOriginal text: For characterizing self-healing using SCXRD, a healed single crystal of\nMNI at RT was carefully mounted on the diffractometer and the incident X-ray beam was\ncentered on the healed region of the crystal. The complete dataset obtained from a healed\ncrystal was compared against that of a pristine crystal to characterize the self-healing\n(see Tables 2-3). The structural models of both the pristine and neatly healed crystals\nexhibited comparable statistical refinement parameters, such as GooF and R1 (Table 2).\nAdditionally, the hydrogen bonding parameters (distances and angles) obtained for both\npristine and healed crystals were consistent, confirming their identical crystalline phase\nbefore and after healing (Table 3).\nRevised text: For characterizing self-healing using SCXRD, a healed single crystal of\nMNI at RT was carefully mounted on the diffractometer and the incident X-ray beam was\ncentered on the healed region of the crystal. The complete dataset obtained from a healed\ncrystal was compared against that of a pristine crystal to characterize the self-healing\n(see Tables 3-4). The structural models of both the pristine and neatly healed crystals\nexhibited comparable statistical refinement parameters, such as GooF and R1 (Table 3).\nThe hydrogen bonding parameters (distances and angles) obtained for both pristine and\nhealed crystals were consistent, confirming their identical crystalline phase before and\nafter healing (Table 4). The mosaicity value of the pristine crystal is 0.63 and that of the\nhealed crystal is 0.85; which might be attributed to stress-induced local defects following\nthe fracture and subsequent self-healing, leading to reduction in the number of\nmeasurable reflections.\nComment 8: The reported R-factors (R1 0.06, wR2 0.20) are slightly higher than ideal\nfor small-molecule crystals. Authors should confirm that refinement strategies (e.g.,\nanisotropic displacement parameters, absorption correction, and twin refinement if\nnecessary) were fully optimized.\nwhat is ideal for small-molecule crystals. However, the SCXRD data reported here is that\nof a mechanically manipulated single crystal (after healing). Since we have performed the\nhealing tests at room temperature (RT), we proceeded to collect its diffraction data also\nat RT. The resulting healing crystal has an elongated habit – long in one direction\ncompared to other two, and it was subjected to handling and mounting from mechanical\ntester setup to the XRD mounting loop, which might be the reason for a slight decrease\nin internal quality. To maintain uniformity, we chose a crystal of similar shape and size as\nour pristine crystal and diffracted it also at RT.\nIf we collect SCXRD data for an ideal-sized pristine crystal of MNI at low temperature\n(100K), we indeed get a good R-factor of ~3% as shown in the below table.\nSample parameter Pristine MNI\nFormula C H N O\n4 5 3 2\nCrystal system monoclinic\nSpace group P2 /n\n1\na/Å 3.6722(1)\nb/Å 15.1483(3)\nc/Å 9.7157(2)\nβ/ 94.722(2)\nTemp [K] 100\nR 0.036\n1\nwR2 0.0993\nHowever, the refinement strategies of reported pristine and healed crystals of MNI were\noptimized. Detailed information regarding crystallographic refinement has now been\nincluded in the Methods section as well as Supplementary Note 7.\nComment 9: The manuscript would benefit from additional discussion comparing this\nmechanism with other reported self-healing crystals, such as piezoelectric, ferroelastic,\nand thermally activated systems. Seems very short explanation in all sections..Improve\nit. This contextualization would strengthen the broader impact of the study.\nResponse: We thank the reviewer for bringing this issue to our attention. The manuscript\nis revised as suggested and the newly added text reads as follows (page 11, lines 29-38).\nscope of self-healing to a large set of organic crystals. For instance, piezoelectric based\nself-healing requires that the crystal structure be non-centrosymmetric.22-23 Other\nreported mechanisms of self-healing involve either ferroelastic detwinning12 or\ntemperature induced phase transitions (during heating-cooling cycles)17 or diffusion-\nbased mechanisms.20 Diffusionless self-healing in centrosymmetric crystals needs further\ninvestigation to shed more light on the mechanism involved and reports along these lines\nare scarce.42 A recent report demonstrating self-healing in conjunction with macroscopic\nelasticity and thermal twisting43 indicates that more examples are needed to draw\nconnections among various behaviours of crystals from the structure-property angle.\nComment 10: The narrative is generally strong, but some sections are wordy.\nStreamlining the introduction and providing a clearer roadmap at the beginning would\nhelp readers follow the story more effectively.\nResponse: We thank the reviewer for this thoughtful feedback. We have carefully revised\nthe manuscript to improve flow and clarity to enhance readability by modifying the\nIntroduction section (highlighted in yellow in revised manuscript).\n19"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have carefully revised the manuscript and conducted a more in-depth discussion on the innovation of the paper\nand the self-healing mechanism. We believe that the readability of the article has been effectively improved after the\nrevision. I think the current version can already be published in the journal Nature Communications.", "reviewer_2": "[Note from the Editor: Reviewer submitted only comments to the editor, but had no further technical concerns]", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts.", "reviewer_4": "Reviewer Report:\n1. The authors have satisfactorily addressed all comments and concerns raised during the review process. The revised\nmanuscript now presents a coherent, well-supported, and mechanistically insightful account of ultrafast autonomous self-\nhealing in a centrosymmetric molecular crystal.\n2. The additional experimental data, expanded discussion on symmetry breaking, fracture mechanics, and crystallographic\nrefinement, as well as the newly included schematic illustrations, significantly strengthen the manuscript. The distinction\nbetween local, stress-induced symmetry breaking and conventional phase-transition-driven symmetry changes is now\nclearly articulated and well justified.\n3. Overall, the work represents a substantial conceptual and experimental advance in the field of molecular crystal self-\nhealing and is well suited for publication in Nature Communications."}, "author_response": ""}]

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