Structural Origin of Crystallographic Face‐Dependent Elastoplastic and Plastic Bending in an Organic Molecular Crystal

Priyasha Harsha; Shamim Ahmad; Tapaswini Sethi; C Malla Reddy; Dinabandhu Das

doi:10.1002/chem.71133

. 2026 May 15;32(32):e71133. doi: 10.1002/chem.71133

Structural Origin of Crystallographic Face‐Dependent Elastoplastic and Plastic Bending in an Organic Molecular Crystal

Priyasha Harsha ¹, Shamim Ahmad ², Tapaswini Sethi ¹, C Malla Reddy ^2,^3,^✉, Dinabandhu Das ^1,^✉

PMCID: PMC13505656 PMID: 42139719

ABSTRACT

Crystalline materials that exhibit attractive mechanical responses under external stresses have attracted considerable interest due to their potential applications across multiple fields. Here, we present a crystalline form of 9‐fluorenone hydrazone (9‐FH) that exhibits dual mechanical stress response, that is, plasticity and elastoplastic behavior, on different crystallographic faces. This dual mechanical response arises from specific molecular packing features, wherein intermolecular interactions play a key role in modulating the crystal's mechanical properties from the different faces. Here, large 9‐FH single crystals demonstrate excellent plastic behavior under applied forces, attributed to the reorganization of weak interactions across the slip planes. Moreover, this comprehensive analysis offers detailed insights into the distinct mechanical responses of 9‐FH and the underlying mechanism.

Keywords: elastic bending, elastoplastic, flexible crystal, plastic bending, Schiff base

Single crystals of of 9‐fluorenone hydrazone (9‐FH) exhibits plastic and elastoplastic behaviour on different crystallographic faces. The dual mechanical response arises from specific molecular packing features. The plastic behavior of large 9‐FH single crystals has been attributed to the reorganization of weak interactions across the slip planes in the crystal structure.

graphic file with name CHEM-32-e71133-g005.webp

1. Introduction

Crystallographers and structural chemists have always been fascinated by crystals because of their interesting properties, which depend on crystal engineering, arising from interactions between the constituent elements/molecules in a 3D periodic arrangement [1, 2, 3, 4, 5]. Among the different properties of crystals, recently, mechanical properties have drawn significant attention from materials scientists, owing to their various applications [6, 7, 8, 9, 10, 11, 12]. Recent research on the mechanical properties of single crystals revealed plastic and elastic behavior, besides the most common brittle nature of the crystals [13, 14, 15, 16, 17]. Elasticity and plasticity of crystals depend upon the reversible or irreversible deformation of the crystal upon application of an external stress, respectively [18, 19]. However, all materials show some degree of elasticity. Researchers have recently focused on flexible organic crystals, which can undergo significant deformation without fracturing [20, 21]. The flexibility allows them to function in diverse applications, including optical devices [22, 23, 24], artificial mechanosensors [25], molecular machineries [26], muscle‐mimetic biomaterials [27], and pharmaceuticals [28].

In order to understand the full potential of flexible organic crystals, it is crucial to understand the underlying mechanisms that govern their mechanical behavior [29, 30]. Different research groups have proposed mechanisms pertaining to elastically and plastically flexible crystals largely based on crystal packing and nature of intermolecular interactions, such as hydrogen bonding [17, 31, 32, 33], halogen bonding [34, 35, 36, 37, 38], van der Waals interaction [39, 40], and π stacking [41, 42, 43]. Generally, elastic flexibility is observed in crystals with a combination of weak and dispersive forces between the molecules [44, 45]. In contrast, plastic deformation is found in the organic crystals with anisotropic crystal packing and slip planes, although there are a few cases where the nature of packing and slip plane topology appears to deviate from the general observations [46]. Recently, a detailed mechanistic study of elasto‐plastic mechanical flexibility in a molecular crystal has been reported by Emmerling et al. [47]. Several studies have explored the mechanical properties and flexibility of crystals by modulating halogen and hydrogen bonding interactions in halogenated Schiff bases [48, 49], halogenated esters [50], and co‐crystals [51]. Studies focusing on the mechanisms of crystals with elastic and plastic deformation behavior revealed a strong correlation with the crystal packing and nature of intermolecular interactions with respect to the applied external stress [17, 31, 32, 33, 47, 52]. In recent years, dual mechanical behavior has been reported, combining both elastic and plastic deformations of a single crystal [13, 53, 54, 55, 56, 57, 58]. Such dual behavior, however, requires careful tuning of the crystallographic structure to maintain a balance between the elastic threshold and the point of rupture. This complex balance is challenging to achieve, and only a limited number of molecular materials exhibit detectable elasto‐plasticity at the macroscopic level [59]. Reddy et al. have reported the elastoplastic behavior of a single crystal involving a higher transition metal, emphasizing the structural basis of mechanical flexibility in the crystals [53]. Zhang et al. have reported a flexible organic crystal based on (E)‐1‐(((3,5‐dimethoxyphenyl)imino)methyl)‐naphthalen‐2‐ol, which exhibits both elastic bending and plastic twisting, offering potential applications in polarized rotators [13]. Hao et al. further expanded on this concept by synthesizing 9‐anthraldehyde‐based organic crystals that displayed plastic twisting, two‐dimensional elastic bending, and light‐responsive bending [56]. These studies underscore the importance of developing a deeper understanding of elasto‐plasticity. While some studies have explored the relationship between intermolecular forces and flexibility, a comprehensive understanding of the design principles for achieving both elastic and plastic deformation in single crystals remains elusive [40, 53, 60, 61, 62, 63].

In this study, we report a dual mechanical response of the molecular crystal of an organic compound 9‐fluorenone hydrazone (9‐FH) (Scheme 1), which exhibits both plastic and elastoplastic behavior under applied stress on two distinct crystallographic faces. Although the crystal structure of 9‐FH was reported before, the mechanical properties have not been studied [64]. Mechanistic insight for the dual mechanical property has been provided with the help of single‐crystal x‐ray diffraction analysis. To understand the role of intermolecular interactions in mechanical performance, energy‐frameworks analysis has been performed. Additionally, nanoindentation experiments provided quantitative data on mechanical robustness from the major and minor faces of the single crystals, offering insights into the implications of molecular packing and intermolecular interactions on the different mechanical properties of both faces of the single crystals [65, 66].

2. Experimental Section

2.1. Synthesis and Crystal Growth

The compound 9‐FH has been synthesized by following the previously reported procedure via Schiff base condensation between the corresponding ketone and hydrazine hydrate [64]. Crystallization was carried out using the slow evaporation method in a 1:1 mixture of CHCl₃ and hexane. Colorless needle‐shaped single crystals suitable for x‐ray diffraction of 9‐FH were obtained after 2–3 days.

2.2. Characterization

The synthesized bulk compound is characterized by FT‐IR, ¹H‐NMR, and powder x‐ray diffraction (See Supporting Information for details) and matched with the earlier report. The force‐induced mechanical behavior was visualized in Euromax, Nexius Zoom, and an optical microscope outfitted with polarized plates and a digital camera. The mechanical behavior of the single crystals has been observed by a three‐point bending test in which crystals were supported by two contact points, while force was applied in the middle from opposite directions.

2.3. Face Indexing and X‐Ray Crystallography

Single crystal x‐ray diffraction data were collected on a Bruker D8 Quest single crystal x‐ray diffractometer equipped with a microfocus anode (MoKα) and a PHOTON 100 CMOS detector. The data were integrated and scaled using the Bruker suite programs [67, 68]. The redetermined structure was solved by the direct method and refined using SHELXL. Crystallographic data are given in Table S1.

Face indexing of 9‐FH crystals has been performed on a single crystal. The major face is (001) perpendicular to the c‐axis. The minor side face and bottom minor face are (100) and (010) perpendicular to a‐axis and b‐axis, respectively (Figure 2b).

(a) Schematic representation of **9‐FH** crystal for showing plastic bending at two different faces. (b) Face indexing of the **9‐FH** showing the prominent faces of the crystal. (c) Optical microscopy images showing plastic bending of the **9‐FH** crystal under force applied perpendicular to the (001) plane. (d) Crystal packing of **9‐FH** viewed down a axis with the active slip plane, parallel to (001) plane highlighted by red strips. (e) Optical microscopy images showing elastoplastic bending of the crystal under force applied perpendicular to the (100) plane. (f) Crystal packing of **9‐FH** viewed down c‐axis with strong N─H···N hydrogen bonds highlighted as red dotted lines.

2.4. Theoretical Calculations

The intermolecular interactions in the crystal structure were analyzed using Crystal Explorer 21.5 [69], and pairwise interaction energies were calculated by generating atoms within a radius of 3.8 Å. The tube size (scale factor) used in all energy frameworks was set to 80, and the lower energy threshold (cutoff) was selected at 1 kJ mol⁻¹ for better visualization. The degree of strength of the interactions is related to the thickness of the tube connecting nearby molecules. The interaction energy of 9‐FH is estimated using the accurate unperturbed technique CE‐B3LYP/6‐31 g (d, p) energy model.

2.5. Nanoindentation

Nanoindentation on the major/minor faces of the 9‐FH single crystals was carried out using a state‐of‐the‐art instrument at a maximum 3 mN load from the Hysitron Triboindentor, TI Premier, Minneapolis, USA, equipped with a scanning probe microscope (SPM), using a three‐sided pyramidal Berkovich tip of radius 150 nm in load control mode. Elastic modulus (E) and hardness (H) for both faces were determined using the standard Oliver–Pharr method [70, 71]. The single crystals were glued to a cover slip on their major or minor faces that adhered to a steel stub for the indentation.

3. Results and Discussion

3.1. Crystal Structure Analysis

The crystal structure of 9‐FH is solved in the P2₁ space group, consisting of a full molecule in the asymmetric unit (Figure 1a). The molecules related by 2₁ screw axes are connected by N─H···N (3.085(2) Å) hydrogen bond with the formation of columnar arrangements parallel to the b‐axis (Figure 1b). Weak π···π interactions (3.865 Å) are present between the molecules in the columns. Neighboring columns are connected via C─H···π interactions. Analysis of the crystal structure shows a herringbone pattern viewed down the a‐axis (Figure 1b).

(a) Thermal ellipsoid plot of the asymmetric unit of **9‐FH**; (b) Herringbone pattern in the crystal structure of **9‐FH**, N─H···N hydrogen bond along a column (light green shaded portion), and C─H···π interactions between the neighboring columns are shown by dotted lines (maroon).

3.2. Dual Mechanical Properties of 9‐FH and the Bending Mechanism

The mechanical behavior of the needle‐shaped single crystals of 9‐FH has been studied by the three‐point bending method under polarized microscopy. 9‐FH crystals could be readily bent by applying force perpendicular to the (001) plane, without any fracture. Crystal does not regain its original shape after the removal of forces—that confirms the plastic bending behavior of the crystal (Figure 2a,c, and Video S1). On the other hand, the three‐point bending test on the (100/−100) face shows elasticity up to a certain stress limit. A crystal with a thickness of 20.3 µm displayed elastic strain of approximately 3% (Figure S12), recovering its original shape upon release of the applied stress, which indicates elastic deformation. Beyond this limit, the crystal underwent plastic deformation upon the application of more stress. Notably, within the elastic strain limit, the crystal maintained its elasticity even over multiple deformation steps, as demonstrated in Video S2. A similar observation was made in another crystal with a thickness of 24.2 µm, where elastic bending was clearly maintained up to a strain limit. However, when the elastic strain exceeded approximately 4%–6%, a significant irreversible (plastic) deformation occurred, attributed to the presence of slip planes (Figure 2a,e and Video S3). Despite this, a partial elastic recovery of about 2%–3% was still observed upon removal of the external stress (Figure S14). Face indexing of the 9‐FH indicates that the planes parallel to (001) and (100) are the plastic bending faces (Figure 2b). Plastic bending usually occurs with a narrow curve, while elastoplastic bending shows a smoother and wider curve. This difference is common in many flexible crystals but is rarely pointed out [53].

Analysis of the crystal structure of 9‐FH revealed the rationality behind the plastic and elastoplastic bending behavior observed from two distinct crystallographic faces (bending planes) (Figure 2c,e). In the first bending plane, that is, the (001) plane, when force is applied perpendicular to it, the plastic deformation occurs due to the slippage of molecules parallel to the a‐axis (Figure 2c,d). This mechanism is common in organic molecular crystals and is called the slip plane mechanism [39]. In 9‐FH, the presence of weak C–H···π (3.715 Å) interactions generates active slip planes parallel to the (001) planes (Figure 2d). Thus, when force is applied perpendicular to this plane, that is, along the c‐axis, the weak C─H···π interactions reorganize, resulting in permanent deformation of the crystal. The plastic deformation for the major (001) face occurs with a narrow bending curve (Figure 2c). On the other hand, when force is applied perpendicular to (100) plane, that is, along a‐axis, stronger N─H···N (3.085(2)Å) hydrogen bonding resists the stress to some extent against permanent deformation, due to which, the crystals initially show the elastic bending. Further application of forces surpasses the elastic limit due to rupture of N─H···N hydrogen bonds and other weak interactions, resulting in permanent deformation of the crystal (Figure 2e,f). The bending behavior, depending upon the crystallographic faces, highlights the interplay between weak and strong intermolecular interactions in governing the dual mechanical response of 9‐FH crystals. Interestingly, elastoplastic deformation from the major (100) face creates a smooth and wide bending curve shown in the snapshots of Figure 2e.

Furthermore, the crystals could be twisted manually by putting forces in opposite directions at both ends of the crystal without fracturing (Figure 3). The twisting of the crystals is attributed to plastic bending that occurs when the force is applied perpendicular to two faces, such as (001) and (100) (Figure 2c,e). Notably, N─H···N hydrogen bonding and C─H···π interactions facilitate the slipping of molecules. In the twisting process, both N─H···N hydrogen bonding and C─H···π interactions play a significant role, being the most influential interactions for twisting.

(a) Schematic representation of twisting behavior of **9‐FH** crystal. (b) Twisted crystal of the **9‐FH**.

The structure–mechanical property relationship of 9‐FH crystals was examined using a nanoindentation experiment (Figure 4). In these tests, representative load (P) and depth of penetration (h) response curves were obtained from the major face (001) and minor faces (100) (Figure 4). The E and H values were extracted using the standard Oliver–Pharr method based on the P‒h response curves [38]. As widely recognized, E indicates the crystal's resistance to elastic deformation, while H reflects its resistance to plastic deformation. Here, nanoindentation experiments were done to assess the mechanical properties of a crystal by indenting with a three‐sided pyramidal Berkovich indenter tip with an applied maximum load of 3 mN at load control mode. The penetration depth or displacement of the indenter is measured as a function of this applied load, and at 3 mN, the crystals exhibited an average elastic modulus E (GPa) = 8.03 ± 0.61 and hardness is H (MPa) = 314.12 ± 15.7 (Table 1) from their major face (001), while indenting on the minor face (100) revealed a relatively softer nature with E = 6.72 ± 0.96 GPa and H = 313.18 ± 50.56. The softer nature of the minor face is also reflected in a higher average contact depth (h_c ) of 604.69 ± 50.69 nm (Table 1). The P‒h responses showed significant residual penetration depths, indicating predominant plastic deformation in the crystal and suggesting that the 9‐FH crystals are soft rather than brittle (Figure 4b). The loading curves showed occasional “pop‐ins” for the major face (Figure 4b), which can be attributed to the abrupt penetration of the indenter tip. The softer nature of the minor face (100) may be ascribed to the penetration of the indenter tip nearly parallel to the molecular planes (Figure 5b). Scattered, histogram plots for the H and E are shown in Figure 4; the impressions of the indentation crater are presented in Figure S8.

(a) Schematic representation of the nanindentation experimental setup, multiple P‒h curves from (b) the major (001) and (c) minor (100) faces of the **9‐FH** single crystals, (d) scattered plot of H versus E for both the faces, and histogram plots for (e) E and (f) H of major/minor faces.

TABLE 1.

Nanoindentation property parameters from the major (001) and minor (100) faces of the 9‐FH single crystals.

Faces	Elastic modulus (E, GPa)	Hardness (H, MPa)	Contact depth (h_c , nm)
Major (001)	8.04 ± 0.61	314.12 ± 15.7	598.57 ± 15.33
Minor (100)	6.72 ± 0.96	313.18 ± 50.56	604.69 ± 50.69

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Crystal packing views depict the direction of the indentation on (a) major and (b) minor faces.

Based on these observations, the (001) face with higher E exhibits plastic deformation under applied stress, while the (100) face with lower E demonstrates elastoplastic behavior. Since both faces have nearly similar hardness values, the difference in their deformation behavior mainly arises from their elastic modulus, confirming that the (100) face is more strain‐accommodating than the (001) face.

The energy frameworks analysis reveals that the herringbone arrangement of molecules in the crystal structure of 9‐FH provides an intracolumnar interaction energy of −34.4 kJ/mol (Figure 6). The intercolumnar interaction energy along the c‐axis, which includes weak C─H···π interactions, is −16.2 kJ/mol (see Figure 6). This indicates that the plane parallel to (001) serves as an active slip plane responsible for plastic deformation. In the point bending test, when a force is applied perpendicular to the slip plane (001), the deformation occurs. In contrast, for (100) face, that is, the minor face, the presence of N─H···N interactions allows the crystal to undergo elastic bending up to a certain limit. The total interaction energy for this face is −41.0 kJ/mol (see Figure 6). However, when the applied force exceeds the elastic limit, the crystal undergoes permanent deformation (Figure 6).

The energy frameworks diagram of **9‐FH,** showcasing all the interaction energies and slip plane, that is, (001) plane is indicated by the green shaded portion.

4. Conclusion

In summary, 9‐FH crystals exhibit notable plastic and elastoplastic bending properties along distinct crystallographic faces. The rare combination of plasticity and elasticity in the crystal of 9‐FH arises from the interplay between weak intermolecular forces (C─H···π and C─H···N interactions) and relatively stronger N─H···N and π···π interactions, which collectively contribute to the crystal's flexibility. During the bending process, both N─H···N hydrogen bonding and C─H···π interactions significantly influence crystal deformation, in which the N─H···N hydrogen bonds maintain structural integrity up to a certain stress. However, these bonds break under excessive force, leading to the crystal's permanent deformation. Energy frameworks analysis and nanoindentation experiments were performed to quantify the hardness and stress‐bearing capacity of 9‐FH, providing further insights into the mechanical resilience of the single crystals. Quantitative mechanical property assessment through nanoindentation experiments reveals that the major (001) face has more resistance to elastic deformation, supporting its plastic nature. The crystals undergo plastic deformation from the (001) face, indicating that once the elastic limit is surpassed, it deforms permanently. Conversely, the minor (100) face shows a lower elastic modulus, suggesting decreased stiffness and can exhibit elastoplastic behavior, implying that it can recover partially after deformation. These can be attributed to the relatively parallel penetration of the tip concerning molecular packing.

Conflicts of Interest

The authors have no conflicts of interest.

Supporting information

Supporting File 1: chem71133‐sup‐0001‐SuppMat.docx

CHEM-32-e71133-s002.docx^{(4.6MB, docx)}

Supporting File 2: chem71133‐sup‐0002‐VideoS1.mp4

Download video file^{(2.3MB, mp4)}

Supporting File 3: chem71133‐sup‐0002‐VideoS2.mp4

Download video file^{(8.9MB, mp4)}

Supporting File 4: chem71133‐sup‐0002‐VideoS3.mp4

Download video file^{(14MB, mp4)}

Acknowledgments

PH and TS are grateful to JNU for the fellowship. SA thanks CSIR India for the fellowship (File No. 09/921(0264)/2019‐EMR‐I). CMR thanks SERB for the funding (File No. CRG/2021/004992). DD acknowledges DST for funding (File No. DST/INT/South Africa/P‐02/2021). DD is thankful to DST for providing Single‐Crystal X‐ray diffraction, TGA, and DSC facilities in the School of Physical Sciences, JNU, under the FIST program (File No. SR/FST/PSII‐032/2013(G).

Contributor Information

C. Malla Reddy, Email: cmallareddy@gmail.com.

Dinabandhu Das, Email: jnu.dinu@gmail.com.

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PERMALINK

Structural Origin of Crystallographic Face‐Dependent Elastoplastic and Plastic Bending in an Organic Molecular Crystal

Priyasha Harsha

Shamim Ahmad

Tapaswini Sethi

C Malla Reddy

Dinabandhu Das

ABSTRACT

1. Introduction

SCHEME 1.