Mechanoresponsive Flexible Crystals

Abstract

Flexible crystals have gained significant attention owing to their remarkable pliability, plasticity, and adaptability, making them highly popular in various research and application fields. The main challenges in developing flexible crystals lie in the rational design, preparation, and performance optimization of such crystals. Therefore, a comprehensive understanding of the fundamental origins of crystal flexibility is crucial for establishing evaluation criteria and design principles. This Perspective offers a retrospective analysis of the development of flexible crystals over the past two decades. It summarizes the elastic standards and possible plastic bending mechanisms tailored to diverse flexible crystals and analyzes the assessment of their theoretical basis and applicability. Meanwhile, the compatibility between crystal elasticity and plasticity has been discussed, unveiling the immense prospects of elastic/plastic crystals for applications in biomedicine, flexible electronic devices, and flexible optics. Furthermore, this Perspective presents state-of-the-art experimental avenues and analysis methods for investigating molecular interactions in molecular crystals, which is vital for the future exploration of the mechanisms of crystal flexibility.

Figure 1

Mechanical plastic crystals. (A) 2-(Methylthio)nicotinic acid crystal bent on the (010) face. The arrows show the point of impact of forceps and needles. The polarizing microscopic images on the (10–1) face and SEM images showed the changes of the bending section. Reproduced from ref (26). Copyright 2006 American Chemical Society. (B) Plastic bending of N-substituted naphthalene diimide derivatives with methyl-vdW groups. Reproduced from ref (48). Copyright 2016 American Chemical Society. (C) 2D plastic bending of the crystal achieved by introducing C–H···O interactions into 1,4-dibromobenzene. Reproduced from ref (49). Copyright 2017 American Chemical Society. (D) Anisotropic packed dimethyl sulfone crystal exhibits plastic bending along the (1–10) crystal plane. Reproduced with permission from ref (50). Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (E) Crystal structures and morphology of Cu-Trz. The blue lines represent the C–H···π interactions and the orange dash lines represent the Cu···Cu interactions. Reproduced with permission from ref (42). Copyright 2022 The Authors, Published by Springer Nature.

Figure 2

Mechanical elastic crystals. (A) Preparation of the elastic cocrystal of methanol, caffeine, and 4-chloro-3-nitrobenzoic acid. Reproduced with permission from ref (27). Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (B) Elastic bending of Schiff base crystals with abundant intermolecular interactions. Reproduced with permission from ref (28). Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) Elastic bending of the cocrystal formed by probenecid and 4,4′-azopyridine. Reproduced with permission from ref (60). Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (D) The elasticity of 1D cadmium(II) halide polymer crystal. Reproduced with permission from ref (61). Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (E) High elasticity of the (E)-1-(4-(dimethylamino)phenyl)iminomethyl-2-hydroxyl-naphthalene crystal. Reproduced with permission from ref (62). Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (F) Elastic crystal of tetrafluorophenyl derivative with slip planes. Reproduced with permission from ref (63). Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 3

Superelastic crystals. (A) The terephthalamide crystal exhibits pseudoelasticity with phase transitions under mechanical stress. Reproduced with permission from ref (68). Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (B) CuQ2-TCNQ crystal shows phase transition induced elongation under mechanical press. Reproduced from ref (69). Copyright 2014 American Chemical Society. (C) Self-healing of the dipyrazolethiuram disulfide crystal under mechanical stress. Reproduced with permission from ref (70). Copyright2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 4

Crystals with both elasticity and plasticity. (A) 2D elastic bending and reconfigurable plastic twisting of 6,6′-((1E,1’E)-hydrazine-1,2-diylidenebis(ethan-1-yl-1-ylidene))bis(3-methylphenol) crystal. Reproduced with permission from ref (72). Copyright 2022 Wiley-VCH GmbH. (B) Plastic twisting and elastic bending of (E)-1-(((3,5-dimethoxyphenyl)imino)methyl)naphthalene-2-ol crystals. Reproduced with permission from ref (73). Copyright 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) Elastic bending plastic twisting of (E)-2-((4-(trifluoromethoxy)phenyl)imino)methyl-6-methoxyphenol crystal. Reproduced with permission from ref (75). Copyright 2023 Elsevier Ltd.

Figure 5

Anisotropic packing model mechanism of plastic deformation. (A) Stacking of dimers along the longest crystal dimension. Notice the weak Me···Me interactions and π–π molecular stacking. (B) 2-(Methylthio)nicotinic acid crystal fracture at the (001) plane and bent on the (010) face. (C) Half-planed surface plots of undeformed and deformed crystals. Comparing the stacked blocks marked in red shows the change in the relative position of the molecules due to the bending model. (D) Lateral expansion and internal contraction of local π–π accumulation in a single molecular column. (E) Bending results in an angle change in the position of the crystal tip. Reproduced from ref (26). Copyright 2006 American Chemical Society.

Figure 6

Slip plane model mechanism of plastic deformation. (A) Strategy of preparing plastic crystals by strong interaction and weak interaction is introduced. (B) Slip planes and plastic bending of naphthalene diimides derivatives. Reproduced from ref (48). Copyright 2016 American Chemical Society. (C) Schematic representation of the process of the fracture and recombination of intermolecular interactions during the plastic bending of the Cu-Trz crystal. Reproduced with permission from ref (42). Copyright 2022 The Authors, Published by Springer Nature.

Figure 7

“Spaghetti” model mechanism of plastic deformation. Schematic representation of the “spaghetti” model for the bending of the Zn-CP crystal. Note that each bundle of straws represents a cluster of CP chains. Referenced from 40.

Figure 8

Interlocking stacking model mechanism of elastic deformation. (A) Crystal ternary eutectic packing in interlocking of the comb-like 2D sheets (blue) and the formation of channels (red) with disordered methanol along the crystallographic c-axis. Formation of dimers by caffeine and 4-chloro-3-nitrobenzoic acid occurs through O–H···N and C–H···O interactions; the caffeine molecules form 1D tapes. Views perpendicular to (100) and the 2D layers in (010). Reproduced with permission from ref (27). Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (B) 2,3-Dichlorobenzylidine-4-bromoaniline crystal stack forms a herringbone stacking. Reproduced with permission from ref (28). Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) (E)-1-(2,3-Dichlorophenyl)-N-(2,5-dichlorophenyl) methanimine crystal stack forms a nearly parallel chain-like structure. Reproduced from ref (84). Copyright 2019 American Chemical Society. (D) First elastic bending mechanism model of interlocking stacking. The change in the angle of herringbone stacking leads to expansion and contraction at the crystal bend. (E) Change in the π–π plane distance causes expansion and contraction at the crystal bend.

Figure 9

Fibril lamella morphology model mechanism of elastic deformation. (A) Molecular structure of BMTP and its molecular columns formed by stacking. (B) Illustration of the common organic crystal breakage and the process of BMTP single crystal’s mechanically induced splitting into fibrous layers when applied stress. Reproduced with permission from ref (66). Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) Schematic representation of the fiber structure change on bending process. (D) Schematic representation of the distance change between molecules. Reproduced with permission from ref (89). Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 10

Reversible molecular rotation model mechanism of elastic deformation. (A) Molecular structure of [Cu(acac)₂]. (B) Crystal structure of unbent [Cu(acac)₂] viewed along the [101] and [010] directions. (C) Schematic representation of the reversible molecular movement mechanism as applied to [Cu(acac)₂]. (D) Schematic representation of compression and expansion for the molecular staking, resulting in the bending of the crystal. Reproduced with permission from ref (90). Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 11

Dynamic physical cross-link frameworks. (A) Molecular structure of PhOH-Cz. (B) PhOH-Cz molecules accumulate to form molecular layers along the (010) crystal face, with colored rectangles indicating areas of weak molecular interaction. (C, D) SEM and AFM images of the PhOH-Cz crystal along the (010) crystal face. The layered structure can be clearly observed, corresponding to the accumulation of microscopic molecules. (E) XRD patterns of U-shaped bent crystals at bent and straight regions (left) and microscopic images of a bent section in U-shaped crystal. Side section view of the trajectory of the line map from the cx to cc area in the crystal. Diagram of the structural perturbation along the crystallographic a-, b-, and c-axes in the unit cell. The corresponding blue, green, and pink arrows in (E) represent slight changes along the different axes. (F) Crystalline structure and molecular packing of the bent PhOH-Cz crystals at the cc and cx points. Reproduced with permission from ref (67). Copyright 2022 Elsevier Inc.

Figure 12

Flexible mechanism of elastic-plastic combination. (A) Schematic illustration of the proposed elasto-plastic bending mechanism of the dimethylammonium perrhenate ionic salt crystal. The reversible rotation of the molecule causes elastic bending of the crystal. However, when the degree of molecular rotation exceeds a certain limit, the molecular columns slide relative to one another, leading to irreversible plastic bending. Reproduced with permission from ref (97). Copyright 2022 2021 Wiley-VCH GmbH. (B) Diagram of elastic and plastic bending of anisotropic response types and their results at different structural levels. (C, D) Two crystals with both elastic bending and plastic twisting responses and their molecular structures and packing patterns. The packing of the molecules creates potential slip planes with cross arrangements, which can allow for molecular slip and relative motion, leading to plastic twisting of the crystal. Reproduced with permission from ref (73), Copyright 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim; ref (75), Copyright 2023 Elsevier Ltd. All rights reserved.

Figure 13

Characterization methods for crystal flexibility. (A) Three-point bending method proves the crystal flexibility. (B) Schematic diagram of nanoindentation technology. Reproduced with permission from ref (67). Copyright 2022 Elsevier Inc. (C) SCXRD characterization of the integrity of the bending crystals. Reproduced with permission from ref (27). Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (D) Schematic representation of narrow φ-scans (yellow circles indicate X-ray beams perpendicular to the page). (E) Optical configuration of MPFM. Reproduced from ref (111). Copyright 2023 American Chemical Society.

Figure 14

Typical applications of flexible molecular crystals. (A) Application of crystal flexibility in the pharmaceutical industry. Changes in drug tablet properties by preparing the flexible crystals blend. Reproduced from ref (154). Copyright 2015 American Chemical Society. (B) Flexible optical waveguides based on single-component organic bulk crystals. Reproduced from ref (149). Copyright 2019 American Chemical Society. (C) Bending promotes the fluorescence enhancement of crystals. Reproduced with permission from ref (169). Copyright 2020 Wiley-VCH GmbH. (D) Bending promotes electrical performance of crystal. Reproduced with permission from ref (19). Copyright 2021 Wiley-VCH GmbH.