Self-Healing Molecular Crystals

Trimodal operation of a robust smart organic crystal

We describe a dynamic crystalline material that integrates mechanical, thermal, and light modes of operation, with unusual robustness and resilience and a variety of both slow and fast kinematic effects that occur on very different time scales. In the mechanical mode of operation, crystals of this material are amenable to elastic deformation, and they can be reversibly morphed and even closed into a loop, sustaining strains of up to about 2.6%. Upon release of the external force, the crystals resume their original shape without any sign of damage, demonstrating outstanding elasticity. Application of torque results in plastic twisting for several rotations without damage, and the twisted crystal can still be bent elastically. The thermal mode of operation relies on switching the lattice at least several dozen times. The migration of the phase boundaries depends on the crystal habit. It can be precisely controlled by temperature, and it is accompanied by both slow and fast motions, including shear deformation and leaping. Parallel boundaries result in a thermomechanical effect, while non-parallel boundaries result in a thermosalient effect. Finally, the photochemical mode of operation is driven by isomerization and can be thermally reverted. The structure of the crystal can also be switched photochemically, and the generation of a bilayer induces rapid bending upon exposure to ultraviolet light, an effect that further diversifies the mechanical response of the material. The small structural changes, low-energy and weak intramolecular hydrogen bonds, and shear deformation, which could dissipate part of the elastic energy, are considered to be the decisive factors for the conservation of the long-range order and the extraordinary diversity in the response of this, and potentially many other dynamic crystalline materials.

Ferroelastic Control of the Multicolor Emission from a Triply Doped Organic Crystal

Emission from crystalline organic solids is often quenched by nonemissive energy-transfer deexcitation processes. While dispersion of fluorophores in polymers or other hosts has been used to enhance the emission intensity, this strategy results in randomization of guest orientation and optical losses at grain boundaries. Here, we report the doping of inherently nonemissive single crystals of anilinium bromide with three fluorescent organic molecules. The doping process equips the crystal with emission characteristics that tune from blue to deep orange. The emission intensity can be reversibly modulated by ferroelastic twinning, which causes the material to function as a multiemissive force sensor. This approach opens up new pathways in the manipulation of emissive properties in organic crystals and may have substantial implications for optoelectronic devices and sensors.

Strategies to Diversification of the Mechanical Properties of Organic Crystals

Structurally ordered soft materials that respond to complementary stimuli are susceptible to control over their spatial and temporal morphostructural configurations by intersectional or combined effects such as gating, feedback, shape-memory, or programming. In the absence of general and robust design and prediction strategies for their mechanical properties, at present, combined chemical and crystal engineering approaches could provide useful guidelines to identify effectors that determine both the magnitude and time of their response. Here, we capitalize on the purported ability of soft intermolecular interactions to instigate mechanical compliance by using halogenation to elicit both mechanical and photochemical activity of organic crystals. Starting from (E)-1,4-diphenylbut-2-ene-1,4-dione, whose crystals are brittle and photoinert, we use double and quadruple halogenation to introduce halogen-bonded planes that become interfaces for molecular gliding, rendering the material mechanically and photochemically plastic. Fluorination diversifies the mechanical effects further, and crystals of the tetrafluoro derivative are not only elastic but also motile, displaying the rare photosalient effect.

Ultradynamic Isoreticularly Expanded Porous Organic Crystals

Porous organic materials showcasing large framework dynamics present new paths for adsorption and separation with enhanced capacity and selectivity beyond the size-sieving limits, which is attributed to their guest-responsive sorption behaviors. Porous hydrogen-bonded crosslinked organic frameworks (HCOFs) are attractive for their remarkable ability to undergo guest-triggered expansion and contraction facilitated by their flexible covalent crosslinkages. However, the voids of HCOFs remain limited, which restrains the extent of the framework dynamics. In this work, we synthesized a series of HCOFs characterized by unprecedented size expansion capabilities induced by solvents. These HCOFs were constructed by isoreticularly co-crystallizing two complementary sets of hydrogen bonding building blocks to generate porous molecular crystals, which were crosslinked through thiol-ene/yne single-crystal-to-single-crystal transformations. The generated HCOFs exhibit enhanced chemical durability, high crystallinity, and extraordinary framework dynamics. For instance, HCOF-104 crystals featuring a pore diameter of 13.6 Å expanded in DMF to 300 ± 10% of their original lengths within just 1 min. This expansion allows the HCOFs to adsorb guest molecules that are significantly larger than the pore sizes of their crystalline states. Through methanol-induced contraction, these large guests were encapsulated in the fast-contracted HCOFs. These advancements in porous framework dynamics pave the way for new methods of encapsulating guests for targeted delivery.

Light-Induced Living Polymer Networks with Adaptive Functional Properties

The advent of covalent adaptable networks (CANs) through the incorporation of dynamic covalent bonds has led to unprecedented properties of macromolecular systems, which can be engineered at the molecular level. Among the various types of stimuli that can be used to trigger chemical changes within polymer networks, light stands out for its remote and spatiotemporal control under ambient conditions. However, most examples of photoactive CANs need to be transparent and they exhibit slow response, side reactions, and limited light penetration. In this vein, it is interesting to understand how molecular engineering of optically active dynamic linkages that offer fast response to visible light can impart "living" characteristics to CANs, especially in opaque systems. Here, the use of carbazole-based thiuram disulfides (CTDs) that offer dual reactivity as photoactivated reshuffling linkages and iniferters under visible light irradiation is reported. The fast response to visible light activation of the CTDs leads to temporal control of shape manipulation, healing, and chain extension in the polymer networks, despite the lack of optical transparency. This strategy charts a promising avenue for manipulating multifunctional photoactivated CANs in a controlled manner.

Highly efficient in crystallo energy transduction of light to work

Various mechanical effects have been reported with molecular materials, yet organic crystals capable of multiple dynamic effects are rare, and at present, their performance is worse than some of the common actuators. Here, we report a confluence of different mechanical effects across three polymorphs of an organic crystal that can efficiently convert light into work. Upon photodimerization, acicular crystals of polymorph I display output work densities of about 0.06-3.94 kJ m-3, comparable to ceramic piezoelectric actuators. Prismatic crystals of the same form exhibit very high work densities of about 1.5-28.5 kJ m-3, values that are comparable to thermal actuators. Moreover, while crystals of polymorph II roll under the same conditions, crystals of polymorph III are not photochemically reactive; however, they are mechanically flexible. The results demonstrate that multiple and possibly combined mechanical effects can be anticipated even for a simple organic crystal.

Shape memory and self-healing in a molecular crystal with inverse temperature symmetry breaking

Mechanically responsive molecular crystals have attracted increasing attention for their potential as actuators, sensors, and switches. However, their inherent structural rigidity usually makes them vulnerable to external stimuli, limiting their usage in many applications. Here, we present the mechanically compliant single crystals of penciclovir, a first-line antiviral drug, achieved through an unconventional ferroelastic transformation with inverse temperature symmetry breaking. These crystals display a diverse set of self-restorative behaviors well above room temperature (385 K), including ferroelasticity, superelasticity, and shape memory effects, suggesting their promising applications in high-temperature settings. Crystallographic analysis reveals that cooperative molecular displacement within the layered crystal structure is responsible for these unique properties. Most importantly, these ferroelastic crystals manifest a polymer-like self-healing behavior even after severe cracking induced by thermal or mechanical stresses. These findings suggest the potential for similar memory and restorative effects in other molecular crystals featuring layered structures and provide valuable insights for leveraging organic molecules in the development of high-performance, ultra-flexible molecular crystalline materials with promising applications.

Room-Temperature Anisotropic Actuation Driven by a Synergistic Order-Disorder and Displacive Phase Transition in a Ferroelectric Crystal

Actuating materials convert different forms of energy into mechanical responses. To satisfy various application scenarios, they are desired to have rich categories, novel functionalities, clear structure-property relationships, fast responses, and, in particular, giant and reversible shape changes. Herein, we report a phase transition-driven ferroelectric crystal, (rac-3-HOPD)PbI3 (3-HOPD = 3-hydroxypiperidine cation), showing intriguingly large and anisotropic room-temperature actuating behaviors. The crystal consists of rigid one-dimensional [PbI3] anionic chains running along the a-axis and discrete disk-like cations loosely wrapping around the chains, leaving room for anisotropic shape changes in both the b- and c-axes. The shape change is switched by a ferroelectric phase transition occurring at around room temperature (294 K), driven by the exceptionally synergistic order-disorder and displacive phase transition. The rotation of the cations exerts internal pressure on the stacking structure to trigger an exceptionally large displacement of the inorganic chains, corresponding to a crystal lattice transformation with length changes of +24.6% and -17.5% along the b- and c-axis, respectively. Single crystal-based prototype devices of circuit switches and elevators have been fabricated by exploiting the unconventional negative temperature-dependent actuating behaviors. This work provides a new model for the development of multifunctional mechanically responsive materials.

Mechanoresponsive Flexible Crystals

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.

Hybrid and composite materials of organic crystals

Organic molecular crystals have historically been viewed as delicate and fragile materials. However, recent studies have revealed that many organic crystals, especially those with high aspect ratios, can display significant flexibility, elasticity, and shape adaptability. The discovery of mechanical compliance in organic crystals has recently enabled their integration with responsive polymers and other components to create novel hybrid and composite materials. These hybrids exhibit unique structure-property relationships and synergistic effects that not only combine, but occasionally also enhance the advantages of the constituent crystals and polymers. Such organic crystal composites rapidly emerge as a promising new class of materials for diverse applications in optics, electronics, sensing, soft robotics, and beyond. While specific, mostly practical challenges remain regarding scalability and manufacturability, being endowed with both structurally ordered and disordered components, the crystal-polymer composite materials set a hitherto unexplored yet very promising platform for the next-generation adaptive devices. This Perspective provides an in-depth analysis of the state-of-the-art in design strategies, dynamic properties and applications of hybrid and composite materials centered on organic crystals. It addresses the current challenges and provides a future outlook on this emerging class of multifunctional, stimuli-responsive, and mechanically robust class of materials.

Collective photothermal bending of flexible organic crystals modified with MXene-polymer multilayers as optical waveguide arrays

The performance of any engineering material is naturally limited by its structure, and while each material suffers from one or multiple shortcomings when considered for a particular application, these can be potentially circumvented by hybridization with other materials. By combining organic crystals with MXenes as thermal absorbers and charged polymers as adhesive counter-ionic components, we propose a simple access to flexible hybrid organic crystal materials that have the ability to mechanically respond to infrared light. The ensuing hybrid organic crystals are durable, respond fast, and can be cycled between straight and deformed state repeatedly without fatigue. The point of flexure and the curvature of the crystals can be precisely controlled by modulating the position, duration, and power of thermal excitation, and this control can be extended from individual hybrid crystals to motion of ordered two-dimensional arrays of such crystals. We also demonstrate that excitation can be achieved over very long distances (>3 m). The ability to control the shape with infrared light adds to the versatility in the anticipated applications of organic crystals, most immediately in their application as thermally controllable flexible optical waveguides for signal transmission in flexible organic electronics.

Modulating Thermal Properties of Polymers through Crystal Engineering

Crystal engineering has exclusively focused on the development of advanced materials based on small organic molecules. We now demonstrate how the cocrystallization of a polymer yields a material with significantly enhanced thermal stability but equivalent mechanical flexibility. Isomorphous replacement of one of the cocrystal components enables the formation of solid solutions with melting points that can be readily fine-tuned over a usefully wide temperature range. The results of this study credibly extend the scope of crystal engineering and cocrystallization from small molecules to polymers.

A Nonswelling Hydrogel with Regenerable High Wet Tissue Adhesion for Bioelectronics

Reducing the swelling of tissue-adhesive hydrogels is crucial for maintaining stable tissue adhesion and inhibiting tissue inflammation. However, reported strategies for reducing swelling always result in a simultaneous decrease in the tissue adhesive strength of the hydrogel. Furthermore, once the covalent bonds break in the currently reported hydrogels, they cannot be rebuilt, and the hydrogel loses its tissue adhesive ability. In this work, a nonswelling hydrogel (named as "PAACP") possessing regenerable high tissue adhesion is synthesized by copolymerizing and crosslinking poly(vinyl butyral) with acrylic acid, gelatin, and chitosan-grafted N-acetyl-l-cysteine. The tissue adhesive strength of the obtained PAACP reaches 211.4 kPa, which is approximately ten times higher than that of the reported nonswelling hydrogels, and the hydrogel can be reused for multiple cycles. The as-prepared hydrogel shows great potential in soft bioelectronics, as muscle fatigue is successfully monitored via the electrode array and strain sensor integrated on PAACP substrates. The success of these bioelectronics offers potential applicability in the long-term diagnosis of muscle-related health conditions and prosthetic manipulations.

Crystal regeneration - a unique growth phenomenon observed in organic crystals post breakage

Crystal regeneration has been observed in macroscopic paracetamol crystals post breakage along their cleavage plane. High resolution imaging confirmed regeneration rates to be 3-fold faster than growth prior to breakage. Further analysis of the solute-solvent interactions is required to elucidate the process which currently lacks linearity with traditional growth theories.

Autonomous and directional flow of water and transport of particles across a subliming dynamic crystal surface

Chemical and morphological traits of natural substrates that can propel and transport fluids over their surfaces have long provided inspiration for the engineering of artificial materials that can harvest and collect water from aerial humidity. Here we report that the gradual widening of parallel microchannels on a surface of a slowly subliming hexachlorobenzene crystal can promote the autonomous and bidirectional transduction of condensed aerial water. Driven by topology changes on the surface of the crystal and water exchange with the gas phase, droplets of condensed water migrate over the crystal. These droplets are also able to transport silver particles and other particulate matter, such as dust. The velocity of the particles was shown to be dependent on both the sublimation rate of the crystal and the relative humidity of its environment. This example of a sublimation-powered water flow demonstrates that topological surface changes accompanying crystal phase transitions can be harnessed to transport liquid and solid matter over surfaces.

Programming Hydrogels with Complex Transient Behaviors via Autocatalytic Cascade Reactions

It is challenging to design complex synthetic life-like systems that can show both autoevolution and fuel-driven transient behaviors. Here, we report a new class of chemical reaction networks (CRNs) to construct life-like polymer hydrogels. The CRNs are constituted of autocatalytic cascade reactions and fuel-driven reaction networks. The reactions start with only two compounds, that is, thiol of 4-arm-PEG-SH and thiuram disulfides, and undergo thiol oxidation (k₁), disulfide metathesis (k₂), and thionate hydrolysis-coupling reactions (k₃) subsequently, leading to a four-state autonomous transition of sol(I) → soft gel → sol(II) → stiff gel. Moreover, thiuram disulfides can be applied as a fuel to drive the repeated occurrence of metathesis and hydrolysis-coupling reactions, generating dissipative stiff gel → sol(II) → stiff gel cycles. Systematic kinetics studies reveal that the event and lifetime of every transient state could be delicately tailored-up by varying the thiuram disulfide concentration, pH of the system, and thiuram structures. Since the consecutive transient behaviors are precisely predictable, we envision the strategy's potential in guiding the molecular designs of autonomous and adaptive materials for many fields.

Autonomous Reconstitution of Fractured Hybrid Perovskite Single Crystals

The outstanding performance and facile processability turn hybrid organic-inorganic perovskites into one of the most sought-after classes of semiconducting materials for optoelectronics. Yet, their translation into real-world applications necessitates that challenges with their chemical stability and poor mechanical robustness are first addressed. Here, centimeter-size single crystals of methylammoniumlead(II) iodide (MAPbI₃ ) are reported to be capable of autonomous self-healing under minimal compression at ambient temperature. When crystals are halved and the fragments are brought in contact, they can readily self-repair as a result of a liquid-like behavior of their lattice at the contact surface, which leads to a remarkable healing with an efficiency of up to 82%. The successful reconstitution of the broken single crystals is reflected in recuperation of their optoelectronic properties. Testing of the healed crystals as photodetectors shows an impressive 74% recovery of the generated photocurrent relative to pristine crystals. This self-healing capability of MAPbI₃ single crystals is an efficient strategy to overcome the poor mechanical properties and low wear resistance of these materials, and paves the way for durable and stable optoelectronic devices based on single crystals of hybrid perovskites.

Global Analysis of the Mechanical Properties of Organic Crystals

Organic crystals, although widely studied, have not been considered nascent candidate materials in the engineering design. Here we summarize the reported mechanical properties of organic crystals reported over the past three decades, and we establish a global mechanical property profile that can be used to predict and identify mechanically robust organic crystals. Being composed of light elements, organic crystals populate a narrow region in the mechanical property-density space between soft, disordered organic materials and stiff, ordered materials. Two subsets of extraordinarily stiff and hard organic crystalline materials were identified and rationalized by the normalized number density, strength and directionality of their intermolecular interactions. We conclude that the future light-weight, soft, all-organic components in devices should capitalize on the combination of long-range structural order and softness as the greatest asset of organic single crystals.

Understanding stress-induced disorder and breakage in organic crystals: beyond crystal structure anisotropy

Crystal engineering has advanced the strategies for design and synthesis of organic solids with the main focus being on customising the properties of the materials. Research in this area has a significant impact on large-scale manufacturing, as industrial processes may lead to the deterioration of such properties due to stress-induced transformations and breakage. In this work, we investigate the mechanical properties of structurally related labile multicomponent solids of carbamazepine (CBZ), namely the dihydrate (CBZ·2H2O), a cocrystal of CBZ with 1,4-benzoquinone (2CBZ·BZQ) and the solvates with formamide and 1,4-dioxane (CBZ·FORM and 2CBZ·DIOX, respectively). The effect of factors that are external (e.g. impact stressing) and/or internal (e.g. phase transformations and thermal motion) to the crystals are evaluated. In comparison to the other CBZ multicomponent crystal forms, CBZ·2H2O crystals tolerate less stress and are more susceptible to breakage. It is shown that this poor resistance to fracture may be a consequence of the packing of CBZ molecules and the orientation of the principal molecular axes in the structure relative to the cleavage plane. It is concluded, however, that the CBZ lattice alone is not accountable for the formation of cracks in the crystals of CBZ·2H2O. The strength and the temperature-dependence of electrostatic interactions, such as hydrogen bonds between CBZ and coformer, appear to influence the levels of stress to which the crystals are subjected that lead to fracture. Our findings show that the appropriate selection of coformer in multicomponent crystal forms, targetting superior mechanical properties, needs to account for the intrinsic stress generated by molecular vibrations and not solely by crystal anisotropy. Structural defects within the crystal lattice, although highly influenced by the crystallisation conditions and which are especially difficult to control in organic solids, may also affect breakage.

Solvate sponge crystals of (DMF)3NaClO4: reversible pressure/temperature controlled juicing in a melt/press-castable sodium-ion conductor

A new type of crystalline solid, termed "solvate sponge crystal", is presented, and the chemical basis of its properties are explained for a melt- and press-castable solid sodium ion conductor. X-ray crystallography and atomistic simulations reveal details of atomic interactions and clustering in (DMF)3NaClO4 and (DMF)2NaClO4 (DMF = N-N'-dimethylformamide). External pressure or heating results in reversible expulsion of liquid DMF from (DMF)3NaClO4 to generate (DMF)2NaClO4. The process reverses upon the release of pressure or cooling. Simulations reveal the mechanism of crystal "juicing," as well as melting. In particular, cation-solvent clusters form a chain of octahedrally coordinated Na+-DMF networks, which have perchlorate ions present in a separate sublattice space in 3 : 1 stoichiometry. Upon heating and/or pressing, the Na+⋯DMF chains break and the replacement of a DMF molecule with a ClO4 - anion per Na+ ion leads to the conversion of the 3 : 1 stoichiometry to a 2 : 1 stoichiometry. The simulations reveal the anisotropic nature of pressure induced stoichiometric conversion. The results provide molecular level understanding of a solvate sponge crystal with novel and desirable physical castability properties for device fabrication.

Anisotropic Dynamics and Mechanics of Macromolecular Crystals Containing Lattice-Patterned Polymer Networks

The mechanical and functional properties of many crystalline materials depend on cooperative changes in lattice arrangements in response to external perturbations. However, the flexibility and adaptiveness of crystalline materials are limited. Additionally, the bottom-up, molecular-level design of crystals with desired dynamic and mechanical properties at the macroscopic level remains a considerable challenge. To address these challenges, we had previously integrated mesoporous, cubic ferritin crystals with hydrogel networks, resulting in hybrid materials (polymer-integrated crystals or PIX) which could undergo dramatic structural changes while maintaining crystalline periodicity and display efficient self-healing. The dynamics and mechanics of these ferritin-PIX were devoid of directionality, which is an important attribute of many molecular and macroscopic materials/devices. In this study, we report that such directionality can be achieved through the use of ferritin crystals with anisotropic symmetries (rhombohedral or trigonal), which enable the templated formation of patterned hydrogel networks in crystallo. The resulting PIX expand and contract anisotropically without losing crystallinity, undergo prompt bending motions in response to stimuli, and self-heal efficiently, capturing some of the essential features of sophisticated biological devices like skeletal muscles.

Multifunctional Features of Organic Charge-Transfer Complexes: Advances and Perspectives

The recent progress of charge-transfer complexes (CTCs) for application in many fields, such as charge transport, light emission, nonlinear optics, photoelectric conversion, and external stimuli response, makes them promising candidates for practical utility in pharmaceuticals, electronics, photonics, luminescence, sensors, molecular electronics and so on. Multicomponent CTCs have been gradually designed and prepared as novel organic active semiconductors with ideal performance and stability compared to single components. In this review, we mainly focus on the recently reported development of various charge-transfer complexes and their performance in field-effect transistors, light-emitting devices, lasers, sensors, and stimuli-responsive behaviors.

Photoreversible Birefringence Change of Diarylethene Single Crystals as Revealed by Change in Molecular Polarizability Anisotropy

Stimuli-responsive organic crystals represent a new frontier of material chemistry. Recently, we have reported photoreversible interference color change to multicolor in single crystals composed of a photochromic diarylethene derivative, 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a), accompanied by the photochromic reaction. The origin of the interference color change is due to the photoinduced birefringence change in the photoisomerization of diarylethenes. In this study, we newly found that single crystals composed of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (2a) also exhibit a photoreversible interference color change. The birefringence value for crystal 2a increased with the photocyclization conversion, while that for crystal 1a decreased. The relationship between the photoinduced birefringence changes for crystals 1a and 2a and their molecular structures was discussed based on the change in the molecular polarizability anisotropy accompanied by the photochromic reaction. These results would provide not only new opportunities for the application of photochromic crystals but also useful strategies for the design of crystalline materials that exhibit the desired birefringence change.

High mobility of lattice molecules and defects during the early stage of protein crystallization

Protein crystals are expected to be useful not only for their molecular structure analysis but also as functional materials due to their unique properties. Although the generation and the propagation of defects during crystallization play critical roles in the final properties of protein crystals, the dynamics of these processes are poorly understood. By time-resolved liquid-cell transmission electron microscopy, we observed that nanosized crystal defects are surprisingly mobile during the early stages of the crystallization of a lysozyme as a model protein. This highly dynamic behavior of defects reveals that the lattice molecules are mobile throughout the crystal structure. Moreover, the disappearance of the defects indicated that intermolecular bonds can break and reform rapidly with little energetic cost, as reported in theoretical studies. All these findings are in marked contrast to the generally accepted notion that crystal lattices are rigid with very limited mobility of individual lattice molecules.

Efficiently self-healing boronic ester crystals

The perception of organic crystals being rigid static entities is quickly eroding, and molecular crystals are now matching a number of properties previously thought to be unique to soft materials. Here, we present crystals of a boronate ester that encompass many of the elastic and plastic mechanical properties of polymers such as bending, twisting, coiling and highly efficient self-healing of up to 67%, while they maintain their long-range structural order. The approach utilizes the concept of dynamic covalent chemistry and proves it can be applied towards ordered materials. This work expands our current understanding of the properties of crystalline molecular materials, and it could have implications towards the development of mechanically robust organic crystals that are capable of self-repair for durable all-organic electronics and soft robotics.

Organic crystal is described that can be bent plastically and twisted elastically, and can self-heal to 67%, an efficiency that is an order-of-magnitude higher compared to the only previously reported example.

Shape-memory effects in molecular crystals

Molecular crystals can be bent elastically by expansion or plastically by delamination into slabs that glide along slip planes. Here we report that upon bending, terephthalic acid crystals can undergo a mechanically induced phase transition without delamination and their overall crystal integrity is retained. Such plastically bent crystals act as bimorphs and their phase uniformity can be recovered thermally by taking the crystal over the phase transition temperature. This recovers the original straight shape and the crystal can be bent by a reverse thermal treatment, resulting in shape memory effects akin of those observed with some metal alloys and polymers. We anticipate that similar memory and restorative effects are common for other molecular crystals having metastable polymorphs. The results demonstrate the advantage of using intermolecular interactions to accomplish mechanically adaptive properties with organic solids that bridge the gap between mesophasic and inorganic materials in the materials property space.

Molecular crystals can be bent elastically by expansion or contraction on opposite faces, or plastically by delamination into slabs that glide along slip planes. Here the authors report crystals that can be bent plastically while undergoing a mechanically induced phase transition without delamination.

Thermodynamically stable whilst kinetically labile coordination bonds lead to strong and tough self-healing polymers

There is often a trade-off between mechanical properties (modulus and toughness) and dynamic self-healing. Here we report the design and synthesis of a polymer containing thermodynamically stable whilst kinetically labile coordination complex to address this conundrum. The Zn-Hbimcp (Hbimcp = 2,6-bis((imino)methyl)-4-chlorophenol) coordination bond used in this work has a relatively large association constant (2.2 × 10¹¹) but also undergoes fast and reversible intra- and inter-molecular ligand exchange processes. The as-prepared Zn(Hbimcp)₂-PDMS polymer is highly stretchable (up to 2400% strain) with a high toughness of 29.3 MJ m⁻³, and can autonomously self-heal at room temperature. Control experiments showed that the optimal combination of its bond strength and bond dynamics is responsible for the material’s mechanical toughness and self-healing property. This molecular design concept points out a promising direction for the preparation of self-healing polymers with excellent mechanical properties. We further show this type of polymer can be potentially used as energy absorbing material.

There is often a trade-off between mechanical properties (modulus and toughness) and dynamic self-healing in materials. Here the authors design and synthesize a polymer containing thermodynamically stable whilst kinetically labile coordination complexes to address this conundrum.

Advanced Materials for Next-Generation Spacecraft

Spacecraft are expected to traverse enormous distances over long periods of time without an opportunity for maintenance, re-fueling, or repair, and, for interplanetary probes, no on-board crew to actively control the spacecraft configuration or flight path. Nevertheless, space technology has reached the stage when mining of space resources, space travel, and even colonization of other celestial bodies such as Mars and the Moon are being seriously considered. These ambitious aims call for spacecraft capable of self-controlled, self-adapting, and self-healing behavior. It is a tough challenge to address using traditional materials and approaches for their assembly. True interplanetary advances may only be attained using novel self-assembled and self-healing materials, which would allow for realization of next-generation spacecraft, where the concepts of adaptation and healing are at the core of every level of spacecraft design. Herein, recent achievements are captured and future directions in materials-driven development of space technology outlined.

Elastic Self-Doping Organic Single Crystals Exhibiting Flexible Optical Waveguide and Amplified Spontaneous Emission

Organic crystals are generally brittle and tend to crack under applied stress. Doped organic crystals are even more brittle because of lattice defects. Herein, the first doped organic crystals 1d@2d, which display elastic bending ability under applied stress, are reported. Moreover, the potential applications of elastic-doped crystals 1d@2d in flexible optoelectronics are impressively demonstrated. The elastic crystals 1d@2d with high quality and large size are crystalized by a simple and unique "self-doping" process, which is a regular solution evaporation of crude product 1d (2,5-dihydro-3,6-bis(octylamino)terephthalate) containing a minute amount of 2d (3,6-bis(octylamino)terephthalate) as the oxidized byproduct. The host 1d is easily crystallized to form elastic crystals but is nonfluorescent, while the guest 2d has poor crystallinity and is highly emissive. The doping approach integrates the advantages of both 1d and 2d, and thus endows doped crystals 1d@2d with good elasticity as well as intense orange fluorescence. Taking these advantages, the application potentials of these doped crystals 1d@2d are evaluated by measuring optical waveguide and amplified spontaneous emission in both the straight and bent states.

Hyperexpandable, self-healing macromolecular crystals with integrated polymer networks

The formation of condensed matter typically involves a trade-off between structural order and flexibility. As the extent and directionality of interactions between atomic or molecular components increase, materials generally become more ordered but less compliant, and vice versa. Nevertheless, high levels of structural order and flexibility are not necessarily mutually exclusive; there are many biological (such as microtubules1,2, flagella 3 , viruses4,5) and synthetic assemblies (for example, dynamic molecular crystals6-9 and frameworks10-13) that can undergo considerable structural transformations without losing their crystalline order and that have remarkable mechanical properties8,14,15 that are useful in diverse applications, such as selective sorption 16 , separation 17 , sensing 18 and mechanoactuation 19 . However, the extent of structural changes and the elasticity of such flexible crystals are constrained by the necessity to maintain a continuous network of bonding interactions between the constituents of the lattice. Consequently, even the most dynamic porous materials tend to be brittle and isolated as microcrystalline powders 14 , whereas flexible organic or inorganic molecular crystals cannot expand without fracturing. Owing to their rigidity, crystalline materials rarely display self-healing behaviour 20 . Here we report that macromolecular ferritin crystals with integrated hydrogel polymers can isotropically expand to 180 per cent of their original dimensions and more than 500 per cent of their original volume while retaining periodic order and faceted Wulff morphologies. Even after the separation of neighbouring ferritin molecules by 50 ångströms upon lattice expansion, specific molecular contacts between them can be reformed upon lattice contraction, resulting in the recovery of atomic-level periodicity and the highest-resolution ferritin structure reported so far. Dynamic bonding interactions between the hydrogel network and the ferritin molecules endow the crystals with the ability to resist fragmentation and self-heal efficiently, whereas the chemical tailorability of the ferritin molecules enables the creation of chemically and mechanically differentiated domains within single crystals.

Crystals on the move: mechanical effects in dynamic solids

When exposed to external stimuli such as heat or light, certain single crystals can acquire momentum and undergo motion. On a molecular scale, the motility of such dynamic single crystals is triggered by a phase transition or chemical reaction without gaseous products, and macroscopically manifests as either slow (reversible or irreversible) deformation, or as rapid, almost instantaneous propulsion of the crystals that is oftentimes accompanied by disintegration. While the elastic energy of the slow reconfiguration processes such as bending, twisting and coiling can be utilized for actuation of other objects, the fast disintegrative processes could be exploited to initiate pressure-sensitive applications. This short review intends to summarize recent developments in the growing research on dynamic crystals, especially aspects of the mechanism of rapid motion of thermosalient and photosalient (leaping) crystals. The collective evidence indicates that these solids are organic-based analogues of the inorganic martensitic materials. While qualitative explanation of the molecular processes that lead to the related dynamic phenomena can be provided, quantification of their kinematics, estimation of the useful work that can be extracted, and prediction of their occurrence are yet to be established. Harnessing the potential of these materials to rapidly and efficiently perform the fundamentally important process of transduction of heat or light into kinetic energy appears as a prospective basis for their application in motion gears and devices.

Cluster-model DFT simulations of the infrared spectra of triazine-based molecular crystals

A cluster-model approach is developed to simulate the IR spectra of triazine-based molecular crystals, and the distinct hydrogen-bonding environments of the crystallographically independent molecules can be clearly revealed.

Self-Healing Behavior in a Thermo-Mechanically Responsive Cocrystal during a Reversible Phase Transition

The molecular-level motions of a coronene-based supramolecular rotator are amplified into macroscopic changes of crystals by co-assembly of coronene and TCNB (1,2,4,5-tetracyanobenzene) into a charge-transfer complex. The as-prepared cocrystals show remarkable self-healing behavior and thermo-mechanical responses during thermally-induced reversible single-crystal-to-single-crystal (SCSC) phase transitions. Comprehensive analysis of the microscopic observations as well as differential scanning calorimetry (DSC) measurements and crystal habits reveal that a thermally-reduced-rate-dependent dynamic character exists in the phase transition. The crystallographic studies show that the global similarity of the packing patterns of both phases with local differences, such as molecular stacking sequence and orientations, should be the origin of the self-healing behavior of these crystals.