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The dense and ordered molecular arrangements endow dynamic molecular crystals with fast response, rapid energy conversion, low energy dissipation, and strong coupling between heat/light and mechanical energy. Most of the known dynamic crystals can only respond to a single stimulus, and materials that can respond to multiple stimuli are rare. Here, we report an organic crystalline material that can be bent plastically and is also thermosalient, as its crystals can move when they undergo a reversible phase transition. The crystals transmit light regardless of their shape or crystalline phase. The combination of light transduction and reversible thermomechanical deformation provides an opportunity to switch the waveguiding capability of the material in a narrow temperature range, which holds a tremendous potential for applications in heat‐averse electronic components, such as central processing units. Unlike existing electronics, the material we report here is completely organic and therefore much lighter, potentially reducing the overall weight of electronic circuits. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The cornerstones of the advancement of flexible optoelectronics are the design, preparation, and utilization of novel materials with favorable mechanical and advanced optoelectronic properties. Molecular crystalline materials have emerged as a class of underexplored yet promising materials due to the reduced grain boundaries and defects anticipated to provide enhanced photoelectric characteristics. An inherent drawback that has precluded wider implementation of molecular crystals thus far, however, has been their brittleness, which renders them incapable of ensuring mechanical compliance required for even simple elastic or plastic deformation of the device. It is perplexing that despite a plethora of reports that have in the meantime become available underpinning the flexibility of molecular crystals, the "discovery" of elastically or plastically deformable crystals remains limited to cases of serendipitous and laborious trial-and-error approaches, a situation that calls for a systematic and thorough assessment of these properties and their correlation with the structure. This review provides a comprehensive and concise overview of the current understanding of the origins of crystal flexibility, the working mechanisms of deformations such as plastic and elastic bending behaviors, and insights into the examples of flexible molecular crystals, specifically concerning photoelectronic changes that occur in deformed crystals. We hope this summary will provide a reference for future experimental and computational efforts with flexible molecular crystals aimed towards improving their mechanical behavior and optoelectronic properties, ultimately intending to advance the flexible optoelectronic technology. [ABSTRACT FROM AUTHOR]

The capability of stimulated response by mechanical deformation to induce motion or actuation is the foundation of lightweight organic, dynamic materials for designing light and soft robots. Various biomimetic soft robots are constructed to demonstrate the vast versatility of responses and flexibility in shape-shifting. We now report that the integration of organic molecular crystals and polymers brings about synergistic improvement in the performance of both materials as a hybrid materials class, with the polymers adding hygroresponsive and thermally responsive functionalities to the crystals. The resulting hybrid dynamic elements respond within milliseconds, which represents several orders of magnitude of improvement in the time response relative to some other type of common actuators. Combining molecular crystals with polymers brings crystals as largely overlooked materials much closer to specific applications in soft (micro)robotics and related fields. Response by mechanical deformation to give actuation is essential in the design of soft robots. Here, the authors demonstrate that combining organic molecular crystals with polymers makes it possible to create materials with susceptible properties to changes in temperature and humidity. [ABSTRACT FROM AUTHOR]

Reshaping of elongated organic crystals that can be used as semiconductors, waveguides or soft robotic grippers by application of force or light is now a commonplace, however mechanical response of organic crystals to changes in humidity has not been accomplished yet. Here, we report a universal approach to instigating a humidity response into elastically bendable organic crystals that elicits controllable deformation with linear response to aerial humidity while retaining their physical integrity entirely intact. Hygroresponsive bilayer elements are designed by mechanically coupling a humidity‐responsive polymer with elastic molecular crystals that have been mechanically reinforced by a polymer coating. As an illustration of the application of these cladded crystalline actuators, they are tested as active optical transducers of visible light where the position of light output can be precisely controlled by variations in aerial humidity. Within a broader context, the approach described here provides access to a vast range of mechanically robust, lightweight hybrid hygroresponsive crystalline materials. [ABSTRACT FROM AUTHOR]

This review presents a critical and comprehensive overview of current experimental measurements of complete elastic constant tensors for molecular crystals. For a large fraction of these molecular crystals, detailed comparisons are made with elastic tensors obtained using the corrected small basis set Hartree–Fock method S‐HF‐3c, and these are shown to be competitive with many of those obtained from more sophisticated density functional theory plus dispersion (DFT‐D) approaches. These detailed comparisons between S‐HF‐3c, experimental and DFT‐D computed tensors make use of a novel rotation‐invariant spherical harmonic description of the Young's modulus, and identify outliers among sets of independent experimental results. The result is a curated database of experimental elastic tensors for molecular crystals, which we hope will stimulate more extensive use of elastic tensor information—experimental and computational—in studies aimed at correlating mechanical properties of molecular crystals with their underlying crystal structure. [ABSTRACT FROM AUTHOR]

Molecular crystals are promising anisotropic optical transducing media for next‐generation optoelectronic microdevices that will be capable of secure transduction of information and impervious to external electromagnetic interference. However, their full potential has not been explored yet due to their poor processability, low mechanical compliance, pronounced brittleness and high proneness to cracking that often result in irrecoverable damage. These issues are detrimental to their ability to transduce light. Here, a novel strategy is presented based on 3D epitaxial crystal growth of organic/inorganic crystals based on charge‐assisted hydrogen bonds that can be used to efficiently weld broken molecular single‐crystalline optical waveguides and restore their light‐transducing capability. This approach can also be applied to prepare asymmetric multidomain crystalline heterostructures starting from isostructural molecular tectons, resulting in novel opto/electro/mechanical functionalities in the hybrid materials. It also removes an important obstacle toward wider application of molecular crystals in the next‐generation optoelectronics. [ABSTRACT FROM AUTHOR]

The mention of the word "crystal" invokes images of minerals, gems, and rocks, all of which are inevitably solid, hard, and durable entities with well‐defined smooth faces and straight edges. With the discovery in the first half of the 20th century that many molecular crystals are soft and can be deformed in a similar way as rubber or plastic, this perception is changing, and both the concept and formal definition of what a crystal is may require reinterpretation. The seemingly naïve question posed in the title of this Minireview does not have a simple answer. Here, we discuss how the effects of the elastic and plastic deformation of molecular crystals on the diffraction signature give primary evidence of their degree of crystallinity. In most cases, the definition of a crystal holds for both elastically and plastically deformed crystals and, unless there is significant or complete physical separation of the crystal during the deformation, they can safely be considered (deformed) single crystals with a high concentration of defects. [ABSTRACT FROM AUTHOR]

An anthracene derivative, 9,10‐dicyanoanthracene, crystallizes as fluorescent needle‐like single crystals that can be readily plastically bent in two directions. Spatially resolved photoluminescence analysis revealed that this material has robust optoelectronic properties that are preserved upon extreme crystal deformation. The highly flexible crystals were successfully tested as efficient switchable optical waveguiding elements for both active and passive light transduction, and the mode of operation depends on the wavelength of the incident light. This prototypical dual‐mode organic optical crystalline fiber brings mechanically compliant molecular organic crystals closer to applications as novel light‐transducing media for wireless transfer of information in all‐organic micro‐optoelectronic devices. Mechanically compliant organic crystals from a simple anthracene derivative were tested as optical waveguides for both active and passive light transduction. The mode of operation depends on the wavelength of the incident light. [ABSTRACT FROM AUTHOR]

Various aspects of molecular motion in crystals have been extensively studied in different research fields of chemistry as a valuable source of structural and dynamic information en route to new smart materials and solid-state molecular machines. Recent research efforts have been directed towards engineering crystalline media with specific motile components, namely, amphidynamic crystals, whose dynamics can be exploited to achieve specific functions such as sensing, gas separation and switchable dielectrics. The most promising structural models within this line of pursuit are based upon anisotropic Brownian rotary trajectories. In this highlight, we review the recent advances in this field, with particular emphasis on potential applications. This summary should provide useful guidelines for further development of this remarkable class of materials. [ABSTRACT FROM AUTHOR]

Abstract: Mechanically reconfigurable molecular crystals—ordered materials that can adapt to variable operating and environmental conditions by deformation, whereby they attain motility or perform work—are quickly shaping a new research direction in materials science, crystal adaptronics. Properties such as elasticity, superelasticity, and ferroelasticity, which are normally related to inorganic materials, and phenomena such as shape‐memory and self‐healing effects, which are well‐established for soft materials, are increasingly being reported for molecular crystals, yet their mechanism, quantification, and relation to the crystal structure of organic crystals are not immediately apparent. This Minireview provides a condensed topical overview of elastic, superelastic, and ferroelastic molecular crystals, new classes of materials that bridge the gap between soft matter and inorganic materials. The occurrence and detection of these unconventional properties, and the underlying structural features of the related molecular materials are discussed and highlighted with selected prominent recent examples. [ABSTRACT FROM AUTHOR]

Abstract: The first example of a smart crystalline material, the 2:1 cocrystal of probenecid and 4,4′‐azopyridine, which responds reversibly to multiple external stimuli (heat, UV light, and mechanical pressure) by twisting, bending, and elastic deformation without fracture is reported. This material is also able to self‐heal on heating and cooling, thereby overcoming the main setbacks of molecular crystals for future applications as crystal actuators. The photo‐ and thermomechanical effects and self‐healing capabilities of the material are rooted in reversible trans–cis isomerization of the azopyridine unit and crystal‐to‐crystal phase transition. Fairly isotropic intermolecular interactions and interlocked crisscrossed molecular packing secure high elasticity of the crystals. [ABSTRACT FROM AUTHOR]

One of the most inevitable limitations of any material that is exposed to mechanical impact is that they are inexorably prone to mechanical damage, such as cracking, denting, gouging, or wearing. To confront this challenge, the field of polymers has developed materials that are capable of autonomous self-healing and recover their macroscopic integrity similar to biological organisms. However, the study of this phenomenon has mostly remained within the soft materials community and has not been explored by solid-state organic chemists. The first evidence of self-healing in a molecular crystal is now presented using crystals of dipyrazolethiuram disulfide. The crystals were mildly compressed and the degree of healing was found to be 6.7 %. These findings show that the self-healing properties can be extended beyond mesophasic materials and applied towards the realm of ordered solid-state compounds. [ABSTRACT FROM AUTHOR]

Thermosalient crystals are solids that exhibit motion at the macroscale as a consequence of a thermally induced phase transition. They represent an interesting scientific phenomenon and could be useful as actuators for the conversion of thermal energy into motion or mechanical work. The potential utilization of these miniature transducers in real-world devices requires a controllable phase transition (i.e. a predetermined temperature). While it is difficult to control these performances with a single-component molecular crystal, “tunable” properties could be accomplished by solid solutions. To verify this hypothesis, the thermosalient material [Zn(bpy)Br2] (bpy = 2,2′-bipyridine) was selected and its synthesis was performed in the presence of chloride ions. The resulting mixed crystals ([Zn(bpy)Br2xCl2(1−x)]) show that the product undergoes the expected thermosalient phase transition, and the temperature of the onset of the phase transition and the transition enthalpy depend on the Cl/Br ratio. [ABSTRACT FROM AUTHOR]

The article offers information on the current state of the emerging field of mechanically responsive single crystals. It summarizes observations of single crystals that can bend, twist, hop, spin, explode, split, roll, or respond in other ways to stimuli, such as light or heat. A general model for the mechanical effects in molecular crystals is also presented.

The exceptional mechanical flexibility observed with certain organic crystals defies the common perception of single crystals as brittle objects. Here, we describe the morphostructural consequences of plastic deformation in crystals of hexachlorobenzene that can be bent mechanically at multiple locations to 360° with retention of macroscopic integrity. This extraordinary plasticity proceeds by segregation of the bent section into flexible layers that slide on top of each other, thereby generating domains with slightly different lattice orientations. Microscopic, spectroscopic and diffraction analyses of the bent crystal showed that the preservation of crystal integrity when stress is applied on the (001) face requires sliding of layers by breaking and re-formation of halogen-halogen interactions. Application of stress on the (100) face, in the direction where π···π interactions dominate the packing, leads to immediate crystal disintegration. Within a broader perspective, this study highlights the yet unrecognized extraordinary malleability of molecular crystals with strongly anisotropic supramolecular interactions. [ABSTRACT FROM AUTHOR]

The classical perception of single crystals of molecular materials as rigid and brittle entities has downsized the research interest in their mechanical effects that had been initiated and was active back in the 1980s. More recently, the modern analytical techniques for their mechanical, electron-microscopic, structural, spectroscopic and kinematic characterization have contributed to accumulate compelling evidence that under certain circumstances, even some seemingly rigid single crystals can deform, bend, twist, hop, wiggle or perform other 'acrobatics' that are atypical for non-soft matter. These examples contribute to a paradigm shift in our understanding of the elasticity of molecular crystals and also provide direct mechanistic insight into the structural perturbations at the limits of the susceptibility of ordered matter to internal and external mechanical forces. As the relevance of motility and reshaping of molecular crystals is being recognized by the crystal research community as a demonstration of a very basic concept-conversion of thermal or light energy into work-a new and exciting crystal chemistry around mechanically responsive single crystals rapidly unfolds. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Plastic bending of organic crystals is a well-known, yet mechanistically poorly understood phenomenon. On three structurally related epimers, derivatives of galactose, glucose, and mannose, it is demonstrated here that small changes in the molecular structure can have a profound effect on the mechanical properties. While the galactose derivative affords crystals which can be easily bent, the crystals of the derivatives of glucose and mannose are brittle and do not bend. Structural, microscopic, and mechanical evidence is provided showing that hydrogen bonding of water molecules is the key element for sliding over the slip planes in the crystal and accounts for the plastic bending. [ABSTRACT FROM AUTHOR]

The range of unit cell orientations generated at the kink of a bent single crystal poses unsurmountable challenges with diffraction analysis and limits the insight into the molecular-scale mechanism of bending. On a plastically bent crystal of hexachlorobenzene, it is demonstrated here that spatially resolved microfocus infrared spectroscopy using synchrotron radiation can be applied in conjunction with periodic density functional theory calculations to predict spectral changes or to extract information on structural changes that occur as a consequence of bending. The approach reproduces well the observed trends, such as the wall effects, and provides estimations of the vibrational shifts, unit cell deformations, and intramolecular parameters. Generally, expansion of the lattice induces red-shift while compression induces larger blue-shift of the characteristic ν(C-C) and ν(C-Cl) modes. Uniform or non-uniform expansion or contraction of the unit cell of 0.1 Šresults in shifts of several cm-1, whereas deformation of the cell of 0.5° at the unique angle causes shifts of <0.5 cm-1. Since this approach does not include parameters related to the actual stimulus by which the deformation has been induced, it can be generalized and applied to other mechanically, photochemically, or thermally bent crystals. [ABSTRACT FROM AUTHOR]

The thermosalient crystals of terephthalic acid are extraordinarily mechanically compliant and reversibly shape-shift between two forms with different crystal habits. While the transition of form II to form I is spontaneous, the transition of form I to form II is latent and can be triggered by applying local mechanical stress, whereby crystals leap several centimeters in air. This mechanosalient effect (mechanically stimulated motility) is due to sudden release of strain that has accrued in the crystal of form I, which is a metastable structure at ambient conditions. High-speed optical analysis and serial scanning electron microscopy reveal that the mechanical effect is due to rapid reshaping of crystal domains on a millisecond time scale triggered by mechanical stimulation. Mechanically pre-deformed crystals taken over the thermal phase transition exhibit memory effects and partially regain their shape, while cracked, sliced, or otherwise damaged crystals tend to recover their macroscopic integrity by restorative action of intermolecular π-π interactions in a manner which resembles the behavior of shape-memory and self-healing polymers. These observations provide additional evidence that the thermo-/photo-/mechanosalient effects are macroscopic manifestations of martensitic-type transitions in molecular solids. [ABSTRACT FROM AUTHOR]

The increasing number of reports on photomechanical effects in molecular crystals necessitates systematic studies to understand the intrinsic and external effectors that determine and have predictive power of their type and magnitude. Differential light absorption and product gradient between the surface and the bulk of the crystal are often invoked to qualitatively explain the mechanical response of crystals to light; however, the details on how this difference in photochemical response accounts for macroscopic effects such as surface modification, deformation, or disintegration of crystals are yet to be established. Using both bulk- and surface-sensitive analytical techniques, a rare instance of benzylidenefuranone crystals is studied here, and it is capable of several distinct types of photomechanical response including surface striation and delamination, photosalient effect (ballistic disintegration and motion), and photoinduced bending by dimerization. The results provide a holistic view on these effects and set the stage for the development of overarching theoretical models to describe the photomechanics in the ordered solid state. [ABSTRACT FROM AUTHOR]

The growing realization that photoinduced bending of slender photoreactive single crystals is surprisingly common has inspired researchers to control crystal motility for actuation. However, new mechanically responsive crystals are reported at a greater rate than their quantitative photophysical characterization; a quantitative identification of measurable parameters and molecular-scale factors that determine the mechanical response has yet to be established. Herein, a simple mathematical description of the quasi-static and time-dependent photoinduced bending of macroscopic single crystals is provided. This kinetic model goes beyond the approximate treatment of a bending crystal as a simple composite bilayer. It includes alternative pathways for excited-state decay and provides a more accurate description of the bending by accounting for the spatial gradient in the product/reactant ratio. A new crystal form (space group P21/n) of the photoresponsive azo-dye Disperse Red 1 (DR1) is analyzed within the constraints of the aforementioned model. The crystal bending kinetics depends on intrinsic factors (crystal size) and external factors (excitation time, direction, and intensity). [ABSTRACT FROM AUTHOR]