The anticipated shift in the focal point of interest of solid-state chemists, crystal engineers, and crystallographers from structure to properties to function parallels the need to apply our accumulated understanding of the intricacies of crystal structure to explaining the related properties, with the ultimate goal of harnessing that knowledge in applications that require soft, lightweight, or biocompatible organic solids. In these developments, the adaptive molecular crystals warrant particular attention as an alternative choice of materials for light, flexible, and environmentally benign devices, primarily memories, capacitors, sensors, and actuators. Some of the outstanding requirements for the application of these dynamic materials as high-efficiency energy-storage devices are strongly induced polarization, a high switching field, and narrow hysteresis in the case of reversible dynamic processes. However, having been studied almost exclusively by chemists, molecular crystals still lack the appropriate investigations that reliably evaluate their reproducibility, scalability, and actuating performance, and some important drawbacks have diverted the interest of engineers from these materials in applications. United under the umbrella term crystal adaptronics, the recent research efforts aim to realistically assess the appositeness of dynamic crystals for applications that require fast, reversible, and continuous operation over prolonged periods of time. With the aim of highlighting the most recent developments, this Perspective discusses their assets and pitfalls. It also provides some hints on the likely future developments that capitalize on the untapped, sequestered potential of this distinct materials class for applications.

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.

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.

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.

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.

An anthracene derivative,9 ,10-dicyanoanthracene, crystallizes as fluorescent needle-like single crystals that can be readily plastically bent in two directions.S patially 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.Unlike conduction of electrons through metal conductors, transduction of light is inherently impervious to interference with external electromagnetic fields,a nd this calls for new organic materials as light-weight, cost-effective and secure optical transducers of information. Thef avorable optical properties and long-range order of molecular crystals is increasingly being recognized as an ew platform for construction of metal-free,a ll-organic electronics and soft robotics.P oor processing ability and less-than-optimal mechanical properties of the organic crystals,and particularly their pronounced brittleness and fragility,h owever,a re usually taken as major impediments against their implementation in flexible devices,inwhich thin organic films have long been superior and the preferred phase of choice.T he recent advent of methodologies for controlled crystal growth has provided strategies for fairly good control over the habit, the aspect ratio,a nd mosaic spread of molecular crystals. [1] Moreover,t he burgeoning research into mechanical properties of molecular crystals has revealed that certain organic crystals can be extraordinarily mechanically compliant;t hey are endowed with atypical properties,s uch as elasticity and plasticity that are comparable to those of metallic conduc-

Flexible organic single crystals are evolving as new materials for optical waveguides that can be used for transfer of information in organic optoelectronic microcircuits.I ntegration in microelectronics of such crystalline waveguides requires downsizing and precise spatial control over their shape and sizea tt he microscale,h owever that currently is not possible due to difficulties with manipulation of these small, brittle objects that are prone to cracking and disintegration. Here we demonstrate that atomic force microscopy(AFM) can be used to reshape,r esizea nd relocate single-crystal microwaveguides in order to attain spatial control over their light output. Using an AFM cantilever tip,m echanically compliant acicular microcrystals of three N-benzylideneanilines were bent to an arbitrary angle,s liced out from ab undle into individual crystals,cut into shorter crystals of arbitrary length, and moved across and aboveasolid surface.W hen excited by using laser light, such bent microcrystals act as active optical microwaveguides that transduce their fluorescence,w ith the total intensity of transduced light being dependent on the optical path length. This micromanipulation of the crystal waveguides using AFM is non-invasive,a nd after bending their emissive spectral output remains unaltered. The approach reported here effectively overcomes the difficulties that are commonly encountered with reshaping and positioning of small delicate objects (the "thick fingers" problem), and can be applied to mechanically reconfigure organic optical waveguides in order to attain spatial control over their output in two and three dimensions in optical microcircuits.

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.