A Multidirectional Superelastic Organic Crystal by Versatile Ferroelastical Manipulation

Mechanical twinning changes atomic, molecular, and crystal orientations along with directions of the anisotropic properties of the crystalline materials while maintaining single crystallinity in each domain. However, such deformability has been less studied in brittle organic crystals despite their remarkable anisotropic functions. Herein we demonstrate a direction‐dependent mechanical twinning that shows superelasticity in one direction and ferroelasticity in two other directions in a single crystal of 1,3‐bis(4‐methoxyphenyl)urea. The crystal can undergo stepwise twinning and ferroelastically forms various shapes with multiple domains oriented in different directions, thereby affording a crystal that shows superelasticity in multiple directions. This adaptability and shape recoverability in a ferroelastic and superelastic single crystal under ambient conditions are of great importance in future applications of organic crystals as mechanical materials, such as in soft robotics.     

Ferroelasticity in an Organic Crystal: A Macroscopic and Molecular Level Study

Ferroelasticity has been relatively well‐studied in mechanically robust inorganic atomic solids but poorly investigated in organic crystals, which are typically inherently fragile. The absence of precise methods for the mechanical analysis of small crystals has, no doubt, impeded research on organic ferroelasticity. The first example of ferroelasticity in an organic molecular crystal of 5‐chloro‐2‐nitroaniline is presented, with thorough characterization by macro‐ and microscopic methods. The observed cyclic stress–strain curve satisfies the requirements of ferroelasticity. Single‐crystal X‐ray structure analysis provides insight into lattice correspondence at the twining interface, which enables substantial crystal bending by a large molecular orientational shift. This deformation represents the highest maximum strain (115.9 %) among reported twinning materials, and the associated dissipated energy density of 216 kJ m⁻³ is relatively large, which suggests that this material is potentially useful as a mechanical damping agent.     

Crystal Adaptronics: Mechanically Reconfigurable Elastic and Superelastic Molecular Crystals

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.     

A Crystalline π‐Stack Containing Five Stereoisomers: Insights into Conformational Isomorphism,Chirality Inversion,and Disorder

An unprecedented crystal‐packing arrangement of a tetramethoxy‐bay‐substituted perylene bisimide (PBI) consists of three crystallographically independent molecules, that is, an achiral ( AC ) PBI of saddle‐shaped geometry along with two pairs of propeller‐like twisted (P )‐ and (M )‐enantiomeric PBI frameworks. All these five conformations are observed within a single π‐stack revealing an intriguing packing sequence with an inversion of chirality from P to M via AC . Nudged elastic band calculations for the isolated molecule show that AC is a local minimum of the P to M interconversion path. In addition, two minor conformations were observed in the crystal, one of which resembles a transition‐state molecule. Theoretical studies of dimeric and trimeric stacks reveal that the coexistence of all these structures in the crystal lattice is aided by the strong dispersion interactions between PBI cores and perfectly interdigitated dodecyl chains which stabilize energetically higher conformations.     

Elastic Organic Crystals of a Fluorescent π‐Conjugated Molecule

An elastic organic crystal of a π‐conjugated molecule has been fabricated. A large fluorescent single crystal of 1,4‐bis[2‐(4‐methylthienyl)]‐2,3,5,6‐tetrafluorobenzene (over 1 cm long) exhibited a fibril lamella morphology based on slip‐stacked molecular wires, and it was found to be a remarkably elastic crystalline material. The straight crystal was capable of bending more than 180° under applied stress and then quickly reverted to its original shape upon relaxation. In addition, the fluorescence quantum yield of the crystal was about twice that of the compound in THF solution. Mechanical bending–relaxation resulted in reversible change of the morphology and fluorescence. This research offers a more general approach to flexible crystals as a promising new family of organic semiconducting materials.     

Superelastic Organic Crystals

Superelastic materials (crystal‐to‐crystal transformation pseudo elasticity) that consist of organic components have not been observed since superelasticity was discovered in a Au‐Cd alloy in 1932. Superelastic materials have been exclusively developed in metallic or inorganic covalent solids, as represented by Ti‐Ni alloys. Organosuperelasticity is now revealed in a pure organic crystal of terephthalamide, which precisely produces a large motion with high repetition and high energy storage efficiency. This process is driven by a small shear stress owing to the low density of strain energy related to the low lattice energy.     

Room‐Temperature,Strain‐Tunable Orientation of Magnetization in a Hybrid Ferromagnetic Co Nanorod–Liquid Crystalline Elastomer Nanocomposite

Hybrid nanocomposites based on magnetic nanoparticles dispersed in liquid crystalline elastomers are fascinating emerging materials. Their expected strong magneto‐elastic coupling may open new applications as actuators, magnetic switches, and for reversible storage of magnetic information. We report here the synthesis of a novel hybrid ferromagnetic liquid crystalline elastomer. In this material, highly anisotropic Co nanorods are aligned through a cross‐linking process performed in the presence of an external magnetic field. We obtain a highly anisotropic magnetic material which exhibits remarkable magneto‐elastic coupling. The nanorod alignment can be switched at will at room temperature by weak mechanical stress, leading to a change of more than 50 % of the remnant magnetization ratio and of the coercive field.     

Acoustic Emission from Organic Martensites

In salient effects, still crystals of solids that switch between phases acquire a momentum and are autonomously propelled because of rapid release of elastic energy accrued during a latent structural transition induced by heat, light, or mechanical stimulation. When mechanical reconfiguration is induced by change of temperature in thermosalient crystals, bursts of detectable acoustic waves are generated prior to self‐actuation. These observations provide compelling evidence that the thermosalient transitions in organic and organic‐containing crystals are molecular analogues of the martensitic transitions in some metals, and metal alloys such as steel and shape‐memory alloys. Within a broader context, these results reveal that, akin to metallic bonding, the intermolecular interactions in molecular solids are capable of gradual accrual and sudden release of a substantial amount of strain during anisotropic thermal expansion, followed by a rapid transformation of the crystal packing in a diffusionless, non‐displacive transition.     

Photoluminescent Ferroelastic Molecular Crystals

Ferroelasticity has been reported for several types of molecular crystals, which show mechanical‐stress‐induced shape change under twinning and/or spontaneous formation of strain. Aiming to create materials that exhibit both ferroelasticity and light‐emission characteristics, we discovered the first examples of ferroelastic luminescent organometallic crystals. Crystals of arylgold(I)(N‐heterocyclic carbene)(NHC) complexes bend upon exposure to anisotropic mechanical stress. X‐ray diffraction analyses and stress‐strain measurements on these ferroelastic crystals confirmed typical ferroelastic behavior, mechanical twinning, and the spontaneous build‐up of strain. A comparison with single‐crystal structures of related gold‐NHC complexes that do not show ferroelasticity shed light on the structural origins of the ferroelastic behavior.     

Elasticity in Macrophage‐Synthesized Biocrystals

Supramolecular crystalline assembly constitutes a rational approach to bioengineer intracellular structures. Here, biocrystals of clofazimine (CFZ) that form in vivo within macrophages were measured to have marked curvature. Isolated crystals, however, showed reduced curvature suggesting that intracellular forces bend these drug crystals. Consistent with the ability of biocrystals to elastically deform, the inherent crystal structure of the principal molecular component of the biocrystals—the hydrochloride salt of CFZ (CFZ‐HCl)—has a corrugated packing along the (001) face and weak dispersive bonding in multiple directions. These characteristics were previously found to be linked to the elasticity of other organic crystals. Internal stress in bent CFZ‐HCl led to photoelastic effects on the azimuthal orientation of polarized light transmittance. We propose that elastic, intracellular crystals can serve as templates to construct functional microdevices with different applications.     

Super‐ and Ferroelastic Organic Semiconductors for Ultraflexible Single‐Crystal Electronics

Like silicon, single crystals of organic semiconductors are pursued to attain intrinsic charge transport properties. However, they are intolerant to mechanical deformation, impeding their application in flexible electronic devices. Such contradictory properties, namely exceptional molecular ordering and mechanical flexibility, are unified in this work. We found that bis(triisopropylsilylethynyl)pentacene (TIPS‐P) crystals can undergo mechanically induced structural transitions to exhibit superelasticity and ferroelasticity. These properties arise from cooperative and correlated molecular displacements and rotations in response to mechanical stress. By utilizing a bending‐induced ferroelastic transition of TIPS‐P, flexible single‐crystal electronic devices were obtained that can tolerate strains (?) of more than 13 % while maintaining the charge carrier mobility of unstrained crystals (μ>0.7 μ₀). Our work will pave the way for high‐performance ultraflexible single‐crystal organic electronics for sensors, memories, and robotic applications.     

Hypergolic Triggers as Co‐crystal Formers: Co‐crystallization for Creating New Hypergolic Materials with Tunable Energy Content

We demonstrate a co‐crystal‐based strategy to create new solid hypergols, that is, materials exhibiting spontaneous ignition when in contact with an oxidant, from typically non‐hypergolic fuel molecules. In these materials, the energy content and density can be changed without affecting the ignition delay. The use of an imidazole‐substituted decaborane as a hypergolic “trigger” component in combination with energy‐rich but non‐hypergolic nitrobenzene or pyrazine yielded hypergolic co‐crystals that combine improved combustion properties with ultrashort ignition delays as low as 1 ms.     

Versatile Ferroelastic Deformability in an Organic Single Crystal by Twinning about a Molecular Zone Axis

Ferroelasticity involves the generation of spontaneous strain in a solid by the application of mechanical stress. The phenomenon has been well‐studied in metal alloys but relatively neglected in organic solid‐state chemistry. Herein we present multiple discrete modes of mechanical twinning and a mechanistic analysis of ferroelasticity in 1,4‐diethoxybenzene. Single crystals of the compound can be almost freely deformed, as multiple different twin domains are generated simultaneously. Within each domain, single‐crystal character is preserved. Such extremely versatile, ferroelastic deformability is unprecedented in single crystals of any kind and defies the fragility and anisotropic mechanical behaviour of most organic crystals. The dissipated energy and critical stress associated with twinning deformation in 1,4‐diethoxybenzene suggests that organic solids could be developed for absorbing weak mechanical shocks in such applications as mechanical damping and soft robotics.     

Low‐Temperature Anharmonicity in Cesium Chloride (CsCl)

Anharmonic lattice vibrations govern heat transfer in materials, and anharmonicity is commonly assumed to be dominant at high temperature. The textbook cubic ionic defect‐free crystal CsCl is shown to have an unexplained low thermal conductivity at room temperature (ca. 1 W/(m K)), which increases to around 13 W/(m K) at 25 K. Through high‐resolution X‐ray diffraction it is unexpectedly shown that the Cs atomic displacement parameter becomes anharmonic at 20 K.     

Turning Regioselectivity into Stereoselectivity: Efficient Dual Resolution of P‐Stereogenic Phosphine Oxides through Bifurcation of the Reaction Pathway of a Common Intermediate

Synthetic routes that provide facile access to either enantiomeric form of a target compound are particularly valuable. The crystallization‐free dual resolution of phosphine oxides that gives highly enantioenriched materials (up to 94 % ee) in excellent yields is reported. Both enantiomeric oxides have been prepared from a single intermediate, (R_P)‐alkoxyphosphonium chloride, which is formed in the course of a selective dynamic kinetic resolution using a single enantiomer of menthol as the chiral auxiliary. The origin of the dual stereoselectivity lies in bifurcation of the reaction pathway of this intermediate, which works as a stereochemical railroad switch. Under controlled conditions, Arbuzov‐type collapse of this intermediate proceeds through C O bond fission with retention of the configuration at the phosphorus center. Conversely, alkaline hydrolysis of the P O bond leads to the opposite S_P enantiomer.     

Coupling of Magnetic and Elastic Domains in the Organic–Inorganic Layered Perovskite‐Like (C6H5C2H4NH3)2FeIICl4 Crystal

Multiferroic materials coupling ferroelasticity and ferromagnetism show strong magnetoelastic effects as magnetization is induced by mechanical stress or alternately strain induced by applying a magnetic field. These effects were reported for inorganic multiferroics such as LaCo_x Sr_1−x O₃. (C₆H₅C₂H₄NH₃)₂Fe^IICl₄ is the first example of an organic–inorganic perovskite to exhibit such effects below the canted antiferromagnetism at T _C=98 K and ferroelasticity at T _C=433 K. This is shown by switching the magnetic hysteresis on and off by uniaxial pressure through the strong coupling of the magnetic and elastic domains. The spin‐canting direction was controlled by mechanical stress in the heating and cooling cycles. This unique observation gives additional impetus in the search for coupled hysteretic effect in organic–inorganic multiferroics.     

Diferrocenophosphaborin: A Planar‐Chiral,Redox‐Active and Anion‐Responsive Ambiphilic Ligand

A new class of Janus‐like ambiphilic ligands is introduced. The rigid diferrocene backbone in heterocycles 4‐SnP and 4‐BP creates an unprecedented chiral environment as demonstrated by multinuclear NMR and single‐crystal X‐ray studies. In addition, the ligands are redox‐responsive and the Lewis acidic borane moiety in 4‐BP can be exploited to further tune the properties: a clear decrease in the CO stretching frequency of a Vaska‐type Rh^I complex 5‐BP is observed upon addition of fluoride ions. Thus, the Lewis acid and Lewis base sites influence each other and their strength can be modulated by redox chemistry and anion binding.     

A Three‐Dimensional Dynamic Supramolecular “Sticky Fingers” Organic Framework

Engineering high‐recognition host–guest materials is a burgeoning area in basic and applied research. The challenge of exploring novel porous materials with advanced functionalities prompted us to develop dynamic crystalline structures promoted by soft interactions. The first example of a pure molecular dynamic crystalline framework is demonstrated, which is held together by means of weak “sticky fingers” van der Waals interactions. The presented organic‐fullerene‐based material exhibits a non‐porous dynamic crystalline structure capable of undergoing single‐crystal‐to‐single‐crystal reactions. Exposure to hydrazine vapors induces structural and chemical changes that manifest as toposelective hydrogenation of alternating rings on the surface of the [60]fullerene. Control experiments confirm that the same reaction does not occur when performed in solution. Easy‐to‐detect changes in the macroscopic properties of the sample suggest utility as molecular sensors or energy‐storage materials.     

Docking Strategy To Construct Thermostable,Single‐Crystalline,Hydrogen‐Bonded Organic Framework with High Surface Area

Enhancing thermal and chemical durability and increasing surface area are two main directions for the construction and improvement of the performance of porous hydrogen‐bonded organic frameworks (HOFs). Herein, a hexaazatriphenylene (HAT) derivative that possesses six carboxyaryl groups serves as a suitable building block for the systematic construction of thermally and chemically durable HOFs with high surface area through shape‐fitted docking between the HAT cores and interpenetrated three‐dimensional network. A HAT derivative with carboxybiphenyl groups forms a stable single‐crystalline porous HOF that displays protic solvent durability, even in concentrated HCl, heat resistance up to 305 °C, and a high Brunauer–Emmett–Teller surface area [SA_(BET)] of 1288 m² g⁻¹. A single crystal of this HOF displays anisotropic fluorescence, which suggests that it would be applicable to polarized emitters based on robust functional porous materials.     

Enhanced Photoactivity from Single‐Crystalline SrTaO2N Nanoplates Synthesized by Topotactic Nitridation

There are few methods yielding oxynitride crystals with defined shape, yet shape‐controlled crystals often give enhanced photoactivity. Herein, single‐crystalline SrTaO₂N nanoplates and polyhedra are achieved selectively. Central to these synthetic advances is the crystallization pathways used, in which single‐crystalline SrTaO₂N nanoplates form by topotactic nitridation of aerosol‐prepared Sr₂Ta₂O₇ nanoplates and SrTaO₂N polyhedra form by flux‐assisted nitridation of the nanoplates. Evaluation of these materials for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) showed improved performance for the SrTaO₂N nanoplates, with a record apparent quantum efficiency (AQE) of 6.1 % for OER compared to the polyhedra (AQE: 1.6 %) and SrTaO₂N polycrystals (AQE: 0.6 %). The enhanced performance from the nanoplates arises from their morphology and lower defect density. These results highlight the importance of developing new synthetic routes to high quality oxynitrides.