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.     

Molecular Spring‐like Triple‐Helix Coordination Polymers as Dual‐Stress and Thermally Responsive Crystalline Metal–Organic Materials

Elastic metal–organic materials (MOMs) capable of multiple stimuli‐responsiveness based on dual‐stress and thermally responsive triple‐helix coordination polymers are presented. The strong metal‐coordination linkage and the flexibility of organic linkers in these MOMs, rather than the 4 Å stacking interactions observed in organic crystals, causes the helical chain to act like a molecular spring and thus accounts for their macroscopic elasticity. The thermosalient effect of elastic MOMs is reported for the first time. Crystal structure analyses at different temperatures reveal that this thermoresponsiveness is achieved by adaptive regulation of the triple‐helix chains by fine‐tuning the opening angle of flexible V‐shaped organic linkers and rotation of its lateral conjugated groups to resist possible expansion, thus demonstrating the vital role of adaptive reorganization of triple‐helix metal–organic chains as a molecular spring‐like motif in crystal jumping.     

Molecular‐Level Understanding of Structural Changes of Organic Crystals Induced by Macroscopic Mechanical Stimulation

Structural changes to molecular crystals upon mechanical stimulation have attracted attention for sensing, recording, and microactuation. Comprehensive structure information is required to understand relationships between the mechanical force applied, the crystal structure, and the bulk property changes in order to develop general design concepts for mechanoresponsive compounds. Unfortunately, mechanical stimulation of organic crystals typically deteriorates their integrity, preventing detailed structure analyses by single‐crystal X‐ray diffraction (XRD) methods. However, in the past three years, several interesting studies have been reported in which molecular crystals retain their integrity even after a mechanically induced crystalline structure change. These materials have allowed us to investigate how macroscopic mechanical forces affect the microscopic structures of molecular crystals by single‐crystal XRD analyses. This Minireview summarizes current knowledge of mechanically induced structure changes in molecular crystals, which will facilitate research in this field.     

Anisotropic Dissociation of π–π Stacking and Flipping‐Motion‐Induced Crystal Jumping in Alkylacridones and Their Dicyanomethylene Derivatives

Ethylacridone ( 1 b ) and dicyanomethylenated acridones 2 a , b , d showed crystal‐jumping activity upon heating. This is the first example of thermosalient behavior in a simple aromatic ketone and its derivatives. A systematic investigation of the jumping behavior of derivatives with different alkyl chains by variable‐temperature X‐ray crystal‐structure analyses revealed the mechanism of this phenomenon. Anisotropic dissociation of π stacking in a dimer was important for inducing crystal jumping in 1 b , whereas the collective fluctuation/flipping motion of a dicyanomethylene unit induced crystal jumping in 2 .     

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.     

Molecular Spring-like Triple-Helix Coordination Polymers as Dual-Stress and Thermally Responsive Crystalline Metal–Organic Materials

Elastic metal–organic materials (MOMs) capable of multiple stimuli-responsiveness based on dual-stress and thermally responsive triple-helix coordination polymers are presented. The strong metal-coordination linkage and the flexibility of organic linkers in these MOMs, rather than the 4 Å stacking interactions observed in organic crystals, causes the helical chain to act like a molecular spring and thus accounts for their macroscopic elasticity. The thermosalient effect of elastic MOMs is reported for the first time. Crystal structure analyses at different temperatures reveal that this thermoresponsiveness is achieved by adaptive regulation of the triple-helix chains by fine-tuning the opening angle of flexible V-shaped organic linkers and rotation of its lateral conjugated groups to resist possible expansion, thus demonstrating the vital role of adaptive reorganization of triple-helix metal–organic chains as a molecular spring-like motif in crystal jumping.     

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.     

Fully polarizable QM/MM calculations: An application to the nonbonded iodine–oxygen interaction in dimethyl‐2‐iodobenzoylphosphonate

The compound dimethyl‐2‐iodobenzoylphosphonate is unusual in that it forms well‐ordered crystals that clearly show short iodine‐oxygen interactions in which both the iodine and the oxygen are in their normal oxidation states. These interactions were studied using a new hybrid quantum mechanical–molecular mechanical approach that employs a polarizable molecular mechanics component. The electric field at the molecular mechanics atoms was calculated from a distributed multipole expansion of the wave function; this induced dipoles on the molecular mechanics atoms. The electrostatic potential in a spherical shell around the induced dipoles was reproduced through induced charges on the atomic center and those bonded to it using an analytical (rather than numerical) procedure. The new atomic charges (induced charges plus permanent charges) were then able to interact with the quantum mechanical entity and polarize the wave function. The procedure was iterated to convergence. The calculations show that the iodine atom becomes more positive in the crystal environment (modeled by a chain of three molecules of dimethyl‐2‐iodobenzoylphosphonate). Thus, while the cooperative effects of the crystal environment may not be the only feature stabilizing this unusual interaction, they do play a significant role in reducing the otherwise unfavourable iodine–oxygen monopole–monopole interaction. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 478–482, 2000     

Detailed Investigation of the Structural,Thermal, and Electronic Properties of Gold Isocyanide Complexes with Mechano‐Triggered Single‐Crystal‐to‐Single‐Crystal Phase Transitions

Mechano‐induced phase transitions in organic crystalline materials, which can alter their properties, have received much attention. However, most mechano‐responsive molecular crystals exhibit crystal‐to‐amorphous phase transitions, and the intermolecular interaction patterns in the daughter phase are difficult to characterize. We have investigated phenyl(phenylisocyanide)gold(I) ( 1 ) and phenyl(3,5‐dimethylphenylisocyanide)gold(I) ( 2 ) complexes, which exhibit a mechano‐triggered single‐crystal‐to‐single‐crystal phase transition. Previous reports of complexes 1 and 2 have focused on the relationships between the crystalline structures and photoluminescence properties; in this work we have focused on other aspects. The face index measurements of complexes 1 and 2 before and after the mechano‐induced phase transitions have indicated that they undergo non‐epitaxial phase transitions without a rigorous orientational relationship between the mother and daughter phases. Differential scanning calorimetry analyses revealed the phase transition of complex 1 to be enthalpically driven by the formation of new aurophilic interactions. In contrast, the phase transition of complex 2 was found to be entropically driven, with the closure of an empty void in the mother phase. Scanning electron microscopy observation showed that the degree of the charging effect of both complexes 1 and 2 was changed by the phase transitions, which suggests that the formation of the aurophilic interactions affords more effective conductive pathways. Moreover, flash‐photolysis time‐resolved microwave conductivity measurements revealed that complex 1 increased in conductivity after the phase change, whereas the conductivity of complex 2 decreased. These contrasting results were explained by the different patterns in the aurophilic interactions. Finally, an intriguing disappearing polymorphism of complex 2 has been reported, in which a polymorph form could not be obtained again after some period of time, even with repeated trials. The present studies provide us with a variety of hitherto unknown insights into mechano‐responsive molecular crystals, which help us to understand the phase transition behaviors upon mechanical stimulation and establish rational design principles.     

Effect of Alkyl Groups in Pyrene Chromophore on the Mechanical Response of Pyrene‐Octafluoronaphthalene Co‐Crystals

Changes in the photophysical properties of pyrene ( Py )‐octafluoronaphthalene ( OFN ) co‐crystals ( Py ? OFN ) upon mechanical stimuli are described herein. The Py ? OFN co‐crystal showed a mechano‐induced bathochromic shift in emission, and a similar tendency was observed for the 1,3,6,8‐tetramethylpyrene‐ OFN co‐crystal. These shifts are due to disruption of the microscopic molecular orientation in the co‐crystal, which allows for excimer formation. In sharp contrast to the parent Py ? OFN and methyl‐substituted Py ‐ OFN co‐crystals, no mechano‐induced bathochromic shift was observed when longer alkyl chains were introduced to the 1‐, 3‐, 6‐, and 8‐positions of the Py chromophore. This photophysical opposability against mechanical stimuli could be explained by the orthogonally oriented alkyl groups on the Py ring, which existed between two Py cores like pillars. This fixed OFN to maintain the face‐to‐face alternatively stacked structure of the co‐crystal and thus prevented the formation of the Py excimer. The pillar effect demonstrated herein provides a rational design for co‐crystalline systems that are photophysically stable against mechanical stresses.     

Thermally Driven Polymorphic Transition Prompting a Naked‐Eye‐Detectable Bending and Straightening Motion of Single Crystals

The amplification of molecular motions so that they can be detected by the naked eye (10⁷‐fold amplification from the ångström to the millimeter scale) is a challenging issue in the development of mechanical molecular devices. In this context, the perfectly ordered molecular alignment of the crystalline phase has advantages, as demonstrated by the macroscale mechanical motions of single crystals upon the photochemical transformation of molecules. In the course of our studies on thermoresponsive amphiphiles containing tetra(ethylene glycol) (TEG) moieties, we serendipitously found that thermal conformational changes of TEG units trigger a single‐crystal‐to‐single‐crystal polymorphic phase transition. The single crystal of the amphiphile undergoes bending and straightening motion during both heating and cooling processes at the phase‐transition temperatures. Thus, the thermally triggered conformational change of PEG units may have the advantage of inducing mechanical motion in bulk materials.     

Photoinduced Ratchet‐Like Rotational Motion of Branched Molecular Crystals

Photomechanical molecular crystals can undergo a variety of light‐induced motions, including expansion, bending, twisting, and jumping. The use of more complex crystal shapes may provide ways to turn these motions into useful work. To generate such shapes, pH‐driven reprecipitation has been used to grow branched microcrystals of the anthracene derivative 4‐fluoroanthracenecarboxylic acid. When these microcrystals are illuminated with light of λ=405 nm, an intermolecular [4+4] photodimerization reaction drives twisting and bending of the individual branches. These deformations drive a rotation of the overall crystal that can be repeated over multiple exposures to light. The magnitude and direction of this rotation vary because of differences in the crystal shape, but a typical branched crystal undergoes a 50° net rotation after 25 consecutive irradiations for 1 s. The ability of these crystals to undergo ratchet‐like rotation is attributed to their chiral shape.     

Thermally Induced Intra‐Carboxyl Proton Shuttle in a Molecular Rack‐and‐Pinion Cascade Achieving Macroscopic Crystal Deformation

Proton transport via dynamic molecules is ubiquitous in chemistry and biology. However, its use as a switching mechanism for properties in functional molecular assemblies is far less common. In this study, we demonstrate how an intra‐carboxyl proton shuttle can be generated in a molecular assembly akin to a rack‐and‐pinion cascade via a thermally induced single‐crystal‐to‐single‐crystal phase transition. In a triply interpenetrated supramolecular organic framework (SOF), a 4,4′‐azopyridine (azpy) molecule connects to two biphenyl‐3,3′,5,5′‐tetracarboxylic acid (H₄BPTC) molecules to form a functional molecular system with switchable mechanical properties. A temperature change reversibly triggers a molecular movement akin to a rack‐and‐pinion cascade, which mainly involves 1) an intra‐carboxyl proton shuttle coupled with tilting of the azo molecules and azo pedal motion and 2) H₄BPTC translation. Moreover, both the molecular motions are collective, and being propagated across the entire framework, leading to a macroscopic crystal expansion and contraction.     

Visualization of the Recrystallization of Solution‐Grown Poly[(R)‐3‐hydroxybutyrate] Lamellar Crystals

Morphological changes of solution‐grown poly[(R)‐3‐hydroxybutyrate] lamellar crystals during heating were directly investigated by atomic force microscopy. The thickening of lamellar crystals was further visualized by enzymatic degradation of less‐ordered crystal regions in thermally treated lamellar crystals. The morphological changes of lamellar crystals induced by thermal treatment are due to recrystallization.     

Mechano‐Responsive,Thermo‐Reversible,Luminescent Organogels Derived from a Long‐Chained,Naturally Occurring Fatty Acid

The gelating ability of an α‐diketo derivative of oleic acid, 9,10‐dioxooctadecanoic acid ( DODA ), is investigated. DODA can gelate aromatic liquids and many other organic liquids. By contrast, none of the liquids examined can be gelated by the methyl ester of DODA. DODA is a more efficient gelator than stearic acid and the monoketo derivative due to its more extensive intermolecular dipole–dipole interactions. Formation of organogels of DODA can be induced by both thermal and mechanical stimuli, during which the luminescent and mechanical properties can be modulated significantly. The emission from DODA in 1‐octanol exhibits a large, reversible, hypsochromic shift (≈25 nm) between its thermally cycled gel and sol states. The emission changes have been exploited to probe the kinetics of the aggregation and deaggregation processes. DODA is the simplest gelator of which we are aware that exhibits a reversible shift in the emission. Although the self‐assembled fibrillar networks of the DODA gels in 1‐octanol, benzonitrile, or silicone oil are crystalline, isothermal mechanical cycling between the gel and the sol states is rapid and can be repeated several times (i.e., they are thixotropic). The single‐crystal structure of DODA indicates that extended intermolecular dipole–dipole interactions are crucial to the thermal and mechanical formation of DODA gels and the consequential changes in emissive and mechanical properties. From analyses of structural information, gelator packing, and morphology differences, we hypothesize that the mechanical destruction and reformation of the gel networks involves interconversion between the 3D networks and 1D fiber bundles. The thermal processes allow the fibrillar 3D networks and their 0D components (i.e., isolated molecules or small aggregates of DODA ) to be interconverted. These results describe a facile approach to the design of mechano‐responsive, thermo‐reversible gels with control over their emission wavelengths.     

Fabrication of a Hydrogen‐Bonded Organic Framework Membrane through Solution Processing for Pressure‐Regulated Gas Separation

Ordered and flexible porous frameworks with solution processability are highly desirable to fabricate continuous and large‐scale membranes for the efficient gas separation. Herein, the first microporous hydrogen‐bonded organic framework (HOF) membrane has been fabricated by an optimized solution‐processing technique. The framework exhibits the superior stability because of the abundant hydrogen bonds and strong π–π interactions. Thanks to the flexible HOF structure, the membrane possesses the unprecedented pressure‐responsive H₂/N₂ separation performance. Furthermore, the scratched membrane can be healed by the treatment of solvent vapor, achieving the recovery of separation performance.     

Laser Induced Transient Structures in a 150 nm Gold Crystal

The evolution of transient structures induced in gold crystals by irradiation with 400 nm, 100 fs laser pulses has been measured by means of time resolved x‐ray diffraction. The data show that the photon/electron interaction generates a hot electron wave on a 150 nm Au crystal, which is responsible for a “blast force” that generates an intense shock wave that alters the lattice structure as it propagates, with sound velocity, through the bulk of the crystal. This mechanical force is developed within 1‐2 ps after the laser pulse interacts with the gold surface electrons. Other major effects that have contributed to the strain in the lattice have also been evaluated.     

Crystallization Force—A Density Functional Theory Concept for Revealing Intermolecular Interactions and Molecular Packing in Organic Crystals

Organic molecules are prone to polymorphic formation in the solid state due to the rich diversity of functional groups that results in comparable intermolecular interactions, which can be greatly affected by the selection of solvent and other crystallization conditions. Intermolecular interactions are typically weak forces, such as van der Waals and stronger short‐range ones including hydrogen bonding, that are believed to determine the packing of organic molecules during the crystal‐growth process. A different packing of the same molecules leads to the formation of a new crystal structure. To disclose the underlying causes that drive the molecule to have various packing motifs in the solid state, an electronic concept or function within the framework of conceptual density functional theory has been developed, namely, crystallization force. The concept aims to describe the local change in electronic structure as a result of the self‐assembly process of crystallization and may likely quantify the locality of intermolecular interactions that directs the molecular packing in a crystal. To assess the applicability of the concept, 5‐methyl‐2‐[(2‐nitrophenyl)amino]‐3‐thiophenecarbonitrile, so‐called ROY, which is known to have the largest number of solved polymorphs, has been examined. Electronic calculations were conducted on the seven available crystal structures as well as on the single molecule. The electronic structures were analyzed and crystallization force values were obtained. The results indicate that the crystallization forces are able to reveal intermolecular interactions in the crystals, in particular, the close contacts that are formed between molecules. Strong correlations exist between the total crystallization force and lattice energy of a crystal structure, further suggesting the underlying connection between the crystallization force and molecular packing.     

Solid‐Phase Radical Polymerization of Halogen‐Bond‐Based Crystals and Applications to Pre‐Shaped Polymer Materials

Liquid vinyl monomers were converted into solid crystals via halogen bonding. They underwent solid‐phase radical polymerizations through heating at 40 °C or ultraviolet photo‐irradiation (365 nm). The X‐ray crystallography analysis showed the high degree of monomer alignment in the crystals. The polymerizations of the solid monomer crystals yielded polymers with high molecular weights and relatively low dispersities because of the high degree of the monomer alignment in the crystal. As a unique application of this system, the crystalized monomers were assembled to pre‐determined structures, followed by solid‐phase polymerization, to obtain a two‐layer polymer sheet and a three‐dimensional house‐shaped polymer material. The two‐layer sheet contained a unique asymmetric pore structure and exhibited a solvent‐responsive shape memory property and may find applications to asymmetric membranes and polymer actuators.     

Steric hindrance effect on thermo‐responsive behaviors of well‐defined water‐soluble semi‐rigid polymers

Three series of water‐soluble semi‐rigid thermo‐responsive polymers with well‐defined molecular weights based on mesogen‐jacketed liquid crystal polymers, poly[bis(N‐(2‐hydroxypropyl) pyrrolidone) 2‐vinylterephthalate] [P(2‐HPPVTA)], poly[bis(N‐(1‐methyl‐2‐hydroxyethyl) pyrrolidone) 2‐vinylterephthalate] [P(1‐M‐2‐HEPVTA)] and poly[bis(N‐hydroxypropyl pyrrolidone) 2‐vinylterephthalate] (PHPPVTA) have been synthesized via reversible addition‐fragmentation chain transfer polymerization. The steric hindrance effects on liquid crystalline property and thermo‐responsive behaviors of semi‐rigid water‐soluble polymers (P(2‐HPPVTA), P(1‐M‐2‐HEPVTA), and PHPPVTA) were carefully investigated. From molecular structure, the steric hindrance of P(1‐M‐2‐HEPVTA) is stronger than that of P(2‐HPPVTA). Polarized light microscope and one‐dimensional wide‐angle X‐ray diffraction revealed that both the P(2‐HPPVTA) and P(1‐M‐2‐HEPVTA) display a columnar nematic phase, indicating that the steric hindrance effect do not affect liquid crystalline behavior of the polymers. The dynamic light scattering results demonstrated that P(1‐M‐2‐HEPVTA) exhibited lower cloud point compared with that of P(2‐HPPVTA) at the same mass concentration and the same molecular weight. The more significant molecular weight and concentration dependence on cloud point have been observed in P(2‐HPPVTA) solution than in P(1‐M‐2‐HEPVTA) solution. We also discovered that the cloud points of both P(2‐HPPVTA) and P(1‐M‐2‐HEPVTA) solution are lower in D₂O than in H₂O. It is noted that the cloud point of PM‐2 is 9.9 °C lower in D₂O than in H₂O, much less pronounced than the cloud point difference of PH‐2. The differences of thermo‐responsive behaviors between P(2‐HPPVTA) and P(1‐M‐2‐HEPVTA) were resulted from the steric hindrance effect existed in their side groups. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3429–3438