Thermo-Induced Single-Crystal-to-Single-Crystal Transformations and Photo-Induced [2+2] Cycloaddition Reactions in Polymorphs of Chalcone-Based Molecular Crystals: Multi-Stimuli Responsive Actuators

The polymorphs of 2ClChMe-4 in Form I (ribbon-like crystal) and Form II (block-like crystal) were prepared, and they exhibited curling/flipping and expansion upon heating on account of single-crystal-to-single-crystal transformations. The irreversible phase transformations occurred separately at 53.2 °C and 57.8 °C for the crystals in Form I and Form II, during which the molecular conformation of 2ClChMe-4 changed and the molecules slipped along the (100) plane. Movement at the molecular level resulted in changes of cell parameters, which in turn led to macroscopic motions of the crystals upon heating. Additionally, the ribbon-like crystals of 2ClChMe-4 showed photo-induced bending driven by [2+2] cycloaddition. Accordingly, an actuator showing reversible bending behavior was fabricated triggered by light and heat successively. Like biomimetic self-actuators, such multi-stimuli mechanical responsive molecular crystals might have potential applications in soft robots, artificial muscles and microfluidic systems.     

Exceptionally Plastic/Elastic Organic Crystals of a Naphthalidenimine-Boron Complex Show Flexible Optical Waveguide Properties

The design of molecular compounds that exhibit flexibility is an emerging area of research. Although a fair amount of success has been achieved in the design of plastic or elastic crystals, realizing multidimensional plastic and elastic bending remains challenging. We report herein a naphthalidenimine–boron complex that showed size-dependent dual mechanical bending behavior whereas its parent Schiff base was brittle. Detailed crystallographic and spectroscopic analysis revealed the importance of boron in imparting the interesting mechanical properties. Furthermore, the luminescence of the molecule was turned-on subsequent to boron complexation, thereby allowing it to be explored for multimode optical waveguide applications. Our in-depth study of the size-dependent plastic and elastic bending of the crystals thus provides important insights in molecular engineering and could act as a platform for the development of future smart flexible materials for optoelectronic applications.     

Photoinduced swing of a diarylethene thin broad sword shaped crystal: a study on the detailed mechanism

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light. By contrast in thick crystals, photosalient phenomena were observed. The bending and swinging mechanisms are in fact due to molecular size changes as well as phase transitions. The first slight bending away from the light source is due to photocyclization-induced surface expansion, and the second dramatic bending toward UV incidence is due to single-crystal-to-single-crystal (SCSC) phase transition from the original phase I to phase II_UV. Upon visible light irradiation, the crystal returned to phase I. A similar SCSC phase transition with a similar volume decrease occurred by lowering the temperature (phase III_temp). For both photoinduced and thermal SCSC phase transitions, the symmetry of the unit cell is lowered; in phase II_UV the twisting angle of disordered phenyl groups is different between two adjacent molecules, while in phase III_temp, the population of the phenyl rotamer is different between adjacent molecules. In the case of phase II_UV, we found thickness dependent photosalient phenomena. The thin broad sword shaped crystals with a 3 μm thickness showed no photosalient phenomena, whereas photoinduced SCSC phase transition occurred. In contrast, large crystals of several tens of μm thickness showed photosalient phenomena on the irradiated surface where SCSC phase transition occurred. The results indicated that the accumulated strain, between isomerized and non-isomerized layers, gave rise to the photosalient phenomenon.

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light.     

Hierarchical structures,surface morphology and mechanical elasticity of lamellar crystals dominated by halogen effects

To understand the deformation mechanism of molecular crystals under mechanical forces will accelerate the molecular design and preparation of deformable crystals. Herein, the relationship between structural halogenation and molecular-level stacking, micro/nanoscale surface morphology, and macroscopic mechanical properties are investigated. Elastic crystals of halo-pyrimidinyl carbazoles (CzM-Cl, CzM-Br and CzM-I) with lamellar structure and brittle crystal (CzM-F) were quantitatively analyzed by crystal energy framework (CEF) providing the inter/intralayer interaction energy (Inter/Intra-IE). It is revealed that the elastic crystals bend under external force as a result from stronger Intra-IE to prevent cleavage and weaker Inter-IE for the short-range movement of molecules on the slip plane. This research will provide an insight for the molecular design of flexible crystals and facilitate the development of next-generation smart crystal materials.     

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.     

Optical Waveguiding Organic Single Crystals Exhibiting Physical and Chemical Bending Features

Bendable (elastic and plastic) organic single crystals have been widely studied as emerging flexible materials with highly ordered packing structures. However, even though manifold bendable organic crystals have been recently reported, most of them bend in response to only one stimulus. Herein, we report an organic single crystal of (Z)‐4‐(1‐cyano‐2‐(4‐(dimethylamino)phenyl)vinyl)benzonitrile, which bends under external stress (physical process) and also hydrochloric acid atmosphere (chemical process). This observation indicates that a single organic crystal, whose structure has been optimized simultaneously at both the molecular and supramolecular levels, may display multiple crystal‐bending modes. Furthermore, the crystals exhibit bright orange‐yellow emission and can serve as an active low‐loss optical waveguide in both the straight and the bent state, which indicates a potential optical application.     

Photomechanical Organic Crystals as Smart Materials for Advanced Applications

Photomechanical molecular crystals are receiving much attention due to their efficient conversion of light into mechanical work and advantages including faster response time; higher Young's modulus; and ordered structure, as measured by single-crystal X-ray diffraction. Recently, various photomechanical crystals with different motions (contraction, expansion, bending, fragmentation, hopping, curling, and twisting) are appearing at the forefront of smart materials research. The photomechanical motions of these single crystals during irradiation are triggered by solid-state photochemical reactions and accompanied by phase transformation. This Minireview summarizes recent developments in growing research into photoresponsive molecular crystals. The basic mechanisms of different kinds of photomechanical materials are described in detail; recent advances in photomechanical crystals for promising applications as smart materials are also highlighted.     

Mechanical Motion of Chiral Azobenzene Crystals with Twisting upon Photoirradiation

The photomechanical motion of chiral crystals of trans‐azobenzene derivatives with an (S)‐ and (R)‐phenylethylamide group was investigated and compared with a racemic crystal. Changes in the UV/Vis absorption spectra of the powdered crystals before and after UV irradiation were measured by using an optical waveguide spectrometer, showing that the lifetime of the cis‐to‐trans thermal back‐isomerization of the chiral crystals was faster than that of the racemic crystals. Upon UV irradiation, a long plate‐like chiral microcrystal bent away from the light source with a twisting motion. A square‐like chiral microcrystal curled toward the light with some twisting. Reversible bending of a rod‐like chiral microcrystal was repeatable over twenty‐five cycles. In contrast, bending of a plate‐like racemic microcrystal was small. A possible mechanism for the bending and twisting motion was discussed based on the optimized cis conformer determined by using calculations, showing that the bending motion with twisting is caused by elongation along the b axis and shrinkage along the a axis.     

Structural and compositional effects on thermal expansion behavior in polyimide substrates of varying thicknesses

Polyimide films with thicknesses ranging from 6 μm to 80 μm were prepared with a solvent casting method to explore film thickness effects on the in-plane thermal expansion coefficient (CTE). In the case of polyimide films composed of bulky and flexible molecular units, CTE is consistent regardless of film thickness. In contrast, films with rigid and planar molecular structure show CTE increase according to the increase of film thickness up to 40–50 μm, which then plateau for thicker films. It is apparent that the film thickness dependent thermal expansion originates from complex effects of molecular orientation, charge transfer complex formation, and crystal formation as a function of film thicknesses, through characterization on UV–Vis absorption, crystalline structure, glass transition behavior, and optical retardation. These results provide insight into the design of polymer structures for flexible display substrates that require appropriate CTE values.     

Nonreactive molecular dynamics force field for crystalline hexahydro-1,3,5-trinitro-1,3,5 triazine

An empirical nonreactive force field has been developed for molecular dynamics (MD)/Monte Carlo simulation of the formation, diffusion, and agglomeration of point defects in the crystal lattice of the alpha modification of hexahydro-1,3,5-trinitro-1,3,5 triazine (RDX) using flexible molecules. Bond stretching and angle bending are represented by Morse and harmonic functions, and torsion by a truncated cosine series. Nonbonded interactions, both inter- and intramolecular, are described by Buckingham potentials separately parametrized. Intermolecular electrostatic interactions are treated via a Coulomb term coupled with a smooth 15.0 A cutoff radius. Parameters were taken in part from earlier published works and were determined partly by fitting to known molecular and crystal properties of RDX. In MD simulations at constant pressure and temperature, the model was able to stabilize and maintain the correct crystal structure, symmetry, and molecular conformation of alpha-RDX. Vibrational frequencies, lattice binding energy and dimensions, coefficients of thermal expansion, and several unusually short intermolecular distances are all reproduced in satisfactory agreement with experimental data.     

Mechanically compliant single crystals of a stable organic radical

Mechanically compliant organic crystals are the foundation of the development of future flexible, light-weight single-crystal electronics, and this requires reversibly deformable crystalline organic materials with permanent magnetism. Here, we report and characterize the first instance of a plastically bendable single crystal of a permanent organic radical, 4-(4′-cyano-2′,3′,4′,5′-tetrafluorophenyl)-1,2,3,5-dithiadiazolyl. The weak interactions between the radicals render single crystals of the β phase of this material exceedingly soft, and the S–N interactions facilitate plastic bending. EPR imaging of a bent single crystal reveals the effect of deformation on the three-dimensional spin density of the crystal. The unusual mechanical compliance of this material opens prospects for exploration into flexible crystals of other stable organic radicals towards the development of flexible light-weight organic magnetoresistance devices based on weak, non-hydrogen-bonded interactions in molecular crystals.

Mechanically soft crystals are interesting candidates for single crystal electronics. Here, crystals of a stable dithiadiazolyl radical are shown to be plastically bendable and display a change in their spin density in response to mechanical force.     

Photomechanical Luminescence from Through-Space Conjugated AIEgens

Transforming molecular motions into the macroscopic scale is a topic of great interest to nanoscience. The photomechanical effect is a promising strategy to achieve this goal. Herein, we report an intriguing photomechanical luminescence driven by the photodimerization of 2-phenylbenzo[b]thiophene 1,1-dioxide (P-BTO) in molecular crystals and elucidate the working mechanism and substituent effect through crystallographic analysis and theoretical calculations. Striking splitting, hopping, and bending mechanical behaviors accompanied by a significant blue fluorescence enhancement are observed for P-BTO crystals under UV light, which is attributed to the formation of photodimer 2P-BTO. Although 2P-BTO is poorly π-conjugated because of the central cyclobutane ring, it exhibits prominent through-space conjugation and aggregation-induced emission (AIE), affording strong solid-state blue fluorescence at 415 nm with an excellent quantum yield of up to 96.2 %.     

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.     

Fabrication of flexible light shutter using liquid crystals with polymer structure

We fabricated a light shutter using plastic substrates for high visibility of a flexible see-through display. To achieve a flexible light shutter using liquid crystals (LC), it is essential to maintain the cell gap when the light shutter is bent. We studied methods to fabricate flexible LC light shutters using plastic substrates. We demonstrated light shutters that are initially transparent and flexible with or without polymer walls. We have elucidated that polymer walls and networks provide mechanical stability against the bending of an LC light shutter without any degradation in the electro-optic characteristics. We predict that a flexible light shutter provides not only high visibility but also mechanical stability to a flexible see-through display by positioning it at the back of a flexible see-through display panel.     

Photomechanical Luminescence from Through‐Space Conjugated AIEgens

Transforming molecular motions into the macroscopic scale is a topic of great interest to nanoscience. The photomechanical effect is a promising strategy to achieve this goal. Herein, we report an intriguing photomechanical luminescence driven by the photodimerization of 2‐phenylbenzo[b]thiophene 1,1‐dioxide (P‐BTO) in molecular crystals and elucidate the working mechanism and substituent effect through crystallographic analysis and theoretical calculations. Striking splitting, hopping, and bending mechanical behaviors accompanied by a significant blue fluorescence enhancement are observed for P‐BTO crystals under UV light, which is attributed to the formation of photodimer 2P‐BTO. Although 2P‐BTO is poorly π‐conjugated because of the central cyclobutane ring, it exhibits prominent through‐space conjugation and aggregation‐induced emission (AIE), affording strong solid‐state blue fluorescence at 415 nm with an excellent quantum yield of up to 96.2 %.     

Argentophilicity-dependent colossal thermal expansion in extended prussian blue analogues

The thermal expansion behavior of isostructural variants of the colossal thermal expansion material Ag3[CoIII(CN)6] has been investigated using variable temperature X-ray and neutron powder diffraction. It was found that substitution at the octahedral transition metal site did not strongly affect the thermal expansion behavior, giving Ag3[FeIII(CN)6] as a new colossal thermal expansion material. Substitution at the Ag site (by D) was shown to reduce the thermal expansion coefficients by an order of magnitude. It was proposed that this correlation between the presence of argentophilic interactions and extreme thermal expansion behavior may explain a variety of thermal effects in flexible framework materials containing metallophilic interactions.     

Organic Crystals with Response to Multiple Stimuli: Mechanical Bending,Acid-Induced Bending and Heating-Induced Jumping

Multiple stimuli-responsive molecular crystals are attracting extensive attentions due to their potential as smart materials, such as molecular machines, actuators, and sensors. However, the task of giving a single crystal multiple stimuli-responsive properties remains extremely challenging. Herein, we found two polymorphs (Form O and Form R) of a Schiff base compound, which could respond to multiple stimuli (external force, acid, heat). Form O and Form R have different elastic deformability, which can be attributed to the differences in the molecular conformation, structural packing and intermolecular interactions. Moreover, both polymorphs exhibit reversible bending driven by volatile acid vapor, which we hypothesize is caused by reversible protonation reaction of imines with formic acid. In addition, jumping can be triggered by heating due to the significant anisotropic expansion. The integration of reversible bending and jumping into one single crystal expands the application scope of stimuli-responsive crystalline materials.     

Quantifiable stretching-induced fluorescence shifts of an elastically bendable and plastically twistable organic crystal

Organic crystals with mechanical stimulus-response properties are being developed increasingly nowadays. However, the studies involving tensile-responsive crystals are still lacking due to the strict requirement of crystals with good flexibility. In this work, an organic crystal with the ability of elastic bending and plastic twisting upon loading stress and shearing force, respectively, is reported. The deformability in different directions enables the crystal to be a model for tensile-responsive study. Indeed, blue shifts of fluorescence were observed when the tensile forces loaded upon the needle-shaped crystal were stretched to a certain degree. The mathematical correlation between emission wavelength changes and stretching strain was obtained for the first time, which proves that the crystal has a potential application for tension sensors. In addition, a low detection limit and high sensitivity enabled the crystal to have the ability to detect tension variations in precision instruments. Theoretical calculations and X-ray crystal structure analyses revealed the mechanism of emission wavelength shifts caused by molecular movement during the stretching process. The presented crystal successfully overcame the limitations of traditional mechanochromic organic crystals, which have difficulty in responding to tensile forces.

We present a flexible crystal capable of elastic bending and plastic twisting, and this crystal can undergo fluorescence blue shift induced by tensile force.     

Tension effect in local dynamics of cuprite structures

Cuprite crystals, Cu₂O and Ag₂O, display a negative thermal expansion in a wide temperature range. This work shows, through a phenomenological approach, how the next-nearest-neighbors dynamics observed by EXAFS in both crystals [A. Sanson, F. Rocca, G. Dalba, P. Fornasini, R. Grisenti, M. Dapiaggi, G. Artioli, Phys. Rev. B 73 (2006), 214305] is directly connected to a tension mechanism. As a result, the negative thermal expansion in cuprite crystals could be described within the Rigid Unit Modes formalism, but replacing the typical rigid polyhedral units by rigid rods between nearest-neighbors atoms.     

Vibrational and thermodynamic properties of β-HMX: a first-principles investigation

Thermodynamic properties of β-HMX crystal are investigated using the quasi-harmonic approximation and density functional theory within the local density approximation (LDA), generalized gradient approximation (GGA), and GGA + empirical van der Waals (vdW) correction. It is found that GGA well describes the thermal expansion coefficient and heat capacity but fails to produce correct bulk modulus and equilibrium volume. The vdW correction improves the bulk modulus and volume, but worsens the thermal expansion coefficient and heat capacity. In contrast, LDA describes all thermodynamic properties with reasonable accuracy, and overall is a good exchange-correlation functional for β-HMX molecular crystal. The results also demonstrate significant contributions of phonons to the equation of state. The static calculation of equilibrium volume for β-HMX differs from the room-temperature value incorporating lattice vibrations by over 5%. Therefore, for molecular crystals, it is essential to include phonon contributions when calculated equation of state is compared with experimental data at ambient condition.