The Rise of the Dynamic Crystals

Fluorescence tracking of inter- and intramolecular motion in zwitterionic aggregate

Ionic aggregates are among the most common forms of matter, yet the investigation of their molecular motion is often constrained by the instability of isolated anions and cations, as well as the lack of real-time monitoring techniques. This study presents a zwitterionic strategy that integrates both cations and anions into one fluorescent organic framework, forming a zwitterionic molecule. The zwitterionic strategy simplifies the intricate cation-anion systems that are typically found in conventional inorganic salts and imparts them with fluorescent properties, facilitating real-time tracking of ionic-interaction-induced molecular motion within ionic aggregates. Specifically, a blue shift in the fluorescence wavelength signified changes in aggregate states due to intermolecular motion, whereas a decrease in intensity was linked to intramolecular-motion-caused conformational changes. This spontaneous molecular motion enabled dynamic switching of the excited state energy-decay pathway, leading to switchable color-light responses. Overall, the zwitterionic strategy offers a novel framework for exploring the properties and behaviors of molecules in ionic aggregates.

Regulating Reversible Untwisting and Twisting Motions in Helical Dynamic Molecular Crystals

Dynamic molecular crystals with a desirable morphology and controllable responses to external stimuli are highly desired but remain challenging. Here, we present a new photoactive molecular crystal (MCA) featuring a naphthalene backbone linked with a vinyl structure and an asymmetric tail. It can undergo reversible head-to-tail [2 + 2] photodimerization under visible light illumination and heating. Helicoidally twisted crystalline fibers can be easily prepared by drop-casting an MCA ethanol solution onto a glass surface. The thread length and fiber width can be regulated by changing the solvent evaporation temperature. An MCA fiber untwists upon light irradiation and twists back under mild heating due to the dissociation of the metastable photodimer (d-MCA), enabling a highly reverse transformation. Crystallinity and overall helical morphology can be reserved well during the untwisting and twisting cycles, allowing multiple repetitions of the dynamic motions. Under interval light excitation and continuous heating, an MCA fiber can perform cyclical untwisting-twisting motions over 200 times for 5 h. The amount of photodimer significantly affects the photoresponse, resulting in fully reversible cycles, partial reverse twisting, or complete untwisting at different dimer fractions. We found that about 10% of d-MCA is enough to drive the circular movements. Higher photoproduct conversion makes the fibers prone to fatigue and lose the cyclic responses. Our results provide an excellent example for fabricating new dynamic organic crystals for soft robotics and photoactuators based on spiral twisting movements.

Light-driven structural transformations in isotypical Cd(II) complexes: stereoselectivity and photosalient motion

This work describes the photoreactivity and photosalient (PS) behavior of five new isotypical Cd(II) coordination complexes [Cd(bz)2(4-nvp)2] (1), [Cd(4-clbz)2(4-nvp)2] (2) and (3), and [Cd(4-brbz)2(4-nvp)2] (4) and (5) [Hbz = benzoic acid, H4-clbz = 4-chlorobenzoic acid, and H4-brbz = 4-bromobenzoic acid]. Complexes 2 and 3 are 4-clbz based polymorphs, while 4 and 5 are 4-brbz based polymorphs. Interestingly, complexes 1, 2 and 4 form head-to-head (HH) dimers, while 3 and 5 form head-to-tail (HT) dimers through photodimerization.

One-Dimensional Organic Lead Bromide Elastic Crystals with Strong Electron-Phonon Coupling

Two novel one-dimensional (1D) lead halide elastic crystals, [Pb2Br4(DMA)2]n (1) (DMA = N,N-dimethylacetamide) and [Pb2Br4(DMF)2]n (2) (DMF = N,N-dimethylformamide), were reported. Both compounds 1 and 2 feature a neutral 1D bimetallic axis chain structure. The bending strain of compound 1 is 1.36%, higher than those of all reported 1D single-metal axis chain coordinate polymers, indicating the superior elastic properties of the 1D bimetallic axis chain polymers. Compound 1 exhibits strong red-to-near-infrared (NIR) fluorescence emission below 200 K, with an emission peak at 700 nm and a full width at half maximum (fwhm) of 200 nm, indicating strong electron-phonon coupling in compound 1. The large Stokes shift and broad fwhm of compound 1 may be attributed to its excellent elasticity, as this elasticity allows the molecule's self-trapped exciton state to undergo greater structural distortion. It is suggested that 1D organic lead halide elastic crystals could be promising candidates for emerging applications in efficient NIR light-emitting diodes, supercontinuum sources, and flexible NIR optical waveguides.

Self-healing crystals

Self-healing is an intrinsically exciting concept as it applies to the process of recovery, a commonplace phenomenon found in living organisms. Self-healing of artificial materials is as beneficial to living creatures as it is to materials science, wherein the effect can considerably prolong lifetimes. Although self-healing sodium chloride crystals were discovered in the 1980s, the field entered a renaissance when healing was observed in the emerging materials class of molecular crystals in 2016. Self-healing properties in polymers, cementitious materials, and coatings have already found commercial applications. The reinvigorated interest in self-healing molecular crystals stems from their prospects as durable, lightweight and flexible emissive or electronic materials. Ideally being defectless and ordered media, organic crystals have unique optical, mechanical and electrical properties, and the possibility of self-healing substantially increases their viability for smart devices.

Elusive Interplay of 3D Structural Similarity and Twinning in Mechanical Flexibility of Luminescent Organic Crystals

The properties of molecular crystalline solids are critically dependent on the spatial arrangement of molecules and underlying noncovalent interactions. Here, two new isoelectronic cocrystals of a substituted carbazole-based emitter are presented, with bipyridyl ethylene and azene molecules, namely, cocrystal 1 and cocrystal 2, respectively. Though both isoelectronic cocrystals are also three-dimensional (3D) isostructural at the supramolecular level, they show similar photoluminescence properties as anticipated, but irreconcilable macroscopic mechanical properties. Upon applying external stress on their respective major crystal faces, cocrystal 1 is elastically flexible, while the 3D isostructural cocrystal 2 exhibits brittle fracture. Remarkably, elastic flexibility in cocrystal 2 can be induced through twinning-mediated crystal face modification, without any change in crystal structure.

Mechanical Transitions in Crystals: The Low-Temperature Thermosalient Transition of a Mesogenic Polyphenyl

Thermosalient transitions are a subset of single-crystal-to-single-crystal (SCSC) transitions, in which the change of lattice parameters is highly anisotropic and very fast. As a result, crystals at the transition undergo macroscopic dynamical effects (hopping, jumping, and shattering). These transitions feature a conversion of heat to mechanical energy that can be exploited in the realization of advanced materials. Most thermosalient transitions are observed at temperatures higher than room temperature. Examples of low-temperature thermosalient transitions are rare. We describe a new example of a low-temperature thermosalient transition in a sexiphenyl compound. At about -40 °C, the parent single crystal (phase I) shatters into single crystal fragments of the new phase (phase II). The two phases have been studied by single-crystal X-ray analysis using a synchrotron source, variable-temperature Raman spectroscopy, and computational analysis of lattice normal vibration modes. A mechanism of the transition is proposed. We confirm colossal thermal expansion coefficients and supercells as reliable features of thermosalient transitions and add as a third feature a low-frequency principal optical vibration of the crystal lattice prompting the transition. Based on this, a roadmap for the automated prediction of thermosalient transitions in molecular crystals is also outlined.

Broadband-Light-Induced [2+2] Cycloaddition and Thermoinduced Cycloreversion-Powered Dynamic Molecular Crystals

Photomechanically responsive dynamic molecular crystals are central to developing efficient, rapid, and robust materials capable of conversion of light energy to mechanical work. However, unlike some other, mainly photochromic molecular solar thermal energy storage (MOST) systems, solids that undergo photoinduced [2+2] cycloaddition have not been thoroughly explored for powering reversible actuation, despite that this reaction system carries potential in the heavily strained bonds of the cyclobutane ring. In this study, we propose that broadband-light-induced [2+2] cycloaddition can be used to store energy and actuate dynamic organic crystals by irradiation with visible light. The prototypical material, pyrenylvinylpyrylium tetrafluoroborate (1-PVPyL), undergoes a topochemical [2+2] cycloaddition induced not only by ultraviolet radiation (365 nm) but also by monochromatic green light (532 nm), red light (620 nm) and broadband visible light in a single-crystal-to-single-crystal (SCSC) manner, causing its crystals to bend. The crystals effectively act as energy depots, where the reverse deformation can be initiated by heating and the stored energy is released via thermal cycloreversion reaction. Given the ubiquity of the [2+2] cycloaddition in the solid state, the current study invites the development of new dimeric MOST architectures that utilize sunlight for energy storage and thermal triggers for energy release. Within a broader scope, this approach provides a platform for fabrication of visible-light-driven crystal actuators capable of harnessing sunlight.

Capturing the Progressive Conformational Evolutions of Sterically-Congested Dihydrophenazines via Crystallization

To gain a deeper understanding of the sequential multistep excited-state structural evolutions of N,N'-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC) luminophores, we strategically freeze distinct conformations by crystallization, allowing to capture the progressive conformational transformations within a DPAC-based framework by utilizing single-crystal X-ray diffractometry. Our focus lies in the innovative modification of DPAC via the synthesis of cyano (CN)-substituted derivatives DPAC-nCN (n=1-4, with n indicating the number of CN groups). The incorporation of electron-withdrawing CN groups modulates electron delocalization and lowers energy barriers, facilitating access to conformational polymorphism within the crystals. Unlike the limited diversity observed in the crystallization behaviour of DPAC, the DPAC-2CN to DPAC-4CN derivatives exhibit distinct crystalline forms, with conformational diversity increasing in tandem with the number of CN substituents. Notably, the single DPAC-4CN molecule features multi-colored crystals transitioning from blue to red, with the folding angle of the polycyclic dihydrodibenzo[a,c]phenazine ring progressive varying from ~130° to ~172°. Additionally, DPAC-4CN's red crystals with high-energy planar conformation (~172°) can experience a sudden jumping when subjected to stimuli. This study not only advances the understanding of conformational dynamics in dihydrophenazines but also paves a new way for the development of dynamic crystal materials.

Dynamic Molecular Cocrystals with Alkyl Chain Dependent Thermosalient Phase Transitions

Thermally responsive molecular crystals exhibiting programmable mechanical motions hold significant promise for applications in smart actuators, sensors, and drug delivery systems. However, achieving precise control over their phase transition thermodynamics remains a fundamental challenge. A series of isomorphic 5-fluorocytosine/fatty acid cocrystals is reported where the phase transition temperatures vary across an interval of 100 K with increasing alkyl chain. Two distinct transition pathways are unveiled: i) a cooperative single-crystal-to-single-crystal transition (II-III) accompanied by explosive mechanical motions, and ii) a reconstructive transition (I-III) following classical nucleation-growth mechanisms. The cooperative phase transition (II-III) induces remarkable expansion, with a striking +64.4% expansion along the layer stacking direction and a -16.9% contraction perpendicular to the (001) plane, leading to dynamic phenomena such as jumping, rotating, and splitting. Notably, the transition temperatures (Tt, II-III) exhibit linear dependence on coformer chain length (from C10 to C18), a correlation attributed to interlayer hydrophobic interactions. This work provides a versatile approach for designing molecular crystals with tunable thermo-mechanical properties, offering new opportunities for advanced applications in dynamic functional materials.

Flexible Organic Crystalline Fibers and Loops with Strong Second Harmonic Generation

Flexible organic crystals represent a novel class of smart materials that open many opportunities for optical applications. While it has been established that elastic or plastic deformation of slender molecular crystals can be commonly induced by external intervention, crystals that grow in bent or curled shapes naturally are rarely reported. This study introduces an extraordinarily flexible organic crystalline fibrous material, (Z)-3-(2,3-dichloropyridin-4-yl)-2-(3,5-dimethylphenyl)acrylonitrile (DPA), that crystallizes both as straight and curled crystals. Crystals of DPA are easily obtained from solution either as long fibers or as crystals that are curled to various extent, and sometimes even closed into a loop. The straight crystalline fibers can be bent mechanically by applying force or photochemically by exposure to ultraviolet light. The straight and curled crystals are both polar and capable of highly efficient second harmonic generation (SHG) with respective intensities of 2.03 ± 0.15 and 1.52 ± 0.12 times (equivalent strain ≈ 1%) that of urea. Curling during crystal growth provides direct access to curved SHG-active flexible organic optical waveguiding elements, such as crystalline optical ring resonators, thereby circumventing the necessity for manual crystal bending, which is usually not readily scalable. This work highlights the unconventional properties and capabilities that fibrous molecular crystalline materials bring to the global materials space and their potential applications as shape-conforming, nonlinear organic materials.

Chemiluminescent 2-Coumaranones: Synthesis, Luminescence Mechanism, and Emerging Applications

Recently, 2-Coumaranones have emerged as a highly promising class of chemiluminescent compounds, distinguished by their unique structural properties that facilitate efficient light emission. This review provides a comprehensive analysis of their synthesis, structural characteristics, and chemiluminescence mechanisms, integrating historical perspectives with the latest advancements in the field. Beyond their intrinsic photophysical and chemical properties, 2-coumaranones have demonstrated broad utility across bioanalytical and material sciences. Notable applications include enzyme-catalyzed chemiluminescence in aqueous systems, glucose and urease-triggered detection assays, and mechano-base-responsive luminescence for stress sensing. Additionally, recent developments in chemiluminescent protective groups and their incorporation into advanced functional materials underscore the versatility of these compounds. Despite significant progress, key challenges remain, particularly in optimizing quantum yield, emission properties, and solvent compatibility for practical applications. Future research should prioritize the development of highly tunable 2-coumaranone derivatives with enhanced spectral and kinetic properties, further expanding their potential in diagnostics, bioimaging, and mechanoluminescent sensing. By addressing these challenges, 2-coumaranones could pave the way for next-generation chemiluminescent technologies with unprecedented sensitivity and adaptability.

Organic Self-Healing Single Crystals

Self-healing single crystals, which possess the ability to recover from damage, represent an emerging filed within dynamic single-crystal materials. These materials not only deepen our understanding of the flexible structures inherent in single crystals but also offer a novel pathway for the development of smart materials, including soft robots, microelectronic devices, and optical devices. In this perspective, we provide a comprehensive summary of recent advancements in organic self-healing single crystals, highlighting various self-healing mechanisms, typical molecular structures, and the testing methods utilized to investigate these materials. We hope that our systematic overview of this field will significantly contribute to the advancement of self-healing single crystal materials as a new class of molecular-based functional materials, particularly in the integration of self-healing properties with innovative optoelectronic functionalities.

Light-Driven Adaptive Molecular Crystals Activated by [2+2] and [4+4] Cycloadditions

Photomechanical crystals act as light-driven material-machines that can convert the energy carried by photons into kinetic energy via shape deformation or displacement, and this capability holds a paramount significance for the development of photoactuated devices. This transformation is usually attributed to anisotropic expansion or contraction of the unit cell engendered by light-induced structural modifications that lead to accumulation and release of stress that generates a momentum, resulting in readily observable mechanical effects. Among the available photochemical processes, the photoinduced [2+2] and [4+4] reactions are known for their robustness, predictability, amenability to control with molecular and supramolecular engineering approaches, and efficiency that has already been elevated to a proof-of-concept smart devices based on organic crystals. This review article presents a summary of the recent research progress on photomechanical properties of organic and metal-organic crystals where the mechanical effects are based on [2+2] and [4+4] cycloaddition reactions. It consolidates the current understating of the chemical strategies and structure-property correlations, and highlights the advantages and drawbacks of this class of adaptive crystals within the broader field of crystal adaptronics.

Manipulating a Thermosalient Crystal Using Selective Deuteration

The thermosalient transformation in nickel(II) bis(diisopropyl)dithiocarbamate has been investigated using selective deuteration. The deuterated crystals undergo a reversible displacive phase transition that is ∼4 K higher in temperature compared to the protonated analogue. Neutron, synchrotron, density-functional theory, and calorimetric techniques were utilized to demonstrate the substantial effect of deuterium. All techniques demonstrated the equivalence of the mechanism on an atomic scale between the protonated and deuterated complexes. The data collected in this study reveal details of the changes of atomic motion that underpin the thermosalience inherent in this system. Deuterium decreased the frequency of atomic vibrations thus increasing the temperature of the observed transformation. This study represents a key advancement in the field of thermosalient molecular systems and provides insights into the control and manipulation of thermosalient materials.

Thiophene Sulfone Single Crystal as a Reversible Thermoelastic Linear Actuator with an Extended Stroke and Second-Harmonic Generation Switching

Dynamic organic crystals are becoming recognized as some of the fastest materials for converting light or heat to mechanical work. The degree of deformation and the response time of any actuating material are often exclusive of each other; however, both factors influence the material's overall performance limits. Unlike polymers, whose disordered structures are not conducive to rapid energy transfer, cooperative phase transitions in dynamic molecular crystals that are amenable to rapid and concerted martensitic-like structure switching could help circumvent that limitation. Here, we report that single crystals of a dibenzothiophene sulfone derivative exhibit extraordinarily large, rapid, and reversible elongation when they undergo a thermally induced phase transition. The value for the linear stroke of ∼15% along the long crystal axis with retention of macroscopic integrity of this material is remarkable and capitalizes on an anisotropic lattice switching with relative changes of 14.8% and -9.5% along its crystallographic a and c axes, respectively, resulting in a visible macroscopic elongation of the crystal. The transitioning crystals deliver forces ranging from 0.19 to 15 μN and a work density of ∼7 × 10-3 J m-3. The phase transformation is accompanied by a change in symmetry between centrosymmetric and noncentrosymmetric space groups and a significant change in both the fluorescence and the second-order nonlinear optical (NLO) response. The combination of these properties makes this material a favorable choice for low-power, precise, and small-scale NLO actuation applications.

Structural Evolution Leading to the Thermosalient Phase Transition of Oxitropium Bromide

This study investigates the thermosalient effect in oxitropium bromide, with a focus on the role of anisotropic thermal expansion, elastic properties, and sound propagation in driving this phenomenon. Variable-temperature X-ray powder diffraction (VTXRPD) revealed significant anisotropic thermal expansion, including negative thermal expansion (NTE) along the c-axis in the low-temperature Form A. Density functional theory (DFT) calculations were used to analyze elastic properties of oxitropium bromide and confirmed that it does not exhibit negative compressibility, emphasizing thermal anisotropy as the primary factor in the phase transition. Studies of elastic constants and sound propagation demonstrated a preferred pathway for energy transfer along the z-direction, enabling rapid strain release during the phase transition. These findings confirmed that the thermosalient effect arises from cooperative molecular motion, resulting in an abrupt and energetic transformation driven by the interplay of structural anisotropy and elastic properties.

Thermoelastic twisting-assisted crystal jumping based on a self-healing molecular crystal

Adaptive crystals have attracted significant attention from solid-state chemists and crystal engineers for their promising applications in memories, capacitors, sensors, and actuators. Among them, thermosalient crystals are particularly favored thanks to their efficient energy conversions and rapid responses. However, the mechanisms for the mechanical responses of thermosalient crystals remain largely unclear. Herein we demonstrate that thermosalient effects of molecular crystals could be driven by thermoelastic twisting behaviors. The crystal, based on a model compound with rigid dibenzothiophene sulfone planes and flexible ethoxy chains, can spontaneously self-heal from mechanical fractures. Upon heating, the crystal undergoes remarkable thermosalient behaviors driven by a distinctive left- or right-handed twisting. This thermoelastic twisting converts thermal energy into elastic potential energy, which is further released as kinetic energy upon untwisting to drive the crystal jump. Our demonstration on thermoelastic twisting-induced crystal jumping offers a different perspective on the origins of thermosalient crystals and could provide inspiration for future engineering and application of dynamic molecular crystals.

In Situ Generation of Microwire Heterojunctions with Flexible Optical Waveguide and Hydration-Mediated Photochromism

Flexible heterojunctions based on molecular systems are in high demand for applications in photonics, electronics, and smart materials, but fabrication challenges have hindered progress. Herein, we present an in situ approach to creating optical heterojunctions using hydration-mediated flexible molecular crystals. These hydrated multi-component molecular solids display strong blue emitting optical waveguides with minimal optical loss (0.005 dB/μm) and excellent flexibility (elastic modulus: 3.87 GPa). The water-mediated process enables the molecular microwires with tunable elastic and plastic deformation, as well as reversible uptake and release of lattice water, facilitating the formation of flexible heterojunctions. Spectral analysis and theoretical modeling reveal that these microwires exhibit both photochromism and color-tunable dual emission (fluorescence and phosphorescence), expanding their utility in photonic information encoding. Therefore, this work introduces a hydration-mediated molecular engineering strategy for fabricating crystalline heterojunctions with on-demand processability and controllable emission sequences, enabling optical signal manipulation at the micro/nanoscale.

Precise Photochemical Post-Processing of Molecular Crystals

Molecular crystals carry a great potential as new soft smart materials, with a plethora of recent examples overcoming the major obstacle of mechanical flexibility, and this research direction holds enormous potential to revolutionize optics, electronics, medicine, and space exploration. However, shaping organic crystals into desired shapes and sizes remains a major practical challenge due to the lack of control over the crystallization process, and the difficulties in mechanical post-processing without introduction of defects that are usually imparted by their soft nature. Here we present an innovative approach that employs photochemical processing for precise and nondestructive cutting of a molecular crystal. Our proposed method uses light to post-process crystals of the desired size and shape, similar to using light to cut other materials. This reaction induces strain, ensuring sharp cleavage without the need for melting or other processes. We further demonstrate the potential of this approach by producing crystals of arbitrary size, which can be used as controllable optical waveguides. Among other potential applications, this method can be used to prepare dynamic crystals, particularly those with aspect ratios crucial for mechanical deformation, such as flexible electronics, soft robotics, and sensing.

Combining Simple Deformations to Elicit Complex Motions and Directed Swimming of Smart Organic Crystals with Controllable Thickness

The lack of control over the crystal growth in a systematic way currently stands as an unsurmountable impediment to the preparation of dynamic crystals as soft robots; in effect, the mechanical effects of molecular crystals have become a subject of scattered reports that pertain only to specific crystal sizes and actuation conditions, often without the ability to establish or confirm systematic trends. One of the factors that prevents the verification of such performance is the unavailability of strategies for effectively controlling crystal size and aspect ratio, where crystals of serendipitous size are harvested from crystallization solution. Here we devised a water-assisted precipitation method to prepare crystals of chemical variants of 9-anthracene derivatives with various thicknesses that respond to ultraviolet light with simple mechanical effects, including bending, splintering, and rotation. By capitalizing on the robust mechanical flexibility and deformability of crystals, we demonstrate systematic variations in crystal deformation that are further elevated in complexity to construct crystal-based robots capable of controllable motions reminiscent of sailing and humanoid movements. The results illustrate an approach to eliminate one of the critical obstacles towards complete control over the motility of dynamic molecular crystals as microrobots in non-aerial environments.

Hybrid Strategies for Enhancing the Multifunctionality of Smart Dynamic Molecular Crystal Materials

Dynamic molecular crystals are an emerging class of smart engineering materials that possess unique ability to convert external energy into mechanical motion. Moreover, they have being considered as strong candidates for dynamic elements in applications such as flexible electronic devices, artificial muscles, sensors, and soft robots. However, the inherent defects of molecular crystals like brittleness, short-life and fatigue, have significantly impeded their practical applications. Inspired by the concept of "the whole is greater than the sum of its parts" in the field of biology, building stimuli-response composites materials can be regarded as one of the ways to break through the current limitations of dynamic molecular crystals. Moreover, the hybrid materials can exhibit new functionalities that cannot be achieved by a single object. In this review, the focus was placed on the analysis and discussion of various hybrid strategies and options, as well as the functionalities of hybrid dynamic molecular crystal materials and the important practical applications of composite materials, with the introduction of photomechanical molecular crystals and flexible molecular crystals as a starting point. Moreover, the efficiency, limitations, and advantages of different hybrid methods were compared and discussed. Furthermore, the promising perspectives of smart dynamic molecular crystal materials were also discussed and the potential directions for future work were suggested.

Bidirectional strain release in a thermosalient organic salt crystal induced by anisotropic thermal expansion and phase transformation

The jumping of crystals in the presence of external stimuli such as temperature, light, and mechanical forces is observed due to the release of accumulated strain in the crystal. Thermosalience in molecular crystals is generally induced by anisotropic thermal expansion accompanied by phase transformation. However, a thermosalient effect has not been observed in the same crystal at two different temperature zones caused by two distinct mechanisms. Here, we report the bidirectional release of strain leading to jumping by heating and cooling of an organic salt crystal of SQIM. Jumping of the crystals by heating is driven by anisotropic thermal expansion coupled with a change in hydrogen bonding and contraction of the imidazolium cation, whereas jumping of crystals by cooling is attributed to the contraction of inter-layer distance followed by shear-induced phase transformation.

Toward On-Demand Polymorphic Transitions of Organic Crystals via Side Chain and Lattice Dynamics Engineering

Controlling polymorphism, namely, the occurrence of multiple crystal forms for a given compound, is still an open technological challenge that needs to be addressed for the reliable manufacturing of crystalline functional materials. Here, we devised a series of 13 organic crystals engineered to embody molecular fragments undergoing specific nanoscale motion anticipated to drive cooperative order-disorder phase transitions. By combining polarized optical microscopy coupled with a heating/cooling stage, differential scanning calorimetry, X-ray diffraction, low-frequency Raman spectroscopy, and calculations (density functional theory and molecular dynamics), we proved the occurrence of cooperative transitions in all the crystalline systems, and we demonstrated how both the molecular structure and lattice dynamics play crucial roles in these peculiar solid-to-solid transformations. These results introduce an efficient strategy to design polymorphic molecular crystalline materials endowed with specific molecular-scale lattice and macroscopic dynamics.

Easy Processable Photomechanical Thin Film Involving a Photochromic Diarylethene and a Thermoplastic Elastomer in Supramolecular Interaction

A novel supramolecular photoactuator in the form of a thin film of centimetric size has been developed as an alternative to traditional liquid crystal elastomers (LCE) involving azobenzene (AZO) units or photochromic microcrystals. This thin film is produced through spin coating without the need for alignment or crosslinking. The photoactuator combines a photochromic dithienylethene (DTE) functionalized with ureidopyrimidinone (UPy) units, and a telechelic thermoplastic elastomer, also functionalized with UPy, allowing quadruple hydrogen bonding between the two components. Upon alternating ultraviolet (UV) and visible light exposure, the film exhibits reversible bending and color changes, studied using displacement and absorption tracking setups. For the first time, the photomechanical effect (PME) is quantitatively correlated with photochromism, showing that DTE units drive the movement under both UV (photocyclization) and visible (photoreversion) light. In situ illumination techniques reveal that the PME arises from photoinduced strain within 160 nm UPy-bonded DTE domains, which expand and contract by approximately 50% under UV and visible light, respectively. The semicrystalline nature of the elastomer and a robust supramolecular network connecting both components are critical in converting microscopic photostrain into macroscopic actuation.

Dynamic organic crystals as exceptionally efficient artificial natural light-harvesting actuators

Dynamic organic crystal materials that can directly convert solar energy into mechanical work hold the potential to be efficient artificial actuators. However, developing dynamic organic crystals that can efficiently transform natural light energy into mechanical energy is still quite challenging. Herein, a novel dynamic organic crystal whose two polymorphs (Form I and Form II) are both capable of effectively converting natural light into work was successfully synthesized. Under the irradiation of ultraviolet (UV), blue and natural light, the on/off toggling of a photosalient effect could be triggered. Specifically, under UV light irradiation, Form I demonstrates output work densities of about 4.2-8.4 × 104 J m-3 and 1.6-4.9 × 102 J m-3 before and after disintegration, respectively. Form II exhibits output work densities of about 1.3 × 102 to 1.9 × 103 J m-3 by means of photoinduced bending, suggesting that controllable bending may be more favorable for energy harvesting than the photosalient effect. Utilizing the exceptionally high energy transduction efficiency of Form I, we developed a natural light-driven micro-actuator that can realize output work densities of 2.8-5.0 × 104 J m-3. The natural light-harvesting performance of this actuator significantly surpasses those of previously reported photomechanical crystals and could even be comparable to thermal actuators.

Giant Thermosalient Effect in a Molecular Single Crystal: Dynamic Transformations and Mechanistic Insights

The exploration of mechanical motion in molecular crystals under external stimuli is of great interest because of its potential applications in diverse fields, such as electronics, actuation, or sensing. Understanding the underlying processes, including phase transitions and structural changes, is crucial for exploiting the dynamic nature of these crystals. Here, we present a novel organic compound, PT-BTD, consisting of five interconnected aromatic units and two peripheral alkyl chains, which forms crystals that undergo a drastic anisotropic expansion (33% in the length of one of its dimensions) upon thermal stimulation, resulting in a pronounced deformation of their crystal shape. Remarkably, the transformation occurs while maintaining the single-crystal nature, which has allowed us to follow the crystal-to-crystal transformation by single-crystal analysis of the initial and expanded polymorphs, providing valuable insights into the underlying mechanisms of this unique thermosalient behavior. At the molecular level, this transformation is associated with subtle, coordinated conformational changes, including slight rotations of the five interconnected aromatic units in its structure and increased dynamism in one of its peripheral alkyl chains as the temperature rises, leading to the displacement of the molecules. In situ polarized optical microscopy reveals that this transformation occurs as a rapidly advancing front, indicative of a martensitic phase transition. The results of this study highlight the crucial role of a soft and flexible structural configuration combined with a highly compact but loosely bound supramolecular structure in the design of thermoelastic materials.

Coordination Site Selective Occupation Strategy for Tuning the Photosalient Effects of Photoactive Cd Complexes

The application of photo responsive crystals to useful actuation demands a rational design to elicit controllable movement. The [2+2] photocycloaddition reaction triggers mechanical motion using associated photosalient (PS) effects. We herein report a coordination site selective occupation strategy to modulate the arrangement of C=C bonds and thereby tune the PS effect. Replacing or repositioning the donor atom at one end of the linear ligand allowed for a greater level of molecular structural flexibility, facilitating [2+2] photocycloaddition. The distance between photoreactive centres and coordination sites was adjusted by ligand design to regulate PS behaviour. This work suggests new avenues for modulating PS movement to achieve useful motion.

Capturing the Interplay Between TADF and RTP Through Mechanically Flexible Polymorphic Optical Waveguides

Polymorphism plays a pivotal role in generating a range of crystalline materials with diverse photophysical and mechanical attributes, all originating from the same molecule. Here, we showcase two distinct polymorphs: green (GY) emissive and orange (OR) emissive crystals of 5'-(4-(diphenylamino)phenyl)-[2,2'-bithiophene]-5-carbaldehyde (TPA-CHO). These polymorphs display differing optical characteristics, with GY exhibiting thermally activated delayed fluorescence (TADF) and OR showing room temperature phosphorescence (RTP). Additionally, both polymorphic crystals display mechanical flexibility and optical waveguiding capabilities. Leveraging the AFM-tip-based mechanophotonics technique, we position the GY optical waveguide at varying lengths perpendicular to the OR waveguide. This approach facilitates the exploration of the interplay between TADF and RTP phenomena by judiciously controlling the optical path length of crystal waveguides. Essentially, our approach provides a clear pathway for understanding and controlling the photophysical processes in organic molecular crystals, paving the way for advancements in polymorphic crystal-based photonic circuit technologies.

Methods in molecular photocrystallography

Over the last three decades, the technology that makes it possible to follow chemical processes in the solid state in real time has grown enormously. These studies have important implications for the design of new functional materials for applications in optoelectronics and sensors. Light-matter interactions are of particular importance, and photocrystallography has proved to be an important tool for studying these interactions. In this technique, the three-dimensional structures of light-activated molecules, in their excited states, are determined using single-crystal X-ray crystallography. With advances in the design of high-power lasers, pulsed LEDs and time-gated X-ray detectors, the increased availability of synchrotron facilities, and most recently, the development of XFELs, it is now possible to determine the structures of molecules with lifetimes ranging from minutes down to picoseconds, within a single crystal, using the photocrystallographic technique. This review discusses the procedures for conducting successful photocrystallographic studies and outlines the different methodologies that have been developed to study structures with specific lifetime ranges. The complexity of the methods required increases considerably as the lifetime of the excited state shortens. The discussion is supported by examples of successful photocrystallographic studies across a range of timescales and emphasises the importance of the use of complementary analytical techniques in order to understand the solid-state processes fully.

Photo-controllable microcleaner: photo-induced crawling motion and particle transport of azobenzene crystals on a liquid-like surface

Organic crystals of 3,3'-dimethylazobenzene (DMAB) exhibit photo-induced crawling motion on solid surfaces when they are simultaneously irradiated with ultraviolet and visible light from opposite directions. DMAB crystals are candidates for light-driven cargo transporters, having simple chemical compositions and material structures. However, fast crawling motion without significant shape deformation has not yet been achieved. In this study, compared with hydrophilic glass and conventional hydrophobic surfaces with alkyl chains, siloxane-based hybrid surfaces, which are "liquid-like surfaces," result in the fastest crawling motion (4.2 μm min-1) while the droplet-like shape of DMAB crystals is maintained. Additionally, we successfully demonstrate that the DMAB crystals are capable of capturing and carrying silica particles on the hybrid surface. The transport direction is changed on demand without releasing the particles by simply changing the irradiation direction. The particles can be left on the substrate by removing the DMAB crystals via sublimation at room temperature. This result showcases a new concept of "photo-controllable microcleaner" that can operate a series of cargo capture-carry-release tasks. We expect this transporter to contribute to the development of crystal actuators, microfluidics, and microscale molecular flasks/reactors.

Macroscopic Twisting of Chiral Metal Halide Single Crystals Driven by Thermo-Induced Topochemical Dehydration

Dynamic twisting crystals, combining the features of dynamic crystals and twisting crystals, promise advanced applications in targeted drug delivery, biosensors, microrobots, and spiral optoelectronics. However, the determination of dynamic twisting crystals with specific directions remains a formidable challenge in practical applications. Herein, based on organic-inorganic hybrid metal halide (OIHMH) single crystals, we have realized the chirality-induced macroscopic twisting of single crystals driven by a thermo-induced topochemical dehydration reaction. These crystals exhibit molecular-chirality-induced twisting upon heating, along with reversals in their linear chiroptical circular dichroism and nonlinear chiroptical second harmonic generation circular dichroism. Such an induced twisting has been attributed to the alteration of the helical arrangement of chiral cation post-topochemical dehydration. The feasibility of tuning the macroscopic twisting of OIHMH single crystals and the switching in their linear and nonlinear chiroptical properties might open up new avenues for developing dynamic crystals for microactuating and optoelectronic applications.

Maneuverability and Processability of Molecular Crystals

Crystal adaptronics, a burgeoning field at the intersection of materials science and engineering, focuses on harnessing the unique properties of organic molecular crystals to achieve unprecedented levels of maneuverability and processability in various applications. Increasingly, ordered stacks of crystalline materials are being endowed with fascinating mechanical compliance changes in response to external environments. Understanding how these crystals can be manipulated and tailored for specific functions has become paramount in the pursuit of advanced materials with customizable properties. Simultaneously, the processability of organic molecular crystals plays a pivotal role in shaping their utility in real-world applications. From growth methodologies to fabrication techniques, the ability to precisely machine these crystals opens new avenues for engineering materials with enhanced functionality. These processing methods enhance the versatility of organic crystals, allowing their integration into various devices and technologies, and further expanding the potential applications. This review aims to provide a concise overview of the current landscape in the study of dynamic organic molecular crystals, with an emphasis on the interconnected themes of operability and processability.

Shape Shifting and Locking in Mechanically Responsive Organic-Inorganic Hybrid Materials for Thermoelastic Actuators

The construction of mechanically responsive materials with reversible shape-shifting, shape-locking, and stretchability holds promise for a wide range of applications in fields such as soft robotics and flexible electronics. Here, we report novel thermoelastic one-dimensional organic-inorganic hybrids (R/S-Hmpy)PbI3 (Hmpy=2-hydroxymethyl-pyrrolidinium) to show mechanical responses. The single crystals undergo two phase transitions at 310 K and 380 K. When heated to 380 K, they show shape-shifting and expansion along the b-axis by about 13.4 %, corresponding to a larger deformation than that of thermally activated shape memory alloys (8.5 %), and exhibit a strong actuation force. During the cooling process, the stretched crystal shape maintains and a shape-locking phenomenon occurs, which is lifted when the temperature decreases to 305 K. Meanwhile, due to the introduction of chiral ions, the thermal switching shows a 10-fold second-order nonlinear switching contrast (common values typically below 3-fold). This study presents a thermoelastic actuator based on shape-shifting and -locking of organic-inorganic hybrids for the first time. The dielectric and nonlinear optical switching properties of organic-inorganic hybrids broaden the range of applications of mechanically responsive crystals.

Pressure-induced shape and color changes and mechanical-stimulation-driven reverse transition in a one-dimensional hybrid halide

Dynamic crystals with directional deformations in response to external stimuli through molecular reconfiguration, are observed predominantly in certain organic crystals and metal complexes. Low-dimensional hybrid halides, resemble these materials due to the presence of strong hydrogen bonds between bulky organic moieties and inorganic units, whereas their dynamic behavior remains largely unexplored. Here we show that a one-dimensional hybrid halide (MV)BiBr5 (MV = methylviologen) undergoes an isosymmetric phase transition at hydrostatic pressure of 0.20 GPa, accompanied by a remarkable length expansion of 20-30% and red to dark yellow color change. Unexpectedly, the backward transition can be fully reversed by mechanical stimulation rather than decompression. In the high-pressure phase, the coexistence of strong Bi3+ lone pair stereochemical activity and large reorientations of the planar MV2+ cations, together with the newly formed CH···Br hydrogen interactions, are the structural features that facilitate microscopic changes and stabilize the metastable high-pressure phase at ambient conditions.

Gas Release as an Efficient Strategy to Tune Mechanical Properties and Thermoresponsiveness of Dynamic Molecular Crystals

Dynamic molecular crystals combining multiple and finely tunable functionalities are attracting and an increasing attention due to their potential applications in a broad range of fields as efficient energy transducers and stimuli-responsive materials. In this context, a multicomponent organic salt, piperazinium trifluoroacetate (PZTFA), endowed with an unusual multidimensional responsive landscape is reported. Crystals of the salt undergo smooth plastic deformation under mechanical stress and thermo-induced jumping. Furthermore, via controlled crystal bending and release of trifluoroacetic acid from the lattice, which is anticipated from the design of the material, both the mechanical response and the thermoresponsive behavior are efficiently tuned while partially preserving the crystallinity of the system. In particular, mechanical deformation hampers guest release and hence the macroscopic jumping effect, while trifluoroacetic acid release stiffens the crystals. These complex adaptive responses establish a new crystal engineering strategy to gain further control over dynamic organic crystals.

Photomechanical properties in metal-organic crystals

The emergence of materials that can effectively convert photon energy (light) into motion (mechanical work) and change their shapes on command is of great interest for their potential in the fabrication of devices (powered by light) that will revolutionize the technologies of optical actuators, smart medical devices, soft robotics, artificial muscles and flexible electronics. Recently, metal-organic crystals have emerged as desirable smart hybrid materials that can hop, split and jump. Thus, their incorporation into polymer host objects can control movement from molecules to millimetres, opening up a new world of light-switching smart materials. This feature article briefly summarizes the recent part of the fast-growing literature on photomechanical properties in metal-organic crystals, such as coordination compounds, coordination polymers (CPs), and metal-organic frameworks (MOFs). The article highlights the contributions of our group along with others in this area and aims to provide a consolidated idea of the engineering strategies and structure-property relationships of these hybrid materials for such rare phenomena with diverse potential applications.

Frustration of H-Bonding and Frustrated Packings in a Hexamorphic Crystal System with Reversible Crystal-Crystal Transitions

The study of transitions between polymorphic phases is a less investigated chapter of the widely studied book of polymorphism. In this paper, we discuss the phase behavior of a new compound that has been rationally designed to show frustration of H-bonds for the strong amide N-H donor, which cannot be involved in H-bonding nor in van der Waals interactions. The compound (ImB) is a showcase of almost all possible cases of transitions between polymorphs [monotropic/enantiotropic, fast/slow, diffusive/displacive, and single-crystal-to-single-crystal (SCSC)] and of relation between polymorphs with different Z'. Six crystal phases (I, II, III, IV, V, and VI) were identified for it with five crystal-crystal transitions. Two transitions are reversible/SCSC/fast. Of the three monotropic transitions, all non-SCSC, one is slow, and the others are fast. Of the two enantiotropic SCSC transitions, one does not exhibit undercooling, while the other shows strong undercooling. Phase III, with Z' = 6, is stable at room temperature between phase II (Z' = 1), stable at high temperature, and phase IV (Z' = 2), stable at low temperature. All six polymorphs are based on the same O-H···O═C H-bonding synthon, which produces infinite chains in five polymorphs and ring tetramers in one. The sequence of reversible SCSC transitions IV ⇆ III ⇆ II involves a remarkable ping pong of the symmetry rules by which H-bonded chains are built. Based on all of this, a possible roadmap for prediction of SCSC transitions in crystals is shortly outlined.

Photo- and Stress-Induced Bending of (E)-1,2-Bis(pyridinium-4-yl)ethene Dinitrate Crystals

We report on the elastic and photodynamic properties of (E)-1,2-bis(pyridinium-4-yl)ethene dinitrate [H2Ebpe](NO3)2, whose needle-like crystals can be reversibly deformed by applying external mechanical stress. The macro-scale mechanical properties of [H2Ebpe](NO3)2 crystals were quantified by a three-point bending test, which gave a stress-strain curve with an elastic modulus of 1.18 GPa, and its values are lower than those of other flexible elastic organic crystals. It can also be reversibly bent through the [2+2] cycloaddition reaction of the olefin moiety, depending on the direction of UV irradiation.

Surface Engineering of the Mechanical Properties of Molecular Crystals via an Atomistic Model for Computing the Facet Stress Response of Solids

Dynamic molecular crystals are an emerging class of crystalline materials that can respond to mechanical stress by dissipating internal strain in a number of ways. Given the serendipitous nature of the discovery of such crystals, progress in the field requires advances in computational methods for the accurate and high-throughput computation of the nanomechanical properties of crystals on specific facets which are exposed to mechanical stress. Here, we develop and apply a new atomistic model for computing the surface elastic moduli of crystals on any set of facets of interest using dispersion-corrected density functional theory (DFT-D) methods. The model was benchmarked against a total of 24 reported nanoindentation measurements from a diverse set of molecular crystals and was found to be generally reliable. Using only the experimental crystal structure of the dietary supplement, L-aspartic acid, the model was subsequently applied under blind test conditions, to correctly predict the growth morphology, facet and nanomechanical properties of L-aspartic acid to within the accuracy of the measured elastic stiffness of the crystal, 24.53±0.56 GPa. This work paves the way for the computational design and experimental realization of other functional molecular crystals with tailor-made mechanical properties.

Chromic and dynamic: soft crystals of platinum(II) complexes pave the way for multi-responsive materials

The development of smart, stimuli-responsive materials has received increased attention in the past decade for their applications as sensing technologies. This commentary discusses a timely topical review by Kato [(2024). IUCrJ, 11, 442-452] on the fabrication of multi-stimuli responsive crystals comprised of luminescent platinum(II) complexes, which exhibit intriguing chromic phenomena in response to stimuli.

Trimodal operation of a robust smart organic crystal

We describe a dynamic crystalline material that integrates mechanical, thermal, and light modes of operation, with unusual robustness and resilience and a variety of both slow and fast kinematic effects that occur on very different time scales. In the mechanical mode of operation, crystals of this material are amenable to elastic deformation, and they can be reversibly morphed and even closed into a loop, sustaining strains of up to about 2.6%. Upon release of the external force, the crystals resume their original shape without any sign of damage, demonstrating outstanding elasticity. Application of torque results in plastic twisting for several rotations without damage, and the twisted crystal can still be bent elastically. The thermal mode of operation relies on switching the lattice at least several dozen times. The migration of the phase boundaries depends on the crystal habit. It can be precisely controlled by temperature, and it is accompanied by both slow and fast motions, including shear deformation and leaping. Parallel boundaries result in a thermomechanical effect, while non-parallel boundaries result in a thermosalient effect. Finally, the photochemical mode of operation is driven by isomerization and can be thermally reverted. The structure of the crystal can also be switched photochemically, and the generation of a bilayer induces rapid bending upon exposure to ultraviolet light, an effect that further diversifies the mechanical response of the material. The small structural changes, low-energy and weak intramolecular hydrogen bonds, and shear deformation, which could dissipate part of the elastic energy, are considered to be the decisive factors for the conservation of the long-range order and the extraordinary diversity in the response of this, and potentially many other dynamic crystalline materials.

Strategies to Diversification of the Mechanical Properties of Organic Crystals

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.

Molecular Packing Topology and Interactions to Decipher Mechanical Compliances in Dicyano-Distyrylbenzene Derivatives

Flexible optoelectronics is the need of the hour as the market moves toward wearable and conformable devices. Crystalline π-conjugated materials offer high performance as active materials compared to their amorphous counterpart, but they are typically brittle. This poses a significant challenge that needs to be overcome to unfold their potential in optoelectronic devices. Unveiling the molecular packing topology and identifying interaction descriptors that can accommodate strain offers essential guiding principles for developing conjugated materials as active components in flexible optoelectronics. The molecular packing and interaction topology of eight crystal systems of dicyano-distyrylbenzene derivatives are investigated. Face-to-face π-stacks in an inclined orientation relative to the bending surface can accommodate expansion and compression with minimal molecular motion from their equilibrium positions. This configuration exhibits good compliance towards mechanical strain, while a similar structure with a criss-cross arrangement capable of distributing applied strain equally in opposite directions enhances the flexibility. Molecular arrangements that cannot reversibly undergo expansion and compression exhibit brittleness. In the isometric CT crystals, the disproportionate strength of the interactions along the bending plane and orthogonal directions makes these materials sustain a moderate bending strain. These results provide an updated explanation for the elastic bending in semiconducting π-conjugated crystals.

Bending, Twisting, and Propulsion of Photoreactive Crystals by Controlled Gas Release

The rapid release of gas by a chemical reaction to generate momentum is one of the most fundamental ways to elicit motion that could be used to sustain and control the motility of objects. We report that hollow crystals of a three-dimensional supramolecular metal complex that releases gas by photolysis can propel themselves or other objects and advance in space when suspended in mother solution. In needle-like regular crystals, the reaction occurs mainly on the surface and results in the formation of cracks that evolve due to internal pressure; the expansion on the cracked surface of the crystal results in bending, twisting, or coiling of the crystal. In hollow crystals, gas accumulates inside their cavities and emanates preferentially from the recess at the crystal terminus, propelling the crystals to undergo directional photomechanical motion through the mother solution. The motility of the object which can be controlled externally to perform work delineates the concept of "crystal microbots", realized by photoreactive organic crystals capable of prolonged directional motion for actuation or delivery. Within the prospects, we envisage the development of a plethora of light-weight, efficient, autonomously operating robots based on organic crystals with high work capacity where motion over large distances can be attained due to the large volume of latent gas generated from a small volume of the crystalline solid.

Highly efficient in crystallo energy transduction of light to work

Various mechanical effects have been reported with molecular materials, yet organic crystals capable of multiple dynamic effects are rare, and at present, their performance is worse than some of the common actuators. Here, we report a confluence of different mechanical effects across three polymorphs of an organic crystal that can efficiently convert light into work. Upon photodimerization, acicular crystals of polymorph I display output work densities of about 0.06-3.94 kJ m-3, comparable to ceramic piezoelectric actuators. Prismatic crystals of the same form exhibit very high work densities of about 1.5-28.5 kJ m-3, values that are comparable to thermal actuators. Moreover, while crystals of polymorph II roll under the same conditions, crystals of polymorph III are not photochemically reactive; however, they are mechanically flexible. The results demonstrate that multiple and possibly combined mechanical effects can be anticipated even for a simple organic crystal.

Engineering mechanical compliance in polymers and composites for the design of smart flexible sensors

Polymers are one of the most popular materials for next-generation flexible sensing device fabrication due to their tunable mechanical and electrical properties. A series of prior research studies in the field of smart flexible and wearable sensing illustrates the potential of various polymer and composite materials to be applied in sensor development. In this direction, mechanical compliance plays a vital role as it ensures the stability and reliability of the fabricated sensor. Therefore, engineering mechanical compliance for the development of smart flexible solutions has emerged as a significant area of research. Furthermore, the usage of flexible sensing devices is rapidly increasing in the field of healthcare devices and robotic automation. This feature article summarizes the relevant contributions of the authors in the field of engineered polymers and composites for flexible sensor development with a focus on healthcare and physical sensing applications. We discuss the polymer and composite materials, their characteristics, fabrication technologies, finite element method analysis, and examples of flexible physical sensors, i.e. pressure, strain, and temperature sensors, for various wearable healthcare applications and robotic automation. Finally, we discuss examples of multi-sensory systems having flexible sensors.

Determining the quantum yield of photochemical reactions in crystals from simultaneous effects of photothermal and photochemical bending of needle-shaped crystals

Photoinduced bending of needle crystals caused by photochemical transformation can be used as an extremely sensitive method for studying the kinetics of the transformation. However, the determination of the absolute value of the quantum yield of the reaction requires an accurate value of the intensity of light penetrating the crystal, in contrast to reactions in solutions where only the value of the total absorbed irradiation dose is sufficient. To address this problem, this study utilizes the effect of photothermal bending of a crystal due to its heating by light, occurring simultaneously with the bending due to transformation and proportional to the same value of light intensity. The ratio of the amplitudes of the two effects is independent of the light intensity, which allows the quantum yield to be determined without knowledge of the intensity value. In addition, the method allows the light intensity and thermal conductivity of the crystal to be estimated. The method is applied to measure wavelength dependence of the quantum yield of nitro-to-nitrito photoisomerization in [Co(NH3)5NO2]Cl(NO3) crystals. A monotonically decreasing value of the quantum yield φ from 0.19 to 0.04 in the range of λ from 403 to 523 nm was obtained. This result indicates the qualitative differences in the transformation mechanism in crystals and in solutions, where φ = 0.03 independent of λ in the same wavelength range.

Directional Crystal Jumping Controlled by Chirality

Jumping crystals of racemic mixtures of asparagine monohydrate have been presented in this contribution to emphasize the key role of molecular chirality in governing the direction of macroscopic motions. When heated at the specific faces of the single crystals, a pair of enantiomorphs jump in opposite directions, which are further utilized for chiral resolution. The hydrogen-bonded networks between asparagine molecules in a specific direction provide oriented channels for the escape of water molecules during the dehydration, serving as a foundation for the directional crystal jumping. Our findings not only lay the foundation for the future creation of directed actuation systems based on dynamic crystals but shall also guide the efforts to reveal the correlation between chirality and motion across diverse realms of knowledge.

The Photosalient Effect and Thermochromic Luminescence Based on o-Carborane-Assisted π-Stacking in the Crystalline State

Herein, we report the unique multiple-stimuli responsiveness of anthracene-tethered o-carborane derivatives. We designed and synthesized anthracene derivatives with different substitution positions and numbers of the o-carborane units. Two compounds had characteristic crystal structures involving the columnar π-stacking structures of the anthracene units. From the analysis of crystalline-state structure-property relationships, it was revealed that the crystals exhibited the photosalient effect accompanied by photochemical [4+4] cycloaddition reactions and temperature-dependent photophysical dual-emission properties including excimer emission of anthracene. Those properties were considered as non-radiative and radiative deactivation pathways through the excimer formation in the excited state and the formation of excimer species was facilitated by the π-stacking structure of anthracene units. Moreover, we found unusual temperature dependency on the occurrence of the photosalient effect. According to the data from variable temperature X-ray crystallography, a strong correlation between lattice shrinkage and strain accumulation is suggested.