ABSTRACT Flexible organic crystals offer significant opportunities for organic electronics; however, their potential applications in fields like flexible sensors are at present limited by poor charge transport that result in failure under reversible conditions. In this report, we propose a universal interfacing design strategy for the fabrication of versatile conductive MXene coatings on the surface of flexible organic crystals that preserve the electric conductivity of the MXenes while they also remain mechanically flexible and robust across a wide temperature range of nearly 300°C both above and below ambient temperature. This new sub‐class of hybrid materials based on adaptive organic crystals undergoes reversible changes in the crystal structure upon deformation, while they also engender stable electrical conductivity and retain deformation sensitivity from about −196°C to 100°C. A wireless sensing system based on these materials was constructed to monitor bionic finger movements that demonstrate the potential for human‐machine interfacing and medical monitoring applications. The structural stability of this and similar devices that utilize crystal‐MXene hybrid materials under non‐ambient temperatures provide an innovative solution to the current challenges with occasional failure of low‐temperature mechanical sensing systems in extreme environments, and specifically such that are used in lunar and deep‐space explorations.

The relationship between single-crystal-to-single-crystal (SCSC) transformations in solid-state [2 + 2] photocycloaddition and light-induced motion of crystals, known as the photosalient effect, remains poorly understood. Here, we report a two-dimensional zinc(II) coordination polymer (1) that exhibits both reversible SCSC [2 + 2] photocycloaddition reactivity and macroscopic photomechanical motions (jumping, splitting, and other effects). 1 contains three crystallographically distinct olefin pairs, enabling stepwise [2 + 2] cycloaddition under ultraviolet light, and the reverse reaction proceeds by heating. Single-crystal X-ray diffraction was used to capture the intermediate states during stepwise photodimerization and thermal ring cleavage, revealing the preferred reactivity of the olefin pairs in the structure. This work provides an example of fully reversible, photosalient SCSC [2 + 2] photocycloaddition, demonstrating that macroscopic dynamics and precise molecular-scale transformations can coexist in dynamic crystals.

Narrowband near-infrared (NIR) emissive materials are indispensable for advancing optical communications, biological imaging, and precision sensing technologies. Despite their potential, achieving precise control over the emission profile while ensuring the long-term stability of the material remains a significant challenge. In this study, we utilized a diaminomaleonitrile derivative to demonstrate an approach that combines geometric (cis-trans) isomerism and crystal size to control the ultra-narrowband NIR emission of an organic crystalline material. By systematically optimizing key parameters, including light exposure, temperature, and solvent diffusion dynamics, we established a robust platform for controlled and preferential growth of its two isomers with specific optical properties. Remarkably, the crystals of the cis-isomer exhibited size-dependent narrowing of the NIR emission band, with an impressive narrow full-width-at-half-maximum of 40 nm (0.096 eV), while the trans-isomer exhibited ultranarrowband NIR emission with outstanding photostability attributed to its rigid, planar molecular structure. Comprehensive theoretical and experimental analyses revealed the critical role of intermolecular interactions and vibrational relaxation in modulating their emission characteristics. This study establishes a robust methodology for the preparation of narrowband organic NIR-emissive materials.

The thermosalient effect is a rarely observed, potentially very useful and at the present, unpredictable mechanical response during a phase transition that is thought to hold the potential for rapid and clean energy conversion devoid of gaseous products. Here, we report the serendipitous discovery of a rare instance of a thermosalient organic solid that exhibits the effect below room temperature. The crystals of this carbazole-based material are dynamic at both molecular and macroscopic scales. Using variable temperature synchrotron X-ray diffraction and variable-temperature solid-state nuclear magnetic resonance (ssNMR), we thoroughly examined the hysteretic structural transition in this material, emphasizing its macroscopic reconfigurability. We discovered unexpected large-amplitude molecular oscillations in the low-temperature phase, which challenge conventional assumptions about salient materials. Notably, we combined 2H ssNMR with computational modeling to reveal this dual-scale dynamism, setting the groundwork for advancements in energy-efficient actuators, sensors, and intelligent materials. This work might open new avenues for developing crystalline materials that can be implemented in innovative devices operating seamlessly across various scales.

Flexible organic crystals have been recently explored for optical transduction in flexible small-scale optical devices; however, most studies are limited to the visible range (400-600 nm). Due to the photonic well confinement and low optical losses, optical telecommunications require transducive media with tunable optical emission in the 850-1550 nm region. While chemical substitution is a viable option to shift the emissive properties of organic crystals to the telecom regime, depending on the target modification, this could be a resource-intensive approach. Here, with two cocrystals of 1,1'-biphenyl (BP), 4,4'-dimethylbiphenyl (DM), and 7,7,8,8-tetracyanoquinodimethane (TCNQ), we demonstrate that cocrystallization provides an alternative and straightforward access to organic materials with emissions in the NIR region. The crystal between DM and TCNQ is mechanically plastic and emissive in the NIR region and can be used as an NIR optical waveguide. Both experimental and computational analyses point to the structure of the component molecules as determining factors of the CT interaction and, therefore, the emission window. The optical band gap is conducive to narrowing by strong CT interaction, resulting in emission in the NIR region.

Soft organic crystals that combine high strength and toughness are essential for flexible electronics and bioinspired devices, but they often compromise one property for the other. Here, we demonstrate a visible-light-driven, single-crystal-to-single-crystal photopolymerization of 1,1'-dioxo-1H,1'H-[2,2'-biindene]-3,3'-diyl-bis(decanoate) (B10) into a polymeric crystal (PB10) that simultaneously with polymerization enhances its mechanical strength and toughness. Under white-light irradiation (2.5 W cm-2), centimeter-long B10 needles exhibit splitting, coiling, and straightening, accompanied by a color change from red to colorless. This transformation is accompanied by a molecular reorganization, where the weak (π···π stacking) interactions are replaced by stronger (C-C) bonds, resulting in a drastic change in mechanical properties. As a result, upon photopolymerization, the PB10 crystals transition from purely elastic to elastic/plastic, with a nearly 228-fold increase in toughness. This polymerization is also accompanied by increases in tensile modulus and a nearly 81-fold increase in tensile toughness. Remarkably, the PB10 crystals exhibit a load-bearing capacity exceeding 1 × 105 times their own mass, additionally reflecting the dramatic enhancement in mechanical strength.

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.

Common self-healing mechanisms rely on the diffusion of chemical entities across a fissure to rebuild the interface. As diffusion is temperature-controlled, cryogenic conditions are prohibitive to self-healing. Here we report a molecular crystal that heals at ambient and high temperature (298 and 423 K) but that is also capable of autonomous recovery at 77 K. The efficiency of this process depends on dipole-dipole interactions as the dominant mechanism that reduces the separation between the interfaces. Comparative optical transmission measurements confirm that healed crystals are approximately 99% transparent relative to the same material before cracking. This cryogenic self-healing capability is used to design an autonomously reparative, all-organic, crystalline optical transmission system and enables substantial recovery of the optical losses due to the material's ability to recover after damage. This and possibly other similar materials overcome the natural limitations of macromolecular self-healing media at cryogenic temperatures, opening opportunities for developing materials that can operate practically indefinitely under extreme conditions.

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.

Amplifying microscopic or molecular perturbations to induce macroscopic mechanical effects in well-ordered molecular crystals is the foundation of the newly recognized potential of organic crystalline smart materials for soft organic electronics, optics, actuators, switches, and robots. Diverse molecular crystal actuators that transform external energy into mechanical motions have been prepared in the past decade, yet their spatiotemporal operational capability, adaptability, reversibility, and durability have not been fully explored. In this study, we present adaptive molecular single crystals that can respond to force, heat, and light, demonstrating mechanical flexibility, reversible expansion, and complex movements, including rolling and climbing locomotion. These crystals exhibit significant anisotropic thermal expansion within the temperature range from 303 to 413 K, expanding by approximately 4.8% along their longest axis. The exceptional flexibility and thermal expandability of this material enable quick and sustained locomotion of the single crystals when exposed to ultraviolet light. Our findings highlight the considerable yet underexplored potential of adaptive organic crystals that can be used as lightweight thermomechanical and photomechanical actuators.

Concomitant long-lived phosphorescence and cryogenic elasticity in soft matter is an immensely challenging endeavor due to the contrasting effect of low temperatures on these properties. While the low temperature normally extends and enhances phosphorescence, it typically compromises mechanical elasticity by freezing the molecular motion, inevitably leading to brittleness and cracking of soft materials. In this work, we posit that the emerging class of organic crystals can overcome this intrinsic disparity and describe an organic crystalline material that meets both requirements─an exceptional elasticity of its crystals at 77 K and ultralong afterglow of up to about 30 s, the longest lifetime of a flexible organic crystal reported to date. The material, triphenylene, was prepared as elastic crystals, where the molecular rigidity and dense packing enable reversible lattice deformation and mechanical robustness on cooling, while they also result in prolonged phosphorescence at low temperatures. Crystals of this material act as dynamic phosphorescent waveguides, with their emission persisting in low temperatures and dark, demonstrating both sustained signal transmission capabilities and a unique opportunity for spatiotemporal control of the optical output. At a conceptual level, the results introduce organic crystals for time-encoded biological information transmission, providing a novel material platform for flexible, lightweight optical devices and sensors that can function in extreme environments.

The recent discovery of the self-healing capabilities of molecular crystals has shown significant efficiency, approaching nearly 100%, particularly when this process is coupled with a phase transition. This places these materials on par with other, better-studied soft materials, such as polymers. However, the physical inaccessibility of the contact interface with common analytical methods hinders direct experimental observation of the critical molecular-scale processes responsible for the recovery of the interfacial gap; as a result, this effect has largely remained phenomenological. This report employs molecular dynamics simulations and mechanical analysis to unravel the mechanistic details behind the remarkably efficient (95%) self-healing observed in the ferroelastic crystal anilinium bromide. Bulk simulations successfully reproduce the experimentally observed phase transitions under both uniaxial and biaxial loading conditions, while slab model calculations with free surfaces capture crack formation and self-healing phenomena associated with twinning and detwinning. The atomistic insights establish a two-step model: external mechanical loading first activates molecular slip along the (110)[11̄0] path, providing a periodic impulse force that encourages molecular reorientation, leading to twinning and detwinning, along with subsequent ferroelastic and self-healing behaviors. These findings underscore the critical roles of crystal packing and mechanical loading and offer clear design principles for developing new organic self-healing materials with enhanced mechanical properties.

Topological [2+2] cycloaddition is known to provide a convenient synthetic route for cyclobutane derivatives from favorably dispositioned dienes. In this study, new (2Z,4E)-2-(2,4-difluorophenyl)-5-phenylpenta-2,4-dienenitrile (HDE), (2Z,4E)-2-(2,4-difluorophenyl)-5-(p-tolyl)penta-2,4-dienenitrile (MeDE), (2Z,4E)-5-(4-chlorophenyl)-2-(2,4-difluorophenyl)penta-2,4-dienenitrile (ClDE), (2Z,4E)-5-(4-bromophenyl)-2-(2,4-difluorophenyl)penta-2,4-dienenitrile (BrDE), (2Z,4E)-2-(2,4-difluorophenyl)-5-(4-methoxyphenyl) penta-2,4-dienenitrile (MeODE), and (2Z,4E)-2-(2,4-difluorophenyl)-5-(4-(dimethylamino)phenyl)penta-2,4-dienenitrile (MeNDE) were synthesized, and their reactivity and selectivity were investigated in relation to their molecular packing in the respective crystals. HDE and MeDE, with head-to-tail (HT) arrangement, yielded only one type of photodimer. On the contrary, ClDE and BrDE, with head-to-head (HH) packing, and where the "olefin pairsα,β-α,β" and "olefin pairsγ,δ-γ,δ" satisfy Schimdt's criteria, reacted to a mixture of photoproducts. Kinetics analysis suggests that the reaction rates of HDE and MeDE are higher than those of ClDE and BrDE. This observation may be due to the strong non-covalent interactions between the potentially reactive olefin pairs as suggested by energy decomposition analysis. Furthermore, the reaction activation energies for photodimerization of the HT-packed olefin pairs are indeed lower than those of the HH-arranged ones. The HT packing of the diphenyldienes not only enhances the reactivity in the topological [2+2] cycloaddition but also contributes chemospecificity, regiospecifity, and stereospecificity, all of which are essential for the preparation of specific cyclobutanes derivatives based on photodimerization.

Circular organic crystals are essential as optically transducive components in flexible organic optoelectronics, yet this crystal habit is not easily obtained through traditional crystallization approaches. Here, we present a photoresponsive organic crystalline material that when exposed to ultraviolet or visible light, initially undergoes photoinduced bending, followed by photosalient effect and accompanied by delamination to elastic quasicircular microcrystals. Curvature analysis under different conditions confirms the controllability of this process. Light at 365 nm, 405 nm, and 445 nm generates microcrystals with high curvatures (11-12 mm-1), while 470 nm light produces lower curvature (5 mm-1), aligning with the absorption profile. Increasing the excitation power from 15 mW to 150 mW results in increase of the yield of microcrystals with high curvatures (10-20 mm-1) from 20% to 94%. This light-driven fabrication method provides a controlled and reproducible means of realizing rare crystal morphologies, highlighting the potential for exploring quantitative relationships between such morphologies and their unconventional optical properties.

Organic molecular crystals capable of mechanical adaptation are poised to revolutionize soft advanced materials with potentially immense implications from optics to electronics and biomedicine. While these prospects have guided studies into both fundamental and performance aspects, currently available methodologies for the design of organic crystalline matter with specific mechanical properties based on classical crystal engineering principles lack reliability and generality in application, and this significantly limits consideration of organic crystals as materials of choice. To address this challenge, we apply deep learning models for the design of mechanically compliant organic crystalline materials. We introduce CrystalGAN, a deep generative framework based on a generative adversarial network (GAN), designed to efficiently generate flexible molecular crystals with desired mechanical properties. CrystalGAN leverages a graph convolutional network (GCN) to construct both the generator and discriminator of a Wasserstein GAN (WGAN), enhancing the validity of the generated molecules. A convolutional neural network (CNN) was trained to predict and discriminate the mechanical properties of unknown molecules, based on the data collected from the extant literature and compared with multilayer perceptron (MLP) with backpropagation algorithm. The CNN showed favorable performance with high accuracy in various computational evaluations and successfully predicted the mechanical response of the flexible crystals. The inferences are complemented by case studies that employed CrystalGAN and CNN to generate molecules that are expected to crystallize as flexible crystals with improved tableting properties. This work overcomes one of the current major challenges with the lack of discovery and prediction of organic crystals with specific mechanical properties.

Owing to the rich physical-chemistry properties, phosphates have found diverse applications. Their behaved interesting characteristics mainly benefit from the flexibility of PO4 tetrahedra. The development of new phosphates is of special significance to enrich the relationship between the structure and properties of phosphates. Here, we present a new phosphate Sr2In(PO4)(P2O7) which crystallizes to a monoclinic cell (S.G. P2/c), with the lattice parameters of a=6.5832(3) Å, b=6.8984(3) Å, c=19.813(1) Å, β=99.562(1)°, V= 887.3(1) Å3, and Z=2. In the structure, there are 11 independent 2i lattice sites for one In, two Sr, three P, and eleven O, respectively. The three-dimensional frame of this structure is made of InO6, PO4, P2O7, SrO8, and SrO9 units. The co-existence of P2O7 and PO4 was confirmed by IR spectra assignment. This new phosphate shows up the large band gap and high physicochemical stability catering for the material requirement in futural optical or other functional application fields.

Optically transmissive materials are indispensable for the transmission of light or light -encoded signals in telecommunications and optobiomedical techniques. Here, we propose that slender crystals of small organic molecules can be used as optically transparent, flexible, lightweight, and emissive media to deliver photons into or through biological tissues as an alternative to silica- or polymer -based light waveguides. We demonstrate that organic crystals remain transmissive in various tissues, and their efficiency in light transduction depends on the intrinsic optical properties of the crystal, optical path, geometry of excitation, and the type of tissue. Moreover, elastically or plastically deformable organic crystals remain mechanically compliant and can be bent after they have been embedded in the tissue, opening prospects for designing a new class of biocompatible light waveguiding elements based on crystalline organic materials. In vivo implantation and toxicity assays capitalize on mechanical flexibility and biocompatibility in animal models. Within a broader context, the high transparency, anisotropy, and biocompatibility of some organic crystals turn this emerging class of materials into a prospective platform for delivering photons for specific interaction with target cells in tissues for applications such as photodynamic therapy and optogenetics.

The dense and ordered molecular arrangements endow dynamic molecular crystals with fast response, rapid energy conversion, low energy dissipation, and strong coupling between heat/light and mechanical energy. Most of the known dynamic crystals can only respond to a single stimulus, and materials that can respond to multiple stimuli are rare. Here, we report an organic crystalline material that can be bent plastically and is also thermosalient, as its crystals can move when they undergo a reversible phase transition. The crystals transmit light regardless of their shape or crystalline phase. The combination of light transduction and reversible thermomechanical deformation provides an opportunity to switch the waveguiding capability of the material in a narrow temperature range, which holds a tremendous potential for applications in heat-averse electronic components, such as central processing units. Unlike existing electronics, the material we report here is completely organic and therefore much lighter, potentially reducing the overall weight of electronic circuits.