Remote and precise control over morphology and motion of organic crystals by using magnetic field

Molecular Thermal Engine Based on a Highly Flexible Elastic Crystal

Materials that exhibit actuation behavior in response to external stimuli have a wide range of applications owing to their ability to convert input energy into mechanical work. Light and chemicals are common sources of input energy. However, actuation using thermal energy from ambient-temperature sources remains challenging. In this study, we introduce novel elastic crystals composed of dodecylated porphyrin molecules that exhibit high flexibility and deformation in response to temperature changes. When a crystal is loaded with a small weight and positioned between high- and low-temperature heat sources, it exhibited continuous, large, and rapid oscillations. These oscillations persisted for at least 160 h, corresponding to 3.9 million deformation cycles, as long as the temperature difference was maintained. This study presents the first example of a molecular crystal functioning as an engine that can extract kinetic energy from static and ambient-temperature sources.

Highly Emissive Flexible Organic Crystals with Broad Piezochromism and Optical Waveguides

The integration of mechanical flexibility with stimuli-responsive optical properties in organic single crystals (OSCs) remains a formidable challenge due to the intrinsic rigidity of π-conjugated systems. Herein, we report a rationally designed OSC, FBCF, that synergistically combines elastic deformability, broad piezochromism, and optical waveguides via a planar benzothiadiazole (BTA) core and fluoroarene termini. The molecular architecture enforces face-to-face slip-stacking via multiple noncovalent interactions (N•••H, π-π, and C-F•••π), yielding centimeter-scale crystals with exceptional elastic bending and full recovery. These crystals exhibit high photoluminescence (PL) efficiency (PLQY = 75.8%) and serve as optical waveguides, maintaining performance even when bent (optical loss coefficients (OLCs): 0.286-0.529 dB mm-1). Crucially, FBCF exhibits broad pressure-responsive spectral shifts (Δλ = 171 nm), achieving a piezochromic sensitivity of 16.7 nm GPa-1, exceeding that of most organic systems. Single-crystal X-ray diffraction analyses reveal hierarchical packing: rigid lamellar layers (stabilized by strong π-π/dipole interactions, -118.56 to ‑58.45 kcal/mol) interconnected via weaker interlayer C-F•••π bonds (-18.8 kcal/mol). This architecture enables elastic deformation through interlayer slippage while preserving intralayer order.

From Flexible Crystals to Piezoelectrics: The Advent of a New Class of Flexible Functional Molecular Materials

The recent discoveries of mechanically flexible molecular crystals have fuelled a resurgence of research interest in molecular piezoelectrics. This has raised the quest to explore structure-property relations in molecular piezoelectric crystals, which remain largely obscure. Here, the fundamental structural features associated with organic molecular piezoelectric crystals are explored in relation to their mechanical and supramolecular flexibility. Along with the electrostatic properties such as molecular dipole moments and spontaneous crystal polarization, possible correlations of piezoelectric coefficients with intermolecular interaction topologies and their anisotropy point toward their link with mechanical flexibility in molecular crystals. In addition, the possible roles of crystal packing efficiency, lattice cohesive energies, Young's moduli, and its anisotropy from elastic tensors have been examined. This quantitative overview suggests that piezoelectric response in molecular materials is a complex interplay of several structural and electrostatic factors. Based on these analyses and the fundamental aspects of electromechanical coupling, it becomes apparent that combining mechanical flexibility and supramolecular chirality/polarity can be a promising approach to discovering soft molecular piezoelectrics for novel actuators and energy-harvesting materials.

Head-to-Tail Packing to Facilitate [2+2] Cycloaddition for Green Synthesis of Cyclobutane Derivatives in Specific Configuration

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.

A high-frequency nanoscale positioner driven by an external electric field: a molecular dynamics study

The precise localization at the nanoscale plays a crucial role in mass transfer, nanostructure reconstructions, and nanofabrication processes. This study presents a model for a nano-positioner via strain engineering that can be precisely controlled by an external electric field (EF). The model consists of three major components: a graphene kirigami (GK) nanospring for achieving large elastic deformation, a charged carbon nanotube (CNT) connecting the GK, and a graphene substrate for controlling the movement path of the CNT. Upon activation of the EF, the charged CNT moves along the substrate, stretches the GK, and eventually settles at a desired position after undergoing damped oscillations. When turning off the EF, the CNT returns to its initial position. Molecular dynamics simulations are employed to evaluate the safety, stability, precision, and response speed of this system in a pulsed EFs while taking account of GK geometry and EF mode effects. Within a certain range of EF intensity, this nano-positioner operates safely with tunable positioning capability while maintaining high precision and response speed. Furthermore, this nanosystem can work smoothly at gigahertz in a pulsed EF.

Spatiotemporal Control of Photoisomerization Dynamics via Domino Barriers for Programmatically Responsive Heterostructures

Controlling the photoisomerization reaction at the micro-/nanoscale is important for the realization of high-end photonic components. Unfortunately, spatiotemporal manipulation of the photoisomerization dynamics still faces a significant challenge. Here, we propose an effective strategy to control the photoisomerization reaction spatiotemporally through introducing a steric-hindrance effect by the aid of alloy engineering. The external guest molecules behave like domino barriers and efficiently regulate the photoisomerization dynamics. Moreover, the flexible assembly of the organic heterostructures with different steric-hindrance degrees enabled us to spatiotemporally modulate the photoisomerization dynamics in 1D, 2D, and even annular morphologies. Interestingly, the photoisomerization reaction exhibits anisotropic change characteristics in 2D microcrystals. Our work provides deep insight into the modulation of the photoisomerization reaction and would promote the development of smart responsive barcodes with improved security level toward advanced anti-counterfeiting applications.

Cornu-Spiral-Like Organic Crystal Waveguide Providing Discriminatory Optical Pathway for Smart Organic Photonic Circuit

In the era of artificial intelligence, developing advanced and intelligent photonic circuits has become essential. In this work, the fabrication of a smart organic photonic circuit (OPC), is illustrated which utilizes a Cornu-spiral-like waveguide (CSW) to produce discriminating optical pathways in the circuit. The mechanical flexibility of Schiff base, (E)-1-(((5-iodopyridin-2-yl)imino)methyl)naphthalen-2-ol (IPyIN) facilitates the fabrication of a first-of-its-kind, two-ring-based CSW via the atomic force microscopy cantilever tip-assisted mechanophotonics approach. The photonic studies suggest that the CSW structure routes optical signals in discriminating trajectories. To capitalize on the discriminatory properties of the CSW, two linear waveguides are integrated onto both rings of the CSW to create a smart OPC. This smart OPC can selectively route photons depending on the pathways determined by the CSW to switch it ON or OFF completely depending on partial or complete flow of optical signals in the circuit. Such intelligent photonic circuits are essential for advancing smart technologies.

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.

Ultra-Wide Modulation and Reversible Reconfiguration of a Flexible Organic Crystalline Optical Waveguide Between 645 and 731 nm

Flexible organic crystalline optical waveguides, which deliver input or self-emit light through various dynamic organic crystals, have attracted increasing attention in the past decade. However, the modulation of the waveguide output relies on chemical design and substituent modification, being time-consuming and laborious. Here we report an elastic organic crystal that displays long-distance light transduction up to 2.0 cm and an ultra-wide modulation of crystalline optical waveguides between red (645 nm) and near infrared (731 nm) in both the pristine and the elastically bent states based on a pre-designed self-absorption effect. The flexible organic crystalline optical waveguides can be precisely and reversibly reconfigured by controlling the irradiation point. In addition, deep-red amplified spontaneous emission (ASE) that is able to transduce through a 5.0 mm bent crystal with an ultra-low optical loss coefficient of 0.093 dB/mm has been attained. To the best of our knowledge, this is the first report of flexible organic ASE waveguides. The present study not only provides a simple yet effective strategy to remarkably modulate flexible organic crystalline optical waveguides but also demonstrates the superiority of lasing over normal emission as flexible optical communication elements.

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.

Efficient Aerial Water Harvesting with Self-Sensing Dynamic Janus Crystals

Water scarcity is one of the most pressing issues of contemporary societal development that requires innovative technologies where the material not only harvests water but also plays an active role in the process. Here, we demonstrate a highly efficient optical self-sensing approach to humidity capture from the air, where both humidity-harvesting and water-transduction functionalities are imparted on slender organic crystals by partial silanization via layer-by-layer hybridization. We report that due to the integration of the harvesting of aerial moisture and the collection of the condensed water, the ensuing Janus-type crystals capture humidity with the highest-to-date water collection efficiency of 15.96 ± 0.63 g cm-2 h-1. The water-collecting elements are also capable of delivering the water by reversible and periodic elastic deformation, and their high optical transparency allows real-time monitoring of the periodic fog collection process by deformational modulation of passively or actively transduced light that outcouples at the crystal-droplet interface. The results could inspire sophisticated approaches to humidity harvesting where optically transparent crystals combine fog capture with self-sensing capabilities for continuous and optimized operation to maximize the cost-gain balance of aerial fog capture.

Large Local Internal Stress in an Elastically Bent Molecular Crystal Revealed by Raman Shifts

The structural dynamics involved in the mechanical flexibility of molecular crystals and the internal stress in such flexible materials remain obscure. Here, the study reports an elastically bending lipidated molecular crystal that shows systematic shifts in characteristic vibrational frequencies across the bent crystal region - revealing the nature of structural changes during bending and the local internal stress distribution. The blueshifts in the bond stretching modes (such as C═O and C-H modes) in the inner arc region and redshifts in the outer arc region of the bent crystals observed via micro-Raman mapping are counterintuitive to the bending models based on intermolecular hydrogen bonds. Correlating these shifts with the trends observed from high-pressure Raman studies on the crystal reveals the local stress difference between the inner arc and outer arc regions of the bent crystal to be ≈2 GPa, more than an order of magnitude higher than the previously proposed value in elastically bending crystals. High local internal stress can have direct ramifications on the properties of molecular piezoelectric energy harvesters, actuators, semiconductors, and flexible optoelectronic materials.

Mechanical Tuning of Fluorescence Lifetime and Bandgap in an Elastically Flexible Molecular Semiconductor Crystal

Despite having superior transport properties, lack of mechanical flexibility is a major drawback of crystalline molecular semiconductors as compared to their polymer analogues. Here single crystals of an organic semiconductor are reported that are not only flexible but exhibit systematic tuning of bandgaps, fluorescence lifetime, and emission wavelengths upon elastically bending. Spatially resolved fluorescence lifetime imaging and confocal fluorescence microscopy reveals systematic trends in the lifetime decay across the bent crystal region along with shifts in the emission wavelength. From the outer arc to the inner arc of the bent crystal, a significant decrease in the lifetime of ≈1.9 ns is observed, with a gradual bathochromic shift of ≈10 nm in the emission wavelength. For the crystal having a bandgap of 2.73 eV, the directional stress arising from bending leads to molecular reorientation effects and variations in the extent of intermolecular interactions- which are correlated to the lowering of bandgap and the evolution of the projected density of states. The systematic changes in the interactions quantified using electron density topological analysis in the compressed inner arc and elongated outer arc region are correlated to the non-radiative decay processes, thus rationalizing the tuning of fluorescence lifetime.

Logarithmic and Archimedean organic crystalline spirals

Crystals can be found in many shapes but do not usually grow as spirals. Here we show that applying a non-uniform layer of a polymer blend onto slender centimeter-size organic crystals prestrains the crystals into hybrid dynamic elements with spiral shapes that respond reversibly to environmental variations in temperature or humidity by curling. Exposure to humidity results in partial uncurling within several seconds, whereby a logarithmic-type spiral crystal is transformed into an Archimedean one. Conical helices obtained by lateral pulling of the spirals can wind around solid objects similar to plant tendrils or lift suspended objects with a positive correlation between the actuator's elongation and the cargo mass. The morphological, kinematic, and kinetic attributes turn these hybrid materials into an attractive platform for flexible sensors and soft robots, while they also provide an approach to morph crystalline fibers in non-natural spiral habits inaccessible with the common crystallization approaches.

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.

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.

Spatially controllable and mechanically switchable isomorphous organoferroeleastic crystal optical waveguides and networks

The precise, reversible, and diffusionless shape-switching ability of organic ferroelastic crystals, while maintaining their structural integrity, positions them as promising materials for next-generation hybrid photonic devices. Herein, we present versatile bi-directional ferroelasticity and optical waveguide properties of three isomorphous, halogen-based, Schiff base organic crystals. These crystals exhibit sharp bending at multiple interfaces driven by molecular movement around the CH = N bond and subsequent 180° rotational twinning, offering controlled light path manipulation. The ferroelastic nature of these crystals allowed the construction of robust hybrid photonic structures, including Z-shaped configurations, closed-loop networks, and staircase-like hybrid optical waveguides. This study highlights the potential of shape-switchable organoferroelastic crystals as waveguides for applications in programmable photonic devices.

Crystal Engineering-Driven Sunlight Responsiveness and Flexible Waveguide Transmission

Here, a barbituric acid derivative containing pyrene rings (DPPT) was successfully synthesized, and two types of crystals were prepared by using crystal engineering methods. Orange sheet-like crystals (DPPT-O, observed in visible light), prepared in a DCM/CH3OH solution, exhibited brittleness and weak fluorescence emission, along with sunlight-induced bending and fracturing. Red needle-like crystals (DPPT-R, also observed in visible light), synthesized in a DCM/CH3CN solution, demonstrated elastic properties, strong fluorescence emission, and excellent optical waveguide performance (with an optical loss coefficient of 0.23-0.30 dB mm-1). Single-crystal data analysis revealed that the stacking arrangement of molecules critically influenced the elasticity of the crystals, while the reaction cavity size regulated the photomechanical properties of the crystals. This study achieved effective control over sunlight responsiveness and flexible optical waveguide transmission for the first time, providing innovative insights for the application of homogeneous organic polycrystalline molecular crystals in this field.

Surface coating induced room-temperature phosphorescence in flexible organic single crystals

Materials exhibiting room temperature phosphorescence (RTP) are in high demand for signage, information encryption, sensing, and biological imaging. Due to weak spin-orbit coupling and other non-radiative processes that effectively quench the triplet excited states, RTP is sparsely observed in organic materials. Although the incorporation of a heavy atom through covalent or non-covalent modification circumvents these drawbacks, heavy-atom-containing materials are undesirable because of their deleterious side effects. Here, we designed and synthesized a new naphthalidenimine-boron complex as a coating material for the single crystals of 4,4'-dimethoxybenzophenone. The coated surface was observed to exhibit yellowish-green phosphorescence with ms lifetimes at ambient conditions through Förster resonance energy transfer (FRET). Importantly, the mechanical flexibility of the single crystals was observed to be retained after coating. The fluorescence-phosphorescence dual emission was utilised for colour-tunable optical waveguiding and anti-counterfeiting applications. As organic single crystals that can sustain mechanical deformations are emerging as the next-generation materials for electronic device fabrication, the flexible RTP organic crystals showing colour-tuneable optical waveguiding could be omnipotent in electronics.

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.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Efficient Screening of Pharmaceutical Cocrystals by Microspacing In-Air Sublimation

Cocrystal screening and single-crystal growth remain the primary obstacles in the development of pharmaceutical cocrystals. Here, we present a new approach for cocrystal screening, microspacing in-air sublimation (MAS), to obtain new cocrystals and grow high-quality single crystals of cocrystals within tens of minutes. The method possesses the advantages of strong designable ability of devices, user-friendly control, and compatibility with materials, especially for the thermolabile molecules. A novel drug-drug cocrystal of favipiravir (FPV) with salicylamide (SAA) was first discovered by this method, which shows improved physiochemical properties. Furthermore, this method proved effective in cultivating single crystals of FPV-isonicotinamide (FPV-INIA), FPV-urea, FPV-nicotinamide (FPV-NIA), and FPV-tromethamine (FPV-Tro) cocrystals, and the structures of these cocrystals were determined for the first time. By adjusting the growth temperature and growth distance precisely, we also achieved single crystals of 10 different paracetamol (PCA) cocrystals and piracetam (PIR) cocrystals, which underscores the versatility and efficiency of this method in pharmaceutical cocrystal screening.

Flexible molecular crystals for optoelectronic applications

The cornerstones of the advancement of flexible optoelectronics are the design, preparation, and utilization of novel materials with favorable mechanical and advanced optoelectronic properties. Molecular crystalline materials have emerged as a class of underexplored yet promising materials due to the reduced grain boundaries and defects anticipated to provide enhanced photoelectric characteristics. An inherent drawback that has precluded wider implementation of molecular crystals thus far, however, has been their brittleness, which renders them incapable of ensuring mechanical compliance required for even simple elastic or plastic deformation of the device. It is perplexing that despite a plethora of reports that have in the meantime become available underpinning the flexibility of molecular crystals, the "discovery" of elastically or plastically deformable crystals remains limited to cases of serendipitous and laborious trial-and-error approaches, a situation that calls for a systematic and thorough assessment of these properties and their correlation with the structure. This review provides a comprehensive and concise overview of the current understanding of the origins of crystal flexibility, the working mechanisms of deformations such as plastic and elastic bending behaviors, and insights into the examples of flexible molecular crystals, specifically concerning photoelectronic changes that occur in deformed crystals. We hope this summary will provide a reference for future experimental and computational efforts with flexible molecular crystals aimed towards improving their mechanical behavior and optoelectronic properties, ultimately intending to advance the flexible optoelectronic technology.

Mechanoresponsive Flexible Crystals

Flexible crystals have gained significant attention owing to their remarkable pliability, plasticity, and adaptability, making them highly popular in various research and application fields. The main challenges in developing flexible crystals lie in the rational design, preparation, and performance optimization of such crystals. Therefore, a comprehensive understanding of the fundamental origins of crystal flexibility is crucial for establishing evaluation criteria and design principles. This Perspective offers a retrospective analysis of the development of flexible crystals over the past two decades. It summarizes the elastic standards and possible plastic bending mechanisms tailored to diverse flexible crystals and analyzes the assessment of their theoretical basis and applicability. Meanwhile, the compatibility between crystal elasticity and plasticity has been discussed, unveiling the immense prospects of elastic/plastic crystals for applications in biomedicine, flexible electronic devices, and flexible optics. Furthermore, this Perspective presents state-of-the-art experimental avenues and analysis methods for investigating molecular interactions in molecular crystals, which is vital for the future exploration of the mechanisms of crystal flexibility.

Hybrid and composite materials of organic crystals

Organic molecular crystals have historically been viewed as delicate and fragile materials. However, recent studies have revealed that many organic crystals, especially those with high aspect ratios, can display significant flexibility, elasticity, and shape adaptability. The discovery of mechanical compliance in organic crystals has recently enabled their integration with responsive polymers and other components to create novel hybrid and composite materials. These hybrids exhibit unique structure-property relationships and synergistic effects that not only combine, but occasionally also enhance the advantages of the constituent crystals and polymers. Such organic crystal composites rapidly emerge as a promising new class of materials for diverse applications in optics, electronics, sensing, soft robotics, and beyond. While specific, mostly practical challenges remain regarding scalability and manufacturability, being endowed with both structurally ordered and disordered components, the crystal-polymer composite materials set a hitherto unexplored yet very promising platform for the next-generation adaptive devices. This Perspective provides an in-depth analysis of the state-of-the-art in design strategies, dynamic properties and applications of hybrid and composite materials centered on organic crystals. It addresses the current challenges and provides a future outlook on this emerging class of multifunctional, stimuli-responsive, and mechanically robust class of materials.

Mechanism of Phase Transition in dl-Methionine: Determining Cooperative and Molecule-by-Molecule Transformations

The solid-state phase transition in dl-methionine has been extensively studied because of its atypical behavior. The transition occurs through changes in the molecular conformation and 3D packing of the molecules. Phase transitions in racemic aliphatic amino acid crystals are known to show different behaviors depending on whether conformational changes or packing changes are involved, where the former is thought to proceed through a nucleation-and-growth mechanism in a standard molecule-by-molecule picture, and the latter through a cooperative mechanism. The phase transition of dl-methionine resembles the thermodynamic, kinetic, and structural features of both categories: a conformational change and relative shifts between layers in two directions. The present paper presents molecular dynamics simulations of the phase transition to examine the underlying mechanism from two perspectives: (i) analysis of the scaling behavior of the free energy barriers involved in the phase transition and (ii) a structural inspection of the phase transition. Both methods can help to distinguish between a concerted phase change and a molecule-by-molecule or zip-like mechanism. The free energy predominantly scales with the system size, which suggests a cooperative mechanism. The structural changes draw, however, a slightly more complex picture. The conformational changes appear to occur in a molecule-by-molecule fashion, where the rotational movement is triggered by movement in the same layer. Conformational changes occur on a time scale nearly twice as long as the shifts between layers. Shifts in one direction appear to be less concerted than shifts in the perpendicular direction. We relate this to the edge-free energy involved in these shifts. We believe that the behavior observed in dl-methionine is likely applicable to phase transitions in other layered systems that interact through aliphatic chains as well.

Magnetically controlled assembly: a new approach to organic integrated photonics

Hierarchical self-assembly of organic molecules or assemblies is of great importance for organic photonics to move from fundamental research to integrated and practical applications. Magnetic fields with the advantages of high controllability, non-contact manipulation, and instantaneous response have emerged as an elegant way to prepare organic hierarchical nanostructures. In this perspective, we outline the development history of organic photonic materials and highlight the importance of organic hierarchical nanostructures for a wide range of applications, including microlasers, optical displays, information encoding, sensing, and beyond. Then, we will discuss recent advances in magnetically controlled assembly for creating organic hierarchical nanostructures, with a particular focus on their potential for enabling the development of integrated photonic devices with unprecedented functionality and performance. Finally, we present several perspectives on the further development of magnetically controlled assembly strategies from the perspective of performance optimization and functional design of organic integrated photonics.

Collective photothermal bending of flexible organic crystals modified with MXene-polymer multilayers as optical waveguide arrays

The performance of any engineering material is naturally limited by its structure, and while each material suffers from one or multiple shortcomings when considered for a particular application, these can be potentially circumvented by hybridization with other materials. By combining organic crystals with MXenes as thermal absorbers and charged polymers as adhesive counter-ionic components, we propose a simple access to flexible hybrid organic crystal materials that have the ability to mechanically respond to infrared light. The ensuing hybrid organic crystals are durable, respond fast, and can be cycled between straight and deformed state repeatedly without fatigue. The point of flexure and the curvature of the crystals can be precisely controlled by modulating the position, duration, and power of thermal excitation, and this control can be extended from individual hybrid crystals to motion of ordered two-dimensional arrays of such crystals. We also demonstrate that excitation can be achieved over very long distances (>3 m). The ability to control the shape with infrared light adds to the versatility in the anticipated applications of organic crystals, most immediately in their application as thermally controllable flexible optical waveguides for signal transmission in flexible organic electronics.

Bioinspired soft robots based on organic polymer-crystal hybrid materials with response to temperature and humidity

The capability of stimulated response by mechanical deformation to induce motion or actuation is the foundation of lightweight organic, dynamic materials for designing light and soft robots. Various biomimetic soft robots are constructed to demonstrate the vast versatility of responses and flexibility in shape-shifting. We now report that the integration of organic molecular crystals and polymers brings about synergistic improvement in the performance of both materials as a hybrid materials class, with the polymers adding hygroresponsive and thermally responsive functionalities to the crystals. The resulting hybrid dynamic elements respond within milliseconds, which represents several orders of magnitude of improvement in the time response relative to some other type of common actuators. Combining molecular crystals with polymers brings crystals as largely overlooked materials much closer to specific applications in soft (micro)robotics and related fields.

Two-Dimensional Optical Waveguides at Telecom Wavelengths Based on Organic Single-Crystal Microsheets of a Charge Transfer Complex

Organic charge transfer (CT) cocrystals open a new door for the exploitation of low-dimensional near-infrared (NIR) emitters by a convenient self-assembly approach. However, research about the fabrication of sheet-like NIR-emitting microstructures that are significant for structural construction and integrated application is limited by the unidirectional molecular packing mode. Herein, via regulation of the biaxial intermolecular CT interaction, single-crystalline microsheets with remarkable NIR emission from 720 to 960 nm were synthesized via the solution self-assembly process of dithieno[3,2-b:2',3'-d]thiophene and 7,7,8,8-tetracyanoquinodimethane. The expected sheet-like structure is conducive to achieving a two-dimensional (2D) optical waveguide with an ultralow optical loss rate of 0.250 dB/μm at 860 nm. More significantly, these as-prepared organic microsheets with tunable thicknesses (h) from 100 to 1100 nm exhibit thickness-dependent NIR optical transportation performance. These findings could pave the way to a new class of low-dimensional NIR emitters for 2D photonics at telecom wavelengths.

Host-Guest Doping in Flexible Organic Crystals for Room-Temperature Phosphorescence

Organic single crystals (OSCs) with excellent flexibility and unique optical properties are of great importance due to their broad applicability in optical/optoelectronic devices and sensors. Nevertheless, fabricating flexible OSCs with room-temperature phosphorescence (RTP) remains a great challenge. Herein, we propose a host-guest doping strategy to achieve both RTP and flexibility of OSCs. The single-stranded crystal is highly bendable upon external force application and can immediately return to its original straight shape after removal of the stress, impressively emitting bright deep-red phosphorescence. The theoretical and experimental results demonstrate that the bright RTP arises from Förster resonance energy transfer (FRET) from the triphenylene molecules to the dopants. This strategy is both conceptually and synthetically simple and offers a universal approach for the preparation of flexible OSCs with RTP.

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.

Electrically conductive hybrid organic crystals as flexible optical waveguides

Hybrid materials capitalize on the properties of individual materials to attain a specific combination of performance assets that is not available with the individual components alone. We describe a straightforward approach to preparation of sandwich-type hybrid dynamic materials that combine metals as electrically conductive components and polymers as bending, momentum-inducing components with flexible organic crystals as mechanically compliant and optically transducive medium. The resulting hybrid materials are conductive to both electricity and light, while they also respond to changes in temperature by deformation. Depending on the metal, their conductivity ranges from 7.9 to 21.0 S µm^‒1. The elements respond rapidly to temperature by curling or uncurling in about 0.2 s, which in one typical case corresponds to exceedingly fast deformation and recovery rates of 2187.5° s^‒1 and 1458.3° s^‒1, respectively. In cyclic operation mode, their conductivity decreases less than 1% after 10,000 thermal cycles. The mechanothermal robustness and dual functionality favors these materials as candidates for a variety of applications in organic-based optics and electronics, and expands the prospects of application of organic crystals beyond the natural limits of their dynamic performance.

Low-Dimensional Organic Crystals: From Precise Synthesis to Advanced Applications

Low-dimensional organic crystals (LOCs) have attracted increasing attention recently for their potential applications in miniaturized optoelectronics and integrated photonics. Such applications are possible owing to their tunable physicochemical properties and excellent charge/photon transport features. As a result, the precise synthesis of LOCs has been examined in terms of morphology modulation, large-area pattern arrays, and complex architectures, and this has led to a series of appealing structure-dependent properties for future optoelectronic applications. This review summarizes the recent advances in the precise synthesis of LOCs in addition to discussing their structure-property relationships in the context of optoelectronic applications. It also presents the current challenges related to organic crystals with specific structures and desired performances, and the outlook regarding their use in next-generation integrated optoelectronic applications.