Control of Photomechanical Crystal Twisting by Illumination Direction

Crystal Structure Landscape of Diarylethene-Based Crystalline Solids: A Comprehensive CSD Analysis

Diarylethenes (DAEs) are an exciting class of stimulus-responsive organic molecules that exhibit electrocyclization reactions upon exposure to light, heat, or other stimuli. The rational design of DAE-based crystalline materials is, however, complicated by the presence of DAE atropisomers, only one of which is photoactive. Data mining of the CSD produced 1349 unique molecular DAE structures that were subsequently analyzed according to selected chemical and geometric attributes. Additional analyses were performed on 1078 dithienylethene (DTE) structures-the largest subgroup within the ensemble. The crystal structure landscape, based upon D-D parameterization and analysis, revealed a vast array of molecular geometries, many of which may not correspond to energetic minima. The analyses link various chemical and geometric parameters to isomers observed in the lattice and their reactivity; however, potential biases intrinsic to this ensemble of structures complicate the determination of causal relationships. We believe that this retrospective comprehensive analysis of DAE structures represents an important step for understanding more broadly the crystal landscape of this class of materials.

Solar-Powered Molecular Crystal Motor Based on an Anthracene-Thiazolidinedione Photoisomerization Reaction

Assembling molecular machines into crystals provides a way to harness their power on large length scales, but the development of a crystal analogue to a molecular motor remains a challenge. The molecule (Z)-5-(anthracen-9-ylmethylene)-3-butylthiazolidine-2,4-dione (C4-ATD) has E and Z isomers with strongly overlapping absorption spectra. This spectroscopic property allows both Z → E and E → Z photoisomerization reactions to be driven by a single light source, and simulations indicate this property can provide a route to robust oscillatory motion. Reprecipitation in an aqueous surfactant enables the growth of single crystal microwires that exhibit continuous mechanical oscillations under a wide range of illumination conditions, including ambient solar irradiation. Molecular crystal motors provide a new approach for transforming continuous light into oscillatory mechanical motion.

Supramolecular Chiral Aggregation of Porphyrin Induced by Photo-Generated Triphenylamines Radical Cations

Here, it is shown that photoirradiation triggered chiral J-aggregates formation of an achiral anionic porphyrin, TPPS (tetrakis(4-sulfonatophenyl) porphyrin), in the presence of chiral triphenylamine (TPA) derivatives. A series of chiral triarylamines linked with aromatic rings is designed through urea or amide bonds. UV-irradiation of self-assembled urea-linked triphenylamine derivatives causes the formation of persistent radical cations in the chlorinated solvents, which subsequently induces the aggregation of TPPS. Transferring chirality of TPA derivatives to achiral TPPS J-aggregates leads to the chiral assemblies with remarkable chiroptical signals. The experimental results demonstrate that, TPA derivatives linked by the urea bond can effectively promote the aggregation of TPPS rather than those with the amide bond although the photo-generated radical cations are both produced. It is suggested that the urea-linked TPA derivatives are more favorable to stable radical cations and thus cause the formation of TPPS chiral J-aggregation. This work may open up an avenue for designing photo-modulated chiral supramolecular assemblies.

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.

[2 + 2] cycloaddition and its photomechanical effects on 1D coordination polymers with reversible amide bonds and coordination site regulation

Photo-responsive materials can convert light energy into mechanical energy, with great application potential in biomedicine, flexible electronic devices, and bionic systems. We combined reversible amide bonds, coordination site regulation, and coordination polymer (CP) self-assembly to synthesize two 1D photo-responsive CPs. Obvious photomechanical behavior was observed under UV irradiation. By combining the CPs with PVA, the mechanical stresses were amplified and macroscopic driving behavior was realized. In addition, two cyclobutane amide derivatives and a pair of cyclobutane carboxyl isomers were isolated through coordination bond destruction and amide bond hydrolysis. Therefore, photo-actuators and supramolecular synthesis in smart materials may serve as important clues.

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.

Fabrication of Hyperbranched Photomechanical Crystals Composed of a Photochromic Diarylethene

We report the fabrication of hyperbranched hollow crystals of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene on a concave surface of the spherical glass substrate by sublimation and their practical photomechanical behaviors. The number of units of the branched structure of the hollow crystals composed of this compound is proportional to the substrate curvature of the substrate. Compared with the sublimation process of the same compound on the flat glass substrate, two kinds of the thin film domains are generated separately in the center and around the edge of the spherical glass substrate. Especially under the high relative humidity condition, the boundaries between these thin film domains move gradually around the edge through the center during as long as 6 h of sublimation time so that the hyperbranched hollow crystals are densely produced on the entire surface of the substrate. These hyperbranched hollow crystals can be prepared with the highly ordered molecular packing due to the very slow formation process of the crystalline walls of the hollow structures. Furthermore, the photo-induced bending behaviors in the few- and highly-branched hollow crystals have the practical roles in moving and bending the minute objects according to their characteristics of these branched shapes.

Intersectional Effects of Crystal Features on the Actuation Performance of Dynamic Molecular Crystals

Despite being researched for decades, shape-shifting molecular crystals have yet to claim their spot as an actuating materials class among the primary functional materials. While the process for developing and commercializing materials can be lengthy, it inevitably starts with building an extensive knowledge base, which for molecular crystal actuators remains scattered and disjointed. Using machine learning for the first time, we identify inherent features and structure-function relationships that fundamentally impact the mechanical response of molecular crystal actuators. Our model can factor in different crystal properties in tandem and decipher their intersectional and combined effects on each actuation performance. This analysis is an open invitation to utilize interdisciplinary expertise in translating the current basic research on molecular crystal actuators into technology-based development that promotes large-scale experimentation and prototyping.

Molecular Twisting Affects the Solid-State Photochemical Reactions of Unsaturated Ketones and the Photomechanical Effects of Molecular Crystals

Three groups of chalcone derivatives and their analogues involving halogen atoms (X=F, Cl, Br) have been synthesized. Firstly, the nearly planar acyclic chalcone derivatives were inclined to undergo photo-induced stereospecific [2+2] cycloaddition, which triggered the crystals to exhibit macroscopic motions of bending or cracking. In particular, the single-crystal-to-single-crystal transformation happened upon UV irradiation of the crystals, which was helpful for the understanding photomechanical effects. Cyclic 3,4-dihydronaphthalene-based chalcone analogues possess a more twisted conformation, and they tend to undergo trans-cis isomerization. No photomechanical effect was observed for the crystals of the cyclic chalcone analogues due to the lower isomerization rate. The twist degree of chroman-based molecules was in between of the first two, [2+2] cycloaddition and trans-cis isomerization simultaneously took place in crystals. Photo-induced bending and twisting were observed for the crystals of chroman-based chalcone analogues. Therefore, the differences in molecular dihedral angles in α,β-unsaturated ketones were responsible for their photochemical characters and in turn to tune the photomechanical effects. In this work, a bridge between the molecular structures and solid-state photochemical reactions triggered photomechanical crystals is built.

A theoretical framework for the design of molecular crystal engines

Photomechanical molecular crystals have garnered attention for their ability to transform light into mechanical work, but difficulties in characterizing the structural changes and mechanical responses experimentally have hindered the development of practical organic crystal engines. This study proposes a new computational framework for predicting the solid-state crystal-to-crystal photochemical transformations entirely from first principles, and it establishes a photomechanical engine cycle that quantifies the anisotropic mechanical performance resulting from the transformation. The approach relies on crystal structure prediction, solid-state topochemical principles, and high-quality electronic structure methods. After validating the framework on the well-studied [4 + 4] cycloadditions in 9-methyl anthracene and 9-tert-butyl anthracene ester, the experimentally-unknown solid-state transformation of 9-carboxylic acid anthracene is predicted for the first time. The results illustrate how the mechanical work is done by relaxation of the crystal lattice to accommodate the photoproduct, rather than by the photochemistry itself. The large ∼10⁷ J m^-3 work densities computed for all three systems highlight the promise of photomechanical crystal engines. This study demonstrates the importance of crystal packing in determining molecular crystal engine performance and provides tools and insights to design improved materials in silico.

Edge-to-Center Propagation of Photochemical Reaction during Single-Crystal-to-Single-Crystal Photomechanical Transformation of 2,5-Distyrylpyrazine Crystals

Photomechanical molecular crystals are promising materials for photon-powered artificial actuators. To interpret the photomechanical responses, the spatiotemporal distribution of photoproducts in crystals could be an important role in addition to molecular structures, molecular packings, illumination conditions, crystal morphology, crystal size, and so on. In this study, we have found that single crystals of 2,5-distyrylpyrazine show a smooth single-crystal-to-single-crystal photomechanical expansion, and the photochemical reaction propagates from the edge to the center of the single crystal. We revealed that the surface effect (special reactivity at the crystal surface) in addition to the cooperative effect (the reaction is facilitated by neighboring molecules) is essential for the edge-to-center propagation of the photochemical reaction. Our results would provide a foundation for future studies of the photochemical reaction dynamics in photomechanical molecular crystals.

Plastic bending in a semiconducting coordination polymer crystal enabled by delamination

Coordination polymers (CPs) are a class of crystalline solids that are considered brittle, due to the dominance of directional coordination bonding, which limits their utility in flexible electronics and wearable devices. Hence, engineering plasticity into functional CPs is of great importance. Here, we report plastic bending of a semiconducting CP crystal, Cu-Trz (Trz = 1,2,3-triazolate), that originates from delamination facilitated by the discrete bonding interactions along different crystallographic directions in the lattice. The coexistence of strong coordination bonds and weak supramolecular interactions, together with the unique molecular packing, are the structural features that enable the mechanical flexibility and anisotropic response. The spatially resolved analysis of short-range molecular forces reveals that the strong coordination bonds, and the adaptive C–H···π and Cu···Cu interactions, synergistically lead to the delamination of the local structures and consequently the associated mechanical bending. The proposed delamination mechanism offers a versatile tool for designing the plasticity of CPs and other molecular crystals.

Patterning submicron photomechanical features into single diarylethene crystals using electron beam lithography

Lithography methods are commonly used to create structures in inorganic semiconductors like silicon but have not been widely applied to organic crystals. In this work, electron beam lithography (EBL) is used to pattern structures into single organic photomechanical crystals composed of 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene. The electron beam creates amorphous regions of decomposed molecules, while the reactivity of the unexposed crystal regions is preserved under a removable Au coating. Exposure of the patterned crystal to 365 nm light causes ridges of amorphous material to increase in height by 30-70%. This height increase can be reversed by visible light exposure and undergo multiple cycles. The reversible surface morphology changes are strong enough to rupture a sheet of graphene placed on top of the patterned crystal. Surprisingly, the change in dimensions of the EBL features is an order of magnitude larger than the changes in overall crystal dimensions as deduced from X-ray diffraction experiments and microscopy observations. A dynamic extrusion model is presented to explain how nanoscale features imprinted into single crystals can amplify molecular-level photomechanical changes. This work demonstrates the capability of EBL methods to produce sub-micron structural features on single photomechanical crystals, providing a new route to monolithic light-powered actuator devices.

Photomechanical Structures Based on Porous Alumina Templates Filled with 9-Methylanthracene Nanowires

9-Methylanthracene (9MA) undergoes a concerted [4 + 4] photodimerization in its crystal form that can be harnessed in order to generate photomechanical motions such as bending, twisting, and expansion. As described in this paper, 9MA nanowires were grown in anodic aluminum oxide (AAO) templates with the goal of using the crystal expansion to generate a net increase in the height of the composite disk. The growth conditions were optimized in order to raise the filling amount from 28% to 77% of the available volume in the porous AAO. A new experimental method for detecting motion, based on the analysis of data from a dynamically misaligned Michelson interferometer, was developed. Template bending was observed, showing that the photodimerization of the confined nanowires generated mechanical work, but no conclusive evidence for surface disruption or vertical translation was observed. Optical measurements, as well as atomic force and scanning electron microscopy, showed that incomplete filling, crystal orientation, and debris from template polishing likely prevented the observation of vertical actuation in these nanocrystal composites. This work highlights some of the practical challenges that are involved in creating photomechanical actuators using the organic–inorganic composite approach, with the two most significant being (1) the uniform filling of the porous template with the organic active material and (2) the removal of excess organic material from the template’s surface.

Performance of Composite Glass-Diarylethene Crystal Photomechanical Actuator Membranes

Hybrid organic-inorganic composites based on organic photochromic crystals embedded in inorganic templates provide a new approach to photomechanical materials. Diarylethene (DAE) nanowire crystals grown in Al2O3 membranes have exhibited reversible photoinduced bending and lifting [Dong, X., Chem. Mater. 2019, 31, 1016-1022]. In this paper, the hybrid approach is extended to porous SiO2 membranes. Despite the different template material (SiO2 instead of Al2O3) and much larger channels (5 μm diameter instead of 0.2 μm diameter), similar photomechanical behavior is observed for this new class of organic-inorganic hybrid actuators. The ability to reuse individual glass templates across different DAE filling cycles allows us to show that the DAE filling step is crucial for determining the mechanical work done by the bending template. The bending curvature also depends quadratically on the template thickness, in good agreement with theory. The light-induced bending can be repeated for up to 150 cycles without loss of performance, suggesting good fatigue resistance. The results in this paper demonstrate that the hybrid organic-inorganic approach can be extended to other host materials and template geometries. They also suggest that optimizing the organic filling and template thickness could improve the work output by an order of magnitude.

Controllable photomechanical bending of metal-organic rotaxane crystals facilitated by regioselective confined-space photodimerization

Molecular machines based on mechanically-interlocked molecules (MIMs) such as (pseudo) rotaxanes or catenates are known for their molecular-level dynamics, but promoting macro-mechanical response of these molecular machines or related materials is still challenging. Herein, by employing macrocyclic cucurbit[8]uril (CB[8])-based pseudorotaxane with a pair of styrene-derived photoactive guest molecules as linking structs of uranyl node, we describe a metal-organic rotaxane compound, U-CB[8]-MPyVB, that is capable of delivering controllable macroscopic mechanical responses. Under light irradiation, the ladder-shape structural unit of metal-organic rotaxane chain in U-CB[8]-MPyVB undergoes a regioselective solid-state [2 + 2] photodimerization, and facilitates a photo-triggered single-crystal-to-single-crystal (SCSC) transformation, which even induces macroscopic photomechanical bending of individual rod-like bulk crystals. The fabrication of rotaxane-based crystalline materials with both photoresponsive microscopic and macroscopic dynamic behaviors in solid state can be promising photoactuator devices, and will have implications in emerging fields such as optomechanical microdevices and smart microrobotics.

Engineering Photomechanical Molecular Crystals to Achieve Extraordinary Expansion Based on Solid-State [2 + 2] Photocycloaddition

Photomechanical molecular crystals are promising candidates for photoactuators and can potentially be implemented as smart materials in various fields. Here, we synthesized a new molecular crystal, (E)-3-(naphthalen-1-yl)acrylaldehyde malononitrile ((E)-NAAM), that can undergo a solid-state [2 + 2] photocycloaddition reaction under visible light (≥400 nm) illumination. (E)-NAAM microcrystals containing symmetric twinned sealed cavities were prepared using a surfactant-mediated crystal seeded growth method. When exposed to light, the hollow microcrystals exhibited robust photomechanical motions, including bending and dramatic directional expansion of up to 43.1% elongation of the original crystal length before fragmentation due to the photosalient effect. The sealed cavities inside the microcrystals could store different aqueous dye solutions for approximately one month and release the solutions instantly upon light irradiation. A unique slow-fast-slow crystal elongation kinematic process was observed, suggesting significant molecular rearrangements during the illumination period, leading to an average anisotropic crystal elongation of 37.0% (±3.8%). The significant molecular structure and geometry changes accompanying the photocycloaddition reaction, which propels photochemistry to nearly 100% completion, also facilitate photomechanical crystal expansion. Our results provide a possible way to rationally design molecular structures and engineer crystal morphologies to promote more interesting photomechanical behaviors.

Future-Oriented Advanced Diarylethene Photoswitches: From Molecular Design to Spontaneous Assembly Systems

Diarylethene (DAE) photoswitch is a new and promising family of photochromic molecules and has shown superior performance as a smart trigger in stimulus-responsive materials. During the past few decades, the DAE family has achieved a leap from simple molecules to functional molecules and developed toward validity as a universal switching building block. In recent years, the introduction of DAE into an assembly system has been an attractive strategy that enables the photochromic behavior of the building blocks to be manifested at the level of the entire system, beyond the DAE unit itself. This assembly-based strategy will bring many unexpected results that promote the design and manufacture of a new generation of advanced materials. Here, recent advances in the design and fabrication of diarylethene as a trigger in materials science, chemistry, and biomedicine are reviewed.

Cold photo-carving of halogen-bonded co-crystals of a dye and a volatile co-former using visible light

The formation of co-crystals by the assembly of molecules with complementary molecular recognition functionalities is a popular strategy to design or improve a range of solid-state properties, including those relevant for pharmaceuticals, photo- or thermoresponsive materials and organic electronics. Here, we report halogen-bonded co-crystals of a fluorinated azobenzene derivative with a volatile component-either dioxane or pyrazine-that can be cut, carved or engraved with low-power visible light. This cold photo-carving process is enabled by the co-crystallization of a light-absorbing azo dye with a volatile component, which gives rise to materials that can be selectively disassembled with micrometre precision using low-power, non-burning laser irradiation or a commercial confocal microscope. The ability to shape co-crystals in three dimensions using laser powers of 0.5-20 mW-substantially lower than those used for metals, ceramics or polymers-is rationalized by photo-carving that targets the disruption of weak supramolecular interactions, rather than the covalent bonds or ionic structures targeted by conventional laser beam or focused ion beam machining processes.

Relating Excited States to the Dynamics of Macroscopic Strain in Photoresponsive Crystals

Exposure of a photoreactive single crystal to light with a wavelength offset from its absorption maximum can have two distinct effects. The first is the "direct" effect, wherein the excited state generated in individual chemical species is influenced. The second is the "indirect" effect, which describes the penetration of light into the crystal and hence the spatial propagation and completeness of transformation. We illustrate using the nitro-nitrito isomerization of [Co(NH₃)₅NO₂]Cl(NO₃) as an example that the direct and indirect effects can be independently determined. This is achieved by comparing the dynamics of macroscopic crystal deformation (bending curvature and crystal elongation) induced by the photochemical reaction when irradiating a crystal at the absorption maximum and at different band edges (above or below the maximum) of the same band. Quantitative description of the macroscopic strain dynamics in comparison with experiments allowed us to suggest that irradiation at different tails of the same absorption band causes isomerization to proceed via different excited states and an additional photochemical reaction (presumably, reverse nitrito-nitro isomerization) can occur on irradiation at the ligand-field band edges.

Superelasticity of a photo-actuating chiral salicylideneamine crystal

Superelasticity is a type of elastic response to an applied external force, caused by a phase transformation. Actuation of materials is also an elastic response to external stimuli such as light and heat. Although both superelasticity and actuation are deformations resulting from stimulus-induced stress, there is a phenomenological difference between the two with respect to whether force is an input or an output. Here, we report that a molecular crystal manifests superelasticity during photo-actuation under light irradiation. The crystal exhibits stepwise twisted actuation due to two effects, photoisomerization and photo-triggered phase transition, and the actuation behavior is simulated based on a dynamic multi-layer model. The simulation, in turn, reveals how the photoisomerization and phase transition progress in the crystal, indicating superelasticity induced by modest stress due to the formation of photoproducts. This work provides not only a successful simulation of stepwise twisted actuation, but also to the best of our knowledge the first indication of superelasticity induced by light.

Three-level hierarchical self-assembly of azobenzene conjugated phenylalanines into superhelical nanostructures with light-switchable helicity

Photosensitive azobenzene conjugated phenylalanines can self-assemble into a series of hierarchical superhelices, which can reversibly transform into achiral nanorods via light stimuli.

Photomechanical crystalline materials: new developments, property tuning and applications

This highlight gives an overview of the mechanism development, property tuning and application exploration of photomechanical crystalline materials.

Mechanical Deformation and Multiple Thermal Restoration of Organic Crystals: Reversible Multi-Stage Shape-Changing Effect with Luminescence-Color Changes

Shape-memory materials can be mechanically deformed and subsequently reverse the deformation upon changing the temperature. Shape-memory materials have attracted considerable attention for basic research and industrial applications, and polymer and...

Reversible Photochromic Coordination Polymer by Phototriggered Subtle Molecular Conformation Variations

Photochromic materials are constructed with molecules accompanied by structural change after triggering by light, which are of great importance and necessity for various applications. However, because of space-confinement effects, molecule stacking of these photoresponsive chromophores within coordination polymers (CPs) always results in an efficiency decrement and a response delay, and this phenomenon will lead to a poor photochromic property. Herein, a CP (named CIT-E) with a 3-fold-interpenetrating network structure, which was prepared with (Z)-1,2-diphenyl-1,2-bis[4-(pyridin-3-ylmethoxy)phenyl]ethene (1Z) and a CuI cluster, showed fast reversible photochromic behavior. Under UV-light illumination, the color of CIT-Z changed from pale yellow to reddish brown. With the illumination of green light, the polymer could return to its initial color within 10 s. To reveal the mechanism of reversible photochromic behavior of CIT-Z, single-crystal structures of each color state were fully studied, and other scientific study methods were also used, such as time-dependent density functional theory calculation and control experiments. It was found that, with light illumination, this behavior of CIT-Z was the result of a ligand-to-metal charge-transfer process, and this process was triggered by subtle molecular conformation variation of tetraphenylethylene. It should be noted that CIT-Z has high thermal and chemical stability, which are excellent advantages as smart photoresponsive materials. As a proof of concept, a uniform thin film with such a fascinating photochromic property allows applications in invisible anticounterfeiting and dynamic optical data storage. Overall, the present study opens up a new avenue toward reversible photochromic materials.

Programmable photoresponsive materials based on a single molecule via distinct topochemical reactions

Engineering the preorganization of photoactive units remains a big challenge in solid-state photochemistry research. It is of not only theoretical importance in the construction of topochemical reactions but also technological significance in the fabrication of advanced materials. Here, a cyanostilbene derivative, (Z)-2-(3,5-bis(trifluoromethyl)phenyl)-3-(naphthalen-2-yl) acrylonitrile (BNA), was crystallized into two polymorphs under different conditions. The two crystals, BNA-α and BNA-β, have totally different intra-π-dimer and inter-π-dimer hierarchical architectures on the basis of a very simple monomer, which provides them with distinct reactivities, functions and photoresponsive properties. Firstly, two different types of solid-state [2 + 2] photocycloaddition reaction: (i) a typical olefin-olefin cycloaddition reaction within the symmetric π-dimers of BNA-α and (ii) an unusual olefin-aromatic ring cycloaddition reaction within the offset π-dimers of BNA-β have been observed, respectively. Secondly, the crystal of BNA-α can be bent to 90° without any fracture, exhibiting outstanding flexibility upon UV irradiation, while the reversible photocycloaddition/thermal cleavage process (below 100 °C) accompanied by unique fluorescence changes can be achieved in the crystal of BNA-β. Finally, micro-scale photoactuators and light-writable anti-counterfeiting materials have been successfully fabricated. This work paves a simple way to construct smart materials through a bottom-up way that is realized by manipulating hierarchical architectures in the solid state.

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

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

Recent Development of Photodeformable Crystals: From Materials to Mechanisms

Photodeformable materials are a class of molecules that can convert photon energy into mechanical energy, which have attracted tremendous attention in the last few decades. Owing to their unique photoinduced deformable properties, including fast light-response and diverse mechanical behaviors, photodeformable materials have exhibited great potential in many practical applications such as actuators, photoswitches, artificial muscles, and bioimaging. In this review, we sort out the current state of photodeformable crystals and classify them into six categories by molecular structures: diarylethenes, azobenzenes, anthracenes, olefins, triarylethylenes, and other systems. Three distinct light-responsive mechanisms, photocyclization, trans-cis isomerization, and photodimerization, are revealed to play significant roles in the molecular photodeformation. Their corresponding photodeformable behaviors such as twisting, bending, hopping, bursting, and curling, as well as the potential applications, are also discussed. Furthermore, the challenges and prospective development directions of photodeformable crystals are highlighted.

Light-Responsive Soft Actuators: Mechanism, Materials, Fabrication, and Applications

Soft robots are those that can move like living organisms and adapt to the surrounding environment. Compared with traditional rigid robots, the advantages of soft robots, in terms of material flexibility, human–computer interaction, and biological adaptability, have received extensive attention. Flexible actuators based on light response are one of the most promising ways to promote the field of cordless soft robots, and they have attracted the attention of scientists in bionic design, actuation implementation, and application. First, the three working principles and the commonly used light-responsive materials for light-responsive actuators are introduced. Then, the characteristics of light-responsive soft actuators are sequentially presented, emphasizing the structure strategy, actuation performance, and emerging applications. Finally, this review is concluded with a perspective on the existing challenges and future opportunities in this nascent research frontier.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Mechanical Motion in Crystals Triggered by Solid State Photochemical [2+2] Cycloaddition Reaction

Some special crystals respond to light by jumping, scattering or bursting just like popping of popcorn kernels on a hot surface. This rare phenomenon is called the photosalient (PS) effect. Molecular level control over the arrangement of light-responsive molecules in microscopic crystals for macroscale deformation or mechanical motion offers the possibility of using light to control smart material structures across the length scales. Photochemical [2+2] cycloaddition has recently emerged as a promising route to obtain photoswitchable structures and a wide variety of frameworks, but such reaction in crystals leading to macroscopic mechanical motion is relatively less explored. Study of chemistry of such novel soft crystals for the generation of smart materials is an imperative task. This minireview highlights recent advances in solid-state [2+2] cycloaddition in crystals to induce macroscale mechanical motion and thereby transduction of light into kinetic energy.

Photothermally Driven High-Speed Crystal Actuation and Its Simulation

Mechanically responsive crystals have been increasingly explored, mainly based on photoisomerization. However, photoisomerization has some disadvantages for crystal actuation, such as a slow actuation speed, no actuation of thick crystals, and a narrow wavelength range. Here we report photothermally driven fast-bending actuation and simulation of a salicylideneaniline derivative crystal with an o-amino substituent in enol form. Under ultraviolet (UV) light irradiation, these thin (<20 μm) crystals bent but the thick (>40 μm) crystals did not due to photoisomerization; in contrast, thick crystals bent very quickly (in several milliseconds) due to the photothermal effect, even by visible light. Finally, 500 Hz high-frequency bending was achieved by pulsed UV laser irradiation. The generated photothermal energy was estimated based on the photodynamics using femtosecond transient absorption. Photothermal bending is caused by a nonsteady temperature gradient in the thickness direction due to the heat conduction of photothermal energy generated near the crystal surface. The temperature gradient was calculated based on the one-dimensional nonsteady heat conduction equation to simulate photothermally driven crystal bending successfully. Most crystals that absorb light have their own photothermal effects. It is expected that the creation and design of actuation of almost all crystals will be possible via the photothermal effect, which cannot be realized by photoisomerization, and the potential and versatility of crystals as actuation materials will expand in the near future.

Light-Driven Crystal-Polymer Hybrid Actuators

Recently, soft robots, which are made of soft and light organic materials, have attracted much attention because of improved safety for daily interactions with humans. Mechanically responsive materials that can move macroscopically by external stimuli, such as light and heat, have been studied extensively over the past two decades, and they are expected to be applicable to soft robots. Among them, mechanically responsive crystals are attractive in terms of a larger Young’s modulus and faster response speed compared with polymers and gels. However, it is impractical to use one piece of a single crystal as a crystal machine; it is difficult to control the size of crystals and obtain large crystals. Hybridization of crystals with polymers is one way to create actuators with more realistic movements. Herein, we report a hybrid crystal assembly in which plate-like salicylideneaniline crystals are aligned in polymer films by a “rubbing” technique, a new approach which is inexpensive, easy, and applicable to a wide range of crystals and polymers. The hybrid films bent reversibly upon alternate irradiation with ultraviolet and visible light. The hybrid films bent as fast as single crystals, even when larger than single-crystal size, showing great mechanical performance originating from the advantages of both molecular crystals (fast response time) and polymers (large size). This work enriches the development of light-driven hybrid actuators composed of molecular crystals and polymers.

Fully Controllable Structural Phase Transition in Thermomechanical Molecular Crystals with a Very Small Thermal Hysteresis

The construction of a practical crystalline molecular machine faces two challenges: to realize a collective molecular movement, and to amplify this movement into a precisely controlled mechanical response in real time and space. Thermosalient single crystals display cooperative molecular movements that are converted to strong macroscopic mechanical responses or shape deformations during temperature-induced structural phase transitions. However, these collective molecular movements are hard to control once initiated, and often feature thermal hystereses that are larger than 10 °C, which greatly hamper their practical applications. Here, it is demonstrated that the phase boundaries of the thermomechanical molecular crystal based on a fluorenone derivative 4-DBpFO can be used to finely control its structural phase transition. When this phase transition is triggered at two opposite crystal faces, it is accompanied by two parallel phase boundaries that can be temperature controlled to move forward, backward, or to halt, benefitting from the stored elastic energy between the parallel boundaries. Moreover, the thermal hysteresis is greatly decreased to 2-3 °C, which allows for circular heating/cooling cycles that can produce a continuous work output.

Synthesis of Anthracene-Based Cyclic π-Clusters and Elucidation of their Properties Originating from Congested Aromatic Planes

Synthesis and properties of anthracene-based cyclic π-clusters which possess two and four anthracene units are discussed. The optimal cyclization conditions were determined based on a nickel(0)-mediated reaction that afforded a cyclic anthracene dimer as the major product. Bringing two anthracene planes in close proximity in a face-to-face manner resulted in red-shifted absorption owing to the narrowing of the HOMO-LUMO gap. The cyclic anthracene dimer exhibits multi-stimuli responsiveness due to high π-congestion. For example, photoirradiation on the anthracene dimer affords its photoisomer having C-C bonds that are longer than 1.65 Å, which can undergo thermal reversion under gentle heating. This enabled mechanochromism of the photoisomer (colorless) to the original anthracene dimer (red). Photoisomerization was also observed in the crystalline state, accompanied by crystal jumping or collapsing, that is, the photosalient effect.

Light-Powered Autonomous Flagella-Like Motion of Molecular Crystal Microwires

The ability to exhibit life-like oscillatory motion fueled by light represents a new capability for stimuli-responsive materials. Although this capability has been demonstrated in soft materials like polymers, it has never been observed in molecular crystals, which are not generally regarded as dynamic objects. In this work, it is shown that molecular crystalline microwires composed of (Z)-2-(3-(anthracen-9-yl)allylidene)malononitrile ((Z)-DVAM) can be continuously actuated when exposed to a combination of ultraviolet and visible light. The photo-induced motion mimics the oscillatory behavior of biological flagella and enables propagation of microwires across a surface and through liquids, with translational speeds up to 7 μm s^-1 . This is the first example of molecular crystals that show complex oscillatory behavior under continuous irradiation. A model that relates the rotation of the transition dipole moment between reversible E→Z photoisomerization to the microscopic torque can qualitatively reproduce how the rotational frequency depends on light intensity and polarization.

Dynamic effects in crystalline coordination polymers

Dynamic macroscopic behaviour of single crystals of coordination polymers when subjected to light, heat, and mechanical force.

Anisotropic bending and twisting behaviour of a twin crystal composed of a diarylethene

The unusual photomechanical behaviour of a “twin crystal” consisting of a diarylethene derivative was investigated.

Photomechanical response of sulfonylhydrazone molecular crystals

Photomechanical responses of the single crystals of a series of sulfonylhydrazones are explored for the first time.