Photomechanical Organic Crystals as Smart Materials for Advanced Applications

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

Liquid‐Crystalline Ordering as a Concept in Materials Science: From Semiconductors to Stimuli‐Responsive Devices

While the unique optical properties of liquid crystals (LCs) are already well exploited for flat‐panel displays, their intrinsic ability to self‐organize into ordered mesophases, which are intermediate states between crystal and liquid, gives rise to a broad variety of additional applications. The high degree of molecular order, the possibility for large scale orientation, and the structural motif of the aromatic subunits recommend liquid‐crystalline materials as organic semiconductors, which are solvent‐processable and can easily be deposited on a substrate. The anisotropy of liquid crystals can further cause a stimuli‐responsive macroscopic shape change of cross‐linked polymer networks, which act as reversibly contracting artificial muscles. After illustrating the concept of liquid‐crystalline order in this Review, emphasis will be placed on synthetic strategies for novel classes of LC materials, and the design and fabrication of active devices.     

Photomechanics of liquid-crystalline elastomers and other polymers

Muscle is a transducer that can convert chemical energy into mechanical motion. To construct artificial muscles, it is desirable to use soft materials with high mechanical flexibility and durability rather than hard materials such as metals. For effective muscle-like actuation, materials with stratified structures and high molecular orders are necessary. Liquid-crystalline elastomers (LCEs) are superior soft materials that possess both the order of liquid crystals and the elasticity of elastomers (as they contain polymer networks). With the aid of LCEs, it is possible to convert small amounts of external energy into macroscopic amounts of mechanical energy. In this Review, we focus on light as an energy source and describe the recent progress in the area of soft materials that can convert light energy into mechanical energy directly (photomechanical effect), especially the photomechanical effects of LCEs with a view to applications for light-driven LCE actuators.     

Photomechanical Luminescence from Through‐Space Conjugated AIEgens

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

Electro‐Optic (E‐O) Molecular Glasses

This Focus Review describes molecular glasses as a new class of materials for nonlinear optical (NLO) applications, especially for electro‐optic (E‐O) devices. Examples of E‐O molecular glasses are reviewed with a focus on the molecular design of NLO chromophores and solid‐state engineering of molecular glasses. Molecular glasses based on dendrimers of multiple chromophores, molecular glass blends of chromophores, and molecular glasses based on reversible self‐assembly of chromophores are introduced as promising architectures to prepare morphologically stable molecular glasses with large E‐O activities and improved material properties for device applications. Future directions to fully exploit the potential of molecular glasses for NLO materials are presented.     

Molecular Spring‐like Triple‐Helix Coordination Polymers as Dual‐Stress and Thermally Responsive Crystalline Metal–Organic Materials

Elastic metal–organic materials (MOMs) capable of multiple stimuli‐responsiveness based on dual‐stress and thermally responsive triple‐helix coordination polymers are presented. The strong metal‐coordination linkage and the flexibility of organic linkers in these MOMs, rather than the 4 Å stacking interactions observed in organic crystals, causes the helical chain to act like a molecular spring and thus accounts for their macroscopic elasticity. The thermosalient effect of elastic MOMs is reported for the first time. Crystal structure analyses at different temperatures reveal that this thermoresponsiveness is achieved by adaptive regulation of the triple‐helix chains by fine‐tuning the opening angle of flexible V‐shaped organic linkers and rotation of its lateral conjugated groups to resist possible expansion, thus demonstrating the vital role of adaptive reorganization of triple‐helix metal–organic chains as a molecular spring‐like motif in crystal jumping.     

Organic Photomechanical Materials

Organic molecules can transform photons into Angstrom‐scale motions by undergoing photochemical reactions. Ordered media, for example, liquid crystals or molecular crystals, can align these molecular‐scale motions to produce motion on much larger (micron to millimeter) length scales. In this Review, we describe the basic principles that underlie organic photomechanical materials, starting with a brief survey of molecular photochromic systems that have been used as elements of photomechanical materials. We then describe various options for incorporating these active elements into a solid‐state material, including dispersal in a polymer matrix, covalent attachment to a polymer chain, or self‐assembly into molecular crystals. Particular emphasis is placed on ordered media, such as liquid‐crystal elastomers and molecular crystals, that have been shown to produce motion on large (micron to millimeter) length scales. We also discuss other mechanisms for generating photomechanical motion that do not involve photochemical reactions, such as photothermal expansion and photoinduced charge transfer. Finally, we identify areas for future research, ranging from the study of basic phenomena in solid‐state photochemistry, to molecular and host matrix design, and the optimization of photoexcitation conditions. The ultimate realization of photon‐fueled micromachines will likely involve advances spanning the disciplines of chemistry, physics and engineering.     

Functional Liquid‐Crystalline Assemblies: Self‐Organized Soft Materials

In the 21st century, soft materials will become more important as functional materials because of their dynamic nature. Although soft materials are not as highly durable as hard materials, such as metals, ceramics, and engineering plastics, they can respond well to stimuli and the environment. The introduction of order into soft materials induces new dynamic functions. Liquid crystals are ordered soft materials consisting of self‐organized molecules and can potentially be used as new functional materials for electron, ion, or molecular transporting, sensory, catalytic, optical, and bio‐active materials. For this functionalization, unconventional materials design is required. Herein, we describe new approaches to the functionalization of liquid crystals and show how the design of liquid crystals formed by supramolecular assembly and nano‐segregation leads to the formation of a variety of new self‐organized functional materials.     

Self‐Healing Molecular Crystals

One of the most inevitable limitations of any material that is exposed to mechanical impact is that they are inexorably prone to mechanical damage, such as cracking, denting, gouging, or wearing. To confront this challenge, the field of polymers has developed materials that are capable of autonomous self‐healing and recover their macroscopic integrity similar to biological organisms. However, the study of this phenomenon has mostly remained within the soft materials community and has not been explored by solid‐state organic chemists. The first evidence of self‐healing in a molecular crystal is now presented using crystals of dipyrazolethiuram disulfide. The crystals were mildly compressed and the degree of healing was found to be 6.7 %. These findings show that the self‐healing properties can be extended beyond mesophasic materials and applied towards the realm of ordered solid‐state compounds.     

Light‐Directed Dynamic Chirality Inversion in Functional Self‐Organized Helical Superstructures

Helical superstructures are widely observed in nature, in synthetic polymers, and in supramolecular assemblies. Controlling the chirality (the handedness) of dynamic helical superstructures of molecular and macromolecular systems by external stimuli is a challenging task, but is of great fundamental significance with appealing morphology‐dependent applications. Light‐driven chirality inversion in self‐organized helical superstructures (i.e. cholesteric, chiral nematic liquid crystals) is currently in the limelight because inversion of the handedness alters the chirality of the circularly polarized light that they selectively reflect, which has wide potential for application. Here we discuss the recent developments toward inversion of the handedness of cholesteric liquid crystals enabled by photoisomerizable chiral molecular switches or motors. Different classes of chiral photoresponsive dopants (guests) capable of conferring light‐driven reversible chirality inversion of helical superstructures fabricated from different nematic hosts are discussed. Rational molecular designs of chiral molecular switches toward endowing handedness inversion to the induced helical superstructures of cholesteric liquid crystals are highlighted. This Review is concluded by throwing light on the challenges and opportunities in this emerging frontier, and it is expected to provide useful guidelines toward the development of self‐organized soft materials with stimuli‐directed chirality inversion capability and multifunctional host–guest systems.     

Bioinspired Actuators Based on Stimuli‐Responsive Polymers

Organisms exhibit strong environmental adaptability by controllably adjusting their morphologies or fast locomotion; thus providing constant inspiration for scientists to develop artificial actuators that not only have diverse and sophisticated shape‐morphing capabilities, but can also further transfer dynamic and reversible shape deformations into macroscopic motion under the following principles: asymmetric friction, the Marangoni effect, and counteracting forces of the surrounding conditions. Among numerous available materials for fabricating bioinspired artificial actuators, stimuli‐responsive polymers are superior in their flexible features and the ability to change their physicochemical properties dynamically under external stimuli, such as temperature, pH, light, and ionic strength. Herein, different mechanisms, working principles, and applications of stimuli‐responsive polymeric actuators are comprehensively introduced. Furthermore, perspectives on existing challenges and future directions of this field are provided.     

Light‐Powered Self‐Healable Metallosupramolecular Soft Actuators

Supramolecular functional materials able to respond to external stimuli have several advantages over their classical covalent counterparts. The preparation of soft actuators with the ability to respond to external stimuli in a spatiotemporal fashion, to self‐repair, and to show directional motion, is currently one of the most challenging research goals. Herein, we report a series of metallopolymers based on zinc(II)–terpyridine coordination nodes and bearing photoisomerizable diazobenzene units and/or solubilizing luminescent phenylene–ethynylene moieties. These supramolecular polymers act as powerful gelating agents at low critical gelation concentrations. The resulting multiresponsive organogels display light‐triggered mechanical actuation and luminescent properties. Furthermore, owing to the presence of dynamic coordinating bonds, they show self‐healing abilities.     

Topochemical Polymerization of 1,3‐Diene Monomers and Features of Polymer Crystals as Organic Intercalation Materials

From the viewpoint of controlled polymer synthesis, topochemical polymerization based on crystal engineering is very useful for controlling not only the primary chain structures but also the higher‐order structures of the crystalline polymers. We found a new type of topochemical polymerization of muconic and sorbic acid derivatives to give stereoregular and high‐molecular weight polymers under photo‐, X‐ray, and γ‐ray irradiation of the monomer crystals. In this article, we describe detailed features and the mechanism of the topochemical polymerization of diethyl‐(Z,Z)‐muconate as well as of various alkylammonium derivatives of muconic and sorbic acids, which are 1,3‐diene mono‐ and dicarboxylic acid derivatives, to control the stereochemical structures of the polymers. The polymerization reactivity of these monomers in the crystalline state and the stereochemical structure of the polymers produced are discussed based on the concept of crystal engineering, which is a useful method to design and control the reactivity, structure, and properties of organic solids. The reactivity of the topochemical polymerization is determined by the monomer crystal structure, i.e. the monomer molecular arrangement in the crystals. Polymer crystals derived from topochemical polymerization have a high potential as new organic crystalline materials for various applications. Organic intercalation using the polymer crystals prepared from alkylammonium muconates and sorbates is also described.     

Ultrasound‐Assisted Construction of Halogen‐Bonded Nanosized Cocrystals That Exhibit Thermosensitive Luminescence

Multi‐component organic nanocrystals that are comprised of two or more supramolecular building blocks can be used to extend the design and assembly scope of solid molecular materials. Herein, we report the use of ultrasonication to prepare halogen‐bonded stilbene‐based nano‐cocrystals that exhibit different photoemission properties, including one‐ and two‐phonon emission and fluorescence lifetimes, relative to those of macrodimensional crystals. The structural transformation from nano‐cocrystals into nanocrystals upon heating results in a luminescence red‐shift from greenish blue to yellow. The temperature‐dependent ratiometric luminescence may allow such nano‐cocrystals to be used as fluorescent sensors and thermosensitive materials.     

Synthesis of Spin‐Crossover Nano‐ and Micro‐objects in Homogeneous Media

New methods are proposed for the synthesis of spin‐crossover nano‐ and micro‐objects. Several nano‐objects that are based upon the spin‐crossover complex [Fe(hptrz)₃](OTs)₂ (hptrz=4‐heptyl‐1,2,4‐triazole, Ts=para‐toluenesulfonyl) were prepared in homogeneous media. The use of various reagents (Triton X‐100, PVP, TOPO, and PEGs of different molecular weights) as stabilizing agents yielded materials of different size (6 nm–2 μm) and morphology (nanorods, nanoplates, small spherical particles, and nano‐ and micro‐crystals). In particular, when Triton X‐100 was used, a variation in the morphology from nanorods to nanoplates was observed by changing the nature of the solvent. Interestingly, the preparation of the nanorods and nanoplates was always accompanied by the formation of small spherical particles. Alternatively, when PEG was used, 200–400 nm crystals of the complex were obtained. In addition, a very promising polymer‐free synthetic method is discussed that was based on the preparation of relatively stable Fe^II–triazole oligomers in CHCl₃. Their specific treatment led to micro‐crystals, small nanoparticles, or gels. The size and morphology of all of these objects were characterized by TEM and by dynamic light scattering (DLS) where possible. Their spin‐crossover behavior was studied by optical and magnetic measurements. The spin‐transition features for large particles (>100 nm) were very similar to that of the bulk material, that is, close to room temperature with a hysteresis width of up to 8 K. The effects of the matrix and/or size‐reduction led to modification of the transition temperature and an abruptness of the spin transition for oligomeric solutions and small nanoparticles of 6 nm in size.     

Self‐Immolative Poly(4,5‐dichlorophthalaldehyde) and its Applications in Multi‐Stimuli‐Responsive Macroscopic Plastics

End‐capped poly(4,5‐dichlorophthalaldehyde) (PCl₂PA), which is a new self‐immolative CD_r polymer with the unique capability of depolymerizing continuously and completely in the solid state when an end cap is cleaved from the polymer by reaction with a specific molecular signal, is described. End‐capped poly(4,5‐dichlorophthalaldehyde) is sufficiently stable to enable patterning of three‐dimensional macroscopic polymeric materials by selective laser sintering. These unique materials are capable of 1) autonomously amplifying macroscopic changes in the material in response to specific molecular inputs, and 2) altering their responses depending on the identity of the applied signal. Thus, not only does end‐capped PCl₂PA provide new and unique capabilities compared to the small subset of existing CD_r polymers, but it also provides access to a new class of stimuli‐responsive materials.     

烯碳材料在人工肌肉领域的应用进展

随着仿生机器人、智能控制及人工智能等领域的发展,传统的机械驱动方式已无法满足相关领域对致动系统提出的柔性、高效及多源刺激响应性等要求,因此需发展新型的人工肌肉材料。以碳纳米管和石墨烯为代表的烯碳材料具有轻质、高强、高电导率和柔性等特征,在人工肌肉领域展现出了巨大的应用潜力。以烯碳材料为基元构筑宏观组装体材料,或以烯碳材料为添加相制备纳米复合材料,可在微观和宏观架起桥梁,实现烯碳材料在人工肌肉领域的应用。本文基于上述两种应用形式,综述了烯碳材料在人工肌肉领域的应用进展。首先从一维纤维和二维薄膜的烯碳人工肌肉宏观表现形态出发,介绍了既作为结构材料,又提供了响应、驱动功能的烯碳材料在人工肌肉中的应用。接着从机电性能、可编程的响应形变以及传感功能三个方向,介绍了烯碳材料作为增强赋能相在人工肌肉材料中的功能性应用。最后阐述了基于烯碳材料人工肌肉的机遇与挑战。     

Phase Transition-Promoted Rapid Photomechanical Motions of Single Crystals of a Triene Coordination Polymer

Molecular crystals with the ability to transform light energy into macroscopic mechanical motions are a promising class of materials with potential applications in actuating and photonic devices. In regard to such materials, coordination polymers that exhibit dynamic photomechanical motion, associated with a phase transition, are unknown. Herein, we report an intriguing photoactive, one-dimensional Zn^II coordination polymer, 1 , derived from 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene and 3,5-difluorobenzoate. Single crystals of 1 under UV light irradiation exhibit rapid shrinking and bending, violent bursting-jumping, splitting, and cracking behavior. Single-crystal X-ray diffraction analysis and ¹H NMR spectroscopy reveal an unusual photoinduced phase transition involving a single-crystal-to-single-crystal [2+2] cycloaddition reaction that results in photomechanical responses. Interestingly, crystals of 1 , which are triclinic with space group , are transformed into a higher symmetry, monoclinic cell with space group C2/c. This process represents a rare example of symmetry enhancement upon photoirradiation. The photomechanical activity is likely due to the sudden release of stress associated with strained molecular geometries and significant solid-state molecular movement arising from cleavage and formation of chemical bonds. A composite membrane fabricated from 1 and polyvinyl alcohol (PVA) also displays interesting photomechanical behavior under UV light illumination, indicating the material's potential as a photoactuator.     

Photomechanical polymer hydrogels based on molecular photoswitches

As intelligent materials responsive to light, photomechanical hydrogels not only possess high-water content, excellent softness and biocompatibility, but also can accomplish various mechanical motions upon spatiotemporal stimulation of external light, which exhibit great potential in biomedical and underwater bionic fields. Molecular photoswitches have been used broadly in preparation of photomechanical hydrogels owing to their high photosensitivity and reversible molecular structure transformations induced by light. Herein, the current progress of photomechanical hydrogels based on typical molecular photoswitches such as spiropyran, azobenzene, and hexaarylbiimidazole (HABI) are introduced. Especially, as a promising building unit for photomechanical hydrogels, HABI has been highlighted due to the unique molecular structures and reversible photoswitching capability. HABI-derived polymer hydrogels demonstrate flexible mechanical behaviors upon localized light irradiation. The characteristics and challenges of photomechanical hydrogels based on molecular photoswitches are also prospected.     

Infrared Fingerprint Engineering: A Molecular‐Design Approach to Long‐Wave Infrared Transparency with Polymeric Materials

Optical technologies in the long‐wave infrared (LWIR) spectrum (7–14 μm) offer important advantages for high‐resolution thermal imaging in near or complete darkness. The use of polymeric transmissive materials for IR imaging offers numerous cost and processing advantages but suffers from inferior optical properties in the LWIR spectrum. A major challenge in the design of LWIR‐transparent organic materials is that nearly all organic molecules absorb in this spectral window which lies within the so‐called IR‐fingerprint region. We report on a new molecular‐design approach to prepare high refractive index polymers with enhanced LWIR transparency. Computational methods were used to accelerate the design of novel molecules and polymers. Using this approach, we have prepared chalcogenide hybrid inorganic/organic polymers (CHIPs) with enhanced LWIR transparency and thermomechanical properties via inverse vulcanization of elemental sulfur with new organic co‐monomers.