Stimuli-Responsive Crystalline Smart Materials: From Rational Design and Fabrication to Applications

Template-Directed In Crystallo Photopolymerization of a Donor-Acceptor Cyclopropane: When Everything Falls into Place!

A single-crystal-to-single-crystal solid-state reaction of vinylogous donor-acceptor cyclopropanes is documented. The enantiospecific synthesis of new products, distinct from those obtained in solution, is achieved for the target compounds. Photopolymerization occurred upon X-ray exposure to the crystals. Notably, in one case, this reactivity exhibits selectivity since an ordered arrangement of polymers and unreacted cocrystallized monomeric conformers has been observed. Structural characterization of the complete transformation monitored through single-crystal X-ray diffraction and supported by molecular dynamics simulations sheds light on the subtle role of crystal packing in the reaction process. Moreover, the X-ray diffraction (XRD)-resolved structure of a donor-acceptor cyclopropane intermediate reveals an elongation in bond length that corroborates the existence of the so-called "push-pull effect".

Fabrication of Large-Area Multi-Stimulus Responsive Thin Films via Interfacially Confined Irreversible Katritzky Reaction

Fabrication of large-area thin films through irreversible reactions remains a formidable task. This study reports a breakthrough strategy for in situ synthesis of large-area, free-standing, robust and multi-stimulus responsive thin films through a catalyst-free and irreversible Katritzky reaction at a liquid-liquid interface. The as resulted films are featured with adjustable thickness of 1-3 μm and an area up to 50 cm2. The thin films exhibit fast photo-mechanical motions (a response time of ca 0.1 s), vapor-mechanical motions, as well as photo-chromic and solvato-chromic behaviors. It was revealed that the reason behind the observable motions is proton transfer from the imine groups to the carbonyl structures within the film induced by photo- and/or dimethyl sulfoxide-stimulus. In addition, the films can harvest anionic radicals and the radicals as captured can be efficiently degraded under UV light illumination. This study provides a new strategy for fabricating smart thin films via interfacially confined irreversible Katritzky reaction.

Efficient Capture and Low Energy Release of NH3 by Azophenol Decorated Photoresponsive Covalent Organic Frameworks

In NH3 capture technologies, the desorption process is usually driven by high temperature and low pressure (such as 150-200 °C under vacuum), which accounts for intensive energy consumption and CO2 emission. Developing light responsive adsorbent is promising in this regard but remains a great challenge. Here, we for the first time designed and synthesized a light responsive azophenol-containing covalent organic framework (COF), COF-HNU38, to address this challenge. We found that at 25 °C and 1.0 bar, the cis -COF exhibited a NH3 uptake capacity of 7.7 mmol g-1 and a NH3/N2 selectivity of 158. In the adsorbed NH3, about 29.0 % could be removed by vis-light irradiated cis-trans isomerization at 25 °C, and the remaining NH3 might be released at 25 °C under vacuum. Almost no decrease in adsorption capacity was observed after eight adsorption-desorption cycles. As such, an efficient NH3 capture and low energy release strategy was established thanks to the multiple hydrogen bond interactions (which are strong in total but weak in individuals) between NH3 and the smart COF, as well as the increased polarity and number of hydrogen bond sites after the trans-cis isomerization.

Multi Stimuli Responsive Dual Aggregation-Induced Emission and Photochromic Behavior of a Tetraphenyl Substituted Triphenylamine Derivative and its Application as Anti-counterfeiting Agent

A multi-stimuli responsive tetraphenyl substituted tripehnylamine-based aggregation induced emissive (AIE) material coupled with spiropyran was prepared. Owing to the presence of AIE and photochromic moiety, the molecule exhibits emissive aggregates, photochromism, and acidochromism. The multiple stimuli sensitive behavior of the molecule was explored for anti-counterfeiting behavior on TLC plate and commercial banknotes. The fluorogenic and photogenic response under UV and visible light established the potential of the candidate as a new generation encryption material.

A comparison of the isostructural [Co(NH3)5NO2]XNO3 and [Co(NH3)5ONO]XNO3, X = Cl- or Br- in relation to nitro-nitrito linkage isomerization and photomechanical effects

A new photoactive cobalt coordination compound, [Co(NH3)5NO2]BrNO3 (I), was obtained. Its crystal structure was shown to be isostructural with previously known [Co(NH3)5NO2]ClNO3 (II) for which linkage isomerization accompanied with mechanical response of the crystal has been already reported. Single crystals of I are transformed into nitrito isomer [Co(NH3)5ONO]BrNO3 (III) on irradiation with blue light (λ = 465 nm) without being destroyed. The crystal structure of III was also solved using single-crystal X-ray diffraction and compared with previously known [Co(NH3)5ONO]ClNO3 (IV). A detailed comparison of the structures of I, II, III and IV, including unit-cell parameters, the distribution of free space (in particular, reaction cavities around the nitro ligand), the lengths of hydrogen bonds, coordination and Voronoi-Dirichlet polyhedra has been performed. Single-crystal X-ray diffraction data were complemented with IR spectra. The effect of the replacement of Cl- by Br- on the crystal structure and on the nitro-nitrito photoisomerization is discussed.

Multifunctional Polyoxometalates-Based Ionohydrogels toward Flexible Electronics

Multifunctional flexible electronics present tremendous opportunities in the rapidly evolving digital age. One potential avenue to realize this goal is the integration of polyoxometalates (POMs) and ionic liquid-based gels (ILGs), but the challenge of macrophase separation due to poor compatibility, especially caused by repulsion between like-charged units, poses a significant hurdle. Herein, the possibilities of producing diverse and homogenous POMs-containing ionohydrogels by nanoconfining POMs and ionic liquids (ILs) within an elastomer-like polyzwitterionic hydrogel using a simple one-step random copolymerization method, are expanded vastly. The incorporation of polyzwitterions provides a nanoconfined microenvironment and effectively modulates excessive electrostatic interactions in POMs/ILs/H2O blending system, facilitating a phase transition from macrophase separation to a submillimeter scale worm-like microphase-separation system. Moreover, combining POMs-reinforced ionohydrogels with a developed integrated self-powered sensing system utilizing strain sensors and Zn-ion hybrid supercapacitors has enabled efficient energy storage and detection of external strain changes with high precision. This work not only provides guidelines for manipulating morphology within phase-separation gelation systems, but also paves the way for developing versatile POMs-based ionohydrogels for state-of-the-art smart flexible electronics.

Synthesis of Room Temperature Curable Polymer Binder Mixed with Polymethyl Methacrylate and Urethane Acrylate for High-Strength and Improved Transparency

Urethane acrylate (UA) was synthesized from various di-polyols, such as poly(tetrahydrofuran) (PTMG, Mn = 1000), poly(ethylene glycol) (PEG, Mn = 1000), and poly(propylene glycol) (PPG, Mn = 1000), for use as a polymer binder for paint. Polymethyl methacrylate (PMMA) and UA were blended to form an acrylic resin with high transmittance and stress-strain curve. When PMMA was blended with UA, a network structure was formed due to physical entanglement between the two polymers, increasing the mechanical properties. UA was synthesized by forming a prepolymer using di-polyol and hexamethylene diisocyanate, which were chain structure monomers, and capping them with 2-hydroxyethyl methacrylate to provide an acryl group. Fourier transform infrared spectroscopy was used to observe the changes in functional groups, and gel permeation chromatography was used to confirm that the three series showed similar molecular weight and PDI values. The yellowing phenomenon that appears mainly in the curing reaction of the polymer binder was solved, and the mechanical properties according to the effects of the polyol used in the main chain were compared. The content of the blended UA was quantified using ultravioletvisible spectroscopy at a wavelength of 370 nm based on 5, 10, 15, and 20 wt%, and the shear strength and tensile strength were evaluated using specimens in a suitable mode. The ratio for producing the polymer binder was optimized. The mechanical properties of the polymer binder with 5-10 wt% UA were improved in all series.

Molecular-Sieving Separation of Methanol/Benzene Azeotrope by a Flexible Metal-Organic Framework

Separation of methanol/benzene azeotrope mixtures is very challenging not only by the conventional distillation technique but also by adsorbents. In this work, we design and synthesize a flexible Ca-based metal-organic framework MAF-58 consisting of cheap raw materials. MAF-58 shows selective methanol-induced pore-opening flexibility. Although the opened pores are large enough to accommodate benzene molecules, MAF-58 shows methanol/benzene molecular sieving with ultrahigh experimental selectivity, giving 5.1 mmol g-1 high-purity (99.99%+) methanol and 2.0 mmol g-1 high-purity (99.97%+) benzene in a single adsorption/desorption cycle. Computational simulations reveal that the preferentially adsorbed, coordinated methanol molecules act as the gating component to selectively block the diffusion of benzene, offering a new gating adsorption mechanism.

Precise Post-Synthetic Modification of Heterometal-Organic Capsules for Selectively Encapsulating Tetrahedral Anions

Post-synthetic modification plays a crucial role in precisely adjusting the structure and functions of advanced materials. Herein, we report the self-assembly of a tubular heterometallic Pd3Cu6L16 capsule that incorporates Pd(II) and CuL1 metalloligands. This capsule undergoes further modification with two tridentate anionic ligands (L2) to afford a bicapped Pd3Cu6L16L22 capsule with an Edshammer polyhedral structure. By employing transition metal ions, acid, and oxidation agents, the bicapped capsule can be converted into an uncapped one. This uncapped form can then revert back to the bicapped structure on the addition of Br- ions and a base. Interestingly, introducing Ag+ ions leads to the removal of one L2 ligand from the bicapped capsule, yielding a mono-capped Pd3Cu6L16L2 structure. Furthermore, the size of the anions critically influences the precise control over the post-synthetic modifications of the capsules. It was demonstrated that these capsules selectively encapsulate tetrahedral anions, offering a novel approach for the design of intelligent molecular delivery systems.

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.

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

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

Ultra-Strong Janus Covalent Organic Framework Membrane with Smart Response to Organic Vapor

Vapor-driven smart Janus materials have made significant advancements in intelligent monitoring, control, and interaction, etc. Nevertheless, the development of ultrafast response single-layer Janus membrane, along with a deep exploration of the smart response mechanisms, remains a long-term endeavor. Here, the successful synthesis of a high-crystallinity single-layer Covalent organic framework (COF) Janus membrane is reported by morphology control. This kind of membrane displays superior mechanical properties and specific surface area, along with excellent responsiveness to CH2Cl2 vapor. The analysis of the underlying mechanisms reveals that the vapor-induced breathing effect of the COF and the stress mismatch of the Janus structure play a crucial role in its smart deformation performance. It is believed that this COF Janus membrane holds promise for complex tasks in various fields.

Nanoparticle-Empowered Core-Shell Microcapsules: From Architecture Design to Fabrication and Functions

Compartmentalization is a powerful concept to integrate multiscale components with diverse functionalities into miniature architectures. Inspired by evolution-optimized cell compartments, synthetic core-shell capsules enable storage of actives and on-demand delivery of programmed functions, driving scientific progress across various fields including adaptive materials, sustainable electronics, soft robotics, and precision medicine. To simultaneously maximize structural stability and environmental sensitivity, which are the two most critical characteristics dictating performance, diverse nanoparticles are incorporated into microcapsules with a dense shell and a liquid core. Recent studies have revealed that these nano-additives not only enhance the intrinsic properties of capsules including mechanical robustness, optical behaviors, and thermal conductivity, but also empower dynamic features such as triggered release, deformable structures, and fueled mobility. In this review, the physicochemical principles that govern nanoparticle assembly during microencapsulation are examined in detail and the architecture-controlled functionalities are outlined. Through the analysis of how each primary method implants nanoparticles into microcapsules, their distinct spatial organizations within the core-shell structures are highlighted. Following a detailed discussion of the specialized functions enabled by specific nanoparticles, the vision of the required fundamental insights and experimental studies for this class of microcarriers to fulfill its potential are sketched.

Stimuli-Responsive Properties of a Zinc(II) Salen-Type Schiff-Base Complex and Vapochromic Detection of Volatile Organic Compounds

This contribution reports, through a combined thermogravimetric analysis, differential scanning calorimetry, UV-vis, powder X-ray diffraction, and Rietveld refinement analysis, on the stimuli-responsive chromic properties of a substituted Zn(salmal) Schiff-base Lewis acidic complex with unique and distinct thermo- and vapochromic characteristics. The solid complex obtained in air or by evaporation of the solvent from their THF solutions shows a marked thermochromism associated with a phase transition, unusually triggered by the reversible desorption/adsorption of one lattice water molecule. In contrast, the anhydrous solid, achieved from THF solutions of the complex by evaporation of the solvent under anhydrous conditions, behaves very differently as it does not show any absorption of water or thermochromism and exhibits varied vapochromic properties. Detection of volatile organic compounds having Lewis basicity is demonstrated by using the anhydrous solid or the related cast films on glass or paper substrates. In both cases, a marked vapochromism is observed upon exposure to vapors of various volatile species and involves well-defined optical absorptions and naked-eye color changes, also allowing the discrimination of primary aliphatic amines. Vapochromic behavior with the formation of stable, stoichiometric adducts is also demonstrated for both the solid obtained in air and the anhydrous solid or for the related cast films after exposure to vapors of pyridine.

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.

Stimuli-fluorochromic smart organic materials

Smart materials based on stimuli-fluorochromic π-conjugated solids (SFCSs) have aroused significant interest due to their versatile and exciting properties, leading to advanced applications. In this review, we highlight the recent developments in SFCS-based smart materials, expanding beyond organometallic compounds and light-responsive organic luminescent materials, with a discussion on the design strategies, exciting properties and stimuli-fluorochromic mechanisms along with their potential applications in the exciting fields of encryption, sensors, data storage, display, green printing, etc. The review comprehensively covers single-component and multi-component SFCSs as well as their stimuli-fluorochromic behaviors under external stimuli. We also provide insights into current achievements, limitations, and major challenges as well as future opportunities, aiming to inspire further investigation in this field in the near future. We expect this review to inspire more innovative research on SFCSs and their advanced applications so as to promote further development of smart materials and devices.

Smart Home Sleep Respiratory Monitoring System Based on a Breath-Responsive Covalent Organic Framework

A smart home sleep respiratory monitoring system based on a breath-responsive covalent organic framework (COF) was developed and utilized to monitor the sleep respiratory behavior of real sleep apnea patients in this work. The capacitance of the interdigital electrode chip coated with COFTPDA-TFPy exhibits thousands-level reversible responses to breath humidity gases, with subsecond response time and robustness against environmental humidity. A miniaturized printed circuit board, an open-face-mask-based respiratory sensor, and a smartphone app were constructed for the wearable wireless smart home sleep respiratory monitoring system. Leveraging the sensitive and rapid reversible response of COFs, the COF-based respiratory monitoring system can effectively record normal breath, rapid breath, and breath apnea, enabling over a thousand cycles of hour-level continuous monitoring during daily wear. Next, we took the groundbreaking step of advancing the humidity sensor to the clinical trial stage. In clinical experiments on real sleep apnea patients, the COF-based respiratory monitoring system successfully recorded hour-level sleep respiratory data and differentiated the breathing behavior characteristics and severity of sleep apnea patients and subjects with normal sleep function and primary snoring patients. This work successfully advanced humidity sensors into clinical research for real patients and demonstrated the enormous application potential of COF materials in clinical diagnosis.

Interfacial Water Molecules as Agents for Phase Change Control and Proton Conductivity Enhancement in the Ammonium Vanadyl Tartrate System

This study demonstrates the reversible structural transformation, single-crystal-to-single-crystal, of the ammonium vanadyl (L-tartrate) complex salt from the hydrate phase to the anhydrous phase. The transformation can be initiated by stimuli, such as temperature, humidity, or vacuum conditions. The hydrate and anhydrous phases exhibit a tetragonal structure (P41212), with marked differences in hydrogen bonding due to the presence or absence of one water molecule per asymmetric unit. The intricate relationship between crystal packing and intermolecular interactions in the hydrate phase was investigated by crystallographic charge density analysis revealing, at the molecular level, the reasons for the observed 5 orders of magnitude higher proton conductivity of the hydrate phase compared to that of the anhydrous phase. To gain further insight into the processes occurring at the surfaces of grain boundaries and the proton transfer mechanisms in this system, rehydration of the complex salt was carried out by using D2O instead of H2O and monitored by in situ ATR-FTIR spectroscopy. The results highlight the critical role of interfacial water molecules in driving structural transformations and influencing proton conductivity.

Responsive macrocyclic and supramolecular structures powered by platinum

Humankind's manipulation of platinum dates back more than two millennia to burial objects. Since then, its use has evolved from purely decorative purposes in jewelry to more functional applications such as in catalysts, pharmaceuticals, and bioimaging agents. Platinum offers a range of properties arguably unmatched by any other metal, including electroactivity, photoluminescence, chromic behaviour, catalysis, redox reactivity, photoreactivity, and stimuli-controlled intermetallic interactions. The vast body of knowledge generated by the exploration of these and other properties of platinum has recently merged with other areas of chemistry such as supramolecular and host-guest chemistry. This has shown us that platinum can incorporate its responsive character into supramolecular assemblies (e.g., macrocycles and polymers) to produce materials with tailorable functions and responses. In this Perspective Article, we cover some platinum-powered supramolecular structures reported by us and others, hoping to inspire new and exciting discoveries in the field.

Photo-Responsive Dynamic Organic Room-Temperature Phosphorescence Materials Based on a Functional Unit Combination Strategy

A rational molecular design strategy facilitates the development of a purely organic room-temperature phosphorescence (RTP) material system with precisely regulated luminescence properties, which surely promotes its functional integration and intelligent application. Here, a functional unit combination strategy is proposed to design novel RTP molecules combining a folding unit with diverse luminescent cores. The different luminescent cores are mainly responsible for tunable RTP properties, while the folding unit contributes to the spin-orbit coupling (SOC) enhancement, which makes the RTP material design as workable as the building block principle. By this strategy, a series of color/lifetime-tunable RTP materials is achieved with unique photo-responsive RTP enhancement when subjected to UV irradiation, which expands their application scenarios in reusable privacy tags, advanced "4D" encryption, and phase separation analysis of blended polymers. This work suggests a simple and effective strategy to design purely organic RTP materials with tunable color and lifetime, and also provides new application options for photo-responsive dynamic RTP materials.

Thermo-Electro-Responsive Redox-Copolymers for Amplified Solvation, Morphological Control, and Tunable Ion Interactions

Electro-responsive metallopolymers can possess highly specific and tunable ion interactions, and have been explored extensively as electrode materials for ion-selective separations. However, there remains a limited understanding of the role of solvation and polymer-solvent interactions in ion binding and selectivity. The elucidation of ion-solvent-polymer interactions, in combination with the rational design of tailored copolymers, can lead to new pathways for modulating ion selectivity and morphology. Here, we present thermo-electrochemical-responsive copolymer electrodes of N-isopropylacrylamide (NIPAM) and ferrocenylpropyl methacrylamide (FPMAm) with tunable polymer-solvent interactions through copolymer ratio, temperature, and electrochemical potential. As compared to the homopolymer PFPMAm, the P(NIPAM0.9-co-FPMAm0.1) copolymer ingressed 2 orders of magnitude more water molecules per doping ion when electrochemically oxidized, as measured by electrochemical quartz crystal microbalance. P(NIPAM0.9-co-FPMAm0.1) exhibited a unique thermo-electrochemically reversible response and swelled up to 83% after electrochemical oxidation, then deswelled below its original size upon raising the temperature from 20 to 40 °C, as measured through spectroscopic ellipsometry. Reduced P(NIPAM0.9-co-FPMAm0.1) had an inhomogeneous depth profile, with layers of low solvation. In contrast, oxidized P(NIPAM0.9-co-FPMAm0.1) displayed a more uniform and highly solvated depth profile, as measured through neutron reflectometry. P(NIPAM0.9-co-FPMAm0.1) and PFPMAm showed almost a fivefold difference in selectivity for target ions, evidence that polymer hydrophilicity plays a key role in determining ion partitioning between solvent and the polymer interface. Our work points to new macromolecular engineering strategies for tuning ion selectivity in stimuli-responsive materials.

Stimuli-Responsive, Dynamic Supramolecular Organic Frameworks

Supramolecular organic frameworks (SOFs) are a class of three-dimensional, potentially porous materials obtained by the self-assembly of organic building blocks held together by weak interactions such as hydrogen bonds, halogen bonds, π⋅⋅⋅π stacking and dispersion forces. SOFs are being extensively studied for their potential applications in gas storage and separation, catalysis, guest encapsulation and sensing. The supramolecular forces that guide their self-assembly endow them with an attractive combination of crystallinity and flexibility, providing intelligent dynamic materials that can respond to external stimuli in a reversible way. The present review article will focus on SOFs showing dynamic behaviour when exposed to different stimuli, highlighting fundamental aspects such as the combination of tectons and supramolecular interactions involved in the framework formation, structure-property relationship and their potential applications.

Tunable Chromic Properties of Viologen-Metal Polymers Modulated by Coordination Modes for Inkless Erasable Printing

Inkless and erasable printing (IEP) based on chromic materials holds great promise to alleviate environmental and sustainable problems. Metal-organic polymers (MOPs) are bright platforms for constructing IEP materials. However, it is still challenging to design target MOPs with excellent specific functions rationally due to the intricate component-structure-property relationships. Herein, an effective strategy was proposed for the rational design IEP-MOP materials. The stimuli-responsive viologen moiety was introduced into the construction of MOPs to give it potential chromic behaviors and two different coordination models (i. e. bilateral coordination model, M1 ; unilateral coordinated model, M2 ) based on the same viologen ligand were designed. Aided by theoretical calculations, model M1 was recommended secondarily as a more suitable system for IEP materials. Along this line, two representative viologen-ZnII MOPs 1 and 2 with models M1 and M2 were synthesized successfully. Experiments exhibit that 1 does have quicker stimuli response, stronger color contrast and longer radical lifetime compared to 2. Significantly, the obtained 1-IEP media brightly inherits the excellent chromic characteristics of 1 and the flexibility of the paper at the same time, which achieves most daily printing requirements, as well as enough resolution and durability to be used in identification by smart device.

Predicting photoactivity in dithienylethene crystalline solids

This commentary discusses the design of stimuli-responsive materials, specifically, light-responsive dithienylethene-based compounds. Recent progress in predicting photoactivity using a combination of theory and crystal structure landscape experiments is highlighted.

Photosalience and Thermal Phase Transitions of Azobenzene- and Crown Ether-Based Complexes in Polymorphic Crystals

Stimuli-responsive molecular crystals have attracted considerable attention as promising smart materials with applications in various fields such as sensing, actuation, and optoelectronics. Understanding the structure-mechanical property relationships, however, remains largely unexplored when it comes to functionalizing these organic crystals. Here, we report three polymorphic crystals (Forms A, B, and C) formed by the non-threaded complexation of a dibenzo[18]crown-6 (DB18C6) ether ring and an azobenzene-based ammonium cation, each exhibiting distinct thermal phase transitions, photoinduced deformations, and mechanical behavior. Structural changes on going from Form A to Form B and from Form C to Form B during heating and cooling, respectively, are observed by single-crystal X-ray crystallography. Form A shows photoinduced reversible bending, whereas Form B exhibits isotropic expansion. Form C displays uniaxial negative expansion with a remarkable increase of 44% in thickness under photoirradiation. Force measurements and nanoindentation reveal that the soft crystals of Form A with a low elastic modulus demonstrate a significant photoresponse, attributed to the non-threaded molecular structure, which permits flexibility of the azobenzene unit. This work represents a significant advance in the understanding of the correlation between structure-thermomechanical and structure-photomechanical properties necessary for the development of multi-stimulus-responsive materials with tailored properties.

One stimulus-induced two-step photophysical response with high contrast and tunable switching time for dynamic displaying

One stimulus-induced two-step photophysical response, especially with tunable switching time, is a great challenge for organic chromophores. Herein, a polymorphic material 2,7-DCF could undergo in situ two sequential dual-channel responses upon dichloromethane fuming. Both the appearance color and the fluorescence change from red to yellow to deep red with high contrast. The first step corresponds to a fast amorphous-to-crystalline transformation, while the second is a slow solid-state cocrystallization process. Based on single crystal structures and theoretical calculations, such distinct color changes are mainly attributed to conformation twisting and the electron coupling with incorporated solvent molecule through C-H⋅⋅⋅O interaction. Importantly, the second slow photophysical response could be drastically sped up by seeding strategy, or be totally inhibited. Such characteristics pave a way for the potential applications in dynamic anti-counterfeiting and data encryption. Based on the two-step transformation, polymorph 2,7-DCF-a could achieve a successive four-level response to external stimuli. In contrast, polymorph 2,7-DCF-d exhibits a stepwise hypsochromic fluorescence shift over 100 nm. This study would significantly promote the development of stimuli-sensitive systems from "one stimulus, one-step response" to "one stimulus, two or multi-step response".

Advanced stimuli-responsive membranes for smart separation

Membranes have been extensively studied and applied in various fields owing to their high energy efficiency and small environmental impact. Further conferring membranes with stimuli responsiveness can allow them to dynamically tune their pore structure and/or surface properties for efficient separation performance. This review summarizes and discusses important developments and achievements in stimuli-responsive membranes. The most commonly utilized stimuli, including light, pH, temperature, ions, and electric and magnetic fields, are discussed in detail. Special attention is given to stimuli-responsive control of membrane pore structure (pore size and porosity/connectivity) and surface properties (wettability, surface topology, and surface charge), from the perspective of determining the appropriate membrane properties and microstructures. This review also focuses on strategies to prepare stimuli-responsive membranes, including blending, casting, polymerization, self-assembly, and electrospinning. Smart applications for separations are also reviewed as well as a discussion of remaining challenges and future prospects in this exciting field. This review offers critical insights for the membrane and broader materials science communities regarding the on-demand and dynamic control of membrane structures and properties. We hope that this review will inspire the design of novel stimuli-responsive membranes to promote sustainable development and make progress toward commercialization.

Control over Phase Transformations in a Family of Flexible Double Diamondoid Coordination Networks through Linker Ligand Substitution

In this work, we present the first metal-organic framework (MOF) platform with a self-penetrated double diamondoid (ddi) topology that exhibits switching between closed (nonporous) and open (porous) phases induced by exposure to gases. A crystal engineering strategy, linker ligand substitution, was used to control gas sorption properties for CO₂ and C3 gases. Specifically, bimbz (1,4-bis(imidazol-1-yl)benzene) in the coordination network X-ddi-1-Ni ([Ni₂(bimbz)₂(bdc)₂(H₂O)]_n, H₂bdc = 1,4-benzenedicarboxylic acid) was replaced by bimpz (3,6-bis(imidazol-1-yl)pyridazine) in X-ddi-2-Ni ([Ni₂(bimpz)₂(bdc)₂(H₂O)]_n). In addition, the 1:1 mixed crystal X-ddi-1,2-Ni ([Ni₂(bimbz)(bimpz)(bdc)₂(H₂O)]_n) was prepared and studied. All three variants form isostructural closed (β) phases upon activation which each exhibited different reversible properties upon exposure to CO₂ at 195 K and C3 gases at 273 K. For CO₂, X-ddi-1-Ni revealed incomplete gate-opening, X-ddi-2-Ni exhibited a stepped isotherm with saturation uptake of 3.92 mol·mol^-1, and X-ddi-1,2-Ni achieved up to 62% more gas uptake and a distinct isotherm shape vs the parent materials. Single-crystal X-ray diffraction (SCXRD) and in situ powder X-ray diffraction (PXRD) experiments provided insight into the mechanisms of phase transformation and revealed that the β phases are nonporous with unit cell volumes 39.9, 40.8, and 41.0% lower than the corresponding as-synthesized α phases, X-ddi-1-Ni-α, X-ddi-2-Ni-α, and X-ddi-1,2-Ni-α, respectively. The results presented herein represent the first report of reversible switching between closed and open phases in ddi topology coordination networks and further highlight how ligand substitution can profoundly impact the gas sorption properties of switching sorbents.

Self-Adaptive Electronic Structure of Amphoteric Conjugated Ligand-Modified 3 d Metal-C3 N4 Smart Electrocatalyst by pH Self-Response Realizing Electrocatalytic Self-Adjustment

Constructing pH-responsive smart material provides a new opportunity to address the problem that traditional electrocatalysts cannot achieve both alkaline oxygen evolution reaction (OER) and acidic hydrogen evolution reaction (HER) activities. In this study, amphoteric conjugated ligand (2-aminoterephthalic acid, BDC-NH₂ )-modified 3d metal-anchored graphitic carbon nitride (3d metal-C₃ N₄ ) smart electrocatalysts are constructed, and self-adaptation of the electronic structure is realized by self-response to pH stimulation, which results in self-adjustment of alkaline OER and acidic HER. Specifically, the amino and carboxyl functional groups in BDC-NH₂ undergo protonation and deprotonation respectively under different pH stimulation to adapt to environmental changes. Through DFT calculations, the increase or decrease of electron delocalization range brought by the self-response characteristic is found to lead to redistribution of the Bader charge around the modified active sites. The OER and HER activities are greatly promoted roughly 4.8 and 8.5 times over Co-C₃ N₄ after BDC-NH₂ -induced self-adaptive processes under different environments, arising from the reduced energy barrier of O* to OOH* and ΔG_H* . Impressively, the proposed BDC-NH₂ -induced smart regulation strategy is applicable to a series of 3d metal anchors for C₃ N₄ , including Co, Ni and Fe, providing a general structural upgrading method for constructing smart electrocatalytic systems.

Engineering Flexible Metal-Phenolic Networks with Guest Responsiveness via Intermolecular Interactions

Flexible metal-organic materials are of growing interest owing to their ability to undergo reversible structural transformations under external stimuli. Here, we report flexible metal-phenolic networks (MPNs) featuring stimuli-responsive behavior to diverse solute guests. The competitive coordination of metal ions to phenolic ligands of multiple coordination sites and solute guests (e.g., glucose) primarily determines the responsive behavior of the MPNs, as revealed experimentally and computationally. Glucose molecules can be embedded into the dynamic MPNs upon mixing, leading to the reconfiguration of the metal-organic networks and thus changes in their physicochemical properties for targeting applications. This study expands the library of stimuli-responsive flexible metal-organic materials and the understanding of intermolecular interactions between metal-organic materials and solute guests, which is essential for the rational design of responsive materials for various applications.

Propagating MOF flexibility at the macroscale: the case of MOF-based mechanical actuators

Shapeshifting materials have captured the imagination of researchers for their myriad potential applications, yet their practical development remains challenging. These materials operate by mechanical actuation: their structural responses to external stimuli generate mechanical work. Here, we review progress on the use of flexible metal-organic frameworks (MOFs) in composite actuators that shapeshift in a controlled fashion. We highlight the dynamic behaviour of flexible MOFs, which are unique among materials, even other porous ones, and introduce the concept of propagation, which involves the efficient transmission of flexible MOF deformations to the macroscale. Furthermore, we explain how researchers can observe, measure, and induce such effects in MOF composites. Next, we review pioneering first-generation MOF-composite actuators that shapeshift in response to changes in humidity, temperature, pressure, or to other stimuli. Finally, we allude to recent developments, identify remaining R & D hurdles, and suggest future directions in this field.

Controllable Transition Metal-Directed Assembly of [Mo2O2S2]2+ Building Blocks into Smart Molecular Humidity-Responsive Actuators

Smart molecular actuators have become a cutting-edge theme due to their ability to convert chemical energy into mechanical energy under external stimulations. However, realizing actuation at the molecular level and elucidating the mechanisms for actuating still remain challenging. Herein, we design and fabricate a novel nanoscaled polyoxometalate-based humidity-responsive molecular actuator {Bi₈Mo₄₈} through the assembly of [Mo₂O₂S₂]²⁺ units, transition metals, and flexible phosphonic acid ligands. {Bi₈Mo₄₈} exhibits a semi-flexible cage-like architecture with oxygen-rich surfaces and highly negative charges 72-. The nanoscaled molecular actuator shows reversible expansion and contraction behavior under humidity variations due to lattice expansion and contraction induced by hydrogen bonding and solvation interactions between {Bi₈Mo₄₈} and water molecules. Molecular dynamics simulation was further employed to study these processes, which provides a fundamental understanding for the mechanism of humidity actuation at the molecular level.

Supramolecular Polymers with Photoswitchable Multistate Fluorescence for Anti-Counterfeiting and Encryption

Photoswitchable fluorescent materials are desirable for many applications because their emission signals can be easily modulated on demand. In this study, novel photoswitchable multistate fluorescent supramolecular polymers (PMFSPs) were prepared via host-guest interactions under a facile ultrasonication strategy. In the system, photochromic fluorescent diarylethylene monomer (SDTE, donor) and adamantane-containing monomer (BAC) were covalently combined into the backbone of the guest polymer (P1) via radical copolymerization. Meanwhile, the host moiety (CDSP, acceptor) was synthesized by covalent incorporation of photochromic spiropyran dye (SPCOOH) with β-cyclodextrin. By adjusting the stimulation wavelength and utilizing photoinduced fluorescence resonance energy transfer (FRET), the supramolecular polymers can undergo reversible tristate fluorescence switching among none, red, and green. In addition, due to the high contrast, rapid photoresponsiveness and prominent photoreversibility of the prepared PMFSPs, we demonstrated that they have great potential in advanced anti-counterfeiting and multilevel information encryption.

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.

Photomechanical Motions in Organoboron-Based Phosphorescent Molecular Crystals Driven by a Crystal-State [2 + 2] Cycloaddition Reaction

Photoluminescent molecular crystals integrated with the ability to transform light energy into macroscopic mechanical motions are a promising choice of materials for both actuating and photonic devices. However, such dynamic photomechanical effects, based on molecular organoboron compounds as well as phosphorescent crystalline materials, are not yet known. Here we present an intriguing example of photomechanical molecular single crystals of a newly synthesized organoboron containing Lewis acid-base molecular adduct (BN1, substituted triphenylboroxine and 1,2-di(4-pyridyl)ethylene) having a capsule shape molecular geometry. The single crystals of BN1 under UV light exhibit controllable rapid bending-shape recovery, delamination, violent splitting-jumping, and expanding features. The detailed structural investigation by single-crystal X-ray diffraction and ¹H NMR spectroscopy reveals that the photosalient behavior of the BN1 single crystals is driven by a crystal-to-crystal [2 + 2] cycloaddition reaction, supported by four donor-acceptor type B←N bonds. The instant photomechanical reaction in the BN1 crystals occurs under UV on account of sudden release of stress associated with the strained molecular geometry, significant solid-state molecular movements (supramolecular change), and cleavage of half intermolecular B←N linkages to result in a complete photodimerized single-crystalline product via the existence of two other intermediate photoproducts. In addition, the BN1 crystals display short-lived room temperature phosphorescence, and the photodynamic events are accompanied by the enhancement of their phosphorescence intensity to yield the photoproduct. Interestingly, the molecular crystals of the final photoproduct polymerize at ambient conditions when recrystallized from the solution forming a 2D supramolecular crystalline polymer stabilized by the retention of all B←N coordination modes.

Delicate and Independent Manipulation of Dynamic Fluorescence Behavior of Polymer Nanoparticles Based on a Core-Shell Strategy

Fluorescent polymer nanomaterials with dynamic fluorescence properties hold great potential in many advanced applications, including but not limited to information encryption, adaptive camouflage, and biosensors. The key to improving the application value of materials is to establish an accurate control strategy for dynamic fluorescence behavior. Herein, we develop a core-shell engineering strategy to precisely and independently manipulate the dynamic fluorescence behavior through the shell polymeric matrix. The core-shell fluorescent polymer nanoparticles (CS-FPNPs) are constructed through a sequential process of miniemulsion polymerization and seeded emulsion polymerization. Taking advantage of the core-shell structure, the rigid core matrix ensures the strong initial emission of AIE units, while the photoisomerization behavior of spiropyrane (SP) units is delicately and independently regulated by the rigidness of the shell matrix. Thereby, CS-FPNPs exhibit bright time-dependent reversible dynamic fluorescence behavior under alternating UV/vis irradiation. Benefited from the excellent processability and film formation ability, we have successfully applied CS-FPNPs to dynamic decorative painting, warning labels, and dynamic QR code security. Impressively, the fluorescence manipulation strategy based on core-shell engineering allows the independent regulation of specific luminescent units in complicated emission systems to accurately embody designed emission behavior.

Photoresponsive Metal-Organic Frameworks as Adjustable Scaffolds in Reticular Chemistry

The easy and remote switching of light makes this stimulus an ideal candidate for a large number of applications, among which the preparation of photoresponsive materials stands out. The interest of several scientists in this area in order to achieve improved functionalities has increase parallel to the growth of the structural complexity of these materials. Thus, metal-organic frameworks (MOFs) turned out to be ideal scaffolds for light-responsive ligands. This review is focused on the integration of photoresponsive organic ligands inside MOF crystalline arrays to prepare enhanced functional materials. Besides the summary of the preparation, properties and applications of these materials, an overview of the future outlook of this research area is provided.