Stimuli-responsive Materials Research Articles

Dynamic, multi-color, multi-level stimulus-responsive luminescence in a co-crystallized coordination polymer: Modulation of energy transfer by acid-induced ligand protonation and photo-induced radical generation

Stimulus-responsive luminescent materials (SLMs), which can respond to external stimuli by switching their luminescence properties, hold great promise for a variety of applications. Particularly, multi-level SLMs with a logical stimulus sequence and a dynamic, multi-color transition are highly desirable for anti-counterfeiting and information encryption, but an effective design strategy is lacking. Herein, a co-crystallized coordination polymer multi-level SLM [Cd(9-AC)4]·(HAD)2·(9-HAC)2 (9-HAC for anthracene-9-carboxylic acid and AD for acridine) with dual emission centers of coordination unit ([Cd(9-AC)4]2-) and exciplex ([9-HAC/HAD]+) has been successfully constructed by a stepwise solvent process. This SLM possesses a variety of stimulus-responsive properties, including mechanochromic (MC), mechanochromic luminescence (MCL), reversible acidochromic luminescence (ACL), and reversible photochromic luminescence (PCL). Notably, both ACL and PCL properties are achieved by modulating the energy transfer process between the coordination unit and the exciplex, while there is a logical sequence between these two stimulus actions. The acid-induced activation and radical-regulated PCL performance has been exploited in the development of anti-counterfeiting and information encryption applications. This work provides an effective strategy to realize dynamic, multi-color, multi-level stimuli-responsive luminescence by modulating the energy transfer through acid-induced ligand protonation and photo-induced radical generation.

Photodegradable glyco-microfibers fabricated by the self-assembly of cellobiose derivatives bearing nitrobenzyl groups

Stimuli-responsive materials constructed via the self-assembly of small biomolecules are attracting increasing attention because of their biocompatibility, sustainability, and variety of (bio)applications. Nevertheless, the research on oligosaccharide-based molecular designs for such stimuli-responsive materials (or stimuli-responsive glyco-materials) is limited, partly due to the intrinsic structural diversity of oligosaccharides and the difficulty associated with their selective chemical syntheses. Herein, we report the construction of photodegradable glyco-microfibers by the self-assembly of cellobiose derivatives bearing nitrobenzyl groups. The atomic-scale, self-assembled architecture of the photodegradable glyco-microfibers is unveiled and compared with those of pristine cellobiose and cellulose polymorphs in previous reports.

Oxindolyl-Based Radicals with Tunable Mechanochromic and Thermochromic Behavior.

Organic radicals based dynamic covalent chemistry is promising in preparing stimuli-responsive chromic materials, due to their simplicity of dissociation/association, accompanied with distinct color changes during the process. However, suitable organic radicals for dynamic covalent chemistry have not been widely explored yet. Herein, a series of oxindolyl-based mono-radicals (OxRs) with different substituents were successfully synthesized and studied systematically as potential building blocks for stimuli-responsive chromic materials. These OxRs would dimerize spontaneously to form their corresponding dimers. The structures of dimers were unambiguously confirmed through low-temperature 1H NMR and single-crystal X-ray diffraction analyses. Dynamic interconversion between monomers and dimers was achieved by reversible cleavage and recovery of the σ-bond upon soft external stimuli (temperature, pressure, and solvent polarity), accompanied by significant color changes. It is interesting that the stability of the mono-radical could be tuned through changing different substituents, and consequently altering the bond dissociation energy of the dynamic covalent bond between monomers. These new OxRs characterized by appreciable properties are entitled to more opportunities in developing mechanochromic and thermochromic materials, where their responsiveness to stimuli can be readily controlled by the substituents adhered.

A multi-dimensional anti-counterfeiting nanocomposite based on fluorescent CDs and Eu-MOFs with dual function for continuous detection of Cu2+ and Bacillus anthracis

Traditional luminescent materials for anti-counterfeiting usually adopt encryption of low-level systems with limited security, which seriously hinders their application in preventing counterfeiting and information leakage. Therefore, it is urgent to develop materials for higher-level anti-counterfeiting. In this work, a stimulus-responsive intelligent luminescent material (AC@CDs-Eu-MOFs) was prepared by co-loading green-fluorescence CDs and red-fluorescence Eu-MOFs on Amino clay (AC). 5D security barcodes, which were easy to observe while difficult to clone, were ulteriorly designed based on the nanocomposite owing to the tunable fluorescence by optical stimulation and chemical stimulus. Owing to the large capacity, low cost, and easy authentication, the 5D security barcodes possess enormous potential in optical data storage and multi-dimensional information encryption. In addition, the chemical stimulus-response enables the nanocomposite achievable in detection of Cu2+ and Bacillus anthracis. Particularly, quantitative determination of copper in environmental water samples could be realized with a detection limit as low as 10.67 nM. Therefore, the material shows potential for detection of environmental pollutants besides advanced anti-counterfeiting.

Dual-response signal under mechanical stimulation: Alkyl substituent effect on the fine-tuning of molecular packing

Mechanical stimulus-responsive materials are generally considered to be closely related to molecular packing in the solid state, underscoring the significance of regulating molecular packing via precise molecular design. In this study, flexible alkyl substituents were employed to adjust the molecular packing. By modifying alkyl substituents, predictable changes in the molecular packing of their derivatives were observed, resulting in adjustable ML properties. The DPA-PYZ-TB with a tert-butyl-side chain exhibited strong ML and MC stimulus response signals. The enhanced ML and MC performance can be attributed to the effective suppression of non-radiation through the helical crystal arrangement and strong intermolecular interactions. The relationship between molecular arrangement and ML/MC properties was validated through analysis of the single crystal structure and corresponding experimental results. This study offers valuable insights into the enigmatic ML process and the development of efficient luminous materials that utilize both MC and ML.

Investigating the nonlinear dynamics of photosensitive Belousov-Zhabotinsky gels via bifurcation analyses.

Controlling the dynamics of active stimuli-responsive smart materials is essential to replicate the biomimetic functionalities at different length scales for a variety of biological systems-based applications. Photosensitive Belousov-Zhabotinsky (BZ) gels, powered by a nonlinear chemical oscillator, called a BZ reaction are one of the stimuli-responsive smart materials in demand due to their ability to continuously transduce chemical oscillations into mechanical deformations. The chemical oscillations in a BZ reaction and subsequent mechanical oscillations in photosensitive BZ gels occur due to the redox cycle of photosensitive ruthenium complex-based catalysts. In this work, our objective is to identify how the behavior of photosensitive BZ gels can be tuned and used for biomimetic applications by investigating its dynamical characteristics using bifurcation analyses. Specifically, we use the normal form approach and perform linear and nonlinear stability analyses to identify high-order bifurcations by computing higher-order Lyapunov and frequency coefficients. We revealed the existence of domains that encompass coexisting stable and unstable limit cycles (LCs), which merge to form a semi-stable LC at the limit point of cycle (LPC). Their existence shows how a slight variation in the BZ gel recipe can significantly alter its dynamics. Subsequently, we quantify the amplitude and frequency of oscillations in different domains under the effect of variation of BZ reaction formulations. We believe that the outcomes of our work serve as an efficient template for the design and control of BZ gel-based applications. The usage of a normal form and a systematic representation of nonlinear dynamics allow our framework to be extended for other nonlinear dynamical systems.

Retinomorphic Color Perception Based on Opponent Process Enabled by Perovskite Bipolar Photodetectors.

The ability to perceive color by the retina can be attributed to both its trichromatic photoreceptors and the antagonistic neural wiring known as the opponent process. While neuromorphic sensors have been shown to demonstrate memory and adaptation capabilities, color perception is still challenging due to the intrinsic lack of spectral selectivity in narrow bandgap semiconductors. Furthermore, research on emulating neural wiring is severely lacking. The combination of halide perovskite materials with a tunable bandgap and a novel bipolar photodetector design emulates the efficiency of the retina in processing color information. The stimuli-responsive material is also responsible for maintaining partial color constancy-an adaptation feature. Leveraging the unique enhancement of color contrasts, an in-sensor data compression and edge detection can also be demonstrated. The color perception, chromatic adaptation, and color contrast enhancement make perovskite bipolar photodetectors a unique example where the sensor and neural wiring can be co-developed in conjunction.

Ceramic/Polymer Nanodielectrics: Towards a Multifunctional or Smart Performance

Ceramic nanoparticles/polymer composite nanodielectrics constitute a novel class of engineering materials, which exhibit tunable mechanical, thermal, electrical and magnetic properties, energy storing/recovering ability in tandem with processability and corrosion resistance. By these means the produced nanocomposites are characterized by multifunctionality since structural components are able to perform a wide range of desirable functions. When the embedded nanoparticles are polar oxides and/or piezo/ferro-electrics, or any kind of stimuli responsive materials additional functional performance is induced in the nanocomposites, providing a suitable basis for the development of smart materials.

A water strider-inspired intestinal stent actuator for controllable adhesion and unidirectional biofluid picking

Soft-bodied aquatic organisms exhibit extraordinary navigation and mobility in liquid environments which inspiring the development of biomimetic actuators with complex movements. Stimulus-responsive soft materials including hydrogels and shape-memory polymers are replacing traditional rigid parts that leading to dynamic and responsive soft actuators. In this study, we took inspiration from water strider to develop a biomimetic actuator for targeted stimulation and pH sensing in the gastrointestinal tract. We designed a soft and water-based Janus adhesive hydrogel patch that attaches to specific parts of the intestine and responds to pH changes through external stimulation. The hydrogel patch that forms the belly of the water strider driver incorporates an inverse opal microstructure that enables pH responsive behavior. The hydrogel patch on the water strider's leg uses a sandwich structure of Cu particles to convert light into heat and bend under infrared light to mimic the water strider's leg simulating the efficient and steady movement of the water strider's leg which transporting the biological fluid in one direction. This miniature bionic actuator demonstrates controlled adhesion and unidirectional biofluid delivery capabilities, proving its potential for targeted stimulus response and pH sensing in the gastrointestinal tract, thus opening up new possibilities for medical applications in the growing field of soft actuators.

Design, synthesis, solvatochromism, and colorimetric detection of amines based on isolated perylene bisimide anionic radical π-dimers

Some π-molecular radicals typically exist in solution as π-dimers with metastable characteristics. Therefore, attempting to improve the stability of radical π-dimers is a fascinating endeavor. Herein, a series of hydroxyl-functionalised isolated perylene bisimide anionic radical π-dimers were obtained by using intramolecular hydrogen bond assistance in a one-step process for the first time. It is also found that these radical π-dimers exhibit solvatochromism and acid-base discoloration, when the intermolecular hydrogen bonds are disrupted. Perylene bisimide anionic radical π-dimers exhibit unique properties such as solvatochromism and acid-base discoloration. These characteristics make them suitable for ultra-sensitive colorimetric and fluorescent dual-signal detection of amines. Among them the π-dimer 3 achievable the lowest detection limits of 0.13 ppb, 0.02 ppb and 0.049 ppb for primary, secondary and tertiary amines, respectively. Additionally, colorimetric films prepared using these radical π-dimers demonstrate high sensitivity, enabling detection of amines at ppb levels. We believe that this class of radical π-dimers could be develop into a new class of environmental stimuli-responsive materials.

Photo/thermal dual-responsive azobenzene-based photosensitive resin for 4D printing

4D printing, combining the advantages of 3D printing and stimuli-responsive materials, provides a precise and rapid fabrication method for complex-shaped smart objects. In this study, the photoisomerization characteristic of azobenzene is used to develop 4D printable resins composed of azobenzene-based acrylates (AZO) and other acrylate compounds, which can be UV-cured and 3D printed into objects with photo and thermal dual responsiveness. UV-curing kinetics reveal that AZO competes with the photoinitiator for photons during photopolymerization, reducing the generation of reactive oxygen species and subsequently affecting the mechanical properties of UV-cured resin. The optimum content of AZO in 4D printing resins is 0.5 wt%, producing 3D printed objects with excellent mechanical properties, including a maximum stress of 25.2 MPa and a strain of 100 %. This ensures the successful formation of complex-shaped objects in the 3D printing process. Moreover, the objects printed with this photosensitive resin demonstrate exceptional shape memory capabilities, achieving a 100 % shape recovery ratio over three cycles of thermal stimulation. The printed openwork spheroid with a concave shape can successfully revert to its original form after 48 s of irradiation with 365 nm UV light. The degree of shape recovery can be precisely controlled by adjusting the UV irradiation time and position. Therefore, these photo/thermal dual-responsive 4D objects show great application prospects in medical devices and smart materials.

Mechanochromic Displays Based on Photoswitchable Cholesteric Liquid Crystal Elastomers

Stimuli responsive optical materials are attractive for many areas, from healthcare to art design. However, creating intricate color‐changing patterns for visual information is still a challenge. This work describes the preparation of mechanochromic structural colored intricate pictures imprinted in cholesteric liquid crystal elastomers by using a chiral isosorbide molecular photo‐switch. The photo‐switch contains a photoisomerizable cinnamate moiety and was incorporated in a main chain liquid crystal oligomer with photopolymerizable acrylate end groups. After coating, the structural colored film was irradiated with ultraviolet (UV) light in air causing E/Z isomerization of the cinnamate units leading to a redshift of the structural color of the film. A grayscale photomask was used to spatially control the photoisomerization reaction and imprint colorful pictures such as portraits and landscapes, in the cholesteric liquid crystal films with high resolution. Photopolymerization in a nitrogen atmosphere led to a mechanochromic cholesteric liquid crystal elastomer with striking structural colors that blueshift upon strain independently. The sharp details of the patterns were preserved even under deformation and the system returned to the initial state upon strain removal. Our work offers a simple photo‐switch approach to prepare stimuli responsive optical polymers imprinted with color‐changing pictures of unprecedented complexity.

Mechanochromic Displays Based on Photoswitchable Cholesteric Liquid Crystal Elastomers.

Stimuli responsive optical materials are attractive for many areas, from healthcare to art design. However, creating intricate color-changing patterns for visual information is still a challenge. This work describes the preparation of mechanochromic structural colored intricate pictures imprinted in cholesteric liquid crystal elastomers by using a chiral isosorbide molecular photo-switch. The photo-switch contains a photoisomerizable cinnamate moiety and was incorporated in a main chain liquid crystal oligomer with photopolymerizable acrylate end groups. After coating, the structural colored film was irradiated with ultraviolet (UV) light in air causing E/Z isomerization of the cinnamate units leading to a redshift of the structural color of the film. A grayscale photomask was used to spatially control the photoisomerization reaction and imprint colorful pictures such as portraits and landscapes, in the cholesteric liquid crystal films with high resolution. Photopolymerization in a nitrogen atmosphere led to a mechanochromic cholesteric liquid crystal elastomer with striking structural colors that blueshift upon strain independently. The sharp details of the patterns were preserved even under deformation and the system returned to the initial state upon strain removal. Our work offers a simple photo-switch approach to prepare stimuli responsive optical polymers imprinted with color-changing pictures of unprecedented complexity.

Functional Upcycling of Polyurethane Thermosets into Value-Added Thermoplastics via Small-Molecule Carbamate-Assisted Decross-Linking Extrusion.

The cross-linked structures of most commodity polyurethanes (PUs) hinder their recycling by common mechanical/chemical approaches. Catalyzed dynamic carbamate exchange emerges as a promising PU recycling strategy, which converts traditional static PU thermosets into reprocessable covalent adaptable networks (CANs). However, this approach has been limited to thermoset-to-thermoset reprocessing of PU CANs, accompanied by their well-preserved network structures and extremely high viscosities, which pose challenges to processing and certain applications. This study reports a catalytic decross-linking extrusion process aided by small-molecule carbamates, which can upcycle PU thermosets into easily processable and functional PU thermoplastics in a solvent-free and high-throughput manner. Key to this process is the employment of small-molecule carbamates as decross-linkers to simultaneously deconstruct cross-linked PUs and functionalize the decross-linked PU chains, through catalyzed carbamate exchange reactions in a twin-screw extruder. This strategy applies to both aromatic and aliphatic cross-linked PU films and foams, and the amount of small-molecule carbamates required to decross-link PU networks is determined through thermal, chemical, and structural analyses of the resulting extrudates. This approach is generalizable to small-molecule carbamates with various steric/electronic structures and chemical functionalities including methacrylate, anthracene, and stilbene groups. The chain-end functionalization is confirmed by analyzing the purified decross-linked extrudates after dialysis. This thermoset-to-thermoplastic extrusion process represents a powerful approach for upcycling postconsumer PU thermosets into a library of thermoplastic PUs with controlled molecular weights and chain-end functionalities for diverse applications, including adhesives, photoresins, and stimuli-responsive materials, as demonstrated herein. In the future, this strategy could be extended to upcycle many other step-growth networks capable of undergoing catalytic bond exchange reactions, such as cross-linked polyureas and polyesters, contributing to plastic waste management in general.

Numerical model of the locomotion of oscillating ‘robots’ with frictional anisotropy on differently-structured surfaces

In engineering materials, surface anisotropy is known in certain textured patterns that appear during the manufacturing process. In biology, there are numerous examples of mechanical systems which combine anisotropic surfaces with the motion, elicited due to some actuation using muscles or stimuli-responsive materials, such as highly ordered cellulose fiber arrays of plant seeds. The systems supplemented by the muscles are rather fast actuators, because of the relatively high speed of muscle contraction, whereas the latter ones are very slow, because they generate actuation depending on the daily changes in the environmental air humidity. If the substrate has ordered surface profile, one can expect certain statistical order of potential trajectories (depending on the order of the spatial distribution of the surface asperities). If not, the expected trajectories can be statistically rather random. The same presumably holds true for the artificial miniature robots that use actuation in combination with frictional anisotropy. In order to prove this hypothesis, we developed numerical model helping us to study abovementioned cases of locomotion in 2D space on an uneven terrain. We show that at extremely long times, these systems tends to behave according to the rules of ballistic diffusion. Physically, it means that their motion tends to be associated with the “channels” of the patterned substrate. Such a motion is more or less the same as it should be in the uniform space. Such asymptotic behavior is specific for the motion in model regular potential and would be impossible on more realistic (and complex) fractal reliefs. However, one can expect that in any kind of the potential with certain symmetry (hexagonal or rhombic, for example), where it is still possible to find the ways, the motion along fixed direction during long (or even almost infinite) time intervals is possible.

Aggression to Biomembranes by Hydrophobic Tail Chains under the Stimulus of Reductant.

Stimulus-responsive materials hold significant promise for antitumor applications due to their variable structures and physical properties. In this paper, a series of peptides with a responsive viologen derivative, Pep-CnV (n = 1, 2, 3) were designed and synthesized. The process and mechanism of the interaction were studied and discussed. An ultraviolet-visible (UV) spectrophotometer and fluorescence spectrophotometer were used to study their redox responsiveness. Additionally, their secondary structures were measured by Circular Dichroism (CD) in the presence or absence of the reductant, Na2SO3. DPPC and DPPG liposomes were prepared to mimic normal and tumor cell membranes. The interaction between Pep-CnV and biomembranes was investigated by the measurements of surface tension and cargo leakage. Results proved Pep-CnV was more likely to interact with the DPPG liposome and destroy its biomembrane under the stimulus of the reductant. And the destruction increased with the length of the hydrophobic tail chain. Pep-CnV showed its potential as an intelligent antitumor agent.

Stimuli-responsive materials in oral diseases: a review.

Oral diseases, such as dental caries, periodontitis, and oral cancers, are highly prevalent worldwide. Many oral diseases are typically associated with bacterial infections or the proliferation of malignant cells, and they are usually located superficially. Articles were retrieved from PubMed/Medline, Web of Science. All studies focusing on stimuli-responsive materials in oral diseases were included and carefully evaluated. Stimulus-responsive materials are innovative materials that selectively undergo structural changes and trigger drug release based on shifts at the molecular level, such as changes in pH, electric field, magnetic field, or light in the surrounding environment. These changes lead to alterations in the properties of the materials at the macro- or microscopic level. Consequently, stimuli-responsive materials are particularly suitable for treating superficial site diseases and have found extensive applications in antibacterial and anticancer therapies. These characteristics make them convenient and effective for addressing oral diseases. This review aimed to summarize the classification, mechanism of action, and application of stimuli-responsive materials in the treatment of oral diseases, point out the existing limitations, and speculate the prospects for clinical applications. Our findings may provide useful information of stimuli-responsive materials in oral diseases for dental clinicians.

Aggregation-Induced Emission Enhancement and Solid-State Photoswitching of Crystalline Carbazole N-Salicylidene Anilines.

The development of fluorescent stimuli-responsive organic materials has attracted substantial interest due to their increasing optoelectronic applications. This study systematically introduces fluorine atoms on one end of carbazole-based N-salicylidene anilines 5a-5f to elucidate the impact in their solution and solid-state photophysics. The addition of fluorine atoms at one end of the molecule induced significant changes, for example, a reduction in the quantum yield (QY) fluorescence emission in solution, going from QY near unity in compound 5a (QY ∼ 100%) to a negligible emission in 5f (QY < 1%). Similarly, compound 5a showed a very strong aggregation-induced enhancement emission behavior, whereas compounds with a higher fluorine content were almost quenched. Furthermore, the crystalline solid-state photoisomerization in N-salicylidene anilines is not trivial, and only compounds with three (5e) and five fluorine atoms (5f) exhibited reversible solid-state photoisomerization under 405 nm light source irradiation. We propose that the presence of the arene-perfluoroarene interaction in the crystalline array facilitates the latter behavior. Our findings present a comprehensive study of crystal engineering for the obtention of photoswitchable crystalline materials and adjustable photophysics response, paving the way for its implementation in other systems.

Hierarchical Dual-Mode Efficient Tunable Afterglow via J-Aggregates in Single-Phosphor-Doped Polymer.

The development of polymer-based persistent luminescence materials with color-tunable organic afterglow and multiple responses is highly desirable for applications in anti-counterfeiting, flexible displays, and data-storage. However, achieving efficient persistent luminescence from a single-phosphor system with multiple responses remains a challenging task. Herein, by doping 9H-pyrido[3,4-b]indole (PI2) into an amorphous polyacrylamide matrix, a hierarchical dual-mode emission system is developed, which exhibits color-tunable afterglow due to excitation-, temperature-, and humidity-dependence. Notably, the coexistence of the isolated state and J-aggregate state of the guest molecule not only provides an excitation-dependent afterglow color, but also leads to a hierarchical temperature-dependent afterglow color resulting from different thermally activated delayed fluorescence (TADF) and ultralong organic phosphorescence (UOP) behaviors of the isolated and aggregated states. The complex responsiveness based on the hierarchical dual-mode emission can serve for security features through inkjet printing and ink-writing. These findings may provide further insight into the regulated persistent luminescence by isolated and aggregated phosphors in doped polymer systems and expand the scope of stimuli-responsive organic afterglow materials for broader applications.

Narrowband Blue Circularly Polarized Luminescence Emitter Based on BN-Doped Benzo[6]helicene with Stimuli-Responsive Properties.

Boron-doped helicenes, known for their unique electronic and photophysical properties, are of great interest for numerous applications. This research introduces two new azabora[6]helicenes, H[6]BN1 and H[6]BN2, synthesized through an efficient method. These molecules have boron and nitrogen atoms in opposing positions, enhancing their distinctive attributes. Both helicenes show excellent emission properties, with H[6]BN1 and H[6]BN2 exhibiting narrowband blue fluorescence and circularly polarized luminescence (CPL), achieving glum values of 4~5×10-4 which is beneficial for chiroptical applications. The addition of a donor group, 3, 6-di-tert-butyl-9H-carbazole, in H[6]BN2 improves luminescence, likely due to enhanced molecular orbital overlap and electron delocalization. H[6]BN1's needle-like single crystals exhibit mechanochromism, changing luminescent color from yellow to green under mechanical stress, which is promising for stimulus-responsive materials. In conclusion, this study presents a novel class of BN[6]helicenes with superior chiroptical properties. Their combination of electronic features and mechanochromism makes them ideal for advanced chiroptical materials, expanding the potential of helicene-based compounds and offering new directions for the synthesis of molecules with specific chiroptical characteristics.