Structural Design and Application of Azo-based Supramolecular Polymer Systems

Study on Photodeformation of Solvent Resistance in Hydrogen-Bonded Cross-Linked Main-Chain Azobenzene Films

Hydrogen-bonded cross-linked main chain azobenzene (azo) photoactive polymers have broad application prospects in flexible actuators, optical actuators, and other fields. Most of the research on this kind of photoresponsive material is mainly focused on air, and exploration in solvents remains underexplored. In this paper, azobenzene polyamide ester semicrystalline polymer (PEA-6T) with hydrogen-bond cross-linking was synthesized by Michael addition polymerization. The uniaxially oriented polymer film with high orientation (48.85%) and fast response (5 s under UV light and 55 s under visible light) was obtained by a simple solution casting/mechanical stretching method. Compared with PEA-2T and PEA-4T, PEA-6T exhibits enhanced mechanical properties (elastic modulus increased by 17.4%; yield strength increased by 34.1%; breaking strength increased by 75.4%; elongation at break increased by 33.8%; toughness increased by 101.3%; photoinduced stress increased by 43.5%) and reduced light response time (decreased by 58.3% in ultraviolet light and 50% in visible light) due to the elongation of the compliant chain length. The thin PEA-6T film exhibited light-induced deformation not only in air but also in polar solvents such as water, methanol, ethanol, butanol, and saline solutions (e.g., normal saline, 0.9 wt% NaCl, and simulated seawater, 3.5 wt% NaCl). In addition, polarizing optical microscope (POM) observations showed that the brightness and texture direction of the films remained stable (ΔBrightness < 5%), the light response time was consistent (6 s under UV light, 65 s under visible light), the light-induced stress retention rate was 95%, and the films exhibited good solvent resistance. This study bridges the research gap in azobenzene photoresponsive materials in solvent environments, and the material shows potential for applications in marine equipment coatings or biomedical actuators.

Visual Sensor with Host-Guest Specific Recognition and Light-Electrical Co-Controlled Switch

Perception of UV radiation has important applications in medical health, industrial production, electronic communication, etc. In numerous application scenarios, there is an increasing demand for the intuitive and low-cost detection of UV radiation through colorimetric visual behavior, as well as the efficient and multi-functional utilization of UV radiation. However, photodetectors based on photoconductive modes or photosensitive colorimetric materials are not conducive to portable or multi-scene applications owing to their complex and expensive photosensitive components, potential photobleaching, and single-stimulus response behavior. Here, a multifunctional visual sensor based on the "host-guest photo-controlled permutation" strategy and the "lock and key" model is developed. The host-guest specific molecular recognition and electrochromic sensing platform is integrated at the micro-molecular scale, enabling multi-functional and multi-scene applications in the convenient and fast perception of UV radiation, military camouflage, and information erasure at the macro level of human-computer interaction through light-electrical co-controlled visual switching characteristics. This light-electrical co-controlled visual sensor based on an optoelectronic multi-mode sensing system is expected to provide new ideas and paradigms for healthcare, microelectronics manufacturing, and wearable electronic devices owing to its advantages of signal visualization, low energy consumption, low cost, and versatility.

New Prospects Arising from Dynamically Crosslinked Polymers: Reprogramming Their Properties

Dynamically crosslinked polymers (DCPs) have gained significant attention owing to their applications in fabricating (re)processable, recyclable, and self-healable thermosets, which hold great promise in addressing ecological issues, such as plastic pollution and resource scarcity. However, the current research predominantly focuses on redefining and/or manipulating their geometries while replicating their bulk properties. Given the inherent design flexibility of dynamic covalent networks, DCPs also exhibit a remarkable potential for various novel applications through postsynthesis reprogramming their properties. In this review, the recent advancements in strategies that enable DCPs to transform their bulk properties after synthesis are presented. The underlying mechanisms and associated material properties are overviewed mainly through three distinct strategies, namely latent catalysts, material-growth, and topology isomerizable networks. Furthermore, the mutual relationship and impact of these strategies when integrated within one material system are also discussed. Finally, the application prospects and relevant issues necessitating further investigation, along with the potential solutions are analyzed.

Tunable Fluorescence, Morphology, and Antibacterial Behaviors of Conjugated Oligomers via Host-Guest Supramolecular Self-Assembly

Host-guest supramolecular self-assembly has become one facile but efficient way to regulate the optical properties of conjugated oligomers and construct promising photofunctional materials. Herein, we design two linear conjugated oligomers terminated with two or four pyridinium moieties, which show different 1:1 'head-to-tail' binding patterns with cucurbit[8]uril (CB[8]) to form host-guest supramolecules. After being encapsulated in the hydrophobic cavity of the CB[8] host, the fluorescence emission of the conjugated oligomers undergoes significant changes, resulting in tunable fluorescence color with enhanced quantum yields. Triggered by the aggregation of supramolecules, the regular or rigid binding modes lead to the formation of cuboids and spheroids in nanoscale, respectively. Due to the macrocyclic-confinement effect, the light-driven reactive oxygen species (ROS) production of the host-guest complex is increased significantly, thereby improving the photodynamic antibacterial performance toward Staphylococcus aureus (S. aureus).

Programming actuation onset of a liquid crystalline elastomer via isomerization of network topology

Tuning actuation temperatures of liquid crystalline elastomers (LCEs) achieves control of their actuation onsets, which is generally accomplished in the synthesis step and cannot be altered afterward. Multiple actuation onsets in one LCE can be encoded if the post-synthesis regulation of actuation temperature can be spatiotemporally achieved. This would allow realizing a logical time-evolution of actuation, desired for future soft robots. Nevertheless, this task is challenging given the additional need to ensure mesogen alignment required for actuation. We achieved this goal with a topology isomerizable network (TIN) of LCE containing aromatic and aliphatic esters in the mesogenic and amorphous phases, respectively. These two ester bonds can be distinctly activated for transesterification. The homolytic bond exchange between aliphatic esters allows mechanically induced mesogen alignment without affecting the mesogenic phase. Most importantly, the heterolytic exchange between aromatic and aliphatic esters changes the actuation temperature under different conditions. Spatial control of the two mechanisms via a photo-latent catalyst unleashes the freedom in regulating actuation temperature distribution, yielding unusual controllability in actuation geometries and logical sequence. Our principle is generally applicable to common LCEs containing both aromatic and aliphatic esters.

Progress in Research and Application of Metal-Organic Gels: A Review

In recent years, metal-organic gels (MOGs) have attracted much attention due to their hierarchical porous structure, large specific surface area, and good surface modifiability. Compared with MOFs, the synthesis conditions of MOGs are gentler and more stable. At present, MOGs are widely used in the fields of catalysis, adsorption, energy storage, electrochromic devices, sensing, analysis, and detection. In this paper, literature metrology and knowledge graph visualization analysis are adopted to analyze and summarize the literature data in the field of MOGs. The visualization maps of the temporal distribution, spatial distribution, authors and institutions' distribution, influence of highly cited literature and journals, keyword clustering, and research trends are helpful to clearly grasp the content and development trend of MOG materials research, point out the future research direction for scholars, and promote the practical application of MOGs. At the same time, the paper reviews the research and application progress of MOGs in recent years by combining keyword clustering, time lines, and emergence maps, and looks forward to their challenges, future development trend, and application prospects.

Rapidly and Repeatedly Reprogrammable Liquid Crystalline Elastomer via a Shape Memory Mechanism

Realization of muscle-like actuation for a liquid crystal elastomer (LCE) requires mesogen alignment, which is typically achieved/fixed chemically during the synthesis. Post-synthesis regulation of the alignment in a convenient and repeatable manner is highly desirable yet challenging. Here, a dual-phase LCE network is designed and synthesized with a crystalline melting transition above a liquid crystalline transition. The crystalline phase can serve as an "alignment frame" to fix any mechanical deformation via a shape memory mechanism, leading to corresponding mesogen alignment in the liquid crystalline phase. The alignment can be erased by melting, which can be the starting point for reprogramming. This strategy that relies on a physical shape memory transition for mesogen alignment permits repeated reprogramming in a timescale of seconds, in stark contrast to typical methods. It further leads to unusual versatility in designing 3D printed LCE with unlimited programmable actuation modes.

Multi-photoresponsive triphenylethylene derivatives with photochromism, photodeformation and room temperature phosphorescence

A series of triphenylethylene derivatives exhibited multi-photoresponsive properties, including photochromism, photodeformation and room temperature phosphorescence (RTP), which are strongly related to molecular conformations and packing in the aggregated states. Accordingly, these properties can be subtly adjusted by substituents to the center double bond and/or peripheral phenyl moieties. The introduction of bromine atom was beneficial to photochromism and photodeformation properties as a result of the additional C-H⋯Br interactions and electron-withdrawing property. Also, it can promote the RTP effect via heavy atom effect, resulting in persistent afterglow lasting up to 150 min as detected by chemiluminescent imaging system.

Photoisomerization Kinetics of Photoswitchable Thin Films Based on Nanostructure/Molecular Layers of AlN-AO7

The photoisomerization kinetics of photoswitchable thin films based on nanostructure/molecular layers of AlN-AO7 have been studied, investigated and reported. The trans → cis isomerization process occurs by UV-light irradiation. The cis-isomer could be turned back to the trans-isomer by either thermal or optical relaxation. The kinetics and time-evolution of the photoisomerization and reverse isomerization mechanism of AlN-AO7 thin films are investigated by UV-Vis absorbance spectra using relevant models. All phases of AlN-AO7 thin film, initial trans-, cis-, optical trans-, thermal trans-phases, were investigated using UV-Vis absorbance spectra, FTIR spectra, XRD and SEM. Transforming AlN-AO7 thin film from the initial trans-phase into cis-phase leads to curvature in the AO7 leaves and increases in the strain inside the structure. Going back to the trans-phase by either optical or thermal relaxation leads to vanishing the curvature and decreasing the structure's strain. Finally, the energy storage capacity was calculated using DSC and was found to be 36.38 J g^-1 , simultaneously realizing the multisource solar energy storage and environmental heat.

Mesomeric Effects of Azobenzene Bearing Natural Product-Based Molecules for Liquid Crystal Materials: An Overview

Latest progress in the liquid crystal (LC) field related to azo molecules incorporated into natural product- based moieties for the improvement of LC texture and mesomeric phases has received great interest among researchers. A LC containing natural product-based moieties i.e. menthol, kojic acid, cholesterol and chalcone with stable azo and azobenzene scaffolds with specific optical tunability, has been widely used in photo-active materials such as Liquid Crystal Display (LCD), LC films, smart windows and other devices. This review discusses the influence of azobenzene, a renowned photo-responsive and stable LC scaffold, in mesogenic phases due to photo-isomerization and optical switching. The incorporation of mesomeric phases of natural product moieties to azo molecules has improved the properties of LC, i.e, from the nematic phase to the smectic phase with proper magnetic field alignment. Natural product-based LC can be useful in numerous applications, especially practical electronic or optic devices such as optical image storage, display devices, solar cells, optical switching.

Reprocessable Photodeformable Azobenzene Polymers

Photodeformable azobenzene (azo) polymers are a class of smart polymers that can efficiently convert light energy into mechanical power, holding great promise in various photoactuating applications. They are typically of crosslinked polymer networks with highly oriented azo mesogens embedded inside. Upon exposure to the light of appropriate wavelength, they experience dramatic order parameter change following the configuration change of the azo units. This could result in the generation and accumulation of the gradient microscopic photomechanical force in the crosslinked polymer networks, thus leading to their macroscopic deformation. So far, a great number of photodeformable azo polymers have been developed, including some unoriented ones showing photodeformation based on different mechanisms. Among them, photodeformable azo polymers with dynamic crosslinking networks (and some uncrosslinked ones) have aroused particular interest recently because of their obvious advantages over those with stable chemical crosslinking structures such as high recyclability and reprocessability. In this paper, I provide a detailed overview of the recent progress in such reprocessable photodeformable polymers. In addition, some challenges and perspectives are also presented.

A multi-responsive organogel and colloid based on the self-assembly of a Ag(i)-azopyridine coordination polymer

In this work, through the coordination of C₃ symmetric azopyridine ligands and Ag(i), coordination polymers with azo groups on the main chain were prepared. The trans coordination polymer formed an organogel with a network of nanofibers at low critical gelation concentrations, and it exhibited the abilities of self-healing and multi-stimuli response to heating, light, mechanical shearing, and chemicals due to the presence of dynamic coordinating bonds. On the other hand, the cis coordination polymer was found to assemble into nanoparticles to give a responsive colloid, which can produce fibrous precipitation in several days upon visible light irradiation due to the isomerization of the azo groups. This work provides a novel example for the design of a multi-responsive organogel and colloid based on the structural transformation of coordination polymers.

Geminate labels programmed by two-tone microdroplets combining structural and fluorescent color

Creating a security label that carries entirely distinct information in reflective and fluorescent states would enhance anti-counterfeiting levels to deter counterfeits ranging from currencies to pharmaceuticals, but has proven extremely challenging. Efforts to tune the reflection color of luminescent materials by modifying inherent chemical structures remain outweighed by substantial trade-offs in fluorescence properties, and vice versa, which destroys the information integrity of labels in either reflection or fluorescent color. Here, a strategy is reported to design geminate labels by programming fluorescent cholesteric liquid crystal microdroplets (two-tone inks), where the luminescent material is 'coated' with the structural color from helical superstructures. These structurally defined microdroplets fabricated by a capillary microfluidic technique contribute to different but intact messages of both reflective and fluorescent patterns in the geminate labels. Such two-tone inks have enormous potential to provide a platform for encryption and protection of valuable authentic information in anti-counterfeiting technology.

Fabrication of Photoresponsive Crystalline Artificial Muscles Based on PEGylated Covalent Organic Framework Membranes

Seeking new photoresponsive materials with high energy conversion efficiency, good mechanical properties, as well as well-defined photoactuation mechanisms is of paramount significance. To address these challenges, we first introduced crystalline covalent organic frameworks (COFs) into the photoactuator field and created a facile fabrication strategy to directly install photoresponsive functional groups (i.e., acylhydrazone) on the skeletons of COFs. Herein, an approach to use polyethylene glycol (PEG) cross-linked dimers as the building blocks of the COF-42 platform was developed and afforded a series of uniform and freestanding membranes (PEG-COF-42) with outstanding mechanical properties (e.g., high flexibility and mechanical strength). Notably, these membranes possessed a fast mechanical response (e.g., bending) to UV light and good reversibility upon blue light or heating. After an in-depth investigation of the photoactuation mechanism via various techniques, we proposed a mechanism for the photoresponsive performance of PEG-COF-42: configurational change of acylhydrazone (i.e., E ↔ Z isomerization) accompanied by an excited-state intramolecular proton transfer (ESIPT) process intramolecularly transferring hydrogens from hydrogen donors (N-H) to hydrogen acceptors (oxygen in PEG). Moreover, attributed to the PEG moieties, PEG-COF-42 also demonstrated a vapor-responsive performance. This study not only broadens the application scopes of COFs but also provides new opportunities for the construction of multi-stimuli-responsive materials.

Azopyridine: a smart photo- and chemo-responsive substituent for polymers and supramolecular assemblies

This mini-review summarizes key features of the photoisomerization of polymer-tethered azopyridine in aqueous media and describes recent accomplishments on the fast thermal cis-to-trans relaxation of azopyridinium or H-bonded azopyridine.