Aggregation-Induced Emission-Active Gels: Fabrications, Functions, and Applications

Chromophores that exhibit aggregation-induced emission (i.e., aggregation-induced emission luminogens [AIEgens]) emit intense fluorescence in their aggregated states, but show negligible emission as discrete molecular species in solution due to the changes in restriction and freedom of intramolecular motions. As solvent-swollen quasi-solids with both a compact phase and a free space, gels enable manipulation of intramolecular motions. Thus, AIE-active gels have attracted significant interest owing to their various distinctive properties and promising application potential. Herein, a comprehensive overview of AIE-active gels is provided. The fabrication strategies employed are detailed, and the applications of AIEgens are summarized. In addition, the gel functions arising from the AIE moieties are revealed, along with their structure-property relationships. Furthermore, the applications of AIE-active gels in diverse areas are illustrated. Finally, ongoing challenges and potential means to address them are discussed, along with future perspectives on AIE-active gels, with the overall aim of inspiring research on novel materials and ideas.

Catalytic Gels for a Prebiotically Relevant Asymmetric Aldol Reaction in Water: From Organocatalyst Design to Hydrogel Discovery and Back Again

This paper reports an investigation into organocatalytic hydrogels as prebiotically relevant systems. Gels are interesting prebiotic reaction media, combining heterogeneous and homogeneous characteristics with a structurally organized active "solid-like" catalyst separated from the surrounding environment, yet in intimate contact with the solution phase and readily accessible via "liquid-like" diffusion. A simple self-assembling glutamine amide derivative 1 was initially found to catalyze a model aldol reaction between cyclohexanone and 4-nitrobenzaldehyde, but it did not maintain its gel structure during reaction. In this study, it was observed that compound 1 could react directly with the benzaldehyde to form a hydrogel in situ based on Schiff base 2 as a low-molecular-weight gelator (LMWG). This new dynamic gel is a rare example of a two-component self-assembled LMWG hydrogel and was fully characterized. It was demonstrated that glutamine amide 1 could select an optimal aldehyde component and preferentially assemble from mixtures. In the hunt for an organocatalyst, reductive conditions were applied to the Schiff base to yield secondary amine 3, which is also a highly effective hydrogelator at very low loadings with a high degree of nanoscale order. Most importantly, the hydrogel based on 3 catalyzed the prebiotically relevant aldol dimerization of glycolaldehyde to give threose and erythrose. In buffered conditions, this reaction gave excellent conversions, good diastereoselectivity, and some enantioselectivity. Catalysis using the hydrogel of 3 was much better than that using non-assembled 3-demonstrating a clear benefit of self-assembly. The results suggest that hydrogels offer a potential strategy by which prebiotic reactions can be promoted using simple, prebiotically plausible LMWGs that can selectively self-organize from complex mixtures. Such processes may have been of prebiotic importance.

Flexible Cathode Materials Enabled by a Multifunctional Covalent Organic Gel for Lithium-Sulfur Batteries with High Areal Capacities

Foldable lithium-sulfur (Li-S) batteries have captured considerable interest in advanced flexible energy storage systems. However, sulfur utilization, cycling stability, and mechanical durability are still not satisfactory for flexible batteries with high sulfur loadings. Herein, we present one type of new freestanding electrode material derived from a thiourea-based covalent organic gel (COG). COG can accommodate high loading of carbon nanotubes (CNTs) and sulfur with the concomitant formation of an embedded conductive CNT network. The unique performance of the COG not only facilitates ion transfer and electrolyte infiltration but also effectively confines polysulfides in the internal cavities. These advantages endow the freestanding CNT/S/COG electrodes with high reversible capacity, good rate performance, excellent cycling stability, and superior structural integrity. CNT/S/COG with an ultrahigh sulfur loading of 12.6 mg cm^-2 delivers a high discharging capacity of 13.7 mA h cm^-2 (1097 mA h g^-1) at 0.1 C; the capacity retention is as high as 83.9% after 100 cycles. Moreover, CNT/S/COG could be processed into foldable pouch cells. This study has demonstrated great potential of COGs for the fabrication of advanced flexible energy storage devices with high energy density and long cycling life.

Dynamic self-assembly of ions with variable size and charge in solution

Recently it was found that at ambient temperatures and in specific ternary solvents a cationic macrocyclic tetraimidazolium molecular box and small dianionic salts can self-assemble into highly defined, colloid-like ionic clusters, called ionoids. Here, we present evidence that the solution-based ionic self-assembly process leading to ionoids is a general phenomenon by characterizing new ionic building blocks which are capable of generating loosely bound globular and anisotropic structures similar to those in the established system. Using new cationic and anionic molecules, we show that variations in the size ratio between cationic and anionic component mainly affect size, shape and durability of the ionic clusters. Utilizing dynamic light scattering (DLS), continuously monitored phase-analysis light scattering (cmPALS) and continuous wave electron paramagnetic resonance (CW EPR) spectroscopy, we can thus define generalized ionic ratios, in which specific combinations of ionic compounds with certain size and charge densities are able to form these soft yet durable and long-lived ionic clusters. Furthermore, we characterize the temporal development of our dynamically self-assembled structures in solution from the level of the individual ionic building blocks to stable clusters with minimum lifetimes of months through previously established ionoid evolution diagrams (IEDs). The direct comparison of various cluster systems with respect to their shape, size and charges allows correlations of structural changes of the individual building blocks with the fate of self-assembled entities inside the crafted IEDs. This work generalizes the concept of ionoid formation to ions of specific sizes and charge densities, which may broaden the scope of this new type of highly dynamic and soft yet remarkably durable structures in the field of supramolecular chemistry.

Recently it was found that at ambient temperatures and in specific ternary solvents a cationic macrocyclic tetraimidazolium molecular box and small dianionic salts can self-assemble into highly defined, colloid-like ionic clusters, called ionoids.

An AIEgen-based 3D covalent organic framework for white light-emitting diodes

The design and synthesis of three-dimensional covalent organic frameworks (3D COFs) have still been considered as a big challenge. Here we report the design and synthesis of an AIEgen-based 3D COF (3D-TPE-COF), with a high surface area (1084 m² g⁻¹). According to powder X-ray diffraction and continuous rotation electron diffraction analyses, 3D-TPE-COF is identified to adopt a seven-fold interpenetrated pts topology. Interestingly, 3D-TPE-COF emits yellow fluorescence upon excitation, with a photoluminescence quantum yield of 20%. Moreover, by simply coating 3D-TPE-COF onto a commercial blue light-emitting diode (LED), a prototype white LED (WLED) under continuously driving without degradation for 1200 h was demonstrated. The present work suggests the possibility of using COF materials for stable WLEDs, which will greatly inspire us to design and synthesize fluorescent 3D COFs and facilitate the development of COF-based WLEDs in future.

3D covalent organic frameworks (COF) show interesting hierarchical arrangements of nanopores and open sites but their synthesis remains challenging. Here the authors report a fluorescent AIEgen-based 3D COF and demonstrate application as a coating material for white LEDs and for sensing of picric acid

In Situ Water-Compatible Polymer Entrapment: A Strategy for Transferring Superhydrophobic Microporous Organic Polymers to Water

Microporous organic polymer nanoparticles bearing tetraphenylethylene moieties (MOPTs) were prepared in the presence of poly(vinylpyrrolidone) (PVP). The PVP was entrapped into the microporous network of MOPT to form MOPT-P and played the roles of size control, porosity enhancement, and surface property management. MOPT materials without PVP showed superhydrophobicity with a water contact angle of 151°. In comparison, the MOPT-P showed excellent water compatibility. Moreover, due to the aggregation-induced emission property of tetraphenylethylene moieties, the MOPT-P showed emission and excellent emission-based sensing of nitrophenols in water with K_sv values in the range of 1.26 × 10⁴ ∼ 3.37 × 10⁴ M^-1. It is noteworthy that the MOPT-P used water only as a sensing medium and did not require additional organic solvents to enhance water dispersibility of materials. The MOPT-P could be recovered and reused for the sensing at least five times.

Tetraphenylethylene-Based AIE-Active Probes for Sensing Applications

This Review provides a comprehensive analysis of recent development in the field of aggregation-induced emission (AIE)-active tetraphenylethylene (TPE) luminophores and their applications in biomolecular science. It begins with a discussion of the diverse range of structural motifs that have found particular applications in sensing, and demonstrates that TPE structures and their derivatives have been used for a diverse range of analytes such as such as H⁺, anions, cations, heavy metals, organic volatiles, and toxic gases. Advances are discussed in depth where TPE is utilized as a mechanoluminescent material in bioinspired receptor units with specificity for analytes for such as glucose or RNA. The rapid advances in sensor research make this summary of recent developments in AIE-active TPE luminophores timely, in order to disseminate the advantages of these materials for sensing of analytes in solution, as well as the importance of solid and aggregated states in controlling sensing behavior.

A tetraphenylethylene-based acylhydrazone gel for selective luminescence sensing

A supramolecular gel based on dynamic covalent acylhydrazone bonding for selective and sensitive Cu²⁺ and subsequent CN⁻ detection has been reported.

Covalent organic frameworks (COFs): perspectives of industrialization

In this highlight, we review the state-of-the-art development of COFs from an industrial point of view in five aspects, including their types, growth mechanisms, synthetic methods, processability and applications.

Facile synthesis of novel porous self-assembling hydrogen-bonding covalent organic polymers and their applications towards fluoroquinolone antibiotics adsorption

A series of porous hydrogen-bonding covalent organic polymers (H_COPs) have been synthesized based on three-composite building blocks through a quick and succinct method for fluoroquinolone antibiotics adsorption from aqueous solutions. The porous properties of the H_COPs were regulated and controlled by adjusting the lengths of linkers, and the crystallinity and stability were strengthened due to the introduction of hydrogen bonds in H_COPs. Taking advantage of the porous properties and π-conjugated phenyl rings, as well as functional –CO–NH– and –COOH groups, H_COPs removed organic pollutants from wastewater effectively and showed good reusability. The external adsorption behavior was analyzed using both kinetic analysis and isotherm analysis. The results showed that the adsorption obeys the pseudo-second order kinetic model and follows the Langmuir isotherm model. The obtained maximum adsorption capacity of the four H_COPs was arranged in sequence according to the specific surface areas and pore sizes. Furthermore, the internal mechanisms involving perforated porousness, electrostatic interaction, hydrophobic interaction, π–π electron-donor–acceptor (EDA) interaction and hydrogen bonding formation, were investigated in detail. We envisage broadly applying the H_COPs in the facile and effective management of environmental pollution.

A series of porous hydrogen-bonding covalent organic polymers (H_COPs) have been synthesized based on three-composite building blocks through a quick and succinct method for fluoroquinolone antibiotics adsorption from aqueous solutions.

Recyclable Polydimethylsiloxane Network Crosslinked by Dynamic Transesterification Reaction

This article reports preparation of a crosslinked polydimethylsiloxane (PDMS) network via dynamic transesterification reaction between PDMS-diglycidyl ether and pripol 1017 with Zn(OAc)2 as the catalyst. The thermal dynamic nature of the network was investigated by the creep-recovery and stress-relaxation tests. The synthesized PDMS elastomer showed excellent solvent resistance even under high temperature, and could be reprocessed by hot pressing at 180 °C with the mechanical properties maintained after 10 cycles. Application of the PDMS elastomer in constructing micro-patterned stamps repeatedly has been demonstrated. The high plastic temperature and good solvent resistance distinguish the research from other reported thermoplastic PDMS elastomers and broaden the practical application areas.

Hydrogen-Bond and Supramolecular-Contact Mediated Fluorescence Enhancement of Electrochromic Azomethines

An electronic push-pull fluorophore consisting of an intrinsically fluorescent central fluorene capped with two diaminophenyl groups was prepared. An aminothiophene was conjugated to the two flanking diphenylamines through a fluorescent quenching azomethine bond. X-ray crystallographic analysis confirmed that the fluorophore formed multiple intermolecular supramolecular bonds. It formed two hydrogen bonds involving a terminal amine, resulting in an antiparallel supramolecular dimer. Hydrogen bonding was also confirmed by FTIR and NMR spectroscopic analyses, and further validated theoretically by DFT calculations. Intrinsic fluorescence quenching modes could be reduced by intermolecular supramolecular contacts. These contacts could be engaged at high concentrations and in thin films, resulting in fluorescence enhancement. The fluorescence of the fluorophore could also be restored to an intensity similar to its azomethine-free counterpart with the addition of water in >50 % v/v in tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), and acetonitrile. The fluorophore also exhibited reversible oxidation and its color could be switched between yellow and blue when oxidized. Reversible electrochemically mediated fluorescence turn-off on turn-on was also possible.

Solid-state emissive cyanostilbene based conjugated microporous polymers via cost-effective Knoevenagel polycondensation

Solid-state emissive cyanostilbene-based conjugated porous polymers with different topological structure and photophysical properties were newly synthesized via cost-effective Knoevenagel polycondensation.

A catalytic chiral gel microfluidic reactor assembled via dynamic covalent chemistry

A novel dynamic covalent gel strategy is reported to immobilize an asymmetric catalyst within the channels of a microfluidic flow reactor. A layer of a catalytically active Mn-salen dynamic covalent imine gel matrix was coated onto a functionalized capillary. Mn-salen active moiety was incorporated into dynamic covalent imine gel matrix via the reaction of a chiral Mn-salen dialdehyde unit with a tetraamine linker. The catalytic activity of the capillary reactor has been demonstrated in enantioselective kinetic resolution of secondary alcohols.

Phenothiazine and pyridine-N-oxide-based AIE-active triazoles: synthesis, morphology and photophysical properties

AIE triazoles based on phenothiazine and pyridine-N-oxide units provided micro- and nano-crystals with an emission enhancement of 233-fold higher.

Hierarchical supramolecules and organization using boronic acid building blocks

Current progress on hierarchical supramolecules using boronic acids has been highlighted in this feature article. The feasibility of the structure-directing ability is fully discussed from the standpoint of the generation of new smart materials.

Thermo-responsive white-light emission based on tetraphenylethylene- and rhodamine B-containing boronate nanoparticles

White-light emissive boronate nanoparticles have been prepared, which exhibit reversible and thermo-responsive emission in the investigated temperature range (5–65 °C) with a temperature sensitivity of 1.1% K⁻¹ in water.