Fluorescence color change of supramolecular polymer networks controlled by crown ether-cation recognition

We report a fluorescent supramolecular polymer networks (SPNs) system based on crown ether-cation recognition. The polymer side chains bear ammonium cations, which can be recognized by host molecules with a B15C5 unit and a quinoline group at each end. The quinoline group makes the host molecule exhibit blue fluorescence. After the formation of SPNs, the recognition of the crown ether-cation transforms the blue fluorescence into yellow fluorescence. The accompanying fluorescence color change during the formation of SPNs makes it with potential applications in the fields of display, printing, information storage, and bioimaging.

Ion recognition properties of 2,2'-bibenzimidazole regulated by ammonium-modified pillar[5]arenes

With the increasing issues of environmental degradation and health problem, the selective detection of toxic ions has attracted considerable attention from researchers. Chemical fluorescent sensors with the advantages of facile operation, high sensitivity, rapid response, and easy visualization are emerging as powerful detection tools towards ions. However, the selective recognition of ions is always hindered by the presence of other interfering substances. Herein, we show that supramolecular host-guest interaction based on a pillar[5]arene provides a new opportunity to regulate the ionic recognition properties of guest molecules. A pillar[5]arene-based host-guest complex HG was constructed through the host-guest interaction between ammonium functionalized pillar[5]arene (HAP5) and 2,2'-bibenzimidazole (G). The host-gust complex HG can realize the successive, highly selective, and sensitive detection of specific ions. It was found that only in the presence of HAP5, the sensitivity towards cations was evidently enhanced, and selective successive recognition for I^- and HSO₄^- was achieved. Those results indicate that the introduction of HAP5 can effectively improve the ion recognition performance of 2,2'-bibenzimidazole, so it is a feasible strategy using supramolecular host-guest interaction to regulate the ionic recognition properties of guest molecules.

Dimeric Pillar[5]arene as a Novel Fluorescent Host for Controllable Fabrication of Supramolecular Assemblies and Their Photocatalytic Applications

A dimeric fluorescent macrocycle m-TPE Di-EtP5 (meso-tetraphenylethylene dimeric ethoxypillar[5]arene) is synthesized based on the meso-functionalized ethoxy pillar[5]arene. Through the connectivity of two pillar[5]arenes by CC double bond, the central tetraphenylethylene (TPE) moiety is simultaneously formed. The resultant bicyclic molecule not only retains the host-guest properties of pillararenes but also introduces the interesting aggregation-induced emission properties inherent in the embedded TPE structure. Three dinitrile derivatives with various linkers are designed as guests (G1, G2, and G3) to form host-guest assemblies with m-TPE Di-EtP5. The morphological control and fluorescence properties of the assemblies are successfully realized. G1 with a shorter alkyl chain as the linker completely threads into the cavities of the host. G2, due to its longer chain length, forms a linear supramolecular polymer upon binding to m-TPE Di-EtP5. G3 differs from G2 by possessing a bulky phenyl group in the middle of the chain, which can be further assembled with m-TPE Di-EtP5 to form supramolecular layered polymer and precipitated out in solution, and can be efficiently applied to photocatalytic reactions.

Aggregation-induced emission enhancement (AIEE) of tetrarhenium(I) metallacycles and their application as luminescent sensors for nitroaromatics and antibiotics

The self-assembly of tetrarhenium metallacycles [{Re(CO)₃}₂(μ-dhaq)(μ-N-N)]₂ (3a, N-N = 1,3-bis(1-butylbenzimidazol-2-yl)benzene; 3b, N-N = 1,3-bis(1-octylbenzimidazol-2-yl)benzene), (H₂-dhaq = 1,4-dihydroxy-9,10-anthraquinone) and [{Re(CO)₃}₂(μ-thaq)(μ-N-N)]₂ (4, N-N = 1,3-bis(1-butylbenzimidazol-2-yl)benzene), (H₂-thaq = 1,2,4-trihydroxy-9,10-anthraquinone) under solvothermal conditions is described. The metallacycles 3a,b and 4 underwent aggregation-induced emission enhancement (AIEE) in THF upon the incremental addition of water. TEM images revealed that metallacycle 3a in a 60% aqueous THF solution formed rectangular aggregates with a wide size distribution, while a 90% aqueous THF solution resulted in the formation of a mixture of nanorods and amorphous aggregates due to rapid and abrupt aggregation. UV-vis and emission spectral profiles supported the formation of nanoaggregates of metallacycles 3a,b and 4 upon the gradual addition of water to a THF solution containing metallacycles. Further studies indicated that these nanoaggregates were excellent probes for the sensitive and selective detection of nitro group containing picric acid (PA) derivatives as well as antibiotics.

Covalently bridged pillararene-based polymers: structures, synthesis, and applications

Covalently bridged pillararene-based polymers (CBPPs) are a special class of macrocycle-based polymers in which multiple pillararene monomers are attached to the polymer structures by covalent bonds. Owing to the unique molecular structures including the connection components or the spatial structures, CBPPs have become increasingly popular in applications ranging from environmental science to biomedical science. In this review, CBPPs are divided into three types (linear polymers, grafted polymers, and cross-linked polymers) according to their structural characteristics and described from the perspective of synthesis methods comprehensively. In addition, the applications of CBPPs are presented, including selective adsorption and separation, fluorescence sensing and detection, construction of supramolecular gels, anticancer drug delivery, artificial light-harvesting, catalysis, and others. Finally, the current challenging issues and comprehensive prospects of CBPPs are discussed.

Pillar[5]arene Derivatives Embedded with Aggregation-Induced Emission Luminogens and Their Fluorescence Regulation

Through McMurry coupling reaction, three meso-position functionalized pillar[5]arene derivatives (H-1, H-2, and H-3) have been successfully prepared by embedding aggregation-induced emission luminogens (AIEgens, diphenyldibenzofulvene (DPDBF) and tetraphenylethylene (TPE)) into the skeleton of supramolecular macrocycles. H-1, bearing [1₅ ]paracyclophane ([1₅ ]PCP) and DPDBF moiety, exhibits yellow emission and demonstrates obvious AIE effect. In order to further improve the host-guest properties of this type of structure, H-2 and H-3 are prepared by replacing the [1₅ ]PCP moiety with pillar[5]arene backbone, both of which show significant AIE effect and excellent host-guest complexation properties with pyrazine salt guest G-1 and 1,4-dicyanobutane G-2. Our findings indicate that G-1 can decrease the fluorescence intensity of the AIE macrocycles, while G-2 can increase their fluorescence intensity in solution.

Modular Platform of Carbohydrates-modified Supramolecular Polymers Based on Dendritic Peptide Scaffolds

Glycopeptide supramolecular polymers displaying multivalent carbohydrates are particularly suitable for immune-relevant biomaterials, due to the important functions of carbohydrates in mediating cell-cell communication and modulating immune responses. However, the diversity and complexity of carbohydrates limited the generation of glycopeptide supramolecular monomers. Thereby, a modular platform of presenting various carbohydrates, especially more complex oligosaccharides, is highly desirable but remains underexplored. Here, we first prepared the linear amphiphilic glycopeptides that self-assembled into spherical nanoparticles and worm-like nanoparticles. Furthermore, the dendritic glycopeptides that self-assembled into uniform nanorods were designed to generate modular supramolecular polymers with variable functionality, via redesigning the molecular backbone. With various functional oligosaccharide-modified supramolecular polymers, the in vitro studies further indicated that these polymers were not cytotoxic to macrophages, and significantly modulated the production of proinflammatory cytokines. These findings provide a promising platform to develop supramolecular glycopeptide biomaterials with potential applications in immunomodulation and immunotherapy.

Enhancing Mechanical Performance of a Polymer Material by Incorporating Pillar[5]arene-Based Host–Guest Interactions

Polymer gels have been widely used in the field for tissue engineering, sensing, and drug delivery due to their excellent biocompatibility, hydrophilicity, and degradability. However, common polymer gels are easily deformed on account of their relatively weak mechanical properties, thereby hindering their application fields, as well as shortening their service life. The incorporation of reversible non-covalent bonds is capable of improving the mechanical properties of polymer gels. Thus, here, a poly(methyl methacrylate) polymer network was prepared by introducing host–guest interactions between pillar[5]arene and pyridine cation. Owing to the incorporated host–guest interactions, the modified polymer gels exhibited extraordinary mechanical properties according to the results of the tensile tests. In addition, the influence of the host–guest interaction on the mechanical properties of the gels was also proved by rheological experiments and swelling experiments.

A Fluorescent Linear Conjugated Polymer Constructed from Pillararene and Anthracene

Over the past few years, conjugated polymers (CPs) have aroused much attention owing to their rigid conjugated structures, which can perform well in light harvesting and energy transfer and offer great potential in materials chemistry. In this article, we fabricate a new luminescent linear CP p(P[5](OTf)₂-co-9,10-dea) via the Sonogashira coupling of 9,10-diethynylanthracene and trifluoromethanesulfonic anhydride (OTf) modified pillar[5]arene, generating enhanced yellow-green fluorescence emission at around 552 nm. The reaction condition was screened to get a deeper understanding of this polymerization approach, resulting in an excellent yield as high as 92% ultimately. Besides the optical properties, self-assembly behaviors of the CP in low/high concentrations were studied, where interesting adjustable morphologies from tube to sheet were observed. In addition, the fluorescence performance and structural architecture can be disturbed by the host–guest reorganization between the host CP and the guest adiponitrile, suggesting great potential of this CP material in the field of sensing and detection.

Pillararene-containing polymers with tunable fluorescence properties based on host-guest interactions

Linear polymers containing pillar[5]arenes as the pendant groups were designed and synthesized via a ring-opening metathesis polymerization. Such polymers could form supramolecular brush polymers and exhibited tunable fluorescence properties based on the host-guest interactions.

Bio-inspired AIE pillar[5]arene probe with multiple binding sites to discriminate alkanediamines

Two functionalized pillar[5]arenes (H1 and H2) with significant AIE properties were synthesized. H2 is an excellent probe to selectively detect specific alkanediamines owing to its multiple binding sites, which result in the enhancement of emission based on the AIE mechanism and the induced-fit mechanism, and provides a new strategy to develop probes with high selectivity and sensitivity.

Recent Applications of Pillar[n]arene-Based Host-Guest Recognition in Chemosensing and Imaging

Pillar[n]arene is a novel kind of synthetic supramolecular macrocyclic host characterized by its particular pillar-shaped structure consisting of an electron-rich cavity and two finely adjustable rims. Benefiting from its rigid structure, facile synthesis, ease of functionalization, and outstanding host-guest chemistry, pillar[n]arene shows great potential for diverse applications. Significantly, the host-guest recognition of pillar[n]arene provides a novel approach for chemosensing and imaging. Herein, this Review critically and comprehensively reviews the applications of pillar[n]arene-based host-guest recognition in chemosensing and imaging. The sensing and imaging mechanisms as well as the unique roles and advantages of pillar[n]arene-based host-guest recognition are summarized. In addition, preparations of hybrid materials based on pillar[n]arene and inorganic materials are also introduced comprehensively in the light of chemosensing and imaging. Finally, current challenges and perspectives on pillar[n]arene-based host-guest recognition in chemosensing and imaging are outlined.

Efficient artificial light-harvesting systems based on aggregation-induced emission in supramolecular gels

Based on a new designed acyl hydrazone gelator (G2), we developed a supramolecular organogel in glycol with two different hydrophobic fluorescent dyes, namely rhodamine B (RhB) and acridine red, as acceptors. Both the G2@gel-RhB and G2@gel-acridine red systems showed high levels of energy-transfer efficiency and high fluorescence quantum yields.

Selective recognition of tryptophan by a methylpillar[5]arene-based supramolecular fuorescent probe

Herein we present a simple fluorescence quenching method to selectively recognise and determine L-tryptophan (L-Trp) out of other 19 natural amino acids. Methylpillar[5]arene (MeP5), which is employed as a macrocyclic fluorescent probe, exhibits fluorescence activity in the solution of poor solvents because of aggregation-induced emission (AIE) effect. Fluorescence quenching of MeP5 in the solution of EtOH/CH₂Cl₂ (98/2, v/v) was observed upon the addition of L-Trp whereas other 19 natural amino acids did not bring about obvious change in fluorescence intensity. ¹H NMR titration, fluorescence spectroscopy, mass spectrometry and theoretical analysis revealed that L-Trp can be encapsulated into the cavity of MeP5 to form a stable 1:1 host-guest inclusion complex which accounts for the quenching characteristics. The proposed procedure in this investigation offers an attractive and promising method for the selective detection of L-Trp in a mixture of natural amino acids.

Recent Advances on Fabrication of Polymeric Composites Based on Multicomponent Reactions for Bioimaging and Environmental Pollutant Removal

As the core of polymer chemistry, manufacture of functional polymers is one of research hotspots over the past several decades. Various polymers are developed for diverse applications due to their tunable structures and unique properties. However, traditional step-by-step preparation strategies inevitably involve some problems, such as separation, purification, and time-consuming. The multicomponent reactions (MCRs) are emerging as environmentally benign synthetic strategies to construct multifunctional polymers or composites with pendant groups and designed structures because of their features, such as efficient, fast, green, and atom economy. This mini review summarizes the latest advances about fabrication of multifunctional fluorescent polymers or adsorptive polymeric composites through different MCRs, including Kabachnik-Fields reaction, Biginelli reaction, mercaptoacetic acid locking imine reaction, Debus-Radziszewski reaction, and Mannich reaction. The potential applications of these polymeric composites in biomedical and environmental remediation are also highlighted. It is expected that this mini-review will promote the development preparation and applications of functional polymers through MCRs.

Supramolecular Engineering of Efficient Artificial Light-Harvesting Systems from Cyanovinylene Chromophores and Pillar[5]arene-Based Polymer Hosts

Enhanced emission and adjustable wavelength for single luminogen systems are highly desirable in the scope of photoluminescent materials. Herein, a supramolecular strategy has been proposed for supramolecular assembly-induced enhanced emission and valid emission manipulation by fabricating an amphiphilic copolymer host material with pillar[5]arene units as the side chains, whereby cyanovinylene-based (CV) derivatives are anchored to the polymer hosts via host-guest interactions. The guest-bearing copolymers can further form luminescent supramolecular polymer nanoparticles (SPNs). Remarkably, the as-prepared SPNs exhibit dramatic emission enhancement and tunable fluorescence wavelength, ascribing to the synergetic effects involving the restriction of intramolecular motions and the prevented excimer formation for CV moieties, as endowed by host-guest interactions and the entanglement of the polymer chains. Furthermore, the SPNs can be established as efficient artificial light-harvesting systems via the inclusion of Nile red into the particles for broadened emission spectra. As a proof-of-concept study, the use of pillar[5]arene-containing polymer hosts largely facilitates the emission enhancement and wavelength adjustment for the inherent luminogens, setting the basis for the supramolecular design of highly tunable luminescent systems.

Pillararene-based AIEgens: research progress and appealing applications

The pillararene-based AIEgens and AIE materials, constructed using different assembly forms, show attractive applications in various areas.

A triphenylamine derived fluorescent probe for efficient detection of H2S based on aggregation-induced emission

The probe TPA-HS can quickly identify H₂S (20 minutes) and release TPA-CHO with aggregation-induced emission properties.

Highly Efficient Förster Resonance Energy Transfer Modulations of Dual-AIEgens between a Tetraphenylethylene Donor and a Merocyanine Acceptor in Photo-Switchable [2]Rotaxanes and Reversible Photo-Patterning Applications

A series of novel photo-switchable [2]rotaxanes (i.e., Rot-A-SP and Rot-B-SP before and after shuttling controlled by acid-base, respectively) containing one spiropyran (SP) unit (as a photochromic stopper) on the axle and two tetraphenylethylene (TPE) units on the macrocycle were synthesized via click reaction. Upon UV/visible light exposure, both mono-fluorophoric rotaxanes Rot-A-SP and Rot-B-SP with the closed form (i.e., non-emissive SP unit) could be transformed into the open form (i.e., red-emissive merocyanine (MC) unit) to acquire their respective bi-fluorophoric Rot-A-MC and Rot-B-MC reversibly. The aggregation-induced emission (AIE) properties of bi-fluorophoric TPE combined with MC AIEgens of these designed rotaxanes and mixtures in semi-aqueous solutions induced interesting ratiometric photoluminescence (PL) and Förster resonance energy transfer (FRET) behaviors, which were further investigated and verified by dynamic light scattering (DLS), X-ray diffraction (XRD), and time-resolved photoluminescence (TRPL) measurements along with theoretical studies. Accordingly, in contrast to the model axle (Axle-MC) and the analogous mixture (Mixture-MC, containing the axle and macrocycle components in a 1:1 molar ratio), more efficient FRET behaviors and stronger red PL emissions were obtained from dual-AIEgens between a blue-emissive TPE donor (PL emission at 468 nm) and a red-emissive MC acceptor (PL emission at 668 nm) in both novel photo-switchable [2]rotaxanes Rot-A-MC and Rot-B-MC under various external modulations, including water content, UV/Vis irradiation, pH value, and temperature. Furthermore, the reversible fluorescent photo-patterning applications of Rot-A-SP in a powder form and a solid film with excellent photochromic and fluorescent behaviors are first investigated in this report.

One-Step Multicomponent Polymerizations for the Synthesis of Multifunctional AIE Polymers

As a new class of functional luminescent materials, polymers with aggregation-induced emission (AIE) feature attract much attention because of their advantages of efficient solid-state fluorescence, excellent processability, structural diversity, and multifunctionalities. Among all polymerization methods toward AIE polymers, multicomponent polymerizations (MCPs) exhibit the merits of simple operation, good atom economy, high polymerization efficiency, broad functional-group tolerance, etc. In this feature article, the recent progress on the development of one-step MCPs for the synthesis of AIE polymers is highlighted. The representative functionalities of the resulting AIE polymers are illustrated. Perspectives on the challenges and future development directions of this field are also discussed.

Fluorescent Metallosupramolecular Elastomers for Fast and Ultrasensitive Humidity Sensing

Fluorescent supramolecular polymers that can respond to subtle external stimuli to generate luminescence signals are promising in a wide range of applications, including probes, anti-counterfeiting materials, and sensors. However, complicated preparative procedures, limited responsive speed, and relatively low sensitivity still limit their practical sensing applications. Herein, we report europium-containing metallosupramolecular (PU-Eu) elastomers for fast and ultrasensitive humidity sensing by employing hygroscopic polyurethane (PU), whose urethane groups can coordinate with europium ions (Eu³⁺), emitting a strong luminescent signal by ligand-to-metal energy transfer. The variant of the coordination bond strength triggered by external humidity imparts the PU-Eu elastomer with a fast (∼1.1 s) and ultrasensitive response to the humid condition, where the external humidity increases by ∼1% and the corresponding fluorescence intensity will drop by ∼421.98 a.u. By a dip-coating process, PU-Eu elastomers can be conveniently coated on a hydrophilic and porous cellulose acetate nanofiber membrane, and the resulting composite membrane can achieve real-time and reversible monitoring of environmental humidity and human respiration. Given the versatility of PU-Eu elastomers, this study provides a low-cost and facile route of obtaining fluorescent metallosupramolecular polymers for fast and ultrasensitive humidity sensing.

Constructing Cross-Linked Nanofibrous Scaffold via Dual-Enzyme-Instructed Hierarchical Assembly

To explore the potential of step-by-step assembly in the fabrication of biological materials, we designed and synthesized two peptide-based molecules for enzyme-instructed hierarchical assembly. Upon the treatment of alkaline phosphatase, one molecule undergoes enzyme-instructed self-assembly forming uniformed nanofibers. The other one that can self-assemble into vesicles undergoes enzyme-induced transformation of self-assembly converting vesicles into irregular aggregates upon the treatment of carboxylesterase. Coadministration of two enzymes to a mixture of these two molecules in a stage-by-stage fashion leads to a physically knotted nanofibrous scaffold that is applicable as a nanostructured matrix for cell culture.

Th4+ tuned aggregation-induced emission: A novel strategy for sequential ultrasensitive detection and separation of Th4+ and Hg2

A novel strategy, Th⁴⁺ tuned aggregation-induced emission, for sequential ultrasensitive detection and separation of Th⁴⁺ and Hg²⁺ was developed successfully. For demonstration this strategy, we designed and synthesized two tripodal gelators TH (tri-(isoniazid-4-yl)-functionalized trimesic acylhydrazine) and TA (tri-(pyridine-4-yl)-functionalized trimesic amide). The TH and TA could assemble into a stable supramolecular polymer hydrogel THTA-G in DMSO/H₂O (3.3:6.7, v/v) binary-solution. The THTA-G does not show aggregation-induced emission (AIE) effect. However, after addition of Th⁴⁺ into the THTA-G, the obtained metallogel THTA-GTh shows strong green AIE effect, which indicated that Th⁴⁺ could tune the gel generation of AIE effect. Interestingly, the THTA-G could ultrasensitive fluorescently detect Th⁴⁺, and the corresponding metallogel THTA-GTh could ultrasensitively detect Hg²⁺. The detection limits of THTA-G and THTA-GTh for Th⁴⁺ and Hg²⁺ are 8.61 × 10^-11 mol/L and 1.08 × 10^-11 mol/L, respectively. Additionally, the xerogels of THTA-G and THTA-GTh could separate Th⁴⁺ and Hg²⁺ from aqueous solution with excellent ingestion capacity, and the THTA-G could be used as a writable smart light-emitting material.

Supramolecular materials based on AIE luminogens (AIEgens): construction and applications

The emergence of aggregation-induced emission luminogens (AIEgens) has significantly stimulated the development of luminescent supramolecular materials because their strong emissions in the aggregated state have resolved the notorious obstacle of the aggregation-caused quenching (ACQ) effect, thereby enabling AIEgen-based supramolecular materials to have a promising prospect in the fields of luminescent materials, sensors, bioimaging, drug delivery, and theranostics. Moreover, in contrast to conventional fluorescent molecules, the configuration of AIEgens is highly twisted in space. Investigating AIEgens and the corresponding supramolecular materials provides fundamental insights into the self-assembly of nonplanar molecules, drastically expands the building blocks of supramolecular materials, and pushes forward the frontiers of supramolecular chemistry. In this review, we will summarize the basic concepts, seminal studies, recent trends, and perspectives in the construction and applications of AIEgen-based supramolecular materials with the hope to inspire more interest and additional ideas from researchers and further advance the development of supramolecular chemistry.

Host-Guest Complexation of Perethylated Pillar[6]arene toward Ferrocene Derivatives Both in Solution and Solid State: Different Binding Modes Induced by Minor Structural Changes of Guests

Herein, novel host-guest properties between perethylated pillar[6]arene and four kinds of ferrocene derivatives were fully investigated. NMR titrations, 2D NOESY NMR spectroscopy, and ESI-MS are used to confirm that they indeed formed stable inclusion complexes. Two precious single-crystal structures were obtained and showed that ferrocene derivatives with different chemical structures exhibit different binding modes with perethylated pillar[6]arenes.

A super-stretchable, self-healing and injectable supramolecular hydrogel constructed by a host–guest crosslinker

Supramolecular hydrogels based on host–guest interactions have drawn considerable attention due to their unique properties and promising applications.