Functional supramolecular gels based on pillar[n]arene macrocycles

Tough Hydrogels for Load-Bearing Applications

Tough hydrogels have emerged as a promising class of materials to target load-bearing applications, where the material has to resist multiple cycles of extreme mechanical impact. A variety of chemical interactions and network architectures are used to enhance the mechanical properties and fracture mechanics of hydrogels making them stiffer and tougher. In recent years, the mechanical properties of tough, high-performance hydrogels have been benchmarked, however, this is often incomplete as important variables like water content are largely ignored. In this review, the aim is to clarify the reported mechanical properties of state-of-the-art tough hydrogels by providing a comprehensive library of fracture and mechanical property data. First, common methods for mechanical characterization of such high-performance hydrogels are introduced. Then, various modes of energy dissipation to obtain tough hydrogels are discussed and used to categorize the individual datasets helping to asses the material's (fracture) mechanical properties. Finally, current applications are considered, tough high-performance hydrogels are compared with existing materials, and promising future opportunities are discussed.

Constructing a solid-state supramolecular polymer based on host-guest recognition between perethylated pillar[5]arene and tetrathiafulvalene

Herein we present a novel linear supramolecular polymeric structure formed in both the solution and solid state, utilizing the host-guest recognition motif between perethylated pillar[5]arene and tetrathiafulvalene.

Highly selective Cu2+ detection with a naphthalimide-functionalised pillar[5]arene fluorescent chemosensor

Ligand 1, a rim-differentiated pillar[5]arene macrocycle modified with five naphthalimide groups through click chemistry, serves as an effective ratiometric fluorescent chemosensor for Cu2+. In contrast to the monomeric naphthalimide control compound 2, which shows only monomer emission, ligand 1 demonstrates dual emission characteristics encompassing both the monomer and excimer of the naphthalimide moieties. The binding properties of ligand 1 toward 15 different metal ions were systematically investigated in CH2Cl2/CH3CN (v/v, 1 : 1) by UV-vis and fluorescence spectroscopy. Remarkably, ligand 1 exhibits exceptional selectivity for Cu2+ ions. Upon complexation with Cu2+, the excimer emission of ligand 1 diminishes, concomitant with an enhancement of its monomer emission. The binding ratio for 1·Cu2+ was determined to be 1 : 1, with an association constant of (3.39 ± 0.40) × 105 M-1 calculated using a nonlinear least-squares curve-fitting method. Furthermore, the limit of detection (LOD) was found to be 185 ± 7 nM. Our results from 1H NMR titration, high-resolution mass spectrometry analysis and density functional theory calculations of 1·Cu2+ suggest synergistic coordination between Cu2+ and the triazole groups on ligand 1.

Editorial on Special Issue "Design and Optimization of Pharmaceutical Gels"

The efficacy of many bioactive agents, including drugs, food supplements, and vaccines, is limited because of their poor chemical stability, low water solubility, and low oral bioavailability [...].

Calix[4]arene-based Supramolecular Gels for Mercury Ion Removal in Water

A calix[4]arene-based gelator 1, with lower-rim mono triazolylpyridine group, capable of spontaneous self-assembly into microspheres in different ethanol/H2 O mixtures, is synthesized. The concentration-dependent 1 H NMR spectra and X-ray single-crystal structure of 1 provided evidence for self-assembly of gelator 1 via cooperative interactions of intermolecular noncovalent forces. Furthermore, metallogels by self-assembly of 1 was found to exhibit remarkable selectivity toward Hg2+ ions. 1 H NMR spectra support that Hg2+ ion was bound to the nitrogen atoms of two coordination sites of 1, which composed of triazole and pyridine. Moreover, the results of field emission scanning electron microscopy and rheology experiments indicated that Hg2+ ions not only enhanced the gelling ability of gelator 1 in ethanol but also led to morphological change of its self-assembly through metal-ligand interactions. Finally, the in situ gelation, triggered by mixing a gelator solution of 1 in ethanol with water samples such as deionized (DI), tap, and lake water, leads to the effective removal of Hg(II) from a water sample which reduced from 400 to 1.6 ppm.

Novel Biocompatible Trianglamine Networks for Efficient Iodine Capture

Herein, we report for the first time a biocompatible cross-linked trianglamine (Δ) network for the efficient iodine removal from the vapor phase, water, and seawater. In the vapor phase, the cross-linked network could capture 6 g g-1 of iodine, ranking among the most performant materials for iodine vapor capture. In the liquid phase, this cross-linked network is also capable of capturing iodine at high rates from aqueous media (water and seawater). This network displayed fast adsorption kinetics, and they are fully recyclable. This study reveals the high affinity of iodine for the intrinsic cavity of the trianglamine. The synthesized materials are extremely interesting since they are environmentally friendly and inexpensive and the synthesis could easily be scaled up to be used as the material of choice in response to accidents in the nuclear industry.

Self-assembly of β-cyclodextrin-pillar[5]arene molecules into supramolecular nanoassemblies: morphology control by stimulus responsiveness and host-guest interactions

Macrocyclic molecules have attracted considerable attention as new functional materials owing to their unique pore size structure and excellent host-guest properties. With the development of macrocyclic compounds, the properties of mono-modified macrocyclic materials can be improved by incorporating pillar[n]arene or cyclodextrin derivatives through bridge bonds. Herein, we report the self-assembly of amphiphilic di-macrocyclic host molecules (H1-2) based on β-cyclodextrin and pillar[5]arene units linked by azophenyl or biphenyl groups. In a H₂O/DMSO (19 : 1, v/v) mixed polar solvent, an amphiphile H1 with an azophenyl group self-assembled into unique nanorings and exhibited an obvious photoresponsive colour change. This photochromic behaviour makes H1 suitable for application in carbon paper materials on which arbitrary patterns can be erased and rewritten. The amphiphile H2, with a biphenyl unit, self-assembled into spherical micelles. These differences indicate that various linker units lead to changes in the intermolecular and hydrophilic-hydrophobic interactions. In a CHCl₃/DMSO (19 : 1, v/v) mixed low-polarity solvent, the amphiphile H1 self-assembled into fibrous aggregates, whereas the molecule H2 assembled into unique nanoring aggregates. In this CHCl₃/DMSO mixed solvent system, small nanosheet aggregates were formed by the addition of a guest molecule (G) composed of tetraphenylethene and hexanenitrile groups. With prolonged aggregation time, the small sheet aggregates further aggregated into cross-linked nanoribbons and eventually formed large nanosheet aggregates. The data reveal that the morphology of H1-2 can be controlled by tuning the intermolecular interactions of the molecules via the formation of host-guest complexes. Moreover, the polyhydroxy cyclodextrin unit on H1-2 can be strongly adsorbed on the stationary phase in column chromatography via multiple hydrogen bonds, and the singly modified pillar[5]arenes can be successfully separated by host-guest interactions.

Pillararene-Based Supramolecular Polymers for Cancer Therapy

Supramolecular polymers have attracted considerable interest due to their intriguing features and functions. The dynamic reversibility of noncovalent interactions endows supramolecular polymers with tunable physicochemical properties, self-healing, and externally stimulated responses. Among them, pillararene-based supramolecular polymers show great potential for biomedical applications due to their fascinating host-guest interactions and easy modification. Herein, we summarize the state of the art of pillararene-based supramolecular polymers for cancer therapy and illustrate its developmental trend and future perspective.

Cation controlled rotation in anionic pillar[5]arenes and its application for fluorescence switch

Controlling molecular motion is one of hot topics in the field of chemistry. Molecular rotors have wide applications in building nanomachines and functional materials, due to their controllable rotations. Hence, the development of novel rotor systems, controlled by external stimuli, is desirable. Pillar[n]arenes, a class of macrocycles, have a unique planar chirality, in which two stable conformational isomers pR and pS would interconvert by oxygen-through-the-annulus rotations of their hydroquinone rings. We observe the differential kinetic traits of planar chirality transformation in sodium carboxylate pillar[5]arene (WP5-Na) and ammonium carboxylate pillar[5]arene (WP5-NH₄), which inspire us to construct a promising rotary platform in anionic pillar[5]arenes (WP5) skeletons. Herein, we demonstrate the non-negligible effect of counter cations on rotational barriers of hydroquinone rings in WP5, which enables a cation grease/brake rotor system. Applications of this tunable rotor system as fluorescence switch and anti-counterfeiting ink are further explored.

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.

Pillararene-Inspired Macrocycles: From Extended Pillar[n]arenes to Geminiarenes

chemistry since their establishment due to their innate functional features of molecular recognition and complexation. The rapid development of modern supramolecular chemistry has also significantly benefited from creating new macrocycles with distinctive geometries and properties. For instance, pillar[n]arenes (pillarenes), a relatively young generation of star macrocyclic hosts among the well-established ones (e.g., crown ethers, cyclodextrins, cucurbiturils, and calixarenes), promoted a phenomenal research hotspot all over the world in the past decade. Although the synthesis, host-guest properties, and various supramolecular functions of pillarenes have been intensively studied, many objective limitations and challenges still cannot be ignored. For example, high-level pillar[n]arenes (n > 7) usually do not possess applicable large-sized cavities due to structural folding and cannot be synthesized on a large scale because of the uncompetitive cyclization process. Furthermore, two functional groups must be covalently para-connected to each repeating phenylene unit, which severely limits their structural diversity and flexibility. In this context, we have developed a series of pillarene-inspired macrocycles (PIMs) using a versatile and modular synthetic strategy during the past few years, aiming to break through the synthetic limitations in traditional pillarenes and find new opportunities and challenges in supramolecular chemistry and beyond. Specifically, by grafting biphenyl units into the pillarene backbones, extended pillar[n]arenes with rigid and nanometer-sized cavities could be obtained with reasonable synthetic yields by selectively removing hydroxy/alkoxy substitutes on pillarene backbones, leaning pillar[6]arenes and leggero pillar[n]arenes with enhanced structural flexibility and cavity adaptability were obtained. By combining the two types of bridging modes in pillarenes and calixarenes, a smart macrocyclic receptor with two different but interconvertible conformational features, namely geminiarene, was discovered. Benefiting from the synthetic accessibility, facile functionalization, and superior host-guest properties in solution or the solid state, this new family of macrocycles has exhibited a broad range of applications, including but not limited to supramolecular assembly/gelation/polymers, pollutant detection and separation, porous organic polymers, crystalline/amorphous molecular materials, hybrid materials, and controlled drug delivery. Thus, in this Account, we summarize our research efforts on these PIMs. We first present an overview of their design and modular synthesis and a summary of their derivatization strategies. Thereafter, particular attention is paid to their structural features, supramolecular functions, and application exploration. Finally, the remaining challenges and perspectives are outlined for their future development. We hope that this Account and our works can stimulate further advances in synthetic macrocyclic chemistry and supramolecular functional systems, leading to practical applications in various research areas.

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.

Stabilization of Antitumor Agent Busulfan through the Encapsulation within a Water-Soluble Pillar[5]arene

The complexation of antitumor agent busulfan by negatively charged carboxylatopillar[5]arenein water is reported. The encapsulation within carboxylatopillar[5]arenein reduces the hydrolytic degradation of busulfan from 90.7 % to 25.2 % after 24 days and accordingly enhances its stability by providing a hydrophobic shelter for busulfan in water. Moreover, the complexation result in 12 times improvement of water solubility for busulfan. Our result provides a supramolecular approach for stabilizing the anticancer agent busulfan.

Evidenced cucurbit[n]uril-based host-guest interactions using single-molecule force spectroscopy

In this work, enhanced guest-pair interactions in the cavity of cucurbit[8]uril (CB[8]) are quantitatively determined using single-molecule force spectroscopy (SMFS). Significantly, the light-driven dynamic conformational change of guest pairs leads to a rupture force switching between the connected and broken CB[8]-mediated heteroternary complexation with viologen and bis(azobenzene) derivatives. SMFS is further utilized to detect methyl viologen based on the competitive host-guest interaction toward the guest in CB[8] or CB[7]. These findings highlight the extraordinary power of SMFS in supramolecular chemistry and will contribute to the fundamental understanding of the mechanochemical behavior of host-guest interactions.

GOx-assisted synthesis of pillar[5]arene based supramolecular polymeric nanoparticles for targeted/synergistic chemo-chemodynamic cancer therapy

Background

Cancer is the most serious world's health problems on the global level and various strategies have been developed for cancer therapy. Pillar[5]arene-based supramolecular therapeutic nano-platform (SP/GOx NPs) was constructed successfully via orthogonal dynamic covalent bonds and intermolecular H-bonds with the assistance of glucose oxidase (GOx) and exhibited efficient targeted/synergistic chemo-chemodynamic cancer therapy.

Methods

The morphology of SP/GOx NPs was characterized by DLS, TEM, SEM and EDS mapping. The cancer therapy efficinecy was investigated both in vivo and in vitro.

Results

SP/GOx NPs can load drug molecules (Dox) and modify target molecule (FA-Py) on its surface conveniently. When the resultant FA-Py/SP/GOx/Dox NPs enters blood circulation, FA-Py will target it to cancer cells efficiently, where GOx can catalyst the overexpressed glucose to generate H₂O₂. Subsequently, the generated H₂O₂ in cancer cells catalyzed by ferrocene unit to form •OH, which can kill cancer cells. Furthermore, the loaded Dox molecules released under acid microenvironment, which can further achieve chemo-therapy.

Conclusion

All the experiments showed that the excellent antitumor performance of FA-Py/SP/GOx/Dox NPs, which provided an new method for pillar[5]arene-based supramolecular polymer for biomedical applications.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-01237-0.

Supramolecular systems prepared using terpyridine-containing pillararene

Recent progresses about the preparation of terpyridine-containing pillararene, as well as the utilization of those building blocks for making external stimulud-responsive supramolecular systems were summarized in this review.

Neutral Isocyanide-Templated Assembly of Pillar[5]arene [2] and [3]Pseudorotaxanes

Unprecedented pillar[5]arene–isocyanide pseudorotaxane inclusion complexes are reported. Extensive 1H-NMR experiments reveal remarkably strong binding affinities of alkyl diisocyanide guests (Ka >105 M-1 in CDCl3) by pillar[5]arenes. Characterised by multinuclear 1H...

Pillar[5]arene based supramolecular polymer for a singlet oxygen reservoir

A novel type of supramolecular polymer based on pillararene for the storage and control release of singlet oxygen.

Aggregation Mode, Host-Guest Chemistry in Water, and Extraction Capability of an Uncharged, Water-Soluble, Liquid Pillar[5]arene Derivative

An uncharged, water-soluble per-ethylene-glycol pillar[5]arene derivative (1) was synthesized and its aggregation mode, host-guest chemistry in water and extraction ability was explored. Compound 1 is a liquid at room temperature; in water, limited self-aggregation occurred at high concentrations as deduced from diffusion NMR and dynamic light scattering. Compound 1 forms pseudo-rotaxane-like 1 : 1 host-guest complexes with 1,ω-di-substituted alkanes with association constants on the order of 103 -104  m-1 . Interestingly, NMR experiments showed that the guest location relative to the host ring system differs among the different complexes. In proof-of-concept experiments, compound 1 was shown to extract structurally related organic compounds from benzene into water with significant selectivity. Compound 1, which is a liquid at room temperature and has only limited interactions with its side arms, can, in principle, be regarded as a complement to or as a kind of type I porous liquid.

MOF-based multi-stimuli-responsive supramolecular nanoplatform equipped with macrocycle nanovalves for plant growth regulation

Controllable and on-demand delivery of agrochemicals such as plant hormones is conducive to improving agrochemicals utilization, tackling water and environmental pollution, reducing soil acidification, and realizing the goals of precision agriculture. Herein, a smart plant hormone delivery system based on metal-organic frameworks (MOFs) and supramolecular nanovalves, namely gibberellin (GA)-loaded CLT6@PCN-Q, is constructed through supramolecular host-guest interaction to regulate the growth of dicotyledonous Chinese cabbage and monocotyledonous wheat. The porous nanoscale MOF (NMOF) with a uniform diameter of 97 nm modified by quaternary ammonium (Q) stalks is served as a cargo reservoir, followed by the decoration of carboxylated leaning tower[6]arene (CLT6) based nanovalves on NMOF surfaces through host-guest interactions to fabricate CLT6@PCN-Q with a diameter of ∼101 nm and a zeta potential value of -13.2 mV. Interestingly, the as-fabricated supramolecular nanoplatform exhibits efficient cargo loading and multi-stimuli-responsive release under various external stimuli including pH, temperature, and competitive agent spermine (SPM), which can realize the on-demand release of cargo. In addition, GA-loaded CLT6@PCN-Q is capable of effectively promoting the seeds germination of wheat and stem growth of dicotyledonous Chinese cabbage and monocotyledonous wheat (1.86 and 1.30 times of control groups, respectively). The smart supramolecular nanoplatform based on MOFs and supramolecular nanovalves paves a way for the controlled delivery of plant hormones and other agrochemicals for promoting plant growth, offering new insights and methods to realize precision agriculture. STATEMENT OF SIGNIFICANCE: To achieve controllable and sustainable release of cargos such as agrochemicals, a smart MOF-based multi-stimuli-responsive supramolecular nanoplatform equipped with supramolecular nanovalves was fabricated via the host-guest interaction between quaternary ammonium stalks-functionalized nanoMOFs and water-soluble leaning tower[6]arene. The as-prepared supramolecular nanoplatform with uniform diameter distribution demonstrated good cargo release in response to various external stimuli. The installation of synthetic macrocycles could effectively reduce cargo loss in the pre-treatment process. This type of supramolecular nanoplatform exhibited good promoting effect on seed germination and plant growth dicotyledonous Chinese cabbage and monocotyledonous wheat. As an eco-friendly, controlled, and efficient cargo delivery system, this supramolecular nanoplatform will be a promising candidate in precision agriculture and controlled drug release to attract the broad readership.

GSH/ROS Dual-Responsive Supramolecular Nanoparticles Based on Pillar[6]arene and Betulinic Acid Prodrug for Chemo-Chemodynamic Combination Therapy

Chemodynamic therapy (CDT) based on intracellular Fenton reactions is attracting increasing interest in cancer treatment. A simple and novel method to regulate the tumor microenvironment for improved CDT with satisfactory effectiveness is urgently needed. Therefore, glutathione (GSH)/ROS (reactive oxygen species) dual-responsive supramolecular nanoparticles (GOx@BNPs) for chemo–chemodynamic combination therapy were constructed via host–guest complexation between water-soluble pillar[6]arene and the ferrocene-modified natural anticancer product betulinic acid (BA) prodrug, followed by encapsulation of glucose oxidase (GOx) in the nanoparticles. The novel supramolecular nanoparticles could be activated by the overexpressed GSH and ROS in the tumor microenvironment (TME), not only accelerating the dissociation of nanoparticles—and, thus, improving the BA recovery and release capability in tumors—but also showing the high-efficiency conversion of glucose into hydroxyl radicals (·OH) in succession through intracellular Fenton reactions. Investigation of antitumor activity and mechanisms revealed that the dramatic suppression of cancer cell growth induced by GOx@BNPs was derived from the elevation of ROS, decrease in ATP and mitochondrial transmembrane potential (MTP) and, finally, cell apoptosis. This work presents a novel method for the regulation of the tumor microenvironment for improved CDT, and the preparation of novel GSH/ROS dual-responsive supramolecular nanoparticles, which could exert significant cytotoxicity against cancer cells through the synergistic interaction of chemodynamic therapy, starvation therapy, and chemotherapy (CDT/ST/CT).

Recyclable Supramolecular Assembly-Induced Emission System for Selective Detection and Efficient Removal of Mercury(II)

An efficient strategy for simultaneously detecting and removing Hg²⁺ from water is vital to address mercury pollution. Herein a supramolecular assembly G⊂H with photoluminescent properties is facilely constructed through the self-assembly of a functional pillar[5]arene bearing two N,N-dimethyldithiocarbamoyl binding sites (H) and an AIE-active tetraphenylethene derivative (G). Remarkably, the fluorescence of G⊂H can be exclusively quenched by Hg²⁺ among the 30 cations due to the formation of non-luminous ground state complex and only L-cysteine can restore fluorescence in the common 20 amino acids. Meanwhile, the probe G⊂H has a considerable thermal and pH stability, a good anti-interference property from various cations, and a satisfactory sensitivity. More importantly, G⊂H exhibits a prominent capability of Hg²⁺ removal with rapid capture rate (within 1 h) and excellent adsorption efficiency (98 %), as well as a highly efficient recyclability without losing any adsorption activity.

Supramolecular Sensing of Chemical Warfare Agents

Chemical warfare agents are a class of organic molecules used as chemical weapons due to their high toxicity and lethal effects. For this reason, the fast detection of these compounds in the environment is crucial. Traditional detection methods are based on instrumental techniques, such as mass spectrometry or HPLC, however the use of molecular sensors able to change a detectable property (e. g., luminescence, color, electrical resistance) can be cheaper and faster. Today, molecular sensing of chemical warfare agents is mainly based on the "covalent approach", in which the sensor reacts with the analyte, or on the "supramolecular approach", which involves the formation of non-covalent interactions between the sensor and the analyte. This Review is focused on the recent developments of supramolecular sensors of organophosphorus chemical warfare agents (from 2013). In particular, supramolecular sensors are classified by function of the sensing mechanism: i) Lewis Acids, ii) hydrogen bonds, iii) macrocyclic hosts, iv) multi-topic sensors, v) nanosensors. It is shown how the supramolecular non-covalent approach leads to a reversible sensing and higher selectivity towards the selected analyte respect to other interfering molecules.

"Texas-Sized" Molecular Boxes: From Chemistry to Applications

The design and synthesis of novel macrocyclic host molecules continues to attract attention because such species play important roles in supramolecular chemistry. However, the discovery of new classes of macrocycles presents a considerable challenge due to the need to embody by design effective molecular recognition features, as well as ideally the development of synthetic routes that permit further functionalization. In 2010, we reported a new class of macrocyclic hosts: a set of tetracationic imidazolium macrocycles, which we termed “Texas-sized” molecular boxes (TxSBs) in homage to Stoddart’s classic “blue box” (CBPQT⁴⁺). Compared with the rigid blue box, the first generation TxSB displayed considerably greater conformational flexibility and a relatively large central cavity, making it a good host for a variety of electron-rich guests. In this review, we provide a comprehensive summary of TxSB chemistry, detailing our recent progress in the area of anion-responsive supramolecular self-assembly and applications of the underlying chemistry to water purification, information storage, and controlled drug release. Our objective is to provide not only a review of the fundamental findings, but also to outline future research directions where TxSBs and their constructs may have a role to play.

Molecular Triangles: A New Class of Macrocycles

ConspectusMacrocycles have had a profound influence on the establishment of supramolecular chemistry because of their abundant molecular recognition and self-assembly characteristics. The design of new macrocyclic receptors that can be tailored by synthesis to display new and exotic properties is an important research objective for chemists and materials scientists. Rigid macrocycles with π-conjugated aromatic units, in contrast with flexible ones, tend to possess large interior and exterior π-surfaces in addition to persistent shapes. These features not only endow these macrocycles with a wide range of host-guest properties but also render them ideal building blocks for constructing a diverse variety of supramolecular architectures. The incorporation of π-conjugated units into macrocycles also imbues them with a wealth of optical, electronic, and magnetic properties, resulting in their broad application in materials science and molecular nanotechnology.Recently, we have designed and synthesized a new class of macrocycles, namely, molecular triangles, which have rigid structures with triangular geometries. They consist of three chiral trans-1,2-cyclohexano apexes and three aromatic tetracarboxylic diimide linkers, such as pyromellitic diimide, naphthalene diimide, and perylene diimide. Benefiting from the availability of facile synthetic protocols, the geometries and properties of these rigid molecular triangles can be altered at will. By combining these tetracarboxylic diimide linkers, we have been able to synthesize both molecular equilateral and isosceles triangles. During the past few years, we have conducted research in a systematic manner on the structural features and self-assembly characteristics of these molecular triangles. The following points are worthy of note regarding these molecular triangles: (i) They possess shape-persistent inner cavities of a highly electron-deficient nature. These features endow them with the ability to complex with anions and electron-rich molecules, forming supramolecular nanotubes and two-dimensional tilings. (ii) Those with intrinsic chirality are able to self-assemble into solid-state nonhelical or single-handed helical superstructures, inducing selective chirality transfer from the macrocycles to their crystalline supramolecular assemblies. (iii) The triangular arrangement of aromatic tetracarboxylic diimide linkers contributes to through-space electron delocalization encompassing the entire macrocycle, conferring exotic electronic and spin properties. To date, the family of molecular triangles has exhibited a range of physicochemical properties, such as anion recognition, chiral assembly, supramolecular gelation, energy storage, solid-state luminescence, and nonlinear optical response.In this Account, we summarize our recent progress in research into these molecular triangles. We present an overview of their design and synthesis, as well as a general summary of their structural features. Thereafter, we discuss state-of-the-art developments in relation to their molecular recognition properties and their assembly characteristics. In addition, we highlight the potential applications of these molecular triangles and their complexes with a range of solvents and electron-rich molecules. Finally, we speculate on further structural modifications and application-oriented explorations based on this class of molecular triangles.

Design Guidelines for Cationic Pillar[n]arenes that Prevent Biofilm Formation by Gram-Positive Pathogens

Bacterial biofilms are a major threat to human health, causing persistent infections that lead to millions of fatalities worldwide every year. Biofilms also cause billions of dollars of damage annually by interfering with industrial processes. Recently, cationic pillararenes were found to be potent inhibitors of biofilm formation in Gram-positive bacteria. To identify the structural features of pillararenes that result in antibiofilm activity, we evaluated the activity of 16 cationic pillar[5]arene derivatives including that of the first cationic water-soluble pillar[5]arene-based rotaxane. Twelve of the derivatives were potent inhibitors of biofilm formation by Gram-positive pathogens. Structure activity analyses of our pillararene derivatives indicated that positively charged head groups are critical for the observed antibiofilm activity. Although certain changes in the lipophilicity of the substituents on the positively charged head groups are tolerated, dramatic elevation in the hydrophobicity of the substituents or an increase in steric bulk on these positive charges abolishes the antibiofilm activity. An increase in the overall positive charge from 10 to 20 did not affect the activity significantly, but pillararenes with 5 positive charges and 5 long alkyl chains had reduced activity. Surprisingly, the cavity of the pillar[n]arene is not essential for the observed activity, although the macrocyclic structure of the pillar[n]arene core, which facilitates the clustering of the positive charges, appears important. Interestingly, the compounds found to be efficient inhibitors of biofilm formation were nonhemolytic at concentrations that are ∼100-fold of their MBIC₅₀ (the minimal concentration of a compound at which at least 50% inhibition of biofilm formation was observed compared to untreated cells). The structure–activity relationship guidelines established here pave the way for a rational design of potent cationic pillar[n]arene-based antibiofilm agents.

Metallo-Supramolecular Coordination Polymers Based on Amidopyridine Derivatives of Pillar[5]arene and Cu(II) and Pd(II) Cations: Synthesis and Recognition of Nitroaromatic Compounds

Decasubstituted pillar[5]arenes containing amidopyridine fragments have been synthesized for the first time. As was shown by UV-vis spectroscopy, the pillar[5]arenes with p-amidopyridine fragments form supramolecular associates with Cu(II) and Pd(II) cations in methanol in a 2:1 ratio. Using a sol-gel approach these associates are transformed into metallo-supramolecular coordination polymers (supramolecular gels) which were characterized as amorphous powders by scanning electron microscopy (SEM) and dynamic light scattering (DLS). The powders are able to selectively adsorb up to 46% of nitrophenols from water and were incorporated into an electrochemical sensor to selectively recognize them in aqueous acidic solution.

Supramolecular AIE polymer-based rare earth metallogels for the selective detection and high efficiency removal of cyanide and perchlorate

Two novel supramolecular AIE polymer-based rare earth metallogels (PT-GEu and PT-GTb) have been rationally designed and synthesized for the efficient detection and removal of cyanide (CN⁻) and perchlorate (ClO₄⁻).

Crown ether-pillararene hybrid macrocyclic systems

A combination of Nobel macrocycle-crown ether and star macrocycle-pillararenes together in organic synthesis and material science is significant in obtaining hybrid systems, with rigid/flexible structural architecture, induced planar chirality, a negative cooperative effect and multiple fused cyclic hosts. In this review, we will discuss the synthesis/preparation of crown ether-pillararene hybrid macrocyclic systems by covalent bonds, supramolecular interactions and mechanical bonds, leading to hybrid compounds, supramolecular assemblies and mechanically interlocked molecules. The practical applications of crown ether-containing pillararenes will also be discussed in diverse areas, such as molecular recognition via fused multiple macrocycles and ion channels as well as external stimuli-responsive smart materials. We also call the attention of related researchers towards academic and technical issues about topological structures and applied functions in this fresh new fused macrocyclic field.

Simultaneous determination of Acetaminophen and dopamine based on a water-soluble pillar[6]arene and ultrafine Pd nanoparticle-modified covalent organic framework nanocomposite

A novel electrochemical sensing platform for the simultaneous determination of AP and DA based on a water-soluble pillar[6]arene and ultrafine Pd nanoparticle-modified covalent organic framework nanocomposite.

Pillararene-based conjugated porous polymers

The synthesis and application of promising polymeric materials–pillararene-based conjugated porous polymers–are discussed and summarized in this review.

Stimuli-Responsive Internally Ion-Paired Supramolecular Polymer Based on a Bis-pillar[5]arene Dicarboxylic Acid Monomer

A novel bis-pillar[5]arene dicarboxylic acid self-assembles in the presence of 1,12-diaminododecane to yield overall neutral, internally ion-paired supramolecular polymers. Their aggregation, binding mode, and morphology can be tuned by external stimuli such as solvent polarity, concentration, and base treatment.

Supramolecular Tessellations via Pillar[n]arenes-Based Exo-Wall Interactions

Supramolecular tessellation is a newly emerging and promising area in supramolecular chemistry because of its unique structural aesthetics and potential applications. Herein, we investigate the "exo-wall" interactions of pillar[n]arenes and prepare fantastic hexagonal supramolecular tessellations based on perethylated pillar[6]arenes (EtP6) with electron-deficient molecules 1,5-difluoro-2,4-dinitrobenzene (DFN) and tetrafluoro-1,4-benzoquinone (TFB). The crystal structures clearly confirm that EtP6 can form highly ordered hexagonal 2D tiling patterns with DFN/TFB as linkers through cocrystallization. Moreover, the self-assembled packing arrangements in the ultimate cocrystal superstructures can be adjusted under different crystallization conditions. This work not only explores the rare exo-wall interactions based on pillar[n]arenes but also reports the fabrication of supramolecular tessellations based on pillararenes for the first time, showing a new perspective in supramolecular chemistry.