Molecular-Scale Porous Materials Based on Pillar[n]arenes

Supramolecular polymer networks based on pillar[5]arene: synthesis, characterization and application in the Fenton reaction

New supramolecular networks with ferrocene units were efficiently constructed via orthogonal pillar[5]arene-based host–guest and H-bonding interaction. It can be applied in Fenton-like reaction in water.

Morphology transformation of pillararene-based supramolecular nanostructures

In this feature article, the construction methods and the factors that influence the morphological transformation of pillararene-based supramolecular nanostructures are reviewed.

The synthesis and applications of porphyrin-containing pillararenes

Recent progress regarding the combination of porphyrins and pillararenes into hybrid compounds and supramolecular systems is summarized in this review.

A reversible, switchable pH-driven quaternary ammonium pillar[5]arene nanogate for mesoporous silica nanoparticles

Here we describe the assembly and pH-driven operation of two nanocarriers based on non-functionalized (MCM-41) and carboxylate-functionalized (MCM-41-COOH) containers loaded with the anticancer drug doxorubicin (DOX) and capped by quaternary ammonium pillar[5]arene (P[5]A) nanogates. MCM-41 and MCM-41-COOH containers were synthesized and transmission and scanning electron microscopies showed nanoparticles with spherical morphology and dimensions of 85 ± 13 nm. The nanochannels of MCM-41 loaded with DOX were gated through the electrostatic interactions between P[5]A and the silanolate groups formed at the silica-water interface, yielding the MCM-41-DOX-P[5]A nanocarrier. The second nanocarrier was gated through the electrostatic interactions between the carboxylate groups mounted on the surface of MCM-41 and P[5]A, resulting in the MCM-41-COO-DOX-P[5]A nanocarrier. The DOX release profiles from both nanocarriers were investigated by UV-vis spectroscopy at different pH values (2.0, 5.5 and 7.4) and also in the presence of ions, such as citrate^3- (19 mmol L^-1) and Zn²⁺ (1.2 and 50 mmol L^-1) at 37 °C. MCM-41-COO-DOX-P[5]A can be turned on and off eight times through the formation and breaking of electrostatic interactions. In vitro studies show that MCM-41-COO-DOX-P[5]A can penetrate and release DOX in the nucleus of human breast adenocarcinoma MCF-7 cancer cells leading to a pronounced cytotoxic effect. Therefore, the fabricated nanocarrier based on a water-soluble cationic pillar[5]arene nanogate, which is reversibly opened and closed by electrostatic interactions, can be considered as a promising drug transport and delivery technique for future cancer therapy.

Novel Amino-pillar[5]arene as a Fluorescent Probe for Highly Selective Detection of Au3+ Ions

A novel fluorescent probe, amino-pillar[5]arene (APA), was prepared via a green, effective, and convenient synthetic method, which was characterized by nuclear magnetic resonance (NMR), infrared (IR), and high-resolution mass spectrometry. The fluorescence sensing behavior of the APA probe toward 22 metal ions in aqueous solutions were studied by fluorescence spectroscopy. The results showed that APA could be used as a selective fluorescent probe for the specificity detection of Au3+ ions. Moreover, the detection characteristics were investigated by fluorescence spectral titration, pH effect, fluorescence competitive experiments, Job's plot analysis, 1H NMR, and IR. The results indicated that detection of Au3+ ions by the APA probe could be achieved in the range of pH 1-13.5 and that other coexisting metal ions did not cause any marked interference. The titration analysis results indicated that the fluorescence intensity decreased as the concentration of Au3+ ions increased, with an excellent correlation (R 2 = 0.9942). The detection limit was as low as 7.59 × 10-8 mol·L-1, and the binding ratio of the APA probe with Au3+ ions was 2:1. Therefore, the APA probe has potential applications for detecting Au3+ ions in the environment and in living organisms.

Conformational dynamics and interfacial interactions of peptide-appended pillar[5]arene water channels in biomimetic membranes

Peptide appended pillar[5]arene (PAP) is an artificial water channel resembling biological water channel proteins, which has shown a significant potential for designing bioinspired water purification systems. Given that PAP channels need to be incorporated at a high density in membrane matrices, it is critical to examine the role of channel-channel and channel-membrane interactions in governing the structural and functional characteristics of channels. To resolve the atomic-scale details of these interactions, we have carried out atomistic molecular dynamics (MD) simulations of multiple PAP channels inserted in a lipid or a block-copolymer (BCP) membrane matrix. Classical MD simulations on a sub-microsecond timescale showed clustering of channels only in the lipid membrane, but enhanced sampling MD simulations showed thermodynamically-favorable dimerized states of channels in both lipid and BCP membranes. The dimerized configurations of channels, with an extensive buried surface area, were stabilized via interactions between the aromatic groups in the peptide arms of neighboring channels. The conformational metrics characterizing the orientational and structural changes in channels revealed a higher flexibility in the lipid membrane as opposed to the BCP membrane although hydrogen bonds between the channel and the membrane molecules were not a major contributor to the stability of channels in the BCP membrane. We also found that the channels undergo wetting/dewetting transitions in both lipid and BCP membranes with a marginally higher probability of undergoing a dewetting transition in the BCP membrane. Collectively, these results highlight the role of channel dynamics in governing channel-channel and channel-membrane interfacial interactions, and provide atomic-scale insights needed to design stable and functional biomimetic membranes for efficient separations.

Redox-Induced Molecular Actuators: The Case of Oxy-Alternate Bridged Cyclotetraveratrylene

We report a practical two-step approach for the synthesis of hybrid-bridge macrocyclic molecules that has been used to synthesize two novel oxy-alternate-bridged macrocyclic molecules, oxy-alternate cyclotetraveratrylene (^O-altCTTV) and oxy-alternate cyclohexaveratrylene (^O-altCHV). Electrochemistry, absorption spectroscopy, X-ray crystallography, and DFT calculations demonstrate that ^O-altCTTV acts as a redox-induced molecular actuator, as its switches from the open conformation in the neutral state to the closed conformation in the cation-radical state.

Construction of Type III-C Rotaxane-Branched Dendrimers and Their Anion-Induced Dimension Modulation Feature

Starting from a novel rotaxane building block with dendrimer growth sites being located at both the wheel and axle component, we realized the successful construction of a new family of rotaxane-branched dendrimers, i.e., Type III-C rotaxane-branched dendrimers, up to fourth generation as a highly branched [46]rotaxane through a controllable divergent approach. In the resultant rotaxane-branched dendrimers, the wheel components of the rotaxane units are located on the branches as well as at the branching points, making them excellent candidates to mimic the amplified collective molecular motions. Thus, taking advantage of the urea moiety inserted into the axle components of the rotaxane units as the binding sites, the addition or removal of acetate anion as stimulus endows the individual rotaxane unit a switchable feature that lead to a collective expansion-contraction motion of the integrated rotaxane-branched dendrimers, thus allowing for the remarkable and reversible size modulation. Such a three-dimensional size switching feature makes Type III-C rotaxane-branched dendrimers a very promising platform toward the fabrication of novel dynamic smart materials.

Supramolecular trap for catching polyamines in cells as an anti-tumor strategy

Polyamines are essential for the growth of eukaryotic cells and can be dysregulated in tumors. Here we describe a strategy to deplete polyamines through host-guest encapsulation using a peptide-pillar[5]arene conjugate (P1P5A, P1 = RGDSK(N₃)EEEE) as a supramolecular trap. The RGD in the peptide sequence allows the molecule to bind to integrin α_vβ₃-overexpressing tumor cells. The negative charged glutamic acid residues enhance the inclusion affinities between the pillar[5]arene and cationic polyamines via electrostatic interactions and facilitate the solubility of the conjugate in aqueous media. The trap P1P5A efficiently encapsulates polyamines with association constants of 10⁵-10⁶ M^-1. We show that P1P5A has a wide spectrum of antitumor activities, and induces apoptosis via affecting the polyamine biosynthetic pathway. Experiments in vivo show that P1P5A effectively inhibits the growth of breast adenocarcinoma xenografts in female nude mice. This work reveals an approach for suppressing tumor growth by using supramolecular macrocycles to trap polyamines in tumor cells.

Recognition Selectivities of Lasso-Type Pseudo[1]rotaxane Based on a Mono-Ester-Functionalized Pillar[5]arene

Two types of mono-ester-functionalized pillar[5]arenes, P1 and P2, bearing different side-chain groups, were synthesized. Their host–guest complexation and self-inclusion properties were studied by ¹H NMR and 2D nuclear overhauser effect spectroscopy (NOESY) NMR measurements. The results showed that the substituents on their phenolic units have a great influence on the self-assembly of both pillar[5]arenes, although they both could form stable pseudo[1]rotaxanes at room temperature. When eight bulky 4-brombutyloxy groups were capped on the cavity, instead of methoxy groups, pseudo[1]rotaxane P1 became less stable and its locked ester group in the inner space of cavity was not as deep as P2, leading to distinctly different host–guest properties between P1 and P2 with 1,6-dibromohexane. Moreover, pillar[5]arene P1 displayed effective molecular recognition toward 1,6-dichlorohexane and 1,2-bromoethane among the guest dihalides. In addition, the self-complex models and stabilities between P1 and P2 were also studied by computational modeling and experimental calculations.

Functionalization at Will of Rim-Differentiated Pillar[5]arenes

The development of an efficient synthetic route toward rim-differentiated C₅-symmetric pillar[5]arenes (P[5]s), whose two rims are decorated with different chemical functionalities, opens up successive transformations of this macrocyclic scaffold. This paper describes a gram-scale synthesis of a C₅-symmetric penta-hydroxy P[5] precursor, and a range of highly efficient reactions that allow functionalizing either rim at will via, e.g., sulfur(VI) fluoride exchange (SuFEx) reactions, esterifications, or Suzuki–Miyaura coupling. Afterward, BBr₃ demethylation activates another rim for similar functionalizations.

One-, Two-, and Three-Dimensional Supramolecular Assemblies Based on Tubular and Regular Polygonal Structures of Pillar[n]arenes

Pillar[ n]arenes, which were first reported by our group in 2008, are promising macrocyclic compounds in supramolecular chemistry. The simple, tubular, and highly symmetrical shape of pillar[ n]arenes has allowed various supramolecular assemblies with well-defined structures to be constructed. The pillar-shaped structures of pillar[ n]arenes are suitable for surface modification and formation of one-dimensional (1D) channels. The regular polygonal prism shape of organized pillar[ n]arenes contributes to the construction of highly assembled structures such as two-dimensional (2D) sheets and three-dimensional (3D) spheres. In this minireview, we describe supramolecular assemblies with various dimensions. First, we discuss 1D supramolecular assemblies based on tubular structures of pillar[ n]arenes. Second, 2D supramolecular sheet formation based on regular polygonal structures is described. Finally, 3D supramolecular assemblies such as vesicles and 3D frameworks constructed from pillar[ n]arenes are discussed.

Conjugated Macrocycle Polymer Nanoparticles with Alternating Pillarenes and Porphyrins as Struts and Cyclic Nodes

Conjugated macrocycle polymers (CMPs) integrated using the macrocyclic confinement effect make imposing restrictions feasible on the growth of metal nanoparticles with confined size and high dispersion. For a proof-of-concept exploration, a novel nanoscale CMP is reported, denoted as DMP[5]-TPP-CMP, comprising two representative types of macrocyclic compounds, i.e., pillararene and porphyrin, as alternating strut/node components in the skeleton. With abundant anchoring sites, CMP implanted with Pd nanoparticles (Pd@DMP[5]-TPP-CMP, Pd@CMP for short) is successfully obtained through a simple post-treatment, exhibiting remarkable catalytic activity in Suzuki-Miyaura coupling (SMC) and nitrophenol reduction. The as-prepared Pd@CMP material shows favorable performance in expediting the process of SMC with an appreciable yield even under mild conditions, as well as in facilitating the electron transfer process from borohydride to nitrophenol through metal-hydride complex to produce aminophenol with a very short transformation time of 3 min and superior apparent kinetic rate constant k _app of 1.9 × 10^-2 s^-1 , higher than most palladium supports. Significantly, this multifunctional Pd@CMP composite material not only enriches the family of CMPs, but also sheds light on the development of green catalysts with excellent stability and easy recyclability without deactivation.

Molecular weight fractionation by confinement of polymer in one-dimensional pillar[5]arene channels

Confinement of polymers in nano-spaces can induce unique molecular dynamics and properties. Here we show molecular weight fractionation by the confinement of single polymer chains of poly(ethylene oxide) (PEO) in the one-dimensional (1D) channels of crystalline pillar[5]arene. Pillar[5]arene crystals are activated by heating under reduced pressure. The activated crystals are immersed in melted PEO, causing the crystals to selectively take up PEO with high mass fraction. The high mass fractionation is caused by the greater number of attractive CH/π interactions between PEO C-H groups and the π-electron-rich 1D channel of the pillar[5]arene with increasing PEO chain length. The molecular motion of the confined PEO (PEO chain thickness of ~3.7 Å) in the 1D channel of pillar[5]arenes (diameter of ~4.7 Å) is highly restricted compared with that of neat PEO.

Confinement of polymers in nano-spaces can induce unique molecular dynamics and properties. Here the authors show high mass fractionation by the confinement of single polymer chains of poly(ethylene oxide) in the one-dimensional channels of crystalline pillar[5]arene.

Water-Soluble Pillar[n]arene Mediated Supramolecular Self-Assembly: Multi-Dimensional Morphology Controlled by Host Size

We report tunable supramolecular self-assemblies formed by water-soluble pillar[n]arenes (WPns, n=5-7) and bipyridinium-azobenzene guests. Nanoscale or microscale morphology of self-assemblies in water was controlled by the host size of WPn. Supramolecular self-assemblies could undergo morphology conversion under irradiation with UV light.

Pillararene-based supramolecular polymers

Pillararenes, as a new type of macrocyclic hosts, possess columnar structures and electron-rich cavities. Pillararenes not only recognize suitable cations, but also bind many neutral molecules. Due to the easy modification of pillararenes, various functional groups can be conveniently attached to the rim of pillararenes to provide suitable interaction sites, and the modified pillararenes even bind anionic guests. Thus, pillararenes and their derivatives have presented intriguing and unique host-guest recognition nature in the past few years, which make them ideal building blocks for the preparation of supramolecular polymers. Pillararene-based supramolecular polymers (PSPs) not only possess many merits of traditional covalent polymers but also have many specific properties, such as self-reparability, degradability, and self-adaptation. This feature paper gives an overview of the preparation of PSPs and covers recent research advance and future trends of pillararene-based host-guest pairs, assembly methods, topological architectures, stimuli-responsiveness, and functional features. We expect that the review will be helpful to researchers working in the fields of supramolecular chemistry and polymer science.

Semi-Rigid Molecular-Clip-Based Molecular Crystal Gearshift

A new version of molecular clip, with a semi-rigid symmetrical crab-type architecture and flexible cavity size, has been successfully designed and synthesized via a one-pot Friedel-Crafts alkylation reaction. The X-ray single-crystal diffraction data provide a simple and intuitive explanation, not only for its well-preorganized and regulated conformation but also for its selective and tunable guest-binding capability. For the first time, the newly designed molecular clip was demonstrated to be not only a controllable variable-speed nonporous adsorption material in solution iodine capture, but also capable of on-off switching in volatile iodine capture. The presented new concept of molecular crystal gearshift directly from the molecular clip crystals represents an important advance in the development of synthetic receptor chemistry, which will exert a significant influence on small-molecule crystallography.

A fluorescent pillarene coordination polymer

A fluorescent pillarene-based coordination polymer was constructed from dicarboxylic acid-functionalized copillar[5]arene and Cr³⁺, for sensing of Fe³⁺, acetone, and nitrophenols.

Functionalization of inorganic nanomaterials with pillar[n]arenes

Pillar[n]arenes, which consist of hydroquinone units linked by -CH2- bridges at 2,5-positions, are a relatively new class of synthetic macrocycles since 2008. Their facile preparation and flexible modification properties make them ideal stabilizers for inorganic nanomaterials. Furthermore, their symmetrical and columnar architectures with very rigid and π-rich cavities endow them with rich host-guest properties. This Feature Article provides an overview of the functionalization of inorganic nanomaterials with pillar[n]arenes and their applications. These inorganic nanomaterials are classified into three major classes according to different types of compositions: (1) novel metal nanomaterials; (2) hybrid metal nanomaterials; and (3) porous materials. The applications of these nanomaterials such as catalysis, drug delivery, cancer therapy, and sensing have been comprehensively discussed.

Fluorescent supramolecular polymers with aggregation induced emission properties

This review summarizes the recent developments in AIE fluorescent supramolecular polymeric materials based on different types of intermolecular noncovalent interactions, and their wide ranging applications as chemical sensors, organic electronic materials, bio-imaging agents and so on.

Red-light-responsive molecular encapsulation in water: an ideal combination of photochemistry and host–guest interaction

Red-light-responsive CB[8]-mediated host–guest system featured with high photoisomerization ratio was fabricated which could be used as molecular container with red-light-activated release ability.

A pillar[5]arene with an amino-terminated arm stabilizes the formation of aliphatic hemiaminals and imines

A self-included mono-amino substituted pillar[5]arene efficiently stabilizes the hemiaminal and imine formation from the reaction of aliphatic amines and aldehydes.

New opportunities in synthetic macrocyclic arenes

This feature article summarizes the latest research progress in the design and development of new synthetic macrocyclic arenes.

From Supramolecular Vesicles to Micelles: Controllable Construction of Tumor-Targeting Nanocarriers Based on Host-Guest Interaction between a Pillar[5]arene-Based Prodrug and a RGD-Sulfonate Guest

The targeting ability, drug-loading capacity, and size of the drug nanocarriers are crucial for enhancing the therapeutic index for cancer therapy. Herein, the morphology and size-controllable fabrication of supramolecular tumor-targeting nanocarriers based on host-guest recognition between a novel pillar[5]arene-based prodrug WP5-DOX and a Arg-Gly-Asp (RGD)-modified sulfonate guest RGD-SG is reported. The amphiphilic WP5-DOX⊃RGD-SG complex with a molar ratio of 5:1 self-assembles into vesicles, whereas smaller-sized micelles can be obtained by changing the molar ratio to 1:3. This represents a novel strategy of controllable construction of supramolecular nanovehicles with different sizes and morphologies based on the same host-guest interactions by using different host-guest ratios. Furthermore, in vitro and in vivo studies reveal that both these prodrug nanocarriers could selectively deliver doxorubicin to RGD receptor-overexpressing cancer cells, leading to longer blood retention time, enhanced antitumor efficacy, and reduced systematic toxicity in murine tumor model, suggesting their potential application for targeted drug delivery.

The synthesis of water-soluble phosphate pillar[5]arenes functionalized graphene as a fluorescent probe for sensitive detection of paraquat

We describe a selective and sensitive fluorescence platform for the detection of paraquat (PQ) based on competitive host-guest recognition between phosphate pillar[5]arenes (PWP5) and probe (Safranine T, ST) with using PWP5 functionalized reduced graphene (PWP5-rGO) as the receptor. PQ is a positive charge molecule that is captured by PWP5 via electrostatic interactions. The host-guest interaction between PWP5 and PQ is studied by ¹H NMR. Therefore, a selective and sensitive fluorescence sensing of detection PQ is developed. It has a linear response ranges of 0.01-2.0 and 2.0-50.0 μM and a low detection limit of 0.0035 μM (S/N = 3) for PQ. The sensing platform is also used to test PQ in two water samples with satisfying results. It suggests that this approach has potential applications for the determination of PQ.

Gated Materials: Installing Macrocyclic Arenes-Based Supramolecular Nanovalves on Porous Nanomaterials for Controlled Cargo Release

Supramolecular nanovalves are an emerging class of important elements that are functionalized on the surfaces of inorganic or hybrid nanocarriers in the constructions of smart cargo delivery systems. Taking advantage of the pseudorotaxane structure via host-guest complexation and the dynamic nature of supramolecular interactions, macrocyclic arene-based supramolecular nanovalves have shown great promise in the applications of drug delivery and controlled release. Careful selection of diverse external stimuli, such as pH variations, temperature changes, redox, enzymes, light irradiation, and competitive binding, can activate the opening and closing of the nanovalves by altering the supramolecular structure or binding affinities. Meanwhile, the porous solid supports in controlled release systems also play an important role in the functionalities of the nanocarriers, which include, but not limited to, mesoporous silica nanoparticles (MSNs), metal-organic frameworks (MOFs), core-shell nanomaterials, and rare-earth porous nanomaterials. The elaborate decoration by macrocyclic arenes-based supramolecular nanovalves on porous nanomaterials has provided intelligent controlled release platforms. In this review, we will focus on the overview of supramolecular nanovalves based on two typical macrocyclic arenes, that is, calixarenes and pillarenes, and their operation manners in the controlled release processes.

Multifunctional Supramolecular Materials Constructed from Polypyrrole@UiO-66 Nanohybrids and Pillararene Nanovalves for Targeted Chemophotothermal Therapy

Multifunctional supramolecular nanomaterials capable of targeted and multimodal therapy hold great potential to improve the efficiency of cancer therapeutics. Herein, we report a proof-of-concept nanoplatform for effective chemophotothermal therapy via the integration of folic acid-based active targeting and supramolecular nanovalves-based passive targeting. Inspired by facile surface engineering and designable layer-by-layer assembly concept, we design and synthesize PPy@UiO-66@WP6@PEI-Fa nanoparticles (PUWPFa NPs) to achieve efficient synergistic chemophotothermal therapy, taking advantage of the desirable photothermal conversion capability of polypyrrole nanoparticles (PPy NPs) and high drug-loading capacity of hybrid scaffolds. Significantly, pillararene-based pseudorotaxanes as pH/temperature dual-responsive nanovalves allow targeted drug delivery in pathological environment with sustained release over 4 days, which is complementary to photothermal therapy, and folic acid-conjugated polyethyleneimine (PEI-Fa) at the outmost layer through electrostatic interactions is able to enhance tumor-targeting and therapeutic efficiency. Such PUWPFa NPs showed efficient synergistic chemophotothermal therapy of cervical cancer both in vitro and in vivo. The present strategy offers not only the distinctly targeted drug delivery and release, but also excellent tumor inhibition efficacy of simultaneous chemophotothermal therapy, opening a new avenue for effective cancer treatment.

Antiadhesive and antibacterial properties of pillar[5]arene-based multilayers

A new type of coating, based on carboxylato-pillar[5]arene/poly(allylamine hydrochloride) multilayer films, for the sustained release of antibiotics with in vitro antiadhesive and antimicrobial activity against Gram-positive and Gram-negative bacteria is described.

Comparative Study on the Supramolecular Assemblies Formed by Calixpyridinium and Two Alginates with Different Viscosities

In this work, a comparative study on the supramolecular assemblies formed by calixpyridinium and two alginates with different viscosities was performed. We found that sodium alginate (SA) with medium viscosity (SA-M) had a better capability to induce aggregation of calixpyridinium in comparison with SA with low viscosity (SA-L) because of the stronger electrostatic interactions between calixpyridinium and SA-M. Therefore, the morphology of calixpyridinium-SA-M supramolecular aggregates was a compact spherical structure, while that of calixpyridinium-SA-L supramolecular aggregates was an incompact lamellar structure. As a result, adding much more amount of 1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt to calixpyridinium-SA-M solution was required to achieve the balance of the competitive binding, and in comparison with calixpyridinium-SA-L supramolecular aggregates, calixpyridinium-SA-M supramolecular aggregates were more sensitive to alkali. However, for the same reason, in comparison with calixpyridinium-SA-M supramolecular aggregates, calixpyridinium-SA-L supramolecular aggregates were much more stable in water not only at room temperature but also at a higher temperature, and even in salt solution. Therefore, in comparison with calixpyridinium-SA-L supramolecular aggregates, calixpyridinium-SA-M supramolecular aggregates exhibited a completely opposite response to acid because of the generation of salt. Because SA is an important biomaterial with excellent biocompatibility, it is anticipated that this comparative study is extremely important in constructing functional supramolecular biomaterials.