Supramolecular polymers constructed from macrocycle-based host-guest molecular recognition motifs

Controlled Formation of a Main Chain Supramolecular Polymer Based on Metal-Ligand Interactions and a Thiol-Ene Click Reaction

Supramolecular polymers with multiple functionalities and hierarchical structures have received considerable attention and become a hot research topic over the past years. Herein, a main-chain supramolecular polymer has been successfully fabricated by using metal-ligand interactions and a thiol-ene click reaction. ¹ H NMR, UV/Vis, DOSY, and viscosity measurements were carried out to investigate the molecular recognition and the process of supramolecular polymerization. From the study, the orthogonality between thiol-ene click reactions and the terpyridine-metal ions complexation behavior was testified, and supramolecular polymeric assemblies could be constructed by a one pot method. In the meantime, due to the incorporation of metal-ligand interactions, the supramolecular polymer shows stimuli-responsive properties toward chemical stimuli. Hence, this work could provide a methodology for developing supramolecular polymers as smart materials.

Cucurbit[8]uril-Based Polymers and Polymer Materials

Cucurbit[8]uril (CB[8]) is unique and notable in the cucurbit[n]uril family, since it has a relatively large cavity and thus is able to simultaneously accommodate two guest molecules. Typically, an electron-deficient first guest and an electron-rich second guest can be bound by CB[8] to form a stable 1:1:1 heteroternary supramolecular complex. Additionally, two homo guests can also be strongly dimerized inside the cavity of CB[8] to form a 2:1 homoternary supramolecular complex. During the past decade, by combining polymer science and CB[8] host-guest chemistry, a variety of systems have been established to construct supramolecular polymers with polymer chains typically at the nanoscale/sub-microscale, and CB[8]-based micro/nanostructured polymer materials in the form of polymer networks and hydrogels, microcapsules, micelles, vesicles, and colloidal particles, normally in solution and occasionally on surfaces. This Review summarizes the noncovalent interactions and strategies used for the preparation of CB[8]-based polymers and polymer materials with a focus on the representative and latest developments, followed by a brief discussion of their characterization, properties, and applications.

Preparation of cross-linked supramolecular polymers based on benzo-21-crown-7/secondary ammonium salt host-guest interactions

We found that TC7 not only self-assembles into one-dimensional supramolecular aggregates in chloroform, but also forms cross-linked supramolecular polymers via host-guest complexation between benzo-21-crown-7 and secondary ammonium salts. Compared with one-dimensional linear supramolecular polymers, soft and long viscous fibers were pulled out from a concentrated solution of cross-linked supramolecular polymers as a result of higher viscosity and lower diffusion coefficients.

Dynamic hydrogels mediated by macrocyclic host-guest interactions

Hydrogels have attracted increasing research interest in recent years due to their dynamic properties and potential applications in biomaterials. Concurrently, macrocycle-based host-guest interactions have played an important role in the development of supramolecular chemistry. Recently, research towards dynamic hydrogels mediated by various macrocyclic host-guest interactions has been gradually disclosed. In this review, we will outline the burgeoning progress in the development of functional hydrogels mediated by various host molecules, such as cyclodextrins, cucurbit[n]urils, calix[n]arenes, pillar[n]arenes, and other macrocycles. Smart hydrogels with outstanding properties, like biocompatibility, toughness, and self-healing, are mainly focused. We believe that this review will highlight the potential of dynamic hydrogels mediated by macrocycle-based host-guest interactions.

Alleviation of Polycation-Induced Blood Coagulation by the Formation of Polypseudorotaxanes with Macrocyclic Cucurbit[7]uril

Hexadimethrine bromide (HB), a polycation to neutralize heparin and control internal bleeding, may also cause serious blood coagulation that may be life-threatening. Reversal of HB by heparin and vice versa might potentially lead to a vicious circle of alternative bleeding and blood coagulation. In this Letter, a biocompatible synthetic nanoreceptor, cucurbit[7]uril (CB[7]), was demonstrated to dramatically alleviate HB-induced blood coagulation in vitro as well as in vivo, through the formation of HB@CB[7] polypseudorotaxanes. This discovery, for the first time, suggests the significant clinical potential of a synthetic receptor in alleviating the side effects and toxicity of a polycationic drug.

Insight into Water-Soluble Highly Fluorescent Low-Dimensional Host-Guest Supramolecular Polymers: Structure and Energy-Transfer Dynamics Revealed by Polarized Fluorescence Spectroscopy

Water-soluble, highly fluorescent host-guest chromophore-cucurbit[8]uril supramolecular polymer bundles are investigated by polarized time-resolved photoluminescence spectroscopy, structural methods, and quantum chemistry to fully reveal structural organization and heterogeneity but, in particular, energy-transfer dynamics, being of crucial importance for the design of supramolecular artificial light-harvesting systems.

Pillararene-Based Two-Component Thixotropic Supramolecular Organogels: Complementarity and Multivalency as Prominent Motifs

Rationally designed two-component supramolecular organogels based on multiple chemical interactions between percarboxylato- and peramino-pillararenes are described. Mixing low concentration solutions (<1 % w/v) of decacarboxylato-pillar[5]arene (1) with decaamino-pillar[5]arenes (2 b-d) affords, rapidly and without heating, organogels displaying an exceptional combination of properties. These supramolecular organogels, the characteristics of which are tunable, were found to be thixotropic and thermally stable, with T_gel values in some cases exceeding the boiling point of the embedded solvent. It is demonstrated that both structural complementarity and multivalency are important determinants in the gelation process of these attractive soft materials.

Biomimetic Supramolecular Fibers Exhibit Water-Induced Supercontraction

Spider silk is a fascinating material, combining high strength and elasticity that outperforms most synthetic fibers. Another intriguing feature of spider silk is its ability to "supercontract," shrinking up to 50% when exposed to water. This is likely on account of the entropy-driven recoiling of secondary structured proteins when water penetrates the spider silk. In contrast, humidity-driven contraction in synthetic fibers is difficult to achieve. Here, inspired by the spider silk model, a supercontractile fiber (SCF), which contracts up to 50% of its original length at high humidity, comparable to spider silk, is reported. The fiber exhibits up to 300% uptake of water by volume, confirmed via environmental scanning electron microscopy. Interestingly, the SCF exhibits tunable mechanical properties by varying humidity, which is reflected by the prolonged failure strain and the reversible damping capacity. This smart supramolecular fiber material provides a new opportunity of fabricating biomimetic muscle for diverse applications.

Terminal Molecular Isomer-Effect on Supramolecular Self-Assembly System Based on Naphthalimide Derivative and Its Sensing Application for Mercury(II) and Iron(III) Ions

A series of naphthalimide derivative gelators (G-o, G-m, and G-p) with three molecular isomers as their terminal groups were designed and synthesized. Only G-m and G-p could form stable organogels in some solvents including methanol, acetonitrile, n-hexane, toluene, ethanol, DMSO, DMF, and mixed solvents of acetonitrile/H₂O (1/1, v/v). The different self-assembly structures were obtained from the self-assembly process of G-o, G-m, and G-p such as structures like a Chinese chestnut formed by irregular micrometer pieces, microbelts, and microbelt structures mingled with the bird's nest structures which exhibited different surface hydrophobicity with water contact angles of 121-139° due to their different intermolecular noncovalent interactions. To our surprise, G-p acetonitrile solution emitted 492 nm light with a red-shift of 72 nm compared with that emitted from G-o and G-m acetonitrile solution under 350 nm light excitation. Three gelators showed different detection abilities toward metal ions. G-o did not have any ability for sensitive and selective detection toward any ion. In contrast, G-m and G-p could sensitively and selectively detect Hg²⁺ and Fe³⁺. The detection limits for Fe³⁺ and Hg ²⁺ by G-m were 4.76 × 10^-5 M and 7.01 × 10^-6 M with the corresponding association constants ( K) of 1.64 × 10⁴ and 3.79 × 10⁴ M^-1, respectively. The detection limits for Fe³⁺ and Hg²⁺ by G-p were 3.26 × 10^-5 and 1.77 × 10^-6 M with the corresponding K of 1.44 × 10⁵ and 1.99 × 10⁴ M^-1, respectively. More interestingly, the back-titration of SCN^- could distinguish Hg²⁺ from Fe³⁺. At the same time, xerogels G-m and G-p also exhibited responsiveness toward Fe³⁺ and Hg²⁺ through fluorescence changes. The photophysical properties, gel formation, hierarchical structures, surface wettability, and their function in this self-assembly system could be tuned through the molecular isomer effect. This work provides a new research paradigm for molecular isomer tuned supramolecular self-assembly materials from noncovalent interaction to molecular function.

Molecular modelling of supramolecular one dimensional polymers

Supramolecular polymers exemplify the need to employ several computational techniques to study processes and phenomena occuring at varied length and time scales. Electronic processes, conformational and configurational excitations of small aggregates of chromophoric molecules, solvent effects under realistic thermodynamic conditions and mesoscale morphologies are some of the challenges which demand hierarchical modelling approaches. This review focusses on one-dimensional supramolecular polymers, the mechanism of self-assembly of monomers in polar and non-polar solvents and properties they exhibit. Directions for future work are as well outlined.

Reductive-Responsive, Single-Molecular-Layer Polymer Nanocapsules Prepared by Lateral-Functionalized Pillar[5]arenes for Targeting Anticancer Drug Delivery

Herein, a new reductive-responsive pillar[5]arene-based, single-molecule-layer polymer nanocapsule is constructed for drug delivery. The functionalized system shows good biocompatibility, efficient internalization into targeted cells and obvious triggered release of entrapped drugs in a reducing environment such as cytoplasm. Besides, this smart vehicle loaded with anticancer drug shows excellent inhibition for tumor cell proliferation and exhibits low side effect on normal cells. This work not only demonstrates the development of a new reductive-responsive single molecular layer polymer nanocapsule for anticancer drug targeting delivery but also extends the design of smart materials for biomedical applications.

Tetraphenylethylene-Interweaving Conjugated Macrocycle Polymer Materials as Two-Photon Fluorescence Sensors for Metal Ions and Organic Molecules

A luminescent conjugated macrocycle polymer (CMP) with strong two-photon fluorescence property, namely, P[5]-TPE-CMP, is constructed from ditriflate-functionalized pillar[5]arene and a 1,1,2,2-tetrakis(4-ethynylphenyl)ethylene (TPE) linker through a Sonogashira-Hagihara cross-coupling reaction. Significantly, in sharp contrast with the corresponding conjugated microporous polymer without synthetic macrocycles, P[5]-TPE-CMP shows an outstanding stability against photobleaching and exhibits highly selective cation sensing capability toward Fe³⁺ at different excitation wavelengths (both UV and red-near-infrared regions). Meanwhile, its fluorescence could also be sufficiently quenched by 4-amino azobenzene, a frequently used organic dye that is certified to be carcinogenic, as compared with a group of common organic compounds. This work paves a new way for enhancing the properties of porous organic polymers through the introduction of supramolecular macrocycles like macrocyclic arenes.

Effect of Magnesium Bond on the Competition Between Hydrogen and Halogen Bonds and the Induction of Proton and Halogen Transfer

HOX (X=Cl, Br, I, and At) can engage in either a H-bond (HB) or halogen bond (XB) with a base-like HCN, NH₃ , and imidazole. Although the former is energetically preferred for X=Cl and Br, it is the XB that is more stable for At, with I showing little preference. MgY₂ forms a Mg-bond with the O atom of HOX, which grows stronger in the order X=Cl<Br<I<At and Y=F<Cl<Br. When all three molecules are combined, both the Mg and the H/X bonds are cooperatively strengthened to a large degree. Rather than causing a reversal in the HB/XB competition, the Mg-bond acts primarily to amplify the natural preference within the dimer. The Mg-bond induces a certain degree of transfer from O to N of the bridging atom in the H/X bond. Comparison is also made with the effects of a Be-bond.

Chemistry and engineering of cyclodextrins for molecular imaging

Cyclodextrins (CDs) are naturally occurring cyclic oligosaccharides bearing a basket-shaped topology with an "inner-outer" amphiphilic character. The abundance of hydroxyl groups enables CDs to be functionalized with multiple targeting ligands and imaging elements. The imaging time, and the payload of different imaging elements, can be tuned by taking advantage of the commercial availability of CDs with different sizes of the cavity. This review aims to offer an outlook of the chemistry and engineering of CDs for the development of molecular probes. Complexation thermodynamics of CDs, and the corresponding implications for probe design, are also presented with examples demonstrating the structural and physiochemical roles played by CDs in the full ambit of molecular imaging. We hope that this review not only offers a synopsis of the current development of CD-based molecular probes, but can also facilitate translation of the incremental advancements from the laboratory to real biomedical applications by illuminating opportunities and challenges for future research.

Glycidyl Triazolyl Polymers: Poly(ethylene glycol) Derivatives Functionalized by Azide-Alkyne Cycloaddition Reaction

Glycidyl triazolyl polymer (GTP), which is the product of the Huisgen dipolar cycloaddition reaction between glycidyl azide polymer and alkyne derivatives, is featured here. GTP is the multifunctionalized poly(ethylene glycol) (PEG). The drawback of PEG is that linear PEG has the functional group only at both ends. The low loading capability of the functional groups limits the possibilities of PEG applications. GTP facilitates the synthesis of multifunctionalized PEG derivatives. In this article, 74 examples of GTP homopolymers and copolymers are introduced. The synthetic protocols and work-up processes of GTP are summarized. In addition, application studies are reviewed: for example, stimuli-responsive and self-healing materials, materials for electrical memory devices, ion-conductive materials, and biomedical materials. Finally, some issues on GTP synthesis and future directions for GTP-based polymer materials are proposed.

Progress in bio-inspired sacrificial bonds in artificial polymeric materials

Mimicking natural structures has been highly pursued in the fabrication of synthetic polymeric materials due to its potential in breaking the bottlenecks in mechanical properties and extending the applications of polymeric materials. Recently, it has been revealed that the energy dissipating mechanisms via sacrificial bonds are among the important factors which account for strong and tough attributes of natural materials. Great progress in synthesis of polymeric materials consisting of sacrificial bonds has been achieved. The present review aims at (1) summarizing progress in the mechanics and chemistry of sacrificial bond bearing polymers, (2) describing the mechanisms of sacrificial bonds in strengthening/toughening polymers based on studies by single-molecule force spectroscopy, chromophore incorporation and constitutive laws, (3) presenting synthesis methods for sacrificial bonding including dual-crosslink, dual/multiple-network, and sacrificial interfaces, (4) discussing the important advances in engineering sacrificial bonding into hydrogels, biomimetic structures and elastomers, and (5) suggesting future works on molecular simulation, viscoelasticity, construction of sacrificial interfaces and sacrificial bonds with high dissociative temperature. It is hoped that this review will provide guidance for further development of sacrificial bonding strategies in polymeric materials.

Supramolecular assemblies through host-guest complexation between cucurbiturils and an amphiphilic guest molecule

We report the formation of supramolecular complexation between cucurbit[n]urils (CBn) and an amphiphilic pyridinium-functionalized anthracene (AnPy) in aqueous solution. The CB7 cavity is capable of accommodating the pyridinium moiety, while CB8 can encapsulate the pyridinium and anthracene moieties at once. The encapsulation of AnPy by CB7 leads to the formation of nanoparticles, while the complexation of AnPy with CB8 leads to the formation of nanorods.

The chaotropic effect as an orthogonal assembly motif for multi-responsive dodecaborate-cucurbituril supramolecular networks

In aqueous solution, host-guest complexation is frequently driven by the hydrophobic effect, which also constitutes a popular approach in the design of supramolecular assemblies. Herein, we report an orthogonal assembly motif, the chaotropic effect, which exploits the tendency of chaotropic anions to associate with hydrophobic surfaces.

Laterally functionalized pillar[5]arene: a new building block for covalent self-assembly

Laterally functionalized pillar[5]arenes were synthesized for the first time by bromination at the methylene bridge of dimethoxypillar[5]arene. The synthesized molecule was then used as a novel building block by being covalently self-assembled into polymer nanocapsules and 2D polymer films.

Host-guest chemistry in two-dimensional supramolecular networks

Nanoporous supramolecular networks physisorbed on solid surfaces have been extensively used to immobilize a variety of guest molecules. Host-guest chemistry in such two-dimensional (2D) porous networks is a rapidly expanding field due to potential applications in separation technology, catalysis and nanoscale patterning. Diverse structural topologies with high crystallinity have been obtained to capture molecular guests of different sizes and shapes. A range of non-covalent forces such as hydrogen bonds, van der Waals interactions, coordinate bonds have been employed to assemble the host networks. Recent years have witnessed a surge in the activity in this field with the implementation of rational design strategies for realizing controlled and selective guest capture. In this feature article, we review the development in the field of surface-supported host-guest chemistry as studied by scanning tunneling microscopy (STM). Typical host-guest architectures studied on solid surfaces, both under ambient conditions at the solution-solid interface as well as those formed at the ultrahigh vacuum (UHV)-solid interface, are described. We focus on isoreticular host networks, hosts functionalized pores and dynamic host-guest systems that respond to external stimuli.

Reversible 2D Supramolecular Organic Frameworks encompassing Viologen Cation Radicals and CB[8]

Reversible 2D supramolecular organic frameworks encompassing branched viologen architectures and cucurbit[8]uril (CB[8]) were constructed and investigated. UV-vis investigation clearly indicates the formation and intermolecular dimerization of monocation radicals and their encapsulation into the hydrophobic CB[8] cavity which is further complemented by EPR (electron paramagnetic resonance) spectroscopy. Particle size measurements by dynamic light scattering method showed particle sizes in the range of several µm indicating larger aggregates. Zeta potential measurements suggested the instability of these particles and their tendency to form aggregates. TEM (transmission electron microscope) analysis further revealed the formation of supramolecular polymer (monocation radical with cucurbit[8]uril) whose diameter were in the range of several µm as indicated by DLS measurements; however the oxidized form, i.e., the viologen dication with cucurbit[8]uril showed dotted spots in the range of sub nanometer level. The internal periodicities of the supramolecular polymers were analyzed by SAXs (small angle X-ray scattering) measurements. Additionally, we have demonstrated that these supramolecular organic frameworks can be depolymerized by oxidation in air and again can be polymerized (intermolecular radical dimerization) by reduction under inert atmosphere demonstrating that these systems will be of broad interest.

Molecular recognition and activation by polyaza macrocyclic compounds based on host-guest interactions

The design and syntheses of supramolecular hosts for the recognition and activation of molecules and anions are one of the most active research fields in supramolecular chemistry, in which polyaza macrocyclic ligands and their complexes have drawn particular attention due to their strong host-guest interactions. This review mainly focuses on the recent progress in the recognition of molecules and anions by polyaza macrocyclic compounds including polyaza macrocycles, polyaza macrobicycles and polyaza macrotricycles, as well as the activation of molecules by polyaza macrocyclic ligands and their metal complexes.

Fabrication of POSS-embedded supramolecular hyperbranched polymers with multi-responsive morphology transitions

We demonstrate a simple approach to prepare POSS-embedded supramolecular hyperbranched polymers with multiple stimulus morphology transitions driven by triple supramolecular driving forces in selective solvents.

Competition of cation–π and exo-wall π–π interactions: a novel approach to achieve ultrasensitive response

A novel approach to achieve ultrasensitive response was successfully developed by rationally introducing the competition between cation–π and exo-wall π–π interactions into a pillar[5]arene-based supramolecular organogel (P5N-OG).

A multi-responsive cross-linked supramolecular polymer network constructed by mussel yield coordination interaction and pillar[5]arene-based host–guest complexation

A novel cross-linked supramolecular polymer network was prepared inspired by mussel yield catechol–Fe³⁺ coordination and pillar[5]arene-based host–guest recognition.

A H2S and I− dual-responsive supramolecular polymer constructed via pillar[5]arene-based host–guest interactions and metal coordination

A supramolecular polymer was designed and prepared by self-assembly of a pillar[5]arene dimer (AA-type), constructed from Ag-coordination, and a homoditopic (BB-type) guest (G). The supramolecular polymer displayed H₂S and I⁻ dual responsiveness due to the sensitivity of Ag⁺ to H₂S and I⁻.

An AIEE fluorescent supramolecular cross-linked polymer network based on pillar[5]arene host–guest recognition: construction and application in explosive detection

A novel fluorescent supramolecular cross-linked polymer network with aggregation induced enhanced emission properties was constructed and further used for explosive detection.

Supramolecular motifs for the self-assembly of monosubstituted pillar[5]arenes with an amide fragment: from nanoparticles to supramolecular polymers

The effects of solvents on the aggregation properties of novel monosubstituted pillar[5]arenes containing an N-alkylamide fragment have been investigated.

Tunable white-light emission by supramolecular self-sorting in highly swollen hydrogels

Fluorescence-tunable hydrogels especially emitting white-light were achieved by swelling hydrogels in solutions containing two kinds of dyes.

Composite Membrane with a Calixarene-Containing Polyamide Functional Layer

Monolayer thin films were prepared at the interface of hexane and water to investigate the film formation ability of monomers through interfacial polymerization (IP). A tetra-calix[4]arene chloride derivative (CC) and a diamino-terminated PEG-1000 (DAP) produced a high strength membrane among the tested monomers. IP is consequently proposed to prepare a composite membrane with CC and DAP on a polysulfone (PSF) bulk membrane used for ultrafiltration. The top layer was cross-linked by heat-treating at 60°C for 2 min, with DAP (2 wt.-%) in water and CC (0.05 wt.-%) in hexane. Attenuated total reflectance (ATR)-FTIR and X-ray photoelectron spectroscopy data confirmed that a polyamide was formed on the surface of the PSF substrate. The skin layer was a 3 μm thick smooth thin-film as determined by field emission scanning electron microscopy (FE-SEM), and was also compact without gaps. Pure water flux was ~80.5 L m−2 h−1 under 0.5 MPa. Rejection of MgSO4 was round 22 %, since the calixarene-containing network was a sparse grid, and also had an affinity for metal cations. Although the skin of the composite membrane was compact under SEM, it was easy for metal cations to transfer through. This composite membrane might have good performance in other separation areas as a result of the special structure imparted by using the calixarenes as cross-linking knots.

Terpyridine-based metallo-organic cages and supramolecular gelation by coordination-driven self-assembly and host–guest interaction

Dimetallo-cages connected with dialkylammonium salts via host–guest interactions resulted in thermodynamic polymer metallo-gels.

Dendronised diazapyrenium derivatives: host–guest complexes in aqueous solution

Water soluble dendrimers, containing a diazapyrenium core, bind CB[7] with a higher association constant than CB[8].

Novel 2-(hydroxy)-naphthyl imino functionalized pillar[5]arene: a highly efficient supramolecular sensor for tandem fluorescence detection of Fe3+ and F− and the facile separation of Fe3+

A novel pillar[5]arene-based supramolecular sensor (AP5N) for tandem fluorescence detection of Fe³⁺ and F⁻ was successfully prepared. Interestingly, the sensor AP5N shows excellent facile separation property for Fe³⁺.