Responsive Gel-like Supramolecular Network Based on Pillar[6]arene–Ferrocenium Recognition Motifs in Polymeric Matrix

The role of macrocycles in supramolecular assembly with polymers

Recently, supramolecular self-assembly has attracted the attention of researchers worldwide because it enables the creation of nanostructures with unique properties without additional costs. Spontaneous organization of molecules allows the design and development of new nanostructures that can interact with drugs and living cells and generate a response. Therefore, supramolecular structures have enormous potential and can be in demand in various fields of healthcare and ecology. One of the widely used building blocks of such supramolecular assemblies is polymers. This review examines the joint aggregation behavior of various macrocycles (cyclodextrins, calixarenes, cucurbiturils, porphyrins, and pillararenes) with polymers, the functional properties of these supramolecular systems and their potential applications.

Ferrocene: an exotic building block for supramolecular assemblies

Ferrocene (Fc), a classical organometallic complex, has found potential applications in ligand design, catalysis, and analytical, biological, medicinal and materials chemistry. In recent years, the use of Fc as a building block in supramolecular chemistry has emerged. The molecular shape, size, and hydrophobicity of Fc make it an ideal guest for a variety of macrocyclic host molecules to form stable host-guest complexes. The vertical distance (3.3 Å) between two cyclopentadienyl rings and molecular "ball bearing" property in Fc support the formation of intramolecular π-π stacking, H-bonding and metallophilic interactions between two appropriate substituents in 1,n'-disubstituted ferrocenes. Along with these molecular features, the rigidity along with rotational flexibility, redox reversibility and oxidation-triggered tunable hydrophobicity of Fc have led to its use as an exotic building block for the development of a wide range of supramolecular assemblies such as smart molecular receptors, intricate metal-organic assemblies, supramolecular polymers, and gels including out-of-equilibrium assemblies and metal nanoparticle assemblies. This review highlights the concepts behind the design and development of these assemblies, where the Fc unit has a direct and defined role in their formation and function. The use of Fc in supramolecular assembly is still a relatively young field and set to be the subject of increasing research interest towards the development of fascinating supramolecular structures with tailored properties and programmable functions towards applications in materials and biological sciences.

Recent Advances in Stimuli-Responsive Metallogels

Recently, stimuli-responsive supramolecular gels have received significant attention because their properties can be modulated through external stimuli such as heat, light, electricity, magnetic fields, mechanical stress, pH, ions, chemicals and enzymes. Among these gels, stimuli-responsive supramolecular metallogels have shown promising applications in material science because of their fascinating redox, optical, electronic and magnetic properties. In this review, research progress on stimuli-responsive supramolecular metallogels in recent years is systematically summarized. According to external stimulus sources, stimuli-responsive supramolecular metallogels, including chemical, physical and multiple stimuli-responsive metallogels, are discussed separately. Moreover, challenges, suggestions and opportunities regarding the development of novel stimuli-responsive metallogels are presented. We believe the knowledge and inspiration gained from this review will deepen the current understanding of stimuli-responsive smart metallogels and encourage more scientists to provide valuable contributions to this topic in the coming decades.

Multistep sequence-controlled supramolecular polymerization by the combination of multiple self-assembly motifs

The precise sequence control of polymer chain is an important research topic of polymer chemistry. Although some methods such as iterative synthesis and supramolecular polymerization have been developed to fabricate sequence-controllable polymer, it is still a great challenge to consecutively prepare multiple supramolecular polymers with different sequence structures. In this work, through the reasonable utilization of assembly motifs, we integrated multiple host-guest recognitions and metal coordination interactions to prepare different sequence-controlled supramolecular polymers by a multistep assembly strategy. This research provides inspiration for the design and preparation of supramolecular polymers with different sequence structures.

Supramolecular Polymer Gel Lubricant with Excellent Mechanical Stability and Tribological Performances

Lubricants performing better in machinery systems would lead to the remarkable reduction of environmental pollution problems and the significant improvement of fuel economy. A new family of supramolecular polymer gel lubricants with urea groups has been successfully prepared via self-assembling noncovalent bonds. These newly designed supramolecular polymer gels were well characterized with field-emission scanning electron microscopy, proton nuclear magnetic resonance, attenuated total reflection-Fourier transform infrared spectroscopy, a rheometer, oscillating reciprocating friction, and a wear tester. Compared to low molecular weight supramolecular gels, the covalent and noncovalent bonds cooperated in the supramolecular polymer gel based on macromolecules. Hence, the mechanical properties and viscoelasticity of gel lubricants are greater than those of the low molecular weight supramolecular gels. Furthermore, owing to the longer chain length of polymer gelators, the thickness of the adsorbed film formed on the surface lubricated by macromolecules is thicker than that on the surface lubricated by low molecular weight supramolecular gels, which positively correlates with the lubricating property, making supramolecular polymer gels based on macromolecules better than low molecular weight supramolecular gels. Excitingly, the supramolecular polymer gels based on macromolecules exhibit more excellent thermal reversibility, creep recovery, and thixotropic properties, which not only achieve the lubricating property but also lead to the remarkable reduction of environmental pollution problems due to oil creeping.

"Rim-Differentiated" Pillar[6]arenes

A "rim-differentiated" pillar[6]arene (RD-P[6]) was obtained successfully, with the assistance of a dimeric silver trifluoroacetate template, among eight different constitutional isomers in a direct and regioselective manner. The solid-state conformation of this macrocycle could switch from the 1,3,5-alternate to a truly rim-differentiated one upon guest inclusion. This highly symmetric RD-P[6] not only hosts metal-containing molecules inside its cavity, but also can form a pillar[6]arene-C₆₀ adduct through co-crystallization on account of donor-acceptor interactions. The development of synthetic strategies to desymmetrize pillararenes offers new opportunities for engineering complex molecular architectures and organic electronic materials.

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.

Supramolecular polymer gels: from construction methods to functionality

Supramolecular polymer gels (SPGs) are precisely designed gels brought together by noncovalent interactions to form three-dimensional network structures of polymers. SPGs combine the merits of supramolecular polymers and gels, such as stimuli-responsiveness, self-healing, and self-adaptation, which endows SPGs with potential application value in the fields of biomaterials, etc. Recently, much effort has been made to design new SPGs and related materials with high performance. Herein, we review the research endeavor and future directions of SPGs depending on the construction methods, topological structures, stimuli-responsiveness, and functionality. We hope that the review will provide reference values for the researchers working in supramolecular chemistry and gels.

Paper without a Trail: Time-Dependent Encryption using Pillar[5]arene-Based Host-Guest Invisible Ink

A stimuli-responsive invisible ink for time-dependent encryption of information is reported. Consisting of a pillar[5]arene-based supramolecular network grafted with spiropyran moieties, these materials display time-dependent photochromic behavior with tailorable fading rates. Ultraviolet (UV) light results in isomerization of the colorless spiropyran to the corresponding colored merocyanine, while visible light or heat causes the reverse isomerization with a rate that is dependent on the density of host-guest crosslinks. The kinetics of discoloration are a function of merocyanine aggregation, which becomes more pronounced as the host-guest crosslink density is increased, leading to a reduced conversion rate and slower time-dependent fading. The degree of crosslinking, and hence the fading rate, may be modulated via the addition of unbound pillar[5]arene host or nitrile guest as competitors. Time-dependent information encryption is enabled by combining selective placement of host and guest competitors and UV patterning. UV patterning provides an initially "false" image that does not reveal the desired information, and it is only after a given time that the encrypted data appears. This work provides a unique approach to enhance the security of information storage associated with offline portable data encryption.

Supramolecular polymer networks crosslinked by crown ether-based host-guest recognition: dynamic materials with tailored mechanical properties in bulk

Supramolecular polymer networks (SPNs) based on host-guest recognition have attracted much research attention to develop smart supramolecular materials. However, these researches mainly focus on SPNs in solution or in gel...

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

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

Thermally Responsive Poly(ethylene oxide)-Based Polyrotaxanes Bearing Hydrogen-Bonding Pillar[5]arene Rings*

Poly(ethylene oxide)s (PEOs) are useful polymers with good water solubility, biological compatibility, and commercial availability. PEOs with various end groups were threaded into pillar[5]arene rings in a mixture of water and methanol to afford pseudopolyrotaxanes. Corresponding polyrotaxanes were also constructed by capping COOH-terminated pseudopolyrotaxanes with bulky amines, in which multiple hydrogen bonds involving the pillar[5]arene OH groups were critically important to prevent dethreading. The number of threaded ring components could be rationally controlled in these materials, providing a simple and versatile method to tune the mechanical and thermal properties. Specifically, a polyrotaxane with a high-molecular-weight axle became elastic upon heating above the melting point of PEOs and exhibited temperature-dependent shape memory property because of the topological confinement and crosslinked hydrogen bonds.

A Stretchable Pillararene-Containing Supramolecular Polymeric Material with Self-Healing Property

Constructing polymeric materials with stretchable and self-healing properties arise increasing interest in the field of tissue engineering, wearable electronics and soft actuators. Herein, a new type of supramolecular cross-linker was constructed through host-guest interaction between pillar[5]arene functionalized acrylate and pyridinium functionalized acrylate, which could form supramolecular polymeric material via photo-polymerization of n-butyl acrylate (BA). Such material exhibited excellent tensile properties, with maximum tensile strength of 3.4 MPa and strain of 3000%, respectively. Moreover, this material can effectively dissipate energy with the energy absorption efficiency of 93%, which could be applied in the field of energy absorbing materials. In addition, the material showed self-healing property after cut and responded to competitive guest.

Pillararene-based self-assemblies for electrochemical biosensors

The ingenious design and synthesis of novel macrocycles bring out renewed vigor of supramolecular chemistry in the past decade. As an intriguing class of macrocycles, pillararene and pillararene-based functional materials that are constructed through the noncovalent bond self-assembly approach have been undergoing a rapid growth, benefiting from their unique structures and physiochemical properties. This review elaborates recent significant advances of electrochemical studies based on pillararene systems. Fundamental electrochemical behavior of pillar[n]arene[m]quinone and pillararene-based self-assemblies as well as their applications in electrochemical biosensors are highlighted. In addition, the advantages and functions of pillararene self-assembly systems resulted from the unique molecular architectures are analyzed. Finally, current challenges and future development tendency in this burgeoning field are discussed from the viewpoint of both fundamental research and applications. Overall, this review not only manifests the main development vein of pillararene-based electrochemical systems, but also conquers a solid foundation for their further bioelectrochemical applications.

Polymerization of a biscalix[5]arene derivative

Recent decades have seen an increased interest in the preparation of polymers possessing host or guest moieties as the end group, which has enabled new polymeric materials such as self-healable, shape-memory, and stimuli-responsive materials. Such polymers are commonly synthesized by tethering the host or guest moieties to polymers. On the other hand, there are limited reports demonstrating the preparation of host- or guest-appended polymers by directly polymerizing the corresponding host- or guest-appended monomers, which is valuable for easy access to diverse polymers from single molecular species. However, reactive host and/or guest moieties of the monomer interfere with the polymerization reaction. Here, we report that a biscalix[5]arene host-appended molecule can be polymerized with various monomers to form the corresponding host-appended polymers. The host–guest complexation behavior of calix[5]arene-appended polymers with fullerene derivatives was studied by ¹H NMR and UV/Vis spectroscopic techniques, which revealed that the long polymer chains did not prevent host–guest complexation even when the fullerene derivative was equipped with a polymer chain. Thus, the present study shows the potential for developing polymers that have various combinations of polymer chains.

A calix[5]arene appended monomer molecule was subjected to polymerization reaction to yield corresponding methacrylate polymers. The calix[5]arene appended polymers showed excellent encapsulation capability for fullerene molecules.

Pillar[n]arene-Based Supramolecular Switches in Solution and on Surfaces

The design and synthesis of new synthetic macrocycles has driven the rapid development of supramolecular chemistry and materials. Pillar[n]arenes, as a new type of macrocyclic compounds, are used as a promising type of building blocks for switchable supramolecular systems due to their versatile functionalization and the ability of binding toward various guest molecules. A number of guests can form inclusion complexes with pillar[n]arenes and their derivatives in solution, which are sensitive to different external triggers. Interestingly, the pursuit of complex stimuli-responsive functional materials and devices has largely motivated the shift of pillar[n]arene-based switches from solution media to surfaces for controllable macroscopic motions on solid platforms. Facilitated by the facile modification of pillar[n]arenes on various solid supports and the dynamic binding of host-guest complexes, numerous functional hybrid materials with adjustable physical or chemical properties and integrated functionalities have been reported in the last decade. Here, the advance of supramolecular switches in solution and on surfaces based on pillar[n]arenes and derivatives with an emphasis on the efforts and the latest contributions from the field is discussed.

Temperature-Dependent and pH-Responsive Pillar[5]arene-Based Complexes and Hydrogen-Bond-Based Supramolecular Pentagonal Boxes in Water

Supramolecular systems in water are of paramount importance and those based on hydrogen bonds are both intriguing and scarce. Here, after studying the peculiar host-guest complexes formed between per-dimethylamino-pillar[5]arene (1) and the bis-sulfonates 2 a-c, we describe the formation of the first hydrogen-bond-based supramolecular pentagonal boxes (SPBs), which are stable in water. These pH-responsive SPBs are constructed from 1 as a body, benzene polycarboxylic acids 3 a,b as lid compounds, and 2 a-c as guests. We demonstrate that encapsulation of 2 a-c in pillar[5]arene 1 and in the highly stable water-soluble SPBs, that is, 1(3 a)₂ and 1(3 b)₂ , is both temperature and pH dependent and, quite interestingly, depends, on the nature of the lid compounds used for capping the boxes even at high pH. We also highlight the difference in the ¹ H NMR characteristics of 2 b and 2 c in the cavity of 1 and the SPBs.

Supramolecular Vesicles Based on Amphiphilic Pillar[n]arenes for Smart Nano-Drug Delivery

Supramolecular vesicles are the most popular smart nano-drug delivery systems (SDDs) because of their unique cavities, which have high loading carrying capacity and controlled-release action in response to specific stimuli. These vesicles are constructed from amphiphilic molecules via host-guest complexation, typically with targeted stimuli-responsive units, which are particularly important in biotechnology and biomedicine applications. Amphiphilic pillar[n]arenes, which are novel and functional macrocyclic host molecules, have been widely used to construct supramolecular vesicles because of their intrinsic rigid and symmetrical structure, electron-rich cavities and excellent properties. In this review, we first explain the synthesis of three types of amphiphilic pillar[n]arenes: neutral, anionic and cationic pillar[n]arenes. Second, we examine supramolecular vesicles composed of amphiphilic pillar[n]arenes recently used for the construction of SDDs. In addition, we describe the prospects for multifunctional amphiphilic pillar[n]arenes, particularly their potential in novel applications.

Transparency and AIE tunable supramolecular polymer hydrogel acts as TEA-HCl vapor controlled smart optical material

Stimuli-responsive optical materials attract lots of attention due to their broad applications. Herein, a novel smart stimuli-responsive supramolecular polymer was successfully constructed using a simple tripodal quaternary ammonium-based gelator (TH). The TH self-assembles into a supramolecular polymer hydrogel (TH-G) and shows aggregation-induced emission (AIE) properties. Interestingly, the transparency and fluorescence of the TH-G xerogel film (TH-GF) could be reversibly regulated by use of triethylamine (TEA) and hydrochloric acid (HCl) vapor. When alternately fumed with TEA and HCl vapor, the optical transmittance of the TH-GF was changed from 8.9% to 92.7%. Meanwhile, the fluorescence of the TH-G shows an "ON/OFF" switch. The reversible switching of the transparency and the fluorescence of the TH-GF is attributed to the assembly and disassembly of the supramolecular polymer TH-G. Based on these stimuli-response properties, the TH-GF could act as an optical material and shows potential applications as smart windows or fluorescent display material controlled by TEA and HCl vapor.

Supramolecular self-healing materials from non-covalent cross-linking host-guest interactions

The introduction of non-covalent bonds is effective for achieving self-healing properties because they can be controlled reversibly. One approach to introduce these bonds into supramolecular materials is use of host-guest interactions. This feature article summarizes the development of supramolecular materials constructed by non-covalent cross-linking through several approaches, such as host-guest interactions between host polymers and guest polymers, 1 : 2-type host-guest interactions, and host-guest interactions from the polymerization of host-guest inclusion complexes. Host-guest interactions show self-healing functions while also enabling stimuli-responsiveness (redox, pH, and temperature). The self-healing function of supramolecular materials is achieved by stress dispersion arising from host-guest interactions when stress is applied. Reversible bonds based on host-guest interactions have tremendous potential to expand the variety of functional materials.

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

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

Functional supramolecular gels based on pillar[n]arene macrocycles

Supramolecular gels constructed from low-molecular-weight gelators via noncovalent interactions have received increasing attention. The rapid development of stimuli-responsive supramolecular gels with attractive properties is highly desirable to meet the ever-growing demand of materials science and chemistry. The inherent reversible and dynamic nature of noncovalent interactions in supramolecular gels endows the materials with sensing, processing, and actuating functions in response to specific environmental changes and offers them great potential in flexible biomaterials and intelligent devices. In particular, pillar[n]arenes with symmetrical pillar-shaped architectures have been recognized as an emerging class of synthetic macrocycles after crown ethers, cyclodextrins, calixarenes, and cucurbiturils, and proven to be excellent candidates for the fabrication of functional supramolecular gels due to their many advantages including facile synthesis, diverse functionalization, and appealing host-guest properties. This review provides a comprehensive overview of recent progress in supramolecular gels involving pillar[n]arenes and their derivatives as synthetic macrocyclic arenes, from the viewpoints of the synthetic approach, controllable assembly, stimuli-responsiveness, and functions. Perspectives of this burgeoning field of research are also given at the end.

Pillararene-Containing Polymers with Tunable Conductivity Based on Host-Guest Complexations

Regulating the conductivity of conducting polymers has spurred increasing studies, aiming at meeting different demands in various fields, including chemosensors, photovoltaic cells, and so on. Herein, linear pillar[5]arene-containing conjugated polymers were designed and synthesized via metathesis cyclopolymerization of pillar[5]arene-functionalized 1,6-heptadiyne. Upon addition of an ionic guest, such polymers could form inclusion complexes, of which the glass transition temperature decreased dramatically. With the aid of ionic guest and host-guest complexations between the pendant pillararenes and guest, these supramolecular materials exhibited tunable conductivity from 10^-12 to 10^-3 S·cm^-1 at 30 °C. In addition, compared with the polymers without pendant pillar[5]arenes, such polymers showed better compatibility with the ionic guest, which could prevent the leakage of the latter one and was good for the conductivity of the material.

Efficient Aggregation-Induced Emission Manipulated by Polymer Host Materials

Linear copolymer hosts bearing a number of pillar[5]arene dangling side chains are synthesized for the facile construction of highly emissive supramolecular polymer networks (SPNs) upon noncovalently cross-linking with a series of tetraphenyethylene (TPE)-based tetratopic guests terminated with different functional groups through supramolecular host-guest interactions. An extremely high fluorescence quantum yield (98.22%) of the SPNs materials is obtained in tetrahydrofuran (THF) by fine-tuning the parameters, and meanwhile supramolecular light-harvesting systems based on spherical supramolecular nanoparticles are constructed by interweaving 9,10-distyrylanthracene (DSA) and TPE-based guest molecules of aggregation-induced emission (AIE) with the copolymer hosts in the mixed solvent of THF/H₂ O. The present study not only illustrates the restriction of the intramolecular rotations (RIR)-ruled emission enhancement mechanism regulated particularly by macrocyclic arene-containing copolymer hosts, but also suggests a new self-assembly approach to construct high-performance light-harvesting materials.

Stimuli-responsive supramolecular nano-systems based on pillar[n]arenes and their related applications

Stimuli-responsive supramolecular nano-systems (SRNS) have been a trending interdisciplinary research area due to the responsiveness upon appropriate stimuli, which makes SRNS very attractive in multiple fields where precise control is vital.

Stimuli-Responsive Supramolecular Assemblies Constructed from Pillar[ n]arenes

Supramolecular assemblies are constructed from at least two molecules through various noncovalent bonding modes such as hydrogen bonding, cationic-anionic electrostatic interactions, aromatic interactions, metal-ligand bonding, hydrophobic-hydrophilic interactions, and charge-transfer interactions. Owing to the dynamic and reversible nature of these noncovalent bonds, the assembly and disassembly of these molecules are dynamic and reversible. Molecules self-assemble to form the most conformationally and thermally stable structures through these noncovalent interactions. The formation of these noncovalent interactions is affected by the properties of the environment such as its polarity, temperature, and pressure; thus, the structure of the assembled compounds is determined by the environment. The sizes and shapes of the supramolecular assemblies play an important role in determining their functions. Therefore, controlling their size and shape is important. Introducing stimuli-responsive groups into supramolecular assemblies is a useful way to control their size and shape. Controlling supramolecular structures and motions with external stimuli, i.e., periodic and rotational motions on the molecular scale, structures, and molecular weights at the nano- and micrometer scales, visible shrinking/expansion, and adhesive behavior at a macroscopic scale, is very useful. Macrocyclic host molecules are useful building blocks for the construction of stimuli-responsive supramolecular assemblies because their host ability can be tuned by changing the shape and electron density of the cavity. The size-dependent hosting ability of the cavity is similar to the lock-and-key model in biological systems. Stimuli-responsive supramolecular assemblies have been developed by using macrocyclic compounds such as cyclodextrins, cucurbit[ n]urils, calix[ n]arenes, crown ethers, and related macrocycles. We successfully developed new pillar-shaped macrocyclic hosts in 2008, which were coined pillar[ n]arenes. The unique structural features of pillar[ n]arenes allowed new properties. This year, 2018, marks one decade of research into pillar[ n]arene chemistry, and in that time the properties of pillar[ n]arenes have been widely investigated by various scientists. Thanks to their efforts, the characteristic properties of pillar[ n]arenes that result from their pillar-shaped structures have been elucidated. Their host ability, the chirality of their pillar-shaped structure, and their versatile functionality are unique features of pillar[ n]arenes not seen in other well-known hosts, and these properties are very useful for the creation of new stimuli-responsive supramolecular assemblies. In this Account, we describe photo-, pH- and redox-responsive supramolecular assemblies based on pillar[ n]arenes. First, we discuss molecular-scale stimuli-responsive supramolecular assemblies, i.e., pseudorotaxanes, pseudocatenanes, and supramolecular polymers. We also highlight subnanometer- and micrometer-scale stimuli-responsive supramolecular assembles such as particles and vesicles. Finally, we discuss the macroscopic stimuli-responsive structural changes of surfaces and gels. This Account will provide useful information for researchers working on not only pillar[ n]arene chemistry but also the chemistry of other macrocyclic hosts, and it will inspire new discoveries in the field of supramolecular assemblies and systems containing macrocyclic hosts.

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.

Self-assembled orthoester cryptands: orthoester scope, post-functionalization, kinetic locking and tunable degradation kinetics

Dynamic adaptability and biodegradability are key features of functional, 21st century host-guest systems. We have recently discovered a class of tripodal supramolecular hosts, in which two orthoesters act as constitutionally dynamic bridgeheads. Having previously demonstrated the adaptive nature of these hosts, we now report the synthesis and characterization - including eight solid state structures - of a diverse set of orthoester cages, which provides evidence for the broad scope of this new host class. With the same set of compounds, we demonstrated that the rates of orthoester exchange and hydrolysis can be tuned over a remarkably wide range, from rapid hydrolysis at pH 8 to nearly inert at pH 1, and that the Taft parameter of the orthoester substituent allows an adequate prediction of the reaction kinetics. Moreover, the synthesis of an alkyne-capped cryptand enabled the post-functionalization of orthoester cryptands by Sonogashira and CuAAC "click" reactions. The methylation of the resulting triazole furnished a cryptate that was kinetically inert towards orthoester exchange and hydrolysis at pH > 1, which is equivalent to the "turnoff" of constitutionally dynamic imines by means of reduction. These findings indicate that orthoester cages may be more broadly useful than anticipated, e.g. as drug delivery agents with precisely tunable biodegradability or, thanks to the kinetic locking strategy, as ion sensors.

Controlled Self-Assembly of Multiple-Responsive Superamphiphilc Polymers Based on Host-Guest Inclusions of a Modified PEG with β-Cyclodextrin

Superamphiphilic polymers (SAPs) constructed by host-guest inclusion can self-assemble into various nanostructures in solution, which can find applications in many fields such as nanodevices, drug delivery, and template synthesis. Herein, we report the controlled self-assembly of multiple-responsive SAP based on a selective host-guest inclusion of β-cyclodextrin (β-CD) with a modified poly(ethylene glycol) (PEG) (FcC₁₁AzoPEG) consisting of a ferrocene (Fc) end group, a C₁₁ alkyl chain, an azobenzene (Azo) block, and a poly(ethylene glycol)methyl ether (PEG) chain. These SAPs can self-assemble into interesting nanostructures in water upon exposure to different stimuli because β-CD can be selectively included with different guests, such as Fc, Azo, and C₁₁ alkyl chain, under different stimuli. The inclusion complex of Fc with β-CD (Fc@β-CD SAP) can form nanowire micelles in aqueous solution. The nanowire micelles can be transformed into spindle micelles with the addition of oxidant because the majority of β-CDs dissociated from the complex Fc@β-CD SAP due to a conversion of Fc to Fc⁺ and will preferentially include with Azo group to form another dominant inclusion complex (Azo@β-CD SAP). After UV irradiation, the spindle micelles can be further transformed into spherical micelles because most of β-CDs are excluded from the complex Azo@β-CD SAP due to a trans- to cis-Azo conversion and then form a dominant inclusion complex with C₁₁ alkyl chains (C₁₁@β-CD SAP). This work not only demonstrates the selective host-guest inclusion of stimuli-responsive groups modified PEG with β-CD but also provides a useful approach for construction of diverse morphologies.

Warm/cool-tone switchable thermochromic material for smart windows by orthogonally integrating properties of pillar[6]arene and ferrocene

Functional materials play a vital role in the fabrication of smart windows, which can provide a more comfortable indoor environment for humans to enjoy a better lifestyle. Traditional materials for smart windows tend to possess only a single functionality with the purpose of regulating the input of solar energy. However, different color tones also have great influences on human emotions. Herein, a strategy for orthogonal integration of different properties is proposed, namely the thermo-responsiveness of ethylene glycol-modified pillar[6]arene (EGP6) and the redox-induced reversible color switching of ferrocene/ferrocenium groups are orthogonally integrated into one system. This gives rise to a material with cooperative and non-interfering dual functions, featuring both thermochromism and warm/cool tone-switchability. Consequently, the obtained bifunctional material for fabricating smart windows can not only regulate the input of solar energy but also can provide a more comfortable color tone to improve the feelings and emotions of people in indoor environments.

Materials for smart windows usually possess single functionality, thus developing materials that regulate solar energy whilst changing color to affect human emotion is desirable. Here the authors combine pillar[6]arenes and ferrocene/ferrocenium groups to produce warm/cool tone-switchable thermochromic materials.

Recent advances in pillar[n]arenes: synthesis and applications based on host-guest interactions

Pillar[n]arenes (n = 5-15) are a novel class of macrocyclic molecules with hydroquinone as the repeating unit linked by methylene bridges at para-positions. Introduced by T. Ogoshi for the first time in 2008, pillararenes have attracted increasing interest and have been widely studied during the last eight years, due to their unique structural advantages as host molecules, such as symmetrical rigid architecture, electron-rich cavities and facile functional modification. In this review, we first describe the syntheses of pillar[n]arenes including cyclooligomerization of pillar[n]arenes and modification of pillar[n]arenes after cyclooligomerization, summarising almost twenty different kinds of guest motifs and dividing them into three types: cationic, neutral and anionic motifs. The main section of this review examines the applications of pillar[n]arenes based on the host-guest interactions in different research fields, including biology, materials science and environmental science. Finally, future research directions and potential for novel applications are discussed.

A cyanide-triggered hydrogen-bond-breaking deprotonation mechanism: fluorescent detection of cyanide using a thioacetohydrazone-functionalized bispillar[5]arene

Bispillar[5]arene-based fluorescent sensor was used for fluorescent detection of cyanide anions through deprotonation accompanied by intermolecular hydrogen bond breakage.

A novel supramolecular polymer network based on a catenane-type crosslinker

A novel supramolecular mechanically interlocked crosslinker was designed and used to prepare a supramolecular polymer network.

Secondary dialkylammonium salt/crown ether [2]pseudorotaxanes as nanostructured platforms for proton transport

Secondary dialkylammonium salt/crown ether [2]pseudorotaxanes can be confined into two-dimensional nanochannels, leading to remarkable enhancements and rational control of proton conductivity.