Bispillar[5]arene-Based Slide-Ring Polyrotaxanation Enables Enhanced Toughness, Recyclability, Impact, and Puncture Resistance of Polyisoprene Elastomers

A series of slide-ring polyrotaxanes (SRPs) have been constructed by the solvent-free blending of a ditopic pillar[5]arene (DP5A) and polyisoprene (PIP) after thermal annealing. Solid-state 13C NMR experiments supported the fact that the pillar[5]arene rings of DP5A were threaded by PIP chains to afford physically interlocked networks. Tensile tests revealed that 1% of DP5A can improve the elongation at break from 50 to 239%, the tensile modulus from 2.1 to 3.9 MPa, and the toughness from 0.35 to 4.5 MJ/m3. Impact and puncture resistance experiments show that the DP5A-doped materials exhibit remarkable enhancement of protective and impalement-resistant performance. The samples can be also recycled repeatedly due to their physical crosslinking nature. The important stress delocalization effects have been attributed to the pulley effect of DP5A in the SRP materials, which represents a supramolecular approach for improving the performance of PIP elastomers.

Pillar[5]arenes decorated with six-membered-ring aromatics at all the substitution positions

Macrocyclic molecules have characteristic properties different from linear ones, such as high symmetry and guest-inclusion ability. To bring drastic changes to these properties, direct introduction of many substituents is a challenging but effective tool. Herein, we attain direct installation of ten six-membered-ring aromatic π-units into both rims of a pillar[5]arene. In contrast to previous pillar[n]arenes with less hindered five-membered-ring units, which showed conformational complexity and crushed crystal structures, the per-phenyl-substituted pillar[5]arene has a cylinder-shaped crystal structure with a dichloromethane inside the cavity and is obtained as a single pair of D 5-symmetric enantiomers. The average dihedral angles between the core and peripheral benzene rings sharply increase from 38° to 66°. These differences indicate the importance of local steric repulsion on both rims for determining the structures and properties of macrocycles.

Per-Arylation of Pillar[n]arenes: An Effective Tool to Modify the Properties of Macrocycles

Installation of various substituents is a reliable and versatile way to alter the properties of macrocyclic molecules, but high-yield and controlled methods are not always available especially for multifold reactions. Herein, we report 10- and 12-fold introduction of aryl substituents onto both rims of cylinder-shaped pillar[n]arenes, which usually have alkoxy substituents slanting to the cylinder axes. Although alkoxy pillar[5]arenes exist as D₅-symmetric enantiomeric pairs, arylated pillar[5]arenes provide crushed single-crystal structures and stereoisomerism including C₂-symmetric conformations depending on the aryl groups. Pillar[n]arenes with 2-benzofuranyl groups display bright fluorescence with quantum yields of 88-90% and no host-guest complexation with electron-deficient molecules in solution due to large deviation from alkoxy compounds. A benzofuran-appended pillar[6]arene instead captures small gaseous molecules in the solid state, probably owing to outside spaces surrounded by aromatic rings.

Copillar[5]arene Chemistry: Synthesis and Applications

Research on pillar[n]arenes has witnessed a very quick expansion. This emerging class of functionalized macrocyclic oligoarenes not only offers host–guest properties due to the presence of the central cavity, but also presents a wide variety of covalent functionalization possibilities. This short review focuses on copillararenes, a subfamily of pillar[n]arenes. In copillararenes, at least one of the hydroquinone units bears different functional groups compared to the others. After having defined the particular features of copillararenes, this short review compares the different synthetic strategies allowing their construction. Some key applications and future perspectives are also described.

1 Introduction

2 General Features of Pillar[5]arenes

3 Synthesis of Functionalized Copillar[4+1]arenes

4 Concluding Remarks

Tetraphenylethene-Embedded Pillar[5]arene and [15]Paracyclophane: Distorted Cavities and Host-Guest Binding Properties

Two aggregation-induced emission (AIE) macrocycles (DMP[5]-TPE and PCP[5]-TPE) were prepared by embedding Tetraphenylethene (TPE) unit into the skeletons of Dimethoxypillar[5]arene (DMP[5]) and [15]Paracyclophane ([15]PCP) at meso position, respectively. In crystal, the PCP[5]-TPE showed a distorted cavity, and the incubation of hexane inside the DMP[5]-TPE cavity caused a distinct change in the molecular conformation compared to PCP[5]-TPE. There was no complexation between PCP[5]-TPE and 1,4-dicyanobutane (DCB). UV absorption experiments showed the distorted cavity of DMP[5]-TPE hindered association with DCB.

Cavity-promotion by pillar[5]arenes expedites organic photoredox-catalysed reductive dehalogenations

The efficiency of the photo-induced electron transfer in photoredox catalysis is limited by the diffusional collision of the excited catalyst and the substrate. We herein present cavity-bound photoredox catalysts, which preassociate the substrates, leading to significantly shortened reaction times. A pillar[5]arene serves as the cavity and phenothiazine as a catalyst in the reductive dehalogenation of aliphatic bromides as a proof of concept reaction.

Tiara[5]arenes: Synthesis, Solid-State Conformational Studies, Host-Guest Properties, and Application as Nonporous Adaptive Crystals

Tiara[5]arenes (T[5]s), a new class of five‐fold symmetric oligophenolic macrocycles that are not accessible from the addition of formaldehyde to phenol, were synthesized for the first time. These pillar[5]arene‐derived structures display both unique conformational freedom, differing from that of pillararenes, with a rich blend of solid‐state conformations and excellent host–guest interactions in solution. Finally we show how this novel macrocyclic scaffold can be functionalized in a variety of ways and used as functional crystalline materials to distinguish uniquely between benzene and cyclohexane.

Synthesis, functionalization, and isolation of planar-chiral pillar[5]arenes with bulky substituents using a chiral derivatization agent

Bulky perneopentyloxy-pillar[5]arene (Pillar-1) was synthesized and its conformational mobility was investigated using variable-temperature ¹H NMR spectroscopy. The host–guest interactions between Pillar-1 and n-octyltrimethylammonium hexafluorophosphate (OMA) were investigated, and the formation of a 1 : 1 complex was revealed via¹H NMR. Planar-chiral isomers were synthesized via the reaction of a hydroxy-functionalized pillar[5]arene with chiral derivatization agent (S)-(+)-MTPA-Cl. The (Sp, R)-and (Rp, R)-forms of the pillar[5]arene diastereomers were isolated by HPLC, and their structures were analyzed by ¹⁹F NMR. HPLC measurements indicated that racemization did not take place at 40 °C for 72 h.

Bulky perneopentyloxy-pillar[5]arene was synthesized. Complexation behavior and conformational mobility were investigated using ¹H NMR spectroscopy. Isolation of planar-chiral pillar[5]arenes using a chiral derivatization agent were carried out.

Chiral discrimination of 2-heptlyaminium salt by planar-chiral monohydroxy-functionalized pillar[5]arenes

A chiral receptor was synthesized based on monohydroxy-functionalized pillar[5]arenes and its ability to discriminate alkyl aminium salts is demonstrated.

Monosubstituted pillar[5]arene functionalized with (amino)phosphonate fragments are “smart” building blocks for constructing nanosized structures with some s- and p-metal cations in the organic phase

Pillar[5]arenes with phosphonate- and 1-aminophosphonate-substituents form complexes with Na⁺, K⁺, Cs⁺ and Pb²⁺ with a 1 : 1 stoichiometry and lg K_a values between 2.0 and 4.8, and that Pb²⁺ is the most effectively bound cation.

Multi-stimuli-responsive supramolecular gel constructed by pillar[5]arene-based pseudorotaxanes for efficient detection and separation of multi-analytes in aqueous solution

Here, a novel pseudorotaxanes-type crosslinker of a supramolecular polymer network (WP5-PN) has been constructed from a host water-soluble pillar[5]arene (WP5) and a guest naphthalene dimethylamine derivative (PN) via a stepwise process involving multiple non-covalent interactions. The obtained supramolecular polymers were able to transform into a supramolecular polymer gel (WP5-PN-G) and show AIE properties in DMSO-H2O binary solution. Interestingly, due to the dynamic and reversible nature of non-covalent interactions, the resultant supramolecular polymer gels exhibited external stimuli-responsiveness to different parameters, such as temperature, acid-base, competitive guest and mechanical stress. Moreover, WP5-PN-G showed fluorescent response for Fe3+ and Cu2+, while its xerogel showed excellent recyclable separation properties for these metal ions with adsorption rates up to 98.07% and 95.38%, respectively. Moreover, by rational introduction of these metal ions into the WP5-PN-G, corresponding metal ion coordinated metallogels, such as WP5-PN-FeG and WP5-PN-CuG were obtained. These metallogels could selectively and sensitively sense F- and CN-, respectively. The detection limits of these metallogels for F- and CN- were about 1 × 10-8 M. The WP5-PN-G has potential applications in multi-analytes detection and separation as well as fluorescent display materials.

Monosulfonicpillar[5]arene: Synthesis, Characterization, and Complexation with Tetraphenylethene for Aggregation-Induced Emission

A pillar[5]arene derivative with a hydrophilic sulfonic group, i.e., monosulfonicpillar[5]arene (MSP5), has been successfully synthesized for the first time, which exhibited strong binding affinity towards alcohol analogs. Significantly, fluorescent supramolecular ensemble was fabricated from the supramolecular complexation of MSP5 and a neutral guest with tetraphenylethene core. Enhanced fluorescent emission of this system can be detected both in dilute solution and the solid state, and its temperature and competitive guest multi-responsive properties suggest its promising application as a chemical sensor towards alcohol analogs, ethylenediamine, and temperature variations.

Biological and related applications of pillar[n]arenes

Pillar[n]arenes are a new class of synthetic supramolecular macrocycles streamlined by their particular pillar-shaped architecture which consists of an electron-rich cavity and two fine-tuneable rims. The ease and diversity of the functionalization of the two rims open possibilities for the design of new architectures, topological isomers, and scaffolds. Significantly, this emerging class of macrocyclic receptors offers a unique platform for biological purposes. This review article covers the most recent contributions from the pillar[n]arene field in terms of artificial membrane transport systems, controlled drug delivery systems, biomedical imaging, biosensors, cell adhesion, fluorescent sensing, and pesticide detection based on host-guest interactions. The review also uniquely describes the properties of sub-units that make pillar[n]arenes suitable for biological applications and it provides a detailed outline for the design of new innovative pillar-like structures with specific properties to open up a new avenue for pillar[n]arene chemistry.

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 new approach for the synthesis of mono- and A1/A2-dihydroxy-substituted pillar[5]arenes and their complexation with alkyl alcohols in solution and in the solid state

A new method for synthesizing hydroxy-functionalized pillar[5]arenes by catalytic hydrogenation to remove the benzyl protecting groups of a pillararene precursor has been employed.

A novel supramolecular polymer gel based on naphthalimide functionalized-pillar[5]arene for the fluorescence detection of Hg2+ and I- and recyclable removal of Hg2+via cation-π interactions

The development of novel materials for the detection and removal of Hg²⁺ is a very important issue due to the acute toxicity of Hg²⁺. Herein, a novel supramolecular polymer P5BD-DPHB has been constructed by the collaboration of a naphthalimide functionalized-pillar[5]arene host (P5BD) and a bis-bromohexane functionalized-pillar[5]arene guest (DPHB). P5BD-DPHB could form a stable supramolecular gel (P5BD-DPHB-G). Interestingly, P5BD-DPHB-G shows selective fluorescent "turn-on" detection for Hg²⁺via cation-π interactions with high selectivity and sensitivity. Furthermore, the Hg²⁺ coordinated supramolecular gel P5BD-DPHB-HgG can detect I^- successively. The detection limits for Hg²⁺ and I^- are 1.65 × 10^-9 and 1.84 × 10^-8 mol L^-1, respectively. Even more significantly, the xerogel of P5BD-DPHB-G could remove Hg²⁺ from aqueous solution with excellent recyclability and ingestion capacity, and with a Hg²⁺ removal rate of 98%.

A pillar[5]arene-based multiple-stimuli responsive metal-organic gel was constructed for facile removal of mercury ions

A thioacetohydrazide functionalized pillar[5]arene was synthesized, which could further assemble into a linear supramolecular metal-organic polymer upon adding Zn²⁺. Furthermore, the obtained linear supramolecular metal-organic polymer could self-assemble to form a fluorescent supramolecular metal-organic gel at high concentration. When TBAOH was added to the viscous solution at high temperature, the obtained solution could not form a supramolecular metal-organic gel upon cooling. More importantly, when Hg²⁺ ions are added to the metal-organic gel, the strong blue fluorescence is clearly quenched, and this metal-organic gel (xerogel) could effectively remove Hg²⁺ from water. Simultaneously, a thin film based on the metal-organic gel was prepared, which was confirmed to be a convenient test kit for detecting Hg²⁺.

A novel water soluble chemosensor based on carboxyl functionalized NDI derivatives for selective detection and facile removal of mercury(ii)

A novel water-soluble Hg²⁺ sensor M2 has been designed and synthesized, which can provide a fluorescent “turn-on” response when it detects Hg²⁺. More meaningfully, the sensor M2 can remove Hg²⁺ from water effectively.

Monoaminophosphorylated pillar[5]arenes as hosts for alkaneamines

New synthesized aminophosphonated pillar[5]arenes form host–guest complexes with aliphatic amines contrary to monoamine macrocycles that tend to self-assemble.

A novel iodination-triggered competitive coordination mechanism: indirect detection of Hg2+and I−using a simple copillar[5]arene-based fluorometric sensor

Proposed mechanism for the detection of Hg²⁺and I⁻byDBP5.

A copillar[5]arene-based fluorescence “on–off–on” sensor is applied in sequential recognition of an iron cation and a fluoride anion

A copillar[5]arene-based [c2]daisy-chain dimer is applied in sequential detection of Fe³⁺ and F⁻ through a competitive complexation reaction.

Mercaptooxazole–phenazine based blue fluorescent sensor for the ultra-sensitive detection of mercury(ii) ions in aqueous solution

Herein, a fluorescent sensor based on the mechanism of the deprotonation process was designed and synthesized, which could detect Hg²⁺ in aqueous solution with remarkable fluorescence color changed (from yellow to light blue).

A novel functionalized pillar[5]arene-based selective amino acid sensor forl-tryptophan

The sensing mechanism of the sensorBTAP5forl-Trp.

Pillar[5]arene-based fluorescent polymer for selective detection and removal of mercury ions

A novel pillar[5]arene-based fluorescent polymer has been synthesized, and it is used for fluorescence detection and removal of the toxic mercury ions.

Pillar-Shaped Macrocyclic Hosts Pillar[n]arenes: New Key Players for Supramolecular Chemistry

In 2008, we reported a new class of pillar-shaped macrocyclic hosts, known as "pillar[n]arenes". Today, pillar[n]arenes are recognized as key players in supramolecular chemistry because of their facile synthesis, unique pillar shape, versatile functionality, interesting host-guest properties, and original supramolecular assembly characteristics, which have resulted in numerous electrochemical and biomedical material applications. In this Review, we have provided historical background to macrocyclic chemistry, followed by a detailed discussion of the fundamental properties of pillar[n]arenes, including their synthesis, structure, and host-guest properties. Furthermore, we have discussed the applications of pillar[n]arenes to materials science, as well as their applications in supramolecular chemistry, in terms of their fundamental properties. Finally, we have described the future perspectives of pillar[n]arene chemistry. We hope that this Review will provide a useful reference for researchers working in the field and inspire discoveries concerning pillar[n]arene chemistry.

A novel functionalized pillar[5]arene for forming a fluorescent switch and a molecular keypad

The sensing mechanism of the sensor PC5 to Fe³⁺.

Selective precipitation of alkyl dihalides using a newly synthesized water-soluble bisphosphorylpillar[5]arene

Bisphosphorylpillar[5]arene H2 binds strongly to alkyldiammonium ions and alkyl dihalides. The halogen bonds between H2 and alkyl dihalides lead to the rapid precipitation of the complex in water.

Functionalizing pillar[n]arenes

Macrocyclic chemistry has relied on the dominance of some key cavitands, including cyclodextrins, calixarenes, cyclophanes, and cucurbiturils, to advance the field of host-guest science. Very few of the many other cavitands introduced by chemists during these past few decades have been developed to near the extent of these four key players. A relatively new family of macrocycles that are becoming increasingly dominant in the field of macrocyclic chemistry are the pillar[n]arenes composed of n hydroquinone rings connected in their 2- and 5-positions by methylene bridges. This substitution pattern creates a cylindrical or pillar-like structure that has identical upper and lower rims. The preparation of pillar[n]arenes is facile, with pillar[5]- through pillar[7]arene being readily accessible and the larger macrocycles (n = 8-14) being accessible in diminishing yields. The rigid pillar[n]arene cavities are highly π-electron-rich on account of the n activated aromatic faces pointing toward their centers, allowing the cavities to interact strongly with a range of π-electron-deficient guests including pyridiniums, alkylammoniums, and imidazoliums. The substitution pattern of pillar[n]arenes bestows chirality onto the macrocycle in the form of n chiral planes. The absolute configuration of the chiral planes in pillar[n]arenes can be either fixed or rapidly undergoing inversion. The future of pillar[n]arenes is going to be dependent on their ability to fulfill specific applications. Chemical modification of the parent pillar[n]arenes lets us create functionalized hosts with anticipated chemical or physical properties. The featured potential applications of pillar[n]arenes to date are far reaching and include novel hosts with relevance to nanotechnology, materials science, and medicine. Pillar[n]arenes have an overwhelming advantage over other hosts since the number of ways available to incorporate handles into their structures are diverse and easy to implement. In this Account, we describe the routes to chemically modified pillar[n]arenes by discussing the chemistry of their functionalization: monofunctionalization, difunctionalization, rim differentiation, perfunctionalization, and phenylene substitution. We assess the synthetic complications of employing these functionalization procedures and survey the potential applications and novel properties that arise with these functionalized pillar[n]arenes. We also highlight the challenges and the synthetic approaches that have yet to be fully explored for the selective chemical modification of these hosts. Finally, we examine a related class of macrocycles and consider their future applications. We trust that this Account will stimulate the development of new methods for functionalizing these novel hosts to realize pillar[n]arene-containing compounds capable of finding applications.