Supramolecular Enhancement of Charge Transport through Pillar[5]arene-Based Self-Assembled Monolayers

Intermolecular charge transport is one of the essential modes for modulating charge transport in molecular electronic devices. Supermolecules are highly promising candidates for molecular devices because of their abundant structures and easy functionalization. Herein, we report an efficient strategy to enhance charge transport through pillar[5]arene self-assembled monolayers (SAMs) by introducing cationic guests. The current density of pillar[5]arene SAMs can be raised up to about 2.1 orders of magnitude by inserting cationic molecules into the cavity of pillar[5]arenes in SAMs. Importantly, we have also observed a positive correlation between the charge transport of pillar[5]arene-based complex SAMs and the binding affinities of the pillar[5]arene-based complexation. Such an enhancement of charge transport is attributed to the efficient host–guest interactions that stabilize the supramolecular complexes and lower the energy gaps for charge transport. This work provides a predictive pattern for the regulation of intermolecular charge transport in guiding the design of next generation switches and functional sensors in supramolecular electronics.     

Water-soluble pillar[4]arene[1]quinone: Synthesis,host-guest property and application in the fluorescence turn-on sensing of ethylenediamine in aqueous solution,organic solvent and air

Water-soluble pillar[5]arenes are a class of typical macrocycles and have aroused tremendous attention for its easy to modify, abundant host-guest properties and extensive applications. However, up to now, all the reported water-soluble pillar[5]arenes acted as the host molecules, whereas they failed to be postsynthetically modified, which seriously impeded the development of the pillar[5]arene-based supramolecular chemistry. In this work, a new water-soluble pillar[5]arene, pillar[4]arene[1]quinone, was designed and synthsized with eight quaternary ammonium groups as well as a quinone units. Such a new water-soluble pillar[4]arene[1]quinone was capable of forming 1:1 stable complex with sodium 1-octanesulfonate in aqueous solution. Since the 1,4-quinone unit of WP[4]Q[1] could react with ethylenediamine (EDA) to form a conjugated quinoxaline structure, so pillar[4]arene[1]quinone could apply to the facile fluorescence turn-on sensing of EDA in aqueous solution, organic solvent and air.     

Polycationic Pillar[5]arene Derivatives: Interaction with DNA and Biological Applications

Dendritic pillar[5]arene derivatives have been efficiently prepared by grafting dendrons with peripheral Boc‐protected amine subunits onto a preconstructed pillar[5]arene scaffold. Upon cleavage of the Boc‐protected groups, water‐soluble pillar[5]arene derivatives with 20 ( 13 ) and 40 ( 14 ) peripheral ammonium groups have been obtained. The capability of these compounds to form stable nanoparticles with plasmid DNA has been demonstrated by gel electrophoresis, transmission electron microscopy (TEM), and dynamic light scattering (DLS) investigations. Transfection efficiencies of the self‐assembled 13 /pCMV‐Luc and 14 /pCMV‐Luc polyplexes have been evaluated in vitro with HeLa cells. The transfection efficiencies found for both compounds are good, and pillar[5]arenes 13 and 14 show very low toxicity if any.     

Construction and investigation of photo-switch property of azobenzene-bridged pillar[5]arene-based [3]rotaxanes

Series of azobenzene-bridged pillar[5]arene-based [3]rotaxanes with different alkyl chain length of guest molecules were constructed by threading-endcapping method with alkylenetriazole as axile and tetrahydrochromene as endcapping group.The encapsulation of pillar[5]arenes were proved by highresolution mass,¹ H NMR and NOESY spectra.The photo-responsive property were examined by irradiation of the synthesized [3]rotaxanes with 365 nm and blue light LED,which caused trans to cis and cis to trans isomerization,respectively.Irradiation of corresponding model guest compounds without pillar[5]arene encapsulation resulted in near completely trans to cis and cis to trans isomerization,indicating the existence of pillar[5]arenes is the determining factor for the comprised photo isomerization efficiency.     

Chemical conjugation with cyclodextrins as a versatile tool for drug delivery

A pillar[5]arene-based N-heterocyclic carbene ligand was prepared by reaction of bromoethoxy pillar[5]arene with excess 1-methylimidazole at 130 °C in the absence of solvent and used as a catalyst for the Suzuki coupling reaction. Excellent yields were obtained when the Suzuki reactions were carried out under ambient atmosphere in ethanol, employing 0.2 mol% ligand, 1 mol% PdCl₂(CH₃CN)₂ and 1.5 mmol of K₂CO₃. The novel pillar[5]arene-based imidazolium salt is a promising material for the construction of highly active supramolecular catalytic systems.     

Recent progress in the research on the host-guest chemistry of pillar[n]arenes

Pillar[n]arene, as a new kind of macrocyclic host molecule, is a cyclic oligomer, which has a unique rigid structure with a hydrophobic cavity and can interact with many size-matched guest molecules. In this review, the molecular recognition, self-assembly and applications of the pillar[n]arenes in the past two years were described. On the basis of previous scientific research, a variety of pillar[n]arene-based supramolecular systems responsive to specific external stimuli such as pH, redox, gas, light, etc. has been constructed. Pillar[n]arenes have exhibited great potential in constructing these fantastic supramolecular systems based on host-guest recognition, including nanomaterials, controllable drug delivery, transmembrane channels, chemosensors and catalytic entities. These supramolecular systems have a wide range of applications in the material, biology, detection and catalysis field, but their applications are not limited to these fields.     

Recent advances of functional gels controlled by pillar[n]arene-based host–guest interactions

Functional gels fabricated from supramolecular host–guest interactions exhibit outstanding characteristics including stimuli-responsiveness, self-healing and adaptability. Pillar[n]arenes are new generation of supramolecular macrocyclic host, which displayed excellent host–guest recognition properties. In the last few years, pillar[n]arene-based gels that include both hydrogels and organogels have been attracted more and more attention. In this digest, the recent advances in this field are reviewed, with special emphasis on the multistimuli responsive pillar[n]arene gels. It is anticipated that more and more pillar[n]arenes-based gel materials with smart properties will be developed in the near future.     

per-Hydroxylated pillar[6]arene: synthesis, X-ray crystal structure, and host-guest complexation

A per-hydroxylated pillar[6]arene was prepared. Single-crystal X-ray analysis demonstrated that its molecules are arranged in an up-to-down manner to form infinite channels in the solid state. Its host-guest complexation with a series of bispyridinium salts in solution was further investigated. It was found that the per-hydroxylated pillar[6]arene could form a 1:1 complex with paraquat in acetone with an association constant of 2.2 × 10(2) M(-1). This complex is a [2]pseudorotaxane as shown by its crystal structure, which is the first pillar[6]arene-based host-guest complex crystal structure.     

基于柱[5]芳烃主客体包结构筑分子响应型超分子水凝胶

主客体相互作用是在水溶液中与大环主体分子形成稳定的包结物的理想驱动力.以功能化的苯并咪唑衍生物为客体(M),水溶性柱[5]芳烃为主体构建了一种分子响应型超分子水凝胶.通过1H NMR, 2D NOESY和扫描电子显微镜(SEM)研究了水凝胶的成凝胶机理.有趣的是,主客体包结作用、柱[5]芳烃间有序的"外腔"π-π相互作用和分层堆积对于获得超分子水凝胶是必不可少的,非共价键相互作用的动态可逆性使凝胶体系对温度变化/化学刺激产生响应.此外,加入竞争性客体己二腈(ADN)/百草枯(PQ)后,柱[5]芳烃基水凝胶可转化为溶胶.因此,该超分子水凝胶可以选择性识别有机分子.     

Temperature-Driven Planar Chirality Switching of a Pillar[5]arene-Based Molecular Universal Joint

The study of an enantiopure bicyclic pillar[5]arene-based molecular universal joint (MUJ) by single-crystal X-ray diffraction allowed for the first time the unequivocal assignment of the absolute configuration of a planar chiral pillar[5]arene by circular dichroism spectroscopy. Crucially, the absolute configuration of the MUJ was switched reversibly by temperature, with an accompanying sign inversion of the anisotropy factor that varied by as much as 0.03, which is the largest value ever reported. Mechanistically, the reversible chirality switching of the MUJ is driven by the threading/dethreading motion of the fused ring and hence is dependent on both the size and nature of the ring and the solvent employed, reflecting the critical balance between the self-complexation of the ring by pillar[5]arene, the solvation to the excluded ring, and the inclusion of solvent molecules in the cavity.     

Open‐tubular capillary electrochromatography using carboxylatopillar[5]arene as stationary phase

Pillar[n]arenes have achieved much interest in material chemistry and supramolecular chemistry due to unusual pillar shape structure and high selectivity toward guest. However, pillar[n]arenes have not yet been applied in capillary electrochromatography. This work at first time reports that carboxylatopillar[5]arene is used as a stationary phase in open‐tubular capillary electrochromatography. Carboxylatopillar[5]arene not only possess the advantages of pillar[n]arenes but also provide free carboxy groups for immobilizing on the inner wall of capillary column via covalent bonding. The characterization of SEM and FT‐IR indicated that carboxylatopillar[5]arene was successfully grafted on the inner wall of capillary. The baseline separation of model analytes including neutral, basic, and acidic compounds, nonsteroidal anti‐inflammatory drugs and dansyl‐amino acids have been achieved thanks to the electron‐rich cavity of carboxylatopillar[5]arene and hydrophobic interactions between the analytes and stationary phase. The intraday, interday, and column‐to‐column precisions (RSDs) of retention time and peak area for the neutral analytes were all less than 3.34 and 9.65%, respectively. This work indicates that pillar[n]arenes have great potential in capillary electrochromatography as novel stationary phase.     

Monofunctionalized pillar[5]arenes: synthesis and supramolecular structure

The first synthesis of a monohydroxy pillar[5]arene has been carried out, by controlling the de-O-methylation of per-methylated pillar[5]arene. Using monohydroxy pillar[5]arene as an intermediate, mono-guest-functionalized pillar[5]arene was prepared. It formed a self-inclusion complex in CDCl(3) whereas in acetone-d(6) dethreading of the guest moiety took place.     

Recent development of pillar[n]arene-based amphiphiles

Pillar[n]arene-based amphiphiles,mainly including amphiphilic pillar[n]arenes and supra-amphiphilic pillar[n]arenes,have obtained considerable interests in recent years due to their fascinating chemical structures,various self-assembly behaviors,and widely applications.Thanks to the pillar-like frameworks and the rich host-guest recognitions of the cavities,these amphiphiles can be easily controlled to form dimensional and morphologic assemblies for multiple applications.Compared with traditional linear covalent amphiphiles,the introduction of host-guest recognitions facilitated the preparation and controllability of these supramolecular amphiphilic systems.Moreover,the host-guest recognitions endow the assemblies from pillar[n]arene-based amphiphiles with stimuli-responsive functions.In this mini-review,we summarized the chemical structures,self-assembly features,and the applications of pillar[n]arene-based amphiphiles.However,several research topics of pillar[n]arenebased amphiphiles can be further developed in the future,such as larger cavity amphiphilic pillar[n]arenes,co-assembly with 2 D materials and utilization of the host-guest interactions.     

High-yield diastereoselective synthesis of planar chiral [2]- and [3]rotaxanes constructed from per-ethylated pillar[5]arene and pyridinium derivatives

Planar chiral [2]- and [3]rotaxanes constructed from pillar[5]arenes as wheels and pyridinium derivatives as axles were obtained in high yield using click reactions. The process of rotaxane formation was diastereoselective; the obtained [2]rotaxane was a racemic mixture consisting of (pS, pS, pS, pS, pS) and (pR, pR, pR, pR, pR) forms of the per-ethylated pillar[5]arene (C2) wheel, and other possible types of the [2]rotaxane did not form. Isolation of the enantiopure [2]rotaxanes with one axle through (pS, pS, pS, pS, pS)-C2 or (pR, pR, pR, pR, pR)-C2 wheels was accomplished. Furthermore, pillar[5]arene-based [3]rotaxane was successfully synthesized by attachment of two pseudo [2]rotaxanes onto a bifunctional linker. [3]Rotaxane formed in a 1:2:1 mixture with one axle threaded through two (pS, pS, pS, pS, pS)-C2, one (pS, pS, pS, pS, pS)-C2 and one (pR, pR, pR, pR, pR)-C2 (meso form), or two (pR, pR, pR, pR, pR)-C2 wheels. The [3]rotaxane enantiomers and the meso form were successfully isolated using appropriate chiral HPLC column chromatography. The procedure developed in this study is the starting point for the creation of pillar[5]arene-based interlocked molecules.     

Piling Up Pillar[5]arenes To Self‐Assemble Nanotubes

New liquid‐crystalline pillar[5]arene derivatives have been prepared by grafting first‐generation Percec‐type poly(benzylether) dendrons onto the macrocyclic scaffold. The molecules adopt a disc‐shaped structure perfectly suited for self‐organization into a columnar liquid‐crystalline phase. In this way, the pillar[5]arene cores are piled up, thus forming a nanotubular wire encased within a shell of peripheral dendrons. The capability of pillar[5]arenes to form inclusion complexes has been also exploited. Specifically, detailed binding studies have been carried out in solution with 1,6‐dicyanohexane as the guest. Inclusion complexes have also been prepared in the solid state. Supramolecular organization into the Col_h mesophase has been deduced from X‐ray diffraction data and found to be similar to that observed within the crystal lattice of a model inclusion complex prepared from 1,4‐dimethoxypillar[5]arene and 1,6‐dicyanohexane.     

Porous Carbon Fibers Containing Pores with Sizes Controlled at the Ångstrom Level by the Cavity Size of Pillar[6]arene

We report a new synthesis method of fibrous carbon material with pores sizes that are precisely controlled at the Ångstrom level, by carbonization of two dimensional (2D) porous sheets of pillar[6]arenes. The 2D porous sheets were prepared by 2D supramolecular polymerization induced by oxidation of hydroquinone units of pillar[6]arenes. Owing to the hexagonal structure of pillar[6]arene, the assembly induced by 2D supramolecular polymerization gave hexagonal 2D porous sheets, and the highly ordered structure of the 2D porous sheets formed regular fibrous structures. Then, carbonization of the 2D porous sheets afforded fibrous carbon materials with micropores. The micropore size of the fibrous porous carbon prepared from pillar[6]arene was the same size as that of the starting material pillar[6]arene assembly.     

Efficient enhancement of fluorescence emission via TPE functionalized cationic pillar[5]arene-based host–guest recognition-mediated supramolecular self-assembly

A tetraphenylethene (TPE) functionalized cationic pillar[5]arene (CWP5-TPE) was successfully synthesized, and the intramolecular rotation of the TPE motif was restricted via cationic pillar[5]arene-based host–guest recognition-mediated supramolecular self-assembly in water, resulting in the efficient enhancement of fluorescence emission based on the aggregation induced emission (AIE) mechanism. CWP5-TPE self-assembled into nanoribbons while the host–guest inclusion complex formed into supramolecular amphiphile nanoparticles in water.     

Cucurbit[n]uril-calix[n]arene-based supramolecular frameworks assembled using the outer surface interactions of cucurbit[n]urils

Based on the crystal structures of two cucurbit[6]uril/calix[n]arene-based supramolecular frameworks reported by Long and co-workers,we further investigated the interactions of cucurbit[6]uril with 4-sulfocalix[4]arene and 4-sulfocalix[6]arene using ¹H NMR spectroscopy and isothermal titration calorimetry(ITC),respectively.Moreover,solid fluorescent materials were prepared via the adsorption of fluorescent dyes by these porous supramolecular frameworks,which exhibit a selective response to certain volatile organic compounds.     

Comparison of pillar[5]arene and calix[4]arene anion receptor ability in aqueous media

Abstract

Recognition ability of both cationic pillar[5]arene and calix[4]arene has been studied in aqueous media. Anion complexation can be evaluated from their ability to complex their counterions as well as an added external organic anion. DOSY NMR experiments and fluorescence quenching show that pillararenes have a larger ability for including their own counterions than calixarenes irrespective of the anion (tetrafluoroborate or chloride or bromide) and the structure of the cationic moiety (trimethylammonium or methylimidazolium). Counterion complexation shows a picture where four to five positive charges of the pillar[5]arene are neutralised, meanwhile only one positive charge of the calixarene is neutralised for a 1 mM solution of the macrocycle. Irrespective of the smaller net positive charge in the pillar[5]arene, its binding ability for organic anions (toluenesulfonate or hydroxybenzoate) is larger than for calix[4]arene allowing a better accommodation of the guest in its cavity. The larger separation between the cationic groups of the receptor and its electron-rich aromatic region improves the anion recognition ability for pillar[5]arene.     

Syntheses of a pillar[4]arene[1]quinone and a difunctionalized pillar[5]arene by partial oxidation

A pillar[4]arene[1]quinone and a difunctionalized pillar[5]arene have been synthesized by partial oxidation.