Pillar[6]arenes: From preparation, host-guest property to self-assembly and applications

Pillar[6]arenes: From preparation, host-guest property to self-assembly and applications

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Ruowen Tang^a, Yanpeng Ye^a, Shajun Zhu^b, Yang Wang^a, Bing Lu^a^,^*, Yong Yao^a^,^*

Chinese Chemical Letters | 2023, 34(3) : 107734

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Chinese Chemical Letters | 2023, 34(3): 107734

• Review •

Pillar[6]arenes: From preparation, host-guest property to self-assembly and applications

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Ruowen Tang^a, Yanpeng Ye^a, Shajun Zhu^b, Yang Wang^a, Bing Lu^a^,^*, Yong Yao^a^,^*

Affiliations

^a School of Chemistry and Chemical Engineering, Nantong University, Nantong 226019, China

^b Department of Hepatobiliary and Pancreatic Surgery, Affiliated Hospital of Nantong University, Nantong 226001, China

Published: 2023-03-15 doi: 10.1016/j.cclet.2022.08.014

Outline

Abstract

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Pillar[n]arenes primarily comprise pillar[5]arenes and pillar[6]arenes, which belong to the new class of supramolecular macrocyclic hosts. Pillar[n]arenes have aroused wide attention because of their highly rigid and symmetrical architectures, controllable cavity size, and wide applications in a wide variety of areas. Although pillar[6]arene is difficult to synthesize, numerous studies have been conducted on it. In this review, the strategies to synthesize and functionalize pillar[6]arenes are investigated systematically. In addition, their host-guest properties in organic solvents and in aqueous solution are described. Moreover, pillar[6]arenes applied in different fields (e.g., molecular recognition, drug release, cancer therapy, and gas separation) are clarified. Hopefully, this study is capable of arousing more attention from increasing scientists to study large-cavity pillar[n]arenes.

Key words

Pillar[6]arenes / Host-guest interactions / Drug release / Cancer therapy / Supramolecular chemistry

Cite this Article

Ruowen Tang, Yanpeng Ye, Shajun Zhu, Yang Wang, Bing Lu, Yong Yao. Pillar[6]arenes: From preparation, host-guest property to self-assembly and applications[J]. Chinese Chemical Letters, 2023 , 34 (3) : 107734 - . DOI: 10.1016/j.cclet.2022.08.014

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1 Introduction

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It is generally known that the design and synthesis of macrocyclics with multiple functions are of great significance in the development of supramolecular chemistry [1,2]. In nature, a highly selective multi-molecule recognition system between two or more molecules exhibits high functionality and complex supramolecular assembly [3,4]. To date, numerous macrocyclic hosts (e.g., calixarenes [5], cucurbiturils [6,7], and cyclodextrins [8]) have been synthesized and applied to a wide variety of aspects (e.g., molecular recognition, gas separation, energy storage, drug delivery, and cancer therapy) thus far. Pillar[n]arenes refer to a new type of macrocyclic compounds that exhibit highly and rigid symmetrical architectures and comprise electron-donating dialkoxybenzene units connected by methylene bridges in the para-positions. Pillar[n]arenes have aroused wide attention since 2008 [9-14]. Pillar[n]arenes can more easily adjust their cavity size, and the functional modification of pillar[n]arenes are also easier, as compared with the traditional macrocyclic hosts. Pillar[n]arenes primarily consist of five or six units. To be specific, the syntheses, host-guest interactions and wide applications of pillar[5]arenes have advancing rapidly [15-17]. In contrast, rare studies on pillar[6]arenes have been conducted [18,19]. However, pillar[6]arene (ca. 7.5 Å) has a larger cavity size than pillar[5]arene (ca. 5.5 Å) [3,4]. Accordingly, larger-size guests (e.g., numerous positively charged polyaromatic compounds, bulky hydrocarbons, and ferrocene [20,21]) can be associated with pillar[6]arenes efficiently.

In this review, the strategies to synthesize and functionalize pillar[6]arenes are discussed systematically [22]. Moreover, their host-guest properties in organic solvents and in aqueous solution are described. Furthermore, pillar[6]arenes applied in different areas (e.g., molecular recognition, drug release, cancer therapy, and gas separation) are also clarified.

2 Syntheses of pillar[6]arenes

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The successful preparation of pillar[6]arenes lays a foundation for the investigation of their properties and applications. As mentioned above, the most significant obstacle to the preparation of pillar[6]arenes is that pillar[5]arenes are more stable in the products generated by the reaction, thus leading a low yield of pillar[6]arenes. After the development of nearly a decade, four main methods have been proposed for the syntheses of pillar[6]arenes (Scheme 1).

2.1 Monomer direct cyclocondensation

The first pillar[6]arene was discovered by Cao et al. [23] through the monomer direct cyclocondensation. As depicted in Scheme 2a, the first pillar[6]arene was synthesized by p-toluenesulfonic acid catalyzed cyclocondensation of 2, 5-bis(benzyloxymethyl)-1, 4-diethoxybenzene in CH₂Cl₂. Then in 2011, Huang' group also prepared pillar[6]arenes successfully by Lewis acid-promoted self-condensation of 2, 5-alkoxybenzyl alcohols in CH₂Cl₂ (Scheme 2b) [24]. Yao prepared bromoethyl-modified pillar[6]arene (P6-4) by condensation 1, 4-bis(2-bromoethoxy)benzene and trioxymethylene with boron trifluoride ether (BF₃·(Et₂O)₂) as catalyst in ClCH₂CH₂Cl successfully (Scheme 2c) [25]. Besides, pillar[6]arenes are often the by-products of pillar[5]arenes in the monomer direct cyclocondensation method, so the yield of pillar[6]arene is very low.

2.2 Template syntheses

In order to improve the yield of pillar[6]arene, templated syntheses were developed successfully. Fox example, in 2014, Ogoshi et al. demonstrated the template effect of different solvents on the formation of pillar[5]arenes and pillar[6]arenes, and discovered that chlorocyclohexane (Cl-CyC6) is a good solvent for synthesis of pillar[6]arenes (Scheme 3a) [26]. Besides large size solvent, guest molecules were also added to improve the synthesis of pillar[6]arenes. In 2015, Chen et al. discovered a facile method for selective synthesis of pillar[6]arenes. Due to a strong donor-acceptor interaction between 2, 7-dipentylbenzo[lmn][3, 8]phenanthroline-1, 3, 6, 8(2H, 7H)-tetraone (NDI) and the precursors, NDI has been demonstrated to serve as a template for selective synthesis of pillar[6]arenes in a modest yield (Scheme 3b) [27]. In 2016, Scarso et al. used a range of small organic and organometallic cations as the of cation templated to prepare pillar[6]arenes [28], and the yields of pillar[6]arenes could be improved greatly and up to 38% (Scheme 3c). In 2022, Sue and co-workers first made use of a silver salt template to develop "Rim-Differentiated" pillar[6]arenes (P6-16). The desired production was obtained by oligomerizing 2, 5-dialkoxybenzyl alcohol monomers in a head-to-tail fashion and RD-pillar[6]arenes were expected as a side production during this process [29].

2.3 Free solvent synthesis

Although several synthetic strategies of pillar[6]arenes have been developed, the synthetic process and yield are not satisfactory. In 2016, Santra et al. reported an efficient solvent-free procedure for the synthesis of pillar[6]arenes with H₂SO₄ as catalyst [30]. This method not only avoided the use of organic solvents, but also possessed mild reaction conditions and high yields (Scheme 4). The approach is not only simple, high yield, energy efficient, but also applicable to plenty of 1, 4-dialkoxybenzenes as raw materials.

2.4 Fragment coupling method

Due to the highly symmetrical structures generated from one-pot syntheses, the partial functionalization of pillar[6]arene is usually beset with low yields and onerous purifications of the target multifunctional pillar[6]arene. In 2021, Huang's group reported a method of two-step fragment coupling for synthesizing symmetrically tetra-functionalized pillar[6]arenes (Scheme 5) [31]. Besides be simple and versatile, this method makes hetero-fragment coupling and pre-functionalization available. Yang et al. also used a distinct and facile two-step strategy to synthesize leaning pillar[6]arene. 1, 4-Bis(2, 5-dimethoxybenzyl)benzene (MDM) and paraformaldehyde were mixed in CH₂Cl₂ under the circumstance of the Lewis acid BF₃·O(Et)₂ by the fragment coupling method [32].

Besides the above synthetic methods, Ogoshi et al. also demonstrated that pillar[6]arene can be obtained from the expanding the ring size of pillar[5]arene in chloroform with BF₃·OEt₂ as the catalyst at 20 ℃ [33].

3 Functionalized derivations of pillar[6]arenes

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In order to further investigate the properties of pillar[6]arenes and develop their applications in a wide variety of fields, the functional modification of pillar[6]arenes takes on a great significance. According to the different type of functional groups and the modification positions, pillar[6]arenes are divided into cation-based pillar[6]arenes (CP6), anion-based pillar[6]arenes (AP6), neutral groups-based pillar[6]arenes (NP6) and partially functionalized pillar[6]arenes (PP6).

3.1 Cation-based pillar[6]arenes

Cation-based pillar[6]arenes (CP6) were primarily formed from the key intermediate P6-4 or P6-14 containing 12 bromo-alkyl moieties. For instance, Chen et al. reported the novel synthesis of CP6-1 containing multiple pyridinium moieties (Scheme 6a) by the reflux of P6-14 and excess pyridine in CH₃CN in 2013 [34]. In 2014, Yao et al. used a simple method to prepare a type of cation pillar[6]arenes (CP6-2) [25] as a colorless solid by treating P6-4 with excess N-methylimidazole (30 equiv.) in ethanol (Scheme 6b). Subsequently, cation pillar[6]arenes CP6-3 (Scheme 6c) [35] and CP6-5 (Scheme 6e) [36] with 12 trimethyl-ammonium groups at upper and lower rims were synthesized by refluxing bromo-alkyl modified pillar[6]arene with an excessive amount of trimethylamine in the CH₃CH₂OH/toluene mixture. On that basis, Ran et al. introduced a hydroxyl group at two upper and lower positions and developed a water-soluble macrocyclic synthetic receptor (CP6-4) in 2020 (Scheme 6d) [37]. Furthermore, tertiary-amine groups modified pillar[6]arene (NP6-9) was well prepared from P6-4 reacting with ethylenediamine. Subsequently, HCl or CO₂ was added to the solution of NP6-9, which led to the development of two novel cation-based pillar[6]arenes CP6-6 and CP6-7 (Schemes 6f and g) [38].

3.2 Anion-based pillar[6]arenes

Yu et al. first prepared Anion-based pillar[6]arene in 2012 by introducing carboxylate anionic groups on both rims of pillar[6]arene. As depicted in Scheme 7a, ester-substituted pillar[6]arene (NP6-2) was synthesized through the etherification of NP6-1. Subsequently, NP6-2 was hydrolyzed, which led to the production of carboxylic acid-substituted pillar[6]arene (NP6-3). Lastly, anion pillar[6]arene (AP6-1) was obtained using 1 equiv. of sodium hydroxide in aqueous solution [39]. Next, in 2016, phosphate-based pillar[6]arene (AP6-2) was synthesized in a classical manner [40]. The Arbuzov reaction between triethyl phosphite and bromo-substituted pillararene derivative (P6-4) and then followed by the Mckenna reaction using bromotrimethylsilane (TMSBr) to react with the generated pillararene-based phosphonate ester (NP6-4). Afterward, the target AP6-2 was produced through the addition of 1 equiv. NaOH into the solution of NP6-5 (Scheme 7b). In 2020, Isaacs et al. used parent hydroxylated pillar[6]arene (NP6-1) to react with SO₃ in pyridine at 90 ℃ for the generation of anion pillar[6]arene sulfate (AP6-3) in a 66.9% yield (Scheme 7c) [41].

3.3 Neutral groups-based pillar[6]arenes

Some useful neutral groups, besides cation and anion units, are also employed to modify pillar[6]arenes. In 2012, Ogoshi et al. synthesized a new water-soluble pillar[6]arene with 12 tri(ethylene oxide) substituents (NP6-6B) by etherifying the per-hydroxylated pillar[6]arene (Scheme 8a). It is confirmed as a novel thermo-responsive pillar[n]arenes and can exhibit lower critical solution temperature behavior in aqueous solution [20].

Primary amine is recognized as a vital functional group, which can further react with a considerable of compounds. In 2017, Duan et al. synthesized a primary amine modified pillar[6]arene (NP6-8) successfully through two steps [8]. As shown in Scheme 8b, pillar[6]arene derivative P6-4 was reacted with sodium azide in N, N-dimethylformamide (DMF) at 100 ℃ to produce NP6-7 (89% yield). Then NP6-8 (99% yield) was obtained by palladium-catalyzed hydrogenation of NP6-7 in methanol at 50 ℃.

In 2021, Li et al. synthesized another ethyl urea group-based pillar[6]arene (NP6-10) based on NP6-8 (Scheme 8c). The NP6-8 was stirred in tetrahydrofuran (THF), followed by the addition of ethyl isocyanate. After 12 h, pillar[6]arene derived with ethyl urea (NP6-10) was obtained as white solid by column chromatography [42].

3.4 Partially functionalized pillar[6]arenes

Besides the above-mentioned per-types of pillar[6]arenes derivatives, some researchers also designed and constructed partial modified pillar[6]arenes. For instance, in 2012, Ogoshi performed mono-reactive pillar[6]arenes (PP6-0) refluxing with excess 1, 6-dibromohexane for 24 h to obtain an important partially functionalized pillar[6]arene (PP6-1, Scheme 9a). PP6-1 has been found as a critical compound for other monofunctionalized pillar[6]arenes [43]. For instance, it is capable of reacting with pyridine in N₂ atmosphere to produce PP6-1A.

In 2013, Han et al. reported the first difunctionalized pillar[6]arenes by partial oxidation. As shown in Scheme 9b, through partial oxidation with (NH₄)₂[Ce(NO₃)₆] as the oxidizing agent, pillar[5]arene[1]quinones (PP6-2) were prepared from per-propyloxy-pillar[6]arene (P6-15). Subsequently, dihydroxyl functionalized pillar[6]arene (PP6-3) was prepared through the reduction of PP6-2 with Na₂S₂O₄ [44].

In 2019, Xiao et al. synthesized a chiral electrochemically responsive molecular universal joint (Scheme 9c) [45] by fusing a macrocyclic pillar[6]arene to a ferrocene-based side ring. The compound (PP6-4) can be synthesized by reacting A1/A2-diaminopropy-pillar[6]arene in five steps with 1, 1'-ferrocenedicarboxylic acid under the action of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide and 1-hydroxybenzotriazole in CHCl₃.

4 Host-guest properties of pillar[6]arenes

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The hydroxyl groups of pillar[6]arenes are easier to functionalize with different substituents, which can control the binding characteristics between hosts and guests effectively [46]. Through the selection of different building blocks, pillar[6]arene based host-guest complexes process different sizes and structures [47]. The cavity size of pillar[6]arenes have been found to be larger than those of pillar[5]arenes, so pillar[6]arenes are capable of performing better host-guest complexion with the large-size guests. In 2010, Han et al. [48] reported the first pillar[6]arene-based host-guest complexion, after that, the host-guest properties of pillar[6]arenes have been aroused a lot of attention. The guest molecules for pillar[6]arenes primarily consist of the categories below.

4.1 Cation guests

In 2012, Ogoshi's group developed a new water-soluble pillar[6]arene with tri(ethylene oxide) groups (NP6-6B). They found that the trans form of an azobenzene guest (trans-CG0) could form a stable 1:1 complex with NP6-6B. Irradiation with UV light induced a conformation change for the azobenzene guest from the trans to cis form, and dethreading occurred due to a size mismatch between the cis form and the pillar[6]arene cavity [20]. Subsequently, Huang's group [49] and Wang' group [50] all found that trans form of azobenzene (trans-CG1) could form a stable host-guest complex with both P6-1 and AP6-1 while cis-CG1 could not (Fig. 1).

In 2013, Xia et al. demonstrated another complexion between per-butylated pillar[6]arenes (P6-2) and a ferrocenium cation (CG2) [51]. They could form a novel and highly stable inclusion complexion. The reduced form of CG2 (ferrocene) exhibits extremely weak binding affinity, whereas its oxidized form shows much stronger binding ability with P6-2 (Fig. 2).

There were also some pyridinium salts and their derivatives as cationic guests (Scheme 10). In 2014, Huang's group demonstrated that large size guest 3, 5-dimethhylpyridinium (CG3) can be complex with anions modified pillar[6]arene (AP6-1) with a high association constant [52]. Besides, Wang' group presented the complexion of AP6-1 and guest molecule CG4 bearing one pyridinium group and two alkyl chains [53]. In 2019, Wang' group presented a [2]pseudorotaxane with the use of a mono (ethylene oxide) substituted pillar[6]arene (NP6-6A) and a paraquat derivative guest (CG5) [54].

Furthermore, some other large size guests (e.g., n-octyltriethyl ammonium hexafluoro-phosphate (CG6) [48], 1-adamantylammonium tetrakis[3, 5-bis(trifluoromethyl)phenyl]borate (CG8) [44], diquat (CG9) [55], benzimidazolium (CG10) [56], photosensitizer methylene Blue (CG11) [57], and phenylboronic acid pinacol ester derivative carrying alkyl chain (CG12) [58]) are capable of forming stable complex with pillar[6]arenes (Scheme 10).

4.2 Anion guests

Some classical anion guests for pillar[6]arene were shown in Scheme 11. For example, in 2013, Li et al. investigated the host-guest interactions between pyridinium-modified pillar[6]arenes (CP6-1) and two anionic naphthalenesulfonate substrates, i.e., sodium 2-naphthalenesulfonate (AG1) and its derivatives (AG2). CP6-1 exhibited strong binding affinities. Through study, it was found that the binding affinity for disulfonated naphthalene (AG2) was much stronger than that for monosulfonated naphthalene (AG1), which was due to the electrostatic attraction interactions of the positively charged pillar[6]arenes with negatively charged sulfonate units were the decisive driving forces of the electrostatic inclusion complexes [34]. Besides, Xue et al. also found that cationic water-soluble pillar[6]arene (CP6-3) could complex with AG1 with a strong binding affinity [35].

In 2014, Yao et al. demonstrated the complexion of pillar[6]arenes (CP6-2) and a sodium p-hydroxybenzoate derivative (AG3) which had a 1:1 stoichiometry with an association constant about (1.63 ± 0.03) × 10⁶ L/mol [25]. It was mainly driven by multiple electrostatic interactions, hydrophobic interactions, and π-π stacking interactions between the benzene rings on the pillar[6]arene host and AG3 in water. This recognition between CP6-2 and AG3 could be used to control the aggregation of a complex between CP6-2 and an amphiphilic sodium p-hydroxybenzoate acid derivative (AG4) in water (Fig. 3).

In 2016, the complex between CP6-5 and 2-anthracenecarboxylic acid (AG5) was reported by Gui et al. [36]. Comparing different complexion models, it was proved that the cationic groups of pillar[6]arene could improve the electrostatic attraction and reduce the electrostatic repulsion between carboxylate anions of HH-stacked AG5 pairs under the condition of 1:2 (Fig. 4).

4.3 Neutral guests

Besides complex with charged guests, pillar[6]arenes could also complex with neutral guests (Scheme 12). In 2015, Yuan et al. reported the selective binding of a series of nitrile derivatives (i.e., NG1) by ethylated pillar[6]arene (P6-1). Through dipole–dipole forces and C–H···O, C–H···N and C–H···π bonds, suitable nitriles having good size/shape-fit effect could complex with pillar[6]arenes efficiently [59].

Numerous reports have been made on the selective recognition of basic amino acids by anionic macrocycles, whereas rare studies have been conducted on the selective recognition of acidic amino acids based on pillar[6]arenes in water. Thus, Duan et al. demonstrated that the complexions of NP6-8 and acidic amino acids (i.e., NG2 and its derivatives) in 2017 (Fig. 5). ¹H NMR spectroscopy [8] indicated the formation of a strong threaded host–guest complex between NP6-8 and NG2s.

Moreover, Han et al. reported the host-guest complexions between partially modified pillar[6]arene (PP6-5) and neutral pentaerythritol derivatives (NG3, NG4 and NG5) in 2020 (Fig. 6). Whether in solution or solid state, multiple C-H···π interactions are considered the main driving forces [60].

5 Applications

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Pillar[6]arenes have developed rapidly over the past few years and have aroused more attention. They have been applied to a wide variety of fields (e.g., molecular detection, drug release, cancer therapy) due to their excellent characteristics.

5.1 Signal and molecular detection

In 2018, Yu et al. developed a supramolecular hybrid material comprising graphene oxide (GO) and a pillar[6]arene-based host-guest complex (CP6-7⊃AG1-PyN) (Fig. 7), which can be applied in an ultrasonic (US) and photoacoustic (PA) signal nanoamplifier [38]. The surface of GO@CP6-7⊃AG1-PyN collects a considerable number of CO₂ nanobubbles due to the decomposition of the bicarbonate counterions, which can enhance the performances of US and PA excited by the near-infrared (NIR) light that mediates the photothermal effect.

In 2019, Tan et al. used an approach based on fluorescence resonance energy transfer (FRET) which recognized competitively between CP6-5 functionalized reduced graphene oxide (CP6-5@rGO) and probe/insulin molecules [61]. Accordingly, Tan et al. developed a supramolecular recognition sensor which was well used to determine insulin in artificial serum (Fig. 8).

Cao and Meier reported a new, promising branch about fluorescent chemo-sensors based on pillararene complexs in sensor technology in 2019 [62]. Aliphatic chains could be combined with the cavity of pillar[6]arenes through the interaction of C-H···π and bulky or flaky (sub-) structures could be arranged on the portals. Pillar[6]arenes could be used as receptors for chemical sensors by studying the fluorescence changes after complexation.

In 2020, Ran et al. synthesized gold nanoparticles (AuNPs) using hydroxylatopillar[6]arene (CP6-4) as a stable ligan. CP6-4@Au can increase the electron transfer rate and capture considerable primary antibodies (AbI). The surface of N-doped carbon quantum dots and cobalt oxide (N-CQDS@Co₃O₄) can be employed to deposit CP6-4@Au through π-π interaction. An electrochemical immunosensor with significant stability was constructed by CP6-4@Au/N-CQDS@Co₃O₄, which was used to detect human epidldymis protein (HE4) [37]. In the same year, Tan et al. constructed a sensitive and selective electrochemical sensing strategy for l-ascorbic acid (AA) based on a covalent organic framework (COF)-loading non-noble transition metal Co ion and macrocyclic cationic pillar[6]arene (CP6-5) nanocomposite (CP6-5-COF-Co) [63].

5.2 Drug release

Wang's group have conducted numerous studies on drug release. In 2013, they initially presented novel supramolecular prodrug vesicles through the host-guest interaction of water-soluble pillar[6]arene (AP6-1) and hydrophobic CG2-derivative in water (Fig. 9) [64]. The above vesicles exhibit pH-responsive behavior in aqueous solution and can encapsulate mitoxantrone (MTZ) to produce MTZ-loaded vesicles, which particularly show a rapid MTZ release in the low-pH environment. In the following years, they have also presented more achievements in drug release [40,53,65-67].

Based on the same host-guest interaction between AP6-1 and CG2-derivative, a general nanosponge was fabricated by Ma et al. in 2020 [19]. The obtained nanosponges were prepared and can be used to load dyes or antitumor drugs. The cargo was efficiently loaded and stably encapsulated through host−guest interaction. Drug-loaded nanosponges could efficiently deliver the chemotherapeutic agent in vitro and overcome multidrug resistance.

Both pillar[6]arene-based organic vesicles and pillar[6]arene modified inorganic nanomaterials can be employed to control trug release. For instance, in 2018, Pei et al. constructed a novel supramolecular hybrid material ZIF-8@DOX@AP6-4@CG7 which was complexed by carboxylated pillar[6]arene (AP6-4) and a galactose derivative (CG7) (Fig. 10). This material not only could be loaded with drugs, but also maintained the pH-sensitive drug release properties of ZIF-8 [68].

In addition, AP6-1-valved mesoporous silica nanoparticles (MSN) functionalized with dimethylbenzimidazolium and bipyridinium stalks were constructed, respectively, for multiresponsive controlled release. The controlled release of the AP6-1-valved MSN delivery systems can meet diverse requirements and has promising biological applications in targeted drug therapy [69].

5.3 Cancer therapy

In recent years, pillar[6]arenes were applied in cancer therapy have become the general trend. Inspired by facile surface engineering and designable layer-by-layer assembly concept, Yang et al. design and synthesize PPy@UiO-66@AP6-1@PEI−Fa nanoparticles (PUAPFa NPs) to achieve efficient synergistic chemo/photo/thermal therapy, taking advantage of the desirable photothermal conversion capability of polypyrrole nanoparticles (PPy NPs) and high drug-loading capacity of hybrid scaffolds. Significantly, pillararene-based pseudorotaxanes as pH/temperature dual-responsive nanovalves allow targeted drug delivery in pathological environment with sustained release over 4 days, which is complementary to photothermal therapy, and folic acid-conjugated polyethyleneimine (PEI−Fa) at the outmost layer through electrostatic interactions is able to enhance tumor-targeting and therapeutic efficiency. Such PUAPFa NPs showed efficient synergistic CPT of cervical cancer both in vitro and in vivo (Fig. 11) [70].

Moreover, Liu et al. synthesized a ternary pillar[6]arene-based supramolecular nanocatalyst for chemodynamic therapy (CDT). It not only broadened the use of CDT materials and demonstrated new developments in cancer therapy based pillar[6]arenes in 2021 [71]. Meng's group demonstrated the improvement of anti-tumor drugs and the treatment of tumor cells based on pillar[6]arenes-based Intelligent drug delivery systems (DDSs) (Fig. 12) [72].

5.4 Separation and purification

Huang's group was the first to apply pillar[6]arenes to separation and purification. They found that pillar[6]arenes could be used to capture iodine [73], separate the mixture of styrene (St) and ethylbenzene (EB) [74], separate methylcyclohexane from toluene with 99.2% purity [75] and so on [76-82]. For instance, in 2021, they made used nonporous adaptive crystals of four pillararenes, per-ethylated pillar[5]arene, per-ethylated pillar[6]arene (P6-1), per-bromoethylated pillar[5]arene, and per-bromoethylated pillar[6]arene (P6-4) to separate Isopropylbenzene (IPB) and α-methylstyrene (AMS). Due to the stability of final crystal structures loaded with different guest molecules, they found that P6-4 selectively adsorbs IPB from an equal volume mixture of IPB and AMS with 95.43% purity, whereas the other three pillararenes, per-ethylated pillar[5]arene, per-ethylated pillar[6]arene (P6-1), and per-bromoethylated pillar[5]arene could not separate it (Fig. 13) [83].

Ogoshi's group suggested in 2018 that activated crystals of P6-1 produced by removing the solvent upon heating can take up branched and cyclic alkane vapors [84]. Accordingly, they used host–guest complexation of cyclic and branched alkane vapors through crystal state P6-1 to separate linear and branched alkanes.

Besides pillar[6]arene-based crystal materials, pillar[6]arene-based polymers have been also used for the removal of organic dye from water. In 2021, Yao prepared CP6-5-based supramolecular polymeric material from electrostatic interactions. The obtained material can be easily regenerated, whereas it can effectively remove organic dyes from water as an adsorbent [85]. In 2014, Zhang used AP6-1 and methyl viologen with a photoreactive polyelectrolyte, diazoresin to prepare a type of multilayer films. They can be used to enrich molecular dyes and purify methyl viologen-polluted water [86]. Liu used the property of the confinement effect of the macrocyclic cavity of pillar[6]arene (CP6-2) on the layered arrangement, which can significantly enhance the catalytic ability of the nanosheets for dye degradation [87].

5.5 Solubility and activity enhancement

Pillar[6]arenes can also be serve as macrocycles in solubility and activity enhancement. For instance, in 2012, Huang's group synthesized the first water-soluble pillar[6]arene (AP6-1) and facilitated the dispersion of multiwalled carbon nanotubes in water by adjusting its water solubility [39]. They employed the confined hydrophobic cavity of AP6-1 to interact with a neutral guest NG6 containing a π-rich pyrenyl ring through hydrophobic interactions. The solubility of this host−guest system can be controlled by adjusting the solution pH. Accordingly, the solubility of the MWNTs can be controlled easily, and this process can also be achieved reversibly (Fig. 14a). Besides, in 2014, they also constructed a UV-responsive supra-amphiphile based on host–guest interactions between a pillar[6]arene (AP6-1) containing carboxylate anionic groups on both rims and a UV-responsive guest bearing the 2-nitrobenzyl ester moiety (CG3-Py) (Fig. 14b) [52]. Furthermore, the above researchers applied this supramolecular system to the UV-responsive dispersion of MWNTs in water.

The property of activity enhancement can also be expressed in biological aspects. Besides Huang's group [9,88], Zhang et al. conducted studies in this area. In 2018, they prepared the host-guest complex of carboxylated pillar[6]arene (AP6-4) with oxaliplatin (OxPt) [89]. Encapsulated OxPt is completely released from the host-guest complex through competitive substitution with spermine (SPM) since the AP6-4 exhibits a higher binding affinity for SPM than OxPt. Thus, this complex has 20% higher anticancer activity than OxPt (Fig. 15).

Pillar[6]arenes can also enhance activity in other aspects. In 2018, Pei constructed a supramolecular photosensitizer system based on water-soluble pillar[6]arene (AP6-4) and the photosensitizer methylene blue (MB) through host–guest interaction. This system can effectively overcome photobleaching and prolong time MB to produce singlet oxygen when exposed to light, which takes on a great significance in durable photodynamic therapy [41].

5.6 Mechanical interlocking structures

Pillar[6]arenes can serve as important macrocyclic hosts for the construction of rotaxanes, supramolecular polymers, and mechanical interlocking structures because of their excellent host-guest binding properties. For instance, in 2012, Ogoshi's group initially prepared [2]rotaxanes with P6-1 as a wheel and a pyridinium derivative as an axle (Fig. 16). The wheel segments of P6-1 can be moved along the axle from one station to another under thermal stimulation [90]. In the following years, Ogoshi's group had conducted several investigations in this aspect [3,91].

Besides the above simplest rotaxane, pillar[6]arenes and guest molecules can also form pseudorotaxane. In 2014, Huang's group constructed a host-guest complexion between AP6-4 and a tetraphenylethene derivative in water, which can also form pseudorotaxane units spontaneously [92]. In 2019, Xia et al. prepared a novel metallosupramolecular poly-pseudorotaxane through metal coordination and NP6-6A⊃CG5 molecular recognition [54].

In 2018, Wang's group constructed highly efficient light-harvesting systems through the supramolecular self-assembly of AP6-1, a salicylaldehyde azine derivative, and two different fluorescence dyes, including Nile Red or Eosin Y [93]. In 2021, Li's group used a supramolecular host–guest interaction to construct a visible-light-regulated Cl⁻-transport membrane channel based on NP6-10 [40]. Moreover, they designed a l-/d-N-acetyl-cysteine-pillar[6]arene functionalized Si surface (l-/d-NACP surface) and investigated the effect of superficial chirality to selective adsorption of R-adrenaline [94].

5.7 Catalysis and supramolecular polymer gel

Through modification, assembly, etc., pillar[6]arenes composites can possess strong catalytic performance. In 2019, Yang et al. synthesized an anionic water-soluble pillar[6]arene (LP6-5) which could be used as both the reductant and stabilizer for the one-pot synthesis of gold nanoparticles (LP6-5-AuNPs) (Fig. 17). They could efficiently catalyze the hydrogenation of p-nitrophenol [95].

At the same year, Zhao and co-workers had also done plenty of researches on the application of pillar[6]arenes in catalysis [96-98]. For example, they studied a class of supramolecular host water-soluble pillar[6]arene-modified Ag nanoparticle-functionalized two-dimensional (2D) covalent organic framework (COF) composite in 2019 [96]. This kind of material had great electrocatalytic activity and could recognize paraquat in electrochemical detection highly.

6 Conclusion

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In brief, since the researches of pillar[5]arenes have formed a complete system, more researchers have paid attention to pillar[6]arenes. This review elaborates the existing research on pillar[6]arenes in the aspects of synthesis, functionalization, host-guest complexation and application. They have gradually become vital research objects since their larger cavities can be complex with larger guests. Although pillar[6]arenes have been used in various fields with remarkable results, there are still some problems that need to be further investigated and solved. The first is how to synthesize a large scale of functionalized pillar[6]arenes, such as water-soluble pillar[6]arene and rim-differentiated pillar[6]arene, effectively. The second is that the synthesis and properties of amphiphilic pillar[6]arenes have not been achieved. Finally, the preparation of pillar[6]arene-based dimers and trimers is in urgent need of development. In the future, we believe that more researchers will find and investigate the better performance and wide applications of pillar[6]arenes.

Declaration of competing interest

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

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This work was supported by the National Natural Science Foundation of China (No. 22007052), the Natural Science Foundation of Jiangsu Province (No. BK20190917), the project of "Six Talent Peaks in Jiangsu Province" (No. XCL-085), Science and Technology Project of Nantong City (No. JC2020055), and China Postdoctoral Science Foundation (No. 2020M680071). We also thank Nantong University Analysis & Testing Center for characterization.

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Year 2023 volume 34 Issue 3

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doi: 10.1016/j.cclet.2022.08.014

Receive Date：2022-06-05
Online Date：2025-11-21
Published：2023-03-15

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Received：2022-06-05
Revised：2022-08-02
Accepted：2022-08-05

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^a School of Chemistry and Chemical Engineering, Nantong University, Nantong 226019, China

^b Department of Hepatobiliary and Pancreatic Surgery, Affiliated Hospital of Nantong University, Nantong 226001, China

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Scheme 1 The application of pillar[6]arenes.

Scheme 2 Syntheses of (a) P6-1 and P6-2; (b) P6-3; (c) P6-4 from monomer direct cyclocondensation.

Scheme 3 Syntheses of (a) P6-5; (b) P6-6; (c) P6-7, P6-8 and P6-9; (d) P6-16 through templated syntheses.

Scheme 4 Syntheses of P6-10 and P6-11 through free solvent synthesis.

Scheme 5 Syntheses of P6-12, P6-13 and P6-17 from fragment coupling method.

Scheme 6 Structures of cation pillar[6]arenes (a) CP6-1; (b) CP6-2; (c) CP6-3; (d) CP6-4; (e) CP6-5; (f) CP6-6; (g) CP6-7.

Scheme 7 Syntheses of anion pillar[6]arenes (a) AP6-1; (b) AP6-2; (c) AP6-3.

Scheme 8 (a) Structures of NP6-6A and NP6-6B; (b) Synthesis of NP6-10; (c) Structure of NP6-10.

Scheme 9 Syntheses of pillar[6]arenes: (a) PP6-0, PP6-1 and PP6-1A; (b) PP6-2. (c) Structure of PP6-4.

Scheme 10 Structures of cation guests for pillar[6]arenes.

Scheme 11 Structures of anion guests for pillar[6]arenes.

Fig. 3. The illustration of the aggregate transformation from AG4 based micelles to vesicles based on CP6-2⊃AG4. Reproduced with permission [25]. Copyright 2014, the Royal Society of Chemistry.

Scheme 12 Structures of neutral guests for pillar[6]arenes.

Fig. 7. Schematic representation of the preparation of the supramolecular hybrid material (GO@CP6-7⊃AG1-PyN) exhibiting NIR light-triggered PA and US imaging enhancement. Reproduced with permission [38]. Copyright 2018, the Royal Society of Chemistry.

Fig. 10. Schematic of the construction of a supramolecular hybrid targeted drug delivery system based on host–guest complexation and ZIF-8@DOX@AP6-4. Reproduced with permission [68]. Copyright 2018, the Royal Society of Chemistry.

Fig. 11. Schematic illustration of the preparation of PUWPFa nanoplatform (A) and such nanoplatform for dual targeted chemophotothermal therapy of cervical cancer and the structures of representative building blocks (B). Reproduced with permission [70]. Copyright 2018, American Chemical Society.

Fig. 12. Schematic illustration of the fabrication of DOX@AG6/CP6-1 and tumor-selective treatment performed on tumor-bearing nude mice. Reproduced with permission [72]. Copyright 2021, American Chemical Society.

Fig. 13. Schematic illustration of highly selective separation of isopropylbenzene (IPB) and α-methylstyrene (AMS) by nonporous adaptive crystals of perbromoethylated pillar[6]arene (P6-4) via vapor- and liquid-phase adsorptions. Reproduced with permission [83]. Copyright 2021, American Chemical Society.

Fig. 14. (a) Illustration of the pH-responsive solubility of the MWNTs in the presence of AP6-1⊃NG6. Reproduced with permission [39]. Copyright 2012, American Chemical Society. (b) Chemical structures of AP6-1, CG3-Py and schematic representation of UV-responsive self-assembly in water. Reproduced with permission [52]. Copyright 2014, the Royal Society of Chemistry.

Fig. 15. Schematic illustration of supramolecular chemotherapy based on host−guest complexation between OxPt (CG8) and AP6-4, and the thorough release of OxPt (CG8) from AP6-4 through a competitive binding with spermine. Reproduced with permission [89]. Copyright 2018, American Chemical Society.

Fig. 17. Schematic illustration of LP6-5-AuNPs and their self-assembly, label-free detection of cationic methyl viologen (MV), and efficient catalysis for the hydrogenation of p-nitrophenol. Reproduced with permission [95]. Copyright 2019, American Chemical Society.

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