Stimuli-Responsive Supramolecular Assemblies Constructed from

Article Cite This: Acc. Chem. Res. 2018, 51, 1656−1666

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Stimuli-Responsive Supramolecular Assemblies Constructed from Pillar[n]arenes Takahiro Kakuta,† Tada-aki Yamagishi,† and Tomoki Ogoshi*,†,‡ †

Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan WPI Nano Life Science Institute, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan

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‡

CONSPECTUS: 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 chargetransfer 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. which we termed “pillar[n]arenes”.18 Pillar[n]arenes have been widely used because their special features enable new properties.19−21 Pillar[n]arenes are useful building blocks for the creation of stimuli-responsive supramolecular assemblies for the following reasons: (1) Superior host ability and planar chirality: Figure 1a shows the chemical and crystal structures of pillar[5]arene. Owing to their para-bridge linkage, the macrocycles have a symmetrical pillar-shaped structure. Because of the electron-

1. INTRODUCTION Stimuli-responsive supramolecular assemblies have received a great deal of interest because their structures can be changed by applying a stimulus, and in some cases, reverted back to their original structures by applying another stimulus.1−5 Macrocyclic hosts are useful building blocks for designing stimuliresponsive supramolecular assemblies because they exhibit cavity size-dependent host abilities.6−11 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.12−17 In 2008, our group developed new pillar-shaped host molecules, © 2018 American Chemical Society

Received: April 10, 2018 Published: June 11, 2018 1656

DOI: 10.1021/acs.accounts.8b00157 Acc. Chem. Res. 2018, 51, 1656−1666

Article

Accounts of Chemical Research

Figure 1. (a) Chemical and crystal structures of pillar[5]arene. (b) Cavity sizes and suitable guests of pillar[5]- and pillar[6]arenes and stimuliresponsive host−guest system using pillar[6]arenes. (c) Planar chirality.

Figure 2. (a) Mono-, (b) di-, (c) tri-, (d) tetra-, and (e) per-functionalized pillar[5]arenes. (f) Rim-different and (g) methylene bridge substituted pillar[5]arenes. The other isomers are omitted in di-, tri-, tetra-, and rim-different pillar[5]arenes.

Figure 3. (a) Schematic representation of pseudo[2]rotaxanation using a pH-responsive guest. Reproduced with permission from ref 27. Copyright 2010 Royal Society of Chemistry. (b) Schematic representation of photoresponsive pseudo[2]rotaxane based on [5]1 and an azobenzene (G3). Reproduced with permission from ref 28. Copyright 2011 American Chemical Society.

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Accounts of Chemical Research rich cavity of pillar[n]arenes, neutral molecules are the preferred guests because multiple CH/π interactions can form to stabilize the host−guest complex (Figure 1b).22,23 The cavity size of pillar[5]arenes is ca. 4.7 Å, which accommodates linear molecules. In contrast, bulky molecules such as cyclic and branched molecules and aromatic compounds containing trans-azobenzene and ferrocenium groups are suitable guests for pillar[6]arenes because the cavity of pillar[6]arenes (ca. 6.7 Å) can accommodate these molecules. Pillar[6]arenes rarely form complexes with cisazobenzene because their size does not match the cavity of pillar[6]arenes.24 Pillar[6]arenes also rarely form host−guest complexes with neutral ferrocene.25 The host ability and photoand redox-responsive hostguest systems can contribute to the design of stimuli-responsive supramolecular assemblies. Furthermore, pillar[n]arenes have planar chirality (Figure 1c)26 owing to the position of the alkoxy substituents. The stimulicontrol of the planar chirality is a characteristic of pillar[n]arene chemistry. (2) Versatile functionalization: Pillar[n]arenes have alkoxy substituents. Synthetic procedures for the preparation of mono-, di-, tri-, tetra- and per-functionalized pillar[n]arenes have been developed.20 Therefore, stimuli-responsive groups can be installed to develop stimuli-responsive supramolecular assemblies (Figure 2). In this Account, we describe photo-, pH-, electric-field and redox-responsive supramolecular assemblies based on pillar[n]arenes.

Figure 4. Schematic representation of the redox-responsive pseudo[2]rotaxane induced by the redox conversion between [5]2 and [5]3. Reproduced with permission from ref 29. Copyright 2016 Royal Society of Chemistry.

cavity was decreased by the generation of the benzoquinone. Upon addition of a reductant, [5]3 was converted to [5]2, and [5]2 formed a stable host−guest complex with G4. Thus, the complexation behavior can be controlled by the redox interconversion between [5]2 and [5]3. 2.2. Chiral Inversion of Pseudo[1]catenanes

Pillar[n]arenes have planar chirality (Figure 1c), and the interconversion between pS and pR was inhibited by the formation of a [2]rotaxane structure, because the presence of the axle in the pillar[5]arene cavity prevented the rotation of the 1,4-dialkoxybenzene units. 30 Stimuli-control of the interconversion is a key feature of pillar[n]arene chemistry. We synthesized a pseudo[1]catenane structure containing a pillar[5]arene ring and an alkyl chain ring ([5]4, Figure 5a).31

2. STIMULI-RESPONSIVE SUPRAMOLECULAR ASSEMBLIES 2.1. Pseudo[2]rotaxane

Pillar[n]arenes show stronger host−guest interactions with cationic molecules than with neutral molecules because cationπ interactions can form between the electron-rich pillar[n]arene cavity and cationic molecules. Li and co-workers reported a pHresponsive pseudo[2]rotaxane (Figure 3a).27 Pillar[5]arene containing 10 phenolic groups ([5]1) formed a pseudo[2]rotaxane with dication G1 because G1 was bound in the cavity through cation−π interactions. In contrast, neutral G2, produced by the addition of a base, hardly associated with [5]1. The complexation behavior was reversible by the addition of base or acid. We reported a photocontrolled pseudo[2]rotaxanation (Figure 3b).28 The axle contains azobenzene and adamantyl ends (G3), and the adamantine end serves as a permanent stopper. Therefore, the wheel of [5]1 could slip over the trans-azobenzene. The time to reach equilibrium was short (half-life = 3.33 ± 0.21 s). The trans-azobenzene was converted to cis-azobenzene by UV-light irradiation. Even in the cis-azobenzene, the threading of axle 3 through the wheel of [5]1 was possible, but it took a long time (9.13 ± 1.2 h) because the bulky cis end increases the free energy of activation of the wheel slipping over the end. We reported a redox-responsive pseudo[2]rotaxanation using pillar[5]arene containing one hydroquinone ([5]2, Figure 4).29 An n-butylene bridged by triazole units (G4) is a good guest for generating a pseudo[2]rotaxane with pillar[5]arenes. A pseudo[2]rotaxane was also formed using [5]2 (K ∼ 460 M−1). A pillar[5]arene containing one benzoquinone ([5]3) was generated from the oxidation of a hydroquinone. The oxidized form ([5]3) did not form a stable host−guest complex with G4 (K ∼ 30 M−1) because the π-electron density of the

Figure 5. (a) Schematic representation of the chiral inversion of pseudo[1]catenanes induced by (a) host−guest complexation and (b) temperature change. Reproduced with permission from refs 31 and 32. Copyright 2013 and 2017 Wiley-VCH Verlag GmbH & Co. KGaA.

[5]4 mainly formed a self-inclusion complex; thus, we successfully separated planar-chiral pseudo[1]catenane [5]4 by chiral column chromatography, because the inclusion of the alkyl chain prevented the rotation of the benzene units. The addition of a competitive guest, 1,4-dicyanobutane, to chiral pseudo[1]catenane [5]4 triggered planar-chiral inversion, because 1,4-dicyanobutane displaced the alkyl chain inside the cavity. Thermally induced planar-chirality switching using a pseudo[1]catenane structure was reported by Yang and coworkers (Figure 5b).32 They used an ethylene glycol chain instead of an alkyl chain to form pseudo[1]catenane [5]5. The ethylene glycol groups showed different desolvation energies in 1658


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molecular polymers via the dimerization and dissociation of an anthracene moiety.34 Anthracene photodimerizes upon irradiation with 350 nm UV light, and dissociation of the dimer occurs upon irradiation with 300 nm UV light or heating. They synthesized pillar[5]arene bearing an anthracene group ([5]7, Figure 6b). This compound formed a host dimer structure by 350 nm UV-light irradiation owing to the photodimerization of the anthracene groups. Mixing the host dimer and guest dimer G6 resulted in the formation of a supramolecular polymer (Figure 6c). The supramolecular polymer was dissociated by 300 nm UV-light irradiation or heating, because these conditions dissociated the anthracene dimers that were acting as the linker. Then, the supramolecular polymer was reformed upon irradiation with 350 nm UV light because of the redimerization of the anthracene groups. Ying and co-workers reported temperature- and solvent-responsive fluorescent supramolecular polymers containing tetraphenylethene moieties, which exhibit aggregation-induced emission.35

various solvents. In nonpolar solvents, the ethylene glycol chains in [5]5 exhibited stable solvation; thus, [5]5 formed a dethreaded species. At high temperatures, the ethylene glycol chains moved into the cavity owing to desolvation. 2.3. Supramolecular Polymers

Supramolecular polymers can be constructed through physical interactions; thus, supramolecular polymer association/dissociation can be regulated by stimuli. We reported a photoresponsive supramolecular polymer using the photoisomerization of an azobenzene (Figure 6a).33

2.4. LCST Behavior

Pillar[n]arenes have a hydrophobic core; thus, the introduction of hydrophilic moieties enables construction of amphiphilic molecules that exhibit lower critical solution temperature (LCST) behavior. We synthesized pillar[5]arene bearing 10 tri(ethylene oxide) groups (Figure 7a, [5]9).36 [5]9 could be dissolved in water at room temperature because of the tri(ethylene oxide) groups, which make the molecule amphiphilic. However, the transparent aqueous solution containing [5]9 turned turbid upon heating and regained its transparency upon cooling, which indicated that [5]9 exhibited LCST behavior. The LCST behavior can be attributed to the combination of the hydrophilic tri(ethylene oxide) groups and the hydrophobic pillar[5]arene core. At room temperature, water molecules were solvated by the tri(ethylene oxide) chains; therefore, [5]9 could be dispersed in aqueous media. At high temperature, the desolvation of the water molecules from the tri(ethylene oxide) groups triggered intermolecular association of the hydrophobic pillar[5]arene cores. The clouding points could be controlled by the host− guest system (Figure 7a). The clouding point could be increased by adding a cationic guest, such as G7, and it was decreased upon the addition of cucurbit[7]uril. Because cucurbit[7]uril forms a strong host−guest complex with G7 (>105 M−1), cucurbit[7]uril worked as a competitive host to dissociate the host−guest complex between [5]9 and G7, and thus the clouding point of [5]9 reverted to its initial value. To control the clouding point by photostimuli, we designed a photoresponsive guest composed of an azobenzene with two bulky cations (Figure 7b, G8).37 When trans-G8 was added to an aqueous solution containing tri(ethylene oxide)-substituted pillar[6]arene [6]1, the mixture had a clouding point of 57 °C, which was higher than that of [6]1 alone (41 °C), indicating the formation of a host−guest complex between [6]1 and trans-G8. Upon UV-light irradiation of the mixture, G8 was converted from the trans to the cis form, which triggered the dissociation of the complex and decreased the clouding point to 52 °C. The change in the clouding point was completely reversible by alternating UV- and visible-light irradiation. A redox-responsive clouding point change using pillar[5]arene containing a benzoquinone unit was also achieved by altering the oxidation state of the benzoquinone unit.38

Figure 6. (a) Photoresponsive supramolecular polymer constructed from host dimer [5]6 and guest dimer G5. Reproduced with permission from ref 33. Copyright 2013 Royal Society of Chemistry. (b) Photo- and thermoresponsive host dimer [5]7 and guest dimer G6. (c) Schematic representation of photo- and thermoresponsive supramolecular polymerization. Reproduced with permission from ref 34. Copyright 2013 American Chemical Society.

Supramolecular polymer formation was observed by mixing a pillar[5]arene host dimer connected by a trans-azobenzene derivative ([5]6) and a bipyridinium guest dimer (G5) in a 1:1 ratio. The molecular weight of the supramolecular polymer was decreased by UV-light irradiation. Dissociation of the supramolecular polymer was caused by the steric hindrance generated by the photoisomerization from the trans- to the cis-azobenzene in [5]6. The molecular weight of the supramolecular polymer was increased by visible-light irradiation because the steric hindrance was reduced by photoisomerization from the cis- to the trans-azobenzene. Therefore, the molecular weight of the supramolecular polymer was reversibly controlled by UV- and visible-light irradiation. Yang and coworkers reported dual photo- and thermoresponsive supra1659


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Figure 7. (a) Assembly and disassembly of amphiphilic pillar[5]arene ([5]9) controlled by host−guest complexation. Reproduced with permission from ref 36. Copyright 2012 American Chemical Society. (b) Photoresponsive assembly and disassembly of amphiphilic pillar[6]arene ([6]1) with a photoresponsive guest. Reproduced with permission from ref 37. Copyright 2012 American Chemical Society.

Figure 8. (a) Schematic representation of photoresponsive vesicle and particle formation based on [6]2 and G9. Reproduced with permission from ref 39. Copyright 2014 Royal Society of Chemistry. (b) Schematic representation of the photo- and thermoresponsive size-switchable supramolecular assembly between [5]10 and G10. Reproduced with permission from ref 40. Copyright 2016 Springer Nature.

2.5. Supramolecular Assemblies in Aqueous Media

assembled from the complexes. The nanoparticles were 800 nm in size when the trans form was used but 200 nm when the cis form was used. The change in the particle size was reversible by photoisomerization of G10. The redox-responsive host−guest system formed by ferrocene and pillar[6]arenes can also be used to control the assembled structure. Pei and co-workers used solubility switching to control the release of the drug doxorubicin (DOX; Figure 9).41 First, they synthesized a pillar[5]arene bearing 10 ferrocene groups ([5]11). [5]11 was an amphiphilic molecule when ferrocene was oxidized because the pillar[5]arene core and ferrocenium cations were hydrophobic and hydrophilic, respectively, resulting in the formation of vesicles in water. However, dissociation of the vesicles was observed when the ferrocenium cations were reduced to neutral ferrocene. The drug DOX was incorporated into the vesicles consisting of [5]11 with ferrocenium cations. The release of DOX was observed upon reduction of the ferrocenium cations because of the dissociation of the vesicles.

Photoresponsive interactions between pillar[6]arenes and azobenzenes can also be used to develop photoreversible assemblies such as micelles and vesicles. Huang and co-workers reported vesicle formation of an amphiphilic inclusion complex between water-soluble pillar[6]arene ([6]2) and an azobenzene (Figure 8a; G9).39 [6]2 formed a host−guest complex with trans-G9. [6]2 is hydrophilic and G9 is hydrophobic; thus, the complex had an amphiphilic structure, which allowed vesicle formation. However, the assembled structures of cis-G9, which was produced by UV-light irradiation, were nanoparticles because [6]2 did not form a stable host−guest complex with cis-G9. The formation of vesicles and nanoparticles was reversible by alternating UV- and visible-light irradiation. Liu and co-workers developed photoregulated size-controlled nanoparticles consisting of a host−guest complex between [5]10 and azobenzene derivative G10 (Figure 8b).40 transAzobenzene G10 threaded into the cavity of [5]10, but cisazobenzene G10 did not. The difference in the size of the host−guest complex determined the size of the nanoparticles 1660


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Figure 9. Schematic representation of drug delivery systems controlled by the reduction and oxidation of the ferrocene moieties in [5]11. Reproduced with permission from ref 41. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 10. (a) Schematic representation of redox-responsive organogel formation. Reproduced with permission from ref 42. Copyright 2015 American Chemical Society. (b) Illustration of the dramatically promoted swelling of G12 by the [6]2−ferrocene host−guest interactions and subsequent pH-responsive swelling−shrinking transition. Reproduced with permission from ref 43. Copyright 2016 American Chemical Society.

2.6. Supramolecular Gel

G11 and the pillar[6]arenes in [6]3. However, the addition of an oxidant to the mixture triggered the formation of a supramolecular gel because the ferrocenes in G11 were converted to ferrocenium cations, and the ferrocenium cations were moved into the cavities of the pillar[6]arenes in [6]3 to cross-link the polymeric chains. The same research group prepared a hydrogel using a cross-linked hydrophilic polymer with hydrophobic ferrocenes (Figure 10b, G12).43 The hydrophobic ferrocenes inhibited the swelling of the hydrogel in water. When the material was immersed in an aqueous solution containing water-soluble anionic pillar[6]arene ([6]2),

Stimuli-responsive supramolecular gels can be created by using stimuli-responsive host−guest systems based on pillar[n]arenes. Wang and co-workers reported the preparation of stimuliresponsive supramolecular organogels using inclusion complexes of pillar[6]arene and ferrocenium cations as cross-linkers to connect the polymeric chains (Figure 10a).42 Copolymers containing pillar[6]arene moieties ([6]3) and ferrocene derivatives (G11) were prepared. No supramolecular gels were formed by mixing [6]3 and G11 in an organic solvent because no complexes were formed between the ferrocenes in 1661


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Figure 11. (a) Self-assembled hexakis-pillar[5]arene metallacycles. (b) Schematic representation of the disassembly and reassembly of supramolecular polymer gels induced by competition between a guest and a bromide anion. Reproduced with permission from ref 46. Copyright 2014 American Chemical Society.

Figure 12. (a) Synthetic route to mechanized silica nanoparticles functionalized with [5]10 [2]pseudorotaxanes. (b) Nanovalves on the surface of the mesoporous nanoparticles can be operated either by pH changes or by competitive binding to regulate the release of the cargo molecules. Reproduced with permission from ref 47. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

prepared by metal−ligand bonds between pillar[5]arene bearing one dipyridyl unit ([5]12) and two linear di-Pt(II) acceptors of different lengths (Figure 11a). The addition of ditopic guest G13 resulted in the formation of supramolecular gels because G13 served as a cross-linker. The supramolecular gel was converted to a sol upon the addition of a competitive guest, 1,4-dicyanobutane, and the sol reverted to a gel upon the addition of a competitive host, a native pillar[5]arene. The gel to sol transition behavior was also observed when bromide anions were added, because the metal−ligand bonds were dissociated by the formation of Pt−Br complexes. The removal of the bromide anions triggered the reformation of supra-

the ferrocene groups were encapsulated into the cavities of [6]2, which triggered a dramatic swelling of the hydrogels. Shrinking and swelling of the gel occurred by the addition of acid or base to the solution. The pH-responsive shrinking/ swelling was attributed to the protonation and deprotonation of the carboxylic acid groups of [6]2. The metal−ligand bonds possess relatively moderate bond energies and thus useful for creating hierarchical assemblies.44,45 Yang and co-workers prepared multiple-stimuli-responsive supramolecular gels using metal−ligand bonds and host− guest interactions (Figure 11).46 First, discrete hexakispillar[5]arene metallacycles with different diameters were 1662


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Figure 13. (a) Schematic diagram of a temperature-responsive switch constructed using a [5]13-based host−guest interaction. (b) Cycling experiment of the wettability switching behavior of IL-modified Au surfaces with [5]13 and water. Reproduced with permission from ref 49. Copyright 2016 American Chemical Society. (c) Chemical structure of azobenzene-modified pillar[5]arene [5]14. (d) Schematic illustration of the switchable on−off cycling of the photoalignment in a [5]14 thin film. Reproduced with permission from ref 50. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 14. (a) Chemical structures of cationic ([5]15) and anionic ([5]10) pillar[5]arenes and a cationic pillar[5]arene bearing one azobenzene moiety ([5]16). (b) LbL assembly by consecutive adsorption of cationic and anionic pillar[5]arenes. (c) Size-selective molecular recognition of multilayer films. (d) Schematic representation of photoresponsive guest uptake, release and storage regulated by an azobenzene valve. Reproduced with permission from refs 51 and 52. Copyright 2015 and 2018 American Chemical Society.

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monolayer because of electrostatic interactions between the anionic inorganic surface and the cationic pillar[5]arenes. The surface of the film was positively charged; therefore, a bilayer structure could be produced by dipping the monolayered film into a solution containing anionic pillar[5]arene [5]10 (Figure 14a). Overall, alternate dipping of the substrate in both solutions afforded a multilayered pillar[5]arene film (Figure 14b). The film took up a guest molecule, i.e., paradinitrobenzene (DNB), into the spaces constructed from the pillar[5]arenes, but ortho- and meta-DNB were not suitable guests (Figure 14c). Access of the guest could be regulated by using photoirradiation by introducing an azobenzene gate on the outer surface.52 The thin film with four layers had an anionic surface. To attach azobenzene gates, the substrate was dipped into an aqueous solution of a cationic pillar[5]arene containing an azobenzene moiety [5]16. In the trans form, the guest could enter the spaces constructed from the pillar[5]arenes and then freely exit the space (Figure 14a). The trans form was converted to the cis form by UV-light irradiation. No guest access and release were observed in the cis form, indicating that cis-azobenzenes worked as gates to prevent the guests from accessing the cavities. The opening/closing of the gate could be reversibly controlled by alternate UV- and visiblelight irradiation.

molecular gels, because the metal−ligand bonds were reformed to afford the discrete hexakis-pillar[5]arene metallacycles. 2.7. Drug Delivery Systems

The storage and release of drugs by stimuli is an important technique for drug delivery systems. Host−guest complexes that are pH-responsive can be used as nanovalves on porous surfaces to control the release of drugs from porous materials. Yang, Stoddart, and co-workers prepared a porous silica nanoparticle coating by forming host−guest complexes between a pyridinium salt and water-soluble anionic pillar[5]arenes ([5]10, Figure 12).47 First, pyridinium guests were incorporated into the surface of the porous silica nanoparticles by a sol−gel reaction (i, ii, and iii). Then, the drug, i.e., the cargo, was loaded into the pores of the silica nanoparticles (iv). Finally, the host−guest complexes were formed upon the addition of water-soluble pillar[5]arene [5]10 to prevent the release of the cargo (v). The host−guest complexes on the surface worked as nanovalves to store the cargo in the porous silica. A change in the pH or the addition of a competitive guest induced the release of the cargo via the dissociation of the complexes (Figure 12b). Release of cargo using metal−organic frameworks (MOFs) gated by pHresponsive host−guest complexes has also been reported.48 2.8. Surfaces

3. CONCLUSIONS AND OUTLOOK In only 10 years, significant progress in the use of pillar[n]arene-based stimuli-responsive assembly systems has been made. Compared with stimuli-responsive assemblies based on well-known host molecules, the versatile functionality and unique host−guest properties of pillar[n]arenes enable the creation of hierarchical stimuli-responsive assemblies, even though this field is still in its infancy. The regular polygonal structures of these compounds are very useful for surface modification and the creation of regularly assembled structures. Recently, pillar[n]arenes have been used for the preparation of molecular-scale porous materials for capturing gases, organic vapors, and pollutants.53−56 The combination of these new applications and the stimuli-responsive systems presented in this Account will inspire new research directions. New functionalization methods, i.e., syntheses of pillar[n]arenes with different rims (Figure 2f)57 and substituents at the methylene bridges with moderate yields (Figure 2g),58,59 have recently been developed. We envisage that the continued development of new functionalization approaches for pillar[n]arenes will open up new opportunities to create stimuliresponsive supramolecular assemblies.

Modification of surfaces by organic molecules can be a useful way to introduce new functions on the surfaces. Pillar[n]arenes have a symmetrical tubular structure, which is suitable for the functionalization of surfaces. Li and co-workers developed a temperature-responsive surface by using host−guest complexation (Figure 13a).49 First, an ionic liquid-modified gold surface was synthesized. A decrease in the contact angle from that of a hydrophobic (132.4 ± 2.3°) to that of a hydrophilic surface (37.4 ± 2.0°) indicated the successful modification of the gold surface with the ionic liquid moieties. The ionic liquid-modified surface was hydrophilic, but could be changed to a superhydrophobic surface (146.2 ± 1.8°) by dipping the substrate in a solution containing a hydrophobic pillar[5]arene bearing two anthracene moieties [5]13 via the binding of the ionic liquid moieties in the cavity of [5]13. The superhydrophobic surface could be converted to a hydrophilic surface by heating, which caused the dissociation of the complex. The surface was reversibly converted back to the superhydrophobic surface by cooling because the host−guest complex could reform at room temperature (Figure 13b). A photoresponsive surface was prepared using a pillar[5]arene bearing 10 azobenzene moieties ([5]14, Figure 13c) by Wang and co-workers.50 The structure assembled by surface modification with [5]14 by casting was random (Figure 13d). UV-light irradiation or annealing induced a uniform orientation of [5]14. Photoisomerization of the azobenzene moieties from trans to cis by visible-light irradiation triggered the formation of a randomly oriented structure. The switching of the alignment was completely reversible. Pillar[5]arenes [5]14 on the surface could be uniformly oriented on the surface by UV-light irradiation or annealing and then changed to a randomly oriented structure by visible-light irradiation. Our group reported the preparation of pillar[5]arene films using a layer-by-layer assembly method (Figure 14a).51 The surface of the inorganic substrates, i.e., silica, was negatively charged. Immersion of the inorganic substrate into a solution containing cationic pillar[5]arene bearing 10 ammonium moieties ([5]15, Figure 14a) afforded a pillar[5]arene

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-76-234-4775. Fax: +81-76-234-4800. E-mail: [email protected]. ORCID

Tomoki Ogoshi: 0000-0002-4464-0347 Funding

T.O. gratefully appreciates the financial support from JSPS KAKENHI Grant Numbers JP15H00990, JP15KK0185, JP16H04130, JP17H05148, and JP18H04510; JST PRESTO (JPMJPR1313); and Kanazawa University CHOZEN Project. NanoLSI is supported by the World Premier International Research Center (WPI) Initiative, Japan. 1664


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Accounts of Chemical Research Notes

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The authors declare no competing financial interest. Biographies Takahiro Kakuta is currently an assistant professor at Kanazawa University. He received his PhD (2014) from Osaka University under the supervision of Prof. Akira Harada. He served as a postdoctoral fellow (2014−2016) at Kyoto University in the group of Prof. Yoshiki Chujo. His current interests include the synthesis of polymers, supramolecular materials, and functional materials. Tada-aki Yamagishi received his PhD (1989) in polymer chemistry from Kyoto University. He has worked at Kanazawa University since 1988; first as an assistant professor (1988−1997), then an associate professor (1997−2009), and then as a professor (2009−present). His current interests include the synthesis of high-performance polymers and supramolecular and hybrid materials. Tomoki Ogoshi received his PhD (2005) from Kyoto University under the supervision of Prof. Yoshiki Chujo. Thereafter, he served as a Japan Society for the Promotion of Science Postdoctoral Research Fellow (2005−2006) at Osaka University in the group of Prof. Akira Harada. In 2006, he moved to Kanazawa University as an assistant professor (2006−2010) and was later promoted to an associate professor (2010−2015) and then to a full professor. His current research interests focus on developing functional materials based on polymer and supramolecular chemistry.

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