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FEATURE ARTICLE

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Pillararene-based supramolecular polymers: from molecular recognition to polymeric aggregates Chunju Li Pillar[n]arenes (P[n]As) and their derivatives, consisting of (substituted) hydroquinone units linked by methylene bridges at para-positions, are new type of cyclophane hosts developed in 2008. Their intrinsic characteristics and properties, such as facile preparation and flexible modification, symmetrical and columnar architectures, very rigid and p-rich cavities, as well as intriguing and peculiar guest complexation capability, make them ideal building blocks for the fabrication of polymeric supramolecules. This Feature Article provides an overview of the construction of pillararene-based supramolecular polymers and covers recent

Received 29th April 2014, Accepted 11th July 2014

research endeavors of the marriage between pillararene-based host–guest pairs and polymeric aggregates.

DOI: 10.1039/c4cc03170a

(1) supramolecular polymers relying on pillararene-based cationic guest recognition; (2) supramolecular

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polymers relying on pillararene-based neutral guest recognition. The host–guest motifs, building strategies, topological architectures, stimuli-responsiveness and functionalities are comprehensively discussed.

These polymers are classified into two major classes according to the different types of guest species:

1. Introduction Supramolecular polymers,1 with monomeric subunits held together by noncovalent complexation and/or dynamic covalent bonds, have attracted considerable attention in recent years.2–4 The reversible and tunable nature of noncovalent interactions and dynamic covalent bonds endows supramolecular polymers with some intrinsic properties including degradability, selfhealing, easy recycling, adaptation and stimuli-responsiveness, Department of Chemistry, Shanghai University, Shanghai, 200444, P. R. China. E-mail: [email protected]

Chunju Li was born in Heilongjiang, China, in 1979. He received his PhD degree from Nankai University under the supervision of Prof. Yu Liu in 2007. After a year as a group leader at Sundia MediTech Company, LTD., he joined Shanghai University in 2008, and was promoted as an Associate Professor in 2010. Currently, he is also a shortterm postdoctoral fellow with Chunju Li Prof. Jonathan L. Sessler at The University of Texas at Austin. His research is focused on pillararene-based molecular recognition and self-assembly.

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which usually could not be provided by conventional covalently bonded polymers. Thereby, the marriage of supramolecular chemistry and polymer chemistry makes supramolecular polymers unique and ideal candidates for smart materials. Multiple hydrogen bonds, p-stacking interactions and metal–ligand coordination have been extensively used to bring the building blocks together to produce supramolecular polymers.5–7 Prof. Meijer is a pioneer behind the development of supramolecular selfassembling polymers, and has constructed a variety of supramolecular polymers based on multiple hydrogen bonds.6 In addition, host–guest interactions have also attracted great interest because there are multiple and cooperative noncovalent interactions in the inclusion complexes of hosts and guests. It is well-established that the host–guest recognition motifs based on crown ethers,8 cyclodextrins,9 calixarenes,10 cucurbiturils11 and etc.12 can be employed to efficiently fabricate supramolecular polymeric assemblies. And some perfect host–guest pairs for supramolecular polymerization have been created, including 24-crown-8 (or 21-crown-7) and secondary ammonium salt, calixarene and organic cation, b-cyclodextrin and adamantane, as well as a cucurbit[8]uril-based ternary complex. To design and explore novel and functional recognition motifs is a permanent and challenging topic in supramolecular polymer chemistry. Pillar[n]arenes13 (P[n]As), firstly reported by Ogoshi and coworkers in 2008,13a are a new type of cyclophane hosts. They are made up of (substituted) hydroquinone units linked by methylene bridges at the 2 and 5 positions, and form symmetrical pillar architectures, which are different from the ‘‘basket’’ structures of conventional calixarenes (Fig. 1). Since the pioneering discovery by Ogoshi et al.,13a pillararenes and

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Fig. 1

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Chemical structures of calix[n]arenes and pillar[n]arenes.

their derivatives have recently stimulated a tremendous upsurge of interest in supramolecular chemistry and materials science, not only due to their interesting host–guest properties but also for the potential applications in the construction of vesicles,14 molecular machines such as nanovalves,15 artificial transmembrane channels,16 metal–organic frameworks,17 liquid crystals,18 and supramolecular polymers. Pillararenes possess some typical characteristics as follows: (i) Facile preparation and flexible modification. Pillararenes are inherently easy to prepare; per-alkylated pillararenes can be achieved by a one-step condensation reaction with good yield using commercial reagents and further deprotection of the alkoxy groups could quantitatively yield native (per-hydroxylated) pillararenes. On the other hand, it is facile to afford pillararene derivatives, and the developed strategies include direct cyclization, cooligomerization, as well as derivation of pillararene skeletons. Various functional P[5]A derivatives, such as monofunctionalized pillararenes, fully functionalized pillararenes, copillararenes, and pillararene dimers, have been designed and successfully synthesized. These pillararene derivatives can not only improve the original binding abilities of native pillararenes and simple alkylsubstituted pillararenes, but also provide new functionalities. (ii) Symmetrical and columnar architectures. Different from the ‘‘basket’’ geometry of calixarenes, pillararenes have two identical cavity portals and possess symmetrical structures, and thus are ideal host molecules for the construction of (pseudo)rotaxanes, catenanes, tubular assemblies, ion channels and etc. (iii) Relatively rigid and p-rich cavities. Pillararenes have more rigid and p-rich cavities than the traditional calixarenes, which may afford highly efficient binding affinities to specially designed guests. For example, alkyl-substituted P[5]As can form stable complexes with 1,4-disubstituted butane guests, with association constants (Ka) up to 104–105 M1.19 The strong binding affinities of P[5]As towards neutral guests in organic media are very unique considering that other supramolecular macrocycles, i.e., crown ethers, calixarenes and resorcarenes, generally interact strongly with cationic guest molecules. (iv) Six members of the P[n]A (n = 5–10) family. Six P[n]A homologues, from P[5]A to P[10]A, have been successfully

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Fig. 2

Chemical structures of typical P[n]A host molecules.

synthesized and isolated so far. P[5]A, P[6]A and P[7]A have pentagonal, hexagonal and heptagonal pillar structures, with diameters of the internal cavities of B4.7 Å, B6.7 Å, and B8.7 Å.13c,20,21 The cavity sizes of P[5,6,7]As are similar with those of cucurbit[6,7,8]urils (and a-, b- and g-cyclodextrins), respectively.22 More interestingly, large P[8,9,10]As possess two pseudo-cavities with well arranged columnar conformations: two pentagons, one pentagon and one hexagon, and two hexagons for P[8], P[9] and P[10]A, respectively. Due to their different cavity features of size and topology, P[n]A members can provide diverse recognition/assembly behavior. Larger homologues of pillararenes are still under development and hopefully will be published in the near future. In the past six years, P[n]As have exhibited intriguing and peculiar host–guest properties. A series of cationic and neutral guest moieties have been designed and proven to be suitable for pillararene cavities of different sizes. Fig. 2 shows the chemical structures and abbreviations of some typical P[n]A host molecules used in this article. OHP[n]A, MeP[n]A, EtP[n]A, and BuP[n]A represent per-hydroxylated, per-methylated, perethylated, and per-butylated pillar[n]arenes, respectively. Based on these host–guest motifs, various supramolecular polymers with different topologies and functionalities have been successfully fabricated. Herein, two major classes of pillararene-based supramolecular polymers are illustrated according to the different types of guest species: (1) supramolecular polymers relying on pillararene-based cationic guest recognition; (2) supramolecular polymers relying on pillararene-based neutral guest recognition.

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This Feature Article provides an overview of these two types of supramolecular polymers and covers recent studies on the marriage between pillararene-based host–guest pairs and polymeric assemblies.

2. Supramolecular polymers relying on pillararene-based cationic guest recognition As mentioned above, the P[n]A hosts provide three-dimensional, rigid and p-rich cavities, so they are perfect molecular containers for cationic guests with suitable size and shape via cation–p-electron interactions. Huang, Ogoshi, Wang, and our group have demonstrated many kinds of cationic guest molecules for P[n]As, including paraquat (N,N0 -dimethyl-4,40 -bipyridinium) derivatives (1a–c),23 bis(pyridinium) derivatives (2a–g),23a,24 1,4-bis(imidazolium)butanes (3a–b),25 pyrazinium cation (4),26 secondary ammonium salts (5, 6a–b),27,28 quaternary ammonium salts (8a–b, 9),29,30 tropylium (10),31 and ferrocenium cation (11).32 Fig. 3 shows the structures of some representative cationic guest molecules. 2.1

Paraquat guests

Paraquat derivatives have been widely employed to build functional supramolecular systems with crown ethers, calixarenes and cucurbiturils. We found that parent OHP[5]A formed 2 : 1 external complexes with guests 1a–c.23a The OHP[5]A wheel does not reside on the viologen nucleus of the axles, and the main binding site is the joint of alkyl and aromatic viologen residues. Interestingly, the introduction of ethylene oxide groups onto P[5]A’s portals would result in the formation of 1 : 1 [2]pseudorotaxanes.23b The ethylene oxide substituted groups play a pivotal role in such a binding geometry. On one hand, the resulting P[5]A-based hosts have deeper cavities than OHP[5]A; on the other hand, there exist multiple hydrogen bonds between the guest’s hydrogens and the oxygen atoms of the host’s ethylene oxide chains.

Fig. 3

Fig. 4 Chemical structure of compound 12 and schematic illustration of its self-complexation in dilute solutions and self-assembly at high concentrations to form oligomers and polymers.

Base on the paraquatCP5[n]A recognition motif, Stoddart’s group33 synthesized a monofunctionalized pillar[5]arene bearing a viologen group (12) through the copper(I)-catalyzed Huisgen alkyne– azide 1,3-dipolar cycloaddition (CuAAC ‘click’ reaction), and studied its self-complexation and self-assembly behaviour. This AB-type heteroditopic subunit exhibits self-complexation in a very dilute CH2Cl2 solution (0.1 mM), and assembles to oligomers and polymers in concentrated solutions. Notably, sealed solutions of 12 at concentrations above 25 mM would yield an organogel (Fig. 4). The marriage of two or more kinds of noncovalent interactions in an orthogonal way could produce complicated, hierarchically ordered polymeric materials with novel architectures and properties. By the combination of the paraquatCP5[n]A host–guest pair and quadruple hydrogen bonds, Wang’s group34 reported the construction of two kinds of supramolecular aggregates: one is a linear supramolecular polymer; the other is a supramolecular polypseudorotaxane network. In the quest for the linear supramolecular polymer,34a they grafted a ureidopyrimidinone (UPy) unit on the P[5]A framework

Chemical structures of representative cationic guests.

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Fig. 5 Chemical structures of 13 and 14 and schematic illustration of linear supramolecular polymers.

to afford 13, and chose a ditopic guest containing two viologen moieties (14) as a connecting molecule (Fig. 5). The dimerization of the UPY units led to the formation of a P[5]A dimer bridged by quadruple hydrogen bonds, which could be regarded as an AA-type homoditopic monomer. Further inclusion complexation with bisparaquat 14 generated a linear supramolecular polymer at high concentrations. The same authors further achieved the conversion from linear supramolecular aggregates based on mono-functionalized 13 to supramolecular polymeric networks from a difunctionalized UPY P[5]A (15).34b Compound 15 solely self-assembles into linear supramolecular polymers at high concentrations via quadruple hydrogen bonding interactions. The linear polymeric backbones with many free pillararene cavities could interact with connector 16 to result in novel polypseudorotaxane networks (Fig. 6). Furthermore, a translucent film could be produced through the combination of these networks with PEG-2000 as the polymer matrix. It should be pointed out that since paraquat derivatives are important redox couples, and can exist in the dicationic, radical cationic and neutral form through a reversible redox process, there is a possibility that the inclusion–dissociation between paraquats and pillararenes and the assembly–disassembly of the aforementioned supramolecuar aggregates based on paraquatCpillararene host–guest pairs could be reversibly controlled by electrochemical switching and redox control.10b 2.2

Fig. 6 Chemical structures of 15 and 16 and schematic illustration of the polypseudorotaxane network from the subunit 15 and cross-linker 16.

linker. For the first time, we found that a linear guest with fourcarbon chain (2c) is the most suitable one for the P[5]A cavity. The nature of the substituents attached to the pyridinium rings noticeably affects the binding behavior. For example, the introduction of electronwithdrawing pyridyl groups (4g) not only enhances the association ability, but also increases the charge transfer (CT) absorption. These inclusion complexes can be even stable in the very high-polarity solvent of DMSO-d6, giving moderate Ka values (740  30 M1 for 2gCOHP[5]A). By the substitution of the pyridinium on the 1,4-bis(pyridinium)butane dications for a similar sized imidazolium, we25 designed a new templating axle, i.e., 1,4-bis[N-(N0 -hydroimidazolium)]butane (3a), that could interact with the P5A host through cation–p interactions. The effects of both the solvent and counterion have been examined, revealing that OHP[5]A could form a stable inclusion complex with 1,4-bis(imidazolium)butanes containing strongly coordinating anions (such as Cl) in high polarity solvents (such as 3 : 2 acetone-d6 : DMSO-d6). And more interestingly, the resulting [2]pseudorotaxane could be reversibly switched off (and back on) manipulated by the addition of base (n-Bu3N) and acid (CF3COOH), i.e., by deprotonation and protonation of the guest (Fig. 7).

Pyridinium, imidazolium and pyrazinium cations

Since viologen is relatively bulky for the P[5]A cavity, and would not be fully engulfed in some cases, our group23a designed a series of bis(pyridinium) bishexafluorophosphate salts in which two pyridinium units are connected by methylene [–(CH2)n–] bridges (n = 2–6, 2a–e, Fig. 3) with different lengths. OHP[5]A forms 1 : 1 pseudorotaxane-type inclusion complexes with 2b–e, and could not interact with 2a bearing a very short

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Fig. 7 Formation of [2]pseudorotaxanes between 3a and OHP[5]A and the base–acid controlled dethreading–rethreading movements.

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Fig. 8 Chemical structures of 17 and 18 and schematic illustration of linear supramolecular polymers based on cationic monomer 17.

The fundamental knowledge gained from this study on the fascinating pH-switchable [2]pseudorotaxane complex can be utilized to produce responsive and functional supramolecular assemblies. Huang’s group35 constructed a pH-responsive supramolecular polymer based on the imidazolium cationCP[5]A motif, where the polymerization degree can be reversibly controlled by adding base and acid to switch the monomer between the neutral state and the cationic state. At 200 mM, for example,

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the maximum possible polymerization degree (nmax) of cationic monomer 17 was calculated to be 38, corresponding to the polymer with a molar mass of 52 kDa. While for neutral 18 at the same concentration, the nmax value is only 9.43, corresponding to a relatively low molar mass of 11.8 kDa (Fig. 8). Wang and coworkers26 designed and synthesized a linear pillar[5]arene-modified conjugated polymer (19), and fabricated fantastic side-chain polypseudorotaxanes by threading the n-octylpyrazinium axle (4) into the pillar[5]arene units. Polymer 19 is strongly fluorescent, whereas the resulting polypseudorotaxanes 4nC19 exhibited very weak fluorescence. This is owing to the efficient electron transfer from the conjugated backbone to the cation guest. Upon further addition of Cl, the solution fluorescence recovered due to the dissipation of the polypseudorotaxanes. It is well known that ion-pairing effects hamper the host–guest binding of charged species by neutral receptors. A strongly coordinating anion, Cl, would destroy the host–guest complexation of P[5]A with 4, leading to the transition of the side-chain polypseudorotaxanes to the original conjugated host polymer followed by the fluorescence recovery. These observations show pronounced potential in the design of anion-switchable fluorescent sensors (Fig. 9). 2.3.

Organic ammonium cations

Huang and coworkers29a,b have demonstrated the binding events of P[5,6]As towards quaternary ammonium salts, i.e., n-octyltrimethyl ammonium hexafluorophosphate (8a) and

Fig. 9 Chemical structure of 19 and graphical representation of the formation of P[5]A-modified polypseudorotaxanes and their disassembly induced by a chloride anion.

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n-octyltriethyl ammonium hexafluorophosphate (8b). The smaller guest, 8a, was found to fit the cavity of P[5]A, showing a Ka value in the magnitude of 103 M1 in CDCl3.29a And the larger one, 8b, is proved to be a suitable guest for P[6]A. For example, per-isobutylated P[6]A could engulf 8b to form an interpenetrated complex with an association constant of 334  24 M1 in CDCl3, but no complexation between per-isobutylated P[5]A and 8b was observed.29b The same research group27 has also reported the binding behavior of secondary ammonium salt with P[5]A. n-Octylethyl ammonium hexafluorophosphate (5) can be included in the MeP[5]A cavity to form a [2]pseudorotaxane-type complex, in which the dethreading would be realized by addition of a chloride anion. For the first time, Hou et al.29c synthesized the larger homologues of pillararenes, i.e., PA[8,9,10]s, with unique double-pillar cavities, and investigated their interactions with quaternary ammonium 8a. No complexation for EtP[8]A was observed; EtP[9]A could complex one guest and EtP[10]A could complex two, as shown in Fig. 10. Such interesting binding modes can be used to build some special supramolecular architectures such as doubly threaded rotaxanes and catenanes. Self-sorting, whereby molecules can efficiently distinguish their corresponding recognition motifs within complex mixtures, is a fundamental property of biological systems, such as DNA replication, translation, and transcription. Our group28 has established a four-component self-sorting system from P[5,6]A-based selective molecular recognition. The secondary ammonium salt 6a bearing a n-butyl group was designed due to the well documented size-fit relationship between P[5]A and the alkyl chain; 6b is promising as an ideal guest for P[6]A because P[6]A has a similar cavity size to b-cyclodextrin and cucurbit[7]uril which can strongly interact with adamantane guests. It was found that 6b could not interact with EtP[5]A and the Ka value of 6a with EtP5A is 890 times larger than that for EtP[6]A. Based on such high binding selectivity, a high-fidelity self-sorting system consisting of two cationic guests and two pillararene hosts is successfully created, as shown in Fig. 11. Likewise, Ogoshi et al.30 also developed a four-component self-sorting system. According to the different sizes of P[5]A and P[6]A, they chose a pyridinium cation and a 1,4-diazabicyclo[2.2.2]octane cation as the guest moieties. Molecular binding behavior studies indicate that (i) the binding affinity

Fig. 10 Schematic representation of the different inclusion complexation properties of PA[n]s (n = 8–10) with guest 8a.

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Fig. 11 Cartoon illustration of a four-component self-sorting system based on EtP[5]A, EtP[6]A, 6a and 6b.

Fig. 12 Chemical structures of 20 and 21 and schematic illustration of selfsorted supramolecular alternating copolymers from AB-type 20 and 21.

for MeP[5]A and the pyridinium moiety is much stronger than that for EtP[6]A; (ii) MeP[5]A cannot interact with the large 1,4-diazabicyclo[2.2.2]octane cation; and (iii) the Ka value for EtP[6]A with the 1,4-diazabicyclo[2.2.2]octane cation is 6 times larger than that for the pyridinium moiety. Accordingly, the mixture of four components selectively produced pyridinium cationCP[5]A and 1,4-diazabicyclo[2.2.2]octane cationCP[6]A host–guest pairs. They further synthesized a monofunctionalized P[5]A with a 1,4-diazabicyclo[2.2.2]octane cation unit (20) and a monofunctionalized P[6]A with a pyridinium unit (21), and built a linear supramolecular polymer with alternating P[5]A and P[6]A moieties (Fig. 12). This building strategy based on self-sorted recognition and assembly provides a promising new method to create sophisticated supramolecular host–guest architectures. Inspired by the switchable host–guest recognition of P[5]A and organic ammonium cations, Yang’s group36 has demonstrated an AB-type multi-responsive supramolecular polymer (Fig. 13). At low concentrations, P[5]A 22 assembles into two types of dimers, a head to tail structure and a double-threaded [c2]daisy chain. The subunit forms linear supramolecular aggregates at high concentrations, which can be disaggregated under the stimulation of tetrabutylammonium chloride, silver trifluoromethanesulfonate, as well as the solvent composition. Notably, a mat of supramolecular polymer nanofibers could be drawn during an electrospinning process. Although the molecular recognition of P[5]As and P[6]As has been widely conducted, host–guest complexation based on P[7]As has been rarely demonstrated. The only example was the highly efficient complexation of EtP[7]A towards the 3,5-dimethyl-1adamantyl ammonium cation with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (7).37 Since our previous work28 has validated

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stability of 11 is highly improved upon complexation with the P[6]A host, and the resulting inclusion complex showed a good redox-response. These two motifs, 10CP[6]A and 11CP[6]A, are potentially available for the future construction of smart supramolecular materials with stimuli-responsiveness and special functions.

3. Supramolecular polymers relying on pillararene-based neutral guest recognition

Fig. 13 Chemical structure of 22 and graphical representation of its selfaggregation at low concentration and self-assembly at high concentration.

the size- and shape-fit relationship between P[6]A and the adamantane unit, we envisioned that the substituted adamantyl ammonium cation with a larger volume would match P[7]A with a larger cavity. As a result, 3,5-dimethyl-1-adamantyl ammonium guest 7 was designed and demonstrated to form an inclusion complex with EtP[7]A, giving a very large Ka value of (1.2  0.3)  105 M1 in CDCl3. To date, however, supramolecular aggregates from P[7]A-based molecular recognition have not been reported yet. 2.4.

Tropylium and ferrocenium cations

All the aforementioned cationic guests (1–9) belong to the organic nitrogen-containing cations. Recently, we31 have introduced a carbonium ion, i.e., tropylium tetrafluoroborate (10) to pillararene chemistry. Tropylium is a heptagonal, planar, cyclic ion with resonance structures resulting from the presence of six p-electrons in the conjugated unsaturated seven-membered ring. It is a novel p-acceptor and an ideal guest for CT interactions. Both EtP[6]A and OHP[6]A could efficiently interact with 10 to form novel CT inclusion complexes. In contrast, EtP[5]A and EtP[7]A cannot form such complexes, which may be attributed to the strict size-fit effects. Medium-sized P[6]As are nicely suitable hosts for the cationic heptatomic ring, and neither smaller P[5]A nor larger P[7]A led to the inclusion complexation. Another interesting example is a redox-responsive inclusion complex between P[6]A and ferrocenium guest (11).32 Ferrocene is one of the commonly used redox molecules. It could be oxidized into ferrocenium via chemical or electrochemical oxidation. Wang’s group has reported that P[6]A could form a stable inclusion complex with ferrocenium, while interacts weakly with the reduced counterpart ferrocene. More interestingly, the

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Compared with other macrocycles, the most peculiar host–guest properties of pillararenes are the strong binding affinities of P[5]As towards neutral guests in organic solvents, since calixarenes and crown ethers generally interact strongly with cationic guest molecules. Our group19 has exploited a series of neutral molecular recognition motifs based on 1,4-disubstituted butane axles, as shown in Fig. 14. In 2011, we19a reported the surprising host–guest properties of simple per-alkylated P[5]As towards the neutral 1,4-bis(imidazol-1-yl)butane (23a) guest and the formation of interpenetrated complexes both in solution and the solid state. The association constant for 23aCEtP[5]A pair is (2.0  0.4)  104 M1 in CDCl3. Multiple hydrogen bonding and C–H  p interactions play an important role in the strong binding stability. Thermodynamically, the negative values of enthalpy changes (DH1), and entropy changes (TDS1) suggest that the complexation events are enthalpy-driven processes. To further clarify the binding mechanisms and driving forces for the unique abilities of P[5]As to recognize neutral molecules, a series of nitrogen heterocycle substituted 1,4-butylene guests (23a–d) with similar structures were then examined,19g showing strikingly different association abilities with P[5]A hosts. The Ka value of 23b containing 1-substituted 1,2,3-triazole moieties with EtP5A is 1500 times larger than that for 23c bearing 2-substituted 1,2,3triazole moieties. C–H  N(O) hydrogen-bonding interactions and C–H  p interactions play important roles in the formation of the complex; multiple weak C–H  N interactions between the alkyl groups of the wheel and the outer nitrogen atoms of the heterocycles of the axle are the dominant driving forces for complexation.19g Even stronger binding affinities were found for dinitrile guests.19b Slow exchange on the NMR timescale was observed for the complexation between 23e and EtP[5]A, while the free 23e was never observed in the presence of up to one equivalent of EtP5A wheel in CDCl3. The Ka value for this complex is 4105 M1 19b,38 in CDCl3, and (1.5  0.3)  104 M1 in 1 : 9 (v : v) DMSO-d6–CDCl3. When using (CD3)2CO as the solvent, the association constant for 23eCEtP[5]A is determined to be (2.4  0.3)  103 M1, which is 14 times higher than that of 23aCEtP[5]A [(1.7  0.1)  102 M1], and 6.7 times larger than that of the typical N,N-dibenzylammonium hexafluorophosphateCdibenzo-24-crown-8 complex (360 M1)39 in the same solvent. Dipole–dipole forces between the guest’s cyano groups and the host’s oxygen atoms play a vital role in the host–guest

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Fig. 14

Chemical structures of neutral 1,4-disubstituted butane guests (23a–o) and their association constants (M1) with EtP[5]A in a CDCl3 solution.

complexation;19b C–H  p and hydrogen bonding interactions are two other kinds of important driving forces. For the 1,4-dihalobutaneCP[5]A motifs,19c 23f–i, 1,4difluorobutane 23f was initially assumed to have a larger binding strength because the high electronegativity of fluorine would result in the enhanced dipole–dipole, C–H  p, and hydrogen bonding interactions. However, the association constant of 23f with EtP[5]A [(8.6  0.5)  10 M1] is the smallest one, and the value increases in the order of F o Cl o Br o I, i.e., with the increasing polarizability of the guest, revealing that the dominant driving force for the interaction is van der Waals dispersion forces (also known as London forces). Iodine is highly polarizable; the dispersion forces between 23i and P[5]A host are significantly strong. That is why 23i displays the largest association constant of (1.0  0.1)  104 M1, up to 120 fold compared with 23f. Besides the above good guest moieties, some other 1,4-butylene derivatives including 1,4-butanediamine (23j),19e 1,4-diazidobutane (23k),19c 1,4-butanediol (23l),19c n-octane (23m),19f 1,7-octadiene (23n)19f and 1,7-octadiyne (23o)19f have also been examined. Although relatively weak affinities (Ka o 103 M1) were observed, some of these guest moieties, such as 23j, 23k, 23l and 23o, can be conveniently applied in the production of rotaxanes, catenanes, as well as large supramolecular aggregates, since there are reactive sites located at the ends of the resulting interpreted complexes. Based on pillararenes’ neutral molecular recognition behavior, some novel neutral supramolecular polymers exhibiting beautiful geometries and interesting properties have thus been created. 3.1

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Alkane and haloalkane guests

Huang et al.40 reported the first example using host–guest recognition of pillararenes in the fabrication of supramolecular polymers in 2011. Self-complementary copillar[5]arene (CoP[5]A) 24a by incorporating an n-octyl group onto the P[5]A portal, which was prepared through a cooligomerization strategy, could self-organize to form linear supramolecular

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Fig. 15 Chemical structures of 24 and 25 and schematic illustration of linear supramolecular polymers formed by 24 and [c2]daisy chain formed by 25.

polymers both in solution and in the solid state (Fig. 15). Although the binding of the alkaneCP[5]A pair is weak (Ka o 100 M1), it can efficiently induce the aggregation of AB-type 24a at high concentrations. For example, at 768 mM, the nmax value was calculated to be 36.8 (p = 97.2%), corresponding to a polymer with a molar mass of 31.2 kDa. The crystal structures of 24a and 24b suggested that the aggregation of these two copillararene subunits to form linear supramolecular polymers was driven by the quadruple C–H  p interactions. And this is the first example of a supramolecular polymer driven by C–H  p interactions. In contrast, copillararene 25 by introducing a bromo atom at the end of the alkyl guest part of 24b selfassembled in completely different manners in the solid state to yield [c2]daisy-chain dimers,41 which is mainly driven by van der Waals forces between the exo cavity parts of the alkyl groups. Recently, we42 designed a double-threaded dimer by means of an AB2-type heterotritopic CoP[5]A (26, Fig. 16), containing a

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Fig. 16 Chemical structures of 26 and 27 and illustration of the supramolecular polymerization between [c2]daisy-chain dimer 2626 and covalent dimer 27.

P[5]A unit and two 4-bromobutyl [Br(CH2)4] guest units. Besides multiple C–H  p/O/Br interactions between host–guest moieties, interestingly, there exist double C–H  O hydrogen bonds between the two host moieties with a distance of 2.71 Å. To the best of our knowledge, this affords the first example of [c2] daisy chains from heterotritopic monomers, although cyclic dimers based on AB-type heteroditopic monomers are much documented. Very different from traditional [c2]daisy chains, 2626 has two additional free guest moieties at both ends (Fig. 17), and can be regarded as a nontypical homoditopic BB-type monomer. As shown in Fig. 16, a combination of this doublethreaded dimer and a homoditopic AA-type monomer (27) should lead to supramolecular copolymers with alternating dimer blocks of the [c2]daisy-chain dimer and the pillar[5]arene dimer (Fig. 16). These results provided a facile method for building novel [c2]daisy chain based supramolecular alternating copolymers.

Fig. 17

Side and top views of the crystal structure of the 2626 dimer.

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3.2

Amines and imidazole derivatives

Wang’s group43 has combined alkylamineCP[5]A host–guest interactions and quadruple hydrogen bonding to create linear supramolecular polymers by virtue of an orthogonal strategy (Fig. 18). Based on alkylamineCMeP[5]A complexes, they designed and synthesized a series of [2]rotaxanes with terminal UPy units, which played a dual role as both the end-capping and the interlocking units. Due to the dimerization of UPy motifs, [2]rotaxane 28 would efficiently self-assemble to supramolecular polymers in chloroform. Dynamic covalent bonds have gained much attention in the fields of self-assembly and functional materials due to their dual nature. They can break and reform reversibly under suitable conditions, and can also remain as permanent and strong as covalent bonds under certain conditions. Yang et al.44 constructed responsive double-dynamic polymers by the combination of the

Fig. 18 Synthesis of [2]rotaxane 28 and cartoon illustration of linear supramolecular polymers based on 28.

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Fig. 19 Chemical structures of 29, 292 and 30 and cartoon representation of the formation and response of the linear supramolecular polymers and the reversible photocontrolled and thermally controlled gel–sol transition.

imidazoleCP[5]A recognition system and dynamic covalent bonds supplied by reversible anthracene dimerization (Fig. 19). Photodimerization of anthracene-terminated supramolecular monomers, i.e., [3]pseudorotaxane 29*30C29, in a 1,2-dichloroethane–cyclohexane (1 : 6, v : v) solution leads to the formation of supramolecular polymers and organogels. This is an elegant example in which host–guest interactions and dynamic covalent bonds, in an orthogonal way, led to stepwise polymerization. Heating the gels could disrupt the host–guest complexes alone or both the complexes and the anthracene dimers, and therefore resulted in the destruction. The polymers could reform readily upon cooling or irradiation. Such a strategy based on both host– guest interaction and dynamic covalent bond will be useful for the production of various smart supramolecular materials with switchable properties. 3.3

Nitrile guests

Fig. 20 shows a series of efficient neutral 1,4-disubstituted butane guest moieties for P[5]As with Ka values of 103–105 M1. However, it is impracticable or difficult to utilize these symmetric moieties

Fig. 20 Some efficient neutral guest moieties for P[5]As and the redesign of asymmetric 31.

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to create supramolecular assemblies because they are impossible or require great efforts to be functionalized or modified to get self-complementary heteroditopic (AB-type) monomers, or homoditopic (B2-type) guests. If using monosubstituted butanes as the guest moieties, however, the host–guest binding abilities are usually very weak, giving Ka values only in the vicinity of 10–102 M1.19a–c,g Based on these results, our group45 designed an asymmetric guest (31) possessing a cyano site and a triazole site at its ends (Fig. 20). As anticipated, axle 31 interacts strongly with P[5]As, which was studied by 1H NMR, 2D NOESY and X-ray crystallographic analysis. The Ka value for 31CMeP[5]A is (1.2  0.2)  104 M1 in CDCl3, which is strong enough to join the host and guest units together efficiently. And more importantly, this moiety can be easily prepared/ modified and it is facile to synthesize AB monomers and B2-type guests by the CuAAC ‘‘click’’ reaction. As a result, the 31CP[5]A host–guest pair is expected to become a star pillararene-based motif for supramolecular polymerization and self-assembly. Encouraged by this excellent motif, our group45 has explored an AA/BB-type and an A2/B3-type supramolecular polymer from P[5]A dimer 27 and ditopic/tritopic guests (32 and 33, Fig. 21). These two connectors were easily achieved through CuAAC ‘‘click’’ chemistry. The resulting supramolecular polymerizations are highly efficient due to the strong binding between host and guest moieties, affording large polymerization degrees and number-average molecular weights. For example, at 100 mM (27), nmax was calculated to be 89 for 32C27 and 113 for 33C27, corresponding to polymers with molar mass of 98 and 127 kDa, respectively. As expected, the assembled morphology of A2/B3-type 33C27 was proved as a 2D net-like polymer by transmission electron microscopy (TEM) images. While for 32C27, no linear objects were observed after several

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TEM and atomic force microscopy (AFM) measurements, and regular 0D globular morphologies with an average diameter of 260–350 nm were always found. Since the spacer (ten methylenes) of 32 is very flexible, the morphological folding for the linear aggregate 32C27, driven by the p-stacking between P[5]A units, presumably results in the conversion from linear structures to spherical nanoparticles. Additionally, we46 also demonstrated a supramolecular hyperbranched polymer from a CoP[5]A based on the host– guest motif of 31CP[5]A (Fig. 22). Hyperbranched polymers are one important kind of functional materials exhibiting some unique properties different from linear polymers. Supramolecular hyperbranched polymers can be facilely established from ABn-type (n Z 2) heteromultitopic monomers. CoP[5]A 26,42 containing complementary P[5]A and 4-bromobutyl [Br(CH2)4] recognition unimers, is a typical AB2-type monomer and could be prepared by a one-step cooligomerization. However, it does not assemble to hyperbranched polymeric supramolecules, but forms cyclic interwoven dimers both in solution and in the solid state due to the specially designed recognition site and the host–host C–H  O hydrogen bonds.42 It is well

Feature Article

documented that (i) prevention of the host–host (or guest– guest) interaction, (ii) introduction of strong host–guest pairs, and (iii) using flexible linkers could favor the formation of supramolecular polymers. Thus, heterotritopic (AB2-type) CoP[5]A (34) by introducing two 31 moieties to P[5]A portals was designed and prepared. Compared with 26, 34 has long and flexible linkers and strong binding motifs. Self-assembly of this well-designed AB2-type subunit produces supramolecular hyperbranched polymers. Particularly, a competitive guest (butanedinitrile) would efficiently depolymerize the treelike aggregate, leading ultimately to [2]pseudorotaxane butanedinitrileC34 as shown in Fig. 22. The coordination chemistry has recently been involved in building pillararene-based supramolecular polymeric materials by Yang’s group.47 The authors fabricated supramolecular polymer gels with multiple responsibilities based on the complexation of hexakis-P[5]A metallacycles with a neutral ditopic nitrile guest. A monofunctionalized P[5]A with 1201 dipyridyl donor (35) was firstly synthesized; two beautiful hexagonal metallacycles 36 and 37 (Fig. 23) were then constructed by coordination with two 1801 linear di-Pt(II) acceptors. Upon complexation with ditopic

Fig. 21 Chemical structures of 27, 32 and 33 and schematic illustration of an AA/BB-type supramolecular polymer formed by 27 and 32, and an A2/B3type polymer formed by 27 and 33.

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Fig. 22

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Synthesis of CoP[5]A 34 and schematic representation of supramolecular hyperbranched polymers based on AB2-type monomer 34.

guest 38, cross-linked supramolecular polymer networks (38)3C36 and (38)3C37 were successfully created, and they could further form gels in concentrated solutions. More interestingly, the reversible gel–sol transitions of such polymer gels were successfully realized by virtue of the disassembly and reassembly stimulated by different external stimuli, such as temperature,

bromide anion, and competitive adiponitrile guest. By taking advantage of the hierarchical self-assembly, this work not only enriches the library of higher-order supramolecular topologies but also outlines a new strategy to construct stimuli-responsive smart soft materials (Fig. 24).

Fig. 23 Chemical structures of 35, 36 and 37 and cartoon illustration of the construction of hexakis-P[5]A metallacycles 36 and 37.

Fig. 24 Schematic illustration of the formation of supramolecular polymer gels by 36 and 38.

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4. Conclusions and perspectives In this Feature Article we have discussed the construction of a variety of pillararene-based supramolecular polymeric assemblies with different topologies and properties. It is an efficient approach to achieve supramolecular polymers by employing the molecular recognition of pillararenes due to their facile modification and intrinsically intriguing inclusion properties. More excitingly, some of these assemblies are mildly responsive to external stimuli, such as pH, concentration, solvent polarity, temperature, photochemistry and competitive components. It is evident that these studies have pushed forward the research of supramolecular polymer science. However, since pillararene chemistry has only developed for six years, it is still an immature project to build pillararene-based supramolecular polymers, compared to crown ethers, cyclodextrins, calixarenes, cucurbiturils, etc. There are a lot of challenges and opportunities in this emerging research field. In our opinion, the following three important aspects should be considered in the development of supramolecular polymer chemistry based on pillararenes: (1) To date, the construction of supramolecular polymers based on P[7–10]As has not been realized yet. On one hand, the synthetic yields for P[7–10]As are very low; on the other hand, host–guest properties of P[7–10]A homologs have been rarely studied, although molecular recognition of P[5,6]As has been relatively well conducted. Thereby, both their synthetic methodologies and binding properties need urgent exploration. Noticeably, P[8–10]A members with unique double-pillar cavities, possessing two pseudo-cavities, can provide special and diverse self-assembly behavior. For example, they could achieve a U-shaped complex, 2 : 2 (or n : n) cyclic oligomer, and linear supramolecular polymers by inclusion complexation with ditopic guests (Fig. 25). (2) It should be pointed out that major efforts in this area have been focused on the fabrication of structurally novel assemblies in the past four years, while relatively less attention has been paid to the properties and applications of these polymers. There is no doubt that the investigations of their potential applications in materials science, biology and environmental science should gain more attention in the future.

Fig. 25 Cartoon illustration of possible self-assembly behavior between P[10]As and ditopic guests. (A) U-shaped complex, (B) 2 : 2 (or n : n) cyclic oligomer, and (C) linear supramolecular polymers.

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(3) Some types of supramolecular polymeric assemblies with potentially special properties, such as water-soluble polymers and chiral polymers have not been produced so far. Altogether, we believe that supramolecular polymerization utilizing pillararene-based molecular recognition will be quite prosperous in the near future, and lots of assemblies with novel structures and functionalities will be fabricated.

Acknowledgements We thank all excellent authors whose names appear in the references. This work was supported by the National Natural Science Foundation of China (No. 21272155 and 21272147) and Innovation Program of Shanghai Municipal Education Commission (No. 13YZ010).

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