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Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Nonporous Adaptive Crystals of Pillararenes Published as part of the Accounts of Chemical Research special issue “Supramolecular Chemistry in Confined Space and Organized Assemblies”. Kecheng Jie, Yujuan Zhou, Errui Li, and Feihe Huang*

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State Key Laboratory of Chemical Engineering, Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China CONSPECTUS: Porous materials with high surface areas have drawn more and more attention in recent years because of their wide applications in physical adsorption and energy-efficient adsorptive separation processes. Most of the reported porous materials are macromolecular porous materials, such as zeolites, metal−organic frameworks (MOFs), or porous coordination polymers (PCPs), and porous organic polymers (POPs) or covalent organic frameworks (COFs), in which the building blocks are linked together by covalent or coordinative bonds. These materials are barely soluble and thus are not solutionprocessable. Furthermore, the relatively low chemical, moisture, and thermal stability of most MOFs and COFs cannot be neglected. On the other hand, molecular porous materials such as porous organic cages (POCs), which have been developed very recently, also show promising applications in adsorption and separation processes. They can be soluble in organic solvents, making them solution-processable materials. However, they are usually sensitive to acid/base and humid environments since most of them are based on dynamic covalent bonding. These macromolecular and molecular porous materials usually have two similar features: high Brunauer−Emmett−Teller (BET) surface areas and rigid pore structures, which are stable during adsorption and separation processes. In this Account, we describe a novel class of solid materials for adsorption and separation, nonporous adaptive crystals (NACs), which function at the supramolecular level. They are nonporous in the initial crystalline state, but the intrinsic or extrinsic porosity of the crystals along with a crystal structure transformation is induced by preferable guest molecules. Unlike solventinduced crystal polymorphism phenomena of common organic crystals that occur at the solid−liquid phase, NACs capture vaporized guests at the solid−gas phase. Upon removal of guest molecules, the crystal structure transforms back to the original nonporous structure. Here we focus on the discussion of pillararene-based NACs for adsorption and separation and the crystal structure transformations from the initial nonporous crystalline state to new guest-loaded structures during the adsorption and separation processes. Single-crystal X-ray diffraction, powder X-ray diffraction, gas chromatography, and solution NMR spectroscopy are the main techniques to verify the adsorption and separation processes and the structural transformations. Compared with traditional porous materials, NACs of pillararenes have several advantages. First, their preparation is simple and cheap, and they can be synthesized on a large scale to meet practical demands. Second, pillararenes have better chemical, moisture, and thermal stability than crystalline MOFs, COFs, and POCs, which are usually constructed on the basis of reversible chemical bonds. Third, pillararenes are soluble in many common organic solvents, which means that they can be easily processed in solution. Fourth, their regeneration is simple and they can be reused many times with no decrease in performance. It is expected that this class of materials will not only exert a significant influence on scientific research but also show practical applications in chemical industry.

1. INTRODUCTION

more and more attention in recent years because of their wide applications in physical adsorption and energy-efficient adsorptive separation processes.7−11 To date, various porous materials have been intensively investigated in such areas. Most of the reported porous materials are macromolecular porous materials, such as zeolites,12,13 metal−organic frameworks

Adsorption and separation are of significance in modern life and industry. Nowadays, adsorption technologies are widely applied in processes varying from water/environment treatment1 to enrichment of nuclear fuel and electronics,2,3 while separation processes are extensively applied in various areas such as petroleum refining, mining, paper manufacturing, and chemical and pharmaceutical production.4−6 In such circumstances, porous materials with high surface areas have drawn © XXXX American Chemical Society

Received: June 1, 2018

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DOI: 10.1021/acs.accounts.8b00255 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research (MOFs), or porous coordination polymers (PCPs),14−19 and porous organic polymers (POPs)20−25 or covalent organic frameworks (COFs),26−31 in which the molecular building blocks are linked together by covalent or coordinative bonds (Figure 1a). Most of these materials are barely soluble because

separation of methane and propane.49 Pillararene-based porous polymers were also fabricated for water purification.50 In this Account, we describe a novel class of materials, nonporous adaptive crystals (NACs) of pillararenes, for adsorption and separation (Figure 1c). In contrast to traditional robust porous materials, NACs are nonporous in the initial crystalline state, but intrinsic or extrinsic porosity with various sizes and shapes inside pillararene crystals is induced by preferable vaporized guest molecules along with crystal structure transformations. Unlike solvent-induced crystal polymorphism phenomena of common organic crystals that occur at the solid−liquid phase, NACs can capture vaporized guests at the solid−gas phase. Upon removal of the guest molecules, the crystal structure transforms back to the original nonporous structure. In the following, we focus on the discussion of pillararene-based NACs for adsorption and separation and the crystal structure transformations from the initial nonporous crystalline state to new guest-loaded structures in the adsorption and separation processes. Another feature that distinguishes NACs from traditional porous materials is that the final structures of the complexed pillararenes play a vital role in the adsorptive separation processes.

Figure 1. Schematic representations of (a) macromolecular-level porous materials such as zeolites and MOFs and (b) molecular-level porous materials such as POCs. (c) Chemical structures and top view cartoon representations of pillararenes used in the construction of NACs. Adapted with permission from ref 11. Copyright 2015 American Association for the Advancement of Science.

2. NONPOROUS ADAPTIVE CRYSTALS OF PILLAR[6]ARENE To date, perethylated pillar[6]arene (EtP6) is the only pillar[6]arene that has been used to construct NACs.51−54 2.1. Crystal Structures of Desolvated EtP6

of their rigid cross-linked macromolecular skeletons, thus leading to the loss of solution-processability. Nevertheless, some of the macromolecular porous materials with ordered pore structures such as MOFs and COFs are built on the basis of reversible chemical and coordinative bonds.14,16,27 Their relatively low chemical, moisture, and thermal stability cannot be neglected when they are applied under critical conditions. Very recently, molecular porous materials such as porous organic cages (POCs) have been developed, which also show promising applications in adsorption and separation processes (Figure 1b).32−35 They can be soluble in organic solvents, making them solution-processable materials. However, most of them are still sensitive to acid/base and humid environments because of the reversible chemical bonds that form the cages. These macromolecular and molecular porous materials usually have two similar features: high Brunauer−Emmett−Teller (BET) surface areas and rigid pore structures, which are stable during adsorption and separation processes. Pillararenes,36−45 a new generation of supramolecular hosts, have attracted much attention from supramolecular chemists during the past decade. In contrast to meta-bridged calixarenes with basket-shaped structures, pillararenes are linked by methylene bridges at the para positions of 2,5-dialkoxybenzene rings, forming a unique pillar structure. Besides the investigation of their abundant host−guest properties, our group initiated research using pillararenes to construct porous materials. In 2011, our group reported the first pillararenebased porous material, a hybrid porous material constructed from an anionic pillar[5]arene and a poly(ionic liquid), for selective adsorption of n-alkylene diols.46 Afterward, the Yang group47 and Ogoshi et al.48 reported pillararene-based supramolecular organic frameworks for gas adsorption and separation. Coskun and co-workers reported conjugated microporous polymers with pillararene skeletons for the

Desolvation of EtP6 at different temperatures produced different polymorphs of EtP6. Upon desolvation at room temperature, a metastable phase, EtP6α, was obtained.51 As characterized by single-crystal X-ray diffraction (SC-XRD) and powder X-ray diffraction (PXRD), EtP6α is a honeycomb-like 1D channel structure (Figure 2a). Desolvation at a higher temperature (>140 °C) led to the formation of a thermodynamically stable guest-free phase, EtP6β.52 The single-crystal structure of EtP6β contains a new EtP6 conformer in which the aromatic pillars are no longer aligned, and this rearrangement results in loss of the EtP6 cavity (Figure 2b). N2 sorption experiments confirmed the nonporous nature of EtP6β, while EtP6α is switched to EtP6β during the sample preparation process because of its instability (Figure 3). 2.2. Styrene Purification

Styrene (St), a very important aromatic chemical feedstock, is mainly produced by dehydrogenation of ethylbenzene (EB). After dehydrogenation, the product stream still contains a large fraction (20−40%) of unreacted EB that must be removed.53 However, the separation of St and EB by distillation is difficult because of their similar boiling points. The current preferred technologies, such as extractive distillation and vacuum distillation, are energetically intensive. Moreover, the energyefficient adsorptive separation of St and EB using a suitable porous material is also difficult because of their similar molecular sizes.54 Our group first exploited the applications of EtP6 NACs as adsorptive separation materials to purify St.51 EtP6α captures single-component St and EB vapors with very similar uptake amounts and rates, leading to structural transformations from EtP6α to St-loaded St@EtP6 and EB-loaded EB@EtP6, respectively. The adsorbed St molecules are located in the B


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Accounts of Chemical Research

captures St from an St/EB mixture with a crystal structure transformation from EtP6α to St@EtP6. The selectivity mechanism can be explained by both experimental and computational methods: St@EtP6 is more stable than EB@ EtP6; St molecules diffuse into EB@EtP6, displacing EB molecules and transforming EB@EtP6 into St@EtP6. Upon heating, St molecules were released from St@EtP6 with >99% purity while St@EtP6 was transformed to a new phase, EtP6β, which still selectively captured St from the St/ EB mixture without loss of performance after many cycles (Figure 3). On the contrary, NACs of perethylated pillar[5]arene (EtP5) do not have the ability to separate the St/EB mixture. This work opens up an avenue for applications of NACs in molecular separation areas. 2.3. Xylene Separation

Xylene isomers are important chemical feedstocks in the chemical industry that are produced from crude oil as a mixture. In particular, p-xylene (pX) is the most important isomer as a feedstock, with purity requirement of >99%, for terephthalic acid and dimethyl terephthalate production.55,56 However, the separation of xylenes is very challenging because of their similar boiling points and molecular sizes, and it has been classified by Sholl and Lively6 as one of the “seven chemical separations to change the world”. Besides styrene purification, our group found that the xylene separation by NACs of EtP6 could also be achieved with remarkable pX selectivity.52 EtP6β was able to adsorb the vapors of the three single-component xylene isomers with crystal structure transformations from EtP6β to an o-xylene (oX)-loaded EtP6 structure (oX@EtP6), a m-xylene (mX)loaded EtP6 structure (mX@EtP6), and a pX-loaded EtP6 structure (pX@EtP6) (Figure 4). These three crystal structures are totally different: in pX@EtP6, EtP6 molecules form 1D channel structures with the EtP6 pillars angled to maximize π−π stacking interactions with the pX guest. By contrast, in oX@EtP6 with a honeycomb 1D channel structure, the pillars are all aligned and the oX guest is disordered. In mX@EtP6, the EtP6 structure is distorted with mX outside the cavity, which is different from solution-grown 2(mX)@EtP6 structure. Upon exposure to a mixture of xylene isomer vapors, EtP6β adsorbed only pX with the structural transformation to pX@EtP6. This shows a remarkable pX selectivity (Figure 4). Upon heating, pX molecules were released from pX@EtP6 with >99% purity, while pX@EtP6 was transformed to EtP6β, which could be recycled many times without loss of performance. On the other hand, NACs of EtP5 cannot discriminate xylene isomer mixtures. Selective adsorption of pX is an intrinsic feature of the EtP6 host, as the flexible EtP6 cavities adapt during absorption, thus enabling preferential absorption of pX in the crystalline state. In this work, the selectivity mechanism of EtP6 toward pX was rationalized computationally by exploring the conformational energy landscape and crystal structure prediction. The conformer corresponding to the pX@EtP6 crystal structure led to the densest and most stable crystal structure in the computational study, which is the main reason for the pX selectivity.

Figure 2. Single-crystal structures of (a) EtP6α and (b) EtP6β.

Figure 3. Schematic representation of the styrene purification procedure using EtP6 NACs and the reversible transformations between EtP6β and St@EtP6.

extrinsic pores between distorted EtP6 molecules in a crystal structure that transforms to accommodate the St guests, while EB molecules are located in the intrinsic 1D channels of EtP6 (Figure 3). However, upon exposure to a mixture of St and EB, EtP6α adsorbed both St and EB in the first hour but only St at last. Crystal structure analyses confirmed the formation of St@ EtP6 and EB@EtP6 in the first hour, but only St@EtP6 was found at last (Figure 3). This showed that EtP6α selectively

2.4. Iodine Capture

Radiological iodine species are volatile radionuclide waste pollutants that are harmful to human beings.57 129I is a hazardous iodine isotope with a particularly long half-life (∼107 years), which means that it must be captured and C


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Figure 4. Scheme summarizing the interconversion of various pillar[6]arene−xylene host−guest crystal structures in solution and the solid state.

Figure 5. (a) Pictures showing the time-dependent color changes of EtP6β upon exposure to I2 vapor. (b) Schematic representation of the structural transformations that occur upon uptake of iodine in EtP6β crystals, release of iodine in cyclohexane from I2@EtP6, and removal of cyclohexane from Cy@EtP6.

reliably stored. Although 131I is a short-lived radionuclide (halflife of 8.02 days), it also needs immediate trapping because of its high volatility and its effect on human metabolic processes.58 Thus, volatile iodine must be captured efficiently and reliably stored. In addition to styrene purification and xylene separation, our group reported that EtP6 NACs can work as adsorbents to capture iodine.59 EtP6 NACs can capture not only volatile iodine in the air but also iodine in both organic and aqueous solutions with a structural transformation from EtP6β to I2loaded I2@EtP6 (Figure 5a). Each adsorbed iodine molecule was observed to be located between two adjacent EtP6 molecules, driven by typical charge-transfer interactions between I2 and benzene rings (Figure 5b). Iodine was released spontaneously from I2@EtP6 crystalline solids when I2@EtP6 solid was immersed in cyclohexane, with a structural

transformation from I2@EtP6 to an EtP6−cyclohexane complex (Cy@EtP6). Upon removal of cyclohexane, EtP6β was recovered, and it could be reused many times without losing its iodine capture ability (Figure 5b). In comparison, the NACs of EtP5 and EtP7 are not able to capture iodine. 2.5. Branched Alkane Separation

Branched alkanes with five to seven carbons are major components in high-octane gasoline because of their high research octane numbers (RONs). In contrast, RONs of linear alkanes are quite low. Thus, the separation of branched alkanes from linear alkanes is necessary to produce gasoline with a high RON.60 Ogoshi and co-workers reported the improvement of RONs by selective uptake of branched alkanes from a mixture of isooctane and n-heptane using NACs of EtP6.61 EtP6β took D


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behavior in the crystalline state. The gate opening of n-hexane (C6) induces a crystal structural transformation from EtP5α to C6-loaded C6@EtP5, which has a herringbone structure (Figure 8). The pores in C6@EtP5 are discrete, totally

up cyclic and branched alkane vapors at a gate-opening pressure with a structural transformation from EtP6β to 1D channel Cy@EtP6, but linear alkane vapors were not taken up (Figure 6). Significant improvement of RONs can be achieved:

Figure 6. Schematic representation of the separation of branched alkanes from linear alkanes using EtP6β crystals. Adapted with permission from ref 61. Copyright 2018 John Wiley and Sons.

the RON value of a mixture of isooctane and n-heptane can be improved from 17% to higher than 99% through vapor uptake by EtP6β. Isooctane encapsulated in 1D channel EtP6 structures could be released with a purity of more than 99% by heating, and the reactivated EtP6β crystals could be reused for the improvement of the RON (Figure 6).

Figure 8. Schematic representation of the adsorption of linear alkanes with gate-opening behavior using EtP5α crystals. Adapted with permission from ref 63. Copyright 2015 John Wiley and Sons.

different from the continuous pore structures in porous materials, indicating a new guest adsorption behavior. The same behavior was also observed for n-pentane and n-heptane. However, linear alkanes with fewer than four carbons and cyclic or branched alkanes cannot be adsorbed by EtP5α (Figure 8). The pillar[5]arene host−guest property facilitates the unexpected alkane-length- and shape-selective gate-opening behavior in the crystalline state. Moreover, alkanes adsorbed in the NACs of EtP5 are very stable and cannot be desorbed even under reduced pressure. They can only be released at temperatures higher than their boiling points. This work offers the first example of gate-opening behavior using organic crystals based on macrocyclic compounds.

3. NONPOROUS ADAPTIVE CRYSTALS OF PILLAR[5]ARENE To date, perethylated pillar[5]arene (EtP5) and pillar[4]arene[1]quinone (EtP4Q1) are the only two pillar[5]arene derivatives that have been used to construct NACs. Only the crystal structure of activated EtP5 has been obtained. 3.1. Crystal Structure of Activated EtP5

When EtP5 was crystallized from oX, no oX was found in the structure. Instead, the structure, hereafter called EtP5α, was guest-free (Figure 7).52 The simulated PXRD pattern of

3.3. Linear Positional Isomer Sorting

The separation of olefin positional isomers with straight carbon chains is of significance in chemical industry. This is conventionally processed by energy-intensive extractive distillation columns.64 Inspired by the alkane vapor uptake behavior, our group successfully used NACs of EtP5 to separate linear olefin isomers.65 EtP5α crystals adsorb all three single-component linear pentene isomers, namely, 1-pentene (1-Pe), trans-2-pentene (trans-2-Pe), and cis-2-pentene (cis-2Pe), with gate-opening behavior (Figure 9a), transforming EtP5α to different structures (Figure 9b). Upon exposure to mixtures of linear pentene vapors, EtP5α selectively adsorbs 1Pe over its positional isomers 2-Pe, leading to a structural change from EtP5α to a 1-Pe-loaded structure (1-Pe@EtP5). What is really intriguing is that the NACs of EtP5α distinguish molecules having minor differences in their gas sorption isotherms with remarkable selectivity (Figure 9a), while this is difficult for traditional porous materials. This selectivity generally arises from the stability of the final crystal structure of EtP5 upon guest capture rather than suitable pore size/ shape. The purity of 1-Pe reaches 98.7% in just one cycle after release from 1-Pe@EtP5. Meanwhile, the removal of 1-Pe transforms 1-Pe@EtP5 crystals back to EtP5α, which can be reused without loss of separation performance (Figure 9c). Moreover, this linear positional isomer sorting in EtP5α is also efficient in the separation of α/β-haloalkane isomers, that is,

Figure 7. Single-crystal structure of EtP5α.

activated EtP5 material matches the simulated PXRD pattern of EtP5α, indicating that they have the same structure. In this structure, the rearrangement results in a loss of the EtP5 cavity. N2 sorption has shown that EtP5α is nonporous, in agreement with its crystal structure. 3.2. Alkane Adsorption

During the investigation of pillararene-based host−guest chemistry, our group found that pillar[5]arenes encapsulate linear alkanes into their cavities both in solution and in the solid state as a result of CH/π interactions between alkanes and the benzene rings on pillar[5]arenes.62 Inspired by this unique feature, Ogoshi et al.63 reported that exhibits EtP5α length- and shape-selective gate-opening alkane uptake E


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Figure 9. (a) Vapor sorption isotherms of EtP5α toward linear pentenes: 1-Pe (blue squares), trans-2-Pe (magenta triangles), cis-2-Pe (red diamonds). Solid symbols denote adsorption, and open symbols denote desorption. (b) PXRD patterns of EtP5: (I) original EtP5α; (II) after adsorption of 1-Pe vapor; (III) after adsorption of trans-2-Pe vapor; (IV) after adsorption of cis-2-Pe vapor. (c) Structural representation of the transformation from EtP5α to 1-Pe@EtP5 upon uptake of 1-Pe/2-Pe vapor mixture and the release of 1-Pe by heating.

the separation of 1-chlorobutane from 2-chlorobutane with over 99% purity in one cycle. This work demonstrated that not only the NACs of EtP6 but also the NACs of EtP5 function as adsorptive separation materials. 3.4. Alkane Length Sorting

On the basis of the host−guest complexation of pillar[5]arene with alkanes, Ogoshi et al.66 further discovered an alkane length sorting behavior using NACs of EtP5. In contrast to previous vapor-phase separation, the separation of a mixture of n-alkanes with various chain lengths was difficult to achieve by a solid−vapor adsorption. They exploited a new method to separate n-alkanes with different chain lengths in a solid−liquid phase. EtP5α crystals do not dissolve in bulk n-alkane solutions. Immersing EtP5α in liquid n-alkanes led to the fast quantitative uptake of single-component n-alkanes into the crystals and afforded host−guest complex crystals (Figure 10a). As a result, the host−guest complex crystals are easily isolated by filtration after immersion of EtP5α crystals in bulk n-alkane solutions (Figure 10a). The solid−liquid adsorption of EtP5α with n-alkanes exhibited alkane length sorting behavior. EtP5α tended to form complexes with longer n-alkanes when immersed in a mixture of n-alkanes with various chain lengths. In particular, selective uptake of the longer n-alkanes was achieved even from a mixture containing two n-alkanes (Cn, CnH2n+2) that are different by only one CH2 unit, such as n-heptane (C7) and C8, C12 and C13, and C15 and C16. Crystal structural analyses showed that the selectivity comes from the structural differences. When EtP5α was immersed in C6 or C7, EtP5α was transformed to similar herringbone C6-loaded EtP5 (C6@ EtP5) or C7-loaded EtP5 (C7@EtP5) structures (Figure 11b). However, immersion of EtP5α in C8 transformed EtP5α to a 1D channel C8-encapsulated EtP5 (C8@EtP5) structure (Figure 10b). The crystal structure of C16-loaded EtP5

Figure 10. (a) Schematic representation of the procedure to obtain alkane-loaded EtP5 crystals. Alkane-loaded host−guest complex crystals were prepared by immersion of EtP5α in bulk n-alkanes followed by filtration of the resulting crystals. (b) Single-crystal structures of C6@EtP5, C7@EtP5, C8@EtP5, and C16@EtP5. Adapted with permission from ref 66. Copyright 2017 Royal Society of Chemistry.

(C16@EtP5) showed that two pillar[5]arene molecules cooperatively cover one C16 molecule to form 1D channel structures (Figure 10b). Generally, n-alkanes with longer chains form more stable host−guest complexes with EtP5 because of the greater number of CH/π interactions between the alkanes and the aryl rings on EtP5, thus determining the selectivity. 3.5. Alkane-Shape-Selective Vapochromic Behavior of Pillar[4]arene[1]quinone NACs

Besides EtP5 and EtP6, Ogoshi et al.67 discovered a pillararene derivative that could be used to construct NACs. Pillar[4]arene[1]quinone, EtP4Q1, obtained by oxidation of EtP5 (Figure 11a), possesses a dark-brown color in the initial crystalline state after activation due to the inter- and intramolecular charge-transfer interactions between 1,4-diethF


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upon capture of different guests. On one hand, only guest molecules that have supramolecular interactions with the host molecules can trigger the structural transformations by penetrating into the NACs. This unique feature renders one possibility for guest selectivity and separation by extrusion of undesired guest molecules. On the other hand, guests that have stronger supramolecular interactions with host molecules are more easily adsorbed by NACs, thus forming more stable complexes. This offers another selectivity possibility. All of these features make these materials work at the supramolecular level. Compared with traditional macromolecular or molecular porous materials, whose pore structures usually do not change upon guest capture, NACs of pillararenes have several advantages. First, their preparation is simple and cheap, and they can be synthesized on a large scale to meet practical demands. Second, pillararenes have better chemical, moisture, and thermal stability than crystalline MOFs, COFs, and POCs, which are constructed from reversible chemical bonds. Third, pillararenes are soluble in many common organic solvents, which means that they can be easily processed in solution. Fourth, they are highly recyclable with no decrease in performance. In the near future, we will try to explore more pillararene members to construct NACs for targeted applications. Also, other macrocyclic or acyclic supramolecular hosts with inherent flexibility in their structures may also have the potential to form NACs with multiple functions. Other significant adsorption processes such as organic pollutant removal and demanding separations such as chiral separations and configurational isomer separations by postmodified pillararene NACs are also a target for future investigations. It is expected that this class of novel materials will not only exert a significant influence on scientific research but also show practical applications in chemical industry.

Figure 11. (a) Chemical structures of EtP5, EtP4Q1, and EtP4H1. (b, c) Crystal structures of EtP4Q1 based on crystals prepared from (b) methanol and (c) n-hexane. In the X-ray structures, C = gray and O = red; H atoms have been omitted for clarity. Adapted from ref 67. Copyright 2017 American Chemical Society.

oxybenzene units and benzoquinone units. Very similar to the NACs of EtP5, the NACs of EtP4Q1 adsorbed neither CO2 nor N2 but had the ability to capture n-alkane vapors in their cavities. The uptake of n-alkane vapors resulted in a color change of these crystals from dark brown to red, indicating a vapochromic behavior (Figure 11). On the contrary, cyclic and branched alkanes do not lead to a color change of EtP4Q1 NACs, and thus, EtP4Q1 displays alkane-shape-selective vapochromic behavior. Interestingly, exposing NACs of EtP4Q1 to methanol vapor induced a color change from dark brown to black, which is the same color as the assynthesized methanol-containing EtP4Q1 crystals (Figure 11). After removal of adsorbed n-alkane or methanol, the color of EtP4Q1 crystals went back to dark brown. The reactivated crystals were able to take up n-hexane and methanol vapor again; thus, the processes were completely reversible. Crystal structural analyses further demonstrated the mechanism of the color changes. In the crystal structure of n-hexane-loaded EtP4Q1, partial intermolecular π stacking between adjacent benzoquinone and 1,4-diethoxybenzene units was observed because the structure of EtP4Q1 has a highly symmetrical shape (Figure 11c). In the crystal structure of methanol-loaded EtP4Q1 (Figure 11b), no π stacking was observed between the benzoquinone and 1,4-diethoxybenzene units because the shape of EtP4Q1 is slightly distorted by inclusion of methanol in the cavity. The difference in the πstacking arrangements in the crystal structures after exposure to n-hexane and methanol vapors affects the charge-transfer complexation, thus resulting in vapor-dependent color change. Such vapochromic behavior is switched off when EtP4Q1 is reduced to pillar[4]arene[1]hydroquinone (EtP4H1) (Figure 11a). Intra- or intermolecular hydrogen bonding of hydroquinone prevents the crystal transformation that induces the vapor adsorption behavior. The vapochromic behavior is switched on again when EtP4H1 is oxidized to EtP4Q1. This work thus opens up a way to use pillararene-based NACs in the chemical sensing area.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Feihe Huang: 0000-0003-3177-6744 Notes

The authors declare no competing financial interest. Biographies Kecheng Jie was born in China in 1989. He obtained his B.S. from Zhejiang University in 2012. Then he joined the laboratory of Prof. Feihe Huang at Zhejiang University and got his Ph.D. in supramolecular chemistry in 2017. Now he is working as a postdoctoral researcher in Prof. Sheng Dai’s group at The University of Tennessee, Knoxville. His current research interests are porous liquids and porous organic polymers. Yujuan Zhou was born in China in 1992. She got her B.S. at Northwest A&F University in 2013. Then she joined the laboratory of Prof. Feihe Huang at Zhejiang University to pursue her Ph.D. in chemistry. Her current research is focused on pillararene-based host− guest chemistry.

4. CONCLUSIONS AND OUTLOOK The NACs of pillararenes have been demonstrated to be novel materials for adsorption and separation. Generally, the flexibility or adaptivity of these pillararene crystals stems from the soft −CH2− bridges, which renders on−off porosity

Errui Li was born in China in 1993. He got his B.S. at Hefei University of Technology in 2016. Then he joined the laboratory of Prof. Feihe Huang at Zhejiang University to pursue his Ph.D. in G


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chemistry. His current research is focused on pillararene-based host− guest chemistry. Feihe Huang was born in China in 1973. He obtained his Ph.D. in Chemistry from Virginia Polytechnic Institute and State University under the guidance of Prof. Harry W. Gibson in March 2005. Then he joined Prof. Peter J. Stang’s group at the University of Utah as a postdoctoral researcher. He became a Professor of Chemistry at Zhejiang University in December 2005. His current research interests are pillararene supramolecular chemistry, supramolecular polymers, and nonporous adaptive crystals.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (21434005, 91527301).

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