Guest Vapor-Induced State Change of Structural Liquid Pillar[6]arene

Jan 6, 2019 - to-structural liquid state changes. We observed that a macrocyclic compound, a pillar[6]arene derivative bearing. 12 n-hexyl substituent...
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Guest Vapour-Induced State Change of Structural Liquid Pillar[6]arene Tomoki Ogoshi, Keisuke Maruyama, Yuma Sakatsume, Takahiro Kakuta, Tada-aki Yamagishi, Takahiro Ichikawa, and Motohiro Mizuno J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12253 • Publication Date (Web): 06 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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Journal of the American Chemical Society

Guest Vapour-Induced State Change of Structural Liquid Pillar[6]arene Tomoki Ogoshi*,†,‡,§, Keisuke Maruyama†, Yuma Sakatsume†, Takahiro Kakuta†,‡, Tada-aki Yamagishi†, Takahiro Ichikawa§,|| and Motohiro Mizuno† †Graduate

School of Natural Science and Technology and ‡WPI Nano Life Science Institute, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan §JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan ||Department of Biotechnology, Faculty of Engineering, Tokyo University of Agriculture and Technology, Nakacho, Koganei, Tokyo 184-8588, Japan Supporting Information Placeholder ABSTRACT: State change is a key phenomenon in material science. We report the first observation of vapourresponsive reversible structural liquid-to-solid and solid-tostructural liquid state changes. We observed that a macrocyclic compound, a pillar[6]arene derivative bearing 12 n-hexyl substituents, is a room temperature structural liquid with unique properties. Formation of a host-guest complex between the pillar[6]arene cavity and the n-hexyl substituent results in a structural liquid with nanoscale structural heterogeneities. The structural liquid solidifies when exposed to competitive cyclohexane guest vapour, whereupon cyclohexane replaces the n-hexyl substituents in the pillar[6]arene cavity and the n-hexyl substituents located outside of the cavity crystalize into distinct nanolayer assemblies. The solid reverts back to the structural liquid when the cyclohexane guest is removed through heating under reduced pressure because of re-threading of the n-hexyl substituents into the cavity. The structural liquid-to-solid and solid-to-structural liquid changes are reversible through the uptake and release of cyclohexane guest vapour.

State change between solids and liquids is a fundamental and important phenomenon in material science. Temperature and pressure changes generally cause the state change. The state change behaviour between solid and liquid phases can be used for adhesion materials and latent heat storage. Thus, study of reversible state change by stimuli other than temperature and pressure changes has received a great deal of interest.1 Molecules exhibiting reversible state changes have previously been prepared by introducing stimuli-responsive functional groups. For example, photo-responsive solid-to-liquid and liquid-tosolid switchable materials have been synthesized by introducing azobenzene groups.1c,d In these systems, dynamic structural changes induced by photo-isomerization of the azobenzene moiety between the trans and cis forms contribute to the reversible photo-responsive state changes. Pillar[n]arenes, which were reported by our group in 2008,2 are a new type of pillar-shaped macrocyclic host in supramolecular chemistry.3 Owing to the para-bridge

connections, their structures are highly symmetrical polygonal structures. An important feature of pillar[n]arenes is their high functionality.3c Functional groups can be installed in both rims of pillar[n]arenes, and significantly affect physical properties. In this study, we observed that a pillar[6]arene was a room temperature structural liquid after introducing 12 n-hexyl chains (C6[6], Figure 1a). Structural liquids have nanoscale structural heterogeneities but do not have regular periodical structures.4 Reports of liquids containing host molecules are very limited. James and co-workers reported liquids containing organic-cages.5a Our group reported liquid state

Figure 1. (a) Chemical structures and (b) melting (open circles) and glass (closed circle) transitions of pillar[6]arenes bearing various linear alkyl chains. (c) Photos and (d) transmittance at 500 nm of C6[6] by (i) exposing structural liquid C6[6] to cyclohexane guest vapour at 25 oC for 30 min and (ii) heating the solid C6[6] at 80 oC under reduced pressure for 30 min.

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pillar[n]arenes by introducing ionic liquid groups and tri(ethylene oxide) groups.5b,c However, to the best of our knowledge, this paper is first example of liquids containing pores with nanoscale structural heterogeneities. Interestingly, the structural liquid C6[6] changed to a solid by expose to a competitive guest vapour. It took several seconds for the state change, thus we could visually follow the event. Cyclohexane was used as the competitive guest vapour because cyclohexane fits into the cavity size of pillar[6]arenes, and is taken by pillar[6]arene crystals containing no solvates.6 Removing the cyclohexane guest in the solid by heating under reduced pressure resulted in the solid-to-structural liquid state change. The structural liquidto-solid and solid-to-structural liquid state changes were completely reversible through the uptake and release of cyclohexane guest vapour, which means that cyclohexane guest vapour is the key to induce the state changes.

substituents. Imidazolium ionic liquids carrying various linear alkyl chains also showed the similar phase transition change.8 C6[6] was a room temperature structural liquid (Figure 1b). It was previously reported that C6[6] was obtained as a solid with a melting point of 85-86 oC.7d We observed that the transition temperature was largely influenced by the presence of solvates. We chose cyclohexane as the guest vapour because pillar[6]arene crystals containing no solvates are able to capture cyclohexane vapour, owing to the size similarity between the cavity of pillar[6]arenes and cyclohexane molecules.6 By exposing the structural liquid C6[6] to cyclohexane guest vapour, the structural liquid quickly took up the cyclohexane guest vapour, and the transparent structural

Figure 2. DSC first heating and cooling curves (scanning rate: 10 °C/min) of (a) the structural liquid C6[6] without solvate and (b) the solid C6[6] prepared by exposing the structural liquid C6[6] to cyclohexane vapour. n-Alkyl chain-substituted pillar[6]arenes (Cm[6], where m is the carbon numbers of the n-alkyl chain, m = 2-4 and 6-8, Figure 1a) were synthesized according to the previously reported procedures.7 The detailed synthesis of new pillar[6]arenes bearing 12 n-alkyl chains (m = 5 and 9-11) is provided in the Supporting Information. The transition temperatures of the pillar[6]arenes are summarized in Figure 1b. The melting points of pillar[6]arenes with short ethyl and n-propyl substituents were over 100 oC, but decreased as the lengths of the linear alkyl chain increased up to n-hexyl length chains. This is because - stacking between the benzene rings of pillar[6]arene molecules would be inhibited when the length of the n-alkyl substituents is increased. When the length of the n-alkyl substituent became longer than n-heptyl chains, the melting points of the pillar[6]arenes increased again because intermolecular packing between the long n-alkyl chains was favourable. Overall, pillar[6]arene with n-hexyl substituents C6[6] had the lowest transition temperature (-12 oC) among all of the pillar[6]arenes with various n-alkyl

Figure 3. X-ray diffraction patterns of (a) C2[6], (b) structural liquid C6[6] (upper) and solid C6[6] after exposing to cyclohexane vapour (bottom) and (c) C11[6]. (d) Carbon signals of n-hexyl chains of structural liquid C6[6] (upper) and solid C6[6] after exposing to cyclohexane vapour (bottom) in solid state 13C NMR. (e) Schematic illustration of the guest vapour-induced state change of C6[6].

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Journal of the American Chemical Society liquid changed to a turbid solid within several seconds (Figure 1c(i) and Movie S1). The uptake of the cyclohexane guest vapour was confirmed by 1H NMR measurements and the sorption isotherms of C6[6] (Figures S16 and S17). The whole sample immediately solidified after exposing the structural liquid C6[6] to cyclohexane vapour. From sorption isotherm of C6[6], uptake of cyclohexane vapour suddenly occurred at ca. 2.0 KPa, which is gate-opening pressure. The gate-opening behaviour is also observed in cyclohexane vapour uptake by crystal state of C2[6].6 Heating the solid to 80 oC under reduced pressure resulted in the removal of the cyclohexane guest in the solid, which was also confirmed by 1H NMR measurements (Figure S16). By removing cyclohexane guest, the solid was changed back to the structural liquid (Figure 1c(ii)). The structural liquid-to-solid and solid-to-structural liquid state changes were completely reversible through the uptake and release of the cyclohexane guest vapour. The cyclohexane vapour sorption event could be directly monitored by human eye. The reversible state changes could be also monitored by the transmittance change between the solid and structural liquid phases (Figure 1d). Figure 2a shows the DSC trace of the structural liquid C6[6] in the absence of cyclohexane solvate. In the first heating and cooling processes, a baseline shift was observed at –12 oC. The baseline shift was reproduced in the second heating and cooling processes (Figure S10). These data clearly indicated that the glass transition temperature of C6[6] was –12 oC. However, in the first heating process, the solid state C6[6] containing cyclohexane solvate, which was prepared by exposing the structural liquid C6[6] to cyclohexane guest vapour, did not show the baseline shift at –12 oC, and exhibited new endothermic peaks from 73 to 87 oC (Figure 2b), which corresponded to the melting point of the C6[6]/cyclohexane host-guest complex and the boiling point of cyclohexane. After the release of the cyclohexane solvate in the first heating process, the DSC curves were the same as the first cooling and second heating/cooling cycles of C6[6] in the absence of solvate molecules (Figure S11), which clearly indicated that the presence of cyclohexane solvate in C6[6] affects the transition temperature of C6[6]. The structural liquid state was extremely stable for long time at room temperature. Even after annealing at – 40 oC for 3 h, the liquid state was retained (Figure S15). We could not find new melting point even after annealing at – 40 oC for 12 h with slow scanning rate (Figure S12). These results indicate that C6[6] should be not supercooled state at room temperature, and extremely stable liquid for long time at room temperature.9 To understand the nanoscale structural heterogeneities of the structural liquid Cm[6] and the mechanism of the cyclohexane guest vapour induced state change, X-ray diffraction (XRD) measurements were performed. For the short n-alkyl chains (Figure 3a shows the XRD pattern of C2[6], the other XRD patterns of C3[6] and C4[6] are shown in the SI), clear diffraction patterns were found. However, the peak from the layer-assembled structure (001) was not clearly observed, which indicated that clear layer structure did not form by packing of the n-alkyl chains because n-alkyl chains are too short to be packed. In contrast, for the long n-alkyl chains over n-pentyl length chains, a clear (001) peak from the layer-assembled structure was observed (Figure 3c shows XRD pattern of C11[6], the other XRD patterns of C5[6], and from C7[6] to C10[6] are shown in the SI). The peaks observed from

packing between n-alkyl chains were also observed at 20o. These results indicated that the alkyl chains over n-pentyl length are sufficiently long to facilitate the alkyl chain packing. In the structural liquid C6[6], broadening of the Figure 4. The structural liquid-to-solid state change of C6[6] after exposure to (i) 1,4-dioxane vapour. Little change occurred when the structural liquid C6[6] was exposed to (ii) methanol vapour. (iii) Solubilization occurred when the structural liquid C6[6] was exposed to chloroform vapour. XRD peaks was observed at around 5 and 20o, which was assigned to d-spacing of 1D layer and packing of the n-hexyl chains, respectively. Owing to the observation of the XRD patterns and no birefringence of C6[6] (Figure S15), we assigned C6[6] as a structural liquid. The nano-layer structure remained even at high temperatures (Figure S14a). Similar diffraction patterns were observed in the melted state of pillar[5]arenes and pillar[6]arenes bearing n-alkyl chains at high temperatures (Figure S14), thus the formation of structural liquids are original features of melted state pillar[n]arenes. The 1D layer d-spacing in the structural liquid of C6[6] (16.1 Å) was shorter than that observed with molecular modelling (ca. 22 Å, Figure S18). The solid C6[6] containing solvate cyclohexane molecules clearly showed the (001) peak from the layer structure, and the peak at 20o from the packing of alkyl chains. The 1D layer d-spacing in the solid state (19.7 Å) was larger than that in the structural liquid C6[6] (16.1 Å), and this matched with the molecular modelling (ca. 22 Å), which indicated a packing of the n-hexyl substituents at the outside of the cavity after the uptake of cyclohexane guest vapour. The cavity size of pillar[6]arenes is 6.7 Å, which fits branched and cyclic alkanes, but is too large for linear alkanes.5 Branched and cyclic alkanes are better guests for pillar[6]arenes than linear alkanes. In the structural liquid C6[6] without cyclohexane solvate, the n-hexyl substituents worked weak guests, and thus would be loosely incorporated in the cavity of pillar[6]arenes. Therefore, the PXRD patterns were broadened, and the d-spacing was smaller than that observed with molecular modelling. In contrast, incorporation of cyclohexane guest vapour in the cavity of pillar[6]arene triggered de-threading of the n-hexyl substituent from the cavity, and induced crystallization of

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the n-hexyl substituents. The other possible reason would be - interactions after inclusion of cyclohexane vapour because the pentagonal structures of C6[6] are easy-toassemble. To confirm the threading/de-threading of the nhexyl substituent by the uptake and release of cyclohexane guest vapour, we measured the solid state 13C NMR spectra in the range of the signals from the n-hexyl substituents (Figure 3d). The solid C6[6] containing cyclohexane solvate exhibited clear carbon signals from the n-hexyl substituents. Compared with C6[6] in the solid state, up-field shifts of the signal from the n-hexyl substituents were observed in the structural liquid C6[6]. The up-field shift arose from the shielding effect. The results supported that the n-hexyl substituent was included in cavity of C6[6] in the structural liquid, and de-threading of the n-hexyl substituent occurred by the uptake of cyclohexane guest vapour. We investigated other organic vapour uptakes by the structural liquid C6[6]. The structural liquid took up organic vapours such as 1,4-dioxane, THF, cyclooctane, acetone and ethyl acetate, as confirmed by 1H NMR measurements (Figures S21-S26), and the uptake of these organic vapour triggered the structural liquid-to-solid state change as was observed with cyclohexane guest vapour (Figure 4(i)). Clear state changes were not observed when certain organic vapours, such as ethanol and methanol, were used (Figure 4(ii)) because the structural liquid C6[6] hardly took up these organic vapours, as confirmed by 1H NMR measurements (Figures S19 and S20). Solubilization of C6[6] was observed when the structural liquid C6[6] was exposed to chloroform, diethyl ether and dichloromethane vapours (Figure 4(iii)). In conclusion, we observed that pillar[6]arene bearing 12 n-hexyl chains C6[6] was a room temperature structural liquid with nanoscale structural heterogeneities owing to the formation of host-guest complexes between the n-hexyl substituent and the cavity. By exposing the structural liquid to cyclohexane guest vapour, the structural liquid changed to a solid because the cyclohexane guest expelled the nhexyl chain from the cavity, and the n-hexyl substituents located outside of the cavity crystalized. The structural liquid-to-solid and solid-to-structural liquid state changes were completely reversible through the uptake and release of cyclohexane guest vapour, respectively. Vapochromism is defined as a changing of colour and/or emission properties on exposure to vapour.10 In this study, organic vapour can be directly detected by the state change from a structural liquid to solid. Volatile organic amines, acids, alcohols, ketones, ethers, nitriles, halogenated alkanes, aromatics and water have been used to induce vapour-responsive materials.11 However, alkane-vapour-responsive materials are very rare12 because alkanes contain only C−C and C−H groups, which exhibit little affinity for adsorption materials. Vapour-induced state change behaviour is also very rare,1b and from the view point of host-guest chemistry, direct detection of a guest vapour uptake event using state change is very rare. Because of the vapour selectivity, we believe the vapour-induced state change can be applied for new vapour detection systems. Another application is adhesion materials using the guest-vapour induced state change. Supporting Information. Experimental section, 1H NMR, movie, DSC curves, variable temperature XRD patterns, polarized optical micrograph image, sorption isotherm, optimized structure by DFT calculations and monitoring uptake of organic vapours. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

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*Department of Chemistry and Chemical Engineering, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan TEL: +81-76-234-4775; FAX: +81-76-234-4800 E-mail: [email protected] Note The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by Grant-in-Aid for Scientific Research on Innovative Areas: π-System Figuration (JP15H00990 and JP17H05148) and Soft Crystals (JP18H04510), JST PRESTO Grant Number (JPMJPR1313 for T.O. and JPMJPR1413 for T.I.), JST CREST Grant Number (JPMJCR18R3 for T.O.) and Kanazawa University CHOZEN Project. NanoLSI is supported by the World Premier International Research Centre (WPI) Initiative, Japan.

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