Alkane-Shape-Selective Vapochromic Behavior Based on Crystal

Apr 17, 2017 - Alkane-Shape-Selective Vapochromic Behavior Based on Crystal-State Host–Guest Complexation of Pillar[5]arene Containing One Benzoquin...
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Alkane-Shape-Selective Vapochromic Behavior Based on CrystalState Host−Guest Complexation of Pillar[5]arene Containing One Benzoquinone Unit Tomoki Ogoshi,*,†,‡ Yasuo Shimada,† Yoko Sakata,† Shigehisa Akine,† and Tada-aki Yamagishi† †

Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

‡

S Supporting Information *

cooperate to efficiently capture and store the n-alkanes; this cannot be achieved by normal complexation in the solution state. Furthermore, the crystal-state complexation shows alkane-shapeselective adsorption behavior; crystals of 1 took up linear alkanes but not branched or cyclic alkanes. This trend is also observed in the host−guest behavior of pillar[5]arenes with alkanes in solution.11c,d Overall, crystal-state complexation using pillar[5]arene 1 has enhanced alkane-recognition ability compared with solution-state complexation of the same compounds. But, pillar[5]arene crystals are colorless solids; thus, we cannot detect encapsulation of n-alkane vapor. Pillar[n]arenes are easily functionalized, which enables researchers to synthesize tailormade macrocycles with targeted applications.10b−d In this study, we used crystals of functionalized pillar[5]arene 2 (Figure 1a), which contains one benzoquinone unit that endows the molecule with alkane-vapor-induced vapochromic properties. Similar to crystals of pillar[5]arene 1, crystals 2 showed efficient alkane vapor adsorption and storage abilities, and alkane-shape selectivity. Furthermore, owing to the formation of a chargetransfer complex between the 1,4-diethoxybenzene and benzoquinone units, crystals 2 are colored solids. We discovered that the color of the solid changed on uptake of n-alkane vapors. Exposing the crystals to methanol vapor resulted in a different color change compared with that observed on n-alkane exposure. Furthermore, the vapor uptake ability can be switched off by reduction of the benzoquinone unit in pillar[5]arene 2 to a hydrouinone unit in pillar[5]arene 3 (Figure 1a). Pillar[5]arene 2 was prepared by oxidation of pillar[5]arene 1. Crystals 2 were grown by slow evaporation of a solution of 2 in methanol at room temperature.13 Owing to the charge-transfer complexation between benzoquinone and 1,4-diethoxybenzene units, the color of the crystals was black (Figure 1b). In the crystal structure, methanol molecules were included in the cavity of 2, and the complexes assembled to form one-dimensional channels along the a axis. The black crystals were dried at 25 °C for 48 h under reduced pressure to obtain activated crystals 2β that did not contain methanol (Figure 1, process I), which was confirmed by 1H NMR measurement (Figure S1). The powder X-ray diffraction (PXRD) pattern of these activated crystals (2β, Figure 1e) was different from that of the original methanol-containing crystals (2α, Figure 1d), which indicates that a crystal transformation occurred during the activation process. The

ABSTRACT: Colored crystals of pillar[5]arene containing one benzoquinone unit were found to exhibit alkaneshape-selective vapochromic behavior. Activated pillar[5]arene crystals, prepared by removing solvated methanol from pillar[5]arene crystals, changed color from darkbrown to light-red after exposure to linear alkane vapors; however, no color changes were observed on exposure to branched or cyclic alkanes. Uptake of methanol vapor by the activated crystals induced a different color change, from dark-brown to black. This multi-vapochromism results from the different intermolecular π-stacking interactions between the benzoquinone and 1,4-diethoxybenzene units in the alkane- and methanol-containing crystals. Unlike most known vapochromic materials, these pillar[5]arene-based materials were highly stable; after uptake of n-alkanes or methanol the color of the crystals did not change after storage in air for 3 weeks. This is because the included guests were stabilized in the cavity by multiple CH/π interactions.

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apochromic materials, which change color and/or emission properties on exposure to certain vapors, have recently received considerable attention because they can be used to detect analytes, even with the naked eye.1 Vapor-induced rearrangement of the molecular packing in the solid state is the main driving force for the optical property changes. Volatile organic amines,2 alcohols,3 ketones,3a,4 ethers,5 water,6 nitriles,7 halogenated alkanes,5a,7b,8 and aromatics8d,9 have been used to induce vapochromic behavior. However, alkane-vapor-responsive vapochromic materials are rare3a because alkanes contain only C−C and C−H groups, which exhibit little affinity for adsorption materials. Even by macrocyclic hosts, in which guest molecules are captured by means of multiple interactions, complexation of alkanes is difficult. Pillar[5]arenes,10 a new type of macrocyclic host in supramolecular chemistry, can form host− guest complexes with n-alkanes, but the association constants of the complexes in solution are rather low (K = 10−20 M−1).11 Recently, we reported that crystals of pillar[5]arene 1 (Figure 1a), which bears 10 ethyl groups, were able to efficiently take up n-alkane vapors into the pillar[5]arene cavity as a consequence of a crystal transformation, and these crystals could store the uptaken n-alkane even under reduced pressure.12 Multiple CH/π interactions in the crystals between pillar[5]arene and n-alkanes © 2017 American Chemical Society

Received: January 19, 2017 Published: April 17, 2017 5664

DOI: 10.1021/jacs.7b00631 J. Am. Chem. Soc. 2017, 139, 5664−5667

Communication

Journal of the American Chemical Society

Figure 1. (a) Chemical structures of pillar[5]arenes bearing 10 ethyl groups (1), containing one benzoquinone unit (2), and containing one hydroquinone unit (3). Crystal structures of 2 prepared from (b) methanol and (c) n-hexane. In the X-ray structures, C = gray, O = red; H atoms are omitted for clarify. Powder X-ray diffraction patterns of (d) 2α, (e) 2β, and (f) 2γ. Black lines, observed; red lines, simulated from X-ray crystal structures.

containing crystals 2γ were also activated again by heating at 80 °C for 12 h under reduced pressure (Figure 1, process IV, and Figure S5). The reactivated crystals 2β were able to take up nhexane and methanol vapor again; thus, the processes were completely reversible (Figure S5). These color changes were clearly measured by diffuse reflectance spectroscopy (Figure 2). The main absorption of activated crystals 2β was observed at 500−700 nm (Figure 2a). After exposure to n-hexane, the absorption bands in the crystals at 600−700 nm decreased in intensity. We could also monitor the time-dependence of the vapochromic behavior (Figure S6). It took ca. 100 min to complete the color change. In contrast, the absorption bands at

color of the crystals also changed during this process: Black crystals 2α changed to dark-brown crystals 2β on removal of the methanol. We then investigated the gas and vapor adsorption properties of activated crystals 2β. The activated crystals of 2β adsorbed neither CO2 nor N2 (Figure S2). Pillar[5]arene cavity size is sufficient to encapsulate CO2;14 thus, the structure of 2β must not be suitable for the gas encapsulation. Surprisingly, exposing activated crystals 2β to n-hexane vapor (Figure 1, process III) resulted in a distinct color change from dark-brown to light-red 2γ. The same color change was observed on exposing activated crystals 2β to other n-alkanes, such as n-heptane, noctane, n-nonane, and n-decane. In contrast, the dark-brown color was retained when activated crystals 2β were exposed to branched and cyclic alkanes, 2,2′- and 2,3-dimethylbutane and cyclohexane (Figure 1, process V). By exposing activated crystals 2β to an equimolar mixture containing n-hexane, 2,2′-, 2,3dimethylbutane, and cyclohexane, 2β selectively took up nhexane vapor among the alkane mixture with color change from brown to red (Figure S3). These results show that the alkanevapor-induced vapochromic behavior selectively occurred with alkane-shape recognition. Moreover, a different color change was observed when activated crystals 2β were exposed to methanol vapor (Figure 1, process II). Exposing activated crystals 2β to methanol vapor induced a color change from dark-brown to black, which is the same color as as-synthesized methanolcontaining crystals 2α. Methanol-containing crystals 2α were able to activate again by drying at 25 °C for 48 h under reduced pressure (Figure 1, process I, and Figure S4). n-Hexane-

Figure 2. Diffuse reflectance spectra of (a) activated crystals 2β (brown line) and crystals after exposure to n-hexane 2γ (red line) and methanol 2α (black line), and (b) after exposure to cyclohexane (CyC6, purple line), 2,2′-dimethylbutane (2,2′, green line), and 2,3-dimethylbutane (2,3, blue line) vapors. 5665

DOI: 10.1021/jacs.7b00631 J. Am. Chem. Soc. 2017, 139, 5664−5667

Communication

Journal of the American Chemical Society

containing crystals 2γ were exposed to methanol vapor, the crystals did not show color change, and methanol uptake was negligible (Figure S10). From the X-ray crystal structure of the nhexane⊂2 complex (Figure 1c), n-hexane included in the cavity of 2 is stabilized by multiple CH/π interactions. Furthermore, heating at 80 °C under reduced pressure (Figure 1, process IV) was required to remove n-hexane, whereas methanol in crystals 2α was able to be removed by drying at 25 °C under reduced pressure (Figure 1, process I), indicating that n-hexane was more stabilized in the cavity of 2 than methanol. Thus, crystals of 2 have potential to selectively capture n-hexane vapor. Next, the vapor adsorption ability of activated crystals 2β was investigated in detail. In the process of adsorption of n-hexane (Figure 4a, blue circles), uptake of vapor did not occur at low

600−800 nm increased in intensity in the crystals after exposure to methanol vapor (Figure 2a). There were no clear differences between activated crystals 2β and the same crystals after exposure to branched and cyclic alkanes (Figure 2b). To better understand the mechanism of the vapor-dependent color change, crystals 2 were grown by slow evaporation of a solution of 2 in n-hexane. The simulated PXRD pattern determined from the single X-ray crystal structure (Figure 1f) was almost same as that of the crystals obtained after exposure of activated crystals 2β to n-hexane (2γ, Figure 1f), n-heptane, noctane, n-nonane, and n-decane vapors (Figure S7). Thus, the molecular arrangement in the crystals obtained from n-hexane is the same as that in the crystals obtained after exposure to these nalkane vapors. In the X-ray crystal structure (Figure 1c), one nhexane molecule is included in the cavity of pillar[5]arene 2, and n-hexane included in the cavity is stabilized by multiple CH/π interactions. Because the cavity of 2 is filled with linear n-hexane, the shape of 2 forms highly symmetrical structure. The 1:1 host− guest complexes assemble into one-dimensional channels along the b axis. Partial intermolecular π-stacking between adjacent benzoquinone and 1,4-diethoxybenzene units were observed because the shape of 2 forms high symmetrical structure. The powder X-ray pattern of the crystals obtained after exposure to methanol vapor (Figure 1d) was also the same as the simulated powder pattern from the X-ray crystal structure of the methanol⊂2 complex; thus, the X-ray crystal structure is the same as that of crystals 2α obtained after exposing activated crystals 2β to methanol vapor. In the crystal structure of 2α (Figure 1b), no π-stacking was observed between the benzoquinone and 1,4-diethoxybenzene units because the shape of 2 is slightly distorted by including methanol into the cavity. The difference in the π-stacking arrangements in the crystal structures after exposure to n-hexane and methanol vapors will affect the charge-transfer complexation. This structural difference is thus the main reason for the vapor-dependent color change. We investigated selective uptake of guest vapors by activated crystals 2β (Figure 3). By exposing activated crystals 2β to 1:1

Figure 4. Sorption isotherms of activated crystals 2β (blue circles) and 3 (orange triangles) toward (a) n-hexane and (b) methanol vapors. Solid symbols, adsorption; open symbols, desorption.

pressures, but a sudden uptake of vapor occurred at a given pressure, which means that these crystals have a pronounced gate-opening behavior. The same gate-opening behavior was observed for the adsorption of other n-alkane vapors, including nheptane and n-octane (Figure S11) and in the adsorption of these n-alkanes using activated crystals 1.12 In contrast, uptake of methanol was observed at low pressures (Figure 4b, blue circles). We then calculated the encapsulation efficiencies from the uptaken amount of n-alkane or methanol, as determined by 1H NMR spectroscopy, vapor sorption experiments, and TGA study (Table S1). Over ca. 90% of the molecules of pillar[5]arene 2 formed 1:1 host−guest complexes with n-alkanes or methanol. In the desorption process, minimal release of the included n-alkanes and methanol was observed (Figure 4a,b, blue circles), which indicates that the included n-alkanes and methanol can be stored in the pillar[5]arene cavity, even under reduced pressure. This is a result of the multiple CH/π interactions in the crystal state, which were revealed by Huang and co-workers.11a,b Because the included guests were not released in air, no color change was observed when the crystals were stored in air for more than 3 weeks (Figure S12). Activated crystals 2β showed minimal uptake of branched and cyclic alkanes, such as 2,3- and 2,2′dimethylbutane and cyclohexane (Figure S13). This alkaneshape-selective uptake was also observed for activated crystals 1.12 Benzoquinone can be readily reduced to hydroquinone, which can be converted back to benzoquinone by oxidation.15 Therefore, we envisaged that activated crystals 2β could be used as a redox-responsive adsorption material. Activated crystals 3 were prepared by reduction of 2 followed by heating the resulting crystals at 50 °C for 12 h under reduced pressure. The activation was confirmed by 1H NMR measurement (Figure S14). The powder X-ray pattern of the activated crystals 3 was

Figure 3. Structural representation of the selective transformation from 2β to 2γ by exposing 2β to 1:1 mixture of methanol and n-hexane, and irreversible transformation between 2α and 2γ.

equimolar mixture of n-hexane and methanol, the activated crystals selectively took up n-hexane and showed color change brown to red (Figure S8). By exposing methanol-containing crystals with black color 2α to n-hexane vapor, guest exchange occurred from methanol to n-hexane with color change from black to red (Figure S9). On the other hand, when n-hexane5666

DOI: 10.1021/jacs.7b00631 J. Am. Chem. Soc. 2017, 139, 5664−5667

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Journal of the American Chemical Society different from the activated crystals 2β (Figure S15), indicating that molecular packing structures were different between activated crystals 2β and 3. Figure 4 shows sorption isotherms of these activated crystals of 3. Activated crystals 3 hardly took up n-hexane (Figure 4a, orange triangles) or methanol (Figure 4b, orange triangles), whereas activated crystals 2β quantitatively took up vapors of these compounds (blue circles). This indicates that the uptake of these vapors can be switched on/off by oxidation/reduction of the benzoquinone/hydroquinone of the pillar[5]arene. In reduced form 3, the phenolic moieties are expected to participate in intra- or intermolecular hydrogen bonding.16 This hydrogen bonding stabilizes the molecular packing and prevents the crystal transformation that induces the gate-opening behavior. In conclusion, we have demonstrated that combination of the vapor-adsorption ability of crystalline pillar[5]arene and the charge-transfer complexation between the benzoquinone and 1,4-diethoxybenzene units of pillar[5]arenes can be used to obtain a new vapochromic system. Although there are many vapochromic materials containing transition metal ions such as platinum(II) and gold(I) ions,1 examples of vapochromic organic compounds that do not contain metal ions are limited.5a,7a,8b,c To the best of our knowledge, this is the first example of vapochromic behavior of organic crystals containing macrocyclic compounds. There are only a few occasions where pillar[n]arenes have been used gas and vapor sorption materials.12,14 This vapochromic system features alkane-vapor shape-selective color changes; vapors of n-alkanes with low affinities for common adsorption materials induced a color change, but those of branched and cyclic alkanes did not. The other characteristic feature of this system is the ultrahigh color stability after vapor exposure. After exposure to n-alkane and methanol vapors, the crystals retained their red and black colors, respectively, even when the crystals were stored in air for more than 3 weeks. Many-known vapochromic systems suffer from poor stability, and thus the ultrahigh stability of this vapochromic system is very rare.8a Our alkane-shape-selective and highly stable vapochromic system is made possible by the efficient shapeselective and stable host−guest complexation behavior of pillar[5]arenes in the crystal state. This vapochromic system, based on host−guest complexation of crystalline pillar[5]arenes, thus provides a new direction for the design of vapochromic systems with novel functions.

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Shigehisa Akine: 0000-0003-0447-5057 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grant Numbers 15H00990, 16H04130, and 17H05148 and the Kanazawa University CHOZEN Project.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b00631. Experimental section, 1H NMR, monitoring of vapochromic behavior by diffuse reflectance spectroscopy, PXRD patterns after uptake of n-alkanes, sorption isotherms of activated crystals 2β toward linear, cyclic and branched alkanes, complexation efficiency, and stability of color induced by uptake of n-hexane and methanol (PDF) X-ray crystallographic data for 2γ (CIF)

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AUTHOR INFORMATION

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Tomoki Ogoshi: 0000-0002-4464-0347 5667

DOI: 10.1021/jacs.7b00631 J. Am. Chem. Soc. 2017, 139, 5664−5667