78 | December 2021 www.facs.website Formation of three-dimensional (3D) spheres: Fullerene (C60), which is constructed from 12 pentagons and 20 hexagons, has a 3D soccer-ball spherical structure. In this structure, the pentagons provide curvature for 3D sphere formation. We successfully obtained a 2D sheet by assembly of pillar[6]arenes (Figure 9), therefore our next challenge was construction of 3D spherical structures by incorporation of pentagonal pillar[5]arenes into the 2D sheet (Figure 10).39 Figure 10 SEM images and schematic representation of 1D tube formation by pillar[5]quinone, 2D hexagonal sheet formation by pillar[6]arene and the vesicle formation by co-assembly of pentagon pillar[5]quinones with hexagon pillar[6]arenes. However, a 2D sheet constructed from pillar[6]arenes was formed by simply mixing pillar[5]arene and pillar[6]arene because the more highly symmetric pillar[6]arene is easier to assemble than pillar[5] arene. Mixing of pillar[5]arene and pillar[6]arene at the molecular level was achieved by using the inter-molecular charge-transfer complex between hydroquinone and benzoquinone. Pillar[5]quinone, which was prepared by oxidation of pillar[5]arene, was mixed with pillar[6]arene. In this case, pillar[5]quinone was completely mixed with pillar[6]arene via the inter-molecular charge-transfer complex. The assembled structures were tubular that consisted of pillar[5]quinone alone, and co-assembled samples containing excess pillar[5]quinone. Disk-shaped hexagonal assemblies of pillar[6]arene alone were observed along with co-assembled samples with excess pillar[6]arene. At a 12:20 pillar[5]quinone:pillar[6]arene molar ratio, which is the magic ratio for C60, 3D spheres were formed by co-assembly of pillar[5]quinone with pillar[6]arene. How can bulk pillar[n]arene assemblies be used? Host–guest complexation events are generally performed in the solution state because the host molecules are solids in most cases. However, when we synthesized a pillar[5]arene with 10 tri(ethylene oxide) chains, the obtained macrocyclic compound was liquid at room temperature (Figure 11a).40 We therefore used the liquid pillar[5]arene not only as a macrocyclic compound but also as a solvent for synthesis of the interlocked molecule [2]rotaxane. An axle with azide groups at both ends and a stopper with an alkyne group were directly dissolved in bulk liquid pillar[5]arene (Figure 11b). The end-capping reaction was achieved by a Husigentype copper(I)-catalyzed alkyne–azide cycloaddition “click” reaction. Surprisingly, [2]rotaxane was obtained in high yield (>88%) in the bulk system, whereas the yield of [2]rotaxane was quite low when a normal solvent system was used. In a normal solvent system, at the molecular level, the macrocyclic host and guest molecules are dispersed in a good solvent, therefore host–guest complexation takes place when the guest molecules meet the host molecules. However, host and guest solvation decrease the chance of these molecules meeting each other and decrease the stability of the host–guest complex. The yield of [2]rotaxane is therefore low in normal solvent systems. In contrast, in a bulk system, the guest molecules are directly surrounded by an excess of liquid pillar[5]arene. Inclusion of the guest molecules into the pillar[5]arene cavity is therefore maintained, which results in high-yield synthesis of [2]rotaxane. We realized that a host–guest complexation system that uses bulk liquid pillar[5]arene is more efficient than a normal solvent system. We therefore next investigated host–guest complexation of crystalline pillar[n]arenes (Figure 12).41,42 Figure 12 Alkane-shape selective vapor uptake by crystalline (a) pillar[5]arenes and (b) pillar[6]arenes. When pillar[5]arene crystals were exposed to linear alkane vapors such as n-hexane vapor, the crystals took up the linear alkane vapor (Figure 12a). However, no uptake of cyclic alkanes including cyclohexane and branched alkane vapors was observed. The converse results were obtained when pillar[6]arene crystals were used (Figure 12b). Pillar[6]arene crystals took up cyclic and branched alkane vapors, but did not take up linear alkane vapors. The selectivity is the same as that for the host–guest complexation in normal solvent systems. We therefore discovered that host–guest complexation events occur even in crystalline pillar[n]arenes. Furthermore, pillar[n]arene crystals quantitatively took up alkane vapors into the crystals, whereas the association constants for host–guest complexes between pillar[n] arene and alkane guests are quite low in normal solvent systems, as a result of solvation. Crystal-state complexation is therefore superior for host–guest complex formation even in the low association constants in solvent systems. Figure 11 (a) Solid to liquid transition by modification of tri(ethylene oxide) chains on the rims of pillar[5]arene. (b) High yield synthesis of [2]rotaxane by Huisgen reaction in the liquid pillar[5]arene.
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