AsiaChem | Chemistry in Japan | December 2021 Volume 2 Issue 1

76 | December 2021 www.facs.website In contrast to point chirality, one of the interesting aspects of planar chirality is the dynamic chirality changes caused by the unit rotation. By controlling the unit rotation, the pS and pR forms can be switched. To control the chirality, we designed a new catenane-like structure (Figure 5d).31 Catenanes are compounds in which two or more macrocycles are mechanically interlocked. In this molecule, the guest is an alkyl chain connected to one pillar[5]arene unit. Rotation of the unit connecting the alkyl chain therefore switches inclusion and dethreading of the alkyl chain. In the inclusion form, the structure is similar to that of [2]catenane. In the dethreaded form, two rings are connected covalently, but are not interlocked with each other. The molecule can therefore be described as a pseudo[1]catenane. The pS and pR enantiomers of the inclusion form were successfully separated by chiral column chromatography because of inhibition of the unit rotation in the inclusion form. On addition of a competitive guest, the planar chirality was converted from pS to pR or pR to pS because the unit rotation occurred via structural changes from the inclusion form to the dethreading form. How can pillar[n]arenes be assembled? Formation of one-dimensional (1D) tubes: Pillar[n]arenes are highly symmetric polygonal structures. The pillar-shaped structures differ from those of other macrocyclic hosts and should be suitable for the construction of 1D tubes. The versatile functionality of pillar[n]arenes enables 1D tube formation from inter-molecular assemblies of pillar[n] arenes. Pillar[n]arenes have two faces of the same size, and this introduces sites for interactions such as hydrogen-bonding and ionic interactions on both rims, which leads to the formation of continuous 1D tubes. For example, pillar[5]arene with 10 hydroxy groups can form 1D tubes via inter-molecular hydrogen bonds, and this assembly can form bundle structures eventually (Figure 6a).32 The formation of 1D tubular structures has also been induced by host–guest complexation of pillar[5]arene crystals with long, linear n-alkane guests (Figure 6b).33 A pillar[5]arene with 10 ethoxy groups formed herringbone assemblies by complexation with short n-alkane guests such as n-hexane C6 (Cn, CnH2n+2), and C7 because C6 and C7 are shorter than the height of pillar[5]arene. The formation of 1D channels was achieved by complexation with long n-alkanes with chains containing more than eight carbon atoms (>C8). This is because the pillar[5]arene height is less than the length of C8. The C8 molecule is not completely covered by a single molecule of pillar[5]arene, therefore a neighboring pillar[5]arene needs to cooperatively cover the protruding part of C8, which results in 1D channel formation. The length of the n-alkane guest determines the supramolecular assembly of pillar[5] arenes in the crystalline state. Complexation with a linear polymer also triggers 1D channel formation because polymer chains are much longer than the height of pillar[5] arene.34 Poly(ethylene oxide) (PEO) has been used as the linear polymeric chain. The melting point of PEO is approximately 60ºC, therefore PEO is in the molten state at 80ºC. Immersion of pillar[5]arene crystals in molten PEO affords 1D channel structures via complexation of PEO with pillar[5]arene. Pillar[5]arene crystals selectively took up high-molecular-weight PEO from a mixture of PEOs of various molecular weights (Figure 6c). This high mass fractionation resulted from the increasing number of attractive CH-π interactions between PEO C-H groups and the π-electron-rich 1D channel of pillar[5]arene with increasing PEO chain length. This was determined by molecular mechanics simulations. Length-controlled 1D tubes can be produced by layer-by-layer assembly of cationic and anionic pillar[5]arenes (Figure 7).35 Figure 7 Layer-by-layer assembly by consecutive adsorption of cationic and anionic pillar[5]arenes. Reproduced with permission from reference.35 Normally, cationic and anionic polymers are used for layer-by-layer assembly because there are multiple cationic–anionic interactions between cationic and anionic polymeric chains. Pillar[n]arenes have two faces, and the two faces have multiple interaction sites. The formation of 1D channels on the surface via layer-by-layer assembly is therefore possible. Immersion of an inorganic substrate with anionic charges on the surface in a cationic pillar[5]arene solution leads to cationic pillar[5] arene assembly on the surface. An important point regarding this system is that pillar[5]arene has two cationic rims; one cationic rim is used Figure 6 (a) 1D tube formation by inter-molecular hydrogen bonding between pillar[5]arenes with 10 hydroxy groups. (b) Guest length selective supramolecular assemblies of crystalline pillar[5]arenes. (c) High mass fractionation by 1D pillar[5]arene channels. Liquid chromatography traces of an equal-weight mixture of PEOs (upper) and host–guest complex crystals after the immersion in the mixture (lower). Reproduced with permission from reference.34

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