www.asiachem.news December 2021 | 77 for adsorption on the inorganic surface by cationic–anionic interactions, but the other rim still has positive charges. Dimeric tubular structures can therefore be formed on the surface by immersing the cationic pillar[5]arene film in an anionic pillar[5]arene solution. At this stage, the surface is anionic, therefore immersing the resulting film into a cationic pillar[5]arene solution results in formation of trimeric tubular structures. Eventually, by alternating immersions of the film, length-controlled 1D tubes can be obtained. Pillar[n]arenes have two interaction faces, and therefore form continuous 1D channel structures. Pillar[n]arenes with one interactive face and one face with no interaction sites can be used as the ends of 1D tubes to obtain length-controlled discrete 1D tubes (Figure 8a).36 Figure 8 Rational design of discrete tubes by dimerization and trimerization of pillar[5]arenes. Reproduced with permission from reference.36 Use of an improved procedure for synthesizing rim-differentiated pillar[5]arenes, enabled the synthesis of new rim-differentiated pillar[5] arenes bearing benzoic acid groups on one rim and alkyl chains on the other rim. Benzoic acids form dimeric structures at high concentrations, therefore the rim-differentiated pillar[5]arenes form dimeric structure at high concentrations. The formation of dimeric structures was confirmed by single-crystal analysis of the rim-differentiated pillar[5]arene. Twomolecules of the rim-differentiated pillar[5]arene interact with each other headto-head via hydrogen bonds, which results in a dimeric tubular structure. Discrete dimers can act as transportation channels for water molecules, but not for larger cations such as sodium or potassium cations (Figure 8b).37 The normal pillar[5]arene cavity size is ca. 4.7 Å, but a discrete dimer has a narrow cavity on the benzoic acid group rim. The inter-molecular hydrogen bonds between the benzoic acid groups on the rim narrow the cavity size to ca. 2.8 Å; this can act as a water channel but blocks the passage of sodium or potassium cations. Another interesting feature is a rapid water transportation ability. These dimers can transport ca. 107 water molecules s-1/channel, which is only one order of magnitude lower than the value for the natural membrane protein aquaporin (ca. 108-9 water molecules s-1/channel). The dimer can be converted to a discrete trimer (Figure 8c).36 Mixing a rim-differentiated pillar[5]arene and peralkylamino-substituted pillar[5] arene in a 2:1 feed ratio resulted in formation of a discrete tubular trimer via multiple ionic interactions. Formation of two-dimensional (2D) sheets: Pillar[5]arenes and pillar[6] arenes are regular pentagonal and hexagonal molecules, respectively. Hexagonal molecules are good building blocks for obtaining well-defined 2D supramolecular assemblies because the structures are more highly symmetric than pentagonal molecules, this is known as molecular tiling. We therefore decided to use hexagonal pillar[6]arene as a building block for 2D sheet synthesis (Figure 9).38 As the driving force for assembling pillar[6]arenes, we used formation of the intermolecular supramolecular charge-transfer complex between hydroquinone and benzoquinone, which is generated by oxidation of hydroquinone. Chemical or electrochemical oxidation of pillar[6]arene resulted in formation of 2D hexagonal sheets. The 2D hexagonal sheets have pores (4.04 Å) that arise from the pillar[6]arene assembly (4.10 Å). We speculated that the 2D porous sheets could potentially be used as a source for synthesizing carbon materials by carbonization at 900ºC because they have many phenolic groups similarly to good carbon sources such as phenolic resins. Carbonization of the 2D sheet gave carbon material in relatively high yield (54%). The carbon material had pores of size 4.09 Å, which was similar to those of the 2D sheet (4.04 Å) and the pillar[6]arene assembly (4.10 Å). A porous carbon material with pores of the same size as those of the organic building block can therefore be produced by the assembly and subsequent carbonization. Angstrom-level pore control of carbon materials has been investigated by using porous coordination polymers or metal–organic frameworks and covalent organic frameworks. However, their original porous structures were destroyed during the carbonization process in most cases. Porous material synthesis from the pillar[6]arene assembly is therefore a new strategy for creating carbon materials with pores controlled at the angstrom level from organic building blocks. Figure 9 2D supramolecular polymerization by oxidation OH[6] and porous carbon (PC[6]) by carbonization of CT[6].
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