Assembled structures pillar arenes

In this chapter, first, the assembled structures of the cyclic pentamers, pillar[5]arenes, and cyclic hexamers, pillar[6]arenes, in the crystal state are discussed. They have been most widely used because they can be obtained in relatively good yields. Pillar[5]- and pillar[6]arenes possess clear cylindrical pillar-shaped structures when compared with other macrocyclic compounds. Therefore, it is envisioned that the assembled structures of pillar[n]arenes in the crystal state should be easily analyzed and this pre-organized conformation will facilitate the formation of herringbone, one-dimensional channel and slipped-stacked pillar[w]arenes. It has been revealed that the assembled structures of pillar[n]arenes largely depend on the ring size, substituents and solvents used to obtain the single crystal. The assembled structures of pil-lar[ ]arenes contribute to the gas and organic vapor adsorption properties. 

Second, we describe crystal state-assembled structures of pillar[5]arenes, pillar[6]arenes, and larger pillar[n]arene homologs (n = 8, 9, 10). Furthermore, the bulk materials formed using pillar[ ]arenes are discussed. 

Huang et al. also reported the assembled structures of an amphiphilic pillar[5]arene having all five ester groups on the same side (6.20) in the crystal state. The amphiphilic pillar[5]arene also formed onedimensional channels with a rotation angle of 36° (Figure 6.12). The trend is the same as for the non-symmetrical pillar[5]arenes having all five alkoxy groups on the same side, and for a pillar[5]arene with 10 ester moieties. 

Host-guest complexes also form various assembled structures in the solid state. Host-guest complexes between per-ethylated pillar[5]arene 6.3 and 1,4-dichlorobutane were arranged perpendicular to each other (Figure 6.13a, herringbone structure). ... 

We show examples of the assembled structures of pillar[5]arene molecules in the solid state. They mainly show three assembled structures onedimensional channels, slipped-stacked and herringbone structures. Considering the structural difference between cyclic pentagonal pillar[5]ar-enes and hexagonal pillar[6]arenes, the assembly of pillar[6]arenes is more regular than that of pillar[5]arenes. This is because pillar[6]arenes are highly symmetrical hexagonal structures. 

The assembled structure of pillar[6]arene with one benzoquinone unit (6.26 Figure 6.16) was reported by Huang et... 

In this case, even with the highly symmetrical hexagonal pillar[6]arene structure, the molecules were slipped-stacked. This is because either the introduction of one benzoquinone unit reduces the symmetry of pillar[6]-arene, or the formation of the inter-molecular charge-transfer complex between the benzoquinone and l,4-dipropo ybenzene units disturbs the regular hexagonal assembly of pillar[6]arene molecules. 

The assembled structures of these large pillar[ ]arene homologs are shown in the right-hand column of Figure 6.21. Pillar[9]- and pillar[10]arene assembled to form one-dimensional channels, but the packing mode of pillar[8]arene exhibited slipped stacking of pillar[8]arene molecules. 

In their crystalline state, pillar[5]arenes mainly form three different kinds of assembled structure, including 3D structures (e.g., herringbone), ID channels and slipped-stack structures. The ring size, nature of the substituents on the rims of the pillar[ ]arene structure and type of solvent used for the crystallization aU play a critical role in dictating the assembled structures of pillar[/i]arenes. 

We previously reported that a pillar[5]arene bearing ten ethoxy groups on its rims could form two different assembled structures depending on the type of solvent used for the crystallization (Fig. 19.13) [52]. 

Similar to calix[n]arenes, porphyrins have been known to be one of the support pillars of supramolecular chemistry attributing suitable photoactive and electroactive properties to the molecular structures designed around them for building artificial molecular devices. Thus, various metallated and free base porphyrin-calixarene assemblies could afford attractive scaffolds for application in the areas of multipoint molecular recognition, receptors, host-guest chemistry, catalysis and photoinduced electron transfers. 

A one-dimensional channel structure, formed by the assembly of pil-lar[5]arene derivatives, was first reported by Hou et al Pillar[5]arene deca-ester (6.2) formed two kinds of one-dimensional channel. Single crystals were obtained by slow evaporation of its solution in both chloroform and a mixture of chloroform and ethylene glycol. When the crystal was grown in chloroform, the molecules of deca-ester 6.2 were aligned to form infinite channels with a central pore size of 6.50 A, in which the adjacent molecules... 

Pan and Xue also synthesized pillar[5]arenes with three different kinds of substituent unit. Pillar[5]arenes containing one (6.16) and two (6.17) benzoquinone units assembled into one-dimensional channels by the stacking of pillar[5]arene molecules (Figure 6.10). The channels arranged into honeycomb-like structures. In contrast, two overlapped tubes were observed in pillar[5]arene with three benzoquinone units (6.18). 

However, the assembly of pentagonal pillar[5]arene 6.9, induced by oxidation, afforded an amorphous structure because packing of the pentagonal molecules cannot afford a well-defined structure (Figure 6.18a). 

Recently, Zhao and co-workers constructed a supramolecular polymer network by mixing the pillar[5]arene trimer 7.35 with a bisparaquat linker (7.36). In particular, as the concentration of the mixture increased, the supramolecular assemblies obtained transformed from vesicular structures, to tubular objects, layers and staeked layers (Figure 7.20). 

In 2012, Huang et al. prepared a new type of amphiphilic pillar[5]arene (11.7) which can self-assemble to form vesicles in water (Figure 11.7). The vesicles could encapsulate calcein, a model hydrophilic guest, within their interiors under neutral conditions. The vesicular structure appeared to transform into a micellar structure with a decrease in pH. Owing to this transition, the calcein was released into the solution in response to a decrease in pH (Figure 11.7). This is very interesting, because the stimuli-responsive nanocapsule may have potential applications in selective drug delivery in tissues with a lower pH, such as infected tissues and tumor tissues. 

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