Polypseudorotaxane Constructed from Pillar[5]arene and Viologen Polymer

Abstract

Introduction. Polypseudorotaxanes and polyrotaxanes constructed through complexation of covalent polymers with macrocyclic molecules have attracted tremendous interest in the fields of supramolecular, material and biomedical chemistry because the macrocyclic rings move freely and rotate on the polymer axis. 1 For construction of polypseudorotaxanes and polyrotaxanes, cyclodextrins 2-4 and crown ethers 5 were initially utilized as the macrocyclic components. Harada et al. first reported the formation of polypseudorotaxanes consisting of cyclodextrins and linear polymers such as poly(ethylene glycol) and poly(propylene glycol). 2 Cyclodextrin molecules are spontaneously threaded onto the polymer chain in water due to hydrophobic interactions between the linear polymer chain and the inner cavity of the cyclodextrin. Moreover, by addition of bulky end-capping groups to the polypseudorotaxanes, cyclodextrin-based polyrotaxanes were successfully synthesized. 3 Since cucurbiturils can also form very stable host-guest complexes with positively charged molecules in aqueous media, polypseudorotaxanes and polyrotaxanes consisting of cucurbiturils and positively charged polymers have been prepared. 6 We recently synthesized a new type of host molecule and named it "pillar[5]arene" (Figure 1a). 7,8 The composition of pillar[5]arene is almost the same as that of typical calixarenes. 9 However, because its repeating units are connected by methylene bridges at the para-position, pillar[5]arene has a unique symmetrical pillar architecture, which is different from the basketshaped structure of the meta-bridged calixarenes. Since pillar[5]arene is composed of an electron donor of hydroquinone, it exhibits very interesting host-guest properties with electron accepting molecules such as viologen and pyridinium derivatives. 7 Therefore, in the present communication we report a novel polypseudorotaxane constructed from pillar[5]arene as the cyclic rings and viologen polymer as the polymeric chain ( Figure 1b). We investigated the effects of the end-group and the length of the linker between the viologen moieties on complexation of the viologen polymers with pillar-[5]arene. Moreover, intriguing solvent-dependent shuttling behavior of pillar[5]arene along the polymer axis is reported.

Synthesis of novel pillar-shaped cavitands “Pillar[5]arenes” and their application for supramolecular materials

“…In contrast, mixing 6 with the viologen polymer with adamantyl groups at both ends did not give a polypseudorotaxane because of the bulkiness of the adamantyl stopper. This observation implies that 6 was threaded onto the viologen polymer axis from its both ends [102]. The adamantyl group is a suitable stopper for synthesis of the polyrotaxane from 6 and viologen polymer.…”

Pillar-Shaped Macrocyclic Hosts Pillar[n]arenes: New Key Players for Supramolecular Chemistry

“…Pillar [5]arenes form stable host−guest complexes with cationic molecules; thus, we investigated host−guest complexation between per-hydroxylated pillar [5]arene 12 [5] and a viologen polymer (Figure 72a). 277 When per-hydroxylated pillar [5]arene 12 [5] was mixed with viologen polymer, formation of a polypseudorotaxane was observed (Figure 72b). By capping the chain ends with bulky adamantyl moieties through cationization, the first pillar[5]-arene-based polyrotaxane 49 was successfully prepared ( Figure 72c).…”

Polyrotaxanes

“…Formation of the hostguest complex depended on the length of the linker between viologen moieties. Adamantyl moiety was found to be large enough to prevent the threading of pillar [5]arene (23). Formation of intermolecular hydrogen bonds between the OH moieties of pillar [5]arenes stabilized the structure and reduced the solubility.…”

“…These examples include various macrocycles, for example, crown ethers (CEs) (6)(7)(8)(9), cyclophanes (CPs) (10,11), calix[n]arenes (12)(13)(14), CDs (15)(16)(17)(18)(19), cucurbiturils (CBs) (20), cyclobis(paraquat-p-phenylene) (21), and pillararenes (22)(23)(24), and numerous types of axes depending on the macrocycles. As shown in Figure 1, polyrotaxanes are categorized into two classes based on their structure: main-chain polyrotaxanes and side-chain polyrotaxanes.…”