No 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.

New polyrotaxane composed of pillar[5]arene and viologen polymer was successfully prepared with extremely high yield by capping the chain ends with adamantyl moieties. Pillar[5]arene and viologen polymer are soluble in various solvents such as acetone, acetonitrile, methanol, DMF, and DMSO, while the polyrotaxane was soluble in DMF and DMSO and insoluble in the other organic solvents. Formation of inter-molecular hydrogen bonds between the OH moieties of pillar[5]arenes stabilized the structure and reduced the solubility. The polyrotaxane exhibited a thermally induced color change from yellow to violet. On heating, the inter-molecular hydrogen bond became weakened, and the shuttling motion of pillar[5]arenes on the polymer axis was fast. Thus, efficient electron transfer from the electron donors of pillar[5]arenes to the electron acceptor of viologen polymer occurred in the whole polymer chain and the radical cation species were stabilized.

Cyclic ketones reacted with N-bromosuccinimide (NBS) catalysed by NH(4)OAc in Et(2)O at 25 degrees C to give the corresponding alpha-brominated ketones in good yields, while acyclic ketones were efficiently brominated in CCl(4) at 80 degrees C.