Columnar and smectic liquid crystals based on crown ethers

Unsymmetrical benzo[15]crown-5 ethers 5 with one lateral ortho-terphenyl unit bearing alkoxy side chains of varying chain lengths (C5-C14) were prepared from 3,4-dialkoxyphenylbromides 2. Complexation with metal salts NIX (M = Na, Cs) afforded the corresponding derivatives MX-5. The uncomplexed crown ethers 5h and 5i, with dodecyloxy and tetradecyloxy side chains, respectively, exhibit liquid crystalline properties. In the series of complexed crown ethers, liquid crystal properties appeared as early as NaI center dot 5f with C9H19 side chains. Whereas the uncomplexed 5h,i form smectic mesophases, the complexed Nal-5g and NaI center dot 5h exhibit textures typical of columnar mesophases. These results were supported by X-ray diffraction measurements (WAXS, SAXS), which revealed smectic (5h,i), rectangular columnar (NaI center dot 5g), and hexagonal columnar (NaI center dot 5h) meso- phases. As the liquid crystalline phase might retain packing features of the solid-state structure, single-crystal Xray analyses were also performed for some of the uncomplexed and complexed crown ether derivatives. The complex NaI3 center dot 5a displays a sandwichtype structure, with the crown ether cores mutually antiperiplanar and maintaining an almost perfect crown conformation. In contrast, non-mesogenic uncomplexed crown ether 5b displays a layer-type ordering in the solid phase.