Syntheses and Thermal Properties of New Liquid Crystals Bearing a Crown Ether Ring: Cation Binding in the Nematic Phase

A series of crown ether derivatives containing a biphenyl or cholesteryl moiety have been synthesized for the first time. The structural effects of these compounds on the formation of liquid-crystalline phases are discussed. We have found that benzo crown ether derivatives bearing a p-cyano- or p-(octyloxy)biphenyl moiety linked by an ester or ethyne group display a nematic liquid-crystalline phase, and 4’-((cholesteryloxy)carbonyl)benzo crown ethers exhibit a cholesteric phase; similar compounds with an oxymethyl-15-crown-5 or azacrown ether unit show no mesophases. For the biphenyl-type liquid crystals the effect of a 15-crown-5 ring on the thermal properties is similar to that seen with one lateral methoxy group, i.e., it decreases the thermal stability of the liquid-crystalline state, but the presence of an 18-crown-6 ring can exert a relatively large effect. These observations may be explained by the conformational rigidity of benzo crown rings, especially for benzo-15-crown-5, and their efficient participation in the entire conjugated system. Picrate doping experiments provided clear evidence for the selective binding of alkali-metal cations by the crown ethers in the nematic liquid-crystalline phase, although the complexation behavior of the crown ethers in this microenvironment is different from that seen in typical solvents. This is the first example of the formation of a liquid-crystalline phase in crown ether compounds and of host-guest-type behavior for such compounds in the liquid-crystalline state.