Hydrogen-Bonded Liquid Crystals. Novel Mesogens Incorporating Nonmesogenic Bipyridyl Compounds through Complexation between H-Bond Donor and Acceptor Moieties

Mesogenic structures have been built from 2:1 (molar ratio) mixtures of 4-alkoxy- or 4-alkylbenzoic acid (nOBA or nBA; n is the carbon number of the alkyl chain) and 4,4′-bipyridine (BPy) or trans-1,2-bis(4-pyridyl)ethylene (BPyE). In these complexes, the benzoic acid derivative functions as an H-bond donor and the bipyridyl compound operates as a functional H-bond acceptor. Well-defined structures of the mesogenic complexes are formed from independent and different molecules. These complexes exhibit stable mesophases that are not observed for each of the single components. For example, a 2:1 (molar ratio) complex prepared from 4-ethoxybenzoic acid (2OBA) and 4,4′-bipyridine (BPy) shows a nematic phase from 150 to 169 °C while both individual compounds are nonmesogenic. The liquid-crystalline phase is induced by the hydrogen bonding. The 2:1 complex of 4-butoxybenzoic acid (4OBA) and trans-1,2-bis(4-pyridyl)ethylene (BPyE) exhibits a smectic A phase (146–168 °C) and a nematic phase (168–177 °C). The smectic phase displayed by the complex is not observed for 40BA and BPyE. The isotropization temperature is increased by the complexation through the H bonds. The effect of the terminal alkyl chain length on thermal properties has been examined for the 2:1 H-bonded complexes of a series of the benzoic acids and the bipyridyl compound. The type of mesophases obtained is affected by the alkyl chain length. Infrared study suggests that the hydrogen bond is an un-ionic type with a double minimum potential energy and its stability is greatly dependent on the order of the molecular complexes. Phase diagrams have been obtained for the binary mixture of H-bond donor and acceptor moieties. The isotropization temperature curves show significant positive deviations because of the intermolecular H-bond interaction.