Molecular order and dynamics of liquid crystals formed from hydrogen-bonded networks of 5-octadecyloxyisophthalic acid

Abstract

The supramolecular organization of 5-octadecyloxyisophthalic acid (C18ISA) and its complexes with different bases has been studied by polarization microscopy, differential scanning calorimetry (DSC), IR spectroscopy and X-ray diffraction (XRD). The molecular dynamics have been elucidated using solid-state 1H NMR and 2H NMR spectroscopy of C18ISA selectively deuteriated in the 2-, 4- and 6-positions of the aromatic ring. These systems form novel mesogenic structures. The stability of the liquid-crystalline phase depends on intermolecular interactions such as hydrogen and ionic bonding, as well as hydrophobic interactions due to the side-chain packing between the complementary components of the system. On cooling from the melt, C18ISA shows a monotropic liquid-crystalline phase with a non-interdigitated lamellar structure. Optical textures on hydrophobic substrates exhibit a highly fluid smectic phase. On heating, a new type of crystalline mesophase in the temperature range 385–433 K is observed that rests on the hydrophobic interactions of the alkyl chains, while the hydrogen-bonded layer breaks down at the phase transition at 386 K. Similar behaviour is observed for most of the C18ISA-4,4′-bipyridyl complexes. The C18ISA-piperazine complex forms a different enantiotropic liquid-crystalline phase with a broad mesophase range of 150 K. Solid-state 2H NMR spectroscopy proves the high molecular order of the complex, which can be macroscopically oriented in the magnetic field. IR spectral data of the complex indicate both ionic and hydrogen-bonding character. The conformational order of the alkyl chains is reduced at the phase transition into the liquid-crystalline phase so that for this complex the mesophase is stabilized by hydrogen and ionic bonds.