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.