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Free keywords:
Discotic Liquid Crystals, Pyramidic Liquid Crystals, Dynamic Carbon-13 Mas Nmr, Molecular Reorientation, Tribenzocyclononene, Saddle-crown Isomerization
Abstract:
A new homologous series of compounds, nonaalkanoyloxy tribenzocyclononene (TBCN-n, where n is the number of carbons per side chain) with n=2 to 14 was synthesized. The compounds exist in two isomeric forms, crown and saddle. For sufficiently long side chains (n S 4 for the saddle and n S 5 for the crown) the homologues of both series are mesogenic, exhibiting columnar hexagonal mesophases. The mesomorphic properties of the compounds were studied by differential scanning calorimetry, optical polarizing microscopy, X-ray diffraction and carbon-13 solid state NMR spectroscopy. No interconversion between the crown and saddle isomers is observed in the solid and mesophases (except very slow saddle to crown transformation, close to the clearing points of the mesophase), but isomerization is fast in the melt. This leads to peculiar phase diagrams which are discussed in the text. Analysis of the carbon-13 NMR spectra indicates that in the crown mesophases there is rapid reorientation of the molecules about the columnar axes, while no such motion is found in the mesophases of the saddle isomers. The latter appear, however, to undergo thermally activated high-amplitude librations, which freeze out on cooling to room temperature and below.
