Abstract
A series of new supramolecular three-ring bent-shape Schiff base liquid crystal (LC) complexes were prepared and studied. On one side, two alkoxy chain lengths of the carboxylic acids were used, namely eight and sixteen carbons. Moreover, on the other side, terminal small compact groups, which substituted aniline, with different polarities were utilized. Furthermore, the hydrogen-bonding interactions in the formed complexes were elucidated by Fourier-transform infrared (FT–IR) spectroscopy. The mesomorphic thermal and optical characteristics of the samples were determined by differential thermal analysis (DSC) and polarized optical microscopy (POM). The complexes exhibited enantiotropic and dimorphic mesophase behaviors. The results indicate that the polarity of the compact groups and the lengths of the alkoxy chains greatly impacted the mesomorphic characteristics and thermal stabilities of the mesophases. The observed values of the enthalpy changes (ΔH) associated with the crystalline smectic-A (TCr-SmA) transitions were extremely small compared with the conventional values that characterize supramolecular hydrogen-bonded liquid crystalline complexes. ΔH, which corresponded to the nematic isotropic transitions (TN-I), varied from 0.13 to 9.54 kJ/mol depending mainly on the polarity of the groups that were para-attached to the aniline moiety. Finally, the theoretical results obtained by density functional theory (DFT) calculations were discussed. The DFT geometrical structures showed non-coplanar structures. The mesomorphic range was correlated with the calculated dipole moment, polarizability and the aspect ratios of the investigated compounds.
Highlights
Supramolecular aggregation arises from the binding of a group of molecules with welldefined structures
Liquid crystals (LCs) are among the most valuable formative materials that can be produced by supramolecular assembling [4,5,6,7], and these generated assemblies in liquid crystal (LC) have been employed in diverse applications [8,9,10,11,12]
The formations of the complexes were proven by Fourier-transform infrared (FT–IR) and nuclear magnetic resonance (NMR) spectral analyses [52,53,54,55,56]
Summary
Supramolecular aggregation arises from the binding of a group of molecules with welldefined structures These molecules are grouped by second-order (non-covalent) bonds, e.g., hydrogen bonds, halogen bonds, pi agglutination, van der Waals forces, coordination bonds, and dipole–dipole interactions [1,2,3]. The term “hydrogen bond” was first employed by Pauling [13], who postulated that hydrogen bonds exhibited an electrostatic characteristic. This concept evolved over six decades before Steiner and Singer [14] (in the early 1990s) defined hydrogen bonding as any attraction of the type “X–H···A”, where H and A possess partially positive and partially negative charges, respectively. This concept indicates the possibility of variations in hydrogen bond strengths depending on the properties of the atoms in the bond (electronegativity)
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