Abstract
A series of non-symmetrical Schiff base liquid crystals were prepared and investigated. Schiff bases of p-alkyloxy aniline derivatives and 4-phenyl pyridine-4′-carbaldehyde were synthesized. The terminal alkoxy groups substituting aniline are of varied chain length, namely C6, C8, and C16. The structures of the compounds were confirmed via 1H NMR and 13C NMR spectroscopy. Different mesophases of the samples were thermally and optically characterized by differential thermal analysis (DSC) and polarized optical microscopy (POM). All samples revealed enantiotropic smectic B (SmB) and smectic A (SmA) mesophases. The results obtained were further correlated with the density functional theory (DFT) theoretical calculations. The results are compared to a series of compounds bearing biphenyl moiety in their mesogens. The thermal stabilities of the different mesophase reduced upon the increment of the alkoxy chain length. The temperature ranges of both the smectic mesophases of new compounds bearing the 4-phenyl pyridine moiety are generally expanded higher than the other series. In addition, the total mesophase range is greater in the new compounds when compared to their biphenyl analogues. The DFT results were investigated in terms of the molecular geometries and the frontier molecular orbitals as well as the charge distribution mapping to show and illustrate the difference in the mesomorphic properties.
Highlights
A unique state of matter called the Liquid crystalline state gained much importance and attracted considerable attention both in the basic sciences and technological applications because of a variety of advanced applications including digital displays, sensors, liquid crystal displays (LCDs), high-performance polymers, transporting of electron, ion or molecule and drug-delivery systems, and hybrid composites [1,2,3,4,5]
It is observed that the transition temperatures of smectic B (SmB)/smectic A (SmA) mesophase transition decreases as alkoxy chain length increase
The thermal stabilities of the different mesophase reduced upon the increment of the alkoxy chain length
Summary
A unique state of matter called the Liquid crystalline state gained much importance and attracted considerable attention both in the basic sciences and technological applications because of a variety of advanced applications including digital displays, sensors, liquid crystal displays (LCDs), high-performance polymers, transporting of electron, ion or molecule and drug-delivery systems, and hybrid composites [1,2,3,4,5]. Rod-like liquid crystalline materials called calamitic mesogens (possessing a rigid core with two or more aromatic rings linked by connecting groups and flexible terminal chains), have attracted remarkable attention over recent decades due to their ability to display a wide range of technologically important liquid crystal phases [6,7,8,9]. With a variety of structures of rod-like molecules, they display some potential applications by changing their physical properties including dielectric anisotropy (∆ε), birefringence (∆n), rotational viscosity and mesophase range for liquid crystal compounds [10,11,12,13]. A variety of materials synthesized based on five and six membered heterocyclic rings comprising one, two, or three nitrogen atoms gave rise to attracting mesomorphic properties. In the design of new mesogenic compounds the linking unit, terminal groups and flexible hydrocarbon chains are crucial
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