Induced mesomorphism in supramolecular structures of H-bonded binary mixtures containing fluoro and chloro substituted benzoic acids

In order to realize the mesomorphism through supramolecular structures, an attempt is made to synthesize new proton acceptors viz., (4-pyridyl)-benzylidene-4′-n-octylaniline (PyB8A) which can form the hydrogen bond with the carboxylic acids. The non-mesogenic halo substituted aromatic carboxylic acids are used as proton donors. The aromatic carboxylic acids used are o-chlorobenzoic acid, m-chlorobenzoic acid, p-chlorobenzoic acid, o-fluorobenzoic acid, m-fluorobenzoic acid and p-fluorobenzoic acid. It is observed that the proton acceptor is involved in the formation of hydrogen bond with the aromatic carboxylic acids. The chemical characterization of the synthesized compounds is carried out by FTIR and ¹H NMR spectroscopy techniques. The phase characterization is done by Polarizing Optical Microscope and the phase transition temperatures are obtained by Differential Scanning Calorimetry (DSC) (Shimadzu). X-ray powder diffraction (XRD) is carried out for phase confirmation and the structures of molecular complexes are optimized using density functional theory. It is noticed that the polarity, size and position (w.r.t. proton donor) of the substituent is playing an important role in stabilizing the mesomorphism. Longitudinal and transverse dipole moment (µ_l) of the molecules is influenced by the terminal and lateral substituents of the proton donors. Para chloro/fluoro-substituted H-bonded binary mixtures are exhibiting highest range of mesomorphism, while the lowest range is shown by meta substituted binary mixtures. The ortho substituted binary mixtures are intermediary of para and meta substituted binary mixtures. The mesomorphism is more appreciable in fluoro-substituted binary mixtures than the chloro-substituted mixtures.