Mesomorphic and geometrical orientation study of the relative position of fluorine atom in some thermotropic liquid crystal systems

New series of the Schiff base/esters, 4-((2 '-or 3 '-fluorophenylimino)methyl)phenyl-4 ''-alkoxy benzoates were prepared and their mesophase behaviour investigated. The type of the mesophase and its transition temperature were determined by differential scanning calorimetry (DSC) and polarised optical microscopy (POM). The optimised structures of Schiff base/ester and their azo/ester groups have been deduced theoretically by density functional theory (DFT). Each group of homologues differs from the other by the position of the fluorine atom on the terminal benzene ring, in addition to laterally neat homologues. Comparative studies revealed that the position and orientation of the lateral fluorine atom as well as the type of the linking core not only affects the melting temperature but also the mesophase stability. DFT results showed that the type and stability of the mesophase could be illustrated in the terms of calculated dipole moment, polarisability, thermal energy and molecular electrostatic potential.