End Group Effect on the Thermal and Gas Chromatographic Properties of Some para-Substituted Non-Symmetrical Nematogens

Abstract

Non-symmetrically substituted nematic liquid crystals with different moieties as terminal substituents at one end and n-heptyloxy group at the other end were synthesized and characterized. The common structural features of the compounds are that they consist of the same terminal substituent at one end. The five present nematogens structurally related nematogens are compared to evaluate the effect of the type of end group on the thermal and analytical properties. The compounds were characterized by elemental analysis, FTIR, 1H-NMR spectroscopy and chemical ionization mass spectrometry. The materials’ thermal behavior and phase characterization were investigated by differential scanning calorimetry and polarizing optical microscopy. All the compounds are mesomorphic in nature, showing the nematic phase over a wide temperature range. Differential scanning calorimetry showed that the transition temperatures as well as the nematic range are strongly influenced by the type of end group attached at only one end of the nematic core molecule. Gas chromatographic properties of the compounds were determined and shown to have excellent separation capabilities of various classes of compounds, including polycyclic aromatic hydrocarbons (PAH), substituted benzenes and volatile organic compounds present in the essential oil. The separation mechanism of only some isomers was discussed in detail and, however, a relative basis for given isomers, taking into account the type of chemical bonding, molecular shape and size, the intermolecular forces (IMFs) and other factors has been proposed.