Liquid crystalline behaviour of symmetrical thermotropic Schiff base compounds

Abstract

A series of new symmetrical liquid crystal compounds with Schiff base as a linking unit attached to different terminal alkyl chains were synthesized and characterized. The intermediates of 4-(decyloxy)benzaldehyde, 4-(dodecyloxy)benzaldehyde, and 4-(tetradecyloxy)benzaldehyde were synthesized through alkylation reaction between 4-hydroxybenzaldehyde with three series of bromoalkanes. Next, it further reacted with 1,4-phenylenediamine through condensation reaction to produce symmetrical Schiff base linkage with different terminal group chain which indicated as N-(substitutedbenxylidene)benzene-1,4-diamine. The compounds were characterized using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and CHN elemental analysis. The Schiff base linkages are detectable in the FTIR spectra within the range of 1601–1604 cm⁻¹ and in the NMR spectra at δ 8.47–8.48 ppm. The mesophase transition phase of this compounds were determined using polarized optical microscope (POM) and further confirmed using differential scanning calorimetry (DSC). The Schiff base compounds were found to be mesogenic with smectic C and nematic transitions phases. These compounds attached to decyl, dodecyl, and tetradecyl terminal chains were found to exhibit smectic C phase at 142.47, 135.60, and 131.90 °C and nematic phase at 199.62, 197.11, and 188.76 °C, respectively. The incorporation of a Schiff base linking unit with a symmetrical alkyl chain induces the mesogenic effect in the molecules. A long alkyl chain allows the compounds to adopt a lamellar packing, facilitating the formation of a smectic phase. Incorporating the Schiff base into the molecules maintains an appropriate permanent dipole moment, which enables the homologs to arrange in a parallel orientational order, thereby adopting a nematogenic character.