Cyanostilbene-based ester & Schiff base containing liquid crystals: Synthesis, mesomorphic studies and DFT calculations

Abstract

In this study, Schiff base derivatives of cyanostilbene ester with varying alkoxy chain lengths were designed and synthesized. Different analytical techniques such as FT-IR, ¹H NMR, ¹³C NMR, and ESI-MS were used to characterize newly synthesized mesogens. Using DSC and POM, the compounds were studied for their mesomorphic behaviour. Along with the nematic mesophase, this homologous series exhibited a stable enantiotropic SmC mesophase with the increase of the alkoxy chain length at the ester end of the molecules. Compounds SB2 and SB3, which have shorter alkoxy chain lengths, exhibited a less stable SmC mesophase and a more stable enantiotropic nematic mesophase in this homologous series. Variation of alkyl chain length in compounds SB6-18 resulted in the stability of the SmC mesophase. The tilted SmC mesophase of compound SB10 was further validated using VT-XRD investigation. In order to demonstrate the mesomorphic behaviour of the compounds under study, DFT calculations were carried out and compared with the experimental results in terms of aspect ratio, dipole moment, and polarisability. Furthermore, a projection of the molecular electrostatic potential and frontier molecular orbitals was used to show how the polarity of the central linkage and terminal group affects the energy gap of the FMOs and the distribution of electrostatic charges on the studied compounds.