Biphenyl based schiff base derivatives of 6-aminocoumarin: Design, synthesis, mesomorphic properties and DFT studies

Abstract

The study focused on synthesizing two series of coumarin Schiff base liquid crystals. These series involved molecules with variations in alkoxy chains at aldehyde ends: one being the Schiff base with 4′‑hydroxy-[1,1′]-biphenyl-4-carbaldehyde (ASBP series) and the other being alkyl 4-formyl-[1,1′]-biphenyl-4-carboxylate (AEBP series). The compounds were characterized using FTIR, NMR, and Mass spectral analyses. All the compounds were studied for their mesomorphic properties, differential scanning calorimetry (DSC) and polarized optical microscopy (POM). In both the homologous series, variation of alkyl chain length showed intriguing phase transitions. In the first homologous series ASBP, compound with hexyloxy chain displayed an enantiotropic nematic (N) phase, while those with chain lengths n = 8–16 exhibited enantiotropic smectic A (SmA) mesophase. Similarly, in the AEBP series, compounds with n = 6,8 chain length showed an enantiotropic nematic (N) phase, while those with higher chain lengths n = 10–16 exhibited enantiotropic smectic A (SmA) phases. SmA mesophase was further confirmed by powdered XRD analysis. The study included DFT theoretical calculations to comprehend the compounds' mesomorphic behavior, comparing them with analogous reported compounds. Additionally, the investigation correlated experimental findings of individual compounds with calculated parameters like polarizability, dipole moment, and aspect ratio. Molecular electrostatic potential projections and analysis of frontier molecular orbitals were also employed to elucidate how polarity variations in terminal chains and mesogenic cores influenced the energy gap of FMOs and the distribution of electrostatic charges on these compounds. It has been found that the ASBP series has lower energy gap and higher dipole moment than the AEBP series.