Coumarin Schiff base derivatives with alkyl 4’-formyl-biphenyl-4-carboxylate: design, synthesis, mesomorphic studies and DFT calculations

Abstract

ABSTRACT Symmetric and unsymmetric homologous series of coumarin Schiff base derivatives with alkyl 4’-formyl-biphenyl-4-carboxylate have been synthesised and studied for their mesomorphic behaviour. In the symmetric homologous series of Schiff base derivatives BPES(6–16), alkoxy chain length was varied at both ends of the molecules with n = 6,8,10,12,14,16, while in the unsymmetric homologous series of Schiff base derivatives 10BPEU(2–12), chain length was varied at coumarin end only (n = 2–8,10,12). In symmetric homologous series, compounds BPES(6–10) (n = 6,8,10) exhibited enantiotropic smectic A (SmA) mesophase, while smectic C (SmC) mesophase in cooling cycle only. SmC mesophase became prominent on increasing the chain length n = 12,14,16 in BPES(12–16). In the unsymmetric homologous series 10BPEU(2–12), compound with n = 2 showed monotropic nematic mesophase and highly stable (~165 °C) SmA mesophase. Higher analogues in unsymmetric series were turned biphasic and showed SmA as well as SmC mesophase during cooling scan only. Compound with n = 12 showed stable enantiotropic SmC mesophase (~92 °C). VT-XRD analysis of symmetric compound with n = 12, showed the tilted SmC mesophase. Further DFT calculations were investigated for un/symmetric series along with the literature molecules with similar variations and compared for various structural (length, aspect ratio) and electronic (dipole moment, polarisability) parameters to establish correlation with observed mesomorphic behaviour.