Coumarin Schiff base derivatives with 4′-alkoxy biphenyl-4-carboxaldehyde: Synthesis, mesomorphic studies and DFT calculations

Abstract

Two homologous series of coumarin Schiff base derivatives with 4′-alkoxy biphenyl-4-carboxaldehyde have been designed and synthesized. All the compounds were analyzed IR, 1H NMR, 13C NMR, ESI-MS and CHN analysis followed by mesomorphic properties polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) analysis. In the first homologous series BPS(6–16), Schiff base of 7-alkoxy-3-aminocoumarin with 4′-alkoxy biphenyl-4-carboxaldehyde, alkoxy chain length was varied systematically at both end of the molecules such as n = 6,8,10,12,14,16. In this series, BPS(6–10) (n = 6,8,10) exhibited enantiotropic nematic(N) and smectic C (SmC) mesophase and turned to monophasic on increasing the chain length n = 12,14,16 in BPS(12–16) with stable SmC mesophase. While in the second homologous series 10BPU(2–16), systemic variation of alkoxy chain length was carried out at coumarin end only (n = 2–18) and 4′-decyloxy-biphenyl-4-carboxaldehyde was used for Schiff base formation. In the this series, early member of the series with n = 2,3 showed enantiotropic very stable (∼ 145 °C) nematic mesophase, while N as well as SmC mesophase were observed for n = 4–8 at, whereas with increase of chain length stability of nematic mesophase was decreased and that of SmC was increased. Higher membered analogous 10BPU(12–16) exhibited SmC mesophase with stability in the rage of 130–160 °C. VT-XRD analysis was also confirmed the tilted SmC mesophase in this class of compounds. For both the series, all compounds were studied for the DFT calculations as well as literature molecules with similar variations. Based on the calculated structural (length, aspect ratio) and electronic (dipole moment, polarizability) parameters were correlated with mesomorphic behaviour observed for literature compounds and compounds from both the series followed by the detailed discussion.