Coumarin Schiff base-esters liquid crystals with symmetrical and unsymmetrical alkoxy chains: Synthesis, mesomorphic properties and DFT approach

Abstract

Herein, two series of Coumarin Schiff base-esters liquid crystals have been prepared. The first series, Schiff base of 7-alkoxy-3-aminocoumarin with 4-formylphenyl 4-alkoxy benzoate with symmetric variation of alkoxy chain at the both end of the molecules were designed and synthesized. The second is Schiff base of 7-alkoxy-3-aminocoumarin with 4-formylphenyl 4-dodecyloxybenzoate. All these compounds were characterized using different analytical techniques such as FTIR, NMR and Mass spectral analysis. Both series were studied for their mesomorphic behaviour using differential scanning calorimetry (DSC) for the phase transitions and polarised optical microscopy (POM) to identify the type of mesophase. On varying the alkyl chain length at the both end of molecules systematically in the SCE series, compounds of chain lengths 2–8 exhibited an enantiotropic nematic (N) phase whereas SCE-8 exhibited enantiotropic smectic C (SmC) phase in addition to enantiotropic nematic (N) phase. Compounds of the symmetrical series n = 10–18 showed enantiotropic SmC phase. In the unsymmetrical series with n = 12 at aldehyde end of the molecule, enantiotropic nematic mesophase was observed for n = 2–14, while enantiotropic SmC mesophase was appeared from n = 6–18. The DFT theoretical calculations have been conducted for both the series to understand the mesomorphic behaviour of the investigated compounds. Moreover, they are compared with similar reported compounds to explain the relationship between the structural parameters and the mesomorphic behaviour.