Terminal methylene biphenyl derived coumarin Schiff base-esters: Synthesis, mesomorphic behaviour and DFT investigations

Abstract

In the present investigation new unsymmetrical coumarin-methylene biphenyl MBPCE-(2–16) mesogens were designed and synthesized. Methylene biphenyl end group at one end of molecules have been linked to the 7-alkyloxy substituted coumarin core through imine and ester mesogenic connecting units. The structures of new hybrids have been elucidated employing different analytical techniques including FT-IR, 1H NMR, 13C NMR, Mass & CHN elemental analysis. New molecules have been investigated for their mesomorphic behaviour using DSC (Measurement of transition temperature and enthalpy associated) and POM (Texture study) techniques and the existence of mesophase was established by VT-XRD analysis. Compounds with n = 2 (-OC2H5) to n = 7 (-OC7H15) chain length exhibited enantiotropic nematic mesophase with good thermal stability. As the chain length increases, compounds from n = 8 (-OC8H17) and n = 10 (-OC10H21) displayed enantiotropic nematic as well as smectic-A mesophase whereas compound with n = 12 (-OC12H25) showed only interdigitated SmA phase. Next two members of the series failed to show any mesomorphism. The commencement of SmA phase and disappearance of nematic could be attributed to more layer arrangement with increased van der waals interactions. The density functional theory (DFT) calculations were employed to complement the experimental results concerning the mesomorphic behaviour. Development of smectic phase could be due to the augmentation in the area of the intermolecular interaction which increases the degree of molecular packing. Conversely, it was discovered that higher polarizability leads to lower nematic thermal stability because it strengthens parallel intermolecular interactions, which promotes a higher degree of molecular order while reducing the development of lower-order nematic mesophase.