Studies in Synthesis of 6-Aminocoumarin Schiff Base Derivatives as Mesogens and their DFT Approach

In first series, 6-amino coumarin Schiff base derivatives 10a-g were synthesized with alkoxy chain n=6-18 and studied for their mesomorphic properties. In this series, compounds 10b with n=8 showed monotropic nematic mesophase, while enantiotropic smectic A mesophase for higher analogues compounds 10c-g (with n=10-18) was observed. In another series of Schiff base derivatives 12a-h additional ester linkage was introduced at the aldehyde end and were studied for their mesomorphic behavior. In second series, compounds 12a-c (with n=4-8) showed enantiotropic nematic mesophase with very good thermal stability. Higher analogues of the second series with n=12-18 showed stable enantiotropic smectic A mesophase. Compounds of both series were studied by DFT calculations. Different electronic parameters such as dipole moment, polarizability, molecular electrostatic potentials, frontier molecular orbitals, and thermodynamic variables were calculated and compared with the observed mesomorphic trends in both the studied series. Further, electronic parameters of compounds 10b and 12c and that of reference compound 4 were calculated and discussed to understand the effect of the removal of a hydroxyl group on the ortho position of the aldehyde part.