Structural investigation of resorcinol based symmetrical banana mesogens by XRD, NMR and polarization measurements

Abstract

Synthesis and structural characterization of two novel symmetrical banana mesogens built from resorcinol with seven phenyl rings linked by ester and imine with a terminal dodecyl/dodecyloxy chain has been carried out. Density functional theory (DFT) has been employed for obtaining the geometry optimized structures, the dipole moments and ¹³C NMR chemical shifts. The HOPM and DSC studies revealed enantiotropic B₂ and B₇ phases for the dodecyl and dodecyloxy homologs respectively. The powder X-ray studies of both the mesogens indicate the presence of layer ordering. The polarization measurements reveal an anti-ferroelectric switching for the B₂ phase of the dodecyl homolog whose structure has been identified as SmC_SP_A. The B₇ phase of the dodecyloxy homolog was found to be non-switchable. High resolution ¹³C NMR study of the dodecyl homolog in its mesophase has been carried out. ¹³C–¹H dipolar couplings obtained from the 2-dimensional separated local field spectroscopy experiment were used to obtain the orientational order parameters of the different segments of the mesogen. Very large ¹³C–¹H dipolar couplings observed for the carbons of the central phenyl ring (9.7–12.3 kHz) in comparison to the dipolar couplings of those of the side arm phenyl rings (less than 3 kHz) are a direct consequence of the ordering in the banana phase and the shape of the molecule. From the ratio of the local order parameter values, the bent-angle of the mesogen could be determined in a straight forward manner to be 120.5°.