¹³C CSA Tensors and Orientational Order of Model and Dimer Mesogens Comprising of Phenyl Benzoate

A Model mesogen and its symmetrical Dimer made up of phenyl benzoate core unit are investigated by ¹³C NMR spectroscopy. The existence of layer order in smectic A and smectic C phases of Dimer mesogen is established by powder X-ray diffraction. The chemical shift anisotropy (CSA) tensors of Model mesogen are determined by 2D separation of undistorted powder patterns by effortless recoupling (SUPER) experiment and are utilized for calculating the order parameters employing the alignment-induced chemical shifts (AIS). Additionally, 2D separated local field (SLF) NMR is availed for extracting ¹³C-¹H dipolar couplings for both mesogens and used for computing the order parameters. A good agreement in the order parameters calculated from ¹³C-¹H dipolar couplings and AIS is observed. Accordingly, the main order parameter (S_zz) for the phenyl rings of the Model mesogen is found to be in the range 0.54-0.82, and for the Dimer mesogen, the values span 0.64-0.82 across mesophases. Since the phenyl benzoate core unit is frequently employed structural moiety for constructing the main chain as well as side chain liquid crystalline polymers and liquid crystalline elastomers, the CSA tensors reported here will be of immense utility for the structural characterization of these materials.