13C NMR Studies, Molecular Order, and Mesophase Properties of Thiophene Mesogens

Three-ring mesogens with a core comprising thiophene linked to one phenyl ring directly and to the other via flexible ester are synthesized with terminal alkoxy chains to probe the mesophase properties and find the molecular order. The phenyl thiophene link in the core offers a comparison of the mesophase features with the molecular shape of the mesogen. The synthesized mesogens display enantiotropic polymesomorphism and accordingly nematic, smectic A, smectic C and smectic B mesophases are perceived depending upon the terminal chain length. For some of the homologues, monotropic higher order smectic phases such as smectic F and crystal E are also witnessed. The existence of polymesomorphism are originally observed by HOPM and DSC and further confirmed by powder X-ray diffraction studies. For the C8 homologue, high resolution solid state C-13 NMR spectroscopy is employed to find the molecular structure in the liquid crystalline phase and using the 2D SLF technique, the C-13-H-1 dipolar couplings are extracted to calculate the order parameter. By comparing the ratio of local order of thiophene as well as phenyl rings, we establish the bent-core shape of the mesogen. Importantly, for assigning the carbon chemical shifts of the core unit of aligned C8 mesogen, the C-13 NMR measured in mesophase of the synthetic intermediate is employed. Thus, the proposed approach addresses the key step in the spectral assignment of target mesogens with the use of C-13 NMR data of mesomorphic intermediate.