Mesophase Properties and Molecular Order of Shape Anisometrically Divergent Thiophene Mesogens

The shape anisometrically variant thiophene mesogens are investigated by ¹³C NMR spectroscopy for the structural and orientational information in the liquid crystalline phase. These mesogens feature thiophene positioned at different locations within the core unit and include distinct substitutions. The mesogenic core is built with phenyl and thiophene rings directly connected or joined via linking units. Depending on the structure of the core unit, the occurrence of nematic and smectic C mesophases is recognized. The 1D and 2D ¹³C NMR studies in the liquid crystalline phase reveal interesting orientational information. The ¹³C¹H dipolar couplings measured by the 2D separated local field technique offer precise structural information about thiophene and its orientation concerning the long molecular axis. The large variation in the magnitude of ¹³C¹H dipolar couplings of thiophene is found to be extremely sensitive to molecular shape and is analyzed with respect to individual ring geometries. Thus, for a rod-like mesogen, the thiophene ring main order parameter in the nematic phase is 0.45, whereas, for a mesogen with a flexible spacer, it decreases to 0.17 in the same phase. In contrast, the thiophene main order parameter for bent-core mesogen reaches 0.82 in the smectic C phase.