Influence of Thiophene Ring on Molecular Order of Structurally Simple π–Conjugated Smectogens: 13C NMR Study

Structurally simple rod-like π–conjugated mesogens with thiophene directly connected to phenyl, biphenyl, and fluorenone rings with terminal chains are synthesized respectively.  The occurrence of smectic A/smectic C phases is concurred by a hot-stage optical polarising microscope (HOPM), differential scanning calorimetry (DSC), and X-Ray diffraction (XRD). The static 1D and 2D 13C nuclear magnetic resonance (NMR) studies in the liquid crystalline phase are carried out to find the alignment-induced chemical shifts (AIS) and 13C-1H dipolar couplings. The orientational order parameters of the mesogens determined from 13C-1H dipolar couplings disclose that the long axis is not only collinear to the C3-C4 bond of the thiophene ring but also for the local axes of phenyl and biphenyl rings.  For fluorenone-based mesogen, the molecular biaxiality is found to be high owing to the increased breadth of the molecule. The study unveils that the orientation of thiophene and the phenyl rings is similar in the current mesogens in stark contrast to mesogens, where thiophene is connected to phenyl rings through linking groups.