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

Abstract

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 13 C nuclear magnetic resonance (NMR) studies in the liquid crystalline phase are carried out to find the alignment-induced chemical shifts (AIS) and 13 CÀ 1 H dipolar couplings. The orientational order parameters of the mesogens determined from 13 CÀ 1 H dipolar couplings disclose that the long axis is not only collinear to the C3À 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.

13C Nuclear Magnetic Resonance Investigations of Molecular Mesogens with a Terminal Phenyl Ring Linked via a Spacer: Odd–Even Effect

“…The mesophase properties of the synthesized mesogens are determined, and the detailed one-dimensional (1D) and two-dimensional (2D) 13 C NMR investigations in the liquid crystalline phase have been carried out. Indeed, the 13 C NMR studies empower the determination of site-specific orientation of moieties that are part of the mesogenic core. − Further, the investigation offers direct experimental evidence of the orientation of the terminal phenyl ring connected to the mesogenic core through a spacer unit that holds odd/even number of atoms. Since the stability of the N TB phase is known to depend on the average bend angle between the two mesogenic units, the present study where the average angle between the rodlike core and the terminal phenyl ring is examined bears greater implication.…”