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

Abstract

A set of mesogens considered as model molecules to the technologically important twist-bend (NTB) nematogens are investigated. They consist of a three-ring core connected to a phenyl ring via a flexible spacer and displayed enantiotropic nematic and smectic C mesophases. In such systems, odd or even number of atoms present in the spacer could influence the orientation of the terminal phenyl ring and thus have a bearing on designing the NTB phase, considered as intermediate between the nematic and the cholesteric phases. One-dimensional (1D) and two-dimensional (2D) 13C NMR spectra have been recorded in the liquid crystalline phases and the alignment-induced chemical shifts (AIS) and the 13C-1H dipolar couplings obtained. The order parameters of the phenyl rings reveal features relatable to the odd/even number of atoms of the flexible spacer and the type of linkage. The AIS plots of the phenyl rings of the even spacer-based mesogen showed the usual behavior for all of the phenyl rings with a decrease in AIS with increasing temperature. However, for the odd-spacer mesogens, unusual behaviors are noted for the terminal phenyl ring. Thus, two of the mesogens showed an increase of AIS in the smectic C phase that continued till the middle of the nematic phase temperature range, followed by a decrease. The other two odd-spacer mesogens also showed different behaviors. These observations indicate that the terminal phenyl ring is oriented at an angle with respect to the long molecular axis for the odd-spacer mesogens that changes as a function of temperature. The angles have been found to depend on the nature of the atom/group connecting the spacer to the terminal ring and the spacer length. Thus, the present study provides critical information on the design of the odd dimers that are recognized to generate fascinating NTB mesophases.