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 (N TB ) 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 N TB phase, considered as intermediate between the nematic and the cholesteric phases. Onedimensional (1D) and two-dimensional (2D) 13 C NMR spectra have been recorded in the liquid crystalline phases and the alignment-induced chemical shifts (AIS) and the 13 C− 1 H 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 N TB mesophases.

Effortless Extraction of Structural and Orientational Information from 13C–1H Dipolar Couplings for Thiophene Mesogens

“…For the mesogens in which thiophene is linked to the rest of the core unit without the spacer moiety, the S zz values of the thiophene ring are the same (∼0.63) whereas in mesogens where thiophene is separated by the rest of the core via a spacer, a dramatic reduction in S zz values (∼0.16–0.18) is witnessed. In a recent work, we elucidated the molecular dynamics and orientational preferences of mesogens in which phenyl rings are separated by the rest of the core with spacer units that differ in the odd–even atoms . Even though a slight variation in molecular structure, as well as the mesophase transition temperatures, exists between the mesogens, the gross changes in the order parameters offer reliable ordering preferences of the thiophene ring.…”

“…In a recent work, we elucidated the molecular dynamics and orientational preferences of mesogens in which phenyl rings are separated by the rest of the core with spacer units that differ in the odd−even atoms. 24 Even though a slight variation in molecular structure, as well as the mesophase transition temperatures, exists between the mesogens, the gross changes in the order parameters offer reliable ordering preferences of the thiophene ring. It is noted that the biaxial order parameter is on the order of 10% or less of the major order parameter.…”

“…31 This is perhaps due to the orientation of a particular molecular fragment with reference to the overall orienting axis. In a couple of earlier studies, for example with a phenyl ring connected by a flexible ring 24 or occurring in a lateral arm, 32 it is noticed that the major and minor order parameter values are interchanged.…”

“…21,22 In this context, mesogens comprising bulky terminal groups attached to mesogenic cores through a flexible spacer are considered model systems and their studies attained significant importance. 23,24 Nuclear magnetic resonance spectroscopy played a central role in probing the geometry and dynamics of molecules in the liquid crystalline phase. 25,26 The accessibility of probe nuclei like 1 H, 2 H, 13 C, and 19 F empowers the NMR to extract the relevant information from the mesogens in either isotopelabeled form or natural abundance.…”

“…Thermotropic liquid crystals are generally built with core units made up of phenyl rings directly linked or via connecting units along with terminal chains on both sides or at one terminus. , This design framework led to numerous molecular mesogens displaying various nematic and a wide range of smectic mesophases . The replacement of phenyl rings with five-membered heterocyclic rings led to molecular architectures slightly deviating from rod-like shapes. , Among the five-membered heterocyclic rings exploited for generating the mesogens, thiophene, a five-membered ring with a sulfur atom, engrossed principal attention due to exciting reasons. − The interest in π-conjugated liquid crystalline semiconductors unequivocally motivated the researchers to incorporate thiophene with other aromatic rings for applications in organic electronics. , Fundamentally, thiophene as well as phenyl rings are aromatic systems by sp 2 -hybridized carbons with significant differences in their geometry. , When thiophene is part of the core unit, besides contributing to a change in the geometry of the molecule, it greatly influences the material properties. − It has been well recognized that the replacement of the phenyl ring with thiophene in a mesogenic core leads to changes in properties like an increase in optical anisotropy and a decrease in melting point, offers negative dielectric anisotropy, reduces the viscosity, and favors fast switching times. , In these molecular systems, thiophene can connect to other segments of the core unit directly or through the linking groups or via a flexible spacer and greatly impact the geometry and shape of the molecule. − The discovery of the twist-bend (N TB ) nematic mesophase in odd dimers has drawn the attention of researchers in which a spacer is incorporated between two mesogenic core units. , In this context, mesogens comprising bulky terminal groups attached to mesogenic cores through a flexible spacer are considered model systems and their studies attained significant importance. , …”