¹³C NMR investigations of molecular order of rod‐like, bent‐core, and thiophene mesogens

Abstract

In this review, methods to obtain the orientational order of topologically variant molecular mesogens using by one-and two-dimensional (2D) solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy are described. Besides 13 C chemical shifts, the 13 C─ 1 H dipolar couplings measured from 2Dseparated local field (SLF) technique are used for computing the order parameters of a variety of mesogens. The investigated molecules are composed of a variable number of rings in the core, that is, core ranging from simply one ring to five rings. Among the mesogens investigated, a special focus has been placed on mesogens with thiophene rings, which are gaining popularity as liquid crystalline organic semiconductors. The replacement of a phenyl ring by thiophene in the core has a dramatic influence on molecular topology, as observed from the measured order parameters. The review highlights the advantages of the 2D SLF method for understanding the local dynamics and for mapping the topology of mesogens through the measured order parameters. SLF NMR studies of as many as 24 molecular mesogens that vary in terms of the molecular structure as well as topology are covered in the review. Order parameters of the rings have been estimated from the 13 C─ 1 H dipolar couplings in the nematic, smectic A, smectic C, and tilted hexatic phases as well as in B 1 and B 2 mesophases of various mesogens. It is anticipated that, in the years to come, the 2D SLF method would provide advanced molecular information on structurally complex mesogens that are emerging in liquid crystal science through the incessant efforts of synthetic chemists. The mini review covers the orientational order of topologically variant molecular mesogens determined by 1D and 2D solid-state 13 C NMR spectroscopy. Accordingly, rod-like, bent-core, and thiophene mesogens were subjected to 2D SLF measurements to get the order parameters from which the topology was established. The replacement of phenyl ring by thiophene and its influence on order parameters as well as on molecular topology is also discussed.

Direct method to grasp molecular topology of mesogens through13C–1H dipolar couplings

“…Generally, for the SAMPI-4 experiment, the 13 C– 1 H dipolar couplings need to be extracted from experimental dipolar oscillation frequencies whenever carbon couples to more than one proton. 23,24 Accordingly, by following the established protocols, 8,13,25,26 the order parameters of the phenyl rings of the core unit are determined by employing the following equation 27,28 D CH = K [ S zz (3 cos 2 θ z − 1)/2 + ( S xx − S yy )(cos 2 θ x − cos 2 θ y )/2 + S xz (cos θ x cos θ z )]where constant K = − hγ H γ C /4π 2 r CH 3 , with h is the Planck constant, γ H ( 1 H) and γ C ( 13 C) are gyromagnetic ratios, θ is the angle formed by CH bond vector r CH with the respective coordinate axis of the phenyl ring under consideration. The S zz , S xx – S yy , and S xz are the components of the order parameter tensor.…”

“…7 The advent of 2D separated local field (SLF) NMR experiments further enhanced the supremacy of the NMR technique, as witnessed in recent literature. 8,9 These experiments offer a 2D plot of 13 C chemical shifts and 13 C-1 H dipolar couplings sensitive to orientational order and molecular topology. [10][11][12] The molecular structures under scrutiny include conventional rod-like molecule, mesogens with lateral phenyl ring, and unsymmetrical bent-core (banana) shape molecules.…”

Molecular Order of Topologically Variant Flexible Mesogens by 13C Nuclear Magnetic Resonance

“…The SAMPI-4 experiment provides the experimental dipolar oscillation frequencies, and by employing them, the local order parameters of the phenyl rings of the core unit can be determined by adopting the established protocols. ,,− Since 1,4-disubstituted phenyl rings undergo π flips about the para axis, D 2h symmetry is assumed and the two local order parameters namely, S zz and ( S xx – S yy ) are sufficient to represent its molecular order, which are related to dipolar couplings by the following equation , where K = − h γ H γ C /4π 2 r 3 CH , with γ H and γ C being the gyromagnetic ratios of protons and carbons, respectively; r CH is the CH bond distance; and θ x , θ y , and θ y are angles between vector r CH and the respective coordinate axes. The standard bond distances r CH = 1.1 Å for the C–H bond and r CC = 1.4 Å for the C–C bond are used for the calculations.…”

“…13 C NMR as a standard tool for structural characterization of liquid crystals is becoming popular in recent years, being used for the study of a variety of systems such as the odd dimeric mesogens that exhibit a twist–bend nematic mesophase, the banana mesogens, polycatenar systems, and various phases including those that exhibit partial interdigitisation. − In this study, we demonstrate the utility of 13 C NMR spectroscopy for deducing the molecular topology through the conformations in the liquid crystalline phase for three mesogens in which four phenyl rings are linked by connecting units and a spacer. The common structural feature of the investigated mesogens is that the rigid core consists of three phenyl rings joined by linking groups, while the fourth ring is connected via a flexible spacer to the rigid core.…”

“…In recent years, 13 C NMR spectroscopic studies have become an important tool for characterization of a variety of different types of mesogens and to infer such information such as the bent angle of banana mesogens, partial interdigitization in smectic phases, ordering in polycatenar systems, organization in twist bend nematic phase, and so on. − In the present study, topologically variant three mesogens in which a three phenyl ring core is connected to a fourth phenyl ring through a flexible spacer are synthesized and detailed 13 C NMR investigations were carried out to determine the order parameters. The HOPM and DSC studies establish existence of an enantiotropic nematic phase for all molecules with additional smectic mesophases in either a monotropic or enantiotropic manner.…”