Monolayer to Interdigitated Partial Bilayer Smectic C Transition in Thiophene-Based Spacer Mesogens: X-ray Diffraction and ¹³C Nuclear Magnetic Resonance Studies

Abstract

Mesophase organization of molecules built with thiophene at the center and linked via flexible spacers to rigid side arm core units and terminal alkoxy chains has been investigated. Thirty homologues realized by varying the span of the spacers as well as the length of the terminal chains have been studied. In addition to the enantiotropic nematic phase observed for all the mesogens, the increase of the spacer as well as the terminal chain lengths resulted in the smectic C phase. The molecular organization in the smectic phase as investigated by temperature dependent X-ray diffraction measurements revealed an interesting behavior that depended on the length of the spacer vis-a-vis the length of the terminal chain. Thus, a tilted interdigitated partial bilayer organization was observed for molecules with a shorter spacer length, while a tilted monolayer arrangement was observed for those with a longer spacer length. High-resolution solid state (13)C NMR studies carried out for representative mesogens indicated a U-shape for all the molecules, indicating that intermolecular interactions and molecular dynamics rather than molecular shape are responsible for the observed behavior. Models for the mesophase organization have been considered and the results understood in terms of segregation of incompatible parts of the mesogens combined with steric frustration leading to the observed lamellar order.

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

“…Significant effort has been made to investigate thiophene‐based mesogens by our group using 13 C NMR measurements . It was well recognized that the replacement of phenyl ring with thiophene in mesogen results in decrease of melting point, increase of optical anisotropy, reduction in viscosity, promotion of negative dielectric anisotropy, and fast switching times .…”

“…2,5 thiophene dicarboxylic acid‐based mesogens in which side arms are constructed with two or three rings are investigated by 13 C NMR . For those mesogens in which sidearms were constructed with three rings, a flexible spacer was inserted between the center thiophene and the side core units . Quite interestingly both mesogens, that is, those with flexible spacer and directly connected to thiophene exhibited nematic and SmC mesophases.…”

“…Two‐dimensional SAMPI‐4 spectra of M‐22 in (a) smectic C phase at 140°C and (b) nematic phase at 175°C (Adapted with permission from Kesava Reddy et al Copyright 2015 American Chemical Society)…”

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.…”