Molecular and Segmental Orientational Order in a Smectic Mesophase of a Thermotropic Ionic Liquid Crystal

Jing Dai,Sergey Dvinskikh,Boris Kharkov

doi:10.3390/cryst9010018

Abstract

We investigate conformational dynamics in the smectic A phase formed by the mesogenic ionic liquid 1-tetradecyl-3-methylimidazolium nitrate. Solid-state high-resolution 13C nuclear magnetic resonance (NMR) spectra are recorded in the sample with the mesophase director aligned in the magnetic field of the NMR spectrometer. The applied NMR method, proton encoded local field spectroscopy, delivers heteronuclear dipolar couplings of each 13C spin to its 1H neighbours. From the analysis of the dipolar couplings, orientational order parameters of the C–H bonds along the hydrocarbon chain were determined. The estimated value of the molecular order parameter S is significantly lower compared to that in smectic phases of conventional non-ionic liquid crystals.

Highlights

Ionic liquids that form ordered mesophases in temperature ranges between solid and isotropic liquid phases belong to a class of ionic liquid crystal (ILC) materials [1]
The molecular and conformational dynamics of flexible organic cations in the smectic A phase of the ionic liquid C14 mimNO3 was studied by means of 1 H–13 C dipolar
We demonstrated that the solid-state dipolar Nuclear magnetic resonance (NMR) spectroscopy in ILC samples is a powerful tool sensitive to details of the molecular conformation and dynamics in the organic smectic layer

Summary

Introduction

Ionic liquids that form ordered mesophases in temperature ranges between solid and isotropic liquid phases belong to a class of ionic liquid crystal (ILC) materials [1]. Molecules and ions in liquid crystals (LC) exhibit a high molecular translational and rotational mobility combined with orientational and positional order. Nuclear magnetic resonance (NMR) spectroscopy is widely used to study molecular conformational and rotational dynamics and orientational order in LC [3,4]. Structural and order parameters are obtained through the measurements of orientation- and distance-dependent dipole couplings. Due to the orientational ordering, the dipolar coupling in mesophase is not averaged to zero, in contrast to isotropic phase. 13 C–1 H heteronuclear dipolar couplings in LC are measured by two-dimensional (2D) separated local field spectroscopy technique [5] Due to the orientational ordering, the dipolar coupling in mesophase is not averaged to zero, in contrast to isotropic phase. 13 C–1 H heteronuclear dipolar couplings in LC are measured by two-dimensional (2D) separated local field spectroscopy technique [5]

Methods

Results

Conclusion