Order and dynamics of rod-like and banana-shaped liquid crystals by 2H NMR

Abstract

Deuterium NMR spectroscopy is a very powerful technique for studying partially or totally ordered systems, such as liquid crystals (LCs) and polymers. LCs represent a branch of the most general class of soft materials, with peculiar physical and chemical properties which attracted scientific attention for their potentiality for technological applications. From a chemical point of view, there are three aspects in which (2)H NMR could provide significant insights: (i) the conformational and structural properties; (ii) the molecular dynamics and mobility; and (iii) the orientational order and aggregation/distribution of molecules in the different liquid-crystalline phases. In this work, some of the recent developments in this field are discussed, focusing on two main topics: (1) the molecular dynamics of the smectic liquid-crystalline phases formed by rod-like molecules and (2) the unusual orientational and dynamic properties of the new liquid-crystal line mesophases formed by banana-shaped molecules (BLCs)

Direct Measure of the Tilt Angle in de Vries‐Type Liquid Crystals through NMR Spectroscopy

“…In fact, other than for standard SmA phases, [25,[27][28][29] in the present case, the values of the S zz , defined in the local frame of the deuterated phenyl ring, are quite different at the two magnetic fields. Taking into account the known de Vries character [7][8][9] of 9 HL, we can explain this behavior by assuming the presence of a tilt angle of the para axis of the labeled phenyl ring (n p in Scheme 1) with respect to the magnetic field also in the SmA phase.…”

Nuclear magnetic resonance: a powerful tool to study liquid crystals

“…One of the most interesting properties of liquid crystals, namely, the phase and molecular orientational order can be obtained by analysing the temperature dependence of the 2 H quadrupolar splittings [22] or the trend of 1 H-13 C NMR dipolar splitting obtained by the separated local field (SLF) or the proton-detected local field (PDLF) NMR experiments [23]. In case of fluorinated compounds, 19 F is also a useful nucleus to investigate.…”

“…All this considered, the measured quantities, Δν d and Δν q , can be related to the local orientational order parameters, S zz and Δ (=S xx −S yy ), referred to the deuterated aromatic ring, through the following equations [22]:…”

“…3.3. In typical rod-like liquid crystals, the orientational properties of the aromatic core are usually assumed to be comparable to those of the whole molecule [22]; this approximation can be accepted in particular when the aromatic core is rigid. In the case of 9HL, the aromatic core is constituted by three phenyl rings connected each other by ester groups.…”

Nuclear Magnetic Resonance: Basic Principles and Liquid State Spectroscopy

“…Although this has been demonstrated for 13 C 1D-spectral data (Emsley, 2005), the spectral analysis is oftentimes too complex for obtaining dipolar couplings. Instead deuterium NMR spectra can be used to obtain orientational order information as these are dominated by nuclear electric quadrupole interactions (Emsley, 2005;Domenici, 2007). A further interesting aspect is to obtain information on the molecular orientation of small molecules embedded in a liquid crystalline medium.…”