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

Abstract

De Vries-type phase transitions in liquid crystalline (LC) smectogens have recently attracted the attention of the liquid crystal community due to their potential in the development of new ferroelectric (FLC) and antiferroelectric (AFLC) electrooptic systems. Indeed, one of the major problems to the application of ferroelectric smectic materials is related to the layer shrinkage at the transition from the not-tilted SmA phase to the polar SmC* phase. This shrinking normally produces zigzag defects due to the opposite distribution of chevron configurations, thus limiting the performance and quality of electrooptic devices. In the last decades, several compounds have been found to behave differently from standard ones. In particular, they do not show any layer shrinkage at the SmA–SmC* transition, and the layer spacing remains substantially constant within the ferroelectric phase. Several models have been proposed to explain such an unconventional feature. The first interpretation, still partially valid, is the one described by the crystallographer de Vries, known as the “diffuse cone model”. 5] He proposed that in these systems LC molecules are tilted, with respect to the layer normal (“l” in Scheme 1), also in the SmA phase, but that the azimuthal angle is randomly distributed within the smectic layer. The SmA–SmC* phase transition is thus seen as a disorder–order transition in the azimuthal directions of the molecular tilt. Several experimental works on different liquid crystals showing de Vries-type transitions, basically confirmed this hypothesis, even though this does not exclude other possible explanations, such as the presence of partially interdigitation among consecutive smectic layers as well as conformational changes at the SmA–SmC* phase transition. For this reason, the nature of the de Vries transition remains a challenging aspect for researchers active in the material science field. However, in the recent years, fundamental progresses have been done in the comprehension of the de Vries -type materials. In addition to layer shrinkage (less than 5%) at the SmA–SmC* transition, the SmA phases formed by de Vries LCs have an uncommonly large electroclinic effect, which is strongly connected to the presence of significant tilt of molecules. The application of external electric fields determines an increase of the induced tilt angle, often defined as “optical tilt” to distinguish it from the molecular tilt, whose temperature dependence is well described by the Landau mean field theory. Moreover, the large soft-mode absorption detected by dielectric measurements, and the high birefringence are additional characterizing features of de Vries SmA phases. To our best knowledge, NMR spectroscopy has never been applied to de Vries-type LC systems, despite its great potential in the determination of both local and molecular properties, namely orientational and conformational ones. Herein, the first H NMR investigation of a de Vries liquid crystal compound [the (S)-hexyl-lactate derivative abbreviated as 9HL] selectively labeled in the aromatic core (Scheme 1), is reported. We monitored the trend of the tilt angle of the deuterated moiety within the SmA and SmC* phases with an NMR study at high magnetic field. This approach, recently used to investigate standard ferroelectric LCs, 28] takes advantage of the ability of high magnetic fields (H) in unwinding the supramolecular heliScheme 1. Molecular structure of the 9HL-d2 sample under investigation. Optimized geometry is displayed with the orientation of the local director of the deuterated phenyl fragment (np) and the layer normal (l) to the SmA planes (p).