Conformational Properties and Orientational Order of a de Vries Liquid Crystal Investigated through NMR Spectroscopy

Abstract

Solid-state and liquid-state NMR spectroscopic techniques are used to describe at molecular level the behaviour of a de Vries liquid crystal (namely the mesogen 9HL) at the SmA-SmC* transition, which is characterized by the absence of the layer shrinkage, typical of non-de Vries smectogens. Previous (2)H NMR studies on the same smectogen, performed at a different magnetic field (from 4.70 to 18.80 T), provided evidence of the occurrence of a tilt of one of the three phenyl rings, constituting the aromatic core of 9HL, at the SmA-SmC* phase transition. In this work, the study is extended to the whole rigid aromatic core of the 9HL. In particular, the variable temperature behavior of the mesogen studied by 1D (13)C NMR cross-polarization (CP) and 2D (1)H-(13)C PDLF (proton-encoded (13)C-detected, local field) NMR experiments made possible the characterization of the conformational and orientational properties in the two smectic phases. These results are compared with various proposed models invoked to describe the SmA-SmC* transition in de Vries smectogens at a molecular level.

Nuclear magnetic resonance: a powerful tool to study liquid crystals

“…In this way, the main orientational order parameters, Szz, of the three rings were determined, and their trend within the mesophases was further confirmed by an additional NMR experiment: the 1 H-13 C NMR PDLF [18]. From this comparative study, the behaviour of the three-ring core of the 9HL molecule was fully understood showing in the SmA phase that the three rings are almost collinear and they have similar values of Szz with an average tilt of 5-11°w ith respect to the normal to the SmA layers.…”

“…In this respect, NMR gave a contribution with several studies [18,27,33,34] performed on a rod-like 'de Vries' compound, namely 9HL (see Figure 4). One of the most fascinating properties of 'de Vries' systems is the absence of layer-shrinkage at the SmA-SmC* transition, coupled with a high response to the electric field in the SmA phase, the so called electroclinic effect.…”

“…In the last 10 years, in fact, the multinuclear approaches were applied more than in the past (see Figure 2(a) and (b)). Some recent examples of original studies applying multinuclear approaches are reported in references [17][18][19][20].…”

“…Multinuclear NMR experiments of 9HL in the isotropic, in the liquid crystalline phases and in the solid state were performed [18] starting from the assignment of the carbons of the 9HL through a combination of 2D NMR experiments ( 1 H-13 C HETCOR, 1 H-13 C PDLF and 13 C- 13 C INADEQUATE). The combination of these techniques allowed us the determination of the isotropic δ iso and anisotropic δ aniso components of the 13 C chemical shift hδi for each carbon site of the aromatic core.…”

“…where the 13 C chemical shift tensor principal elements (δ 11 , δ 22 and δ 33 ) and their relative orientation (through the angle β) were determined by combining data from the literature and from the 2D PASS NMR experiment [18] performed in the solid state of 9HL.…”