Mario Cifelli

2H-NMR and SAXS of a ferroelectric liquid crystal : unwinding of the ferroelectric chiral helix by high magnetic fields

Abstract 2 H-NMR in three different magnetic fields and small angle X-ray scattering (SAXS) are employed to investigate the orientational order, molecular organisation and phase transitions of the chiral smectic liquid crystal (−)-(S)-[4-(2-methylbutyloxycarbonyl)phenyl] 4- n -heptyl-biphenyl-carboxylate (MBHB) suitably deuterated for 2 H-NMR measurements. Unwinding of the ferroelectric chiral helix by high magnetic fields is observed and discussed. Tilt angle values are obtained with the two different techniques and compared. The results show the importance of molecular conformation for the structural properties of this mesogen.

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

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.

NMR studies of the ferroelectric SmC* phase

In this work an attempt is made to compare and rationalise the structural and dynamic behaviour of some ferroelectric rod-like mesogens studied by our group in recent years, mainly by means of 2H- and 13C-nuclear magnetic resonance (NMR) spectroscopy. This comparison concerning the local orientational order, the average molecular conformation and the chiral smectic C (SmC*) phase structure, with a brief overview on the reorientational dynamic properties, allowed us to identify some common behaviour of these ferroelectric rod-like mesogens when decreasing the temperature from lower ordered phases to and within the SmC* phase. The main results obtained by means of NMR studies on these mesogens are rationalised and discussed in comparison with detailed discrete Fourier transform computations of their molecular conformations (in the limit of isolated molecules) with the purpose of enlightening the role of intermolecular and packing interactions in the ferroelectric phase with respect to the smectic A and nematic phases.

Direct measure of the tilt angle in de Vries-type liquid crystals through NMR spectroscopy.

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).

From the Synclinic to the Anticlinic Smectic Phases: a Deuterium NMR and Diffusion NMR Study

ABSTRACT The behaviour of the 2H NMR line-width throughout the whole mesophasic range of the ferroelectric liquid crystal “1-methylheptyl 4′-(4″-n-decyloxybenzoyloxy) biphenyl-4-carboxylate” (10B1M7) is here reported and discussed. Static Fringe Field NMR Diffusometry measurements from the isotropic to the anticlinic phase have been performed leading to a significant result concerning the tumbling diffusional coefficient (D⊥). These results have been compared to X-Ray measurements and 2H NMR line-widths in order to explain the peculiar behaviour occurring at the transition between the ferroelectric (FLC) and the antiferroelectric (AFLC) liquid crystalline phases.

Order and dynamics of a liquid crystalline dendrimer by means of 2H NMR spectroscopy

A complete Deuterium NMR study performed on partially deuterated liquid crystalline carbosilane dendrimer is here reported. The dendrimer under investigation shows a SmA phase in a large temperature range from 381 to 293 K, and its mesophasic properties have been previously determined. However, in this work the occurrence of a biphasic region between the isotropic and SmA phases has been put in evidence. The orientational order of the dendrimer, labeled on its lateral mesogenic units, is here evaluated in the whole temperature range by means of (2)H NMR, revealing a peculiar trend at low temperatures (T < 326 K). This aspect has been further investigated by a detailed analysis of the (2)H NMR spectral features, such as the quadrupolar splitting, the line shape, and the line-width, as a function of temperature. In the context of a detailed NMR analysis, relaxation times (T(1) and T(2)) have also been measured, pointing out a slowing down of the dynamics by decreasing the temperature, which determines from one side the spectral changes observed in the NMR spectra, on the other the observation of a minimum in the T(1).

Chemical-physical and in vivo evaluations of a self-assembling amphiphilic peptide as an injectable hydrogel scaffold for biomedical applications

The self-aggregation and gelation of an amphiphilic peptide (C17H35CONH–A4G3ERGD, peptide amphiphile) were studied by light scattering, viscometry, nuclear magnetic resonance diffusometry, and atomic force microscopy. The peptide amphiphile critical aggregation concentration was evaluated to be 16 and 60 µM by light scattering and viscometry, respectively. The observed difference was attributed to the larger sensitivity of the latter technique to the presence of long fibrils. The addition of one equivalent or more of divalent cations (Ca2+ and Mg2+) to peptide amphiphile formed dense incoherent hydrogels. Based on the atomic force microscopy and nanoindentation data, both the hydrogel morphology and stiffness were independent of the cation type and peptide amphiphile concentration. However, gel stiffness increased on increasing Ca2+/peptide amphiphile molar ratio while a parallel decrease in the apparent water diffusion rate was observed by nuclear magnetic resonance diffusometry. The dispersions of endothelial progenitor cells in the peptide amphiphile hydrogels were evaluated in vivo on a rat tissue hypoxia model. Significant capillary formation at the injection site was observed by tissue appearance and histological examination, which indicated endothelial progenitor cell/peptide amphiphile hydrogel-enhanced angiogenesis in ischemic tissue.

Translational self-diffusion in the smectic phases of ferroelectric liquid crystals: An overview

In this work, we present our recent results obtained in the field of translational self-diffusion studies by means of 1H NMR diffusometry in smectic phases formed by calamitic chiral liquid crystals. In particular, the diffusional behavior of chiral smectic phases with different clinicity, such as the ferroelectric and antiferroelectric phases, will be discussed on the basis of the results obtained on three different chiral smectogens. This research demonstrates that, despite previous assertions, out-of-plane translational diffusion in smectic phases is poorly affected by the clinicity of the phases, showing no discontinuity at the transition from synclinic to anticlinic structures, typical of the smectic C* phases. We can conclude that diffusion basically reflects the layered smectic structure where in-plane diffusion is much faster and less hindered than the out-of-plane one.

Dynamics of a Liquid Crystal in its Smectic A Phase from Angle Dependent Deuterium Spin Relaxation Measurements

The Zeeman and quadrupolar spin-lattice relaxation times of the aromatic deuterons of OAB-d 12 were measured throughout the smectic A phase at two different frequencies (15 and 46.04 MHz). At 15 MHz measurements were performed as a function of the angle between the phase director and the external magnetic field. Eight spectral densities were consequently determined, and were analyzed by means of a global target fitting procedure using diffusional models for the overall molecular reorientations and the internal motions. Diffusional coefficients of about 10 9 s −1 were found for molecular spinning and phenyl ring rotations, three orders of magnitude higher than for molecular tumbling.

Microscopic Organization and Tilt angle in Smectic A and Chiral Smectic C* Phases: Characterization and Orientational Order by 2H-NMR and Electric Polarization Measurements

Abstract 2H-NMR spectroscopy is employed to investigate the orientational order, tilt angle and layers organization of the chiral smectic liquid crystal (-)-(S)-[4-(2-methylbutyloxy carbonyl) phenyl] 4-n-heptyl-biphenyl carboxylate-d8 (MBHB-d8) in the magnetic field. The electrooptical behaviour of this material has also been investigated. The results from these two different techniques show the importance of molecular conformation for the electric and structural properties of this mesogen.