C. A. Veracini

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.

Dynamics of 4,4'-di-n-heptylazoxybenzene (HAB) studied using dielectric and 2H NMR relaxation measurements

Results of studies of 4,4′-di-n-heptylazoxybenzene (HAB) in the isotropic, nematic and smectic A phases are presented. Two experimental methods were employed: broad band dielectric spectroscopy and nuclear magnetic resonance spectroscopy. The complex dielectric permittivity, ϵ*(ω)=ϵ′(ω)−iϵ″(ω), was measured in the frequency range 1 kHz–4 GHz. This allowed two main relaxation processes to be separated in all the phases studied: the low frequency (l. f.) process connected with molecular reorientations around the short axes, and the high frequency (h. f.) process connected with the rotations around the long axes. The corresponding relaxation times and activation enthalpies were obtained. The l. f. relaxation time changes step-wise at the phase transitions, whereas the h. f. relaxation time passes smoothly through all the phases. The measurement of 2H spin-lattice relaxation times was carried out throughout the mesophase range at 61.38 MHz. These data were analysed together with the relaxation times measured at 10.00 and 46.04 MHz, available from previous studies. Using suitable theoretical models the principal components of the diffusional tensor, D ‖ and D ⊥, as well as the diffusion coefficients D R relative to the internal rotation of the phenyl rings, were determined. The results of both studies are compared and discussed.

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.

Orientational order and dynamics of fluorescent probes in aligned lyotropic phases by fluorescence depolarization spectroscopy. Perylene in a nematic discotic phase. A comparison with2H NMR

Abstract The orientational order and dynamics of perylene in the nematic discotic phase (NdII) of the potassium laurate/KCl/decanol/water system has been investigated by fluorescence depolarization (FD) spectroscopy. The order parameters of perylene-d 12 in the same phase had been previously determined by 2H NMR. FD experiments were performed with the phase-modulation technique in the temperature range between 10°C and 50°C. At each temperature we could determine and for the axis of the transition moments of perylene, together with the rotational correlation time ΘR. All these quantities assume reasonable values and show regular trends as functions of temperature. In particular, is in good agreement with that determined by 2H NMR.

2H NMR spectra of 2-(p-heptylbenzoyloxy)-5-(p-heptylbenzenazo)tropone. A new sigmatropic liquid crystal

Abstract Liquid crystals incorporating in their molecular framework a seven-membered ring are still relatively rare [1]. Recently the synthesis and thermal behaviour of a series of liquid crystal materials having a tropone moiety in their mesogenic core have been reported [2–4]. These mesogens, based on a 2-(acyloxy)tropone core structure, show intramolecular migration of the acyl substituents between the two oxygen atoms at C-1 and C-2, an effect already known for simple 2-(acyloxy)tropones in their isotropic solutions [5]. This migration involves a concerted [1, 9]-sigmatropic rearrangement [2]. This rearrangement could play a major role in determining the properties of the mesophases: it has been suggested in fact that, because of this rearrangement, the mesogenic molecules acquire a mean rod-like shape which can sustain the mesophase formation [2].

Deuterium Spin-Lattice Relaxation in Thermotropic Liquid Crystals. Study of 4,4′-di-n-heptylazobenzene, 4,4′-di-n-heptylazoxybenzene and 4,4′-di-n-octylazoxybenzene

Abstract The deuterium Zeeman spin-lattice relaxation times TIZ in 4,4′-di-n-heptylazobenzene (HB), 4,4′-di-n-heptylazoxybenzene (HAB), and 4,4′-di n-octylazoxybenzene (OAB) deuterated on the aromatic rings and on the first methylene group of the chains were measured at 10 and 46.04 MHz by means of the inversion-recovery technique. Measurements were made throughout the mesomorphic range and also in the isotropic phase. The relaxation rates show a discontinuity at the isotropic-nematic transition for all deuterons not ascribable to the onset of orientational order. The trends of InR are linear within the different phases except near phase transitions, and allow to obtain apparent activation energies for the overall motions of the different C—D bonds.

Rotational dynamics of a chiral mesogen by 2H NMR study: can it be anomalous?

Deuterium NMR spectroscopy is used to study a ring-deuteriated chiral liquid crystal 4-(2-methylbutyl)oxycarbonylphenyl 4-(10-undecenyloxy)benzoate. The quadrupolar and proton-deuteron dipolar splittings, and deuteron quadrupolar and Zeeman spin-lattice relaxation times were measured as a function of temperature in the smectic A phase at two different Larmor frequencies. The derived spectral densities of motion at different temperatures were analysed simultaneously using a rotational diffusion model which also includes internal ring rotations. Motional parameters (D ⊥, D ∥, D R) and order parameter tensors (Szz , Sxx -Syy ) were obtained. Although the present data seem insufficient to draw a definitive conclusion, we believe that it is possible for this particular chiral molecule to have D⊥ >D∥ , which is different from non-chiral rod-like liquid crystals.