Molecular dynamics in the smectic a and C phases in a long-chain ferroelectric liquid crystal: 2H NMR, dielectric properties, and a theoretical treatment

In this work, the rotational-diffusion coefficients D(parallel) and D(perpendicular) for the ferroelectric smectogen (+)-(S)-4-[4'-(1-methylheptyloxy)] biphenyl 4-(10-undecenyloxy)benzoate have been studied by means of 2H NMR spectroscopy in the smectic C phase, using a new theoretical approach (Domenici,V.; Geppi, M.; Veracini, C. A. Chem. Phys. Lett. 2003, 382, 518). The analysis of spin-lattice relaxation times has been performed in terms of the diffusional constant and the activation energy of the internal and overall molecular-reorientational motions, and the results are compared to the smectic A (SmA) phase. Moreover, from the 2H NMR data in the SmA phase, the dielectric permittivity and the dielectric relaxation time functions are investigated using a theoretical approach. The longitudinal and transverse components of the real Rchigammaomega and imaginary chigammaomega (gamma = parallel, perpendicular) parts of the complex susceptibility tensor and the nematic-like rotational-viscosity coefficients, lambda2 and lambda5, are calculated.     

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).     

Collective dynamics of a ferroelectric smectogen in geometrical confinement

We investigate the influence of geometric confinement on the dynamics in a ferroelectric smectic C* material. Molecular and collective relaxation processes are studied by means of broadband dielectric spectroscopy. In microporous material (pore size 200nm) the Goldstone mode (GM) is still observed, but it is shifted to lower frequencies compared with bulk samples. In nanoporous material (pore size 7.5 nm), the collective reorientation modes are completely suppressed. In all samples, the molecular processes seem to be uninfluenced by the confinement.     

2H NMR spectroscopy of liquid crystals: structure and orientational order of a chiral smectogen in its A,C* and J* phases

²H NMR spectroscopy is employed to investigate the orientational order, molecular structure and phase transitions of the chiral smectic liquid crystal 1-methylheptyl 4'-(4-n-decyloxybenzoyloxy)biphenyl-4-carboxylate (10B1M7), showing smectic A, C* and J phases, as well as several sub-smectic C* phases. Two optically pure, differently deuteriated isotopomers have been purposely synthesized and studied.     

2H NMR spectroscopy of liquid crystals: structure and orientational order of a chiral smectogen in its A, C* and J* phases

²H NMR spectroscopy is employed to investigate the orientational order, molecular structure and phase transitions of the chiral smectic liquid crystal 1-methylheptyl 4'-(4-n-decyloxybenzoyloxy)biphenyl-4-carboxylate (10B1M7), showing smectic A, C* and J phases, as well as several sub-smectic C* phases. Two optically pure, differently deuteriated isotopomers have been purposely synthesized and studied.     

Solubility of thioindigo dopants in a smectic liquid crystal host evaluated by 2H NMR spectroscopy

The solubility of two partially deuterated thioindigo dopants in a smectic liquid crystal host was evaluated by variable temperature ²H NMR spectroscopy and polarized microscopy. 2H NMR spectra showed that the dopant (±)-6,6'-bis(2-octyloxy)-5,5-dinitrothioindigo-d ₆ forms a homogeneous solution with the smectic phases of the liquid crystal host (±)-4-(4-methylhexyloxy)phenyl 4-decyloxybenzoate (PhB) up to its saturation point of 3 mol %. These results are consistent with polarized microscopy observations of the dopant crystallizing out of solution upon reaching a concentration of 3 mol %. On the other hand, ²H NMR spectra of (±)-5,5'-dichloro-6,6'-bis(2-octyloxy)thioindigo-d ₆ dissolved in PhB showed evidence of a partitioning of the solution between smectic and isotropic microdomains, which increases with increasing dopant concentration—from 1.2 to 9.1 mol %. To a large extent, this smectic/isotropic microphase separation could not be detected by polarized microscopy. These results suggest that ²H NMR spectroscopy can provide a more accurate determination of the occurrence and extent of microphase separation in doped liquid crystal samples.     

Molecular dynamics and ordering of pyridine in its cyclophosphazene inclusion compound as evaluated by solid state 2H NMR spectroscopy

Variable temperature 2H NMR experiments (line shape analysis, relaxation studies) were carried out on the pyridine-d5-tris-(1,2-dioxyphenyl)-cyclotriphosphazene inclusion compound in the temperature range between 110-300 K. It is found that the pyridine guests are highly mobile throughout the whole temperature range covered here. The observation of three superimposed 2H NMR signals can be understood in terms of a particular (motionally averaged) orientation of the pyridine molecules, which is a consequence of the molecular symmetry of the pyridine guests and the imposed channel restrictions. The experimental data are consistent with a combined rotation on cone-small angle fluctuation model, which assumes a fast molecular reorientation between two superimposed cones with an opening angle for the inner cone between 59-73 degrees (angle of fluctuation between 1-3 degrees ). On the basis of this model assumption it is possible to reproduce both the experimental 2H NMR line shapes and the spin-lattice relaxation data in a quantitative way. The analysis of the partially relaxed spectra (inversion recovery experiments) yields the correlation times for this overall motional process. They follow an Arrhenius behavior from which an activation energy of 8.7 +/- 0.4 kJ mol(-1) is derived. The results are discussed in the framework of the published data for related systems.     

Molecular dynamics of discotic charge-transfer complexes, dielectric spectroscopy and 2H NMR studie

Using a combination of solid state ²H NMR spectroscopy on selectively deuteriated samples and dielectric spectroscopy, the molecular dynamics of discotic charge-transfer (CT) complexes were investigated. These complexes show particular thermodynamic and flow properties. Considered were mixtures of low molar mass donors and acceptors, low molar mass donors with main chain acceptor polymers and covalently linked donor-acceptor twins with different lengths of the spacer. A main result is that correlated rotational motions of discotic molecules or groups about the columnar axis are observed in all systems except for the twin with the short spacer. This type of motion seems to be a general feature of columnar phases. The non-discotic acceptor which is incorporated in the columns participates in this motion. The twin possessing a long spacer displays at high temperatures an additional process: it performs a diffusion process between the columns. A further result is that broad biphasic regions exist in CT mixtures at the transition from the discotic to the isotropic state.     

The temperature behaviour of a ferroelectric smectic mixture and the detection of ferroelectric switching by NMR

The orientational order of a ferroelectric mixture has been measured by proton NMR. The alignment of the mesogenic units is reflected in the splitting of the NMR signal into a doublet. Up to a cell thickness of 200 μm, it was possible to produce well-oriented layers in the bookshelf geometry by magnetic orientation of the substances in the B₀-field. The angular dependences of the line width of the NMR signal on the tilt angle of the director have been calculated. A ferroelectric switching was detected by measuring the angular dependence of the line width in the switched state. The tilt angle and the orientational order parameter S of the bookshelf samples were estimated at various temperatures.     

Molecular dynamics in paramagnetic materials as studied by magic-angle spinning 2H NMR spectra

A magic-angle spinning (MAS) 2H NMR experiment was applied to study the molecular motion in paramagnetic compounds. The temperature dependences of 2H MAS NMR spectra were measured for paramagnetic [M(H2O)6][SiF6] (M=Ni2+, Mn2+, Co2+) and diamagnetic [Zn(H2O)6][SiF6]. The paramagnetic compounds exhibited an asymmetric line shape in 2H MAS NMR spectra because of the electron-nuclear dipolar coupling. The drastic changes in the shape of spinning sideband patterns and in the line width of spinning sidebands due to the 180 degrees flip of water molecules and the reorientation of [M(H2O)6]2+ about its C3 axis were observed. In the paramagnetic compounds, paramagnetic spin-spin relaxation and anisotropic g-factor result in additional linebroadening of each of the spinning sidebands. The spectral simulation of MAS 2H NMR, including the effects of paramagnetic shift and anisotropic spin-spin relaxation due to electron-nuclear dipolar coupling and anisotropic g-factor, was performed for several molecular motions. Information about molecular motions in the dynamic range of 10(2) s(-1)相似文献   

Epitaxial growth alignment of ferroelectric smectic C phases on rubbed polymers

Several of the polymers reported for surface alignment of ferroelectric smectic C phases in the literature are tested with a ferroelectric room temperature mixture. The results for this material and the reported findings in the literature are compared to the known crystal structure of the polymers. It is found that polymers with triclinic or monoclinic crystals give good alignment and bistability for the smectic C phase. Other crystal structures or non-crystalline polymers give a poorer performance. The mechanisms for creating a highly crystalline polymer surface are discussed, and the epitaxial growth of smectic phases on the crystalline surface is shown to be in accord with experiment.     

Molecular dynamics simulations of a liquid crystalline molecule in the smectic A and E phases and in vacuum

Molecular dynamics simulations are performed in this work at 393 and 323 K for a mesogenic molecule ( R )-1-methylheptyl 4\[4-(2-allyloxyethoxy)biphenyl-4-carbonyloxy]benzoate in the simulated smectics A and E, respectively, and in a vacuum at 300 K, for a period of 1.0ns. The trajectories obtained from molecular dynamics simulations allow us to investigate the dynamical behaviour of this mesogenic molecule in the simulated smectic phases. This dynamical behaviour of a single molecule is presented using the distributions of dihedral angles and rotational diffusion around the C-axis defined by the simulated cells. Simulation results indicate that, except for the bonds near the end of the spacer segment, the dihedral angles all exhibit a single Gaussian-like distribution in the smectic A and E phases. Fluctuations of a dihedral angle about its mean value are more restricted in the smectics A and E than in those simulated in a vacuum. The average value of the fluctuations of the dihedral angles at the bonds in the spacer is found to be about 2 fold larger than that of fluctuations in the tail of the same molecule in the smectic A and E phases. In the smectic A phase, the distribution of orientations of a molecule about its long axis in a 36 molecule cell in which the outer molecules are fixed is found to have three distinct peaks. This result shows that the orientational fluctuations of single molecules are limited by confinement due to neighbouring molecules, i.e. that the layers have short-range structural correlations. The orientational distributions show larger fluctuations at the ends of the molecules.     

Study of water dynamics and distances in paramagnetic solids by variable-temperature two-dimensional 2H NMR spectroscopy

A recently proposed two-dimensional (2)H NMR experiment is used to measure the (2)H (spin I=1) quadrupolar and paramagnetic shift anisotropy interactions in powdered CuCl(2).2D(2)O as a function of temperature. The principal components of the quadrupolar and paramagnetic shift anisotropy tensors and the Euler angles describing the orientations of the tensors in the molecular frame are determined at each temperature. For this purpose an analytical approach is introduced to extract desired parameters from motionally averaged two-dimensional line shapes where the averaging is introduced by rapid 180 degrees flips around C(2) axes of D(2)O molecules. This approach can be readily applied to study various materials containing water of crystallization. It is also clearly shown that the rapid continuous rotation of D(2)O molecules around their C(2) axes is not taking place in the studied solid in the range of temperatures between 209 and 344 K. Once the paramagnetic shift anisotropy of a deuterium atom is measured accurately it is used to estimate the distance between deuterium and the nearest copper atom bearing an unpaired electron. Excellent agreement is found between structural parameters obtained in this study and those provided by neutron and x-ray diffraction, showing that the paramagnetic shift anisotropy is a sensitive probe of distances in paramagnetic solids.     

Recent advancements in understanding thermotropic liquid crystal structure and dynamics by means of NMR spectroscopy

Recent progresses of NMR spectroscopy to the study of liquid crystals and related ordered systems are surveyed. Particular attention will be devoted to review suitably tailored NMR experiments and their applications on those systems (e.g. biaxial nematics, chiral smectics, V shaped mesogens, liquid crystalline elastomers) which are presently subject of active and innovative research.     

Orientational order and dynamics of fluorescent probes in aligned lyotropic phases by fluorescence depolarization spectroscopy. Perylene in a nematic discotic phase. A comparison with 2H NMR

Abstract

The orientational order and dynamics of perylene in the nematic discotic phase (N_dII) 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 <P ₂> and <P ₄> for the axis of the transition moments of perylene, together with the rotational correlation time Θ_R. All these quantities assume reasonable values and show regular trends as functions of temperature. In particular, <P ₂> is in good agreement with that determined by ²H NMR.     

Molecular arrangement in a chiral smectic liquid crystal cell studied by polarized infrared spectroscopy

The structure of the chiral smectic C phase (SmC * ) of the mesogen MHP10CBC in a homogeneously aligned thin cell, that exhibits V-shaped-like switching in a certain frequency range, was studied using polarized Fourier-transform infrared spectroscopy and optical microscopy. The molecular orientational distributions were analysed quantitatively in terms of dichroic parameters of the absorbance profiles, by taking into account the orientational properties of the transition moments for several phenyl and carbonyl bands. The polar angles of the transition moments, with respect to the molecular long axis and their azimuthal orientational parameters used in calculations, were determined from the infrared dichroic data for helical and electrically unwound structures in a thicker cell of this material. For a sufficiently thin cell in the SmC * phase at zero electric field, the results on the azimuthal orientational distribution of the director over a ferroelectric liquid crystal cone, with respect to the substrate normal, are in agreement with the model of a partly twisted SmC * structure. The voltage-dependent dichroic parameters in relation to the direction and the degree of the preferable orientation of the molecules in a sample are compared for the SmC * and the SmC * A phases.     

Molecular arrangement in a chiral smectic liquid crystal cell studied by polarized infrared spectroscopy

The structure of the chiral smectic C phase (SmC*) of the mesogen MHP10CBC in a homogeneously aligned thin cell, that exhibits V-shaped-like switching in a certain frequency range, was studied using polarized Fourier-transform infrared spectroscopy and optical microscopy. The molecular orientational distributions were analysed quantitatively in terms of dichroic parameters of the absorbance profiles, by taking into account the orientational properties of the transition moments for several phenyl and carbonyl bands. The polar angles of the transition moments, with respect to the molecular long axis and their azimuthal orientational parameters used in calculations, were determined from the infrared dichroic data for helical and electrically unwound structures in a thicker cell of this material. For a sufficiently thin cell in the SmC* phase at zero electric field, the results on the azimuthal orientational distribution of the director over a ferroelectric liquid crystal cone, with respect to the substrate normal, are in agreement with the model of a partly twisted SmC* structure. The voltage-dependent dichroic parameters in relation to the direction and the degree of the preferable orientation of the molecules in a sample are compared for the SmC* and the SmC* A phases.