NMR field-cycling study of proton and deuteron spin relaxation in the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl

NMR field-cycling measurements of the deuteron spin relaxation dispersion T₁(v) for the fully deuteriated nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB-d₁₉) over a broader Larmor frequency range (v≈10 kHz to 30 MHz) than reported so far in the literature basically confirm the magnetic relaxation mechanisms previously observed by frequency dependent proton spin studies of various nematogenic molecules, namely collective nematic modes of the director field in the kilohertz regime, and anisotropic reorientations of individual molecules (mainly self-diffusion for the protons and mainly rotations about the long axis for the deuterons) in the megahertz range. Within the experimental error limits such a model allows a self-consistent interpretation of the available deuteron and proton T₁(v) results for deuteriated or protonated 5CB, respectively. In particular, the magnitudes of the measured order fluctuation contributions are in approximate accordance, i.e. within a factor of less than two, with theoretical estimates from NMR line splittings and the relevant material parameters. More exact and more extensive deuteron studies are needed to locate the origin of the observed minor inconsistency.     

Angular and frequency dependent spin relaxation study of liquid crystalline cyanobiphenyls

NMR field-cycling measurements of the Larmor frequency (v) and angular (Δ) dependences of the longitudinal proton spin relaxation time T₁ for the nematic liquid crystals 5CB and 8CB allow a more detailed analysis of the underlying molecular motions than data available previously. All T₁ (v, Δ) dispersion profiles essentially distinguish three frequency ranges where T₁ is governed by either local field effects, collective motions (director order fluctuations), or rotational and translational diffusion of individual molecules or molecular groups, respectively. The angular dependence supports and extends previous conclusions about the significance of the order fluctuation term at low (kHz) and high (MHz) Larmor frequencies; in addition it is the basis for the disentanglement of local field effects, which involve Jeener's dipolar relaxation, and of the sophisticated rotational relaxation models suggested in the literature by Dong, Nordio and Vold. It is found that Vold's third rate concept gives the best explanation of the measurements. The results on the rotational diffusion processes essentially agree with deuteron studies from the literature, but also reveal clear distinctions with regard to the anisotropy parameter σ, essentially due to the improved separation from the order fluctuation contribution.     

The influence of director fluctuations on molecular reorientation of a small probe molecule in a liquid-crystalline environment

The influence of director fluctuations on a probe molecule dissolved in a nematic liquid crystal is calculated to all orders using the gaussian properties of the director field. Consequences for the N.M.R. spectrum and relaxation through quadrupolar interaction of the probe molecule are indicated. It is found that the equilibrium distribution of molecular orientations, and consequently quadrupolar spectral splittings are virtually unchanged by fluctuations in the director field. The spectral densities are also obtained and it is shown that in addition to J₀₁ also J₀₀ and J₀₂ become frequency dependent in the N.M.R. frequency range. The frequency dependence of J₀₀ and J₀₂ has a logarithmic contribution; the magnitude of the frequency dependent contribution to J₀₂ is, however, insufficient to explain experimental results.     

Statistical model of polar nematic polymers

The interaction in specially designed polymers may be dominated by the first rank P₁(cos β) type potential instead of the second rank P₂(cos β) type, as in conventional liquid crystal polymers (P₁(cos β) and P₂(cos β) are the first and second Legendre polynomials, respectively of the angle made by the polymer segments with respect to the preferred direction or director). The ordering of the polymers is envisaged in terms of the worm-like theory. The polymers show a polar nematic phase, that is a ferroelectric phase. These polymers have certain interesting properties, such as a second order polar nematic—isotropic transition, and related critical features. These materials are expected to exhibit giant dielectric responses. For more general polymer systems with interactions of both P₁(cos β) and P₂(cos β) types, we predict a peculiar phase behaviour.     

Viscoelastic coefficients of glass-forming nematics

In this paper, we report measurements of the viscoelastic properties of nematic liquid crystals which exhibit a glass transition in the nematic phase. We have studied the Freedericksz transition in planar cells with a magneto-optical method. K₁ was determined from the critical field, and the rotational viscosity, γ₁, from the response time for the director orientation by the external field. We found a temperature dependence of γ₁ of the Vogel type, with absolute values ranging over several orders of magnitude, and K₁ values similar to those of conventional thermotropic low molar mass nematics.     

Light scattering from a nematic monodomain in an electric field Twist elastic constant and viscosity coefficient of nematic polymer-solvent mixtures

The light scattering technique was used to investigate the viscoelastic parameters characterizing director twist distortions in miscible nematic mixtures of 5CB (pentacyanobiphenyl) with two side chain liquid crystal polymers and a main chain liquid crystal polymer. By applying an AC electric field to homeotropically-aligned nematic monodomains of the mixtures, the field-dependent scattering intensities and director orientation fluctuation relaxation rates yield, respectively, the twist elastic constant K₂₂ and viscosity coefficient γ₁. The results directly demonstrate that the addition of liquid crystal polymers causes substantial decreases of the relaxation rates for dynamic light scattering from the twist mode and these changes are due to small decreases in K₂₂ coupled with large increases in γ₁. The decrements in K₂₂ are comparable for both side chain and main chain liquid crystal polymers. The relative increase in the twist viscosity for the side chain liquid crystal polymers is much smaller than those of main chain polymers. A theoretical model is used to qualitatively interpret the difference between the viscous behaviour of the twist mode for both side chain and main chain liquid crystal polymers in a nematic solvent.     

Molecular motion and phase transition in perovskite-type (CH3)nNH4−nSnCl3 (n=1–4) studied by proton NMR spin–lattice relaxation

Powder X-ray diffraction, ¹¹⁹Sn NMR spectra, and ¹H NMR spin–lattice relaxation times, T₁, were measured for (CH₃)_nNH_4−nSnCl₃ (n=1–4). From the Rietveld analysis, it is shown that all four compounds crystallize into deformed perovskite-type structures at room temperature. The temperature dependence of ¹H T₁ was analyzed in terms of the CH₃ reorientation and other motions of the whole cation. Except for the phase transition in CH₃NH₃SnCl₃, which is from monoclinic to rhombohedral at 331 K, ¹H T₁ was continuously changed at other phase transitions in this compound as well as in the n=2–4 compounds, suggesting that the transitions are not caused by the change of the motional state of the cation but by an instability of the [SnCl₃]_nⁿ⁻ perovskite lattice.     

H and H NMR studies of cyclohexane nanocrystals in controlled pore glasses

The formation and behaviour of cyclohexane and cyclohexane-d₁₂ nanocrystals in mesoporous solids of well-defined dimensional constraints are studied by ¹H and ²H NMR. The NMR line widths, spin–spin relaxation times (T₂), spin–lattice relaxation times (T₁) and diffusivities (D) were measured as a function of temperature, and the results are discussed with reference to the values obtained for the bulk materials. The confined solids exhibit substantial changes in the phase behaviour and molecular dynamics. Thus, the line-shape measurements reveal a two-phase system consisting of a highly mobile component at the surface of the pore and a plastically crystalline phase in the centre of the pore. The liquid-like surface layer in the mesopores is observable well below the reduced transition temperature of the confined cyclohexane. However, the T₂ and diffusion measurements show that the mobile phase also embraces a minor component attributed to non-frozen liquid in pockets or offshoots.     

NMR relaxation of rigid molecules in the nematic phase of a discotic liquid crystal

Frequency and temperature dependent NMR relaxation measurements were performed on deuteriated benzene, pyrene and triphenylene dissolved in the nematic phase of a discotic liquid crystal. The results show a strong frequency dependence of the spectral densities. Based on the symmetries of the system and the usual model for director fluctuations this frequency dependence should be equal for J₁ and J₂. From fitting the commonly used model of rotational diffusion and director fluctuations to the data we see that this is not the case for benzene and triphenylene, even though the fits themselves are satisfactory. Values for the elastic constants, effective viscosity and translational diffusion in similar discotic liquid crystals do not account quantitatively for the frequency dependence of benzene. For both pyrene and triphenylene quantitative comparison was impossible due to lack of translational diffusion data. We also find that the so-called cut-off wave-length is of the order of the dimensions of the liquid crystal molecules, just as in ordinary nematics.     

Molecular dynamics study of the ferroelectric liquid crystal CI IPNOC by proton spin-lattice relaxation

Proton spin-lattice relaxation studies were carried out in the S_A and S*_C phases of the liquid crystal CI IPNOC using both conventional and fast field cycling NMR techniques. T₁ dispersion curves were obtained at two different temperatures for each mesophase covering frequencies from 10² to 3 × 10⁸ Hz. In both mesophases the T₁ data can be described assuming the presence of three different relaxation mechanisms, namely local molecular rotations, molecular self-diffusion and collective motions. The self-diffusion constant D₁ was evaluated for several temperatures and the activation energy associated with the diffusion process was obtained. The expected contribution of the soft-mode for the spin-lattice relaxation could not be separated from the contribution of other collective motions. The correlation times associated with the rotations around the molecular long axis and with the fluctuations of this axis were evaluated for both the S_A and the S*_C phases.     

Molecular geometry and excited electronic states : Part XIII. Conformational geometries of 9,10-diphenyl anthracene in different electronic states and its electronic spectral behaviour

The conformational geometries of 9,10-diphenyl anthracene in the S₀, S₁, and T₁ electronic states determined by means of the Warshel—Karplus method are presented. Based on the theoretical equilibrium geometries of the con- and dis-twisted conformations the calculated electronic transition energies and oscillator strengths are used for an interpretation of the electronic spectral behaviour in absorption and fluorescence. The corresponding theory-experiment comparison performed is satisfactory.     

Dielectric and elastic properties of a nematic phase induced by charge transfer interaction in a binary system containing disc-shaped molecules

The dielectric constants and the elastic coefficients for splay (K₁) and bend (K₃) of the charge transfer induced nematic (N_c) phase of tridecyl pentakis(phenylethynyl)phenyl ether (1) doped with different amounts of 2,4,7-trinitrofluorenone (2) were determined by studying the electric field induced bend deformation using the capacitance method. A negative dielectric anisotropy was observed. For the bend elastic constant K₃ values up to 22 × 10^-12 N are found which are one order of magnitude higher than the respective values of discotic nematic (N_D) phases. Values of 0·6-0·8 are obtained for the ratio K₁/K₃; these show a minimum for the equimolar complex.     

X-ray study of the nematic and smectic A phases of the cyano-substituted pyridines

We report an X-ray study of lamellar ordering in the nematic and smectic A phases of a homologous series of polar liquid crystals, the 2-(4-alkylphenyl)-5-cyanopyridines (nCP). Experiments were carried out using a diffractometer with a linear position sensitive detector. In the nematic and smectic A phases of the nCP and their mixtures with non-polar 4-n-butyl-4'-methoxyazoxybenzene two types of layering were found. One corresponds to the fluctuations of the smectic density wave with a monolayer wavevector q₁, and the other is due to the partial bilayer fluctuations with the incommensurate wavevector q₂ ≠ q₁/2. The temperature dependences of the X-ray scattering intensity and the longitudinal correlation length for both types of layering in the nematic phase are presented. The critical behaviour in the vicinity of the smectic A-nematic phase transition occurs for a fluctuation mode, either q₁ or q₂, depending on the position on the liquid crystal phase diagram. The influence of the molecular structure of cyano-substituted pyridines on the formation of layered structures of different types is also discussed.     

γ-巴豆酰内酯激发态动力学的共振拉曼光谱和完全活性空间自洽场(CASSCF)计算研究

采用共振拉曼光谱和完全活性空间自洽场(CASSCF)方法研究了γ-巴豆酰内酯的光吸收S₂态的结构动力学和衰变机制. 采用含时密度泛函理论方法结合光谱实验确认了紫外光谱和振动光谱. 获得了涵盖A-带吸收的4个激发波长下的共振拉曼光谱. 用CASSCF计算得到了S_1,min, S_2,min, T_1,min, T_2,min和T_3,min及其相关势能面交叉点的结构与能量. 研究了A-带共振拉曼光谱强度模式与S_2,min和CI(S₂/S₁)交叉点结构的关系. 借助El-Sayed规则分析了各系间窜跃路径的效率, 提出了γ-巴豆酰内酯从S_2,FC弛豫到基态S₀的2条主要路径: 内转换路径和系间窜跃路径.     

Gradient expansion of the kinetic energy density functional: Local behavior of the kinetic energy density

Calculations with Hartree—Fock electron densities for the rare gas atoms He through Xe show that the gradient expansion for the kinetic energy functional, T[] = T₀[] + T₂[] + T₄[] + … = ∫t() dτ, approximates the kinetic energy by averaging over the shell structure present in the true local kinetic energy density, t(), and that the accuracy of the gradient expansion improves with increasing atomic number. Components of t(), t₀(), t₂() and t₄(), are exhibited and discussed. The defined function t() is everywhere positive.     

Intersystem crossing at singlet conical intersections

The energy of the lowest triplet state of organic molecules is intermediate between the ground state and the first excited singlet. At the S₁/S₀ conical intersection, the two singlet states are degenerate. It is shown that for some molecules (ethylene, benzene, toluene and pyrrole) the T₁ state is also degenerate with the two singlet states. Moreover, the spin orbit coupling matrix element at this structure is necessarily large, so that intersystem crossing can be quite efficient. If the lowest triplet state is repulsive (as in the studied molecules) it may significantly contribute to the dissociation yield under certain experimental conditions.     

X-ray investigation of the nematic and reentrant nematic phases of N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4'-cyanoaniline

We have performed an X-ray scattering study of the nematic-smectic A and reentrant nematic-smectic A phase transitions in N-[(4-n-octyloxybenzoyloxy)-salicilidene]-4'-cyanoaniline (OOBOSCA). A diffractometer with a linear position sensitive detector was used. The results show that the smectic phase in OOBOSCA is of the A_d type with an interlayer spacing incommensurate with the molecular length L; d ∽ 1·2L. In the reentrant nematic phase two types of fluctuation modes were found. One of them corresponds to the monolayer wavevector q₁ ∽ 2π/L, and the other is due to the partial bilayer fluctuations with the wavevector q₂ ∽ 0·8q₁. The temperature dependences of the interlayer spacing, X-ray scattering intensity and longitudinal correlation length for both types of layering in the reentrant nematic phase are presented. The change of the fluctuation regime from S_Ad, to S_Cd type with decreasing temperature in the reentrant nematic phase of OOBOSCA was found. The results are discussed on the basis of models with competing order parameters. The influence of alkyl chain flexibility on the stability of a partial bilayer smectic phase is also considered.     

Molecular vibrations of complexes with trigonal ligands. Part V. Symmetry coordinates for tetrahedral complexes

Ligands of C_3V symmetry are assumed to be a part of a T_d complex. Suitable combinations of C_3v (a₂ + e) symmetry coordinates, which fit into the T_d model, are studied. The results constitute a useful supplement to the C_3V (a₁ + e) type coordinates studied previously. Torsional coordinates in the Ni(PF₃)₄ type model are treated in particular.     

On the nematic-nematic phase transition in mixtures composed of sheet-shaped palladium organyls and apolar organic solvents

The three sheet-shaped palladium organyls 1a-c exhibit lyotropic nematic properties in apolar organic solvents. The occurrence of two nematic lyomesophases was verified by the investigation of the orientational properties of these liquid crystalline systems. In cells with obliquely deposited SiO_x layers, the director of the high temperature phase N₂ is aligned in a homogeneous, planar manner. In the low temperature phase N₁, a different orientational pattern is observed. The specific pattern type depends on the deposition conditions of the SiO_x layers. In samples homeotropically aligned in the N₂ phase, the director is tilted away from the cell normal after the transition into the N₁ phase. These changes of orientation are reversible. The experimental observations presented here support our proposed model for the structural change at the nematic-nematic phase transition.     

Influence of cell and LC parameters on the dynamics of STN liquid crystal displays

The dynamics of super-twisted nematic (STN) liquid crystal displays have rarely been studied. In this article, the dynamic response of STN is analysed in detail. The evolution of director configuration with time was obtained by solving Ericksen-Leslie hydrodynamic equations. The time varying midlayer tilt angle is presented as a measure of dynamic response. The influence on STN dynamics of cell parameters including pretilt angle, twist angle, cell thickness, and of material parameters including d/p, K₂₂, K₃₃, were studied.