2H NMR Spectroscopic Investigation of para-Nitroazobenzene Liquid Crystals

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Magnetic alignment study of rare-earth-containing liquid crystals

The liquid-crystalline rare-earth complexes of the type [Ln(LH)3(DOS)3]-where Ln is Tb, Dy, Ho, Er, Tm, or Yb; LH is the Schiff base N-octadecyl-4-tetradecyloxysalicylaldimine; and DOS is dodecylsulfate-exhibit a smectic A phase. Because of the presence of rare-earth ions with a large magnetic anisotropy, the smectic A phase of these liquid crystals can be easier aligned in an external magnetic field than smectic A phases of conventional liquid crystals. The magnetic anisotropy of the [Ln(LH)3(DOS)3] complexes was determined by measurement of the temperature-dependence of the magnetic susceptibility using a Faraday balance. The highest value for the magnetic anisotropy was found for the dysprosium(III) complex. The magnetic alignment of these liquid crystals was studied by time-resolved synchrotron small-angle X-ray scattering experiments. Depending on the sign of the magnetic anisotropy, the director of the liquid-crystalline molecules was aligned parallel or perpendicular to the magnetic field lines. A positive value of the magnetic anisotropy (and parallel alignment) was found for the thulium(III) and the ytterbium(III) complexes, whereas a negative value of the magnetic anisotropy (and perpendicular alignment) was observed for the terbium(III) and dysprosium(III) complexes.     

Dielectric investigations on oriented phases formed by banana-shaped molecules

Dielectric investigations on a magnetically oriented sample forming N, SmA, SmC and B 2 phases were carried out. The sample shows a negative dielectric anisotropy in the N state and a strong increase of the static dielectric constants perpendicular to the director in the ensuring 'classical' phases. This proves directly the strong and continuous increase of the positive dipole correlation in the lateral direction due to the steric interactions of the bent-shape molecules. From a dynamical point of view, the SmA and SmC phases of this sample show deviations from the classical phases of rod-like molecules. Crystallization did not allow us to extend the dielectric measurements into the B 2 state.     

Magnetic resonance study of a vanadium pentoxide gel

In this work we report results from continuous-wave (CW) and pulsed electron paramagnetic resonance (EPR) and proton nuclear magnetic resonance (NMR) studies of the vanadium pentoxide xerogel V₂O₅:nH₂O (n ≈ 1.6). The low temperature CW-EPR spectrum shows hyperfine structure due to coupling of unpaired V⁴⁺ electron with the vanadium nucleus. The analysis of the spin Hamiltonian parameters suggests that the V⁴⁺ ions are located in tetragonally distorted octahedral sites. The transition temperature from the rigid-lattice low-temperature regime to the high temperature liquid-like regime was determined from the analysis of the temperature dependence of the hyperfine splitting and the V⁴⁺ motional correlation time. The Electron Spin Echo Envelope Modulation (ESEEM) data shows the signals resulting from the interaction of ¹H nuclei with V⁴⁺ ions. The modulation effect was observed only for field values in the center of the EPR absorption spectrum corresponding to the single crystals orientated perpendicular to the magnetic field direction. At least three protons are identified in the xerogel by our magnetic resonance experiments: (I) the OH groups in the equatorial plane, (ii) the bound water molecules in the axial V=O bond and (iii) the free mobile water molecules between the oxide layers. Proton NMR lineshapes and spin-lattice relaxation times were measured in the temperature range between 150 K and 323 K. Our analysis indicates that only a fraction of the xerogel protons contribute to the measured conductivity.     

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.     

Dielectric investigations on oriented phases formed by banana-shaped molecules

Dielectric investigations on a magnetically oriented sample forming N, SmA, SmC and B 2 phases were carried out. The sample shows a negative dielectric anisotropy in the N state and a strong increase of the static dielectric constants perpendicular to the director in the ensuring 'classical' phases. This proves directly the strong and continuous increase of the positive dipole correlation in the lateral direction due to the steric interactions of the bent-shape molecules. From a dynamical point of view, the SmA and SmC phases of this sample show deviations from the classical phases of rod-like molecules. Crystallization did not allow us to extend the dielectric measurements into the B 2 state.     

Pulsed gradient NMR study of anisotropic surfactant diffusion in the caesium perfluoro octanoate/D2O system

We use pulsed field gradient ¹⁹F NMR to measure the diffusion coefficients of surfactant molecules in the isotropic and lamellar phases of the caesium perfluoro octanoate (CsPFO)/D₂O system. An aligned lamellar sample is created by cooling through the nematic phase in the presence of a 1·4 T magnetic field. The director in the lamellar phase does not respond to ordinary field strengths, thus the aligned sample can be rotated clockwise or counterclockwise to place the director at a magic angle, where measurement of diffusion coefficients becomes possible. From a pair of so-obtained coefficients, we derive the principal values of the diffusion tensor corresponding to the directions parallel and perpendicular to the director (D_⊥ and D_⊥). We found D_∥ to be at least 20 times D_∥ a much larger anisotropy than is seen in electrical conductivity and water diffusion in similar systems. These results are compared to electrical conductivity, water and dye diffusion measurements.     

Molecular dynamics simulation of the nematic liquid crystal phase in the presence of an intense magnetic field

The influence of an intense external field on the dynamics of the nematic liquid crystal phase is investigated using a molecular dynamics simulation for the Gay-Berne nematogen under isobaric-isothermal conditions. The molecular dynamics as a function of the second-rank orientational order parameter P<2> for a system consisting of a nematic liquid crystal in the presence of an intense magnetic field is compared with that of a similar system without the field. The translational motion of molecules is determined as a function of the translational diffusion coefficient tensor and the anisotropy and compared with the values predicted theoretically. The rotational dynamics of molecules is analyzed using the first- and the second-rank orientational time correlation functions. The translational diffusion coefficient parallel with respect to the director is constrained by the intense field, although the perpendicular one is decreased as the P<2> is increased, just as it is in the system without the field. However, no essential effect of the strong magnetic field is observed in the rotational molecular dynamics. Further, the rotational diffusion coefficient parallel with respect to the director obtained from the first-rank orientational time correlation function in the simulation is qualitatively in agreement with that in the real nematic liquid crystalline molecules. The P<2> dependence of the rotational diffusion coefficient for the system with the intense magnetic field shows a tendency similar to that for the system without the field.     

Electric-field driven director oscillations in a nematic liquid crystal: a NMR investigation

We have investigated the oscillatory behavior of the nematic director for 4-pentyl-4'-cyanobiphenyl (5CB) when it is subjected to a static magnetic field and a sinusoidal electric field. In these experiments the two fields were inclined at about 50 degrees and the frequency of the electric field was varied from several hertz to approximately 1000 Hz. The director orientation was measured using time-resolved deuterium NMR spectroscopy since this has the advantage of being able to determine the state of director alignment in the sample. In fact, for all of the frequencies studied the director is found to remain uniformly aligned. Since the diamagnetic and dielectric anisotropies are both positive the director oscillates in the plane formed by the two fields. These oscillations were observed to continue for many cycles, indicating that the coherence in the director orientation was not lost during this motion. The maximum and minimum angles made by the director with the magnetic field were determined, as a function of frequency, from the NMR spectrum averaged over many thousand cycles of the oscillations. At low frequencies (several hertz) these limiting angles are essentially independent of frequency but as the frequency increases the two angles approach each other and become equal at high frequencies, typically 1000 Hz. Our results are well explained by a hydrodynamic theory in which the sinusoidal time dependence of the electric field is included in the torque-balance equation. This analysis also shows that, for a range of frequencies between the high and low limits, these NMR experiments can give dynamic as well as static information concerning the nematic phase.     

Simulation of EPR and time resolved EPR lineshapes in partially ordered glasses

Simulation of magnetic resonance spectra of probes in partially ordered glasses requires in principle a numerical integration on the full set of three Euler angles omega=(alpha beta gamma) from a laboratory fixed to a molecule fixed reference frame. It is shown that it is possible to manage efficiently this problem by using the algebraic properties of the Wigner matrix elements. This analysis is applied to time resolved EPR (TREPR) spectra of a series of bis-adducts of C60 in the ordered glass of a nematic liquid crystal solvent. A paramagnetic triplet state is created by light excitation and TREPR spectra are obtained with the external magnetic field set parallel or perpendicular to the director n of the mesophase. The preferred orientation in the mesophase of the triplet state zero field tensor is determined.     

A small angle neutron scattering study of the conformation of a side chain liquid crystal poly(methacrylate) in the smectic C phase

The conformation of the backbone in the side chain liquid crystal polymer poly\[ omega (4-methoxybiphenyl-4-yloxy)butyl methacrylate] has been studied in the smectic C and nematic phases. Small angle neutron scattering experiments were performed on mixtures of molecules with perdeuteriated backbones and unlabelled molecules. The polymer is found to adopt an oblate conformation in the smectic C phase. The components of the radius of gyration parallel and perpendicular to the director are determined as a function of temperature from Guinier plots of the SANS data. The radii of gyration do not vary across the smectic phase and are found to be Rg,||=(27+/-1)A, Rg, =(42+/-1)A. These results are compared with recent SANS results for other side chain liquid crystal polymers.     

A study of magnetic liquids with the help of a paramagnetic sensor

The paramagnetic sensor method was used to study local magnetic fields in a magnetite aqueous suspension. The sensor was 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl, a stable nitroxide radical. The lines in the EPR spectrum of the sensor were demonstrated to be broadened due to the dipole-dipole interaction with magnetite nanoparticles. It was established that no spin exchange occurred between sensor molecules and magnetite nanoparticles. The g-factor was found to decrease with the concentration of magnetite nanoparticles in the suspension. The mean strengths of the local magnetic fields calculated from changes in the EPR spectrum of the sensor proved to be substantially lower than the values determined from magnetic measurements. This difference was accounted for by the formation of linear aggregates of magnetite nanoparticles under the action of a magnetic field.     

New 1,3-dioxan-type ionic liquid crystal compounds having a large dielectric constant

New pyridinium-type thermotropic ionic liquid crystal materials having a 1,3-dioxan ring in its central core: N-substituted-4-(5-alkyl-1,3-dioxan-2-yl)pyridinium bromides (6) were synthesized. The mesomorphic behaviour of these compounds and dielectric constant perpendicular to the molecular axis were measured. The principal features of these compounds are that they exhibit a smectic A phase over a wide temperature range including room temperature - for example 6f: g -9 SmA 181 I (°C)-and they have a large dielectric constant perpendicular to the molecular axis.     

Photochemistry of artificial photosynthetic reaction centers in liquid crystals probed by multifrequency EPR (9.5 and 95 GHz)

Photoinduced charge separation and recombination in a carotenoid-porphyrin-fullerene triad C-P-C(60)(1) have been followed by multifrequency time-resolved electron paramagnetic resonance (TREPR) at intermediate magnetic field and microwave frequency (X-band) and high field and frequency (W-band). The electron-transfer process has been characterized in the different phases of two uniaxial liquid crystals (E-7 and ZLI-1167). The triad undergoes photoinduced electron transfer, with the generation of a long-lived charge-separated state, and charge recombination to the triplet state, localized in the carotene moiety, mimicking different aspects of the photosynthetic electron-transfer process. Both the photoinduced spin-correlated radical pair and the spin-polarized recombination triplet are observed starting from the crystalline up to the isotropic phase of the liquid crystals. The W-band TREPR radical pair spectrum has allowed unambiguous assignment of the spin-correlated radical pair spectrum to the charge-separated state C(.+)-P-C(60)(.-). The magnetic interaction parameters have been evaluated by simulation of the spin-polarized radical pair spectrum and the spin-selective recombination rates have been derived from the time dependence of the spectrum. The weak exchange interaction parameter (J = +0.5 +/- 0.2 G) provides a direct measure of the dominant electronic coupling matrix element V between the C(.+)-P-C(60)(.-) radical pair state and the recombination triplet state (3)C-P-C(60). The kinetic parameters have been analyzed in terms of the effect of the liquid crystal medium on the electron-transfer process. Effects of orientation of the molecular triad in the liquid crystal are evidenced by simulations of the carotenoid triplet state EPR spectra at different orientations of the external magnetic field with respect to the director of the mesophase. The order parameter (S = 0.5 +/- 0.05) has been evaluated.     

Orientational Behaviour of the Nematic Director upon Macroscopic Rotation at the Magic Angle

The ¹⁹F N.M.R. spectrum of 1,2,2,2-tetrachloro-l,l-difluoroethane has been studied in the nematic liquid crystal ZLI1167 (Merck) upon rotation at the magic angle. The director of the liquid crystal is oriented perpendicular to the spinning axis when the angle between the rotation axis and the magnetic field is less than the magic angle and parallel when this angle is more than the magic angle. It is shown that exactly at the magic angle the spectrum corresponds to a frequency modulated powder pattern. This powder pattern leads to an understanding of the orientational behaviour of the director when a nematic is spun at the magic angle.     

Electron paramagnetic resonance study of a Eu2+ related defect in CsBr:Eu needle image plates for computed radiography

The electron paramagnetic resonance (EPR) spectrum of needle image plates of CsBr doped with Eu(2+), which are proposed as new X-ray storage phosphors for computed radiography, is studied at room temperature and Q-band microwave frequencies (34 GHz). X-ray diffraction analysis demonstrates that the CsBr:Eu(2+) needles have an 001 out of plane (perpendicular to the plate) orientation, and contrary to expectation that the in plane orientation is not random. The room temperature EPR spectrum is attributed to a single centre which is related to Eu(2+) with axial 001 symmetry. Using the spin Hamiltonian parameters extracted from the spectrum recorded with the magnetic field parallel to the needles' axes, we convincingly simulate both the spectrum of a powdered image plate and the single crystal like angular dependence of intact pieces of image plate. The knowledge of the symmetry of this centre, which appears to be related with the radiation sensitivity of the plate, presents a first step in finding its model and role in the X-ray storage process.     

Effects of a transverse electric field in nematics: Induced biaxiality and the bend Fréedericksz transition

We have studied the effects of a transverse electric field on director fluctuations in the nematic liquid crystal 5CB (4-n-pentyl-4′-cyanobiphenyl) in the bend Fréedericksz geometry. The sample was homeotropically aligned by surface treatment of the glass cell walls and an additional magnetic field was applied perpendicular to the walls. An electric field was then applied parallel to the walls; below the bend Fréedericksz transition, director fluctuations parallel to the electric field are enhanced. This field-induced biaxiality was observed and studied by monitoring the intensity of light transmitted by the sample placed between crossed polarizers. Landau theory for 5CB predicts the electric field induced bend transition to be first order. Our observations of the transmitted intensity are consistent with this prediction. We have also observed that this transition is to a modulated rather than to a uniform phase.     

Magnetic field-induced switching of the radical-pair intersystem crossing mechanism in a donor-bridge-acceptor molecule for artificial photosynthesis

A covalent, fixed-distance donor-bridge-acceptor (D-B-A) molecule was synthesized that upon photoexcitation undergoes ultrafast charge separation to yield a radical ion pair (RP) in which the spin-spin exchange interaction (2J) between the two radicals is sufficiently large to result in preferential RP intersystem crossing to the highest-energy RP eigenstate (T(+1)) at the 350 mT magnetic field characteristic of X-band (9.5 GHz) EPR spectroscopy. This behavior is unprecedented in covalent D-B-A molecules, and is evidenced by the time-resolved EPR (TREPR) spectrum at X-band of (3*)D-B-A derived from RP recombination, which shows all six canonical EPR transitions polarized in emission (e,e,e,e,e,e). In contrast, when the RP is photogenerated in a 3400 mT magnetic field, the TREPR triplet spectrum at W-band (94 GHz) of (3*)D-B-A displays the (a,e,e,a,a,e) polarization pattern characteristic of a weakly coupled RP precursor, similar to that observed in photosynthetic reaction center proteins, and indicates a switch to selective population of the lower-energy T(0) eigenstate.     

Side group functionalized liquid crystalline polymers and blends VIII. Nematic and SmAre phase formation in hydrogen-bonded blends of smectic side group LC polymers and a low molar mass dopant

Hydrogen-bonded blends based on smectic side group functionalized LC copolymers containing 4-alkyloxybenzoic acid fragments (proton donor) and a non-mesogenic low molecular mass dopant 4-cyanophenyl pyridine-4-carboxylate or 4-methoxyphenyl-d₄ pyridine-4-carboxylate (proton acceptor) were obtained. The blends containing 10-35 mol % of low molecular weight dopant form nematic (I-N-SmA) or re-entrant SmA phases (I-SmA-N-SmAre). The temperature dependence of the order parameter S, the birefringence Δn, and the splay K ₁ and bend K ₃ elastic constants of the nematic phase were studied by ²H NMR spectroscopy and the Fréedericksz method of threshold transitions in a magnetic field. A mechanism for the destruction of the SmA phase and the formation of the nematic phase in the hydrogen-bonded blends is suggested.     

New 1,3-dioxan-type ionic liquid crystal compounds having a large dielectric constant

New pyridinium-type thermotropic ionic liquid crystal materials having a 1,3-dioxan ring in its central core: N-substituted-4-(5-alkyl-1,3-dioxan-2-yl)pyridinium bromides (6) were synthesized. The mesomorphic behaviour of these compounds and dielectric constant perpendicular to the molecular axis were measured. The principal features of these compounds are that they exhibit a smectic A phase over a wide temperature range including room temperature - for example 6f: g -9 SmA 181 I (°C)-and they have a large dielectric constant perpendicular to the molecular axis.