Cyanobiphenyl Liquid Crystals Research Articles

Distribution Function of Relaxation Times for a 4-n-Pentyl-4ʹ-Cyanobiphenyl Liquid Crystal

The distribution function of relaxation times is reconstructed from the dielectric loss spectrum measured over a wide range of frequencies for a 4-n-pentyl-4′-cyanobiphenyl (5CB) liquid crystal. It is demonstrated that the distribution function for the isotropic and nematic phases is asymmetric in shape. Comparison shows that the reconstructed distribution function is in qualitative agreement with similar functions that are analytically derived from the Cole-Davidson and Havriliak-Negami empirical equations. The specific features observed in the behavior of the distribution function with a variation in the angle between the direction of polarization of a microwave electric field and the director of the liquid-crystal molecules are analyzed. A complex dependence of the permittivity for the liquid crystal in the range of ultrahigh frequencies is explained in terms of additional relaxation mechanisms associated with different motions of molecular fragments of alkyl chains.

Role of Delocalized Electrons in Polyaniline−Nematogen Cyanobiphenyls Interaction

Applying an electric field across a nematic liquid crystal display changes the texture of the thin nematic film. When the field is switched off, the texture relaxes to the initial one within seconds. Experimental evidence that very fast relaxation times could appear in some particular cases is reported. A thin film of plasma polymerized aniline induces a stronger planar anchoring of cyanobiphenyl liquid crystals. This strong anchoring appears when, due to an asymmetric conduction of the liquid crystal cell, alternating voltages create a transient electrically charged layer at the polyaniline/liquid crystal interface. This layer favors an expansion of anchoring toward the bulk, thus lowering dramatically the relaxation time and increasing the contrast ratio.

Mapping molecular conformation and orientation of polyimide surfaces for homeotropic liquid crystal alignment by nonlinear optical spectroscopy.

Surface-specific sum-frequency vibrational spectroscopy and second-harmonic generation were used to study the structures of polyimide (PI) surfaces for homeotropic liquid crystal (LC) alignment and the molecular orientation of LC adsobates on these surfaces. The imide ring was perpendicular to the surface with one of CO bonds protruding out of the surface and the other pointing into the bulk rather than flat on the surface. The ester CO bond in the side chain was sticking out of the surface with a tilt angle of about 45 degrees -55 degrees from the surface normal, indicating that the rigid side chain core was, more or less, along the surface normal. The part of alkyl chain on the top of the side chain followed the orientation of the side chain core and protruded out of the surface with some gauche defects. The cyano biphenyl LC molecules were adsorbed on the PI preferentially with the terminal cyano group facing the PI surface.

Hydrogen-bonding between a dichroic dye and a liquid crystal-forming molecule, for application to LCDs

The performance of guest–host type liquid crystal displays (LCDs) is governed by the solubility and dissolution stability of dichroic dyes in the liquid crystal medium in guest–host kind of LCDs. The solubility and stability is influenced by hydrogen-bonding between the dye and the liquid crystal host. An earlier work was carried out to study the hydrogen-bonding behavior of 4-pentyl-4′-cyanobiphenyl (5CB) (liquid crystal), benzonitrile, and acetonitrile, with 1-hexanol in CCl4 using FTIR spectroscopy, ab initio and equation of state calculations [Ind. Eng. Chem. Res. 42 (2003) 1129]. To gain further thermodynamic understanding, hydrogen-bonding behavior of 5CB (liquid crystal), benzonitrile, and acetonitrile, with a dichroic dye Disperse Red 1 (DR1) {2-[ethyl[4-[(4-nitrophenyl)azo]phenyl]amino]ethanol} in CCl4 were studied in this work. In all three mixtures, -OH acts as a hydrogen-bond donor and -CN acts as hydrogen-bond acceptor. Based on FTIR analysis for DR1/acetonitrile, DR1/benzonitrile, and DR1/5CB, the enthalpies of hydrogen-bond formation are −24.2, −23.7, and −3.7kJ/mol, respectively; and the entropies of hydrogen-bond formation are −58.9, −58.1, and −0.4J/(molK), respectively. Though, for DR1/5CB, enthalpy of H-bonding is not too negative, the entropy of H-bonding is more favorable; hence the H-bonding appears to be entropically driven. Lattice fluid hydrogen-bonding (LFHB) parameters obtained from the spectroscopic data were observed to match well with the experimental trends. LFHB equation of state was then used to predict hydrogen-bonding for a wider range of conditions.

Parallel Synthesis of 4-alkyl-4'-cyanobiphenyl Liquid Crystals

In the search for novel nematic liquid crystal materials for display applications, it is common to investigate several different alkyl chain homologues of a particular core structure. It is sometimes possible to develop a synthetic pathway that allows homologous variation in the ultimate step, but the majority of published methods for liquid crystal synthesis begin from an alkyl chain containing intermediate. This results in a separate pathway for each homologue. A “one-pot” technique has been developed for the parallel synthesis of up to five homologues of a given core structure, exemplified by “the 4-alkyl-4′-cyanobiphenyl series, together with their isolation in gram quantities and high purity by preparative high performance liquid chromatography. The technique demonstrates that it is possible to identify and monitor each intermediate throughout the synthetic pathway, using gas chromatography/mass spectrometry. It is also shown that there are no substantive losses in yield for any homologue throughout the pathway. It is concluded that this technique is a viable method for the synthesis of novel liquid crystal materials.

Original Magnetic Alignment of a Nematic Phase Containing Single-Walled Nanotubes

Dispersions of single-walled carbon nanotubes in a 4-pentyl-4'-cyano-biphenyl nematic liquid crystal matrix are studied by X-ray scattering. For some mixtures, the addition of nanotubes strongly modifies the properties of the matrix: under a magnetic field, the nematic director is aligned perpendicular to the field, whereas it is parallel to it for pure 4-pentyl-4'-cyano-biphenyl. This original modification of liquid crystal alignment by the addition of nanotubes raises new questions about nanotubes and liquid crystal interactions.

Molecular dynamics and alignment studies of silica-filled 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal

A comprehensive study of the dielectric properties of 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal filled with silica particles (particle size 30–80 nm, concentration 2, 3, 5, 10 and 15 wt%). Dielectric spectroscopy in the frequency range 100 to 10 7 Hz was applied to investigate the influence of the filler on the dynamic behaviour of the liquid crystal molecules in both the nematic and isotropic phases. In this frequency range one relaxation process is observed (at f>106 Hz). The dynamical behaviour of the 5CB liquid crystal is described by the Cole-Cole relaxation function. The temperature dependence of the relaxation time obeys the empirical Arrhenius equation. The activation energies are approximately 75 kJ mol1 for the pure 5CB sample in the nematic phase and 50 kJ mol1 for the 5 wt% silica-filled 5CB sample. These values are compared with the corresponding literature values. The reversible electro-mechanical response of these samples under the influence of an applied a.c. electric field is investigated.

The dynamic electric birefringence of homeotropic aligned liquid crystals

Dynamic Kerr effect behaviour and transient molecular reorientation were investigated in thin layer of some alkyl cyanobiphenyl liquid crystals in the nematic phase at several temperatures and in an electro-optical system based on homeotropic-homogeneous alignment. All of the Kerr effect transients were obtained using homeotropically aligned samples. The variation of the relaxation decay time with temperature is very small, and low activation energies are obtained for the cooperative reorientation of the whole samples. The shapes of the Kerr decay transient for large retardations and the static Kerr constants of the liquid crystals were also studied using a computer program of MathCad 4.0.

Hydrogen-Bonding Effects in Liquid Crystals for Application to LCDs

Hydrogen bonding plays an important role in the performance of liquid crystal displays (LCDs) by controlling the solubility and dissolution stability of dichroic dyes in the liquid crystal medium, for guest-host type LCDs. The hydrogen-bonding behaviors of 4-pentyl-4‘-cyanobiphenyl (5CB) (liquid crystal), benzonitrile, and acetonitrile, with 1-hexanol in CCl4 were studied using FTIR spectroscopy, equations of state, and ab initio calculations. In all three mixtures, the −OH group acts as a hydrogen-bond donor, and the −CN group acts as a hydrogen-bond acceptor. On the basis of FTIR analysis, the enthalpy of hydrogen-bond formation is observed to be −14.7 kJ/mol for 1-hexanol/acetonitrile, −5.4 kJ/mol for 1-hexanol/benzonitrile, and −4.6 kJ/mol for 1-hexanol/5CB. The entropy of hydrogen-bond formation is more favorable for 1-hexanol/5CB than for 1-hexanol/acetonitrile or 1-hexanol/benzonitrile. Trends in the spectroscopic results agree with ab initio calculations. Lattice fluid hydrogen-bonding parameters ...

Perdeuterated cyanobiphenyl liquid crystals for infrared applications

Perdeuterated 4’-pentyl-4-cyanobiphenyl (D5CB) was synthesized and its physical properties evaluated and compared to those of 5CB. D5CB retains physical properties similar to those of 5CB, such as phase transition temperatures, dielectric constants, and refractive indices. An outstanding feature of D5CB is that it exhibits a much cleaner and reduced infrared absorption. Perdeuteration, therefore, extends the usable range of liquid crystals to the mid infrared by significantly reducing the absorption in the near infrared, which is essential for telecom applications.

Effect of surface and molecular structure on the dielectric behaviour in thick layers of cyanobiphenyl liquid crystals

The static dielectric studies of hexoxy heptoxy and octoxy cyanobiphenyl liquid crystals (6OCB, 7OCB and 8OCB) and the euteric nematic mixture of 6OCB/7OCB indicate that molecular alignment can be induced by the steel electrod surface of a dielectric thick cell with an electrode gap of about 1 mm. Whereas the permittivity values of alkyl cyanobiphenyls (6CB, 7CB and 8CB) in the nematic phase indicate imperfect alignment of the liquid crystal molecules. The good parallel molecular alignment of alkoxy cyanobiphenyls was to existence of oxygen atom in their molecular structures. These observations demonstrated a sensitive relationship between the molecular alignment, molecular structure and surface effect. Analysis of the dielectric data re-confirmed that the dipole components of the alkoxy cyanobiphenyls along the long molecular axes have a strong tendency for anti-parallel alignment which increases with increasing nematic order, whereas the dipole components perpendicular to the long molecular axes indicate a tendency for parallel alignment.

Volume and polarisability anisotropy of hexoxy and heptoxy cyanobiphenyl liquid crystals and the eutectic mixture of E 607

The phase transitions, effective molecular polarisibilities, molar volumes and order parameters of hexoxy and heptoxy cyanobiphenyl (6OCB and 7OCB) and the eutectic mixture E 607, composed of hexoxy and heptoxy cyanobiphenyls (41.5:58.5), were calculated and compared using accurate optical and density data measured at several temperatures. The order parameters calculated using the different methods of Vuks, Neugebauer, Saupe-Maier and Palffy-Muhoray are nearly the same for the liquid crystals considered in the present study.

The Effect of Quasispherical Solutes on the Smectic-A−Nematic and Nematic−Isotropic Phase Equilibria in p-n-Alkyl-p‘-Cyanobiphenyl Liquid Crystals

Density as a function of temperature was measured in the nematic and isotropic phases in the immediate vicinity of the nematic-to-isotropic transition temperature (TNI) of p-n-hexyl-p‘-cyanobiphenyl (6CB) and p-n-octyl-p‘-cyanobiphenyl (8CB) and in the smectic-A and nematic phases in the immediate vicinity of the smectic-A-to-nematic transition temperature (TAN‘) of 8CB. Similar measurements were made on dilute solutions of the quasispherical solutes Et4C (tetraethylmethane) and R4Sn (R = C2H5, n-C3H7, and n-C4H9) in 6CB and 8CB. Addition of these solutes led, as is expected for first-order phase transitions, to a nematic−isotropic two-phase region bounded by temperatures TN and TI, which are lower than TNI, and in the case of the 8CB systems to a smectic-A−nematic two-phase region bounded by temperatures TA and TN‘, which are lower than TAN‘. Volume changes at the transitions of the liquid crystals confirm that they are weakly first-order. βN and βI, the moduli of the slopes of the nematic and isotropic boundary lines, respectively, of the nematic−isotropic two-phase regions in the 6CB and 8CB systems were obtained assuming that the respective reduced temperatures TN* (= TN/TNI) and TI* (= TI/TNI) versus solute mole fraction (x2) diagrams are linear. βA and βN‘, similarly defined for the smectic-A−nematic two-phase regions in the 8CB systems, were obtained from the respective linear TA* (= TA/TAN‘) and TN‘* (= TN‘ / TAN‘) versus x2 diagrams. The dependencies of βN and βA on solute and solvent sizes and the nature of the liquid crystalline phase are discussed.

Surface orientation of liquid crystals: director fluctuation and optical flickering effect in nematic liquid crystals studied by total reflection spectroellipsometry

The director fluctuation and associated optical flickering effect in a nematic 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal has been studied by time-resolved spectroellipsometry (TRSE). The comparison of the results of total reflection TRSE (surface dynamics) with those of transmission TRSE (bulk dynamics) indicates that the flickering effect has its origin in director fluctuation near the substrate surface caused by a change in internal electric field due to ion migration. The shape and intensity of the flickering response curve shows a characteristic frequency dependence, indicating that several different ionic species contribute to director fluctuation. It is also demonstrated that the observed line shape can be reproduced by a numerical simulation that assumes ion migration.

Surface ordering above the isotropic-smectic-A transition at a silane-treated substrate.

Surface ordering in a homologous series of alkyl cyanobiphenyl (nCB) liquid crystals having a direct isotropic-smectic-A (I-A) transition was investigated using evanescent-wave ellipsometry. The liquid crystal was bounded by a solid substrate treated with a silane surfactant which induced homeotropic (perpendicular) ordering of the liquid crystal molecules in the smectic-A phase. In the isotropic phase, one of the liquid crystals (10CB) partially wet the interface with an orientationally ordered, homeotropically aligned layer. The ordered interfacial layer grew without layering transitions but remained finite in thickness as the bulk I-A transition was approached. The interfacial layer has significantly lower orientational order than is observed in the smectic phase, indicating the possibility that the surface region of 10CB may be in a surface-induced, nonspontaneous nematic phase. The other liquid crystals (11 and 12CB) showed no surface ordering behavior whatsoever. Models describing the ordered surface layer of 10CB are presented. The results can be interpreted as a sharp transition in the surface ordering behavior as the chain length of the liquid crystal is varied, at the I-A transition of a liquid crystal.

Calculation of vibrational spectra for cyanobiphenyl liquid crystals

Density functional theory, B3-LYP with the 6-31G ∗ basis set was applied to study the structures and vibrational infrared (IR) spectra of biphenyl derivatives. The calculated force fields were scaled using the scaled quantum mechanical force field method. The predicted vibrational frequencies were compared with the experimental IR and Raman spectra (500–4000 cm −1). The spectra were interpreted and vibrational assignments were reported. This study shows that the scaled density functional force field approach enables, through the transferability of scale factors, good interpretation of vibrational spectra of large molecules.

NONLINEAR OPTICAL RESPONSE OF CYANOBIPHENYL LIQUID CRYSTALS TO HIGH-POWER, NANOSECOND LASER RADIATION

Results from investigations are summarized into: (1) transient refractive and absorptive (two-photon) nonlinearities at 0.532 μm by the Z-scan method, and (2) reflective nonlinearity in the near-IR, of linearly nonabsorbing cyanobiphenyl liquid crystals under nanosecond laser irradiation. (1) For isotropic liquid crystals at the several-nanosecond time scale and several tens-micrometers beam-waist-diameter, transient molecular-reorientation and thermal/density refractive nonlinearities compete in changing the sign of the total transient refractive nonlinearity. For the different, given pulse durations, the influence of coupled thermal and density effects on nonlinear refraction depends, through buildup time, on the beam-waist diameter. Nonlinear absorption coefficients depend on the incident intensity. For planar nematic layers, cumulative effects in heating (and in refractive nonlinearity) were observed even at low, 2–10 Hz pulse repetition rate. These results are useful for optical power limiting applications, and for intensity and beam-quality sensors of pulsed, high-power lasers. (2) Reflective nonlinearity of chiral-nematic (cholesteric) mirrors near selective reflection conditions for circular polarized light at λ=1.064 μm was studied both under free space irradiation and inside a laser resonator. Specially chosen experimental irradiation conditions make it possible to attribute the observed changing of reflectivity to athermal helix unwinding by the optical field. The results can find applications in laser-resonator mirrors, Q-switches and soft apertures for beam-profile formation, and also in showing the limits of use cholesteric optical elements in high-power laser beams.

Effects of cation composition in MCM-41 mesoporous molecular sieves on the structures of encapsulated 4-pentyl-4′-cyanobiphenyl liquid crystals

The proportions of the various states are determined by the cation composition of the matrix for liquid-crystal material encapsulated in the pore spaces of mesoporous molecular sieves of MCM-41 type having high aluminum contents. The liquid-crystal properties are retained in the encapsulated state.

Generation of Maxwell Displacement Current across Liquid Crystal Monolayers and Control of Liquid Crystal Alignment

The orientational transition process of p-pentyl-p'-cyano-biphenyl (5CB) liquid crystal monolayers at the air-water surface during monolayer compression and the cis-trans photoisomerization process of surface azobenzene monolayer were examined using the Maxwell displacement current measurement. Using this measurement, the orientational transition process of 5CB molecules in monolayers mixed with azobenzene molecules was examined by alternating irradiation with ultra-violet and visible light. Subsequently, we examined the orientational transition process of 5CB molecules in Liquid crystal (LC) cells triggered by the conformational change of the surface azobezene monolayer on polyimide (PI) Langmuir-Blodgett (LB) films. It was shown that the optical transmittance and the capacitance of the LC cells with 1-layer PI LB films changed reversibly by the alternating irradiation of ultra-violet (UV) and visible light, whereas those of LC cells with more than 3-layer PI LB layers did not change reversibly. These results were discussed taking into account the physico-chemical property of PI LB films.

Coupling between molecular vibrations and liquid crystalline order parameters.

Specific Raman active modes in two prototype cyanobiphenyl liquid crystals are shown to display a temperature dependent softening proportional to either the nematic or smectic order parameters, while other vibrations (like the C identical withN stretch mode) remain unaltered. This selective coupling between intramolecular vibrations and the liquid crystalline order is related to the intrinsic symmetry of the modes. The method provides a simple, microscopic, noninvasive optical technique with which the liquid crystalline order parameters can be qualitatively mapped out.