Polar 2‐alkoxyethoxy‐substituted nematic liquid crystals

The synthesis is reported of a series of polar nematic liquid crystals incorporating an alkoxyalkoxy chain attached in a terminal position to a molecular core consisting of two aromatic rings. The dependence of the mesomorphism and phase transition temperatures on the nature of this terminal chain was studied. The shape anisotropy of the core is increased by the presence of an additional 1,4‐disubstituted phenyl unit in a number of p‐terphenyls. A number of highly polar esters with a terminal methoxyethoxy chain have been prepared as dopants to lower the threshold voltage of LCDs. The presence of the methoxyethoxy chain leads to an increase in the molecular dipole moment, the dielectric anisotropy and the dielectric constant perpendicular to the molecular long axis. Several compounds with a four‐unit diether central linkage were also prepared.     

High figure‐of‐merit nematic mixtures based on totally unsaturated isothiocyanate liquid crystals

High birefringence and low viscosity isothiocyanate liquid crystal single compounds, and eutectic mixtures based solely on unsaturated rigid core structures, are reported. Extraordinarily high values of figure‐of‐merit were observed at room temperature for the formulated nematic mixtures. Potential applications of such mixtures for laser beam steering at λ = 1.55 µm using optical phased arrays are emphasized.     

Synthesis and mesomorphic behaviour of wedge‐shaped nematic liquid crystals with flexoelectric properties

The synthesis and mesomorphic behaviour of a series of wedge‐shaped liquid crystals and some reference compounds are reported. These unusual liquid crystals possess smectic C, smectic A and nematic phases. These new wedge‐shaped materials with a high degree of shape anisotropy and a large dipole moment can be used to induce an increase in the flexoelectric effects of nematic guest–host mixtures as dopants at low concentrations.     

Third order optical non–linearities of nematic liquid crystals

Abstract

Liquid crystals can exhibit large third order optical non-linearities. Using the Z-scan technique, we have measured the intensity dependence of the non-linear absorption and non-linear refractive indices on time-scales ranging from milliseconds to picoseconds for nematic liquid crystals. This method allows the determination of the non-linear absorption coefficients as well as the signs and magnitudes of the non-linear refractive indices for different polarizations. A two pulse technique further enables estimation of the response time of the dominant mechanism. Using CW argon and Q-switched and mode-locked pumped dye lasers, we have carried out Z-scan measurements on aligned liquid crystal samples as a function of temperature, as well as wavelength. In the geometries studied, director reorientation is not expected to take place. On the nanosecond time-scale, all materials studied were self-focusing for polarization perpendicular to the director, and self-defocusing for parallel polarization. On the picosecond timescale, the samples were self-focusing for all polarizations, but strong non-linear birefringence was typically observed. An attempt is made to relate the bulk response to the molecular structures.     

A novel vertical alignment technology for nematic liquid crystals based on electrostatic self‐assembled method

A novel vertical alignment film for nematic liquid crystals is reported based on electrostatic self‐assembly of alkyl sulfonic salts in aqueous solution. A series of self‐assembled films with different alkyl chain lengths were prepared and used as alignment films. It was revealed that only when the number of carbon atoms in the alkyl chain approaches 11 or larger, could the self‐assembled film induce vertical alignment of liquid crystals. We also found that the homeotropic alignment of liquid crystals was related to the surface roughness of self‐assembled films. In addition, this vertical alignment film showed good electro‐optical characteristics and excellent thermal stability.     

Thermotropic biaxial nematic liquid crystals: spontaneous or field stabilized?

An intermediate nematic phase is proposed for the interpretation of recent experimental results on phase biaxiality in bent-core nematic liquid crystals. The phase is macroscopically uniaxial but has microscopic biaxial, and possibly polar, domains. Under the action of an electric field, the phase acquires macroscopic biaxial ordering resulting from the collective alignment of the domains. A phenomenological theory is developed for the molecular order in this phase and for its transitions to purely uniaxial and to spontaneously biaxial nematic phases.     

Symmetrical carbazole–fluorene–carbazole nematic liquid crystals as electroluminescent organic semiconductors

A series of novel symmetrical carbazole–fluorene–carbazoles has been synthesised using simple and highly efficient reaction pathways. The correlation between their molecular structures, differing in the length of the alkyl chains in the lateral position attached to a 2,7-disubstituted fluorene moiety in the molecular core and/or differing in the length of alkoxy chains at the terminal positions of 2,7-disubstituted carbazole moieties, was investigated. Their mesomorphic behaviour, transition temperatures, energy levels and band gaps were measured. A test organic light-emitting diode incorporating one of these compounds exhibits blue electroluminescence.     

Exact tilt angle profiles for splay–bend deformations in nematic liquid crystals

The exact tilt angle profiles for splay–bend deformations, in nematic liquid crystal samples limited by inhomogeneous surfaces, are determined in the one‐constant approximation. The boundary value problem concerning the situation of strong anchoring at the surfaces of a sample of slab shape of thickness d (Dirichlet's problem) is analytically solved in the presence of an external uniform field. The boundary value problem concerning the weak anchoring situation (mixed problem) is also exactly solved in the absence of an external field. The results are used to obtain the thickness dependence of the optical path difference between the ordinary and extraordinary rays, from which the physical properties of the sample can be deduced.     

High birefringence and high resistivity isothiocyanate‐based nematic liquid crystal mixtures

The molecular structures and physical properties of several single‐ and double‐fluorinated isothiocyanatotolane, isothiocyanatocyclohexyltolane, and isothiocyanatoterphenyl compounds are reported. Two eutectic mixtures comprising these compounds are formulated and their properties evaluated. These mixtures exhibit a high birefringence, relatively low viscosity, high resistivity, and good photo and thermal stabilities. Potential applications of these mixtures for spatial light modulators, optical phased arrays, and high speed photonics are discussed.     

Synthesis and ferroelectric properties of chiral liquid crystals derived from (S)‐1‐propyloxy‐2‐propanol

A new optically active chiral moiety, (S)‐1‐propyloxy‐2‐propanol, was designed and synthesized by the treatment of 1‐propanol with (S)‐propylene oxide under basic conditions. Its derivatives, the (R)‐1‐propyloxy‐2‐propyl 4‐[4‐(4‐alkoxyphenyl)phenoyloxy]benzoates, PPmPPB (m = 8–12), were prepared for the investigation of mesomorphic properties. All of the chiral materials displayed enantiotropic SmA? and SmC? phases, and the shorter alkyl chain members (m = 8–11) displayed an additional unidentified SmX? phase. The switching current, spontaneous polarization, tilt angle, dielectric constant and electro‐optical response for the materials in the ferroelectric SmC? phase were measured. The electro‐optic responses in polyimide film‐coated homogeneously aligned cells exhibit thresholdless, V‐shaped switching in the ferroelectric phase.     

Twin‐like domains in chiral smectic C liquid crystals

In the chiral smectic C phase of liquid crystals with the phase transition N*–SmC*, texture development depending on the sample thickness is reported. In very thin samples, domains of rectangular‐like shape are observed. As two possible tilts of smectic layers are possible for one anchoring direction, smectic layers inside a domain, called twin‐like domains, are tilted with respect to layers in outer regions, similarly to crystalline planes in solid crystalline twins. An elastic model of such a twin domain is proposed and its energy determined.     

Dynamics of the acousto‐optic effect in a nematic liquid crystal

In a nematic liquid crystal cell, the application of an ultrasonic wave induces a rotation of the director, leading to a change in the optical transmission through the cell. In this study, we investigate the dynamic response of the optical intensity after the ultrasonic wave is switched on or off. Our experiments show that the optical intensity follows a double‐exponential function of time, indicating that the system has two relaxation modes with widely different time scales. The fast mode has an amplitude and time scale qualitatively consistent with the dynamics of the Fréedericksz transition, but the slow mode shows novel behaviour associated with the acousto‐optic effect.     

Nonlinear optical properties of dye-doped E7 liquid crystals at the nematic–isotropic transition

This work shows the influence of a 2,1,3-benzothiadiazole-based dye in the nonlinear optical refraction and nonlinear optical absorption of the thermotropic liquid crystal E7 at the nematic–isotropic transition in the ms time-scale using the Z-scan technique. The addition of dye does not modify the critical exponent of the nonlinear birefringence observed for the undoped sample at the transition, confirming the tricritical character. Also, the order parameter based in the nonlinear absorption shows, for the samples with higher dopings, a critical exponent that deviates from the expected value in the tricritical hypothesis.     

Measurement of the rotational viscosity, γ1 of nematic liquid crystals under high pressure

Abstract

A method for the measurement of the rotational viscosity, γ₁ of nematic liquid crystals under high pressure is described. First measurements for the liquid crystals 4'-methoxybenzylidene-4-n-butylaniline, a broad range nematic mixture of substituted cyclohexyl-phenyls and a re-entrant nematic mixture are presented.     

Alignment of liquid crystals induced by a photopolymerized self‐assembled film

An alignment film derived from a photopolymerized self‐assembled film may be used to orient nematic liquid crystals after irradiating the film with linearly polarized UV (LPUV). A photosensitive cationic amphiphile was first synthesized containing two double bonds and which could be polymerized by UV. A layer‐by‐layer self‐assembled multilayer film was next prepared in an aqueous solution of the cationic amphiphile and poly(sodium 4‐styrenesulphonate); the UV‐Vis spectra showed that each layer of the LBL multilayer film was uniform. When the film was irradiated by LPUV, the photosensitive double bonds underwent [2+2] cycloaddition along the vector direction of LPUV. The polarized UV‐Vis absorption spectra also provided evidence that the film was anisotropic, i.e. the photopolymerization was along a certain direction. The anisotropic film was used as an alignment layer for nematic liquid crystals, and observations under a polarizing microscope indicated that the alignment of the liquid crystals was good, as expected, and that the orientation direction of the liquid crystals was always perpendicular to the electric vector of the irradiating LPUV.     

Electro‐optical cells using a cellulose derivative and cholesteric liquid crystals

A cellulose derivative/liquid crystal composite‐type electro‐optical cell using a commercial cholesteric liquid crystal (CLC) was investigated. The electro‐optical properties of the system were examined, i.e. the dependence on applied voltage of the reflected wavelength and the minimum and maximum transmissions. A thin film of the CLC was dispersed with a cross‐linked cellulose film of 25 µm thickness. In the voltage dependence of the reflected wavelength it was verified that there is a hysteresis in the reflected wavelength. The variation of the reflected wavelength with temperature was also determined. The results are analysed in the framework of similar systems described in the literature for CLC dispersed in a polymer matrix.     

High birefringence and large negative dielectric anisotropy phenyl‐tolane liquid crystals

Two phenyl‐tolane compounds with two pairs of (2,3) lateral difluoro substitutions were synthesized and their properties evaluated. These compounds exhibit a high birefringence (Δn ～ 0.35) and a large negative dielectric anisotropy (Δε～ ?8). These two important features make the phenyl‐tolane compounds useful as a dopant in a negative Δε liquid‐crystal mixture to enhance the performance of the host mixture, or as a negative component in a dual‐frequency liquid‐crystal mixture to improve the dielectric anisotropy and birefringence and lower the crossover frequency.     

Synthesis of N‐Aryl‐2‐substituted Tetrahydrobenzimidazoles via Direct N‐Arylation

A series of novel N‐aryl‐2‐substituted tetrahydrobenzimidazoles has been synthesized via direct N‐arylation of 2‐substituted tetrahydrobenzimidazoles, which was accomplished by a medium aryl electrophile, 4‐methylsulfonylfluorobenze, in the presence of 37% KF/Al₂O₃ and 18‐crown‐6 in fair yields under mild reaction conditions. Meanwhile, the hydrogenation of 2‐phenylbenzimidazole was studied.     

Rotational viscosity γ1 of nematic liquid crystals as a function of temperature,pressure and density

Abstract

Measurements of the rotational viscosity γ₁ and the density are presented for a mixture of 4′-methoxybenzylidenebutylaniline (MBBA) and its ethoxy homologue EBBA and a mixture of cyclohexylphenylnitriles (ZLI 2413 from Merck AG) as a function of temperature and pressure. A new set-up for the measurement of densities under pressures of up to 3kbar is described. It is shown that the pressure dependence of the kinematic rotational viscosity γ₁/ρ and the temperature dependence of γ₁ under isobaric and isochoric conditions have common features with that of the shear viscosity of isotropic liquids. Furthermore, it is found that the curves γ₁ = f(1/T) for constant p and γ₁ = g(ρ) for constant T can be shifted one onto another by an appropriate shift of the scale of the independent variable.