Synthesis and properties of novel liquid crystalline materials with super high birefringence: styrene monomers bearing diacetylenes, naphthyl, and nitrogen-containing groups

Three new liquid crystal asymmetrical styrene monomers bearing diacetylenes, naphthyl, and nitrogen-containing groups were successfully synthesized from 2-(bromoethynyl)-6-(hexyloxy)naphthalene, 4-(4-bromo-2-vinylphenyl)-2-methylbut-3-yn-2-ol, and derivatives of 4-ethynylaniline. The molecular structures of these compounds were confirmed by FTIR, ¹H NMR spectroscopy, elemental analysis, and mass spectrometry. The liquid crystalline properties of monomers were characterized by differential scanning calorimetry and polarized light microscopy. Results indicated that all the compounds exhibited the nematic phase in liquid crystal state and super high optical birefringence of 0.5-0.8. The change of terminal nitrogen-containing group affected the birefringence values in the order of -N(CH₃)₂<-NH₂<-NCS. Moreover, measurements using UV-vis and fluorescence spectroscopy showed their good photoluminescence properties and high quantum efficiency of 0.4-1.0.     

The effect of phenyl ring on the physical properties of liquid crystals containing 4-pyridyl terminal group

ABSTRACT

Liquid crystals with 4-pyridyl end group containing one triple bond and phenylenes have been synthesised. Their structures were confirmed by traditional spectroscopic methods. The properties were measured by differential scanning calorimetry (DSC), polarising optical microscopy (POM), Abbe refractometer and an electrical constants instrument. The results show that all of the compounds exhibit nematic phase. The addition of a phenylene to the molecular skeleton brings about an obvious increase of the mesophase intervals (from 8.8 to 95.5) and birefringence (up to 0.33), as well as minus or plus effect of dielectric anisotropies depending on the position of the phenylene. Density Functional Theory (DFT) calculations of molecular conformation, polarisability and dipole moment were used to further study the molecular structure–property relationship.     

Dielectric relaxation in two liquid crystalline polyacrylates under high hydrostatic pressure

For two polyacrylates with p-cyano-phenyl benzoate side groups and different spacer lengths the dielectric relaxation was measured in the nematic and isotropic phase at pressures up to 5000 bar. The relaxation of the cyano groups was observed as a separate relaxation process; it seems to be closely correlated with the glass relaxation. The pressure dependence of the glass temperature and of the clearing temperature could be derived from the experiments.     

Dielectric properties of liquid crystalline composites doped with nano-dimensional fragments of shungite carbon

A cholesteric liquid crystal (CLC) – cholesteryl tridecylate (X-20) was doped with nanoparticles of shungite carbon (Sh) to effectively improve some physicochemical properties of the CLC matrix for the further use in electronic devices. The influence of Sh (concentration of 0.005 and 0.02 wt. %) on phase transition temperatures of X-20 was studied. Addition of 0.005 wt. % of Sh shifts phase transition temperatures upward, while the concentration increase to 0.02 wt. % leads to the opposite effect. These data were taken into account during the study of dielectric properties in different phase states. The dielectric properties were studied in the frequency range from 20 Hz to 10 MHz. Only for the system X-20/Sh (0.02 wt. %), dispersion of the dielectric permittivity was observed. The dispersion was caused by the appearance of additional relaxation processes and it was substantially more extended than the classical Debye theory suggests. The results of the research show that the ‘CLC – Sh nanoparticles’ composites can be used as promising materials to increase the efficiency of radio electronics devices.     

Selected fluorosubstituted phenyltolanes with a terminal group: NCS,CN, F,OCF3 and their mesogenic and dielectric properties and use for the formulation of high birefringence nematic mixtures to GHz and THz applications

Thirty laterally fluorosubstituted new 4-isothiocyanato-, 4-cyano-, 4-fluoro- and 4-(1,1,1-trifluorometoxy)-4′-(4-alkylphenyl)tolanes have been synthesised and their phase transition temperatures and enthalpies and electric permittivity have been measured. These data have been analysed together with data for earlier prepared similar compounds and with recently prepared analogously substituted 4″-alkyl-terphenyls. It was shown that some di-, tri- and tetra-fluorosubstituted 4-isothiocyanato- and 4-cyano-4′-(4-alkylphenyltolanes) are very useful components for the formulation of high birefringence (Δn up to 0.5 measured at 589 nm) and large temperature range nematic mixtures for photonic applications, especially in the infra-red, GHz and THz range of electromagnetic radiation. Three ways of formulation of high birefringence mixtures were proposed.     

The physical properties of alkene-terminated liquid crystal molecules/E8 mixture and the electro-optical properties as they doped in polymer-dispersed liquid crystal systems

Effects of the content of fluorinated alkene-terminated liquid crystal (LC) molecules on the physical properties of the fluorinated alkene-terminated LC/E8 mixture were studied. The morphology and electro-optical properties as they doped in polymer-dispersed liquid crystal (PDLC) films were investigated. The detailed discussion of the obtained results is given. As a result, comparing with the physical properties of the series of LC mixtures with the same content of the analogous fully saturated compounds doped with E8, we find that the birefringence is significantly larger for the LC mixture with the alkene-terminated materials. Both fluorinated alkene-terminated LC molecules and the analogous fully saturated compounds doped with E8 reduce the driving voltage of PDLC films. Moreover, PDLC films with the fluorinated alkene-terminated LC molecules possessed higher contrast ratio and faster response time than that of the PDLC films prepared by adding the same mass fraction of the analogous fully saturated compounds. Thus, the ability to manipulate physical properties of LC mixture and electro-optical properties of PDLC films by changing the LC molecular structures may have future relevance for new LC structures design and applications of PDLC films.     

Novel high birefringence bistolane liquid crystals with lateral fluorosubstituent

Twelve compounds based on lateral fluorinated bistolane moieties with cyano or trifluoromethyl terminal groups were synthesised with Hagihara–Sonogashira cross-coupling reaction. The molecular structures of the compounds were confirmed by Fourier transform infrared, ¹H-nuclear magnetic resonance and mass spectrometry. The transition temperatures, optical textures and optical anisotropies were determined by differential scanning calorimetry, polarizing optical microscopy and ultraviolet microscopy. The effects of the terminal substituents and lateral fluorosubstituent group on their physical properties were investigated and discussed. The compounds with lateral double fluorosubstituent and cyano terminal group exhibited nematogenic texture, while other compounds showed smectic phase. All the compounds exhibited wide liquid crystal temperature ranges and high clearing points. Interestingly, the compounds have the high birefringence in the range of between 0.480 and 0.633.     

Dielectric and elastic properties of fluorine-containing tricyclic isothiocyanate liquid crystal

The dielectric and elastic properties of liquid crystals (LCs) generally depend on the molecule structure, such as polar group and carbon chain length. For further investigation of the influence of molecular structure on the dielectric and elastic properties of fluorine-containing tricyclic isothiocyanate LC, the experimental temperature was controlled at 25°C by precision hot stage, and a precision LCR meter was used to measure the capacitance of six LC cells under the voltage from 0.1 to 20 V at 1 kHz. An LC cell capacitance model and a dual-cell model were adopted to obtain the dielectric anisotropy, and the capacitance–voltage curves of six LC materials were plotted. The threshold voltage of Fréedericksz transition was analysed, and a finite difference iterative method was used to attain specific values of three elastic constants. The influence of molecular structure on the dielectric and elastic constants was finally analysed. Experimental results showed that the introduction of meta-difluoro group would increase the dielectric anisotropy and reduce the threshold voltage of LC. As the length of the alkyl carbon chain increased, the dielectric anisotropy would have an odd–even alternation effect, which would lead to changes in the elastic constants of LC.     

Asymmetrical phenyldiacetylenes liquid crystalline compounds with high birefringence and characteristics of selective reflection

A series of asymmetrically disubstituted liquid crystalline compounds with high birefringence based on phenyldiacetylenes was synthesised by coupling of intermediate molecules with a phenylacetylene fragment. The structures of the intermediates and resulting compounds were confirmed by Fourier transform infrared, ¹H nuclear magnetic resonance and Matrix assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. Their liquid crystalline behaviour, including transition temperatures and phase sequences, was investigated by differential scanning calorimetry and polarising optical microscopy. As an important parameter, the birefringence was also measured, using polarising light interferometry. The results indicated that the compounds exhibited different liquid crystalline phases in the high temperature region, with high birefringence values. In addition, the selective reflection characteristics associated with birefringence under conditions which maintained the matrix nematic liquid crystal and the concentration of chiral dopant were studied.     

Dielectric spectroscopy of unsymmetrical liquid crystal dimers showing wide temperature range TGBA and TGBC* phases

The electrical properties of frustrated twist grain boundary (TGB) phase are a matter of curiosity. Some studies have indicated the existence of soft and Goldstone modes in TGBA and TGBC* phases respectively. However, the experimental results are still not very conclusive. In the present work, we report dielectric studies of wide temperature range TGBA and TGBC* phases of an optically active dimeric compound 4‐n‐decyloxy‐4′‐(cholesteryloxycarbonyl‐1‐butyloxy) chalcone in the frequency range of 1 Hz to 35 MHz for the planar and homeotropic anchoring of the molecules. Two different relaxation processes have been detected for the planar anchoring of molecules in the TGBA and TGBC^* phases. The soft mode like behaviour is obtained due to tilt fluctuation of molecules in the megahertz region for both TGBA and TGBC* phases. Goldstone mode like behaviour due to phase fluctuation of molecules has been detected for the TGBC* phase in the low frequency region (～200–300 Hz). Activation energies for DC conductivity have also been determined for various phases of the material.     

Dielectric properties of a strongly polar nematic liquid crystal compound doped with gold nanoparticles

This study focuses on the electrical characteristics of a strongly polar nematic liquid crystal, Hexyloxy-cyanobiphenyl (6OCB), doped with a low concentration (2% by weight) of citrate buffer stabilised gold nanoparticles (GNPs) at low frequencies between 20 Hz and 35 MHz. The doped samples have lower values of nematic–isotropic transition temperature, permittivity (both parallel and perpendicular to the field direction) and dielectric anisotropy; however, relaxation time and activation energy were increased. The observed results could be explained on the basis of weakly anisotropic nature of GNPs and a local rearrangement of liquid crystal molecules surrounding the nanoparticles. Moreover, a complimentary suggestion on a possible change in the dipole–dipole correlation is made to explain the difference in changes (qualitative and quantitative) observed for permittivity of the host nematic liquid crystal doped with GNP. Temperature dependent dielectric relaxation studies indicate an increase in viscosity and potential barrier; and hence a change in strength of inter-molecular and intra-molecular interactions is suggested.     

Relaxation frequencies for flip-flop rotation of three- and four-ring compounds and dual frequency addressing mixtures with high birefringence

Relaxation frequencies f_r characterising the molecular rotation around the short axes (flip-flop movement) of 45 three- and four-ring polar compounds with high birefringence (mainly cyclohexyl, phenyl and biphenyl tolanes and quaterphenyls terminated with F, OCF₃, NCS group) have been measured. These compounds exhibit a large positive dielectric anisotropy Δε = ε_║ ? ε_⊥ in the nematic phase and relatively low f_r values (in the range of dozens kHz). The dependence of f_r upon molecular structures and physical properties of systems (molecular rigid core length, polarity and viscosity) is discussed. Mixtures for dual frequencies addressing display with the crossover frequency f_co of the order of ~1 kHz have been prepared by joining the above compounds with a mixture consisted of two, three and four fluoro-substituted in the lateral position tolanes and phenyltolanes showing negative dielectric anisotropy. It was stated that relaxation frequency is correlated mainly with their length and polarity.     

The birefringence and extinction coefficient of positive and negative liquid crystals in the terahertz range

Birefringence and extinction coefficients of positive nematic liquid crystal (NLC) MLC-2142 and negative NLC BHR28000-400 are measured by terahertz time-domain spectroscopy system (THz-TDS). Frequency ranging from 0.1 to 0.6 THz, the birefringence of positive NLC MLC-2142 increases with the increase of frequency, and keeps larger than 0.23, which exhibits application potential in tunable broadband LC THz device where a large birefringence is required. In contrast, the birefringence of BHR28000-400 decreases with the increased frequency, which shows a completely different optical property from the positive NLC. The extinction coefficient of the above two kinds of liquid crystals are also investigated.     

Dielectric and visco-elastic properties of laterally fluorinated liquid crystal single compounds and their mixture

The physical properties of three laterally fluorinated liquid crystalline compounds with negative dielectric anisotropy have been studied from static dielectric permittivity, optical birefringence, bend elastic constant, relaxation time and rotational viscosity measurements. Such negative dielectric anisotropy materials find use as components of mixtures for application in vertically aligned mode liquid crystal displays. Moreover, the physical properties of one phenyl cyclohexane compound with positive dielectric anisotropy have also been studied. A five-component mixture comprising these four mesogens and a non-mesogenic component has been formulated and its physical properties have been thoroughly investigated. An attempt has been made to strike a balance between the optical birefringence of the mixture to adjust the cell gap and the dielectric anisotropy and threshold voltage (V_th ) to ensure low driving voltages. The pretilt angle effect on the threshold voltage and the relaxation time has also been studied. At T?=?20°C, the response time decreases to 22% and 41% for the mixture for 2° and 5° pretilt as compared to zero pretilt. On the other hand, at the same temperature the V_th values are decreased by 5% and 9%, respectively.     

Photoswitching properties of photonic band gap material containing azo-polymer liquid crystal

A photochemically tunable photonic crystal was prepared by infiltration of the polymer liquid crystal (LC) having azo-chromophores in a SiO₂ inverse opal structure. The SiO₂ inverse opal film infiltrated with the polymer LC reflected a light corresponding to the periodicity as well as the refractive indices of the inverse opal structure. Linearly polarized light irradiation caused the shift of the reflection band to longer wavelength more than 15 nm. This is caused by the formation of uniaxially anisotropic molecular orientation of the polymer LC. The switched state was stable under interior light, and reversible switching of the reflection band can be achieved by the linearly and circularly polarized light irradiation. This photoswitching property will be suitable not only for various optical materials, but also for introduction of defects in the photonic crystals.     

Investigation of dielectric and electro-optical properties of nematic liquid crystal with the suspension of biowaste-based porous carbon nanoparticles

ABSTRACT

The effect of biowaste porous carbon nanoparticles (PCNPs) on the dielectric and electro-optical properties of nematic liquid crystal (LC) mixture (1823A) of 4-(4-alkyl-cyclohexyl) benzene isothiocyanates and 4-(4-alkyl-cyclohexyl) biphenyl isothiocyanates has been studied. The dielectric permittivity of nematic LC has been increased with the dispersion of carbon NPs. The dielectric anisotropy has been calculated and found to be decreased with the dispersion of PCNPs into the pure nematic LC. The response time and birefringence have been also observed with the variation of temperature, frequency as well as the concentrations of carbon NPs. After the dispersion of PCNPs achieved better birefringence and faster response in the dispersed system, which is the significant application in display devices. Threshold voltage splay elastic coefficient and rotational viscosity have been calculated for both pure and NPs dispersed nematic system. Its value is increased with the dispersion of NPs. Additionally, photoluminescence and figure of merit have investigated as a comparative study of nematic matrix as well the dispersed system. The experimental results have been found to have good agreement with the theoretical data of nematic LC. An effort has been made to explain these experimental results on the basis of interaction between nematic molecules and carbon NPs.     

Dielectric relaxation study of a H shaped liquid crystal dimer

The present article reports dielectric relaxation study of an unusual shaped liquid crystal dimer (H shape) in the temperature range 40–85°C. The study of this liquid crystal dimer is important due to the presence of azo central linkage, which is photosensitive. The dielectric relaxation study indicates coupling between the electric field and the liquid crystal molecule of H shaped dimer. Various dielectric properties for this liquid crystal dimer are quite different from the conventional liquid crystals. The dielectric relaxation observed in the smectic phase is due to the reorientation of molecule along the director in the presence of an applied electric field. The temperature dependence of the dielectric parameters, such as relaxation frequency, relaxation strength and distribution parameter have been evaluated for this liquid crystal dimer for both planar as well as homeotropically aligned cell and then compared. The Cole–Cole equation has been used to evaluate the aforementioned parameters.     

Dielectric and electro-optical properties of a photosensitive liquid crystal

The temperature dependencies of the dielectric and electro-optical properties of a pure photo-ferroelectric liquid crystal have been investigated, in the chiral smectic C (SmC*) phase, with and without ultraviolet (UV) illumination. The SmC* dielectric Goldstone mode characteristics, the spontaneous polarization, the tilt angle and the rotational viscosity are affected by UV irradiation. Under increasing UV light intensities the dielectric strength Δ?_G increases whereas the relaxation frequency f_G , the spontaneous polarization P _s, the electro-optical rotational viscosity γ_eo and the tilt angle θ decrease. The twist elastic constant K ₃ and the Goldstone mode rotational viscosity γ_G of the studied compound are deduced from dielectric experimental results. Good agreement was found between both viscosities. The observed dielectric behaviours are controlled by the elastic constant and the rotational viscosity variations.     

The synthesis of liquid crystalline lanthanide complexes and their magnetic birefringence

Liquid crystalline complexes of the type [ML(LH)₂](NO₃)₂, where LH=C₇H₁₅O-C₆H₃(OH)CH=N-C₁₈H₃₇ and M=Gd, Dy, and La, have been obtained. All of them exhibit the S_A phase and have high magnetic birefringence constants due to the association of the complexes in solution. In the case of the dysprosium derivative, the effect is much stronger (four orders of magnitude greater than the usual value) because of the high magnetic anisotropy of the particles of the complex.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1003–1005, June, 1994.The authors are grateful to Prof. S. G. Vul'fson for helpful advice and discussions.     

Influence of floating-ring-electrode on large-aperture liquid crystal lens

Large-aperture liquid crystal (LALC) lens with hole-patterned electrodes possesses small lens power and high addressing voltage because of the thick dielectric layer inserted between the hole-patterned electrode and LC layer. With an embedded narrow floating ring electrode (FRE), the lens power and addressing voltage of the LALC lens could be effectively increased and decreased, respectively. In this study, we analyse the electro-optic performance of LALC lens upon variation of the diameter of the embedded FRE. Results reveal that the FRE diameter determines the electric-field distribution and hence the electro-optic behaviour of the LALC lens. The LALC lens with embedded 2-mm-diameter FRE has excellent lens properties, such as low aberration, high focal quality and modulation transfer function performance comparable with solid glass lens.