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.     

Electrooptical properties of new type fluorinated phenyl-tolane isothiocyanate liquid crystal compounds

A series of 4?-(4-alkyl-phenyl)-2?,6?-difluorotolane isothiocyanate liquid crystal (LC) compounds were synthesised, and their phase transitions and electrooptical properties were investigated. These compounds exhibit high birefringence, about 0.47–0.52. As the number of carbon atoms in the alkyl chains increases, these LC compounds can exhibit smectic phases. When these LCs were mixed into the commercial LCs, the birefringence values of LC mixtures become higher than pure commercial LCs, and the visco-elastic coefficients of five LC mixtures are very close to each other at every test temperature. The results of response properties indicate that the compounds with a tri-fluorine substitution and n-propyl end group possess better response performance than the others. These LC compounds are particularly attractive for achieving fast response times in LC optic devices.     

New terphenyl liquid crystals terminated by 2-chloro-3,3,3-trifluoropropenyl group

New liquid crystal (LC) compounds composed of terphenyl as a core and 2-chloro-3,3,3-trifluoropropenyl terminal group were synthesised. Their structures were confirmed by nuclear magnetic resonance, mass spectra and infrared spectrometry. The mesomorphic and physical properties were evaluated. The results show that these compounds have a smectic phase, large dielectric anisotropy, high birefringence and suitable rotational viscosity. The lateral fluorine substitutions lead to a large perpendicular component of the dielectric constant, and a small value of Δε/ε_⊥. These characteristics are significant for the fringe-field switching LC display mode.     

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.     

New isothiocyanato liquid crystals containing thieno[3,2-b]thiophene central core

New isothiocyanato liquid crystal (LC) materials based on thieno[3,2-b]thiophene core have been synthesised and characterised. Their mesomorphic and physical properties were evaluated. The results indicate that most of the new compounds exhibit only enantiotropic smectic A phases with high clearing points, and the difluorinated compounds (A3 and A6), in particular, also display a stable nematic phase with low melting points. Compared with the corresponding biphenyl and thiophene-phenyl analogues, these thieno[3,2-b]thiophene-based compounds possess increased birefringence (the highest value found to be 0.40) and mesogenic phase intervals. These new thieno[3,2-b]thiophene-based LCs exhibit a high birefringence and a high clearing point and are of potential use as components of high birefringence mixtures.     

Synthesis and evaluation of dinaphthylacetylene nematic liquid crystals for high-birefringence materials

We synthesised 1,2-bis(6-alkoxy-2-naphthyl)acetylene as a new dinaphthylacetylene liquid crystal (LC) compound with alkoxy tails of length m (DNA–OCm) and characterised their phase behaviour by polarising optical microscopy and differential scanning calorimetry, where all compounds exhibited a nematic phase. Also, we measured the actual, and not extrapolated, Δn values of this target compound, and our results show the birefringence potential of the pure LC compound. They have high birefringence values, with the highest value of approximately 0.5 obtained for DNA–OC2 at 550 nm at 10°C above the nematic?crystal transition temperature.     

Synthesis,characterisation and liquid crystal properties of 2,5‐bis[5‐alkyl(alkoxy)phenyl‐1,3,4‐oxadiazole]bromobenzenes

A series of new thermal bilateral liquid crystal compounds with the phenylenebis‐1,3,4‐oxadiazole structure was synthesised. The molecular structures of the oxadiazole compounds were confirmed by FT‐IR and ¹H NMR spectroscopy, elemental analysis and mass spectrometry. Thermogravimetric analysis indicates that the compounds in an atmosphere of nitrogen have good thermal stability. Measurements using differential scanning calorimetry, polarising optical microscopy and temperature‐dependent wide‐angle X‐ray diffraction indicated that the liquid crystalline properties are related to the length of the end groups. When the end group was tert‐butyl or alkoxy with four and six carbons, the compounds exhibit no liquid crystal phase. However, compounds with end groups containing more than eight carbons show significant bidirectional thermally‐induced liquid crystal properties and the structure of the liquid crystal phase is the lamellar smectic A phase. All the compounds exhibit blue fluorescence.     

Synthesis of new wide nematic diaryl-diacetylenes containing thiophene-based heteromonocyclic and heterobicyclic structures,and their birefringence properties

New monocyclic and bicyclic thiophene-based diaryl-diacetylene liquid crystal (LC) materials were synthesised and demonstrated to exhibit wide enantiotropic nematic phases. One thiophenyl-benzene derivative, in particular, displayed a stable nematic phase across a temperature range of over 100°C. The birefringence properties of the compounds were measured using a multiple-beam interference (MBI) method, with the highest value found to be for the same thiophenyl-benzene molecule (Δn = 0.57, 550 nm) at 10°C above the crystal-to-nematic transition temperature. Furthermore, the effects of the cyclic structure on the mesomorphic behaviours, transition temperatures, and birefringence properties were investigated in detail from both experimental and theoretical viewpoints.     

High birefringent reactive discotic liquid crystals based on asymmetrical triphenylene with phenyl-acetylene moieties

Triphenylene (TP)-based liquid crystals, a category of discotic liquid crystals (DLCs), are easy to synthesise, thermally stable and can undergo self-assembly. A new DLC compound, 2,7,10-tris[4-(6-acryloyloxyhexyloxy)benzoate]-3,6,11-tris[(4-hexylphenyl)ethynyl]-triphenylene, was obtained with a 49% yield using catechol as a starting material and features acetylene groups on its TP core. This material can form films with high birefringence through its cross-linkable acrylate ends. Here, we synthesised this new compound and characterised it using nuclear magnetic resonance spectroscopy, mass spectrometry and elemental analysis. The thermal behaviour was also investigated using differential scanning calorimetry and polarised optical microscopy. This new asymmetric TP-based DLC compound exhibited a nematic liquid-crystalline phase between ?20 and 170°C and formed an optical anisotropic film with a high birefringence (Δn = 0.21–0.25).     

Novel diphenylacetylene-based room-temperature liquid crystalline molecules with alkylthio groups,and investigation of the role for terminal alkyl chains in mesogenic incidence and tendency

In this study, we offer a new family of alkylthio-containing diphenylacetylene-based liquid crystalline molecules (the so-called tolanes) showing nematic phases stable down to room temperature and high birefringence, with insights into the role for terminal alkyl chains in mesogenic incidence and tendency. A number of asymmetric tolane homologues having various alkyl chain lengths in the alkylthio and alkyl groups at each p-, p’-position were synthesised, and their phase transition behaviour was investigated by polarising optical microscope observation, differential scanning calorimetry and wide-angle X-ray diffraction measurement. Consequently, several homologues exhibited monotropic nematic or highly ordered smectic (soft crystal) phases stable down to room temperature. It is found that a long alkyl group on the side opposite to an alkylthio group is prerequisite for mesogenic incidence. In addition, a nematogenic homologue exhibited a higher birefringence value of 0.20 compared to that of 0.19 for an alkoxy counterpart.     

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 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.     

Synthesis and the effect of 2,3-difluoro substitution on the properties of diarylacetylene terminated by an allyloxy group

A series of 2,3-difluoro tolane allyloxy-based liquid crystals (LCs), which are composed of the tolane unit, lateral fluoro substituents, alkoxy and allyloxy terminal groups, were synthesised with 4-bromo-2,3-difluorophenol as the starting material via a five-step reaction. Their structures were confirmed by infrared spectroscopy (IR), mass spectroscopy (MS) and nuclear magnetic resonance (NMR). The mesomorphic properties were investigated by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). The dielectric measurement was performed using an independent analyser EC-1 and the birefringence was determined using an Abbe refractometer. The results reveal that LCs have large negative dielectric anisotropy and high birefringence, which are potentially important for the fast response displays and liquid crystal photonic devices (LCDs).     

Liquid crystal birefringence for millimeter wave radar

Abstract

Many liquid crystals are found to have relatively high birefringence (Δn) values in the microwave and millimeter wave regions, as calculated from the phase shift induced by their reorientation by magnetic or electric fields. At 30 GHz, Δn values were obtained in the range of 0.08 to 0.18 for eleven liquid crystal mixtures of various types. The most favourable liquid crystal structures for high millimeter wave birefringence are highly conjugated rod-like molecules containing biphenyl, terphenyl, phenylpyrimidine, biphenylpyrimidine, and tolane groups in nematics of positive dielectric anisotropy (Δε). However, other liquid crystal structures including Schiffs base, azoxybenzene, and aromatic ester groups also have substantial birefringence, including nematics with negative and crossover Δε, as well as cholesteric nematics. The Δn varied only slightly at different frequencies of microwave millimeter wave in the 15–94 GHz range. Studies on magnetic and electrical field liquid crystal orientation in specially designed waveguides provide a basis for new types of modulators and scanning array antennae in the millimeter wave region, where more compact liquid crystal modulation media can be used than in the microwave region. These scanners can be used for both sending and receiving radar signals for potentially low cost radar systems.     

Synthesis, characterisation and liquid crystal properties of 2,5-bis[5-alkyl(alkoxy)phenyl-1,3,4-oxadiazole]bromobenzenes

A series of new thermal bilateral liquid crystal compounds with the phenylenebis-1,3,4-oxadiazole structure was synthesised. The molecular structures of the oxadiazole compounds were confirmed by FT-IR and ¹H NMR spectroscopy, elemental analysis and mass spectrometry. Thermogravimetric analysis indicates that the compounds in an atmosphere of nitrogen have good thermal stability. Measurements using differential scanning calorimetry, polarising optical microscopy and temperature-dependent wide-angle X-ray diffraction indicated that the liquid crystalline properties are related to the length of the end groups. When the end group was tert-butyl or alkoxy with four and six carbons, the compounds exhibit no liquid crystal phase. However, compounds with end groups containing more than eight carbons show significant bidirectional thermally-induced liquid crystal properties and the structure of the liquid crystal phase is the lamellar smectic A phase. All the compounds exhibit blue fluorescence.     

Synthesis and mesomorphic properties and optical behaviour analysis of homologous series azobenzene liquid crystal monomer with polar substituents

ABSTRACT

Twenty novel azobenzene liquid crystal micromolecular compounds named ω-[4-(p-substituted azobenzeneoxy carbonyl]acid (X-ABCnA) have been designed and synthesised, followed by studies on the thermal performance and mesomorphic properties of the compounds. The liquid crystal compounds were divided into five homologous series based on the terminal substituents R (R = CH₃O, CH₃, H, Cl, NO₂). In each series, the number of carbons on flexible chain was 4, 6, 8 and 10, respectively. Fourier-transform infrared, proton nuclear magnetic resonance and elementary analysis demonstrated that the structure of the synthesised azobenzene liquid crystal compounds was consistent with the molecular design. The mesomorphic properties were tested, analysed and characterised by using differential scanning calorimetry and polarised optical microscopy. The melting transition (T _m) of all the compounds in homologous series with different substituents appeared to decrease with the increase of carbon numbers on flexible chains. The same held true for the temperature of isotropic-mesophase/crystalline transition. The compounds with stronger polarity of terminal substituents were more likely to form broader mesogenic ranges. The liquid crystal compounds discussed in this work can be regarded as a reference for the synthesis of mesogenic arms participating in the synthesis of novel multi-arm liquid crystalline macromolecules and polymers.     

Tolane liquid crystals bearing fluorinated terminal group and their mid-wave infrared properties

This paper describes the synthesis and characterisation of several liquid crystal (LC) compounds having a tolane core structure and a trifluoromethoxy terminal group. The mesomorphic properties, mid-wave infrared (IR) absorption and birefringence were investigated. These materials exhibit low absorption in mid-wave IR regions (MWIR) and high birefringence (Δn ~ 0.26–0.29). The impact of fluorine substitution was investigated. Compounds with monofluorine exhibit wide mesophase temperature (88.5°C), and the mesomorphic behaviour weakened with increasing fluorine atom. The results show that they are promising LC materials for applications in the MWIR.     

New isothiocyanatotolane liquid crystals with terminal but-3-enyl substitute

Four kinds of new but-3-enyl-based isothiocyanate liquid crystals composed of tolane core and but-3-enyl terminal group (A1–A4) were synthesised via seven step reactions based on 2-(4-bromophenethyl)-1,3-dioxolane, and four n-butyl analogues B1–B4 as comparison structures were also prepared. The mesomorphic properties and physical properties of the compounds were investigated. Single fluorinated compounds A2 and A3 exhibit monotropic nematic phase, and they have lower melting enthalpy and higher clearing points than those of the comparison compounds B2 and B3. The non-fluoro-substituted compound A1 and difluorinated compound A4 exhibit no nematic phase. Replacement of n-butyl chain by but-3-enyl as terminal group is enabled to increase birefringence (Δn ~ 0.394–0.430) and reduce rotational viscosity. These isothiocyanatotolanes with terminal but-3-enyl substitution have potential application for high birefringence mixtures.     

Syntheses of new diluents for medium birefringence liquid crystals materials

New liquid crystals of 3HEHV ((4-trans-propyl-cyclohexyl-1), (4?-vinyl-cyclohexyl-1?)-ethane-1,2) and 3HB2V ((4-trans-propyl-cyclohexyl-1), (4-butenyl-3,4)-benzene) were synthesised and evaluated as the diluents for medium birefringence liquid crystal mixtures. The procedures transferring cis-isomers into trans-isomers of the cyclohexane were described. The results showed that both 3HEHV and 3HB2V decrease the crystallisation temperature and the viscosities by doping them with the medium birefringence mixtures in a fraction of 15%. The characteristics of the structures are correlated with the properties of the mixtures.     

Synthesis and mesomorphic properties of super high birefringence isothiocyanato bistolane liquid crystals

Four series of high birefringence bistolane liquid crystals containing isothiocyanato termal groups were synthesized and characterized. As well as the phenyl group, both biphenyl and naphthyl moieties were introduced to enhance the birefringence. These bistolane compounds exhibit reasonably low melting points and high birefringence of 0.5-0.8. A eutectic mixture was formulated from these compounds exhibiting a wide nematic range, high figure-of-merit and low viscosity.