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).     

Synthesis and physical properties of tolane liquid crystals containing 2,3-difluorophenylene and terminated by a tetrahydropyran moiety

Four series of tolane liquid crystals containing 2,3-difluorophenylene and terminated by a tetrahydropyran (THP) moiety have been synthesised via multistep reactions based on 4-alkylcyclohexanecarboxylic acids and 4-bromo-2,3-difluorophenol. Their properties were measured by differential scanning calorimetry, polarising optical microscopy, Abbe refractometer and an electrical constants instrument. The results show that the THP-based liquid crystals nT reveal a relatively high value of the birefringence, negative dielectric anisotropy, good solubility and a broad nematic mesophase. The effects of the terminal alkyl chain, the lateral fluoro substituents and the THP terminal group on the mesomorphic and physical properties are discussed. Meanwhile, density functional theory calculations of molecular conformation and polarisability were conducted to correlate the experimental findings.     

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.     

Cyano terminated tolane compounds for polymer dispersed liquid crystal application: relationship between cyano terminated tolane based molecular structures and electro-optical properties

ABSTRACT

The structures of the liquid crystal (LC) molecules have a key role in impacting the electro-optical performance of a polymer dispersed liquid crystal (PDLC) film. In this paper, the relationship between the LC molecular structures and the electro-optical properties of PDLC films is investigated based on an unexplored cyano-terminated tolane compounds (CTTCs) doped E8 LCs/UV polymers system. Due to the high polarity of CTTCs, LCs doped with the cyano-terminated tolane (CTT) molecules exhibit high birefringence and large positive dielectric anisotropy. On the whole, PDLC films doped with the CTT molecules exhibit a lower driving voltage than that doped with the pure E8. More excitingly, PDLC films based on CTT molecules with larger length-to-width ratio and longer conjugated system show higher contrast ratio (CR) and faster response time. Eventually, the mechanism of the effects of CTT-based molecular structures and the relationship between the electro-optical performance of PDLC films and CTT molecules are illustrated. This work paves a new way for optimising the electro-optical properties of PDLC films.     

Nematogenic laterally fluorinated biphenyls with polar terminal groups

Abstract

The physical properties are reported for a series of positive dielectric anisotropy (δε) alkylbiphenyls possessing a halogen-containing terminal group and incorporating varying degrees of lateral fluorination. The nematic range and dielectric and optical anisotropies can be varied by the appropriate choice of terminal group and extent of lateral substitution. These materials possess greater stability than the corresponding cyano compounds and are suitable for use in applications that require high stability.     

Synthesis of cyclopentyloxy terphenyl liquid crystals with negative dielectric anisotropy

Liquid crystalline materials possessing negative dielectric anisotropy (Δε) have attracted considerable attention because they can be used to formulate eutectic mixture for several display applications. The negative Δε can be achieved by introducing a lateral polar substituent onto the mesogen core of a liquid crystalline material, and fluorine atoms are usually used because of the small size and high electronegativity. 2,3-ortho-Difluoro substituent liquid crystals exhibit stable and profound physical properties such as the low viscosity, strong negative Δε, and high solubility. To avoid the decrease of the mesophase range, 2,3-ortho-difluorophenyl is often placed next to the terminal chain. In this paper, we have synthesised a new series of 2,3-ortho-difluoroterphenyl liquid crystals with a negative dielectric anisotropy. Ether oxygen is introduced to the structure because it can further increase the lateral dipole moment. 3-Alkylcyclopentane end group decreases both the melting point and the viscosity, which makes it an appropriate substituent for negative Δε 2,3-ortho-difluoroterphenyl liquid crystals. The mesomorphic properties as well as the optical anisotropies of the synthesised liquid crystals are discussed in this paper. All compounds in the series are found to possess highly negative dielectric anisotropy, wide mesophase ranges; therefore, they have a great potential to formulate eutectic mixture for liquid crystal displays.     

Synthesis and physical properties of laterally fluorinated liquid crystals containing 1,3,2-dioxaborinane and cyclohexyl units

Six series of laterally fluorinated liquid crystals containing 1,3,2-dioxaborinane and cyclohexyl units have been synthesized and characterized. Their mesomorphic properties were characterized by polarizing optical microscopy and differential scanning calorimetry. All these compounds are thermotropic liquid crystalline materials. All three-ring compounds (series A, B and C) and most four-ring compounds (series D, E and F) exhibit only a nematic phase, while some four-ring compounds with long terminal alkyl chains show nematic, smectic A and even smectic B phases. In addition, four compounds containing a 1,3-dioxane unit were prepared, and their mesomorphic properties compared with their 1,3,2-dioxaborinane analogues. The dielectric anisotropies of selected target compounds were determined; some showed a negative anisotropy. The relationships between the properties and chemical structures of these new compounds are discussed.     

Synthesis and mesomorphic properties of fluoro and isothiocyanato biphenyl tolane liquid crystals

Four series of high birefringence biphenyl tolane liquid crystals having a terminal isothiocyanato or fluoro group were synthesized; their mesomorphic properties were studied by optical polarizing microscopy and DSC. These biphenyl tolane compounds exhibit reasonably low melting points and high birefringence of 0.37-0.49. By using these compounds a eutectic mixture was formulated exhibiting a wide nematic range, high figure-of-merit and low viscosity.     

Synthesis and mesomorphic properties of fluoro and isothiocyanato biphenyl tolane liquid crystals

Four series of high birefringence biphenyl tolane liquid crystals having a terminal isothiocyanato or fluoro group were synthesized; their mesomorphic properties were studied by optical polarizing microscopy and DSC. These biphenyl tolane compounds exhibit reasonably low melting points and high birefringence of 0.37–0.49. By using these compounds a eutectic mixture was formulated exhibiting a wide nematic range, high figure‐of‐merit and low viscosity.     

Dielectric studies of tolane derivatives exhibiting the E and K phases

Results of dielectric studies for three tolane derivatives containing the isothiocyanato (NCS) and alkyl or alkoxy groups in the terminal positions are presented. A relaxation process associated with molecular rotations around the short axes has been observed in the crystal-like smectic phases (B, E, K). The K phase was characterized for the first time by dielectric spectroscopy. The results are discussed by taking into account the dielectric relaxation data obtained recently for the biphenyl analogues in the E phase. The available X-ray data allow us to calculate the so-called packing parameter which characterizes the free volume of the molecules in the unit cells.     

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.     

Dual frequency liquid crystals: a review

We review the recent progress in developing high performance dual frequency liquid crystal (DFLC) materials. A new figure of merit is derived for evaluating the DFLC performance. The dielectric relaxation and electro-optical properties of several high performance compounds, which possess a positive dielectric anisotropy at low frequencies and a relaxation frequency in the kilohertz region, are presented. Potential applications of these compounds are discussed. Compounds with a negative dielectric anisotropy were also compared and discussed. Two experimental DFLC mixtures were demonstrated with one intended for room temperature operation and another for elevated temperature operation.     

The effect of intermolecular actions on the nematic phase range of tolane-liquid crystals

An approach to understand the effect of intermolecular actions on the nematic stability, a series of tolane compounds nH containing intermolecular π–π stacking, dipole–dipole and hydrogen bond interactions, are developed and investigated. Their mesophase behaviour were measured by differential scanning calorimetry (DSC) and polarising optical microscopy (POM), the results show that carboxylic acids nH exhibit high melting points and narrow nematic phase intervals, which are attributed to the too strong intermolecular actions. By disrupting the intermolecular hydrogen bond, their corresponding methyl esters nC have been designed and synthesised. It is interesting to note that these compounds exhibit broad nematic mesophase intervals and low melting points. The above results demonstrate that the interruption of intermolecular actions is an effective way to improve the nematic stability. In addition, the effects of the terminal alkyl chains and the terminal polar groups on the nematic stability were also discussed. Finally, DFT calculations of molecular conformation and dipole moment were conducted to better understanding of the molecular structure–mesomorphic property relationship.     

新颖的含乙炔桥键液晶分子设计与合成

为降低液晶分子的双折射率并获得宽液晶相区, 用环己基替代传统二苯乙炔液晶分子中的一个苯环, 得到一类结构新颖的含乙炔桥键的负介电各向异性液晶分子4a, 4b. 目标化合物采用1-trans-4-(trans-4-正戊基环己基)环己基乙炔与4-碘-2,3-二氟苯基醚进行Sonogashira偶联反应制备, 反应总产率27%～28%. 产物结构经MS, IR, NMR鉴定确认. 采用DSC结合偏光显微镜对液晶相变温度进行了测试, 结果表明新化合物4a, 4b清亮点分别为212, 216 ℃, 向列相温区均达到140 ℃, 与传统二苯乙炔类液晶的相变温区基本相当. 物理性能测试表明, 分子骨架共轭程度的削弱不仅使双折射率大幅降低, 而且介电各向异性绝对值还有所增大, 为分子设计提供了新的思路. 新化合物综合性能得到显著改善, 在大尺寸液晶电视领域具有非常好的应用前景.     

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.     

Amide as Terminal Groups: Synthesis and Properties as New Tolane‐Type Liquid Crystals

A series of novel tolane‐type liquid crystals with amide group as terminal group have been prepared. The terminal amide groups were modified, and the influence of these structural parameters on liquid crystal phases was investigated by polarizing optical microscope (POM) and differential scanning calorimetry (DSC). Three of these new compounds exhibit nematic phase, good thermal stabilities. In general, these liquid crystals with amide as end groups have high melting points and phase transition temperatures, which result from the hydrogen bonds. Based on theoretical calculations, these new molecules with strong electron donating amide as end group have narrower HOMO‐LUMO energy gap and higher dipole moment than tolane.     

Synthesis of UV‐curable liquid crystalline diacrylates for the application of polarized electroluminescence

Two series of UV‐curable rod‐like diacrylates containing bis‐tolane (M1–M5) and bis‐stilbene (M6–M10) mesogens were synthesized. All of the diacrylates were prepared through convergent syntheses involving palladium‐catalysed coupling reactions. The thermal transitions and mesomorphic properties of the diacrylates were characterized by differential scanning calorimetry and optical microscopy. All the bis‐tolane‐based monomers reveal nematic phases, except M5 which shows no mesomorphic behaviour. For the bis‐stilbene based monomers, most show no mesomorphic behavior and only M7 and M9 exhibit a SmA phase.     

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.     

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.     

Synthesis and physical properties of laterally fluorinated liquid crystals containing 1,3,2-dioxaborinane and cyclohexyl units

Six series of laterally fluorinated liquid crystals containing 1,3,2-dioxaborinane and cyclohexyl units have been synthesized and characterized. Their mesomorphic properties were characterized by polarizing optical microscopy and differential scanning calorimetry. All these compounds are thermotropic liquid crystalline materials. All three-ring compounds (series A, B and C) and most four-ring compounds (series D, E and F) exhibit only a nematic phase, while some four-ring compounds with long terminal alkyl chains show nematic, smectic A and even smectic B phases. In addition, four compounds containing a 1,3-dioxane unit were prepared, and their mesomorphic properties compared with their 1,3,2-dioxaborinane analogues. The dielectric anisotropies of selected target compounds were determined; some showed a negative anisotropy. The relationships between the properties and chemical structures of these new compounds are discussed.