Laterally dibenzyloxy-branched nematogens with large nematic range and rich solid polymorphism

Mesogenic compounds containing four rings in the core usually have very high melting points. However, when two identical lateral benzyloxy groups are introduced on the same side of one of the central rings, the melting point is lowered dramatically and a large nematic range is retained. This range is affected by the bulkiness of the para-substituents in the lateral rings. Methyl groups can be introduced in the ortho- or meta-positions with a consequent decrease in the melting temperature without much affecting the nematic range. These compounds exhibit a rich solid polymorphism which is certainly related to the effect of the conformations of the lateral substituent on the molecular arrangment in the solid phase. Some preliminary NMR experiments on the nematic phase indicate that the molecular long axis coincides with the core axis, whereas the para-axis of the lateral fragment makes an angle close to the magic angle with respect to the molecular long axis.     

Hydroxy-functionalized liquid-crystalline polyazomethines I. Synthesis, characterization and structure-mesogenic behaviour relationship

A new series of polyazomethines based on hydroxy-functionalized mesogenic units has been synthesized and characterized. Different structural variations have been checked in order to obtain mesogenic polyazomethines with low melting temperatures and a broad nematic range. Completely conjugated mesogenic units with a lateral substituent or alteration of coaxiality are shown to be the most suitable structural variations. No mesogenic behaviour was observed for polyazomethines with a flexible mesogenic core due to unfavourable conformational equilibrium. Polyazomethines have complex thermal behaviour and show an increase in the degree of polymerization on annealing.     

Laterally dibenzyloxy-branched nematogens with large nematic range and rich solid polymorphism

Mesogenic compounds containing four rings in the core usually have very high melting points. However, when two identical lateral benzyloxy groups are introduced on the same side of one of the central rings, the melting point is lowered dramatically and a large nematic range is retained. This range is affected by the bulkiness of the para-substituents in the lateral rings. Methyl groups can be introduced in the ortho- or meta-positions with a consequent decrease in the melting temperature without much affecting the nematic range. These compounds exhibit a rich solid polymorphism which is certainly related to the effect of the conformations of the lateral substituent on the molecular arrangment in the solid phase. Some preliminary NMR experiments on the nematic phase indicate that the molecular long axis coincides with the core axis, whereas the para-axis of the lateral fragment makes an angle close to the magic angle with respect to the molecular long axis.     

Synthesis of novel liquid crystal compounds with aromatic amide mesogenic cores

The synthesis and properties of a new series of compounds having aromatic amide mesogenic cores are reported. Most of these new compounds are thermotropic mesogens. In consideration of the fact that aromatic amides form crystals of high melting point which is unfavourable for the formation of thermotropic liquid crystals, we make use of lateral substitution to decrease both the packing efficiency and the hydrogen bonding, so that the melting temperature of the aromatic amides is sufficiently depressed. The lateral substituent used in these new compounds is bromine. In order to investigate the influence on properties of the end groups, different alkoxy, alkyl and other groups are used at the two ends of the rod-like molecules. The two ends are either identical or different, with an electron-donating alkoxy as one end and the electron-accepting cyano group as the other. The results indicate that appropriate lateral and terminal substitution is essential for the aromatic amides to form thermotropic liquid crystals. The peculiar mesophase characterized by an X-ray diffraction pattern of a SmC phase, but a texture of a nematic phase is also noted.     

The effect of a lateral aromatic branch on the orientational ordering of laterally alkoxy substituted nematics

Two new mesogenic series have been synthesized. They have the same main core which contains four aromatic rings, with two lateral substituents on the same side of one of the inner rings. One of the substituents is a 4-chlorobenzyloxy group and the second is an alkoxy chain. The positions of the lateral substituents are different in the two series. Despite the large aromatic branch, an enantiotropic nematic phase is obtained for most of the compounds. The orientational ordering behaviour of these new compounds has been investigated by C-13 NMR, and two isomeric compounds containing a lateral dodecyloxy chain were chosen for the study. The order parameters of the chain were obtained by a 2D C-13 NMR technique with variable angle spinning. The temperature dependence of the order parameters was determined using the C-13 chemical shifts with slow spinning of the sample parallel to the magnetic field. The first methylene fragment in each lateral substituent of each compound has a positive C-H bond order parameter, implying that this fragment adopts a cis conformation in the nematic phase. The two lateral substituents are folded back along the mesogenic core in opposite directions. The lateral chain is found to be roughly aligned along the molecular long axis imposed by the core, whereas the para -axis of the phenyl ring in the lateral aromatic branch makes a considerable angle with the molecular long axis. It was also found that the different patterns of substitution do not affect the position of the molecular long axis to any large extent.     

Fluorinated tolan-type nematic mesogens containing a 1,3-dioxan unit

Twenty new 2,5-disubstituted 1,3-dioxans were prepared, and their mesomorphic properties were investigated by optical polarizing microscopy and DSC; all of the compounds exhibit the nematic phase only. With fluorination in the mesogenic core, the melting and clearing points were reduced, but their nematic phase range only slightly shortened.     

Fluorinated tolan-type nematic mesogens containing a 1,3-dioxan unit

Twenty new 2,5-disubstituted 1,3-dioxans were prepared, and their mesomorphic properties were investigated by optical polarizing microscopy and DSC; all of the compounds exhibit the nematic phase only. With fluorination in the mesogenic core, the melting and clearing points were reduced, but their nematic phase range only slightly shortened.     

Improved nematic mesophase stability of benzoxazole-liquid crystals via modification of inter-ring twist angle of biphenyl unit

A series of nematic heterocyclic liquid crystals, 2-(2?-fluoro-4?-alkoxy-1,1?-biphenyl-4-yl)-benzoxazole derivatives (coded as nPF(2)PBx) bearing hydrogen (nPF(2)PBH), methyl (nPF(2)PBM) and nitro (nPF(2)PBN) terminal groups, respectively, are developped and investigated. The results show that the nematic mesophase stability is obviously improved via a slightly increase in the inter-ring twist angle between the planes of two phenyl rings in the biphenyl unit, where modification of the twist angle can be achieved by adjusting the position of the lateral monofluorine substituent from the outer to the inter ring of the biphenyl mesogenic core unit. Meanwhile, the inter-ring lateral fluorine substituent results in a decrease in both melting and clearing points but only a nematic mesophase with an acceptably wide mesophase range for nPF(2)PBx, and thus a potential application in liquid crystal display mixtures is expected, especially for the compounds nPF(2)PBM and nPF(2)PBN. The reduced π–π interaction/conjugation caused by the increased twist angle is considered as one of the possible reasons for the formation of a nematic mesophase.     

Nematogens incorporating a lateral flexible ring

A lateral flexible ring can be introduced on a mesogenic core containing four rings. Lateral aliphatic rings containing up to 12 atoms have been synthesized. Despite this large lateral protruding substituent, a large nematic range is obtained. A compound with a benzo15-crown-5 lateral ring has been successfully obtained and shows the possibility of designing nematic liquid crystals with a crown ether moiety at a lateral position.     

Synthesis of side chain liquid-crystalline polysiloxane containing trans-cyclohexane-based mesogenic side groups

The synthesis and characterization of nine new side chain liquid-crystalline polysiloxanes containing one cyclohexyl ring and another 2-4 aromatic rings in their mesogenic side groups are described. All synthesized polymers displayed nematic mesomorphism. Most of the polymers showed a very wide mesomorphic temperature range. The mesogenic core length has profound influence on the phase transitions of the polymers. The mesomorphic temperature range increased with increasing mesogenic core length.     

Nematogens incorporating a lateral flexible ring

A lateral flexible ring can be introduced on a mesogenic core containing four rings. Lateral aliphatic rings containing up to 12 atoms have been synthesized. Despite this large lateral protruding substituent, a large nematic range is obtained. A compound with a benzo15-crown-5 lateral ring has been successfully obtained and shows the possibility of designing nematic liquid crystals with a crown ether moiety at a lateral position.     

Synthesis of side chain liquid-crystalline polysiloxane containing trans-cyclohexane-based mesogenic side groups

Abstract

The synthesis and characterization of nine new side chain liquid-crystalline polysiloxanes containing one cyclohexyl ring and another 2-4 aromatic rings in their mesogenic side groups are described. All synthesized polymers displayed nematic mesomorphism. Most of the polymers showed a very wide mesomorphic temperature range. The mesogenic core length has profound influence on the phase transitions of the polymers. The mesomorphic temperature range increased with increasing mesogenic core length.     

Effect of the chain length on the thermal and analytical properties of laterally biforked nematogens

Three laterally substituted liquid crystals were synthesized in order to investigate the effect of a lateral biforked chain on the thermal and analytical properties. The mesogenic molecules have the same core containing four aromatic rings connected by two ester and one diazo linkages, they differ by the length of one chain within the lateral biforked substituent. The phase transition temperatures were obtained by polarized light microscopy and differential scanning calorimetry (DSC). The clearing temperature and the nematic range decrease with increasing length of the lateral biforked chain. The stationary phases derived from these nematogens provide excellent resolution of various classes of compounds, including aromatic hydrocarbons (AH), substituted benzenes, polycyclic aromatic hydrocarbons (PAH), phenols and volatile organic compounds (VOC) present in the essential oils. The selectivities of the stationary phases were found to decrease according to the length of the side chain.     

A novel series of styrene-based liquid crystal monomers displaying either nematic or chiral nematic phases

A new series of liquid crystalline styrene-based monomers is described. These monomers are prepared by the DCC-mediated esterification reaction between 4-[11-(4-vinylphenoxy)undecyloxy]benzoic acid and a range of phenols chosen due to their proven utility in the synthesis of liquid crystals. Most members of the series display thermally stable (enantiotropic) nematic phases, although a few give only monotropic nematic phases. By incorporating the (S)-2-methylbutyl side chain, monomers that exhibit the chiral nematic phase can be obtained. Predictably, monomers derived from phenols containing an additional ring as substituent (e.g. 4-cyano-4'-hydroxybiphenyl) display relatively high transition temperatures. In contrast, monomers derived from simple 4-n-alkylphenols possess a nematic phase, which is accessible at moderate temperatures. In addition, a eutectic mixture derived from these monomers has a melting point only just above room temperature, which is an advantage for the fabrication of robust films via the in situ photopolymerization process. Standard free radical polymerization of a number of these monomers provides side chain liquid crystal polymers, SCLCPs, with mesophases that are stable over a wide temperature range. For a homologous series of SCLCPs containing a terminal n-alkyl chain on the mesogenic group, an unexpected but distinct odd-even effect is observed.     

含薄荷基的手性液晶单体的合成、结构与性能研究

合成了五种新型含薄荷基的手性单体(M1～M5), 它们的结构、纯度及旋光性质通过了1H NMR, FT-IR、元素分析仪及旋光仪等手段的表征, 采用DSC, POM, UV/Vis/NIR等研究了单体的介晶性能、相行为及选择反射性能. 结果表明: 单体的比旋光度值随苯环数目的增加而降低, 通过在薄荷基与液晶核之间引入柔性间隔基元, 实现了含薄荷基单体具有液晶性能的目的. 除M1外, 其余四种单体均呈现手性近晶C (SC*)相和胆甾(Ch)相, 此外M5还出现了蓝相织构. M2～M4只在SC*相区能观察到选择反射现象, 而M5在SC*相区和Ch相区均出现明显的选择反射现象, 且随温度的升高, SC*相区的反射波长发生“红移”, 而Ch相区的反射波长则发生“蓝移”. 随着液晶核刚性的增加, 对应单体的熔点和清亮点增大, 液晶相范围变宽. 液晶核中的酯基桥键与组合方式也对单体的熔点和清亮点具有一定的影响.     

Conformational behaviour of laterally dialkoxy branched mesogens. Part one

The crystal structures of two mesogenic compounds having two neighbouring lateral alkoxy chains in the central part of the molecules have been solved. In both structures, the common four ring central core is extended and quite rigid with a total length close to 28.5 Å. All the molecules are strictly parallel because of the P1 space group; the molecular arrangement is characteristic of a nematogenic system. In these structures, the lateral alkoxy chains are folded back to the core; they exhibit slightly different conformations and are quite disordered. The ¹³C chemical shift of the first -OCH₂- within the lateral chains can probe the mean conformation of the chain in the nematic phase. This chemical shift is independent of the compound. Nevertheless, in the solid phase, this chemical shift is dependent on small geometrical changes due to the influence of the oxygen in the neighbouring lateral chain. In addition, temperature cycling of the sample leads to different crystalline solid forms as evidenced by DSC and NMR studies.     

Solid-state NMR characterization and determination of the orientational order of a nematogen

Thermotropic liquid crystalline compounds are of considerable importance due to their potential applications as advanced functional materials. A mesogen consisting of a terminal dimethylamino group, which can act as a charge-transfer donor, is particularly valuable for its light emission and nonlinear optical properties. In this study, we report the solid-state NMR investigation of the nematic behavior of one such novel mesogen (4-(dodecyloxy)benzoic acid 4-[((4-(dimethylamino)phenyl)imino)methyl]phenyl ester). Static and MAS experiments were performed on nematic and crystalline phases of the compound to measure (13)C chemical shift, (13)C-(1)H dipolar coupling, and (1)H chemical shift values. 2D chemical shift correlation of (1)H and (13)C nuclei confirmed the (13)C chemical shift values determined from 1D CPMAS experiments. The appearance of more peaks in both CPMAS and (13)C-(1)H HETCOR spectra of a crystalline solid suggests the heterogeneous orientations of phenyl rings of the mesogenic core. Variable-temperature experiments infer the motional averaging of these orientations before melting. The (1)H-(13)C dipolar coupling values, measured by 2D PITANSEMA experiments, were used to determine the orientational order of the mesogenic core at various temperatures. The influence of the linking unit and terminal substituents on the order parameter values of the mesogenic core is discussed.     

Synthesis and study the liquid crystalline properties of compounds containing benzoxazole core and terminal vinyl group

Terminal vinyl-based benzoxazole liquid crystalline compounds, 2-(3-fluoro-4?-alkoxy-1,1?-biphenyl ?4-yl)-5-(2-propenyloxymethyl)-benzoxazole (nPPF(2)BP), were synthesised and their structures were confirmed by infrared (IR) spectra, proton nuclear magnetic resonance (¹H-NMR) spectra, gas chromatography with electron impact-mass spectrometry (GC/EI-MS), matrix-assisted laser desorption/ionisation-time of flight (MALDI-TOF) mass spectrometry and elemental analysis (EA). The compounds show enantiotropic smectic/nematic phases with mesophase ranges are 71–97 °C and 87–136°C on heating and cooling processes for nPPF(2)BP, respectively. They give low melting points due to lateral fluoro substituent and flexible terminal 2-propenyloxymethyl chain. It is found that the compounds nPPF(2)BP with shorter alkoxy chain (n = 3, 4) exhibit a wide range of nematic mesophase, which is ascribed to enhanced π–π interaction caused by terminal vinyl moiety, whereas further elongation of the terminal alkoxy chain results in supressing nematic phase and increasing smectic mesophase. Compared with methyl terminated analogues, 2-propenyloxymethyl terminated compounds nPPF(2)BP display much lower melting points and wider or comparable mesophase range both in heating and cooling.     

Orientational ordering in some nematogens deviating from the classical rod-shape

Several new laterally substituted liquid crystalline compounds have been synthesized. They have the same main core which contains four rings (two aromatic, two alicyclic) with two lateral substituents introduced on the same side of one of the inner rings. One of the substituents is a 4-X-benzyloxy group (X=CH3,CN,Cl) and the other is a hexyloxy chain. The presence of the lateral aromatic substituent makes these compounds deviate markedly from the classical rod-shape. However, a wide enantiotropic nematic phase is present for all the compounds. The order parameters of the chain and the para -disubstituted aromatic rings were obtained by using a 2D 13C NMR technique with variable angle spinning. The temperature dependence of the order parameters was estimated using 13C chemical shifts with slow spinning of the sample parallel to the magnetic field. The results indicate that the two lateral substituents are more or less folded back along the mesogenic core. Thus, the flexible lateral chain is found to be roughly aligned with the molecular long axis, whereas the para axis of the less flexible aromatic branch makes a considerable angle with the molecular long axis imposed by the core, substantially increasing the mean width of the molecule. The core ordering does not seem to be influenced by the type and position of the substituents. The folding back of the lateral chain and the substantial tilt of the lateral aromatic branch with respect to the core main axis are confirmed by the X-ray structure of a parent compound.     

Orientational ordering in some nematogens deviating from the classical rod-shape

Several new laterally substituted liquid crystalline compounds have been synthesized. They have the same main core which contains four rings (two aromatic, two alicyclic) with two lateral substituents introduced on the same side of one of the inner rings. One of the substituents is a 4-X-benzyloxy group (X=CH3,CN,Cl) and the other is a hexyloxy chain. The presence of the lateral aromatic substituent makes these compounds deviate markedly from the classical rod-shape. However, a wide enantiotropic nematic phase is present for all the compounds. The order parameters of the chain and the para -disubstituted aromatic rings were obtained by using a 2D 13C NMR technique with variable angle spinning. The temperature dependence of the order parameters was estimated using 13C chemical shifts with slow spinning of the sample parallel to the magnetic field. The results indicate that the two lateral substituents are more or less folded back along the mesogenic core. Thus, the flexible lateral chain is found to be roughly aligned with the molecular long axis, whereas the para axis of the less flexible aromatic branch makes a considerable angle with the molecular long axis imposed by the core, substantially increasing the mean width of the molecule. The core ordering does not seem to be influenced by the type and position of the substituents. The folding back of the lateral chain and the substantial tilt of the lateral aromatic branch with respect to the core main axis are confirmed by the X-ray structure of a parent compound.