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 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.     

Three ring dioxanes as dopants enhancing the stability of the smectic C phase

Abstract

Searching for compounds which could be useful as modifiers of smectic C mixtures, we have synthesized four homologous series of three ring dioxanes, 2BBD, 5BBD, 2CBD, and 5CBD. Their phase transition temperatures and enthalpies were measured and their liquid crystal phases identified. Compounds belonging to series n-BBD form smectic B_cr phases for shorter alkyl chains, and smectic B_cr and A phases, for longer chains. Compounds belonging to the n-CBD series exhibit the smectic A phase, but those with longer alkyl chains have exclusively smectic B phases and those with short tails have other low temperature, highly ordered smectic phases. The compounds were added to smectic C mixture and it was found that some can be useful as dopants. Compounds with longer alkyl tails in the molecule are more suitable for this purpose; the type of ring in the core is less important.     

Synthesis and properties of fluorine tail-terminated cyanobiphenyls and terphenyls for chemoresponsive liquid crystals

ABSTRACT

A series of fluorine tail-terminated alkoxy and alkyl cyanobiphenyl compounds and some cyano-p-terphenyl derivatives were synthesised and mesogenic properties described. Comparison with the non-fluorinated K series and M series indicates that the terminal fluorine atom generally decreases the transition temperatures and, more interestingly, depresses the formation of a smectic phase. Several binary LC mixtures formed by the fluorine tail-terminated compounds were found exhibiting promising room temperature nematic phases with wide ranges. The mixture F7OCB and F8OCB shows homeotropic ordering at the metal salts-decorated surfaces and planar ordering at the free surface, which may have potential application in designing a more sensitive and faster LC system to targeted analytes.     

三氟丙炔基类液晶的合成及性能

合成了16个三氟丙炔为端基的液晶化合物,以苯甲醛衍生物与1,1,1-三氯三氟乙烷的有机锌试剂进行加成脱水成烯,再通过Suzuki偶联反应后脱去HCl制得目标产物,总产率67%,气相色谱(GC)纯度99.5%,通过红外光谱(IR)、核磁共振谱(NMR)和质谱(MS)确定其结构。利用DSC和POM对该类化合物的热性能进行了测试,结果表明,联苯类的化合物不具有液晶相,而三联苯类的化合物具有近晶A相,且熔点随烷基链碳原子的增加呈下降趋势。物理性能测试结果表明,该类化合物的Δε值为12.61~21.94,末端三氟甲基基团和侧氟的引入可以有效的增加Δε值;Δn值0.19~0.29,随分子内苯环数目的增加而变大;γ1值为49.9~468.0 m Pa·s,随分子内苯环数量的增加而增加。化合物4n具有高Δε(19.2)、高ε_⊥(10.3)及低Δε/ε_⊥(1.9)。通过模拟计算讨论了炔键位置对此类液晶材料性质的影响。该类化合物在调节混合液晶介电各向异性和双折射率方面有很好的应用价值,同时通过分子结构的改进也可以很好地应用于FFS(fringe-field switching)显示模式。     

The effect of olefinic terminal chains on the mesomorphic properties of 4,4′-disubstituted diphenyldiacetylenes

New diphenyldiacetylenes of the type with A, B = H and/or F; m = 0, 1; n = 1–4; and X = C _n H_2n+1, F, CF₃ or CN were synthesized and their mesomorphic properties determined by hot stage polarizing microscopy and DSC. When m = 0, all of these compounds showed only a nematic phase except when X = CF₃ when both nematic and smectic A phases were seen. Both clearing and melting temperatures were higher than those reported for substitution with the corresponding alkyl chains but the much larger increase in clearing temperatures produced considerably wider nematic phases. Eutectic mixtures of a few of these olefins yielded nematic materials also having much wider temperature ranges and higher clearing temperatures than the eutectic mixtures of the alkyl compounds, while retaining their high birefringence and low viscosities. Such materials are of interest for beam-steering devices. Four of the diacetylenes with m = 1 (A, B = H) were also prepared (X = C₆H₁₃, F, n= 2, 3). When X was C₆H₁₃ (n=2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X=F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X=C₆H₁₃ and n=2 had a melting temperature below room temperature.     

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (S_A1) or monolayer and partially bilayer (S_Ad) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A₁ phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC _m H_{2m + 1} terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A₁ phase slowly changes to be more like the smectic A_d phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (S_A1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A₁ phase is rapidly transformed into the smectic A_d phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.     

Chiral liquid crystalline m-nitrotolans and tolans: synthesis and mesomorphic properties

Two homologous series, 4'-(4-n-alkoxybenzoyloxy)-4-substituted 3-nitrotolans (Ia-d) and 4'-(4-n-alkoxybenzoyloxy)-4-substituted tolans (IIa-c), with substituents (S)-2-methyl-1-butyl and n-alkyl, were synthesized by Sonogashira's coupling. Their mesomorphic behaviour is reported. The thermal stability of the series II is higher than that of series I. Series I melting points and clearing points are lower than those of series II. None of the chiral tolans or m-nitrotolans have an enantiotropic smectic C phase. When the chiral chains are changed to n-alkyl groups in compounds Ia,b and IIa, an enantiotropic smectic C phase is seen. All compounds have a nematic or cholesteric phase, and two homologues of series II present a monotropic smectic C phase with mosaic texture. The mesophases were characterized using optical polarized light microscopy and differential scanning calorimetry.     

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.     

Synthesis and properties of (−)-menthol-derived chiral liquid crystals by introducing adipoyloxy spacer between mesogenic core and chiral menthyl

A homologous series of new chiral liquid crystal compounds, M6BnB (n = 2, 4, 6, 8, 10, 12, 14, 16, 18), with varying length of n-alkoxy chains at one end, was formed by covalently linking a chiral (?)-menthyl with biphenyl-benzoate via adipoyloxy spacer group. A combination of analysis methods such as Fourier transform infrared, ¹H NMR spectra, differential scanning calorimetry, polarised optical microscopy and X-ray diffraction was carried out to systematically investigate their phase structures and phase transition behaviours. The length of the flexible terminal alkoxy group has a profound influence on the clearing points (T_iso) and T_iso decrease with the increase of alkoxy chain (n). In addition, increasing the length of the terminal alkoxy group tends to narrow the temperature range of the N* phase and favour the development of chiral smectic C phase.     

Unsymmetry effect of hexa-peri-hexabenzocoronenes on columnar mesophase behaviour

To address the effects of molecular symmetry on the phase of columnar liquid crystals, a series of unsymmetry (C₁) and symmetry (D_3h) hexa-peri-hexabenzocoronene derivatives was prepared. Compounds 3 and 4, as well as a mixture of compounds 3+4, all had a significantly lower melting temperature and clearing temperature than high-symmetry compounds (D_6h), and all results show a strong dependence on molecular symmetry.     

Bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments: synthesis and characterization of mesomorphic behaviour

A series of bent-core V-shaped mesogens consisting of salicylaldimine mesogenic segments have been synthesized and their mesomorphic behaviour characterized. In an attempt to understand structure–property relationships, the lengths of the terminal alkoxy chains have been varied from C₂ to C₁₂, C₁₆ and C₁₈, resulting in 13 new bent-core V-shaped molecules. The thermal behaviour of these new compounds has been investigated by optical microscopy, calorimetry and X-ray diffraction studies. In general the compounds show conventional mesophases similar to those shown by calamitic LCs. The materials exhibit good thermal stability, even though their melting and clearing transition temperatures are high as a result of the presence of intramolecular hydrogen bonding between the H-atom of the hydroxyl group and the N-atom of the imine functionality. The phase appearing in the first member of the series with ethoxy chains is a nematic, while the C₃ to C₆ derivatives exhibit a smectic A phase as well as a nematic phase. The higher homologues, C₇ to C₁₂, C₁₆ and C₁₈, show only the smectic A phase. X-ray studies reveal that the SmA phase has a partially bilayer (interdigitated) structure. Remarkably, in some cases, the smectic A phase supercools well below room temperature. It is apparent from our studies that increasing the length of the alkoxy chains promotes smectic behaviour, in agreement with the general observation made for such bent-core molecules.     

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.     

Liquid crystalline properties of 4-alkoxyphenyl 3-fluoro- and 3-methyl-4-thiocyanatophenyl terephthalates

The preparation and thermal properties of homologous series of 4-alkoxyphenyl 3-fluoroand 3-methyl-4-thiocyanatophenyl terephthalates are described. Both homologous series show nematic phases; smectic A and C phases are also formed in the higher homologous. The methyl group and fluorine atom adjacent to the thiocyanate group lower the clearing points, in comparison with the unsubstituted compounds. The layer structures for both smectic A and C phases were characterized by X-ray diffraction experiments. The effects of the substituents on liquid crystalline properties are discussed.     

The mesophases of octa-alkanoyloxy-9,10-anthraquinone

Abstract

Nine members of the octa-n-alkanoyloxy-9,10-anthraquinone series, ranging from octanoyloxy (n = 8) to hexadecanoyloxy (n = 16) (where n is the number of carbon atoms per chain), were prepared and their mesomorphic properties studied by differential scanning calorimetry and optical microscopy. All the compounds studied are mesomorphic with the higher homologues exhibiting two (for n ≧ 12) or even three (for n ≧ 14) columnar mesophases. For n = 8 to 11, the phase sequence is C-D_B-I, where D_B is an optically biaxial mesophase with a two-dimensional pgg symmetry. For n = 12, 13, the phase sequence is C-M₃-D_B-I, where M₃ is a highly ordered phase whose structure has not been identified. Finally for n = 14 to 16, the phase sequence is C-M₃-D_B-D_A-I, where D_A is an hexagonal phase. In all cases, the phases change from high to low order with increasing temperature and all transitions are first order. Compared with the corresponding hexa-substituted naphthoquinones the present series generates larger mesomorphic ranges and the higher homologues exhibit a uniaxial phase which is lacking in the hexa-substituted series. The increase in volume occupied by the aliphatic chains in the octa-substituted series is evidently sufficient to reduce the interaction between the aromatic cores of the different columns to permit the stability of a uniaxial phase.     

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.     

Substituted 1,4-dicyclohexyl-butan-1-ols A new class of liquid-crystalline compounds

Abstract

1,4-dicyclohexyl-butan-1-ols with a lateral hydroxyl group in the middle part of the molecule exhibit higher clearing temperatures than those of the respective compounds whithout this lateral group. A smectic B-like texture and a low temperature additional phase X were found by microscopic and calorimetric investigations. A further unexpected result was the strong increase of the dielectric constant at the S_B/X transition. These preliminary results are explained in terms of hydrogen bridges.     

Non-conventional multi-arm ester liquid crystal derived from cholic acid: synthesis,thermal and optical behaviour

ABSTRACT

With cholic acid as the core, five multi-arm ester liquid crystals were synthesised with different terminal substituents. B₁–B₅ were mesogenic arms which were linked to multifunctional chiral core cholic acid. The effect of electron-withdrawing and electron-donating groups on the mesogenic behaviour of the compounds was discussed. All products were structurally well characterised by elemental analysis, ¹HNMR, and FT-IR. The phase behaviours were investigated by means of polarised optical microscopy and differential scanning calorimetry. B₂, B₄, B₅ and C₂, C₄, C₅ and D₂, D₄, D₅ exhibited typical nematic texture and belonged to thermotropic enantiotropic nematic liquid crystals. B₃, C₃ and D₃ also displayed typical nematic texture of thermotropic monotropic nematic liquid crystal. B₁, C₁ and D₁ did not have mesogenic performance. Wider mesogenic range for 81.6°C of C₅ whose terminal substituent is nitro on heating cycle while 121.1°C on cooling cycle. The results indicated that terminal substituents have a pretty important effect on the mesogenic phase and range of multi-arm liquid crystal compounds. The mesomorphic behaviour of compounds with polar groups terminally substituted performance much better than those without polar groups. In this ester multi-arm liquid crystal system, electron-withdrawing groups terminally substituted behave better than those electron-donating groups substituted.     

Synthesis and properties of antiferroelectric and/or ferroelectric compounds with the –CH2O group close to chirality centre

ABSTRACT

Novel chiral three-ring compounds with the –CH₂O group close to chirality centre were synthesised and their properties were studied. The phase transitions and phase sequences were observed using a polarising optical microscope. The phase transition temperatures and enthalpies were checked by differential scanning calorimetry. A broad-frequency dielectric spectroscopy was also used to confirm the phase transition temperatures as well as the phase sequence. The helical pitch was measured by the spectrophotometry method. It was found that the compounds differing only in one lateral substituted fluorine atom create two different chiral tilted smectic phases, one a ferroelectric phase (SmC*) and the other an antiferroelectric phase (SmC_A*).     

Phase-transition studies of 5O.5 and 9O.4

As part of our detailed comparative studies of groups of liquid-crystalline compounds that belong to a homologous series, we present phase-transition studies of the compounds N-(4-n-pentyloxybenzylidene)4′-n-pentylaniline (5O.5) and N-(4-n-nonyloxybenzylidene)4′-n-butylaniline (9O.4) using different experimental techniques. The compound 5O.5 is reported to exhibit a phase sequence N, S_A, S_C, S_B and S_G, while 90.4 shows the sequence S_A, S_F and S_G. The salient features of our work on 5O.5 are (i) a new smectic F phase is found in place of the reported smectic B phase, which is confirmed by both miscibility and X-ray studies; (ii) the formation of smectic-C-like short-range order in the nematic phase well above the S_A-N transition; and (iii) a large tilt-angle variation in the smectic C phase (0–23·5°C) in a small temperature range (4·1°C). The phase changes across the S_A-I transition, and for the first time across S_F-S_A transition, are carried out by volumetric studies. The detailed inferences of these are also presented.