Synthesis and characterization of wide range mesogenic esters based on asymmetrical 2,5-disubstituted 1,3,4-thiadiazole

A homologous series of new 13 esters, 4-(5-(p-tolyl)-1,3,4-thiadiazol-2-yl)-phenyl-4-alkoxybenzoate, (IV_n), based on 1,3,4-thiadiazole core has been synthesized. The structures of these esters were confirmed by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance and mass techniques. Their mesophases behavior was investigated with hot-stage polarizing optical microscope and differential scanning calorimetry. The thermal stability for most of these derivatives was measured by thermal gravimetric analysis. All the target esters showed enantiotropic mesomorphic behaviors with nematic and nematic/smectic C phases. The phase transition temperatures and liquid crystalline properties were affected by the nature of heterocyclic ring and the length of the alkoxy chain. Only the nematic phase was observed in the first 10 derivatives, (n = 1–10), while the last 3, (n = 12, 16 and 18) showed nematic and smectic C phases. These compounds demonstrated high liquid crystalline ranges, both in heating and cooling cycles. The mesomorphic results obtained were compared with the reported analogs of similar constituents.     

Bent-shaped mesogenic oxadiazoles and thiadiazoles with terminal methyl group

Rod-shaped 4-methylbenzoic acid-N′-(4′-n-alkoxybenzoyl) hydrazide (series I) have been synthesized by the Schotten–Baumann reaction of 4-methylbenzhydrazide with 4-n-alkoxy benzoyl chloride using dry pyridine, as a solvent. The series I compounds have been cyclized to bent-shaped mesogenic 2-(4′-methylphenyl)-5-(4″-n-alkoxyphenyl)-1,3,4-oxadiazoles (series II) and 2-(4′-methylphenyl)-5-(4″-n-alkoxyphenyl)-1,3,4-thiadiazole (series III) using POCl₃ and Lawesson's reagent, respectively. The synthesized compounds are characterized by the combination of elemental analysis and standard spectroscopic methods. In series II, lower and middle members are non-mesogenic. n-Dodecyloxy to n-hexadecyloxy derivatives exhibit enantiotropic nematic mesophase. In series III, all the compounds synthesized exhibit enantiotropic nematic mesophase. n-Tetradecyloxy and n-hexadecyloxy derivatives also exhibit enantiotropic SmA mesophase. The mesomorphic properties of the series II and III used in this study are compared with each other and with other structurally related compound to evaluate the effect of different heterocyclic moieties as well as terminal substituents on mesomorphism.     

Synthesis of novel fluorinated bis(biphenyl)diacetylene derivatives as highly birefringent nematic liquid crystalline materials

Three novel fluorinated bis(biphenyl)diacetylene (1,4-bis(biphenyl-4-yl)buta-1,3-diyne) derivatives, with alkoxy tails at the terminal positions, were prepared for use as high-birefringent nematic materials. To lower the thermal transition temperatures and determine the origin of the nematic phase of these compounds, fluorine atoms were introduced at lateral positions. Thermal transition temperatures and mesomorphic properties were investigated by differential scanning calorimetry and polarized optical microscopy. In addition, birefringence was measured using a microscopic method based on the interference method. Both the nematogenic properties and optical performances depended not only on the number of fluorine substituents, but also on the position of the substituents. The compound having the highest fluorine content exhibited the lowest birefringence, and the compound bearing fluorine atoms only on the inner benzene rings of the biphenyl cores exhibited an enantiotropic nematic phase with a maximum birefringence value of 0.48 at 550 nm and 140 °C.     

Synthesis and phase-transition behaviors of new symmetrical dimer liquid crystals containing benzothiazole unit

This article reports the synthesis and characterization of a series of homologous symmetrical dimers α,ω-bis[4-(6′-ethoxybenzothiazol-2′-yl)iminomethylphenoxy]alkane. Five members with different lengths of alkyl spacer groups of even parity varying from butyl (C₄H₈) to dodecyl (C₁₂H₂₄) were synthesized. Spectroscopic analysis (IR, ¹H and ¹³C NMR) along with electron-ionization mass spectrometric techniques were employed to verify the molecular structures of the dimers. Differential scanning calorimeter was used for determination of the phase-transition temperature and associated enthalpy changes. Optical studies and mesophase identification were carried out using a polarizing optical microscope attached to hotstage. A diversity of phase-transition behavior was observed as the length of the alkyl spacer increased from C₄H₈ to C₁₂H₂₄. Almost all title compounds exhibited nematic phase except the dimer containing butyl spacer in which the mesomorphic property was absent.     

Fan-shaped,toric and spherulitic textures of mesomorphic oxadiazoles

When a family of non-symmetrical heterocycled compounds is investigated, a variety of mesophases can be observed with rather different features. Here we report the behaviour of seven different members among a family of such materials, which consists of mesomorphic oxadiazole compounds. In two of these compounds, the optical microscope investigation shows very interesting behaviours. In their smectic phases, fan-shaped and toric textures, sometimes with periodic instability, are observed. Moreover, the nematic phase displays a texture transition. Texture transitions have been previously observed only inside the nematic phase of some compounds belonging to the families of the oxybenzoic and cyclohexane acids. In these two oxadiazole compounds we can observe a nematic phase with a spherulitic texture, heating the samples from the smectic phase. This nematic texture disappears as the sample is further heated, changing into a smooth texture.     

Mesomorphic behaviors of a series of heterocyclic benzothiazole-imine-ester-based liquid crystals

A series of new calamitic liquid crystals, 2-[4-(4-alkoxybenzoyloxy)benzylidenamino]benzothiazoles comprising a heterocyclic (benzothiazole) and two phenyl rings core system, terminal alkoxy chain, imine and ester linkers were prepared and characterized. This series comprises eight members wherein the members vary by the length of alkoxy chain (C_nH_2n+1O-, where n?=?4, 6, 8, 10, 12, 14, 16, 18). Spectral analysis results were in conformity with the expected structure. Their thermotropic behaviors were studied by using differential scanning calorimetry, optical polarizing microscopy and powder X-ray diffraction techniques. A single mesophase (nematic) was observed for the first member of the series (n?=?4). As the alkoxy chain increased to n?=?6, 8, 10, 12, 14, the nematic phase appeared together with an additional SmA phase. When moving from n?=?16 until the highest member (n?=?18), the nematic phase disappeared and these compounds only exhibited a single mesophase (SmA).     

两种不同液晶相肉桂酸酯在溶液态及液晶态的光化学反应

采用紫外可见分光光度计,研究了两种肉桂酸酯类化合物在液晶态及溶液态的光化学行为. 结果表明,在液晶态下紫外曝光,向列相液晶同时发生光二聚和光异构;而胆甾相液晶将会发生可以被分为三个阶段的相对比较复杂的光化学反应. 同时,溶液态光化学反应的结果表明,紫外光谱分析可以对肉桂酸酯光化学反应进行动力学分析.     

Synthesis and mesomorphic behaviors of a series of heterocyclic pyridine-imine-ester based liquid crystals

ABSTRACT

A series of new calamitic liquid crystals, 4-{[(5-methylpyridin-2-yl)imino]methyl}phenyl 4-alkoxybenzoates were synthesized. This series consists of nine members wherein they are differed by the length of alkoxy chain. Spectral analysis results were in accordance with the expected structure. Their thermotropic behaviors were analyzed with Differential Scanning Calorimetry (DSC), Polarizing Optical Microscopy (POM) and powder X-ray diffraction techniques. First member with the shortest alkoxy chain (n?=?2) is a non-mesogen. As the alkoxy chain increased to n?=?4, the monotropic nematic phase appeared. An enantiotropic mesophase (nematic) was observed for the following three members (n?=?6, 8, 10). As the alkoxy chain increased to n?=?12, enantiotropic nematic phase exhibited together with monotropic smectic A (SmA) phase. As the alkoxy chain continuously increased to n?=?14 and 16, enantiotropic phases were observed for both N and SmA. When moving to n?=?18, the nematic phase disappeared and this compound only exhibited a single mesophase (SmA).     

Texture transitions in binary mixtures of 6OBAC with compounds of its homologous series

Recently the authors have observed in compounds of the 4,n-alkyloxybenzoic acid series, with the homologous index n ranging from 6 to 9, a texture transition in the nematic range which subdivides the nematic phase in two sub-phases displaying different textures in polarised light analysis. To investigate a persistence of texture transitions in nematic phases, we prepared binary mixtures of 4,6-alkyloxybenzoic acid (6OBAC) with other members (7-, 8-, 9-, 12-, 16OBAC) of its homologous series. Binary mixtures exhibit a broadening in the temperature ranges of both smectic and nematic phases. A nematic temperature range of 75°C is observed. In the nematic phase, in spite of the microscopic disorder introduced by mixing two components, the polarised light optics analysis of the liquid crystal cells reveals a texture transition. In the case of the binary mixture of 6OBAC with 12OBAC and with 16OBAC, that is of compounds with monomers of rather different lengths, the texture transition temperature is not homogeneous in the cell, probably due to a local variation in the relative concentrations of compounds.     

Influences of nanoparticles and chain length on thermodynamic and electrical behavior of fluorine liquid crystals

Hydrogen-bonded polar nematic liquid crystal series with the general formula nOBAF (n = 7—12) is studied. The mesomorphic characterization is demonstrated through differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The complexes with short alkyl chains (n=7, 8) present a wide nematic range and monotropic smectic F mesophase, whereas the longer alkyl chain (n=10—12) analogues show high melting and low clearing mesomorphic liquid crystals. The thermal range of the mesophase and the birefringence increase with chain length decreasing. Furthermore, the effect of the nanoparticles (LiNbO₃) on the thermal and the electrical behavior of 8OBAF are investigated. The presence of LiNbO₃ nanoparticles increases the conductivity and reduces the resistivity of the complex.     

Growth of toric domains in mesophases of oxadiazoles

In a previous paper (Phase Transitions, 80(9), 987, 2007), we discussed textures of some mesomorphic oxadiazole compounds with terminal Cl-substituent. Optical microscope investigations showed a very interesting behaviour of smectic and nematic phases: the smectic phase has fan-shaped and toric textures and the nematic phase has spherulitic domains, which disappear as the sample is further heated, the texture changing into a smooth one. Here, the focus is on the growth of toric domains from the nematic phase and on the role of defects in domain structure. We use compounds with the same structure of oxadiazoles discussed in the previous paper but with terminal Br-substituent.     

Synthesis and phase transition studies on non-symmetric liquid crystal dimers: N-(4-(n-(4-(benzothiazol-2-yl)phenoxy)alkyloxy)-benzylidene)-4-chloroanilines

A new series of non-symmetric liquid crystal dimers N-(4-(n-(4-(benzothiazol-2-yl)phenoxy)alkyloxy)benzylidene)-4-chloroaniline containing benzothiazole and benzylideneimine units connected by a flexible alkyl spacer, –(CH₂) _n –, with n ranging from 4 to 12 in even parity have been prepared. All five members of this homologous series exhibit an enantiotropic nematic phase. The compounds with greater n of 8–12 exhibit both nematic and smectic phases upon cooling. A notable feature among this series is that for the member with n?=?10, the smectic–nematic transition is also present. The nematic–isotropic transition temperatures and associated entropy changes with respect to all compounds in this series exhibit a dramatic dependence on the length of the flexible spacer. A comparison of the transitional properties of this series with those of α-(4-benzylidenechloroaniline-4′-oxy)-ω-[4-(thiophene-2-carboxyl)benzylideneaniline-4′-oxy]alkanes reveals that replacing benzothiazole moiety at one side of the flexible alkyl spacer reduces the nematic–isotropic transition temperature.     

Syntheses and mesomorphic properties of laterally fluorinatedphenyl isothiocyanatotolanes and their high birefringent mixtures

Syntheses and mesomorphic properties of di-, tri- and tetrafluorosubstituted 4′-(4-alkylphenyl)-4-isothiocyanatotolanes are described. Four fluorine substituted compounds in the position 2′, 3′, 3, and 5 show stronger nematic character than difluorosubstituted ones in the position 3, 5 or 2′, 3′. Example of high birefringence nematic mixture and its properties are given.     

Azo substituted V-shaped liquid crystalline compounds: synthesis and mesophase characterisation

The first examples of V-shaped liquid crystalline materials made of non-symmetrical molecules are reported. They belong to a homologous series of azo substituted compounds, wherein the two arms attached to the 1,2-positions of the benzene ring are different. The new compounds are characterised by the spectroscopic methods. The liquid crystalline properties are investigated using polarising optical microscopy, differential scanning calorimetry and X-ray diffraction studies. All the compounds synthesised here are found to be liquid crystalline. They exhibit nematic and smectic A mesophases, which are uniaxial in nature. The preliminary qualitative experiments showed that the compounds are photosensitive. Based on the X-ray results, the molecular arrangement of these compounds in their mesophases is illustrated. A relationship between molecular structures to mesomorphic properties in such systems has also been predicted.     

Mesomorphic and optical properties of undoped and doped azomethines

Three calamitic azomethines were prepared by the condensation of the benzene-1,4-dicarboxaldehyde with the three aliphatic amines (LCOA1–LCOA3). The structures of the azomethines were characterized by means of FTIR, ¹H, ¹³C NMR spectroscopies and elemental analysis; the results showed an agreement with the proposed structure. The azomethines emitted blue or green light. An influence of n-decyl sulfonic acid (DSA) on mesomorphic behaviors, absorption and emission wavelengths was investigated. The liquid crystalline properties of the azomethines were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and wide and small angle X-ray diffraction (WAXS and SAXS). The mesomorphic properties of the compounds depend on both the length of the outer flexible spacers and the kind of connection between the alkyl groups and the imine group, i.e. directly via aliphatic chain or by phenyl ring. LCOA1–LCOA2 exhibited smectic and nematic phases, whereas the LCOA3 did not exhibit thermotropic liquid crystal properties. Basing on the comparison of thermal behaviors and optoelectronic properties of undoped (LCOAX) and doped (LCOAX/DSA) azomethines it was shown that ionic self-organized processes are powerful and are able to tune: i) the processability, ii) the LC behavior, and iii) the maximum of emission band of the synthesized azomethines.     

Synthesis and phase transition behaviours of laterally substituted liquid crystals containing methylhydroquinone: emerging of smectic C phase for higher homologues

A series of laterally substituted low-molar-mass liquid crystals with molecular geometry were constructed with three phenyl rings bridged through ester central groups as the rigid core and a lateral methyl group as the flexible part of a molecule, with a view to understanding and establishing the effect of molecular structure on liquid crystal behaviour. Low-molar-mass mesogens known as 1,4-bis[(4′-n-alkyloxybenzoyl)oxy]toluene with different number of carbon (n) at the alkyl chain have been prepared. Their molecular structures were proposed via physical measurements and spectroscopic techniques. Mesomorphic properties were studied by using differential scanning calorimetry, optical polarizing microscopy and powder X-ray diffraction techniques. The results showed that the melting points as well as the clearing temperatures decreased upon lengthening of the terminal alkyloxy chain lengths. Members with the shorter chain (n = 2–10) exhibited nematic phase. As for higher homologues, members with n = 12, 14, 16 and 18 showed polymorphism, whereby these compounds displayed both smectic and nematic properties.     

Mesomorphic behaviors of a series of heterocyclic thiophene-imine-ester-based liquid crystals

The synthesis and characterization of a series of heterocyclic liquid crystal, 4-{[(thiophen-2-yl)methylidene]amino}phenyl 4-alkoxybenzoates possessing even number of carbon atoms at the alkoxy chain (C_nH_2n+1O-, n = 6, 8, 10, 12, 14, 16, 18) are reported. The molecular structures of title compounds were elucidated using Fourier-transform infrared spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopic techniques along with mass spectrometric analysis. The phase behavior of these compounds was characterized and studied by differential scanning calorimetry and polarizing optical microscopy. All members exhibited enantiotropic nematic phase except for the highest member (n = 18) which is a non-mesogen. Influence of alkoxy chain length on the transition temperatures of crystal-to-nematic (melting point) and nematic-to-isotropic (clearing point) was studied. Nematic phase range was found to increase from n = 6 to n = 10, then it started to descend from n = 12 to n = 16 and finally the nematic phase disappeared when n changed to 18.     

Synthesis and mesomorphic properties of nonsymmetric liquid crystalline dimers containing azobenzene groups

Four novel nonsymmetric dimers containing azobenzene mesogenic groups were synthesized. The nonsymmetric dimers compounds namely, ethyl 4-[(4-{4-(4-((4-nitrophenyl)diazenyl)phenoxy)alkyloxy}phenyl)diazenyl]benzoate were obtained from the alkylation of ethyl 4-[(4-(4-bromoalkyloxy)phenyl)diazenyl]benzoate with 4-[(4-nitrophenyl)diazenyl]phenol. The mesomorphic properties of the compounds were determined by DSC and polarizing optical microscopy. The first member of the series was nonliquid crystalline while all other homologues display nematic and smectic A phases. The trans-azobenzene groups of the dimers display a high-intensity π–π* transition at about 365 nm and a low-intensity n–π* transition at around 465 nm, therefore, photochromism can be achieved by the introduction of the azo linkage to the dimeric liquid crystalline molecules.