Nematic mesophase enhanced via lateral monofluorine substitution on benzoxazole-liquid crystals

Fluorine is widely used as a lateral substituent to modify the physical properties of liquid crystals. Here, laterally monofluorinated compounds, 2-(4?-alkoxy-2-fluorobiphenyl-4-yl)-benzoxazole derivatives (nPPF(2)Bx) bearing different substituents (H, CH₃, NO₂, coded as nPPF(2)BH, nPPF(2)BM and nPPF(2)BN, respectively) at 5-position, were synthesised and characterised. It is interesting to note that these only display enantiotropic nematic mesophases with mesophase ranges of 12–28°C and 13–45°C on heating and cooling for nPPF(2)BH, 46–97°C and 62–120°C for nPPF(2)BM and 82–108°C and 87–113°C for nPPF(2)BN, which are very different from the corresponding monofluorine-substituted analogue (compounds I) with enantiotropic smectic or smectic/nematic mesophases. The enhanced nematic mesophase is attributed to the reduced π–π interaction/conjugation resulting from the twisted structure of the molecule caused by the introduction of a fluorine atom into the inter-ring of the biphenyl unit. These results suggest that modification of the monofluorine substituent position is an effective method to improve the nematic mesophase in benzoxazole-liquid crystals.     

Molecular relaxations in dual-frequency nematic liquid crystals

The preparation of nematic liquid crystals mixtures results in changing of molecular relaxations in comparison to pure substances. Typical example is the creation of dual-frequency nematic liquid crystals using a base mixture and functional admixtures. In this paper, we present how dielectric properties of starting compounds change at mixture preparation. Three dual-frequency nematic mixtures of different composition were prepared and examined by means of dielectric spectroscopy in a wide frequency (100 Hz to 10 MHz) and temperature range (170°C to ?60°C). Parameters of detected modes for pure compounds and final mixtures were calculated and their relationships with crossover frequency are discussed.     

Star-shaped supramolecular ionic liquid crystals based on pyridinium salts

Series of novel star-shaped liquid crystals containing [1,1?-biphenyl]-4,4?-diyl diisonicotinate mesogens with various counterions (Br^?, B-SO₃^?,C-SO₃^?,H₂PO₄^?, BF₄^?) have been synthesized and characterized, which display a nematic phase. The molecular structures of the liquid crystals, thereof consisting of rod-like mesogens linked together by different long alkyl spacers to a small disc-like core of central benzene-1,3,5-triyl triisonicotinate, have been fully characterized by ¹H NMR. Their thermal and supramolecular organization behaviours have been studied by combining polarized optical microscopy, differential scanning calorimetry and small-angle X-ray scattering. These investigations showed that the mesophase temperature range increased with increasing alkyl chain length and the clearing point decreased with increase of anion size. These star-shaped liquid crystals have a long and ordered molecular structure, the electron delocalization of the π-π stacking conjugation effect and the electrostatic attraction of ionic make them have good ferroelectric properties and ionic conductivity properties. The interest in preparing ionic liquid crystal with a nematic phase lies in the technological applications as it is well known that the nematic phase has the highest fluidity of all liquid crystalline phases and hence the possibility to align it by applying an external electric/magnetic field, commonly used in electro-optical devices.     

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.     

Novel codeinone derivatives via Michael addition of barbituric acids

Two novel adducts of codeinone with barbituric and 2-thiobarbituric acids have been synthesized via Michael addition. The compounds were spectroscopically elucidated by means of IR-LD spectroscopy of oriented samples as a suspension in nematic liquid crystals, UV spectroscopy and ¹H and ¹³C NMR spectroscopy. The 2-thiobarbituric adduct was characterized by X-ray crystallography. HPLC tandem mass spectrometry (HPLC ESI MS/MS) and thermal methods were also employed. Quantum chemical calculations have been performed with a view to obtaining the electronic structure and vibrational properties of the novel compounds.     

Highly fluorinated liquid crystals with wide nematic phase interval and good solubility

A series of new liquid crystal compounds with both ethylene and difluoromethyleneoxy linking groups have been synthesised and their physical properties were measured. The compounds exhibit lower melting points and broader nematic mesophase temperature range than that of corresponding four-rings compound. Their anisotropic properties are Δε of about 25 and Δn of 0.135. The investigation shows that their solubilities are much better than that of reference compound. A mixture made by equivalent mass ratio of the compounds with side chain of ethyl, propyl and pentyl is found to keep nematic phase from 12°C°C to 109°C, which is rarely observed in highly fluorinated liquid crystals.     

Mesogenic 4-alkoxy-2-hydroxy-4’-formylazobenzenes

Homologous 4-alkoxy-2-hydroxy-4’-formylazobenzenes (Alk = C₃H₇, C₆H₁₁, C₈H₁₇) were synthesized and were shown to produce monotropic nematic liquid crystalline phase. The products were characterized by electron absorption and ¹H and ¹³C NMR spectra. The effect of lateral hydroxy group on their mesomorphic properties was analyzed.     

Synthesis and mesomorphic properties of new azine-type liquid crystals

A series of symmetrical azine-type liquid crystals were synthesized. The characteristic of these liquid crystals is that they had high clearing point ( ～320 ℃) and broad thermal range of nematic phase ( ～154 ℃). It was also found that the end groups of the liquid crystals had effect on the mesomorphic properties.     

Synthesis of a New Liquid Crystalline Polymer as a Highly Active β-Nucleator and Its Effect on Melting and Crystallization Behaviors of Isotactic Polypropylene

A new nematic liquid crystalline polymer as a highly active β-nucleator (LCP-N) of isotactic polypropylene (iPP) was synthesized and characterized. The effect of LCP-N on thermal behavior of the iPP was investigated with differential scanning calorimetry. LCP-N showed a melting transition at 85.0°C and a nematic to isotropic phase transition at 278.0°C. The incorporation of LCP-N could lead to substantial changes in the thermal behavior of the iPP. The nucleating activity of LCP-N mainly depended on its content, mesogenic molecular structure, and thermal history of processing. A high content of β-form could be obtained by the combined effect of the optimum LCP-N concentration and crystallization temperature and time. The Φ_β reached 77% when the LCP-N content, crystallization temperature, and crystallization time were 0.4 wt.%, 125°C, and 1 h, respectively.     

Wide-Line 1H NMR and DSC study of phase states of mesomorphic states of 4′-n-alkoxy-4-biphenylcarbonitrile mesogens (n = 3 and 7)

4′-(Propyloxy)-4-cyanobiphenyl (3OCB) and 4′-(heptyloxy)-4-cyanobiphenyl (7OCB) have been studied in the temperature range 20–80°C by wide-line ¹H NMR and DSC. The ¹H NMR line shapes, half-widths, and second moments, as well as the relaxation characteristics of separate lines, are discussed. The DSC data are compared with the specific features of mesogenic transformations of these homologues and ¹H NMR data. At certain temperatures depending on the composition of alkyl moieties, a dynamically disordered fraction of molecules appear in solid crystals. These molecules are involved in the thermotropic transitions of 3OCB to isotropic liquid and of 7OCB to nematic liquid crystal. The temperature ranges, stabilities, and other characteristics of the transitions of 3OCB to isotropic liquid and of 7OCB to nematic liquid crystal were determined.     

Pyrolysis of black cumin seed: Significance of catalyst and temperature product yields and chromatographic characterization

In this study, black cumin seed cake was transformed into liquid and solid products via pyrolysis method at 300, 400 and 500?°C with Ca(OH)₂, Al₂O₃, SnCl₄.5H₂O catalysts and without catalyst. The obtained liquid and solid products were examined with GC-MS, Elemental, FT-IR, ¹H NMR and SEM analysis methods. The highest liquid product yield was obtained as 30% in the presence of Al₂O₃ catalyst. All catalysts were found to be effective on conversion. According to the results of the elemental analysis of the liquid products obtained at the end of the experiments, HHV values were found between 32.44 and 36.19?Mj/kg. According to the results of GC-MS analysis of the liquid products, in total, 77, 61 and 66 pieces of compounds consist of monoaromatics, aliphatics, oxygenated compounds with different properties were obtained at 300, 400 and 500?°C reaction temperatures respectively. The results of the FT-IR and ¹H NMR that are used to examine the liquid products support the results of GC-MS.     

Synthesis and liquid crystalline properties of substituted 2,5‐diaryl 1,3,4‐oxadiazole derivatives without flexible chains

A series of new compounds based on aromatically 2,5‐disubstituted 1,3,4‐oxadiazoles without flexible chains, formulated as p‐R–C₆H₄–(OC₂N₂)–(p‐C₆H₄)₂–R′ with (i) R = CH₃O, R′ = CH₃O, CH₃S, F, H (Ia–Id), (ii) R = CH₃S, R′ = CH₃O, CH₃S, F, H (IIa–IId) and (iii) R = F, R′ = CH₃O, CH₃S, F, H (IIIa–IIId) (p‐C₆H₄ and OC₂N₂ represent a p‐phenylene spacer and a 1,3,4‐oxadiazole ring, respectively), were synthesised and characterised by ¹H and ¹³C NMR, MS and HRMS techniques. Mesomorphic properties were investigated using differential scanning calorimetry and polarizing optical microscopy. All of the target compounds (except Id, IId, IIIc and IIId) exhibited an enantiotropic nematic mesophase with high melting temperatures. The liquid crystalline properties of these compounds were influenced greatly by polarity, steric factors and positions of the terminal groups. The effect of the terminal groups on the liquid crystal properties 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.     

Synthesis and properties of liquid crystal monomers containing a reactive group in the lateral substituent

A series of nematic liquid crystal (LC) monomers containing a reactive group (double bonds) in the lateral substituent was designed and synthesised. Length of the lateral substituted groups that have one double bond varied from 1 to 4 methylene units. Length of the terminal substituted groups varied from 2 to 5 methylene units. The molecular structures of the intermediates and the LC monomers were characterised by Fourier transform infrared spectroscopy (FT-IR), elemental analysis and nuclear magnetic resonance (NMR) spectroscopy. The thermal phase behaviour of the monomers was investigated by differential scanning calorimetry (DSC) and polarised optical microscopy (POM) coupled with hot stage. Some molecules (V₁₅, V₂₅) with high aspect ratio exhibit enantiotropic nematic mesophase. The other compounds (V₁₂, V₂₂, V₄₂, V₄₃) show monotropic nematic mesophase during cooling. The relationship between the structure and mesomorphic property is also discussed.     

A novel phosphorus‐containing thermotropic liquid crystalline poly(ester‐imide) with high flame retardancy

A novel phosphorus–nitrogen thermotropic liquid crystalline poly(ester‐imide) (PN‐TLCP) derived from p‐acetoxybenzoic acid (ABA), terephthalic acid (TPA), acetylated 2‐(6‐oxide‐6H‐dibenz<c,e><1,2>oxa phosphorin‐ 6‐yl)‐1,4‐dihydroxy phenylene (DOPO‐AHQ) and N,N'‐hexane‐1,6‐diylbis(trimellitimide) was prepared by melt transesterification. The chemical structure, the mesophase behavior, and the thermal properties of the copolymer were investigated with Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance spectroscopy (¹H NMR), elemental analysis, wide‐angle X‐ray diffraction (WAXD), hot‐stage polarized light microscopy (PLM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). PN‐TLCP exhibited a nematic mesophase with a strong birefringence at a low and broad mesomorphic temperature ranging from 220 to 350°C, an initial flow temperature as low as about 190°C and a glass transition temperature of about 160°C. PN‐TLCP has also good thermal stability, high char residues and excellent flame retardancy (limiting oxygen index, LOI = 71 and UL‐94 V‐0 rating). Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis,Characterization, Thermal Analyses,and Spectroscopic Properties of Novel Naphthyl-Functionalized Imidazolium Ionic Liquids

A series of novel ionic liquids based on naphthyl-functionalized imidazolium cation have been prepared. Their structure was characterized by NMR. The thermal stabilities of the prepared liquids were studied by thermal gravimetric analysis. The new ionic liquids containing NTf^-₂ anion display significantly higher thermal stabilities (>400°C). Anion exchange to PF^-₆, BF^-₄, and Br^– decreases the thermal stabilities of such ionic liquids. Fluorescence and UV–Vis absorption spectroscopy were used to study the spectroscopic properties of the ionic liquids. Compared with common ionic liquids, the described ionic liquids provide robust fluorescence properties and remarkably increased UV–Vis absorption. This research may enrich the field of functionalized ionic liquids and provide a platform for extension of ionic liquid applications.     

19F Chemical shift tensor in fluorobenzene compounds

Fluorine-19 NMR multiple-pulse experiments have been applied to a series of meta-, para- and ortho-substituted fluorobenzene compounds in the solid state. The principal elements of the ¹⁹F chemical shift tensor (σ₁₁, σ₂₂, σ< ₃₃) were determined and the orientation of the tensor axes was inferrred from secondary information like molecular motion, related compounds and liquid crystal studies. Comparison with anisotropies obtained from molecules dissolved in liquid crystals in the nematic phase is discussed where data are available. Using the Gicrke-Flygarc approach we were able to exctract the spin-rotation interaction tensor elements C_??, C_tcy;tcy; and C_zz of ¹⁹F in sever; fluorobenzene compounds.     

Non-covalent dendrimer-based liquid crystalline complexes: Synthesis and characterization

The synthesis and structural characterization of dendritic macromolecules based on 3,5-dihydroxybenzoic acid are described. The molecular structures and purity of all new compounds were confirmed by ¹H NMR, ¹³C NMR spectroscopy and elemental analysis. The dendritic non-covalent liquid crystalline complexes were prepared through the formation of hydrogen bonds between different generation of dendritic acids (proton donor) and stilbazole derivative containing bipyridyl units (proton acceptor). We found that, the hydrogen-bonded dendritic liquid crystals supramolecules (G1-2py, G2-2py and G3-2py) exhibits nematic and semectic phase. The polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) were used for investigation of the liquid crystalline properties of the hydrogen-bonded dendritic supramolecular complexes.     

CO<Subscript>2</Subscript> capture by Mg–Al and Zn–Al hydrotalcite-like compounds

Hydrotalcite-like compounds (HTC) are distinguished by their properties for CO₂ capture, like high surface area, basic sites, thermal stability and good adsorption/desorption efficiency. Mg–Al e Zn–Al HTCs with Al³⁺ molar ratios x = 0.20, 0.28 and 0.33 were synthesized by coprecipitation, and subsequently calcined at 400 °C. For both HTCs, X-ray diffraction patterns have attested the formation of mixed oxides through calcination. The amount of basic sites, measured by temperature-programmed desorption of CO₂, decreases as x increases. The CO₂ adsorption was performed in a thermogravimetric balance using an adsorption temperature of 50 °C. Mg–Al and Zn–Al samples with x = 0.33 molar composition presented the highest CO₂ adsorption, 0.91 and 0.21 mmol g^?1, respectively. The Langmuir isotherm fitted well to the experimental data. It was also found that increasing the number of adsorption/desorption cycles the CO₂ adsorption decreases, which is associated with the irreversible chemisorption.     

Macrocyclised pheynyl cinnamate dimer utilisable as photoresponsive chiral dopant for nematic liquid crystals

A macrocyclised phenyl cinnamate dimer with a chiral spacer was prepared, and its photochemistry was compared with that of a precursor linear dimer and corresponding monomeric compounds. Although ultraviolet irradiation of the monomer resulted merely in cis–trans isomerisation of a cinnamate, irradiation of the cyclic and linear dimers induced intra-molecular [2 + 2] photodimerisation of cinnamate groups. Photodimerisation in the cyclic dimer proceeded 20 times faster than in the linear dimer. In a nematic liquid crystal, the cyclic dimer exhibited a high helical twisting power of 27.5 μm^?1, which decreased to 6.7 μm^?1 with dimerisation. A macrocyclised dimer such as this can be used as a photoresponsive chiral dopant for nematic liquid crystals.