Mesophase Transition Temperatures Research Articles

Synthesis, mesomorphic behaviour, and DFT studies of biphenyl bis-ester Schiff base liquid crystals

A novel series of calamitic liquid crystals with rod-like structures, derived from biphenyl bis-ester, has been successfully synthesized. This was achieved through the condensation reaction of [1,1′-biphenyl]-4-yl 4-formylbenzoate with n-alkyl 4-aminobenzoate, where n ranges from 2 to 8, 10, 12, 14, 16, and 18. The synthesized compounds were characterized using FT-IR, 1H NMR, 13C NMR, and mass spectroscopy techniques. The liquid crystal behaviour of all synthesized compounds was investigated using Polarizing Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC). It was found that all compounds within the homologous series exhibited mesogenic properties. The effect of molecular flexibility on the mesophase type and transition temperature range was systematically explored. Some compounds displayed a nematic phase, while others showed characteristics of the smectic A phase; additionally, certain compounds exhibited properties of both the nematic and smectic A phases. Furthermore, utilizing B3LYP/6–31++G (d,p) basis sets DFT level calculations and complete geometric optimization of the synthesized compounds was conducted. This optimization facilitated the examination of optimized geometrical structures, aspect ratios, dipole moments, polarizabilities, Frontier molecular orbitals, and molecular electrostatic potentials through DFT calculations.

Generation of mono-mesophase in allylidene-based homologues with a longer molecular axis

In this article, we have synthesized twenty-eight nematogenic homologous derivatives of allylidene base with cinnamates and benzoates linkages. All nematogenic homologous derivatives were characterized by ultraviolet spectroscopy, Infrared and proton magnetic resonance with study of their mesophase properties by the polarized optical microscope (POM) where they were found to be enantiotropically nematogenic. The thermograms of mesophase transition enthalpies and entropies were recorded by differential scanning calorimetry (DSC). The geometries of all molecules were optimized by DFT/B3LYP method using a basis set def2-SVP, the result of these calculated molecule's geometries for all molecules showed to have an electron-rich longer molecular axis, which enables the molecule to form a stronger pi interaction with adjacent molecules and induces the nematogenic mesophase behavior. The effect of an additional diene system on mesophase behavior of a stepped type of molecules was also studied. The role of the number of rings on mesophase behavior as an odd-even effect on mesophase transition temperature as well as the role of the substitution on the terminal ring for the same was considered.

Optical mesophase textures and the enthalpy changes with temperature of a liquid crystal compound 9OBA

In this article, our intention is to observe the mesophase textures of a liquid crystal (LC) compound 4-nonyloxybenzoic Acid (9OBA) in an attempt to understand the nature of dramatic changes of the mesophase transition textures with temperature. The studies of mesophase textures were carried out by using polarizing optical microscope (POM). Mesophase transition temperatures and their corresponding enthalpy changes (during heating) are experimentally deduced from differential scanning calorimetry (DSC). Higher value of enthalpy changes has been observed for the solid crystalline to Smectic C (SmC) transition. The compound 9OBA is found to exhibit nematic and SmC mesosphase and may be useful for the projection display since it exhibits high clearing temperature. Copyright © 2017 VBRI Press.

Effect of replacing the terminal phenyl ring with 3-pyridyl and inversion of imine linkage on the mesophase behaviour of four-ring azo/ester/Schiff base compounds

ABSTRACT In the present work two homologue series of four-ring azo/ester/imine compounds; namely, 4-(4′–alkoxy–phenylazo) phenyl–4′′–(benzylidene–amino) benzoate, I n, and 4–(4′–alkoxy–phenylazo) phenyl–4′′–[(pyridin-3–ylmethylene)–amino] benzoate, II n, were prepared and investigated for their mesophase behaviour. In these two types of homologue series, the length of the terminal alkoxy group (n) varies between 6 and 16 carbons. Compounds prepared in both series were structurally characterised via thermogravimetric and elemental analyses, FTIR, 1H-NMR, and mass spectroscopy. Their mesophase transition temperatures were determined by differential scanning calorimetry (DSC), whereas, the type of the mesophase was identified by polarised light microscopy (PLM). The molecular structure–property relationship was conducted aiming to investigate the effect of replacing the terminal phenyl group in I n with a pyridyl one in II n, in addition to the effect of variation of the alkoxy chain length. Aiming to investigate the effect of inversion of the methineazo group on the mesophase behaviour of the compounds, a comparison was made between the prepared series of compounds (I n) with previously prepared four rings 4-(4′-alkoxyphenylazo)–phenyl 4′′–phenyliminomethyl–benzoate phenyl-4ʹ-(4′′-alkoxyphenylazo) benzoate, III n. Finally, the mesophases of series II n were compared with their corresponding three-ring analogues, 4-(4′-alkoxy phenyl −4′′-[(pyridin-4-ylmethylene)–amino] benzoate analogues (IV n).

First columnar rufigallol liquid crystals with high fluorescence at aggregated states

The design and synthesis of columnar liquid crystals with high fluorescence at aggregated states (eliminating the ACQ effect) was an important challenge in the field of liquid crystalline materials. In this work, the first rufigallol columnar liquid crystals with AIE groups were prepared and exhibited good fluorescence in aggregated states. A series of diphenylacrylonitrile-rufigallol-diphenylacrylonitrile trimers was designed and prepared in 70–78% yields. They showed the ordered hexagonal columnar liquid crystals based on the induction of planar rufigallol structures. They also exhibited the good fluorescence in aggregated states with the fluorescence quantum yields of 0.21–0.26 based on the AIE effect of diphenylacrylonitrile units. The length of bridging alkyl chains between rufigallol and diphenylacrylonitrile units contributed to decrease the mesophase transition temperature and increase the fluorescence quantum yields.

Synthesis and liquid crystal behavior of main-chain aliphatic carbonate copolymers derived from diosgenin

To synthesize new main-chain aliphatic carbonate liquid crystal (LC) copolymers Dios-(CH2)6-(TMC)x-(DLLA)y, a compound of diosgenyl derivative Dios-(CH2)6-OH was used as an initiator and reactant through the ring-opening polymerization of 1,3-trimethylene carbonate and d,l-lactide without other catalyst. The chemical structures of the copolymers obtained in this study were characterized by Fourier transform infrared spectrum and proton nuclear magnetic resonance spectrum. Their LC properties were investigated by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction measurement. The effect of the diosgenyl derivative content on mesophase and phase transition temperature of the copolymers was discussed. It was found that three copolymers all displayed fan-shaped texture of a smectic A phase. The results seemed to show an increased tendency toward the isotropic temperature for the LC copolymers with increasing the content of the diosgenyl derivative.

Designing Eu-β-diketonate complexes as a support of ionic liquid crystals (ILCs) with additional luminescent properties

Here we describe the preparation, characterisation, liquid crystal properties and photophysical studies of new potassium salts of tetrakis(β-diketonate)europium(III) complexes based on long-chained symmetrical and unsymmetrically substituted 1,3-dialkyloxyphenyl-β-diketones ligands, K[Eu(OOR(n)R(m))4] (R(n) = OC6H4OCnH2n+1; R(m) = OC6H4OCmH2m+1; n = m = 8, 10, 12, 14, 16; n = 4, 8, m = 12, 16; n = 12, m = 16). All compounds exhibit mesomorphic and luminescence behaviour, regardless the presence of symmetrical or unsymmetrical ligands. It has been shown that the shortest molecular lengths, established by the summation of the two alkyl chains at the substituents of the β-diketonate ligand, give rise to more effective mesophase transition temperatures, while the asymmetry favours higher clearing temperatures. Related to the ionic salts based on long-chained pirazolium cations [H2pzR(n)]+ (R(n) = OC6H4OCnH2n+1; n = 4, 6, 8, 10, 12, 14, 16, 18) and tetrakis(1,3-bis(methoxyphenyl)propane-1,3-dionate)europium(III) as anion, they have also been studied and proved to be luminescent materials. By contrast, none of them was found to be mesomorphic. Therefore, the strategically increased chain length at the cation did not show to be effective for achieving liquid crystal properties. Integration of one of these luminescent europium complexes onto a polymeric PMMA matrix was positively evaluated towards its use as a temperature sensor and fluoride or cyanide ion sensor.

Synthesis and mesomorphic properties of calix[4]resorcinarene–triphenylene oligomers

ABSTRACTBy controlling the mol ratios of reactants, novel calix[4]resorcinarene–triphenylene monomer, dimer and tetramer were designed and synthesised in yields of 50–60% via Click chemistry. Their structures were characterised by NMR and MS. Their liquid crystalline behaviours were studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction analysis. The more triphenylene units on calix[4]resorcinarene resulted in the wider temperature scopes of mesophase and higher phase transition temperatures. The monomer 6 and dimer 7 showed the mixed columnar mesophase with hexagonal columnar structure and disordered lamellar columnar structure, and compound 8 possessed only disordered lamellar columnar mesophase. These research results suggest that calix[4]resorcinarene was a good platform to construct columnar liquid crystal and the mesomorphic properties were greatly influenced by the substituted numbers of mesogen units on calix skeleton.

Enhancement in the thermodynamic, electrical and optical properties of hexabutoxytriphenylene due to copper nanoparticles

In the present article, we have studied the effect of copper nano-particles on the thermal, optical and dielectric parameters of a liquid crystalline material 2,3,6,7,10,11-hexabutyloxytriphenylene (in short HAT4) showing wide temperature range (~65°C) hexatic columnar mesophase. A composite has been prepared by dispersing 0.6wt.% of copper nano-particles. UV–vis spectroscopy has been used to record the absorption spectra. It has been observed that the presence of copper nano-particles introduces surface plasmon resonance and reduces the optical band gap of HAT4. Though isotropic to mesophase transition temperature is unaffected but mesophase-crystal transition temperature has decreased and hence range of the mesophase has enhanced due to the presence of copper nano-particles. While dc conductivity has increased by about two orders of magnitudes, dielectric permittivity has also moderately increased. With the enhanced properties, HAT4-copper nano-particle composite is useful for one dimensional conduction and photovoltaic applications.

Chain length mismatch and packing effects on the thermotropic phase behavior of salt-free catanionic surfactants

Thermotropic liquid crystal formation by salt-free catanionic surfactants (alkyltrimethylammonium alkylsulfonates, herein designated as TAmSon) has been investigated as a function of chain length mismatch (asymmetry). Previous studies on these compounds have revealed an unusual and rich asymmetry-dependent lyotropic phase behavior. Herein, phase transition temperatures and transition enthalpies/entropies were determined by differential scanning calorimetry, while mesophases were assigned by polarized light microscopy. Three series of compounds were investigated, namely: the TA16Son series, where n=6–10; the TAmSo8 series, where m=12–16; and a constant m+n series, TAmSon where m+n=22. Typically, several solid phases and two smectic mesophases are found prior to isotropization to the liquid phase. As asymmetry decreases, two somewhat counterintuitive tendencies emerge: a general decrease in enthalpy/entropy for solid–solid and solid–first mesophase transitions, and an increase in solid–first mesophase transition temperatures. Yet, solid phases are seen to be more stable for the most asymmetric compounds, while the second mesophase is more stable for the least asymmetric ones, in what appears to be a more complex behavior than expected. The results are globally interpreted in terms of subtle differences in chain interdigitation and packing, and odd–even chain effects.

Bent-core compounds with two branched chains: evidence of a new dark conglomerate mesophase

Biphenyl-3,4′-diol derived bent-core compounds with branched side chains have been synthesized and characterized to study the influence of terminal branched groups on mesophase properties and transition temperatures of bent-core mesogens. The liquid crystalline properties of the compounds were investigated by differential scanning calorimetry, optical polarizing microscopy and X-ray scattering. Depending on the length of the chain, the size of molecular branches and the position of branched chains, either enantiotropic or monotropic rectangular columnar phases were observed. For one of the compounds combining two different branched side chains, a new type of optically isotropic dark conglomerate phase (MIso[*]) was found. It is proposed to be a distorted version of a non-tilted or only slightly tilted B6-like intercalated smectic phase with additional in plane order.

Deviation of phase transition process in surface-tethered thermotropic liquid crystalline polymer nanocomposites with graphene oxide: a spectroscopic study

Temperature-resolved FTIR spectroscopy in combination with two-dimensional correlation spectroscopy (2Dcos) was employed to elucidate the dynamic mesophase transition behavior of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS) blended with PMPCS grafted graphene oxide (GO-PMPCS) nanofiller. After integration with 2.0 wt% nanofiller, the columnar nematic phase of polymer nanocomposites was retained while deviation of the mesophase transition temperature was identified, which was ascribed to the π–π stacking interactions and charge transfer effect between polymer and GO sheets revealed by Raman and X-ray photoelectron spectroscopy. Based on 2Dcos analysis, a sequential group motion from aromatic rings to carbonyl groups and main chains was discerned at the early stage of the phase transition. It was concluded that surface-tethering of the PMPCS matrix onto GO sheets may be preferred before the mesophase development. A two-step mesophase transition process defined as a physical adsorption process and a mesophase development process under physical cross-linking was illustrated.

Identification of phase transition temperatures by statistical image analysis

Analysis of the textures and phase transitions of thermotropic liquid crystals p-ethoxybenzylidene-p-n-butylaniline, cholesteryl myristate and cholesteryl decanoate was carried out using a polarising optical microscope under crossed polarisers attached with a hot stage and a high-resolution camera. Using MATLAB software, the statistical parameters, such as mean, standard deviation, root mean square, contrast and entropy of the textures as a function of temperature, are computed. These results show abrupt changes as a function of temperature, indicating the phase transition of the sample. The statistical image analysis of various liquid crystal samples are in good agreement with the literature results and the proposed methodology offers a reliable and very sensitive technique to identify mesophase transition temperatures.

The nanoscale engineering of nematic liquid crystals for displays

Liquid crystal displays, LCDs, are the world's ubiquitous communication devices. In only 50 years, from the late 1960s to the present day, we have seen the birth of a new industry and its accession to dominance across the globe. How was this achieved? Imagination and innovation from device engineers? Yes, but the dreams could only be realised via equally extraordinary developments in the nanoscale engineering of molecular structures of materials through rational design. This article describes how the chemistry of liquid crystals developed so that mesophase structure, transition temperatures and physical properties could be predicted by the best practitioners of the subject so that new device concepts could be realised.

Polystyrene–Polyperfluorooctylethyl acrylate Diblock Copolymers: The Effect of Dilution of the Fluorinated Mesogenic Chains on Bulk and Surface Properties

AbstractSummary: Three smectic poly(styrene‐b‐perfluorooctylethyl acrylate) block copolymers (S‐b‐AF8) with different degrees of polymerization (n, m) of the relative blocks were synthesized by atom transfer radical polymerization (S, n = 25; AF8 m = 2, 6, 23). The mesophase structure and transition temperatures were investigated by DSC and WAXD. The block copolymer having the shortest fluorinated block was blended with a thermoplastic elastomer SEBS in different proportions, in order to look at the effect of a further dilution of the perfluorinated groups on non‐wetting properties. Thin films of the block copolymers as well as the blends exhibited large contact angles with both water and n‐hexadecane, which resulted in low solid surface tensions. XPS findings at different photoemission angles confirmed the effective surface segregation of the mesogenic chains of the fluorinated polymer block.

Synthesis and liquid crystalline behavior of azulene-based liquid crystals with 6-hexadecyl substituents on each azulene ring

Abstract Hexakis(6-hexadecyl-2-azulenyl)benzene (1b) has been synthesized by Co2(CO)8-catalyzed cyclotrimerization reaction of bis(6-hexadecyl-2-azulenyl)acetylene (2b). The mesomorphic behaviors of 1b, 2b, and 6-hexadecyl-2-phenylazulene (3b) were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) techniques and their mesomorphic properties were compared with those of their 6-octyl derivatives 1a, 2a, and 3a. Increase of the number of carbon atoms in the peripheral side chains drops the isotropization temperatures of 1b, 2b, and 3b by 56.9 °C, 33 °C, and 23.6 °C, respectively. Additionally, the phase-transition behavior varied with increase of the number of the peripheral chains, as well as decrease of the crystalline–mesophase transition temperatures, except for compound 3b. As the results, spontaneous monodomain homeotropic molecular alignment was revealed by compound 1b in its Colhd mesophase on non-treated glass substrate, which would be attracted to the application for the device fabrication of molecular materials.

The dramatic influence of the location of bend and of lateral fluoro substitution on the mesomorphic properties of angular chiral esters based on a 1,3-disubstituted benzene ring

The synthesis and mesomorphic properties of a range of 3-ring and 4-ring chiral esters based on a 1,3-disubstituted benzene unit are detailed. These materials all deviate from the usual linear molecular architecture of liquid crystals, and hence are all angular in nature. Some of these materials have the bend right at the end of the molecule where the chain deviates from the normal linear arrangement, and hence a ‘hockey stick’ molecular architecture is perhaps an accurate description. Other materials have a genuine bent-core construction where the bend is towards the centre of the molecule, and hence are best termed as ‘boomerang’ shape. In all cases, interesting comparisons of mesophase morphology and transition temperatures were found, both between the various angular materials and with their linear analogues. In particular, the influence on transition temperatures of lateral fluoro substitution in the novel angular materials was found to be wholly different to that found in the known linear analogues. The work forms part of a larger on-going research programme to investigate the mesomorphic and chirality-dependent properties of angular liquid crystals. The research revealed that no mesomorphism in parent compounds is possible when the bend is close to the centre of the molecule, however, lateral fluoro substitution of such compounds facilitates the generation of liquid crystal phases. Where the molecular bend is as a consequence of the terminal unit at the end of the core, then surprisingly high clearing points resulted, and such materials were found to show the potential for a high tilt angle, and a strong tendency towards helical mesophases. Lateral fluoro substitution of these latter examples resulted consistently in significantly higher clearing points, which is in marked contrast to the behaviour reported previously in known liquid crystals of linear molecular architectures.

Solution processable lutetium phthalocyanine organic field-effect transistors

Spun films of novel thermotropic liquid crystalline lutetium bisphthalocyanine sandwich complexes substituted with sixteen octyl chains are employed as active organic semiconducting layers in the fabrication of organic field-effect transistors (OFET). When the device is annealed under vacuum at 70 °C, an increase in the field effect mobility in the saturated regime by a factor of five is observed while the threshold voltage is reduced to half the value obtained for as-deposited films. The annealed devices exhibit an increase in on/off ratio by two orders of magnitude. An improvement in performance of OFETs annealed above the crystal to mesophase transition temperature is consistent with atomic force microscopic images indicating increases in grain sizes and decreases in mean surface roughness of the organic bisphthalocyanine films.

Low Surface Energy Properties of Smectic Fluorinated Block Copolymer/SEBS Blends

A smectic polystyrene–poly(perfluorooctylethyl acrylate) block copolymer was synthesized by atom transfer radical polymerization and blended with a thermoplastic elastomer SEBS in different compositions. The mesophase structure and transition temperatures were investigated by DSC and WAXD. Thin films of the block copolymer and the blends exhibited large contact angles with both water and n-hexadecane, which resulted in low solid surface tensions. XPS findings at different photoemission angles confirmed the effective surface segregation of the mesogenic chains of the fluorinated polymer block.

Phase behaviors of comb-like liquid crystalline polysiloxanes containing fluorinated mesogenic units

Several liquid crystalline polysiloxanes (Pa–Pf) bearing fluorinated mesogenic units were synthesized using poly(methylhydrogeno)siloxane, 4′-(4-undec-10-enoyloxy-benzoyloxy)-biphenyl-4-yl 4-fluoro-benzoate and 4′-propionyloxy-biphenyl-4-yl 4-allyloxy-benzoate, and effect of fluorinated mesogenic units on phase behaviors of the fluorinated LC polysiloxanes was studied as well. The samples Pa and Pb showed single nematic mesophase when they were heated and cooled, but Pc, Pd, Pe and Pf exhibited both smectic and nematic phases. The glass transition temperature and smectic A–nematic mesophase transition temperature of polymers increased slightly with increase of fluorinated units in the polymer systems, but mesophase–isotropic phase transition temperature decreased slightly. In XRD curves, the intensity of sharp reflections at low angle increased with increase of fluorinated mesogenic units in the polymers' systems, indicating that the longer spacer and the fluorophobic effect of fluorinated mesogenic units could lead to a significant stabilization and even to modifications of smectic mesophases.