Nickel-catalyzed synthesis of phosphonium salts

The nickel-catalyzed synthesis of phosphonium salts from aryl halides and triphenylphosphine is reported.The formation of phosphonium salts occurs by the oxidative addition reaction of Ni[P(C₆H₅)₃]₃ on the aryl halides and reaction of the coordinated aryl groups with triphenylphosphine.The influence of substituents on the aromatic nucleus is studied.     

Photochromism in mixtures of liquid crystalline chiral copolymers with a photosensitive chiral dopant

Mixtures of a photosensitive chiral dopant based on ( )-menthone with left-handed and righthanded cholesteric copolymers were prepared. The phase behaviour and optical properties of the mixtures prepared were studied. The action of UV radiation on planar oriented films of such systems was shown to induce dramatic changes in the maximum reflection wavelength as a result of E-Z isomerization of the dopant molecules. The kinetics of photoisomerization of such mixtures in solution and in the bulk were investigated at different temperatures. The above mixtures can be considered as promising and uprecedented materials for coloured data recording and storage.     

Anomalous phase sequence in new chiral liquid crystalline materials

We have prepared and studied a series of rod-like liquid crystalline compounds with the molecular core laterally substituted by bromine atom and with the lactate unit in the chiral terminal chain. Microscopic observation under the polarising microscope, x-ray and dielectric spectroscopy results enable us to establish the TGBA–SmA–TGBA_RE–TGBC phase sequence. The presence of the re-entrant TGBA phase below the regular SmA phase, found for three homologues, is quite uncommon. Results are discussed and compared with the previously studied compounds with regular phase sequences.     

Solvent-free mechanochemical synthesis of phosphonium salts

Phosphonium salts have been prepared during high-energy ball-milling of triphenylphosphine with solid organic bromides; the reactions occur at ambient conditions without a solvent; in the case of 2-bromo-2-phenylacetophenone the reaction in a solution usually produces a mixture containing both the C-phosphorylated and O-phosphorylated compounds, while the solvent-free mechanically induced transformation results in the thermodynamically favorable C-phosphorylated product; the occurrence of the observed transformations during mechanical processing of solid reactants is confirmed by the solid-state 31P NMR spectroscopy and X-ray powder diffraction.     

Nematic liquid crystalline structures in dilute surfactant solutions

The nematic phase and the viscous isotropic (VI) phase formed by solutions of CTAB and SHNC are investigated using light scattering and viscosity techniques. It is argued that the turbidity in the nematic phase is due to orientational fluctuations in the director and therefore the intensity scales as Q^-2. It is shown that with dilution of the nematic phase with water the turbidity disappears with a simultaneous increase of viscosity.     

Luminescent lanthanide-containing chiral liquid crystalline polymers in the side chain

Luminescent lanthanide-containing chiral liquid crystalline polymers are graft-copolymerised using poly(methylhydrogeno)siloxane (PMHS), crosslinking agent, liquid crystalline monomer and lanthanide complexes. The chemical structures of the monomers are characterised by FTIR, ^1?H NMR and elemental analyses. The mesomorphic properties and phase behaviour are investigated by differential scanning calorimetry, thermogravimetric analysis, polarising optical microscopy and X-ray diffraction. The polymers containing less than 9 mol% of the crosslinking units reveal reversible mesomorphic phase transition, wide mesophase temperature ranges and high thermal stability. With the introduction of lanthanide complex units, the polymers are enabled with the significant luminescent properties. The temperature dependence of fluorescence intensity was studied in the liquid crystalline phase. The IR imaging shows that the lanthanide complex units evenly distribute in polymers.     

Side-chain liquid crystalline polysiloxanes containing a cyanohydrin chiral centre

A number of new ferroelectric side chain liquid crystal homopolymers derived from poly(hydromethylsiloxane), their copolymers with different mesogens and with poly(dimethylsiloxane) backbones have been prepared. The transition temperatures of the monomers, homo and copolymers have been determined. The homopolymers exhibit S*_C and N* phases, while copolymers show only a S*_C phase. A measured spontaneous polarization value of monomer, 9b, is larger than 500 nC cm^-2.     

Collective fluctuation in chiral smectic phases of main-chain liquid crystalline polymers

We have investigated the collective fluctuation in the chiral smectic phases of main-chain polyesters having a mesogenic p,p'-dibenzoate unit via dielectric and dynamic electrooptical measurements. A relaxation due to the Goldstone mode was observed at several tens of Hz in the chiral smectic C (SmC*) phase in both measurements, where another relaxation associated with the soft mode was also confirmed around the SmA-SmC* phase transition temperature. Although the mesogens are linked together in a main-chain liquid crystalline polymer, the observed soft mode is associated with the tilt fluctuation of the average direction of the mesogens.     

Analysis of the vibrational spectra of chiral liquid crystalline thioesters

Vibrational spectra of (S)-(+)-4-(1-methylheptyloxy)biphenyl 4-pentylphenylthiobenzoate (MHOBS5) were calculated using the density functional theory method, with two different basis sets, the 6-31G* and the 6-31+G*. The force fields were scaled using the scaled quantum-mechanical force field (SQM) procedure. Various sets of scaling factors for the SQM procedure are discussed. Results of simulations were used to analyse the experimental data obtained from infrared and Raman experiments for MHOBS5 and the homologous MHOBS4.     

Nano-biphasic ionic liquid systems composed of hydrophobic phosphonium salts and a hydrophilic ammonium salt

A combination of a phosphonium-type-zwitterions-lithium bis(trifluoromethanesulfonyl)imide complex and a hydrophilic ammonium salt provides a nanosegregated liquid-crystalline matrix consisting of hydrophilic ionic liquid (IL) domains and hydrophobic IL domains.     

Synthesis and SmC* properties of chiral liquid crystalline terephthalates

Chiral ester derivatives of terephthalic acid containing one, three, or five phenyl rings were prepared by using (S)-2-methylbutanol or (R)-2-chloropropanol as starting materials. The one-ring terephthalates did not exhibit any liquid crystalline phases, but one of them was used as a chiral dopant in ferroelectric mixtures. Elongation of the aromatic core structure to three and five phenyl rings stabilized the SmA phase, whereas a (monotropic) SmC* phase was detected in some three-ring and five-ring derivatives. In ferroelectric mixtures based on the terephthalates, spontaneous polarizations up to 340 nC cm2 were measured.     

Photo-optical properties of new combined chiral photochromic liquid crystalline copolymers

For the first time, a series of cholesteric copolymers containing combined chiral photochromic side groups has been synthesized and the phase behaviour and optical properties of the copolymers have been characterized. Specific features of the photochemical and photo-optical behaviour of such systems were studied, and the quantum yields of the photo-induced process in solution and in the condensed state of the cholesteric copolymers were calculated. The selective light reflection wavelength was found to be controlled by the UV radiation. The synthesized polymers were shown to be promising candidates for colour data recording.     

Synthesis and characterization of chiral smectic C liquid crystalline terpolysiloxanes

A number of tailor-made side chain liquid crystalline terpolysiloxanes, containing chiral cyanohydrin ester, phenyl pyrimidine and fluorinated phenylbenzoate as the side groups, have been synthesized for potential applications as ferroelectric liquid crystal polymers. All the polysiloxanes exhibit the mesomorphic sequence of crystal-chiral smectic C-smectic A-isotropic phases.     

Dielectric relaxation in a novel tapered chiral photochromatic liquid crystalline dendrimer

Temperature-dependent dielectric spectroscopic measurements of newly synthesised ribbon-shaped chiral liquid crystalline dendrimer with photochromic azobenzene mesogens and an isosorbide chiral centre on planar anchoring cell have been performed in the frequency range of 1.0 Hz to 5.5 MHz. Three dielectric dispersions in the relaxation frequency range of 10–80 Hz, 80–130 kHz and ~3.5 MHz have been observed not only in chiral smectic phase but also in isotropic phase in which two lower-frequency processes are retarded while the other one remained at same relaxation rate from isotropic to chiral smectic phase. Based on the dielectric and optical polarising microscopic results, the chiral smectic phase has been identified as SmC* phase. The relaxation mode observed at low-frequency region in the SmC* phase followed the dielectric characteristics of pinned Goldstone mode. Whereas the dielectric dispersions observed at high-frequency region have been analysed in the framework of the model given by Maier and Meier.     

Helical twisting power and compatibility in twisted nematic phase of new chiral liquid crystalline dopants with various liquid crystalline matrices

ABSTRACT

Four chiral dopants exhibiting smectic LC phases themselves were prepared and their helical twisting power (HTP) and thermal phase behaviour in mixtures with four various LC hosts were studied. The influence of host liquid crystal on HTP was evaluated and generally higher values were found for hosts with high birefringence. Unexpectedly, high enhancement was found for an LC-chiral dopant pair, both having a similar aromatic core – biphenyl ring substituted with polar group. All studied chiral dopants exhibited limited compatibility with the LC hosts in twisted nematic phase at room temperature. For one of the studied mixtures, it was able to obtain single twisted nematic phase with selective light reflection band with maximum at wavelength about 1.0 µm. Carboxylic acid-type dopants exhibited total compatibility with the studied host in single twisted nematic phase at elevated temperatures, allowing preparation of mixtures with reflection band in the visible range. In case of the carboxylic acid dopants, blue phases for optimised compositions were observed. Intermolecular hydrogen bonding between carboxylic acid proton and pyridine nitrogen of chiral dopants was found. Doping the LC host with these dopants led to slight enhancement of HTP value and higher solubility in the LC host.     

Visible light-induced thiocyanation of gem-difluorinated phosphonium salts

gem-Difluorinated phosphonium salts were generated from nitrones by methylation and reaction with (triphenylphosphonio)difluoroacetate. The phosphonium salts were subjected to visible light-induced phosphonium–thiocyanate exchange in the presence of ammonium thiocyanate, copper thiocyanate, and an iridium photocatalyst. The sequence of the reactions was performed in one pot affording thiocyanates attached to the difluoromethylene fragment.