Thermal and optical properties of newly synthesized dicholesteryl esters with a phenylene oxide link in the normal and solidified cholesteric phases

A series of liquid crystalline compounds with a central phenylene oxide group to which two cholesteryl groups are attached via two alkanoate spacers has been synthesized and investigated. All the compounds except the one with ethanoate spacers showed a cholesteric phase as a single mesophase. The phase transition temperatures, the corresponding enthalpy changes and the wavelengths of selective reflection associated with the cholesteric phases showed a strong odd-even effect as a function of the spacer length. By rapid cooling of the compounds from the cholesteric phase to 0°C, solid films maintaining a cholesteric molecular order were obtained. At room temperature, the solid film showed stable cholesteric colours controlled by changing the temperature at which the rapid cooling was begun. Heating the cholesteric solid film of the compound with hexanoate spacers gave two forms of crystals above 80°C, whose ratio changed depending on the colour of the starting solid films. This result suggests the existence of two conformational isomers in the liquid crystalline state. Since it is possible repeatedly to fix stable red, green and blue colours by thermal treatment of this compound, we may apply it to a rewritable full colour recording in the thermal mode.     

Enhanced optical oxygen sensing using a newly synthesized ruthenium complex together with oxygen carriers

In this article, an emission based, simple and fast method is proposed for the determination of gaseous oxygen. A newly synthesized fluorophore, dichloro-{2,6-bis[1-(4-dimethylamino-phenylimino) ethyl]pyridine}ruthenium(II) has been used for oxygen sensing together with oxygen carrier perfluorochemicals (PFCs) in silicon matrix. It should be noted that the solubility of oxygen in fluorocarbons is about three to ten times large as that observed in the parent hydrocarbons or in water, respectively. Employed PFCs are chemically and biochemically inert, have high dissolution capacities for oxygen, and, once doped into sensing film, considerably enhance the response of sensing agent.     

Structure and electronic properties for a newly synthesized red fluorescent material

The ground-state structure and electronic properties of a newly synthesized red fluorescent material, 2-[3-(2-{4-[(2-hydroxy-ethyl)-methyl-amino]-phenyl}-vinyl)-5, 5-dimethyl-cyclohex-2-enylidene]-malononitrile (A31), are investigated using a hybrid density-functional theory (DFT) approach, B3LYP, and the 6-31G* basis set. We have obtained four geometrical isomers. The theoretical infrared (IR) spectrum calculated by B3LYP/6-31G* level of theory is in very good agreement with our experimental measurement. The cation and anion are optimized to clarify the effects of the hole and electron injections and the energies needed by injections are estimated.     

pH-dependent optical properties of a poly(phenylene ethynylene) conjugated polyampholyte

A poly(phenylene ethynylene) conjugated polymer (PPE-NMe(3)(+)-COO(-)) containing tetraalkylammonium groups and carboxylate groups has been synthesized by Sonogashira coupling. Due to the presence of the strong cationic and weak anionic pendant units, the polymer undergoes a pH-induced transition from cationic polyelectrolyte to polyampholyte due to deprotonation of the carboxylic acid units in basic solution. Studies of the pH dependence of the polymers' optical properties reveal changes in absorption oscillator strength and fluorescence quantum efficiency that are triggered by the transition from cationic polyelectrolyte to polyampholyte nature. Stern-Volmer fluorescence quenching of PPE-NMe(3)(+)-COO(-) with a negatively charged quencher 1,4,5,8-naphthalenediimide-N,N-bis(methylsulfonate) (NDS) shows that the polymer fluorescence quenching is amplified at low pH where the polymer is a polycation, whereas the quenching efficiency is considerably less at high pH.     

Physical characterization of B1 and B2 phases in a newly synthesized series of banana shaped molecules

A new homologous series of achiral banana-shaped mesogens ('Cn') has been synthesized and studied by classical techniques (optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optic investigations). Complete miscibility with compounds of the n-OPIMB series shows that the short homologues (C5-C9) exhibit a frustrated B1 antiphase, while the longer members (C10-C14) present a B2 smectic C phase. Electro-optic measurements confirm this identification.     

Measurement of the Kerr effect in cholesteric blue phases

The electric field induced birefringence within the blue phases and the isotropic phase of several different liquid crystals has been measured. The temperature dependences within the blue phases are found to be strong, characteristically different for each phase, and unlike what is predicted by simple theoretical arguments.     

Study of electronic and spectroscopic properties on a newly synthesized red fluorescent material

The ground state (S(0)) and the lowest singlet excited state (S(1)) of a newly synthesized red fluorescent material, 2-[3-(2-{4-[(2-Hydroxy-ethyl)-methyl-amino]-phenyl}-vinyl)-5,5-dimethyl-cyclohex-2-enylidene]-malononitrile (A31), are investigated. The S(0) and S(1) geometries are optimized at the ab initio Hartree-Fock and the singles configuration interaction (CIS) levels of theory, respectively. The CIS and semiempirical Zerner's Intermediate Neglect of Differential Overlap (ZINDO) methods provide the results for the absorption (S(0)-->S(1)) and emission (S(1)-->S(0)) transition energies. The Stokes shifts calculated at the CIS and ZINDO levels of theory are obtained. The absorption spectra in various solvents are calculated using the time-dependent density-functional theory method in combination with the polarized continuum model, which are in very good agreement with our experimental measurements. The solvent effects are discussed.     

Anomalous optical properties of photoactive cholesteric liquid crystal doped with single-walled carbon nanotubes

Optical transmission and selective reflection data are reported for suspensions of single-walled carbon nanotubes (CNTs) in photoactive nematic material ZhK-440 with a mesogenic chiral dopant M5. At small concentrations of CNTs (C ≈ 0.01–0.05%), the preferential localisation of CNTs at oily sticks (cholesteric topological defects) and suppression of the network of oily streaks by CNTs were observed. At the same time, the optical density D was shown to be essentially non-linear and a minimum at certain concentration of CNTs, C ≈ 0.05–0.08%, was observed. This anomalous behaviour was explained by the presence of the structural transition from the loose (ramified) aggregates with highly anisotropic shape (oriented along the anchoring direction on rubbed polyvinyl alcohol) to the compact aggregates with denser packing. The location of this minimum, as well as the selective reflection maximum (helical pitch), was sensitive to partially reversible UV-induced trans–cis–trans isomerisation effects. The UV-controlled helical pitch variation was shown to be only slightly affected by introduction of CNTs.     

Synthesis and characterization of two series of pressure-sensitive cholesteric liquid crystal elastomers with optical properties

ABSTRACT

We synthesized two series of cholesteric liquid-crystal elastomers by hydrosilylation among monomers MA containing a cholesteryl group, MB (MC) containing a phenolic hydroxyl group and MD as the crosslinker. The chemical structures of all the monomers and LCEs were confirmed by ¹H NMR and FT-IR. We explored the mesomorphic properties and phase behaviours by TGA, DSC, POM, and XRD. All the LCEs presented elasticity, reversible phase transition, and high thermal stability. For two series of LCEs, the glass transition temperature increased slowly, and the isotropic transition temperature increased obviously. PA-I-PA-V and PB-I-PB-IV displayed selective reflection and colourful Grandjean texture, but PA-VI, PB-V, and PB-VI needed external pressure to show them.     

Photoinduced textural and optical changes in a cholesteric copolymer with azobenzene-containing side groups

Textural changes induced by UV irradiation of planarly oriented films of a cholesteric copolymer were investigated. The copolymer is composed of photosensitive ethoxyazobenzene and chiral cholesterol-containing side groups, and displays a chiral nematic phase with a left-handed helical supramolecular structure in the temperature range 84-128°C. UV light with a wavelength of 365 nm leads to a deformation of the planar texture at temperatures higher than 100°C and to a reduction of the absorbance peak for left-handed circularly polarized light. Polarizing optical microscopy revealed the formation of a focal-conic texture in the irradiated regions. The texture induced by irradiation is very stable but the initial planarly oriented state can be recovered by mechanical stress. The observed phenomenon is attributed to the E-Z photoisomerization of azobenzene groups leading apparently to the formation of a thin isotropic layer in the films which in turn causes a destabilization of the planar texture. At temperatures below 100°C the textural transformations are accompanied by small shifts of the selective reflection maxima to longer wavelengths. The kinetics of the observed phenomenon were studied in detail and the influence of different parameters, such as light intensity and temperature, examined.     

Selective reflection in the cholesteric and blue phases of a chiral-racemic mixture of CE6

The reflection spectrum for visible light is examined for the cholesteric and blue phases of chiral CE6. Pronounced side band oscillations are observed. The Bragg wavelength for total reflection diverges towards the smectic phase with an exponent v = 0·71±0·05. Going from the cholesteric phase to BPI, the lattice parameter increases by (2)^1/2. Evidence is given for the existence of a long-lived supercooled blue phase (BPS).     

Selective reflection in the cholesteric and blue phases of a chiral-racemic mixture of CE6

Abstract

The reflection spectrum for visible light is examined for the cholesteric and blue phases of chiral CE6. Pronounced side band oscillations are observed. The Bragg wavelength for total reflection diverges towards the smectic phase with an exponent v = 0·71±0·05. Going from the cholesteric phase to BPI, the lattice parameter increases by (2)^1/2. Evidence is given for the existence of a long-lived supercooled blue phase (BPS).     

Ultrasonic absorption studies in the cholesteric and isotropic phases of cholesteryl valerate

Ultrasonic absorption has been measured in the cholesteric and isotropic phases of cholesteryl valerate as a function of both temperature and frequency. The anomalous absorption observed in the cholesteric phase has been interpreted by considering the coupling of the sound wave to the director fluctuations. In the isotropic phase coupling of the sound wave to the tensor order parameter is considered to analyse the absorption results.     

Thermal degradation of polymers with phenylene units in the chain. I. Polyphenylenes and poly(phenylene oxides)

The thermal degradation of polyphenylenes and poly(phenylene oxides) was studied under vacuum at temperatures between 350 and 620°C. The volatile and solid degradation products were analyzed by mass spectroscopy, infrared spectroscopy, and elemental analysis. Overall mechanisms for the thermal breakdown have been proposed. Polyphenylene decomposes to form polymer carbon, while hydrogen is the major volatile product. Some ring breakdown occurs with evolution of methane. Poly(phenylene oxide) forms mainly low molecular weight chain fragments, partially with hydroxyl endgroups. Some of the ether linkages decompose with ring breakdown, yielding carbon monoxide, water, and some carbon dioxide. Pendent groups on polyphenylenes and poly(phenylene oxides) are removed at the lower temperatures. The hydroxyl group yields essentially carbon monoxide and dioxide (the carbon being supplied by the rings), the methyl group methane, and the methoxy group methane and some methanol.     

Influence of the position and number of fluorine atoms and of the chiral moiety on a newly synthesized series with anticlinic properties

A series of trifluoro-substituted benzoate derivatives: (S)-1-ethylheptyl 4-[4-(4-alkyloxy-3- fluorobenzoyloxy)-3-fluorobenzoyloxy]-2-fluorobenzoates is reported. The short chain members (n = 8 to n = 11) display a direct SmC*_A-SmA transition, whereas for longer chains a SmC* phase appears, but no ferrielectric phases are present, and a direct SmC_A*-SmC* transition is obtained. The mesomorphic properties were studied by optical microscopy and DSC, and by electro-optical, helical pitch and optical rotatory power measurements. The effect of the number and position of the fluoro substituents, and the influence of the chiral moiety on the mesomorphic behaviour are discussed.     

Influence of the position and number of fluorine atoms and of the chiral moiety on a newly synthesized series with anticlinic properties

A series of trifluoro-substituted benzoate derivatives: (S)-1-ethylheptyl 4-[4-(4-alkyloxy-3- fluorobenzoyloxy)-3-fluorobenzoyloxy]-2-fluorobenzoates is reported. The short chain members (n = 8 to n = 11) display a direct SmC*_A-SmA transition, whereas for longer chains a SmC* phase appears, but no ferrielectric phases are present, and a direct SmC_A*-SmC* transition is obtained. The mesomorphic properties were studied by optical microscopy and DSC, and by electro-optical, helical pitch and optical rotatory power measurements. The effect of the number and position of the fluoro substituents, and the influence of the chiral moiety on the mesomorphic behaviour are discussed.     

Interaction and optical properties of phases in the PbF2-PbO-NaPO3 system

Interactions of PbO, PbF₂, and their mixtures with sodium metaphosphate are studied by thermal analysis and X-ray powder diffraction. Considerable distinctions are found between the diffuse reflectance spectra of phases formed by various lead compounds. The compositions of phases are indicative of complex interactions; lead fluorapatite Pb₁₀(PO₄)₆F₂ is formed in some cases.     

Thermal properties of systems containing cholesteryl esters and triglycerides.

Binary and ternary systems of the three cholesteryl esters, linoleate, oleate, and stearate and the two triglycerides, triolein and tristearin were studied in order to determine the phase transitions and the conditions for the cholesteric and smectic mesophases. Phase transitions were determined using differential thermal analysis, melting point determination, and polarizing microscopy. Of the cholesterol esters the linoleate-oleate system showed complete miscibility in both the liquid and solid phases. The linoleate-stearate and oleate-stearate systems are of the eutectic type with limited solid solubility. The mesophases are monotropic as to the crystalline state and exist over the entire composition interval in all cholesteryl ester systems studied.     

Chirality,twist and structures of micellar lyotropic cholesteric liquid crystals in comparison to the properties of chiralic thermotropic phases

Phase chirality in disk-like lyotropic cholesteric phases (Ch(D)) was investigated, which was induced by addition of center and axial chiral dopants to achiral lyotropic nematic host phases (N(D)). In a lyotropic nematic matrix of the disk-like N(D) phase in the ternary system hexadecyldimethylethyl ammonium bromide (C16Me2EABr)/water/n-decanol, a disk-like lyotropic cholesteric phase Ch(D) was induced by addition of the axial optically active compound R(-)-1,1'-binaphthalene-2,2'-diyl-hydrogen-phosphate (BDP). The helical twisting power (HTP) of the BDP is generally lower than the HTP value of inducing substances with center chirality as cholesterol, prednisolone and taurocholic acid. At constant composition of the N(D) phase, the helix lengths were determined in dependence on the BDP and steroid concentration by means of evaluation of the 'spaghetti-like' texture using polarizing microscopy. The reciprocal helix lengths are changing linearly with rising BDP concentration. The properties of the Ch(D) phase (textures, helix lengths, structural parameters of the micelles) induced by the chiral compounds and changed by the composition of host phases can give information to the mechanism of chirality transfer from the molecular level to that of the micellar aggregates and finally, to the liquid crystalline superstructure. Furthermore, the matrix influence of the N(D) phase on the helix formation was examined at constant BDP and steroid concentration. The structure in the Ch(D) phase was described in terms of micelle parameters. Finally, the inducing properties of a center chiral optically active compound such as cholesterol, prednisolone and taurocholic acid were compared with those of the axial chiral compound BDP. Last but not least, the situation of the theoretical and structural background for helix formation in liquid crystals, e.g. the explanation of chiralic transfer between micelles is analyzed and discussed. Two main conditions are necessary to build up the helix in the Ch(D) phase: the formation of H-bridges; and the existence of a specific chiralic interaction energy between neighboring micelles in the cholesteric superstructure.     

Rheological properties in cholesteric and blue phases of cholesteryl isostearyl carbonate: viscosity and effect of electric field on the rheology

Rheological property (shear stress versus shear rate) of cholesteryl isostearyl carbonate (CISC) is measured as a function of temperature, finding that, like some other cholesteryl derivatives, CISC has blue phases (BPs) between the cholesteric (Ch) and the isotropic (I) phases. In the Ch and the BP phases, measurements of the electrorheological (ER) effect, of the electric field on the rheology, are made; a slight increase of the viscosity is observed in the BPs and no ER effect in the Ch phase, which is contrasted to the distinct ER effect in the nematic (N) and the smectic A (SmA) phases.