Roman Dąbrowski

Influence of alkoxy chain length and fluorosubstitution on mesogenic and spectral properties of high tilted antiferroelectric esters

The synthesis and properties of a few dozen highly evolved analogues of (S)-MHPOBC are reported. The liquid crystal phases, and phase transition temperatures and enthalpies, as evidenced by polarized optical microscopy and differential scanning calorimetry, respectively, are given. The helical pitch in pure compounds and some mixtures is estimated using the selective reflection method. All of the compounds show antiferroelectric phases, and some key structure–property relationships for the new materials are reported. Specifically, it is found that the length of the oligomethylene segment of a novel partially fluorinated alkoxy ether tail incorporated into the structures strongly influences the stability of the antiferroelectric phase and the observed phase sequences, as well as the values of the helical pitch and its temperature dependence. The formulation of a family of broad temperature range eutectic mixtures possessing the highly desirable orthoconic antiferroelectric phases with long helical pitch is described.

New Liquid Crystalline Materials for Photonic Applications

The mesogenic and optical properties of few families of nematic liquid crystals with low, medium and high birefringence as well as high tilted chiral anticlinic(antiferroelectric)smectics recently developed have been shortly characterized and reviewed. They are:alkyl 4-trans-(4-trans-alkylcyclohexyl)cyclohexyl carbonates being liquid crystals with ordinary index nolower than silica glass in broad temperature range,high birefringence two ring 4′-alkyl- and 4′-alkoxy-4′-isothiocyanatotolanes and three ring 4′-(4-alkyl)phenyl-, 4′-(trans-4-alkyl)cyclohexyl- or 4-(4′-trans-alkyl)cyclohexylethyl-4-isothiocyanatotolanes unsubstituted or laterally substituted by fluorine or chlorine atoms,(S)1-methylheptyloxyphenyl 4-(perfluoroalkanoyloxyalkoxy)biphenylates and(S)1-methylheptyloxybiphenylyl-4-yl 4-(perfluoroalkanoyloxyalkoxy)benzoates having the broad temperature range of antiferroelectric phase.The examples of mixtures with low noandΔn, highΔn as well as orthoconic antiferroelectric mixtures are given.

Novel high birefringent isothiocyanates based on quaterphenyl and phenylethynyltolane molecular cores

We report the synthesis of new laterally alkyl substituted high birefringent quaterphenyl and phenylethynyltolane liquid crystals with isothiocyanate terminal group. The thermal and optical properties such as birefringence are measured and discussed based on their molecular structures. The compounds exhibit birefringence (Δn) in the range of 0.44–0.63, and are expected to be appropriate components of high birefringent nematic mixtures for various applications.

Nematic Phases Created in Binary Mixtures of Smectics A

Abstract It is shown that the forming of nematic mixtures composed uniquely of compounds with the smectic phase A is possible. The nematic phase is created as a result of the weakening of intermolecular attraction forces in the disturbed smectic layers. Many examples are given of binary mixtures in which induction of the nematic phase is observed or an enhancement of the nematic phase at the cost of the smectic phase takes place. The effect of the molecular structure and of the properties of the smecticphase A on its destabilization in a mixture and the creation of the nematic gap between the smectic A regions are discussed. The most characteristic features of the phase diagrams related to the S→I and S→N phase transitions and the accompanying changes of enthalpy of these phase transitions, of density and viscosity are shown in detail by way of example of binary mixtures composed of 5-n-alkyl-2-(4′-isothiocyanatophenyl)-1,3-dioxanes (smectic A1) and 80CB, or 120CB or 12CB (smectic Ad).

From the discovery of the partially bilayer smectic A phase to blue phases in polar liquid crystals

The significance of Gray’s cyanobiphenyls in the discovery of the partially bilayer smectic A phase (SmAd) and in the formulation of mixtures exhibiting the blue phase is described. The properties of the SmAd phase and its relationships to molecular structures are reviewed. Induced phenomena such as the transformation of nematics into smectics, SmAd or SmA1, as well as the opposite transformation of smectics into nematics are described and discussed.

Polarization Optics of Microstructured Liquid Crystal Fibers

The present paper discusses polarization phenomena occurring in microstructrured liquid crystal fibers and in particular solid-core photonic crystal fibers infiltrated with liquid crystals. We report on the latest experimental polarization characteristics of microstructured photonic crystal fibers filled with prototype nematic liquid crystal guest materials characterized by either extremely low (of the order ∼ 0.05) or medium (of the order ∼ 0.2) material birefringence. Due to anisotropic properties of the microstructrured liquid crystal fibers switching between different guiding mechanisms as well as electrically and temperature-induced tuning of light propagation have been demonstrated. These preliminary results hold great potential for both fiber-optic sensing and in-fiber polarization mode dispersion control and compensation.

Polarization Mode Dispersion in an Elliptical Liquid Crystal-Core Fiber

Polarization phenomena in an elliptical liquid crystal-core fiber at third optical window, i.e., close to 1550 nm have been investigated. This includes direct measurement of polarization mode dispersion and the beat-length parameter. The elliptical liquid crystal-core fiber contained a nematic mixture characterized by extremely low values of refractive indices.

Dielectric properties of ten three-ring LC fluorosubstituted isothiocyanates with different mesogenic cores

Results of the static and dynamic dielectric investigations of 10 liquid crystalline compounds consisting of three rings (phenyl or cyclohexyl) with the n-propyl and isothiocyanato terminals, fluoro lateral substitutes, and different bridging groups (CH2CH2 and/or COO) are presented. These compounds form the nematic phase in a broad temperature range. The dielectric parameters are analyzed in relation to the chemical and dipole structures of the molecules. The influence of different structural elements such as rings, bridging groups and fluorosubstitution on the dielectric properties is discussed. The relaxation time and activation enthalpy characterizing the molecular rotations around the short axes were determined. In the nematic phase, the compounds having two fluorine atoms at the lateral positions exhibit a crossover of the principal permittivity components within the megahertz frequency range. This makes them interesting from the application point of view as the so-called dual frequency materials.

Dielectric studies of tolane derivatives exhibiting the E and K phases

Results of dielectric studies for three tolane derivatives containing the isothiocyanato (NCS) and alkyl or alkoxy groups in the terminal positions are presented. A relaxation process associated with molecular rotations around the short axes has been observed in the crystal‐like smectic phases (B, E, K). The K phase was characterized for the first time by dielectric spectroscopy. The results are discussed by taking into account the dielectric relaxation data obtained recently for the biphenyl analogues in the E phase. The available X‐ray data allow us to calculate the so‐called packing parameter which characterizes the free volume of the molecules in the unit cells.

Compounds with low relaxation frequency and dual frequency mixtures useful for active matrix addressing

Dozens of three- and four-ring liquid crystal derivatives of alkylcyclohexyltolanes, alkylbicyclohexyltolanes and alkylbicyclohexylbiphenyls with a terminal polar group F or trifluoromethoxy (OCF3), cyano (CN), isothiocyanato (NCS) have been synthesised. Their dielectric properties upon temperature and electric field frequency f have been investigated. Compounds dissolved in a negative dielectric medium exhibit strong dispersion of the parallel permittivity ϵ||(f) falling on the kilohertz frequencies with practically constant value of the perpendicular permittivity ϵ⊥ in this frequency range. This causes a change of sign of the dielectric anisotropy from positive at low frequencies to negative at high frequencies. Many so-called dual frequency mixtures, convenient for active matrix addressing, with the fco values in the range of 1–10 kHz at 20°C, have been prepared and investigated. The cross-over frequency fco is close to the relaxation frequency fr. The discussion refers to relations among the relaxation frequency, compound structure and viscosity of a liquid crystal medium.