Phase diagram and electro-optical Kerr effect of nematic mixtures containingtolane and biphenyl cores

An experimental phase diagram for two types of nematic liquid crystals and their binary mixtures was established by polarised optical microscopy and differential scanning calorimetry methods. The mixtures comprised a tolane-base liquid crystal, 4-heptyl-3-fluoro-4-isothiocyanatotolane (7TOLF), and a biphenyl-base nematogen, 4-heptyl-3-fluoro-4-isothiocyanatobiphenyl (7BF). The static Kerr effect and third-order non-linearity were investigated for 7TOLF and 7BF and their nematic mixtures within the isotropic phase. Both the compounds have a positive and large Kerr constant. The second-order phase-transition temperatures, T?, were determined for these mixtures. The linear dependence of (T - T?)^??1 on the Kerr constant is found to be in good agreement with the predications of the Landau–de Gennes model. The third-order non-linear susceptibility,χ⁽³⁾, values were determined for these mixtures.     

H-shaped liquid crystals inducing nematic order in the isotropic liquid

ABSTRACT

We prepared a homologous series of H-shaped liquid crystals I-n and investigated their phase transition properties using optical microscopy and differential scanning calorimetry. All the compounds exhibited a nematic phase at room temperature. The phase transition behaviour is explained in terms of molecular shape anisotropy. Furthermore, those compounds were found to exhibit electro-optical switching in the isotropic liquid in the vicinity of the nematic–isotropic liquid transition, indicating that the microscopic nematic order with a certain coherence length of the molecules exists in the optically isotropic temperature range.     

Optical activity and light scattering in the isotropic and smectic A* phases of a highly chiral smectic liquid crystal

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4'-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral-racemic mixture of S-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic-smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

Determination of the ground- and excited-state dipole moments of 4’-(hexyloxy)-4-biphenylcarbonitrile and 4-isothiocyanatophenyl 4-pentylbicyclo [2,2,2] octane-1-carboxylate nematic liquid crystals and their mixtures

The dipole moments of the ground and excited states of 4′-(hexyloxy)-4-biphenylcarbonitrile and 4-isothiocyanatophenyl 4-pentylbicyclo [2.2.2] octane-1-carboxylate nematic liquid crystals and their mixtures prepared in chloroform and dichloromethane were studied at room temperature. The dipole moments of the ground states of the all samples were calculated according to the Guggenheim–Smith method. The dipole moments of their excited states were determined with the help of the Lippert equation by measuring the absorption and fluorescence spectra, solvent polarity and refractive index values. It was determined that dipole moments of the excited states were higher than those of the ground states. Moreover, the dipole moments of the ground and excited states of two nematic liquid crystals were also estimated by using molecular mechanic method (Gaussian09 program (DFT/B3LYP 6-31G(dp)). The results obtained are interpreted in detail.     

Optical activity and light scattering in the isotropic and smectic A* phases of a highly chiral smectic liquid crystal

Abstract

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4′-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral–racemic mixture of S-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic–smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.     

Three homeotropically aligned nematic liquid crystals: comparison of ultrafast to slow time-scale dynamics

The dynamics of two nematic liquid crystals, 4-(trans-4(')-n-octylcyclohexyl)isothiocyanatobenzene and 4-(4-pentyl-cyclohexyl)-benzonitrile, are investigated as a function of temperature both in the homeotropically aligned nematic phase and in the isotropic phase using optical heterodyne-detected optical Kerr effect experiments, which measures the time derivative of the polarizability-polarizability-correlation function (orientational relaxation). Data are presented over a time range of 500 fs-70 micros for the nematic phase and 500 fs to a few hundred nanoseconds for the isotropic phase. The nematic dynamics are compared with a previously studied liquid crystal in the nematic phase. All three liquid crystals have very similar dynamics in the nematic phase that are very different from the isotropic phase. On the slowest time scale (20 ns-70 micros), a temperature-independent power law, the final power law, t(-f) with f approximately 0.5, is observed. On short time scales (approximately 3 ps to approximately 1 ns), a temperature-dependent intermediate power law is observed with an exponent that displays a linear dependence on the nematic order parameter. Between the intermediate power law and the final power law, there is a crossover region that has an inflection point. For times that are short compared to the intermediate power law (approximately <2 ps), the data decay much faster, and can be described as a third power law, although this functional form is not definitive. The isotopic phase data have the same features as found in previous studies of nematogens in the isotropic phase, i.e., the temperature-independent intermediate power law and von Schweidler power law at short to intermediate times, and a highly temperature-dependent long time exponential decay that is well described by the Landau-de Gennes theory. The results show that liquid-crystal dynamics in the nematic phase exhibit universal behavior.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

Investigations of phase transitions in liquid crystals by means of adiabatic scanning calorimetry

High-resolution calorimetric techniques have substantially contributed in characterising and understanding the delicate thermal behaviour near many phase transitions in liquid crystals. In this paper we describe a high-resolution adiabatic scanning calorimetric technique that has proven to be an important tool in discriminating between first-order and second-order phase transitions in addition to rendering high-resolution information on fluctuations-induced pretransitional specific heat capacity behaviour. The capabilities of adiabatic scanning calorimetry are illustrated with experimental results for the isotropic to nematic and the isotropic to smectic A transitions for a series of alkylcyanobiphenyl compounds. For the nematic to smectic A transition results are presented for pure compounds and mixtures of liquid crystals as well as on the effects of added non-mesogenic solutes and nanoparticles. For chiral molecules results for phase transitions involving blue phases and twist grain boundary phases are considered.     

Dielectric relaxation study of 1-n hexyl-4-(4-isothiocyanatophenyl)bicyclo [2,2,2] octane in the nematic and isotropic phases

Dielectric relaxation investigations on the mesogen 1-n-hexyl-4-(4-isothiocyanato-phenyl)bicyclo[2,2,2]octane in the nematic and isotropic phases have been carried out in the frequency region from about 1 kHz to 1 GHz. Two relaxation processes have been observed in both the isotropic and nematic phases, when the measurements of the electric permittivity versus frequency are made parallel to the orientation axis of the liquid crystal. The possible mechanisms responsible for these two processes are discussed. The height of the potential barrier which hinders the rotation of the liquid crystal molecules around the short axis in the nematic state, and the order parameter of the liquid crystal under investigation have been estimated on the basis of the values for the relaxation times in the nematic and isotropic phases.     

Mesophase behaviour of binary mixtures of bent-core and calamitic compounds

The mesophase behaviour of binary mixtures of bent-core and calamitic liquid crystals is presented. The nematogenic 4,6-dichloro-1,3-phenylene bis[4′-(10-undecen-1-yloxy)-1,1′-biphenyl-4-carboxylate] (I) was the banana-shaped component. As the calamitic compound ethyl 4′-(9-decen-1-yloxy)-1,1′-biphenyl-4-carboxylate (II), similar to one arm of the bent-core molecule, was used which exhibits smectic phases in a wide temperature range. A total of six mixtures with different compositions were prepared and studied by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction on non-oriented samples. In the mixtures, a nematic phase is not concomitant with smectic A phase, and the temperature range of both phases highly depends on the concentration of the comprising compounds. Lowered melting temperatures have been observed for all mixtures with respect to that of the pure compounds. Unforeseen finding is the induction of a monotropic SmC phase in mixtures with lowest concentration of the bent-core compound. Semi-empirical quantum-chemical calculations have also been performed. Based on the calculated molecular conformation, as well as on collected X-ray diffraction data, a model for a possible self-assembly of the banana-shaped and calamitic compounds is proposed.     

双环磷酸酯基氨基甲酸酯类和双酰肼类化合物的合成和生物活性

双环磷酸酯基氨基甲酸酯类和双酰肼类化合物的合成和生物活性     

Short time dynamics in the isotropic phase of liquid crystals: the aspect ratio and the power law decay

Optical heterodyne detected optical Kerr effect (OHD-OKE) experiments are used to study the orientational dynamics of the liquid crystal 4^′-octyl-4-biphenylcarbonitrile (8CB) in the isotropic phase near the isotropic to nematic phase transition. The results are compared to those for three other liquid crystals. The 8CB data display a short time scale temperature independent power law decay and a long time scale exponential decay with a temperature dependence described by Landau–de Gennes theory. The power law exponent is −0.56. Combining this result with previous results for three other liquid crystals [J. Chem. Phys. 116 (2002) 6339; J. Chem. Phys. 116 (2002) 360], it is found that the power law exponent depends linearly on the aspect ratio of the liquid crystal.     

Elastic properties of bent-core nematic liquid crystals: the role of the bend angle

ABSTRACT

The nematic phase formed from bent-core liquid crystals has been the focus of intensive research for more than a decade. With the potential of biaxial nematic phase and other interesting features, such as high Kerr constants, large flexoelectric coefficients and anomalous elastic constants, these bent-core materials have been in the limelight of research. This paper presents a mini-review of the interesting elastic behaviour reported in various bent-core compounds. The review further focusses on two different types of bent-core compounds as exemplars: one derived from an oxadiazole and other a thiadiazole, highlighting the importance of bend angle in bent-core compounds. The universality of the unique behaviour of bend elastic constants via molecular field theory and atomistic modelling has also been summarised.     

The synthesis of [1,2,3]-triazole-based bent core liquid crystals via microwave-mediated ‘Click Reaction’ and their mesomorphic behaviour

A series of [1,2,3]-triazole-based liquid crystal compounds were synthesised via a fast and efficient microwave-mediated ‘Click Reaction’ and their phase behaviour is presented. Most of the 1,4-diaryl-[1,2,3]-triazole compounds exhibited a nematic phase with broad temperature ranges and relatively low transition temperatures compared to previously examined materials. Structural variations, involving regio-isomers and lateral halogenation, gave rise to different phase behaviour of the two families of triazole-based compounds.     

New azobenzene containing bent-core liquid crystals based on disubstituted resorcinol

New bent-core molecules with 4,6-dichlororesorcinol or 4-chloro-2-methylresorcinol as the central unit, and azobenzene with different alkyloxy chain length as side arms were synthesised. The mesophase behaviour of the new compounds was investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction studies and electro-optical measurements. It is found that 4,6-dichlororesorcinol is more conducive towards mesomorphism than 4-chloro-2-methylresorcinol. The liquid crystalline properties of all of the prepared compounds are greatly affected by the lateral substitution on the outer ring. 4,6-Dichlororesorcinol-based compounds without lateral substitution show nematic phases with cybotactic cluster of the SmC-type (N_CybC). Moreover, depending on the chain length, the nematic phase appears as enantiotropic phase for the shortest homologue and as monotropic phase for the higher homologues.     

NMR investigation of the dynamics of banana shaped molecules in the isotropic phase: a comparison with calamitic mesogens behaviour

The first translational self-diffusion NMR measurements in the isotropic phase of banana-shaped liquid crystals are reported. In this paper, two banana-shaped mesogens, having a similar molecular structure and showing a nematic phase, have been investigated by means of translational self-diffusion NMR, (2)H NMR spin-spin and (1)H NMR spin-lattice relaxation measurements in the isotropic phase. While (1)H diffusion and (2)H relaxation times reveal a peculiar slow dynamic behaviour of banana-shaped mesogens compared with calamitic mesogens, the (1)H relaxation times seem to be affected by fast dynamics only. The origin of these dynamic features is discussed in terms of overall and internal molecular motions, in the frame of recent speculations concerning the formation of molecular clusters or aggregates in the isotropic phase of banana-shaped liquid crystals.     

Synthesis and transition temperatures of novel fluorinated chiral liquid crystals containing 1,4-tetrafluorophenylene units

To continue the search for novel series of fluorinated ferroelectric liquid crystals, an additional two series of 4-[(S)-2-methylbutoxy]phenyl 4-[(4-n-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] benzoates (C) and 4-(n-alkoxy)phenyl 4-[(4-(S)-2'-methylbutoxy-2,3,5,6-tetrafluorophenyl)] benzoates (D) have been synthesized. Polarizing microscopic textural observations and DSC measurements of the phase transitions of these novel compounds showed that compounds C were liquid crystals with a chiral nematic (N) phase and a monotropic chiral smectic C phase (S_c), and compounds D exhibited a chiral nematic (N) phase.     

Corresponding states in the smectic-A phase

In the last few years, experimental data of many different sources has been gathered to show that order parameter curves of essentially all nematic liquid crystals (NLCs) compounds can be reduced to a single and universal curve of the corresponding states; experimental data of an impressive number of different nematic compounds has been linearly rescaled in such a way that all them coalesced in a single curve. The surprising novelty observed on NLCs is that this universal behaviour is not restricted to the neighbourhoods of a critical region, as is usually expected from critical phenomena, but it covers the entire domain of the nematic phase, from the nematic isotropic temperature to the nematic crystalline temperature. The aim of this paper is to study the statistical mechanics fundamentals of this behaviour and show that it is a consequence of the universality of the angular part of the potential of the interaction between the particles composing the liquid crystal (LC). We will illustrate this behaviour presenting evidences that it can be found on the smectic-A phase.