Boron clusters as structural elements of novel liquid crystalline materials

closo-Boranes are characterised by high thermal and oxidative stability due in part to highly delocalised bonding within a σ-framework. These boron clusters when employed as structural elements of liquid crystals give rise to novel structures with unique properties and vast potential in applications. The neutral closo-boranes such as the carboranes provide opportunity to perform fundamental structure–property relationship studies that may provide additional insight on the liquid crystalline state. The anionic closo-borates possessing a highly delocalised negative two charge can be used as structural elements of zwitterionic, quadrupolar liquid crystalline materials. Such materials may be of use for understanding the effects of polarity on the liquid crystalline state. Lastly, the anionic closo-monocarbaborates possessing a single highly delocalised negative charge can be envisioned as structural elements of either highly polar or ionic liquid crystalline materials. The highly polar materials may also serve as models for understanding the effects of polarity on the liquid crystalline state. Additionally, they should have application as positive Δ? additives for nematic liquid crystalline mixture formulations. The ionic materials, where the anionic fragment drives liquid crystalline behaviour, lend themselves as potential electrolytes for anisotropic ion transport in battery technologies. The review presented here introduces these concepts and highlights the current status of boron cluster containing liquid crystalline materials.     

Optical Field Induced Scattering in Polymer Dispersed Liquid Crystal Films

Abstract

Polymer dispersed liquid crystals are composite materials consisting of inclusions of liquid crystalline materials dispersed in a polymer binder. If the refractive indices of the constituent liquid crystal and polymer are appropriately matched, then films of these materials may be switched between an optically scattering state and a non–scattering transparent state^1?2 by the application of electric fields which reorient the liquid crystal in the inclusions. In this paper we discuss the response of these materials to intense laser radiation, and examine the mechanisms associated with optical field induced reorientation.     

Reflection image enhancement for photonic crystal devices by using dielectrics

Abstract

We investigated the optical properties of photonic crystal devices (PCDs) using dielectrics. Different dielectrics were injected into a cell gap of the PCDs as a swelling solvent. It is evident that the PCDs reflected a deep blue color when two different materials, chiral ionic liquid (CIL) and photosensitive small molecules, were introduced. To compare the reflection images according to the different dielectrics, a well-known ionic liquid (IL) was used as a control sample. A thinner polymer layer induced a shorter wavelength, which created a strong blue shift phenomenon with a larger refraction index and a larger dielectric constant. In this paper, we obtained 12?nm of reflection enhancement with an applied voltage of 2V using an IL and a deep-blue color image by using the effects of the molecular structure of the CIL and photosensitive materials.     

Synthesis,structure, and mesomorphism of novel liquid crystalline acrylate monomers and polymers

A series of novel liquid crystalline monomers (M₁?M₈) and side chain polymers base polyacrylate backbone were synthesized. The chemical structures were characterized by FT-IR and ¹H-NMR spectra. The mesomorphism and thermal behavior was investigated by polarizing optical microscopy, differential scanning calorimetry, and thermogravimetric analysis. The relationships of structure and mesomorphism are discussed in detail. The eight monomers and their corresponding polymers all show enantiotropic nematic phase. With increasing the spacer length or flexibility of the terminal group, the melting temperature (T_m) and isotropic temperature (T_i) of the corresponding monomers and polymers all decreased. However, with increasing the rigidity of the mesogenic core, T_m and T_i of the corresponding monomers and polymers all increased. TGA showed that all the polymers obtained in this study had excellent thermal stability.     

Electrical and Optical Properties of Conducting Polymer and Carbons in Nano-Scale Periodic Structure and their Intercalation Effects

Abstract

Nano-scale periodic structures of conducting polymer and carbons, which were prepared by infiltration of polymers and carbons in nano-scale interconnected periodic pores in synthetic opals made of regular array of SiO₂ spheres and then removing SiO₂ by etching, have been found to exhibit novel electrical and optical properties. Their electrical and optical properties in thus fabricated conducting polymer and carbon replicas change drastically upon pyrolysis due to progress of carbonization and graphitization. That is, due to the changes in periodicity, pore size, carbonization degree and crystal structure, electrical conductivity, magnetoconductance and their temperature dependences and optical reflection spectra have changed drastically. These replicas with porous nature can be infiltrated and also intercalated with various materials, resulting in also remarkable changes of properties. The synthetic opal infiltrated with conducting polymer can be electrochemically doped, with which remarkable change of optical properties have been observed due to the shift of the diffraction peak accompanying with the change in refractive index. Alkali metal intercalated carbon and graphite with nano-scale periodic structures have been also studied. The applications of these nano-scale periodic structures of conducting polymer and carbon are also discussed.     

Low-Frequency Dielectric Relaxations in Nematics and Dual-Frequency Addressing of Field Effects

Abstract

Novel low threshold nematic mixtures are presented exhibiting very low dielectric crossover frequencies f_c ? 1 kHz at 20°C and unusually large, low- as well as high-frequency dielectric anisotropies ΔEL > 4 and ΔEH < 4 respectively. ΔEL and ΔEH are shown to be independently adjustable. The frequency and temperature dependence of the dielectric constants as well as the optical, elastic and viscous material constants are measured. Approximations are derived which quantitatively describe the influence of the LC material properties on the static electro-optical performance of dual-frequency addressed twisted nematic displays. It is shown that the number of multiplexable lines of dual-frequency addressed TN-LCDs comprising the new materials can be increased by more than a factor of 30 compared with conventional addressing. Moreover, very short turn-off times are reported.     

Can Two-Dimensional Fullerene Polymers Be Intercalated?

Abstract

Two-dimensional C₆₀ polymers can be produced by treatment at high T and high p. Attempts have previously been made to intercalate these layered materials by alkali metals using the gas phase method but the polymers have always decomposed. We investigate here whether intercalated 2D polymers can be formed by other methods, such as by polymerization in the presence of alkali metals. Preliminary x-ray and Raman data can be interpreted to indicate the presence of an alkali metal intercalated tetragonal fullerene polymeric structure.     

Comment on: “Growth,optical, photoluminescence,dielectric, second and third order nonlinear optical studies of benzoyl valine acentric crystal” [(2017) Mol. Cryst. Liq. Cryst., 658, 186–197]

Abstract

In a recent paper [(2017) Mol. Cryst. Liq. Cryst., 658, 186–197], Kumari et al. studied the optical, photoluminescence and dielectric properties of the benzoyl valine acentric crystal. The refractive index of the samples is calculated based on the reflectance spectra and obtained values were used to calculate other optical parameters such as the electric susceptibility, optical and electrical conductivity. It is shown in this comment that the authors have used a wrong approach to calculate the reflectance spectra of the samples and consequently, the mentioned optical parameters are incorrect.     

Growing of bulk crystals and structuring waveguides of fluoride materials for laser applications

We report on recent progress in the synthesis, the crystal growth and the epitaxial growth of fluoride and other laser materials. Results on the fabrication of single crystalline waveguides for dielectric down - and upconversion lasers pumped by semiconductor diode lasers are summarized. Epitaxial growth (molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), pulsed laser deposition (PLD)) and surface modifying techniques (high energy ion implantation, ion diffusion) have been applied in several laboratories. Progress in techniques fabricating optical waveguides from glassy media is addressed as well. Particular emphasis is given on the structuring (wet etching, chemical polishing, ion beam etching) of fluoride crystals for the purpose of obtaining 2-D and 1-D optical waveguides. Results on the structuring of LiYF₄ by wet and ion beam etching are reported. With respect to laser action, the generation of short wavelength light by upconversion (UC) processes, stimulated Raman scattering (SRS) and second harmonic generation (SHG) is discussed. Reports on the first crystalline waveguide lasers of fluoride crystals LiYF₄ and LaF₃, both doped with neodymium, are presented.     

Dielectric Relaxation in Low Temperature Smectic Phases of 4-n-Alkyloxy-benzylidene-amino-4′-cyanobiphenyls

The dielectric behaviour of five homologous 4-n-alkyloxy-benzylidene-amino-4′-cyanobiphenyls was investigated. Especially, we were interested in the dielectric relaxation frequencies of the smectic low temperature phases S_B and S_E as well as the dielectric behaviour in the solid state. We have found in all liquid crystalline phases a low frequency relaxation region explained by a hindered rotation of the molecules around their short axes. By supercooling of the hexyl and heptyl derivatives a new liquid crystalline phase can be obtained. A dielectric relaxation in the solid phase could not be found.     

Synthesis and mesomorphic behavior of symmetrical chiral liquid crystal dimers incorporating ester linked biphenyl- naphthyl cores and terminal vinyl group

A series of symmetrical chiral, liquid crystal dimeric molecules possessing ester- linked, biphenyl-naphthyl cores with varied spacer lengths and terminal vinyl groups have been synthesized using Naproxen as the synthetic precursor. The synthesized symmetrical chiral dimers were characterized by 1H NMR spectroscopy, and their liquid crystalline behavior was confirmed by DSC and HOPM studies. Structural effects on the mesomorphic and physicochemical properties were investigated in terms of variation of chiral chain length. The synthesized dimeric compounds exhibited SmX^*, SmC^*, SmA^*, N^*, BPI^*, and BPII^* mesophase sequences. An odd-even effect was observed in the dimers and the duration of the mesophase decreased with increasing spacer length. The synthesized vinyl substituted liquid crystalline dimers are particularly useful in understanding liquid crystal polymorphism and act as model compounds for liquid crystal polymers.     

Main-chain biodegradable liquid crystal based on diosgenyl end-capped poly(trimethylene carbonate)

Main-chain biodegradable liquid crystal based on c diosgenyl end-capped poly(trimethylene carbonate) [Dios-(TMC)n] was investigated. The novel liquid crystal was synthesized through ring-opening polymerization of trimethylene carbonate initiated by diosgenin, without adding any catalyst. The chemical structure of resulting polymers was confirmed by ¹H NMR. The liquid crystalline properties were validated by X-ray diffraction, differential scanning calorimetry, and polarising optical microscopy. The results showed that the synthesized Dios-(TMC)n exhibited liquid crystallinity in particular temperature ranges because of the incorporation of the diosgenin moieties.     

Synthesis of Novel Ionic Liquid Crystalline Pyrrole Derivatives Having a Viologen Moiety

Abstract

A novel liquid crystalline N-substituted pyrrole having an ionic viologen side chain was synthesized, and was polymerized by a FeCl₃ method. Thermotropic liquid crystalline behavior of the pyrrole and the polymerized material were investigated by DSC, thermo-controlled polarizing optical microscopy, and XRD. The pyrrole derivatives showed two smectic phases between isotropic and crystalline (solid) phases. Viologen functions of redox and cation-radical generation for the pyrrole derivatives were confirmed by cyclic voltammetry with UV-Vis spectroscopy.     

From Chiral Molecules to Chiral Liquid Crystal Phases

Abstract

During the last decade, the topic of chirality and symmetry in physical sciences has become extremely important, particularly as nature itself is often described as being “handed” or chiral. In liquid crystalline systems many important properties are known to be related to chirality, for instance chirality is reflected in helical structures, in special cases of strongly twisted systems novel frustrated phases may appear such as Blue phases and Twist Grain Boundary phases. In addition, many modern high technology applications, such as ferroelectric displays depend entirely on the presence of chirality in the liquid-crystalline materialsemployed.     

Recent advances with electro-optic polymers

Abstract

In recent years the electro-optic polymers emerged as an important branch of material science. This growth and interest is fundamentally motivated by practical application of these materials in second-order nonlinear optics and in waveguiding configuration. Indeed, these materials marry excellent optical quality of amorphous σ bonded polymers with enhanced first hyperpolarizability of imbedded organic nonlinear optical molecules. Although a big progress was achieved with them, concerning particularly the science, understanding and applications, some problems remain still incompletely solved, particularly the stability of induced polar order by the application of external electric field and the molecule aggregation. In this review paper we recall techniques of thin film fabrication, poling, characterization of NLO properties and discuss more precisely problems of molecule aggregation as well as the temporal decay of polar order. A novel 3D second-order NLO chromophores, namely the [2] paracyclophanes, which may help to limit the aggregation, are proposed. We show, in particular, that this molecule can be poled in doped PMMA thin films. Practical applications of electro-optic polymers are also reviewed and discussed.     

Synthesis and Mesomorphic Properties of New T- Shaped Dimesogens

Abstract

Several new T- shaped liquid crystalline dimesogenic compounds were synthesized and their LC properties were characterized. These T-shaped dimesogens consist of substituted fluorene and substituted biphenyl units connected by flexible spacer units of varying lengths. The compounds were characterized for their liquid crystallinity by differential scanning calorimetry (DSC) and polarizing microscopy. All the compounds were found to exhibit enantiotropic nematic phase.     

Optical Materials Based on Nanoscaled Guest/Host Composites

Abstract

Ordered periodic microporous and mesoporous materials allow the construction of composites with many guest types, e. g. organic molecules, inorganic ions, semiconductor clusters or polymers. These guest/host materials combine high stability of the inorganic host system, new structure forming mechanisms due to the confinement of guests in well defined pores, and a modular composition. This could lead to new optical materials which are described in this paper by examples in switches, nonlinear optics and lasers.     

Liquid Crystal Compounds Incorporating the Trans-1,4-Substituted Cyclohexane Ring System

A number of esters and other compounds containing a trans-1,4-substituted cyclohexane ring system-have been prepared, and the liquid crystal thermal stabilities of these new mesogens are discussed in relation to those of their analogues containing the 1,4-phenylene ring. The observed effects are discussed in terms of a possible thermal energy absorption by the cyclohexane ring in causing minor conformational changes.

Other physical properties of these new mesogens are also discussed with particular reference to their potential in applications. In relation to electro-optical display devices, such properties as birefringence, dielectric anistropy, viscosity etc., are important parameters, and the new materials have been assessed in this context as possible additives to the 4-n-alkyl- and 4-n-alkyloxy-4′-cyanobiphenyls for producing a favorable combination of physical parameters.

Also reported is a liquid crystal mixture which has a 100°C nematic range and which contains only materials which are aliphatic. This mixture is assessed for its potential application as an anistropic solvent in analytical techniques such as ultra-violet spectroscopy and nuclear magnetic resonance spectroscopy.     

Trans−1,2-Difluorostilbenes: Promising Materials for High Dielectric Biaxiality Ferroelectric Mixtures

Abstract

A range of two- and three-ring liquid crystals have been synthesised that incorporate a trans?1,2-difluoroethene linking group. Some of the materials have surprisingly low melting points and high smectic C phase stability, and they exhibit the ideal phase sequence for ferroelectric hosts of smectic C to smectic A to nematic. The synthesis was accomplished in a convergent manner through the use of low-temperature lithiations to generate organometallics based on boron and tin. Palladium-catalysed cross-couplings and Michael-type reactions were used to generate the final mesogenic materials. The polar fluoro substituents enable the generation of ferroelectric mixtures with a high dielectric biaxiality which is very important in τV minimum driving schemes. The alkenic linking group in stilbenes normally confers instability, but the difluoro-substituted analogues are stable compounds.