Photochemical reaction of cyclohexyl phenyl ketone within lyotropic liquid crystals

Lyotropic liquid crystals (LCs) formed by sodium dodecyl sulfate (SDS), n-pentanol, and H₂O at room temperature in their tertiary phase diagram have been explored as a confined medium for the typical photochemical reaction of cyclohexyl phenyl ketone (1), which can lead to either intramolecular hydrogen abstraction product 2 or intermolecular reduction product 3 in isotropic solutions upon irradiation. Studies on the product distributions of ketone 1 in the absence and presence of electron donors in this work demonstrate that LC not only restricts the movement of the substrates and intermediates but also encapsulates the substrates and electron donors together during photoirradiation, thereby giving rise to the formation of intermolecular hydrogen abstraction product 3 with high efficiency. A comparison of the same reaction in SDS micelle reveals that LC provides much better constraint than the micelles. The solution-like LC can be used as a microreactor to direct the reaction pathway of ketone 1 by controlling the viscosity and close contact between substrates and electron donors.     

Synthesis and mesomorphic properties of four-ring liquid crystals containing cyclohexyl,phenyl and pyridyl units

Two series of chiral and achiral 4-[(3-n-alkoxycarbonylpyridyl)-6-ethynyl]phenyl 4-(trans-4-n-alkylcyclohexyl)benzoate liquid crystals have been synthesized. Their mesomorphic properties were observed and measured by polarizing optical microscopy and DSC. A broad chiral smectic C phase was observed when terminal chains contained a chiral centre.     

Synthesis and mesomorphic properties of four-ring liquid crystals containing cyclohexyl, phenyl and pyridyl units

Two series of chiral and achiral 4-[(3- n -alkoxycarbonylpyridyl)-6-ethynyl]phenyl 4-( trans -4- n -alkylcyclohexyl)benzoate liquid crystals have been synthesized. Their mesomorphic properties were observed and measured by polarizing optical microscopy and DSC. A broad chiral smectic C phase was observed when terminal chains contained a chiral centre.     

Synthesis and mesomorphic properties of three- and four-ring liquid crystals containing cyclohexyl, phenyl and pyridyl units

Two series of 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl trans-4-n-alkylcyclohexylcarboxylates and 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl 4-(trans-4-n-alkylcyclohexyl)benzoates have been prepared. Their mesmorphic properties were observed and measured by polarizing optical microscopy and differential scanning calorimetry. The relationship between structures and properties is discussed, showing that the ferroelectric phase can be observed when chiral terminal chains exist.     

Synthesis and mesomorphic properties of three- and four-ring liquid crystals containing cyclohexyl,phenyl and pyridyl units

Two series of 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl trans-4-n-alkylcyclohexylcarboxylates and 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl 4-(trans-4-n-alkylcyclohexyl)benzoates have been prepared. Their mesmorphic properties were observed and measured by polarizing optical microscopy and differential scanning calorimetry. The relationship between structures and properties is discussed, showing that the ferroelectric phase can be observed when chiral terminal chains exist.     

Lateral substituted phenyl biphenylcarboxylates ‒ non-chiral analogues of ferroelectric liquid crystals

ABSTRACT

We report mesomorphic properties of non-chiral liquid crystalline molecules and study the effect of lateral halogen (fluoro, chloro) substitution in the molecular core and the length of terminal chains. We have prepared nine homologue series with the molecular core substituted at different positions with respect to the ester linkage group. Additionally, we have modified the length of both terminal alkyl chains (hexyl, octyl, decyl and dodecyl) symmetrically for both terminal chains. The effects of the lateral substitution and the chain length are analysed with respect of the possibility to reduce the transition temperatures and tune the properties of presented liquid crystalline molecules.     

Non-linear optical properties of metallorganic ferroelectric liquid crystals

Second harmonic generation (SHG) efficiencies d eff were determined for Pt-containing ferroelectric liquid crystals (FLCs). A relatively simple method is presented which allows for the simultaneous determination of the second order susceptibility tensor coefficients d ij together with the dispersion and birefringence at the fundamental frequency. The relation between the d ij coefficients is qualitatively interpreted in terms of the molecular shape and of a scheme proposed for the molecular orientation in the ferroelectric phase. Contrary to the usual situation with calamitics, d 23 is much higher than the rest of the coefficients. This peculiarity, which reflects the 3D character of the molecular response, is interesting since d 23 is directly involved in d eff for phase matched SHG.     

Enhancing the electro-optical properties of ferroelectric liquid crystals by doping ferroelectric nanoparticles

The effects on the physical and electro-optical properties of ferroelectric liquid crystals (FLCs) after the doping of a dilute suspension of ferroelectric nanoparticles (BaTiO₃) have been studied. Due to the permanent electric dipole moments of the ferroelectric nanoparticles, the spontaneous polarisation of FLCs with low doping concentration was about twice that of pure FLCs, in addition to a significant improvement in the dielectric properties, the response time and the V-shaped switching in the chiral smectic C (SmC?) phase. The results obtained point the way to an alternative for improving the applicability of FLCs without resorting to chemical synthesis.     

Electro-optic properties of thiol-ene polymer stabilized ferroelectric liquid crystals

Electro-optic properties of polymer stabilized ferroelectric liquid crystal (PSFLC) systems are examined as a function of varying concentrations of either a linear or crosslinked thiol-ene polymer. The thiol-ene method of polymer stabilization is a drastic change from previous studies designed to avert the problem of polymer phase separation. FLC rise time and tilt angle measurements were used to determine the effects of the polymer network on the optical properties of the system. The addition of monomer impurities to both systems demonstrated a reduction in tilt angle, which translated into decreased switching speeds in both systems prior to polymerization. The crosslinked thiol-ene system showed increased switching times due to the creation of polymer in the interlayer spacing of the FLC, but exhibited minimal increase in the rotational viscosity of the system. In addition, the crosslinked polymer systems resulted in an increase in the liquid crystalline order, which produced an increase in the contrast ratio of the system. The linear polymer system showed drastically different results as compared with the crosslinked system. The rise time and tilt angle measurements decreased upon polymerization of the linear thiol-ene and the rotational viscosity and contrast ratio values also decreased. We suggest that the linear thiol-ene polymer phase separation from the interlayer spacing leads to a microscopic misalignment of the FLC molecules, causing a decrease in the optical properties of the LC.     

Electro-optic properties of thiol-ene polymer stabilized ferroelectric liquid crystals

Electro-optic properties of polymer stabilized ferroelectric liquid crystal (PSFLC) systems are examined as a function of varying concentrations of either a linear or crosslinked thiol-ene polymer. The thiol-ene method of polymer stabilization is a drastic change from previous studies designed to avert the problem of polymer phase separation. FLC rise time and tilt angle measurements were used to determine the effects of the polymer network on the optical properties of the system. The addition of monomer impurities to both systems demonstrated a reduction in tilt angle, which translated into decreased switching speeds in both systems prior to polymerization. The crosslinked thiol-ene system showed increased switching times due to the creation of polymer in the interlayer spacing of the FLC, but exhibited minimal increase in the rotational viscosity of the system. In addition, the crosslinked polymer systems resulted in an increase in the liquid crystalline order, which produced an increase in the contrast ratio of the system. The linear polymer system showed drastically different results as compared with the crosslinked system. The rise time and tilt angle measurements decreased upon polymerization of the linear thiol-ene and the rotational viscosity and contrast ratio values also decreased. We suggest that the linear thiol-ene polymer phase separation from the interlayer spacing leads to a microscopic misalignment of the FLC molecules, causing a decrease in the optical properties of the LC.     

From monomeric to polymeric ferroelectric liquid crystals A comparative study of ferroelectric properties

Two new ferroelectric oligosiloxanes, a cyclic tetramer and a twin, have been synthesized. By a comparative study with their corresponding monomer and side chain polysiloxanes, the influence of oligo- and polymerization on the liquid crystalline and ferroelectric properties have been investigated. Polymerization leads to a stabilization of LC phases through increase of the clearing temperatures and suppression of crystallization. Oligomerization also leads to mesophase broadening, but, due to the low degree of polymerization, the effect is inferior to the linear polysiloxanes. The low viscosity of the oligosiloxanes ensures response times in the microsecond region, thus being comparable with their monomer and conventional LMWFLCs. It is found that polymerization increases the spontaneous polarization P_s. This is attributed to the density increase after polymerization, enhancing the inter-mesogenic interactions. The collective and local dynamics of the OFLCs are influenced differently with respect to their molecular structures. Each oligomer is already a good model for its corresponding polymer concerning the soft mode dynamics. For the local β-relaxation a similar temperature dependence of the relaxation times τ for the cyclic tetramer and for the side chain polysiloxanes is observed. The long axial rotation of the twin, having a very efficient decoupling, is significantly faster, thus resembling the monomer.     

From monomeric to polymeric ferroelectric liquid crystals A comparative study of ferroelectric properties

Abstract

Two new ferroelectric oligosiloxanes, a cyclic tetramer and a twin, have been synthesized. By a comparative study with their corresponding monomer and side chain polysiloxanes, the influence of oligo- and polymerization on the liquid crystalline and ferroelectric properties have been investigated. Polymerization leads to a stabilization of LC phases through increase of the clearing temperatures and suppression of crystallization. Oligomerization also leads to mesophase broadening, but, due to the low degree of polymerization, the effect is inferior to the linear polysiloxanes. The low viscosity of the oligosiloxanes ensures response times in the microsecond region, thus being comparable with their monomer and conventional LMWFLCs. It is found that polymerization increases the spontaneous polarization P _s. This is attributed to the density increase after polymerization, enhancing the inter-mesogenic interactions. The collective and local dynamics of the OFLCs are influenced differently with respect to their molecular structures. Each oligomer is already a good model for its corresponding polymer concerning the soft mode dynamics. For the local β-relaxation a similar temperature dependence of the relaxation times τ for the cyclic tetramer and for the side chain polysiloxanes is observed. The long axial rotation of the twin, having a very efficient decoupling, is significantly faster, thus resembling the monomer.     

On the elastic properties of ferroelectric Sc* liquid crystals

On the basis of the symmetry consideration of the S_c* phase, a generalized elastic free energy of ferroelectric S_c* liquid crystals is presented to account for the layer compression or dilatation and the layer distortion as well as c director deformation. The present elastic free energy is expressed in terms of three vectors, i.e. the c director and the wave vector and the spontaneous polarization vector. According to the present S_c* model with C₂ symmetry, it is shown that there may exist 17 non-chiral, 4 chiral terms and 14 flexoelectric terms in the S_c* phase. A few practical applications are also presented to elucidate some interesting elastic properties of S_c* (or S_c) in simplifed geometries.     

Negative birefringence ferroelectric liquid crystals

The synthesis and evaluation of a negative birefringence material which possesses a wide smectic C phase is reported.     

Hydrogen bond-induced ferroelectric liquid crystals

Abstract

Hydrogen-bonded dimers of 4-alkoxy-4′-stilbazole homologues (C _n OSB, proton acceptors) and the mono-(2-methylbutyl)ester of terephthalic acid (MBTA, proton donor) behaved like conventional thermotropic mesogens. For chiral MBTA mixtures, chiral nematic and blue phases were observed with n = 1 and 2, and chiral smectic C phases were observed with n ≥ 5. Achiral phases were observed for mixtures with racemic MBTA as proton donor. Transition temperatures and enthalpies are similar for the chiral and achiral systems.     

Synthesis and properties of new three-ring phenylacetylene liquid crystals containing the cyclohexyl unit

Two series of new three-ring phenylacetylene liquid crystals were synthesized and their physical properties were evaluated. These liquid crystal materials show only a nematic phase, high clearing points and good solubilities. The compounds containing two cyclohexyl rings exhibit moderate birefringence (Δn) values of around 0.14, while those containing one cyclohexyl ring show a broad nematic phase range and high Δn of around 0.33.     

Hydrogen bond-induced ferroelectric liquid crystals

Hydrogen-bonded dimers of 4-alkoxy-4'-stilbazole homologues (C_nOSB, proton acceptors) and the mono-(2-methylbutyl)ester of terephthalic acid (MBTA, proton donor) behaved like conventional thermotropic mesogens. For chiral MBTA mixtures, chiral nematic and blue phases were observed with n = 1 and 2, and chiral smectic C phases were observed with n ≥ 5. Achiral phases were observed for mixtures with racemic MBTA as proton donor. Transition temperatures and enthalpies are similar for the chiral and achiral systems.     

Synthesis and mesomorphic properties of some ferroelectric liquid crystals of lactate

A semiperfluorinated chain has been introduced into three series of lactate‐based molecules, and their mesomorphic properties have been studied by DSC and optical polarized microscope. An abnormal phenomenon has been found that the introduction of a triple bond in the core of the molecule, which lengthened the molecule, gave rise to a decrease in the thermal stability of chiral smectic C phase and smectic A phase. 2, 3‐Difluoro substituents of the phenylenic structure hinder the formation of the chiral smectic C phase because of the steric effect.     

Terahertz properties of liquid crystals doped with ferroelectric BaTiO3 nanoparticles

In this paper, we present the results of the terahertz measurements of liquid crystal (LC) ferroelectric BaTiO₃ nanoparticles (nps) suspensions in the range of frequency from 0.3 up to 3.0 THz. Two different sol-gel methods and the harvesting technique were used to fabricate the nanoparticles. Five LC materials served as hosts for the suspensions: two single compounds: 6CHBT and 2,3′,5′-trifluoro-4-(4-pentylcyclohexyl)-4′-(trifluoromethoxy)-1,1′-biphenyl, and three mixtures: 1867, 2037 and 2020. We characterise, for the first time, the refractive indices and absorption parameters of suspensions with harvested nps in the terahertz range and show how the process of the nps’ preparation affects their response. We observed the increase of birefringence for few LC suspensions in comparison with the pure LCs. The highest increase of birefringence was for 2020 suspension with one kind of ferroelectric nps. On the other hand in most cases the addition of ferroelectric nps to LC causes the increase of its absorption in the THz range. The measurements of LCs terahertz properties by using time-pulsed spectrometer were performed.     

Photorefractive effect of ferroelectric liquid crystals

This paper reviews our recent work on the photorefractive effect of ferroelectric liquid crystals (FLCs). The photorefractive effect is defined as the optical modulation of the refractive index of a medium as a result of a variety of processes. The interference of two laser beams in a photorefractive material establishes a refractive index grating. This phenomenon enables the creation of different types of photonic applications. FLCs exhibit fast electric field response, and the orientation of the molecular axis of FLCs changes its direction according to the change in direction of the spontaneous polarization (Ps). When two laser beams interfere in a photoconductive FLC, an orientational grating is formed. The mechanism of the formation of the grating is based on the response of the Ps to the photoinduced internal electric field. The time of formation of the refractive index grating is significantly shorter in FLC materials.