Properties Of Chiral Liquid Crystals Research Articles

Electro-Optic Effect of Laser Photobleaching on Viscoelastic Properties of Chiral Liquid Crystals

Viscoelastic properties are one of the most fundamental properties of chiral liquid crystals. In general, their determination is not a straightforward task. The main problem is the multitude of physical parameters needed to determine the value of the elasticity and viscosity constants. It is also necessary to consider the character of a respective phase. This problem is particularly important in the case of chiral phases such as ferroelectric and antiferroelectric phases or in the blue phases. There are several experimental methods to measure viscosity and elasticity constants in chiral phases. These methods use various phenomena to detect deformation, e.g., light transmission, polarization current, light modulation, dielectric constant and helix deformation or helix unwinding. Commonly, an external electric field is used to induce deformation, the homogeneity of which inside the cell is essential. This study is focused on the analysis of the effect of laser photobleaching on the electro-optic properties of the antiferroelectric liquid crystal and on the homogeneity of the electric field. The results obtained by confocal microscopy as a function of the cell depth are presented. The influence of the stabilization procedure of the isolated region performed by controlled laser photobleaching on the electro-optic properties has been studied. The observation was conducted using a polarizing microscope, and numerical analysis of two-dimensional colored textures was performed. The obtained results suggest that laser photobleaching can produce an anchoring effect, which has a positive effect on the electro-optic properties of antiferroelectric liquid crystal.

Temperature dependence of bend elastic constant in oblique helicoidal cholesterics

Elastic moduli of liquid crystals, known as Frank constants, are of quintessential importance for understanding fundamental properties of these materials and for the design of their applications. Although there are many methods to measure the Frank constants in the nematic phase, little is known about the elastic constants of the chiral version of the nematic, the so-called cholesteric liquid crystal, since the helicoidal structure of the cholesteric renders these methods inadequate. Here we present a technique to measure the bend modulus K33 of cholesterics that is based on the electrically tunable reflection of light at an oblique helicoidal ChOH cholesteric structure. K33 is typically smaller than 0.6 pN, showing a nonmonotonous temperature dependence with a slight increase near the transition to the twist-bend phase. K33 depends strongly on the molecular composition. In particular, chiral mixtures that contain the flexible dimer 1′′,7′′-bis(4-cyanobiphenyl-4′-yl) heptane (CB7CB) and rodlike molecules such as pentylcyanobiphenyl (5CB) show a K33 value that is 5 times smaller than K33 of pure CB7CB or of mixtures of CB7CB with chiral dopants. Furthermore, K33 in CB11CB doped with a chiral agent is noticeably smaller than K33 in a similarly doped CB7CB which is explained by the longer flexible link in CB11CB. The proposed technique allows a direct in-situ determination of how the molecular composition, molecular structure and molecular chirality affect the elastic properties of chiral liquid crystals.

Effect of molecular structure on dielectric and electro-optic properties of chiral liquid crystals based on lactic acid derivatives

Self-assembling behaviour of a few lactic acid derivatives with several ester linkage groups in the molecular core has been studied. The molecular structure–physical property correlations has been discussed in the light of the temperature dependences of the static permittivity, dielectric anisotropy, spontaneous polarization, response time and torsional bulk viscosity for eight chiral ferroelectric liquid crystalline compounds. Depending on the molecular structure these compounds exhibit the chiral nematic (N*) phase, the paraelectric orthogonal smectic A* (SmA*) phase and the ferroelectric tilted smectic C*(SmC*) phase being thermally stable over relatively broad temperature range. The temperature behaviour of the spontaneous polarization clearly confirms the second order nature of the SmA*-SmC* phase transition and the first order nature of the N*-SmC* phase transition. The effect of temperature and molecular configuration on the polarization and on the dispersion anchoring energy coefficients has been established. The obtained results are discussed in order to contribute to the structure – property relationship for the specific class of soft organic materials which can be used for design of smart multifunctional liquid crystalline mixtures aimed for optoelectronic and photonic applications.

Focus Issue Introduction: Chiral Optical Materials

This special issue, “Chiral Optical Materials,” within the inaugural publication of Optical Materials Express, presents research describing recent advances in the field. Research presented focuses on nonlinear optical properties of chiral organic molecules, optical properties of chiral liquid crystals, and chiral nanomaterials, with a link to advanced optical characterization tools.

Synthesis and mesomorphic properties of chiral liquid crystals derived from (S)‐2‐(6‐hydroxy‐2‐naphthyl)propionic acid with a methyleneoxy linking group

Three homologous series of chiral materials derived from (S)‐2‐(6‐hydroxy‐2‐naphthly)propionic acid with a methyleneoxy linking group were synthesized for the investigation of mesomorphic properties. All the materials displayed enantiotropic SmA* and SmC* phases. The spontaneous polarization (P s) and optical tilt angles in the SmC* phase of the chiral materials were measured. The maximum spontaneous polarizations are in the range 12.3–19.1 nC cm−2. No significant difference could be found in the P s values with respect to the various alkyl chiral chain lengths, n. The maximum optical tilt angles are in the range 25–30°. The optical tilt angles decrease with the increase in alkyl chain length, n.

Synthesis and ferroelectric properties of chiral liquid crystals derived from (S)‐1‐propyloxy‐2‐propanol

A new optically active chiral moiety, (S)‐1‐propyloxy‐2‐propanol, was designed and synthesized by the treatment of 1‐propanol with (S)‐propylene oxide under basic conditions. Its derivatives, the (R)‐1‐propyloxy‐2‐propyl 4‐[4‐(4‐alkoxyphenyl)phenoyloxy]benzoates, PPmPPB (m = 8–12), were prepared for the investigation of mesomorphic properties. All of the chiral materials displayed enantiotropic SmA* and SmC* phases, and the shorter alkyl chain members (m = 8–11) displayed an additional unidentified SmX* phase. The switching current, spontaneous polarization, tilt angle, dielectric constant and electro‐optical response for the materials in the ferroelectric SmC* phase were measured. The electro‐optic responses in polyimide film‐coated homogeneously aligned cells exhibit thresholdless, V‐shaped switching in the ferroelectric phase.

Effect of core structures and swallow-tailed groups on the mesomorphic properties of chiral liquid crystals derived from (L)-lactic acid

A homologous series of chiral swallow-tailed materials, I(p,q,m,n), derived from (L)-lactic acid was synthesized for investigation of the effects of variation of the molecular core structure and the terminal swallow-tailed group on the formation of mesophases. The mesophases and their corresponding transition temperatures were identified by polarizing optical microscopy and differential scanning calorimetry. The results indicate that the variations of the core structure and of the methylene group between the chiral centre and the swallow-tailed group, have a remarkable effect on the appearance of mesophases; change in length of the swallow-tail group has less effect. The spontaneous polarization and dielectric constant in the ferroelectric SmC* phase were also measured.

Synthesis and mesomorphic properties of chiral liquid crystals having thiolactate groups

A homologous series of chiral thiolactate liquid crystals, alkyl 2-[4-(4-decyloxyphenyl)benzoyloxyphenyloxy]thiopropionates, ADBPT-n (n=2–6), was prepared from (R)-2-(4-hydroxyphenoxy)propionic acid. Structural effects on the mesomorphic and physical properties were investigated in terms of variation in the length of alkyl chain attached to the thiolactate group. The mesophases and their corresponding transition temperatures were identified by polarizing optical microscopy and differential scanning calorimetry. Compound ADBPT-4 exhibited SmA* and SmC* phases, whereas other compounds in the series exhibited the SmA* phase along with unidentified SmX1* and SmX2* phases. The maximum P s value measured for ADBPT-4 in the SmC* phase was 17.5 nC cm−2, suggesting the polarization of the material is rather low. The tilt angles were also measured, reaching a maximum of 48 degree.

Synthesis and Mesomorphic Properties of Chiral Liquid Crystals Derived from (R)-3-Ethylmercapto-2-methylpropanol

A new optically active (R)-3-ethylmercapto-2-methylpropanol was designed and synthesized by using (D)-2,10-camphorsultam as a chiral auxiliary. Its derivatives with various core structures in the molecules were prepared for investigating mesomorphic properties. Mesophases and the corresponding transition temperatures of the compounds were identified by the polarizing microscopic textures and DSC carlorimetry. Structurally similar compounds were compared to explore the effect of core structures on the formation of mesophases, especially the frustrated phases.

Liquid Crystals for Charge Transport, Luminescence, and Photonics

AbstractOrdered molecular materials are increasingly used in active electronic and photonic organic devices. In this progress report we discuss whether the self‐assembling properties and supramolecular structures of liquid crystals can be tailored to improve such devices. Recent developments in charge‐transporting and luminescent liquid crystals are discussed in the context of material requirements for organic light‐emitting devices, photovoltaics, and thin film transistors. We identify high carrier mobility, polarized emission, and enhanced output‐coupling as the key advantages of nematic and smectic liquid crystals for electroluminescence. The formation of anisotropic polymer networks gives the added benefits of multilayer capability and photopatternability. The anisotropic transport and high carrier mobilities of columnar liquid crystals make them promising candidates for photovoltaics and transistors. We also outline some of the issues in material design and processing that these applications demand. The photonic properties of chiral liquid crystals and their use as mirror‐less lasers are also discussed.

Synthesis and mesomorphic properties of chiral liquid crystals derived from ( S )-lactic acid with 3-pentanol

The chiral swallow-tailed liquid crystals, 1-ethylpropyl (R)-2-[4-(4′-alkoxybiphenylcarbonyloxy)-phenoxy]propionates, EPmPBPP (m = 8−12), were prepared by using chiral (S)-lactic acid with 3-pentanol as starting materials. Mesophases and their corresponding transition temperatures were determined by polarizing microscopic textures and DSC. The results showed that all the chiral materials exhibited enantiotropic BP, N*, TGBA*, SmA*, and SmC* phases. Spontaneous polarization, dielectric constant and electro-optical response for the materials in the ferroelectric SmC* phase were investigated. It was noted that the electro-optical response of transmittance versus applied voltage obtained from the ferroelectric phase of material EPmPBPP (m = 10) displayed V-shaped switching, while that of other materials displayed the typical characteristics of ferroelectric hysteresis switching or U-shaped switching.

Properties of chiral liquid crystals with inner hydrogen bonds

D. Pociecha, A. Krówczyński, J. Szydłowska, E. Górecka, M. Glogarova and J. Przedmojski, J. Mater. Chem., 1997, 7, 1709 DOI: 10.1039/A701129F