Photochemical tuning of the helical structure of cholesteric liquid crystals by photoisomerization of chiral azobenzenes, and their structural effects

Several chiral azobenzene compounds having different chiral substituents were synthesized. A cholesteric phase was induced by mixing each chiral azobenzene compound with a host non-chiral nematic liquid crystal (E44). The helical twisting power (HTP) as well as the change in HTP by trans-cis photoisomerization of the chiral azobenzene compound was dependent on the structure of the chiral substituents. A compensated nematic phase was induced by combination of E44, a chiral azobenzene compound and a non-photochromic chiral compound. Reversible switching between the compensated nematic phase and cholesteric phase was brought about by trans-cis photoisomerization of the chiral azobenzene compound in the liquid crystalline systems. An azobenzene compound substituted with a menthyl group showed the highest efficiency as the trigger for the switching; this efficiency was related to the compactness of the chiral group substituted within the azobenzene core moiety.     

Induction of the chiral nematic phase in hydrogen-bonded blends of smectic copolymers and low molar mass dopant

A new approach for the preparation of chiral nematic materials is described. The induction of a chiral nematic phase in hydrogen-bonded blends of smectic comb-shaped LC copolymers containing alkyloxy-4-oxybenzoic acid fragments with a low molar mass chiral dopant (a derivative of pyridine) was observed.     

Tuning the helical twisting power of nematic liquid crystals induced by chiral 1, 2-propanediol derivatives using varied substituents

In this study,a novel series of chiral 1,2-propanediol derivatives with different electron-donating and electron-withdrawing groups were synthesized and characterized by FT-IR and ~1H NMR.The helical twisting properties of all the chiral dopants were investigated by doping the chiral dopants into a nematic liquid crystal host(SLC-1717).The results indicate that the donor-acceptor electron effect have a prominent influence on helical twisting property of the chiral nematic phase induced by the chiral dopants. Introducing electron-withdrawing groups into the terminal ends of chiral 1,2-propanediol can decrease the absolute values of the helical twisting power.In addition,the helix inversion temperatures of the induced chiral nematic phase are variational with the change of terminal groups.     

Textures of homologous 4-n-alkyloxybenzoic acids: spontaneous chirality and surface memory

By using polarizing microscopy analysis we have found that several achiral homologues of the 4-n-alkyloxybenzoic acids, displaying only the nematic phase, exhibit the optical properties of a chiral liquid crystal system. These acids possess a mesophase due to the formation of dimers via hydrogen bonding. The microtextural analysis was carried out in the temperature ranges of the isotropic, nematic and crystal phases. The nucleation of a chiral texture in small domains emerging on cooling in the isotropic phase was observed. These small domains are characterized by a conoscopic cross which presents an azimuth of 45° with respect to the polarizer axis, contrary to the usual nematic drops, for which the conoscopic cross is not rotated. On further cooling, these domains coalesce in the nematic phase close to the clearing point, thus building large chiral monodomains. Such coalesced droplets exhibit very thin stripe lines, as in the case of pure cholesterics with a tilted helix axis. Moreover, left- and right-handed chiral domains were observed, combined in regions partially separated by 'oily streaks', also typical of pure cholesterics. On cooling, the chiral nematic (N*) phase transformed through a pronounced texture transition into a normal nematic phase. However, the small chiral grains that formed from the isotropic phase are retained close to the surface, acting as 'memorizing centres'. With suitable boundary conditions, they can provide a macroscopic twist driven by the surface. Moreover, a twisted smectic B not present in the bulk phase diagram was found and interpreted as induced by the surface. Also in the crystal phase a strong memorization of the chiral N* texture was observed.     

Enlargement of blue-phase stability for rod-like low-molecular-weight chiral nematic liquid crystal mixtures

In this study, we investigated the enlargement of liquid crystal (LC) blue-phase (BP) temperature range using the rod-like low-molecular-weight cyano phenyl-type chiral nematic LC with various core group and chiral dopant concentrations. Also, the electro-optic response time was investigated for them. We found that the BP temperature range was strongly dependent upon the core structure and the chiral dopant concentration for the chiral nematic LC mixtures having the same terminal group. Also, we found a stable BP with a wide temperature range (more than 6 K), including a BP-isotropic coexistence state over 13.5 K upon heating and cooling processes and very fast response time (less than 1 ms), by using the cyano phenyl-type chiral nematic LC mixture with a high molecular aspect ratio and a high chiral dopant concentration.     

Synthesis and helical twisting property of polymerizable chiral dopant with temperature-dependent solubility in liquid crystal

In this study,right-handed dicinnamate isosorbide was synthesized via the esterification reaction between optically active isosorbide and cinnamate.The chiral dopant was characterized by FT-IR,~1H NMR,elemental analysis,SEM,UV absorption spectrum.After dissolving in a nematic liquid crystal mixture,the chiral dopant exhibited a temperature-dependent solubility in the chiral nematic liquid crystal mixture.Meanwhile,a relatively high value of helical twisting power of the polymerizable chiral dopant was de...     

Textures of homologous 4-n-alkyloxybenzoic acids: spontaneous chirality and surface memory

By using polarizing microscopy analysis we have found that several achiral homologues of the 4-n-alkyloxybenzoic acids, displaying only the nematic phase, exhibit the optical properties of a chiral liquid crystal system. These acids possess a mesophase due to the formation of dimers via hydrogen bonding. The microtextural analysis was carried out in the temperature ranges of the isotropic, nematic and crystal phases. The nucleation of a chiral texture in small domains emerging on cooling in the isotropic phase was observed. These small domains are characterized by a conoscopic cross which presents an azimuth of 45° with respect to the polarizer axis, contrary to the usual nematic drops, for which the conoscopic cross is not rotated. On further cooling, these domains coalesce in the nematic phase close to the clearing point, thus building large chiral monodomains. Such coalesced droplets exhibit very thin stripe lines, as in the case of pure cholesterics with a tilted helix axis. Moreover, left- and right-handed chiral domains were observed, combined in regions partially separated by 'oily streaks', also typical of pure cholesterics. On cooling, the chiral nematic (N*) phase transformed through a pronounced texture transition into a normal nematic phase. However, the small chiral grains that formed from the isotropic phase are retained close to the surface, acting as 'memorizing centres'. With suitable boundary conditions, they can provide a macroscopic twist driven by the surface. Moreover, a twisted smectic B not present in the bulk phase diagram was found and interpreted as induced by the surface. Also in the crystal phase a strong memorization of the chiral N* texture was observed.     

Enhancement of twisting power in the chiral nematic phase by introducing achiral banana-shaped molecules

Achiral banana-shaped molecules with dodecyloxy tail groups, P-12-O-PIMB, N-12-O-PIMB, and S-12-O-PIMB, have exhibited unusual smectic phases which possess chiral and helical structures. In this work, we mixed these banana-shaped molecules with the chiral molecule forming a chiral nematic liquid crystal and found an exclusive effect of the achiral dopant that the twisting power of the chiral nematic phase in the mixtures is significantly increased with the increase of the content of achiral banana-shaped molecules. This characteristic effect in the chiral nematic field seems to offer the rational evidence for the twist conformation of such banana-shaped molecules, since the chirality should be included intrinsically within each chain. The asymmetric twist conformation in the ester linkage group connecting the central core with the side wings is likely to be the origin of enhanced twisting power.     

Chiralization in mesogenic 1,3-diacylaminobenzenes

The effect of molecular chirality in thermotropic mesomorphic H-bonded polymeric systems has not really been investigated so far. Therefore, five new chiral mesogenic diamides have been prepared and described. Besides more ordered mesophases, the enantiomers exhibit a chiral nematic polymorphism detected by microcalorimetry and probably corresponding to different modes of chiral coiling to give these singular twisted nematic phases.     

Chiralization in mesogenic 1,3-diacylaminobenzenes

The effect of molecular chirality in thermotropic mesomorphic H-bonded polymeric systems has not really been investigated so far. Therefore, five new chiral mesogenic diamides have been prepared and described. Besides more ordered mesophases, the enantiomers exhibit a chiral nematic polymorphism detected by microcalorimetry and probably corresponding to different modes of chiral coiling to give these singular twisted nematic phases.     

Structural Transition in Liquid Crystal Bubbles Generated from Fluidic Nanocellulose Colloids

The structural transition in micrometer‐sized liquid crystal bubbles (LCBs) derived from rod‐like cellulose nanocrystals (CNCs) was studied. The CNC‐based LCBs were suspended in nematic or chiral nematic liquid‐crystalline CNCs, which generated topological defects and distinct birefringent textures around them. The ordering and structure of the LCBs shifted from a nematic to chiral nematic arrangement as water evaporation progressed. These packed LCBs exhibited a specific photonic cross‐communication property that is due to a combination of Bragg reflection and bubble curvature and size.     

Blue phase stability of n-OCB homologue chiral nematic liquid crystal mixtures

Blue phase (BP) stability of a chiral nematic liquid crystal (LC) mixture is dependent upon chemical structure as well as physical properties. In this study, the blue phase temperature range dependent on alkyl chain length was investigated in order to evaluate the relationship between blue phase stability and the molecular structures of four kinds of 4-n-alkyloxy-4'-cyanobiphenyl (n-OCB) homologue chiral nematic LC mixtures composed of rod-like nematic LCs. It was confirmed that the blue phase temperature range was strongly dependent upon the molecular parity, K ₃₃/K ₁₁ and the helical twist power of the n-OCB homologues chiral nematic LC mixtures.     

Novel glassy nematic and chiral nematic oligomers derived from 1,3,5-cyclohexanetricarboxylic and (1R,3S)-( )-camphoric acids

Novel nematic and chiral nematic liquid crystals capable of vitrification have been synthesized using 1,3,5-cyclohexanetricarboxylic and (1R,3S)-(+)-camphoric acids as the base structures to which cyanotolan, cyanobiphenyl, methoxybiphenylbenzoate nematogenic groups, (S)-(-)-1-phenylethylamine and (S)-(+)-1,3-butanediol chiral moieties are attached. Once the glassy state is induced by quenching or controlled cooling from the isotropic state, these liquid crystals showed no detectable tendency towards crystallization upon heating through the glass transition temperature. In all cases, the ΔC_p and ΔH_c values resulting from the DSC heating scans are comparable to those previously reported for polymer analogues and other low molar mass glassy liquid crystals. In the nematic series with varying spacer lengths, both T_g and T_c are consistently lower than the linear polymer counterparts, in contrast to the siloxane-based systems. It was also demonstrated that cholesteric mesomorphism can be induced following one of the three approaches: chiral nematic mixture, chiral nematic cyclic cooligomer, and pendant nematogenic groups attached to a chiral ring.     

Influence of terminal alkyl chain length on helical twisting property of chiral 1,2-propanediol derivatives

In this study,a novel series of chiral 1,2-propanediol derivatives with different terminal alkyl chain length were synthesized and characterized by FT-IR,~1H NMR and DSC.After doped into a nematic liquid crystal host,all the chiral dopants induced chiral nematic liquid crystals exhibiting a helix inversion with temperature variation.The results indicate that terminal alkyl chain length has a prominent influence on helical twisting property of the chiral dopants.With increasing the terminal alkyl chain le...     

Highly birefringent nematic and chiral nematic liquid crystals

We report a simple interference method to determine the dispersion of the extraordinary refractive index and birefringence of highly conjugated and coloured nematic liquid crystals used as light-emitting materials in organic electroluminescent devices. The measurements are made in the nematic glass phase at room temperature. The birefringence is highly dispersive and values up to 1.1 are obtained. Chiral groups are incorporated into the end chains giving a chiral nematic liquid crystal with a very wide stopband in the visible region. The Berreman matrix method is used to simulate transmission through the chiral nematic liquid crystal cell using the refractive index parameters obtained experimentally. Excellent agreement between theory and experiment is found.     

Polymer and Mesoporous Silica Microspheres with Chiral Nematic Order from Cellulose Nanocrystals

Polymer microspheres with chiral nematic order were obtained from an emulsion polymerization technique using cellulose nanocrystals (CNCs) as the template. The growth of the liquid crystals from tiny tactoids to droplets with spherical symmetry was captured and investigated by both optical and electron microscopy for the first time. The size of the microspheres could be tuned between tens and hundreds of micrometers; to obtain single, integrated chiral nematic kernels, the size of water droplets in the emulsion should be similar to that of CNC tactoids. Through a double‐matrix templating method, novel silica microspheres with chiral nematic order were fabricated, which showed a high surface area and mesoporosity. The methods developed here may help to reveal the evolution of other self‐assembling systems, and these materials have potential applications in optical devices and chiral separations.     

仿生介孔二氧化硅用于高效液相色谱拆分手性化合物

仿生介孔硅是以有机物作为模板,可有效复刻模板的独特形貌,从而得到其相同或相似结构孔径的介孔硅。本文从仿生的观点出发,从蟹壳中提取得到几丁质膜,将其用作模板制备了仿生手性向列型介孔二氧化硅,并用其制备了液相色谱柱,进行了手性化合物拆分实验。结果表明,该色谱固定相对10个手性化合物有一定的手性分离效果。     

Synthesis and characterization of chiral compounds containing cationic groups as chiral dopants in liquid crystals

Novel chiral molecules containing cationic groups, (N-[4-triethylammoniomethyl]-benzoyl ester)-ethyl lactate chloride and bi-(N-[4-triethylammoniomethyl]-benzoyl ester)-isosorbide chloride, were designed and synthesized. Chemical structures of the molecules were characterized by elemental analysis, FT-IR, and (1)H NMR. The photochemical properties of the chiral compounds and their textures in nematic liquid crystals (LCs) were investigated by optical rotation, circular dichroism (CD), and polarizing optical microscopy (POM). The novel chiral molecules exhibited good optical activity. The chiral compound based on a L-ethyl lactate chiral center had a left-handed configuration. The chiral compound based on an isosorbide chiral center had a right-handed configuration. The cationic polar groups did not affect the direction of optical rotation, but could effluence the molar rotation of chiral compounds. The mixtures with dopants showed oily streak textures. Doping of a nematic phase liquid crystal with the chiral molecules converted it to the cholesteric phase.     

Thermo-recording in (side chain type smectic A liquid crystal polymer/nematic liquid crystal/chiral dopant/dichroic dye) composite with a low power laser

Thermo-recording in a [side chain type smectic A liquid crystal polymer (SmA-LCP)/nematic liquid crystal (N-LC)/chiral dopant/dichroic dye] composite has been realized by using 2mW of power from a He-Ne laser. The laser irradiation-induced phase transitions of smectic A (SmA) → chiral nematic (N*) → SmA formed the dominant thermo-recording mechanism in the composite. Thermo-recording in the (SmA-LCP/N-LC/chiral dopant/dichroic dye) composite exhibited a higher contrast when compared with the non-polymeric (SmA-LC)/N-LC/chiral dopant/dichroic dye composite.