Synthesis of helical polyacetylene in chiral nematic liquid crystals using crown ether type binaphthyl derivatives as chiral dopants

A series of crown ether type binaphthyl derivatives (CEBDs) were synthesized and used as chiral dopants to induce chiral nematic (N*) liquid crystals (LCs). The twisting powers of the CEBDs for phenylcyclohexane (PCH)-derived nematic LCs were evaluated. It was found that the twisting powers of the CEBDs increased with decreasing ring size of the crown ether. Helical polyacetylenes were synthesized in the N*-LCs induced by the CEBDs. The relationship between the morphology of the helical polyacetylene and the helical structure of the N*-LC was investigated. The result showed that the interdistance between the fibril bundles of the helical polyacetylene was equal to a half-helical pitch of the N*-LC and the screw direction of the polyacetylene fibrils was opposite to that of the N*-LC.     

Dynamic photoswitching of helical inversion in liquid crystals containing photoresponsive axially chiral dopants

Chirality switching is intriguing for the dynamic control of the electronic and optical properties in nanoscale materials. The ability to photochemically switch the chirality in liquid crystals (LCs) is especially attractive given their potential applications in electro-optic displays, optical data storage, and the asymmetric synthesis of organic molecules and polymers. Here, we present a dynamic photoswitching of the helical inversion in chiral nematic LCs (N*-LCs) that contain photoresponsive axially chiral dopants. Novel photoresponsive chiral dithienylethene derivatives bearing two axially chiral binaphthyl moieties are synthesized. The dihedral angle of the binaphthyl rings changes via the photoisomerization between the open and closed forms of the dithienylethene moiety. The N*-LCs induced by the dithienylethene derivatives that are used as chiral dopants exhibit reversible photoswitching behaviors, including a helical inversion in the N*-LC and a phase transition between the N*-LC and the nematic LC. The present compounds are the first chiral dopants that induce a helical inversion in N*-LC via the photoisomerization between open and closed forms of the dithienylethene moiety.     

Helical twisting properties of new chiral dopants with double (S)-1, 2-propanediol units for nematic liquid crystals

<正>A novel series of chiral dopants synthesized from(S)-1,2-propanediol and mesogenic carboxylic acids were characterized by FT-IR,~1H NMR,elemental analysis and their helical twisting properties were investigated by doping the chiral dopants into a nematic liquid crystal host(SLC-1717).The results show that,the helical pitch of N~*-LC mixture exhibited a terminal alkyl chain length dependence and the molecular twisting power β also exhibited a temperature dependence(increasing β with increasing temperature).     

Hierarchically controlled helical graphite films prepared from iodine-doped helical polyacetylene films using morphology-retaining carbonization

One-handed helical graphite films with a hierarchically controlled morphology were prepared from iodine-doped helical polyacetylene (H-PA) films using the recently developed morphology-retaining carbonization method. Results from scanning electron microscopy indicate that the hierarchical helical morphology of the H-PA film remains unchanged even after carbonization at 800 °C. The weight loss of the film due to carbonization was very small; only 10-29% of the weight of the film before doping was lost. Furthermore, the graphite film prepared by subsequent heating at 2600 °C retained the same morphology as that of the original H-PA film and that of the helical carbon film prepared at 800 °C. The screwed direction, twisted degree, and vertical or horizontal alignment of the helical graphite film were well controlled by changing the helical sense, helical pitch, and orientation state of the chiral nematic liquid crystal (N*-LC) used as an asymmetric LC reaction field. X-ray diffraction and Raman scattering measurements showed that graphitic crystallization proceeds in the carbon film during heat treatment at 2600 °C. Transmission electron microscopy measurements indicate that ultrasonication of the helical graphite film in ethanol for several hours gives rise to a single helical graphite fibril. The profound potentiality of the present graphite films is exemplified in their electrical properties. The horizontally aligned helical graphite film exhibits an enhancement in electrical conductivity and an evolution of electrical anisotropy in which conductivity parallel to the helical axis of the fibril bundle is higher than that perpendicular to the axis.     

Helical polyacetylene—Origins and synthesis

We present the origins and synthesis of helical polyacetylene (H‐PA) by focusing on its peculiar spiral morphology. Interfacial polymerization of acetylene was carried out in an asymmetric reaction field consisting of chiral nematic liquid crystal (N*‐LC) and Ziegler–Natta catalyst. As the N*‐LC is composed of nematic liquid crystal and a chiral compound such as a binaphthyl derivative with either the R‐ or S‐configuration, the screw directions of the polyacetylene chain and fibril bundle—and even the spiral morphology—are rigorously controlled by the chirality of the selected compound. Interestingly, the screw directions of the fibril and the bundle in H‐PA were found to be opposite to that of N*‐LC. It is worthwhile to emphasize that the hierarchical spiral morphology involving the primary to higher order structure is generated in a synthetic polymer such as polyacetylene by using N*‐LC as an asymmetric polymerization solvent. © 2008 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 8: 395–406; 2008: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.20163     

Bis-chelated imine-alkoxytitanium complexes: novel chiral dopants with high helical twisting power in liquid crystals

Enantiomerically and diastereomerically pure bis-chelated imine-alkoxytitanium complexes 6 and 7 have been synthesized and used as chiral dopants for converting nematic into cholesteric phases. The dopants were tested in mainly commercially available nematic liquid crystalline compounds or mixtures: LC1 (BASF), ZLI-1695 and ZLI-1840 (Merck), as well as N-(4-methoxybenzylidene)-4'-butylaniline (MBBA). The values of the helical twisting power (HTP) were determined by the Grandjean-Cano method. Exceptionally high helical twisting powers were obtained. Thus, the titanium complex 6 h displayed a HTP value of 740 microm(-1) in MBBA, the highest HTP value reported. The helical twisting power has been found to depend strongly on the structure of the nematic phase and the substitution pattern of the chiral ligand in the titanium complexes 6 and 7. Crystal structure analysis of 6 f confirmed the A,R,R configuration of the metal complex. The chiral imine ligands 4 and 5 were derived from the regioisomeric amino alcohols 1 and 2.     

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.     

Rationally Designed Axially Chiral Diarylethene Switches with High Helical Twisting Power

Three rationally designed axially chiral diarylethene switches were synthesized and their application as chiral dopants for phototunable cholesteric liquid crystal devices was investigated. Design of these molecules was based on the combination of photochromic dithienylcyclopentene core with bridged binaphthyl units as chiral precursors. Aromatic groups were introduced to the molecules at 6,6′‐positions of binaphthyls through a Suzuki–Miyaura coupling reaction. Their helical twisting powers (HTPs) are significantly higher than those of the known chiral diarylethenes reported as chiral dopants so far. Photocyclization of these molecules upon light irradiation brought out dramatic variation in HTPs between different states. The primary colors, red, green, and blue, were obtained in reflection on light irradiation and with thermal stability. Moreover, a multi‐switchable photodisplay was demonstrated using one of these chiral molecular switches.     

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...     

Influence of helical twisting power on the photoswitching behavior of chiral azobenzene compounds: applications to high-performance switching devices

Five photochromic chiral azobenzene compounds and one nonphotochromic chiral compound were synthesized and characterized by IR, 1H NMR spectroscopy, and elemental analysis. Cholesteric liquid crystalline phases were induced by mixing of the nonphotochromic chiral compound and one of the photochromic chiral azobenzene compounds in a host nematic liquid crystal (E44). The helical pitch of the induced cholesteric phase was determined by Cano's wedge method and the helical twisting power (HTP) of each sample was thus determined. The helical twisting powers of azobenzene compounds were decreased upon UV irradiation, due to trans-->cis photoisomerization of azobenzene molecules. Among the azobenzene compounds synthesized in our study, Azo-5, with isomannide (radical) as chiral photochromic dopant, showed the highest HTP and contrast ratio (Tmax/Tmin). Photoswitching between compensated nematic phase and cholesteric phase was achieved through reversible trans<-->cis photoisomerization of the chiral azobenzene molecules through irradiation with UV and visible light, respectively. Transmission rates (contrast ratios) increased with decreasing helical pitch length in the induced cholesteric phase. The influence of helical twisting power on the photoswitching behavior of chiral azobenzene compounds is discussed in detail.     

TiO2 nanorod arrays induced broad-band reflection in chiral nematic liquid crystals with photo-polymerization network

A series of TiO₂ nanorod arrays were obtained by hydrothermal reaction, then the morphologies were characterized and analysed. In the light polymerization condition, different morphologies of TiO₂ nanorod arrays and chiral nematic liquid crystalline (N*-LCs) media were combined to prepare broad-band reflectors. By adjusting the length and interval size of the TiO₂ nanorod arrays, the reflection spectra and the positions of the reflected spectrum and the reflected bandwidth can be controlled accordingly. As a result, the effect of polymerization temperatures and morphologies of TiO₂ nanorod arrays on the broad-band reflection in the N*-LC composite system has been investigated. Potential applications of the composite were related to brightness enhancement films, bandwidth-controllable reflective polarizers and near-infrared light blocking films.     

Novel atropoisomeric binaphthyl-containing liquid crystalline copolymers forming chiral nematic phases

Novel liquid crystalline (LC) acrylate side group copolymers, which consist of nematogenic phenyl 4-methoxybenzoate acrylate monomer (A) and novel chiral binaphthyl (BN) methacrylate monomers (MB-n) have been synthesized. The copolymers prepared differ in the spacer lengths of MB-n (n 3,5,11) and in their compositions. The homopolymers of the three new chiral binaphthyl monomers MB-n were also prepared. Copolymers with a low concentration of binaphthyl monomer units (less than 16 mol%) display a cholesteric mesophase. The induced chirality in the polymers is due to atropoisomerism (C2-symmetry) of the molecules. The helical twisting powers (beta), caused by the atropoisomeric units in the synthesized copolymers, were determined, and their temperature dependencies studied. The unusually high negative temperature coefficient of beta observed above the glass transition temperature is explained in terms of conformational changes of the BN molecules in the copolymers.     

Light-induced structural changes in chiral liquid crystals

Novel light-sensitive chiral dopants are studied as a light-sensitive component in chiral liquid crystals which may be used in tunable optical devices. Light-induced cis-trans- isomerization of chiral dopants results in changes of helical twisting power which translates into variations of helical pitch. Due to the light absorption in the liquid crystal cell the pitch variation is non-uniform across the cell, which leads, at first, to a deformation of cholesteric layers, and then to the formation of cholesteric bubbles. The sequence of structural changes has a distinct visual pattern and occurs at the surface close to the UV light source. Small deformations of cholesteric layers and bubbles are unstable and disappear after removing UV irradiation. The increasing size of the cholesteric bubbles results in better stability; large bubbles do not disappear after removing UV light. A theoretical model is suggested to describe the undulations of cholesteric layers.     

新型P,N配体及基1,4-共轭加成反应

以2-萘酚为起始原料,经过氧化偶联,消旋体的拆分得到手性骨架2-氨基-2'-羟基-1,1'-联萘(NOBIN),并以S-NOBIN为原料,经过六步反应合成了两个新型配体S-(+)-2-(2-吡啶酰胺基)-2'-二苯基膦基-1,1'-联萘(1a)和S-(+)-2-(6-甲基-2-吡啶酰胺基)-2'-二苯基膦基-1,1'-联萘(1b)。并进行了铜配合物催化的二乙基锌对2-环己烯酮的1,4-共轭加成反应的研究。反应产物3-乙基环己酮(13)的e.e.值高达92%。     

Helical Carbon and Graphite Films Prepared from Helical Poly(3,4‐ethylenedioxythiophene) Films Synthesized by Electrochemical Polymerization in Chiral Nematic Liquid Crystals

Helical carbon and graphite films from helical poly(3,4‐ethylenedioxythiophene) (H‐PEDOT) films synthesized through electrochemical polymerization in a chiral nematic liquid‐crystal (N*‐LC) field are prepared. The microscope investigations showed that the H‐PEDOT film synthesized in the N*‐LC has large domains of one‐handed spiral morphology consisting of fibril bundles. The H‐PEDOT films exhibited distinct Cotton effects in circular dichroism spectra. The highly twisted N*‐LC with a helical pitch of smaller than 1 μm produced the H‐PEDOT film with a highly ordered morphology. The spiral morphologies with left‐ and right‐handed screws were observed for the carbon films prepared from the H‐PEDOT films at 800 °C and were well correlated with the textures and helical pitches of the N*‐LCs. The spiral morphologies of the precursors were also retained even in the graphite films prepared from the helical carbon films at 2600 °C.     

New types of multifunctional liquid crystalline photochromic copolymers for optical data recording and storage

A new family of multifunctional chiral‐photochromic liquid crystalline (LC) copolymers containing mesogenic, chiral and photoactive groups were synthesized. The new principles of photo‐regulation of the helical supramolecular structure and optical properties of the binary and ternary chiral‐photochromic LC polymers based on the change of helical twisting power of the chiral‐photochromic monomer units, the dual photochromism and photochemical spectral gap burning were developed. It was shown, that the introduction of small amount of low‐molar‐mass chiral‐photochromic dopants in chiral LC copolymers having different helix signs followed by light irradiation permits one to twist or untwist the helical supramolecular structure. The synthesized polymers are shown to be promising candidates for colour data recording and storage.     

Thermal‐ and Photo‐Induced Phase‐Transition Behaviors of a Tapered Dendritic Liquid Crystal with Photochromic Azobenzene Mesogens and a Bicyclic Chiral Center

A ribbon‐shaped chiral liquid crystalline (LC) dendrimer with photochromic azobenzene mesogens and an isosorbide chiral center (abbreviated as AZ₃DLC) was successfully synthesized and its major phase transitions were studied by using differential scanning calorimetry (DSC) and linear polarized optical microscopy (POM). Its ordered structures at different temperatures were further identified through structure‐sensitive diffraction techniques. Based on the experimental results, it was found that the AZ₃DLC molecule exhibited the low‐ordered chiral smectic (Sm*) LC phase with 6.31 nm periodicity at a high‐temperature phase region. AZ₃DLC showed the reversible photoisomerization in both organic solvents and nematic (N) LC media. As a chiral‐inducing agent, it exhibited a good solubility, a high helical‐twisting power, and a large change in the helical‐twisting power due to its photochemical isomerization in the commercially available N LC hosts. Therefore, we were able to reversibly “remote‐control” the colors in the whole visible region by finely tuning the helical pitch of the spontaneously formed helical superstructures.     

Modification of the pitch of chiral nematic liquid crystals by means of photoisomerization of chiral dopants

New photoisomerizable chiral dopants have been studied. The dopants used were menthone derivatives, a chiral stilbene derivative and a nematic copolymer of a menthone derivative and a benzoyloxybenzonitrile derivative. NMR, HPLC and UV results showed that the E-Z-isomerization of all the compounds indicated proceeded rapidly upon UV exposure, without the formation of undesired by-products. Isomerization of the menthone derivatives, including the copolymer, induced a substantial decrease in the helical twisting power. The changes in helical twisting power induced by the isomerization of the chiral stilbene derivative were limited to a factor of 2. In mixtures of the photoisomerizable dopants with commercial dopants of opposite twisting senses and a nematic host mixture, the sign of the twisting sense could be reversed by illuminating the mixture with UV light. The viewing angle dependence of irradiated regions of a 90 -twisted nematic cell was rotated 90 with respect to the viewing angle dependence of the non-irradiated regions. It is expected that this approach may be useful in the preparation of dual domain TN cells with a reduced viewing angle dependence.     

Comparison of helical twisting powers of novel fluorinated amphiphilic enantiomers with their hydrogenated counterparts

This is the first report on novel amphiphilic enantiomers as esters of alanine and serine with a partially fluorinated octyl chain (L-APFOE and L-SPFOE), which form intrinsic chiral nematic (N*) phases with water only and exhibit very high helical twisting powers (htps) compared to their hydrogenated counterparts, dodecylesters of L-alanine (L-ADDE) and L-serine (l-SDDE). They also exhibit a wider N* range than those known in the literature. Furthermore, the htps of the chiral dopants mandelic acid (MA) and hexahydromandelic acid (HHMA) in the racemic nematic phase of DL-SPFOE and DL-APFOE are remarkably higher than those in the hydrogenated counterparts. The tremendous increase in chirality (htps) of the same single chiral center is a new phenomenon in micellar N* phases and is attributed to a "pivot" effect of the fluoroalkyl chain. As a result, a "skewed micelle" model is proposed to explain why the htp value of L-APFOE is higher than that of L-SPFOE, and accordingly how the intermicellar chirality transfer (chiral induction) may be visualized. The simplicity of these binary mixtures and the enlargement in htp of the single chiral centers visualized as the "skewed micelle" model may provide a basis vital for future computer modeling of chiral lyotropics.     

Twist grain boundary states in non-chiral smectogen-chiral dopant

The liquid crystalline systems studied consisted of non-chiral mesomorphic esters [the eutectic mixture of 4-n-hexyloxyphenyl 4-n-octyloxybenzoate and 4-n-octyloxyphenyl 4-n-hexyloxybenzoate] and structurally similar chiral dopants (N-arylidene derivatives of S-1-phenyl- and S- 1-benzyl-ethylamine). Twist grain boundary phases occur between the cholesteric and smectic C* or smectic A phases in all the investigated systems. The different structures of these TGB phases (TGBA and TGBC) are proved by small angle X-ray scattering and textural studies. The concentration and temperature ranges of the TGB phases are defined by the twisting power of the chiral dopants and their own mesomorphic peculiarities. The experimental dependences of TGB phase temperature range on cholesteric helical twist are influenced by a ratio change of the optically active and racemic forms of the dopant at a constant total concentration. The results obtained are discussed within Renn's theory.