Planar chiral azobenzenophanes as chiroptic switches for photon mode reversible reflection color control in induced chiral nematic liquid crystals

In this report, for the first time, a planar chiral photoresponsive compound has been employed in commercially available nematic liquid crystals to achieve phototunable reflection colors. We designed an azobenzenophane compound having conformational restriction on the free rotation of naphthalene moiety to impose an element of planar chirality and the corresponding enantiomers were resolved by HPLC on chiral column. We have determined the absolute configuration by comparison of density functional theory (DFT) calculations of its electronic circular dichroism (ECD) spectrum and specific rotation [alpha] D to experimental ECD and [alpha] D data. Enantiomers exhibit photochemically reversible isomerization in solution without undergoing thermal or photoinduced racemization. As chiroptic switches in different host nematic liquid crystals, they exhibit good solubility, moderately high helical twisting power, as well as a large change in helical twisting power due to photoisomerization. A unique feature of these chiral photochromic compounds is that no other auxiliary chiral agents is required to achieve a fast photon mode reversible full-range color control in induced cholesterics, that is, both the hypsochromic and bathochromic shift can be obtained from a single LC formulation by reversible photoisomerization of the single chiral compound.     

Structure of liquid crystal droplets with chiral propeller texture

We experimentally studied a nematic liquid crystal whose molecules form twisted head-to-head H-bonded dimers. We observed that when the material transformed from the isotropic to nematic phase, it formed droplets with chiral propeller textures. We carried out a computer simulation to investigate the liquid crystal director configuration inside the droplets and to study the effects of elastic constants and chirality on the droplet texture. Results of our study show it is likely that the material in the droplets had nonzero chirality due to spontaneous chiral phase separation.     

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.     

Synthesis and optical activity of isosorbide chiral derivative containing fluorocarbon group as chiral dopant in liquid crystal materials

Novel isosorbide derivative containing perfluorocarbon group,bi（perfluorooctanesulfonyl）isosorbide ester as chiral dopant in liquid crystal,was synthesized.Chemical structure was characterized by elemental analysis,FT-IR,¹H NMR and ¹⁹F NMR.The optical texture of the mixture was observed by polarized optical microscopy（POM）.Novel chiral dopant containing perfluorocarbon group had excellent optical activity.Its specific rotation and molar rotation were noticeable higher than those of bi（4-chloromethylbenzenecarbonic）isosorbide ester.The fluorocarbon group improved the molar rotation of chiral compound and did not affect optical rotation direction.The texture of the mixture added isosorbide derivative with fluorocarbon group showed the oily streak texture.     

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.     

Comparison of structures and properties of lyotropic cholesteric phases induced by center and axial chiral compounds

We studied the phase chirality in disklike lyotropic cholesteric (Ch_D) phases which were obtained by adding center and axial chiral dopants to achiral lyotropic nematic (N_D) host phases. In a lyotropic nematic matrix of the N_D phase in the hexadecyldimethylethyl-ammonium bromide/water/n-decanol ternary system, a Ch_D phase was induced by adding center chiral sterols (cholesterol, prednisolon, taurocholic acid) and the axial optically active compound R(−)-1,1′- binaphthalene-2,2′-diyl-hydrogen phosphate (BDP). The helical twisting power (HTP) of BDP is generally lower than the HTP of inducing substances with center chirality, such as cholesterol, prednisolon, etc. At constant composition of the N_D phase, the helix lengths were determined from the ordered fingerprint texture, the so-called “spaghetti-like texture” seen in polarizing microscopy. The reciprocal helix lengths change linearly with the BDP concentration. The properties of the Ch_D phase (textures, helix lengths, micelle parameters) induced by the chiral compounds and changed by the composition of host phases give information on the mechanism of chirality transfer from the molecular level to that of the micellar aggregates and, eventually, to the liquid-crystalline superstructure. The structure in the Ch_D phase was described in the form of micelle parameters. For helix formation a minimum concentration of the chiral compounds is necessary. During the helix formation the number of micelles per helix length changes as a function of the concentration of the center and axial chiral molecules. The first step during the formation of the Ch_D phase is the solubilization of dopants into the micelles. Interaction between the optically active molecules then leads to the formation of hydrogen bridges between adjacent optically active molecules in the helical stack. Received: 20 December 1999/Accepted: 22 May 2000     

Synthesis and transition temperatures of novel fluorinated chiral liquid crystals containing 1,4-tetrafluorophenylene units

To continue the search for novel series of fluorinated ferroelectric liquid crystals, an additional two series of 4-[(S)-2-methylbutoxy]phenyl 4-[(4-n-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] benzoates (C) and 4-(n-alkoxy)phenyl 4-[(4-(S)-2'-methylbutoxy-2,3,5,6-tetrafluorophenyl)] benzoates (D) have been synthesized. Polarizing microscopic textural observations and DSC measurements of the phase transitions of these novel compounds showed that compounds C were liquid crystals with a chiral nematic (N) phase and a monotropic chiral smectic C phase (S_c), and compounds D exhibited a chiral nematic (N) phase.     

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.     

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.     

Phase modulation using different orientations of a chiral nematic in liquid crystal over silicon devices

Modulation of the intensity of light by high quality reflective micro-displays is predominantly carried out by liquid crystal over silicon (LCoS) spatial light modulator (SLM) technology for applications such as pico-projectors. Wider use of these devices, in applications such as computer-generated holography and optical correlation, is limited by their phase modulation ability and illumination polarisation state dependence. These devices rely on planar or twisted nematic liquid crystals to modulate the light, but due to their viscoelastic properties they are inherently slow. Research into the use of the polymer stabilised blue phase has already shown that it can offer high speed phase modulation. However, other chiral nematic orientations are yet to be compared in LCoS devices. In this article, we demonstrate that polymer-stabilised chiral nematic liquid crystal electro-optical effects can offer phase modulation in silicon backplane devices. The uniform standing helix and focal conic textures are shown to be independent of the input light polarisation state and the uniform lying helix is shown to be polarisation dependent. These optical responses are then compared with that of the blue phase to identify a suitable orientation for further development in LCoS technology in order to find a high-speed, full phase modulating material.     

The influence of molecular structure on helical twisting power of chiral azobenzene compounds

Photochromic chiral azobenzene compounds with different molecular structures were synthesized, and a cholesteric phase was induced by mixing each chiral azobenzene compound with a non-photochromic chiral compound in a host nematic liquid crystal, E44. Helical pitch and, thus, helical twisting powers (HTP) of the chiral azobenzene compounds and the non-photochromic chiral compound were determined by Cano's wedge method. Molecular structures of the chiral azobenzene compounds were predicted by means of determining their molecular aspect ratio (L/D) with semiempirical molecular calculations (MOPAC at PM3 level). The effects of molecular structure on HTP of the chiral azobenzene compounds are studied in detail. Molecular structures of chiral azobenzene compounds significantly influence their HTPs.     

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.     

Synthesis and lyotropic liquid crystal properties of chiral helical polycarbodiimides

Chiral helical polymers have been expected to exhibit optical activity with a significantly large optical rotation power. In this paper polymethylphenylethylcarbodiimides (Poly-PhEMCDI) with helical structure were synthesized by the insertion polymerization of a corresponding chiral monomeric carbodiimide initiated by a copper complex. The circular dichroism spectra and optical rotation power induced by the ordered rigid main chain helical structure of polycarbodiimides, in solution and as cast films, were studied. Polycarbodiimides have a rigid rod helical structure and form lyotropic liquid crystal (LLC) in organic solvents such as dichloromethane (DCM), chloroform and THF. The LLC phase was studied using polarizing optical microscopy and X-ray diffraction. A chiral nematic phase was formed in DCM and chloroform in the concentration range 20–36%. Spherulites were formed in more concentrated solution. The formation of a LLC phase in polycarbodiimides organic solutions makes it possible to align the polymer helical chain to form an ordered film for electro-optical applications.     

Synthesis and lyotropic liquid crystal properties of chiral helical polycarbodiimides

Chiral helical polymers have been expected to exhibit optical activity with a significantly large optical rotation power. In this paper polymethylphenylethylcarbodiimides (Poly-PhEMCDI) with helical structure were synthesized by the insertion polymerization of a corresponding chiral monomeric carbodiimide initiated by a copper complex. The circular dichroism spectra and optical rotation power induced by the ordered rigid main chain helical structure of polycarbodiimides, in solution and as cast films, were studied. Polycarbodiimides have a rigid rod helical structure and form lyotropic liquid crystal (LLC) in organic solvents such as dichloromethane (DCM), chloroform and THF. The LLC phase was studied using polarizing optical microscopy and X-ray diffraction. A chiral nematic phase was formed in DCM and chloroform in the concentration range 20-36%. Spherulites were formed in more concentrated solution. The formation of a LLC phase in polycarbodiimides organic solutions makes it possible to align the polymer helical chain to form an ordered film for electro-optical applications.     

Optical activity studies in the pretransitional isotropic region of strongly chiral propiolate ester liquid crystals

Optical rotation and circular dichroism measurements performed in the pretransitional isotropic region above the chiral smectic phases TGBA*, TGBC*, S*_C, and S*_A exhibited by 1-alkylalkyl 4'-(4'-n-alkoxyphenylpropioloyloxy)biphenyl-4-carboxylates are reported. These results revealed a high degree of increasing chiral ordering on cooling in the isotropic phase, particularly in the temperature region where a broad DSC peak appears. However, the behaviour of the pretransitional optical activity was unlike that commonly observed for chiral nematic or blue phase compounds and did not follow a simple Landau-de Gennes temperature dependence. Characteristic features of the pretransitional optical activity may aid in understanding the local structure of chiral smectic phases. Electric fields did not seem to influence the results significantly, but strong electric field-induced optical rotation behaviour was demonstrated for a mixture constituted of the laterally fluorinated propiolate ester component and a nematic with a large positive dielectric anisotropy. These field-induced results could be fitted to expressions from the existing theory of pretransitional optical activity. Circular dichroism studies at low temperatures were performed for two compounds in dilute solution and were found to exhibit very large changes in their extinction coefficient at wavelengths in the ultraviolet region, which may indicate strong chiral interactions even in dilute isotropic solutions of these materials.     

Synthesis of chiral azobenzene-based compounds for use in the photochemical tuning of the helical structure of liquid crystals

We have synthesized azobenzene-based molecules containing either one or two chiral groups. A cholesteric phase can be induced by adding the chiral azobenzene compounds to a host nematic liquid crystal. We investigated the effects of the trans - cis photoisomerization of the chiral azobenzene compounds on the properties of the cholesteric phase, such as the helical pitch length. This can be increased or decreased by the photoisomerization of the chiral azobenzene compounds. We discuss the photochemically driven change in the helical pitch of the cholesteric phase with respect to structural effects involving the chiral groups.     

Non-symmetric chiral nematic liquid crystal dimers containing trifluoromethyl and 1,2-propanediol

ABSTRACT

A series of non-symmetric liquid crystal (LC) dimers with the same chiral core 1,2-propanediol (PD) have been synthesised, termed as ABBA-PD-TFBA, PBBA-PD-TFBA, ABA-PD-TFBA, PBA-PD-TFBA and AA-PD-TFBA, respectively, in which one of the two mesogenic groups, the fluorinated mesogenic unit, was kept fix and the other arm was different. The intermediate compounds and LC dimers were characterised by FTIR, ¹H NMR, differential scanning calorimetry, thermogravimetric analysis, polarised optical microscopy and X-ray diffractometer (XRD). The results of the measurements indicated that ABBA-PD-TFBA, PBBA-PD-TFBA and ABA-PD-TFBA displayed optical activity and enantiotropic chiral nematic phase, and PBA-PD-TFBA was an enantiotropic nematic LC while AA-PD-TFBA was a monotropic LC, displaying both nematic phase and smectic A phase on cooling. The results indicated that PD was able to induce the chiral nematic phase, nevertheless, the rigidity of the mesogenic arm, the flexibility of the terminal group and even the type of the terminal chemical bond played an important effect on the thermal properties of the dimers, and even on the formation of the chiral nematic phase. It is also worth noting that C=C at the terminal helped to stabilise the LC phase.     

Liquid crystal dimers derived from naturally occurring chiral moieties: synthesis and characterization

Naturally occurring cholesterol and α-chloroalkanoyl units derived from natural α-amino acids (l-valine, l-leucine, and l-isoleucine) have been utilized to prepare three different series of nonsymmetric liquid crystal dimers. Tolane (diphenylacetylene), which is known to possess several promising structural features, has been chosen as the other mesogenic segment to covalently tether with cholesterol through a flexible spacer. In each series, the terminal α-chloro ester group attached to the tolane unit is kept constant, while the length and parity of the spacer have been varied; specifically, three dimers comprising even-parity spacer of varying length, and one compound with an odd-parity spacer constituted a series. The phase behavior of these dimers has been ascertained mainly by polarizing microscopic and calorimetric studies. Except one, all the 11 dimers display enantiotropic mesomorphism. Within the series, clearing temperatures exhibit a dramatic odd-even effect wherein the even-parity dimers possess higher values. In general, the dimers comprising α-chloro ester group derived from l-valine and l-leucine stabilize chiral nematic and/or smectic phase/s, while the compounds with terminal group resulting from l-isoleucine show twist grain boundary phase additionally; this implies that the nature of the α-chloro ester group influences the phase behavior. Notably, an odd-parity dimer with an α-chloro ester group derived from l-valine exhibits a transition from an intercalated smectic A phase to a monolayer (unknown) smectic phase, as evidenced by optical, calorimetric, and X-ray diffraction studies. As representative case, a dimer has been screened for antimicrobial activity by disc diffusion method; a notable activity has been found against some microbes.     

Synthesis and characterization of new chiral liquid crystalline polyacrylates from L-isoleucine

The synthesis of chiral side chain liquid crystalline polyacrylates with a two-stereogenic centre from L-α-aminoacid is described. The chiral tail is 2-chloroalcohol obtained from L-isoleucine and the spacer group has either four or eleven methylene units. The mesogenic moiety is derived from phenyl benzoate. The stereochemistry of the key intermediate (2S,3S)-(+)-4- [1-(2-chloro-3-methyl)pentyloxy]phenyl benzoate (6) obtained by a Mitsunobu reaction was established by single crystal X-ray analysis. This result indicates that the nucleophilic displacement of chiral diazonium salts proceeds with overall retention of configuration. The liquid crystalline behaviour of polyacrylates P13 and P14 was investigated by DSC, optical microscopy, small angle X-ray scattering and depolarized light scattering. The polyacrylate P13, with eleven methylene units in the spacer, exhibits a chiral smectic A phase whereas the polyacrylate P14, with a spacer containing four methylene units, displays a chiral nematic phase.     

Synthesis and properties of new (−)-menthol-derived chiral liquid crystal compounds with alkyl or alkoxy terminal groups

Two series of chiral mesogenic compounds derived from (?)-menthol with varying length of alkyl or alkoxy terminal groups respectively were designed and synthesised. Their chemical structures were characterised by FT-IR and ¹H-NMR spectra. The thermal properties and optical textures were investigated by differential scanning calorimetry and polarising optical microscopy. Bragg selective reflection spectra of the compounds with the alkoxy chain in the N* phase were measured by ultraviolet/visible spectrometer. The results showed that the alkyl series homologues melt directly to the isotropic phase on heating and display cubic blue phase and focal conic textures of chiral nematic phase on cooling, whereas the alkoxy series displayed oily streak textures with iridescent colours on heating, and platelet textures of blue phase and focal conic textures were observed on cooling cycles. The chain structure and length of the terminal groups have profound influence on the isotropic temperature and a large odd–even effect is observed for the compounds. The selective reflection colours of alkoxy series shifted to longer wavelength with the increasing of temperature.