Photochemical tuning capability of cholesteric liquid crystal cells containing chiral dopants end capped with menthyl groups

In order to investigate the photochemical tuning capability of chiral monomers and polymers containing end‐capped menthyl groups, a new series of chiral dopants was synthesized and added to commercially available nematic liquid crystals to induce cholesteric liquid crystal (LC) phases. The addition of chiral dopants with azo structure led to phototunability of the reflection colour of the LC cells. Photochromic variation of the LC cells due to photoisomerization of the azo compound was investigated. After photopolymerization of the monomers inside the cholesteric LC cells, the centre wavelength of the reflected band of the incident light was found to be fixed and the reflected bandwidth was broadened, resulting in a red shift. A schematic representation of both the photoisomerization of the azo dopants and its effect on variation of twisting pitches is proposed. Real image recording was performed using 365 nm UV through a mask with text. The top and side views of the morphological network structures of a fabricated cholesteric LC cell were investigated using scanning electron microscopy. The results of this investigation demonstrated that RGB reflected colours of LC cells can easily be achieved through the addition of the menthyl‐containing synthesized chiral compounds to nematic LCs. The addition of synthesized AzoM helped further in recording the patterns onto cholesteric LC films using 365 nm UV exposure.     

Optical behaviour of photoimageable cholesteric liquid crystal cells with various novel chiral compounds

In theory, both polarity and steric hindrance are basic factors which affect molecular interactions. To investigate the optical properties and steric structures of chiral compounds having different chiral moieties which affect the wavelength of light reflection in liquid crystal (LC) cells, a series of novel chiral compounds and azobenzene derivatives were synthesized. The liquid crystalline phases of the compounds were identified using small angle X-ray diffraction, differential scanning calorimetry and polarizing optical microscopy. Cholesteric LC cells with various synthesized chiral dopants which selectively reflect visible light were first prepared, the photochemical switching behaviour of colours was then investigated, with special reference to the change in transmittance in cholesteric LC cells containing an azobenzene derivative as a photoisomerizable guest molecule. Reversible isomerization of azobenzene molecules occurred in the cholesteric systems, resulting in a depression of T_ChI and a shift of the selectively reflected wavelength. We discuss the photochemically driven change in the helical pitch of the cholesteric LCs with respect to structural effects involving the chiral moieties. Molecular interactions caused by the added dopants, reliability and stability of the photoisomerization, and UV irradiation effects on the cholesteric LC cells were also investigated. A real image was recorded through a mask on a cholesteric LC cell fabricated in this investigation.     

Optical behaviour of photoimageable cholesteric liquid crystal cells with various novel chiral compounds

In theory, both polarity and steric hindrance are basic factors which affect molecular interactions. To investigate the optical properties and steric structures of chiral compounds having different chiral moieties which affect the wavelength of light reflection in liquid crystal (LC) cells, a series of novel chiral compounds and azobenzene derivatives were synthesized. The liquid crystalline phases of the compounds were identified using small angle X‐ray diffraction, differential scanning calorimetry and polarizing optical microscopy. Cholesteric LC cells with various synthesized chiral dopants which selectively reflect visible light were first prepared, the photochemical switching behaviour of colours was then investigated, with special reference to the change in transmittance in cholesteric LC cells containing an azobenzene derivative as a photoisomerizable guest molecule. Reversible isomerization of azobenzene molecules occurred in the cholesteric systems, resulting in a depression of T _ChI and a shift of the selectively reflected wavelength. We discuss the photochemically driven change in the helical pitch of the cholesteric LCs with respect to structural effects involving the chiral moieties. Molecular interactions caused by the added dopants, reliability and stability of the photoisomerization, and UV irradiation effects on the cholesteric LC cells were also investigated. A real image was recorded through a mask on a cholesteric LC cell fabricated in this investigation.     

Optical behaviour of cholesteric liquid crystal cells with novel photoisomerizable chiral dopants

In order to investigate the effect of photoisomerization of E-Z structures on the optical behaviour of cholesteric liquid crystal (ChLC) cells, a series of novel azo derivatives was synthesized. Molecular structures were identified using ¹H NMR, ¹³C NMR, FTIR and elemental analysis. Thermal properties and the specific rotation of the synthesized chiral azo derivatives were estimated. Rubbed polyvinyl alcohol coated on the inner surface of substrates was used to control the liquid crystal alignment in cells. The effect of chiral dopants on the reflection band of ChLC cells was investigated, as well as the dependence of polarizing optical microscope textures on temperature. The stability and reproducibility of the effect of UV irradiation on the cell reflection band and real image recording were confirmed. Real image recording of the ChLC cells fabricated in this investigation was also studied; a photoinduced image through a mask is given. Photoirradiated and non-irradiated areas appear as different reflected colours leading to the formation of an image. Stacking of the ChLC cells was found to intensify the brightness of the reflection band.     

Optical behaviour of cholesteric liquid crystal cells with novel photoisomerizable chiral dopants

In order to investigate the effect of photoisomerization of E–Z structures on the optical behaviour of cholesteric liquid crystal (ChLC) cells, a series of novel azo derivatives was synthesized. Molecular structures were identified using ¹H NMR, ¹³C NMR, FTIR and elemental analysis. Thermal properties and the specific rotation of the synthesized chiral azo derivatives were estimated. Rubbed polyvinyl alcohol coated on the inner surface of substrates was used to control the liquid crystal alignment in cells. The effect of chiral dopants on the reflection band of ChLC cells was investigated, as well as the dependence of polarizing optical microscope textures on temperature. The stability and reproducibility of the effect of UV irradiation on the cell reflection band and real image recording were confirmed. Real image recording of the ChLC cells fabricated in this investigation was also studied; a photoinduced image through a mask is given. Photoirradiated and non‐irradiated areas appear as different reflected colours leading to the formation of an image. Stacking of the ChLC cells was found to intensify the brightness of the reflection band.     

Thermal tuning band gap in cholesteric liquid crystals

The phase of a liquid crystal (LC) changing from a nematic phase to a cholesteric (Ch) mesophase is achieved by adding different ratios of chiral dopants S811. By studying the transmission spectrum, we are able to measure the helical pitch in cholesteric phase. The pitch in the mixtures of nematic E7 and chiral dopants S811 as a function of the concentration of the dopant and temperature is investigated. The sensitivity of the selective reflection notch of the cholesteric phase to the thermal tuning depends strongly on the ratios of the chiral dopants. It reveals that the influence of temperature is more profound for those cholesteric liquid crystals (CLCs) which exhibit smectic A (SmA) at lower temperatures. When fitted using Keating's formula, the helical pitch calculated from our experimental results lies on the predicted curve. Optimised ratios of the mixture CLCs for the optimised reflection band with the specified wavelength ranging from 467 nm to 2123 nm are suggested.     

Photo-optical properties of photopolymerizable cholesteric compositions

Compositions consisting of nematogenic diacrylate monomer and chiral dopants capable of forming the cholesteric mesophase and of photopolymerizing were obtained and studied. For the first time, the change of optical properties and cholesteric helix pitch during photocrosslinking of diacrylate under UV irradiation (365 nm) was investigated. The kinetics of photopolymerization was studied and several stages of this process were observed. Cholesteric photopolymerizable blends containing chiral photosensitive dopant capable of E–Z isomerizing under UV irradiation (313 nm) were studied. The decrease of the helix twisting power of the dopant during photoisomerization was observed resulting in untwisting of the helix and shift of the selective light reflection peak into the long-wavelength region of the spectrum. The possibility of the photoregulation of optical properties of such blends with following fixation of structure and of these properties by means of photopolymerization under UV irradiation (365 nm) was demonstrated.     

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.     

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.     

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.     

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 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.     

Photo-optical properties of polymer composites based on stretched porous polyethylene filled with photoactive cholesteric liquid crystal

A new type of polymer-liquid crystal composite with photovariable dichroism and birefringence is described. Porous stretched polyethylene films were used as polymer matrices. To induce a cholesteric phase in a commercial nematic host, a chiral photochromic dopant based on sorbide and cinnamic acid capable of E-Z isomerization under UV irradiation was used. A merocianine-type substance was selected as a dichroic dye. Introduction of a dye-doped cholesteric mixture with a helical pitch higher than ∼300 nm to polymer film led to an almost complete transition from a cholesteric to an oriented nematic phase, as well as to an increase in birefringence and the appearance of dichroism. Decrease of the helical pitch by increasing in the chiral dopant concentration in the liquid crystal-polymer composite results in a reduction of the dichroism values. UV irradiation of polymer composite leading to an isomerization of the chiral dopant and helix untwisting induces a noticeable gradual growth of dichroism and birefringence. These new composites can be considered as promising materials for optical applications.     

Chiral side-chain liquid crystalline polysiloxanes bearing fluorinated mesogens and cholesteryl groups

A series of chiral side-chain liquid crystalline (LC) polysiloxanes bearing fluorinated mesogens were synthesized with a cholesteric LC monomer and a fluorinated nematic LC monomer. They were characterized by use of various experimental techniques, and effect of fluorinated mesogens on characteristic of LC polysiloxanes was studied as well. In photoluminescence spectra, a narrow and a broad peak occur at around 270-317 nm, originated, respectively, from fluorinated phenyl groups and the conjugated xenene structure. The specific rotation analysis of all polymers showed negative values, but absolute values were lower than those of the chiral monomers. All polymers showed smectic LC phase with very wide temperature ranges on heating and cooling cycles. Especially, only polymers bearing more fluorinated component exhibited smectic-cholesteric phase transition when they were heated. As the polymers contained more fluorinated mesogens, segregation of the fluorinated segment to the surface should occur at mesomorphic temperature. The highly ordered lamellar mesogen-siloxane matrix systems should be disturbed severely by separation of fluorinated mesogens, suggesting mesogenic orders transition from lamellar smectic to cholesteric phase.     

新型可溶性偶氮聚酰亚胺及其作为液晶光定向层应用的初步研究

以 4 ,4′ 二 (1,2 苯二酸酐 4 羧酸酯 )偶氮苯为二酐单体 ,4 十六烷氧基 4′ ,4″ 二氨基三苯甲烷为二胺单体 ,通过缩聚反应合成了可溶性偶氮聚酰亚胺 .采用红外光谱、氢核磁共振、紫外光谱和热分析等手段 ,对产物的结构、热性能及光学性能等进行了表征 .在紫外光辐照下 ,上述聚合物表现出明显的光色效应 .经线性偏振紫外光 (LPUV)辐射后的上述偶氮聚酰亚胺定向层能诱导液晶盒中液晶分子发生定向沿面排列 ,且取向均匀 .上述实验表明 ,合成的偶氮聚酰亚胺是一类潜在的液晶光定向材料 .     

Polarization holographic grating recording in the cholesteric azobenzene‐containing films with the phototunable helix pitch

The recording of polarization gratings in films of a cholesteric liquid crystalline polymer with different helix pitch was studied in detail. For this purpose, the cholesteric mixture of the nematic azobenzene‐containing copolymer with a chiral‐photochromic dopant was prepared. The utilization of such mixture has made possible to realize dual optical photorecording in one system, first due to the phototuning of the helix pitch by UV light and second the polarization grating recording process by exposure with polarized visible light. The diffraction efficiency strongly depends on the cholesteric helix pitch and films thickness: the increase of the confinement ratio d/p (where d, film thickness; p, helix pitch) results in growth of the diffraction efficiency. Comparison of the induction of polarization gratings in cholesteric, nematic (copolymer without chiral dopant), and amorphous (nonannealed) cholesteric films revealed that only the cholesteric films were characterized by significant oscillations in the diffraction efficiency signal as well as by the presence of the maximum in the first‐order diffraction efficiency in the initial stage of the grating recording process. It was found that in addition to the polarization grating surface relief gratings (SRGs) were also formed in the studied systems, however, the amplitude of the SRG inscribed in the cholesteric films was lower (～20 nm) compared to the grating amplitude obtained in nematic films (～60 nm). Moreover, increasing helix pitch resulted in a decrease of the SRG amplitude. The obtained experimental data demonstrate the great potential of cholesteric LC mixtures of such type for different applications as photoactive materials for photonics. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 773–781     

Broadband reflection in polymer stabilized cholesteric liquid crystal cells with chiral monomers derived from cholesterol

In this study, a series of achiral monomers and chiral monomers of different flexible spacer chains based on cholesteryl moiety were synthesized. Polymer stabilized cholesteric liquid crystal (PSCLC) cells were then created by incorporation of the polymer networks. The influence of the nature of the monomers and the spacer length of chiral monomers on the reflectance properties of PSCLC was investigated as well as the polymerization condition. The results strongly suggest that the chirality of the polymer networks plays an integral role in the observed reflection spectra, and the chiral polymer networks with chiral centers separated well from the polymer backbone induce a greater change in the bulk helix pitch, and produce the broader reflection band in these LC composites. In addition, the temperature dependence of the pitch of the composites before and after polymerization was investigated. To broaden the reflection band further, the experimental processes of thermally induced pitch variation simultaneously with a UV crosslinking reaction of the composites were presented. The morphology of the polymer network in the composites was studied by scanning electron microscopy (SEM). Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

New 2,2'-substituted 4,4'-dimethoxy-6,6'-dimethyl[1,1'-biphenyls], inducing a strong helical twisting power in liquid crystals

Based on the stabilisation of the molecular motion by the chiral residue, novel optically active biphenylic chiral dopants for nematic liquid crystals were developed. This molecular congestion was obtained by introducing mesogenic residues on the 2,2'-positions of the chiral biphenyl; this led to a novel molecular architecture that was found to be efficient. The synthesised optically active biphenyls were characterised with very short cholesteric pitches when used as chiral dopants in nematic liquid crystals. The synthesis of the enatiomerically pure biphenyl dopants and their preliminary physicochemical characterisations are described.     

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.