Laser-induced concentric colour domains in a cholesteric liquid crystal mixture containing a nematic azobenzene dopant

The isothermal formation of concentric colour domains has been observed in a cholesteric mixture doped with 10 wt % of a photochromic nematic material. The host cholesteric mixture included the Merck materials BL131a and BL130, while the photochromic dopant was a mixture of 4-n-butyl-4'-n-alkoxyazobenzenes (BAAB). The helical pitch of the host cholesteric mixture was increased, as expected, on addition of the photochromic nematic material prior to irradiation with the molecules in the purely trans-configuration. On irradiation with low power (0.6 mW) argon ion laser light, cis-isomers formed within the interaction region and concentric colour domains appeared. Selective reflection from the colour domains occurred in the 400-560 nm spectral range. The coloured domains persisted in the time period following irradiation and extended beyond the interaction region because of diffusion of the cis-isomers. Using the diffusion equation, an expression has been obtained for the average concentration of cis-isomers in each of the coloured domains and the dependence of the reflection wavelength upon the concentration of cis-isomers has been determined.     

Photo-induced control of bubble domains in chiral–nematic liquid crystal by a violet laser beam

The stable bubble domains generated by mixing 10% of chiral molecules into an azobenzene liquid crystal (LC)-doped nematic host can be optically controlled by a violet laser beam (415 nm). The photon-induced reversible trans–cis photo-isomerisation of azobenzene changes the helical twisting power (HTP) of LC mixtures in which the HTP of cis-azobenzene LC is lower than trans-azobenzene LC. Under the irradiation of an optical field (>20 mW cm^???2), the helical pitch distance, which is inverted proportional to the HTP, increases and the bubble domains disappear. Immediate obstruction of laser light irradiation initiates cholesteric nucleation, merging of domains and the subsequent generation of stably dispersed bubble domains.     

Chiral,Thermally Irreversible and Quasi-Stealth Photochromic Dopant to Control Selective Reflection Wavelength of Cholesteric Liquid Crystal

A chiral and thermally irreversible photochromic fulgide derivative incorporating an (R)-binaphthol unit in its acid anhydride moiety was used for the photoswitching of the pitch length of cholesteric liquid crystals. Since the absorption maximum wavelengths of both thermally stable photoisomers are nearly in the UV region (quasi-stealth photochromism), it can be exposed to visible light without inducing photochromic reactions. Therefore, when the photoswitching molecule is added to a permanent cholesteric liquid crystal whose reflection light wavelength is in the visible region, the UV light-induced photochromic reaction of the photoswitching molecule changes the wavelength of the reflection light in the visible light region. We have succeeded in regulating the color of cholesteric liquid crystalline cells between red and blue upon UV light irradiation. Attempts to introduce this system in polymer dispersed cholesteric liquid crystals are also described.     

Photosensitive cholesteric polymers with azobenzene-containing chiral groups and mixtures of cholesteric copolymer with chiral-photochromic dopants

New chiral photochromic cholesteric comb-shaped acrylic copolymers and low molecular mass dopants containing azobenzene photosensitive fragments and chiral groups based on menthol and menthone were synthesized. For the copolymers and their mixtures with low molecular mass dopants, the phase behaviour and optical properties were studied. Under irradiation with UV and visible light, the untwisting of cholesteric helix takes place, and the selective light reflection maximum is shifted to the long wavelength spectral region. This shift is related to the E-Z isomerization of the azobenzene chiral groups. For the copolymers and mixtures of the cholesteric polymer with the menthyl-containing dopant, this process is thermally reversible. The specific features of the kinetics of the forward and the reverse thermal processes were characterized. It was demonstrated, that the copolymers and mixtures of the cholesteric copolymer with the menthyl-containing dopant may be used for coloured reversible recording of optical information. For such materials, their resistance with respect to the repeated 'recording-erasing' cycles was tested, and the fatigue resistance was shown to be rather high.     

A comparative study of photo-optical behaviour of photosensitive chiral copolymers with cholesteric mesophases induced in nematogenic and smectogenic matrices

The photo-optical behaviour of two series of chiral photochromic acrylic copolymers with a chiral nematic phase has been studied. These copolymers contain identical chiral photochromic units, but have different structures of the phenyl benzoate mesogenic side groups which are responsible for the development of LC phases. This approach allowed us to examine specific features of the photo-optical behaviour of the copolymers as a function of the nature of the LC 'matrix' in which the cholesteric phase was induced. The action of UV irradiation was shown to lead to the E-Z isomerization of the chiral side groups and, as a consequence, to untwisting of the cholesteric helix of the copolymers. For copolymers of both series, the effective quantum yields of this photochemical process were calculated. In the case of copolymers in which the cholesteric mesophase is induced in a smectogenic matrix, the corresponding values of the quantum yield are lower and depend slightly on temperature. A plausible explanation of the above phenomena is suggested.     

Synthesis and properties of photochromic liquid‐crystalline copolymers containing both spironaphthoxazine and cholesteryl groups

Photochromic liquid‐crystalline copolymers consisting of a photochromic monomeric unit containing both a spironaphthoxazine group and an undecamethylene spacer, and a liquid‐crystalline monomeric unit containing both a cholesteryl group and a decamethylene spacer were prepared to investigate the effect of the thermal properties of the photochromic monomeric unit on the mesomorphic order of the side chain of the related copolymers. The photochromic liquid‐crystalline copolymers containing a photochromic liquid‐crystalline monomeric unit showed only a smectic phase. On the other hand, the photochromic liquid‐crystalline copolymers containing a photochromic non‐liquid‐crystalline monomeric unit showed a chiral nematic phase (cholesteric phase). The photochromic chiral nematic liquid‐crystalline copolymer containing 14 mol % photochromic monomeric unit reflected visible light around 104 °C. To lower the temperature range of reflection of visible light, cholesteryl oleyl carbonate was used as a chiral nematic plasticizer for the photochromic chiral liquid‐crystalline polymer systems. Photo‐induced pitch change of the mixture by means of UV irradiation was investigated and it was concluded that the pitch change observed under UV irradiation was mainly induced by thermal effect in the case of our system. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 887–894, 2000     

Photocontrolled Phase Transitions and Reflection Behaviors of Smectic Liquid Crystals by a Chiral Azobenzene

The photocontrolled phase transitions and reflection behaviors of a smectic liquid crystal, 4‐octyl‐4′‐cyanobiphenyl (8CB), tuned by a chiral azobenzene, are systematically investigated. For the smectic 8CB doped with the chiral azobenzene (1R)‐(?)‐4‐n‐hexyl‐4′‐menthylazobenzene (ABE), the initial smectic phase can be switched to cholesteric and then to isotropic upon UV irradiation due to the trans‐to‐cis photoisomerization of ABE; however, no reflection band is observed. For the smectic 8CB doped with ABE and the chiral agent (S)‐(?)‐1,1′‐binaphthyl‐2,2′‐diol (BN), a reflection band located in the short‐wavelength infrared region is observed, which disappears after further UV irradiation. For the smectic 8CB doped with ABE and a chiral agent with higher helical twisting power, (S)‐2,2′‐methylendioxy‐1,1′‐binaphthalene (DBN), a phototunable system with cholesteric pitch short enough to reflect visible light is demonstrated. With a given concentration of the chiral dopant DBN, a reversible reflection color transition is realized tuned by the isomerization of azobenzene. The reverse phase transition from isotropic to cholesteric and then to smectic can be recovered upon visible irradiation. The photocontrolled phase transitions in smectic liquid crystals and the corresponding changes in reflection band switched by photoisomerization of azobenzene may provide impetus for their practical application in optical memories, displays, and switches.     

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.     

The effect of the position of the methyl group on the helical twisting powers of aldol condensation products of methyl cyclohexanones

The four aldol condensation products of methylcyclohexanone isomers and 4-(4-hexyloxybenzoyloxy)benzaldehyde have been prepared. These chiral products were either obtained with high enantiomeric excess or the enantiomers were separated by means of chiral HPLC. In all cases only the E-isomer was obtained. UV irradiation was used to isomerize the E-isomers to the Z-isomers. Comparison of the helical twisting powers (HTP) of the E-isomers revealed that the derivative in which the methyl group is directly next to the double bond exhibits the highest value. Although the other three E-isomers have lower HTP values, they show large HTP changes and reversal of the helical twist sense upon irradiation. Of these compounds, the one derived from the commercially available (R)-3-methylcyclohexanone is an interesting candidate for use as an alternative to compounds derived from menthone for realizing photochemically induced twist changes in twisted nematic and cholesteric materials.     

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.     

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.     

A Combination of Selective Light Reflection and Fluorescence Modulation in a Cholesteric Polymer Matrix

Summary: The phase behavior and optical properties of a cholesteric ternary copolymer, containing nematogenic phenylbenzoate, cholesteric, and photochromic diarylethene side groups, and its mixture with 2 wt.‐% fluorescent dopant were studied. The investigation of the kinetics of a photochemical opening‐cycle process of the photochromic groups in the cholesteric mixture proved the energy transfer from the fluorescent dopant to the photochromic diarylethene groups. It was shown that the fluorescence intensity of the fluorescent dopant could be controlled by the portion of the “closed” form of the diarylethene groups. During the photocyclization of the photochromic groups a “degeneration” of the selective light reflection of the cholesteric matrix is observed.

Fluorescence‐resonance energy transfer makes possible the process of photosensitization of the back ring‐opening photoreaction of the photochromic diarylethene groups in the cholesteric polymer matrix.     

Cholesteric mixture containing chiral‐photochromic and diarylethene dopants as novel material with dual photochromism

New cholesteric polymeric mixture containing nematic side‐chain homopolymer, 2 wt% of photochromic diarylethene dopant and 5 wt% of chiral‐photochromic dopant based on cinnamic acid and isosorbide was prepared. Upon UV irradiation of planarly‐oriented mixture films at room temperature, a transformation of open colorless form of diarylethene dopant into a closed colored form takes place, which is followed by the appearance of an intense absorption maximum in the visible spectral region and decreasing selective light reflection intensity. This process is photo‐ and thermo‐reversible and many cycles ‘recording‐erasing’ can be realized. UV irradiation and subsequent annealing of the films lead to untwisting of the cholesteric helix and cause an irreversible shift of selective light reflection to the long‐wavelength region. This process is explained by the E‐Z isomerization of chiral photochromic groups of the dopant relative to a C?C bond accompanied by a decrease in their twisting ability. It has been shown, that the use of the material prepared in this work provides an opportunity to record two images at the same time: one image is due to a change in the helical pitch and another (second) image is due to the photochromism of diarylethene dopant. It is pertinent to note that using the approach developed in this work allows one to widely vary the range of materials exhibiting dual photochromism. It was demonstrated that the mixture under study shows advantage as new photosensitive material for optics, optoelectronics and data recording. Copyright © 2002 John Wiley & Sons, Ltd.     

Photoinduced reorientation processes in thin films of photochromic LC polymers on substrates with a photocontrollable command surface

Orientation and reorientation processes that occur in nematic and cholesteric LC polymer systems under irradiation with plane-polarized light are studied. A copolyacrylate containing phenyl benzoate and azobenzene side groups is synthesized as a nematic polymer; the cholesteric mixture is prepared via doping of the nematic copolymer with the chiral dopant, the derivative of D-isosorbide. Thin layers of the azobenzene-containing photoorientant SD-1 are first used as orienting substrates for polymer liquid crystals. Thin layers of the copolymer and of the mixture are spin-coated on the substrate after irradiation of the photoorientant layer with polarized light. It is shown that after annealing phenyl benzoate and azobenzene side groups of the nematic copolymer orient strictly along the direction of orientation of surface molecules, whereas in the case of the cholesteric mixture, a partial formation of the helical structure is observed. It is demonstrated that all the systems under examination can experience the repeated cyclic reorientation of the cooperative type under irradiation and subsequent annealing of the films.     

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     

Hydrogenation of 2- tert -butylphenol over Ni catalyst

The hydrogenation of 2-tert-butylphenol was studied in regard to possibilities of influencing selectivity, namely the ratio ofcis- andtrans-isomers of 2-tert-butylcyclohexanol in the final reaction mixture. The hydrogenation reactions were carried out using the catalyst Ni/Al₂O₃. During the hydrogenations, a higher content of thecis-isomer was attained, when simultaneously the final reaction mixture contained 2-tert-butylcyclohexanone. The content of this intermediate, which primarily hydrogenated to thecis-isomer, increased with a decreased pressure and after the addition of acetic acid into the reaction mixture.     

Large Thermally Irreversible Photoinduced Shift of Selective Light Reflection in Hydrazone-Containing Cholesteric Polymer Systems

Stimuli responsive liquid crystalline polymers are a unique class of so-called “smart” materials demonstrating various types of mesomorphic structures easily controlled by external fields, including light. In the present work we synthesized and studied a comb-shaped hydrazone-containing copolyacrylate exhibited cholesteric liquid crystalline properties with the pitch length of the helix being tuned under irradiation with light. In the cholesteric phase selective light reflection in the near IR spectral range (1650 nm) was measured and a large blue shift of the reflection peak from 1650 nm to 500 nm was found under blue light (428 or 457 nm) irradiation. This shift is related to the Z-E isomerization of photochromic hydrazone-containing groups and it is photochemically reversible. The improved and faster photo-optical response was found after copolymer doping with 10 wt % of low-molar-mass liquid crystal. It is noteworthy that both, the E and Z isomers of hydrazone photochromic group are thermally stable that enable to achieve a pure photoinduced switch without any dark relaxation at any temperatures. The large photoinduced shift of the selective light reflection, together with thermal bistability, makes such systems promising for applications in photonics.     

Cholesteric polymer guest-host mixture with circularly polarized fluorescence: two ways for phototuning of polarization and its intensity

A photosensitive fluorescent cholesteric guest-host mixture consisting of a nematic polyacrylate, a chiral, photochromic dopant sensitive to UV light, and a fluorescent dopant was prepared. The nematic polyacrylate contains 4-phenyl-4'-methoxybenzoate nematogenic side groups and photochromic 4-cyanoazobenzene side groups. The chiral-photochromic dopant formed by isosorbide and cinnamic acid is capable of E-Z photoisomerization and [2 + 2] photo-cycloaddition under light irradiation. The planarly oriented films possess a selective light reflection in the visible spectral region coinciding with the emission peak of the fluorescent dopant. The fluorescence emitted by the planarly oriented films of the mixture is strongly circularly polarized and characterized by a large value of the dis-symmetry factor. At temperatures below glass transition (T(g)) the polarized light action of an Ar(+) laser (488 nm) leads to the photo-orientation of the azobenzene fragments resulting in a strong and reversible disruption of the selective reflection and a decrease of the dis-symmetry factor of fluorescence. UV irradiation leads to E-Z isomerization and/or [2 + 2] cycloaddition of the chiral-photochromic dopant, causing an irreversible shift of the maximum of the dis-symmetry factor to a long-wavelength spectral region under subsequent annealing at temperatures higher than T(g). Such multifunctional glass-forming guest-host mixtures combining photosensitive and fluorescent properties with the unique optical properties of cholesteric liquid crystals can be considered as promising material for optical data processing technologies and photonic applications.     

Photocyclization reactions. Part 6. Solvent and substituent effects in the synthesis of dihydrobenzofuranols using photocyclization of 2-alkoxybenzophenones and ethyl 2-benzoylphenoxyacetates

Photocyclization reactions were carried out on 2-alkoxybenzophenones 1a-h and ethyl 2-benzoyl-phenoxyacetates 5a-e in three solvents of different polarity (benzene, acetonitrile and methanol) to examine solvent and substituent effects on the cyclization of 1,5-biradical intermediates to dihydrobenzofuranols. Irradiation of 1a-f in benzene gave dihydrobenzofuranols 4a-f in 80–94% yields. The ratios of cis-and trans-isomers of 4b-f were 12:1 to 1:0, showing stereoselective formation of cis-isomers. On the other hand, irradiation of 1a-f in acetonitrile and methanol gave 4a-f in 68–81% and 7–75% yields, respectively. However, the ratios of cis- and trans-isomers of 4b-f were 3.5:1 to 1.3:1 in acetonitrile and 2.0:1 to 1:1.7 in methanol, showing decreased stereoselectivity. The decrease in stereoselectivity was attributed to intermolecular hydrogen bonding between the hydroxyl group of 1,5-biradicals and solvents (acetonitrile and methanol). Similarly, irradiation of 5a-e in benzene afforded cis-dihydrobenzofuranols cis- 11a-e stereo-selectively. In contrast, irradiation of 5a-e in acetonitrile and methanol gave a mixture of cis- and trans-isomers of 11a-e because of intermolecular hydrogen bonding between the hydroxyl group of 1,5-biradicals and solvents. The cis and trans ratios of 11a-e varied from 1.5:1 to 17.8:1 in acetonitrile and from 2.6:1 to 1:4.5 in methanol. Solvent and substituent effects on the cyclization of 1,5-biradicals and reaction pathways are discussed.     

Photoinduced anchoring transitions in a nematic doped with azo dyes

Recently, we reported on a light-induced anchoring transition of an azobenzene nematic from planar to homeotropic alignment. In the proposed model of the transition, the changes in shape of the liquid crystal molecules and of their net dipole moment, due to the photoisomerization, were considered to play a vital role in the occurrence of the transition. In order to assess the validity of this model, a study of the anchoring behaviour of nematic guest-host liquid crystal mixtures containing two photochromic dyes, 3,3'- and 4,4'-substituted azobenzenes, was carried out. The dyes have very similar molecular structures to that of the azobenzene nematic previously studied, and their molecules, having a linear shape in the trans-form, maintained this shape after photoisomerization in the case of the 3,3'-azo dye, and changed it to bent in the case of the 4,4'-azo dye. The dyes possessed similar net dipole moments that increased substantially after photoisomerization, resulting in a preferential adsorption of their cis-isomers on the solid substrate. However, only the mixture containing the 4,4'-azo dye exhibited an anchoring transition from planar to homeotropic alignment upon illumination with unpolarized UV light, a behaviour in excellent agreement with the prediction of the model for the light-induced anchoring transition. An anchoring transition from random planar to uniform planar alignment was found to take place in the mixtures when linearly polarized UV light was used, requiring, however, a different exposure time for the two dyes.