Alexey Bobrovsky

Photoactive liquid crystalline polymer systems with light-controllable structure and optical properties

Abstract The state-of-the-art in the field of molecular design, synthesis, phase behaviour and photooptical properties of photochromic liquid crystalline (LC) side chain polymers, their blends with chiral and photochromic low-molar-mass dopants, hydrogen-bonded photochromic LC polymer systems, as well as photoresponsive LC dendrimers, is reviewed. The molecular architecture of the LC polymers and photochemical reactions of the main types of low-molar-mass achiral and chiral photochromes are briefly described. Recent advances in research dealing with the synthesis and study of optical and photooptical properties of a series of photochromic LC polymers bearing nematogenic and photochromic side groups forming nematic, smectic and cholesteric mesophases are considered. Among the various types of photochromic systems used to obtain the photoresponsive LC polymers the main attention focus on the so-called combined multifunctional systems composed of nematogenic, chiral and one or two photochromic fragments incorporated in the same monomer units or entirely into the macromolecule as individual monomers. The systems of both types form chiral nematic (cholesteric) phase with helical supramolecular structure. Polymer cholesteric films with planar orientation exhibit a selective reflection of light in the visible or IR spectral range. The action of light leads to the isomerization of both chiral photochromic groups (or the photochromic group alone), which results in the variation of the helical twisting power of the chiral fragments. This process is accompanied by sharp changes in the supramolecular helical structure and the optical properties of the polymer. By properly selecting the copolymer composition and preparing blends of the LC polymers with low-molar-mass chiral and photochromic dopants, using hydrogen-bonded systems, it is possible to obtain materials with different characters of the light-induced transformations. In particular, this provides the ability of controlling the pitch of the helix, the rate of helix twisting and untwisting, the width of the selective light reflection peak, etc. The last part of review covers a relatively new class of LC compounds—LC photochromic dendrimers, whose photochemical and photooptical properties are briefly discussed. The photochromic LC copolymers under consideration offer new promising materials for reversible and irreversible black/white and colour data recording that can be used in optoelectronics, data storage (in optical memory systems), holography, and colour projection techniques and give rise to a new generation of video discs, flat light guides and coatings with controllable optical properties.

Photochromic azobenzene functionalised banana–calamitic dimers and trimers: mesophase behaviour and photo-orientational phenomena

Azobenzene functionalised banana–calamitic dimers and a novel type of banana–calamitic–banana trimers have been synthesised. Their thermotropic behaviour and the structure of the mesophases have been characterised by differential scanning calorimetry, polarising optical microscopy and X-ray diffractions measurements. Electro-optical studies evidence the polar properties of the smectic phases. A rare anticlinic–synclinic smectic CP (SmCP) phase transition could be proven for two of the twins upon cooling without an electric field although the molecules are not optically active. Remarkably, the phase sequence SmCa–SmCs has been observed upon cooling, the inverse sequence to that reported for non-chiral compounds up to now. A comparative study of the photochemical properties of dilute solutions and of thin films of two compounds has been performed. Irradiation with polarised ultra-violet (365 nm) and visible light (473 nm) induces a photo-orientation process of the chromophores in a direction perpendicular to the polarisation plane of the exciting light. For a trimeric compound bearing an azobenzene bridge between both bent-core mesogenic units this process is associated with cycles of the E–Z–E isomerisation of the azobenzene groups followed by rotational diffusion of molecules in the films. For a bis(azobenzene) containing dimer the E–Z isomerisation in films is strongly suppressed and the photo-induced orientation has a rotational diffusion mechanism.

Light-responsive chiral photochromic liquid crystalline polymer systems

The authors reviewed their results obtained in recent years in the field of development of advanced optically controlled polymer systems based on photochromic cholesteric liquid crystalline (LC) polymers and their blends with each other and with low-molar-mass chiral and photochromic dopants. Recent advances in the research dealing with the design and study of photo-optical properties of photoresponsive LC polymers composed of nematogenic, chiral and one or two photochromic fragments incorporated in the same monomer units or entirely into the macromolecule as individual monomers are discussed. All polymer systems form cholesteric phase with a helical supramolecular structure. The light irradiation leads to the isomerization of one or two photochromic groups, which results in the variation in the helical twisting power of chiral fragments. This process is accompanied by dramatic changes in the supramolecular helical structure and optical properties of the LC polymer. This provides the possibility for controlling the pitch of helix, the rate of helix twisting and untwisting, spectral range, and the width of the selective light reflection. The photochromic LC systems offer new promising materials for reversible and irreversible colour data recording that can be used in optical memory systems (data storage), holography, colour projection techniques and give rise to a new generation of videodiscs, flat light guides and coatings with photocontrollable optical properties.

Optically and Electrically Controlled Circularly Polarized Emission from Cholesteric Liquid Crystal Materials Doped with Semiconductor Quantum Dots

Novel types of electro- and photoactive quantum dot-doped cholesteric materials have been engineered. UV-irradiation or electric field application allows one to control the degree of circular polarization and intensity of fluorescence emission by prepared quantum dot-doped liquid crystal films.

New Chiral-Photochromic Dopant with Variable Helical Twisting Power and its use in Photosensitive Cholesteric Materials

Abstract A new photosensitive chiral dopant based on cinnamic acid and isosorbide was synthesised. UV light action leads to E-Z photoisomerization of the dopant about C=C double bond of cinnamoyl fragment. Mixtures of the dopant with nematic and cholestric polymers were studied. It was shown, that planarly-oriented films of mixtures possess a selective light reflection in ultraviolet, visible and infrared regions of spectrum depending on the dopant concentration. The helical twisting power of the chiral dopant was calculated. UV light action on mixture films leads to a shift of selective light reflection peak to the long wavelength region of spectrum, which is explained by photoisomerization and decrease of the helical twisting power of the dopant. Kinetics of the process of photoinduced untwisting of the cholesteric helix was studied. Advantages of the use of the synthesised dopant and of mixtures based on it were demonstrated in comparison to earlier studied chiral-photochromic materials for optical data recording and storage.

New photosensitive polymer composites based on oriented porous polyethylene filled with azobenzene-containing LC mixture: reversible photomodulation of dichroism and birefringence

A new type of polymer–liquid crystal (LC) composite with photovariable dichroism and birefringence is described. Porous stretched polyethylene (PE) films with pores size in the range 50–500 nm were used as polymer matrices. A photochromic cholesteric mixture based on a commercial nematic mixture doped with azobenzene and merocyanine dopants was prepared. Highly birefringent deeply coloured flexible films possessing strong dichroism were obtained by introduction of the photochromic LC mixture into the stretched PE films by capillary forces. UV irradiation leads to E–Z isomerisation of azobenzene‐containing molecules inside the composite films and results in a significant decrease of the dichroism almost down to zero due to isothermal transition from the mesophase to isotropic melt stimulated by the isomerisation process. This process is reversible and the visible light irradiation or annealing lead to a back transition to the uniaxial birefringent and dichroic state. The composite films show a high fatigue resistance during the alternating cycles of the UV–visible irradiation. These novel composites can be considered as promising materials for photonics and optical information processing.

Novel Generation of Liquid Crystalline Photo-Actuators Based on Stretched Porous Polyethylene Films

The preparation of photo-actuators based on stretched porous polyethylene and an azobenzene-containing liquid crystalline polymer network is reported for the first time. It is revealed that this kind of photo-actuator possesses the following advantages: the lack of a need for using aligning coatings and cells preparation, high deformation of the actuator and its complete reversibility, good mechanical properties, and relatively low cost of fabrication. In addition some kinetic and thermodynamic features of the bending and unbending processes have been studied.

Novel type of combined photopatternable and electro-switchable polymer-stabilized cholesteric materials

A novel type of photopatternable polymer network-stabilized cholesteric materials for creation of electrooptical switching was elaborated. For this purpose cholesteric composites based on a commercial low-molar-mass nematic mixture, a chiral dopant, different amount of polymerizable mesogenic diacrylate (1–6 wt%) and a chiral-photochromic substance consisting cinnamoyl and isosorbide fragments (∼4 wt%) were prepared. A small amount of photoinitiator sensitive mostly to light with wavelength of 365 nm was also added. UV irradiation of the complex mixture with this wavelength light results in the polymerization of the diacrylate and formation of the polymer network stabilizing the initial planar texture in the electrooptical cell. Electrical field application induces a transition from the transparent planar cholesteric texture to a scattered focal conic one; at higher voltage the selective light reflection peak disappears. Irradiation of the cell with shorter wavelength light (313 nm) leads predominantly to the E–Z isomerization of the chiral-photochromic dopant followed by irreversible helix untwisting and to the shift of the selective light reflection from blue to green or red spectral regions. In this case, the competing photopolymerization process almost does not take place. Subsequent irradiation of the cell by longer-wavelength UV light (365 nm) allows one to obtain a polymer network and stabilize the new texture. Electrooptical properties of cells with different values of helix pitch were studied in detail. These data demonstrate the new opportunities for creation of photopatternable electrooptically switchable devices, which can be used in photonics and electrooptics.

Dual photorecording on cholesteric azobenzene-containing LC polymer films using helix pitch phototuning and holographic grating recording

For the first time the possibility to perform dual recording in cholesteric LC polymer films was demonstrated by means of helix pitch photo-tuning and holographic grating recording, as far as these two photo-processes take place under the exposure to light with different wavelengths. For this purpose the cholesteric mixture of the nematic azobenzene-containing copolymer doped with the chiral-photochromic dopant was prepared and the photo-optical properties of thin planarly-oriented films of this mixture were investigated. It was found that the holographic grating recording over ∼1 min allows high diffraction efficiency values to be reached but does not disrupt the supramolecular helical structure that makes possible a two independent images recording.

Photochemistry and photoorientational phenomena in carbosilane dendrimers with terminal azobenzene groups

Photo-optical properties of carbosilane dendrimers of the first, third, and fifth generations with terminal azobenzene groups are presented. The dendrimers synthesized display a crystalline phase, which melts into isotropic liquid at 81-91 °C (depending on the generation number). A comparative analysis of the photo-optical behavior of the dendrimers in dilute solutions and films was carried out. It was shown that under the action of UV irradiation (365 nm) an E/Z photoinduced isomerization of azobenzene groups takes place in the solutions and amorphous films. This process was found to be thermally reversible: annealing leads to a back Z/E isomerization. The processes of photoinduced orientation of terminal azobenzene groups of the dendrimers under the action of polarized UV light (365 nm) were studied.