N. I. Boiko

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.

Liquid crystalline carbosilane dendrimers: First generation

Abstract An approach to the synthesis of a new class of liquid crystalline (LC) compounds, dendrimers of regular structure with terminal mesogenic groups, was elaborated. LC dendrimers based on the carbosilane dendritic matrix of first generation were synthesized. Cyanobiphenyl, methoxyphenyl benzoate and cholesteryl groups were used as mesogenic fragments. Individuality and structure of all compounds obtained was proved by GPC together with 1H- and 29Si NMR methods. The mesomorphic behaviour and structure of the LC dendrimers synthesized were investigated. It is argued that different mesophases of the smectic type are realized in all cases. It is shown that the mesophase type of these compounds essentially depends on the chemical nature of the mesogenic groups.

Liquid crystalline dendrimer of the fifth generation: From lamellar to columnar structure in thermotropic mesophases

The structure of a liquid crystalline (LC) carbosilane dendrimer of the fifth generation bearing 128 terminal cyanobiphenyl mesogenic groups has been studied. This dendrimer was synthesized by a hydrosilylation reaction and then the cyanobiphenyl mesogenic groups were chemically linked to the dendritic matrix via a-OOC-(CH2)10-Si(CH3)2OSi(CH3)2-spacer. Structural studies carried out by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction methods revealed unusual phase behaviour. At room temperature the dendrimer forms a lamellar (smectic A) phase which develops in-plane ordering above 40C.This is due toa tendency to form columns ofmolecules which areprobably perpendicular to the layers. Above 121C the material transforms into another more disordered mesophase which is probably a disordered hexagonal columnar phase. The proposed structures and molecular packing in these different types of mesophase are discussed.

New Chiral Nematic Materials with Photovariable Helical Supramolecular Structure for Reversible Optical Data Recording

Vardeny, Appl. Phys. Lett. 1999, 74, 2590. [22] M. Samoc, A. Samoc, B. Luther-Davies, Z. Bao, L. Yu, B. Hsieh, U. Scherf, J. Opt. Soc. Am. B 1998, 15, 817. [23] D. N. Kumar, J. D. Bhawalkar, P. N. Prasad, Appl. Opt. 1998, 37, 510. [24] Yu. A. Vlasov, M. Deutch, D. J. Norris, Appl. Phys. Lett. 2000, 76, 1627. [25] K. W.-K. Shung, Y. C. Tsai, Phys. Rev. B 1993, 48, 11 265. [26] M. Scalora, J. P. Dowling, C. M. Bowden, M. J. Bloemer, Phys. Rev. Lett. 1994, 73, 1368. [27] W. Stöber, A. Fink, E. Bohn, J. Colloid Interface Sci. 1968, 26, 62. [28] K. Yanagisawa, M. Nishioka, K. Ioku, N. Yamasaki, J. Mater. Sci. Lett. 1991, 10, 7.

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.

Photo-optical properties of new combined chiral photochromic liquid crystalline copolymers

For the first time, a series of cholesteric copolymers containing combined chiral photochromic side groups has been synthesized and the phase behaviour and optical properties of the copolymers have been characterized. Specific features of the photochemical and photo-optical behaviour of such systems were studied, and the quantum yields of the photo-induced process in solution and in the condensed state of the cholesteric copolymers were calculated. The selective light reflection wavelength was found to be controlled by the UV radiation. The synthesized polymers were shown to be promising candidates for colour data recording.

Phase diagrams and optical properties of new menthyl-containing LC copolymers forming chiral mesophases

New comb-shaped LC copolymers based on phenyl benzoate and menthyl-containing monomers were synthesized. Phase behaviour, optical properties and structure of the synthesized copolymers with different compositions were studied. Specific features of the development of helical nematic and smectic mesophases (N*, NB*, Ncyb*, Nre*, SmA*, SmF*, blue phases) were revealed. Special attention was paid to studying the structure and optical properties of the new chiral NB* mesophase. For this unusual type of mesophase, a model accounting for the arrangement of the mesogenic group is suggested. It is stressed that the subscript B does not mean biaxial (a biaxial N phase is usually denoted Nb) and B refers to the hexagonal ordering of the side groups, similar to that in a SmB phase. An alternative nomenclature would be Nh, where subscript h refers to hexagonal as in Colh0 for a hexagonally ordered columnar phase of a discogen.

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 chiral photochromic menthone-containing homopolymers and copolymers - synthesis, phase behaviour and photo-optical properties

A new approach for the preparation of photosensitive materials for coloured data recording and storage is advanced. This approach involves the synthesis of copolymers containing nematogenic and combined (-)-arylidene-p-menthan-3-one chiral photochromic fragments in one monomer unit. The conditions for the formation of an LC phase in the chiral photochromic homopolymers as a function of the structure of the side groups have been identified; for a new series of copolymers, the effect of their composition on the phase behaviour and photooptical properties is considered. Planarly oriented films of the copolymers show selective light reflection in the UV, visible, and near IR spectral regions. The photochemical behaviour of the homopolymers and copolymers in dilute solution and as films has been studied. In these systems, under UV radiation, the E-Z isomerization of the (-)-arylidene-p-menthan-3-one fragment is the dominating process. The kinetic features of the photoprocess are revealed, and the effective qua...

Thermotropic liquid crystalline polymers—22. Optical properties and structure of some cholesteric copolymers

Abstract Copolymers of the nematogenic monomer, p -(ϵ-acryloyloxycaproyloxy)- p ′-methoxyphenylbenzoate, with various cholesterol-containing monomers were synthesized. The copolymers were shown to form mesophases of the cholesteric type. The dependence of the selective reflection of light on copolymer composition was studied. The copolymers with low content of chiral groups form monochromic cholesteric mesophases, with the cholesteric helix pitch being independent of temperature; the helix for copolymers with high content of chiral groups increases its pitch with increase of temperature.