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.