Functional liquid crystalline materials


This research area covers the design and synthesis of liquid crystalline materials with added functional properties or a combination of properties, given by the incorporation of one or more special functional fragments into their molecular structure. The research is running in cooperation with the Liquid Crystals Group of the Department of Dielectrics and other research groups within international cooperation.

The first group of these materials are photosensitive liquid crystals. These are formed by the incorporation of a photosensitive group into the molecules, forming in principle a molecular switch (Fig. 1). The resulting liquid crystalline material responds  not only to conventional electrical, but also to an optical signal, which offers much more possibilities for manipulating the electro-optical and mechanical properties of the material.

From a number of photosensitive groups we design  and synthesise materials based on cinnamic acid, stilbene, azobenzene and also a very recently discovered arylhydrazones. Most of our work is currently focused on materials with an azobenzene photoactive unit. We study the possibilities of using lateral substitution in the vicinity of the azo group, which affects the electron distribution in the molecule, to modify the rate and efficiency of photoinduced changes in the material. In particular, we study the photoisomerisation and subsequent thermal relaxation process using kinetic measurements. In addition to liquid-crystalline materials, we also develop photosensitive chiral additives for liquid crystal mixtures. In the resulting systems, it is possible to modify the supramolecular chirality and the resulting special material properties by means of a light signal.

Example of photosensitive liquid crystal based on azobenzene photosensitive unit and the principle of its function – the photoisomerisation, which leads to the change of molecular shape

The second group of materials we develop is called reactive mesogens and liquid-crystalline polymers. In other words, reactive mesogens are liquid crystalline monomers, i.e. substances in which the added functional element is a polymerisable group (Fig. 2). It may also be another reactive group by which the molecule can be attached by a covalent bond to another molecule, polymer, or surface of a material. In our group, we focus on the design and synthesis of chiral and non-chiral monomers with reactive double bond, acrylate or methacrylate group, as well as diols or thiols.

Selective reflection of light in chiral photosensitive material

The synthesised reactive mesogens are used for preparation of polymeric materials, typically of siloxane, acrylate or methacrylate type, but also other (Fig. 2). This group of materials is very often combined with the first group giving rise so-called photosensitive liquid crystalline polymers. These materials are aimed at modern optics and optoelectronics; for example, as significantly miniaturised elements of optical devices, media for easy rewritable recording and storage of optical data, or for special hidden holographic codes and tags in security technologies and in principle, they can be used as active moving parts of microrobots.

Block structure of a reactive mesogen with examples of reactive groups (RG) and the scheme of the synthesis of photosensitive liquid crystalline elastomer based on polysiloxane

The third type of materials we design and synthesise are liquid crystalline ligands for nanoparticles and nanocomposites. The aim is to design molecules, which owing to their structure not only stabilise the nanoparticle dispersions as conventional surfactants, but also support self-assembly (Fig. 3) in such hybrid nanoparticle system. Ordering nanoparticles is almost unreachable with conventional surfactants. The added functional element of these liquid crystalline compounds is the group by which they selectively bind to the surface of the nanoparticles. As a result, their molecular structure is very similar to the reactive mesogens and, for example, the above-mentioned thiol-type compounds can be used as ligands for silver and gold nanoparticles. For magnetic nanoparticles based on iron oxides, we synthesise for alkylphosphonic acid based ligands. In addition to the synthesis of organic ligands, we developed a method for the synthesis of silver nanoparticles directly in the presence of liquid crystalline ligands.

LC with nanoparticles
Simplified scheme of preparation of liquid crystalline nanocomposite and examples of functional groups of ligands (FG)
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