Azobenzene-containing bent-core liquid crystals: an overview

Azo-functionalised materials are of special interest due to their photochromic nature, i.e. reversible trans–cis isomerisation upon photoirradiation. The combination of photosensitivity and liquid crystalline properties in the same molecule allows the material to be exploited for optical and optoelectronic devices. Azobenzene-based bent-core liquid crystals (BCLCs) have attracted considerable attention in recent years due to their rich mesomorphism. In this review, the main research directions and different molecular structures of bent-core molecules incorporating azobenzene unit and its subtype the so-called hockey-stick molecules are summarised. Additionally, azobenzene-based U-shaped molecules, hydrogen-bonded bent-shaped liquid crystalline materials and some selected examples of two different types of photoswitchable mesogenic dimers are provided. The nature, number and position of the lateral substitutions able to modify the phase behaviour of such BCLCs, affording in turn interesting liquid crystalline phases are discussed. Finally, the isomerisation process of these photosensitive BCLCs in solutions or in mesophases under the effect of UV–visible irradiation is summarised.