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

Abstract

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.