Multi-stage and multi-colour liquid crystal reflections using a chiral triptycene photoswitchable dopant

The photomodulation of the helical pitch of cholesteric liquid crystals results in dynamic and coloured canvases that can potentially be used in applications ranging from energy-efficient displays to colour filters, anti-counterfeiting tags and liquid crystal (LC) lasers. Here we report on the analysis of a series of photoswitchable chiral dopants that combine the large geometrical change and bistability of hydrazone switches with the efficient helical pitch induction of the chiral motif, triptycene. We elucidate the effects that conformational flexibility, dispersion forces and π–π interactions have on the chirality transfer ability of the dopant. We then use the irradiation time with visible light (442 nm) combined with a simple digital light processing microscope projection set-up to draw numerous stable multi-coloured images on an LC canvas, showcasing the fine control this dopant yields over the LC assembly. Understanding how photoswitchable chiral dopants can control the helical pitch of host liquid crystals will aid the development of smart and adaptive soft materials. Now the molecular-level mechanisms that control the chirality transfer in chiral triptycene-containing bistable hydrazones have been elucidated. This enables the preparation of rewritable multi-coloured liquid crystal canvases.