Photoswitchable azobenzene- and stilbene-based chromophores bearing terminal nitro groups: Synthesis, thermal and photochemical properties

This study examined the effects of aromatic cores and spacer lengths of chromophore structures on photoreactivity and mesomorphic properties of a series of azobenzene- and stilbene-based chromophores bearing electron-withdrawing (NO2) terminal groups caused by esterification reactions. The chromophores were composed of liquid crystalline mesophases with six or 11 methylene segments as spacers and with azobenzene (NN) and stilbene (CC) aromatic cores. The target compounds were further characterised using nuclear magnetic resonance spectroscopy, differential scanning calorimetry, polarising optical microscopy, absorption and photoluminescence spectroscopies. All of the azobenzene and stilbene derivatives with six or 11 methylene segments showed enantiotropic mesophases and revealed chiral nematic phases (cholesteric, exhibiting oily streaks textures). S6 and S11 with stilbene cores showed broader phase transition temperature ranges than A6 and A11 with azobenzene cores because of the molecular interaction enhancement of the planar stilbene structure. We investigated the effects of these photochromic compounds' structures on E/Z photoisomerisation under UV irradiation. UV irradiation caused E/Z photoisomerisation at NN and CC segments of the chromophores, leading to reversible and irreversible isomerisation respectively. These synthesised chromophores with functionally photoisomerised groups might have potential application for photoinduced alignments in liquid crystal industries.