Color Tuning in Ferroelectric Nematic Liquid Crystals using Triptycene/Hydrazone Chiral Photoswitches

The development of light-tunable ferroelectric nematic liquid crystals (N_FLCs) will allow for the multi-stimuli control over the properties of this newly discovered LC phase. Here on the development of photoresponsive chiral ferroelectric nematic liquid crystals (N_F ^*LCs) is reported by incorporating chiral triptycene/hydrazone-based bistable photoswitchable dopants into the polar liquid crystal mesogen DIO. The propeller-like dopants exhibit high helical twisting powers with values of 13.5 µm^-1 in DIO, with a further increase in value to 31.0 µm^-1 upon 442 nm light-induced Z → E photoisomerization. This process results in the tuning of the N_F ^*LC's reflection color from near infrared, to red, green, and finally blue at 50 °C. The reverse color transition can be induced by 340 nm light irradiation, which drives the E→Z photoisomerization of the hydrazone. The ferroelectric nature of the N_F ^*LC phase is confirmed by applying an alternating current electric field (0.67 V/ µm, 10 Hz, square wave), which initiated azimuthal deformations of the ferroelectric helix.