Ultrafast photoresponsive phase transitions and reversible color modulation in liquid crystal microdroplets

Light-responsive liquid crystal (LC) microdroplets are promising for next-generation displays, sensors, and anti-counterfeiting technologies, owing to their rapid photoactive behavior. The key to unlocking ultrafast (within seconds) and reversible phase transitions or color shifts lies in the precise molecular design of LC with tailored photoreactivity. In this work, a rapid photoresponsive platform is developed by designing azo-functionalized additives with tailored 3D configurations of LC droplets. A series of alkoxy azobenzene molecules with alkyl spacers (6 to 12 carbons) is synthesized and combined with thermotropic 4'-cyano-4-pentylbiphenyl to produce monodisperse LC droplets via microfluidics. UV-induced trans-cis isomerization of the azobenzene additive drives rapid phase transitions (< 1 s under 20 mW/cm² light), with the trans form preserving nematic order and the cis form disrupting it. Longer alkyl chains broaden and lower photoswitching temperature ranges, while shorter chains narrow them. This effect is amplified with higher fractions of the azo moiety. Incorporating chiral dopants creates photochromic droplets capable of reversible color shifts upon light exposure with different wavelegnths, with full recovery sustained over 40 cycles. The ultrafast thermochromic and photochromic system supports dynamic, orthogonal microscale multicolor photopatterning, offering a versatile platform for designing advanced photo-responsive LC systems for smart materials and photonic technologies.