Fast, Real‐Time, In Situ Monitoring of Solar Ultraviolet Radiation Using Sunlight‐Driven Photoresponsive Liquid Crystals

Abstract Accurate monitoring of ultraviolet (UV) radiation exposure from natural sunlight and artificial light sources is relevant for both human health and safe working environments. In this work, an ultrasensitive visual monitor for solar UV radiation is created using a sunlight‐driven, photoresponsive cholesteric liquid crystal (Ch‐LC) film constructed from cyanostilbene‐based chiral molecular photoswitches. Surprisingly, an ultrafast and irreversible redshift in the reflection color is observed when the obtained Ch‐LC film is irradiated under ambient sunlight, which is ascribed to a large, fast change in the helical twisting upon UV light interacting with the photoswitch in the LC system. The UV monitor exhibited robust optical responses, including color changes, between 100 µW cm −2 and 20 mW cm −2 for 365 nm UV radiation. Based on the experimental data and subsequent simulation data, two detection schemes for solar UV radiation featuring a 10 s color change and the photostationary state time are presented. This work paves the way for a new generation of UV‐radiation monitors that can realize autonomous, ultrafast, and power‐free UV radiation detection of natural sunlight and artificial UV sources.