Photochromic Cholesteric Liquid Crystals via Arylazopyrazole Functionalization of Hydroxypropyl Cellulose

Photochromic materials are of interest for low-power colorimetric sensors, reusable displays, and dynamic optical components. Incorporating photoswitches into cholesteric liquid crystalline mesophases can enable photo-induced shifts in structural color. While cellulose ethers, such as hydroxypropyl cellulose (HPC), constitute a promising basis for photonic functional materials, due to their abundance, water solubility, and ability to form structurally colored cholesteric liquid crystals, they do not innately respond to incident irradiation. Here, we introduce light-responsiveness into cellulosic chiral nematic mesophases to create a novel photochromic material with precise spatial control over the displayed structural color. For this purpose, we substitute HPC with varying amounts of a highly stable arylazopyrazole (AAP) side chain capable of rapid photoswitching and investigate how AAP influences the reflection wavelength. We observe that even a small AAP degree of substitution can lead to reversible and repeatable photoresponsive structural color under alternating UV and green light irradiation. By changing the number of AAP side groups, we adjust the magnitude of the wavelength shift from 20 to 130 nm, revealing the highly tunable behavior of these materials. Ultimately, the color of these AAP HPC cholesteric mesophases can be spatially patterned within 5 min of light exposure, demonstrating their applicability for dynamic photochromic displays.