Tuning the Circularly Polarized Reflection from Cholesteric Hydroxypropyl Cellulose Using Molecular Photoswitches

Abstract

Controlling the ordering of molecular liquid crystals using light has found widespread applications in reflectors, sensors, and tunable optical filters. In chiral liquid crystals such as cholesterics, introducing a molecular switch can alter chiral interactions between mesogens and thus affect the periodicity and photonic band responsible for the structural color of the material. However, analogous photo-control of cellulose-based cholesteric polymers has not been reported. Here, we investigate the addition of achiral and chiral switches to cholesteric mesophases of structurally colored hydroxypropyl cellulose (HPC) and monitor changes in the photonic bandgap upon varying concentration, illumination, or temperature. While an achiral photoswitch enables reversible tuning of the reflected color upon trans-cis photoisomerization, chiral switches (with different pendant groups and R/S enantiomers) lead to substantial differences in pitch and polarized reflection during cycles of illumination or heating/cooling – attributed to the complex interplay between the chiralities of the switch and of the host mesophase. A final combination of chiral photoswitch and spiropyran enables independent control of reflected and transmitted color, which can be relevant for cellulose-based photonic displays, smart labels, or anti-counterfeiting technology.