Light-Driven Handedness Inversion of Cholesteric Liquid Crystals

Light-driven cholesteric liquid crystals are soft intelligent photonic crystal materials, which change their optical properties upon irradiation of light. The molecules are organized into helical superstructures to selectively reflect the circularly polarized light with the same handedness as the light-driven cholesteric liquid crystals. By modulating the helical superstructures with light stimuli, the wavelength or polarization of the selective reflection is tuned. Light-driven handedness inversion of helical superstructures in cholesteric liquid crystals is currently in the limelight. The inversion of handedness alters the chirality of the circularly polarized light, which has wide potential for applications in tunable filters, anti-counterfeiting technologies, circularly polarized lasers, and 3D displays. However, inducing the handedness inversion of cholesteric liquid crystal still remains a challenge because the energy barrier between the opposite twist sense is difficult to overcome. It is necessary to figure out the universal strategies for designing light-driven cholesteric liquid crystal systems with reversible handedness inversion. This review mainly focuses on the development of the light-driven cholesteric liquid crystals with handedness inversion. The reported strategies for controlling the handedness are summarized, including the handedness inversion induced by reverse molecular chirality upon photoirradiation and introduction of chiral conflict. The reverse molecular chirality in different chiral molecular switches with tetrahedral, planar, or axial chirality induced by azobenzene, dithienylethene, overcrowded alkene, or cyano-functionalized diarylethene is concluded. During the photoisomerization process, the changes of conjugation, geometry, and dipole moment are analyzed. The strategies to introduce chiral conflict in cholesteric liquid crystals are demonstrated, and the mechanism of chiral conflict to facilitate handedness inversion is explained. Most importantly, the existing challenges and opportunities toward the systems are discussed.