Polymer stabilization of phototunable cholesteric liquid crystals

Abstract

Prior examinations of azobenzene-based cholesteric liquid crystals (azo-CLCs) have primarily focused on light-directed tuning of the photonic bandgap, with particular attention paid to the magnitude of the shift of the reflection (tuning range). One potential limitation of these materials is the days-long dark restoration of the CLC from the photodisplaced spectra. This work demonstrates that polymer stabilization of azo-CLCs can dramatically reduce the time necessary for restoration from both the photoinduced isotropic (PHI) state and the photodisplaced CLC phase to the original spectral characteristics including position, bandwidth, and reflectivity. Specifically, an unstabilized azo-CLC takes 70 h to restore from the PHI state while a polymer stabilized azo-CLC regenerates the CLC phase in minutes and restores the original spectrum in a couple of hours. It is established herein that polymerization in the CLC phase is of critical importance to the restoration of highly reflective azo-CLC to the original spectral state (position, reflectivity, bandwidth, and baseline transmission). Polymerization in the CLC phase templates the polymer network with the original pitch which effectively guides the restoration of the azo-CLC helix from the photodisplaced state (isotropic or CLC). This work is another step toward the development of light-directed CLC devices capable of direct on–off binary optical switching or instantaneous tuning/relaxation, with potential applications in lasing, photonics, and imaging.