Dicyanodistyrylbenzene-Based Chiral Fluorescence Photoswitches: An Emerging Class of Multifunctional Switches for Dual-Mode Phototunable Liquid Crystals

Two novel enantiomeric chiral fluorescence photoswitches containing two chiroptical (S)-(-)- or (R)-(+)-1,1-bi-2-naphthol covalently bonded to a central dicyanodistyrylbenzene unit are facilely synthesized and efficiently used to construct an optically tunable reflective-photoluminescent cholesteric liquid crystal (LC) device. In comparison with conventional azobenzene-based devices, this new class of chiral photoswitches is found to undergo unprecedented irreversible Z/E photoisomerization in both organic solvent and LC media upon irradiation of 365 nm ultraviolet (UV) light. The isomers on the photostationary state and even the intermediate states after Z/E photoisomerization do not exhibit thermal relaxation behavior and thus are stable in a dark environment. The photo-induced Z/E photoisomerization leads to a large change in the helical twisted power of the chiral switches, enabling a reflection spectral shift of more than 1500 nm in the cholesteric LC. These structural modifications also produce a pronounced fluorescence spectral variation, which makes it possible to simultaneously phototune both the reflection and the fluorescence in the cholesteric LC. We also present proof-of-principle demonstrations of the fact that the phototuning of the cholesteric LC based on the chiral fluorescence photoswitches can be used in irreversible UV light indicators and reflective-photoluminescent LC display devices.