Near-infrared light-induced photoisomerization and photodissociation of a chiral fluorescent photoswitch in cholesteric liquid crystals assisted by upconversion nanoparticles.

Upconversion-luminescence-induced reflective color switching and fluorescence tuning of a cholesteric liquid crystal (CLC) cells were investigated. The CLC system was constructed by co-doping a chiral fluorescence photoswitch, switch 5, and upconversion nanoparticles (UCNPs) into nematic LC media. Under irradiation with 980 nm NIR light, the UCNPs emit both 450 nm blue light and 365 nm UV light to induce the simultaneous Z-to-E and E-to-Z photoisomerization of switch 5. This continuous rotation-inversion movement further leads to an irreversible photoisomerization and photodissociation of dicyanodistyrylthiophene moieties in switch 5. As a result, the reflective color of the CLC cell changed from blue to red and the fluorescence intensity decreased as well when exposed to 980 nm NIR light. Finally, optically written reflective-photoluminescent dual mode CLC cells were further demonstrated.