Photoresponsive circularly polarized long-lived fluorescence in chiral liquid crystals via tunable stepwise energy transfer†

This study achieves efficient photoresponsive circularly polarized long-lived fluorescence by controlling stepwise energy transfer via light exposure. We use cholesteric liquid crystal Chol-C₆-DF as a phosphorescent donor, Br-C₆-Nap as an intermediate fluorescent acceptor, and SP as a photoresponsive fluorescent acceptor to prepare photoresponsive circularly polarized long-lived fluorescent materials DF-Nap-SP(n) via the blending method. Our findings show that before 365 nm light exposure, Chol-C₆-DF and Br-C₆-Nap undergo triplet-to-singlet Förster energy transfer (TS-FRET). However, no singlet-to-singlet Förster energy transfer (SS-FRET) occurs between Br-C₆-Nap and SP. Consequently, upon excitation, long-lived yellow fluorescence from the Br-C₆-Nap component in DF-Nap-SP(n) is observed. After 365 nm light exposure, SP undergoes isomerization to form an MC structure, and its absorption spectrum overlaps significantly with the fluorescence spectrum of Br-C₆-Nap. This allows for both TS-FRET between Chol-C₆-DF and Br-C₆-Nap and SS-FRET between Br-C₆-Nap and MC. As a result, long-lived fluorescence emission from MC can be observed. Additionally, the photophysical properties of DF-Nap-SP(n) can be further tuned by adjusting the SP content. As the SP content increases, the emission color of DF-Nap-SP(n) changes from orange-yellow to red after light exposure, with the fluorescence lifetime decreasing from 69 ms to 8 ms and the luminescence dissymmetry factor decreasing from −0.42 to −0.31.