Photoswitchable Dual-Color Fluorescence With Large Stokes Shift From Dye-Encapsulated Metal-Organic Framework for...

Photoswitchable dual-color fluorescent materials are highly valuable for applications in cellular imaging and super-resolution microscopy. However, realizing these materials, especially broadening their Stokes shift remains a critical challenge. Herein, a novel strategy for achieving photoswitchable dual-emission fluorescence with a large Stokes shift is proposed via a cascade energy transfer (ET) process in metal-organic frameworks (MOFs). By incorporating the photoisomerizable molecule spiropyran as an ET intermediate, and two fluorescent dyes coumarin 153 (Cou153) and methylene blue (MB) within a MOF, rho-ZMOF, the dual-color dynamic luminescent material rho-ZMOF⊃Cou153&SP&MB is achieved. When excited at 400 nm, rho-ZMOF⊃Cou153&SP&MB exhibits coumarin-centered green emission at 516 nm and methylene blue-centered red fluorescence at 700 nm, with a maximum Stokes shift of 300 nm. More importantly, these dual-color emissions can be reversibly switched on or off upon altering UV- and visible-light irradiation because of the photoswitchable cascade energy transfer from coumarin 153 to methylene blue based on the photochromic transformation of spiropyran. As an example, the potential use of rho-ZMOF⊃Cou153&SP&MB in high-resolution and depth-resolved bioimaging is demonstrated.