Design principles for photoswitchable fluorescent dyads

Photoswitchable fluorophores offer precise optical control for advanced imaging, yet the design criteria for an efficient photoswitchable fluorescent dye remain poorly understood. Here, we synthesize four new FRET-based dyads combining different photoswitches with fluorescent dyes and compare them with three previously reported dyads. Dithienylethene-based systems exhibit fluorescence modulation, but diazocine- and fulgimide-based dyads show minimal quenching, despite large FRET overlap. To explain these discrepancies, we develop a model showing that fluorophore absorption at the photoswitching wavelength can induce dye-mediated back-isomerization, reducing the population of the quenching-state. This model allows accurate prediction of photostationary state distributions across all molecular photoswitches and photoswitchable fluorescent dyads and allows us to identify key design principles for high-contrast photoswitchable fluorescent dyes.