Donor engineering on flavonoid-based probes to enhance the fluorescence brightness in water: Design, characterization, photophysical properties, and application for cysteine detection

Increasing demand for detecting environmental and biological analytes in aqueous solutions based on fluorescent probes has raised the bar for the fluorophores used. Especially, the twisted intramolecular charge transfer (TICT) state of fluorophore can trigger the formation of a charge-separated non-emissive specie, leading to a low fluorescence brightness. In this work, we performed a simple donor engineering on the flavonoid-based fluorophores to regulate the TICT formation. A systematic study on relationship between donor structure and photophysical properties has been carried out. Among a series of flavonoids, the methylpiperazine-substituted one exhibited the strongest fluorescence brightness in water since the methylpiperazine could rigidify the rotation of donor moiety to suppress TICT state. Moreover, a novel fluorescent probe based on this flavonoid successfully realized the cysteine detection both in water and in living cells. This work confirmed the validity of design strategy by inhibiting TICT to enhance the fluorescence brightness, and also provided a practical flavonoid-based fluorophore for further bioimaging applications.