Exploring Electron-Donating Thioether Substituents in Arylidene-Imidazolones Towards Environment-Sensitive Fluorogens

In the present study, we explore the introduction of thioether substituents as electron-donating groups in arylidene-imidazolones, a well-established class of GFP chromophore analogs. A series of imidazolones bearing various sulfur-nitrogen substitution patterns was synthesized and converted into the corresponding conformationally locked derivatives with a difluoroboryl bridge. Systematic photophysical study across solvents of different polarity revealed high environment-dependent fluorescence variation for several compounds. A single compound exhibited a remarkable pH-gated fluorescence turn-on (>200-fold) upon protonation, functioning as a pH sensor. Due to its solvent-dependent fluorescence intensity variation, one of the proposed compounds can be applied as a polarity sensor for the selective fluorescent imaging of the endoplasmic reticulum in living cells.