An Indolin-3-imine Photobase and pH Sensitive Fluorophore

This work presents the pH sensing ability of a fluorescent indolin-3-imine derivative. Protonation of the weakly basic imine (pKa=8.3 of its conjugate acid) results in a significant red-shift of the absorption band. The fluorophore acts as a photobase, with a basicity increase of approximately 6 units upon photoexcitation. This behavior promotes excited state proton transfer from weak acids such as protic solvents. The characteristics of the fluorophore enable sensing of water fractions in organic solvents and differentiation between methanol, ethanol, and longer chain alcohols. Initial cell studies indicated the future potential of indolin-3-imines as fluorophores for bioimaging applications. Touching base: The photobasicity and pH sensitivity of a fluorescent indolin-3-imine derivative is described. The fluorophore can deprotonate and sense weakly acidic protic solvents - a process enhanced by excited state proton transfer. Cell imaging experiments revealed that indolin-3-imine is a potential scaffold for fluorescence cell microscopy.image