Halochromism of 2-Indolinone Fluorophore Based on the Deprotonation of an Acidic C–H Bond

We disclosed a new halochromic fluorescent dye based on the deprotonation from an acidic C–H bond. The dye consisting of a 2-indolinone skeleton and a benzothiazole ring exhibited an obvious red shift in its photoluminescence spectra in response to the presence of bases. Structural analyses with 1H NMR measurements and theoretical calculations suggested that the π-conjugated system of the dye was expanded by the deprotonation of the acidic C–H bond on the 2-indolinone skeleton. This structural change contributes to the red shift of photoluminescence and accelerates the radiative decay from the excited state.