Xanthene-based Fluorescence Turn-on Probe for Highly Acidic pH Range in Aqueous Solution.

A xanthene-based probe, Xanth-NPr, is developed as a molecular system that exhibits sensitivity for the highly acidic environments with fluorescence turn-on behavior. Xanth-NPr is designed on the principle of photoinduced electron transfer (PET), which controls the fluorescence profile of the probe. The structure of Xanth-NPr contains the dipropylaniline group as a PET promoting unit. Xanth-NPr exhibited quenched fluorescence as long as it is present in neutral or moderately acidic conditions. However, in the highly acidic pH range, it displayed a strong red-colored fluorescence at 592 nm as the protonation of dipropylaniline moiety inhibits the PET process. A model probe Xanth-M without any PET promoting unit was also synthesized. The model probe along with theoretical calculations was employed to explain the role of the PET process in regulating the fluorescence behavior of Xanth-NPr. Xanth-NPr showed linear fluorescence response as a function of pH in the range of 1 to 4.1 with the pKa value of 2.72. Likewise, its fluorescence profile is not altered by the presence of biologically relevant cations.