Based on the fluorescent spectral changes, we proposed that 1,4-addition was taken placed between 1 and HSO3− rather than 1,2-addition. Moreover, no spectral change was observed upon the addition of HSO3− to the solution of intermediate 2 because of the steric effect. 1H NMR analysis was carried out to demonstrate the proposed addition mechanism. As anticipate, after addition of HSO3− into the solution of 1, the nucleophilic attack of HSO3− toward C-4 disturbed the conjugation and caused an electron transfer in the probe, which converted the N+ in the indolenium to N, and weakened its electron-withdrawing character, leading all the 1H NMR signals up-field shifted (Fig. 4). The proton signal (Hc,at ı 7.7–8.5) that appeared at 5.05 after the HSO3− addition further confirmed the 1,4-addition reaction. Furthermore, this adduct was also characterized by MS analysis (Fig. S6), where the peak at m/z 416.1282 (calc. = 416.1296) corresponding to [M-HSO3+Na]+ was clearly observed.