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.