On the other hand, in acidic solution, the absorption maxima of HBI were slightly red-shifted compared to those in neutral solution, but the shape of the spectrum was almost identical. In principle, ESIPT cannot occur in the cationic form, because the proton acceptor is already protonated (pKa1 = 5.48); however, fluorescence emission spectrum from cationic C form shown in Figure 3d is almost identical to that of K* emission in neutral solution. This predominant but unexpected K* emission in acidic solution was ascribed to excited-state intermolecular proton transfer to water, namely, deprotonation in excited state, since phenolic group is much more acidic in the excited state than in the ground state (Figure 3b,d).
In contrast, π–π* transition peak at 320 nm disappeared while a new band formed at 350 nm in basic solution, which accompanied a blue-shifted intense fluorescence emission (ΦPL = 0.6) with a normal Stokes' shift. This means HBI forms the anion A by deprotonation in basic solution (pKa2 = 8.83), resulting in a new absorption band that is red-shifted and new emission band that lies in the middle of its original E* and K* emission bands