Acid–base behavior of 3-aminoquinoline in its ground and excited states

Electronic absorption and fluorescence spectroscopic techniques reveal that the pKb for neutral form (N) of 3-aminoquinoline (3AQ) is 5–5.7, in the ground as well as the first electronically excited state. However, the case is quite different for the second pKb, involving the monocation and the dication. The emissive state is found to be monocationic, even in highly acidic conditions and in nafion membrane, where the ground state of the molecule is predominantly dicationic. This indicates that the dication loses a proton in its electronically excited state, possibly due to the delocalization of the lone pair on the amino nitrogen atom over the ring, due to an intramolecular charge transfer. The pKb values have been computed using density functional theory (DFT) for the equilibria of neutral ⇆ monocation and monocation ⇆ dication respectively and found to be reasonably in good agreement with the experimental observations.