Photoacidity of the N-salicylidene-5-chloroaminopyridine

Due to the occurrence of excited intramolecular proton transfer (ESIPT) in salicylidenes, in this article the photoacidity of the N-salicylidene-5-chloroaminopyridine (sal-5-Cl-py) was evaluated by electronic absorption and fluorescence spectroscopy. The results strongly suggest that sal-5-Cl-py in aqueous solution behaves as a superacid, ΔpKa=8.83, due to a large change of the dipole moment under electronic excitation between protonated and deprotonated species. Theoretical calculations under the density functional theory - DFT framework showed that as result of the ESIPT the electron density migrates from the phenol to the chloroaminepyridine ring. Such a charge migration is the driving force for the proton transfer reaction.