Excited state prototropism in 2-aminonicotinic acid: effect of solvents and acid-base concentrations

Absorption, fluorescence excitation and fluorescence spectroscopy combined with time-dependent spectrofluorimetry have been used to study the effect of solvents and acid-base concentration on the spectral characteristics of 2-aminonicotinic acid (2-ANA). The 2-ANA exists as a molecular species in non-polar and polar aprotic solvents and zwitterion (ZI-1) in polar protic solvents. Dual emission is observed from ZI and the monoanion (MA) of 2-ANA, suggesting that excited state intramolecular proton transfer (ESIPT) is observed from the amino proton to the carbonyl oxygen, followed by structural reorganization. Single normal Stokes shifted fluorescence is observed from the monocation (MC) of 2-ANA, indicating that the carboxylate ion is more effective than COOH group in ESIPT process. The dication (DC1) is formed by the protonation of N and >CO moieties and dianion by deprotonation of NH2 and COOH groups. Electronic molecular structure calculations were performed on each species using semi-empirical quantum mechanical AM1 method and density functional theory (DFT) B3LYP using 6-31G** basis sets with Gaussian 98 to assign the structure to the various species.