Solvatochromism and prototropism in 6-aminonicotinic acid

Absorption, fluorescence excitation and fluorescence spectroscopic, as well as time-correlated single-photon studies, were carried out on 6-aminonicotinic acid (6-ANA) in different solvents and acid-base concentration in the range of H0/pH/H− from −10 to 16. Observation of single small Stokes shifted emission band in all the solvents confirms the presence of only amine derivative. Comparison of the spectral characteristics of 6-ANA with the methyl 6-aminonicotinate (6-MAN) suggests the presence of undissociated 6-ANA in polar aprotic solvents and zwitterion (ZI) in polar protic ones. Monocation (MC1) is formed by protonating N- atom and dication by further protonating the carbonyl oxygen atom. Monoanion (MA) is formed by deprotonating -COOH group and dianion by the deprotonation of -NH2 group. The electronic structure calculations using AM1 method and density functional theory (DFT) B3LYP with 6-31G** basis set (using Gaussian 98 programs) were carried out to assign the experimental results to the respective species.