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 double bond; length as m-dashN– 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.