Spectral characteristics of methyl 2-aminonicotinate: effect of solvents and acid–base concentrations

Absorption, fluorescence excitation and fluorescence spectroscopy, combined with time-dependent spectroscopy and semi-empirical AM1 and density functional theory using Gaussian 98 program calculations have been used to study the effect of solvent and acid–base concentration on the spectral characteristics of methyl 2-aminonicotinate (2-MAN). These results have shown that amine–imine equilibrium is absent in 2-MAN, both in the ground (S0) and first excited singlet (S1) states. Normal small Stokes shifted emission observed in all the solvents suggest the absence of excited state intramolecular proton transfer (ESIPT) in 2-MAN. Ionic species in S0 and S1 states formed at different acid–base concentrations are: Trication (TC), dication (DC1), monocation (MC1) and monoanion (MA), pKa values for different prototropic equilibrium were determined in S0 and S1 states and discussed.