Spectral characteristics of 2-(4′-amino-3-pyridyl)benzimidazole: Effects of solvent and acid or base concentration

Spectral characteristics of 2-(4′-amino-3-pyridyl)benzimidazole (4-A3PyBI) have been studied in different solvents, as well as at different acid or base concentrations using absorption, fluorescence excitation and fluorescence spectroscopy. Excited singlet state (S1) lifetimes for each species were measured using nanosecond time-dependent spectrofluorimeter. AM1 semi-empirical and density functional theoretical (DFT) calculations were performed on each species for the spectral assignment. From the above results it is concluded that 4-A3PyBI exists only in the amine form. First protonation occurs at pyridineN- atom and second protonation at the benzimidazole (BI)N- atom. When dication (DC) species is excited, two emission bands are observed, having the same fluorescence excitation spectra, suggesting the same ground state (S0) precursor. Short wavelength (SW) emission band is assigned to the π-π* transition and long wavelength (LW) emission to the charge transfer transition. First deprotonation in S0 state occurs from >N-H moiety, whereas in S1 state it is from -NH2 group. Monoanion (MA) so formed in S1 state is non-fluorescent. Dianion (DA) is formed by further deprotonating >N-H moiety in S1 state and it is fluorescent. pKa values were determined and discussed.