Spectral characteristics of various prototropic species of 2-(3′-aminophenyl)pyrido[3,4-d]imidazole

Spectral characteristics of the prototropic species of 2-(3′-aminophenyl)pyrido [3,4-d]imidazole (3-APPI) have been studied in three non-aqueous and aqueous solvents, using absorption, fluorescence excitation and fluorescence spectroscopy, as well as, time dependent spectrofluorimetry. Acid concentration in ether is controlled by trifluoroacetic acid (TFA), whereas in other three solvents H2SO4 is used. Semi-empirical quantum mechanical calculations (AM1) and density functional theory (DFT) calculations have been carried out on all kinds of neutral and ionic species. Combining the experimental and theoretical results, the spectral characteristics have been assigned to various cationic and monoanionic species. It has been shown that: (i) only one kind of monoanion (MA) is formed by the deprotonation of NH moiety and one kind of trication (TC), (ii) two kinds of monocations (MC1 and MC3) in the ground (S0) and first excited singlet (S1) states in polar aprotic and protic solvents, whereas only one monocation (MC3) is formed in ether + TFA system. pKa values for the various prototropic reactions were determined in S0 and S1 states and discussed.