Ground and excited state prototropism in 2-(20-methoxyphenyl)- 1H-imidazo[4,5-c]pyridine

Absorption, fluorescence and fluorescence excitation spectra of 2-(20-methoxyphenyl)-1H-imidazole[4,5-c]pyridine (2-MPIP-c) have been studied in different solvents and at different acid-base concentrations. Single photon counting fluorescence spectroscopic measurements were carried out to find out the excited state lifetimes under the above environments. AM1 semi-empirical quantum mechanical, as well as, electronic structure calculations using the Hartree–Fock (HF), and density functional theory (DFT) B3LYP with 6-31G** basis set using GAUSSIAN 98 program were carried out to supplement the experimental data. Spectral data in different solvents and theoretical calculations suggest the presence of only one species (a-1) of 2-MPIP-c in all the solvents. Out of five different possible monocations (MC’s), only one MC (formed by protonating the benzimidazole N3 atom) is present in the aqueous solution, as well as, in different non-aqueous solvents. Nature of first excited singlet state in neutral 2-MPIP-c and its MC are p pp and charge transfer, respectively. Only one dication (formed by protonating both the basic centers, benzimidazole N3 and pyridyl N5) and one kind of monoanion (formed by deprotonating .N–H) are present in the solution. pKa and pKp a for various equilibriums were determined and discussed. q 2003 Elsevier B.V. All rights reserved.