Ground and excited state prototropic reactions in 2-(2′-methoxyphenyl)-3H-imidazo[4,5-b]pyridine†

Abstract

The absorption, fluorescence and fluorescence excitation spectra of 2-(2′-methoxyphenyl)-3H-imidazo[4,5-b]pyridine (2-MPIP) have been studied in five different solvents and at different acid–base concentrations. Semi-empirical quantum mechanical calculations have been carried out to supplement the experimental results. Spectral characteristics of 2-MPIP in different solvents and semi-empirical quantum mechanical calculations suggest the presence of both a-1 and b-1 isomers. The observation of a single exponential decay and one lifetime suggests an equilibrium between the isomers. Out of five monocations two kinds of monocations (MC’s) are possible, one from protonation of N¹ (MC1) and the second from protonation of N⁴ (MC2) atoms. Spectral characteristics and semi-empirical calculations carried out on MC’s neglect the presence of isomer b-1. Only one kind of dication and one kind of monoanion are formed by protonating both the nitrogen atoms (N¹ and N⁴) and deprotonating the >NH atom respectively. pK_a and pK_a* values for the various prototropic reactions are determined and discussed.