Excited state intramolecular proton transfer in anionic and cationic species of 2-(2′-amino-3-pyridyl)benzimidazole

Excited state intramolecular proton transfer (ESIPT) process has been studied in the monoanionic and monocationic species of 2-(2′-amino-3-pyridyl)benzimidazole (2-A3PyBI) in aqueous and organic solvents. Dual fluorescence is observed from monocationic (aqueous and alcoholic solvents, partly from non-polar and polar aprotic ones) and monoanionic species in aqueous media. Presence of different species has been characterized with the help of absorption, fluorescence excitation and fluorescence spectroscopy, as well as, time resolved fluorimetry. The electronic structure calculations were performed on each species using semi-empirical quantum mechanical AM1 method and density functional theory B3LYP with 6-31G** basis set using Gaussian 98 program to characterize the particular ionic species. Presence of electron withdrawing pyridine N atom and negative charge on the benzimidazole moiety plays the vital role in increasing the rate of ESIPT process.