Prototropism in 2-acetyl benzimidazole and 2-benzoyl benzimidazole

2-Acetyl benzimidazole (2ABI) and 2-benzoyl benzimidazole (2BBI) are found to show excited state prototropic activities in different solvents of varying pH. The absorption, steady state and time-resolved emission studies were made on the basis of a theoretical possibility study of proton transfer in the excited state. The existence of closed ring conformers for both the molecules in the ground state give rise to different emission bands upon excitation in hydroxylic, water and hydrocarbon solvents. Ground state hydrogen abstraction was found with addition of base. Excitation of the closed conformer leads to a band due to excited state intramolecular proton transfer (excited state Zwitterion) across the preexisting intramolecular hydrogen bond in hydrocarbon solvents and an additional intermolecular anionic band only in the hydroxylic solvents. On comparison, the intermolecular proton transfer is more active in 2BBI than 2ABI. An increase or decrease in basicity results in enhancement of emission from ionic conformer with parallel shrinking of the intramolecular proton transfer species.