Photoinduced double proton tautomerism in 4-azabenzimidazole

The proton-transfer tautomerism of 4-azabenzimidazole (4ABI) mediated by hydrogen bonding formation has been studied in the ground as well as in the excited state by means of absorption and emission spectroscopies. Thermodynamics of self-association and hydrogen-bonded complexes in nonpolar solvents were obtained. Proton-transfer isomers of 4ABI have been determined by syntheses and spectral characterization of various 4ABI methyl derivatives. The 4ABI dimer and 1:1 4ABI/acetic acid complex possessing cyclic dual hydrogen bonds undergoes a fast excited-state double proton-transfer reaction, resulting in a proton-transfer tautomer emission. Surprisingly, however, the ESDPT is prohibited in the 4ABI/2-azacyclohexanone cyclic hydrogen-bonded complex. The results render the conclusion that photoinduced double proton transfer in the 4ABI hydrogen-bonded complex can be fine-tuned by its associated guest molecule, as further supported by the molecular modeling as well as ab initio calculations.