The ground- and excited-state tautomerism of 2-(6‘-hydroxy-2‘-pyridyl)benzimidazole (1) and 1-methyl-2-(6‘-hydroxy-2‘-pyridyl)benzimidazole (2) in various solvents was investigated by means of UV−vis absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and quantum-mechanical ab initio calculations. A solvent-dependent tautomeric equilibrium was observed for both compounds in the ground state between the lactim or normal form and the lactam tautomer resulting from a proton translocation between the hydroxyl group and the pyridine nitrogen. Here, we report evidences for a solvent-dependent switching in the nature of the excited-state proton transfer (ESPT) reactions undergone by 1 and 2. In the aprotic solvent acetonitrile, no significant ESPT takes place. The protic solvents ethanol and water facilitate the proton transfer through bridges of solvent molecules, but each solvent catalyzes specifically a different ESPT process. In aqueous solution, the excited lactim species of 1 and 2 undergo a proton transfer from the hydroxyl group to the pyridine nitrogen, favoring the lactam tautomer in the first excited singlet state. In ethanol the ESPT takes place for 1 from the benzimidazole NH to the pyridine nitrogen, originating a new tautomer not observed in the ground state. Compound 2, without a NH group, does not tautomerize in the excited state in ethanol.

Ground-state tautomerism and excited-state proton-transfer processes of 2-(6'-hydroxy-2'-pyridyl)benzimidazolium in H2O and D2O have been studied by means of UV-vis absorption and fluorescence spectroscopy in both steady-state and time-resolved modes. In the ground state, this compound shows a tautomeric equilibrium between the lactim cation, protonated at the benzimidazole N3, and its lactam tautomer, obtained by proton translocation from the hydroxyl group to the pyridine nitrogen. Direct excitation of the lactam tautomer leads to its own fluorescence emission, while as a result of the increase of acidity of the OH group and basicity at the pyridine N upon excitation, the lactim species undergoes a proton translocation from the hydroxyl group to the nitrogen, favoring the lactam structure in the excited state. No fluorescence emission from the initially excited lactim species was detected due to the ultrafast rate of the excited-state proton-transfer processes. The lactim-lactam phototaumerization process takes place via two competitive excited-state proton-transfer routes: a one-step water-assisted proton translocation (probably a double proton transfer) and a two-step pathway which involves first the dissociation of the lactim cation to form an emissive intermediate zwitterionic species and then the acid-catalyzed protonation at the pyridine nitrogen to give rise to the lactam tautomer.