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.