Competition between protonation and deprotonation in the first excited singlet state of 2-(3'-hydroxy-2'-pyridyl)benzimidazole in acidic solutions

The photoinduced proton-transfer processes of 2-(3'-hydroxy-2'-pyridyl)benzimidazole in acidic solutions of acetonitrile, ethanol, and water have been studied by means of UV-vis absorption and fluorescence spectroscopy. In all the solvents considered, the ground-state species under acidic conditions is the enol cation, protonated at the benzimidazole N(3). Upon excitation, both the acidity of the hydroxyl group and the basicity of the pyridyl nitrogen increase, the acid-base properties of the solvent determining if protonation or deprotonation processes will be observed. Thus, in acetonitrile, the excited cation does not suffer any transformation, its fluorescence being observed, whereas in ethanol, some molecules dissociate at the hydroxyl group to yield the neutral keto form, and emission from both this keto species and the cation is observed. Since water can act as both acid and base, the enol cation undergoes in this solvent a two-step tautomerization by two different routes to yield the keto cation protonated at the pyridyl nitrogen. One pathway involves the deprotonation of the enol cation at the hydroxyl group to yield the neutral keto tautomer, followed by protonation of this tautomer at the pyridyl nitrogen to afford the keto cation. A second pathway implies the protonation of the enol cation by H2O (and by H+ at high acidity) at the pyridyl nitrogen to give a dication, which deprotonates at the hydroxyl group, yielding also the keto cation. Only emission from the neutral keto form and the keto cation has been observed, the ratio between them being pH dependent.