| Abstract
| - The photoinduced proton-transfer processes of2-(3‘-hydroxy-2‘-pyridyl)benzimidazole in acidic solutionsofacetonitrile, ethanol, and water have been studied by means of UV−visabsorption and fluorescencespectroscopy. In all the solvents considered, the ground-statespecies under acidic conditions is the enolcation, protonated at the benzimidazole N(3). Upon excitation,both the acidity of the hydroxyl group andthe basicity of the pyridyl nitrogen increase, the acid−baseproperties of the solvent determining if protonationor deprotonation processes will be observed. Thus, inacetonitrile, the excited cation does not suffer anytransformation, its fluorescence being observed, whereas in ethanol,some molecules dissociate at the hydroxylgroup to yield the neutral keto form, and emission from both this ketospecies and the cation is observed.Since water can act as both acid and base, the enol cationundergoes in this solvent a two-step tautomerizationby two different routes to yield the keto cation protonated at thepyridyl nitrogen. One pathway involves thedeprotonation of the enol cation at the hydroxyl group to yield theneutral keto tautomer, followed by protonationof this tautomer at the pyridyl nitrogen to afford the keto cation.A second pathway implies the protonationof the enol cation by H2O (and by H+ at highacidity) at the pyridyl nitrogen to give a dication, whichdeprotonates at the hydroxyl group, yielding also the keto cation.Only emission from the neutral keto formand the keto cation has been observed, the ratio between them being pHdependent.
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