Photoinduced inter- and intramolecular proton transfer in aqueous and ethanolic solutions of 2-(2'-hydroxyphenyl)benzimidazole: Evidence for tautomeric and conformational equilibria in the ground state

Excited-state proton transfer in aqueous and ethanolic solutions of 2-(2'-hydroxyphenyl)benzimidazole (HBI) was investigated by means of UV-vis absorption and fluorescence spectroscopy. The behavior of HBI in water differed from its behavior in ethanol, and in both solvents fluorescence behavior depended on acidity. In both neutral water and neutral ethanol, ground-state HBI exhibits conformational equilibrium between a cis-enol form with an intramolecular hydrogen bond and a trans-enol form that is hydrogen-bonded to the solvent; the ground-state keto tautomer is also present in water but was not detected in ethanol. The excited cis-enol conformer always undergoes ultrafast intramolecular proton transfer to afford the excited keto tautomer. The excited trans-enol form fluoresces in both solvents, and in water it also loses its hydroxyl proton to the solvent, leaving the excited anion. In both acidic aqueous solution and acidic ethanol, excited protonated HBI loses its hydroxyl proton to give the excited keto form, but this process is faster in water than in ethanol, in which fluorescence by the cation is also observed.