PHOTOINDUCED INTERMOLECULAR 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 w ater differed from its behavior in ethanol, and in both solvents fluor escence behavior depended on acidity. In both neutral water and neutra l ethanol, ground-state HBI exhibits conformational equilibrium betwee n a cis-enol form with an intramolecular hydrogen bond and a trans-eno l form that is hydrogen-bonded to the solvent; the ground-state keto t automer is also present in water but was not detected in ethanol. The excited cis-enol conformer always undergoes ultrafast intramolecular p roton 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 aci dic 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.