Solvent-Modulated Ground-State Rotamerism and Tautomerism and Excited-State Proton-Transfer Processes ino-Hydroxynaphthylbenzimidazoles

The ground-state rotamerism and tautomerism and the excited-state proton-transfer processes of 2-(1'-hydroxy2'-naphthyl)benzimidazole (1) and 2-(3'-hydroxy-2'-naphthyl)benzimidazole (2) have been investigated in various solvents by means of UV-vis absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and quantum-mechanical ab initio calculations. For both compounds, a solvent-modulated rotameric equilibrium, and also tautomeric for 1, was observed in the ground state. In apolar solvents, both 1 and 2 exist as planar syn normal forms, with the hydroxyl group hydrogen bonded to the benzimidazole N3. In acetonitrile and ethanol, a rotameric equilibrium is established between the syn form and its planar anti rotamer, with the phenyl ring rotated 180 degrees about the C2-C2' bond. In ethylene glycol, glycerol, and aqueous solution with 40% ethanol, a tautomeric equilibrium was detected for 1 between the syn and anti normal forms and the tautomer form, with the hydroxyl proton transferred to the benzimidazole N3. In all of the solvents studied, the syn normal form of 1 and 2 undergoes an ultrafast excited-state intramolecular proton transfer (ESIPT) to yield the excited tautomer. The anti normal forms of 1 and 2, unable to experience ESIPT, give normal form fluorescence. In addition, the anti normal conformer of 2 partly deprotonates at the hydroxyl group in aqueous solution with 40% ethanol, giving the excited anion. The monocations of 1 and 2, protonated at the benzimidazole N3, are strong photoacids that deprotonate completely in aqueous solution with 40% ethanol and to a great extent in ethanol, giving the excited tautomer.