Conformational effects on the photoinduced proton-transfer processes in 1-methyl-2-(3′-hydroxy-2′-pyridyl)benzimidazole

Abstract

The ground- and excited-state behaviour of 1-methyl-2-(3′-hydroxy-2′-pyridyl)benzimidazole (MeHPyBI) in various solvents has been studied by UV–Vis absorption spectroscopy and by time-resolved and steady-state fluorescence spectroscopy. Three species were detected in the ground state in aqueous solutions of pH near to neutrality: a non-planar enol form, a planar cis-enol form and a planar keto tautomer. Electronic excitation of both the cis-enol form and the keto tautomer yields the excited keto form, the former through an ultrafast excited-state intramolecular proton-transfer (ESIPT). The non-planar enol form cannot undergo ESIPT and partly deprotonates at the hydroxy group to afford the anion. In non-aqueous solvents only the cis-enol form was detected in the ground state, yielding the keto tautomer in the excited state. The first protonation of MeHPyBI takes place at the benzimidazole N(3). Whereas the photoexcited cation shows fluorescence in acidified acetonitrile, it deprotonates at the hydroxy group in acidified ethanol, yielding the keto tautomer. In acidic aqueous solution, the excited cation undergoes a two-step tautomerization by two different routes to afford a keto cation, protonated at the pyridyl N. All the protonated and deprotonated forms of MeHPyBI are non-planar in the ground state both in water and ethanol, but they adopt a planar structure in the excited state. Comparison of the behaviour of MeHPyBI with related molecules reveals that the existence of planar forms and the neutral keto species in the ground state is favoured by the presence of the pyridyl nitrogen. The conformational equilibria in the ground state are crucial for the excited-state behaviour of these types of molecules.

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