Solvent control of molecular structure and excited-state proton-transfer processes of 1-methyl-2-(2′-hydroxyphenyl)- benzimidazole

Abstract

The influence of the solvent on the structure and photoinduced proton-transfer processes of 1-methyl-2- (2′-hydroxyphenyl)benzimidazole (MeHBI) was studied by means of UV–VIS absorption and fluorescence spectroscopy. The steric hindrance of the methyl group determines the non-planar structure of the MeHBI anion and cation in their ground state, but they undergo a rotation towards planarity in their excited state. In hydroxylic solvents, the excited cation loses the hydroxylic proton to the solvent, affording the keto tautomer. The neutral form of MeHBI exhibits conformational equilibrium dependent on the solvent. The cis-enol conformer, with an intramolecular hydrogen bond and a planar structure, is the dominant species in non-hydroxylic solvents and undergoes an excited-state intramolecular proton-transfer (ESIPT) reaction, producing the keto tautomer. The non-planar enol conformer is the only species detected in the ground state in water; in the excited state it loses the hydroxyl proton, leaving the excited anion. Comparable proportions of both enol conformers exist in alcoholic solvents, their relative proportions having been determined. In these solvents the cis-enol form undergoes ESIPT, whereas the non-planar enol rapidly undergoes rotation towards planarity in the excited state, emitting fluorescence from this state. The solvent hydrogen-bond donor acidity determines the ratio of non-planar enol to cis-enol conformers.