Optical behavior of substituted 4-(2'-hydroxyphenyl)imidazoles

A set of tetraarylimidazoles bearing a 2-hydroxyphenyl substituent at position 4, as well as their models lacking intramolecular hydrogen bonds, was efficiently synthesized. Structural investigations proved that the hydrogen bond strength for 4-(2'-hydroxyphenyl)imidazoles is weaker than that for analogous 2-(2'-hydroxyphenyl)imidazoles as estimated from dihedral angles and bond distances. Photophysical investigations revealed that these compounds have other properties than those observed for imidazoles bearing a 2-hydroxyphenyl substituent at position 2. They exhibit a negligible fluorescence quantum yield regardless of the solvent polarity. Additionally, dual fluorescence is observed in nonpolar solvents. Plausibly, although a hydrogen bond is present within their chemical structure in the solid state, it is not clear if excited-state intramolecular proton transfer occurs. The presence of OH groups triggers the radiationless deactivation channel if compared with model imidazole possessing a 2-methoxyphenyl group.