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.