Remarkable Solvatochromic Color Change via Proton Tautomerism of a Phenol-Linked Imidazole Derivative

A unique solvatochromic 2-phenyl-4,5-diarylimidazole
derivative
linked with a phenol moiety and a p-quinonemethide
moiety at the 4- and 5-positions of a imidazole ring, which shows
remarkable color change via proton tautomerism, was synthesized and
the mechanism of the solvatochromic color change was investigated.
The yellow-colored OH tautomeric form (1_OH) exists as
a dominant species in nonpolar solvents, whereas the blue-colored
NH tautomeric form (1_NH) is stabilized in polar solvents.
The molecular structures of these tautomers were determined by X-ray
crystallographic analysis. The p-quinonemethide moiety
and the imidazole ring of 1_OH are coplanar to one another
and possess a planar quinoidal structure. On the other hand, 1_NH has a nonplanar twisted quinoidal structure causing large
bathochromic shift in the visible absorption spectrum. Moreover, the
X-ray crystallographic analysis and the DFT calculations support the
closed-shell singlet character of 1_OH. In contrast 1_NH possesses partial single bond character that leads to
the open-shell singlet biradical character and the decrease in the
singlet–triplet energy gap. The twisting of the π-conjugated
electron system induced by the proton tautomerization was found to
be the origin of the open-shell biradical character of 1_NH and the enhanced solvatochromic color change.