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 <i>p</i>-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 (<b>1_OH</b>) exists as a dominant species in nonpolar solvents, whereas the blue-colored NH tautomeric form (<b>1_NH</b>) is stabilized in polar solvents. The molecular structures of these tautomers were determined by X-ray crystallographic analysis. The <i>p</i>-quinonemethide moiety and the imidazole ring of <b>1_OH</b> are coplanar to one another and possess a planar quinoidal structure. On the other hand, <b>1_NH</b> 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 <b>1_OH</b>. In contrast <b>1_NH</b> 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 <b>1_NH</b> and the enhanced solvatochromic color change