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 (l_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, l_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 l_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 pi-conjugated electron system induced by the proton tautomerization was found to be the origin of the open-shell biradical character of l_NH and the enhanced solvatochromic color change.