3,6-ビス(4,5-ジフェニル-2H-イミダゾール-2-イリデン)-1,4-シクロヘキサジエンのキノイド-ビラジカル互変異性

The mechanistic studies of photochramism, thermochramism and piezochromism of the oxidation products of 2, 4, 5-triarylimidazoles and 2-arylphenanthro[9, 10-d]imidazoles revealed that these reversible color changes were due to dissociation of the oxidation products to radicals. In connection with the investigation, the structure and properties of the oxidation product of1, 4-bis(4, 5-diphenylimidazoly1-2)benzene[1], were studied.By oxidation of[1]with lead dioxlde in benzene or oxidation of a potassium salt of the compound with finely ground powders of potassium ferricyanide in dioxane, an oxidation product, 3, 6-bis(4, 5-diphenyl-2H-imidazol-2-ylidene)-1, 4-cyclohexadiene[2]was obtained as deep greenish-blue fine prisms with a metallic luster. From studies with IR, UV, NMR spectra of the compound and the chemical reactions, it is concluded that[2] assumes predominantly the quinoid structure[2a] at room temperature in both solid and solutions and reversibly isomerizes to the biradical structure[2b]at elevated temperature.The concentration of the biradical structure[2b]in a solid state at 200° C was estimated to be about 0.1% from the spin concentration. Failure to detect ESR signals attributable to a triplet state appears to independence of two unpaired electrons in the biradical structure.