Hexaarylbiimidazolyl: The Choice Initiator for Photopolymers. History of Synthesis and Investigation of Photochemical Properties

In 1957, the late Prof. Hayashi and I began to investigate the chemiluminescence mechanism of lophine (2,4,5-triphenylimidazole), which was the first discrete chemiluminescent organic compound, described by Radziszewski1 in 1877. Because the chemiluminescence intensity of lophine in KOH-ethanol solution with oxidants was not very strong, the chemiluminescence reaction was carried out in a dark room. When an aqueous solution of K3[Fe(CN)6] was added dropwise to the alkaline solution of lophine as an oxidant, a circle of light appeared on the surface of the reaction mixture encircling the area in which the oxidant solution was added, and by stirring the reaction mixture a yellowish light was emitted from the solution. When the chemiluminescence reaction was carried out in daylight, no chemiluminescence light emission was observed, and a purple-colored circle appeared on the surface of the solution encircling the area where the oxidant solution was added. Further stirring of the solution caused the color to disappear and the solution became yellow. We thought that the purple substance should be an intermediate in the chemiluminescent reaction of lophine, and wanted to isolate the purple-colored substance. In further studies to determine the origin of the quickly fading purple color, a significant amount of aqueous solution of K3[Fe(CN)6] was added to the reaction mixture. The color of the solution became purple, and a pale purple precipitate separated out.The precipitate was filtered out and repeatedly washed with water until the washing showed no reaction of Fe(CN) 6 4- ion.