Kinetics of the Photochromic Decay Reaction of Solutions of 2,2′,4,4′,5,5′-Hexaphenyl bi-imidazolyl

Abstract

THE photochromism, in solution and in the solid state, of the compound formed by the oxidation of 2,4,5-tri-phenylimidazole (lophine) using potassium ferricyanide was discovered by Hayashi et al.¹. Thermochromism was also observed in solution and the reddish-purple colour of the solution so formed was attributed to the formation of the radical, 2,4,5-triphenylimidazolyl (L.). After allowing for thermochromism, Hayashi found that the photochromic decay reaction obeyed first order kinetics with an activation energy of 14 kcal/mole, and Ueda² studied the hyperfine splittings of the radical, and confirmed the first order kinetics. Further work³ by Hayashi indicated that the kinetics of the reaction were second order, suggesting that the reaction was a simple bimolecular radical recombination. This communication reports a more detailed examination of the kinetics which shows that the kinetic order is 3/2, falling to first order, which suggests a complex mechanism.