The Mechanism of Photochromism, Thermochromism and Piezochromism of Dimers of Triarylimidazolyl

Abstract A stable free radical, 2,4,5-triphenylimidazolyl, which was produced by oxidation of 2,4,5-triphenylimidazole, dimerizes to yield two dimers, a photochromic dimer and a piezochromic dimer. The photochromic dimer shows thermochromism and photochromism at temperatures between −196°C and about 200°C both in solution and in solid state. The piezochromic dimer shows piezochromism in solid state and thermochromism both in solution and in solid state. Spectro-scopic and kinetic studies showed that photochromism, thermochromism and piezochromism are caused by radical dissociation of the dimers of triphenylimidazolyl on irradiation, heating and grinding. Dimers of substituted triphenylimidazolyl also showed these phenomena which are due to the same mechanism as that of the dimers of triphenylimidazolyl.