Photochromic Color Change of the Dimer of Triphenylimidazolyl at Low Temperatures

Abstract

The photochromic dimer of triphenylimidazolyl which is one of two dimers obtained by oxidation of triphenylimidazole shows photochromism at temperatures between room temperature and −196°C in solutions and in a solid state. At temperatures higher than about −20°C the photochromic color in solutions is reddish-purple, at temperatures lower than about −150°C the color is bluish-purple and at intermediate low temperatures the photochromic color does not appear. Absorption spectrum and ESR measured with the change of temperature showed variations corresponding to the change of color. The photochromic color in a solid state was light reddish-purple above about −70°C and bluish-purple at −196°C. The mechanism of the color change was discussed in connection with that of the photochromism at room temperature and the results obtained from spectroscopic and ESR studies.