Preparation of merocyanine salts by treatment of acids on spiropyrans and investigation on their photochromic behaviors

Merocyanine salts were prepared and their photochromic behavior investigated. The salts were prepared from a solution of spiropyrans in the presence of acids such as toluenesulfonic acid and dodecylbenzene sulfonic acid under irradiation of UV light and heat. The ¹H NMR spectrum confirmed the chemical structure of the merocyanine salts. The salt showed an absorption peak at 422 nm, which was blue-shifted as compared with the absorption peak at 578 nm of the merocyanine form of spiropyran. The salt form was quite stable and virtually exhibited a negative photochromism. The salt showed better fatigue resistance than the spiropyran. The photoinduced refractive index change for the salt films by a prism coupler method was estimated to be as large as 0.0055.