Negative photochromism of water-soluble pyridine-containing nitro-substituted spiropyrans

Negative photochromism of pyridine-containing nitro-substituted spiropyrans in solutions, solid phase, and polymer layers, as well as in complexes with terbium cations, has been studied by a spectral-kinetic method. It has been shown that the concentration of the merocyanine form and the efficiency of photochromic transformations depend on the length of the substituent at the nitrogen of the indoline moiety. A distinctive feature of the photochromic transformations of these compounds is slow spontaneous relaxation of the colorless photoinduced spiropyran form to the colored merocyanine form, which becomes practically acceptable only at a temperature of 80°C.