Spectroscopic Tracking of Salicylideneaniline Photocolored Crystals: An Attempt to Quantify Polymorph-Dependent...

Three polymorphs of salicylideneaniline (SA) were prepared, and their photochromic behavior was examined using a recently developed single-crystal microscopic UV-vis spectroscopy approach. This system enabled us to acquire absorption data during the bleaching process as a function of temperature and visible light intensity. First, we demonstrated that, in contrast to the generally accepted assumption, the bleaching curves were notably influenced by the degree of photosaturation at the initial stage. By modifying our kinetic model to include the term representing the initial degree of photosaturation, we successfully obtained the kinetic parameters intrinsic to each crystal structure. Second, we further analyzed the kinetic parameters to show that the bleaching process was accelerated by visible light irradiation to a significantly higher degree than by thermal relaxation. The two bleaching-prompting effects were quantitatively compared between two photochromic polymorphs, α₁ and α₂; the long life of the photoproduct in α₂ was attributed to efficient self-shielding from visible light irradiation enabled by its structural features. These results prompted us to reexamine the simple dualistic photochemical and thermal classification of photochromic systems and will provide a foundation for the precise structure-function analysis of crystalline materials, including SAs.