ABSTRACT

Hydrogen-atom tautomerism between the enol and keto forms of Schiff bases of salicylaldehyde initiates the phenomena of photochromism and thermochromism in the solid state and in fluid solutions, the generated keto form(s) absorbing at longer wavelength than the enol forms. In addition to the hydrogen transfer, the reaction in the solid state depends on the crystal structure, since it involves structural changes, which lead to the stabilization of the photoproduct. In rigid glasses and in solutions, where the orientation of the molecules is random, all salicylaldehyde Schiff bases exhibit photochromism, thus showing that this property is a characteristic of the molecules. Finally, when salicylaldehyde Schiff bases are used as guests in various restricted environments, they generate new photochromic or thermochromic systems. Depending on the hosts, selective photo- or thermotransformations are taking place, which apart from their fundamental interest, have the potential for various applications.