| Sumario: | Novel photochromic coatings were designed having a remarkable long-term stability of both the coloured state and colourless states in the dark (bistability). The coatings consist of a dispersion of a commercial spirooxazine in a functionalized sol-gel hybrid matrix, acquiring a deep purple coloration upon irradiation with UV-light. In order to stabilize the coloured form of the dye, a zinc salt, capable of forming a chelate with the photochromic molecule when irradiated, was incorporated into the matrix, resulting in a drastic reduction of the thermal bleaching kinetics. The coloured coatings can only be reverted to their original colourless state by irradiation with visible light. This recording-erasing process is reversible and can, therefore, be repeated by alternating irradiation with UV and visible light. Adding the bistable behaviour to the specific properties of reversible photochromic systems makes a new range of applications in the field of optics and light-controlling media available. © 2009 The Royal Society of Chemistry.
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