text: Photochromism is the phenomenon that produces a change of colour in a substance by incident radiation. In other words, Photochromism is a light-induced change of colour of a chemical substance. The spiropyrans are one of the photochromatic molecules that have raised more interest lately. These molecules consist of two heterocyclic functional groups in orthogonal planes bound by a carbon atom. Spiropyrans are one of the oldest families of photochromism. As solids, the spiropyrans do not present photochromism. It is possible in solution and in the dry state that radiation between 250 nm and 380 nm (approximately) is able to, by breaking the C-O binding, transform the spiropyrans into its colour emitting merocyanin-form. The structure of the colourless molecules, the substrate of the reaction (N), is more thermodynamically stable than the product – depending on the solvent in which it is stored. For example in NMP the equilibrium could be switched more toward the merocyanin form (solvatochromic effects). The photoisomers of the spiropyrans have a structure similar to cyanines, even though it is not symmetric about the center of the polymethine chain, and it is classified as a merocyanine (Figure 2).
  Once the irradiation has stopped, the merocyanine in solution starts to discolour and to revert to its original form, the spiropyran (N).
  Procedure:
  * Irradiation of spiropyrans in solution with UV light of wavelength 250–380 nm breaks C-O bonds.
  * Consequently, the structure of the initial molecule changes, the resulting one being merocyanine (MC). Because of the apparent conjugated system after UV illumination the extinction coefficient of the MC-form is significantly higher than the one of the closed spiropyran form.
  * Unlike the initial solution, the product of the photochromism reaction is not colourless.
  *Depending on substituent on the aromatic system the switching behaviour of the derivatives can change in their switching velocity and photo-fatigue resistance.