Spiropyran

A spiropyran is a type of organic chemical compound, known for photochromic properties that provide this molecule with the ability of being used in medical and technological areas.

Spiropyrans generally could be obtained by boiling the aldehyde and the respective benzazolium salts in presence of pyridine or piperidine.

When the spiropyran is in a solution with polar solvents or when it receives heating (thermochromism) or radiation (photochromism) it becomes coloured because its structure has changed and it has been transformed into the merocyanine 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.

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).

Procedure: Photochromic, thermochromic, solvatochromic and electrochromic characteristics of spiropyrans make them especially important in the technology area.

Photochromic compounds based on spiropyrans, spirooxazines, and [2H]chromenes are being investigated because of their silver-free light-sensitive properties that could be used for optical recording data, including thin films, photoswitches (sensors that discern light of certain wavelength), light filters with modulated transmission and miniature hybrid multifunctional materials.

Another group of spiropyrans which contain indoline or nitrogen heterocycles and the indolinospirothiapyrans found their application in film forms of photochromic materials using polyester resins.

Another collection of spiropyrans characterized for their sensitivity to UV radiation are detectors for the protection of organs, for the production of light filters with modulated transmission, or photochromic lenses.

Figure 1: Formation of spiropyran from their fundamental building blocks
Figure 2: Spiropyran ( 1 ) to merocyanine ( 2 )