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 were discovered in the early twentieth century. However, it was in the middle twenties when Fisher and Hirshbergin observed their characteristics and reversible reaction. In 1952, Fisher and co-workers announced for the first time photochromism in spiropyrans. Since then, there have been many studies on photochromic compounds that have continued up to the present.

There are two methods for the production of spiropyrans. The first one can be by condensation of methylene bases with o-hydroxy aromatic aldehydes (or the condensation of the precursor of methylene bases). Spiropyrans generally could be obtained by boiling the aldehyde and the respective benzazolium salts in presence of pyridine or piperidine. The general formula of the synthesis of spiropyrans is shown in the Figure 1.

The second way is by condensation of o-hydroxy aromatic aldehydes with the salts of heterocyclic cations which contains active methylene groups and isolation of the intermediate styryl salts. This second procedure is followed by the removal of the elements of the acid from the obtained styryl salt, such as perchloric acid, with organic bases (gaseous ammonia or amines).

A spiropyran is a 2H-pyran isomer that has the hydrogen atom at position two replaced by a second ring system linked to the carbon atom at position two of the pyran molecule in a spiro way . So there is a carbon atom which is common on both rings, the pyran ring and the replaced ring. The second ring, the replaced one, is usually heterocyclic but there are exceptions.

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

The structural differences between spiropyran and merocyanine form is that, while in the first one the ring is in the closed form, in the other one the ring is opened. The photochromism is due to electrocyclic cleavage of the C-spiro-O bond with photoexcitation.

...
Wikipedia