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Spiropyran
A spiropyran is a type of photochromic organic chemical compound, characterized by their ability to reversibly switch between two structural forms—spiropyran and merocyanine—upon exposure to light or other external stimuli. This reversible transformation alters their optical and electronic properties, making them valuable in various applications, including molecular switches, optical data storage, sensors, and smart materials.
Spiropyrans were discovered in the early twentieth century, but it was not until 1952 that their photochromic properties were formally documented by chemists Fischer and Gerhard Hirshberg. Their pioneering work demonstrated that spiropyrans undergo reversible structural and color changes when exposed to ultraviolet light, a phenomenon that sparked widespread interest in photoresponsive organic compounds. Throughout the latter half of the twentieth century, advancements in synthetic methods enabled the development of a wide range of spiropyran derivatives with enhanced stability and responsiveness. By the 1990s and 2000s, the integration of spiropyrans into polymers, nanomaterials, and biological systems had established them as key components in emerging technologies such as molecular electronics, smart coatings, and environmental sensors. Today, spiropyrans continue to be actively investigated for their potential in dynamic and multifunctional materials.
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:
A second route involves 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.
A solution of the spiropyran in polar solvents upon heating (thermochromism) or radiation (photochromism) becomes coloured owing to formation of the merocyanine isomer. 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 arises from electrocyclic cleavage of the C-spiro-O bond.
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 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. 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 (SP). Procedure:
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Spiropyran
A spiropyran is a type of photochromic organic chemical compound, characterized by their ability to reversibly switch between two structural forms—spiropyran and merocyanine—upon exposure to light or other external stimuli. This reversible transformation alters their optical and electronic properties, making them valuable in various applications, including molecular switches, optical data storage, sensors, and smart materials.
Spiropyrans were discovered in the early twentieth century, but it was not until 1952 that their photochromic properties were formally documented by chemists Fischer and Gerhard Hirshberg. Their pioneering work demonstrated that spiropyrans undergo reversible structural and color changes when exposed to ultraviolet light, a phenomenon that sparked widespread interest in photoresponsive organic compounds. Throughout the latter half of the twentieth century, advancements in synthetic methods enabled the development of a wide range of spiropyran derivatives with enhanced stability and responsiveness. By the 1990s and 2000s, the integration of spiropyrans into polymers, nanomaterials, and biological systems had established them as key components in emerging technologies such as molecular electronics, smart coatings, and environmental sensors. Today, spiropyrans continue to be actively investigated for their potential in dynamic and multifunctional materials.
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:
A second route involves 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.
A solution of the spiropyran in polar solvents upon heating (thermochromism) or radiation (photochromism) becomes coloured owing to formation of the merocyanine isomer. 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 arises from electrocyclic cleavage of the C-spiro-O bond.
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 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. 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 (SP). Procedure: