Spiropyran To Merocyanine
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A spiropyran is a type of photochromic
organic chemical compound Some chemical authorities define an organic compound as a chemical compound that contains a Carbon–hydrogen bond, carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. F ...
, 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.


History

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.


Synthesis

There are two methods for the production of spiropyrans. The first one can be by
condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...
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 In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...
and the respective benzazolium salts in presence of
pyridine Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...
or
piperidine Piperidine is an organic compound with the molecular formula (CH2)5NH. This heterocyclic amine consists of a six-membered ring containing five methylene bridges (–CH2–) and one amine bridge (–NH–). It is a colorless liquid with an odor de ...
: 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).


Structure

A spiropyran is a 2H-
pyran In chemistry, pyran is a six-membered heterocyclic, non-aromatic ring, consisting of five carbon atoms and one oxygen atom and containing two double bonds. The molecular formula is C5H6O. There are two isomers of pyran that differ by the location ...
isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...
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 A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic organic chemistry is the branch of organic chemistry dealing with the synthesis, proper ...
but there are exceptions. A solution of the spiropyran in polar solvents upon heating (
thermochromism Thermochromism is the property of substances to change color due to a change in temperature. A mood ring is an example of this property used in a consumer product although thermochromism also has more practical uses, such as baby bottles, which c ...
) or radiation (
photochromism Photochromism is the reversible change of color upon exposure to light. It is a transformation of a chemical species (photoswitch) between two forms through the absorption of electromagnetic radiation (photoisomerization), where each form has a d ...
) 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 is arises from electrocyclic cleavage of the C-spiro-O bond.


Photochromism

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 A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic organic chemistry is the branch of organic chemistry dealing with the synthesis, proper ...
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 Polymethines are compounds made up from an ''odd'' number of methine groups (CH) bound together by alternating single and double bonds.Kachovski and Dekhtyar, ''Dyes and Pigments'', 22 (1983) 83-97. Compounds made up from an ''even'' number of met ...
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: * 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 that of the closed spiropyran form (SP). * Unlike the initial solution, the product of the photochromism reaction is not colorless. *Depending on substituent on the aromatic system the switching behavior of the derivatives can change in their switching velocity and photo-fatigue resistance.


Applications

Spiropyrans are widely studied for their photochromic properties, which enable reversible transformations between structurally distinct forms in response to external stimuli such as light, heat, pH, or metal ions. This unique behavior has led to their application in a diverse range of fields. In materials science, spiropyrans are incorporated into smart materials and coatings that respond dynamically to environmental changes, offering potential for use in sensors, actuators, and light-responsive surfaces. In electronics, they serve as molecular switches and components in optical data storage systems due to their reversible and controllable optical properties. Spiropyrans also play a significant role in biomedical research, particularly in the development of light-activated drug delivery systems and biosensors. The versatility and tunability of spiropyran derivatives continue to drive research into their integration in emerging technologies across chemistry, physics, and engineering disciplines. Some more detailed examples of the applications of spiropyrans are listed below: * Some photochromic compounds based on spiropyrans, spirooxazines, and Hhromenes were developed as silver-free light-sensitive properties that could be used for self-darkening
glasses Glasses, also known as eyeglasses (American English), spectacles (Commonwealth English), or colloquially as specs, are vision eyewear with clear or tinted lenses mounted in a frame that holds them in front of a person's eyes, typically u ...
as seen in Figure 3, optical recording data, including thin films,
photoswitch A photoswitch is a type of molecule that can change its structural geometry and chemical properties upon irradiation with electromagnetic radiation. Although often used interchangeably with the term molecular machine, a switch does not perform Work ...
es (sensors that discern light of certain
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
), light filters with modulated transmission and miniature hybrid multifunctional materials. * The creation of novel media sensitive to IR radiation and the potential of spiropyrans for optical recording data has influenced the development of semiconductor lasers as an activating source of radiation. *Spiropyrans with ion complexes and spiropyran
copolymers In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are som ...
which are part of powdered and film materials have been used too to record optical data and increase the length of time of its storage. *Another group of spiropyrans which contain
indoline Indoline is an aromatic heterocyclic organic compound with the chemical formulation C8H9N. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing ring. The compound is based on the ind ...
or nitrogen heterocycles and the indolinospirothiapyrans found their application in film forms of photochromic materials using polyester resins. Those resins with a high refractive index were used to make
photochromic lens A photochromic lens is an optical lens that darkens on exposure to light of sufficiently high frequency, most commonly ultraviolet (UV) radiation. In the absence of activating light, the lenses return to their clear state. Photochromic lenses ma ...
es. Moreover, spiropyrans are being used in cosmetics. *New types of modified spiropyrans polymers contained in photochromic compounds found their use in the creation of photoreceptors. The ones with
rhodopsin Rhodopsin, also known as visual purple, is a protein encoded by the ''RHO'' gene and a G-protein-coupled receptor (GPCR). It is a light-sensitive receptor protein that triggers visual phototransduction in rod cells. Rhodopsin mediates dim ...
as a compound are adopted to raise the level of the photosignal. *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. *The determination of peroxidase activity and NO2 levels in the atmosphere are applications of carboxylated spiropyrans. *Spiropyrans can be used to probe the conformational state of DNA, as certain derivatives can
intercalate Intercalation may refer to: *Intercalation (chemistry), insertion of a molecule (or ion) into layered solids such as graphite *Intercalation (timekeeping), insertion of a leap day, week or month into some calendar years to make the calendar follo ...
into DNA when in the open form. *Spiropyrans are used in photo controlled transfer of amino acids across bilayers and membranes because of nucleophilic interaction between zwitterionic merocyanine and polar amino acids. Certain types of spiropyrans display ring opening upon recognition of an analyte, for example zinc ions. *Today, spiropyrans are most used as molecular logic devices, photochromic and electrooptical devices, molecular and supramolecular logic switches, photoswitches and multifunctional artificial receptors.


Further reading

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References

{{Reflist Spiro compounds Pyrans Photochemistry