Photochromism is the reversible change of color upon exposure to light. It is a transformation of a chemical species (
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 ...
) between two forms through the absorption of
electromagnetic radiation
In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...
(
photoisomerization
In chemistry, photoisomerization is a form of isomerization induced by photoexcitation. Both reversible and irreversible photoisomerizations are known for photoswitchable compounds. The term "photoisomerization" usually, however, refers to a rev ...
), where each form has a different absorption spectrum. This reversible structural or geometric change in photochromic molecules affects their
electronic configuration
In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. For example, the electron configuration of the neon atom ...
, molecular strain energy, and other properties.
History
In 1867,
Carl Julius Fritzsche
Carl Julius Fritzsche (17 October 1808 in Neustadt in Sachsen, Neustadt – 8 June 1871) was a German pharmacist and chemist. He was a nephew of pharmacist Friedrich Adolph August Struve (1781–1840).
After five years spent working at his un ...
reported the concept of photochromism, indicating that orange tetracene solution lost its color in daylight but regained it in darkness. Later, similar behavior was observed by both
Edmund ter Meer and Phipson. Ter Meer documented the color change of the potassium salt of dinitroethane, which appeared red in daylight and yellow in the dark. Phipson also recorded that a painted gatepost appeared black during the day and white at night due to a zinc pigment, likely lithopone.
In 1899,
Willy Markwald, who studied the reversible color change of 2,3,4,4-tetrachloronaphthalen-1(4H)-one in the solid state, named this phenomenon “phototropy”. However, this term was later considered misleading due to its association with the biological process “
phototropism
In biology, phototropism is the growth of an organism in response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the ligh ...
”. In 1950, Yehuda Hirshberg (from the
Weizmann Institute of Science
The Weizmann Institute of Science ( ''Machon Weizmann LeMada'') is a Public university, public research university in Rehovot, Israel, established in 1934, fourteen years before the State of Israel was founded. Unlike other List of Israeli uni ...
in Israel) proposed the term “photochromism,” derived from the Greek words ''phos'' (light) and ''chroma'' (color), which remains widely used today.
The phenomenon extends beyond colored compounds, encompassing systems that absorb light across a broad spectrum, from ultraviolet to infrared, and includes both rapid and slow reactions.
Photochromism can take place in both organic and inorganic compounds, and also has its place in biological systems (for example
retinal
Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision).
Some microorganisms use ret ...
in the vision process). The use of photochromic materials has evolved beyond protective eyewear to applications including
3D optical data storage
3D optical data storage is any form of optical data storage in which information can be recorded or read with three-dimensional resolution (as opposed to the two-dimensional resolution afforded, for example, by CD).
This innovation has the p ...
,
photocatalysis
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a photocatalyst, the excited state of which "repeatedly interacts with the reaction partners forming reaction intermediates and regenerates itself after each ...
, and radiation
dosimetry
Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingest ...
.
Principles
:

Photochromism often is associated with
pericyclic reactions
In organic chemistry, a pericyclic reaction is the type of organic reaction wherein the transition state of the molecule has a cyclic geometry, the reaction progresses in a concerted reaction, concerted fashion, and the Localized molecular orbita ...
, cis-trans
isomerizations, intramolecular hydrogen transfer, intramolecular group transfers,
dissociation processes and
electron transfers (
oxidation-reduction).
Transition metal complexes can also display photochromic properties due to
linkage isomerizations.
Important properties of photochromic compounds include
quantum yield
In particle physics, the quantum yield (denoted ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system.
\Phi(\lambda)=\frac
Applications
Fluorescence spectroscopy
The fluorescence ...
, fatigue resistance, and the lifetime of the
photostationary state
The photostationary state of a reversible photochemical reaction is the equilibrium chemical composition under a specific kind of electromagnetic irradiation (usually a single wavelength of visible or UV radiation).
It is a property of particu ...
(PSS). The quantum yield of the photochemical reaction determines the efficiency of the photochromic change relative to the amount of light absorbed.
In photochromic materials, the loss of photochromic component is referred to as fatigue, and it is observed by processes such as
photodegradation
Photodegradation is the alteration of materials by light. Commonly, the term is used loosely to refer to the combined action of sunlight and air, which cause oxidation and hydrolysis. Often photodegradation is intentionally avoided, since it dest ...
,
photobleaching
In optics, photobleaching (sometimes termed fading) is the photochemical alteration of a dye or a fluorophore molecule such that it is permanently unable to fluoresce. This is caused by cleaving of covalent bonds or non-specific reactions between ...
,
photooxidation, and other side reactions. All photochromic compounds suffer from fatigue to some extent, and its rate is strongly dependent on the activating light and the sample conditions.
Photochromic materials have two states, and their interconversion can be controlled using different wavelengths of light. Excitation with any given wavelength of light will result in a mixture of the two states at a particular ratio, called the photostationary state. In a perfect system, there would exist wavelengths that can be used to provide 1:0 and 0:1 ratios of the isomers, but in real systems this is not possible, since the active absorbance bands always overlap to some extent.
Photochromic systems rely on irradiation to induce the isomerization. Some rely on irradiation for the reverse reaction, others use thermal activation for the reverse reaction.
Classes of photochromic materials
Molecular photoswitches
Photochromic quinones
Some quinones, and phenoxynaphthacene quinone in particular, have photochromicity resulting from the ability of the
phenyl
In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula , and is often represented by the symbol Ph (archaically φ) or Ø. The phenyl group is closely related to benzene and can be viewed as a benzene ...
group to migrate from one oxygen atom to another. Quinones with good thermal stability have been prepared, and they also have the additional feature of redox activity, leading to the construction of many-state molecular switches that operate by a mixture of photonic and electronic stimuli.
Inorganic photochromic materials
Many inorganic substances also exhibit photochromic properties, often with much better resistance to fatigue than organic photochromics. In particular,
silver chloride
Silver chloride is an inorganic chemical compound with the chemical formula Ag Cl. This white crystalline solid is well known for its low solubility in water and its sensitivity to light. Upon illumination or heating, silver chloride converts ...
is extensively used in the manufacture of
photochromic lenses. Other
silver
Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...
and
zinc
Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...
halides
In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fluo ...
are also photochromic. Yttrium oxyhydride is another inorganic material with photochromic properties.
Some inorganic photochromic materials include
oxides
An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation state o ...
such as BaMgSiO
4, Na
8 4">lSiO4sub>6Cl
2, and KSr
2Nb
5O
15. Additionally, rare-earth (RE)-doped compounds like CaF
2:Ce, CaF
2:Gd, as well as transition metal oxides such as WO
3, TiO
2, V
2O
5, and Nb
2O
5 have been explored.
Photochromism in transition metal oxides is generally attributed to the redox reactions of the transition metal ion and the resulting electron transfer between its different valence states. When electrons are excited from the valence band to the conduction band, a hole is generated in the valence band. This photo-induced hole can decompose adsorbed water on the material’s surface, producing protons. These protons can react with transition metal ions in different valence states, forming hydrogen-based compounds that exhibit color changes. Upon exposure to light of a different wavelength or an oxidizing atmosphere, the reduced transition metal ion can undergo re-oxidation.
Various forms of
tungsten trioxide
Tungsten(VI) oxide, also known as tungsten trioxide is a chemical compound of oxygen and the transition metal tungsten, with formula WO3. The compound is also called tungstic anhydride, reflecting its relation to tungstic acid . It is a light ...
(WO
3), including bulk crystals, thin films, and quantum dots, have been studied for their photochromic properties. WO
3 transitions between two optical states, shifting from transparent to blue when exposed to light, heat, or electricity. The reversible color change is associated with the tungsten center's ability to undergo oxidation-reduction reactions, alternating between different oxidation states (W⁶⁺ to W⁵⁺ or W⁵⁺ to W⁴⁺).
Molybdenum trioxide
Molybdenum trioxide describes a family of inorganic compounds with the formula MoO3(H2O)n where n = 0, 1, 2. The anhydrous compound is produced on the largest scale of any molybdenum compound since it is the main intermediate produced when molybd ...
(MoO
3) is widely used in UV sensing applications due to its selective absorption of UV light. Upon UV exposure, MoO
3 undergoes a photochromic transformation, which can be reversed in the presence of an oxidizing agent. MoO
3 nanosheets exhibit a stronger photochromic effect than the bulk materials due to enhanced carrier mobility and structural flexibility.
Photochromic coordination compounds
Photochromic coordination complexes are relatively rare compared to the organic compounds listed above. There are two major classes of photochromic coordination compounds: those based on
sodium nitroprusside
Sodium nitroprusside (SNP), sold under the brand name Nitropress among others, is a medication used to lower blood pressure. This may be done if the blood pressure is very high and resulting in symptoms, in certain types of heart failure, and ...
and the ruthenium sulfoxide compounds. The ruthenium sulfoxide complexes were created and developed by Rack and coworkers.
The mode of action is an excited-state isomerization of a sulfoxide ligand on a ruthenium polypyridine fragment from S to O or O to S. The difference in bonding between Ru and S or O leads to the dramatic color change and change in Ru(III/II) reduction potential. The
ground state
The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state ...
is always S-bonded, and the metastable state is always O-bonded. Typically, absorption maxima changes of nearly 100 nm are observed. The metastable states (O-bonded isomers) of this class often revert thermally to their respective ground states (S-bonded isomers), although a number of examples exhibit two-color reversible photochromism. Ultrafast spectroscopy of these compounds has revealed exceptionally fast isomerization lifetimes ranging from 1.5 nanoseconds to 48 picoseconds.
Applications: sunglasses and related materials
Reversible photochromism is the basis of
color changing lenses for
sunglasses
Sunglasses or sun glasses (informally called shades or sunnies; more names Sunglasses#Other names, below) are a form of Eye protection, protective eyewear designed primarily to prevent bright sunlight and high-energy visible light from damagin ...
. The largest limitation in using photochromic technology is that the materials cannot be made stable enough to withstand thousands of hours of outdoor exposure so long-term outdoor applications are not appropriate at this time.
The switching speed of photochromic dyes is highly sensitive to the rigidity of the environment around the dye. As a result, they switch most rapidly in solution and slowest in the rigid environment like a polymer lens. In 2005 it was reported that attaching flexible polymers with low
glass transition temperature
The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rub ...
(for example
siloxanes
In organosilicon chemistry, a siloxane is an organic compound containing a functional group of two silicon atoms bound to an oxygen atom: . The parent siloxanes include the oligomeric and polymeric hydrides with the formulae and . Siloxanes ...
or polybutyl acrylate) to the dyes allows them to switch much more rapidly in a rigid lens. Some spirooxazines with siloxane polymers attached switch at near solution-like speeds even though they are in a rigid lens matrix.
Aspirational applications
Data storage
Photochromic compounds for
data storage
Data storage is the recording (storing) of information (data) in a storage medium. Handwriting, phonographic recording, magnetic tape, and optical discs are all examples of storage media. Biological molecules such as RNA and DNA are con ...
has long been a topic of speculation. The area of
3D optical data storage
3D optical data storage is any form of optical data storage in which information can be recorded or read with three-dimensional resolution (as opposed to the two-dimensional resolution afforded, for example, by CD).
This innovation has the p ...
promises discs that can hold a
terabyte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable un ...
of data.
Solar energy storage
Photochromism is a potential mechanism to store solar energy. The photochromic dihydroazulene–vinylheptafulvene system is a proof-of-concept.
See also
*
Photosensitive glass
Photosensitive glass, also called photostructurable glass (PSG) or photomachinable glass, is a glass in the lithium-silicate family of glasses onto which images can be etched using shortwave radiations, such as ultraviolet. Photosensitive glass ...
*
Hexaarylbiimidazole
References
{{Reflist, 30em
Photochemistry
Chromism
Minerals