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Photochromism
Photochromism
Photochromism
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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 different absorption spectrum.[1][2] This reversible structural or geometric change in photochromic molecules affects their electronic configuration, molecular strain energy, and other properties.[3]

History

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In 1867, Carl Julius Fritzsche 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[4] and Phipson.[5] 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.[6][7] 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".[8] However, this term was later considered misleading due to its association with the biological process "phototropism". In 1950, Yehuda Hirshberg (from the Weizmann Institute of Science in Israel) proposed the term "photochromism", derived from the Greek words phos (light) and chroma (color), which remains widely used today.[6] 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.[6] Photochromism can take place in both organic and inorganic compounds, and also has its place in biological systems (for example retinal in the vision process). The use of photochromic materials has evolved beyond protective eyewear to applications including 3D optical data storage, photocatalysis, and radiation dosimetry.[7]

Principles

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Photoisomerization of [Co(NH3)5NO2]2+.Red-colored isomer (left) converts to the yellow isomer (right) upon UV irradiation.

Photochromism often is associated with pericyclic reactions, cis-trans isomerizations, intramolecular hydrogen transfer, intramolecular group transfers, dissociation processes and electron transfers (oxidation-reduction).[6] Transition metal complexes can also display photochromic properties due to linkage isomerizations.[9][10][11][12]

Important properties of photochromic compounds include quantum yield, fatigue resistance, and the lifetime of the photostationary state (PSS). The quantum yield of the photochemical reaction determines the efficiency of the photochromic change relative to the amount of light absorbed.[13] In photochromic materials, the loss of photochromic component is referred to as fatigue, and it is observed by processes such as photodegradation, photobleaching, 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.[6] 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.[13]

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.[14]

Classes of photochromic materials

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Molecular photoswitches

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Azobenzene The photochromic trans-cis (E/Z) isomerization of azobenzenes has been used extensively in molecular switches. Upon isomerization, azobenzenes experience changes in physical properties, such as molecular geometry, absorption spectra, or dipole moment.[15][16]

Azobenzene groups incorporated into crown ethers give switchable receptors and azobenzenes.[17]

Diarylethenes Diarylethenes undergo a fully reversible transformation between "ring-open" and "ring-closed" isomeric forms when exposed to light of suitable wavelength.[18] Diarylethene-based photoswitches exhibit high photofatigue resistance, enabling them to undergo many photoswitching cycles with minimal degradation.[19] These compounds have been evaluate for long-lasting photochemical memory devices due to the thermal stability of both photoforms of diarylethenes.[18]
Spiropyrans and spirooxazines Spiropyrans, among the oldest photochromic compounds, are closely related to spirooxazines. Irradiation with UV light induce ring-opening, forming a colorful isomer. When the UV source is removed, the chromophore gradually relax to their colorless ground state, the carbon-oxygen bond reforms, and the molecule. This class of photochromes, in particular, is thermodynamically unstable in one form and revert to the stable form in the dark unless cooled to low temperatures.[20][21][22]
Fulgides and Fulgimides Similar to diarylethenes, the photochromic behavior of fulgides and fulgimides is based on 6π-electrocyclic ring-opening and ring-closing reactions.[23] They are highly photochromic photoswitches and reversibly interconvert between two isomeric forms when exposed to light of different wavelengths.[23][24] These compounds exhibit low photochemical fatigue, high thermal stability, as well as high conversion yields.[25][26]
Hydrazones Hydrazone photoswitches can be activated by light and undergo efficient and reversible E/Z isomerization around the C=N double bond.[27][28]
Naphthopyrans Certain naphthopyrans, such as 3,3-diphenyl-3H-naphthopyran, convert from their colorless form to a colored isomer via a ring-opening process. Such materials are used in self-darkening glasses. frameless 345x345px
Azoheteroarenes Azoheteroarenes, structural analogues of azobenzene, are photoswitches capable of reversible E–Z photoisomerization. In these compounds, one or both phenyl rings of azobenzene are replaced by a heterocycle, while maintaining similar structural and mechanistic properties.[29][30] Like azobenzenes, their thermal isomerization follows three main pathways: inversion, rotation, or tautomerization. Typically, the Z-isomer of azoheteroarenes exists as the metastable state.[31] The incorporation of heteroatoms into the ring system enhances functionality as well as improves bioisosterism, polarity, lipophilicity, and solubility, making azoheteroarenes promising alternatives to azobenzenes.[32]

Photochromic quinones

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Some quinones, and phenoxynaphthacene quinone in particular, have photochromicity resulting from the ability of the phenyl 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.[33]

Inorganic photochromic materials

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Many inorganic substances also exhibit photochromic properties, often with much better resistance to fatigue than organic photochromics. In particular, silver chloride is extensively used in the manufacture of photochromic lenses. Other silver and zinc halides are also photochromic. Yttrium oxyhydride is another inorganic material with photochromic properties.[34]

Some inorganic photochromic materials include oxides such as BaMgSiO4, Na8[AlSiO4]6Cl2, and KSr2Nb5O15. Additionally, rare-earth (RE)-doped compounds like CaF2:Ce, CaF2:Gd, as well as transition metal oxides such as WO3, TiO2, V2O5, and Nb2O5 have been explored.[7] 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.[7]

Various forms of tungsten trioxide (WO3), including bulk crystals, thin films, and quantum dots, have been studied for their photochromic properties. WO3 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+).[35][36]

Molybdenum trioxide (MoO3) is widely used in UV sensing applications due to its selective absorption of UV light. Upon UV exposure, MoO3 undergoes a photochromic transformation, which can be reversed in the presence of an oxidizing agent. MoO3 nanosheets exhibit a stronger photochromic effect than the bulk materials due to enhanced carrier mobility and structural flexibility.[37][38]

Photochromic coordination compounds

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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 and the ruthenium sulfoxide compounds. The ruthenium sulfoxide complexes were created and developed by Rack and coworkers.[11][12] 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 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.[12]

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A photochromic eyeglass lens, after exposure to sunlight while part of the lens remained covered by paper.

Reversible photochromism is the basis of color changing lenses for sunglasses. 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.[39] In 2005 it was reported that attaching flexible polymers with low glass transition temperature (for example 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.[40]

Aspirational applications

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Data storage

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Photochromic compounds for data storage has long been a topic of speculation.[41] The area of 3D optical data storage promises discs that can hold a terabyte of data.[42]

Solar energy storage

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Photochromism is a potential mechanism to store solar energy. The photochromic dihydroazulene–vinylheptafulvene system is a proof-of-concept.[43]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Photochromism

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Photochromism is the reversible transformation of a between two forms, A and B, having different absorption spectra, induced in at least one direction by the absorption of . This light-induced change typically alters the color or of materials, enabling applications in adaptive technologies. The phenomenon was first observed in by Carl Julius Fritzsche, who noted the decoloration of under daylight exposure. The term "photochromism" was coined in 1950 by Yehuda Hirshberg during studies on bianthrone derivatives, marking the formal recognition of the field. Early research focused on organic compounds, but inorganic materials soon gained attention for their superior stability and fatigue resistance compared to organics, which often suffer from thermal reversion or degradation. Photochromic systems are classified by their reversion mechanism: (thermal bleaching back to the original state) and P-type (requiring light for reversion), as well as by polarity—positive (colorless to colored) or negative (colored to colorless). Common organic classes include spiropyrans, diarylethenes, fulgides, azobenzenes, and overcrowded alkenes, while inorganic examples encompass oxides like WO₃ and silver halides used in commercial lenses. Mechanistically, photochromism involves photochemical reactions such as , bond cleavage, or charge transfer, often proceeding through excited states and conical intersections with quantum yields ranging from 0 to 1. These processes allow for dynamic control over properties beyond color, including , conductivity, and molecular shape. Notable advancements include the 2016 , awarded to Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Bernard L. Feringa for work on incorporating photochromic elements like overcrowded alkenes as chiroptical switches. Applications of photochromic materials span , , and , with commercial successes like photochromic eyeglass lenses containing silver halides that darken in sunlight, generating billions in annual sales. Emerging uses include optical (e.g., fulgimide-based devices), smart windows for energy-efficient buildings, photopharmacology for light-activated drugs (such as azobenzene-linked antibiotics), sensors, and for and gas adsorption. Recent developments in polymer-integrated systems enhance durability and scalability, promising broader adoption in wearable tech and adaptive displays.

Historical Development

Early Discoveries

The first documented observation of photochromism occurred in 1867 when Johann Fritzsche noted the decoloration of an orange solution under daylight exposure, with color recovery in the dark. This reversible bleaching marked an early recognition of light-induced optical changes in organic materials. In 1899, Willy Markwald demonstrated reversible color changes—termed "phototropy"—in solid-state organic compounds, including 2,3,4,4-tetrachloro-4H-naphthalen-1-one and the anhydrous hydrochloride of benzo-1,8-naphthyridine, upon exposure to or UV light, with thermal reversion in the dark over hours or days. Markwald's experiments, using natural or mercury arc lamps for coloration and ambient conditions for fading, established the reversibility of the process through repeated cycles, distinguishing it from irreversible photodecomposition observed in some earlier cases. Basic setups relied on simple illumination and observation, laying the foundation for systematic photochromic research. While early inorganic examples like films darkening upon light exposure were known from photographic processes since the , their changes were often incomplete or irreversible due to side reactions, with full recognition as photochromic systems emerging later.

Key Milestones and Modern Advances

The term "photochromism" was coined in 1950 by Yehuda Hirshberg during studies on bianthrone derivatives, formalizing the field. In the 1950s, chemists Ernst Fischer and Yehuda Hirshberg pioneered the study of compounds, documenting their photochromic properties and demonstrating fast reversible switching between colorless and colored forms upon UV and visible light exposure, respectively. This breakthrough laid the foundation for synthetic photochromic materials with practical switching speeds on the order of seconds. The marked the commercialization of photochromic technology through the development of silver halide-doped glasses by Corning Glass Works, led by William H. Armistead and S. Donald Stookey, which enabled the production of light-adaptive that darken under UV light and revert indoors. These materials, incorporating microcrystals of or in a glass matrix, achieved response times of 30 seconds for darkening and up to 10 minutes for fading, revolutionizing ophthalmic applications. During the 1970s, patent filings for fulgide-based systems, such as those by researchers at Fuji Photo Film Co., introduced variants with enhanced thermal stability in their colored forms, preventing spontaneous reversion at and enabling applications in optical . These innovations, including isopropylidene-3-furylfulgides, exhibited half-lives exceeding 1,000 hours at ambient conditions, addressing limitations in earlier photochromics. Advances in the and focused on diarylethenes, with Masahiro Irie reporting the first thermally irreversible and fatigue-resistant derivatives in 1988, capable of over 10,000 switching cycles without degradation. Subsequent refinements in the , including dithienylethene structures, improved quantum yields to near unity for both forward and backward reactions, making them ideal for high-endurance optical switches. In the 2020s, significant milestones include recent advances in doped photochromic glasses for rewritable 3D data storage, such as lanthanide-doped variants enabling multicolor and sub-micron resolution via multi-photon processes as of 2025. Additionally, October 2025 research demonstrated 3D-printable resins incorporating and diarylethene moieties, enabling all-optical processors with integrated logic gates that operate at speeds up to 1 GHz under visible light control.

Fundamental Principles

Photochromic Mechanisms

Photochromic behavior arises from the absorption of by molecules in their , promoting electrons to excited electronic states where structural reconfiguration occurs through processes such as or tautomerization. Upon absorption, the molecule transitions vertically to an excited (S₁ or higher) according to the , followed by rapid vibrational relaxation within the manifold. This excitation enables bond rotations, electrocyclic reactions, or proton shifts that lead to a metastable with distinct absorption properties, often via conical intersections that funnel the system back to the (PES). The photophysical processes in photochromic molecules are commonly represented by a , which illustrates the energy levels and transitions between electronic states. In this framework, the (S₀) absorbs a to reach an excited (S₁), from which (ISC) can occur to a (T₁) via spin-orbit coupling, with subsequent or non-radiative decay back to S₀. The key transitions include:
  • Absorption: S₀ → S₁ (rate constant k_abs ∝ ε(λ) I(λ), where ε is the molar absorptivity and I(λ) is the light intensity at λ).
  • (IC): S₁ → S₀ (non-radiative, ultrafast ~10⁻¹² s).
  • : S₁ → T₁ (spin-forbidden, ~10⁻⁹ to 10⁻⁶ s).
  • : T₁ → S₀ (spin-forbidden emission, ~10⁻³ to 10 s).
These pathways highlight how singlet-triplet interconversion influences the efficiency of photochromic switching by competing with reactive decay channels. The reverse transformation, often thermal bleaching, proceeds via barrier crossing on the ground-state PES, where the excited photoproduct relaxes to a higher-energy minimum separated by an activation barrier from the original ground state. This process is thermally activated, with the rate depending on the barrier height (typically 0.5–2 eV) and temperature, allowing reversion to the initial form without light in thermally reversible photochromes. Conical intersections on the PES facilitate efficient non-radiative decay during the forward photoinduced step, ensuring high switching speeds. The efficiency of photochromic reactions is quantified by the (Φ), defined as the number of product molecules formed per absorbed, or Φ = k_r / (k_r + k_nr), where k_r is the rate constant for the reactive pathway (e.g., ) and k_nr encompasses non-radiative and radiative decay rates. Factors affecting Φ include dependence, as absorption cross-sections (ε(λ)) vary, altering the excitation probability and thus the effective flux for reaction; higher Φ often occurs at wavelengths matching the S₀ → S₁ transition maximum. Environmental influences like and temperature also modulate k_r by affecting rotational barriers or thermal back-rates. Fatigue resistance in photochromic systems refers to the ability to undergo numerous switching cycles without degradation, primarily achieved by minimizing side reactions such as oxidation or byproduct formation that irreversibly consume active molecules. Structural modifications, like introducing electron-donating groups or perfluorination in diarylethenes, raise barriers for unwanted pathways on the ground-state PES, preventing oxidative degradation and extending cycle lifetimes beyond 10⁴ switches.

Types of Photochromic Transitions

Photochromic transitions encompass a variety of reaction pathways that induce reversible color changes through structural rearrangements or electronic modifications upon light absorption. These mechanisms are primarily categorized by the nature of the molecular transformation, including , , and proton transfer, each offering distinct shifts and reversibility profiles. Isomerization transitions involve geometric or conformational changes, such as cis-trans isomerization or ring-opening/closing, which alter the conjugation and thus the absorption wavelength. A prominent example is the -to-merocyanine transition, where UV irradiation cleaves the spiro C-O bond in the colorless form, yielding a planar, conjugated merocyanine with intense visible absorption around 550-650 nm; thermal or visible light reverses this to the closed form. Similar cis-trans isomerizations occur in azobenzenes, shifting absorption from UV to visible regions upon trans-to-cis conversion. Electron transfer processes rely on photoinduced charge separation within donor-acceptor systems, generating colored radical ions or charge-transfer complexes. In such systems, excitation of the donor leads to electron donation to the acceptor, producing a new electronic state with extended conjugation and bathochromic shifts. For instance, a supramolecular assembly of electron-rich N,N-dimethylaniline and electron-poor naphthalenediimide exhibits photochromism from colorless to orange via UV-triggered charge separation, with reversal occurring through back electron transfer. These transitions often display high reversibility but can be influenced by solvent polarity. Proton transfer mechanisms, notably excited-state intramolecular proton transfer (ESIPT), feature rapid migration of a proton within the , converting an form to a keto with altered chromophoric properties. Salicylidene compounds, such as salicylideneaniline, exemplify this: UV excitation initiates proton shift from the phenolic OH to the imine nitrogen, resulting in a keto form with red-shifted absorption; the process completes in femtoseconds to picoseconds, followed by ground-state reversion. This tautomerization enables thermochromic gating alongside photochromism. Photochromic systems are further classified as positive or negative based on color intensity changes. Positive photochromism involves enhancement of coloration upon , where a pale or colorless state converts to a deeply colored one, as in spiropyrans shifting from transparent to purple. Negative photochromism, conversely, features decolorization under light, with a colored state bleaching to a colorless form, observed in dihydropyrene derivatives that lose their green hue via ring-opening. Response times for these transitions span orders of magnitude, reflecting the underlying dynamics: ultrafast processes, like ESIPT in salicylideneaniline, occur in picoseconds due to low barriers, while slower isomerizations, such as ring closure, take seconds to minutes. External factors can gate these responses; for example, elevated temperatures accelerate thermal reversion in spiropyrans by increasing the rate constant for merocyanine-to- conversion, and acidic stabilizes the protonated merocyanine form, inhibiting photochromism until neutralization. In inorganic systems, defect-mediated transitions provide analogous but often slower pathways.

Classes of Photochromic Materials

Organic Photochromic Compounds

Organic photochromic compounds are carbon-based molecules that exhibit reversible color changes upon exposure to light through intramolecular structural transformations, such as or cyclization, without involving external . These materials are prized for their potential in optical switching due to their ability to undergo clean, fatigue-resistant photoinduced reactions in solution or solid states. Unlike inorganic counterparts, organic photochromics rely on molecular-level changes, enabling tunability via modifications to adjust spectral response, thermal stability, and quantum yields. Spiropyrans represent one of the earliest and most studied classes of organic photochromics, featuring a spiro carbon atom linking an indoline ring to a chromene moiety, which maintains a colorless, closed-ring structure in the ground state. Upon ultraviolet (UV) irradiation, typically around 365 nm, the C-O bond at the spiro center undergoes heterolytic cleavage, resulting in ring opening to form a colored merocyanine zwitterion with extended conjugation and absorption in the visible range (400-600 nm). The reverse thermal reversion from merocyanine to spiropyran occurs spontaneously, with half-lives on the order of minutes under ambient conditions, though this can be extended to hours or days through steric hindrance or solvent effects. Diarylethenes undergo photochromism via a thermally irreversible, photochemically reversible , where the open-ring (typically colorless, absorbing below 300 nm) cyclizes conrotatorily under UV light to a closed-ring form with visible absorption (500-600 nm). This P-type mechanism ensures high thermal stability of both isomers at , preventing spontaneous reversion. Notably, diarylethenes demonstrate exceptional fatigue resistance, enduring over 10,000 photochromic cycles with minimal degradation, attributed to the absence of reactive intermediates and robust aryl-thiophene frameworks. Azobenzenes exhibit photochromism through trans-cis isomerization, where the stable trans (E) form, with absorption at ~350 nm via a π-π* transition, converts to the bent cis (Z) isomer upon UV exposure, shifting absorption to ~450 nm and altering . The cis form reverts thermally to trans, following Arrhenius kinetics described by the rate constant k=Aexp⁡(−EaRT)k = A \exp\left(-\frac{E_a}{RT}\right) where AA is the , EaE_a is the (typically 80-120 kJ/mol), RR is the , and TT is temperature in . This process enables applications in light-driven actuation, with reversion half-lives ranging from seconds to days depending on substituents. Fulgides, derived from fulgenic acids through dehydration or condensation reactions with anhydrides, display photochromism via 6π electrocyclic ring closure from a colorless open E/Z-isomer (absorption ~400 nm) to a colored closed form (~550 nm) under UV light, with ring opening induced by visible light. Synthesis often involves of benzylidenesuccinic anhydride with aldehydes, yielding isomers separable by , followed by thermal or acid-catalyzed cyclization. The colored form exhibits bathochromic shifts of 100-200 nm upon cyclization, with visible light inducing reversion and good thermal stability in substituted variants. Overcrowded alkenes, featuring sterically hindered central double bonds, undergo photochromism through light-induced between stable (P) and metastable (M) helical configurations, enabling rotary motion and chiroptical switching. These compounds, often substituted with aryl groups, absorb in the UV-visible range (~300-400 nm for forward ) and are noted for their unidirectional rotation under appropriate wavelengths, contributing to applications. Photochromic quinones, such as aryloxyquinones, are synthesized via of to p-benzoquinones or Diels-Alder routes, leading to reversible photoinduced tautomerization or reduction. Upon , these compounds shift from (absorption ~450 nm) to hydroquinone-like forms (~300 nm), involving proton transfer or hydrogen abstraction, with spectral changes enabling detection in photochemical cycles. Recent advances have focused on solid-state spiropyrans with bulky substituents or matrices to suppress aggregation-induced , enabling non-volatile optical devices.

Inorganic Photochromic Materials

Inorganic photochromic materials encompass a range of non-organic solids, including crystalline and amorphous structures, where color changes arise primarily from defect-mediated trapping or intercalation rather than molecular rearrangements. These materials exhibit collective effects in extended lattices, distinguishing them from discrete molecular systems in organic counterparts. Key examples include silver halides and oxides, which demonstrate reversible darkening upon light exposure through the formation of metallic clusters or reduced species. Silver halides such as AgCl and AgBr are foundational inorganic photochromics, embedded as microcrystallites in glass matrices to enable practical applications. Upon ultraviolet irradiation, excitons in the halide lattice dissociate, generating free electrons and holes; the electrons reduce Ag⁺ ions to form metallic silver colloids (Ag⁰ clusters), which scatter and cause visible darkening, while holes oxidize halide ions to form complementary species like Cl₂ or Br₂. This process mimics formation in but occurs reversibly in photochromic formulations. Thermal bleaching restores transparency by recombining electrons and holes, dissolving the silver aggregates back into the lattice. Transition metal oxides, particularly WO₃ and MoO₃, display polychromatic photochromism through the formation of colored bronzes. In WO₃, bandgap excitation injects electrons from the valence band, creating oxygen vacancies or reduced W⁵⁺/W⁴⁺ sites that impart blue hues; subsequent intercalation of cations like H⁺ or Li⁺ from the environment stabilizes these states, forming tungsten bronzes (HₓWO₃) with deeper colors. MoO₃ follows a similar pathway, yielding molybdenum bronzes (HₓMoO₃) with green-to-blue shifts, though its photoresponse is often slower due to higher defect formation energies. These mechanisms enable multi-color tuning but are sensitive to environmental humidity for optimal reversibility. A distinctive in inorganic photochromics is persistent spectral hole burning (PSHB) in amorphous glasses doped with rare-earth ions, such as Eu³⁺ in hosts. irradiation at specific wavelengths perturbs the local coordination environment around the ions via or tautomerization, creating narrow "holes" in the inhomogeneous absorption that persist at . This enables high-density optical , with hole widths as narrow as 10 GHz supporting terabit-scale capacities in three dimensions. Recent advancements include a 2025 development in doped incorporating magnesium and ions, fabricated via direct 3D . Green (532 nm) inscription induces reversible changes for 3D pattern storage, with (417 nm excitation) or (376 nm) emission; erasure occurs via heating at 550°C for 25 minutes, preserving integrity for indefinite rewritability without power consumption. Durability varies significantly across these systems: silver halide-based glasses withstand over 300,000 darkening-bleaching cycles with minimal , attributed to efficient recombination kinetics. In contrast, transition metal photochromics like WO₃ exhibit degradation after 10–15 cycles, due to irreversible trapping of intercalated species or structural in the lattice.

Coordination and Hybrid Compounds

Coordination and hybrid compounds represent a class of photochromic materials where organic ligands are integrated with metal centers, enabling tunable photoresponsive behaviors through metal-ligand interactions that differ from defect-based mechanisms in pure inorganic oxides. These hybrids leverage coordination chemistry to achieve reversible structural or electronic changes upon light exposure, often involving charge transfer or spin state alterations. In metal-organic frameworks (MOFs) incorporating photochromic linkers, reversible color changes arise from ligand-to-metal charge transfer (LMCT) processes. For instance, in MIL-101(Fe), excitation at 355 nm induces LMCT, leading to partial dissociation of the linker from the Fe-oxo node and a shift in to Fe-centered antibonding orbitals, resulting in observable color variation with an lifetime of approximately 30 μs. This mechanism enhances charge separation and reversibility, as the bidentate-to-monodentate carboxylate-Fe bond transition reverts thermally or upon relaxation. Similarly, amino-functionalized MOFs like NH₂-MIL-125() exhibit LMCT-driven charge separation that supports photocatalytic applications, with visible light absorption tuned by ligand modifications. Ruthenium and complexes demonstrate photochromism through spin-crossover (SCO) phenomena or ligand-driven changes, where light induces reconfiguration of d-electrons or structural alterations. In ruthenium-based systems, photoexcitation promotes linkage , altering the metal-ligand geometry and spin configuration, as observed in transient absorption studies showing metal-centered excited states. complexes, often paired with photochromic ligands like dithienylethenes, exhibit photochromism via ligand electrocyclic reactions that modulate the coordination environment and associated magnetic properties, enabling reversible electronic modulation. These interactions are amplified in MOF-embedded forms, where photoactive guests control properties across the framework. Hybrid perovskites exhibit photochromism via photoinduced halide migration, causing compositional shifts that alter bandgap and color. In mixed-halide systems like MAPb(I_{1-x}Br_x)_3, light drives iodide-to-bromide segregation, forming I-rich and Br-rich domains with distinct absorption edges (e.g., red-shift in I-rich regions), confirmed by transient showing reversible trap formation. Recent advancements incorporate spiro-indoline naphthoxazine additives to suppress migration, yielding stable color shifts with surface potential changes of -70 mV under UV , enhancing material durability. A notable development in 2025 involves photoswitchable s for sensor applications, where modulates -metal interactions to enable reversible and detection of analytes like , as reported in coordination polymer frameworks with viologen-based linkers. These materials benefit from tunability, achieved by varying substituents (e.g., electron-donating groups for red-shifted absorption) and metal ions (e.g., Ru vs. Cu for spin-orbit coupling adjustments), allowing photochromic responses from UV to near-IR without relying on high-energy excitation.

Practical Applications

Ophthalmic and Protective Devices

Photochromic lenses represent a primary application of photochromic in ophthalmic devices, enabling eyeglasses and to automatically adjust tint levels in response to ambient conditions. These lenses incorporate photochromic dyes that undergo reversible structural changes upon exposure to (UV) , transitioning from a clear state indoors to a darkened state outdoors for enhanced visual comfort and against . A seminal example is the Transitions lenses developed by , which began commercial production of plastic photochromic eyeglass lenses in 1991. These lenses utilize indeno-naphthopyrans as the key photochromic compounds, which rearrange their molecular structure when activated by UV light, absorbing visible wavelengths to produce the darkening effect. The activation process typically occurs within 30-60 seconds under sufficient UV exposure, allowing rapid adaptation to bright environments. In terms of integration, photochromic dyes are embedded within polymer matrices, such as , to form durable lens materials suitable for and prescription eyewear. This embedding process disperses the dyes uniformly throughout the polycarbonate substrate, ensuring consistent photochromic response while maintaining the material's impact resistance and optical clarity. Such formulations allow the lenses to block up to 100% of UVA and UVB rays in their darkened state, providing essential without the need for separate . Performance standards for photochromic ophthalmic lenses are governed by ANSI Z80.3, which specifies requirements for luminous in both clear and darkened states to ensure safety and efficacy. For instance, the standard mandates that lenses in the clear state achieve at least 75% for general use, while the darkened state should reduce to levels between 8% and 40% depending on the application, preventing excessive darkening that could impair visibility. Compliance with ANSI Z80.3 also includes tests for color perception and UV blockage, confirming that photochromic lenses do not distort traffic signals or essential visual cues. As of 2025, photochromic technology is increasingly integrated into smart glasses for variable tinting, driven by market growth in adaptive solutions. According to industry analysis, the global photochromic lenses market is valued at USD 7.28 billion in 2025, with projections for expansion into intelligent devices that combine photochromic responses with digital features for enhanced user experience. Despite these advances, photochromic lenses exhibit limitations in indoor performance due to reduced UV penetration through window glass, which blocks much of the activating wavelengths. This results in slower or incomplete darkening in environments like vehicles or offices with UV-filtering materials, potentially requiring supplemental lighting or alternative tinting methods for optimal functionality.

Data Storage and Optical Memory

Photochromic materials have been extensively explored for due to their ability to undergo reversible or irreversible structural changes upon light exposure, enabling the encoding of at high densities. In holographic storage systems, diarylethene-based films stand out for their thermal stability and fatigue resistance, allowing multi-bit encoding through polarization holography. For instance, a diarylethene compound doped in a matrix can record polarization holograms by exploiting the photoinduced and dichroism, where orthogonal polarization states represent multiple bits per , achieving diffraction efficiencies up to 20% and enabling rewritable storage with angular . Three-dimensional (3D) optical leverages processes in photochromic glasses to confine writing to focal points, minimizing and enabling volumetric . Early demonstrations used diarylethene-doped materials for bit-wise recording, but recent advances in doped amorphous glasses have realized rewritable 3D patterns via direct at 532 nm, utilizing Tb³⁺/Mn²⁺-induced photochromism for purple coloration that can be thermally erased. This approach supports tunable readout, offering multimodal encoding with stability over extended periods without power consumption, as reported in a 2025 study. In contrast to two-photon systems, spectral hole burning (SHB) in photochromic media provides sub-wavelength resolution by selectively narrowing absorption lines in inhomogeneous broadened spectra, yielding storage densities exceeding 10¹² bits/cm³ through frequency-domain . Bit stability in photochromic memories varies between non-erasable (write-once) and rewritable modes, with the latter requiring high to . Fulgide exemplify rewritable systems, demonstrating over 10⁶ write-erase cycles in matrices due to their robust cyclization without significant , even under repeated UV/vis . Non-erasable modes, often using irreversible photochromes, prioritize long-term archival stability but limit reusability. A key challenge in these systems is achieving readout without inducing erasure, addressed by employing low-intensity probes at wavelengths outside the writing band or via contrasts that avoid photoexcitation of the metastable state.

Emerging and Aspirational Uses

Sensors and Smart Materials

Photochromic materials have been integrated into sensors for , particularly for detecting (UV) radiation through reversible color changes that enable . Spiropyran-based films and textiles, such as those incorporating spiropyran-tetraphenylethene (SP-TPE) electrospun with , undergo rapid upon UV exposure, shifting from colorless to colored states to visually quantify cumulative dose. This color intensity correlates linearly with exposure levels at low doses, providing a reusable platform for personal sun protection monitoring. Recent advancements in photoswitchable sensors leverage coordination compounds for selective detection. Macrocyclic complexes featuring azobenzene-functionalized diaza-crown ethers coordinated with ions (Ln³⁺) exhibit modulated trans-to-cis under UV light (320 nm), resulting in distinct color shifts that are quenched or enhanced based on the metal ion identity due to effects. For instance, (Nd³⁺) complexes show a trans-to-cis rate constant of 7.67 × 10⁻⁵ s⁻¹ and ~90% cis-to-trans recovery upon contact, enabling selective detection over ions like Na⁺. These systems offer high selectivity for specific cations, with photoswitching efficiency decreasing from early (La³⁺ to Tb³⁺) to late s (Dy³⁺ to Lu³⁺). In smart textiles, photochromic dyes derived from organic compounds like spiropyrans and spirooxazines are applied via exhaust dyeing or to fabrics, enabling adaptive responses to light for UV protection and . These dyes change color reversibly under UV (200–400 nm) exposure, darkening fabrics to block harmful rays and alerting users to overexposure. For military applications, such textiles develop patterns that blend with sunlit surroundings through sunlight-induced color shifts, enhancing concealment without electronic components. Integration of photochromic elements into facilitates real-time light intensity feedback. Photochromic fibers, produced via thermal drawing with polymethyl methacrylate cores and fluorescent outer layers, are woven into wristbands or garments to display color changes proportional to UV input, updating every 10 seconds for dynamic environmental awareness. These devices maintain uniform emission over 1–2 m lengths, even under , providing intuitive visual cues for light exposure. Photochromic sensors demonstrate sensitivity thresholds suitable for practical monitoring, such as detecting UV-A intensities as low as 1 mW/cm², with observable color changes within 15 seconds at higher irradiances (5–25 W/m²). This threshold aligns with safety limits for prolonged exposure, ensuring reliable early warnings in wearables.

Energy Conversion and Storage

Photochromic materials play a pivotal role in energy conversion and storage by enabling reversible photochemical reactions that capture and store it in metastable chemical forms for later release as heat. These molecular solar thermal fuels (MSTFs) operate through photoinduced isomerizations, where incident drives a structural change that increases the molecule's , typically without emitting or generating in the process. Upon triggering, the reverse reaction liberates the stored quantitatively as output, offering a closed-cycle complementary to photovoltaic technologies for applications. A seminal example is the norbornadiene (NBD)-quadricyclane (QC) photoswitch, where ultraviolet irradiation of NBD yields QC via a [2+2] , storing approximately 100 kJ/mol of in the strained QC . This forward is highly efficient, with quantum yields often exceeding 0.7, while the reverse QC-to-NBD transformation occurs spontaneously or catalytically at elevated temperatures, releasing the stored energy as heat. Hybrid architectures further advance these capabilities by incorporating photochromic units into metal-organic frameworks (MOFs), facilitating light-to-heat conversion within structured thermal batteries. For instance, azobenzene-functionalized MOFs absorb visible light to trigger trans-to-cis , storing energy while the porous framework enhances heat dissipation and cyclability for repeated solar harvesting. These systems enable long-term storage under ambient conditions, with energy densities comparable to conventional batteries but without degradation from charge-discharge cycles. Energy release in all these photochromic systems is primarily achieved through thermal triggering, where controlled heating lowers the barrier for back-isomerization, enabling on-demand heat output without external catalysts in many designs.

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