Photochromism

{{Short description|Reversible chemical transformation by absorption of electromagnetic radiation}} {{Use Canadian English|date=April 2019}}

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.<ref>{{cite book |doi=10.1002/14356007.t07_t01 |chapter=Chromogenic Materials |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2016 |last1=Lötzsch |first1=Detlef |last2=Eberhardt |first2=Volker |last3=Rabe |first3=Christian |pages=1–26 |isbn=978-3-527-30673-2 }}</ref><ref>{{Cite journal |last=Irie |first=M. |date=2000 |title=Photochromism: Memories and Switches-Introduction |journal=Chemical Reviews |language=en |volume=100 |issue=5 |pages=1683–1684 |doi=10.1021/cr980068l |issn=0009-2665|doi-access=free }}</ref> This reversible structural or geometric change in photochromic molecules affects their electronic configuration, molecular strain energy, and other properties.<ref>{{Citation |last1=Mahitha |first1=P.M. |title=Photochromic molecules and materials: design and development |date=2024 |work=Handbook of Emerging Materials for Sustainable Energy |pages=237–254 |url=https://linkinghub.elsevier.com/retrieve/pii/B9780323961257000150 |access-date=2025-02-18 |publisher=Elsevier |language=en |doi=10.1016/b978-0-323-96125-7.00015-0 |isbn=978-0-323-96125-7 |last2=Nakul |first2=S. |last3=Kumar |first3=Meenu |last4=Kulkarni |first4=Naveen V. |last5=Kharissova |first5=Oxana V. |last6=Kharissov |first6=Boris I.|url-access=subscription }}</ref>

==History== 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<ref>{{Cite journal |last=Ter Meer |first=Edm. |date=1876 |title=Ueber Dinitroverbindungen der Fettreihe |journal=Justus Liebigs Annalen der Chemie |volume=181 |issue=1 |pages=1–22 |doi=10.1002/jlac.18761810102 |issn=0075-4617}}</ref> and Phipson.<ref>{{Cite journal |last=Phipson |first=T. L. |date=1881 |title=Dangers of Pyrogallic Acid |journal=Scientific American |volume=12 |issue=312supp |page=4982 |doi=10.1038/scientificamerican12241881-4982bsupp |issn=0036-8733}}</ref> 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.<ref name=":0">{{Cite journal |last1=Bouas-Laurent |first1=Henri |last2=Dürr |first2=Heinz |date=2016 |title=Books on Photochromism |website=IUPAC Standards Online|doi=10.1515/iupac.73.0019 }}</ref><ref name=":1">{{Cite journal |last1=Du |first1=Jiaren |last2=Yang |first2=Zetian |last3=Lin |first3=Hengwei |last4=Poelman |first4=Dirk |date=2024 |title=Inorganic photochromic materials: Recent advances, mechanism, and emerging applications |url=https://onlinelibrary.wiley.com/doi/10.1002/rpm.20240004 |journal=Responsive Materials |language=en |volume=2 |issue=2 |doi=10.1002/rpm.20240004 |issn=2834-8966|hdl=1854/LU-01HZKMBHJF66CYRDRASR7DYVVN |hdl-access=free }}</ref> 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".<ref>{{Cite journal |last=Marckwald |first=W. |date=1899 |title=Ueber Phototropie |journal=Zeitschrift für Physikalische Chemie |volume=30U |issue=1 |pages=140–145 |doi=10.1515/zpch-1899-3007 |issn=2196-7156}}</ref> 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.<ref name=":0" /> 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.<ref name=":0" /> 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.<ref name=":1" />

== Principles == :[[File:PentammineCoNO2.svg|thumb|220px|Photoisomerization of {{chem2|[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).<ref name=":0" /> Transition metal complexes can also display photochromic properties due to linkage isomerizations.<ref>{{Cite journal |last1=Coppens |first1=Philip |last2=Novozhilova |first2=Irina |last3=Kovalevsky |first3=Andrey |date=2002 |title=Photoinduced Linkage Isomers of Transition-Metal Nitrosyl Compounds and Related Complexes |url=https://pubs.acs.org/doi/10.1021/cr000031c |journal=Chemical Reviews |language=en |volume=102 |issue=4 |pages=861–884 |doi=10.1021/cr000031c |issn=0009-2665|url-access=subscription }}</ref><ref>{{Cite journal |last=Bitterwolf |first=Thomas E. |date=2006 |title=Photochemical nitrosyl linkage isomerism/metastable states |url=https://linkinghub.elsevier.com/retrieve/pii/S0010854505003139 |journal=Coordination Chemistry Reviews |language=en |volume=250 |issue=9–10 |pages=1196–1207 |doi=10.1016/j.ccr.2005.12.016|url-access=subscription }}</ref><ref name=":53">{{Cite journal |last=Rack |first=J |date=2009 |title=Electron transfer triggered sulfoxide isomerization in ruthenium and osmium complexes |url=https://linkinghub.elsevier.com/retrieve/pii/S0010854507002986 |journal=Coordination Chemistry Reviews |language=en |volume=253 |issue=1–2 |pages=78–85 |doi=10.1016/j.ccr.2007.12.021|url-access=subscription }}</ref><ref name=":63">{{Cite journal |last1=McClure |first1=Beth Anne |last2=Rack |first2=Jeffrey J. |date=2010 |title=Isomerization in Photochromic Ruthenium Sulfoxide Complexes |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/ejic.200900548 |journal=European Journal of Inorganic Chemistry |language=en |volume=2010 |issue=25 |pages=3895–3904 |doi=10.1002/ejic.200900548 |issn=1434-1948|url-access=subscription }}</ref>

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.<ref name=":22">{{Cite book |last1=Klán |first1=Petr |title=Photochemistry of Organic Compounds |last2=Wirz |first2=Jakob |date=2009 |publisher=Wiley |doi=10.1002/9781444300017 |isbn=978-1-4051-6173-2}}</ref> 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.<ref name=":0" /> 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.<ref name=":22" />

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.<ref>{{cite journal |title=Photochromism |url=https://goldbook.iupac.org/terms/view/P04589 |website=IUPAC Gold Book|date=2014 |doi=10.1351/goldbook.P04589 |url-access=subscription }}</ref>

==Classes of photochromic materials==

=== Molecular photoswitches === {| class="wikitable" |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.<ref>{{Cite journal |last1=Beharry |first1=Andrew A. |last2=Woolley |first2=G. Andrew |date=2011 |title=Azobenzene photoswitches for biomolecules |url=https://xlink.rsc.org/?DOI=c1cs15023e |journal=Chemical Society Reviews |language=en |volume=40 |issue=8 |pages=4422–4437 |doi=10.1039/c1cs15023e |pmid=21483974 |issn=0306-0012|url-access=subscription }}</ref><ref>{{Cite journal |last1=Merino |first1=Estíbaliz |last2=Ribagorda |first2=María |date=2012 |title=Control over molecular motion using the cis – trans photoisomerization of the azo group |url=https://www.beilstein-journals.org/bjoc/articles/8/119 |journal=Beilstein Journal of Organic Chemistry |language=en |volume=8 |pages=1071–1090 |doi=10.3762/bjoc.8.119 |issn=1860-5397 |pmc=3458724 |pmid=23019434}}</ref> Azobenzene groups incorporated into crown ethers give switchable receptors and azobenzenes.<ref>{{Cite journal |last1=Shiga |first1=Masanobu |last2=Takagi |first2=Makoto |last3=Ueno |first3=Keihei |date=1980 |title=Azo-Crown Ethers. The Dyes with Azo Group Directly Involved in the Crown Ether Skeleton |url=https://academic.oup.com/chemlett/article/9/8/1021/7412590 |journal=Chemistry Letters |language=en |volume=9 |issue=8 |pages=1021–1022 |doi=10.1246/cl.1980.1021 |issn=0366-7022|url-access=subscription }}</ref> |frameless|359x359px |- |Diarylethenes |Diarylethenes undergo a fully reversible transformation between "ring-open" and "ring-closed" isomeric forms when exposed to light of suitable wavelength.<ref name=":3">{{Cite journal |last1=Irie |first1=Masahiro |last2=Mohri |first2=Masaaki |date=1988 |title=Thermally irreversible photochromic systems. Reversible photocyclization of diarylethene derivatives |journal=The Journal of Organic Chemistry |volume=53 |issue=4 |pages=803–808 |doi=10.1021/jo00239a022 |issn=0022-3263}}</ref> Diarylethene-based photoswitches exhibit high photofatigue resistance, enabling them to undergo many photoswitching cycles with minimal degradation.<ref>{{Cite journal |last1=Irie |first1=Masahiro |last2=Fukaminato |first2=Tuyoshi |last3=Matsuda |first3=Kenji |last4=Kobatake |first4=Seiya |date=2014 |title=Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators |url=https://pubs.acs.org/doi/10.1021/cr500249p |journal=Chemical Reviews |language=en |volume=114 |issue=24 |pages=12174–12277 |doi=10.1021/cr500249p |pmid=25514509 |issn=0009-2665|url-access=subscription }}</ref> These compounds have been evaluate for long-lasting photochemical memory devices due to the thermal stability of both photoforms of diarylethenes.<ref name=":3" /> |frameless|362x362px |- |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.<ref>{{Cite journal |last1=Baillet |first1=G. |last2=Giusti |first2=G. |last3=Guglielmetti |first3=R. |date=1993 |title=Comparative photodegradation study between spiro[indoline—oxazine] and spiro[indoline—pyran] derivatives in solution |url=https://linkinghub.elsevier.com/retrieve/pii/1010603093850368 |journal=Journal of Photochemistry and Photobiology A: Chemistry |language=en |volume=70 |issue=2 |pages=157–161 |doi=10.1016/1010-6030(93)85036-8|bibcode=1993JPPA...70..157B |url-access=subscription }}</ref><ref>{{Cite journal |last=Ballet |first=Gilles |date=1997 |title=Photodegradation of Organic Photochromes in Polymers - Naphthopyrans and Naphthoxazines Series - |journal=Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals |volume=298 |issue=1 |pages=75–82 |doi=10.1080/10587259708036145 |bibcode=1997MCLCA.298...75B |issn=1058-725X}}</ref><ref>{{Cite journal |last1=Baillet |first1=Gilles |last2=Giusti |first2=Gérard |last3=Guglielmetti |first3=Robert |date=1995 |title=Study of the Fatigue Process and the Yellowing of Polymeric Films Containing Spirooxazine Photochromic Compounds |journal=Bulletin of the Chemical Society of Japan |volume=68 |issue=4 |pages=1220–1225 |doi=10.1246/bcsj.68.1220 |issn=0009-2673}}</ref> |frameless|367x367px |- |Donor Acceptor Stenhouse Adducts |DASAs are a class of organic photoswitches characterized by their responsiveness to visible light and negative photochromism (the loss of color upon irradiation). Unlike traditional switches like spiropyrans that typically require ultraviolet light for activation<ref>{{Cite journal |last=Keyvan Rad |first=Jaber |last2=Balzade |first2=Zahra |last3=Mahdavian |first3=Ali Reza |date=2022-06-01 |title=Spiropyran-based advanced photoswitchable materials: A fascinating pathway to the future stimuli-responsive devices |url=https://www.sciencedirect.com/science/article/pii/S1389556722000065 |journal=Journal of Photochemistry and Photobiology C: Photochemistry Reviews |volume=51 |article-number=100487 |doi=10.1016/j.jphotochemrev.2022.100487 |issn=1389-5567|url-access=subscription }}</ref><ref>{{Cite journal |last=Klajn |first=Rafal |date=2013-12-02 |title=Spiropyran-based dynamic materials |url=https://pubs.rsc.org/en/content/articlelanding/2014/cs/c3cs60181a |journal=Chemical Society Reviews |language=en |volume=43 |issue=1 |pages=148–184 |doi=10.1039/C3CS60181A |issn=1460-4744|doi-access=free }}</ref>, DASAs can be toggled using visible light, making them more suitable for biological environments where UV exposure can cause cellular damage.<ref>{{Cite journal |last=Clerc |first=Michèle |last2=Sandlass |first2=Sara |last3=Rifaie-Graham |first3=Omar |last4=Peterson |first4=Julie A. |last5=Bruns |first5=Nico |last6=Alaniz |first6=Javier Read de |last7=Boesel |first7=Luciano F. |date=2023-11-27 |title=Visible light-responsive materials: the (photo)chemistry and applications of donor–acceptor Stenhouse adducts in polymer science |url=https://pubs.rsc.org/en/content/articlelanding/2023/cs/d3cs00508a |journal=Chemical Society Reviews |language=en |volume=52 |issue=23 |pages=8245–8294 |doi=10.1039/D3CS00508A |issn=1460-4744|pmc=10680135 }}</ref> Recent research has utilized DASA-functionalized polymers to create light-responsive nanocarriers, such as polymersomes, that allow for the "on-off" switchable release of encapsulated payloads when triggered by visible light.<ref>{{Cite journal |last=Dell |first=Tristan N. |last2=Cammack‐Najera |first2=Ana |last3=Tresa |first3=Rea |last4=Matubbar |first4=Farzina |last5=Kaya |first5=Beyzanur |last6=Lathan |first6=Uthaya |last7=Chami |first7=Mohamed |last8=DiNardi |first8=Ray G. |last9=Rifaie‐Graham |first9=Omar |last10=Wojciechowski |first10=Jonathan P. |last11=Stevens |first11=Molly M. |date=2026-01-31 |title=Hydrogels Incorporating Donor–Acceptor Stenhouse Adducts as a Platform for Photoinduced, On‐Off Switchable Release of Small Molecule Cargos |url=https://onlinelibrary.wiley.com/doi/10.1002/marc.202500868 |journal=Macromolecular Rapid Communications |language=en |volume=47 |issue=8 |doi=10.1002/marc.202500868 |issn=1022-1336 |pmc=13087835 |pmid=41619178}}</ref><ref>{{Cite journal |last=Matubbar |first=Farzina |last2=Kaya |first2=Beyzanur |last3=Ipek |first3=Buse |last4=Dorjee |first4=Tenzin |last5=Hakobyan |first5=Shoghik |last6=Chami |first6=Mohamed |last7=Gautrot |first7=Julien E. |last8=Rifaie‐Graham |first8=Omar |date=2026-02-28 |title=Deciphering Small Molecule Diffusion Parameters Across Light Responsive Polymersome Membranes |url=https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202527008 |journal=Advanced Functional Materials |language=en |doi=10.1002/adfm.202527008 |issn=1616-301X|doi-access=free }}</ref> |frameless|367x367px |- |Fulgides and Fulgimides |Similar to diarylethenes, the photochromic behavior of fulgides and fulgimides is based on 6π-electrocyclic ring-opening and ring-closing reactions.<ref name=":4">{{Cite journal |last1=Renth |first1=F. |last2=Siewertsen |first2=R. |last3=Temps |first3=F. |date=2013 |title=Enhanced photoswitching and ultrafast dynamics in structurally modified photochromic fulgides |url=http://www.tandfonline.com/doi/abs/10.1080/0144235X.2012.729331 |journal=International Reviews in Physical Chemistry |language=en |volume=32 |issue=1 |pages=1–38 |doi=10.1080/0144235X.2012.729331 |issn=0144-235X|url-access=subscription }}</ref> They are highly photochromic photoswitches and reversibly interconvert between two isomeric forms when exposed to light of different wavelengths.<ref name=":4" /><ref>{{Cite book |last1=Browne |first1=Wesley R. |last2=Feringa |first2=Ben L. |date=2011 |chapter=Chapter 5: Chiroptical Molecular Switches |chapter-url=https://doi.org/10.1002/9783527634408.ch5 |title=''In:'' Molecular Switches |volume=1 | edition=2 |pages=121–179 |doi=10.1002/9783527634408.ch5|isbn=978-3-527-31365-5 |chapter-url-access=subscription }}</ref> These compounds exhibit low photochemical fatigue, high thermal stability, as well as high conversion yields.<ref>{{Cite journal |last1=Koshima |first1=Hideko |last2=Nakaya |first2=Hidemitsu |last3=Uchimoto |first3=Hidetaka |last4=Ojima |first4=Naoko |date=2012 |title=Photomechanical Motion of Furylfulgide Crystals |url=https://academic.oup.com/chemlett/article/41/1/107/7388806 |journal=Chemistry Letters |language=en |volume=41 |issue=1 |pages=107–109 |doi=10.1246/cl.2012.107 |issn=0366-7022|url-access=subscription }}</ref><ref>{{Cite journal |last1=Harada |first1=Jun |last2=Taira |first2=Masaya |last3=Ogawa |first3=Keiichiro |date=2017 |title=Photochromism of Fulgide Crystals: From Lattice-Controlled Product Accumulation to Phase Separation |url=https://pubs.acs.org/doi/10.1021/acs.cgd.7b00182 |journal=Crystal Growth & Design |language=en |volume=17 |issue=5 |pages=2682–2687 |doi=10.1021/acs.cgd.7b00182 |bibcode=2017CrGrD..17.2682H |issn=1528-7483|url-access=subscription }}</ref> |frameless|370x370px |- |Hydrazones |Hydrazone photoswitches can be activated by light and undergo efficient and reversible ''E/Z'' isomerization around the C=N double bond.<ref>{{Cite journal |last1=Su |first1=Xin |last2=Aprahamian |first2=Ivan |date=2014 |title=Hydrazone-based switches, metallo-assemblies and sensors |journal=Chemical Society Reviews |language=en |volume=43 |issue=6 |page=1963 |doi=10.1039/c3cs60385g |issn=0306-0012|doi-access=free }}</ref><ref>{{Cite journal |last1=Qi |first1=Qingkai |last2=Huang |first2=Shiqing |last3=Liu |first3=Xiaogang |last4=Aprahamian |first4=Ivan |date=2024 |title=1,2-BF 2 Shift and Photoisomerization Induced Multichromatic Response |url=https://pubs.acs.org/doi/10.1021/jacs.4c00592 |journal=Journal of the American Chemical Society |language=en |volume=146 |issue=10 |pages=6471–6475 |doi=10.1021/jacs.4c00592 |pmid=38428039 |bibcode=2024JAChS.146.6471Q |issn=0002-7863|url-access=subscription }}</ref> |frameless|375x375px |- |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.<ref>{{Cite journal |last=Hartley |first=G. Spencer |date=1938 |title=113. The cis-form of azobenzene and the velocity of the thermal cis→trans-conversion of azobenzene and some derivatives |journal=J. Chem. Soc. |volume=0 |pages=633–642 |doi=10.1039/jr9380000633 |issn=0368-1769}}</ref><ref>{{Cite journal |last1=Crespi |first1=Stefano |last2=Simeth |first2=Nadja A. |last3=König |first3=Burkhard |date=2019 |title=Heteroaryl azo dyes as molecular photoswitches |journal=Nature Reviews Chemistry |volume=3 |issue=3 |pages=133–146 |doi=10.1038/s41570-019-0074-6 |issn=2397-3358}}</ref> Like azobenzenes, their thermal isomerization follows three main pathways: inversion, rotation, or tautomerization. Typically, the ''Z''-isomer of azoheteroarenes exists as the metastable state.<ref>{{Cite journal |last1=Slavov |first1=Chavdar |last2=Yang |first2=Chong |last3=Heindl |first3=Andreas H. |last4=Stauch |first4=Tim |last5=Wegner |first5=Hermann A. |last6=Dreuw |first6=Andreas |last7=Wachtveitl |first7=Josef |date=2018 |title=Twist and Return−Induced Ring Strain Triggers Quick Relaxation of a ( Z )-Stabilized Cyclobisazobenzene |url=https://pubs.acs.org/doi/10.1021/acs.jpclett.8b02159 |journal=The Journal of Physical Chemistry Letters |language=en |volume=9 |issue=16 |pages=4776–4781 |doi=10.1021/acs.jpclett.8b02159 |pmid=30063355 |issn=1948-7185|url-access=subscription }}</ref> 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.<ref>{{Citation |last1=Greenfield |first1=Jake L. |title=Azoheteroarenes |date=2022 |work=Molecular Photoswitches |pages=83–112 |editor-last=Pianowski |editor-first=Zbigniew L. |url=https://onlinelibrary.wiley.com/doi/10.1002/9783527827626.ch5 |access-date=2025-02-19 |edition=1 |publisher=Wiley |language=en |doi=10.1002/9783527827626.ch5 |isbn=978-3-527-35104-6 |last2=Thawani |first2=Aditya R. |last3=Odaybat |first3=Magdalena |last4=Gibson |first4=Rosina S.L. |last5=Jackson |first5=Thomas B. |last6=Fuchter |first6=Matthew J.|url-access=subscription }}</ref> |frameless|366x366px |}

===Photochromic quinones=== 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.<ref>{{Citation |last1=Lvov |first1=Andrey |title=Revisiting peri-aryloxyquinones: From a forgotten photochromic system to a promising tool for emerging applications |date=2023 |url=https://chemrxiv.org/engage/chemrxiv/article-details/6540b9e2c573f893f17f8037 |access-date=2025-02-26 |doi=10.26434/chemrxiv-2023-ds47k |last2=Klimenko |first2=Lyubov |last3=Bykov |first3=Vasily |last4=Hecht |first4=Stefan|doi-access=free }}</ref>

=== Inorganic photochromic materials === 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.<ref>{{cite journal |last1=Mongstad |first1=Trygve |last2=Platzer-Bjorkman |first2=Charlotte |last3=Maehlen |first3=Jan Petter |last4=Mooij |first4=Lennard P A |last5=Pivak |first5=Yevheniy |last6=Dam |first6=Bernard |last7=Marstein |first7=Erik S |last8=Hauback |first8=Bjorn C |last9=Karazhanov |first9=Smagul Zh |year=2011 |title=A new thin film photochromic material: Oxygen-containing yttrium hydride |journal=Solar Energy Materials and Solar Cells |volume=95 |issue=12 |page=3596-3599 |arxiv=1109.2872 |doi=10.1016/j.solmat.2011.08.018 |bibcode=2011SEMSC..95.3596M |s2cid=55961818}}</ref>

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.<ref name=":1" /> 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.<ref name=":1" />

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⁺).<ref>{{Cite journal |last1=Wang |first1=Shufen |last2=Fan |first2=Weiren |last3=Liu |first3=Zichuan |last4=Yu |first4=Aibing |last5=Jiang |first5=Xuchuan |date=2018 |title=Advances on tungsten oxide based photochromic materials: strategies to improve their photochromic properties |url=https://xlink.rsc.org/?DOI=C7TC04189F |journal=Journal of Materials Chemistry C |language=en |volume=6 |issue=2 |pages=191–212 |doi=10.1039/C7TC04189F |issn=2050-7526|url-access=subscription }}</ref><ref>{{Cite journal |last1=Dong |first1=Xu |last2=Shao |first2=Yuankai |last3=Ren |first3=Xiaoning |last4=Li |first4=Zhenguo |last5=Li |first5=Kaixiang |last6=Tong |first6=Yindong |last7=Liu |first7=Xianhua |last8=Lu |first8=Yiren |date=2023 |title=Inserting Bi atoms on the [WO6] framework: Photochromic WO3/Bi2WO6 nanoparticles enable visual sunlight UV sensing |url=https://linkinghub.elsevier.com/retrieve/pii/S0169433223014629 |journal=Applied Surface Science |language=en |volume=636 |article-number=157783 |doi=10.1016/j.apsusc.2023.157783|url-access=subscription }}</ref>

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.<ref>{{Cite journal |last1=Bechinger |first1=C. |last2=Oefinger |first2=G. |last3=Herminghaus |first3=S. |last4=Leiderer |first4=P. |date=1993 |title=On the fundamental role of oxygen for the photochromic effect of WO3 |url=https://pubs.aip.org/jap/article/74/7/4527/393407/On-the-fundamental-role-of-oxygen-for-the |journal=Journal of Applied Physics |language=en |volume=74 |issue=7 |pages=4527–4533 |doi=10.1063/1.354370 |issn=0021-8979|url-access=subscription }}</ref><ref>{{Cite journal |last1=Yao |first1=J. N. |last2=Hashimoto |first2=K. |last3=Fujishima |first3=A. |date=1992 |title=Photochromism induced in an electrolytically pretreated Mo03 thin film by visible light |journal=Nature |volume=355 |issue=6361 |pages=624–626 |doi=10.1038/355624a0 |bibcode=1992Natur.355..624Y |issn=0028-0836}}</ref>

===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 and the ruthenium sulfoxide compounds. The ruthenium sulfoxide complexes were created and developed by Rack and coworkers.<ref name=":53" /><ref name=":63" /> 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.<ref name=":63" />

==Applications: sunglasses and related materials == [[Image:PhotochromicLens.jpg|thumb|upright=1.25|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.<ref>{{Cite journal |last1=Evans |first1=Richard A. |last2=Hanley |first2=Tracey L. |last3=Skidmore |first3=Melissa A. |last4=Davis |first4=Thomas P. |last5=Such |first5=Georgina K. |last6=Yee |first6=Lachlan H. |last7=Ball |first7=Graham E. |last8=Lewis |first8=David A. |date=2005 |title=The generic enhancement of photochromic dye switching speeds in a rigid polymer matrix |url=https://www.nature.com/articles/nmat1326 |journal=Nature Materials |language=en |volume=4 |issue=3 |pages=249–253 |doi=10.1038/nmat1326 |pmid=15696171 |bibcode=2005NatMa...4..249E |issn=1476-1122|url-access=subscription }}</ref> 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.<ref>{{Cite journal |last1=Such |first1=Georgina K. |last2=Evans |first2=Richard A. |last3=Davis |first3=Thomas P. |date=2006 |title=Rapid Photochromic Switching in a Rigid Polymer Matrix Using Living Radical Polymerization |url=https://pubs.acs.org/doi/10.1021/ma052002f |journal=Macromolecules |language=en |volume=39 |issue=4 |pages=1391–1396 |doi=10.1021/ma052002f |bibcode=2006MaMol..39.1391S |issn=0024-9297|url-access=subscription }}</ref>

==Aspirational applications== ===Data storage=== Photochromic compounds for data storage has long been a topic of speculation.<ref>{{Cite journal |last=Hirshberg |first=Yehuda |date=1956 |title=Reversible Formation and Eradication of Colors by Irradiation at Low Temperatures. A Photochemical Memory Model |url=https://pubs.acs.org/doi/abs/10.1021/ja01591a075 |journal=Journal of the American Chemical Society |language=en |volume=78 |issue=10 |pages=2304–2312 |doi=10.1021/ja01591a075 |bibcode=1956JAChS..78.2304H |issn=0002-7863|url-access=subscription }}</ref> The area of 3D optical data storage promises discs that can hold a terabyte of data.<ref>{{Cite journal |last1=Kawata |first1=Satoshi |last2=Kawata |first2=Yoshimasa |date=2000 |title=Three-Dimensional Optical Data Storage Using Photochromic Materials |url=https://pubs.acs.org/doi/10.1021/cr980073p |journal=Chemical Reviews |language=en |volume=100 |issue=5 |pages=1777–1788 |doi=10.1021/cr980073p |pmid=11777420 |issn=0009-2665|url-access=subscription }}</ref>

===Solar energy storage=== Photochromism is a potential mechanism to store solar energy. The photochromic dihydroazulene–vinylheptafulvene system is a proof-of-concept.<ref>{{Cite journal |last1=Cacciarini |first1=Martina |last2=Skov |first2=Anders B. |last3=Jevric |first3=Martyn |last4=Hansen |first4=Anne S. |last5=Elm |first5=Jonas |last6=Kjaergaard |first6=Henrik G. |last7=Mikkelsen |first7=Kurt V. |last8=Brøndsted Nielsen |first8=Mogens |date=2015 |title=Towards Solar Energy Storage in the Photochromic Dihydroazulene–Vinylheptafulvene System |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.201500100 |journal=Chemistry – A European Journal |language=en |volume=21 |issue=20 |pages=7454–7461 |doi=10.1002/chem.201500100 |pmid=25847100 |issn=0947-6539|url-access=subscription }}</ref>

==See also== *Photosensitive glass *Hexaarylbiimidazole

==References== {{Reflist|30em}}

Category:Photochemistry Category:Chromism Category:Minerals