Editing Photoisomerization

{{Short description|Chemical change induced by light}}
{{Use American English|date = April 2019}}
[[File:AzobenzeneIsom,no Engl.png|thumb|300px|Photoisomerization of azobenzene<ref name=azo/>]]
In [[chemistry]], '''photoisomerization''' is a form of [[isomer]]ization induced by photoexcitation.<ref>{{cite book|chapter=Photoisomerization|year=2009|doi=10.1351/goldbook.P04622|title=IUPAC Compendium of Chemical Terminology|isbn=978-0-9678550-9-7}}</ref> Both reversible and irreversible photoisomerizations are known for [[photoswitch]]able compounds. The term "photoisomerization" usually, however, refers to a reversible process.

==Applications==
Photoisomerization of the compound [[retinal]] in the eye allows for vision.

Photoisomerizable substrates have been put to practical use, for instance, in [[pigment]]s for [[CD-RW|rewritable CDs]], [[DVD-RW|DVDs]], and [[3D optical data storage]] solutions. In addition, interest in photoisomerizable molecules has been aimed at molecular devices, such as [[molecular switch]]es,<ref>{{Cite journal |last1=Mammana |first1=A. |year=2011 |title=A Chiroptical Photoswitchable DNA Complex |journal=[[Journal of Physical Chemistry B]] |volume=115 |issue=40 |pages=11581–11587 |doi=10.1021/jp205893y |pmid=21879715 |display-authors=etal|url=https://pure.rug.nl/ws/files/10462706/2011JPhysChemBMammanaSupp.pdf |hdl=11370/cca715c8-861f-4500-943f-028c95e8e55e |s2cid=33375716 |hdl-access=free }}</ref><ref>{{cite journal | last1=Mokdad | first1 = A | last2 = Belof | first2 = J | last3 = Yi | first3 = S | last4 = Shuler | first4 = S | last5 = McLaughlin | first5 = M | last6 = Space | first6 = B | last7 = Larsen | first7 = R | title = Photophysical Studies of the Trans to Cis Isomerization of the Push−Pull Molecule: 1-(Pyridin-4-yl)-2-(N-methylpyrrol-2-yl)ethene (mepepy) | journal=[[Journal of Physical Chemistry B]] | volume = 112 | issue = 36 | pages = 8310–8315 | year = 2008 | doi = 10.1021/jp803268r | pmid = 18700732 | bibcode = 2008JPCA..112.8310M }}</ref> [[molecular motor]]s,<ref>{{Cite journal |last1=Vachon |first1=J. |year=2014 |title=An ultrafast surface-bound photo-active molecular motor |journal=[[Photochemical and Photobiological Sciences]] |volume=13 |issue=2 |pages=241–246 |doi=10.1039/C3PP50208B |pmid=24096390 |s2cid=23165784 |display-authors=etal|url=http://dare.uva.nl/personal/pure/en/publications/an-ultrafast-surfacebound-photoactive-molecular-motor(ca92afa2-9b98-44e8-b06c-c29c41da008d).html }}</ref> and [[molecular electronics]].

Another class of device that uses the photoisomerization process is as an additive in [[liquid crystals]] to change their linear and nonlinear properties.<ref>{{cite journal |last1=Janossy |first1=I. |last2=Szabados |first2=L. |title=Optical reorientation of nematic liquid crystals in the presence of photoisomerization |journal=Physical Review E |date=1 October 1998 |volume=58 |issue=4 |page=4598 |doi=10.1103/PhysRevE.58.4598|bibcode=1998PhRvE..58.4598J |s2cid=26508261 }}</ref> Due to the photoisomerization is possible to induce a molecular reorientation in the liquid crystal bulk, which is used in [[holography]],<ref>{{cite journal |last1=Chen |first1=Alan G |last2=Brady |first2=David J |title=Real-time holography in azo-dye-doped liquid crystals |journal=Optics Letters |date=1992 |volume=17 |issue=6 |pages=441–3 |doi=10.1364/OL.17.000441|pmid=19784354 |bibcode=1992OptL...17..441C |s2cid=20923350 }}</ref> as [[spatial filter]]<ref>{{cite journal |last1=Kato |first1=Jun-ichi |last2=Yamaguchi |first2=Ichirou |title=Nonlinear spatial filtering with a dye-doped liquid-crystal cell |journal=Optics Letters |date=1996 |volume=21 |issue=11 |pages=767–769 |doi=10.1364/OL.21.000767|pmid=19876152 |bibcode=1996OptL...21..767K }}</ref> or optical switching.<ref>{{cite journal |last1=Maly |first1=Kenneth E |last2=Wand |first2=Michael D |title=Bistable ferroelectric liquid crystal photoswitch triggered by a dithienylethene dopant |journal=Journal of the American Chemical Society |date=2002 |volume=124 |issue=27 |pages=7898–7899 |doi=10.1364/OPEX.13.002358|pmid=19495125 |doi-access=free}}</ref>

[[File:Methyl red.svg|thumb|Methyl red molecule, a common azo dye used in liquid crystal doping]]

==Examples==
Azobenzenes,<ref name=azo>{{cite journal|doi=10.1021/cr970155y|pmid=12428986|title=Photoinduced Motions in Azo-Containing Polymers|journal=Chemical Reviews|volume=102|issue=11|pages=4139–4176|year=2002|last1=Natansohn|first1=Almeria|last2=Rochon|first2=Paul}}</ref> [[stilbene]]s,<ref>{{cite journal|doi=10.1021/cr00003a007|title=Photoisomerization dynamics of stilbenes|journal=Chemical Reviews|volume=91|issue=3|pages=415–436|year=1991|last1=Waldeck|first1=David H.}}</ref> [[spiropyran]]s,<ref>{{cite journal|doi=10.1039/C3CS60181A|pmid=23979515|title=Spiropyran-based dynamic materials|journal=Chem. Soc. Rev.|volume=43|issue=1|pages=148–184|year=2014|last1=Klajn|first1=Rafal|doi-access=free}}</ref> are prominent classes of compounds subject to photoisomerism.

:[[File:Synthesis of quadricyclane from norbornadiene.png|400px|center|thumb|Photoisomerization of norbornadiene to quadricyclane.]]
In the presence of a catalyst, [[norbornadiene]] converts to [[quadricyclane]] via ~300nm [[UV radiation]]. When converted back to norbornadiene, quadryicyclane’s ring strain energy is liberated in the form of heat (''ΔH'' = −89 kJ/mol). This reaction has been proposed to store [[solar energy]] ([[photoswitch]]s).<ref>{{cite journal|doi=10.1070/RC2002v071n11ABEH000745|title=Norbornadiene–quadricyclane as an abiotic system for the storage of solar energy|journal=Russian Chemical Reviews|volume=71|issue=11|pages=917–927|year=2002|last1=Dubonosov|first1=Alexander D.|last2=Bren|first2=Vladimir A.|last3=Chernoivanov|first3=V. A.|bibcode=2002RuCRv..71..917D|s2cid=250890545 }}</ref>

Photoisomerization behavior can be roughly categorized into several classes. Two major classes are ''trans''–''cis'' (or ''E''–''Z'') conversion, and open-closed ring transition. Examples of the former include [[stilbene]] and [[azobenzene]]. This type of compounds has a double [[chemical bond|bond]], and rotation or inversion around the double bond affords isomerization between the two states.<ref>{{Cite journal|title=Spectroscopic and computational studies of the photoisomerization|journal=Journal of Molecular Structure|volume=1178|pages=538–543|doi=10.1016/j.molstruc.2018.10.071|bibcode=2019JMoSt1178..538K|year=2019 | last1 = Kazem-Rostami | first1 = Masoud | last2 = Akhmedov | first2 = Novruz G. | last3 = Faramarzi | first3 = Sadegh|s2cid=105312344 }}</ref> Examples of the latter include [[fulgide]] and [[diarylethene]]. This type of compounds undergoes bond cleavage and bond creation upon irradiation with particular wavelengths of light. Still another class is the [[di-π-methane rearrangement]].

==Coordination chemistry==
Many complexes are often photosensitive and many of these complexes undergo photoisomerization.<ref>{{cite book|
title=Photochemistry and Photophysics of Metal Complexes|author=D. M. Roundhill|publisher=Springer|year=1994|isbn= 978-1-4899-1495-8}}</ref>  One case is the conversion of the colorless cis-[[bis(triphenylphosphine)platinum chloride]] to the yellow trans isomer.
[[File:Cis-trans-PtCl2P2.png|thumb|center|Photoisomerization of PtCl<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub>]]

Some [[coordination complex]]es undergo change in their spin state upon illumination, i.e. these are photosensitive [[spin crossover]] complexes.<ref>{{cite journal|doi=10.1016/S0010-8545(01)00381-2|title=Photoswitchable coordination compounds|journal=Coordination Chemistry Reviews|volume=219–221|pages=839–879|year=2001|last1=Gã¼Tlich|first1=P.}}</ref>
[[File:Fe(picNH2)3.png|thumb|center|Light-induced spin-crossover of [Fe(pyCH<sub>2</sub>NH<sub>2</sub>)<sub>3</sub>]<sup>2+</sup>, which switches from high and low-spin]]

==See also==
*[[Photochromism]]

==References==
{{reflist}}

[[Category:Chemical reactions]]