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			{{Short description|Two phenyl rings linked by a N═N double bond}}
	{{chembox
			{{Chembox
	| Verifiedfields = changed
			|Verifiedfields = changed
	| verifiedrevid = 399535705
			|Watchedfields = changed
	| Name = Azobenzene
			|verifiedrevid = 458972978
	| ImageFile_Ref = {{chemboximage|correct|??}}
	| ImageFile = Azobenzene.svg		|Name = Azobenzene
			|ImageFile_Ref = {{chemboximage|correct|??}}
	| ImageName = Skeletal formula of azobenzene
			|ImageFile = (E)-1,2-diphenyldiazene 200.svg
	| ImageFile1 = Azobenzene-trans-3D-balls.png
			|ImageName = Skeletal formula of azobenzene
	| ImageSize1 = 240px
			|ImageFile1 = Azobenzene-trans-3D-balls.png
	| ImageName1 = Ball-and-stick model of azobenzene
			|ImageSize1 = 240px
	| IUPACName = (''E'')-diphenyldiazene
			|ImageName1 = Ball-and-stick model of azobenzene
	| OtherNames = Azobenzene
			|IUPACName = (''E'')-Diphenyldiazene
	| Section1 = {{Chembox Identifiers
			|OtherNames = Azobenzene
	| ChEBI_Ref = {{ebicite|changed|EBI}}
			|Section1={{Chembox Identifiers
	| ChEBI = 58996
			|ChEBI_Ref = {{ebicite|correct|EBI}}
	| SMILES = N(=N/c1ccccc1)\c2ccccc2
			|ChEBI = 58996
	| InChI = 1/C12H10N2/c1-3-7-11(8-4-1)13-14-12-9-5-2-6-10-12/h1-10H/b14-13+
			|SMILES = N(=N/c1ccccc1)\c2ccccc2
	| InChIKey = DMLAVOWQYNRWNQ-BUHFOSPRBP
			|InChI = 1/C12H10N2/c1-3-7-11(8-4-1)13-14-12-9-5-2-6-10-12/h1-10H/b14-13+
	| ChEMBL = 58835
			|InChIKey = DMLAVOWQYNRWNQ-BUHFOSPRBP
	| PubChem = 2272
	| StdInChI_Ref = {{stdinchicite|correct|chemspider}}		|ChEMBL_Ref = {{ebicite|correct|EBI}}
			|ChEMBL = 58835
	| StdInChI = 1S/C12H10N2/c1-3-7-11(8-4-1)13-14-12-9-5-2-6-10-12/h1-10H/b14-13+
			|PubChem = 2272
	| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
			|StdInChI_Ref = {{stdinchicite|correct|chemspider}}
	| StdInChIKey = DMLAVOWQYNRWNQ-BUHFOSPRSA-N
			|StdInChI = 1S/C12H10N2/c1-3-7-11(8-4-1)13-14-12-9-5-2-6-10-12/h1-10H/b14-13+
	| CASNo_Ref = {{cascite|correct|CAS}}
			|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
	| CASNo = 103-33-3
			|StdInChIKey = DMLAVOWQYNRWNQ-BUHFOSPRSA-N
	| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
			|CASNo_Ref = {{cascite|correct|CAS}}
	| ChemSpiderID = 2185
			|CASNo = 103-33-3
	| RTECS = CN1400000
			|EC_number = 203-102-5
	| KEGG_Ref = {{keggcite|changed|kegg}}
			|Beilstein = 742610
	| KEGG = <!-- blanked - oldvalue: C19334 -->
			|Gmelin = 83610
			|UNII_Ref = {{fdacite|correct|FDA}}
			|UNII = F0U1H6UG5C
			|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
			|ChemSpiderID = 2185
			|RTECS = CN1400000
			|KEGG_Ref = {{keggcite|correct|kegg}}
			|KEGG = C19334
			}}
			|Section2={{Chembox Properties
			|C=12 | H=10 | N=2
			|Appearance = orange-red crystals<ref name=crc/>
			|Density = 1.203 g/cm<sup>3</sup><ref name=crc>Haynes, p. 3.32</ref>
			|Solubility = 6.4 mg/L (25 °C)
			|MeltingPt = 67.88 °C (trans), 71.6 °C (cis)
			|MeltingPt_ref =<ref name=crc/>
			|BoilingPtC = 300
			|BoilingPt_ref =<ref name=crc/>
			|pKa = -2.95<ref>{{cite journal|title=Protonation of azo-compounds. Part II: The structure of the conjugate acid of trans-azobenzene|journal=Recl. Trav. Chim. Pays-Bas|volume=88|issue=5|year=1969|pages=562–572|doi=10.1002/recl.19690880507|last1=Hoefnagel|first1=M. A.|last2=Van Veen|first2=A.|last3=Wepster|first3=B. M.}}</ref>
			| MagSus = -106.8·10<sup>−6</sup> cm<sup>3</sup>/mol<ref>Haynes, p. 3.579</ref>
			| RefractIndex = 1.6266 (589 nm, 78 °C)<ref name=crc/>
			}}
			|Section3={{Chembox Structure
			|MolShape = sp<sup>2</sup> at N
			|Dipole = 0 ] (trans isomer)
			}}
			|Section4 ={{Chembox Hazards
			|MainHazards = toxic
			|FlashPtC = 476
			|GHSPictograms = {{GHS07}}{{GHS08}}{{GHS09}}
			|GHSSignalWord = Danger
			|HPhrases = {{H-phrases|302|332|341|350|373|410}}
			|PPhrases = {{P-phrases|201|202|260|261|264|270|271|273|281|301+312|304+312|304+340|308+313|312|314|330|391|405|501}}
			}}
			|Section5={{Chembox Related
			|OtherCompounds = ] ]
	}}		}}
	| Section2 = {{Chembox Properties
	|C=12|H=10|N=2
	| Appearance = orange-red crystals
	| Density = 1.09 g/cm³, solid
	| Solubility = 2.4e-6 g/100mL (25 °C)
	| MeltingPt = 69 °C (342 K)
	| BoilingPt = 293 °C (566 K)
	| pKa = 3.3
	}}
	| Section3 = {{Chembox Structure
	| MolShape = sp<sup>2</sup> at N
	| Dipole = 0 ] (trans isomer)
	}}
	| Section7 = {{Chembox Hazards
	| MainHazards = toxic
	| FlashPt = 476 °C
	| RPhrases = 45-20/22-48/22-50/53-68
	| SPhrases = 53-45-60-61
	}}
	| Section8 = {{Chembox Related
	| OtherCpds = ] ]
	}}
	}}		}}
			'''Azobenzene''' is a ]able ] compound composed of two ] rings linked by a ] ]. It is the simplest example of an aryl ]. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of similar ]. These azo compounds are considered as derivatives of ] (diimide),<ref name="Gold">{{ GoldBookRef | title = azo compounds | file = A00560 | year = 2009 }}</ref> and are sometimes referred to as 'diazenes'. The diazenes absorb light strongly and are common ].<ref>{{cite book|title=Hydrazo, Azo and Azoxy Groups|year=1975|editor=Saul Patai|isbn=9780470023419 |doi=10.1002/0470023414
	]. The ''trans'' form (left) can be converted to the ''cis'' form (right) using an appropriate wavelength (UV at 300-400 nm) of light. A different wavelength (visible blue light >400 nm) can be used to convert the molecule back to the ''trans'' form. Alternately, the molecule will thermally relax to the stable ''trans'' form.]]
			|publisher=John Wiley & Sons|series=PATAI'S Chemistry of Functional Groups|volume=1 }}</ref> Different classes of azo dyes exist, most notably the ones substituted with heteroaryl rings.<ref>{{Cite journal |last1=Crespi |first1=Stefano |last2=Simeth |first2=Nadja A. |last3=König |first3=Burkhard |date=March 2019 |title=Heteroaryl azo dyes as molecular photoswitches |url=https://www.nature.com/articles/s41570-019-0074-6 |journal=Nature Reviews Chemistry |language=en |volume=3 |issue=3 |pages=133–146 |doi=10.1038/s41570-019-0074-6 |issn=2397-3358}}</ref>
			==Structure and synthesis==
	'''Azobenzene''' is a ] compound composed of two ] rings linked by a ] ]. It is the best known example of an ]. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of ]s that share the core azobenzene structure, with different chemical ] extending from the phenyl rings. These azo compounds are considered as derivatives of ] (diimide),<ref name="Gold">{{GoldBookRef|title= azo compounds|url=http://goldbook.iupac.org/A00560.html |year=2009}}</ref> and are sometimes referred to as 'diazenes'. The diazenes strongly absorb light and are used as ] in a variety of industries.
			]
			Azobenzene was first described by ] in 1834.<ref>{{cite journal|author=Mitscherlich, E. |year=1834|title=Ueber das Stickstoffbenzid|journal=Ann. Pharm.|volume= 12|issue=2–3|pages= 311–314|doi=10.1002/jlac.18340120282|url=https://zenodo.org/record/1426912|bibcode=1834AnP...108..225M}}</ref><ref>{{cite journal|last1=Merino |first1=Estíbaliz |last2=Ribagorda Beilstein |first2=María |title=Control over molecular motion using the cis–trans photoisomerization of the azo group|journal=J. Org. Chem.|year=2012|volume= 8|pages= 1071–1090|doi=10.3762/bjoc.8.119|pmid=23019434|pmc=3458724}}</ref> Yellowish-red crystalline flakes of azobenzene were obtained in 1856.<ref>{{ cite journal | title = III. Zur Geschichte des Azobenzols und des Benzidins | year = 1856 | last1 = Noble<!--not Nobel--> | first1 = Alfred | journal = Annalen der Chemie und Pharmacie | volume = 98 | issue = 2 | pages = 253–256|doi=10.1002/jlac.18560980211| url = https://zenodo.org/record/1427078 }}</ref> Its original preparation is similar to the modern one. According to the 1856 method, ] is reduced by iron filings in the presence of ]. In the modern synthesis, ] is the reductant in the presence of a base.<ref>{{OrgSynth
			| author =Bigelow, H. E. |author2=Robinson, D. B.
			| year =1955
			| title =Azobenzene
			| volume =22
			| pages =28
			| collvol =3
			| collvolpages =103
			| prep =CV3P0103}}</ref> Industrial ] using nitrobenzene is also employed.<ref name=Sequeira>{{cite journal
			|last1=Cardoso |first1=D. S. |last2=Šljukić |first2=B. |last3=Santos |first3=D. M. |last4=Sequeira |first4=C. A.
			|date=July 17, 2017
			|title=Organic Electrosynthesis: From Laboratorial Practice to Industrial Applications
			|journal=] |volume=21 |issue=9 |pages=1213–1226
			|doi=10.1021/acs.oprd.7b00004}}</ref>
			''trans''-Azobenzene isomer is planar with an N-N distance of 1.189 Å.<ref>{{cite journal |last1=Harada |first1=J. |last2=Ogawa |first2=K. |last3=Tomoda |first3=S. |year=1997 |title=Molecular Motion and Conformational Interconversion of Azobenzenes in Crystals as Studied by X-ray Diffraction |journal=Acta Crystallogr. B |volume=53 |issue=4 |page=662 |doi=10.1107/S0108768197002772 |doi-access=}}</ref> ''cis''-Azobenzene is nonplanar with a C-N=N-C dihedral angle of 173.5° and an N-N distance of 1.251 Å.<ref>{{cite journal |author1=Mostad, A. |author2=Rømming, C. |year=1971 |title=A Refinement of the Crystal Structure of cis-Azobenzene |journal=Acta Chem. Scand. |volume=25 |page=3561 |doi=10.3891/acta.chem.scand.25-3561 |doi-access=free}}</ref> The trans isomer is more stable by approximately 50 kJ/mol, and the barrier to isomerization in the ground state is approximately 100 kJ/mol.]. The ''trans'' form (left) can be converted to the ''cis'' form (right) using a UV wavelength of 300–400 nm. Visible illumination at >400 nm converts the molecule back to the ''trans'' form. Alternately, the molecule will thermally relax to the stable ''trans'' form.]]
	==Synthesis==
			]
	Azobenzene was first described in 1856 as "gelblich-rote krystallinische Blättchen" ("yellowish-red crystalline flakes" in ]).<ref>Noble, A. “Zur Geschichte des Azobenzols und des Benzidins” Annalen der Chemie und Pharmacie 1856, Volume 98, p 253-256. {{DOI|10.1002/jlac.18560980211 }}.</ref> Its original preparation is similar to the modern one. According to the 1858 method, ] is reduced by iron filings in the presence of ]. In the modern synthesis, ] is the reductant in the presence of a base.<ref>Bigelow, H. E.; Robinson, D. B. "Azobenzene" Organic Syntheses, Collected Volume 3, p.103 (1955). http://www.orgsyn.org/orgsyn/pdfs/CV3P0103.pdf</ref>
			==Reactions==
	==Trans-cis isomerization==
			Azobenzene is a weak base, but undergoes protonation at one nitrogen with a pK<sub>a</sub> = -2.95. It functions as a ], e.g. toward boron trihalides. It binds to low valence metal centers, e.g. Ni(Ph<sub>2</sub>N<sub>2</sub>)(PPh<sub>3</sub>)<sub>2</sub> is well characterized.<ref>{{cite journal|title=Phosphinohydrazines and phosphinohydrazides M(–N(R)–N(R)–PPh2)n of some transition and main group metals: synthesis and characterization: Rearrangement of Ph2P–NR–NR– ligands into aminoiminophosphorane, RNPPh2–NR–, and related chemistry|
	One of the most intriguing properties of azobenzene (and derivatives) is the ] of ] and ] isomers. The two isomers can be switched with particular wavelengths of light: ultraviolet light, which corresponds to the energy gap of the π-π* (''S<sub>2</sub>'' state) transition, for trans-to-cis conversion, and blue light, which is equivalent to that of the n-π* (''S<sub>1</sub>'' state) transition, for cis-to-trans isomerization. For a variety of reasons, the ''cis'' isomer is less stable than the trans (for instance, it has a distorted configuration and is less delocalized than the trans configuration). Thus, cis-azobenzene will thermally relax back to the trans via cis-to-trans isomerization. The trans isomer is more stable by approximately 50 kJ/mol, and the barrier to photo-isomerization is approximately 200 kJ/mol.
			volume=689|issue=19|year=2004|pages=3060–3074|journal=J. Organomet. Chem.|doi=10.1016/j.jorganchem.2004.06.056|
			last1=Fedotova|
			first1=Yana V.|
			last2=Kornev|
			first2=Alexander N.|
			last3=Sushev|
			first3=Vyacheslav V.|
			last4=Kursky|
			first4=Yurii A.|
			last5=Mushtina|
			first5=Tatiana G.|
			last6=Makarenko|
			first6=Natalia P.|
			last7=Fukin|
			first7=Georgy K.|
			last8=Abakumov|
			first8=Gleb A.|
			last9=Zakharov|
			first9=Lev N.|
			last10=Rheingold|
			first10=Arnold L.}}</ref>
			Azobenzene ] to give ]. ] gives ].
	==Spectroscopic classification==
	The wavelengths at which azobenzene isomerization occurs depends on the particular structure of each azo molecule, but they are typically grouped into three classes: the azobenzene-type molecules, the aminoazobenzenes, and the pseudo-]. These azos are yellow, orange, and red, respectively, owing to the subtle differences in their electronic absorption spectra. The compounds similar to the unsubstituted azobenzene exhibit a low-intensity n-π* absorption in the visible region, and a much higher intensity π-π* absorption in the ]. Azos that are ] or ]substituted with ]s (such as ]s), are classified as aminoazobenzenes, and tend to closely spaced n-π* and π-π* bands in the visible. The pseudo-stilbene class is characterized by substituting the 4 and 4' positions of the two azo rings with electron-donating and electron-withdrawing groups (that is, the two opposite ends of the ] system are functionalized). The addition of this ] configuration results in a strongly asymmetric ] distribution, which modifies a host of optical properties. In particular, it shifts the ] of the ''trans'' and the ''cis'' isomers, so that they effectively overlap. Thus, for these compounds a single ] of light in the visible region will induce both the forward and reverse isomerization. Under illumination, these molecules cycle between the two isomeric states.
	==Photophysics of isomerization==		===Trans–cis isomerization===
			Azobenzene (and derivatives) undergo ] of ] and ] isomers. cis-Azobenzene relaxes back, in dark, to the trans isomer. Such thermal relaxation is slow at room temperature. The two isomers can be switched with particular wavelengths of light: ultraviolet light, which corresponds to the energy gap of the π-π* (''S<sub>2</sub>'' state) transition, for trans-to-cis conversion, and blue light, which is equivalent to that of the n-π* (''S<sub>1</sub>'' state) transition, for cis-to-trans isomerization. For a variety of reasons, the ''cis'' isomer is less stable than the trans (for instance, it has a distorted configuration and is less delocalized than the trans configuration). Photoisomerization allows for reversible energy storage (as ]es).
	The photo-isomerization of azobenzene is extremely rapid, occurring on picosecond timescales. The rate of the thermal back-relaxation varies greatly depending on the compound: usually hours for azobenzene-type molecules, minutes for aminoazobenzenes, and seconds for the pseudo-stilbenes.
			====Spectroscopic classification====
	The mechanism of isomerization has been the subject of some debate, with two pathways identified as viable: a ''rotation'' about the N-N bond, with disruption of the double bond, or via an ''inversion'', with a semi-linear and hybridized transition state. It has been suggested that the ''trans''-to-''cis'' conversion occurs via rotation into the ''S<sub>2</sub>'' state, whereas inversion gives rise to the ''cis''-to-''trans'' conversion. It is still under discussion which excited state plays a direct role in the series of the photoisomerization behavior. However, the latest research on ] has suggested that the ''S<sub>2</sub>'' state undergoes internal conversion to the ''S<sub>1</sub>'' state, and then the ''trans''-to-''cis'' isomerization proceeds. Recently another isomerization pathway has been proposed by Diau,<ref>''A New Trans-to-Cis Photoisomerization Mechanism of Azobenzene on the S1(n,π*) Surface'' J. Phys. Chem. A, '''2004''', 108 (6), pp 950–956 {{DOI|10.1021/jp031149a}}</ref> the "concerted inversion" pathway in which both CNN bond angles bend at the same time.
			The wavelengths at which azobenzene isomerization occurs depends on the particular structure of each azo molecule, but they are typically grouped into three classes: the azobenzene-type molecules, the aminoazobenzenes, and the pseudo-]. These azos are yellow, orange, and red, respectively,<ref name=rau>{{ cite book | author = Rau, H. | title = Photochemistry and Photophysics | volume = 2 | editor = Rabek, J. F. | publisher = CRC Press | location = Boca Raton, FL | year = 1990 | pages = 119–141 | isbn = 978-0-8493-4042-0 }}</ref><ref name=shahinpoor>{{ cite book | chapter = Chapter 17 - Azobenzene Polymers as Photomechanical and Multifunctional Smart Materials |author1=Yager, K. G. |author2=Barrett, C. J. | title = Intelligent Materials |editor1=Shahinpoor, M. |editor2=Schneider, H.-J. | publisher = Royal Society of Chemistry | location = Cambridge | year = 2008 | pages = 426–427 | isbn = 978-1-84755-800-8 | doi = 10.1039/9781847558008-00424 | url = http://pubs.rsc.org/en/Content/eBook/978-0-85404-335-4 | chapter-url = https://books.google.com/books?id=Hmq4ctnA1KIC&pg=PA425 }}</ref> owing to the subtle differences in their electronic absorption spectra. The compounds similar to the unsubstituted azobenzene exhibit a low-intensity n-π* absorption in the visible region, and a much higher intensity π-π* absorption in the ]. Azos that are ] or ]substituted with ]s (such as ]s), are classified as aminoazobenzenes, and tend to closely spaced<ref name=rau /> n-π* and π-π* bands in the visible. The pseudo-stilbene class is characterized by substituting the 4 and 4' positions of the two azo rings with electron-donating and electron-withdrawing groups (that is, the two opposite ends of the ] system are functionalized). The addition of this ] configuration results in a strongly asymmetric ] distribution, which modifies a host of optical properties. In particular, it shifts the ] of the ''trans'' and the ''cis'' isomers, so that they effectively overlap.<ref name=shahinpoor /> Thus, for these compounds a single ] of light in the visible region will induce both the forward and reverse isomerization. Under illumination, these molecules cycle between the two isomeric states.
			====Photophysics of isomerization====
	==Photoinduced motions==
			The photo-isomerization of azobenzene is extremely rapid, occurring on picosecond timescales. The rate of the thermal back-relaxation varies greatly depending on the compound: usually hours for azobenzene-type molecules, minutes for aminoazobenzenes, and seconds for the pseudo-stilbenes.<ref name=shahinpoor />
	The photo-isomerization of azobenzene is a form of light-induced molecular motion.<ref>H. Rau, in ''Photochemistry and Photophysics''; Vol. 2, edited by J. Rebek (CRC Press, Boca Raton, FL, 1990), p. 119-141.</ref><ref>A. Natansohn and P. Rochon, ''Chem. Rev.'' 102, 4139-4176 (2002).</ref><ref>Y. Yu, M. Nakano, and T. Ikeda, ''Nature (London, U. K.)'' 425, 145 (2003).</ref> This isomerization can also lead to motion on larger length scales. For instance, ] light will cause the molecules to isomerize and relax in random positions. However, those relaxed (''trans'') molecules that fall perpendicular to the incoming light polarization will no longer be able to absorb, and will remain fixed. Thus, there is a statistical enrichment of chromophores perpendicular to polarized light (orientational hole burning). Polarized irradiation will make an azo-material ] and therefore optically ] and ]. This photo-orientation can also be used to orient other materials (especially in ] systems).<ref>K. Ichimura, ] '''2000''', 100, 1847</ref> For instance, it has been used to selectively orient ] domains, and used to create ] (NLO) materials. Azo isomerization can also be used to photo-switch the liquid crystal phase of a material from ] to ]<ref>S. Tazuke, S. Kurihara, T. Ikeda, ] '''1987''', 911</ref><ref>N. Tamaoki, ] '''2001''', 13, 1135</ref> or to change the ] of a ] phase.<ref>S. Pieraccini, S. Masiero, G. P. Spada, G. Gottarelli, ] '''2003''', 598</ref>
			The mechanism of isomerization has been the subject of some debate, with two pathways identified as viable: a ''rotation'' about the N-N bond, with disruption of the double bond, or via an ''inversion'', with a semi-linear and hybridized transition state. It has been suggested that the ''trans''-to-''cis'' conversion occurs via rotation into the ''S<sub>2</sub>'' state, whereas inversion gives rise to the ''cis''-to-''trans'' conversion. It is still under discussion which excited state plays a direct role in the series of the photoisomerization behavior. However, the latest research utilizing ] has suggested that the ''S<sub>2</sub>'' state undergoes internal conversion to the ''S<sub>1</sub>'' state, and then the ''trans''-to-''cis'' isomerization proceeds. Recently another isomerization pathway has been proposed by Diau,<ref>{{cite journal | doi = 10.1021/jp031149a | title = A New Trans-to-Cis Photoisomerization Mechanism of Azobenzene on the S1(n,π*) Surface | year = 2004 | last1 = Diau | first1 = E. W.-G. | s2cid = 54662441 | journal = The Journal of Physical Chemistry A | volume = 108 | issue = 6 | pages = 950–956 | bibcode = 2004JPCA..108..950W }}</ref> the "concerted inversion" pathway in which both CNN bond angles bend at the same time. There is experimental and computational evidence for the existence of a multistate rotation mechanism involving a triplet state.<ref>{{Cite journal |last1=Reimann |first1=Marc |last2=Teichmann |first2=Ellen |last3=Hecht |first3=Stefan |last4=Kaupp |first4=Martin |date=2022-11-24 |title=Solving the Azobenzene Entropy Puzzle: Direct Evidence for Multi-State Reactivity |url=https://pubs.acs.org/doi/10.1021/acs.jpclett.2c02838 |journal=The Journal of Physical Chemistry Letters |language=en |volume=13 |issue=46 |pages=10882–10888 |doi=10.1021/acs.jpclett.2c02838 |pmid=36394331 |issn=1948-7185}}</ref>
	In 1995, it was reported that exposing a thin film of azo-polymer to a light intensity (or polarization) gradient leads to spontaneous surface patterns. In essence, the polymer material will reversibly deform so as to minimize the amount of material exposed to the light. This phenomenon is not ], since it readily occurs at low power and the transformation is reversible. The mechanism of this surface ] seems related to a new photomechanical effect, involving azobenzene isomerization.<ref>{{cite journal | url=http://pubs.acs.org/doi/abs/10.1021/ma061733s | title=Photomechanical Surface Patterning in Azo-Polymer Materials | author=Kevin G. Yager and Christopher J. Barrett | journal= Macromolecules | year= 2006 |volume= 39 | issue=26 | pages=9320–6 | doi=10.1021/ma061733s}}</ref>
			====Photoinduced motions====
	Bulk expansion and contraction of azobenzene materials have also been observed. In one report, a thin film was made to bend and unbend by exposing it to polarized light. The direction of the macroscopic motion could be controlled by the polarization direction. The bending occurred because the free surface of the material contracted more than the inside of the thin film (due to absorption of laser light as it passes through the film).
			The photo-isomerization of azobenzene is a form of light-induced molecular motion.<ref name=rau /><ref>{{ cite journal |author1=Natansohn A. |author2=Rochon, P. | title = Photoinduced motions in azo-containing polymers | journal = Chemical Reviews | volume = 102 | issue = 11 | pages = 4139–4175 |date=November 2002 | pmid = 12428986 | doi = 10.1021/cr970155y}}</ref><ref>{{ cite journal | doi = 10.1038/425145a | title = Photomechanics: Directed bending of a polymer film by light | year = 2003 | last1 = Yu | first1 = Y. | last2 = Nakano | first2 = M. | last3 = Ikeda | first3 = T. | journal = Nature | volume = 425 | issue = 6954 | pages = 145 | pmid = 12968169 | bibcode = 2003Natur.425..145Y | doi-access = free }}</ref> This isomerization can also lead to motion on larger length scales. For instance, ] light will cause the molecules to isomerize and relax in random positions.<ref>{{Cite journal |last1=Nassrah |first1=Ameer R. K. |last2=Jánossy |first2=István |last3=Kenderesi |first3=Viktor |last4=Tóth-Katona |first4=Tibor |date=January 2021 |title=Polymer–Nematic Liquid Crystal Interface: On the Role of the Liquid Crystalline Molecular Structure and the Phase Sequence in Photoalignment |journal=Polymers |language=en |volume=13 |issue=2 |pages=193 |doi=10.3390/polym13020193 |doi-access=free |issn=2073-4360 |pmc=7825733 |pmid=33430256}}</ref> However, those relaxed (''trans'') molecules that fall perpendicular to the incoming light polarization will no longer be able to absorb, and will remain fixed. Thus, there is a statistical enrichment of chromophores perpendicular to polarized light (orientational hole burning). Polarized irradiation will make an azo-material ] and therefore optically ] and ]. This photo-orientation can also be used to orient other materials (especially in ] systems).<ref>{{ cite journal | doi = 10.1021/cr980079e | title = Photoalignment of Liquid-Crystal Systems | year = 2000 | last1 = Ichimura | first1 = K. | journal = Chemical Reviews | volume = 100 | issue = 5 | pages = 1847–1874 | pmid = 11777423 }}</ref>
	==Other aspects==		==Miscellaneous==
			Azobenzene undergoes ] by metal complexes, e.g. ]:<ref>{{cite journal | last1 = Murahashi | first1 = Shunsuke | last2 = Horiie | first2 = Shigeki|title = The reaction of azobenzene and carbon monoxide | journal = Journal of the American Chemical Society | volume = 78 | pages = 4816 | year = 1956 | doi = 10.1021/ja01599a079 | issue = 18}}</ref>
	Azobenzene molecules can be incorporated into ] matrices as stabilizers. It is also interesting to note that the rigid rod-like structure of azo molecules allows them to behave as ] ]s in many materials.
			]
	The large geometry change associated with azobenzene photoisomerization has also been used to control protein activity with light. Azobenzene has been attached to ligands (drug) to photo-modulate their affinity for proteins. Azobenzene has been employed as a photoswitchable tether between a ligand and the protein: one end of the azobenzene is substituted with a reactive group that attaches to the target protein. The other end displays a ligand for the protein. Depending on where the azobenzene is attached, either the cis or trans isomer will present the ligand to the ligand-binding site, while the other isomer prevents the drug from reaching the site. Again, photoswitching between isomers turns the protein on and off. When applied to ion channels in the nervous system, this approach affords optical control of electrical activity in neurons.
	<ref>Gorostiza P, Isacoff EY. Optical switches for remote and noninvasive control of cell signaling. Science. 2008 Oct 17;322(5900):395-9.</ref><ref>Banghart MR, Volgraf M, Trauner D. Engineering light-gated ion channels. Biochemistry. 2006 Dec 26;45(51):15129-41.</ref>
			Info about the carcinogenicity of Azobenzene can be found on the epa site.
			<ref>{{|first=United States Environmental Protection Agency |date=2024-06-06 |title=Azobenzene |url=https://iris.epa.gov/ChemicalLanding/&substance_nmbr=351}}</ref>
	==References==		==References==
	{{Reflist}}		{{Reflist|30em}}
			==Cited sources==
	]
			*{{RubberBible92nd|page=3.32}}
	]
			==Further reading==
	]
			{{Commons category|Azobenzene}}
	]
			*Of historic interest: {{cite journal | title = The cis form of Azobenzene | author = G. S. Hartley | journal = Nature | year = 1937 | volume = 140 | issue = 3537 | pages = 281 | doi = 10.1038/140281a0| bibcode = 1937Natur.140..281H | doi-access = free }}
	]
			*{{cite journal|last1=Torres-Zúñiga|first1=V. |last2=Morales-Saavedra |first2=O. G. |last3=Rivera |first3=E. |last4=Castañeda-Guzmán |first4=R. |last5=Bañuelos |first5=J. G. |last6=Ortega-Martínez |first6=R. |title=Preparation and photophysical properties of monomeric liquid-crystalline azo-dyes embedded in bulk and film SiO<sub>2</sub>-sonogel glasses|journal=Journal of Sol-Gel Science and Technology|year=2010|volume=56|issue=1|pages=7–18|doi=10.1007/s10971-010-2265-y|s2cid=96304240 }}
	]
			*{{ cite journal | doi = 10.1246/cl.1987.911 | title = Amplified image recording in liquid crystal media by means of photochemically triggered phase transition | year = 1987 | last1 = Tazuke | first1 = S. | last2 = Kurihara | first2 = S. | last3 = Ikeda | first3 = T. | journal = Chemistry Letters | volume = 16 | issue = 5 | pages = 911–914 }}
	]
			*{{ cite journal | doi = 10.1002/1521-4095(200108)13:15<1135::AID-ADMA1135>3.0.CO;2-S | title = Cholesteric Liquid Crystals for Color Information Technology | year = 2001 | last1 = Tamaoki | first1 = N. | journal = Advanced Materials | volume = 13 | issue = 15 | pages = 1135–1147 }}
	]
			*{{ cite journal | doi = 10.1039/b211421f | pmid = 12669843 | title = A new axially-chiral photochemical switch | year = 2003 | last1 = Pieraccini | first1 = S. | last2 = Masiero | first2 = S. | last3 = Spada | first3 = G. P. | last4 = Gottarelli | first4 = G. | journal = Chemical Communications | volume = 2003 | issue = 5 | pages = 598–599 }}
	]
			*{{ cite journal | title = Photomechanical Surface Patterning in Azo-Polymer Materials |author1=Yager, K. G. |author2=Barrett, C. J. | journal = Macromolecules | year = 2006 | volume = 39 | issue = 26 | pages = 9320–9326 | doi = 10.1021/ma061733s |bibcode=2006MaMol..39.9320Y }}
	]
			*{{ cite journal |author1=Gorostiza, P. |author2=Isacoff, E. Y. | title = Optical switches for remote and noninvasive control of cell signaling | journal = Science | volume = 322 | issue = 5900 | pages = 395–399 |date=October 2008 | pmid = 18927384 | doi = 10.1126/science.1166022 |bibcode=2008Sci...322..395G | pmc = 7592022 }}
	]
			*{{ cite journal |author1=Banghart, M. R. |author2=Volgraf, M. |author3=Trauner, D. | title = Engineering light-gated ion channels | journal = Biochemistry | volume = 45 | issue = 51 | pages = 15129–15141 |date=December 2006 | pmid = 17176035 | doi = 10.1021/bi0618058 |citeseerx=10.1.1.70.6273 }}
	]
	]
			]
	]
			]
	]