azobenzene

{{Short description|Two phenyl rings linked by a N═N double bond}}

{{Chembox

|Verifiedfields = changed

|Watchedfields = changed

|verifiedrevid = 458972978

|Name = Azobenzene

|ImageFile_Ref = {{chemboximage|correct|??}}

|ImageFile = (E)-1,2-diphenyldiazene 200.svg

|ImageName = Skeletal formula of azobenzene

|ImageFile1 = Azobenzene-trans-3D-balls.png

|ImageSize1 = 240px

|ImageName1 = Ball-and-stick model of azobenzene

|IUPACName = (E)-Diphenyldiazene

|OtherNames = Azobenzene

|Section1={{Chembox Identifiers

|ChEBI_Ref = {{ebicite|correct|EBI}}

|ChEBI = 58996

|SMILES = N(=N/c1ccccc1)\c2ccccc2

|InChI = 1/C12H10N2/c1-3-7-11(8-4-1)13-14-12-9-5-2-6-10-12/h1-10H/b14-13+

|InChIKey = DMLAVOWQYNRWNQ-BUHFOSPRBP

|ChEMBL_Ref = {{ebicite|correct|EBI}}

|ChEMBL = 58835

|PubChem = 2272

|StdInChI_Ref = {{stdinchicite|correct|chemspider}}

|StdInChI = 1S/C12H10N2/c1-3-7-11(8-4-1)13-14-12-9-5-2-6-10-12/h1-10H/b14-13+

|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}

|StdInChIKey = DMLAVOWQYNRWNQ-BUHFOSPRSA-N

|CASNo_Ref = {{cascite|correct|CAS}}

|CASNo = 103-33-3

|EC_number = 203-102-5

|Beilstein = 742610

|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

|Density = 1.203 g/cm³Haynes, p. 3.32

|Solubility = 6.4 mg/L (25 °C)

|MeltingPt = 67.88 °C (trans), 71.6 °C (cis)

|MeltingPt_ref =

|BoilingPtC = 300

|BoilingPt_ref =

|pKa = −2.95{{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.}}

| MagSus = −106.8·10⁻⁶ cm³/molHaynes, p. 3.579

| RefractIndex = 1.6266 (589 nm, 78 °C)

}}

|Section3={{Chembox Structure

|MolShape = sp² at N

|Dipole = 0 D (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 = Nitrosobenzene aniline

}}

Azobenzene is a photoswitchable chemical compound composed of two phenyl rings linked by a N=N double bond. It is the simplest example of an aryl azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of similar compounds. These azo compounds are considered as derivatives of diazene (diimide),{{ GoldBookRef | title = azo compounds | file = A00560 | year = 2009 }} and are sometimes referred to as 'diazenes'. The diazenes absorb light strongly and are common dyes.{{cite book|title=Hydrazo, Azo and Azoxy Groups|year=1975|editor=Saul Patai|isbn=9780470023419 |doi=10.1002/0470023414

|publisher=John Wiley & Sons|series=PATAI'S Chemistry of Functional Groups|volume=1 }} Different classes of azo dyes exist, most notably the ones substituted with heteroaryl rings.{{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}}

Structure and synthesis

File:AZBENC01.png

Azobenzene was first described by Eilhard Mitscherlich in 1834.{{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}}{{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}} Yellowish-red crystalline flakes of azobenzene were obtained in 1856.{{ cite journal | title = III. Zur Geschichte des Azobenzols und des Benzidins | year = 1856 | last1 = Noble | 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 }} Its original preparation is similar to the modern one. According to the 1856 method, nitrobenzene is reduced by iron filings in the presence of acetic acid. In the modern synthesis, zinc is the reductant in the presence of a base.{{OrgSynth

| author =Bigelow, H. E. |author2=Robinson, D. B.

| year =1955

| title =Azobenzene

| volume =22

| pages =28

| collvol =3

| collvolpages =103

| prep =CV3P0103}} Industrial electrosynthesis using nitrobenzene is also employed.{{cite journal

|date=July 17, 2017

|title=Organic Electrosynthesis: From Laboratorial Practice to Industrial Applications

|journal=Organic Process Research & Development |volume=21 |issue=9 |pages=1213–1226

|doi=10.1021/acs.oprd.7b00004}}

trans-Azobenzene isomer is planar with an N-N distance of 1.189 Å.{{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=}} cis-Azobenzene is nonplanar with a C-N=N-C dihedral angle of 173.5° and an N-N distance of 1.251 Å.{{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}} 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.Image:Azobenzene isomerization.svg. 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.]]

File:Methoxyazobenzene UV2.png

Reactions

Azobenzene is a weak base, but undergoes protonation at one nitrogen with a pK_a = -2.95. It functions as a Lewis base, e.g. toward boron trihalides. It binds to low valence metal centers, e.g. Ni(Ph₂N₂)(PPh₃)₂ is well characterized.{{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|

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.}}

Azobenzene oxidizes to give azoxybenzene. Hydrogenation gives diphenylhydrazine.

=Trans–cis isomerization=

Azobenzene (and derivatives) undergo photoisomerization of trans and cis 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* (S₂ state) transition, for trans-to-cis conversion, and blue light, which is equivalent to that of the n-π* (S₁ 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 photoswitches).

==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-stilbenes. These azos are yellow, orange, and red, respectively,{{ 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 }}{{ 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 }} 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 ultraviolet. Azos that are ortho- or para-substituted with electron-donating groups (such as aminos), 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 aromatic system are functionalized). The addition of this push-pull configuration results in a strongly asymmetric electron distribution, which modifies a host of optical properties. In particular, it shifts the absorption spectra of the trans and the cis isomers, so that they effectively overlap. Thus, for these compounds a single wavelength 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==

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.

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 S₂ 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 femtosecond transient absorption spectroscopy has suggested that the S₂ state undergoes internal conversion to the S₁ state, and then the trans-to-cis isomerization proceeds. Recently another isomerization pathway has been proposed by Diau,{{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 }} 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.{{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}}

==Photoinduced motions==

The photo-isomerization of azobenzene is a form of light-induced molecular motion.{{ 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}}{{ 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 }} This isomerization can also lead to motion on larger length scales. For instance, polarized light will cause the molecules to isomerize and relax in random positions.{{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}} 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 anisotropic and therefore optically birefringent and dichroic. This photo-orientation can also be used to orient other materials (especially in liquid crystal systems).{{ 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 }}

Miscellaneous

Azobenzene undergoes ortho-metalation by metal complexes, e.g. dicobalt octacarbonyl:{{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}}

Cobalt Catalyzed reaction Azobenzene Carbon monoxide Murahashi 1956

Info about the carcinogenicity of Azobenzene can be found on the EPA site, where it has been classified as a "probable human carcinogen" based on evidence of carcinogenicity in animals.

{{|first=United States Environmental Protection Agency |date=2024-06-06 |title=Azobenzene |url=[https://iris.epa.gov/ChemicalLanding/&substance_nmbr=351 https://iris.epa.gov/ChemicalLanding/&substance_nmbr=351}}]

References

Cited sources

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