Азобензол: свойства, структура и фотоизомеризация

Two phenyl rings linked by a N═N double bond
Chembox
|Verifiedfields = changed
|Watchedfields = changed
|verifiedrevid = 458972978
|Name = Azobenzene
|ImageFile Ref =
|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=
|Section2=Chembox Properties
|C=12 | H=10 | N=2
|Appearance = orange red crystals
|Solubility = 6.4 mg/L (25 °C)
|MeltingPt = 67.88 °C (trans), 71.6 °C (cis)
|MeltingPt ref =
| MagSus = 106.8·10−6 cm3/mol
| RefractIndex = 1.6266 (589 nm, 78 °C) and are sometimes referred to as 'diazenes'. The diazenes absorb light strongly and are common dyes. Different classes of azo dyes exist, most notably the ones substituted with heteroaryl rings.

Azobenzene was first described by Eilhard Mitscherlich in 1834. Yellowish red crystalline flakes of azobenzene were obtained in 1856. 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. Industrial electrosynthesis using nitrobenzene is also employed. trans Azobenzene isomer is planar with an N N distance of 1.189 Å. cis Azobenzene is nonplanar with a C N=N C dihedral angle of 173.5° and an N N distance of 1.251 Å. 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.

Azobenzene is a weak base, but undergoes protonation at one nitrogen with a pKa = 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. Azobenzene oxidizes to give azoxybenzene. Hydrogenation gives diphenylhydrazine.

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).

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, 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 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.

The photo isomerization of azobenzene is a form of light induced molecular motion. 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. 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).

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