Azobenzene

Azobenzene
IUPAC name (E)-diphenyldiazene
Other names Azobenzene
Identifiers
CAS number	103-33-3 Y
PubChem	2272
ChemSpider	2185 Y
KEGG	C19334 N
ChEBI	CHEBI:58996 Y
ChEMBL	CHEMBL58835 Y
RTECS number	CN1400000
Jmol-3D images	Image 1
SMILES N(=N/c1ccccc1)\c2ccccc2
InChI InChI=1S/C12H10N2/c1-3-7-11(8-4-1)13-14-12-9-5-2-6-10-12/h1-10H/b14-13+ Y Key: DMLAVOWQYNRWNQ-BUHFOSPRSA-N Y InChI=1/C12H10N2/c1-3-7-11(8-4-1)13-14-12-9-5-2-6-10-12/h1-10H/b14-13+ Key: DMLAVOWQYNRWNQ-BUHFOSPRBP
Properties
Molecular formula	C12H10N2
Molar mass	182.22 g mol−1
Appearance	orange-red crystals
Density	1.09 g/cm³, solid
Melting point	69 °C (342 K)
Boiling point	293 °C (566 K)
Solubility in water	6.4mg/L (25 °C)
Acidity (pKa)	3.3
Structure
Molecular shape	sp2 at N
Dipole moment	0 D (trans isomer)
Hazards
R-phrases	45-20/22-48/22-50/53-68
S-phrases	53-45-60-61
Main hazards	toxic
Flash point	476 °C
Related compounds
Related compounds	Nitrosobenzene aniline
N (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Azobenzene photoisomerization. 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.

Azobenzene is a chemical compound composed of two phenyl rings linked by a N=N double bond. It is the best known example of an azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of molecules that share the core azobenzene structure, with different chemical functional groups extending from the phenyl rings. These azo compounds are considered as derivatives of diazene (diimide),^[1] and are sometimes referred to as 'diazenes'. The diazenes strongly absorb light and are used as dyes in a variety of industries.

Synthesis

Azobenzene was first described in 1856 as "gelblich-rote krystallinische Blättchen" ("yellowish-red crystalline flakes" in German).^[2] Its original preparation is similar to the modern one. According to the 1858 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.^[3]

Trans-cis isomerization

One of the most intriguing properties of azobenzene (and derivatives) is the photoisomerization of trans and cis isomers. 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). 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.

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, 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 on femtosecond transition 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,^[4] the "concerted inversion" pathway in which both CNN bond angles bend at the same time.

Photoinduced motions

The photo-isomerization of azobenzene is a form of light-induced molecular motion.^[5][6][7] 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).^[8] For instance, it has been used to selectively orient liquid crystal domains, and used to create nonlinear optical (NLO) materials. Azo isomerization can also be used to photo-switch the liquid crystal phase of a material from cholesteric to nematic^[9][10] or to change the pitch of a cholesteric phase.^[11]

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 laser ablation, since it readily occurs at low power and the transformation is reversible. The mechanism of this surface holography seems related to a new photomechanical effect, involving azobenzene isomerization.^[12]

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).

Other aspects

Azobenzene molecules can be incorporated into polymer matrices as stabilizers. It is also interesting to note that the rigid rod-like structure of azo molecules allows them to behave as liquid-crystal mesogens 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. ^[13][14]

References

1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2009) "azo compounds".
2. ^ 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 .
3. ^ Bigelow, H. E.; Robinson, D. B. "Azobenzene" Organic Syntheses, Collected Volume 3, p.103 (1955). http://www.orgsyn.org/orgsyn/pdfs/CV3P0103.pdf
4. ^ 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
5. ^ H. Rau, in Photochemistry and Photophysics; Vol. 2, edited by J. Rebek (CRC Press, Boca Raton, FL, 1990), p. 119-141.
6. ^ A. Natansohn and P. Rochon, Chem. Rev. 102, 4139-4176 (2002).
7. ^ Y. Yu, M. Nakano, and T. Ikeda, Nature (London, U. K.) 425, 145 (2003).
8. ^ K. Ichimura, Chem. Rev. 2000, 100, 1847
9. ^ S. Tazuke, S. Kurihara, T. Ikeda, Chemistry Letters 1987, 911
10. ^ N. Tamaoki, Advanced Materials 2001, 13, 1135
11. ^ S. Pieraccini, S. Masiero, G. P. Spada, G. Gottarelli, Chem. Commun. 2003, 598
12. ^ Kevin G. Yager and Christopher J. Barrett (2006). "Photomechanical Surface Patterning in Azo-Polymer Materials". Macromolecules 39 (26): 9320–6. doi:10.1021/ma061733s. http://pubs.acs.org/doi/abs/10.1021/ma061733s. 
13. ^ Gorostiza P, Isacoff EY. Optical switches for remote and noninvasive control of cell signaling. Science. 2008 Oct 17;322(5900):395-9.
14. ^ Banghart MR, Volgraf M, Trauner D. Engineering light-gated ion channels. Biochemistry. 2006 Dec 26;45(51):15129-41.