Azobenzene is a chemical compound composed of two phenyl rings linked by a N=N double bond. 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 (technically, these compounds should be referred to as 'diazenes'). The azobenzene compounds strongly absorb light, and were historically used as dyes in a variety of industries. One of the most intriguing properties of azos is photoisomerization behavior between two isomers, the trans- and cis- configurations. 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 breaks the aromaticity of the trans configuation). 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 on the order of 200 kJ/mol. Azobenzene molecules can be incorporated into polymer matrices in order to stabilize them. It is also interesting to note that the rigid rod-like structure of azo molecules makes them behaves as liquid crystal mesogens in many materials.
Spectroscopic Classification
The exact 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 effectuate both the forward and reverse isomerization. Under illumination, these molecules will be constantly cycling between the two isomeric states.
Photophysics of Isomerization
The photo-isomerization of azobenzene is extremely rapid, occuring on picosecond timescales. 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, which 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.
Photoinduced Motions
The photo-isomerization of azobenzene is a unique form of ligh-induced molecular motion. This motion can also lead to motion on larger lengthscales. 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. Thus, polarized irradiation will make an azo-material anisotropic and optically birefringent. This photo-orientation can also be used to orient other materials (especially in liquid crystal systems). For instance, it has been used to selectively orient liquid crystal domains, and used to create nonlinear optical (NLO) materials.
In 1995, it was discovered that exposing a thin film of azo-polymer with 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. Bulk expansion of azobenzene materials has also been observed. In one report, a thin film was made to bend and unbend by exposing it to polarized light.
References
- H. Rau, in Photochemistry and Photophysics; Vol. 2, edited by J. Rebek (CRC Press, Boca Raton, FL, 1990), p. 119-141.
- A. Natansohn and P. Rochon, Chem. Rev. 102, 4139-4176 (2002).
- Y. Yu, M. Nakano, and T. Ikeda, Nature (London, U. K.) 425, 145 (2003).
