Azobenzene is a photoswitchable chemical compound composed of two phenyl rings linked by a N=N double bond.

[5] Different classes of azo dyes exist, most notably the ones substituted with heteroaryl rings.

These azos are yellow, orange, and red, respectively,[15][16] owing to the subtle differences in their electronic absorption spectra.

Azos that are ortho- or para-substituted with electron-donating groups (such as aminos), are classified as aminoazobenzenes, and tend to closely spaced[15] 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.

[16] Thus, for these compounds a single wavelength of light in the visible region will induce both the forward and reverse isomerization.

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.

There is experimental and computational evidence for the existence of a multistate rotation mechanism involving a triplet state.

[21] However, those relaxed (trans) molecules that fall perpendicular to the incoming light polarization will no longer be able to absorb, and will remain fixed.