Azobenzene is a chemical compound composed of two phenyl rings and one N-N double bond, the former of which are bridged by the latter. One of its most intriguing properties 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. Since the cis- isomer has a distorted configuration and is thus thermally unstable, thermal cis-to-trans isomerization can also occur.
As the mechanism of the photoisomerization behavior, it has been suggested that the rotation of the N-N double bond affords the trans-to-cis conversion via the S2 state (rotational mechanism) and that the inversion of the phenyl ring 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.
Azobenzene was positive in a reverse mutation plate incorporation assay in Salmonella (Ames test) upon addition of hepatic extracts for metabolism.
Azobenzene causes DNA single strand breaks with <30% cytotoxicity by the alkaline elution/rat hepatocyte assay (Sina et al., 1983) and produces unscheduled DNA synthesis in human lymphocytes (Gaudin et al., 1971).
It should be noted that azobenzene may be converted non-enzymatically under the acidic conditions in the stomach to the known human carcinogen benzidine.
Azobenzene occurs as a by-product during the manufacture of benzidine.
Azobenzene itself has no known uses as a dyestuff and is only produced in small quantities for research purposes.
It is concluded that under the conditions of this bioassay, azobenzene was carcinogenic (sarcomagenic) for F344 rats, inducing various types of sarcomas in the spleen and other abdominal organs of both males and females.
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