Azobenzene: cis-trans isomers

A new soluble, light sensitive polymer has been created at the University of Helsinki. The polymer partially dissolves in water or certain alcohols, and, when exposed to light, the polymer switches to its cis isomer and dissolves completely.

This new polymer chain is known to contain azo compounds, and azo compounds are known for this ability to switch between cis and trans isomers using light.

Azo compounds are organic molecules in which two benzene rings (known as phenyl groups) are linked by two nitrogen atoms.
Azobenzene has the IUPAC name diphenyldiazene and is the simplest of the azo compounds.
When the 2 benzene rings (phenyl groups) are on the same side of the N=N bond this is known as the cis isomer. The structural formula for cis-azobenzene is shown below:

When the 2 benzene rings (phenyl groups) are on different sides of the N=N bond this is known as the trans isomer. The structural formula for trans-azobenzene is shown below:

The cis isomer is less stable than the trans isomer. In the presence of light (hν) , the trans isomer will convert to the cis isomer:

hν →

but because the cis isomer is less stable, it will relax back to the trans isomer over time:

→

Cis-azobenzene is more polar than trans-azobenzene so cis-azobenzene is more water soluble than trans-azobenzene.

Reference:
Szymon Wiktorowicz, Heikki Tenhu, Vladimir Aseyev. Using Light To Tune Thermo-Responsive Behavior and Host–Guest Interactions in Tegylated Poly(azocalix[4]arene)s. Macromolecules, 2013; 46 (15): 6209 DOI: 10.1021/ma4011457

Further Reading:
Cis-trans (geometric) Isomers

Suggested Study Questions:

1. Give the molecular formula for cis-azobenzene and trans-azobenzene.
2. Explain why azobenzene displays cis-trans (geometric) isomerism.
3. If the N=N bond in azobenzene was replaced with N-N, would the resulting molecule display cis-trans (geometric) isomerism? Explain your answer.
4.  On a drawing of the structural formula of trans-azobenzene, circle a phenyl group.
5. Diazene (diimine) has the molecular formula (NH)₂ . It contains the same N=N bond as azobenzene or diphenyldiazene. Diazene also displays cis-trans (geometric) isomerism. Draw both cis-diazene and trans-diazene.
6. Hydrazine has the molecular formula N₂H₄. Draw a possible structural formula for hydrazine. 
7. Do you expect hydrazine to display cis-trans (geometric) isomerism? Explain your answer.
8. Draw the molecular formula for the molecule trans-dibutyldiazene.
9. Dibutyldiazene undergoes an addition reaction with hydrogen gas. Draw the structural formula for the product of this reaction.