Azobenzene photochemsitry

Inhibition of Photoisomerization by Hydrogen Bonding

Although originally investigated as a photocage, AzoAMP-1 exhibits interesting photochemistry. Unlike azobenzene AzoAMP-1 does not isomerization when exposed to light; however, AzoAMP-2 where 2 hydrogen atoms are replaced by methyl groups does. This behavior is attributed to intramolecular hydrogen bonding that makes isomerization by concerted inversion energetically unfavorable. Further investigation using AzoAMP-3 suggest that blocking the inversion at either azo-nitrogen is sufficient to prevent isomerization.

Inhibition of Photoisomerization by Electronic Effects

AzoAMP-4 was initially synthesized by an azo coupling reaction as an alternative to AzoAMP-2. Unexpectedly, AzoAMP-4 does not photoisomerize, unlike AzoAMP-2. Acetylation of the phenolic oxygen atom to yields AzoAMP-6, which does isomerize when exposed to light. Investigations with the methoxy ether AzoAMP-5 suggest that the behavior of AzoAMP-4 does not involve tautomerization. Computational studies reveal the presence of a new conical intersection between the excited states of AzoAMP-4 and AzoAMP-5 that dissipate energy without isomerization.