4,4′‐Distyrylazobenzol als Chromophor

4,4′‐Distyrylazobenzene as Chromophore 4,4′‐Distyrylazobenzenes are prepared from 4,4′‐dimethylazobenzene. Whereas the Siegrist method furnishes stereoselectively the ( E,E,E ) configuration, a Wittig olefination leads to a mixture of ( Z,E,Z ), ( Z,E,E ) and ( E,E,E ) isomers which can be easily separated. Irradiation of the intense absorption at λ ≈ 400 nm causes a selective ( E ) → (Z) isomerization of the N = N bond of all three isomers. The reverse reaction takes part in the photochemical equilibration, moreover, it is a thermal process with a relatively low activation barrier.