Photoinduced Conformational Switch of Enantiopure Azobenzenes Controlled by a Sulfoxide

Two series of enantiopure azobenzenes with a p-tolylsulfoxide at the ortho or meta position with
respect to the azo group, have been regioselectively synthesized. Both can act as enantiopure molecular
switches showing different structural features owing to the presence of the stereogenic sulfur. The
photoisomerization process, studied by UV−vis, circular dichroism (CD), NMR, and chiral HPLC evidenced
a double role of the sulfoxide. A transfer of chirality from the sulfoxide to the azo system was observed by
CD in both cis and trans-isomers of the meta sulfinyl derivatives 3, whereas this perturbation was evident
for the ortho sulfinyl series 7 only in the cis isomer. The NMR study evidenced that the s-cis rigid conformation
of the bisaromatic sulfoxide was fixing a different orientation of the overall system in each series both in
the trans and cis isomers, by forcing a final U-shaped structure in cis-3 and an S-shaped structure in cis-7.
Very different values of specific optical rotations were measured in both trans and cis isomers, also reflecting
the existence of distinct chiral entities in the photostationary states. The easy and reversible changes
occurring between different conformational states could find applications in the photocontrol of several
molecular switches.