For stilbene and other chemicals containing a double-bond linker between the two aromatic rings, the excited structure can undergo reversible cis-trans isomerization.
[2] In keeping with the Woodward–Hoffmann rules, molecular orbital symmetry analysis of the photochemical reaction of the six-electron system explains the trans relative configuration at the newly bound centers by a conrotatory process.
However, suitably substituted stilbenes cyclize irreversibly if an aromatizing elimination or hydrogen shift process can take place.
Esters, which exist primarily in the trans conformation about the C-O single bond, do not undergo this process efficiently.
[16] In 2015, Li and Twieg reported a novel derivative of Mallory type photocyclizations and named it as photocyclodehydrofluorination (PCDHF).
[17] Photocyclization can be used as the final step of a sequence to generate a fused aromatic ring at a benzylic position.
After benzylic bromization with N-bromosuccinimide, transformation to the phosphonium salt, and a Wittig reaction with anaromatic aldehyde, photocyclization fuses the aromatic rings.
[18] Several other methods are available to synthesize the phenanthrene ring system; however, most of these are longer or less functional group tolerant than photocyclization.