Regardless of the presence or absence of an oxidant, the first step of the reaction is photochemical excitation of a stilbene or similar structure, leading to formation of a dihydrophenanthrene or similar intermediate. For stilbene and other chemicals containing a double-bond linker between the two aromatic rings, the excited structure can undergo reversible cis-trans isomerization. Although only cis structures can undergo the cyclization step themselves, trans structures can isomerize in situ and then cyclize. 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.
This cyclization is reversible, but several other subsequent reactions can occur instead, depending on structural details and whether certain other reagents are present.