text: In spite of the stepwise, radical mechanism, both stereoselective intra- and intermolecular variants have emerged. Cyclic enones are employed, otherwise competitive cis-trans isomerization ensues.
  The mechanism of [2+2] photocyclization is proposed to begin with photoexcitation of the enone to a singlet excited state. The singlet state is typically very short lived, and decays by intersystem crossing to the triplet state. At this point, the enone forms an exciplex with the ground state alkene, eventually giving the triplet diradical. Spin inversion to the singlet diradical allows closure to the cyclobutane. As an alternative a pericyclic reaction mechanism is proposed, in which after intersystem crossing a radical cation and a radical anion are formed, which then recombine to the cyclobutane.