The Role of Biradicaloid Geometries in Organic Photochemistry

Excited molecules tend to return to the ground state at biradicaloid geometries, i.e. those for which the simple MO description yields two non‐bonding orbitals occupied by only two electrons in the ground state. The nature of electronic states at these geometries and the consequences for the course of photochemical reactions are summarized, using the cases of H2 and H4 as simple models. Several aspects of classification schemes for photochemical reactions are discussed. Detailed mechanisms of photocycloaddition, of adiabatic photochemical rearrangement of Dewar aromatics, and of luminol chemiluminescence are considered as examples.