intermolecular photochemical process leading to carbo- or hetero- cyclic ring systems by formation of two new single bonds, either by a concerted process (e.g., a concerted addition) or by multi-step processes (e.g., the Paternò–Büchi reaction).
@T06503@ photocycloadditions are two-step processes involving the formation of diradical intermediates.
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 388 (