Set of rules proposed in 1975 to classify photochemical reactions based on correlation diagrams and
under the assumption that all photochemical processes are controlled by the generation of primary
products with characteristics of diradicals, i.e., considering the physical fact that the initial single
excitation separates two electrons of a pair into two distinct orbitals. The reactions are classified
according to their topicity, i.e., the total number and nature of available radical sites generated in the
primary photochemical product.