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.
Source:
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 320