From the possible encounter pairs between states of different spin multiplicity, only those conserving multiplicity in going to products are expected to react.
This factor determines the efficiency of @D01716@-controlled reactions, which have an @E02089@. Typical examples are @Q05007@ of @F02453@ of aromatic @H02889@ by O2, @Q05007@ of triplet states by O2, and triplet–triplet @A00366@.
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 428 (https://doi.org/10.1351/pac200779030293)