https://doi.org/10.1351/goldbook.ST07482
From the possible encounter pairs between states of different spin multiplicity, only those conserving multiplicity in going to products are expected to react.
Note: This factor determines the efficiency of diffusion-controlled reactions, which have an encounter-controlled rate. Typical examples are quenching of fluorescence of aromatic hydrocarbons by O2, quenching of triplet states by O2, and triplet–triplet annihilation.