El-Sayed rules

The rate of intersystem crossing is relatively large if the radiationless transition involves a change of orbital type.
A typical case is the transition from the lowest @S05699@ to the tripletmanifold, e.g., 1π,π* → 3n, π* is faster than 1π, π* → 3π, π* and 1n, π* → 3π, π* is faster than 1n, π* → 3n, π*.
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 331 (https://doi.org/10.1351/pac200779030293)