El-Sayed rules

https://doi.org/10.1351/goldbook.ET07369
The rate of @[email protected] is relatively large if the @[email protected] involves a change of orbital type.
Note:
A typical case is the transition from the lowest @[email protected] 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*.
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 331 [Terms] [Paper]