Term: El-Sayed rules
https://doi.org/10.1351/goldbook.ET07369

Definition:
The rate of intersystem crossing is relatively large if the radiationless transition involves a change of orbital type.

Note: 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*.

Related Terms:
1) singlet state (http://doi.org/10.1351/goldbook.S05699).
2) intersystem crossing (http://doi.org/10.1351/goldbook.I03123).
3) radiationless transition (http://doi.org/10.1351/goldbook.R05057).

Source: PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 331 (https://doi.org/10.1351/pac200779030293)

Citation: 'El-Sayed rules' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.ET07369

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