07369 IUPAC Gold Book - El-Sayed rules 10.1351/goldbook.ET07369 ET07369 current 1 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 singlet state to the triplet manifold, 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*. intersystem crossing https://goldbook.iupac.org/terms/view/I03123 radiationless transition https://goldbook.iupac.org/terms/view/R05057 singlet state https://goldbook.iupac.org/terms/view/S05699 PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 331 (https://doi.org/10.1351/pac200779030293) https://goldbook.iupac.org/terms/view/ET07369/html https://goldbook.iupac.org/terms/view/ET07369/json https://goldbook.iupac.org/terms/view/ET07369/plain 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. 10.1351/goldbook.ET07369 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-08-10T11:31:26+00:00