03370 IUPAC Gold Book - Kasha rule 10.1351/goldbook.K03370 K03370 current 1 Polyatomic molecular entities luminesce with appreciable yield only from the lowest excited state of a given multiplicity. There are exceptions to this rule. The concept has been extended to the reaction of excited species, i.e., polyatomic molecular entities react with appreciable yield only from the lowest excited state of a given multiplicity. excited state https://goldbook.iupac.org//terms/view/E02257 molecular entities https://goldbook.iupac.org//terms/view/M03986 PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 360 (https://doi.org/10.1351/pac200779030293) https://goldbook.iupac.org/terms/view/K03370/html https://goldbook.iupac.org/terms/view/K03370/json https://goldbook.iupac.org/terms/view/K03370/plain Citation: 'Kasha rule' 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.K03370 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. 2026-04-19T05:43:21+00:00