Kasha rule

https://doi.org/10.1351/goldbook.K03370
Polyatomic @M03986@ luminesce with appreciable yield only from the lowest @E02257@ of a given multiplicity. There are exceptions to this rule.
Note:
The concept has been extended to the reaction of excited species, i.e., polyatomic molecular entities react with appreciable yield only from the lowest @E02257@ of a given multiplicity.
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 360 [Terms] [Paper]