Editing Fluorescence (section)

=== Kasha's rule ===
[[Kasha's rule]] states that the luminesce (fluorescence or phosphorescence) of a molecule will be emitted only from the lowest excited state of its given multiplicity.<ref>[[International Union of Pure and Applied Chemistry|IUPAC]].PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 360 https://goldbook.iupac.org/terms/view/K03370</ref> Vavilov's rule (a logical extension of Kasha's rule thusly called Kasha–Vavilov rule) dictates that the quantum yield of luminescence is independent of the wavelength of exciting radiation and is proportional to the absorbance of the excited wavelength.<ref>[[International Union of Pure and Applied Chemistry|IUPAC]]. [http://goldbook.iupac.org/K03371.html  – Compendium of Chemical Terminology, 2nd ed. (the "Gold Book")] {{webarchive|url=https://web.archive.org/web/20120321031448/http://goldbook.iupac.org/K03371.html |date=21 March 2012 }}. Compiled by McNaught, A.D. and Wilkinson, A. Blackwell Scientific Publications, Oxford, 1997.</ref> Kasha's rule does not always apply and is violated by simple molecules, such an example is azulene.<ref>Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha's Rule
David Dunlop, Lucie Ludvíková, Ambar Banerjee, Henrik Ottosson, and Tomáš Slanina
Journal of the American Chemical Society 2023 145 (39), 21569-21575
DOI: 10.1021/jacs.3c07625</ref> A somewhat more reliable statement, although still with exceptions, would be that the fluorescence spectrum shows very little dependence on the wavelength of exciting radiation.<ref>{{Cite journal |last1=Qian |first1=Hai |last2=Cousins |first2=Morgan E. |last3=Horak |first3=Erik H. |last4=Wakefield |first4=Audrey |last5=Liptak |first5=Matthew D. |last6=Aprahamian |first6=Ivan |date=January 2017 |title=Suppression of Kasha's rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission |url=https://www.nature.com/articles/nchem.2612 |journal=Nature Chemistry |language=en |volume=9 |issue=1 |pages=83–87 |doi=10.1038/nchem.2612 |pmid=27995926 |s2cid=42798987 |issn=1755-4330}}</ref>