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Kasha's rule
Kasha's rule is a principle in the photochemistry of electronically excited molecules. The rule states that photon emission (fluorescence or phosphorescence) occurs in appreciable yield only from the lowest excited state of a given multiplicity. It is named for American spectroscopist Michael Kasha, who proposed it in 1950.〔(Characterization of Electronic Transitions in Complex Molecules ). Kasha, M. ''Discussions of the Faraday Society'', 1950, 9: p.14-19.〕〔IUPAC. (Kasha rule – Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") ). Compiled by McNaught, A.D. and Wilkinson, A. Blackwell Scientific Publications, Oxford, 1997.〕
==Description and explanation==
The rule is relevant in understanding the emission spectrum of an excited molecule. Upon absorbing a photon, a molecule in its electronic ground state (denoted ''S''0, assuming a singlet state) may – depending on the photon wavelength – be excited to any of a set of higher electronic states (denoted ''S''n where ''n''>0). However, according to Kasha's rule, photon emission (termed fluorescence in the case of an ''S'' state) is expected in appreciable yield only from the lowest excited state, ''S''1. Since only one state is expected to yield emission, an equivalent statement of the rule is that the emission wavelength is independent of the excitation wavelength.〔("Unusual autofluorescence characteristic of cultured red-rain cells" ). Louis, J. and Kumar, A.S. Presented in SPIE Conference 7097, Aug 2008.〕
The rule can be explained by the Franck–Condon factors for vibronic transitions. For a given pair of energy levels that differ in both vibrational and electronic quantum number, the Franck–Condon factor expresses the degree of overlap between their vibrational wavefunctions. The greater the overlap, the quicker the molecule can undergo transition from the higher to the lower level. Overlap between pairs is greatest when the two vibrational levels are close in energy; this tends to be the case when the ''vibrationless'' levels of the electronic states coupled by the transition (where the vibrational quantum number ''v'' is zero) are close. In most molecules, the vibrationless levels of the excited states all lie close together, so molecules in upper states quickly reach the lowest excited state, ''S''1, before they have time to fluoresce. However, the energy gap between ''S''1 and ''S''0 is greater, so here fluorescence occurs, since it is now kinetically competitive with internal conversion (IC).〔(''Photochemistry of Organic Compounds: From Concepts to Practice'' ). Klán, P. and Wirz, J. Wiley-Blackwell, 2009. p.40. ISBN 1-4051-6173-6.〕〔(''Chemistry and Light'' ). Suppan, P. Royal Society of Chemistry, 1994. p.56. ISBN 0-85186-814-2.〕
Exceptions to Kasha's rule arise when there are large energy gaps between excited states. An example is azulene: the classical explanation is that the ''S''1 and ''S''2 states lie sufficiently far apart that fluorescence is observed mostly from ''S''2.〔〔 However, recent research has put forward that this may not be the case, and that fluorescence is seen from ''S''2 because of crossing in the ''N''-dimensional potential surface allowing very fast internal conversion from ''S''1 to ''S''0.
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