20:
400:
of luminescence is generally independent of the excitation wavelength. This can be understood as a consequence of the tendency – implied by Kasha's rule – for molecules in upper states to relax to the lowest excited state non-radiatively. Again there are exceptions: for example
301:. The greater the overlap, the more quickly the molecule can undergo a 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
309:
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,
162:
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454:
282:. 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.
192:
51:
132:
105:
78:
134:) or on one of the vibrational sub-levels. Vibrational relaxation then takes place between excited levels, which leads to dissipation of part of the energy (
164:), taking the form of a transition (internal conversion) towards the lowest excited level. Energy is then dissipated by emission of a photon of energy
434:
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503:
331:
518:
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640:
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218:
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337:
Exceptions to Kasha's rule arise when there are large energy gaps between excited states. An example is
602:
290:
137:
568:
305:
levels of the electronic states coupled by the transition (where the vibrational quantum number
592:. Compiled by McNaught, A.D. and Wilkinson, A. Blackwell Scientific Publications, Oxford, 1997.
470:
461:. Compiled by McNaught, A.D. and Wilkinson, A. Blackwell Scientific Publications, Oxford, 1997.
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26:
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234:
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541:
Dunlop, David; LudvĂková, Lucie; Banerjee, Ambar; Ottosson, Henrik; Slanina, Tomáš (2023).
110:
83:
56:
417:, the difference between the absorption and emission frequencies, related to Kasha's rule.
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is greater, so here fluorescence occurs, since it is now kinetically competitive with
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572:
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297:, the Franck–Condon factor expresses the degree of overlap between their vibrational
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293:. For a given pair of energy levels that differ in both vibrational and electronic
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Kasha–Vavilov rule – Compendium of
Chemical Terminology, 2nd ed. (the "Gold Book")
253:
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of an excited molecule. Upon absorbing a photon, a molecule in its electronic
543:"Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha's Rule"
355:
states lie sufficiently far apart that fluorescence is observed mostly from
275:
state) is expected in appreciable yield only from the lowest excited state,
217:) occurs in appreciable yield only from the lowest excited state of a given
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459:
Kasha rule – Compendium of
Chemical Terminology, 2nd ed. (the "Gold Book")
19:
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316:, before they have time to fluoresce. However, the energy gap between
471:"Unusual autofluorescence characteristic of cultured red-rain cells"
362:. In 2023, an explanation was proposed which pointed out that the
256:– be excited to any of a set of higher electronic states (denoted
474:
435:
Characterization of
Electronic Transitions in Complex Molecules
495:
Photochemistry of
Organic Compounds: From Concepts to Practice
194:, which allows the system to go back to its fundamental state.
523:. Suppan, P. Royal Society of Chemistry, 1994. p.56.
498:. Klán, P. and Wirz, J. Wiley-Blackwell, 2009. p.40.
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excites an electron of fundamental level, of energy
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209:molecules. The rule states that photon emission (
23:Scheme of Kasha's rule. A photon with energy
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267:>0). However, according to Kasha's rule,
221:. It is named after American spectroscopist
233:The rule is relevant in understanding the
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473:. Louis, J. and Kumar, A.S. Presented in
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341:: the classical explanation is that the
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606:Gispert, J.R. Wiley-VCH, 2008. p. 483.
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271:(termed fluorescence in the case of an
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80:, up to an excited energy level (e.g.
7:
392:A corollary of Kasha's rule is the
439:Discussions of the Faraday Society
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14:
285:The rule can be explained by the
252:) may – depending on the photon
1:
396:rule, which states that the
157:{\displaystyle \Delta E_{d}}
229:Description and explanation
225:, who proposed it in 1950.
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477:Conference 7097, Aug 2008.
187:{\displaystyle h\nu _{2}}
46:{\displaystyle h\nu _{1}}
631:Eponymous chemical rules
603:Coordination Chemistry
207:electronically excited
201:is a principle in the
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287:Franck–Condon factors
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127:{\displaystyle E_{2}}
102:
100:{\displaystyle E_{1}}
75:
73:{\displaystyle E_{0}}
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16:Law of photochemistry
556:10.1021/jacs.3c07625
373:character while the
291:vibronic transitions
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520:Chemistry and Light
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369:excited state has
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625:Categories
445:: p.14-19.
422:References
254:wavelength
573:261808767
241:(denoted
176:ν
142:Δ
35:ν
565:37704031
441:, 1950,
409:See also
405:vapour.
382:aromatic
403:benzene
394:Vavilov
339:azulene
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334:(IC).
263:where
586:IUPAC
569:S2CID
455:IUPAC
608:ISBN
561:PMID
525:ISBN
500:ISBN
475:SPIE
348:and
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378:2
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325:S
321:1
318:S
314:1
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307:v
280:1
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265:n
261:n
258:S
246:0
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180:2
172:h
150:d
146:E
120:2
116:E
93:1
89:E
66:0
62:E
39:1
31:h
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