Kasha rule

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Kasha rule in the Jabłoński scheme : fluorescence only from S 1 and phosphorescence only from T 1

The Kasha rule ( English Kasha's rule , after the American photochemist Michael Kasha , who proposed it in 1950), due to its widespread validity sometimes also referred to as photochemical dogma , is a basic principle of photochemistry . It says that the spontaneous emission of a photon comes from the lowest electronically excited state of a given multiplicity . The Kasha rule plays u. a. a role in photosensitized reactions .

description

After absorption of a photon , a molecule is excited from its electronic ground state (in most molecules the singlet state  S 0 ) into an electronically excited state (S n with  n > 0) depending on the wavelength of the incident light . By means of internal conversion, this quickly changes into vibration-excited states S n-1 * of the next lower electronically excited state, which in turn quickly deactivate without radiation to the vibration base state S n-1 of this next-lower electronically excited state .

The Kasha Rule says that these processes follow one another until the lowest electronically excited state is reached, which can then return to the basic electronic state in a radiant manner. The rule applies both to singlet states in which fluorescence from the S 1 is visible and to triplet states which relax by means of phosphorescence from the T 1 .

This observation can using the Franck-Condon principle and the energy gap law ( energy gap law ) will be explained. The Franck-Condon factor describes the overlap of two vibration wave functions ; the greater the overlap, the faster a molecule can change from a high to a low level. The probability of a radiationless transition decreases as the energy difference between the two levels increases. Since the energetic distance between electronically excited states of equal multiplicity is usually much smaller than that between the lowest singlet or triplet state and the electronic ground state, higher excited states relax quickly without radiation to the lowest electronically excited state. Only here is the rate of internal transformation so small that the radiant decay can compete with this process.

Exceptions

Since the Kasha rule is a purely empirical observation, there are a few exceptions where the emission occurs from a more highly excited state. In principle, there are three circumstances that can explain such deviant behavior:

  1. The energetic distance between S 2 and S 1 is so great that the inner transformation is slowed down. If the oscillator strength of the S 0 → S 2 transition is large enough, fluorescence from the S 2 is observed . Examples of this behavior are azulene or thioketones .
  2. Both the oscillator strength of the S 0 → S 1 transition and the energetic distance between S 2 and S 1 are small. As a result, the S 1 state has a long service life and can be thermally excited in the S 2 , which emits.
    However, this process also follows the Kasha rule insofar as the emission takes place from the vibrational ground state of the respective excited state. An example of this is the dual emission in coumarin derivatives or ovals .
  3. If stray light is eliminated during a measurement, emission from a more highly excited state can also be observed in other molecules, e.g. B. in aromatic hydrocarbons such as pyrene or benzo [ a ] anthracene .

Kasha Vavilov rule

One consequence of the Kasha rule is that the fluorescence quantum yield of a molecule is independent of the excitation wavelength. Since, according to Kasha's rule, the emission of a molecule always occurs from the same state, a change in the excitation wavelength (and thus in the excitation energy ) does not change the emission wavelength. This relationship is called the Kasha-Vavilov rule after the Soviet physicist Sergei Wawilow .

There are also exceptions to this rule, e.g. B. the emission of benzene vapor.

Individual evidence

  1. ↑ Michael Kasha: Characterization of electronic transitions in complex molecules . In: Discussions of the Faraday Society . tape 9 , no. 0 , January 1, 1950, ISSN  0366-9033 , doi : 10.1039 / df9500900014 .
  2. ↑ a b Dieter Wöhrle, Michael W. Tausch, Wolf-Dieter Stohrer: Photochemistry: Concepts, Methods, Experiments - Wöhrle - Wiley Online Library . Wiley-VCH, 1998, ISBN 3-527-29545-3 .
  3. ↑ a b Petr Klán, Jakob Wirz: Photochemistry of organic compounds: from concepts to practice . Wiley-Blackwell, 2009, ISBN 978-1-4051-6173-2 .
  4. ↑ Giuseppe Brancato, Giovanni Signore, Paolo Neyroz, Dario Polli, Giulio Cerullo: Dual Fluorescence through Kasha's Rule Breaking: An Unconventional Photomechanism for Intracellular Probe Design . In: Journal of Physical Chemistry B . tape 119 , no. 20 , May 21, 2015, ISSN  1520-6106 , p. 6144-6154 , doi : 10.1021 / acs.jpcb.5b01119 .
  5. ↑ Bernhard Nickel: Delayed Fluorescence from Upper Excited Singlet States Sn (n> 1) of the Aromatic Hydrocarbons 1,2-benzanthracene, fluoranthene, pyrene, and chrysene in methylcyclohexane . In: Helvetica Chimica Acta . tape 61 , no. 1 , January 25, 1978, ISSN  1522-2675 , pp. 198–222 , doi : 10.1002 / hlca.19780610118 .
  6. ^ Robert J. Longfellow, David B. Moss, Charles S. Parmenter: Rovibrational level mixing below and within the channel three region of S1 benzene . In: The Journal of Physical Chemistry . tape 92 , no. 19 , September 1, 1988, ISSN  0022-3654 , pp. 5438-5449 , doi : 10.1021 / j100330a023 .