Jabłoński scheme

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The Jabłoński diagram [ jabwɔɲ̟s̪ki ] or Jabłoński term scheme (named after Aleksander Jabłoński ) illustrates the possible transitions of valence electrons into the various excited states when light is irradiated and back. It provides a clear representation of the phenomena of fluorescence and phosphorescence , which is why it plays an important role in UV / VIS spectroscopy .

Theory and structure

Jabłoński diagram

When electromagnetic waves are irradiated , electrons are excited from their ground state to energetically higher states by absorbing the energy of the irradiated photon .

The relaxation to the ground state can take place in different ways, which are illustrated by the Jabłoński diagram:

  • in most cases by radiationless deactivation ( vibronic relaxation ) of higher oscillation states, whereby the energy is transferred to lower translational , rotational and oscillation states of the surrounding particles (left side in the picture).
  • electronically excited states by inner transformation (engl. internal conversion , IC) in an excited vibrational state of an underlying electronic state (the same multiplicity ) change from where they deactivate from further nonradiative (s. o.) or
  • transition from an excited singlet state to a triplet state (or back) by intersystem crossing (ISC) , or
  • through radiant deactivation (emission of light), which is known as luminescence , change directly to the lower-lying state:
    • The spin-allowed transition from a singlet state is called fluorescence.
    • If the electron is in an excited triplet state , it is called phosphorescence. The excited state is typically long-lived, since the spin reversal required for deactivation is spin- forbidden, i.e. it can only take place with the participation of another particle or through renewed ISC (see above).

history

Term schemes that ordered electronic states vertically according to their energy and horizontally according to their spin multiplicity were already developed by Niels Bohr and Walter Grotrian , but initially only applied to atoms.

The French photophysicist Jean Perrin was the first to use such a scheme for molecules in connection with light absorption and emission . With the help of this scheme he was able to explain the phenomenon of thermally activated delayed fluorescence ; In doing so, he postulated a metastable state , which, however, could not relax itself into the electronic ground state , neither radiating nor non-radiating.

Jabłoński later used a term schema with the help of which he explained why the phosphorescence of organic dye molecules is red-shifted, i.e. low-energy, compared to their fluorescence. In his model there is also a metastable energy level, which, in contrast to Perrin's explanation, could emit light.

This metastable state was later recognized independently by Alexander Terenin as well as Gilbert Newton Lewis and Michael Kasha as a triplet state, an interpretation against which Jabłoński long resisted. The term Jablonski diagram was also used for the first time in the publication by Lewis and Kasha .

In the strict sense, the term Jabłoński diagram is wrong, as today's interpretation of the scheme differs from Jabłoński's own. In French-speaking countries, the term Perrin-Jabłoński diagram is often used to honor Perrin’s preparatory work.

literature

  • Dieter Wöhrle, Michael W. Tausch, Wolf-Dieter Stohrer: Photochemistry - Concepts, Methods, Experiments. Wiley-VCH Verlag, Weinheim 1998, ISBN 3-527-29545-3 , Chapter 2.6.1 The Jablonski diagram, pp. 63-68.
  • Hans Peter Latscha, Uli Kazmaier, Helmut Alfons Klein: Organic Chemistry: Basic Chemistry II . Edition 6. Springer, 2008, ISBN 978-3-540-77106-7 , p. 400 ( limited preview in Google book search).
  • Manfred Hesse, Herbert Meier, Bernd Zeeh: Spectroscopic methods in organic chemistry . Thieme, Stuttgart [a. a.] 2005, ISBN 3-13-576107-X , Figure 1: Jablonski term scheme.

Web links

Individual evidence

  1. ↑ Note: For the pronunciation of jabwɔɲ̟s̪ki see Pronunciation of Polish .
  2. ^ A b New Trends in Fluorescence Spectroscopy - Springer . doi : 10.1007 / 978-3-642-56853-4 .
  3. ^ A. Jabłoński: Efficiency of anti-Stokes fluorescence in dyes . In: Nature . tape 131 , 1933, pp. 839-840 , doi : 10.1038 / 131839b0 .
  4. ↑ Terenine, AN: Photochemical Processes in Aromatic Compounds . In: Acta physicochimica URSS Band 18 , no. 4 , 1943, pp. 210-241 .
  5. ↑ Gilbert N. Lewis, M. Kasha: Phosphorescence and the Triplet State . In: Journal of the American Chemical Society . tape 66 , no. 12 , December 1, 1944, ISSN  0002-7863 , p. 2100–2116 , doi : 10.1021 / ja01240a030 .
  6. ↑ P. Klán, J. Wirz: Photochemistry of Organic Compounds: From Concepts to Practice . 3. Edition. Wiley-Blackwell, 2009, ISBN 978-1-4051-9088-6 , pp. 25 .
  7. ↑ Michael Kasha: The triplet state: An example of GN Lewis' research style . In: Journal of Chemical Education . tape 61 , no. 3 , March 1, 1984, ISSN  0021-9584 , p. 204 , doi : 10.1021 / ed061p204 .