Bathochromic effect

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The bathochromic effect , also known as redshift , describes a deepening of color, i.e. a shift of the absorption spectrum into the longer-wave , lower-energy range of the electromagnetic spectrum .

Basics

In organic dyes, the bathochromic groups −OH , −NH 2 , -NR 2 and −OCH 3 act as auxochromes in a shift in the absorption spectrum from violet to blue, cyan, green and yellow to red. This change in the absorption of the substrate changes the “perceived color” of the colored body correspondingly complementary from red to purple.

The strength of the bathochromic effect is influenced by the structure of the molecule and certain substituents . These influences lead to a greater delocalization of the π electrons. The more the π-electrons of a molecule are delocalized , the less energy is required for their excitation. The light required for excitation is absorbed, the absorbing body appears in the corresponding complementary color .

Cyclic structures have a weaker bathochromic effect than linear ones. Chromophores and especially conjugated systems of chromophores have a stronger bathochromic effect.

Substituents with a bathochromic effect (bathochrome) influence the mesomerism of the chromophore . As a result, the π electrons are in a more delocalized form. Auxochromes like the −NH 2 group cause a deepening of the color. If the H atoms on the −NH 2 group are substituted by alkyl or aryl radicals , the deepening of the color is intensified, since the lone pair of electrons can participate more in the mesomerism. Whether an auxochrome has a bathochromic or hypsochromic (color-enhancing) effect depends on the position on the chromophore. The bathochromic effect becomes particularly intense when an auxochromic (+ M) and the antiauxochromic (−M) functional group complement one another in their effect on the same dye molecule . One functional group increases the electron density in the π-electron system (+ M effect), while the other tries to decrease it at the same time (−M effect). In general, the mesomerism is reinforced by a push-pull system.

With salts of certain dyes , a deepening of color ( halochromism ) occurs. If a hydroxyl group of a phenol is deprotonated , the resulting free electron pairs of the phenolate group participate more in the mesomerism and the absorption maximum is shifted.

application

In general, inserting bathochromic groups produces a dye with a different color. The halochrome effect is used for indicators , as the color change takes place at a certain pH value of the solution .

Related terms

If the bathochromic shift results from a negatively charged (salt-like) group, this is referred to as the halochrome effect . If substituents or functional groups lead to a shift in the absorption spectrum of a substance in the direction of shorter wavelengths, this is, in contrast to the bathochromic effect, a hypsochromic effect .

literature

  • Kurt Nassau: The Physics and Chemistry of Color: The Fifteen Causes of Color . 2nd Edition. Wiley & Sons, New York 2001, ISBN 0-471-39106-9 .
  • Norbert Welsch, Claus Chr. Liebmann: Colors. Nature - technology - art . 2nd Edition. Spectrum Akademischer Verlag, Heidelberg 2004, ISBN 3-8274-1563-2 .
  • Heinrich Zollinger, A. Iqbal: Color Chemistry: Syntheses, Properties, and Applications of Organic Dyes and Pigments . 3. Edition. Helvetica Chimica Acta / Wiley-VCH, Weinheim 2003, ISBN 3-906390-23-3 .
  • Dudley H. Williams; Ian Flemming: Structure Elucidation in Organic Chemistry . 6th edition. Thieme, Stuttgart 1991, ISBN 3-13-437206-1 .

Individual evidence

  1. ↑ How do you explain the colourfulness of organic dyes. Matthias Rinschen at www.deinchemielehrer.de, accessed on June 18, 2016 .
  2. ↑ Structure and color of organic compounds. In: lernhelfer.de. Bibliographisches Institut GmbH Berlin, accessed on June 18, 2016 .
  3. ↑ Bathochromic effect. Ariel Misholi Hamra Webservice - techniklexikon.net, accessed on June 18, 2016 .
  4. ↑ The coloring of organic matter. Märkisches Gymnasium Hamm, accessed on June 18, 2016 .
  5. ↑ a b K. Schwetlick: Organikum . 15th edition, VEB Deutscher Verlag der Wissenschaften, Berlin 1976, p. 513 f.
  6. ↑ R. Winziger: The inversion of the auxochromes . Chimia 15, 1961, pp. 89 f.