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A photolysis or photodissociation is understood to be a cleavage of a chemical bond triggered by electromagnetic radiation , especially light .


During photolysis, the absorption of light in the visible, ultraviolet or even shorter-wave spectrum leads to bond cleavage. The wavelength of the absorbed light must be more energetic than the binding energy so that photolysis can occur.

Unless the molecule is held together by other bonds, the molecular fragments also dissociate as a result . Research into photolysis is a branch of photochemistry that deals with reactions that are triggered by light absorption. In kinetics , flash photolysis is used to study rapid chemical reactions.


The photolysis of small molecules is essential in the upper layers of the atmosphere. These include B. the splitting of molecular oxygen into oxygen atoms (O 2 → 2 O), which form ozone through reaction with molecular oxygen (O 2 + O + M → O 3 + M; M = collision partner).

Photolysis of halogens (especially chlorine, bromine) is used preparatively for radical halogenation. Photopolymerizations can be started by photolysis of suitable photoinitiators . In particular, aliphatic azo compounds , hydroxyketones or (bis) acylphosphine oxides are used .

In organic photochemistry, the α-cleavage of ketones ( Norrish type I reaction ), the N 2 elimination from azo compounds, the N 2 elimination from azides (nitrene formation) and the Barton reaction (photolysis of nitrites ) are preparative important and mechanistically well-studied reactions.

Mechanistic Aspects

Photolysis is triggered by the absorption of light quanta by the molecule to be photolysed. This leads to electronic excitation (raising an electron into an energetically high orbital), which is usually accompanied by vibration excitation (vibronic excitation). Since the excitation usually takes place in antibonding orbitals, the bonds are weakened in the excited state.

The following mechanisms are fundamentally possible for photodissociation:

  1. The excited state is not bound (there is no energetic minimum along the reaction coordinate) and the two molecular fragments drift apart.
  2. The excited state is bound (there is an energetic minimum along the reaction coordinate), but the vibronic excitation is sufficiently large and the two fragments can leave the potential well.
  3. The excited state is bound (there is an energetic minimum along the reaction coordinate), but the vibronic excitation is sufficiently large that a transition to another, dissociative potential curve / surface can take place within the potential well.
  4. The molecule returns to the ground state surface without disintegrating in the excited state and disintegrates there due to the geometry imposed in the excited state and / or the vibronic excitation brought along.

The photolysis of chlorine molecules Cl 2 ( homolytic cleavage ) is a prime example for mechanisms 1 to 3, whereby in this case even the respective proportion of the individual decay channels can be controlled by the choice of the excitation wavelength.

Individual evidence

  1. Robert Guderian: Handbook of environmental changes and ecotoxicology. Springer Verlag, Berlin / Heidelberg 2000, ISBN 978-3-642-63037-8 , p. 220 ( Google books )
  2. Alexander Schönberg : Preparative Organic Photochemistry. With a contribution from GO Schenk. Springer, Berlin / Göttingen / Heidelberg 1958.
  3. Alexander Schönberg: Preparative Organic Photochemistry. In collaboration with GO Schenk, O.-A. Neumüller. 2nd edition of Preparative Organic Photochemistry. Springer, Berlin / Heidelberg / New York 1968.
  4. a b M. Klessinger, J. Michl: Light absorption and photochemistry of organic molecules. VCH, Weinheim, New York 1989.
  5. a b G. of Bühnau, T. Wolff: photochemistry - principles, methods, applications. VCH, Weinheim, New York 1987.
  6. ^ AJ Johnsen, AB Alekseyev, GG Balint-Kurti, M. Brouard, Alex Brown, RJ Buenker, EK Campbell, DB Kokh: A complete quantum mechanical study of chlorine photodissociation . In: The Journal of Chemical Physics . tape 136 , no. 16 , April 28, 2012, p. 164310 , doi : 10.1063 / 1.4704829 .