Exciplex

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An exciplex (from English excited complex ) is an aggregate of two atoms or molecules , one of which is excited.

Basics

By absorbing radiation, matter (atoms, molecules) can be converted into an excited state . This normally has a very short lifespan.

Bimolecular chemical interactions often occur when the electron clouds of both partners collide. Electrons can be exchanged during the spatial overlap of the clouds . This process can affect both electrons at the same time: the electrons of the acceptor and the donor are exchanged simultaneously. The exchange can also take place step by step with the formation of ionic radicals (English charge transfer ).

If the two collision partners - the excited donor molecule and the acceptor molecule in the ground state - are strongly polarized, a metastable complex can form between the two molecules in the excited state. If the two partners are not the same, the complex is called an exciplex , if they are the same, an excimer (from excited dimer ).

If it is possible to form a chemical bond between two particles, there is a minimum of energy depending on the location: The minimum is the so-called equilibrium distance between the two particles that are connected to each other.

Of course, there are also systems in which this is not possible. So are z. B. helium atoms in principle not able to form chemical bonds to other atoms, because this would occupy antibonding molecular orbitals that would destabilize a bond. However, this does not rule out that such systems in the excited state can nevertheless enter into a chemical bond, which, however, breaks again when the particle falls back into the ground state.

meaning

An exciplex is e.g. B. XeCl * in the XeCl excimer laser . The basic state of an exciplex does not exist, which means that there is an automatic population inversion in the excimer laser .

In photochemistry , exciplexes play an important role as starting points for bimolecular reactions.

See also

swell