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Internal conversion (chemistry)

 

 
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Internal conversion (chemistry)
 
 
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Internal conversion is a transition from a higher to a lower electronic state in a molecule. It is sometimes called "radiationless de-excitation", because no photon
Photon

In physics, the photon is an elementary particle, the quantum of the electromagnetic field and the basic unit of light and all other forms of electromagnetic radiation....
s are emitted. It differs from intersystem crossing
Intersystem crossing

Intersystem crossing is a photophysical process. An isoenergetic non-radiative transition between two electronic states having different Term symbol....
 in that, while both are radiationless methods of de-excitation, the molecular spin state for internal conversion remains the same, whereas it changes for intersystem crossing. The energy of the electronically excited state is given off to vibrational modes of the molecule.






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Internal conversion is a transition from a higher to a lower electronic state in a molecule. It is sometimes called "radiationless de-excitation", because no photon
Photon

In physics, the photon is an elementary particle, the quantum of the electromagnetic field and the basic unit of light and all other forms of electromagnetic radiation....
s are emitted. It differs from intersystem crossing
Intersystem crossing

Intersystem crossing is a photophysical process. An isoenergetic non-radiative transition between two electronic states having different Term symbol....
 in that, while both are radiationless methods of de-excitation, the molecular spin state for internal conversion remains the same, whereas it changes for intersystem crossing. The energy of the electronically excited state is given off to vibrational modes of the molecule. The excitation energy is transformed into heat.

A classic example of this process is the quinine
Quinine

Quinine is a natural white crystalline alkaloid having antipyretic , antimalarial drug, analgesic , and anti-inflammatory properties and a bitter taste....
 sulfate fluorescence
Fluorescence

Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of a photon with a longer wavelength....
, which can be quenched
Quenching (fluorescence)

Quenching refers to any process which decreases the fluorescence intensity of a given substance. A variety of processes can result in quenching, such as excited state reactions, energy transfer, complex-formation and collisional quenching....
 by the use of various halide
Halide

A halide is a binary compound, of which one part is a halogen atom and the other part is an chemical element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound....
 salt
Salt

A salt, in chemistry, is defined as the product formed from the neutralisation reaction of acids and base . Salts are ionic compounds composed of cations and anions so that the product is electrically electric charge ....
s. What happens is that the excited molecule can de-excite by increasing the thermal energy of the surrounding solvated ion
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
s.

A general and quantitative discussion of intramolecular radiationless transitions is the subject of an article by M. Bixon and J. Jortner (J. Chem. Phys., 48 (2) 715-726 (1968)).

Several natural molecules perform a very fast internal conversion. This ability to transform the excitation energy of photon into heat can be a crucial property for photoprotection
Photoprotection

The term Photoprotection designates the mechanisms that nature has developed to minimize the damages that the human body suffers when exposed to UV-irradiation....
. Fast internal conversion reduces the excited state lifetime, and thereby prevents bimolecular reactions (oxidative stress
Oxidative stress

Oxidative stress is caused by an imbalance between the production of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or easily repair the resulting damage....
 and free radicals). DNA has an extremely short lifetime due to a fast internal conversion.

Melanin is also a molecule with extremely fast internal conversion. This is what makes it a good photoprotective substance.

Both Melanin and DNA have internal conversion rates that are many orders of magnitude faster than any man-made molecule.

In applications that make use of bimolecular electron transfer the internal conversion is undesirable. For example it is advantageous to have a long lived excited states in Grätzel cells (Dye-sensitized solar cells). Bimolecular electron transfer always produces a reactive chemical species (it produces free radicals)