Internal conversion (chemistry)
Encyclopedia
Internal conversion is a transition from a higher to a lower electronic state in a molecule or atom. It is sometimes called "radiationless de-excitation", because no photon
s are emitted. It differs from intersystem crossing
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 or phonons. The excitation energy is transformed into heat.
A classic example of this process is the quinine
sulfate fluorescence
, which can be quenched
by the use of various halide
salt
s. What happens is that the excited molecule can de-excite by increasing the thermal energy of the surrounding solvated ion
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
by molecules such as melanin
. Fast internal conversion reduces the excited state
lifetime, and thereby prevents bimolecular reactions (oxidative stress
and free radicals).
Nucleic acids (precisely the single, free nucleotides, not those bound in a DNA/RNA strand) has an extremely short lifetime due to a fast internal conversion.
Melanin
is also a molecule with extremely fast internal conversion. This is one property that 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, free radicals.
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s are emitted. It differs from intersystem crossing
Intersystem crossing
Intersystem crossing is a radiationless process involving a transition between two electronic states with different spin multiplicity.-Singlet and triplet states:...
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 or phonons. 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, analgesic , anti-inflammatory properties and a bitter taste. It is a stereoisomer of quinidine which, unlike quinine, is an anti-arrhythmic...
sulfate fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
, 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. As a consequence, quenching is often heavily dependent on...
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 element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
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 in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
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
Photoprotection is a group of mechanisms that nature has developed to minimize the damage that the human body suffers when exposed to UV radiation...
by molecules such as melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
. Fast internal conversion reduces the excited state
lifetime, and thereby prevents bimolecular reactions (oxidative stress
Oxidative stress
Oxidative stress represents an imbalance between the production and manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage...
and free radicals).
Nucleic acids (precisely the single, free nucleotides, not those bound in a DNA/RNA strand) has an extremely short lifetime due to a fast internal conversion.
Melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
is also a molecule with extremely fast internal conversion. This is one property that 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, free radicals.