Photochemistry
Photochemistry
The basic laws of photochemistry
Photochemistry induced by visible and ultraviolet light
Photochemistry is the study of light-induced chemical reactions and physical processes. A photochemical event involves the absorption of light to create an excited species that may subsequently undergo a number of different reactions. These include unimolecular reactions such as dissociation, ionization, and isomerization; bimolecular reactions, which involve a reaction with a second molecule or atom to form a new compound; and reactions producing an emission of light, or luminescence. A photochemical reaction differs notably from a thermally, or heat, induced reaction in that the rate of a photochemical reaction is frequently greatly accelerated, and the products of the photochemical reaction may be impossible to produce otherwise. With the advent of lasers (powerful, single-color light sources) the field of photochemistry has advanced tremendously over the past few decades. An increased understanding of photochemistry has great implications outside of the laboratory, as photochemical reactions are an extremely important aspect of everyday life, underlying the processes of vision, photosynthesis, photography, atmospheric chemistry, the production of smog, and the destruction of the ozone layer.
The absorption of light by atoms and molecules to create an excited species is studied in the field of spectroscopy. The study of the reactions of this excited species is the domain of photochemistry. However, the fields are closely related; spectroscopy is routinely used by photochemists as a tool for identifying reaction pathways and products and, recently, for following reactions as they occur in real time. Some lasers can produce a pulse of light that is only “on” for 1 femto-second (10-15 seconds). A femtosecond laser can be used like an extremely high-speed strobe camera to spectroscopically “photograph” a photochemical reaction.
The basic laws of photochemistry
In the early 1800s Christian von Grotthus (1785-1822) and John Draper (1811-1882) formulated the first law of photochemistry, which states that only light that is absorbed by a molecule can produce a photochemical change in that molecule. This law relates photochemical activity to the fact that each chemical substance absorbs only certain wavelengths of light, the set of which is unique to that substance. Therefore, the presence of light alone is not sufficient to induce a photochemical reaction; the light must also be of the correct wavelength to be absorbed by the reactant species.
In the early 1900s the development of the quantum theory of light—the idea that light is absorbed in discrete packets of energy called photons—led to the extension of the laws of photochemistry. The second law of photochemistry, developed by Johannes Stark (1874-1957) and Albert Einstein (1879-1955), states that only one quantum, or one photon, of light is absorbed by each molecule undergoing a photochemical reaction. In other words, there is a one-to-one correspondence between the number of absorbed photons and the number of excited species. The ability to accurately determine the number of photons leading to a reaction enables the efficiency, or quantum yield, of the reaction to be calculated.
Photochemistry induced by visible and ultraviolet light
Light that can break molecular bonds is most effective at inducing photochemical reactions. The energy required to break a molecular bond ranges from approximately 150 kiloJoules per mole to nearly 1000 kJ/mol, depending on the bond. Visible light, having wavelengths ranging from 400-700 nanometers, corresponds to energies ranging from approximately 300-170 kJ/mol, respectively. Note that this is enough energy to dissociate relatively weak bonds such as the single oxygen (O-O) bond in hydrogen peroxide (HOOH), which is why hydrogen peroxide must be stored in a light-proof bottle.
Ultraviolet light, having wavelengths ranging from 200-400 nm, corresponds to higher energies ranging from approximately 600-300 kJ/mol, respectively. Ultraviolet light can dissociate relatively strong bonds such as the double oxygen (O=O) bond in molecular oxygen (O2) and the double C=O bond in carbon dioxide (CO2); ultraviolet light can also remove chlorine atoms from compounds such as chloromethane (CH3Cl). The ability of ultraviolet light to dissociate these molecules is an important aspect of the stability—and destruction—of ozone molecules in the upper atmosphere.
Reaction Pathways
A photochemical process may be considered to consist of two steps: the absorption of a photon, followed by reaction. If the absorption of a photon causes an electron within an atom or molecule to increase its energy, the species is said to be electronically excited. The absorption and reaction steps for a molecule AB may be written as: AB + hν → AB* AB* → products where hν represents the energy of a photon of frequency ν and the asterisk indicates that the species has become electronically excited. The excited species, AB*, has the additional energy of the absorbed photon and will react in order to reduce its energy. Although the excited species generally does not live long, it is sometimes formally indicated when writing photochemical reactions to stress that the reactant is an electronically excited species. The possible reactions that an electronically excited species may undergo are illustrated below. Note: the symbols * and † denote different levels of electronic excitation.
AB +hv → AB*
Absorption of a photon (electronic excitation)
Followed by:
AB + hv → AB*
Absorption of a photon (electronic excitation)
Followed by:
i) AB* → A + B Dissociation
ii) AB* → AB++ e– Ionization
iii) AB* → BA Isomerization
iv) AB* + C → AC + B or ABC Reaction
v) AB* + DE → AB + DE* Energy Transfer (intermolecular)
vi) AB* + M → AB + M Physical Quenching
vii) AB* → AB† Energy Transfer (intramolecular)
viii) AB* → AB + hυ Luminsecence
Dissociation
The energy of an absorbed photon may be sufficient to break molecular bonds (path i), creating two or more atomic or molecular fragments. An important example of photodissociation is found in the photo-chemistry of stratospheric ozone. Ozone (O3) is produced in the stratosphere from molecular oxygen (O2) through the following pair of reactions: O2 +hν→ O+ O and O + O2→ O3 where hn represents the energy of a photon of ultraviolet light with a wavelength less than 260 nm. Ozone is also dissociated by short-wavelength ultraviolet light (200-300 nm) through the reaction: O3 + hν→ O2 + O. The oxygen atom formed from this reaction may recombine with molecular oxygen to regenerate ozone, thereby completing the ozone cycle. The great importance of stratospheric ozone is that it absorbs harmful short-wavelength ultraviolet light before it reaches Earth’s surface, thus serving as a protective shield.
In recent years, the effect of chlorofluorocarbons, commonly known as Freons or CFCs, on the ozone cycle has become of great concern. CFCs rise into the stratosphere where they are dissociated by ultraviolet light, producing chlorine atoms (Cl) through the reaction: CFC + hν→ Cl + CFC (minus one Cl). These chlorine atoms react with ozone to produce ClO and molecular oxygen: Cl + O3→ ClO + O2. ClO reacts with the oxygen atoms produced from the photodissociation of ozone in reaction 5 to produce molecular oxygen and a chlorine atom: ClO + O→ O2 + Cl. Therefore, the presence of CFCs interrupts the natural ozone cycle by consuming the oxygen atoms that should combine with molecular oxygen to regenerate ozone. The net result is that ozone is removed from the stratosphere while the chlorine atoms are regenerated in a catalytic process to continue the destructive cycle.
Ionization
The separation of an electron from an atom or molecule, leaving a positively charged ion, is a special form of dissociation called ionization. Ionization following absorption of a photon (path ii) usually occurs with light of very short wavelengths (less than 100 nm) and therefore is usually not studied by photochemists, although it is of great importance in x-ray technology. X rays are also sometimes referred to as ionizing radiation.
Isomerization
An excited molecule may undergo a rearrangement of its bonds, forming a new molecule made up of the same atoms but connected in a different manner; this process is called isomerization (path iii). The first step in the vision process involves the light-induced isomerization of pigments in the retina that subsequently undergo a number of thermally and enzymatically driven reactions before ultimately producing a neural signal.
Reaction
An electronically excited species may react with a second species to produce a new product, or set of products (path iv). For example, the products of the
KEY TERMS
Absorption— The acquisition of energy from a photon of light by an atomic or molecular species, often causing electronic excitation.
Electronic excitation— The state of an atom or molecule in which an electron has been given additional energy.
Emission— The generation of a photon of light from an electronically excited atomic or molecular species in order to reduce its total energy.
Photodissociation— The breaking of one or more molecular bonds resulting from the absorption of light energy.
Photon— A quantum, or discrete packet, of light energy.
Quantum yield— In a photochemical reaction, the number of product species divided by the number of photons that were absorbed by the reactant.
Wavelength— The distance between two consecutive crests or troughs in a wave.
ultraviolet dissociation of ozone (reaction 5) are themselves electronically excited: O3 + hν→ O2* + O*. These excited fragments may react with other atmospheric molecules such as water: O* + H2O→ OH + OH. Or they may react with ozone: O2* + O3→ 2O2 + O.These reactions do not readily occur for the corresponding non-excited species, confirming the importance of electronic excitation in determining reactivity.
Energy transfer
In some cases the excited species may simply transfer its excess energy to a second species. This process is called intermolecular energy transfer (path v). Photosynthesis relies on intermolecular energy transfer to redistribute the light energy gathered by chlorophyll to a reaction center where the carbohydrates that nourish the plant are produced. Physical quenching (path vi) is a special case of intermolecular energy transfer in which the chemical behavior of the species to which the energy is transferred does not change. An example of a physical quencher is the walls of a container in which a reaction is confined. If the energy transfer occurs within the same molecule, for example, and if the excess electron energy is transferred into internal motion of the molecule, such as vibration, it is called intramolecular energy transfer (path vii).
Luminescence
Although it is not strictly a photochemical reaction, another pathway by which the excited species may reduce its energy is by emitting a photon of light. This process is called luminescence (path viii). Luminescence includes the processes of fluorescence (prompt emission of a photon) and phosphorescence (delayed emission of a photon). Optical brighteners in laundry detergents contain substances that absorb light of one wavelength, usually in the ultraviolet range, but emit light at a longer wavelength, usually in the visible range— thereby appearing to reflect extra visible light and making clothing appear whiter. This process is called fluorescence and only occurs while the substance is being illuminated. The related process, phosphorescence, persists after the excitation source has been removed and is used in “glow-in-the-dark” items.
Resources
BOOKS
Buchanan, B.B., W. Gruissem, and R L. Jones. Biochemistry and Molecular Biology of Plants. New York: John Wiley and Sons, 2002.
Lide, D.R., ed. CRC Handbook of Chemistry and Physics, 87th ed. Boca Raton: CRC Press, 2006.
PERIODICALS
Li, X. P., Bjorkman, O., Shih, C., et al. “A Pigment-binding Protein Essential for Regulation of Photosynthetic Light Harvesting.” Nature 403; (January 2000): 391-395.
Wayne, Richard. Principles and Applications of Photochemistry Oxford: Oxford Science Publications, 1988.
Karen Trentelman
Photochemistry
Photochemistry
Photochemistry is the study of light-induced chemical reactions and physical processes. A photochemical event involves the absorption of light to create an excited species that may subsequently undergo a number of different reactions. These include unimolecular reactions such as dissociation, ionization, and isomerization; bimolecular reactions, which involve a reaction with a second molecule or atom to form a new compound; and reactions producing an emission of light, or luminescence . A photochemical reaction differs notably from a thermally, or heat , induced reaction in that the rate of a photochemical reaction is frequently greatly accelerated, and the products of the photochemical reaction may be impossible to produce otherwise. With the advent of lasers (powerful, single-color light sources) the field of photochemistry has advanced tremendously over the past few decades. An increased understanding of photochemistry has great implications outside of the laboratory, as photochemical reactions are an extremely important aspect of everyday life, underlying the processes of vision , photosynthesis , photography , atmospheric chemistry , the production of smog , and the destruction of the ozone layer.
The absorption of light by atoms and molecules to create an excited species is studied in the field of spectroscopy . The study of the reactions of this excited species is the domain of photochemistry. However, the fields are closely related; spectroscopy is routinely used by photochemists as a tool for identifying reaction pathways and products and, recently, for following reactions as they occur in real time. Some lasers can produce a pulse of light that is only "on" for 1 femtosecond (10-15 seconds). A femtosecond laser can be used like an extremely high-speed strobe camera to spectroscopically "photograph" a photochemical reaction.
The basic laws of photochemistry
In the early 1800s Christian von Grotthus (1785-1822) and John Draper (1811-1882) formulated the first law of photochemistry, which states that only light that is absorbed by a molecule can produce a photochemical change in that molecule. This law relates photochemical activity to the fact that each chemical substance absorbs only certain wavelengths of light, the set of which is unique to that substance. Therefore, the presence of light alone is not sufficient to induce a photochemical reaction; the light must also be of the correct wavelength to be absorbed by the reactant species.
In the early 1900s the development of the quantum theory of light—the idea that light is absorbed in discrete packets of energy called photons—led to the extension of the laws of photochemistry. The second law of photo-chemistry, developed by Johannes Stark (1874-1957) and Albert Einstein (1879-1955), states that only one quantum, or one photon , of light is absorbed by each molecule undergoing a photochemical reaction. In other words, there is a one-to-one correspondence between the number of absorbed photons and the number of excited species. The ability to accurately determine the number of photons leading to a reaction enables the efficiency, or quantum yield, of the reaction to be calculated.
Photochemistry induced by visible and ultraviolet light
Light that can break molecular bonds is most effective at inducing photochemical reactions. The energy required to break a molecular bond ranges from approximately 150 kiloJoules per mole to nearly 1000 kJ/mol, depending on the bond. Visible light, having wavelengths ranging from 400-700 nanometers, corresponds to energies ranging from approximately 300-170 kJ/mol, respectively. Note that this is enough energy to dissociate relatively weak bonds such as the single oxygen (O-O) bond in hydrogen peroxide (HOOH), which is why hydrogen peroxide must be stored in a light-proof bottle.
Ultraviolet light, having wavelengths ranging from 200-400 nm, corresponds to higher energies ranging from approximately 600-300 kJ/mol, respectively. Ultraviolet light can dissociate relatively strong bonds such as the double oxygen (O=O) bond in molecular oxygen (O2) and the double C=O bond in carbon dioxide (CO2); ultraviolet light can also remove chlorine atoms from compounds such as chloromethane (CH3Cl). The ability of ultraviolet light to dissociate these molecules is an important aspect of the stability—and destruction—of ozone molecules in the upper atmosphere.
Reaction pathways
A photochemical process may be considered to consist of two steps: the absorption of a photon, followed by reaction. If the absorption of a photon causes an electron within an atom or molecule to increase its energy, the species is said to be electronically excited. The absorption and reaction steps for a molecule AB may be written as: AB + hν → AB* AB* → products where hν represents the energy of a photon of frequency ν and the asterisk indicates that the species has become electronically excited. The excited species, AB*, has the additional energy of the absorbed photon and will react in order to reduce its energy. Although the excited species generally does not live long, it is sometimes formally indicated when writing photochemical reactions to stress that the reactant is an electronically excited species. The possible reactions that an electronically excited species may undergo are illustrated below. Note: the symbols * and † denote different levels of electronic excitation.
Absorption of a photon (electronic excitation)
Followed by:
| i) | AB* → A + B | Dissociation |
| ii) | AB* → AB+ + e– | Ionization |
| iii) | AB* → BA | Isomerization |
| iv) | AB* + C → AC + B or ABC | Reaction |
| v) | AB* + DE → AB + DE* | Energy Transfer (intermolecular) |
| vi) | AB* + M → AB + M | Physical Quenching |
| vii) | AB* → AB† | Energy Transfer (intramolecular) |
| viii) | AB* → AB + hν | Luminsecence |
Dissociation
The energy of an absorbed photon may be sufficient to break molecular bonds (path i), creating two or more atomic or molecular fragments. An important example of photodissociation is found in the photochemistry of stratospheric ozone. Ozone (O3) is produced in the stratosphere from molecular oxygen (O2) through the following pair of reactions: O2 + hν → O + O and O + O2 → O3 where hn represents the energy of a photon of ultraviolet light with a wavelength less than 260 nm. Ozone is also dissociated by short-wavelength ultraviolet light (200-300 nm) through the reaction: O3 + hν → O2 + O. The oxygen atom formed from this reaction may recombine with molecular oxygen to regenerate ozone, thereby completing the ozone cycle. The great importance of stratospheric ozone is that it absorbs harmful short-wavelength ultraviolet light before it reaches the Earth's surface, thus serving as a protective shield.
In recent years, the effect of chlorofluorocarbons, commonly known as Freons or CFCs, on the ozone cycle has become of great concern. CFCs rise into the stratosphere where they are dissociated by ultraviolet light, producing chlorine atoms (Cl) through the reaction: CFC + hν → Cl + CFC(minus one Cl). These chlorine atoms react with ozone to produce ClO and molecular oxygen: Cl + O3 → ClO + O2. ClO reacts with the oxygen atoms produced from the photodissociation of ozone in reaction 5 to produce molecular oxygen and a chlorine atom: ClO + O → O2 + Cl. Therefore, the presence of CFCs interrupts the natural ozone cycle by consuming the oxygen atoms that should combine with molecular oxygen to regenerate ozone. The net result is that ozone is removed from the stratosphere while the chlorine atoms are regenerated in a catalytic process to continue the destructive cycle.
Ionization
The separation of an electron from an atom or molecule, leaving a positively charged ion, is a special form of dissociation called ionization. Ionization following absorption of a photon (path ii) usually occurs with light of very short wavelengths (less than 100 nm) and therefore is usually not studied by photochemists, although it is of great importance in x-ray technology. X rays are also sometimes referred to as ionizing radiation .
Isomerization
An excited molecule may undergo a rearrangement of its bonds, forming a new molecule made up of the same atoms but connected in a different manner; this process is called isomerization (path iii). The first step in the vision process involves the light-induced isomerization of pigments in the retina that subsequently undergo a number of thermally and enzymatically driven reactions before ultimately producing a neural signal.
Reaction
An electronically excited species may react with a second species to produce a new product, or set of products (path iv). For example, the products of the ultraviolet dissociation of ozone (reaction 5) are themselves electronically excited: O3 + hν → O *2 + O*. These excited fragments may react with other atmospheric molecules such as water : O* + H2O → OH + OH. Or they may react with ozone: O *2 + O3 → 2O2 + O. These reactions do not readily occur for the corresponding non-excited species, confirming the importance of electronic excitation in determining reactivity.
Energy transfer
In some cases the excited species may simply transfer its excess energy to a second species. This process is called intermolecular energy transfer (path v). Photosynthesis relies on intermolecular energy transfer to redistribute the light energy gathered by chlorophyll to a reaction center where the carbohydrates that nourish the plant are produced. Physical quenching (path vi) is a special case of intermolecular energy transfer in which the chemical behavior of the species to which the energy is transferred does not change. An example of a physical quencher is the walls of a container in which a reaction is confined. If the energy transfer occurs within the same molecule, for example, and if the excess electron energy is transferred into internal motion of the molecule, such as vibration, it is called intramolecular energy transfer (path vii).
Luminescence
Although it is not strictly a photochemical reaction, another pathway by which the excited species may reduce its energy is by emitting a photon of light. This process is called luminescence (path viii). Luminescence includes the processes of fluorescence (prompt emission of a photon) and phosphorescence (delayed emission of a photon). Optical brighteners in laundry detergents contain substances that absorb light of one wavelength, usually in the ultraviolet range, but emit light at a longer wavelength, usually in the visible range—thereby appearing to reflect extra visible light and making clothing appear whiter. This process is called fluorescence and only occurs while the substance is being illuminated. The related process, phosphorescence, persists after the excitation source has been removed and is used in "glow-in-the-dark" items.
Resources
books
Buchanan, B.B., W. Gruissem, and R L. Jones. Biochemistry and Molecular Biology of Plants. Rockville, MD: American Society of Plant Physiologists, 2000.
Lide, D.R., ed. CRC Handbook of Chemistry and Physics Boca Raton: CRC Press, 2001.
Williamson, Samuel J. and Herman Z. Cummins. Light andColor in Nature and Art. New York: John Wiley and Sons, 1983.
periodicals
Li, X. P., Bjorkman, O., Shih, C., et al. "A Pigment-binding Protein Essential for Regulation of Photosynthetic Light Harvesting." Nature 403 (January 2000): 391-395.
Toon, Owen B., and Richard P. Turco. "Polar Stratospheric Clouds and Ozone Depletion." Scientific American no. 264 (1991): 68-74.
Wayne, Richard. Principles and Applications of Photochemistry Oxford: Oxford Science Publications, 1988.
Zewail, Ahmed. "The Birth of Molecules." Scientific American no. 263 (1990): 76-82.
Karen Trentelman
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Absorption
—The acquisition of energy from a photon of light by an atomic or molecular species, often causing electronic excitation.
- Electronic excitation
—The state of an atom or molecule in which an electron has been given additional energy.
- Emission
—The generation of a photon of light from an electronically excited atomic or molecular species in order to reduce its total energy.
- Photodissociation
—The breaking of one or more molecular bonds resulting from the absorption of light energy.
- Photon
—A quantum, or discrete packet, of light energy.
- Quantum yield
—In a photochemical reaction, the number of product species divided by the number of photons that were absorbed by the reactant.
- Wavelength
—The distance between two consecutive crests or troughs in a wave.
Photochemistry
Photochemistry
Photochemistry is the study of chemical changes made possible by light energy. The production of ozone in Earth's upper atmosphere is an example of such a change. Light from the Sun (solar energy) strikes oxygen molecules in the stratosphere, causing them to break down into two oxygen atoms:
O2 + hν → O + O
(The expression hν is commonly used to represent a unit of light energy known as the photon.)
In the next stage of that reaction, oxygen atoms react with oxygen molecules to produce ozone (O3):
O + O2 → O3
Steps in photochemical processes
The excited state. A photochemical change takes place in two steps. Imagine that a light beam is shined on a piece of gold. The light beam can be thought of as a stream of photons, tiny packages of energy. The energy of the photon is expressed by means of the unit hν.
When a photon strikes an atom of gold, it may be absorbed by an electron in the gold atom. The electron then becomes excited, meaning that it has more energy than it did before being hit by the photon. Chemists use an asterisk (*) to indicated that something is in an excited state. Thus, the collision of a photon with an electron (e) can be represented as follows:
e + hν → e*
Once an electron is excited, the whole atom in which it resides is also excited. Another way to represent the same change, then, is to show that the gold atom (Au) becomes excited when struck by a photon:
Au + hν → Au*
Emission of energy. Electrons, atoms, and molecules normally do not remain in an excited state for very long. They tend to give off their excess energy very quickly and return to their original state. When they do so, they often undergo a chemical change. Since this change was originally made possible by absorbed light energy, it is known as a photochemical change.
The formation of ozone is just one example of the many kinds of photochemical changes that can occur. When solar energy breaks an oxygen molecule into two parts, one or both of the oxygen atoms formed may be excited. Another way to write the very first equation above is as follows:
O2 + hν → O* + O
The excited oxygen atom (O*) then has the excess energy needed to react with a second oxygen molecule to form ozone:
O* + O2 → O3
Another way for an excited atom or molecule to lose its energy is to give it off as light. This process is just the reverse of the process by which the atom or molecule first became excited. If the atom or molecule gives off its excess energy almost immediately, the material in which it is contained glows very briefly, a process known as fluorescence. If the excess energy is given off more slowly over a period of time, the process is known as phosphorescence. Both fluorescence and phosphorescence are examples of the general process of light emission by excited materials known as luminescence.
[See also Atom; Luminescence; Photosynthesis ]