Core Concepts
Photochemistry is the study of chemical reactions, processes, and mechanisms initiated by light. This article explores the fundamentals of photochemistry, its applications, and the significance it holds in both nature and technology.
Basics of Photochemistry
When a molecule absorbs light, it gains energy. Light, in the form of photons, excites the molecule from the ground electronic state to an excited state. Eventually, this excited state will relax back to the ground state and dissipate energy. A lot of times, this relaxation is simply done by emitting a photon of the same energy, but sometimes, chemical rearrangements or reactions occur that dissipate some or all of this energy. These reactions are called photochemical reactions because they usually cannot happen from the ground electronic state, so the photons are essential for driving the reactions.
One of the most common photochemical reactions is called photodissociation. In a photodissociation reaction, some of the energy gained through absorption of a photon is dissipated by breaking one or more bonds in the molecule. The photodissociation of ozone, an important reaction that occurs in our atmosphere, is shown below:
This reaction occurs when ultraviolet (UV) light breaks one of the bonds in ozone, thus creating a free oxygen radical. Radicals play a role in photodissociation reactions due to their high reactivity and ability to further drive chemical reactions.
Molecules and Light
Depending on the electronic structure of a molecule, different wavelengths of light can be absorbed. Visible light can be absorbed by many organic molecules, especially those with conjugated double bonds. Oftentimes, these molecules appear colorful to the human eye because of the wavelengths of visible light that they absorb, and subsequently reflect. An example of this is a molecule found in carrots, beta-carotene, which absorbs blue and violet light. Because of these specific colors absorbed, the light we see appears as orange.
Most photochemical reactions are driven by UV light. However, visible light in photochemistry is less damaging and less energetic, offering potential for more controlled and sustainable processes. Because of this, many researchers are targeting photochemical reactions that would use visible light as an alternative.
Classic Reactions in Organic Photochemistry
There are some photochemical reactions that play an essential role in organic synthesis which are used on an industrial scale because light is the only viable method of carrying out certain transformations.
Norrish Reaction
A classic reaction is the Norrish reaction, which involves carbonyl compounds undergoing photochemical cleavage of bonds adjacent to the carbonyl group in the presence of UV light. In a Norrish Type I Reaction, the carbonyl compound absorbs a photon of UV light. This results in a homolytic cleavage of the bond located between the carbonyl carbon and the adjacent carbon, producing two radical species. Ketones undergo this type of reaction to form alkyl and acyl radicals. There are also Norrish Type II Reactions, where an intramolecular hydrogen atom transfer occurs from the gamma carbon to the excited carbonyl oxygen after the exposure to light. A biradical is formed, which can sometimes rearrange. This rearrangement can lead to alkenes or aldehydes to be formed. Both of these are useful reactions in modifying complex molecules. Applications include polymer chemistry and pharmaceuticals.
Type I Reaction
Type II Reaction
[2+2] Cyloaddition Reaction
The [2+2] cycloaddition reaction is another prime example of a photochemical reaction in organic chemistry. This reaction involves two alkenes reacting under UV light to form a cyclobutane ring. This light excites the molecule, allowing for an electronic configuration that allows for new bonds to form that couldn’t do so under normal conditions. Strained four-membered rings are valuable components in pharmaceutical synthesis and material synthesis, giving this reaction an important role in synthetic chemistry. An example of this is the double bonds of two ethylene molecules combining in the presence of UV light to form a cyclobutane.
Paternò–Büchi reaction
Another photochemical [2+2] cycloaddition reaction between a carbonyl compound and an alkene, called the Paternò–Büchi reaction, results in an oxetane being formed. An oxetane is a four-membered ring containing oxygen. When the carbonyl carbon absorbs a photon after being excited through UV light exposure, it reacts with the alkene to form the new bonds.
These organic photochemistry reactions show how light allows for reactions to occur that can’t under normal thermal conditions. These reactions are highly applicable within industries such as pharmaceuticals, polymers, and materials science.
Applications of Photochemistry
Photosynthesis
Photosynthesis is probably the most common example of photochemistry. This process involves the conversion of light energy into chemical energy. Many plants and other bacteria absorb sunlight and convert it into energy stored in glucose, allowing for life to be sustained.
Photomedicine
Light-activated drugs are a way to target and destroy cancerous cells. Photodynamic therapies are based on the knowledge of photochemical reactions and allow for more targeted treatments that are less invasive.
Environmental Chemistry
Atmospheric chemistry relies heavily on photochemical reactions to form and breakdown various pollutants. For example, smog in cities is a direct result of reactions involving nitrogen oxides and other organic compounds that are sunlight induced.
Vision and Biological Processes
Human vision is another prime example of photochemistry. A photochemical reaction in the pigment rhodopsin is triggered when sunlight enters the eye, allowing for the brain to interpret the light as something.
Challenges of Photochemistry
As photochemical reactions occur with light, controlling them with precision poses difficulty. In living organisms and industrial settings, the challenge poses even greater. There is much research to do, and in the future, the goal is that photochemical reactions can be better understood and advanced to allow for the addressing of environmental and medical issues.
Conclusion
Photochemistry is a multidisciplinary field that corresponded to many aspects of science and technology. By harnessing the power of light to drive chemical reactions, photochemistry poses to be a powerful area of science that has the potential to shape the future in regard to understanding the natural world as well as the future of technology.