Singlet oxygen

Singlet oxygen, also known as dioxygen(singlet) or dioxidene, is a highly reactive gaseous inorganic chemical. It consists of two oxygen atoms in an excited quantum state. In this state, all of the electrons are spin-paired. This is different from the most common form of oxygen, known as the triplet ground state. While the physical properties of singlet oxygen are similar to normal oxygen, its chemical reactivity is much higher. It is capable of reacting strongly with many organic compounds. This reactivity makes it both a tool for chemists and a factor in environmental processes.

The behavior of singlet oxygen is defined by its electronic structure. In the standard triplet ground state, denoted as 3Σ, the oxygen molecule has two unpaired electrons. These electrons have the same spin, following Hund's first rule. To create singlet oxygen, these electrons must change their arrangement. One way this happens is when one electron "flips" its spin to pair with the other. This creates the 1Δg state, which is the most stable form of singlet oxygen. Another possible state is the 1Σ state, where both electrons remain in separate orbitals but have opposite spins. The 1Δg state is 94.3 kilojoules per mole higher in energy than the triplet ground state. The 1Σ state is even higher, at 157.0 kilojoules per mole.

Scientists can produce singlet oxygen through several different mechanisms. A common photochemical method involves shining light on oxygen gas. This process requires a sensitizer, which is an organic dye like rose bengal, methylene blue, or porphyrins. Chemical methods can also be used without light. For example, reacting hydrogen peroxide with sodium hypochlorite in an aqueous solution produces singlet oxygen. Another method involves the decomposition of phosphite ozonides. This specific technique is useful because it works well in non-aqueous conditions.

The lifespan of singlet oxygen depends heavily on its environment. In a gas phase, the 1Δg state is relatively long-lived, lasting between 54 and 86 milliseconds. However, when it interacts with solvents, its lifetime drops significantly to microseconds or even nanoseconds. In 2021, researchers measured the lifetime of airborne singlet oxygen at air/solid interfaces to be 550 microseconds. The higher 1Σ state is also moderately short-lived. In gases, it has a mean lifetime of 11.8 seconds, but in solvents like carbon disulfide, it quickly relaxes into the lower 1Δg state.

In the natural world, singlet oxygen plays many complex roles. It is found in trace amounts in the upper atmosphere and in polluted urban areas. In these environments, it can contribute to the formation of nitrogen dioxide, which can damage lungs. It also appears in pine forests where it coexists with ozone during the photodegradation of turpentine. Within plants, singlet oxygen can be produced from light-harvesting chlorophyll molecules during photosynthesis. To prevent damage, plants use carotenoids. These molecules help by removing excess light energy or by quenching the singlet oxygen molecules directly.

In biology and medicine, singlet oxygen has both dangerous and helpful effects. In mammals, it is a reactive oxygen species linked to the oxidation of LDL cholesterol. This process can have cardiovascular effects. On the other hand, doctors use it constructively in photodynamic therapy. This medical treatment uses light-activated pigments to target specific cells.

Additionally, some pigments can cause photosensitivity in the skin when activated by light, which is a concern for both humans and herbivorous animals.

Chemists use the unique reactivity of singlet oxygen to build new molecules through processes like photooxygenation. It can participate in Diels–Alder [4+2] and [2+2] cycloaddition reactions. It also performs formal concerted ene reactions, such as the Schenck ene reaction. For instance, it can react with furans or oxidize thioethers into sulfoxides. When it reacts with certain molecules like citronellol, it can lead to the formation of allyl hydroperoxides. These can then be reduced to create useful allyl alcohols. Even in water, it can form an unusual molecule called trioxidane, which contains three linked oxygen atoms.