Singlet oxygen

Singlet oxygen (or 1O2) is the common name used for an electronically excited state of molecular oxygen (O2), which is less stable than the normal triplet oxygen. Because of its unusual properties, singlet oxygen can persist for over an hour at room temperature, depending on the environment. Because of differences in their electron shells, singlet and triplet oxygen differ in their chemical properties. Singlet oxygen is highly reactive.

Singlet oxygen is usually generated with a photosensitizer pigment. The damaging effects of sunlight on many organic materials (polymers, etc.) are often attributed to the effects of singlet oxygen. In photodynamic therapy, singlet oxygen is produced to kill cancer cells.

The blue color of liquid and solid O2 is actually due to the simultaneous excitation by a single photon of two O2 molecules from their ground states to their excited states, in which the associated energy absorbed corresponds to absorption of light in the red to green region of the visible part of the spectrum, thus the reflected color of liquid and solid O2 appears blue.

Production
Various methods for the production of singlet oxygen exists. A photochemical chemical method involves the irradiation of normal oxygen gas in the presence of an organic dye as a sensitizer, such as rose bengal, methylene blue or porphyrins. Singlet oxygen can also be produced chemically. One of the chemical methods is by the decomposition of hydrogen trioxide, or to react hydrogen peroxide with sodium hypochlorite, which is convenient in school laboratories for demonstrative purposes:


 * H2O2 + NaOCl → O2(1Δg) + NaCl + H2O

Another method is via phosphite ozonides, which in turn is generated in situ. The phosphite ozonide is then catalytically decomposed by pyridine at low temperature to give singlet oxygen:


 * Scheme for preparing singlet oxygen via phosphite ozonides.png

The advantage of this method is that the reaction can be cyclic, which the resulting phosphate ester is reduced to the phosphite ester for further production of singlet oxygen.

Organic chemistry
The chemistry of singlet oxygen is different from that of ground state oxygen. For example, singlet oxygen can participate in Diels-Alder [4+2] and [2+2] cycloaddition reactions, ene reactions, and heteroatom (S, Se, P, N) and organometallic complex oxidation reactions. Singlet oxygen reacts with an alkene -CH=CH-CH2- by abstraction of the allylic proton in an ene reaction type reaction to the allyl hydroperoxide HO-O-R (R = alkyl), which can then be reduced to the allyl alcohol. (This reaction is not actually a true ene reaction, because it isn't concerted: singlet oxygen forms an exciplex that can be called an "epoxide oxide", which then abstracts the hydrogen.) An example is an oxygenation of citronellol:
 * [[Image:Singlet Oxygenation Citronellol.svg|Singlet Oxygenation Citronellol]]

With some substrates 1,2-dioxetanes are formed and cyclic dienes such as 1,3-cyclohexadiene form [4+2]cycloaddition adducts. With water trioxidane, an unusual molecule with three consecutive linked oxygen atoms, is formed.

Biochemistry
In photosynthesis, singlet oxygen can be produced from the light-harvesting chlorophyll molecules. One of the roles of carotenoids in photosynthetic systems is to prevent damage caused by produced singlet oxygen by either removing excess light energy from chlorophyll molecules or quenching the singlet oxygen molecules directly.

In mammalian biology, singlet oxygen is one of the reactive oxygen species, which is linked to oxidation of LDL cholesterol and resultant cardiovascular effects. Polyphenol antioxidants can scavenge and reduce concentrations of reactive oxygen species and may prevent such deleterious oxidative effects.

Ingestion of pigments capable of producing singlet oxygen with activation by light can produce severe photosensitivity of skin. This is especially a concern in herbivorous animals (see Photosensitivity in animals).

Singlet oxygen is the active species in photodynamic therapy.

Orbital states
Molecular orbital theory predicts two low-lying excited singlet states O2(a¹Δg) and O2(b¹Σg+) (for nomenclature see article on Molecular term symbol). These electronic states differ only in the spin and the occupancy of oxygen's two degenerate antibonding πg-orbitals (see degenerate energy level). The O2(b¹Σg+)-state is very short lived and relaxes quickly to the lowest lying excited state, O2(a¹Δg). Thus, the O2(a¹Δg)-state is commonly referred to as singlet oxygen. The energy difference between the lowest energy of O2 in the singlet state and the lowest energy in the triplet state is about 11340 kelvin (Te (a¹Δg <- X³Σg-) = 7882 cm−1, 94.3 kJ/mol, 0.98 eV) Molecular oxygen differs from most molecules in having an open-shell triplet ground state, O2(X³Σg-). Although the three lowest energy states of oxygen can be described by the simple scheme in the figure below, this is a simplification. The excited states of oxygen are made up of combinations of electronic states. The electrons paired in the same orbital, while the first excited state involves states with the electrons in separate degenerate orbitals, as might be expected from Hund's rule.

Chemistry
The energy difference between ground state and singlet oxygen is 94.3 kJ/mol and corresponds to a transition in the near-infrared at ~1270 nm. In the isolated molecule, the transition is strictly forbidden by spin, symmetry and parity selection rules, making it one of nature's most forbidden transitions. In other words, direct excitation of ground state oxygen by light to form singlet oxygen is very improbable. As a consequence, singlet oxygen in the gas phase is extremely long lived (72 minutes). Interaction with solvents, however, reduces the lifetime to microseconds or even nanoseconds.

Direct detection of singlet oxygen is possible using sensitive laser spectroscopy or through its extremely weak phosphorescence at 1270 nm, which is not visible. However, at high singlet oxygen concentrations, the fluorescence of the so-called singlet oxygen dimol (simultaneous emission from two singlet oxygen molecules upon collision) can be observed as a red glow at 634 nm.