Singlet oxygen
It is responsible for the photodegradation of many materials but can be put to constructive use in preparative organic chemistry and photodynamic therapy.
[1]: 355–68 It often appears and coexists confounded in environments that also generate ozone, such as pine forests with photodegradation of turpentine.
These three electronic states differ only in the spin and the occupancy of oxygen's two antibonding πg-orbitals, which are degenerate (equal in energy).
Radiative transitions between the three low-lying electronic states of oxygen are formally forbidden as electric dipole processes.
The energy difference of 94.3 kJ/mol between ground state and singlet oxygen corresponds to a forbidden singlet-triplet transition in the near-infrared at ~1270 nm.
[12] As a consequence, singlet oxygen in the gas phase is relatively long lived (54-86 milliseconds),[13] although interaction with solvents reduces the lifetime to microseconds or even nanoseconds.
In the gas phase, it relaxes primarily to the ground state triplet with a mean lifetime of 11.8 seconds.
[16][17][18] The paramagnetism of the 1Δg state is due to a net orbital (and not spin) electronic angular momentum.
Irradiation of oxygen gas in the presence of an organic dye as a sensitizer, such as rose bengal, methylene blue, or porphyrins—a photochemical method—results in its production.
One chemical method involves the decomposition of triethylsilyl hydrotrioxide generated in situ from triethylsilane and ozone.
A retro-Diels Alder reaction of the diphenylanthracene peroxide can also yield singlet oxygen, along with an diphenylanthracene:[22] A third method liberates singlet oxygen via phosphite ozonides, which are, in turn, generated in situ such as triphenyl phosphite ozonide.
[23][24] Phosphite ozonides will decompose to give singlet oxygen:[25] An advantage of this method is that it is amenable to non-aqueous conditions.
[25][33][34][35] In reactions with water, trioxidane, an unusual molecule with three consecutive linked oxygen atoms, is formed.
[citation needed] In photosynthesis, singlet oxygen can be produced from the light-harvesting chlorophyll molecules.
Polyphenol antioxidants can scavenge and reduce concentrations of reactive oxygen species and may prevent such deleterious oxidative effects.
g singlet oxygen (second excited state), and 3 Σ −
g triplet oxygen (ground state). The lowest energy 1s molecular orbitals are uniformly filled in all three and are omitted for simplicity. The broad horizontal lines labelled π and π* each represent two molecular orbitals (for filling by up to 4 electrons in total). The three states only differ in the occupancy and spin states of electrons in the two degenerate π* antibonding orbitals.
