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 in the same quantum state as most molecules and thus reacts readily with them, thus making singlet oxygen 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.
Read more about Singlet Oxygen: Production, Organic Chemistry, Biochemistry, Orbital States, Chemistry
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