singlet oxygen

Other names: dioxidene

¹O₂

50/50 rare

Singlet oxygen represents the excited state electronic isomer of the oxygen we breathe, contrasting sharply with the triplet oxygen ground state. While triplet oxygen has two unpaired electrons with parallel spins (S=1), singlet oxygen features electrons paired in the same orbital (S=0), eliminating its paramagnetism. This electronic rearrangement makes singlet oxygen an unusual case where the common closed-shell Lewis structure representation actually depicts an excited state rather than the ground state. Singlet oxygen can be generated through photosensitization using dyes like rose bengal or methylene blue, which absorb light and transfer energy to triplet oxygen, promoting it to the singlet state. Unlike its relatively stable triplet counterpart, singlet oxygen is highly reactive, with a lifetime of seconds in the gas phase and microseconds to nanoseconds in solution. Its enhanced reactivity stems from the absence of spin restrictions, allowing it to participate in reactions typically forbidden for triplet oxygen. Synthetic chemists exploit this property in valuable transformations including ene reactions with alkenes, [4+2] cycloadditions with dienes (acting as a dienophile), and [2+2] cycloadditions forming dioxetanes. These reactions have proven invaluable in natural product synthesis, pharmaceutical development, and have been implicated in both photodynamic therapy for cancer treatment and oxidative damage in biological systems.