1O2

1O2 is the designation used in chemistry to refer to singlet molecular oxygen, the electronically excited form of the diatomic oxygen molecule. In this state, O2 is more reactive than its ground-state form (3O2), which is a triplet.

There are two low-lying singlet states commonly encountered: 1Δg and 1Σg+. The 1Δg state is the lowest singlet state, about 0.97 eV above 3O2, and its relaxation to 3O2 can emit phosphorescence at around 1270 nm. The 1Σg+ state lies at higher energy. The term 1O2 is often used to denote singlet oxygen in general, though 1Δg is the traditional reference for the first excited singlet state.

Production of 1O2 occurs mainly by energy transfer from a photosensitizer: a dye or porphyrin absorbs light and transfers energy to ground-state O2 to form 1O2. Common sensitizers include Rose Bengal, methylene blue, and various porphyrins. Singlet oxygen can also be generated photochemically from endoperoxides or by electrical discharges under appropriate conditions.

Reactivity and chemistry: 1O2 is a reactive oxygen species that participates in selective oxidations, notably with alkenes through ene reactions and [4+2] cycloadditions to form endoperoxides. Its reactivity underpins applications in organic synthesis and photodynamic therapy, as well as oxidative processes in materials and biological systems. The lifetime of 1O2 is solvent- and environment-dependent, typically a few microseconds in ordinary solvents, longer in deuterated media, and shorter in solvents or contexts that promote rapid quenching.

Applications and detection: Singlet oxygen is used in synthetic oxidations and photodynamic therapy. Detection often relies on its near-infrared phosphorescence at 1270 nm, or on chemical traps such as DPBF that quantify 1O2 generation.