[22] Synthetic singlet oxygen quenchers

The quenching of singlet oxygen 1O2(1Δg) by synthetic compounds is described in this chapter. The following classes of π- or n-electron containing substrates are considered: olefins, aromatic and heteroaromatic compounds, amines, sulfur compounds, phenols, metal chelates, nonaromatic heterocycles, and indigoids. Singlet oxygen is quenched by or reacts with many organic and bioorganic molecules, which possess, in most cases, reactive π electrons, or n lone pairs of sufficiently low ionization energy. The quenching process can be physical (the quencher enters a vibrational or an electronic excited state) or chemical (the quencher combines with oxygen or is oxidized by oxygen) in nature. Physical quenching may occur via triplet energy transfer or by simple catalysis of the singlet oxygen (1O2) ground state oxygen (3O2) transition via spin-orbit coupling. Charge transfer interactions are important during the quenching process. In many reactions, intermediates are involved—for example, exciplexes, diradicals, or zwitterions.