Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells

Molecular oxygen, O₂, has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O₂(X³Σ_g^-), has garnered much attention, the lowest excited electronic state, O₂(a¹Δ_g), commonly called singlet oxygen, has attracted appreciable interest, principally because of its unique chemical reactivity in systems ranging from the Earth's atmosphere to biological cells. Because O₂(a¹Δ_g) can be produced and deactivated in processes that involve light, the photophysics of O₂(a¹Δ_g) are equally important. Moreover, pathways for O₂(a¹Δ_g) deactivation that regenerate O₂(X³Σ_g^-), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O₂(a¹Δ_g) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past ∼20 years have increased our understanding of O₂(a¹Δ_g) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O₂(a¹Δ_g). One goal of this Review is to summarize recent developments that have broad ramifications, focusing on systems in which oxygen forms a contact complex with an organic molecule M (e.g., a liquid solvent). An important concept is the role played by the M^+•O₂^-• charge-transfer state in both the formation and deactivation of O₂(a¹Δ_g).