Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells.

Molecular oxygen, O 2 , has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O 2 (X 3 Σ g - ), has garnered much attention, the lowest excited electronic state, O 2 (a 1 Δ 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 O 2 (a 1 Δ g ) can be produced and deactivated in processes that involve light, the photophysics of O 2 (a 1 Δ g ) are equally important. Moreover, pathways for O 2 (a 1 Δ g ) deactivation that regenerate O 2 (X 3 Σ g - ), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O 2 (a 1 Δ 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 O 2 (a 1 Δ g ) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O 2 (a 1 Δ 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 +• O 2 -• charge-transfer state in both the formation and deactivation of O 2 (a 1 Δ g ).