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 Sigma(-)(g)), has garnered much attention, the lowest excited electronic state, O-2(a(1)Delta(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)Delta(g)) can be produced and deactivated in processes that involve light, the photophysics of O-2(a(1)Delta(g)) are equally important. Moreover, pathways for O-2(a(1)Delta(g)) deactivation that regenerate O-2(X-3 Sigma(-)(g)), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O-2(a(1)Delta(g)) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past similar to 20 years have increased our understanding of O-2(a(1)Delta(g)) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O-2(a(1)Delta(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+center dot O2-center dot charge-transfer state in both the formation and deactivation of O-2(a(1)Delta(g)).