Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells.

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