Singlet oxygen, O₂(a¹Δ_g), the lowest excited electronic state of molecular oxygen, has been known to the scientific community for ∼80 years. It has a characteristic chemistry that sets it apart from the triplet ground state of molecular oxygen, O₂(X³Σ⁻ _g), and is important in fields that range from atmospheric chemistry and materials science to biology and medicine. For such a “mature citizen”, singlet oxygen nevertheless remains at the cutting-edge of modern science. In this critical review, recent work on singlet oxygen is summarized, focusing primarily on systems that involve light. It is clear that there is indeed still something new under the sun (243 references).

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Singlet molecular oxygen, O₂(a¹Δ_g), has been detected from single neurons and HeLa cells in time-resolved optical experiments by its 1270 nm phosphorescence (a¹Δ_g → X³Σ^– _g) upon irradiation of a photosensitizer incorporated into the cell. The cells were maintained in a buffered medium and their viability was assessed by live/dead assays. To facilitate the detection of singlet oxygen, intracellular H₂O was replaced with D₂O by an osmotic de- and rehydration process. The effect of this insult on the cells was likewise assessed. The data indicate that, in the complicated transition from a “live” to “dead” cell, the majority of our cells have the metabolic activity and morphology characteristic of a live cell. Quenching experiments demonstrate that the singlet oxygen lifetime in our cells is principally determined by interactions with intracellular water and not by interactions with other cell constituents. The data indicate that in a viable, metabolically-functioning, and H₂O-containing cell, the lifetime of singlet oxygen is ∼3 µs. This is consistent with our previous reports, and confirms that the singlet oxygen lifetime in a cell is much longer than hitherto believed. This implies that, in a cell, singlet oxygen is best characterized as a selective rather than reactive intermediate. This is important when considering roles played by singlet oxygen as a signaling agent and as a component in events that result in cell death. The data reported herein also demonstrate that spatially-resolved optical probes can be used to monitor selected events in the light-induced, singlet-oxygen-mediated death of a single cell.