Photorelease of triplet and singlet oxygen from dioxygen complexes

Photolysis of some dioxygen transition metal complexes leads to the release of oxygen molecules in two electronic states, namely, in the ground triplet state 3Σg− and the lowest excited singlet state 1Δg. The reaction involves electron transfer from the dioxygen moiety to the central metal ion. In the reported cases, the photoreaction of peroxo complexes is activated by ligand-to-metal charge transfer excitation. The mechanism of singlet oxygen photorelease relates to intramolecular energy transfer and differs in principle from the photosensitization process. Consequently, the photorelease can be viewed as a new route to dioxygen activation by spin inversion of the ground state oxygen to singlet oxygen. The quantum yields of singlet oxygen formation are mostly low, nevertheless, they can achieve values as high as 0.5. The data collected so far do not allow specifying general conditions for singlet oxygen release, but they do indicate that the electronic state of released dioxygen is interdependent with the nature of permanent ligands and electronic structure of the excited dioxygen complex.

► Dioxygen complexes photorelease oxygen molecules.
► Dioxygen is released in the ground triplet and in the lowest excited singlet states.
► Singlet oxygen formation is principally different from a photosensitization process.
► Dioxygen photorelease was observed for all types of dioxygen complexes.