Oxygen quantification using luminescence has attracted considerable attention in various fields, including environmental monitoring and clinical analysis. Among the reported luminophores, trivalent lanthanide complexes have displayed characteristic narrow emission bands with high brightness. This bright emission is based on photo‐sensitized energy transfer via organic triplet states. The organic triplet states in lanthanide complexes effectively react with the triplet oxygen, enabling oxygen quantification by lanthanide luminescence. Some TbIII and EuIII complexes with slow deactivation processes have also formed the excited state equilibrium, thus resulting in the emission‐lifetime based oxygen sensing property. The combination of TbIII/EuIII emission, EuIII/SmIII emission, EuIII/ligand phosphorescence, and ligand fluorescence/ligand phosphorescence provide the ratiometric oxygen‐sensing properties. Moreover, the reaction generates singlet oxygen species which exhibit numerous applications in the photo‐medical field. The ligands with large π‐conjugated aromatic systems, such as porphyrin, phthalocyanine, and polyaromatic compounds, induces highly efficient oxygen generation. The combination of effective luminescence with singlet‐oxygen generation by the lanthanide complexes render them suitable for photo‐driven theranostics. This review summarizes the research progress of lanthanide complexes with efficient oxygen‐sensing and singlet‐oxygen generation properties.

Luminescent lanthanide complexes containing effective photosensitizers are promising materials for use in displays and sensors. The photosensitizer design strategy has been studied for developing the lanthanide-based luminophores. Herein, we demonstrate a photosensitizer design using dinuclear luminescent lanthanide complex, which exhibits thermally-assisted photosensitized emission. The lanthanide complex comprised Tb(III) ions, six tetramethylheptanedionates, and phosphine oxide bridge containing a phenanthrene frameworks. The phenanthrene ligand and Tb(III) ions are the energy donor (photosensitizer) and acceptor (emission center) parts, respectively. The energy-donating level of the ligand (lowest excited triplet (T1) level = 19,850 cm−1) is lower than the emitting level of the Tb(III) ion (5D4 level = 20,500 cm−1). The long-lived T1 state of the energy-donating ligands promoted an efficient thermally-assisted photosensitized emission of the Tb(III) acceptor (5D4 level), resulting in a pure-green colored emission with a high photosensitized emission quantum yield (73%).

Invited for this month′s cover are the collaborating groups of Yuichi Kitagawa, Yasuchika Hasegawa, and co‐workers at the Hokkaido University. The cover picture shows triplet‐oxygen sensing and singlet‐oxygen generation using lanthanide complexes. Organic ligands play crucial roles in lanthanide complexes as light‐harvesting antennas for lanthanide luminescence and singlet‐oxygen generation. More information can be found in the Review by Y. Kitagawa, Y. Hasegawa, and co‐workers.

A novel design strategy of stacked organic fluorophores using dinuclear lanthanide (Ln(III)) complexes is demonstrated for the formation of excimer. The dinuclear Ln(III) complexes are composed of two Ln(III) (Eu(III) or Gd(III)) ions, six hexafluoroacetylacetonate (hfa), and two pyrene-based phosphine oxide ligands. Single-crystal analysis revealed a rigid pyrene-stacked structure via CH-F (pyrene/hfa) intramolecular interactions. The rigid aggregation structures of the two-typed organic ligands around Ln(III) resulted in high thermal stability (decomposition temperature: 340°C). The aggregated ligands exhibited excimer-type green emission from the stacked pyrene-center. The change in the Ln(III) ion promotes effective shifts of excimer emissions (Gd(III):500 nm, Eu(III):490 nm). The organic aggregation system using red-luminescent Eu(III) also provides temperature-sensitive ratiometric emission composed of π-π* and 4f-4f transitions by energy migration between aggregated ligands and Eu(III).

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