Full Picture\nof Energy Transfer in a Trivalent Europium\nComplex with Bidentate β‑Diketonate Ligand

Trivalent europium (Eu(III)) complexes emit narrow-band\nluminescence\nas a result of energy transfer following the photoexcitation of antenna\nligands. Bidentate β-diketonates are typically used as antenna\nligands; however, their entire energy transfer mechanism to Eu(III)\nremains unknown. We used time-resolved photoluminescence spectroscopy\nand femtosecond transient absorption spectroscopy to map the complete\nintramolecular energy transfer process in the [Eu(hfa)<sub>3</sub>(TPPO)<sub>2</sub>] (hfa = hexafluoroacetylacetonate, TPPO = triphenylphosphine\noxide) complex; hfa is a β-diketonate antenna ligand. Our analysis\nprovides a “full picture” of energy transfer, tracing\neach step from initial photoexcitation to final relaxation: intersystem\ncrossing in the ligands, energy transfer from the ligands to the metal\ncenter, and multiple internal conversion of Eu(III). We demonstrated\nthat the direct bonds of the bidentate ligands enabled a quick and\nnear-unity-efficiency energy transfer from the triplet state of the\nligand to the <sup>5</sup>D<sub>2</sub> state of Eu(III). We also\nrevealed that the loss of the ligand-centered intersystem crossing\nprocess reduces the overall sensitization efficiency. Our findings\nprovide a foundation for the rational design of advanced luminescent\nmaterials, paving the way for advances in lanthanide-based luminescence\nresearch.