Discovery of the Delocalized Nature of the High‐Energy Excited‐State 4f Electron of Eu(III): Spectroscopic Evidence and Applications

Red photoemissions of Eu3+ can be normally found from the lowest excited state of 5D0 level, whereas examples of luminescence from high‐energy excited states such as 5D2 and 5D3 levels are very scarce; the origins of which are still unclear. This study shows, by detailed spectroscopic measurements in CsBr0.9Cl0.1:Eu2+, that exciton lifetime can be controlled by four orders of magnitude between Eu3+ and Eu2+ ions, with 35 ms deriving from parity‐forbidden 5D3→7Fj (j = 1–5) transitions of Eu3+ to 639 ns belonging to the dipole‐allowed 5d→4f transition of Eu2+. The excited‐state lifetime of Eu3+ can be kept for two weeks at room temperature, while maintaining high photoluminescence quantum yield of 93.4% and good thermal stability (85%@433 K). Steady‐state and transient‐state spectroscopies reveal blue emissions from the high‐energy 5D3 excited state of Eu3+ are strongly limited and quenched by oxygen molecules. Electron paramagnetic resonance spectroscopy and theoretical calculations confirm the 4f electrons are highly delocalized due to significantly reduced electron and hole wavefunction overlap, leading to singlet‐oxygen generation via electron transfer. These findings advance the fundamental understanding of the missing luminescence from higher excited states of Eu3+, enabling multifunctional applications such as emission‐lifetime‐based oxygen‐sensing and X‐ray imaging.