Precise Manipulation of Discrete Lanthanide Complexes Vitrification via Flexible Chains and Rigid Modules: Accessing Near‐Unity Photoluminescence Quantum Yields, Upconversion Luminescence, and Ultra‐Bright X‐ray Scintillators

ABSTRACT Weakly bonded discrete lanthanide complexes readily decompose on heating, making conventional melt‐quenching vitrification extremely difficult. Herein, we design rigid‐flexible dual ligands for lanthanide complexes, producing large‐area transparent bulk glasses (Btfa‐Ln‐Glass) with reversible crystalline‐glass‐glass‐ceramic interconversion via “rigid‐flexible synergy”. Notably, precise rigid‐segment conjugation tuning drastically boosts devitrification resistance, yielding stable transparent bulk glasses with ceramization resistance and reversible crystalline–glassy interconversion. The dual‐component ligands’ flexible segments impart molecular mobility, rigid segments ensure network stability, enabling controllable vitrification. Synchrotron radiation x‐ray absorption fine structure (XAFS) analysis confirms that the local coordination geometry, bond lengths, and coordination number of Eu 3+ sites in Btfa‐Eu‐Glass are in excellent agreement with those of the crystalline precursor. After vitrification, the photoluminescence quantum yields (PLQYs) of both Btfa‐Eu‐Glass and NTA‐Eu‐Glass increase dramatically to nearly 100%. Furthermore, Btfa‐Eu 0.5 Yb 0.5 ‐Glass exhibits pronounced two‐photon absorption‐mediated upconversion luminescence (UCL) under 980 nm excitation. As the first intrinsic lanthanide complex glass x‐ray scintillator, Btfa‐Eu‐Glass achieves an exceptional light yield of 60,360 ± 273 photons·MeV −1 , outperforming all reported lanthanide‐doped glass scintillators, and delivers an ultralow limit of detection (LOD) of 124 nGy·s − 1 . This work exploits a rigid‐flexible balancing strategy to overcome the long‐standing challenge of realizing controllable vitrification in discrete lanthanide complexes.