Flexible‐Ligand Rigidification Enables Anti‐Thermal‐Quenching Emission and Cryogenic Afterglow in Antimony‐Based Hybrid Bimetallic Halides

Limited by the flexible and dynamic natures of organic ligands, designing organic‐inorganic hybrid halides with robust luminescence at elevated temperatures remains a critical challenge. Herein, we demonstrate that rigidifying flexible urea‐type ligands within bimetallic hybrid chlorides provides an effective route to anti‐thermal‐quenching (ATQ) self‐trapped‐exciton (STE) emission and cryogenic afterglow in a single crystalline platform. A family of zero‐dimensional hybrid bimetallic chlorides, [RE(DEU) 6 ]SbCl 6 (RE = La, Ce, Eu, Gd, Tb, Y, Lu; DEU = N,N‐diethylurea), is synthesized via low‐temperature solution evaporation. All compounds exhibit broadband STE emissions with high photoluminescence (PL) quantum yields (90.24%–95.32%) and pronounced ATQ luminescence. Temperature‐dependent spectroscopy and structural analyses reveal exceptionally strong electron‐phonon coupling combined with a hydrogen‐bond‐locked, weakly expanding lattice, which together suppress thermally activated PL quenching. Moreover, the DEU network underpins blue prompt emission and green afterglow at cryogenic temperatures, enabling dual‐channel exciton management between organic DEU parts and inorganic [SbCl 6 ] 3− centers. Radius‐encoded tuning across the RE 3+ series links ionic size, lattice rigidity, and ATQ performance, highlighting the role of the RE 3+ ions as an effective structure‐rigidity‐property tuner rather than just a structural linker. This work establishes a flexible‐ligand rigidification paradigm for designing thermally resistant and multifunctional Sb‐based hybrid luminescent materials.