Excitation-Dependent Photoluminescence of Undoped Bimetallic Halides with Dual Emitting Centers for Optical Anticounterfeiting Application

Excitation-dependent (Ex-De) photoluminescence, characterized by distinct emission color changes under varying excitation wavelengths, has been extensively investigated in doped systems, where luminescence relies on a dopant-host synergy. However, Ex-De behavior achieved through two independent luminescent centers in undoped metal halide lattices has scarcely been reported. Herein, we synthesized two novel zero-dimensional (0D) bimetallic halides (BMHs), [Eu(UREA)₇H₂O][SbCl₆] (Eu-U) and [Tb(UREA)₇H₂O][SbCl₆] (Tb-U), via a cooling crystallization method. Structural analyses reveal that Eu³⁺/Tb³⁺ ions coordinate with UREA and water molecules to form cationic complexes [Eu(UREA)₇(H₂O)]³⁺/[Tb(UREA)₇(H₂O)]³⁺, which isolate [SbCl₆]³⁻ anions. Hirshfeld surface analysis further indicates substantial hydrogen bonding between the complex cations and [SbCl₆]³⁻ octahedra, leading to notable structural distortion. Spectroscopic studies confirm that both compounds can exhibit simultaneous broadband emission from Sb³⁺ and narrowband emission from Eu³⁺/Tb³⁺, with no energy transfer between the centers. Leveraging this unique Ex-De behavior, we fabricated anticounterfeiting patterns by embedding Eu-U and Tb-U in silicone, demonstrating effective information concealment and retrieval. This work not only enriches the family of BMHs but also realizes synergistic narrowband and broadband emission, offering new insights for designing Ex-De luminescent materials with independent dual emitting centers.