Tuning Mixed‐Valent Eu2+/Eu3+ in Strontium Formate Frameworks for Multichannel Photoluminescence

Cooperative performance of mixed‐valent Eu²⁺/Eu³⁺ in single‐compound phosphors offers significant advantages in color rendering and luminescence efficiency, but their synthesis is challenging because of Eu²⁺ oxidation. Using the tunable nature of the metal‐ion nodes in metal–organic frameworks (MOFs), we present an in situ reduction and crystallization route for preparing MOFs and doping Eu²⁺/Eu³⁺ with a controlled ratio. These materials exhibit rich photoluminescence, including intrinsic‐ and sensitized‐emissions of Eu²⁺ and Eu³⁺, and long‐lived luminescence from charge transfer. Color rendering can be easily achieved by fine‐tuning the valence states of Eu. A linear relation between temperature and the intensity ratio of Eu²⁺/Eu³⁺ emissions provides outstanding properties for applications as self‐calibrated luminescent thermometers with a wide working temperature range. Further incorporation of Tb³⁺ into the MOFs results in white light, utilizing all Eu²⁺,Tb³⁺, and Eu³⁺ emissions in a single crystalline lattice.