Lanthanide-Doped Bismuth MOFs for Tunable White Light Emission and Ratiometric Cryogenic Thermometry

Abstract

Luminescent metal-organic frameworks (MOFs) have interesting applications as light-emitting devices and optical (temperature) sensors. Bismuth-based MOFs (Bi-MOFs) are not heavily explored yet but are highly promising in this regard. A robust defect-engineered Bi-MOF (namely, Bi-TATAB) is prepared under acid-modulator-free solvothermal conditions. Next, a series of lanthanide (Ln)-doped Bi-MOFs is obtained by incorporating either one or two Ln³⁺ ions (Ln³⁺ = Eu³⁺, Tb³⁺, Eu³⁺/Tb³⁺, Dy³⁺, or Sm³⁺) using a facile one-pot method. The resulting doped analogues show high crystallinity and porosity, good thermal resistance, and highly tunable luminescent properties. The color of the emitted light can be tuned by varying the dopant ions and their ratios. Thus, three white-light-emitting materials, including Eu³⁺ or Eu³⁺/Tb³⁺, are developed. The new material (Eu_0.025Tb_0.05Bi_0.925-TATAB) exhibits temperature sensing capability over a broad temperature range (10–360 K). Two ratiometric temperature sensors are constructed based on two thermometry modes proposed for this MOF: I_Tb/I_Eu and I_Bi-MOF/I_Eu under single-wavelength excitation. Intriguingly, the latter system is among the most sensitive cryogenic MOF-based thermometers reported to date, with a maximum relative sensitivity (S_r) value of 8.84% K⁻¹ (at 60 K). This material is the first example of a Ln/Bi-MOF platform with cryogenic temperature sensing properties.