A Square-Net Europium(III) Metal–Organic Framework for Red-Emitting LEDs and Optical Temperature Sensing

A new europium-based metal–organic framework, denoted UAEU-71, was successfully synthesized through a solvothermal approach using Eu(III) ions as metal nodes and 2,2′-biquinoline-4,4′-dicarboxylate as the organic linker. Structural determination by single-crystal X-ray diffraction reveals that UAEU-71 crystallizes in the triclinic P1 space group and assembles into a two-dimensional square-net (sql) framework. Comprehensive structural, thermal, optical, and electronic characterization supported by PXRD, SEM/EDX, FTIR, TGA, XPS, DRS, and DFT calculations confirms the robustness and semiconducting nature of the framework. Upon ultraviolet excitation, UAEU-71 exhibits intense red photoluminescence emission primarily arising from the Eu(III) ⁵D₀ → ⁷F₂ transition at 618 nm, achieving a solid-state photoluminescence quantum yield (PLQY) of ∼21%. Leveraging its strong UV absorption and efficient linker-to-metal energy transfer, UAEU-71 was successfully used as a red-emitting phosphor for LED applications. Furthermore, temperature-dependent photoluminescence studies ranging from −10 to 100 °C demonstrate reliable ratiometric optical thermometry, with maximum values of 0.0133 °C^1– for absolute sensitivity and 0.51% °C^1– for relative sensitivity. Collectively, these results highlight UAEU-71 as a promising bifunctional material for solid-state lighting and optical temperature sensing applications.