Luminescent Lanthanide Metal–Organic Frameworks for Temperature Sensing in Two Distinct Temperature Regions

Luminescent ratiometric temperature sensors based on lanthanide(III) (Ln³⁺) metal-organic frameworks (LnMOFs) have emerged as promising materials in nanothermometry due to their hybrid nature and unique photophysical and structural properties. These include high chromophore density and well-defined, robust crystalline structures enabling efficient energy transfer and high luminescence performance in various external conditions such as alternating temperatures. However, current LnMOF-based nanothermometers often require high loadings of emissive centers leading to concentration quenching and limited sensitivity and typically operate in a single-temperature region. In this work, we present a new series of LnMOFs based on the ligand 1,4-benzenedicarboxylic acid (H₂BDC) and its 2-amino derivative (H₂ABDC), with the general formula [La_1-<i>x</i>Ln_<i>x</i>(BDC)_1-<i>y</i>(ABDC)_<i>y</i>Cl(DMF)] (Ln = Eu, Tb, Sm, Dy; DMF = <i>N</i>,<i>N</i>-dimethylformamide; <i>x</i> = 0-0.5, <i>y</i> = 0-1). Photophysical studies revealed that (i) the MOF scaffolds can efficiently sensitize the visible-emitting Ln³⁺ ions leading to quantum yields of up to 81%; (ii) mixed-ligand LnMOFs exhibit ligand-centered fluorescence lifetimes of 0.11-9 ns and exceptionally long phosphorescence lifetimes of 0.33 s. Additionally, we demonstrated the remarkable potential of mixed-metal-mixed-ligand LnMOFs, containing very low loadings of emissive components (up to 10 mol %), to function as ratiometric luminescence thermometers (RLTs) across a wide temperature range, including cryogenic (10-110 K) and ambient (70-330 K) temperatures. These LnMOFs exhibit relative sensitivities (<i>S</i>_rel) of up to 11.1% K^-1 in the cryogenic range and 2.2% K^-1 at higher temperatures, comparable to the highest values reported in the literature.