Visible‐Light Excited Luminescent Thermometer Based on Single Lanthanide Organic Frameworks

Isostructural lanthanide organic frameworks (Me 2 NH 2 ) 3 [Ln 3 (FDC) 4 (NO 3 ) 4 ]·4H 2 O (Ln = Eu ( 1 ), Gd ( 2 ), Tb ( 3 ), H 2 FDC = 9‐fluorenone‐2,7‐dicarboxylic acid), synthesized under solvothermal conditions, feature a Ln‐O‐C rod‐packing 3D framework. Time‐resolved luminescence studies show that in 1 the energy difference between the H 2 FDC triplet excited state (17794 cm −1 ) and the 5 D 0 Eu 3+ level (17241 cm −1 ) is small enough to allow a strong thermally activated ion‐to‐ligand back energy transfer. Whereas the emission of the ligand is essentially constant the 5 D 0 → 7 F 2 intensity is quenched when the temperature increases from 12 to 320 K, rendering 1 the first single‐lanthanide organic framework ratiometric luminescent thermometer based on ion‐to‐ligand back energy transfer. More importantly, this material is also the first example of a metal organic framework thermometer operative over a wide temperature range including the physiological (12‐320 K), upon excitation with visible light (450 nm).