Fluorescent Eu/Tb Metal-Organic Frameworks for Ratiometric Temperature Sensing Regulated by Ligand Energy.

This work presents three series of Eu/Tb metal-organic frameworks (MOFs) containing benzophenone-4,4'-dicarboxylic acid (HBPNDC), 4,4'-dicarboxydiphenyl ether (HOBA), and terephthalic acid (HBDC) as the ligands. Eu/Tb MOFs have the same structural features in that their 3D frameworks are simplified as 2,3,10-connected {4.6}{4.6.8.10}{4} topological networks. The solid-state fluorescence spectra of three Eu/Tb MOF series are attributed to the combined emissions of D → F ( = 1-4) transitions in Eu and D → F ( = 6-5) transitions in Tb. The : of Eu/Tb MOFs is optimized as 1:69 based on the relationships between / and :; that is, EuTb-L (L = BPNDC, OBA, and BDC) were selected to carry out the following temperature ()-sensing tests. The fluorescence mechanism of EuTb-L can be explained by a ligand-to-metal charge transfer combined with an intermetallic Tb → Eu energy transfer. The -dependent fluorescence indicates linear relationships with sensitivities of 1.85% K for EuTb-BPNDC, 6.49% K for EuTb-OBA, and 0.28% K for EuTb-BDC. The influence of on the lowest excited triplet energy levels (1 values) of the ligands reveals that the ligand energy regulation impacts their fluorescence properties, including the sensitivity, fluorescence quenching rate, quantum yield, and fluorescence lifetime. This shows that EuTb-BPNDC is sufficiently sensitive to , making it applicable in noncontact measurements.