Fluorescent Eu3+/Tb3+ 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 (H2BPNDC), 4,4'-dicarboxydiphenyl ether (H2OBA), and terephthalic acid (H2BDC) as the ligands. Eu/Tb MOFs have the same structural features in that their 3D frameworks are simplified as 2,3,10-connected {42.6}2{46.618.819.102}{4}2 topological networks. The solid-state fluorescence spectra of three Eu/Tb MOF series are attributed to the combined emissions of 5D0 → 7FJ (J = 1-4) transitions in Eu3+ and 5D4 → 7FJ (J = 6-5) transitions in Tb3+. The nEu:nTb of Eu/Tb MOFs is optimized as 1:69 based on the relationships between ITb(545)/IEu(614) and nEu:nTb; that is, Eu0.0143Tb0.9857-L (L = BPNDC2-, OBA2-, and BDC2-) were selected to carry out the following temperature (T)-sensing tests. The fluorescence mechanism of Eu0.0143Tb0.9857-L can be explained by a ligand-to-metal charge transfer combined with an intermetallic Tb3+ → Eu3+ energy transfer. The T-dependent fluorescence indicates linear relationships with sensitivities of 1.85% K-1 for Eu0.0143Tb0.9857-BPNDC, 6.49% K-1 for Eu0.0143Tb0.9857-OBA, and 0.28% K-1 for Eu0.0143Tb0.9857-BDC. The influence of T on the lowest excited triplet energy levels (T1 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 Eu0.0143Tb0.9857-BPNDC is sufficiently sensitive to T, making it applicable in noncontact T measurements.