Fluorescent Eu<sup>3+</sup>/Tb<sup>3+</sup> Metal–Organic Frameworks for Ratiometric Temperature Sensing Regulated by Ligand Energy
Wei-Ni Miao, Bing Liu, Hong Li, Shu-Jin Zheng, Huan Jiao, Ling Xu
Abstract
This work presents three series of Eu/Tb metal–organic frameworks (MOFs) containing benzophenone-4,4′-dicarboxylic acid (H 2 BPNDC), 4,4′-dicarboxydiphenyl ether (H 2 OBA), and terephthalic acid (H 2 BDC) as the ligands. Eu/Tb MOFs have the same structural features in that their 3D frameworks are simplified as 2,3,10-connected {4 2 .6} 2 {4 6 .6 18 .8 19 .10 2 }{4} 2 topological networks. The solid-state fluorescence spectra of three Eu/Tb MOF series are attributed to the combined emissions of 5 D 0 → 7 F J ( J = 1–4) transitions in Eu 3+ and 5 D 4 → 7 F J ( J = 6–5) transitions in Tb 3+ . The n Eu: n Tb of Eu/Tb MOFs is optimized as 1:69 based on the relationships between I Tb(545) / I Eu(614) and n Eu: n Tb; that is, Eu 0.0143 Tb 0.9857 -L (L = BPNDC 2–, OBA 2–, and BDC 2– ) were selected to carry out the following temperature ( T )-sensing tests. The fluorescence mechanism of Eu 0.0143 Tb 0.9857 -L can be explained by a ligand-to-metal charge transfer combined with an intermetallic Tb 3+ → Eu 3+ energy transfer. The T -dependent fluorescence indicates linear relationships with sensitivities of 1.85% K –1 for Eu 0.0143 Tb 0.9857 -BPNDC, 6.49% K –1 for Eu 0.0143 Tb 0.9857 -OBA, and 0.28% K –1 for Eu 0.0143 Tb 0.9857 -BDC. The influence of T on the lowest excited triplet energy levels ( T 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 Eu 0.0143 Tb 0.9857 -BPNDC is sufficiently sensitive to T, making it applicable in noncontact T measurements.