Ratiometric Luminescent Sensor of Picric Acid Based on the Dual-Emission Mixed-Lanthanide Coordination Polymer

Powerful explosive sensors play a key role in public security and environmental protection. Herein, we report a series of isostructural lanthanide coordination polymers [Ln2L1.5(NMP)2]n (LnL: Ln = Eu, Gd, Tb, Dy, Ho, and Er; H4L = [1,1′:4′,1″-terphenyl]-2′,4,4″,5′-tetracarboxylic acid; NMP = N-methyl-2-pyrrolidone) and mixed-Ln LnL (EuxTb1–xL, EuxGd1–xL, TbxGd1–xL, and EuxTb0.02–xGd0.98L). Luminescence studies show that both H4L and GdL emit strong fluorescence and phosphorescence at 77 K while only fluorescence at room temperature, and TbL exhibits strong Tb3+ characteristic emission, although the energy difference between the triplet excited state of H4L (20661 cm–1) and the 5D4 energy level of Tb3+ (20500 cm–1) is very small. By doping Eu3+ and Tb3+ into GdL, we obtained EuxTb0.02–xGd0.98L emitting warm white light. For TbL and Tb0.01Gd0.99L showing dual emission, upon addition of picric acid (PA) into their suspensions in Tris–HCl buffer, Tb3+ emission decreases slowly; however, the ligand-based emission is sharply quenched, rendering TbL and Tb0.01Gd0.99L excellent single-lanthanide and mixed-lanthanide ratiometric luminescence PA sensor materials, respectively.