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 [Ln ₂ L _1.5 (NMP) ₂ ] _n (LnL: Ln = Eu, Gd, Tb, Dy, Ho, and Er; H ₄ L = [1,1′:4′,1″-terphenyl]-2′,4,4″,5′-tetracarboxylic acid; NMP = N-methyl-2-pyrrolidone) and mixed-Ln LnL (Eu _x Tb _1-x L, Eu _x Gd _1-x L, Tb _x Gd _1-x L, and Eu _x Tb _0.02-x Gd _0.98 L). Luminescence studies show that both H ₄ L and GdL emit strong fluorescence and phosphorescence at 77 K while only fluorescence at room temperature, and TbL exhibits strong Tb ³⁺ characteristic emission, although the energy difference between the triplet excited state of H ₄ L (20661 cm ^-1 ) and the ⁵ D ₄ energy level of Tb ³⁺ (20500 cm ^-1 ) is very small. By doping Eu ³⁺ and Tb ³⁺ into GdL, we obtained Eu _x Tb _0.02-x Gd _0.98 L emitting warm white light. For TbL and Tb _0.01 Gd _0.99 L showing dual emission, upon addition of picric acid (PA) into their suspensions in Tris-HCl buffer, Tb ³⁺ emission decreases slowly; however, the ligand-based emission is sharply quenched, rendering TbL and Tb _0.01 Gd _0.99 L excellent single-lanthanide and mixed-lanthanide ratiometric luminescence PA sensor materials, respectively.