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 [Ln₂L_1.5(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 (Eu _xTb_{1- x}L, Eu _xGd_{1- x}L, Tb _xGd_{1- x}L, and Eu _xTb_{0.02- x}Gd_0.98L). 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 Eu _xTb_{0.02- x}Gd_0.98L emitting warm white light. For TbL and Tb_0.01Gd_0.99L 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 Tb_0.01Gd_0.99L excellent single-lanthanide and mixed-lanthanide ratiometric luminescence PA sensor materials, respectively.