A Series of Rare-Earth Metal-Based Coordination Polymers: Fluorescence and Sensing Studies

Ratiometric fluorescent sensing based on dual-emitting fluorescent coordination polymers (FL-CPs) has attracted intense attention due to their sensing accuracy and easy visualization when compared with sensing relying solely on monochromatic FL-CPs. In this work, a series of rare-earth metal-based CPs, formuled as [(CH₃)₂NH₂][Ln(bpdc)₂] (Ln³⁺ = Y³⁺, Eu³⁺ and Tb³⁺, H₂bpdc = biphenyl-4,4'-dicarboxylic acid), are presented, which show dual emission aroused from the Ln³⁺ ions and the inefficient intermolecular energy transfer from ligands to Ln³⁺ metals. For clarity, the as-made Ln-CPs are named Eu-bpdc, Tb-bpdc, and Y-bpdc based on the corresponding Ln³⁺. Notably, Eu-bpdc, presented as an example, could be used as FL sensing material ratiometric to Fe³⁺ ions. The ratio of FL intensity of Eu³⁺ ions to bpdc^2- ligands (I₄₁₅/I₆₁₅) showed a good linear relationship with the concentrations of Fe³⁺ ions. Moreover, the detection process could be visibly monitored through a change from purple to blue when Eu-bpdc was used as an FL proble. This work provides a good example for exploring visibly ratiometric sensors based on FL-CPs.