An Eu Coordination Polymer: Its Single-Crystal Transformation Synthesis, Fluorescence Detection of Melamine, and Proton Conductivity

In this work, an Eu coordination polymer (1a) was synthesized by a single-crystal-to-single-crystal transformation based upon complex 1 under the stimulation of water molecules ({[Eu(bpdc)1.5(H2O)5]·4H2O}n (1a), {[Eu2(bpdc)3(H2O)2]·5H2O}n (1), and H2bpdc = 2,2′-bipyridine-3,3′-dicarboxylic acid ligand). Complex 1a exhibited considerable pH fluorescence stability in an aqueous solution. Notably, the experiment showed that complex 1a achieved high selectivity and sensitivity for the detection of the notorious food additive melamine (MEL) through a significant fluorescence enhancement response; and yet complex 1 had no fluorescent response with MEL. Furthermore, the recognition mechanism of complex 1a on MEL was explored in detail through the combination of experimental studies and DFT calculations. It was worth pointing out that the proton conductivity of complex 1a had significantly improved compared with that of complex 1 due to the increase in hydrogen-bonding interactions in the framework.