One-Pot Synthesis of Three-Dimensionally Hyperbranched Eu/Fe-Based Heterometallo-Supramolecular Polymers as Thermally Tough Proton-Conducting Nanoparticles

Three-dimensionally hyperbranched Eu/Fe-based heterometallo-supramolecular polymers with carboxylates (polyEuFe) and with carboxylic acid groups (polyEuFe-H) were one-pot-synthesized via the 3:2:6 complexation of an Eu(III) ion, an Fe(II) ion, an asymmetrical ditopic ligand bearing a phenanthroline moiety, and a carboxylic-acid-substituted terpyridine moiety (L). The selective coordination of Eu(III) and Fe(II) ions to the two different binding sites of L was confirmed by ultraviolet-visible (UV-vis) and fluorescence spectroscopic titrations. PolyEuFe and polyEuFe-H were individually obtained by changing the synthetic conditions. The formation of the nanosized polymer particles was confirmed by atomic force microscopy measurement. Thermogravimetric analysis revealed that these polymers have thermal stability higher than 350 °C. PolyEuFe-H exhibited a reversible redox wave of the Fe(II)/Fe(III) couple (E_1/2= 0.76 V). Proton conductivity of the polymers was measured under different relative humidity (RH) conditions. It was proved that the introduction of the carboxylic acid groups to the polymer greatly contributed to the enhancement of the proton conductivity. PolyEuFe-H showed about 10³times higher conductivity (2.36 × 10^-4S/cm) than polyEuFe (3.15 × 10^-7S/cm) at 25 °C (95% RH). The activation energies (E_a) of polyEuFe and polyEuFe-H were 0.89 and 0.29 eV at 95% RH, respectively. The results supported that the proton conduction in polyEuFe is based on the vehicle mechanism, while that in the polyEuFe-H is based on the Grotthuss mechanism. At 90 °C (95% RH), the proton conductivity of polyEuFe-H reached 2.1 × 10^-3S/cm.