Helical Fe(II)-Based Metallo-Supramolecular Polymers: Effect of Crown Ether Groups Located outside the Helix on Hydrous Proton Channel Formation

Helical Fe(II)-based metallo-supramolecular polymers with/without crown ether groups (polyFe-C and polyFe-H, respectively) were synthesized by the 1:1 complexation of an Fe(II) salt and chiral bisterpyridine ligands with/without 18-crown-6. The stoichiometric complexation of Fe(II) and the ligand was confirmed by the UV–vis spectral titration experiment. PolyFe-C and polyFe-H showed a similar molecular weight (9.2 × 104 and 9.83 × 104 Da, respectively) and metal-to-ligand charge transfer absorption at the same wavelength (572 nm) in solution. It was revealed by scanning electron microscopy that polyFe-C with the crown ether groups has a highly porous structure unlike polyFe-H. PolyFe-C showed about 50 times higher proton conductivity (3.0 × 10–4 S cm–1) than polyFe-H (6.4 × 10–6 S cm–1) at 95% RH and 25 °C. The conductivity of polyFe-C was enhanced up to 5.8 × 10–3 S cm–1 with increasing temperature to 75 °C. The calculated activation energy (0.46 eV) suggested that proton transfer in polyFe-C at 95% RH occurred according to the mixed state of the Grotthuss and vehicle mechanisms. The Fourier transform infrared spectrum of polyFe-C unveiled the formation of [H+(H2O)n]. It was concluded that hydronium stabilized by the crown ether groups aligned outside the helical polymer accelerated the incorporation of water molecules to the polymer under humid conditions and assisted the proton channel formation by [H+(H2O)n] in the polymer.