Synthesis, characterization, and electrochemical properties of smectic pyridinium salts with inorganic dihydrogen phosphate ions
文摘
Pyridinium salts with inorganic dihydrogen phosphate ions (CnPy-DHP) were derived from biphenyl benzoate-based precursors bearing terminal alkoxy chains. Molecular structures of the pyridinium salts were characterized by 31P magic-angle spinning nuclear magnetic resonance (31P MAS NMR) spectroscopy, elemental analysis, and Fourier transform infrared (FT-IR) spectroscopy. Differential scanning calorimetry (DSC) measurements and polarizing optical microscopic (POM) observations indicated that the pyridinium salts exhibited smectic A (SA) phase at intermediate temperatures (above 159 °C). X-ray diffraction (XRD) measurements suggested that the pyridinium salts formed a bilayer structure with head-to-head configuration in the SA phase. Electrochemical impedance spectrum (EIS) measurements using indium tin oxide (ITO) electrodes showed that ionic conductivities of the pyridinium salts increased with the decrease in length of alkoxy chains and approached 7.0 × 10−5 S/cm in the SA phase. Wagner’s DC polarization measurements using manganese (IV) oxide (MnO2) electrodes confirmed the presence of proton conduction in the pyridinium salts. The steady state currents resulted from DC polarization revealed that the SA phase favored proton conduction. The temperature dependence of the ionic conductivity followed Arrhenius law, and the proton transport in the SA phase was supposed to occur by hopping of dissociated protons along layered pathways formed by pyridinium cations and dihydrogen phosphate ions.