Preparation and proton conduction in liquid crystals containing sulfonic acid moiety

A series of novel biphenyl benzoate based liquid crystals (1) containing terminal sulfonic acid moieties have been prepared through the etherification reaction for anhydrous proton conduction. The chemical structures of 1 were characterized by nuclear magnetic resonance, Fourier transform infrared spectroscopy and high-resolution mass spectrometry. Differential scanning calorimetry measurements and polarizing optical microscopic observations indicated that 1 exhibited a thermotropic smectic A phase in a temperature range of 188–226°C. X-ray diffraction measurements suggested that the molecules of 1 in the smectic phase formed bilayer nanostructures, wherein the sulfonic acid moieties were lamellar aggregated within the layers to form two-dimensional pathways for proton conduction. Electrochemical impedance spectroscopy measurements suggested that the lamellar assembly of the sulfonic acid moieties in the smectic phase benefited the anhydrous proton conduction of the sulfonic acid derivatives, and proton conductivities on the order of 10⁻⁴ S/cm were achieved. The proton conductivity of 1 in the smectic A phase is higher than that in the crystal state and isotropic liquid state.