Colossal dielectric permittivity and superparaelectricity in phenyl pyrimidine based liquid crystals

Abstract

A set of polar rod-shaped liquid crystalline molecules with dipole moments (m 4 10.4–14.8 D), and molecular structures based on the ferroelectric nematic prototype DIO, are designed, synthesized, and investigated. When the penultimate fluoro-phenyl ring is replaced with a phenylpyrimidine moiety, the molecular dipole moment increases from 9.4 D for DIO to 10.4 D for the new molecule, and when the terminal fluoro-group is additionally replaced by the nitrile group, the dipole moment rises to 14.8 D. Such a replacement enhances not only the net dipole moment of the molecule, but also reduces the steric hindrance to rotations of the moieties within the molecule. The superparaelectric nematic (N) and smectic A (SmA) phases of these compounds are found to exhibit colossal dielectric permittivity, obtained both from dielectric spectroscopy, and capacitance measurements using a simple capacitor divider circuit. The electric polarization is measured vs. the field (E). However, almost no hysteresis in P vs. E is observed in the nematic and smectic A phases. The colossal dielectric permittivity persists over the entire fluidic range. The experimental results lead us to conclude that these materials belong to the class of superparaelectrics (SPE) rather than to ferroelectrics, due to the absence of hysteresis and the linear dependence of P on E. The synthesized organic materials are the first fluids for which superparaelectricity is discovered and furthermore they show great potential for applications in supercapacitors used for storing energy.