Systematic Fluorination Is a Powerful Design Strategy toward Fluid Molecular Ferroelectrics

Abstract

Ferroelectric nematic (N_F) liquid crystals combine liquid-like fluidity and orientational order of conventional nematics with macroscopic electric polarization comparable in magnitude to solid-state ferroelectric materials. Here, we present a systematic study of twenty-seven homologous materials with various fluorination patterns, giving new insight into the molecular origins of spontaneous polar ordering in fluid ferroelectric nematics. Beyond our initial expectations, we find the highest stability of the N_F phase to be in materials with specific fluorination patterns rather than the maximal fluorination, which might be expected based on simple models. We find a delicate balance between polar and apolar nematics, which is entirely dictated by the substitution of the fluorine atoms. Aided by electronic structure calculations, we show this to have its origins in the radial distribution of charge across the molecular surface, with molecules possessing a more oscillatory distribution of electrons across their surfaces and possessing a higher propensity to form polar nematic phases. This work provides a new set of ground rules and design principles that can inform the synthesis of future ferroelectric nematogens.