Ferroelectric nematic and smectic liquid crystals with sub-molecular spatial correlations

Abstract

Nematic liquid crystals are anisotropic fluids which have long-range one-dimensional orientational order and short-range spatial correlations corresponding to the molecular length L. In X-ray studies this is manifested as diffuse peaks along the average direction of the molecular long axis at Q = 2π/L and weaker harmonics at 2Q and 3Q wave numbers. This is the case for the recently discovered ferroelectric nematic (N_F) liquid crystals as well. Here we synthesized highly polar three ring rod-shaped compounds with a terminal thiophene ring which on cooling from the isotropic fluid directly transition to the N_F phase that shows the strongest spatial correlations corresponding to 1/3 of the molecular length (L/3). The set of thiophene compounds reported here have ferroelectric polarizations about 20% larger than that of usual ferroelectric nematic liquid crystal materials. This is the result of the tighter molecular packing and larger mass density, due to the lack of flexible terminal chains of these thiophene compounds compared to most of the N_F materials. Below the N_F phase, compounds with a single nitro or two cyano polar groups on the terminal benzene ring exhibit a so far never observed smectic phase with periodicity ∼1/3 the molecular length. Based on our experimental results, we propose a model of this phase featuring antipolar packing of the molecules within the layers.