The role of fluorine substituents in the formation of the ferroelectric nematic phase

Abstract

The synthesis and characterisation of a group of molecules based on the skeletal structures of 4-[(4-nitrophenoxy)carbonyl]phenyl 4-methoxybenzoate and (4-nitrophenyl)-2-methoxy-4-(4-methoxybenzoyl)oxybenzoate is reported. Fluorine substituents are added to these structures and their liquid crystalline behaviour is characterised. All eight compounds reported exhibit the ferroelectric nematic phase, N_F, and in four of them the antiferroelectric phase labelled N_X is observed. These include rare examples of enantiotropic N_F and N_X phases, and several examples of the direct N_F-isotropic transition. The addition of a fluorine substituent reduces the nematic–isotropic transition temperature, and this is attributed to the reduction in both shape anisotropy and the ability of the molecules to form antiparallel dimers. The effect of the fluorine substituents on the N_F and N_X phases is less regular and interpreted in terms of the changes to the molecular shape and electron distribution. The stability of the N_X phase follows that of the N_F phase in the series without a lateral methoxy group. The addition of a lateral methoxy group reduces the stability of the N_F phase and completely suppresses the N_X phase.