Competition between mirror symmetry breaking and translation symmetry breaking in ferroelectric liquid crystals with increasing lateral substitution

TL;DR

This study explores how molecular structure influences the formation of a heliconical ferroelectric nematic phase in liquid crystals, revealing a competition between different symmetry-breaking mechanisms and identifying phases with exceptionally short helical pitches.

Contribution

It demonstrates how lateral substitution in mesogenic compounds affects phase stability, leading to the emergence of the NTBF phase with unique properties, advancing understanding of symmetry-breaking in ferroelectric liquid crystals.

Findings

Increasing lateral chain length suppresses smectic phases and promotes NTBF formation.

NTBF phase exhibits very short helical pitches of a few hundred nanometers.

AFM imaging shows screw dislocations, indicating complex modulated structures.

Abstract

The recently discovered heliconical ferroelectric nematic (NTBF) phase is a unique example of spontaneous chiral symmetry breaking in a proper ferroelectric fluid. In this study, we investigate four homologous series of mesogenic compounds, differing in the degree of fluorination of the mesogenic core and bearing lateral alkoxy substituents of varying lengths, to understand how molecular architecture influences the formation and stability of the NTBF phase. Increasing the length of the lateral chain lowers the phase transition temperatures and suppresses smectic layer formation, enabling the emergence of the NTBF phase which replaces the orthogonal ferroelectric smectic A (SmAF) phase. This indicates a competition between lamellar and heliconical polar ordering, driven by the interplay of strong molecular dipoles and the self-segregation of chemically incompatible molecular segments that typically favor layered structures. Notably, the NTBF phase in these compounds exhibits exceptionally short helical pitch lengths, on the order of a few hundred nanometers, as revealed by selective light reflection and atomic force microscopy (AFM). Furthermore, for one of the studied compounds AFM imaging of one compound revealed a regular array of screw dislocations within the NTBF phase, suggesting a possible link to more complex modulated or twist-grain-boundary-like structures.