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

Abstract

The recently discovered heliconical ferroelectric nematic (N_TBF) 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 N_TBF phase. Increasing the length of the lateral chain lowers the phase transition temperatures and suppresses smectic layer formation, enabling the emergence of the N_TBF phase which replaces the orthogonal ferroelectric smectic A (SmA_F) 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 favour layered structures. Notably, the N_TBF 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 revealed a regular array of screw dislocations within the N_TBF phase, suggesting a possible link to more complex modulated or twist-grain-boundary-like structures.