Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens

Abstract

Spontaneous mirror symmetry breaking in achiral fluids is of actual interest for technological applications as well as for the understanding of the spontaneous formation of chirality and complexity in soft matter self‐assembly. Here we report a series of achiral bent‐shaped 4‐cyanoresorcinol bisterephthalates terminated by alkyl chains having a length n ranging from 2 to 22. Some of these achiral compounds with a specific chain length ( n = 12‐16) spontaneously form a short‐pitch heliconical lamellar liquid crystalline phase. The formation of this helical superstructure is observed at the paraelectric‐antiferroelectric transition, but only if it coincides with the transition from random to uniform tilt and with the transition from anticlinic to synclinic tilt correlation of the molecules in adjacent layers. For compounds with a bit longer chains ( n = 18‐22) the heliconical phase is only field‐induced, but once formed it is stable in a distinct temperature range, even after switching off the field. The presence of the helix changes the phase properties and the switching mechanism in the LC phases from the naturally preferred rotation around the molecular long axis, which reverses the chirality, to a precession on a cone, which retains the chirality once established. These observations are explained by diastereomeric relations between two different and coexisting modes of superstructural chirality, the layer chirality, and the helical twist evolving between the layers.