Heliconical smectic phases formed by achiral molecules

Jordan P Abberley,Chenhui Zhu,Ewa Gorecka,Mirosław Salamończyk,Damian Pociecha,Rebecca Walker,John M D Storey,Ross Killah,Corrie T Imrie

doi:10.1038/s41467-017-02626-6

Abstract

Chiral symmetry breaking in soft matter is a hot topic of current research. Recently, such a phenomenon was found in a fluidic phase showing orientational order of molecules—the nematic phase; although built of achiral molecules, the phase can exhibit structural chirality—average molecular direction follows a short-pitch helix. Here, we report a series of achiral asymmetric dimers with an odd number of atoms in the spacer, which form twisted structures in nematic as well as in lamellar phases. The tight pitch heliconical nematic (NTB) phase and heliconical tilted smectic C (SmCTB) phase are formed. The formation of a variety of helical structures is accompanied by a gradual freezing of molecular rotation. In the lowest temperature smectic phase, HexI, the twist is expressed through the formation of hierarchical structure: nanoscale helices and mesoscopic helical filaments. The short-pitch helical structure in the smectic phases is confirmed by resonant X-ray measurements.

Highlights

Chiral symmetry breaking in soft matter is a hot topic of current research
The spontaneous formation of chiral structures from effectively achiral molecules is at the heart of intense worldwide research, as the breaking of mirror symmetry is a fundamental issue in chemistry, physics and biology and plays a central role in the origin of biological homochirality
Twisted structures made from chiral building blocks are relatively common in nature, the archetypal examples being DNA or proteins that can exist in helical forms in solids and in the liquid state, whereas examples of achiral molecules that assemble into helical aggregates are much less common, and until recently such helical aggregates were observed only for 3D ordered crystals

Summary

Introduction

Chiral symmetry breaking in soft matter is a hot topic of current research Such a phenomenon was found in a fluidic phase showing orientational order of molecules— the nematic phase; built of achiral molecules, the phase can exhibit structural chirality—average molecular direction follows a short-pitch helix. For the overwhelming majority of twistbend nematogens, the NTB phase is preceded by a conventional nematic (N) phase with uniform director structure, for which the strongly bent molecules give rise to small values of the bend elastic constant[8]. We show that as in the nematic phase, such achiral bent molecules spontaneously form short-pitch length helical structures in smectic phases. The molecular curvature, that is essential for the formation of the NTB phase, can be realised using odd-membered mesogenic dimers. The lowest temperature smectic phase was found to exhibit structural chirality at different length-scales, i.e., layer chirality, nanoscale helices and mesoscopic helical filaments

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