A Twist-Bend Chiral Helix of 8nm Pitch in a Nematic Liquid Crystal of Achiral Molecular Dimers

TL;DR

This study uncovers a nanoscale twist-bend chiral helix in an achiral liquid crystal, combining microscopy, scattering, and simulations to reveal a unique 8 nm pitch structure with nematic order.

Contribution

It provides the first detailed experimental and computational evidence of an 8 nm pitch twist-bend nematic phase in achiral molecules, advancing understanding of liquid crystal nanoscale structures.

Findings

Nanoscale stripe texture with ~8.3 nm spacing observed via FFTEM.

Absence of lamellar x-ray reflection indicating nematic order.

Molecular dynamics simulations confirm a conical twist-bend helix with 8 nm pitch.

Abstract

Freeze Fracture Transmission Electron Microscopy (FFTEM) study of the nanoscale structure of the so-called "twist-bend" nematic (NX) phase of the cyanobiphenyl (CB) dimer molecule CB(CH2)7CB reveals a stripe texture of fluid layers periodically arrayed with a bulk spacing of d ~ 8.3 nm. Fluidity and a rigorously maintained spacing produce long-range-ordered fluid layered focal conic domains. Absence of a lamellar x-ray reflection at wavevector q ~ 2{\pi}/8 nm-1 or its harmonics in synchrotron-based scattering experiments indicates that this periodic structure is achieved with no detectable associated modulation of the electron density, and thus has nematic molecular ordering. A search for periodic ordering with d ~ 8nm in CB(CH2)7CB using atomistic molecular dynamic computer simulation yielded equilibration of a conical twist-bend helixed nematic ground state, of the sort first proposed by Meyer, and envisioned in systems of bent molecules by Dozov and Memmer, We identify {\theta} ~ 33 degree as the cone angle, and p ~ 8nm as the full pitch of the helix, the shortest ever found in a nematic fluid.