Twist-bend nematics and heliconical cholesterics: a physico-chemical analysis of phase transitions and related specific properties

TL;DR

This study investigates the phase transitions and optical properties of twist-bend nematic and heliconical cholesteric phases in specific liquid crystal systems, revealing effects of dopants and temperature on their stability and optical behavior.

Contribution

It provides a detailed physico-chemical analysis of phase transitions in twist-bend nematics and cholesterics, including effects of dopants and electric fields, with experimental observations.

Findings

Chiral dopants induce a chiral twist-bend phase below the nematic phase.

Selective visible light reflection observed in the chiral nematic phase.

Helix unwinding occurs near the N*tb phase, similar to smectic-A transition behavior.

Abstract

Certain nematic liquid crystals, e.g., those formed by banana-shaped molecules, can exhibit a low-temperature twist-bend nematic (Ntb) phase. Upon addition of chiral dopants, a chiral version of the twist-bend phase (N*tb) can be observed below the conventional chiral nematic (N*) phase, while under electric field the N* phase is transformed into a specific state known as "heliconical cholesteric". In this work, we have studied the well-known Ntb-forming system CB7CB:CB6OCB with addition of 5CB and chiral dopants (ChD) using optical microscopy, differential scanning calorimetry (DSC), and measurements of temperature-dependent optical transmission using optical microscopy. Effects of 5CB and different chiral dopants (of steroid or non-steroid nature) upon thermal stability of Ntb phase were analyzed, and selective reflection in the visible range could be observed in the N* phase with a sufficiently high ChD content. As a peculiar feature, sharp unwinding of the cholesteric helix was observed at lower temperatures when approaching the N*tb phase, which appears to be similar to the helix unwinding close to the smectic-A transition.