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All-atom simulations of bent liquid crystal dimers: the twist-bend nematic phase and insights into conformational chirality

Yu, Gary; Wilson, Mark Richard

All-atom simulations of bent liquid crystal dimers: the twist-bend nematic phase and insights into conformational chirality Thumbnail


Authors

Gary Yu



Abstract

The liquid crystal dimer 1,7-bis-4-(4′-cyanobiphenyl)heptane (CB7CB) is known to exhibit a nematic–nematic phase transition, with the lower temperature phase identified as the twist-bend nematic (NTB) phase. Despite the achiral nature of the mesogen, the NTB phase demonstrates emergent chirality through the spontaneous formation of a helical structure. We present extensive molecular dynamics simulations of CB7CB using an all-atom force field. The NTB phase is observed in this model and, upon heating, shows phase transitions into the nematic (N) and isotropic phases. The simulated NTB phase returns a pitch of 8.35 nm and a conical tilt angle of 29°. Analysis of the bend angle between the mesogenic units reveals an average angle of 127°, which is invariant to the simulated phase. We have calculated distributions of the chirality order parameter, χ, for the ensemble of conformers in the NTB and N phases. These distributions elucidate that CB7CB is statistically achiral but can adopt chiral conformers with no preference for a specific handedness. Furthermore, there is no change in the extent of conformational chirality between the NTB and N phases. Using single-molecule stochastic dynamics simulations in the gas phase, we study the dimer series CBnCB (where n = 6, 7, 8 or 9) and CBX(CH2)5YCB (where X/Y = CH2, O or S) in terms of the bend angle and conformational chirality. We confirm that the bent molecular shape determines the ability of a dimer to exhibit the NTB phase rather than its potential to assume chiral conformers; as |χ|max increases with the spacer length, but the even-membered dimers have a linear shape in contrast to the bent nature of dimers with spacers of odd parity. For CBX(CH2)5YCB, it is found that |χ|max increases as the bend angle of the dimer decreases, while the flexibility of the dimers remains unchanged through the series.

Journal Article Type Article
Acceptance Date Mar 24, 2022
Online Publication Date Mar 25, 2022
Publication Date Apr 21, 2022
Deposit Date Mar 4, 2022
Publicly Available Date May 10, 2022
Journal Soft Matter
Print ISSN 1744-683X
Electronic ISSN 1744-6848
Publisher Royal Society of Chemistry
Peer Reviewed Peer Reviewed
Volume 18
Issue 15
Pages 3087-3096
DOI https://doi.org/10.1039/d2sm00291d
Public URL https://durham-repository.worktribe.com/output/1212196

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