Liquid crystal dimers and the twist-bend nematic phase: on the role of spacers and terminal alkyl chains.

Forsyth, Ewan; Paterson, Daniel A.; Cruickshank, Ewan; Strachan, Grant J.; Gorecka, Ewa; Walker, Rebecca; Storey, John M.D.; Imrie, Corrie T.

Abstract

The synthesis and characterisation of four series of liquid crystal dimers based on benzylideneaniline mesogenic units, and in which the lengths of terminal alkyloxy chains are varied are reported. The series differ in terms of their flexible spacers, namely, heptamethylene, nonamethylene, hexyloxy, and oxypentyloxy chains. The heptamethylene- and nonamethylene-linked dimers both show conventional nematic, N, and twist-bend nematic, NTB, phases with short terminal chains, and smectic behaviour emerges on increasing terminal chain length. This is attributed to increased molecular inhomogeneity driving microphase separation. The dimers containing the shorter heptamethylene spacer show a smectic A phase whereas those with the longer nonamethylene spacer exhibit an anticlinic smectic C phase. Smectic behaviour is not observed for the dimers containing either a hexyloxy spacer which exhibit nematic and twist-bend nematic phases, or with an oxypentyloxy spacer which show only a conventional nematic phase. A general observation is that TNTBN and TNI alternate in the same sense in a homologous series on varying the length of the terminal alkyl chains suggesting that the spatial uniformity of the molecular curvature is an important factor in stabilising the NTB phase. The transitional properties of the four corresponding dimers possessing nitrile terminal substituents are also described. These show enantiotropic nematic phases, and in addition, for those containing either polymethylene or hexyloxy spacers, a twist-bend nematic phase is observed. Differences in the thermal behaviour of the dimers may be attributed largely to changes in molecular shape arising from the nature of the link between the spacer and mesogenic units.

Citation

FORSYTH, E., PATERSON, D.A., CRUICKSHANK, E., STRACHAN, G.J., GORECKA, E., WALKER, R., STOREY, J.M.D. and IMRIE, C.T. 2020. Liquid crystal dimers and the twist-bend nematic phase: on the role of spacers and terminal alkyl chains. Journal of molecular liquids [online], 320(part A), article 114391. Available from: https://doi.org/10.1016/j.molliq.2020.114391

• Journal Article Type: Article
• Acceptance Date: Sep 20, 2020
• Online Publication Date: Sep 25, 2020
• Publication Date: Dec 15, 2020
• Deposit Date: Aug 17, 2023
• Publicly Available Date: Aug 29, 2023
• Journal: Journal of Molecular Liquids
• Print ISSN: 0167-7322
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 320
• Issue: part A
• Article Number: 114391
• DOI: https://doi.org/10.1016/j.molliq.2020.114391
• Keywords: Twist-bend nematic; Dimer; Terminal chain; Smectic phase
• Public URL: https://rgu-repository.worktribe.com/output/2043826
• Additional Information: This article has been published with separate supporting information. This supporting information has been incorporated into a single file on this repository and can be found at the end of the file associated with this output.