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

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

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)

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.

Original languageEnglish
Article number114391
Number of pages11
JournalJournal of molecular liquids
Volume320
Early online date25 Sep 2020
DOIs
Publication statusPublished - 15 Dec 2020

Keywords

  • Dimer
  • Smectic phase
  • Terminal chain
  • Twist-bend nematic

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