Synthesis and characterisation of an unsymmetrical, ether-linked, fluorinated bimesogen exhibiting a new polymorphism containing the NTB or ‘twist-bend’ phase

Abstract

In this article we report on the liquid crystal phases and properties of the bimesogen 4-((11-((4'-fluoro-[1,1'-biphenyl]-4-yl)oxy)undecyl)oxy)-2,3-difluoro-4'-(4-propylcyclohexyl)-1,1'-biphenyl. This material was shown to exhibit an Iso Liq-N-NTB-SmA phase sequence, thereby clearly indicating that the NTB phase possesses an ordering of the constituent molecules that is between that of a conventional nematic and the smectic A phase. This compound allows us to better understand the relationship between molecular structure and the NTB phase, and we conclude it is the gross topology that dictates the incidence of this fascinating phase and not molecular properties such as dipole moment and bend angle.

Dependence of Mesomorphic Behaviour of Methylene‐Linked Dimers and the Stability of the NTB/NX Phase upon Choice of Mesogenic Units and Terminal Chain Length

“…In recent years there has been a resurgence of interest in liquid‐crystal dimers and bimesogens, driven by interest in wide temperature range blue phases,1, 2 flexoelectric behaviour3, 4, 5 and the ability of some dimeric materials to exhibit a lower‐temperature mesophase lacking lamellar organisation thereby being described as nematic 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18. This lower temperature ‘nematic’ phase is denoted N X or N TB ,6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 in which ‘X’ and ‘TB’ refer to unknown and twist‐bend, respectively.…”

“…This lower temperature ‘nematic’ phase is denoted N X or N TB ,6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 in which ‘X’ and ‘TB’ refer to unknown and twist‐bend, respectively. The local structure of the N X /N TB phase is still hotly debated, with the heliconical ‘twist‐bend’ model proposed independently by Meyer19 and Dozov20 supported by 2 H NMR studies, measurement of the electroclinic effect, freeze‐fracture transmission electron microscopy (FFTEM) and carbon K‐edge SAXS 21, 22, 23, 24.…”

Polar interactions between bent–core molecules as a stabilising factor for inhomogeneous nematic phases with spontaneous bend deformations

“…In recent years the new inhomogeneous nematic phases have been observed [1][2][3][4][5][6][7][8][9][10][11][12][13], and the study of the detailed structure of these phases is currently the most topical issue in liquid crystals research. The experiments for oligomers and bent-core systems indicate that there are at least two types of modulated nematic structures with one-dimensional periodicity.…”

Developments in liquid-crystalline dimers and oligomers

“…Thus, we opted to prepare a material incorporating the 2,3-difluorobiphenylcyclohexyl (DFBC) unit employed in the first ether-linked bimesogen to exhibit the N TB phase, FBO 11 ODFCB 3 [62]. We have found previously that dimers and bimesogens incorporating the DFBC unit have a strong tendency to exhibit the twist-bend nematic phase [63].…”

“…It has been observed that many functional groups -including the SF 5 groupcan be incorporated into the structure of a suitable dimer without loss of the N TB phase. Last, the twistbend nematic phase -in dimers and bimesogens at least -is found mostly for methylene-linked materials, although a number of other linking groups are known to yield materials that afford this mesophase [7,21,22,33,[44][45][46][47][48][49]. In the first part of this paper, we present the latest part of our study into the structureproperty relationships of bis ether-linked dimers and bimesogens that exhibit the twist-bend phase.…”