Dielectric, calorimetric and mesophase properties of 1′′-(2′,4-difluorobiphenyl-4′-yloxy)-9′′-(4-cyanobiphenyl-4′-yloxy) nonane: an odd liquid crystal dimer with a monotropic mesophase having the characteristics of a twist-bend nematic phase

Abstract

This paper reports a novel liquid crystal phase having the characteristics of a twist-bend nematic phase formed by a non-symmetric ether-linked liquid crystal dimer. The dimer 1''-(2',4-difluorobiphenyl-4'-yloxy)-9''-(4-cyanobiphenyl-4'-yloxy) nonane (FFO9OCB) exhibits two liquid-crystalline phases on cooling at a sufficiently high rate from the isotropic phase. The high temperature mesophase has been reported in the literature as nematic and confirmed in this study. The other mesophase is metastable and can be supercooled giving rise to a glassy state. Its identification and characterization are based on optical textures, broadband dielectric spectroscopy, calorimetry, measurements of both splay and bend elastic constants in the nematic phase and miscibility studies. It is concluded that the low temperature mesophase exhibits the characteristics of a twist-bend nematic phase. Dielectric measurements enable us to obtain the static permittivity and information about the molecular dynamics in the isotropic phase, in the nematic mesophase and across the isotropic-to-nematic phase transition. Two orientations, parallel and perpendicular to the director, have been investigated. In the high temperature nematic mesophase, the dielectric anisotropy is found to be positive. Measurements of the parallel component of the dielectric permittivity are well-explained by the molecular theory of dielectric relaxation in nematic dimers (M. Stocchero, A. Ferrarini, G. J. Moro, D. A. Dunmur and G. R. Luckhurst, J. Chem. Phys., 2004, 121, 8079). The dimer is modelled as a mixture of cis and trans conformers and the model allows an estimate of their relative populations at each temperature. The nematic-to-isotropic phase transition has been exhaustively studied from the accurate evolution of the heat capacity and the static dielectric permittivity data. It has been concluded that the transition is first order in nature, but close to tricritical. The nature of the nematic-to-the novel liquid crystal phase transition is difficult to analyze to the same extent because of insufficient precision. Only observations at cooling rates of 10 K min(-1) or higher were possible because on heating from the glassy state, the twist-bend nematic mesophase crystallizes at temperatures far below the nematic-nematic phase transition.

Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB–N phase transition

“…29 As for the experimental determinations of the bend elastic constant over the uniform nematic phase of twist-bend liquid crystal dimers, it is observed that the bend elastic constant decreases as temperature decreases on approaching the N-N TB phase transition, leading to small values at the phase transition but increasing slightly or being nearly temperature independent before N TB is reached. 25,[30][31][32] In addition, very recently Yun et al 33 have tried to measure K 3 in the N TB phase of CB7CB showing anomalous and positive values.…”

“…The components of the mixtures, for which experimental results are reported, are the classic compound CB7CB and 1 00 -(2 0 ,4-difluorobiphenyl-4 0 -yloxy)-9 00 -(4-cyanobiphenyl-4 0 -yloxy)nonane (hereafter referred to by the acronym FFO9OCB) which has recently been shown to exhibit a monotropic N TB -phase. 32 Measurements are also reported of the splay (K 1 ) and bend (K 3 ) elastic constants in the conventional nematic phase for the pure compounds and some selected mixtures, as a function of both temperature and mole fraction. These experimental results provide further insight into the nature of the N TB -N phase transition, and the validity of our proposed model used to describe the phase transition.…”

The Shape of Things To Come: The Formation of Modulated Nematic Mesophases at Various Length Scales

“…Bend angles were taken to be the all‐ trans conformer that we found to be dominant using solution‐based 1D 1 H NOESY NMR spectroscopy; indeed this is also reported to be the dominant conformer in the bulk N TB phase from 2 H NMR data coupled with DFT calculations 19. The dominance of the all‐ trans conformer over other conformations has been demonstrated for both methylene‐ and ether‐linked dimers using proton‐enhanced local field (PELF) NMR spectroscopy on CB7CB19 and dielectric spectroscopy on 1′′‐(2′,4‐difluorobiphenyl‐4′‐yloxy)‐9′′‐(4‐cyanobiphenyl‐4′‐yloxy)nonane (FFO9OCB), respectively 44. In reality the use of a single conformer to describe the bend angle is insufficient for flexible molecules, such as LC dimers, and so we opted to study a new set of materials computationally, obtaining a library of conformers from which we can then obtain the full distribution of bend angles as well as a weighted average molecular bend 45.…”

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.…”