Phase behavior and properties of the liquid-crystal dimer 1′′,7′′-bis(4-cyanobiphenyl-4′-yl) heptane: A twist-bend nematic liquid crystal

Abstract

The liquid-crystal dimer 1'',7''-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB) exhibits two liquid-crystalline mesophases on cooling from the isotropic phase. The high-temperature phase is nematic; the identification and characterization of the other liquid-crystal phase is reported in this paper. It is concluded that the low-temperature mesophase of CB7CB is a new type of uniaxial nematic phase having a nonuniform director distribution composed of twist-bend deformations. The techniques of small-angle x-ray scattering, modulated differential scanning calorimetry, and dielectric spectroscopy have been applied to establish the nature of the nematic-nematic phase transition and the structural features of the twist-bend nematic phase. In addition, magnetic resonance studies (electron-spin resonance and (2)H nuclear magnetic resonance) have been used to investigate the orientational order and director distribution in the liquid-crystalline phases of CB7CB. The synthesis of a specifically deuterated sample of CB7CB is reported, and measurements showed a bifurcation of the quadrupolar splitting on entering the low-temperature mesophase from the high-temperature nematic phase. This splitting could be interpreted in terms of the chirality of the twist-bend structure of the director. Calculations using an atomistic model and the surface interaction potential with Monte Carlo sampling have been carried out to determine the conformational distribution and predict dielectric and elastic properties in the nematic phase. The former are in agreement with experimental measurements, while the latter are consistent with the formation of a twist-bend nematic phase.

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

“…There had been a number of reports of nematic-nematic phase transitions for bent-core or banana mesogens, but the structural characteristics of the new nematic phases had not been identified. 1,[2][3][4][5][15][16][17] However, recently a very detailed investigation of the flexible bent-shaped liquid crystal dimer 1 00 ,7 00 -bis(4-cyanobiphenyl)-4 0 -yl heptane (CB7CB) 6 concluded that one of the two nematic phases exhibited by this compound is the twistbend nematic phase (N TB ), in agreement with Dozov's predictions. 13 The mesogen CB7CB now appears to be the model liquid crystal on which research about this new phase is mainly focused.…”

“…However, it is known that pure CB7CB dimer exhibits a firstorder N TB -N phase transition. 6 The inset of Fig. 2 shows how the nematic range increases from pure CB7CB dimer when the concentration of CB7CB in the binary mixtures decreases, showing a behaviour analogous to binary mixtures exhibiting the smectic A (SmA) to nematic phase transition.…”

“…1 shows the molecular geometry of both liquid crystal dimers with the spacer in the all-trans conformation. For CB7CB, molecular calculations 6 show that for higher values of the nematic order parameter (low temperatures) the more extended conformers, with angles (w) on average of about 1201 between the long or para axes of the terminal mesogenic groups, are stabilized at the expense of the less extended conformers, with angles on average of about 301. Unfortunately, no calculations are available for the conformational distribution of FFO9OCB, but for the shorter-chain homologue FFO5OCB, calculations made by Cestari et al 29 indicate that the most extended conformers have angles w, on average, of about 1201 for the order parameters comparable to those found from the calculations for CB7CB dimer.…”

“…Similar dimers with odd spacers having a N x -N transition were reported by Panov et al 2 and the behaviour of such dimers has also been studied [3][4][5] but again without the low-temperature nematic being properly identified. Cestari et al 6 have examined an odd liquid crystal dimer and observed a nematic-nematic transition but, as the result of an extensive investigation based on numerous techniques, were able to identify the low-temperature nematic as we shall see shortly. A slightly different dimer again with an odd spacer but having one chiral mesogenic group has been reported by Zep et al,7 surprisingly this system is found to form seven nematic phases over a narrow temperature range.…”

“…One of the most important concerns arises from the fact that the theory needs a drastic simplification of the number of terms and the simplified version predicts a secondorder N TB -N phase transition, which is not supported by experimental determinations. 3,6,26 In the case of flexoelectric coupling, Shamid et al 11 have considered the polar order as an additional order parameter, but again such a Landau theory needs to be simplified and the approximate solution also predicts a secondorder N TB -N phase transition. The same authors 11 proposed the definition of an effective bend elastic constant (K eff 3 ) which could be negative in the N TB phase and positive in the uniform nematic phase.…”