Twist-Bend Nematics

 
In 2013, liquid crystal group members, in close collaboration with colleagues in the SMRC, confirmed the existence and elucidated the structure of the elusive twist-bend nematic, a phase first proposed as a theoretical possibility by Meyer over 40 years earlier [1] and more recently suggested as a candidate for a mysterious chiral nematic phase with smectic-like textures observed in achiral dimeric mesogens by Luckhurst and co-workers [2]. Although no smectic-like peaks were observed by conventional x-ray scattering, freeze-fracture transmission electron microscopy revealed a periodic modulation of the tilted director, with a pitch comparable to the molecular length. This remarkable heliconical structure was also observed in atomistic computer simulations and has since been confirmed experimentally in a number of bent dimer mesogens, as well as in rigid bent-core materials. Remarkably, while there are more than a dozen known smectic phases, the twist-bend nematic was at this time only the fifth nematic phase to be identified [3,4].

chromonics

Figure 1. (A) Structure of CB7CB. The end-to-end length of its van der Waals surface is ~3.0 nm in the all-transmolecular configuration. (B) CB7CB molecule can be viewed as having three parts, each ~1 nm in length: two rigid end groups connected by a flexible spacer. The nematic director field n(r) is the local average orientation of the long axes of the rigid end groups. (C) Schematic illustration of the heliconical TB structures in the layered chiral SmC* phase and in the layerless NTB phase.

 

chromonics

Figure 2. FFTEM image of CB7CB quenched from the NTB phase at T=95°C,with the Pt shadowing direction indicated by the magenta arrow. (A)The fracture face exhibits a 2D texture of curved, periodically arrayed stripes indicative of a bulk,fluid, layered structure of 3D focal conics domains with conic section lines, respectively, parallel to and normal to the fracture surface in B and C.

phase separation

Figure 3. Fully atomistic MD simulations of equilibrium nematic phases at T=370K in CB6CB, a linear molecule (A), and CB7CB, a bent molecule (B),showing a periodic box of a nominally 5.6×5.6×8.0 nm dimension. Initial equilibration is carried out with opposed forces in the z direction on themolecular ends, giving a form of field-induced nematic monodomain. As these forces are relaxed, CB6CB remains a nematic, with n along z (A),whereas CB7CB relaxes into a heliconical NTB structure, of pitch pTB=8.1 nm and cone angle θTB=25° (B). (C and D) Example configurations showing explicitly the orientations ni of the biphenyl groups that determine the half-molecular director field n. CB6CB shows conventional nematic ordering whereas there is clearly heliconical ordering in CB7CB.

References

[1] S. Garoff and R.B. Meyer, "Electroclinic effect at the A-C phase change in a chiral smectic liquid crystal," Phys. Rev. Lett. 38(1), 848–851 (1977).

[2] P.J. Barnes, A.G. Douglass, S.K. Heeks, G.R. Luckhurst, "An enhanced odd-even effect of liquid crystal dimers. Orientational order in theα,ω-bis(4′-cyanobiphenyl-4- yl) alkanes," Liquid Crystals 13(4), 603–613 (1993).

[3] D. Chen et al., "Chiral heliconical ground state of nanoscale pitch in a nematic liquid crystal of achiral molecular dimers," PNAS 110, 15931–15936 (2013). [download]

[4] D. Chen et al., "Twist-bend heliconical chiral nematic liquid crystal phase of an achiral rigid bent-core mesogen," Physical Review E 89, 022506 (2014). [download]

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