Twist-bend heliconical chiral nematic liquid crystal phase of an achiral rigid bent-core mesogen

The chiral, heliconical (twist-bend) nematic ground state is reported in an achiral, rigid, bent-core mesogen (UD68). Similar to the nematic twist-bend ($N_{\mathrm{TB}}$) phase observed in bent molecular dimers, the $N_{\mathrm{TB}}$ phase of UD68 forms macroscopic, smecticlike focal-conic textures and exhibits nanoscale, periodic modulation with no associated modulation of the electron density, i.e., without a detectable lamellar x-ray reflection peak. The $N_{\mathrm{TB}}$ helical pitch is $p_{\mathrm{TB}}$∼14 nm. When an electric field is applied normal to the helix axis, a weak electroclinic effect is observed, revealing 50-μm-scale left- and right-handed domains in a chiral conglomerate.

Figure 1

Chemical structures and phase sequences of the bent-core molecule UD68 (a) and the bent molecular dimer CB7CB (b). (c) Molecular organization of the $N_{\mathrm{TB}}$ and SmCP phases of bent-core molecules. Both the $N_{\mathrm{TB}}$ and SmCP phases are chiral and polar with the molecules forming a heliconical structure. In the $N_{\mathrm{TB}}$ phase, the molecules are intercalated, while in the SmCP phase the bent-core molecules form well-defined smectic layers, the combination of macroscopic polarization and molecular tilt making the layers chiral.

Figure 2

DTLM textures in a weakly rubbed cell and XRD of the mesophases of UD68. (a) Nematic phase (top left) with characteristic Schlieren texture indicating nematic with random planar alignment grows in from the isotropic phase (bottom right), which is dark under crossed polarizers ($T$ = 186 °C). (b) In the $N_{\mathrm{TB}}$ mesophase ($T$ = 167 °C), focal-conic domains (top half) form from the nematic phase (bottom half), with the dark brushes parallel to the polarizers. (c) At lower temperature ($T$ = 155 °C), yellow dendritic aggregates of the Col phase (left) grow into the $N_{\mathrm{TB}}$ texture (right). Images (a)–(c) were obtained in a 2-μm nylon-coated planar cell with weak rubbing. (d) XRD of the $N$, $N_{\mathrm{TB}}$, Col, and Cry phases. The measurements were taken on cooling from the nematic phase at 180 °C, except for the measurement at 24 °C, which was taken after the samples were kept at room temperature for 5 days.

Figure 3

Microscopic textures and electro-optic response of the $N_{\mathrm{TB}}$ phase in a strongly rubbed cell. (a) Uniformly aligned nematic phase at $T=170$ °C with the director along the rubbing direction $R$. (b) Stripes are a characteristic feature of the $N_{\mathrm{TB}}$ phase as it grows in at $T=167$ °C. (c) In an applied $E$ field of 20 V/μm, two regions of different molecular tilt can be distinguished, with the dark and bright regions interchanging when the $E$ field is reversed (d). (e) In the absence of external electric field, the boundaries of domains of opposite chirality are characterized by discontinuities in the stripe pattern, for example, along the dashed green line. (f) The texture of the $N_{\mathrm{TB}}$ phase in a 2-μm planar cell of a bent molecular dimer diluted with its monomer (35 wt %) reported in [9] shows chiral domain boundaries similar to those in (e). Reproduced with permission from Applied Physics Letters 101, 234106 (2012). Copyright 2012 AIP Publishing LLC. Images (a)–(e) were taken in an Instec 4-μm planar cell. The scale bars are 100 μm.

Figure 4

: Freeze-fracture images of the $N_{\mathrm{TB}}$ and Col mesophases. (a) and (b) FFTEM images of the $N_{\mathrm{TB}}$ phase, showing the conical helical structure of period $p_{\mathrm{TB}}$∼14 nm. In both images, the helix axis $h$ is approximately parallel to the fracture plane as modeled in the inset and indicated by the red arrows. No layer surfaces are observed in the $N_{\mathrm{TB}}$ phase. The sample was sandwiched between two clean glass planchettes and quenched at $T$ = 167 °C. (c) and (d) FFTEM images of the Col phase, quenched at $T=155$ °C. The Col phase shows a two-dimensional periodic structure, with layers parallel to the fracture plane and layer modulations along the layer surface. The column orientation is sketched in the inset. Distinct layer steps can be observed in the Col phase. The scale bars are all 100 μm.