Flexoelectrically Driven Electroclinic Effect in the Twist-Bend Nematic Phase of Achiral Molecules with Bent Shapes

Abstract

We extend the twist-bend nematic (N(TB)) model to describe the electro-optics of this novel phase. We predict an electroclinic effect (ECE) subject to a dc electric field E applied perpendicular to the helix axis or wave vector q, with rotation of the N(TB) optic axis around E. This linear effect, with its flexoelectric origin, is a close analog to the electro-optic effects observed for chiral liquid crystals. However, in nematics composed of achiral molecules having a bent shape, it is the electro-optic signature of the N(TB) phase. We test our model experimentally in the low-temperature nematic phase of the odd liquid crystal dimer, CB7CB, with its molecules having, on average, a bent shape. The ECE measurements confirm the previously proposed twist-bend nematic structure of this phase, with its broken chiral symmetry, extremely short (<10 nm) doubly degenerate pitch and ultrafast, submicrosecond response times.

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

“…In our case, we follow Dozov's procedure, 13,21 according to which the elastic distortion causing the director curvature would be the main cause. As an approximation, we suppose that the nematic order parameter related to the director jumps at the N-N TB phase transition by dQ as evidenced experimentally for some systems.…”

“…13 The mesogen CB7CB now appears to be the model liquid crystal on which research about this new phase is mainly focused. [18][19][20][21][22][23][24][25][26] The heliconical twist-bend nematic phase appears as a state in which the spontaneous bend of the director is accompanied by a twist deformation in order to satisfy topological constraints. The induced twist can be left-or right-handed and the sample is expected to form a conglomerate of degenerate domains having opposite chirality even though the constituent molecules are achiral.…”

“…Alternative formulations of the theory only consider purely elastic distortions of the director due to molecular curvature. 13,14,21 Both theoretical approaches can be developed in the framework of a Landau theory on the basis of the Oseen-Frank elastic free energy. Dozov 13 described the twist-bend nematic phase with a negative bend elastic constant (K 3 ) and the purely elastic free energy needs to be developed to fourth order to provide an upper bound to the energy.…”

Twist-bend nematic phases of bent-shaped biaxial molecules

“…The form ofQ U andQ tensors accounts in both models for the global D ∞ symmetry point group of the N TB phase with the (optical) C ∞ axis parallel to the helix axis 11,42 . N TB is also invariant for the twofold rotation around a local vectorm, wherem is perpendicular to the plane containing the helix axisẑ and the local director.…”

Hierarchical elasticity of bimesogenic liquid crystals with twist-bend nematic phase

“…6 In particular, some bimesogenic liquid crystals with an odd number of methylene flexible units or a rigid bent-core mesogen showed an additional nematic phase at temperatures below the conventional nematic phase (N u ). 7,8 Recently, this phase has been identified as the twist-bend nematic phase (N tb ), 9 where the director forms periodic twisted and bent deformations with a nano-scale pitch (8-9 nm). 10 In planar cells, the N tb phase also exhibited spontaneous periodic stripe patterns in micrometer-scale.…”