Leaning-type polar smectic-$C$ phase in a freely suspended bent-core liquid crystal film

A rich variety of responsive behavior occurs in complex structured fluids due to their lower symmetries. On the other hand, fluid disorder tends to increase the symmetry of liquid crystal mesophases. Here, we report direct evidence for the existence of a mesophase with $C_S$ symmetry. The observations are based on optical studies of director inversion walls in freely suspended films in electric fields under obliquely incident light. This phase is distinguished by the polarization lying in the molecular tilt plane in freely suspended films. Such a low-symmetry polar fluid phase has been long predicted to occur in bent-core liquid crystals. The stability of this phase is attributed to the bent shape of the mesogens and dominating dispersion interactions.

Figure 1

(a) Chemical formula of the investigated mesogen CN-1. (b) Depiction of the molecular orientation in a single smectic layer using the angles θ and φ. The $Z$ axis defines the layer normal and p is the layer polarization (the direction of the molecular bow). The projection of the polarization on the layer plane is the $p$ director. The projection of the molecular long axis on the layer plane is the $c$ director c. (c) A cartoon of a leaning phase structure. The leaning phase is obtained by the tilt of the mesogen long axis from the smectic layer normal by an angle θ and followed by a rotation by an angle $\phi ={90}^{\circ }$ about the long axis l. The structure is also obtained directly by leaning molecules in the molecular plane, which is distinguished from tilting.

Figure 2

Films under crossed polarizers and applied dc electric field of $10\mathrm{V}/\mathrm{mm}$ (a)–(d) for the 8° inclined stage with direction marked by “T”; (e), (f) for the noninclined stage $(T=115{}^{\circ }\mathrm{C})$. The polarizers are along the sides of the images.

Figure 3

(a)–(d) Maps of $c$ directors along the cross sections of a $2\pi$ wall for different tilt directions; dependence of the correlation length ${\xi }_p$ on the electric field $E$ at a fixed film thickness 500 nm (e) and on the film thickness at a fixed field $240\mathrm{V}/\mathrm{m}\left(\mathrm{f}\right)(T=115{}^{\circ }\mathrm{C})$.

Figure 4

Coexistence of a nonpolar phase and a polar leaning phase observed in a polarizing microscope with a full wave plate: (a) under ac electric field and (b) dc electric field $(T=119{}^{\circ }\mathrm{C})$. The alignment of the slow axis differs in the polar region.