Electroclinic effect in the chiral lamellarαphase of a lyotropic liquid crystal

Abstract

In thermotropic chiral Sm-A^{*} phases, an electric field along the smectic layers breaks the D_{∞} symmetry of the Sm-A^{*} phase and induces a tilt of the liquid crystal director. This so-called electroclinic effect (ECE) was first reported by Garoff and Meyer in 1977 and attracted substantial scientific and technological interest due to its linear and submicrosecond electro-optic response [S. Garoff and R. B. Meyer, Phys. Rev. A 19, 338 (1979)0556-279110.1103/PhysRevA.19.338]. We now report the observation of an ECE in the pretransitional regime from a lyotropic chiral lamellar L_{α}^{*} phase into a lyo-Sm-C^{*} phase, the lyotropic analog to the thermotropic Sm-C^{*} phase which was recently discovered by Bruckner et al. [Angew. Chem. Int. Ed. 52, 8934 (2013)1433-785110.1002/anie.201303344]. We further show that the observed ECE has all signatures of its thermotropic counterpart, namely (i) the effect is chiral in nature and vanishes in the racemic L_{α} phase, (ii) the effect is essentially linear in the sign and magnitude of the electric field, and (iii) the magnitude of the effect diverges hyperbolically as the temperature approaches the critical temperature of the second order tilting transition. Specific deviations between the ECEs in chiral lamellar and chiral smectic phases are related to the internal field screening effect of electric double layers formed by inevitable ionic impurities in lyotropic phases.

The Lyotropic Analog of the Polar SmC* Phase

“…Since phospholipid cell membranes are in a fluid L * state, this observation might also have certain implications in biophysics. The electroclinic effect in the L α * phase, thus shows all signatures of the electroclinic effect in thermotropic SmA* phases, namely the effect (i) is chiral in nature, (ii) is essentially linear in the sign and magnitude of the electric field, and (iii) shows Curie-Weiss-like behavior in the pretransitional regime of a tilting transition into lyo-SmC* [69]. Beyond these striking similarities between the electroclinic effects in SmA* and L α * there are also specific deviations, namely, the slow decay of the electro-optic response in Figure 24b, after switching and the non-linearity at low field strength in Figure 24a.…”

“…A first indication was the observation by Jakli et al that the chirality of phospholipids makes fluid lamellar phases piezoelectric [67,68]. The discovery of the lyo-SmC* phase however sheds new light on this issue: in 2017 Harjung et al observed an electroclinic response in the L α * phase of the amphiphile 4 with 23 wt% formamide at temperatures close to its transition into the newly discovered lyo-SmC* phase [69]. The samples were poured into a liquid crystal cell and a square-wave electric field was applied to its transparent ITO electrodes (Figure 24a).…”

“…Beyond these striking similarities between the electroclinic effects in SmA* and L α * there are also specific deviations, namely, the slow decay of the electro-optic response in Figure 24b, after switching and the non-linearity at low field strength in Figure 24a. The experiments in [69] indicate that these deviations are related to the comparatively high electric conductivity of lamellar phases which originates from the inevitable presence of ionic impurities in the highly polar and protic solvent layers. Under the action of an electric field, these ionic impurities accumulate at the interfaces between the solid electrodes and the liquid-crystalline electrolyte and form electric double layers which screen the external electric field, and thus reduce the effective field inside the liquid crystal layer.…”

“…direction normal to the layer normal k. (b) Corresponding electro-optic response of the lyotropic samples between crossed polarizers measured by the transmitted light intensity I(t) at a temperature of 0.2 K above the transition into the tilted phase; black line: chiral L * phase of 4 with 23 wt% of formamide; red line: nonchiral L phase of racemic 4 with 23 wt% of formamide. (Reprinted by permission from [69] Copyright 2018 by the American Physical Society.) All in all, 40 years after the discovery of the electroclinic effect in thermotropic SmA* it has now become clear that the same effect exists in chiral lyotropic L * phases as well.…”

“…(a) Square-wave electric field E(t) applied to lyotropic L ( * ) samples in a direction normal to the layer normal k. (b) Corresponding electro-optic response of the lyotropic samples between crossed polarizers measured by the transmitted light intensity I(t) at a temperature of 0.2 K above the transition into the tilted phase; black line: chiral L * phase of 4 with 23 wt% of formamide; red line: nonchiral L phase of racemic 4 with 23 wt% of formamide. (Reprinted by permission from[69] Copyright 2018 by the American Physical Society. )…”