Chiral symmetry breaking by spatial confinement in tactoidal droplets of lyotropic chromonic liquid crystals

Abstract

In many colloidal systems, an orientationally ordered nematic (N) phase emerges from the isotropic (I) melt in the form of spindle-like birefringent tactoids. In cases studied so far, the tactoids always reveal a mirror-symmetric nonchiral structure, sometimes even when the building units are chiral. We report on chiral symmetry breaking in the nematic tactoids formed in molecularly nonchiral polymer-crowded aqueous solutions of low-molecular weight disodium cromoglycate. The parity is broken by twisted packing of self-assembled molecular aggregates within the tactoids as manifested by the observed optical activity. Fluorescent confocal microscopy reveals that the chiral N tactoids are located at the boundaries of cells. We explain the chirality induction as a replacement of energetically costly splay packing of the aggregates within the curved bipolar tactoidal shape with twisted packing. The effect represents a simple pathway of macroscopic chirality induction in an organic system with no molecular chirality, as the only requirements are orientational order and curved shape of confinement.parity breaking | twisted tactoids | geometrical anchoring | nematic-isotropic coexistence I n 1949, Onsager demonstrated theoretically that a dispersion of long rigid rods develops an organized state, a so-called nematic (N) with rods being parallel to each, once their concentration exceeds some critical value (1). This remarkable collective behavior with an establishment of nonpolar axis of alignmentn ¼ −n, called the director, stems from the molecular symmetry alone, as the only requirement of the Onsager model is that the rods do not overlap. The theory was inspired by Bernal and Fankuchen's experiments (2) on aqueous suspensions of tobacco mosaic viruses (TMV) that phase separated into an isotropic (I) phase with a relatively low concentration of TMV rods and an N phase with a high concentration of TMV. The N domains emerged as spindle-like "tactoids" (3, 4) with the axes of rods following the meridians of the spindles. Besides TMV dispersions (2, 5), the N tactoids have been experimentally documented in water dispersions of many other materials, such as vanadium pentoxide (3, 4, 6, 7), bohemite rods (8), fd viruses (9, 10), f-actin (11), carbon nanotubes (12), as well as in lyotropic chromonic liquid crystals (LCLCs) (13,14).Reversible chromonic assembly and ensuing LCLC mesomorphism is displayed broadly by dyes, drugs (15-18), nucleotides (19,20), and DNA oligomers (21,22). Typically, the LCLC molecule is plank-like with aromatic cores and peripheral polar groups. In water, the polyaromatic cores stack face-to-face, forming elongated charged aggregates. The aggregates, self-assembled through weak noncovalent interactions, are polydisperse, with the length distribution that depends on concentration, temperature, ionic strength, and presence of crowding agents. At low concentrations, the aggregates are short and orient randomly. As the volume fraction increases, the aggregates elongate, multiply, and eventually some frac...

Stirring competes with chemical induction in chiral selection of soft matter aggregates

“…In particular, one anticipates that achiral substances can be transformed, at most, into a racemic mixture of equally represented enantiomers, or into an enantiopure system with equal probability for the two enantiomers. Apart from the handedness imparted by proto-chiral organizing surfaces 3 , oriented reacting interfaces 4 , and aligning 5 or confining 6 boundaries, the balance above may be broken under the intervention of one within a few physical forces of signed helicity 7,8 , or through the effect of a chiral chemical modifier. In general terms, there is still the pending issue of whether a chiral physical force may be opposed in an effective way to a chiral chemical intervention.…”

Elasticity-dependent self-assembly of micro-templated chromonic liquid crystal films

“…29 Indeed, studies have demonstrated that LCLCs in micro-scale confinement form unique, elastic-property-dependent configurations. 30,31 In this contribution we manipulate the patterning of LCLC films within micropost arrays by modulating the LCLC mesophases and elastic properties via concentration changes during a drying process. We discover a preferred alignment in nematic films governed by elastic interactions with postplacement geometry, and we observe multiple fascinating stable patterned states of the columnar phase.…”

Chiral structures from achiral liquid crystals in cylindrical capillaries

“…Elasticity-driven chiral symmetry breaking is perhaps most readily manifested in confined LCs (31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43), wherein surface anchoring imposes a preferred angle for LC mesogens at the interface of the confining boundary. Topological defects enforced by boundary conditions can play a key role in the symmetry breaking as well, because energetically costly deformations are often concentrated in the vicinity of the defects (35)(36)(37).…”

“…Liquid crystals (LCs) are soft materials composed of anisotropic mesogens that provide remarkable examples of chiral symmetry breaking arising from elastic anisotropy (29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42). In essence, an LC can minimize elastic free energy by organizing its achiral units into chiral structures, such as helices and chiral layers, that incorporate twist deformation (7,8).…”

“…In essence, an LC can minimize elastic free energy by organizing its achiral units into chiral structures, such as helices and chiral layers, that incorporate twist deformation (7,8). The elastic free energy describing nematic LC deformations depends on so-called splay, twist, bend, and saddle-splay elastic moduli, and when twist deformation is comparatively easy, twisting can relieve strong splay and/or bend deformation and lead to production of equilibrium chiral structures (29,32,(35)(36)(37)(38). Similarly, saddlesplay deformation can stabilize chiral structures (43)(44)(45)(46).…”