Dark conglomerate phases of bent-core liquid crystals

Abstract

Spontaneous or induced chiral symmetry breaking in achiral systems is unusual and understanding the origin of such a phenomenon has been an important area of research for several years. The optically isotropic mesophases exhibited by unconventional liquid crystals are one of the most interesting systems to investigate spontaneous chiral symmetry breaking in liquid crystal mesophases formed by achiral moieties. The dark conglomerate (DC) phases are one such optically isotropic family of phases. In this paper, a detailed account of the tendency of bent-core mesogens to form a variety of polar smectic phases, the formation of DC phases due to layers deformations and the general optical, electrical, physical properties of the DC phases are given. An example of a dark conglomerate phase which exhibit distinct electro-optic properties is described with the nature of dynamics of the response and physical reasons responsible for such behaviour. The challenges and prospects of the dark conglomerate phases are discussed for their potential applications in novel devices.

Emergent chirality in achiral liquid crystals: insights from molecular simulation models of the behaviour of bent-core mesogens

“…We know also that achiral bent-core mesogens can form the twist-bend nematic ðN TB Þ phase [13], where molecules form a short pitch local heliconical organisation, similar to that seen in the N TB phase of achiral liquid crystal dimers [14][15][16][17][18] and the related SmC TB phase [19]. Moreover, for achiral bent-core molecules, the presence of spontaneous saddle splay deformation is sufficient to prevent long-range orientational or translational order, leading to the formation of structures such as the dark conglomerate phase, which exhibits large homochiral domains [20][21][22]. Whilst spontaneous racemisation of a crystal growing from a solution of two distinct enantiomeric forms seems a beautiful natural manifestation of chiral interactions, chirality arising from strictly achiral molecules in a fluid, in which molecules are able to sample a wide range of different conformations, seems truly remarkable!…”

Mirror symmetry breaking in liquids and liquid crystals

“…There is a large number of reports about fluid mirror symmetry broken smectic LC phases of bent-core molecules that are optically isotropic and form macroscopic chiral domains, therefore designated as dark-conglomerate (DC) phases (Figures 4(b,c) and 5(b,c)) [35,36,39,54,55]. Though also the HNF/HNC phases can form optically isotropic (dark) conglomerates we reserve the designation 'DC' here for the fluid DC phases.…”

“…For example, an anticlinic and antiferroelectric SmC a P A structure, instead of the synclinic ferroelectric SmC s P F structure, was suggested for DC phases formed by some oxadiazole based LC compounds (e.g. compound 3 in Figure 6(a)) [55]. Low twist and bend elastic constants (K 22 and K 33 , respectively, see Figure 6) [79] appear to be responsible for layer distortion in oxadiazole based DC phases, whereas for the silylated compounds layer distortion by steric effects, layer decoupling by the formation of nano-segregated silyl layers and developing polar order appear to be more important [69].…”

Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens

“…Rigid layers break into helical nano-ribbons (helicaln anofilamentp hases, HNF,B 4p hases, Figure 1f) [33][34][35][36][37][38] and soft layers form more disordered sponge-like structures, the so-called dark conglomerate phases (DC-phases, Figure 1g). [12,25,30,[39][40][41][42][43][44] Herein we focus on helicity occurringa long the layer normal (longitudinal twist between the short, secondary,m olecular axes, Figure 1a)a nd its effects on the LC self-assembly of bent molecules. The investigated compounds 1/n (Scheme 1) are based on the 1408 bent 4-cyanoresorcinol core (a = 1408)w ith ar educed molecular bent comparedt ot he 1208 angle of ordinary bent-core molecules.…”