Controlling Mirror Symmetry Breaking and Network Formation in Liquid Crystalline Cubic, Isotropic Liquid and Crystalline Phases of Benzil-Based Polycatenars

Abstract

Spontaneous development of chirality in systems composed of achiral molecules is important for new routes to asymmetric synthesis, chiral superstructures and materials, as well as for the understanding of the mechanisms of emergence of prebiotic chirality. Herein, it is shown that the 4,4'-diphenylbenzil unit is a universal transiently chiral bent building block for the design of multi-chained (polycatenar) rod-like molecules capable of forming a wide variety of helically twisted network structures in the liquid, the liquid crystalline (LC) and the crystalline state. Single polar substituents at the apex of tricatenar molecules support the formation of the achiral (racemic) cubic double network phase with Ia $\bar{3}$ d symmetry and relatively small twist along the networks. The combination of an alkyl chain with fluorine substitution leads to the homogeneously chiral triple network phase with I23 space group, and in addition, provides a mirror symmetry broken liquid. Replacing F by Cl or Br further increases the twist, leading to a short pitch double gyroid Ia $\bar{3}$ d phase, which is achiral again. The effects of the structural variations on the network structures, either leading to achiral phases or chiral conglomerates are analyzed.

Keywords: chirality; cubic phases; liquid crystals; mirror symmetry breaking; soft matter.

Figure 1

Schematics showing the networks of a) the Cub_bi/Ia $\bar{3}$ d phase and b) the triple network I23^[*^] phase; c) shows the transient chirality of the benzil unit and d) the development of the helical twist by clashing of bulky end groups attached to the cores. ^[44] The polyaromatic cores are located in the networks and aligned almost perpendicular to the network directions, the continuum between them is filled by the terminal alkyl chains; b) was reproduced from ref. [31] by permission of The Royal Society of Chemistry.

Scheme 1

Structures of the BABH and ANBC based rod‐like hydrogen bonded molecules and supramolecules, and an example of a polycatenar mesogens forming Cub_bi phases.

Scheme 2

Structures of the compounds under investigation, for details of the structures, see Tables 1–4.

Scheme 3

Synthesis of the benzil‐based compounds 2–5. Reagents and conditions: (i) THF, sat. NaHCO₃ solution, [Pd(PPh₃)₄], reflux; (ii) SOCl₂, abs. pyridine, DCM, DMAP, 25 °C.

Figure 2

POM images as observed between crossed (middle) and slightly uncrossed polarizers (left, right) for a–c) the I23^[*^] phase of 1/10 at 105 °C as obtained after cooling from the Iso₁ ^[*^] phase and d–f) the achiral Ia $\bar{3}$ d phase of 3/²Br6 at 70 °C as observed on cooling from Iso₁.

Figure 3

DSC of compound 4/14 showing the Iso‐Iso₁‐Cub_bi/Ia $\bar{3}$ d transition ranges; full heating and cooling scans are shown in Figure S1 a, the DSCs of 4/16 and 4/18 are shown in Figure S1 b,c.

Figure 4

Textures of a) 2/CN at T=147 °C and b) 2/NO₂ at T=167 °C in planar aligned samples showing the growth of the Cub_bi/Ia $\bar{3}$ d phase (dark area) into the fan textures of the birefringent SmA phases on cooling; the insets show the homeotropic textures of the SmA phases at 150 and 170 °C, respectively; the arrows indicate the orientation of polarizer and analyzer.

Figure 5

a) Space filling models of compound 2/CN and b) its antiparallel pair with fully intercalated aromatic cores.

Figure 6

Development of chiral conglomerates of the induced I23^[*^] phase in the contact regions between the achiral Ia $\bar{3}$ d phases of a–c) 1/14 (Ia $\bar{3}$ d _(S)) ^[44] and 2/CN at 149 °C, d‐f) 1/14 and 2/OCF₃ at 121 °C and g–i) 1/14 with 2/F₃ at 126 °C, at the left and right between slightly rotated polarizers and in the middle between crossed polarizers. The induction of a I23^[*^] ribbon indicates that both Ia $\bar{3}$ d phases should have different structure, that is, that they represent Ia $\bar{3}$ d _(L) phases for compounds 2/X. The absence of any birefringence between crossed polarizers indicates the absence of any induced birefringent non‐cubic mesophases and the inversion of the brightness by inverting the direction of the analyzer indicates the presence of a chiral conglomerate in the I23^[*^] ribbons; the phase boundary in a, c) represents the Iso₁–Cub_bi transition of the 1/14+2/CN mixture.

Figure 7

Representative DSC heating and cooling traces of compounds 2/X showing a) the Iso–Iso₁–SmA–Cub_bi/Ia $\bar{3}$ d _(L) transition of 2/NO₂ and b) the Iso–Iso₁–Cub_bi/Ia $\bar{3}$ d _(L) transition of 2/OCF₃; for full scans and the DSC traces of the other compounds, see Figure S1 d,g.

Figure 8

a–c) Representative DSC heating and cooling traces of a) compound 1/6 and b,c) the laterally core halogenated compounds 3/Y6; ^[44] a,b) Iso–Iso₁–Iso₁ ^[*^]‐Cub_bi/I23^[*^] transitions of compounds a) 1/6 and b) 3/F₂6 and c) Iso–Iso₁–Cr_Iso transition of compound 3/²Br6; for full scans and the DSC traces of 3/F₂6 and 3/²Br6, see Figure S1 o,q. d) Schematic sketch showing the transition from Iso via a cybotactic and a percolated liquid to Cub_bi by increasing transient network connectivity, the dots represent locally ordered clusters and the lines indicate the connections between them, the vertical dotted lines indicate phase transitions.

Figure 9

Investigation of compound 3/²F6. a,b) optically active domains in the Iso₁ ^[*^] phase at 100 °C and c,d) in the Cr_iso ^[*^] phase at 60 °C, as observed between slightly uncrossed polarizers after rotation in a), c) anticlockwise and b), d) clockwise direction (contrast enhanced). The different degrees of intermediate brightness result from overlapping with surface films having opposite chirality; e) shows the complete DSC heating and cooling traces (the section indicated by the dashed rectangle is shown in Figure S1 m); f) small angle and g) wide angle X‐ray scattering pattern in the Cr_iso ^[*^] phase at 25 °C (see also Figure S5 for the complete pattern).

Figure 10

Temperature dependence of the d‐value of the small angle scattering and the correlation length (calculated with the Scherrer equation) of the clusters in the Iso phases of compound 3/F₂6, measured on cooling; for the DSC traces see Figure 8 b; the lift‐off temperature of the broad DSC feature on cooling is around 147 °C, see Figure S5 for the related curve of 3/³F6.

Figure 11

Summary of the relations between molecular structure, cubic phase type, cubic phase stability and mirror symmetry breaking in the LC and isotropic liquid phases.