Spontaneous mirror symmetry breaking in benzil-based soft crystalline, cubic liquid crystalline and isotropic liquid phases

Abstract

Benzil (diphenylethane-1,2-dione), which is a long known example for an achiral molecule crystallizing in a chiral space group, can also show mirror symmetry breaking in the fluid state if it is suitably functionalized. For some of the new benzil derivatives even three different subsequent mirror symmetry broken soft matter states with a chiral conglomerate structure can be observed. One is an isotropic liquid, the second one a cubic liquid crystal with a complex network structure and the third is a soft crystalline solid. Chirality develops by helical self-assembly combined with dynamic network formation, thus allowing macroscopic chirality synchronization. These achiral molecules, combining a transiently chiral bent core with multiple alkyl chains, provide a unique link between the mirror symmetry breaking phenomena observed for polycatenar and bent-core mesogens. The homogeneously chiral networks are of interest for application as chiral materials, and as templates for chiral recognition, separation and enantioselective catalysis.

Fig. 1. Schematics showing (a) the transient chirality of benzil derived molecules; and (b) the development of the helical twist by clashing of the bulky end groups attached to the cores and (c and d) the network structure of the Cub_bi phases under discussion. The benzil based polyaromatic cores form the helical networks which are embedded in the continuum filled by the terminal chains. (c and d) were reproduced from ref. 25 by permission of The Royal Society of Chemistry.

Fig. 2. (a and b) SAXS diffractograms of the I23^[*^] (3/4) and the Ia3̄d_(S) (3/16) phases (for the synchrotron source and for numerical data, see Tables S4 and S5; the full indexations of the I23^[*^] phase and the WAXS scans are shown in Fig. S7†); (c and d) reconstructed electron density maps of the I23^[*^] and Ia3̄d_(S) phase; the aromatic cores are located in the networks formed by the green iso-surfaces, the space between is filled by the alkyl chains; for more details, see the ESI, and Fig. S8 for the individual networks; (e–h) the textures of the (e) SmC phase of 3/10 at 230 °C; (f) the Cub_bi/I23^[*^] phase of 3/10 at 121 °C and (g) Cub_bi/Ia3̄d_(S) phase of 3/12 at 120 °C, as observed between slightly uncrossed polarizers; (h) Col_hex phase of 3/10 at 141 °C; the inset shows the texture with an additional λ-plate, indicating that the phase is optically negative, i.e. the orientation of the aromatic cores is perpendicular or only slightly tilted to the column long axis; (i–k) show the induced chiral I23^[*^] phase in the contact region between the achiral Ia3̄d_(L) phase of 3/2 (top) and the Ia3̄d_(S) phase of 3/16 (bottom); the orientation of the polarizers is shown by white arrows; the width of the POM images in (e–k) is 200 μm, and in (f), (i) and (k) the contrast is enhanced; for additional textures, see also Fig. S2–S6.

Fig. 3. (a and b) sections of the DSC traces (10 K min⁻¹) of compounds 3/12 and 3/14 (see Fig. S1 for complete traces); (c) plot of the d-values (black) and the correlation length (ζ, blue) of the small angle scattering of 3/12 in the isotropic liquid phases (see also Fig. S10 and S11†); (d) sketch of the fused helical clusters in the Iso₁^[*^] phase; (e) schematic sketch showing the transition from Iso via a cybotactic and a percolated liquid to Cub_bi by increasing the transient network connectivity; the dots represent locally ordered clusters, the lines indicate the connections between them, and the vertical dotted lines indicate phase transitions.

Fig. 4. The mirror symmetry broken mesophases of compound 3/6. (a–f) Conglomerates of optically active domains: (a and b) in the liquid Iso₁^[*^] phase at 120 °C; (c and d) in the LC Cub_bi/I23^[*^] phase at 110 °C and (e and f) in the soft crystalline Cr_Iso^[*^] phase at 60 °C, as observed on cooling between slightly uncrossed polarizers rotated by 5°; (a, c and e) in the anticlockwise and (b, d and f) in the clockwise direction (contrast enhanced). (g) DSC heating and cooling traces (10 K min⁻¹); (h) small angle and (i) wide angle XRD pattern in the Cr_Iso^[*^] phase at 50 °C (see also Fig. S12 for the complete diffraction pattern and Fig. S13 for the diffraction pattern of 3/10).