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

Abstract

Spontaneousd evelopment of chirality in systems composed of achiral molecules is important for new routes to asymmetricsynthesis, chiral superstructures and materials, as well as for the understanding of the mechanisms of emergence of prebiotic chirality.H erein, it is shown that the 4,4'diphenylbenzil unit is au niversal transiently chiral bent building block for the design of multi-chained (polycatenar) rod-like molecules capable of forming aw idev ariety of helically twisted network structures in the liquid,t he 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 Ia3 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 am irror symmetry broken liquid.R eplacing Fb yC lo rB rf urtheri ncreasest he twist,l eading to as hort pitch doubleg yroid Ia3 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.

Swallow‐Tailed Polycatenars: Controlling Complex Liquid Crystal Self‐Assembly and Mirror Symmetry Breaking at the Lamellae‐Network Cross‐Over

“…The cubic lattice type can be deduced from the relative positions of the most intense small angle scatterings either corresponding to the (211) and (220) indices of the Ia d lattice (Figure 7a and Figure S20a, Supporting Information) or the (321), (400) and (420) indices of the I 23 lattice (Figure 7b and Figure S21a, Supporting Information). [ 15,27a,28–31 ] The assignment to the distinct lattices is for polycatenar compounds easily confirmed by optical investigations between not fully crossed polarizers, where the I 23 phase shows a conglomerate of dark and bright domains which invert their brightness by rotating the analyzer into the opposite direction (Figure 7e). [ 27a,28 ] This indicates the presence of chiral domains as typical for the triple network I 23 structure (Figure 7b).…”

“…This shows that a certain minimum chain length (or chain volume) is required for the formation of the mirror symmetry broken Cub bi [ * ] / I 23 phase. [ 15b,27–32 ] Only in one case, for the compound with the longest total chain length ( B10/22 ), a columnar phase (Col ob ) accompanies the Cub bi [ * ] / I 23 phase as a high temperature phase, resulting from the further increased interface curvature provided by this long chain (see Table 1 and Figure S11, Supporting Information). However, the Col ob phase is metastable and can only be observed on cooling, whereas on heating a direct Cub bi [ * ] / I 23‐Iso transition takes place.…”

“…This chiral liquid formed by achiral molecules is considered as a percolated liquid with a dynamic helical network structure, which provides a long range transmission of uniform chirality in the enantiomeric domains of the conglomerate, but without assuming a long range cubic lattice. [ 29,30–32,34a ] The helical self‐assembly is supported by transiently chiral helical molecular conformations, as provided by the twisted 5,5′‐diphenyl‐2,2′‐bithiophene core of the reported compounds. [ 32,34a ]…”

“…As shown in Figure 8c, the Iso‐Iso 1 [ * ] transition is associated with a small enthalpy in the DSC traces which is attributed to the chirality synchronization. [ 30,31 ] Previous temperature dependent SAXS experiments have indicated a continuously decreasing line width, and hence, growing correlation length of the helical aggregates around the continuous Iso‐Iso 1 [ * ] transition, and a jump to resolution limited Bragg peaks at the discontinuous Iso 1 [ * ] to Cub bi transition. [ 31,34a ] The Iso‐Iso 1 [ * ] transition was also investigated by optical rotation and circular dichroism.…”

“…[ 23 ] There is also a Cub bi phase with more complex triple network structure (previously designated as Im m ) which is only formed in systems involving rod‐like units, the polycatenars being the most prominent, and different models are under discussion, as will be described in Section 3.6. [ 24–26 ] The recent discovery of helix formation, spontaneous mirror symmetry breaking, optical activity and enormous chirality amplification effects, observed in some of the Cub bi phases, [ 27–32 ] in the adjacent non‐cubic tetragonal [ 33 ] and the isotropic liquid phases (Iso 1 [ * ] ) of these achiral polycatenar compounds [ 34–36 ] has renewed the interest in this class of compounds. [ 37–39 ] Such spontaneously chiral soft matter systems are of potential technological interest and might also be of importance for the understanding of the mechanism of emergence of uniform chirality.…”