Systematic exploitation of thermotropic bicontinuous cubic phase families from 1,2-bis(aryloyl)hydrazine-based molecules

Abstract

Rational design of molecules that exhibit a thermotropic bicontinuous cubic (Cub) phase has been earnestly desired. In this work, we describe the suitable selection of a molecular motif that has enabled the systematic exploitation of eight new series of Cub-phase molecules with symmetric molecular cores, N-n (1), PB-n (2), S-n (3), and PEB-n (4), and unsymmetric cores, B-N-n (5), B-PB-n (6), B-S-n (7), and B-PEB-n (8). These eight series all originate from achiral chain-core-chain type rod-like molecules that exhibit two types of Cub phases, an achiral Ia3d phase, and a chiral phase. All the Ia3d phases formed were found to be isomorphous structures, with their cell dimensions being proportional to the core size, and the same was true for the latter chiral phase. We demonstrated that the formation is mainly governed by the segregation between core and alkyl moieties of the molecules, and thus, by the weight fraction of the core portion f. This work also demonstrates that the central dicarbonylhydrazine linkage bearing intermolecular hydrogen bonding ability exhibits a pinning effect that prevents slippage of π-stacks of molecules, which is critical for the formation of the two Cub phases that are composed of chiral networks with twisted molecular arrangements. In each series, the emergence of spontaneous chirality formation that occurred in the chiral phase was limited to between 0.36 and 0.50 in the range of f. An interesting insight was that the introduced unsymmetry of the molecular core strongly influenced the phase behavior, which lowered the temperature range of Cub phases to around that of the smallest core series B-n, while the high temperature limit (T) was roughly proportional to the core size, as determined by the strength of intermolecular π-π interactions.

Helical Networks of π‐Conjugated Rods – A Robust Design Concept for Bicontinuous Cubic Liquid Crystalline Phases with Achiral Ia3¯d and Chiral I23 Lattice

“…They can deviate from these values, for example, if the alkyl chain at the apex is especially long (X = OC 10 H 21 ), or as the length and shape of the core is changed, though the fundamental phase sequence remains the same. [117] Depending on the contribution of the substituent to the volume effect and the twist, the lattice parameter can increase, as observed for the BABHs and ANBCs, [101,103,104,107,108] remain almost constant, as reported here for compounds 3/ m X n and 3a/Y, or even decreases with increasing volume of the substituents, as recently reported for benzil-based polycatenars. [154] However, in all cases, the twist angle increases with the apex volume, leading to the sequence Ia3 d (L) -I23-Ia3 d (S) .…”

“…For example, robust bicontinuous cubic phases were even found for polyaromatic rods with only two alkyl chains (Figure 9a, e.g., BABHs [107,108] and the hydrogen-bonded dimers of the ANBCs, see Figure 1c,d). [16,101,103,104] In these cases, alkyl chain intercalation or replacing the alkyl chains by perfluorinated chains [141,142] or by adding bulky silyl groups [74,108] provides the required interfacial curvature for Cub bi phase formation. Permanent molecular chirality can further support the helical twist, leading to a series of chirality frustrated LC phases.…”

“…[11][12][13][14][15][16] Therefore, Cub bi phases are usually not predictable and appear only in narrow temperature ranges for few well designed compounds (Figure 1c-f). The most prominent are rod-like hydrogen bonded dimers of 4′-n-alkoxy-3′-nitrobiphenyl-4-carboxylic acids (ANBCs), [100][101][102][103][104] the dibenzoylydrazines (BABHs) [105][106][107][108][109][110][111] (Figure 1c,d), [16,17,108,110] and tetracatenar molecules, which can have either a symmetric or a nonsymmetric distribution of the alkyl chains (Figure 1e,f). [112][113][114][115][116][117][118][119][120][121][122][123][124] Nevertheless, even these compounds are very sensitive to minor molecular structural variations.…”

Mirror symmetry breaking in liquids and liquid crystals

“…The required twist is 70°or 250°(π+ 70°) for the Ia3d and 180°for the Im3m [ * ] phases. This explains the phase sequence Ia3d -Im3m [ * ] -Ia3d with re-entrance of the Ia3d phase upon chain elongation usually observed for the cubic phases of rod-like molecules with growing size of the terminally attached alkyl chains [225]. The proper twist can be engineered by molecular design (length and number of terminal alkyl chains) [99,226] or by mixing compounds with short and long chains [8,99].…”

“…Thus the benzoates of diphenylbithiophene (4, Figure 9 (a)) and 15 ( Figure 23) have turned out to be the most efficient presently known, but also phenylbenzoates and benzoates of azobenzenes [233][234][235] are capable of forming the Iso [ * ] phases. In contrast, molecules with shorter core structure or reduced helicity, as for example the compounds 20 and 21 ( Figure 23) [225], though showing chirality in the Im3m [ * ] phases are not capable of forming the Iso [ * ] phases [8,99].…”