Stabilization of Bicontinuous Cubic Phase and Its Two-Sided Nature Produced by Use of Siloxane Tails and Introduction of Molecular Nonsymmetry.

A recent intriguing finding that a helical network arrangement forms the bicontinuous cubic phase is attracting great attention for the possibility of new routes to asymmetric synthesis by achiral molecules. However, the design of the molecular structure for the cubic phase is still unrevealed. In this work, a nonsymmetric core molecule with larger naphthalene and smaller benzene moieties at each side of the central linkage and the same disiloxanyldecyloxy terminal at both terminals is shown to be the first example of molecule forming both single-layered and double-layered core assembly modes in the Ia3d phase as a single molecule system. The molecule forms the former mode at high temperatures as a thermodynamically stable phase, similarly to the symmetric naphthalene core system, whereas, on cooling below a temperature (∼350 K), a metastable Ia3d phase forms a double-layered core state down to room temperature, which is common to the benzene core system. As another effect of the nonsymmetric core, the cubic phase is maintained at room temperature for more than 100 days with slight distortion. Infrared spectral studies and quantum chemical calculations suggested the easy transformation between the two core assembly modes. The core nonsymmetry can be a versatile fine-tuning of the core assembly mode and phase stability for the cubic phase molecules.

Molecular design of anti-spindle-like molecules by use of siloxanyl terminals for a thermotropic bicontinuous cubic phase.

Selecting 1,2-bis(aryloyl)hydrazine as a model molecular framework, this article examines how the combined modification of two molecular moieties, i.e., variation of the molecular core motif (benzene B or naphthalene N as two aromatic rings) and use of bulky and flexible siloxane segments (disiloxane Si2, trisiloxane Si3, or its branched type iSi3) at the end of both chains, affects the phase behavior including the cubic (Cub) phases such as well-known achiral Ia3d or the so-called "Im3m" phase mostly recognized as a chiral one. It was found that the use of a naphthalene core as a larger core effectively provides the compound with improved thermal stability, and the clearing temperature in the N series is ca. 50 K higher than that of the B series. On the other hand, the introduction of siloxane segments at their terminals is effective for lowering the LC-phase temperature range by several tens of K. Focusing on the Cub phases, only the introduction of the disiloxane Si2 segment was useful for their formation, and we envisioned how much degree the anti-spindle shape of the average molecular shape is in the Ia3d phase. The use of the trisiloxane Si3/iSi3 segment primarily led to the formation of columnar (Col) phases. Consideration of the chemical composition revealed that the delicate balance between the three molecular moieties, siloxane terminal, alkyl spacer, and aromatic core part, is critical for the Cub phase formation, and it can be summarized as the threshold weight fraction of the alkyl spacer in the three moieties being 0.284 or larger, which derives an anti-spindle shape favorable for the formation of the Ia3d-Cub phase.

Stabilization of the bicontinuous cubic phase in siloxane-terminated mesogens, 1,2-bis[4'-(n-(oligodimethylsiloxyl)alkoxy)benzoyl]hydrazine.

The introduction of oligodimethyl siloxane segments at the termini of the alkyl tails has been employed to stabilize the bicontinuous cubic (Cub(bi)) phase of a chain-core-chain type molecule having a 1,2-bis(benzoyl)hydrazine central core with two chains attached at the 4' position of each benzoyl moiety. In this study, three silylated molecules, bis-C10Si2, bis-C10Si3, and C10Si2-C8C=C, were synthesized, where "CnSim" represents the number of carbon and silicon atoms in the chain and "bis" indicates the two chains being the same, whereas the last one is asymmetric with respect to the core. The phase behaviors were examined by using polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction techniques. All three compounds form Cub(bi) phases and their syntheses were compared including their parent compound bis-C18. It was clearly revealed that the introduction of oligodimethyl siloxane segments effectively suppresses the crystallization at low temperatures, and as a result stabilizes the Cub(bi) phases, in an extreme case down to room temperature. The semi-quantitative analyses in terms of lattice constant and three-dimensional electron density map help us to better understand the self-assembly process in the Cub(bi) phases. The study also revealed that the introduction of oligodimethyl siloxane segments is not only an alternative for the hydrocarbon segment but also is able to provide a versatile design strategy for obtaining stable Cub(bi) phases.