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.