Molecular packing in two bicontinuous Ia3[combining macron]d gyroid phases of calamitic cubic mesogens BABH(n): roles in structural stability and reentrant behavior.

1,2-Bis(4'-n-alkoxybenzoyl)hydrazine [BABH(n), n is the number of carbon atoms in the alkyl chain] exhibits two different Ia3[combining macron]d cubic phases depending on the chain length (5 ≤n≤ 13 and 15 ≤n≤ 22). The molecular packing modes, not only of molecular cores but also of alkyl chains, are investigated through a maximum entropy method (MEM) and an analysis of the chain-length dependence of the body diagonal of the unit cell. The analyses revealed the difference in molecular packing. The short-chain Ia3[combining macron]d structure of BABH(n) (5 ≤n≤ 13) is constructed by single-layered core motifs and bilayers of alkyl chains, in which the orientation is tilted from the normal to the layer. The long-chain Ia3[combining macron]d structure (15 ≤n≤ 22) is formed by double-layered core motifs and monolayers of alkyl chains, in which the orientation is normal to the layer. Based on the molecular packing modes, the reentrant behavior between the two Ia3[combining macron]d phases was clarified. It was revealed that the alkyl chains of BABH(n) molecules play an essential role in the formation and stability of the two Ia3[combining macron]d structures.