Molecular packing in two bicontinuous Iad gyroid phases of calamitic cubic mesogens BABH(n): roles in structural stability and reentrant behavior

Abstract

1,2-Bis(4′-n-alkoxybenzoyl)hydrazine [BABH(n), n is the number of carbon atoms in the alkyl chain] exhibits two different Iad 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 Iad 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 Iad 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 Iad 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 Iad structures.