Understanding of Molecular Packing in Thermotropic Cubic Phase and Control of the Formation

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The aim of this research project is, first, to understand the molecular packing structure in the thermotropic cubic (Cub) mesophase and, second, to elucidate the phase transition mechanism from a lamellar smectic C (SmC) to Cub phase, and, third, to control the formation of the Cub phase. We focused attention to two homologues, 4'-n-alkoxy-3'-nitrobiphenyl-4-carboxylic acids (ANBC) and 1,2-bis(4'-n-alkoxybenzoyl)hydrazines (BASH). In the two homologous series, the basic units constructing the SmC and Cub phases have a common structure, consisting of an aromatic core moiety with a hydrogen-bonding part at the center and an alkyl tail at each end. The following insights were obtained :For the two homologous series, the phase diagrams as functions of temperature, pressure, and alkyl chain length have been established. The ANBC series shows two types of Cub phases, Ia3d-type and Im3m-type, above the temperature region of the SmC phase. The molecular packing models of both Cub phases were proposed. The BASH series shows also SmC and Cub phases, but the SmC phase are formed only when the number of carbon atoms in the alkoxy chain (n) is 8, 9, or 10. The phase type of the Cub phase belongs to Ia3d for n=7-12. Comparing between the phase behaviors of the two series, the roles of the alkyl tail and central hydrogen-bonding part for the Cub phase formation have been revealed. That is, a key origin for the Cub phase formation is temperature-induced packing frustration within SmC layers due to unsymmetrical shape at the interface between the alkyl tail vs aromatic core parts, just like apolar vs. polar competition in lyotropic systems, and therefore, the quasi-binary picture is quite effective for understanding the whole behaviors. In the thermotropic systems, however, the presence of other contributions of intermolecular interactions such as hydrogen-bonding, dipole-dipole interactions etc. is also important and modifies the phase formation.