Formation of Columnar and Cubic Mesophases by Calamitic Molecules:  Novel Amphotropic Biphenyl Derivatives

Novel amphiphilic 4,4'-disubstituted biphenyl derivatives incorporating a hydrophilic 5,6-dihydroxy-3-oxahexyloxy group and a lipophilic chain have been synthesized by Pd-catalyzed cross-coupling reactions of phenyl boronic acids with 4-(5,6-dihydroxy-3-oxahexyloxy)bromobenzenes. The thermotropic and lyotropic mesomorphic behavior of these compounds were investigated by polarizing microscopy, by differential scanning calorimetry, and by means of X-ray diffraction. Thereby the influence of the length of the alkyl chain, the type of connecting unit between alkyl-chain and the rigid core (O vs CH2), and the influence of lateral substituents and of the molecular chirality on the mesomorphic properties were investigated. All compounds have smectic A-phases as the high-temperature mesophases. Additionally, columnar phases were found for the long chain 4'-alkoxy-4-(5,6-dihydroxy-3-oxahexyloxy)biphenyls 2c-f without lateral substituent and for the 4-(5,6-dihydroxy-3-oxahexyloxy)-4'-dodecyloxy-3'-methylbiphenyl 10. All other laterally substituted biphenyl derivatives 9 and 11-15 and the 4'-alkyl substituted compound 8 display only smectic phases. The influence of protic solvents (formamide and ethylene glycol) on the liquid crystalline properties was studied. The columnar mesophases and Sc-phases are destabilized by addition of protic solvents whereas the S-A- phases are stabilized. Furthermore, binary mixtures of the amphiphilic biphenyl derivatives with each other have been investigated. Complete miscibility was found for the SA-phases and the columnar phases of the compounds 2c-f. However the columnar phases of the nonsubstituted compounds 2c-f and the 3'-methyl substituted derivative 10 do not have an uninterrupted miscibility. Cubic phases (Pn3m or Pn3) and additional columnar phases were induced in the contact regions. Thus, the same diversity of different kinds of mesophases as in lyotropic systems and in the thermotropic phase sequence of polycatenar compounds is found. It is proposed that the cubic and columnar mesophases of these rodlike molecules result from the collapse of the smectic layer structures into ribbons.