FORMATION OF COLUMNAR AND CUBIC MESOPHASES BY CALAMITIC MOLECULES - NOVEL AMPHOTROPIC BIPHENYL DERIVATIVES

Novel amphiphilic 4,4'-disubstituted biphenyl derivatives incorporatin g a hydrophilic 5,6-dihydroxy-3-oxahexyloxy group and a lipophilic cha in have been synthesized by Pd-catalyzed cross-coupling reactions of p henyl boronic acids with 4-(5,6-dihydroxy-3-oxahexyloxy)bromobenzenes. The thermotropic and lyotropic mesomorphic behavior of these compound s 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 later al substituents and of the molecular chirality on the mesomorphic prop erties were investigated. All compounds have smectic A-phases as the h igh-temperature mesophases. Additionally, columnar phases were found f or the long chain 4'-alkoxy-4-(5,6-dihydroxy-3-oxahexyloxy)biphenyls 2 c-f without lateral substituent and for the oxy-3-oxahexyloxy)-4'-dode cyloxy-3'-methylbiphenyl 10. All other laterally substituted biphenyl derivatives 9 and 11-15 and the 4'-alkyl substituted compound 8 displa y only smectic phases. The influence of protic solvents (formamide and ethylene glycol) on the liquid crystalline properties was studied. Th e columnar mesophases and Sc-phases are destabilized by addition of pr otic solvents whereas the S-A- phases are stabilized. Furthermore, bin ary mixtures of the amphiphilic biphenyl derivatives with each other h ave been investigated. Complete miscibility was found for the SA-phase s and the columnar phases of the compounds 2c-f. However the columnar phases of the nonsubstituted compounds 2c-f and the 3'-methyl substitu ted derivative 10 do not have an uninterrupted miscibility. Cubic phas es (Pn3m or Pn3) and additional columnar phases were induced in the co ntact regions. Thus, the same diversity of different kinds of mesophas es as in lyotropic systems and in the thermotropic phase sequence of p olycatenar compounds is found. It is proposed that the cubic and colum nar mesophases of these rodlike molecules result from the collapse of the smectic layer structures into ribbons.