Thermotropic and lyotropic liquid crystalline phases of rigid aromatic amphiphiles

Rodlike amphiphilic molecules that contain exclusively aromatic building-bl ocks and no flexible alkyl chains have been synthesized and their mesomorph ic properties investigated. These novel compounds bear diol head groups of different size (2,3-dihydroxypropyloxy or 5,6-dihydroxy-3-oxahexyloxy group s) at one end of a biphenyl unit, various aromatic segments (banzyloxy, 4-, 3-, or 2-methylbenzyloxy, phenoxy groups) at the other, and additional met hyl substituents in different positions. They were synthesized by using Suz uki cross-coupling reactions as the key steps. Their thermotropic mesomorph ism was investigated by means of polarized light optical microscopy, differ ential scanning calorimetry, and, for enantiotropic phases, by X-ray scatte ring. The liquid crystallinity of this class of compounds is influenced by protic solvents, such as water and glycerol. Dependent on the temperature a nd the solvent content, different S-A phases were found. Several mesophases resulting from the frustration of these layer structures (e.g., different columnar phases, optical isotropic mesophases, and nematic phases) were als o present. The smectic phases have different degrees of intercalation (S-Ad , S-A2) The columnar phases are supposed to be ribbon structures that resul t from the collapse of the smectic layers. They occur in some pure compound s or they are induced upon the addition of protic solvents. The particular phase sequences of the different compounds depend mainly on the position of the methyl substituents at the biphenyl cores and are largely determined b y the degree of intercalation of the aromatic cores.