Molecular design of amphotropic materials: influence of oligooxyethylene groups on the mesogenic properties of calamitic liquid crystals

The syntheses and liquid-crystalline properties of novel oligoethylene glycol derivatives are described. These are amphiphiles and podand-like trimesogens. The hydrophobic sections of the amphiphiles consist of calamitic 4-(5-pentadecyl-1,3,4-thiadiazol-2-yl)-phenyl, 4'-dodecyloxybiphenyl-4-yl or 4-decylphenyl units, which are connected by a hydrophilic 12,13-dihydroxy-1,4,7,10-tetraoxatridecyloxy, 9,10-dihydroxy-1,4,7-trioxadecyloxy, 6,7-dihydroxy-1,4-dioxaheptyloxy or 2,3-dihydroxypropoxy groups. All these amphiphiles contain a 1,2-diol unit. In addition the 12-hydroxy-1,4,7,10-tetraoxadodecyl-, 9-hydroxy-1,4,7-trioxanonyl, 6-hydroxy-1,4-dioxahexyl and 2-hydroxyethoxy derivatives of 4'-dodecyloxybiphenyl are described. These compounds have only a single hydroxy group at their hydrophilic termini. The podand-like trimers consist of three 4'-dodecyloxybiphenyl units which are connected via oligooxyethylene chains with an alpha, alpha('),alpha "-mesitylenetriyl unit. The thermotropic liquid-crystalline properties of these compounds were investigated by polarising microscopy, differential scanning calorimetry and, in some cases by X-ray scattering. Most diol derivatives exhibited an S-A-S-C dimorphism. However, one of the biphenyl derivatives [4-dodecyloxy-4'-(6,7-dihydroxy-1,4-dioxaheptyl)biphenyl] displays another phase instead of the S-C phase, probably a phase with a ribbon structure. The liquid-crystalline phases of these diol derivatives were influenced by the addition of protic solvents. Only lamellar phases were found for the biphenyl derivatives. Some thiadiazole derivatives additionally formed lyomesophases consisting of curved aggregates. No thermotropic liquid crystalline properties could be detected for the oligoethylene glycol monoethers without the 1,2-diol group. However, lyotropic liquid-crystalline phases could be induced by the addition of ethylene glycol or formamide. Only those podands with a medium spacer length were thermotropic liquid crystals and no lyotropic mesophases were detected for the podands.