Binary systems consisting of 2,5-diphenyl-1,3,4-thiadiazole derivatives incorporating an allene unit in one of the terminal chains and the electron acceptor 2,4,7-trinitrofluorenone (TNF) have been investigated. Though the diphenylthiadiazole cores do not represent typical electron donor units, the nematic and smectic C phases observed for the pure compounds were suppressed and replaced by smectic A phases which in most cases have a higher stability than the nematic phases of the pure compounds. The substitution pattern around the allene moiety allowed a systematic study of the influence of steric effects on the mesophase induction by TNF. Compounds with long and especially those with branched terminal chains can take up a larger number of TNF molecules and can reach a higher stability of the induced SmA phase than those with shorter and unbranched chains. The induction of SmA phases is explained as the result of attractive intermolecular interaction between the diphenylthiadiazole rigid cores and TNF molecules provided by donor-acceptor interactions and quadrupole interactions, as well as a consequence of microsegregation and space filling effects.

Several different macrocyclic liquid crystals consisting of two calamitic 2-phenylpyrimidine or 5-phenylpyrimidine units connected at both ends by polyether or alkyl chains have been synthesised by macrocyclisation reactions under high dilution conditions. The nature of the rigid core has a strong impact on the liquid crystalline phases formed. All 2-phenylpyrimidine paracyclophanes show nematic phases, whereas for the series of the 5-phenylpyrimidine derivatives smectic A-phases are exclusively formed. This behaviour is related to conventional calamitic phenylpyrimidine liquid crystals, but the mesophases are strongly stabilised in the macrocycles. An exchange of polyether chains by alkyl chains leads to significant mesophase stabilisation, whereas increasing the spacer length reduces mesophase stability. Pre-organization of the calamitic cores and micro-segregation of chemically distinct molecular parts are discussed as reasons for the observed effects of molecular structure on the mesophase behaviour of these compounds.

The combination of two different molecular architectures, the rather rod-like para-cyclophanes and two half disc-like 1,3-diketonate units fused via ortho-palladation leads to new liquid crystalline materials with a quite unusual molecular shape. By increasing the number of attached alkyl chains a discontinuous transition from a lamellar to a columnar organisation of the molecules was observed. Compounds with twelve alkyl chains are the first para-cyclophane derivatives that show thermotropic columnar mesophase behaviour.

Macrocyclic liquid crystals consisting of a biphenyl rigid core and a 2-phenylpyrimidine unit are fused by cyclopalladation giving a novel type of metallomesogens with nematic and mectic mesophases.

The mesomorphic properties of conventional rod-like liquid crystals (diphenyl-1,3,4-thiadiazoles, diphenylpyrimidines, diphenyltriazines, diphenyltetrazines and p-terphenyl derivatives), of macrocyclic liquid crystals and of dimesogens can be influenced by addition of the electron acceptor 2,4,7-trinitrofluorenone (TNF). Thereby nematic and smectic C phases are suppressed and smectic A phases can be stabilized or induced. Long and branched terminal chains result in a strong stabilization of the S-A phase, whereas no smectic phase is induced to accompany the nematic phases of mesogens with short terminal chains.

Macrocyclic liquid crystals incorporating two different rigid cores are reported for the first time. The synthesis of polyethercyclophanes such as that shown in the Figure is described, together with their phase behavior as investigated by differential scanning calorimetry and polarizing microscopy. The synthesis of macrocycles that combine non‐identical building blocks is describe in order to obtain materials with tailor‐made properties. magnified image

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-l,3,4-thiadiazol-2-yl)-phenyl, 4′-dodecyloxybiphenyl-4-yl or 4-decylphenyl units, which are connected by a hydrophilic 12,13-dihydroxy-l,4,7,10-tetraoxatridecyloxy, 9,10-dihydroxy-l,4,7-trioxadecyloxy, 6,7-dihydroxy-l,4-dioxaheptyloxy or 2,3-dihydroxypropoxy groups. All these amphiphiles contain a 1,2-diol unit. In addition the 12-hydroxy-l,4,7,10-tetraoxadodecyl-, 9-hydroxy-l,4,7-trioxanonyl, 6-hydroxy-l,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 α, α′,α″-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 SA–SC dimorphism. However, one of the biphenyl derivatives [4-dodecyloxy-4′-(6,7-dihydroxy-l,4-dioxaheptyl)biphenyl] displays another phase instead of the SC 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.

Abstract Novel liquid crystalline 2-phenyl-1,3,4-thiadiazole based oligomers with three and four rigid aromatic units linked by a flexible central unit have been investigated by polarizing microscopy. The synthesis of these compounds and the influence of structural variations on the mesomorphic properties are described. The combination of suitable mesogenic moieties with appropriate central units leads to oligomers which exhibit Sc phases.