Detlev Joachimi

Trimeric and tetrameric liquid crystalline thiadiazole derivatives

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.

Amphiphilic propane-l,2-diol derivatives incorporating calamitic structural units

Abstract Novel amphiphilic propane-1,2-diol derivatives incorporating a wide variety of calamitic structural units, like biphenyl, dicyclohexyl. phenylbemzoate, phenyl pyrimidine and phenylthiadiazole, were synthesized and their mesomorphic behaviour studied. Most of these compounds exhibit broad smectic A and C mesophases in the water-free state. These mesophases could be stabilized further by addition of water. The transition temperatures depend mainly on the hydrophilic properties of the rigid core and on the length of the spacer by which the core is connected to the diol unit.

Bulk properties and monolayer behaviour of diol-based mesogens and their acetonides

Amphiphilic 1,2-diol derivatives and their acetonides, incorporating rigid aromatic structural units, have been prepared. They exhibit liquid-crystalline behaviour and form stable monolayers at the air/water interface. The force–area isotherms obtained depend largely on the chemical structure of the rigid core.

The liquid-crystalline behaviour of amphiphilic ethane-1,2-diol derivatives incorporating a six membered ring in their principal structure

Abstract Novel 3-phenyloxy substituted propane-1,2-diol derivatives, 4-(4-n-hexyloxyphenyl)-butane-1,2-diol and 4-(trans-4-n-pyropylcyclohexyl)-butane-1,2-diol have been synthesized and their thermal behaviour has been studied. These compounds exhibit thermotropic and, after addition of water, also lyotropic liquid-crystalline behaviour. The clearing temperatures of the smectic mesophases were found to be strongly dependent on the alkyl chain length and on the structural units that link the aromatic ring to the alkyl chain and to the diol unit. The behaviour of the aromatic compounds is compared with that of the cyclohexane derivative. Thereby it has been realized that the mesophase stability of the amphiphilic diols incorporating a rigid unit is largely determined by both, the molecular geometry (molecular shape and intramolecular flexibility) and the amphiphilic structural pattern.

Heterotrimeric liquid crystalline thiadiazole derivatives

Abstract Various trimeric co-oligomers combining 2-phenyl-1,3,4-thiadiazole mesogenic moieties with a biphenyl mesogenic moiety were synthesized and their mesomorphic behaviour investigated by polarizing microscopy, calorimetry and X-ray scattering. Such co-oligomeric structures provide an opportunity to combine different mesogenic units. Thus readily accessible homochiral biphenyl mesogenic units were connected with thiadiazole mesogenic units leading to an oligomeric liquid crystal material with ferroelectric properties.

Liquid crystalline paracyclophane derivatives

Abstract Various paracyclophane derivatives incorporating 4,4′-biphenyl, 2,5-diphenyl-1,3,4-thiadiazole, phenyl benzoate and 2,6-disubstituted naphthyl rigid cores were synthesized and their mesomorphic behaviour was studied using polarizing microscopy, DSC and X-ray diffraction. Most of these macrocyclic compounds possess liquid crystalline properties with unexpectedly high clearing temperatures compared to those of conventional calamitic mesogens. In this way, the coupling of two appropriate rigid units using flexible chains to form a macrocycle constitutes a new and powerful approach towards mesophase induction and stabilization. The types of mesophase formed by these macrocycles do not depend only on the nature of the bridging chains, but also strongly on the structure of the rigid aromatic system. The smectic A phase and the E phase are formed by polyetherbiphenylophanes. Poly-ethercyclophanes incorporating the 2,5-diphenylthiadiazole rigid core form nematic and smectic C phases. The nematic phase is...

Preliminary Communication : Induction of smectic A phases by electron donor-acceptor interaction between calamitic mesogens and 2,4,7-trinitrofluorenone

The mesomorphic properties of conventional rod-like liquid crystals (diphenyl1,3,4-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 SA phase, whereas no smectic phase is induced to accompany the nematic phases of mesogens with short terminal chains.

Investigation of bulk properties and monolayer behaviour of amphiphilic mesogens: structural variations of the head group

Non-ionic amphiphilic compounds—butane-1.2,3-triol derivatives, 2-hydroxycarboxylic acids and carboxylic acids—incorporating rigid aromatic structural units like 1,4-disubstituted benzene, 4,4′-disubstituted biphenyl or the 2-phenyl-1,3,4-thiadiazole have been prepared. In most cases these compounds exhibit smectic liquid crystalline phases which can be stabilized by the addition of water. Their mesomorphic properties are compared with those of the structurally related 1,2-diol derivatives. Surprisingly, also some ω-(4-alkylphenoxy)alkanoic acids which have no thermotropic mesophases were found to aggregate to lyotropic mesophases in the presence of water. Most of the new compounds form stable monolayers at the air–water interface. The force–area isotherms of the thiadiazole derivatives have been recorded and it was found that they strongly depend on the chemical structure of the head group.

New liquid-crystalline heteroalicyclic compounds

Abstract A wide variety of new sulphur-containing heteroalicyclic liquid-crystalline compounds have been synthesized. Special attention has thus been focussed on the influence of the heteroatoms on the mesomorphic behaviour. 2-Cyclohexyl substituted 1,3-dithianes and 1,3-oxathianes have been found to be superior to those with 2-phenyl substituents. The clearing temperatures of the 2-phenyl-1,3-dithianes depend on the electron donating or electron accepting properties of the substituents attached to the phenyl group. The mesophase stability of the 2-cyclohexyl substituted (hetero)alicycles increases in the order: 1,3-dioxane < 1,3-oxathiane < cyclohexane < 1,3-dithiane. This order is partly reversed for the cyclohexane annulated compounds: 1-oxa-3-thiadecalin < 1,3-dioxadecalin < decalin < 1,3-dithiadecalin. Selected values of the birefrigence and some melting enthalpies have been measured.

New liquid-crystalline materials containing a five-membered heteroalicycle: Trans-2, 4-disubstituted 1, 3-oxathiolanes

Abstract The synthesis of new liquid-crystalline materials containing a five membered heteroalicyclic ring is described. The trans 2,4-disubstituted 1, 3-oxathiolane derivatives exhibit lower clearing temperatures and melting points than the analogous six membered 1, 3-oxathiane compounds. However the mesophase stability of the 1, 3-oxathiolane derivatives is superior to that of comparable cyclopentane derivatives.