Counterion effects on the mesomorphic and electrochemical properties of guanidinium salts.

Ionic liquid crystals (ILCs) combine the ion mobility of ionic liquids with the order and self-assembly of thermotropic mesophases. To understand the role of the anion in ILCs, wedge-shaped arylguanidinium salts with tetradecyloxy side chains were chosen as benchmark systems and their liquid crystalline self-assembly in the bulk phase as well as their electrochemical behavior in solution were studied depending on the anion. Differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS, SAXS) experiments revealed that for spherical anions, the phase width of the hexagonal columnar mesophase increased with the anion size, while for non-spherical anions, the trends were less clear cut. Depending on the anion, the ILCs showed different stability towards electrochemical oxidation and reduction with the most stable being the PF6 based compound. Cyclic voltammetry (CV) and density functional theory (DFT) calculations suggest a possible contribution of the guanidinium cation to the oxidation processes.

First Examples of de†Vries-like Smectic†A to Smectic†C Phase Transitions in Ionic Liquid Crystals.

In ionic liquid crystals, the orthogonal smectic A phase is the most common phase whereas the tilted smectic C phase is rather rare. We present a new study with five novel ionic liquid crystals exhibiting both a smectic A as well as the rare smectic C phase. Two of them have a phenylpyrimidine core whereas the other three are imidazolium azobenzenes. Their phase sequences and tilt angles were studied by polarizing microscopy and their temperature-dependent layer spacing as well as their translational and orientational order parameters were studied by X-ray diffraction. The X-ray tilt angles derived from X-ray studies of the layer contraction and the optically measured tilt angles of the five ionic liquid crystals were compared to obtain their de Vries character. Four of our five mesogens turned out to show de Vries-like behavior with a layer shrinkage that is far less than that expected for conventional materials. These materials can thus be considered as the first de Vries-type materials among ionic liquid crystals.

Photoresponsive ionic liquid crystals based on azobenzene guanidinium salts.

The use of non-ionic LC phases as anisotropic matrices for E/Z-isomerization of azo-guest molecules is often restricted due to limited solubilities and demixing effects. In this study we therefore employed an ionic liquid crystal (ILC) matrix to follow the photo-induced E/Z-isomerization of ionic mesogenic azobenzene guanidinium guests. The latter were prepared from 4-hydroxy-4'-(octyloxy)azobenzene, which was first treated with N-(bromoalkyl)phthalimides to introduce the spacer with varying chain length. Removal of phthalimide and final reaction with a formamidinium salt linked the ionic head group to the photoisomerizable azobenzene unit. Investigation of the mesomorphic behaviour revealed for all azobenzene ILCs smectic A mesophases with high translational order parameters and partial bilayers, as could be stated by layer spacing d. Similar packing behaviour was found for the solid state by X-ray crystal structure analysis. E/Z-isomerization of azobenzene ILCs which were completely miscible with the ionic LC phase of C12MIM-Br as anisotropic host was induced by irradiation with UV light and the reisomerization observed by time-resolved UV-Vis spectroscopy. For comparison, water was used as isotropic host. Z/E-reisomerization activation energies exhibited similar values of 97-100 kJ mol(-1) irrespective of spacer lengths and the type of host. The results demonstrate that a proper match of steric requirements of host and guest as well as layer spacings are needed for a decreased activation energy.

Anomalous odd-even effects in columnar and smectic phases of discotic tetraphenylenes.

Rather uncommon but pronounced odd-even effects in the clearing temperatures of liquid crystals are found for three new homologous series of discotic tetraphenylenes [figure: see text]. Detailed similarities to the well-known odd-even effects in calmitic mesogens point towards a more general understanding of odd-even effects in liquid crystals.The phase transition temperatures in many homologous series of liquid crystals show odd-even effects similar to the well-known odd-even effects in non-mesogenic organic compounds like carboxylic acids. For discotic liquid crystals only a few series with a sufficient number of consecutive homologues are available and only a small part of these series show odd-even effects at all. We now synthesized and investigated three series of discotic tetraphenylenes each ranging from the n=7 to the n=16 homologues. For these compounds an anomalous odd-even effect is found: The ascending and descending transition temperatures with increasing number of methylene groups in the side chains exhibit an inversion of this alternation which is always found between the n=12 and n=14 homologues. An overview of odd-even effects in liquid crystals is presented and we discuss the nature and the origin of odd-even effect in discotic mesogens.

Saddle-shaped tetraphenylenes with peripheral gallic esters displaying columnar mesophases.

Tetraphenylenes 2 with eight peripheral gallic esters were prepared in two steps from octamethoxytetraphenylene 1 in 19-72% yield. Investigation of the mesomorphic properties of 2 by DSC, POM and X-ray diffraction revealed that derivatives 2a-d with short alkoxy chain lengths (C(5)-C(8)) did not show any mesomorphic properties, whereas compounds 2e-i with C(9)-C(13) chains displayed rectangular columnar mesophases and compounds 2j-l with C(14)-C(16) chains displayed hexagonal columnar mesophases. Furthermore an anomalous odd-even effect of the clearing points of compounds 2e-l versus chain length was detected.