Phase Transitions of 4‘-n-Hexacosyloxy-3‘-nitrobiphenyl-4-carboxylic Acid (ANBC-26): Two Types of Thermotropic Cubic Phases

Discotic liquid crystals of transition metal complexes 42: the detailed phase structures and phase transition mechanisms of two Cub mesophases shown by discotic liquid crystals based on phthalocyanine metal complexes

We found in our previous works that the sandwich-type phthalocyanine-based rare earth metal complexes, bis[2,3,9,10,16,17,23,24-octakis(3,4-dialkoxyphenoxy)phthalocyaninato]lanthanoid(III) ({[( C n O )2 PhO ]8 Pc }2 M, M = Eu and Lu , n = 8–16) (1 and 3), exhibited two thermotropic cubic mesophases, Cub1 and Cub2, together with columnar mesophases. It is rare that the discotic liquid crystalline compounds show the cubic mesophase. We revealed that their symmetries of the lower temperature Cub1 mesophase and the higher temperature Cub2 mesophase were [Formula: see text] and [Formula: see text], respectively. However, their detailed phase structures were not revealed in the previous works. In this work, we have synthesized a series of novel sandwich-type phthalocyanine-based terbium complexes, bis[2,3,9,10,16,17,23,24-octakis(3,4-dialkoxyphenoxy)phthalocyaninato]terbium(III) ({[( C n O )2 PhO ]8 Pc }2 Tb , n = 8–16) (2). Their mesomorphic properties have been investigated using polarization microscope, DSC and temperature-dependent X-ray diffraction techniques. As the result, the present Tb complexes (2) also showed two cubic mesophases, [Formula: see text] and [Formula: see text], together with columnar mesophases, as well as the previous Eu homologs (1) and Lu homologs (3). We have furthermore investigated by using temperature-dependent electronic absorption spectroscopy to reveal their detailed phase structures and phase transition mechanism of these two Cub mesophases. We have revealed that the [Formula: see text] mesophase forms a bicontinuous structure consisting of branched columns like jungle gym, but that the [Formula: see text] mesophase forms a discontinuous structure consisting of short columns like drums that resulted from the cutting off of the branched columns.

Galactosyl headgroup interactions control the molecular packing of wheat lipids in Langmuir films and in hydrated liquid-crystalline mesophases

The behavior of the two major galactolipids of wheat endosperm, mono- (MGDG) and di-galactosyldiacylglycerol (DGDG) was studied in aqueous dispersion and at the air/liquid interface. The acyl chains of the pure galactolipids and their binary equimolar mixture are in the fluid or liquid expanded phase. SAXS measurements on liquid-crystalline mesophases associated with the electron density reconstructions show that the DGDG adopts a lamellar phase L(alpha) with parallel orientation of the headgroups with respect to the plane of the bilayer, whereas MGDG forms an inverse hexagonal phase H(II) with a specific organization of galactosyl headgroups. The equimolar mixture shows a different behavior from those previously described with formation of an Im3m cubic phase. In comparing monolayers composed of the pure galactolipids and their equimolar mixtures, PM-IRRAS spectra show significant differences in the optical properties and orientation of galactosyl groups with respect to the interface. Furthermore, Raman and FTIR spectroscopies show that the acyl chains of the galactolipid mixture are more ordered compared to those of the pure components. These results suggest strong interactions between MGDG and DGDG galactosyl headgroups and these specific physical properties of galactolipids are discussed in relation to their biological interest in wheat seed.

Alkyl chains acting as entropy reservoir in liquid crystalline materials

The roles played by the conformational disordering of alkyl chains in determining the aggregation states of matter are reviewed for liquid crystalline materials from a thermodynamic perspective. Entropy, which is one of the most macroscopic concepts but which has a clear microscopic meaning, provides crucial microscopic information for complex systems for which a microscopic description is hard to establish. Starting from structural implication by absolute (third-law) entropy for crystalline solids, the existence of successive phase transitions caused by the successive conformational melting of alkyl chains in discotic mesogens is explained. An experimental basis is given for the "quasi-binary picture" of thermotropic liquid crystals, i.e., the highly disordered alkyl chains behave like a second component (solvent). A novel entropy transfer between the "components" of a molecule and the resulting "alkyl chains as entropy reservoir" mechanism are explained for cubic mesogens.

Degree of disorder in cubic mesophases in thermotropics: thermodynamic study of a liquid crystal showing two cubic mesophases

Heat capacity of a thermotropic mesogen ANBC(22) (4(')-alkoxy-3(')-nitrobiphenyl-4-carboxylic acid with 22 carbon atoms in alkyl chain) showing two cubic mesophases was measured by adiabatic calorimetry between 13 and 480 K. Excess enthalpies and entropies due to phase transitions were determined. A small thermal anomaly due to the cubic Im3m-->cubic Ia3d phase transition was successfully detected. Through an analysis of chain-length dependence of the entropy of transition, the sequence of two cubic mesophases (with space groups Ia3d and Im3m) is deduced for thermotropic mesogens ANBC(n). It is shown that the disorder of the core arrangement decreases in the order of Sm-C-->cubic (Im3m)-->cubic (Ia3d) while that of the chain in the reverse order cubic (Ia3d)-->cubic (Im3m)-->Sm C.