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The strategic tuning of liquid crystalline phase behaviour by adjusting molecular symmetry was investigated. A family of sixteen symmetrical and unsymmetrical 2,6-di(4ʹ-n-alkoxybenzoyloxy) naphthalene derivatives were prepared and their liquid crystal properties examined by differential scanning calorimetry, polarised optical microscopy, and x-ray diffraction. All mesogens formed nematic phases, with longer-chain analogues also exhibiting smectic C phases at lower temperatures. Melting temperatures of the compounds strongly depend on molecular symmetry, whereas clearing transitions are relatively insensitive to this effect. A detailed analysis indicates that the clearing point can be predicted based on the nature of the terminal alkyl chains, with only a secondary effect from molecular symmetry. Moreover, low symmetry molecules showed a greater tendency to form smectic C phases, which was ascribed to the selective depression of the melting point versus the SmC-N transition. This demonstrates that molecular symmetry-breaking is a valuable tool both for tuning liquid crystalline phase range and for increasing a material's polymorphism. [ABSTRACT FROM AUTHOR]

Oligo/polymerisation of known mesogens constitutes a highly efficient strategy in liquid crystal research due to its potential to generate novel liquid crystal materials with intriguing mesomorphic properties. Here we report the synthesis and comparative studies of a synthetic liquid crystal dimer and two of its monomer analogues. By incorporating cholesterol as the mesogenic group, we designed a flexible scaffold consisting of a hybrid of non-polar hydrophobic chain and polar tetraethylene glycol appended to the cholesteryl mesogens. Detailed studies showed that the two classes of mesogens exhibit the same type of liquid crystal phases with similar dimensions but their transition temperatures varied which can be effectively rationalised by the particular chemical functionalities present in each class of materials. [ABSTRACT FROM AUTHOR]

Three series of dibenzo[ a, c]phenazines were prepared in an order to assess the impact of side chain position on the phase stability of columnar liquid crystals. Each series was composed of four isomeric compounds differing only in the disposition of hexyloxy and decyloxy chains around the central aromatic core, giving rise to electronically similar compounds with varying shapes and symmetries. The substitution pattern was found to have a moderate effect on the clearing transition of the liquid crystal but a larger impact on the melting temperatures. These observations suggest a viable strategy for controlling the phase range of liquid crystals via judicious choice of peripheral chain structure and location. [ABSTRACT FROM AUTHOR]

A novel family of amphiphilic cyclodextrin(CD)-based liquid crystals that bear O-acetylated oligoethylene glycol chains at the secondary face is reported. Unlike most of the previously reported liquid crystals (LC) based on chemically modified CDs, which depend on H-bonding as the primary intermolecular forces, the present CD derivatives self-assemble into highly ordered smectic liquid crystal phases via the weaker dipole- dipole intermolecular interactions. The obtained materials are found to display much improved properties such as improved thermostability, reduced clearing temperatures, and better fluidity. The present work opens up new possibilities to design CD-based LC materials. [ABSTRACT FROM AUTHOR]

We report the synthesis of a series of 2,3,5,6-tetraalkoxydibenz[ a, c]anthracenes bearing substituents (H, OCH3, or CN) in the 11- and 12-positions and an investigation of their liquid-crystalline properties. The synthesis involved Suzuki coupling of the appropriate dibromonaphthalene and boronate ester, followed by an oxidative cyclization. Compounds 4 and 5, bearing OCH3 and H, respectively, do not exhibit any liquid-crystalline properties. In contrast, compounds 6a- c, bearing cyano groups, assemble into columnar mesophases, suggesting that electron-withdrawing groups are important for promoting columnar mesophase assembly. Analysis of the XRD revealed that compound 6b exhibits a columnar hexagonal phase, whereas compounds 6a and 6c exhibit columnar rectangular phases. The XRD data of 6a and 6b also show reflections that are consistent with antiparallel dimers within the columnar stacks, and intercolumnar spacings suggest that molecules are tilted within the columns. [ABSTRACT FROM AUTHOR]

Liquid crystalline self-assembly offers the potential to create highly ordered, uniformly aligned, and defect-free thin-film organic semiconductors. Analogues of one of the more promising classes of liquid crystal semiconductors, 5,5"-dialkyl-α-terthiophenes, were prepared in order to investigate the effects of replacing the central thiophene with either an oxadiazole or a thiadiazole ring. The phase behaviour was examined by differential scanning calorimetry, polarized optical microscopy, and variable temperature x-ray diffraction. While the oxadiazole derivative was not liquid crystalline, thiadiazole derivatives formed smectic C and soft crystal lamellar phases, and maintained lamellar order down to room temperature. Variation of the terminal alkyl chains also influenced the observed phase sequence. Single crystal structures revealed the face-to-face orientation of molecules within the layers in the solid-state, a packing motif that is rationalized based on the shape and dipole of the thiadiazole ring, as corroborated by density functional theory (DFT) calculations. The solution opto-electronic properties of the systems were characterized by absorption and emission spectroscopy, cyclic voltammetry, and time-dependent density functional theory (TD-DFT). [ABSTRACT FROM AUTHOR]