Substituted Septithiophenes with End Groups of Different Size: Packing and Frustration in Bulk and Thin Films.

We report on three different liquid crystalline compounds with a central septithiophene core and alkylated end groups of strongly increasing bulkiness. In principle, the thiophene cores prefer to pack parallel to optimize their π-π interactions, which becomes sterically impossible for the bulkier end groups. Using X-ray diffraction, we find that the way out of this packing dilemma is toward liquid-crystal phases of higher dimensionality in the order smectic → columnar ↔ bicontinuous cubic.

Thinning and thickening of free-standing smectic films revisited.

We present a theoretical explanation of the remarkable thickness instabilities that occur in free-standing smectic films (FSSF) upon changing the external conditions: i) upon heating the film above the bulk smectic disordering temperature, generally the film does not rupture but instead shows successive layer-by-layer thinning transitions; ii) thickening of FSSF, which occurs within the thermal range of the smectic phase upon local heating. All observations reported so far can be explained on the basis of the Landau-de Gennes theory of the smectic state in combination with nucleation theory. In overheated smectic films (thinning) or locally heated FSSF (thickening) an additional normal tensile force appears due to a change of the mean density of the film.

Monolayer properties of asymmetrically substituted sexithiophene.

We present a structural comparison of monolayers on a SiO2 substrate of two asymmetrically substituted sexithiophenes (6T). Molecule 1 consists of 6T with a branched alkyl chain at one end only and shows a crystalline structure. In molecule 2, the bifunctional 6T has in addition at the other end a linear alkyl chain.

Order and frustration in liquid-crystalline dendrimers.

X-ray diffraction has been used to elucidate the structure and phase behavior of several liquid-crystalline dendrimers with a different surface topology of the terminal chains. This includes second-generation liquid-crystalline block and statistical dendrimers with mixed aliphatic and mesogenic terminal groups as well as homo-dendrimers of several generations containing only mesogenic end groups. The homo-dendrimers of generation one to four display a monolayer smectic phase, while the fifth generation shows a more ordered columnar phase.

Random disorder and the smectic-nematic transition in liquid-crystalline elastomers.

We report effects of disorder due to random cross-linking on the nematic to smectic-A phase transition in smectic elastomers. Thermoelastic data, stress-strain relations and high-resolution x-ray scattering profiles have been analyzed for two related compounds with a small and a larger nematic range, respectively, each for 5% as well as 10% cross-links. At 5% cross-link density the algebraic decay of the positional correlations of the smectic layers survives in finite-size domains, providing a sharp smectic-nematic transition.