Direct observation of smectic layers in thermotropic liquid crystals.

We demonstrate subnanometer resolution cryo-TEM imaging of smectic layers in the smectic and nematic phases of two bent-core liquid crystals. Our results show perfect periodicity over several hundred layers in the smectic phase and also provide the first direct evidence of smectic clusters on length scales of 30-50 nm in a nematic liquid crystal. The results are corroborated with small angle x-ray scattering measurements. The observation of smectic clusters in the nematic phase is of special interest in bent-core liquid crystals, where the smectic clusters are stable over wide temperature ranges, in contrast to the well-known pretransitional "cybotactic" clusters that appear only in the vicinity of a bulk smectic phase. The means to characterize and manipulate this nanoscale molecular order could open up completely new liquid crystal-based technologies.

Characterization of a chiral phase in an achiral bent-core liquid crystal by polarization studies of resonant x-ray forbidden reflections.

The chiral antiferroelectric structure of an achiral bent-core liquid crystal is characterized by resonant x-ray scattering at chlorine K edge. The "forbidden" reflections resulting from the glide or screw symmetry elements are restored by the anisotropy of the tensor structure factor, which we calculate for two possible structural models. A careful analysis of the polarization states of the restored "forbidden" reflections enables an unambiguous identification of a chiral structure (i.e., the so-called anticlinic, antiferroelectric smectic-C or Sm-C(A)P(A)) coexisting with the achiral synclinic antiferroelectric smectic-C or Sm-C(S)P(A). The method proves to be quite powerful as it identifies the chiral structure within coexisting phases despite an imperfect orientation of the sample. The volume fraction of the chiral phase and the distribution of alignment are extracted from the data.

Multistage polar switching in bent-core mesogens.

We report a novel type of electro-optical switching in a tilted smectic phase of bent-shaped mesogens. The switching consists of a continuous stage and two bistable transitions. Detailed optical and electro-optical measurements using high-speed imaging are given and possible interpretations of the experimental results are discussed.

Influence of the core structure on the development of polar order and superstructural chirality in liquid crystalline phases formed by silylated bent-core molecules: lateral substituents.

The influence of lateral substituents was studied for a new series of bent-core liquid crystals with heptamethyltrisiloxane units at both ends of the aliphatic side chains. These materials were investigated by polarized light microscopy, differential scanning calorimetry, X-ray scattering and electro-optical methods. An antiferroelectric switching columnar phase was found for the non-substituted compound. Different types of non-switching columnar phases were obtained for molecules having substituents (CH, NO) in the bay position, whereas a temperature-dependent transition from polar to non-polar smectic phases was found for molecules having substituents (Cl, CN) adjacent to one of the ester linking units. For the CN-substituted compound a new phase sequence SmCP-SmC-Cub-Iso was observed. The occurrence of the cubic phase is associated with a steric frustration due to the bulky silyl groups. The special rectangular columnar mesophase of the nitro-substituted compound is discussed with respect to its relation to B7-type mesophases.

Formation of novel photodimers from 4-aryl-1,4-dihydropyridines.

N-substituted 3-alkoxycarbonyl-4-aryl-1.4-dihydropyridines have been photochemically investigated for the first time. In contrast to reports of analogous 3,5-dialkoxycarbony] derivatives, they are unreactive in the solid state with shortest distances of potentially reacting double bonds of 6.883(3) A for one derivative examined by x-ray crystal structure analysis. Solution irradiation with unfiltered light (lambda > or = 270 nm) led to novel diazatetrakishomocubanes in 30-50% yields. Diazatetrakishomocubanes were also obtained by irradiation with filtered light (lambda > 313 nm) besides head-to-tail connected syn-dimers. The irradiation of the syn-dimers with unfiltered light led to centrosymmetric cage dimers accompanied by some dimer fragmentation. Formation of the homocubanes via intermediate biradicals is supported by the available data.

The First Functionalized 6,12-Diazatetrakishomocubanes.

The photodimerization of asymmetric 4-aryl-1,4-dihydropyridines results, totally unexpectedly, in the new 6,12-diazatetrakishomocubanes 1. This is in contrast to the previously observed [2+2] photocycloaddition reactions of symmetrical 4-aryl-1,4-dihydropyridines

Solid-state photodimerization of 4-aryl-1,4-dihydropyridines studied by 13C CPMAS NMR spectroscopy.

13C CPMAS NMR spectroscopy has been applied to monitor the solid-state reaction of two different photodimerizing 4-phenyl-1,4-dihydropyridines yielding a cage dimer in one case and an anti-dimer in the other case. The spectra of the reacting monomers exhibit a magnetical inequivalence of chemically equivalent CO and C2/4 carbon atoms caused by a rotation of the pseudoaxially oriented 4-phenyl substituent out off the plane through N1, C3, C8 which could be determined by X-ray crystal structure analyses of the centrosymmetrically arranged monomers. The 13C CPMAS NMR monitoring of the cage dimer formation proves that the reaction takes place in two steps via a syn-dimer for which a non-symmetrical structure was derived from the spectrum. The non-symmetrical structure was confirmed by X-ray crystal structure analysis of one structurally related derivative. A centrosymmetric structure for both the finally formed cage dimer and the anti-dimer of the other monitored photoreaction was proved by their spectra with one set of signals for each half of the dimers. respectively. Thus, conformational properties of the molecules as well as the symmetry of the products can be directly derived from the 13C CPMAS NMR spectra.

[Lipoxygenase inhibitors. IV. Synthesis and cyclization reactions of open-chain N1-aryl-substituted amidrazones]. / Lipoxygenase-Inhibitoren, 4.Mitt.1): Synthese und Cyclisierungsreaktionen offenkettiger N1-arylsubstituierter Amidrazone.

alpha-Carbonyl carboxylic acid arylhydrazonochlorides obtained by Japp-Klingemann reaction are the starting substances for the synthesis of alpha-carbonyl carboxylic acid arylhydrazonoamides, -esters and -thioesters. The inhibiting activity of these compounds against 15- and 5-lipoxygenase is described. Reactions of derivatives of amidrazones with formaldehyde give triazole, triazoline and unexpected benzotriazepine derivatives.