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V Enkelmann

Publications and source records attributed to V Enkelmann.

6 recordsLinked to original sources
PubMed2001

Novel chiral macrocycles containing two electronically interacting arylene chromophores.

Novel chiral macrocycles consisting of two rigid oligoarylene rods and two chiral spiroindane clips have been synthesized by condensation of spiroindane diols and CF3-activated alpha-omega-difluorooligoaryls. Since a broad variety of planar aromatic macrocycles is known, our non-planar, chiral rings represent a new class of macrocyclic compounds. The first two examples, which contain quaterphenylene and diphenylbithiophene rods, are presented in this communication; for one of them a crystal structure is given. The chiroptical properties of the macrocycles can be interpreted as an interplay of the "intra-rod" helicity of individual oligoarylene rods and the "inter-rod" helicity between both chromophores of the macrocycle. The macrocycles can act as chiral dopands of commercially available, and novel, polymeric nematic liquid crystals (emissive polyfluorenes). The "intra-rod" helicity of individual oligoarylene rods is the main feature in determining the resulting helical twisting power (HTP). The cholestric induction in mesogenic, emissive polyfluorenes is of special interest for a realization of electronic devices that have a circularly polarized electroluminescence. The results are also important for an understanding of larger ensembles of chiral rodlike molecules, especially their pi-pi interactions.

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PubMed2001

Polyfluorenes with polyphenylene dendron side chains: toward non-aggregating, light-emitting polymers.

A polyfluorene 12 has been prepared in which bulky polyphenylene dendrimer substituents suppress formation of long wavelength emitting aggregates, thus giving a polymer with pure blue emission. Absorption- and emission spectra and molecular modeling confirm that the bulky dendrimer side chains do not cause extra torsion between the fluorene units. New polyfluorenes with 9,9-diaryl substituents have been prepared to determine the minimum size of substituent necessary for aggregation suppression. An LED using 12 has been demonstrated to produce blue emission with onset voltages below 4 V.

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PubMed2000

EPR probes with well-defined, long distances between two or three unpaired electrons

The synthesis of rod- and star-shaped compounds carrying two or three spin labels as end groups is described. The unpaired electrons are 2.8-5.1 nm apart from each other. The shape-persistent scaffolds were obtained through Pd-Cu-catalyzed alkynyl-aryl coupling and Pd-Cu-catalyzed alkyne dimerization in the presence of oxygen using p-phenyleneethynylene as the basic shape-persistent building block. The spin label 1-oxyl-2,2,5,5-tetramethylpyrroline-3-carboxylic acid (4) was attached through esterification of the terminal phenolic OH groups of the scaffold.

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PubMed2000

Formation, structure and conformational dynamics of highly substituted diphenylcarbonates

The symmetrical carbonates 5-8 were prepared from ethyl 4-hydroxybenzoates with aryl-, 4-(arylethynyl)-phenyl-, arylethynyl- or arylbutadiynyl-substituents in the 3- and 5-positions, by reaction with triphosgene. The choice of base (pyridine, DMAP, NaH) had a strong influence on the conversion: For the synthesis of the carbonates 5 and 6, it was sufficient to use pyridine as the base. However, for the synthesis of the carbonates 7 and 8, NaH had to be used instead. Single-crystal X-ray structure analysis of these carbonates revealed that the substituents point towards the corners of a distorted tetrahedron with the carbonate group sitting in the middle of the tetrahedron and the two angular phenolic building blocks intersecting with an angle of 51-71 degrees. In solution at room temperature, all four substituents are magnetically equivalent as a consequence of conformational flexibility. The two enantiomeric conformers of carbonate 5a interconvert rapidly, probably via a perfect trans conformation with the plane of the Aralpha perpendicular to the carbonate plane. In the case of carbonates 6-8 this process is inhibited by unfavourable interaction of the long substituents at the benzoate moiety. The dynamic process, which has an energy barrier of 8-10 kcalmol(-1), is described in a simplified manner as a continuous rotation of the angular building blocks around the carbonate unit with the C(aryl)-O bonds as the axes of rotation.

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PubMed2000

Synthesis and structural characterization of hexa-tert-butyl-hexa-peri-hexabenzocoronene, its radical cation salt and its tricarbonylchromium complex

The hexa-tert-butyl substituted hexa-peri-hexabenzocoronene was synthesized in an overall yield of 83% from 4-tert-butylphenylacetylene. The key step was the oxidative cyclodehydrogenation of hexa(4-tert-butylphenyl)benzene with anhydrous FeCl3 in CH2Cl2 The high solubility of hexa-tert-butyl-hexa-peri-hexabenzocoronene in common organic solvents allowed a comprehensive spectroscopic characterization of this compound in solution. Electrochemical oxidation at -30 degrees C in the presence of tetrabutylammonium hexafluoroarsenate led to the formation of a stable radical cation salt. Reaction of hexa-tert-butyl-hexa-peri-hexabenzocoronene with an excess of tricarbonyl(naphthalene)chromium in THF/dioxane afforded a mixture of mono- and bis-tricarbonylchromium complexes which could be separated by chromatography. The molecular structures of the parent compound, its radical cation salt and its mono- tricarbonylchromium complex were determined by X-ray analysis and discussed in detail. Remarkably, the crystal structures of these compounds are mainly dominated by the formation of dimers of the aromatic cores.

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