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Biomedical subjects

Volker Enkelmann

Publications and source records attributed to Volker Enkelmann.

11 recordsLinked to original sources
PubMed2004

A multifunctional high-spin biradical pyrazolylbipyridine-bisnitronylnitroxide.

[structure: see text] Synthesis and UV-vis, IR, and EPR spectroscopic characterizations, together with X-ray structural analysis, of functional nitronyl- and iminonitroxides attached to pyrazolylbipyridine are described. The exchange interactions between the nitronylnitroxides are found to be stronger than for the iminonitroxides. Although the substitution of a pyridine with the pyrazole ring leads to shorter distances and larger dipolar couplings, the exchange interaction is diminished. While compound 1 forms dimers in the solid state, the terpyridine 3 leads to supramolecular pi-stacking.

Journal Article↗
PubMed2004

From branched hydrocarbon propellers to C3-symmetric graphite disks.

Branched hydrocarbon propellers (9 and 12) were prepared by stepwise palladium-catalyzed Hagihara-Sonogashira coupling reactions and Diels-Alder cycloadditions and submitted to oxidative cyclodehydrogenation by FeCl3 to afford two new giant graphite disks 2 and 3 with 3-fold symmetry in high yield. One of the precursors, the polyphenylene dendrimer 9 containing 150 carbon atoms, was crystallographically characterized. The poorly soluble graphite disks were characterized by isotope-resolved MALDI-TOF mass spectroscopy, and their electronic and vibrational properties were investigated by solid-state UV-vis and Raman spectroscopy. The effect of molecular size and geometry on their properties was discussed. The molecule 2 represents the largest, 3-fold-symmetric all-benzenoid graphite disk, and the five-membered rings in molecule 3 open the opportunity to synthesize large bowl-shaped PAHs.

Journal Article↗
PubMed2004

Graphitic molecules with partial "zig/zag" periphery.

A new synthetic protocol that affords novel hexa-peri-hexabenzocoronene (HBC) derivatives with partial "zig/zag" periphery and dramatically different electronic properties has been developed, which represents an important step toward understanding structure-property relationships of graphitic materials. The scope of this methodology was broadened by application to a much larger polycyclic aromatic hydrocarbon (PAH) with a more extended "zig/zag" character.

Journal Article↗
PubMed2004

Cruciform pi-systems: effect of aggregation on emission.

The solid state properties of cruciform pentamers are examined in thin film preparation, in the single crystalline state and in nanoparticle formulations; emission behavior was found to vary substantially with the solid state morphology. This type of behaviour is an excellent way to manipulate the emissive properties of conjugated [small pi]-systems.

Journal Article↗
PubMed2004

The role of hydrogen bonding in the crystal structures of zinc phosphate hydrates.

The compounds alpha- and beta-hopeite have been synthesised by hydrothermal crystallisation from aqueous solution at 90 degrees C and 20 degrees C, respectively. The crystal structures of these polymorphic forms of zinc phosphate tetrahydrate (ZPT), Zn(3)(PO(4))(2).4 H(2)O, have been resolved. Single-crystal analysis proves that the main difference between the alpha and beta forms of ZPT is caused by the difference in orientation of one of the water molecules in the ZnO(6) octahedral network, indicating two different hydrogen-bonding patterns. A previously unknown hopeite, Zn(3)(HPO(4))(3).3 H(2)O (ZHPT), has been isolated and analysed. This helps to achieve a better understanding of the mechanism of formation of zinc phosphate compounds. Unambiguous identification of each phase is established by analysis of their unique thermal behaviour and thermodynamic interrelationship.

Crystallization↗
PubMed2003

Study on the heteroatom influence in pyridine-based nitronyl nitroxide biradicals with phenylethynyl spacers on the molecular ground state.

Novel pyridine-based nitronyl nitroxide (NIT) biradicals, 3,5-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (1) and 2,6-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (2), and monoradicals, 4-(5-bromopyridine-3-ylethynyl)-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (3), 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (4), and 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)pyridine (5), were synthesized and investigated by ESR and UV-vis spectroscopy. The solution EPR measurements of the biradicals gave well-resolved, nine-line spectra with exact half line spacing as compared to monoradicals (giso = 2.0067) with isotropic line spacing /aN/= 7.36 G. This indicates strong, intramolecular exchange coupling (J >> 7 x 10(-4) cm(-1); J/aN >> 1) of the biradicals with in the limit of EPR. The temperature dependence on the Deltams = +/-2 signal intensity of biradicals follow Curie behavior down to 4 K ascertaining the triplet ground state or its near-degeneracy with the singlet state. UV-vis studies of 1-5 show characteristic differences in the extinctions of n-pi transitions around 600 nm. Both biradicals 1 and 2 were crystallized in monoclinic space groups C2/c and P2(1)/a with the intraradical distances 1.54 and 1.47 nm, respectively. Computational studies of the biradicals 1, 2, and 1,3-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]benzene (T) were performed by the AM1/CAS(8,8) method to calculate the singlet-triplet (DeltaEST) energy difference and the spin density distribution. Results show that the position of the pyridyl nitrogen in 1 and 2 in comparison with T does not alter the triplet ground-state spin multiplicities supporting the obtained experimental results.

Journal Article↗
PubMed2002

Single-crystal structures of polyphenylene dendrimers.

A series of first-generation polyphenylene dendrimers based on three different cores were prepared by Diels-Alder cycloaddition and their single-crystal structures were determined. Consisting exclusively of interlocked, twisted phenyl rings, these polyphenylene nanostructures have exciting structural and dynamic properties. Single crystals of dendrimers, suitable for X-ray structure analysis, were grown from different solvent mixtures by slow evaporation at room temperature. It should be pointed out that one of the described polyphenylene dendrimers represents up to now the biggest oligophenylene nanostructure from which crystallographic data is available.

Journal Article↗
PubMed2002

Solvent triggering between conformational states in amphiphilic shape-persistent macrocycles.

The amphiphilic shape-persistent macrocycle 1 containing four phenol-OH groups as polar side groups and four hexyloxy groups as nonpolar side groups in an adaptable arrangement was recrystallized from solvents of different polarity. X-ray crystallography reveals that the conformation of the macrocycle is solvent dependent such that in the pyridine solvate only two of the nonpolar side groups point outward while in the THF solvate all four of them point outward. Moreover, in the latter case the three-dimensional packing leads to the formation of a supramolecular channel structure with a large pore size.

Crystallography, X-Ray↗