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Martin Baumgarten

Publications and source records attributed to Martin Baumgarten.

6 recordsLinked to original sources
PubMed2006

Metal-biradical chains from a high-spin ligand and bis(hexafluoroacetylacetonato)copper(II).

The synthesis, X-ray crystal structure, and magnetic studies of a rare example of organic/inorganic spin hybrid clusters extended in infinite ladder-type chain [Cu(C5F6HO2)2]7(C35H35N5O4)2 ([Cu(hfac)2]7(pyacbisNN)2, 2) formed by the reaction of a high spin nitronylnitroxide biradical C35H35N5O4 (pyacbisNN, 1) and bis(hexafluroacetylacetonate)copper(II) = Cu(hfac)2 are described. Single-crystal X-ray structure analysis revealed the triclinic P1 space group of 2 with the following parameters: a = 10.6191(4) A, b = 19.6384(7) A, c = 21.941(9) A, alpha = 107.111(7) degrees, beta = 95.107(8) degrees, gamma = 94.208(0) degrees , Z = 2. Each repeating unit in 2 carries a centrosymmetric cyclic six spin and a linear five spin cluster with four different copper coordination environments having octahedral and square planar geometries. These clusters are interconnected to form infinite chains which are running along the crystallographic b axis. The magnetic measurements show nearly paramagnetic behavior with very small variations over a large temperature range. The magnetic properties are thus result of complex competitions of many weak ferro- and antiferromagnetic interactions, which appear as small deviations from quite linear mu(eff) vs T dependence at low temperature. At high temperature (300-14 K), antiferromagnetic behavior dominates a little, while at very low temperature (14-2 K), a small increase of mu(eff) was observed. The magnetic susceptibility data are described by the Curie-Weiss law [chi = C/(T - theta)] with the optimal parameters C = 4.32 +/- 0.01 emuK/mol and theta = - 0.6 +/- 0.3 K, where C is the Curie constant and theta is the Weiss temperature.

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PubMed2006

Pyrene as chromophore and electrophore: encapsulation in a rigid polyphenylene shell.

Starting from the fourfold ethynyl-substituted chromophore 1,3,6,8-tetraethynylpyrene as core, a series of polyphenylene dendrimers was prepared in high yield by combining divergent and convergent growth methods. The fluorescence quantum yields (Q(f)>0.92) of the encapsulated pyrene chromophore were independent of the size of the polyphenylene shell. Fluorescence quenching studies and temperature-dependent fluorescence spectroscopy were performed to investigate the site isolation of the core. They indicate that a second-generation dendrimer layer is needed to efficiently shield the encapsulated pyrene and prevent aggregate formation. Alkali-metal reduction of the encapsulated pyrene core was carried out to afford the corresponding pyrene radical anions, for which hampered electron transfer to the core was observed with increasing dendrimer generation, which is further proof of the site isolation due to the polyphenylene shell. To improve film formation and solubility of the material, solubilizing alkyl chains were introduced on the periphery of the spherical particles. Furthermore, highly transparent films obtained by a simple drop-casting method showed blue emission mainly from the unaggregated species. The materials presented herein combine high quantum efficiency, good solubility, and improved film-forming properties, which make them possible candidates for several applications in electronic devices.

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PubMed2005

Multiple functionalization of benzophenones inside polyphenylene dendrimers--toward entrapped ions and radicals.

Polyphenylene dendrimers possessing a defined number of keto groups in the dendritic scaffold have been synthesized by using a benzophenone-functionalized tetraphenylcyclopentadienone branching unit. A postsynthetic functionalization of the polyphenylene backbone was achieved by reacting the entrapped keto groups with organolithium reagents yielding monodisperse alcohol products. To investigate the accessibility and reactivity of the embedded groups, many functions of different size and nature, for example, the chromophore pyrene, were introduced. Moreover, suitable precursors for the synthesis of dendrimer entrapped species, trityl cations, trityl radicals, and ketyl radical anions, were obtained. To gain insight into the structure of these newly functionalized dendrimers, UV/vis, EPR, and NMR measurements have been performed. They showed a delocalization of the charge/spin into the polyphenylene dendritic arms leading to a stabilization of the ions/radicals. Remarkably, for the ketyl radicals, EPR measurements indicated the occurrence of intermolecular metal-bridged biradicals. They suggest the existence of a dendritic radical network of the dendrimers themselves.

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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.

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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.

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