The Role of Radical Bridges in Polynuclear Single-Molecule Magnets

Employing radical bridges between anisotropic metal ions has been a viable route to achieve high-performance single-molecule magnets (SMMs). While the bridges have been mainly considered for their ability to promote exchange interactions, the crystal-field effect arising from them has not been taken into account explicitly. This lack of consideration may distort the understanding and limit the development of the entire family. To shed light on this aspect, herein we report a theoretical investigation of a series of N -radical-bridged diterbium complexes. It is found that while promoting strong exchange coupling between the terbium ions, the N -radical induces a crystal field that interferes destructively with that of the outer ligands, and thus reduces the overall SMM behavior. Based on the theoretical results, we conclude that the SMM behavior in this series could be further maximized if the crystal field of the outer ligands is designed to be collinear with that of the radical bridge. This conclusion can be generalized to all exchange-coupled SMMs.     

Rheological and physical properties of aliphatic hyperbranched polyesters

The effects of the size (pseudo‐generation number) and nature of end groups on physical and rheological properties were investigated for a series of hyperbranched polyesters based on an ethoxylated pentaerythritol core and 2,2‐bis‐(hydroxymethyl)propionic acid repeat units. The observed linear dependence of the melt viscosity on the molar mass in the high pseudo‐generation‐number limit indicated that entanglement effects were substantially absent. Moreover, the marked influence of end capping of the end groups on the physical and rheological properties suggested that intermolecular interactions were dominated by contacts between the outer shells of the molecules, in which the end groups were assumed to be concentrated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1218–1225, 2004     

Graphitic nanofillers in PMMA nanocomposites—An investigation of particle size and dispersion and their influence on nanocomposite properties

Mechanical, thermal, and electrical properties of graphite/PMMA composites have been evaluated as functions of particle size and dispersion of the graphitic nanofiller components via the use of three different graphitic nanofillers: “as received graphite” (ARG), “expanded graphite,” (EG) and “graphite nanoplatelets” (GNPs) EG, a graphitic materials with much lower density than ARG, was prepared from ARG flakes via an acid intercalation and thermal expansion. Subsequent sonication of EG in a liquid yielded GNPs as thin stacks of graphitic platelets with thicknesses of ～10 nm. Solution‐based processing was used to prepare PMMA composites with these three fillers. Dynamic mechanical analysis, thermal analysis, and electrical impedance measurements were carried out on the resulting composites, demonstrating that reduced particle size, high surface area, and increased surface roughness can significantly alter the graphite/polymer interface and enhance the mechanical, thermal, and electrical properties of the polymer matrix. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2097–2112, 2007     

Prompt gamma cold neutron activation analysis applied to biological materials

Summary Cold neutrons at the external neutron guide laboratory (ELLA) of the KFA Jülich are used to demonstrate their profitable application for multielement characterization of biological materials. The set-up and experimental conditions of the Prompt Gamma Cold Neutron Activation Analysis (PGCNAA) device is described in detail. Results for C, H, N, S, K, B, and Cd using synthetic standards and the “ratio” technique for calculation are reported for several reference materials and prove the method to be reliable and complementary with respect to the elements being determined by INAA. IAEA fellow at the KFA from 6. 3. 91 to 2. 9. 91 on contract No. VIE/9016R.     

Fluorescence from the second excited singlet state of [18] annulenes

The emission spectra of the second excited singlet state of [18] annulene and of monofluoro [18] annulene (in a 3-methylpentane glass at 4 K) are reported. The large energy gap between the first and second excited singlet states inhibits fast internal conversion and favours the appearance of S₂ → S₀ emission. In addition, fluorescence from the S₁ state can be observed in monofluoro [13] annulene by exciting into the S₂ or directly into the S₁ absorption.     

Synthesis of coumarins by ring-closing metathesis using Grubbs’ catalyst

A novel generally applicable synthesis of coumarins from phenolic substrates utilizing ring-closing metathesis is described. This sequence involves O-allylation of phenols followed by ortho-Claisen rearrangement, subsequent based-induced isomerization affording 2-(1-propenyl)phenols, acylation with acryloyl chloride, and finally ring-closing metathesis (RCM) with Grubbs’ second generation catalyst.     

Factors controlling the regioselectivity of additions to α-enones—III: Reactions of 3-aryl α-enones with phosphorylated anions

Reaction of phosphonoester 2 and phosphononitrile 3 with chalcone and p-methoxychaleone in THF-t-BuOK at room temperature gives only the product resulting from CC double bond attack. The same reagents with benzalacetone lead to mixture of products resulting from CC double bond and carbonyl attack, though phosphine oxide 4 gives only the products of CC attack. Dypnone gives products of carbonyl attack with 3 and does not react with 2.These results are discussed in terms of perturbation theory: C₄ attack increases with delocalization of the reagent's negative charge and lowering of the α-enone LUMO level.     

Action des bromures de perfluoroalkylmagnésiums sur les aldéhydes α,βéthyléniques

Perfluoroalkylmagnesium bromide, C_nF_2n+1MgBr (n = 2, 6, 8), reacts with α,β-unsaturated aldehydes to give allylic alcohols as the sole products. No attack on the β-carbon atom of the carbonyl was detected, even when the reaction was carried out in the presence of CuCl. Part of the Grignard reagent decomposes to trans-R_FCFCFBr and trans-R_FCFCFI. Neither a mechanism involving dissociation into R⁺_F MgX^- nor the formation of a carbene intermediate is capable of explaining the thermal decomposition of the Grignard reagent.