Coordination‐Driven Self‐Assembly of PEO‐Functionalized Perylene Bisimides: Supramolecular Diversity from a Limited Set of Molecular Building Blocks

A limited number of poly(ethylene oxide)‐substituted perylene bisimides, some of which are equipped with terpyridine ligands for transition‐metal coordination (see structure), combine different types of noncovalent interactions to yield optoelectronically active organic materials with different types of supramolecular morphologies.

     

Solid-state synthesis of monazite-type compounds containing tetravalent elements

On the basis of optimized grinding/heating cycles developed for several phosphate-based ceramics, the preparation of brabantite and then monazite/brabantite solid solutions loaded with tetravalent thorium, uranium, and cerium (as a plutonium surrogate) was examined versus the heating temperature. The chemical reactions and transformations occurring when heating the initial mixtures of AnO2/CeO2, CaHPO(4).2H2O (or CaO), and NH4H2PO4 were identified through X-ray diffraction (XRD) and thermogravimetric/differential thermal analysis experiments. The incorporation of thorium, which presents only one stabilized oxidation state, occurs at 1100 degrees C. At this temperature, all the thorium-brabantite samples appear to be pure and single phase as suggested by XRD, electron probe microanalyses, and micro-Raman spectroscopy. By the same method, tetravalent uranium can be also stabilized in uranium-brabantite, i.e., Ca0.5U0.5PO4, after heating at 1200 degrees C. Both brabantites, Ca0.5Th0.5PO4 and Ca0.5U0.5PO4, begin to decompose when increasing the temperature to 1400 and 1300 degrees C, respectively, leading to a mixture of CaO and AnO2 by the volatilization of P4O10. In contrast to the cases of thorium and uranium, cerium(IV) is not stabilized during the heating treatment at high temperature. Indeed, the formation of Ca0.5Ce0.5PO4 appears impossible, due to the partial reduction of cerium(IV) into cerium(III) above 840 degrees C. Consequently, the systems always appear polyphase, with compositions of CeIII1-2xCeIVxCaxPO4 and Ca2P2O7. The same conclusion can be also given when discussing the incorporation of cerium(IV) into La1-2xCeIIIx-yCeIVyCay(PO4)1-x+y. This incomplete incorporation of cerium(IV) confirms the results obtained when trying to stabilize tetravalent plutonium in Ca0.5PuIV0.5PO4 samples.     

High Glass‐Transition Temperature Polymer Networks Harnessing the Dynamic Ring Opening of Pinacol Boronates

Differential scanning calorimetry of high molar mass poly(4‐vinylphenylboronic acid, pinacol ester)s evidenced unusual reactive events above 120 °C, resulting in a high glass‐transition temperature of 220 °C. A reversible ring‐opening reactivity of pinacol boronates is proposed, involving a nucleophilic attack on the sp² boron and subsequent bridging between boron atoms by interconnected pinacol moieties to form a densely crosslinked network with high T_g. FTIR, solid‐state NMR investigations, and rheology studies on the polymer as well as double‐tagging analyses on molecular model structures and theoretical calculations further support this hypothesis and indicate a ring‐opening inducing crosslinking. When diluted in an apolar solvent such as toluene, the polymer network can be resolubilized via ring closing, thus recovering the entropically favored linear chains featuring cyclic boronate esters.     

On the value of the critical point in fractal percolation

We derive a new lower bound p_c>0.8107 for the critical value of Mandelbrot's dyadic fractal percolation model. This is achieved by taking the random fractal set (to be denoted A_∞) and adding to it a countable number of straight line segments, chosen in a certain (nonrandom) way as to simplify greatly the connectivity structure. We denote the modified model thus obtained by C_∞, and write C_n for the set formed after n steps in its construction. Now it is possible, using an iterative technique, to compute the probability of percolating through C_n for any parameter value p and any finite n. For p=0.8107 and n=360 we obtain a value less than 10^?5; using some topological arguments it follows that 0.8107 is subcritical for C_∞ and hence (since _∞ dominates A_∞) for A_∞. ©2001 John Wiley & Sons, Inc. Random Struct. Alg., 18: 332–345, 2001.     

Methylene Bridging Effect on the Structures,Lewis Acidities and Optical Properties of Semi-planar Triarylboranes

Three synthetic methods towards semi-planar triarylboranes with two aryl rings connected by a methylene bridge have been developed. The fine-tuning of their stereoelectronic properties and Lewis acidities was achieved by introducing fluorine, methyl, methoxy, n-butyl and phenyl groups either at their exocyclic or bridged aryl rings. X-ray diffraction analysis and quantum-chemical calculations provided quantitative information on the structural distortion experienced by the near planar hydro-boraanthracene skeleton during the association with Lewis bases such as NH₃ and F⁻. Though the methylene bridge between the ortho-positions of two aryl rings of triarylboranes decreased the Gibbs free energies of complexation with small Lewis bases by less than 5 kJ mol⁻¹ relative to the classical Lewis acid BAr₃, the steric shielding of the CH₂ bridge is sufficient to avoid the formation of Lewis adducts with larger Lewis bases such as triarylphosphines. A newly synthesized spirocyclic amino-borane with a long intramolecular B−N bond that could be dissociated under thermal process, UV-irradiation, or acidic conditions might be a potential candidate in Lewis pairs catalysis.