Photoactive Cryptands. Crystal Structure of the Sodium Cryptate of the Tris(phenanthroline) Macrobicyclic Ligand

The crystal structure of the complex [phen · phen · phen] · NaBr · 2 CHCl₃ of the macrobicyclic ligand 1 has been determined. The complex is of cryptate type, the Na⁺ cation being contained in the molecular cavity of 1 and coordinated to all eight N-atoms. The ligand has a propeller shape and interconverts in solution between the two enantiomeric helical forms.     

Wide-line H NMR: a new application for the study of Brönsted acidity strength of solids with HY zeolite as example

The fitting of 4 K ¹H wide-line NMR spectra of Brönsted-acidic solid samples loaded with known amounts of water allows the determination of the concentration of the following species: H₃O⁺, H₂O…HO; and of the remaining initial species: OH and H₂O.     

Crystal Structures of the Lithium Cryptates of Two Macrobicyclic Ligands Containing Pyridine,Bipyridine, and Biisoquinoline Units

The crystal structures of the LiBr complexes of two macrobicyclic ligands, [py.py.bpy] 1 and [py.biqi.biqi] 2 , have been determined. They confirm the cryptate nature of these species. In 3 , the Li⁺ cation is bound in the shallow cavity of a ligand of flattened shape, whereas in 4 , it is buried inside the macrobicycle and surrounded by its three bridges.     

Classical and Roll-hemming Processes of Pre-strained Metallic Sheets

The goal of this investigation is to gather experimental data on the hemming of concave or convex edge-curved surface samples, to study the influence of geometry and pre-strain on roll-in and load, as well as to assess the validity of the numerical simulation of the process. A specific experimental set-up was designed to study classical and roll-hemming processes. The samples were pre-strained through biaxial tests to obtain a geometry and strain history similar to that of industrial parts. Flanging, pre-hemming, and hemming loads were recorded and geometrical measurements were taken after each step for the 6000 series aluminum alloy samples. It is shown that sample geometry influences both the roll-in and the load, though no clear relationship could be established. Results also show that the roll-in from roll-hemming is lower than from classical hemming and that its evolution greatly differs between the two processes. Tests on the different cases studied also indicate that classical hemming is more likely to cause damage.     

Effects of (multi)branching of dipolar chromophores on photophysical properties and two-photon absorption

To investigate the effect of branching on linear and nonlinear optical properties, a specific series of chromophores, epitome of (multi)branched dipoles, has been thoroughly explored by a combined theoretical and experimental approach. Excited-state structure calculations based on quantum-chemical techniques (time-dependent density functional theory) as well as a Frenkel exciton model nicely complement experimental photoluminescence and one- and two-photon absorption findings and contribute to their interpretation. This allowed us to get a deep insight into the nature of fundamental excited-state dynamics and the nonlinear optical (NLO) response involved. Both experiment and theory reveal that a multidimensional intramolecular charge transfer takes place from the donating moiety to the periphery of the branched molecules upon excitation, while fluorescence stems from an excited state localized on one of the dipolar branches. Branching is also observed to lead to cooperative enhancement of two-photon absorption (TPA) while maintaining high fluorescence quantum yield, thanks to localization of the emitting state. The comparison between results obtained in the Frenkel exciton scheme and ab initio results suggests the coherent coupling between branches as one of the possible mechanisms for the observed enhancement. New strategies for the rational design of NLO molecular assemblies are thus inferred on the basis of the acquired insights.     

Synthesis,Characterization, and Structural Properties of Luminescent Lanthanide Complexes

The crystal structure of the macrobicyclic europium(III) complex [Eu³⁺ ? 1 ]3Cl^? incorporating a N,N′ - dioxide unit has been determined. It confirms the cryptate nature of this species, the included cation being bound to six N- and two O-sites. The efficient shielding of the bound Eu³⁺ ion may be related to the efficient luminescence of this cryptate and points to the role played by the N-oxide sites. To further explore the effect of such binding groups, the two macrocyclic ligands 4 and 5 bearing two bipyridine N,N′ -dioxide lateral arms have been synthesized and their Eu^III and Tb^III complexes prepared.     

Constructing Hierarchical Set Systems

In this note, it is shown that by applying ranking procedures to data that allow, for any three objects a₁, a₂, b in a collection X of objects of interest, to make consistent decisions about which of the two objects a₁ or a₂ is more similar to b, a family of cluster systems can be constructed that start with the associated Apresjan Hierarchy and keep growing until, for k = #X–1, the full set of all subsets of X is reached. Various ideas regarding canonical modifications of the similarity values so that these cluster systems contain as many clusters as possible for small values of k (and in particular for k := 0) and/or are rooted at a specific element in X, possible applications, e.g. concerning (i) the comparison of distinct dissimilarity data defined on the same set X or (ii) diversity optimization, and new tasks arising in ranking statistics are also discussed.Received November 15, 2003     

Epoxide configuration in 3β-acetoxy-7α-hydroxy-14,15-epoxy-apotirucallane prepared from tirucalla-7,24-dien-3-one : X-ray analysis

Epoxidation of 3β - Acetoxy - 7α - hydroxy - Δ¹⁴ - apotirucallane, prepared from Δ7-tirucallone, afforded the corresponding 14α,15α epoxide. The α-configuration of the epoxide function has been established by single-crystal X-ray analysis, representing the first such study of a tirucallol derivative with an intact side-chain. Epoxidation of Apotirucalla - 3,7 - dione gave the corresponding 14α,15α - epoxide as well. Eight new tirucallol derivatives were described.