Conformationally Flexible Bis(9‐fluorenylidene)porphyrin Diradicaloids

A stable 5,10‐bis(9‐fluorenylidene)porphyrin (Por‐Fl) diradicaloid was synthesized. It shows a quinoidal, saddle‐shaped geometry in the single crystal but can be thermally populated to a triplet diradical both in solution and in the solid state. Coordination with the Ni²⁺ ion (Por‐Fl‐Ni) does not significantly change the contorted conformation but reduces the singlet–triplet gap. Heat‐induced geometric change can explain the observed paramagnetic properties as well as unusual hysteresis in SQUID measurements. On the other hand, protonation (Por‐Fl‐2H⁺) dramatically changes the conformation while maintains the closed‐shell electronic structure. Our studies demonstrate how heat, coordination, and protonation affect the geometry, diradical character, and physical properties of conformationally flexible open‐shell singlet diradicaloids.     

Mg2+‐Dependent High Mechanical Anisotropy of Three‐Way‐Junction pRNA as Revealed by Single‐Molecule Force Spectroscopy

Mechanical anisotropy is ubiquitous in biological tissues but is hard to reproduce in synthetic biomaterials. Developing molecular building blocks with anisotropic mechanical response is the key towards engineering anisotropic biomaterials. The three‐way‐junction (3WJ) pRNA, derived from ϕ 29 DNA packaging motor, shows strong mechanical anisotropy upon Mg²⁺ binding. In the absence of Mg²⁺, 3WJ‐pRNA is mechanically weak without noticeable mechanical anisotropy. In the presence of Mg²⁺, the unfolding forces can differ by more than 4‐fold along different pulling directions, ranging from about 47 pN to about 219 pN. Mechanical anisotropy of 3WJ‐pRNA stems from pulling direction dependent cooperativity for the rupture of two Mg²⁺ binding sites, which is a novel mechanism for the mechanical anisotropy of biomacromolecules. It is anticipated that 3WJ‐pRNA can be used as a key element for the construction of biomaterials with controllable mechanical anisotropy.     

Discovery of Hexagonal Structured Pd–B Nanocrystals

We report on hexagonal close-packed (hcp) palladium (Pd)–boron (B) nanocrystals (NCs) by heavy B doping into face-centered cubic (fcc) Pd NCs. Scanning transmission electron microscopy–electron energy loss spectroscopy and synchrotron powder X-ray diffraction measurements demonstrated that the B atoms are homogeneously distributed inside the hcp Pd lattice. The large paramagnetic susceptibility of Pd is significantly suppressed in Pd–B NCs in good agreement with the reduction of density of states at Fermi energy suggested by X-ray absorption near-edge structure and theoretical calculations.     

Porous polycrystals built up by uniformly and axisymmetrically oriented needles: homogenization of elastic propertiesPolycristaux poreux constitués d'aiguilles orientées de façon uniforme ou axisymétrique : homogénéisation des propriétés élastiques

Porous polycrystal-type microstructures built up of needle-like platelets or sheets are characteristic for a number of biological and man-made materials. Herein, we consider (i) uniform, (ii) axisymmetrical orientation distribution of linear elastic, isotropic as well as anisotropic needles. Axisymmetrical needle orientation requires derivation of the Hill tensor for arbitrarily oriented ellipsoidal inclusions with one axis tending towards infinity, embedded in a transversely isotropic matrix; therefore, Laws' integral expression of the Hill tensor is evaluated employing the theory of rational functions. For a porosity lower 0.4, the elastic properties of the polycrystal with uniformly oriented needles are quasi-identical to those of a polycrystal with solid spheres. However, as opposed to the sphere-based model, the needle-based model does not predict a percolation threshold. As regards axisymmetrical orientation distribution of needles, two effects are remarkable: Firstly, the sharper the cone of orientations the higher the anisotropy of the polycrystal. Secondly, for a given cone, the anisotropy increases with the porosity. Estimates for the polycrystal stiffness are hardly influenced by the anisotropy of the bone mineral needles. Our results also confirm the very high degree of orientation randomness of crystals building up mineral foams in bone tissues. To cite this article: A. Fritsch et al., C. R. Mecanique 334 (2006).