Optical Properties of Silver Nanoclusters Synthesized in Surface-Grafted Polyacrylic Acid at Different Grafting Densities

Optics and Spectroscopy - Fluorescent silver nanoclusters immobilized on a plastic substrate are obtained by photoinduced template synthesis using polyacrylic acid chemically grafted to the...     

Dianionic Titanyl and Vanadyl (Cation+)2[MIVO(Pc4−)]2− Phthalocyanine Salts Containing Pc4− Macrocycles

In this study, the titanyl and vanadyl phthalocyanine (Pc) salts (Bu₄N⁺)₂[M^IVO(Pc^4?)]^2? (M=Ti, V) and (Bu₃MeP⁺)₂[M^IVO(Pc^4?)]^2? (M=Ti, V) with [M^IVO(Pc^4?)]^2? dianions were synthesized and characterized. Reduction of M^IVO(Pc^2?) carried out with an excess of sodium fluorenone ketyl in the presence of Bu₄N⁺ or Bu₃MeP⁺ is exclusive to the phthalocyanine centers, forming Pc^4? species. During reduction, the metal +4 charge did not change, implying that Pc is an non‐innocent ligand. The Pc negative charge increase caused the C?N(pyr) bonds to elongate and the C?N(imine) bonds to alternate, thus increasing the distortion of Pc. Jahn–Teller effects are significant in the [eg(π*)]² dianion ground state and can additionally distort the Pc macrocycles. Blueshifts of the Soret and Q‐bands were observed in the UV/Vis/NIR when M^IVO(Pc^2?) was reduced to [M^IVO(Pc ^{. 3?})] ^{. ?} and [M^IVO(Pc^4?)]^2?. From magnetic measurements, [Ti^IVO(Pc^4?)]^2? was found to be diamagnetic and (Bu₄N⁺)₂[V^IVO(Pc^4?)]^2? and (Bu₃MeP⁺)₂[V^IVO(Pc^4?)]^2? were found to have magnetic moments of 1.72–1.78 μ_B corresponding to an S=1/2 spin state owing to V^IV electron spin. As a result, two latter salts show EPR signals with V^IV hyperfine coupling.     

Effect of molecular and lattice structure on hydrogen transfer in molecular crystals of diamino-dinitroethylene and triamino-trinitrobenzene

We have studied the intra- and intermolecular hydrogen transfer in a crystalline 1,1-diamino-2,2-dinitroethylene (DADNE) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) by means of an embedded cluster method and density functional theory (DFT). We found that, even though both of these materials have similar amino- and nitro- functional groups and layered crystalline structures, there are important differences in the mechanisms of hydrogen transfer. In particular, our calculations suggest that the proton migration from an amino-group to a nitro-group of the same molecule is a feasible process in TATB but not in DADNE. At the same time, we have found that no intermolecular hydrogen transfer occurs in either molecular crystal. These results imply that the activation of the decomposition reactions proceeds via different paths in these two materials.     

Self-assembly and high anisotropy thermal expansion of compounds consisting of <Emphasis Type="Italic">T</Emphasis>O<Subscript>3</Subscript> triangular radicals

Thermal motion is the inherent driving force of crystal substances formation. In borates, carbonates and nitrates such an organizing force are the strong anisotropy of thermal vibrations of atoms in the TO₃ triangles (T = B, C, N), flat triborate and other B–O rigid groups containing TO₃ triangles. These triangles and rigid groups, due to the sharp anisotropy of thermal vibrations, tend to be arranged in the parallel (or preferable) orientation. In this case, chemical compound has usually the least energy. The examples of the self-organization of atoms, TO₃ triangles and rigid groups during crystallization process are given.     

Thermal expansion and structural complexity of strontium borates

Thermal expansion of four strontium borates has been studied by high-temperature powder X-ray diffraction. Strong anisotropy of thermal expansion is observed for the structures of Sr₃B₂O₆ (0D) and SrB₂O₄ (1D) based on triangles BO₃ only: The high expansion occurs perpendicular to the BO₃ plane, i.e. along the direction of the less strong bonds in the crystal structure. The monoclinic Sr₂B₁₆O₂₆ (3D) borate expands dramatically due to shear deformations in monoclinic plane: Monoclinic β′ angle changes significantly because it is not fixed by symmetry. In contrast to these borates, SrB₄O₇ (3D) borate built up from tetrahedra only expands almost isotropically. Average value of volume expansion is 36 × 10^?6 K^?1 for studied Sr-borates. Tendency of slight decrease in the volume thermal expansion and sharp decrease in the melting points is observed with an increase in B₂O₃ content as a result of the degree of polymerization increase. As the B₂O₃ content grows in Sr-borates, the structural complexity increases in the SrO–B₂O₃ system due to more complex anionic structure. Inverse “melting temperature decrease—structural complexity increase” correlation is revealed.     

Structure and mechanical property variations in Mg–Gd–Y–Zn–Zr alloy depending on its composition and processing conditions

An effect of alloying element content on mechanical properties and precipitate formation in Mg–RE alloys was studied for Mg–8Gd–4Y–1Zn–0.4Zr (wt%) and Mg–10Gd–5Y–1.8Zn–0.4Zr (wt%). It is shown that small variations in the alloying element concentration can be used to manipulate the alloy microstructure and precipitate formation towards eliminating the asymmetry (tension/compression) and anisotropy of yield stress.     

From insulator to electride: a theoretical model of nanoporous oxide 12CaO.7Al2O3

Recently, a novel inorganic electride stable at room temperatures has been obtained by reducing a complex nanoporous oxide 12CaO.7Al2O3 (C12A7) in a Ca atmosphere (Matsuishi, S.; Toda, Y.; Miyakawa, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Tanaka, I.; Hosono, H. Science 2003, 301, 626). In this system, up to 2.3 x 1021/cm3 electrons can be accommodated in a three-dimensional network of cages formed by a positively charged oxide framework. We demonstrate theoretically that at all concentrations, ne, the electrons are neither associated with specific atoms nor fully delocalized. At low ne, the electrons are isolated from each other and resemble the color centers in insulating materials. They are well localized in some of the lattice cages and yield strong inhomogeneous lattice distortions that provide polaron-type cage-to-cage electron hopping. As ne increases, the electrons form a denser electron gas and become more evenly spread over all available lattice cages. At sufficiently high ne, the system becomes metallic but still retains partially localized character of the conducting electrons. We describe the nature of the electronic states at the Fermi level and predict the changes in the optical and magnetic properties of this system as a function of ne.     

Electronic Communication between S=1/2 Spins in Negatively‐charged Double‐caged Fullerene C60 Derivative Bonded by Two Single Bonds and Pyrrolizidine Bridge

Radical anion salt {cryptand[2.2.2] (K⁺)}₂(bispheroid)^2??3.5C₆H₄Cl₂ ( 1 ) of the double‐caged fullerene C₆₀ derivative, in which fullerene cages are linked by a cyclobutane bridging cycle and additionally by a pyrrolizidine moiety, was obtained. Each fullerene cage in this derivative accepts one electron on reduction, thus forming the (bispheroid)^2? dianions with two interacting S=1/2 spins on the neighboring cages. Low‐temperature magnetic measurements reveal a singlet ground state of the bispheroid dianions whereas triplet contributions prevail at increased temperature. An estimated exchange interaction between two spins J/k_B=?78 K in 1 indicates strong magnetic coupling between them, nearly two times higher than that (J/k_B=?44.7 K) in previously studied (C₆₀^?)₂ dimers linked via a cyclobutane bridge only. The enhancement of magnetic coupling in 1 can be explained by a shorter distance between the fullerene cages and, possibly, an additional channel for the magnetic exchange provided by a pyrrolizidine bridge. Quantum‐chemical calculations of the lowest electronic state of the dianions by means of multi‐configuration quasi‐degenerate perturbation theory support the experimental findings.