Evolution of photoluminescence across dimensionality in lanthanide silicates

The dehydratation process of layered lanthanide silicates K3[LnSi3O8(OH)2], Ln = Y, Eu, Tb, and Er, and the structural characterization of the obtained small-pore framework K3LnSi3O9, Ln = Y, Eu, Tb, and Er solids, named AV-23, have been reported. The structure of AV-23 has been solved by single-crystal X-ray diffraction (XRD) methods and further characterized by chemical analysis, thermogravimetry, scanning electron microscopy, and 29Si MAS NMR. The photoluminescence (PL), radiance, and lifetime values of AV-23 have been studied and compared with those of AV-22. Both materials have a similar chemical makeup and structures sharing analogous building blocks, hence providing a unique opportunity for rationalizing the evolution of the PL properties of lanthanide silicates across dimensionality. Although Tb-AV-23 contains a single crystallographic Tb(3+) site, PL spectroscopy indicates the presence of Ln(3+) centers in regular framework positions and in defect regions. PL evidence suggests that Eu-AV-23 contains a third type of Ln(3+) environment, namely, Eu(3+) ions replacing K(+) ions in the micropores. The radiance values of the Tb-AV-22 and Tb-AV-23 samples are of the same order of magnitude as those of standard Tb(3+) green phosphors. For the samples K3(Y1-aEraSi3O9), a = 0.005-1, efficient emission and larger 4I13/2 lifetimes (ca. 7 ms) are detected for low Er(3+) content, indicating that the Er(3+)-Er(3+) interactions become significant as the Er(3)+ content increases.     

Molecular and Conformational Properties of Dendronized Polymethacrylate in Organic Solvent

The conformational and dynamo-optical properties of a homologous series of polymethacrylate modified by first-generation Fréchet-type dendrons in the molecular mass range of 70–8800 kDa in organic solvents were studied by viscometry, isothermal diffusion, dynamic light scattering, and flow birefringence. The Mark-Kuhn-Houwink equations for this dendronized polymer in toluene and chloroform together with hydrodynamic diameter and Kuhn segment length in toluene are defined as well as the value of intrinsic anisotropy of polarizability for the monomer unit.     

pH-sensitive C-ON bond homolysis of alkoxyamines of imidazoline series with multiple ionizable groups as an approach for control of nitroxide mediated polymerization

Recently, a new concept of pH-switchable agents for reversible addition-fragmentation chain transfer (RAFT) polymerization has been introduced by Benaglia et al. (J. Am. Chem. Soc.2009, 131, 6914-6915). In this paper we extended the concept of pH-switchable mediators to nitroxide mediated polymerization (NMP) by employing nitroxides with basic or acidic groups as controlling agents. Four alkoxyamines, the derivatives of 2-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-2-(pyridin-4-yl)-2,5-dihydro-1H-imidazol-1-oxyl and 2-(2-carboxyethyl)-5,5-diethyl-2,4-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl, have been prepared. The influence of pH on alkoxyamine homolysis rate constants (k(d)) and on the nitroxide-alkyl radical recombination rate constants (k(c)) was studied. All alkoxyamines under study as well as the parent nitroxides have several basic groups, which under pH variation can undergo consecutive protonation. It was shown that the k(d) value under basic conditions are significantly (up to 15-fold) higher than in acidic solution at the same temperature, whereas the k(c) value in basic solutions decrease by a factor of 2 only. The efficiency of NMP is known to be dependent on k(d) and k(c), both constants being dependent on the monomer structure; therefore the performance of NMP of different monomers in the controlled mode requires different conditions. It is shown that the pH value crucially affects the polymerization regime, changing it from the controlled to the uncontrolled mode. The controlled regime of NMP of different hydrophilic monomers (sodium 4-styrenesulphonate and acrylamide) in aqueous solution under mild conditions (90 °C) can be achieved using the same alkoxyamine by the variation of the pH value. The chain length of polymers depends on pH value during the polymerization.     

Kinetics studies of rapid strain-promoted [3 + 2]-cycloadditions of nitrones with biaryl-aza-cyclooctynone

Strain-promoted cycloadditions of cyclic nitrones with biaryl-aza-cyclooctynone (BARAC) proceed with rate constants up to 47.3 M(-1) s(-1), this corresponds to a 47-fold rate enhancement relative to reaction of BARAC with benzyl azide and a 14-fold enhancement over previously reported strain promoted alkyne-nitrone cycloadditions (SPANC). Studies of the SPANC reaction using the linear free energy relationship defined by the Hammett equation demonstrated that the cycloaddition reaction has a rho value of 0.25 ± 0.04, indicating that reaction is not sensitive to substituents and thus should have broad applicability.     

Nonlinear pseudoparabolic equations as singular limit of reaction–diffusion equations

In this article, a solution of a nonlinear pseudoparabolic equation is constructed as a singular limit of a sequence of solutions of quasilinear hyperbolic equations. If a system with cross diffusion, modelling the reaction and diffusion of two biological, chemical, or physical substances, is reduced then such an hyperbolic equation is obtained. For regular solutions even uniqueness can be shown, although the needed regularity can only be proved in two dimensions.     

Photoluminescent layered lanthanide silicates

The hydrothermal synthesis and structural characterization of layered lanthanide silicates, K(3)[M(1-a)Ln(a)Si(3)O(8)(OH)(2)] (M = Y(3+), Tb(3+); Ln = Eu(3+), Er(3+), Tb(3+), and Gd(3+)), named AV-22 materials, are reported. The structure of these solids was elucidated by single-crystal (180 K) and powder X-ray diffraction and further characterized by chemical analysis, thermogravimetry, scanning electron microscopy, (29)Si MAS NMR, and photoluminescence spectroscopy. The Er-AV-22 material is a room-temperature infrared phosphor, while Tb- and Eu-AV-22 are visible emitters with output efficiencies comparable to standards used in commercial lamps. The structure of these materials allows the inclusion of a second (or even a third) type of Ln(3+) ion in the framework and, therefore, the fine-tuning of their photoluminescent properties. For the mixed Tb(3+)/Eu(3+) materials, evidence has been found of the inclusion of Eu(3+) ions in the interlayer space by replacing K+ ions, further allowing the activation of Tb(3+)-to-Eu(3+) energy transfer mechanisms. The occurrence probability of such mechanisms ranges from 0.62 (a = 0.05) to 1.20 ms(-1) (a = 0.1) with a high energy transfer efficiency (0.73 and 0.84, respectively).     

Cp2TiCl2-catalyzed reaction of symmetrical alkynes with α,ω-dicarboxylic acid esters and ЕtAlCl2: An original pathway to С5-С6 cyclic ketones and tetrasubstituted furans

An original method for the synthesis of С₅-С₆ cyclic ketones and tetrasubstituted furans was developed based on the Cp₂TiCl₂-catalyzed reaction of symmetrical alkynes and α,ω-dicarboxylates in the presence of Mg and ЕtAlCl₂. The length of the hydrocarbon chain between carboxyl groups in a,ω-dicarboxylic acid esters affects the chemoselectivity of the reaction. The formation of 2-alkylidene and 2-alkenyl cyclopent(hex)an-1-ones occurred in the reaction with esters of glutaric and adipic acids. α,ω-Dicarboxylic acid esters with more than four methylene units between carboxyl groups gave rise to substituted furans. Stereochemistry of 2-alkenyl cyclopent(hex)an-1-ones was determined based on the analysis of experimental and theoretical data obtained from DFT calculations.     

On the thermodynamic stability of (Eu,Y)-doped barium cerate

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Dramatically Enhanced Solubility of Halide‐Containing Organometallic Species in Diiodomethane: The Role of Solvent⋅⋅⋅Complex Halogen Bonding

In the current study, we evaluated the solubility of a number of organometallic species and showed that it is noticeably improved in diiodomethane when compared to other haloalkane solvents. The better solvation properties of CH₂I₂ were associated with the substantially better σ‐hole‐donating ability of this solvent, which results in the formation of uniquely strong solvent–(metal complex) halogen bonding. The strength of the halogen bonding is attenuated by the introduction of additional halogen atoms in the organometallic species owing to the competitive formation of more favourable intermolecular complex–complex halogen bonding. The exceptional solvation properties of diiodomethane and its inertness towards organometallic species make this solvent a good candidate for NMR studies, in particular, for the acquisition of spectra of insensitive spins.