ZnO-latex hybrids obtained by polymer-controlled crystallization: a spectroscopic investigation

Micro- and submicrosized ZnO-polymer hybrid materials were synthesized by precipitating zinc oxide from an aqueous medium in the presence of poly(styrene-acrylic acid) latex nanoparticles, prepared by miniemulsion polymerization. Up to 10 wt % of the latex becomes incorporated into the crystals. Although the long-range order of the inorganic material is essentially not altered by the polymer, studies by photoluminescence (PL) spectroscopy and electron paramagnetic resonance (EPR) show that the latex particles influence the optical and paramagnetic properties of the hybrids, which can be correlated with changes in the defect structure. Typical PL emission spectra showed a narrow UV peak and a defect-related broad band in the green-yellow spectral region. The former emission is attributed to exciton recombination, whereas the latter seems to be related with deep-level donors. Latex acts as a quencher of the visible emission, and compared to pure ZnO, ZnO-latex hybrids show a significantly lower PL intensity in the visible range. A noticeable dynamic behavior of the PL, clearly more remarkable in the presence of latex, was observed, and it is explained in terms of photodesorption of oxygen adsorbed at surface positions. EPR provided additional information about crystal defects and species with unpaired electrons. All EPR spectra showed a single signal at g approximately 1.96, whose intensity and temperature dependence did not correlate with those of the PL visible band. These findings indicate that the green-yellow emission and the EPR signal of our samples have a different physical origins.     

Femtosecond dynamics of the tert-butyl radical, t-C4H9

The excited-state dynamics of the tert-butyl radical, t-C4H9, was investigated by femtosecond time-resolved photoionization and photoelectron spectroscopy. The experiments were supported by ab initio calculations. tert-Butyl radicals, generated by flash pyrolysis of azo-tert-butane, were excited into the A 2A1 (3s) state between 347 and 307 nm and the 3p band at 274 and 268 nm and ionized by 810-nm radiation, in a [1 + 2'] or [1 + 3'] process. Electronic structure calculations confirm that the two states are of s and p Rydberg characters, respectively. The carbon framework becomes planar and thus ion-like in both states. The photoelectron spectra are broad and seem to be mediated by accidental intermediate resonances in the probe step. All time-resolved photoelectron spectra can be described by a single decay time. For the A 2A1 state, lifetimes between 180 and 69 fs were measured. Surprisingly, a much longer lifetime of around 2 ps was found for the 3p state. To understand the decay dynamics, the potential energy was computed as a function of several important nuclear coordinates. A [1,2] H-atom shift to the isobutyl radical seems not to be important for the excited-state dynamics. Qualitative considerations indicate curve crossings between the ground state, the 3s state, and a valence state along the asymmetric C-C stretch coordinate that correlates to the dimethylcarbene + methyl product channel. The implications of the present study for earlier work on the nanosecond time scale are discussed.     

Perovskite tungsten bronze-type crystals of LixWO3 grown by chemical vapour transport and their characterisation

Crystals of Li_xWO₃ with nominal compositions, x=0.1, 0.25, 0.3, 0.35, 0.4 and 0.45 were grown by chemical vapour transport method using HgCl₂ as transporting agent. A complete transport was achieved with a temperature gradient T₁/T₂=800/700 °C revealing bluish-black crystals of sizes up to a few 10th of a millimeter. X-ray powder diffraction and infrared (IR) absorption spectra show Perovskite tungsten bronze of cubic symmetry (PTB_c) for x=0.45 and 0.4, mixed phase of PTB_c and Perovskite tungsten bronze of tetragonal symmetry (PTB_t) for x=0.35, 0.3 and 0.25 and of PTB_t and Perovskite tungsten bronze of orthorhombic symmetry (PTB_o) for x=0.1. The structure of PTB_t is explained by the off centring of the W-ions along c and tilting of the WO₆ octahedra around c. Crystal slices of mixed phase (i.e. PTB_c and PTB_t) reveal bright and dark areas on a sub-millimeter scale which are separated by sharp interfaces. Laser ablation inductively coupled plasma optical emission (LA ICP OES) analysis on small spot sizes show the separation into Li contents of x=0.18 (bright areas) and x=0.38 (dark areas) as threshold compositions of PTB_t and PTB_c, respectively. Polarized reflectivity using a microscope technique in the bright area of the crystals indicates strong anisotropic absorption effects with maximum between 1000 and 6000 cm⁻¹, which are related to optical excitations of polarons. Crystals of composition x=0.4 and 0.45 appear optically homogeneous and show an effective “free carrier-type plasma frequency” (w_p) of about 12,900 and 13,700 cm⁻¹, respectively.     

Crystal structure, spectroscopic properties, and magnetic behavior of the fluoride-derivatized lanthanoid(III) ortho-oxomolybdates(VI) LnF[MoO4] (Ln=Sm-Tm)

The fluoride-derivatized lanthanoid(III) ortho-oxomolybdates(VI) LnF[MoO₄] (Ln=Sm-Tm) crystallize in the monoclinic space group P2₁/c with four formula units per unit cell (a=516-528 pm, b=1220-1248 pm, c=659-678 pm, β=112.5-113.1°). The structure contains one crystallographically unique Ln³⁺ cation surrounded by two fluoride and six oxide anions in a square antiprism (CN=8). The square antiprisms [LnF₂O₆] are interconnected via three edges to form layers parallel (010), which are cross-linked along [010] by Mo⁶⁺ in tetrahedral oxygen coordination to form the three-dimensional crystal structure. The fluoride anions within this arrangement exhibit a twofold coordination of Ln³⁺ cations in the shape of a boomerang, which is connected to another F⁻ anion to form planar [F₂Ln₂]⁴⁺ rhombuses. Magnetic measurements for GdF[MoO₄], TbF[MoO₄], and DyF[MoO₄] show Curie-Weiss behavior, despite the peculiar arrangement of the lanthanoid(III) cations in layers comparable with those of gray arsenic. Furthermore, Raman, infrared, and diffuse reflectance spectroscopy data for these compounds were recorded and interpreted.     

Supramolecular bidentate ligands by metal-directed in situ formation of antiparallel beta-sheet structures and application in asymmetric catalysis

The principles of protein structure design, molecular recognition, and supramolecular and combinatorial chemistry have been applied to develop a convergent metal-ion-assisted self-assembly approach that is a very simple and effective method for the de novo design and the construction of topologically predetermined antiparallel beta-sheet structures and self-assembled catalysts. A new concept of in situ generation of bidentate P-ligands for transition-metal catalysis, in which two complementary, monodentate, peptide-based ligands are brought together by employing peptide secondary structure motif as constructing tool to direct the self-assembly process, is achieved through formation of stable beta-sheet motifs and subsequent control of selectivity. The supramolecular structures were studied by (1)H, (31)P, and (13)C NMR spectroscopy, ESI mass spectrometry, X-ray structure analysis, and theoretical calculations. Our initial catalysis results confirm the close relationship between the self-assembled sheet conformations and the catalytic activity of these metallopeptides in the asymmetric rhodium-catalyzed hydroformylation. Good catalyst activity and moderate enantioselectivity were observed for the selected combination of catalyst and substrate, but most importantly the concept of this new methodology was successfully proven. This work presents a perspective interface between protein design and supramolecular catalysis for the design of beta-sheet mimetics and screening of libraries of self-organizing supramolecular catalysts.     

Structures,Bonding Analyses and Reactivity of a Dicationic Digallene and Diindene Mimicking trans-bent Ditetrylenes

The reaction of bisdicyclohexylphosphinoethane (dcpe) and the subvalent M^I sources [M^I(PhF)₂][pf] (M=Ga⁺, In⁺; [pf]⁻=[Al(OR^F)₄]⁻; R^F=C(CF₃)₃) yielded the salts [{M(dcpe)}₂][pf]₂, containing the first dicationic, trans-bent digallene and diindene structures reported so far. The non-classical M^I⇆M^I double bonds are surprisingly short and display a ditetrylene-like structure. The bonding situation was extensively analyzed by quantum chemical calculations, QTAIM (Quantum Theory of Atoms in Molecules) and EDA-NOCV (Energy Decomposition Analysis with the combination of Natural Orbitals for Chemical Valence) analyses and is compared to that in the isoelectronic and isostructural, but neutral digermenes and distannenes. The dissolved [{Ga(dcpe)}₂]²⁺([pf]⁻)₂ readily reacts with 1-hexene, cyclooctyne, diphenyldisulfide, diphenylphosphine and under mild conditions at room temperature. This reactivity is analyzed and rationalized.     

Bifunctionalized Intrinsically Microporous Polyimides with Simultaneously Enhanced Gas Permeability and Selectivity

Two novel intrinsically microporous copolyimides synthesized by condensation reaction of 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 3,3,3′,3′‐tetramethyl‐1,1′‐spirobisindane‐5,5′‐diamino‐6,6′‐diol, and 3,5‐diaminobenzoic acid with diamine ratios of 80/20 (Co‐80/20) and 50/50 (Co‐50/50) are reported. Unexpectedly, the Co‐80/20 not only demonstrates higher microporosity (300 m² g⁻¹) than the PIM‐6FDA‐OH homopolymer (190 m² g⁻¹) but also exhibits simultaneously enhanced CO₂ permeability (from 119 to 171 Barrer) and CO₂/CH₄ selectivity (from 35 to 41) after thermal annealing at 250 °C. This higher permeability originates from enhanced diffusivity (D _CO2) and the higher selectivity results from its increased diffusion selectivity (D _CO2/D _CH4). After crosslinking at 300 °C, the Co‐80/20 exhibits an even higher CO₂ permeability (261 Barrer) and almost unchanged CO₂/CH₄ selectivity.

     

Two instructive instabilities in non-linear elasticity: Biaxially loaded membrane,and rubber balloons

Elastic bodies buckle under compressive loads, i.e. solutions become non-unique, they bifurcate and the body becomes unstable. Similar phenomena occur in tension as is evidenced here by the symmetric biaxial loading of a square membrane. Symmetry breaking removes the non-uniqueness. Under non-symmetric loading the load-deformation curves become non-monotone, consequently a hysteresis occurs which is the reflection of a fold-type catastrophy. This instructive instability was discovered by Kearsley [1]. Here we investigate it more fully and present some additional aspects.Balloons have non-monotone pressure-radius relations which suggest non-trivial stability properties. A stability analysis is presented for two interconnected balloons. In this we follow — and expand on — the analyses presented by Dreyer et al. [2] and Kitsche et al. [3].General Invited Lecture presented at AIMETA '95 — 12th Congress of the Italian Association of Theoretical and Applied Mechanics, Napoli, 3–6 October, 1995.