Self-Processing,Diffusion-Based Photopolymers for Holographic Applications

The new photopolymers for holographic applications described herein are based on a cross-linked matrix in which the holographic grating is formed by photo-polymerization of guest monomers in an interference pattern of the recording light. Diffusion of monomer, triggered by this photo-polymerization, from the dark to the bright fringes of the interference pattern is the key parameter for creating high modulation in refractive index Δn during hologram recording. This leads to bright visual volume holograms with high diffraction efficiency. The holographic photopolymers are self-processing. After recording the hologram only (incoherent) light exposure is necessary to bleach the final product and fix the hologram. Unlike earlier photopolymers used in holography, these new materials offer the advantages of no chemical or thermal processing combined with low shrinkage and detuning. Additionally, due to good light sensitivity the formation of the holograms is fast and the film obtained after curing is highly transparent, which makes the material suitable for both, reflection and transmission holography.     

Positive cross correlations in a three-terminal quantum dot with ferromagnetic contacts

We study current fluctuations in an interacting three-terminal quantum dot with ferromagnetic leads. For appropriately polarized contacts, the transport through the dot is governed by dynamical spin blockade, i.e., a spin-dependent bunching of tunneling events not present in the paramagnetic case. This leads, for instance, to positive zero-frequency cross correlations of the currents in the output leads even in the absence of spin accumulation on the dot. We include the influence of spin-flip scattering and identify favorable conditions for the experimental observation of this effect with respect to polarization of the contacts and tunneling rates.     

Synthesis and conformational studies of a beta-turn mimetic incorporated in Leu-enkephalin

A beta-turn mimetic in which the four amino acids of a beta-turn have been replaced by a 10-membered ring has been designed, synthesized, and subjected to conformational studies. In the mimetic, the intramolecular CO(i)-HN(i)(+3) hydrogen bond that is often found in beta-turns has been replaced by an ethylene bridge. In addition, the amide bond between residues i and i + 1 was exchanged for a methylene ether isoster. Such a beta-turn mimetic, based on the first four residues of Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu), was prepared in 15 steps. The synthesis relied on a beta-azido alcohol prepared in five steps from Cbz-Tyr(tBu)-OH as a key, i-position building block. tert-Butyl bromoacetate, glycine, and a Phe-Leu dipetide were then used as building blocks for positions i + 1, i + 2, and i + 3, respectively. Conformational studies based on (1)H NMR data showed that the beta-turn mimetic was flexible, but that it resembled a type-II beta-turn at low temperature. This low energy conformer closely resembled the structure determined for crystalline Leu-enkephalin.     

Single-molecule chemistry and physics explored by low-temperature scanning probe microscopy

It is well known that scanning probe techniques such as scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) routinely offer atomic scale information on the geometric and the electronic structure of solids. Recent developments in STM and especially in non-contact AFM have allowed imaging and spectroscopy of individual molecules on surfaces with unprecedented spatial resolution, which makes it possible to study chemistry and physics at the single molecule level. In this feature article, we first review the physical concepts underlying image contrast in STM and AFM. We then focus on the key experimental considerations and use selected examples to demonstrate the capabilities of modern day low-temperature scanning probe microscopy in providing chemical insight at the single molecule level.     

Computational evidence for a variable first shell coordination of the cadmium(II) ion in aqueous solution

In this paper, we present a state-of-the-art 100 ns molecular dynamics simulation of a cadmium(II) aqueous solution that highlights a very flexible ion first coordination shell which transits between hexa- and heptahydrated complexes. From this investigation, a dynamical picture of the water exchange process emerges that takes place through an associative mechanism for the solvent substitution reaction. Our procedure starts from the generation of an effective two-body potential from quantum mechanical ab initio calculations in which the many-body ion-water terms are accounted for by the polarizable continuum method (PCM). This approach is computationally very efficient and has allowed us to carry out extremely long molecular dynamics simulations, indispensable to reproduce the dynamic properties of the cadmium(II) aqueous solution. Quantum mechanical ab initio calculations of the hexa- and heptahydrated complexes extracted from MD configurations have revealed stable minima for both clusters with the water molecules arranged in T(h)() and C(2) symmetries in the hexa- and heptahydrated complexes, respectively, with a slight energetic preference for the heptahydrated one. Finally, a comparison of the calculated hexa- and heptahydrated cluster IR and Raman spectra with the experimental data in the literature, has demonstrated that the IR spectroscopy is not able to distinguish between the two species, whereas the Raman spectrum of the Cd(2+)-(H(2)O)(7) cluster provides a better agreement with the experimental data.     

Coordination chemistry study of hydrated and solvated lead(II) ions in solution and solid state

The coordination chemistry of lead(II) in the oxygen donor solvents water, dimethylsulfoxide (dmso, Me(2)SO), N,N-dimethylformamide (dmf), N,N-dimethylacetamide (dma), N,N'-dimethylpropyleneurea (dmpu), and 1,1,3,3-tetramethylurea (tmu), as well as in the sulfur donor solvent N,N-dimethylthioformamide (dmtf), has been investigated by extended X-ray absorption fine structure (EXAFS) and/or large angle X-ray scattering (LAXS) in solution, and by single crystal X-ray diffraction and/or EXAFS of solid hydrates and solvates. Lead(II) may either form hemidirected complexes with large bond distance distribution and an apparent gap for excess electron density, or holodirected ones with a symmetric coordination sphere with normal bond distance distribution, depending on the strength of antibonding lead 6s/ligand np molecular orbital interactions and ligand-ligand interactions. The crystallographic data show that the solid lead(II) perchlorate and trifluoromethanesulfonate hydrate structures are hemidirected, while the solid lead(II) solvates of dma and dmpu have regular octahedral configuration with holodirected geometry and mean Pb-O bond distances in the range 2.50-2.52 ?. EXAFS data on the hydrated lead(II) ion in aqueous solution show broad bond distance distribution and a lack of inner-core multiple scattering contributions strongly indicating a hemidirected structure. The Pb-O bond distances found both by EXAFS and LAXS, 2.54(1) ?, point to a six-coordinate hydrated lead(II) ion in hemidirected fashion with an unevenly distributed electron density. The results obtained for the dmso solvated lead(II) ion in solution are ambiguous, but for the most part support a six-coordinate hemidirected complex. The mean Pb-O bond distances determined in dmf and dma solution by LAXS, 2.55(1) and 2.48(1) ?, respectively, indicate that in both solvate complexes lead(II) binds six solvent molecules with the former complex being hemidirected whereas the latter is holodirected. The dmpu and tmu solvated lead(II) ions have a regular holodirected octahedral configuration, as expected given their space-demanding characteristics and ligand-ligand intermolecular interactions. The dmtf solvated lead(II) ion in solution is most likely five-coordinate in a hemidirected configuration, with a mean Pb-S bond distance of 2.908(4) ?. New and improved ionic radii for the lead(II) ion in 4-8-coordination in hemi and holodirected configurations are proposed using crystallographic data.     

Black Silicon nanostructures on silicon thin films prepared by reactive ion etching

In this letter, the application of dry etching to thin films on glass is described. The utilized （ICP-RIE） of SF6 and 02 is discussed and a demonstrated. prepare Black Silicon nanostructures on crystalline silicon reactive ion etching with an inductively coupled plasma remarkable increase in light absorption of about 70% is demonstrated.     

Chelate effect and thermodynamics of metal complex formation in solution: a quantum chemical study

The accuracy of quantum chemical predictions of structures and thermodynamic data for metal complexes depends both on the quantum chemical methods and the chemical models used. A thermodynamic analogue of the Eigen-Wilkins mechanism for ligand substitution reactions (Model A) turns out to be sufficiently simple to catch the essential chemistry of complex formation reactions and allows quantum chemical calculations at the ab initio level of thermodynamic quantities both in gas phase and solution; the latter by using the conductor-like polarizable continuum (CPCM) model. Model A describes the complex formation as a two-step reaction: 1. [M(H2O)x](aq) + L(aq) <==>[M(H2O)x], L(aq); 2. [M(H2O)x], L(aq) <==>[M(H2O)(x-1)L],(H2O)(aq). The first step, the formation of an outer-sphere complex is described using the Fuoss equation and the second, the intramolecular exchange between an entering ligand from the second and water in the first coordination shell, using quantum chemical methods. The thermodynamic quantities for this model were compared to those for the reaction: [M(H2O)x](aq) + L(aq) <==>[M(H2O)(x-1)L](aq) + (H2O)(aq) (Model B), as calculated for each reactant and product separately. The models were tested using complex formation between Zn(2+) and ammonia, methylamine, and ethylenediamine, and complex formation and chelate ring closure reactions in binary and ternary UO(2)(2+)-oxalate systems. The results show that the Gibbs energy of reaction for Model A are not strongly dependent on the number of water ligands and the structure of the second coordination sphere; it provides a much more precise estimate of the thermodynamics of complex formation reactions in solution than that obtained from Model B. The agreement between the experimental and calculated data for the formation of Zn(NH(3))(2+)(aq) and Zn(NH(3))(2)(2+)(aq) is better than 8 kJ/mol for the former, as compared to 30 kJ/mol or larger, for the latter. The Gibbs energy of reaction obtained for the UO(2)(2+) oxalate systems using model B differs between 80 and 130 kJ/mol from the experimental results, whereas the agreement with Model A is better. The errors in the quantum chemical estimates of the entropy and enthalpy of reaction are somewhat larger than those for the Gibbs energy, but still in fair agreement with experiments; adding water molecules in the second coordination sphere improves the agreement significantly. Reasons for the different performance of the two models are discussed. The quantum chemical data were used to discuss the microscopic basis of experimental enthalpy and entropy data, to determine the enthalpy and entropy contributions in chelate ring closure reactions and to discuss the origin of the so-called "chelate effect". Contrary to many earlier suggestions, this is not even in the gas phase, a result of changes in translation entropy contributions. There is no simple explanation of the high stability of chelate complexes; it is a result of both enthalpy and entropy contributions that vary from one system to the other.     

Acceleration of the imine base/isocyanate (IBI)‐mediated polymerization of MMA caused by ionic liquid traces

The brutto rate of the imine base/isocyanate (IBI)‐mediated radical polymerization of methyl methacrylate (MMA) can be significantly increased by use of ionic liquid (IL) traces. At least, catalytic amounts of IL influence both the value of the brutto polymerization rate ν_Br,0 and the necessary reaction temperature of the used IBI mixture. Combinations of 2‐phenyl‐2‐oxazoline (POX) or 1‐methyl pyrazole (1MP) with isocyanates are IBI systems that usually do not initiate MMA at room temperature. By adding traces of 1‐ethyl‐3‐methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([Emim]FAP), polymethyl methacrylate (PMMA) with high average molecular weight can be obtained whereas the initiator mixture (imine base/isocyanate) concentration can be decreased by a factor of 10. The polymerization kinetics of several IBI combinations in the presence of ILs has been determined and a comparison to non‐IL containing initiator mixtures is given. Additionally, the temperature dependence of the IL‐containing polymerizations was measured. The interaction of the IL with MMA and the individual IBI initiator components is studied by means of attenuated total reflection Fourier transformation middle infrared spectroscopy (ATR FT MIR). Furthermore, the IBI brutto polymerization rate constants k_Br,0 were brought into relation to the IL hydrogen bond donating ability α. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

On the structure and dynamics of the hydrated sulfite ion in aqueous solution--an ab initio QMCF MD simulation and large angle X-ray scattering study

Theoretical ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism has been applied in conjunction to experimental large angle X-ray scattering to study the structure and dynamics of the hydrated sulfite ion in aqueous solution. The results show that there is a considerable effect of the lone electron-pair on sulfur concerning structure and dynamics in comparison with the sulfate ion with higher oxidation number and symmetry of the hydration shell. The S-O bond distance in the hydrated sulfite ion has been determined to 1.53(1) ? by both methods. The hydrogen bonds between the three water molecules bound to each sulfite oxygen are only slightly stronger than those in bulk water. The sulfite ion can therefore be regarded as a weak structure maker. The water exchange rate is somewhat slower for the sulfite ion than for the sulfate ion, τ(0.5) = 3.2 and 2.6 ps, respectively. An even more striking observation in the angular radial distribution (ARD) functions is that the for sulfite ion the water exchange takes place in close vicinity of the lone electron-pair directed at its sides, while in principle no water exchange did take place of the water molecules hydrogen bound to sulfite oxygens during the simulation time. This is also confirmed when detailed pathway analysis is conducted. The simulation showed that the water molecules hydrogen bound to the sulfite oxygens can move inside the hydration shell to the area outside the lone electron-pair and there be exchanged. On the other hand, for the hydrated sulfate ion in aqueous solution one can clearly see from the ARD that the distribution of exchange events is symmetrical around the entire hydration sphere.