Electronic Structure of the Positive Radical of 13C-Labeled Poly(3-Octylthienylene Vinylene) Polymer

Poly(3-octylthienylene vinylene) (O-PTV) has a great potential as low-bandgap p-type semiconductor for photovoltaic applications. Here, the positive radical state (positive polaron) is induced chemically in the O-PTV polymer in which the vinylene sites are selectively ¹³C-labeled. Using multi-frequency continuous wave and pulsed electron paramagnetic resonance, the g tensor and maximum ¹H and ¹³C hyperfine couplings are determined. A comparative density functional theory (DFT) analysis of PTV-like oligomers is performed. The experimental parameters suggest a larger localization of the positive polaron than follows from the DFT analysis. The counter anion may play a crucial role in localizing the polaron.     

Efficient Simulation of Markov Chains Using Segmentation

A methodology is proposed that is suitable for efficient simulation of continuous-time Markov chains that are nearly-completely decomposable. For such Markov chains the effort to adequately explore the state space via Crude Monte Carlo (CMC) simulation can be extremely large. The purpose of this paper is to provide a fast alternative to the standard CMC algorithm, which we call Aggregate Monte Carlo (AMC). The idea of the AMC algorithm is to reduce the jumping back and forth of the Markov chain in small subregions of the state space. We accomplish this by aggregating such problem regions into single states. We discuss two methods to identify collections of states where the Markov chain may become ‘trapped’: the stochastic watershed segmentation from image analysis, and a graph-theoretic decomposition method. As a motivating application, we consider the problem of estimating the charge carrier mobility of disordered organic semiconductors, which contain low-energy regions in which the charge carrier can quickly become stuck. It is shown that the AMC estimator for the charge carrier mobility reduces computational costs by several orders of magnitude compared to the CMC estimator.     

Magneto-Sensitive Elastomers: An Experimental Point of View

Magneto-sensitive elastomers (MSEs) are smart materials changing their shape and mechanical properties in the presence of a magnetic field. Focussing on model systems, silicone based MSEs are prepared by a multi-step mixing process and characterised using a rotational rheometer (plate-plate). Data obtained by relaxation tests is used to set-up a material model coupling the theories of viscoelasticity and magnetoelasticity. The behaviour of MSEs in quasi-static and dynamic mechanical shear experiments can be successfully predicted by the analytical model using parameters received by fitting the transient experiments. The model is validated for small shear deformations (γ = 0.02) and low magnetic fields (𝔹 = 0.2 T). (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

New Poincaré-type inequalities

We present some Poincaré-type inequalities for quadratic matrix fields with applications e.g. in gradient plasticity or fluid dynamics. In particular, applications to the pseudostress–velocity formulation of the stationary Stokes problem and to infinitesimal gradient plasticity are discussed.     

Compactness and flow minimization requirements in reforestation initiatives: a heuristic solution method

A heuristic solution method to identify a predefined number of cells in raster maps optimizing both multiple intrinsic cell characteristics and cell patch compactness (Vanegas et al. in Lect. Notes Comput. Sci. 5072:389–404, 2008), is extended so that also spatial interaction is taken into account. Spatial interaction is e.g., present in the flow of sediment between cells in a rasterised watershed as modified by the presence or absence of tree cover in a given cell. The resulting HLC-method is elaborated and tested in the context of a site location problem for reforestation in which the following criteria are considered: (1) the sediment flow reaching the outlet of the watershed is minimized, (2) two intrinsic environmental performances of the cells are maximized/minimized, and (3) the selected cells form a compact patch. To evaluate the performance of the HLC, its results are compared to the ones obtained with an Integer Programming (IP) formulation. This comparison suggests that the HLC identifies high quality, near-to-optimal patches of cells and that its computational efficiency makes it applicable to regional sized cases.     

An infeasible-point subgradient method using adaptive approximate projections

We propose a new subgradient method for the minimization of nonsmooth convex functions over a convex set. To speed up computations we use adaptive approximate projections only requiring to move within a certain distance of the exact projections (which decreases in the course of the algorithm). In particular, the iterates in our method can be infeasible throughout the whole procedure. Nevertheless, we provide conditions which ensure convergence to an optimal feasible point under suitable assumptions. One convergence result deals with step size sequences that are fixed a priori. Two other results handle dynamic Polyak-type step sizes depending on a lower or upper estimate of the optimal objective function value, respectively. Additionally, we briefly sketch two applications: Optimization with convex chance constraints, and finding the minimum ? ₁-norm solution to an underdetermined linear system, an important problem in Compressed Sensing.     

A method for treating the passage of a charged hard sphere ion as it passes through a sharp dielectric boundary

In the implicit solvent models of electrolytes (such as the primitive model (PM)), the ions are modeled as point charges in the centers of spheres (hard spheres in the case of the PM). The surfaces of the spheres are not polarizable which makes these models appropriate to use in computer simulations of electrolyte systems where these ions do not leave their host dielectrics. The same assumption makes them inappropriate in simulations where these ions cross dielectric boundaries because the interaction energy of the point charge with the polarization charge induced on the dielectric boundary diverges. In this paper, we propose a procedure to treat the passage of such ions through dielectric interfaces with an interpolation method. Inspired by the "bubble ion" model (in which the ion's surface is polarizable), we define a space-dependent effective dielectric coefficient, ε(eff)(r), for the ion that overlaps with the dielectric boundary. Then, we replace the "bubble ion" with a point charge that has an effective charge q/ε(eff)(r) and remove the portion of the dielectric boundary where the ion overlaps with it. We implement the interpolation procedure using the induced charge computation method [D. Boda, D. Gillespie, W. Nonner, D. Henderson, and B. Eisenberg, Phys. Rev. E 69, 046702 (2004)]. We analyze the various energy terms using a spherical ion passing through an infinite flat dielectric boundary as an example.     

Synthesen und ungewöhnliche Mesophasensequenzen von Kohlenwasserstoffen eines neuartigen Typs nicht-calamitischer Flüssigkristalle

Abstract

Mehrere Vertreter einer neuen Familie thermotroper, nicht-calamitischer Flüssigkristalle, die p-substituierten, biaxialen und zentrosymmetrischen 1,2,3,5,6,7-Hexakis(phenylethinyl)naphthaline 3a-f, wurden in einer Pd-kataly-sierten CC-Kupplungsreaktion aus dem entsprechenden Hexabromnaphthalin und verschiedenartig monosubstituiertem Acetylen (2a-f) synthetisiert. Diese Synthesemethode stellt ein vielversprechendes, ‘Lego-artiges’ Aufbauprinzip für Mesogene mit flächendeckenden, hoch ungesättigten und konjugierten π-Systemen als neuen Typ von ‘Super-disc-Kernen’ dar. Die stabilen Mesophasenbereiche von 3a-f reichen von c. 42 bis 180 K. Mit einer Ausnahme (3a) zeigen alle neuen Verbindungen eine stabile, fluide N_D-Phase. Während ein Kohlenwasserstoff dieser Serie (3e) sowie der Hexaether 3f nur eine Mesophase ausbilden, zeigen die vier Kohlenwasserstoffe 3a-d Multimesomorphismus mit einer inversen Phasen-sequenz (C → N_D → 2 D → I) bei den mittleren Homologen 3b-d. Beim schnellen Heizen der N_D-Phase des Kohlenwasserstoffs 3d ist kurzfristig eine instabile reentrant-isotrope Phase zu beobachten, die sich in eine columnare Mesophase umwandelt. der Vergleich der Kohlenwasserstoffe 3a-e mit dem Hexaether 3f läßt erstmals bei nicht-calamitischen Flüssigkristallen schlüssig einen elektronischen Effekt von Sauerstoffatomen, den Bindegliedern zwischen den leicht polarisierbaren ‘Super-disc-Kernen’ und den Lateralfunktionen, auf die Stabilität ihrer N_D-Phasen nachweisen. Sauerstoffatome in diesen Positionen stabilisieren die N_D-Phase, oder, mit anderen Worten, destabilisieren columnare Mesophasen.     

Mesophase behaviour and thermal stability of octa-alkoxy substituted phthalocyaninatocobalt (II) complexes

Cobalt (II) phthalocyanines substituted with eight alkoxy chains in the peripheral (2, 3, 9, 10, 16, 17, 23, 24) positions were prepared. The alkoxy chain length was varied between n-butyloxy (C₄H₉O) and n-octadecyloxy (C₁₈H₃₇O). Studies by polarizing optical microscopy and high temperature X-ray diffraction revealed that all the complexes are liquid crystalline and that they exhibit a hexagonal columnar mesophase (Col_h). Transition enthalpies were determined by differential scanning calorimetry. The clearing point could only be observed for compounds with a chain length longer than C₁₃H₂₇O. Both the melting and clearing points decrease with increasing chain length. The transition temperatures of these discotic metallomesogens are higher than those of the corresponding metal-free phthalocyanines, but are comparable with those of the corresponding copper (II) compounds. The thermal decomposition of the compounds was studied by thermogravimetry.     

Unusual Formation of a β-linked Disaccharide in a Nis Glycosylation

Abstract

The N-halosuccinimide glycosylation is a highly selective reaction that leads to trans-configured 1-alkoxy-2-halo-glycosides (halo = bromo, iodo).²³ As an exception to the generally observed exclusive formation of α-linked glycosides in such reactions,² we obtained a 3 / 1 mixture of α- and β-disaccharide 6 and 7, when we treated the silylated glycal 4 with NIS (N-iodosuccinimide) and the glycoside 5.⁴ The similarly protected arabino glycal 9, on the other hand, gave exclusively the expected α-linked saccharide 11, when treated with NIS and the alcohol component 10.⁵ Silylated glycals 4 and 9 were obtained from L-digitoxal 1 ⁶ and L-rhamnal 8 ⁷ by treatment with tert-butyldiphenylchlorosilane⁸ and tert-butyldimethylchlorosilane,⁹ respectively. In both cases the 3-O-silylated derivatives formed in high yields. Only in case of the ribo-configuration minor amounts of a 4-O-silylated product 3 were identified.