Molecular wire formation from viologen assemblies

The adsorption behavior of viologen alpha,omega-dithiols (viologen dithiols) on gold has been investigated. At short exposures, a low-coverage phase consisting of flat-lying molecules has been determined by STM and IR spectroscopy. In contrast, multilayer films are formed after long adsorption times. Single molecular wires could be formed between a gold STM tip and a surface with a low coverage of the adsorbed dithiols, and their electrical behavior was investigated. Molecular conductivity was determined either by the repeated measurement of I(s) curves or by recording I-V curves for different tip-sample separations. These methods concurred in producing a value of (0.5 +/- 0.1) nS for the single-molecule conductivity of the alpha,omega-viologen dithiol molecule HS-6V6-SH. The high conductivity of HS-6V6-SH, as compared to that of HS-C12-SH, may be related to the low-lying LUMO, which provides a barrier indentation for electron transport in a two-step electron-transfer mechanism.     

Structure-property relationships in redox-gated single molecule junctions--a comparison of pyrrolo-tetrathiafulvalene and viologen redox groups

We demonstrate that the electrical "switching" behavior of single molecules connected between two electrode contacts can be controlled by altering their structure and electrochemical characteristics. The electrical properties of gold|molecule|gold single molecule junctions incorporating HS(CH2)6-X-(CH2)6SH, where X = viologen (4,4'-bipyridinium) or pyrrolotetrathiafulvalene, are determined using a scanning tunneling microscopy based technique. The switching behavior, controlled through a tuneable electrochemical gate, changes from an on-off response (viologen) to an off-on-off response (pyrrolotetrathiafulvalene) on changing the central redox group. In contrast, the electrical properties of junctions incorporating redox-inactive HS(CH2)6-1,4-C6H4-(CH2)6SH do not alter significantly as a function of applied potential.     

Evaluation of 1,2,3-Triazoles as Amide Bioisosteres In Cystic Fibrosis Transmembrane Conductance Regulator Modulators VX-770 and VX-809

The 1,2,3-triazole has been successfully utilized as an amide bioisostere in multiple therapeutic contexts. Based on this precedent, triazole analogues derived from VX-809 and VX-770, prominent amide-containing modulators of the cystic fibrosis transmembrane conductance regulator (CFTR), were synthesized and evaluated for CFTR modulation. Triazole 11 , derived from VX-809, displayed markedly reduced efficacy in F508del-CFTR correction in cellular TECC assays in comparison to VX-809. Surprisingly, triazole analogues derived from potentiator VX-770 displayed no potentiation of F508del, G551D, or WT-CFTR in cellular Ussing chamber assays. However, patch clamp analysis revealed that triazole 60 potentiates WT-CFTR similarly to VX-770. The efficacy of 60 in the cell-free patch clamp experiment suggests that the loss of activity in the cellular assay could be due to the inability of VX-770 triazole derivatives to reach the CFTR binding site. Moreover, in addition to the negative impact on biological activity, triazoles in both structural classes displayed decreased metabolic stability in human microsomes relative to the analogous amides. In contrast to the many studies that demonstrate the advantages of using the 1,2,3-triazole, these findings highlight the negative impacts that can arise from replacement of the amide with the triazole and suggest that caution is warranted when considering use of the 1,2,3-triazole as an amide bioisostere.     

Reversible gelation of poly(dimethylsiloxane) with ionic and hydrogen‐bonding substituents

Poly(dimethylsiloxane) copolymers containing a small fraction of carboxylic acid or Zn‐carboxylate groups were prepared and compared regarding reversible gelation by hydrogen‐bonding and ion‐pair interaction. The polymers were synthesized by condensation of a t‐butylcarboxylate functionalized dichlorosilane with an α,ω‐dihydroxy‐poly(dimethylsiloxane), followed by thermal cleavage of the ester bond. Neutralization of the resulting carboxylic acid substituents was achieved by addition of Zn (acac)₂. Reversible crosslinking was investigated by step stress and oscillating shear experiments. The carboxylic acid containing poly(dimethylsiloxane) became rubberlike upon increasing the temperature and liquified again when it was brought back to room temperature. This observation has been explained tentatively by segregation of the carboxylic acid groups into polar domains at high temperatures [i.e., a behavior like it is observed for systems with a lower critical solution temperature (LCST)]. At ambient temperature, the carboxylic acid groups undergo hydrogen bonding to the Si–O–Si backbone. Clustering of the carboxylic acid groups occurs only as these hydrogen bonds break upon raising temperature. Moisture was found to have a strong influence on the reversal of the crosslinking. Addition of zinc acetylacetonate resulted in the formation of an elastic network already at ambient conditions consistent with the concept of ionomers which undergo reversible gelation by formation of ion‐pair multiplets and clusters in the hydrophobic polymer matrix in particularly at low temperatures. At high temperature, both the carboxylic acid and the carboxylate sample exhibited a rather similar viscoelastic behavior consistent with a common structure where transient crosslinks are formed by clusters of the carboxylic acid and the carboxylate groups. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 485–495, 1999     

Deformation and failure kinetics of iPP polymorphs

In this study, the mechanical performance of the different polymorphs of isotactic polypropylene, typically present in iPP crystallized under industrial processing conditions, is assessed. Different preparation strategies were used to obtain samples consisting of almost solely α, β, or γ crystals. X‐Ray measurements were used to validate that the desired phase was obtained. The intrinsic true stress ‐ true strain response of all individual phases was measured in uniaxial compression at several strain rates (deformation kinetics). Moreover, measurements were performed over a wide temperature range, covering the window in between the glass transition and the melting temperature. The relation between obtained yield stress and the strain rate is described with a modification of the Ree‐Eyring model. Differences and similarities in the deformation kinetics of the different phases are presented and discussed. Furthermore, the presence of three deformation processes, acting in parallel, is revealed. The Ree‐Eyring equation enables lifetime prediction for given thermal and mechanical conditions. These predictions were experimentally validated using constant load tests in uniaxial compression. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 729–747     

Postpolymerization modification of poly(pentafluorophenyl methacrylate): Synthesis of a diverse water‐soluble polymer library

This article explores the feasibility of poly(pentafluorophenyl methacrylate) (PPFMA) prepared by reversible addition fragmentation chain transfer (RAFT) polymerization as a platform for the preparation of diverse libraries of functional polymers via postpolymerization modification with primary amines. Experiments with a broad range of functional amines and PPFMA precursors of different molecular weights indicated that the postpolymerization modification reaction proceeds with good to excellent conversion for a diverse variety of functional amines and is essentially independent of the PPFMA precursor molecular weight. The RAFT end group, which was well preserved throughout the polymerization, is cleaved during postpolymerization modification to generate a thiol end group that provides possibilities for further orthogonal chain‐end modification reactions. The degree of postpolymerization modification can be controlled by varying the relative amount of primary amine that is used and random polymethacrylamide copolymers can be prepared via a one‐pot/two‐step sequential addition procedure. Cytotoxicity experiments revealed that the postpolymerization modification strategy does not lead to any additional toxicity compared with the corresponding polymer obtained via direct polymerization, which makes this approach also of interest for the synthesis of biologically active polymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4332–4345, 2009     

一种具有Luneburg透镜特性的轴向GRIN介质透镜

本文给出一种轴向GRIN(梯度折射率)介质透镜。这种透镜具有 Luneburg(卢内堡)透镜的特性。它可以使无限远光线锐成象;亦可用于点—点锐成象系统,光准直或光准直—聚焦系统;在微波频谱区域及有声成象、天线设计方面有一定应用价值。     

The Gaussian Moments Conjecture and the Jacobian Conjecture

We first propose what we call the Gaussian Moments Conjecture. We then show that the Jacobian Conjecture follows from the Gaussian Moments Conjecture. Note that the the Gaussian Moments Conjecture is a special case of [11, Conjecture 3.2]. The latter conjecture was referred to as the Moment Vanishing Conjecture in [7, Conjecture A] and the Integral Conjecture in [6, Conjecture 3.1] (for the one-dimensional case). We also give a counter-example to show that [11, Conjecture 3.2] fails in general for polynomials in more than two variables.     

Novel protective coatings for steel based on a combination of self-assembled monolayers and conducting polymers

Investigations for a new primer system for iron or low alloyed steel have led to first results. Several special phosphonic acids with thiophene derivatives as head groups have been synthesized. They form stable self-assembled monolayers (SAMs) on passivated iron by dipping the substrates into aqueous phosphonic acid solutions. SAM formation was validated by current potential curves and also by contact angle measurements, which showed an intensive hydrophobisation of the iron surface after the dipping process. Finally cyclovoltammetric (CV) experiments after SAM formation indicated the successful polymerisation of the immobilised thiophene derivatives.