Hydrogen-bonded oligo(p-phenylenevinylene) functionalized with perylene bisimide: self-assembly and energy transfer

We describe the synthesis, supramolecular ordering on surfaces and in solution, and photophysical characterization of OPV4UT-PERY, an oligo(p-phenylenevinylene) (OPV) with a covalently attached perylene bisimide moiety. In chloroform, the molecule forms dimers through quadruple hydrogen bonding of the ureido-s-triazine array. This is supported by scanning tunneling microscopy (STM) studies, which reveal dimer formation at the liquid (1,2,4-trichlorobenzene)/solid (graphite) interface. Moreover, contrast reversal in bias-dependent STM imaging provides information on the ordering and different electronic properties of the oligo(p-phenylenevinylene) and perylene bisimide moieties. In dodecane, the molecule self-assembles into H-type aggregates that are still soluble as a result of the hydrophobic shell formed by the dodecyloxy wedges. The donor-acceptor molecule is characterized by efficient energy transfer from the photoexcited OPV to the perylene bisimide. Mixed assemblies with analogous OPVs lacking the perylene bisimide unit have been prepared in dodecane solution and energy transfer to the incorporated perylene bisimides has been studied by fluorescence spectroscopy.     

Phase behavior of mixtures of oppositely charged nanoparticles: heterogeneous Poisson-Boltzmann cell model applied to lysozyme and succinylated lysozyme

We study the phase behavior of mixtures of oppositely charged nanoparticles, both theoretically and experimentally. As an experimental model system we consider mixtures of lysozyme and lysozyme that has been chemically modified in such a way that its charge is nearly equal in magnitude but opposite in sign to that of unmodified lysozyme. We observe reversible macroscopic phase separation that is sensitive not only to protein concentration and ionic strength, but also to temperature. We introduce a heterogeneous Poisson-Boltzmann cell model that generally applies to mixtures of oppositely charged nanoparticles. To account for the phase behavior of our experimental model system, in addition to steric and electrostatic interactions, we need to include a temperature-dependent short-ranged interaction between the lysozyme molecules, the exact origin of which is unknown. The strength and temperature dependence of the short-ranged attraction is found to be of the same order of magnitude as that between unmodified lysozyme molecules. The presence of a rather strong short-ranged attraction in our model system precludes the formation of colloidal liquid phases (or complex coacervates) such as those typically found in mixtures of globular protein molecules and oppositely charged polyelectrolytes.     

Chiral alignment of OPV chromophores: exploitation of the ureidophthalimide-based foldamer

The ability of foldamers to adopt a secondary structure in solution has been exploited to organize peripheral functionality. Our previously reported poly(ureidophthalimide) foldamer proved to be an excellent scaffold for the chiral organization of peripherally positioned oligo(p-phenylenevinylene) (OPV) chromophores. Facile high-yielding synthesis gave access to the required OPV-decorated building blocks. A condensation polymerization provided polymers of sufficient length to allow construction of a helical architecture comprising several turns. Short and long chains were separated by chromatography. Circular dichroism studies in THF of the longer chains indicate the presence of helically arranged OPVs. However, such an effect is not observed in CHCl3. Remarkable are the measurements of the OPV foldamers in heptane. A bisignate Cotton effect is observed in heptane of a sample with a THF history. No Cotton effect is observed in heptane of a sample with a CHCl3 history. In this example of supramolecular synthesis, the solvent dictates the expression of supramolecular chirality in a secondary structure. The short-chain oligomeric fractions that are unable to create a full turn revealed on scanning tunneling microscopy analysis the presence of circular architectures at the graphite/1-phenyloctane interface. This is in full agreement with the proposed conformation of the decorated foldamers.     

Importance of physical vs. chemical interactions in surface shear rheology

The stability of adsorbed protein layers against deformation has in literature been attributed to the formation of a continuous gel-like network. This hypothesis is mostly based on measurements of the increase of the surface shear elasticity with time. For several proteins this increase has been attributed to the formation of intermolecular disulfide bridges between adsorbed proteins. However, according to an alternative model the shear elasticity results from the low mobility of the densely packed proteins. To contribute to this discussion, the actual role of disulfide bridges in interfacial layers is studied. Ovalbumin was thiolated with S-acetylmercaptosuccinic anhydride (S-AMSA), followed by removal of the acetylblock on the sulphur atom, resulting in respectively blocked (SX) and deblocked (SH) ovalbumin variants. This allows comparison of proteins with identical amino acid sequence and similar globular packing and charge distribution, but different chemical reactivity. The presence and reactivity of the introduced, deblocked sulfhydryl groups were confirmed using the sulfhydryl-disulfide exchange index (SEI). Despite the reactivity of the introduced sulfhydryl groups measured in solution, no increase in the surface shear elasticity could be detected with increasing reactivity. This indicates that physical rather than chemical interactions determine the surface shear behaviour. Further experiments were performed in bulk solution to study the conditions needed to induce covalent aggregate formation. From these studies it was found that mere concentration of proteins (to 200 mg/mL, equivalent to a surface concentration of around 2 mg/m(2)) is not sufficient to induce significant aggregation to form a continuous network. In view of these results, it was concluded that the adsorbed layer should not be considered a gelled network of aggregated material (in analogy with three-dimensional gels formed from heating protein solutions). Rather, it would appear that the adsorbed proteins form a highly packed system of proteins with net-repulsive interactions.     

Cu(II) Compounds with Pyrimidine-Based Chelating Ligands,Bridged by a Flexible Alkyl Spacer: Synthesis,Characterisation and X-Ray Structures of Methoxide-Bridged Dinuclear-Based Species

Abstract  The synthesis and full structural characterisation is described of 3 dinuclear-based Cu(II) compounds bridged by methoxide anions, and flexible linear bis(pyrimidine)-α,ω-diaminoalkane ligands. The structures are dinuclear based in all cases, in which the bis(pyrimidine)diaminoalkane ligand chelates to 2 Cu(II) ions in the same dinuclear unit (N–C–C–C–C–C–N linker), or to two different dinuclear units (with the short N–C–C–N linker), thereby generating a polymeric chain. The magnetic exchange in the compounds is dominated by the alkoxido-bridged ligands, which generate a very strong antiferromagnetic coupling between the Cu(II) ions, resulting in diamagnetism at room temperature and below, and EPR silent behaviour. Index Abstract  Dinuclear-based Cu(II) compounds with pyrimidine-based chelating ligands flexible linear bis(pyrimidine)-α,ω-diaminoalkane ligands are described, bridged by methoxide anions. The structures are dinuclear based in all cases, with the ligand chelating to 2 Cu(II) ions in the same dinuclear unit (N–C–C–C–C–C–N linker), or to 2 different dinuclear units (with short N–C–C–N linkers). The very strong antiferromagnetic magnetic exchange in all compounds is caused by the alkoxido-bridged ligands. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

HERFD XAS/ATR-FTIR batch reactor cell

An autoclave reactor was modified to perform simultaneously high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy measurements without altering the reactor design. This operando cell allows one to follow changes in the electronic and geometric structure of the catalyst with HERFD XAS and relate them to the simultaneously detected activity and reaction species with ATR-FTIR formed during the reaction in the liquid phase. The capability of the cell is demonstrated by two studies. The first shows the reduction of Au/CeO(2) precursors in different solvents. The second shows that mainly Au(0) is present in the hydrogenation of nitrobenzene over Au/CeO(2).     

The irreversible formation of palladium carbide during hydrogenation of 1-pentyne over silica-supported palladium nanoparticles: in situ Pd K and L3 edge XAS

The catalytically active phase of silica-supported palladium catalysts in the selective and non-selective hydrogenation of 1-pentyne was determined using in situ X-ray absorption spectroscopy at the Pd K and L(3) edges. Upon exposure to alkyne, a palladium carbide-like phase rapidly forms, which prevents hydrogen to diffuse into the bulk of the nano-sized particles. Both selective and non-selective hydrogenation occur over carbided particles. The palladium carbide-like phase is stable under reaction conditions and only partially decomposes under high hydrogen partial pressure. Non-selective hydrogenation to pentane is not indicative of hydride formation. The palladium carbide phase was detected in the EXAFS analysis and the K edge XANES showed representative features.     

A low-cost 2D fluorescence detection system for μm sized beads on-chip

In this paper we describe a compact fluorescence detection system for on-chip analysis of beads, comprising a low-cost optical HD-DVD pickup. The complete system consists of a fluorescence detection unit, a control unit and a microfluidic chip containing microchannels and optical markers. With these markers the laser beam of the optical pickup can be automatically focused at the centre of the microchannel. With the complete system a two-dimensional fluorescent profile across the channel width can be obtained such that there is no need for hydrodynamic or electrokinetic focusing of the particles in a specific part of the channel. Fluorescent μm sized beads suspended in medium have been detected with the system. Since on both sides of the main beam two additional laser beams at a known distance are generated, also the velocity of individual beads has been determined.     

Thermodynamics of supramolecular naphthalenediimide nanotube formation: the influence of solvents, side chains, and guest templates

Amino-acid functionalized naphthalenediimides self-assemble into hydrogen-bonded supramolecular helical nanotubes via a noncooperative, isodesmic process; the self-assembly of ordered helical systems is usually realized through a cooperative process. This unexpected behavior was rationalized as a manifestation of entropy-enthalpy compensation. Fundamental insights into the thermodynamics governing this self-assembly were obtained through the fitting of the isodesmic model to (1)H NMR spectrometry and circular dichroism spectroscopy measurements. Furthermore, we have extended the application of this mathematical model, for the first time, to quantitatively estimate the effect of guests, solvents, and side chains on the stability of the supramolecular nanotube; most significantly, we demonstrate that C(60) acts as a template to stabilize the nanotube assembly and thereby substantially increase the degree of polymerization.