Phototriggered fluorescence color changes in azobenzene-functionalized conjugated polymers

We report fluorescence studies of phototriggered changes in spectral position and shape for two azobenzene-functionalized poly(p-phenylenevinylene) derivatives, poly(2-methoxy-5-(4-phenylazophenyl-4'-(1,10-dioxydecyl))-1,4-phenylenevinylene) (MPA-10-PPV) and poly(2-hexyloxy-5-(4-phenylazophenyl-4'-(1,10-dioxydecyl))-1,4-phenylenevinylene) (HPA-10-PPV). Upon trans --> cis azobenzene photoisomerization, small (ca. 1 nm) blue shifts in spectral position are observed for MPA-10-PPV in 100% toluene, a good solvent for this polymer. These shifts are reversed upon visible irradiation and can be cycled many times. To probe the dependence of these shifts on initial polymer conformation, a dichloromethane-methanol cosolvent study was performed in which the solvent quality was decreased incrementally to induce a reduction in polymer coil dimensions. Unirradiated dichloromethane solutions of both MPA-10-PPV and HPA-10-PPV showed a red shift and reduction in quantum yield with increasing methanol concentration as expected based on literature results for other poly(p-phenylenevinylene) derivatives. These changes have been attributed to a dramatic conformational collapse by others and occur for these azo polymers over the 30-60% (v/v) methanol range. While little or no light-induced spectral shifting was observed at low (or=70%) methanol concentrations, significant spectral shifts were observed for both polymers upon azobenzene photoisomerization in solutions with 30-60% methanol, the same range over which the polymer undergoes collapse to a highly coiled state. The largest shifts are visible to the eye, with a 65:35 (v/v) dichloromethane-methanol solution of HPA-10-PPV showing yellow-orange fluorescence when the azobenzenes are trans, green fluorescence when they are cis, and yellow-orange again after the azobenzenes are returned to the trans state. We attribute these color changes to a reversible UV-phototriggered expansion of polymer coil dimensions that occurs as a result of trans --> cis azobenzene side chain isomerization and provide temperature data to support this conclusion.     

AN FMN-PHOTOSYSTEM I PHOTOVOLTAIC CELL

Abstract— We have made a photovoltaic cell using Photosystem I subchloroplast particles isolated according to the method of Shiozawa et al. (1974). The particles were placed on a filter between two compartments one of which contained the electron donor, K₄Fe(CN)₆ and the other the electron acceptor, FMN. Upon illumination with white light ( I = 80 W/m²) a potential of 300 mV was observed across a 3000 Ω load resistance. Both Photosystem I photochemistry and direct photoreactions of FMN contribute to the process. A power output of 20 μW was observed for a 2 cm² filter containing 60 μg chlorophyll. This corresponds to 0.1 W/m². The power efficiency was 0.13%. The short circuit current was 108 μA.     

Temporal Control of Gelation and Polymerization Fronts Driven by an Autocatalytic Enzyme Reaction

Chemical systems that remain kinetically dormant until activated have numerous applications in materials science. Herein we present a method for the control of gelation that exploits an inbuilt switch: the increase in pH after an induction period in the urease‐catalyzed hydrolysis of urea was used to trigger the base‐catalyzed Michael addition of a water‐soluble trithiol to a polyethylene glycol diacrylate. The time to gelation (minutes to hours) was either preset through the initial concentrations or the reaction was initiated locally by a base, thus resulting in polymerization fronts that converted the mixture from a liquid into a gel (ca. 0.1 mm min^?1). The rate of hydrolytic degradation of the hydrogel depended on the initial concentrations, thus resulting in a gel lifetime of hours to months. In this way, temporal programming of gelation was possible under mild conditions by using the output of an autocatalytic enzyme reaction to drive both the polymerization and subsequent degradation of a hydrogel.     

Simple Copper Catalysts for the Aerobic Oxidation of Amines: Selectivity Control by the Counterion

We describe the use of simple copper‐salt catalysts in the selective aerobic oxidation of amines to nitriles or imines. These catalysts are marked by their exceptional efficiency, operate at ambient temperature and pressure, and allow the oxidation of amines without expensive ligands or additives. This study highlights the significant role counterions can play in controlling selectivity in catalytic aerobic oxidations.     

Use of reflectance anisotropy spectroscopy for mapping the anisotropy of lyotropic liquid crystal dispersions in formulations

Lyotropic liquid crystal dispersions have commercial importance in the formulation of cleaning products and pharmaceuticals. The anisotropy of such dispersions is an important aspect of their properties and a simple method for measurement would be valuable to formulators. The relatively new optical technique of reflectance anisotropy spectroscopy (RAS) was considered to have potential in this respect. To test the idea, the anisotropy of a binary mixture of an anionic surfactant Aerosol OT (AOT) and glycerol (used as a model for water) was investigated using RAS. The variation in the measured anisotropy parallels the expected behaviour of a dispersion of lyotropic liquid crystals as a function of concentration and temperature. A response surface of anisotropy generated as a function of temperature and surfactant concentration demonstrates the use of RAS as a tool for mapping liquid anisotropy to facilitate the formulation of structured liquids.     

Cyclic peptide inhibitors of staphylococcal virulence prepared by Fmoc-based thiolactone peptide synthesis

Virulence factor production in Staphylococcus aureus is largely under the control of the accessory gene regulator (agr) quorum sensing system. There are four agr groups, all of which exhibit bacterial interference: each agr type synthesizes a cyclic autoinducing peptide (AIP) with a distinct sequence that activates its cognate AgrC receptor and inhibits activation of others. To better understand inhibitory AIP-AgrC interactions, we aimed to identify the minimal molecular determinants required to inhibit both non-cognate and cognate receptors. This minimization of the AIP pharmacophore also may have therapeutic relevance as the use of native AIPs to block virulence of non-cognate agr strains can prevent the establishment of an infection in vivo. We synthesized and evaluated the inhibitory activities of 10 AIP derivatives based on a truncated AIP analogue that inhibits all four agr types. To carry out the rapid, parallel synthesis of these peptides, we employed a new linker for Fmoc-based thioester peptide synthesis. Our results identify key structural elements that are necessary for AgrC inhibition and reveal key differences between non-cognate and cognate inhibitory requirements.     

PNA versus DNA: effects of structural fluctuations on electronic structure and hole-transport mechanisms

The effects of structural fluctuations on charge transfer in double-stranded DNA and peptide nucleic acid (PNA) are investigated. A palindromic sequence with two guanine bases that play the roles of hole donor and acceptor, separated by a bridge of two adenine bases, was analyzed using combined molecular dynamics (MD) and quantum-chemical methods. Surprisingly, electronic structure calculations on individual MD snapshots show significant frontier orbital electronic population on the bridge in approximately 10% of the structures. Electron-density delocalization to the bridge is found to be gated by fluctuations of the covalent conjugated bond structure of the aromatic rings of the nucleic bases. It is concluded, therefore, that both thermal hopping and superexchange should contribute significantly to charge transfer even in short DNA/PNA fragments. PNA is found to be more flexible than DNA, and this flexibility is predicted to produce larger rates of charge transfer.     

Fully automated open access platform for rapid, combined serial evaporation and sample reformatting

This paper reports a novel evaporator and its integration with an automated sample handling system to create a high throughput evaporation platform. The Vaportec V-10 evaporator uses a high speed rotation motor ( approximately 6000 rpm) to spin the vial containing a sample, creating a thin film of solvent which can be readily evaporated by the application of heat to the vial, while the consequent centrifugal force prevents "bumping". An intelligent algorithm controls pressure and temperature for optimum solvent removal conditions and end of run detection, critical for automation. The system allows the option of evaporation directly from a sample source vial, or alternatively, integrated liquid handling facilities provide the capability of transferring samples portionwise from a (large) source vial or bottle to a (small) daughter container, enabling efficient sample reformatting, with minimum user intervention. The open access system makes significant advances over current vacuum centrifugal evaporators in terms of evaporation rate and ease of automation. The evaporator's main features, the integration of robotics to provide automation, and examples of evaporation rates of a wide range of solvents from a variety of containers are described.