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Find video protocols related to scientific articles indexed in Pubmed.
Chemical Vapor Deposition of High Quality Graphene Films from Carbon Dioxide Atmospheres.
ACS Nano
PUBLISHED: 11-15-2014
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The realization of graphene-based, next-generation electronic applications essentially depends on a reproducible, large-scale production of graphene films via chemical vapor deposition (CVD). We demonstrate how key challenges such as uniformity and homogeneity of the copper metal substrate as well as the growth chemistry can be improved by the use of carbon dioxide and carbon dioxide enriched gas atmospheres. Our approach enables graphene film production protocols free of elemental hydrogen and provides graphene layers of superior quality compared to samples produced by conventional hydrogen/methane based CVD processes. The substrates and resulting graphene films were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Raman microscopy, sheet resistance and transport measurements. The superior quality of the as-grown graphene films on copper is indicated by Raman maps revealing average G band widths as low as 18 ± 8 cm(-1) at 514.5 nm excitation. In addition, high charge carrier mobilities of up to 1975 cm(2)/(V s) were observed for electrons in transferred films obtained from a carbon dioxide based growth protocol. The enhanced graphene film quality can be explained by the mild oxidation properties of carbon dioxide, which at high temperatures enables an uniform conditioning of the substrates by an efficient removal of pre-existing and emerging carbon impurities and a continuous suppression and in situ etching of carbon of lesser quality being co-deposited during the CVD growth.
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Bottom-Up Synthesis of Liquid-Phase-Processable Graphene Nanoribbons with Near-Infrared Absorption.
ACS Nano
PUBLISHED: 10-23-2014
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Structurally defined, long (>100 nm), and low-band-gap (?1.2 eV) graphene nanoribbons (GNRs) were synthesized through a bottom-up approach, enabling GNRs with a broad absorption spanning into the near-infrared (NIR) region. The chemical identity of GNRs was validated by IR, Raman, solid-state NMR, and UV-vis-NIR absorption spectroscopy. Atomic force microscopy revealed well-ordered self-assembled monolayers of uniform GNRs on a graphite surface upon deposition from the liquid phase. The broad absorption of the low-band-gap GNRs enables their detailed characterization by Raman and time-resolved terahertz photoconductivity spectroscopy with excitation at multiple wavelengths, including the NIR region, which provides further insights into the fundamental physical properties of such graphene nanostructures.
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Frontispiece: cobalt phenanthroline-indole macrocycles as highly active electrocatalysts for oxygen reduction.
Chemistry
PUBLISHED: 10-21-2014
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Cobalt Electrocatalysts A series of novel mono- and binuclear Co-N4 complexes, based on a new phenanthroline-indole macrocyclic ligand, were synthesized. It was demonstrated that these complexes support the direct four-electron reduction of oxygen to water. Furthermore, electrochemical measurements revealed high electrocatalytic activities, methanol resistance, and a superior long-term stability for the oxygen-reduction reaction (ORR) under alkaline conditions. Thus, these structurally well-defined non-precious metal complexes represent an alternative to platinum-based catalysts in fuel-cell applications. For more details, see the Communication on page 14178 by K. Müllen and co-workers.
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The polar side of polyphenylene dendrimers.
Chem Soc Rev
PUBLISHED: 09-27-2014
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Polyphenylene dendrimers (PPDs) represent a unique class of dendrimers based on their rigid, shape persistent chemical structure. These macromolecules are typically looked at as nonpolar precursors for conjugated systems. Yet over the years there have been synthetic achievements that have produced PPDs with a range of polarities that break the hydrophobic stereotype, and provide dendrimers that can be synthetically tuned to be used in applications such as stable transition metal catalysts, nanocarriers for biological drug delivery, and sensors for volatile organic compounds (VOCs), among many others. This is based on strategies that allow for the modification of PPDs at the core, scaffold, and surface to introduce numerous different groups, such as electrolytes, ions, or other polar species. This review is aimed to demonstrate the versatility of PPDs through their site-specific chemical functionalization to produce robust materials with various polarities.
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Positive magneto-LC effect in conjugated spin-bearing hexabenzocoronene.
J. Am. Chem. Soc.
PUBLISHED: 09-08-2014
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The first neutral spin carrying hexabenzocoronene (HBC) derivative is described. The conjugated phenyl nitroxide substituted HBC with five alkyl chains exhibits a positive magneto-LC effect in columnar hexagonal liquid crystalline phase as probed by differential scanning calorimetry and electron paramagnetic resonance spectroscopy. Surprisingly, at 140 K the ?MS = 2 transition can be observed indicating a thermally accessible triplet state between the neighboring molecules in the columnar arrangements.
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A Crown Ether Decorated Dibenzocoronene Tetracarboxdiimide Chromophore: Synthesis, Sensing, and Self-Organization.
Chem Asian J
PUBLISHED: 09-04-2014
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A macrocyclic dibenzocoronene tetracarboxdiimide containing two benzo-21-crown-7 groups has been synthesized. It shows liquid-crystalline behavior and selectively binds Pb(2+) or K(+) to form 1:2 complexes in solution. The complexation leads to a significant increase of fluorescence; the surface organization of discotic columnar structures, in the solid-state, can be controlled by selective ion binding.
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A Polyphenylene Dendrimer Drug Transporter with Precisely Positioned Amphiphilic Surface Patches.
Adv Healthc Mater
PUBLISHED: 09-02-2014
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The design and synthesis of a polyphenylene dendrimer (PPD 3) with discrete binding sites for lipophilic guest molecules and characteristic surface patterns is presented. Its semi-rigidity in combination with a precise positioning of hydrophilic and hydrophobic groups at the periphery yields a refined architecture with lipophilic binding pockets that accommodate defined numbers of biologically relevant guest molecules such as fatty acids or the drug doxorubicin. The size, architecture, and surface textures allow to even penetrate brain endothelial cells that are a major component of the extremely tight blood-brain barrier. In addition, low to no toxicity is observed in in vivo studies using zebrafish embryos. The unique PPD scaffold allows the precise placement of functional groups in a given environment and offers a universal platform for designing drug transporters that closely mimic many features of proteins.
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Exfoliation of Graphite into Graphene in Polar Solvents Mediated by Amphiphilic Hexa-peri-hexabenzocoronene.
Chem Asian J
PUBLISHED: 08-25-2014
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A water-soluble surfactant consisting of hexa-peri-hexabenzocoronene (HBC) as hydrophobic aromatic core and hydrophilic carboxy substituents was synthesized. It exhibited a self-assembled nanofiber structure in the solid state. Profiting from the ? interactions between the large aromatic core of HBC and graphene, the surfactant mediated the exfoliation of graphite into graphene in polar solvents, which was further stabilized by the bulky hydrophilic carboxylic groups. A graphene dispersion with a concentration as high as 1.1?mg?L(-1) containing 2-6 multilayer nanosheets was obtained. The lateral size of the graphene sheets was in the range of 100-500?nm based on atomic force microscope (AFM) and transmission electron microscope (TEM) measurements.
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Synthesis and optophysical properties of dimeric aza-BODIPY dyes with a push-pull benzodipyrrolidone core.
Chem. Commun. (Camb.)
PUBLISHED: 08-19-2014
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A series of benzodipyrrolidone-based dimeric aza-BODIPY dyes with a push-pull structure are synthesized. Single crystal X-ray diffraction demonstrates these extended aza-BODIPY dyes are planar. The resulting aza-BODIPY chromophores exhibit intense absorption in the 450-800 nm regions and possess lower-lying LUMO energy levels. Furthermore, the push-pull substituents on aza-BODIPYs core have a positive effect on their optophysical properties.
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Transition Between Band and Hopping Transport in Polymer Field-Effect Transistors.
Adv. Mater. Weinheim
PUBLISHED: 08-17-2014
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Hall effect and slightly negative temperature dependence of the mobility in polymeric transistors are demonstrated. The semiconductor channel is based on a polycyclopentadithiophene-benzothiadiazole (CDT-BTZ) donor-acceptor copolymer film whose chain direction is oriented by mechanical compression at the surface of an ionic liquid. The mobility is 5.6 cm(2) V(-1) s(-1) at room temperature, and is further improved to 6.7 cm(2) V(-1) s(-1) at 260 K.
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Hierarchical Supramolecular Assembly of Sterically Demanding ?-Systems by Conjugation with Oligoprolines.
Angew. Chem. Int. Ed. Engl.
PUBLISHED: 08-15-2014
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Self-assembly from flexible worm-like threads via bundles of rigid fibers to nanosheets and nanotubes was achieved by covalent conjugation of perylene monoimide (PMI) chromophores with oligoprolines of increasing length. Whereas the chromophoric ?-system and the peptidic building block do not self-aggregate, the covalent conjugates furnish well-ordered supramolecular structures with a common wall/fiber thickness. Their morphology is controlled by the number of repeat units and can be tuned by seemingly subtle structural modifications.
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Cyclotrimerization of arylalkynes on Au(111).
Chem. Commun. (Camb.)
PUBLISHED: 08-12-2014
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Surface-assisted cyclotrimerization of arylalkynes was studied on Au(111) by means of scanning tunneling microscopy (STM) under ultra-high vacuum (UHV) conditions. Upon thermal activation, cyclotrimerization of 1,3,5-tris-(4-ethynylphenyl)benzene proceeds readily and with high selectivity, and results in two-dimensional covalently bonded polyphenylene nanostructures exhibiting a honeycomb topology.
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Synthesis of nitrogen-doped zigzag-edge peripheries: dibenzo-9a-azaphenalene as repeating unit.
Angew. Chem. Int. Ed. Engl.
PUBLISHED: 08-11-2014
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A bottom-up approach toward stable and monodisperse segments of graphenes with a nitrogen-doped zigzag edge is introduced. Exemplified by the so far unprecedented dibenzo-9a-azaphenalene (DBAPhen) as the core unit, a versatile synthetic concept is introduced that leads to nitrogen-doped zigzag nanographenes and graphene nanoribbons.
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Core-and-Surface-Functionalized Polyphenylene Dendrimers for Solution-Processed, Pure-Blue Light-Emitting Diodes Through Surface-to-Core Energy Transfer.
Macromol Rapid Commun
PUBLISHED: 08-06-2014
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Several pyrene-based polyphenylene dendrimers (PYPPDs) with different peripheral chromophores (PCs) are synthesized and characterized. Deep blue emissions solely from the core are observed for all of them in photoluminescence spectra due to good steric shielding of the core and highly efficient surface-to-core Förster resonant energy transfers (FRETs). Device performances are found in good correlation with the energy gaps between the work function of the electrodes and the frontier molecular orbital (FMO) levels of the PCs. Pure blue emission, luminance as high as 3700 cd m(-2) with Commission Internationale de l'Éclairage 1931 (CIExy ) = (0.16, 0.21), and a peak current efficiency of 0.52 cd A(-1) at CIExy = (0.17, 0.20) are achieved. These dendrimers are among the best dendritic systems so far for fluorescent blue light-emitting materials.
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Potential-driven molecular tiling of a charged polycyclic aromatic compound.
Chem. Commun. (Camb.)
PUBLISHED: 08-01-2014
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Using in situ electrochemical scanning tunnelling microscopy (EC-STM), we demonstrate fully reversible tuning of molecular tiling between self-assembled structures with supramolecular motifs containing 2, 3, 4, 6 or 7 tectons. The structures can be explained by electrocompression of the cationic adlayer at the solid-liquid interface.
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Graphene nanoribbon heterojunctions.
Nat Nanotechnol
PUBLISHED: 07-30-2014
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Despite graphene's remarkable electronic properties, the lack of an electronic bandgap severely limits its potential for applications in digital electronics. In contrast to extended films, narrow strips of graphene (called graphene nanoribbons) are semiconductors through quantum confinement, with a bandgap that can be tuned as a function of the nanoribbon width and edge structure. Atomically precise graphene nanoribbons can be obtained via a bottom-up approach based on the surface-assisted assembly of molecular precursors. Here we report the fabrication of graphene nanoribbon heterojunctions and heterostructures by combining pristine hydrocarbon precursors with their nitrogen-substituted equivalents. Using scanning probe methods, we show that the resulting heterostructures consist of seamlessly assembled segments of pristine (undoped) graphene nanoribbons (p-GNRs) and deterministically nitrogen-doped graphene nanoribbons (N-GNRs), and behave similarly to traditional p-n junctions. With a band shift of 0.5 eV and an electric field of 2?×?10(8) V m(-1) at the heterojunction, these materials bear a high potential for applications in photovoltaics and electronics.
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Cobalt phenanthroline-indole macrocycles as highly active electrocatalysts for oxygen reduction.
Chemistry
PUBLISHED: 07-17-2014
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The replacement of scarce and expensive platinum species poses a challenge in fuel-cell development. The design and synthesis of a novel type of Co(II) -N4 macrocyclic complex, [CoN4 ], based on the phenanthroline-indole macrocyclic ligand (PIM) is reported. This unique ligand allows the formation of mono- and dinuclear complexes with defined active sites that facilitate the direct four-electron reduction of oxygen. Electrochemical measurements revealed that the [CoN4 ]/C (20?wt?%) catalysts have a high activity and long-term stability for the oxygen-reduction reaction (ORR) under alkaline conditions, similar to the Pt/C catalyst. These structurally well-defined complexes represent a nonprecious alternative to platinum species for future fuel-cell applications.
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Poly(isobutylene) Nanoparticles via Cationic Polymerization in Nonaqueous Emulsions.
Macromol Rapid Commun
PUBLISHED: 07-17-2014
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The preparation of poly(isobutylene) (PIB) nanoparticles via cationic emulsion polymerization is presented. As a requirement, an oil-in-perfluoroalkane nonaqueous emulsion is developed, which is inert under the carbocationic polymerization conditions. To stabilize the dichloromethane/hexane droplets in the fluorinated, continuous phase, an amphiphilic block copolymer emulsifier is prepared containing PIB and 1H,1H-perfluoroalkylated poly(pentafluorostyrene) blocks. This system allows for the polymerization of isobutylene with number-average molecular weights (M¯n) up to 27 000 g mol(-1) . The particle morphologies are characterized via dynamic light scattering and electron microscopy. For M¯n > 20 000 g mol(-1) , the particles exhibit shape-persistence at room temperature and are ?100 nm in diameter.
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Evolution of graphene molecules: structural and functional complexity as driving forces behind nanoscience.
ACS Nano
PUBLISHED: 07-11-2014
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The evolution of nanoscience is based on the ability of the fields of chemistry and physics to share competencies through mutually beneficial collaborations. With this in mind, in this Perspective, I describe three classes of compounds: rylene dyes, polyphenylene dendrimers, as well as nanographene molecules and graphene nanoribbons, which have provided a superb platform to nurture these relationships. The synthesis of these complex structures is demanding but also rewarding because they stimulate unique investigations at the single-molecule level by scanning tunneling microscopy and single-molecule spectroscopy. There are close functional and structural relationships between the molecules chosen. In particular, rylenes and nanographenes can be regarded as honeycomb-type, discoid species composed of fused benzene rings. The benzene ring can thus be regarded as a universal modular building block. Polyphenylene dendrimers serve, first, as a scaffold for dyes en route to multichromophoric systems and, second, as chemical precursors for graphene synthesis. Through chemical design, it is possible to tune the properties of these systems at the single-molecule level and to achieve nanoscale control over their self-assembly to form multifunctional (nano)materials.
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Squeezing, Then Stacking: From Breathing Pores to Three-Dimensional Ionic Self-Assembly under Electrochemical Control.
Angew. Chem. Int. Ed. Engl.
PUBLISHED: 06-15-2014
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We demonstrate the spontaneous and reversible transition between the two- and three-dimensional self-assembly of a supramolecular system at the solid-liquid interface under electrochemical conditions, using in situ scanning tunneling microscopy. By tuning the interfacial potential, we can selectively organize our target molecules in an open porous pattern, fill these pores to form an auto-host-guest structure, or stack the building blocks in a stratified bilayer. Using a simple electrostatic model, we rationalize which charge density is required to enable bilayer formation, and conversely, which molecular size/charge ratio is necessary in the design of new building blocks. Our results may lead to a new class of electrochemically controlled dynamic host-guest systems, artificial receptors, and smart materials.
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Dehalogenation and coupling of a polycyclic hydrocarbon on an atomically thin insulator.
ACS Nano
PUBLISHED: 06-13-2014
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Catalytic activity is of pivotal relevance in enabling efficient and selective synthesis processes. Recently, covalent coupling reactions catalyzed by solid metal surfaces opened the rapidly evolving field of on-surface chemical synthesis. Tailored molecular precursors in conjunction with the catalytic activity of the metal substrate allow the synthesis of novel, technologically highly relevant materials such as atomically precise graphene nanoribbons. However, the reaction path on the metal substrate remains unclear in most cases, and the intriguing question is how a specific atomic configuration between reactant and catalyst controls the reaction processes. In this study, we cover the metal substrate with a monolayer of hexagonal boron nitride (h-BN), reducing the reactivity of the metal, and gain unique access to atomistic details during the activation of a polyphenylene precursor by sequential dehalogenation and the subsequent coupling to extended oligomers. We use scanning tunneling microscopy and density functional theory to reveal a reaction site anisotropy, induced by the registry mismatch between the precursor and the nanostructured h-BN monolayer.
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Synthesis and Characterization of Organic Dyes with Various Electron-Accepting Substituents for p-Type Dye-Sensitized Solar Cells.
Chem Asian J
PUBLISHED: 06-12-2014
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Four new donor-?-acceptor dyes differing in their acceptor group have been synthesized and employed as model systems to study the influence of the acceptor groups on the photophysical properties and in NiO-based p-type dye-sensitized solar cells. UV/Vis absorption spectra showed a broad range of absorption coverage with maxima between 331 and 653?nm. Redox potentials as well as HOMO and LUMO energies of the dyes were determined from cyclic voltammetry measurements and evaluated concerning their potential use as sensitizers in p-type dye-sensitized solar cells (p-DSCs). Quantum-chemical density functional theory calculations gave further insight into the frontier orbital distributions, which are relevant for the electronic processes in p-DSCs. In p-DSCs using an iodide/triiodide-based electrolyte, the polycyclic 9,10-dicyano-acenaphtho[1,2-b]quinoxaline (DCANQ) acceptor-containing dye gave the highest power conversion efficiency of 0.08?%, which is comparable to that obtained with the perylenemonoimide (PMI)-containing dye. Interestingly, devices containing the DCANQ-based dye achieve a higher VOC of 163?mV compared to 158?mV for the PMI-containing dye. The result was further confirmed by impedance spectroscopic analysis showing higher recombination resistance and thus a lower recombination rate for devices containing the DCANQ dye than for PMI dye-based devices. However, the use of the strong electron-accepting tricyanofurane (TCF) group played a negative role in the device performance, yielding an efficiency of only 0.01?% due to a low-lying LUMO energy level, thus resulting in an insufficient driving force for efficient dye regeneration. The results demonstrate that a careful molecular design with a proper choice of the acceptor unit is essential for development of sensitizers for p-DSCs.
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pH-responsive self-assembly of fluorophore-ended homopolymers.
Chem. Commun. (Camb.)
PUBLISHED: 06-03-2014
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pH-responsive perylene-ended amphiphilic homopolymers can self-assemble into flower-like structures by pH-adjustment. Both the ?-? stacking of perylene end groups and the electrostatic repulsion effect of the polymer chains contribute to the pH-responsive self-assembly in aqueous solution.
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Toward perylene dyes by the Hundsdiecker reaction.
Org. Lett.
PUBLISHED: 05-16-2014
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An efficient method to synthesize 3,4,9,10-tetrabromoperylenes is reported under optimized Hunsdiecker conditions. Various octasubstituted perylenes were obtained by reaction of 1,6,7,12-tetrachloro-3,4,9,10-tetrabromoperylene with phenol, trimethylsilyl chloride, cooper cyanide, or sulfur via metal-catalyzed couplings or nucleophilic substitutions. These new perylenes show completely different optical and redox properties, thus opening a facile way to develop new chromophophore structures.
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Deposition, characterization, and thin-film-based chemical sensing of ultra-long chemically synthesized graphene nanoribbons.
J. Am. Chem. Soc.
PUBLISHED: 05-15-2014
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Bottom-up synthesis of graphene nanoribbons (GNRs) is an essential step toward utilizing them in electronic and sensing applications due to their defined edge structure and high uniformity. Recently, structurally perfect GNRs with variable lengths and edge structures were created using various chemical synthesis techniques. Nonetheless, issues like GNR deposition, characterization, electronic properties, and applications are not fully explored. Here we report optimized conditions for deposition, characterization, and device fabrication of individual and thin films of ultra-long chemically synthesized GNRs. Moreover, we have demonstrated exceptional NO2 gas sensitivity of the GNR film devices down to parts per billion (ppb) levels. The results lay the foundation for using chemically synthesized GNRs for future electronic and sensing applications.
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Flow-assisted 2D polymorph selection: stabilizing metastable monolayers at the liquid-solid interface.
J. Am. Chem. Soc.
PUBLISHED: 05-14-2014
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Controlling crystal polymorphism constitutes a formidable challenge in contemporary chemistry. Two-dimensional (2D) crystals often provide model systems to decipher the complications in 3D crystals. In this contribution, we explore a unique way of governing 2D polymorphism at the organic liquid-solid interface. We demonstrate that a directional solvent flow could be used to stabilize crystalline monolayers of a metastable polymorph. Furthermore, flow fields active within the applied flow generate millimeter-sized domains of either polymorph in a controlled and reproducible fashion.
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Hierarchically porous carbons with optimized nitrogen doping as highly active electrocatalysts for oxygen reduction.
Nat Commun
PUBLISHED: 05-12-2014
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Development of efficient, low-cost and stable electrocatalysts as the alternative to platinum for the oxygen reduction reaction is of significance for many important electrochemical devices, such as fuel cells, metal-air batteries and chlor-alkali electrolysers. Here we report a highly active nitrogen-doped, carbon-based, metal-free oxygen reduction reaction electrocatalyst, prepared by a hard-templating synthesis, for which nitrogen-enriched aromatic polymers and colloidal silica are used as precursor and template, respectively, followed by ammonia activation. Our protocol allows for the simultaneous optimization of both porous structures and surface functionalities of nitrogen-doped carbons. Accordingly, the prepared catalysts show the highest oxygen reduction reaction activity (half-wave potential of 0.85?V versus reversible hydrogen electrode with a low loading of 0.1?mg?cm(-2)) in alkaline media among all reported metal-free catalysts. Significantly, when used for constructing the air electrode of zinc-air battery, our metal-free catalyst outperforms the state-of the-art platinum-based catalyst.
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A unique perylene-based DNA intercalator: localization in cell nuclei and inhibition of cancer cells and tumors.
Small
PUBLISHED: 05-08-2014
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To date, perylene derivatives have not been explored as DNA intercalator to inhibit cancer cells by intercalating into the base pairs of DNA. Herein, a water-soluble perylene bisimide (PBDI) that efficiently intercalates into the base pairs of DNA is synthesized. Excitingly, PBDI is superior to the commercial DNA intercalator, amonafide, for specific nuclear accumulation and effective suppression of cancer cells and tumors.
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Graphene nanoribbon blends with P3HT for organic electronics.
Nanoscale
PUBLISHED: 04-16-2014
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In organic field-effect transistors (OFETs) the electrical characteristics of polymeric semiconducting materials suffer from the presence of structural/morphological defects and grain boundaries as well as amorphous domains within the film, hindering an efficient transport of charges. To improve the percolation of charges we blend a regioregular poly(3-hexylthiophene) (P3HT) with newly designed N = 18 armchair graphene nanoribbons (GNRs). The latter, prepared by a bottom-up solution synthesis, are expected to form solid aggregates which cannot be easily interfaced with metallic electrodes, limiting charge injection at metal-semiconductor interfaces, and are characterized by a finite size, thus by grain boundaries, which negatively affect the charge transport within the film. Both P3HT and GNRs are soluble/dispersible in organic solvents, enabling the use of a single step co-deposition process. The resulting OFETs show a three-fold increase in the charge carrier mobilities in blend films, when compared to pure P3HT devices. This behavior can be ascribed to GNRs, and aggregates thereof, facilitating the transport of the charges within the conduction channel by connecting the domains of the semiconductor film. The electronic characteristics of the devices such as the Ion/Ioff ratio are not affected by the addition of GNRs at different loads. Studies of the electrical characteristics under illumination for potential use of our blend films as organic phototransistors (OPTs) reveal a tunable photoresponse. Therefore, our strategy offers a new method towards the enhancement of the performance of OFETs, and holds potential for technological applications in (opto)electronics.
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Tailored donor-acceptor polymers with an A-D1-A-D2 structure: controlling intermolecular interactions to enable enhanced polymer photovoltaic devices.
J. Am. Chem. Soc.
PUBLISHED: 04-14-2014
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Extensive efforts have been made to develop novel conjugated polymers that give improved performance in organic photovoltaic devices. The use of polymers based on alternating electron-donating and electron-accepting units not only allows the frontier molecular orbitals to be tuned to maximize the open-circuit voltage of the devices but also controls the optical band gap to increase the number of photons absorbed and thus modifies the other critical device parameter-the short circuit current. In fact, varying the nonchromophoric components of a polymer is often secondary to the efforts to adjust the intermolecular aggregates and improve the charge-carrier mobility. Here, we introduce an approach to polymer synthesis that facilitates simultaneous control over both the structural and electronic properties of the polymers. Through the use of a tailored multicomponent acceptor-donor-acceptor (A-D-A) intermediate, polymers with the unique structure A-D1-A-D2 can be prepared. This approach enables variations in the donor fragment substituents such that control over both the polymer regiochemistry and solubility is possible. This control results in improved intermolecular ?-stacking interactions and therefore enhanced charge-carrier mobility. Solar cells using the A-D1-A-D2 structural polymer show short-circuit current densities that are twice that of the simple, random analogue while still maintaining an identical open-circuit voltage. The key finding of this work is that polymers with an A-D1-A-D2 structure offer significant performance benefits over both regioregular and random A-D polymers. The chemical synthesis approach that enables the preparation of A-D1-A-D2 polymers therefore represents a promising new route to materials for high-efficiency organic photovoltaic devices.
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Exfoliation of graphite into graphene in aqueous solutions of inorganic salts.
J. Am. Chem. Soc.
PUBLISHED: 04-09-2014
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Mass production of high-quality graphene sheets is essential for their practical application in electronics, optoelectronics, composite materials, and energy-storage devices. Here we report a prompt electrochemical exfoliation of graphene sheets into aqueous solutions of different inorganic salts ((NH4)2SO4, Na2SO4, K2SO4, etc.). Exfoliation in these electrolytes leads to graphene with a high yield (>85%, ?3 layers), large lateral size (up to 44 ?m), low oxidation degree (a C/O ratio of 17.2), and a remarkable hole mobility of 310 cm(2) V(-1) s(-1). Further, highly conductive graphene films (11 ? sq(-1)) are readily fabricated on an A4-size paper by applying brush painting of a concentrated graphene ink (10 mg mL(-1), in N,N'-dimethylformamide). All-solid-state flexible supercapacitors manufactured on the basis of such graphene films deliver a high area capacitance of 11.3 mF cm(-2) and an excellent rate capability of 5000 mV s(-1). The described electrochemical exfoliation shows great promise for the industrial-scale synthesis of high-quality graphene for numerous advanced applications.
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Chemical vapor deposition of N-doped graphene and carbon films: the role of precursors and gas phase.
ACS Nano
PUBLISHED: 03-26-2014
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Thermally induced chemical vapor deposition (CVD) was used to study the formation of nitrogen-doped graphene and carbon films on copper from aliphatic nitrogen-containing precursors consisting of C1- and C2-units and (hetero)aromatic nitrogen-containing ring systems. The structure and quality of the resulting films were correlated to the influence of the functional groups of the precursor molecules and gas phase composition. They were analyzed with SEM, TEM, EDX, XPS, and Raman spectroscopy. The presence of (N-doped) graphene was confirmed by the 2D mode of the Raman spectra. The isolated graphene films obtained from nitrogen-containing precursors reveal a high conductivity and transparency compared to standard graphene CVD samples. Precursors with amine functional groups (e.g., methylamine) can lead to a direct formation of graphene even without additional hydrogen present in the gas phase. This is not observed for, e.g., methane under comparable CVD conditions. Therefore, the intermediate gas phase species (e.g., amine radicals) can significantly enhance the graphene film growth kinetics. Kinetic and thermodynamic effects can be invoked to discuss the decay of the precursors.
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Effects of chemical structure on the dynamic and static surface tensions of short-chain, multi-arm nonionic fluorosurfactants.
J Colloid Interface Sci
PUBLISHED: 03-18-2014
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Fluorinated surfactants with short perfluoroalkyl chains (R(F)) as potential substitutes for the environmentally questionable, long R(F) systems are presented. Three types of nonionic hydrophilic-fluorophilic amphiphiles are synthesized and evaluated based on surface activity in equilibrated (static) and non-equilibrated (dynamic) states. Furthermore, several mono- and disaccharide-based fluorosurfactants are also examined as potential non-bioaccumulative alternatives. A correlation between the chemical structure and resulting surface properties is made by comparing R(F) length, number and size, alkyl-spacer, and hydrophilic moieties. Based on dynamic and static surface tension experiments, the effects of surfactant structure are summarized to provide a basis for the future design of fluorosurfactants. We have found that surfactants with more perfluorinated chains tend to have a higher surface tension reduction, but typically result in slower dynamic behaviors. Using the presented structural characteristics, surfactants with R(F)<4 can be prepared with static surface tensions as low as 18.1 mN/m or reduce surface tension within milliseconds.
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Layer-by-layer assembled heteroatom-doped graphene films with ultrahigh volumetric capacitance and rate capability for micro-supercapacitors.
Adv. Mater. Weinheim
PUBLISHED: 03-18-2014
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Highly uniform, ultrathin, layer-by-layer heteroatom (N, B) co-doped graphene films are fabricated for high-performance on-chip planar micro-supercapacitors with an ultrahigh volumetric capacitance of ?488 F cm(-3) and excellent rate capability due to the synergistic effect of nitrogen and boron co-doping.
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The guanidinium group as a key part of water-soluble polymer carriers for siRNA complexation and protection against degradation.
Macromol Rapid Commun
PUBLISHED: 02-20-2014
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Here, the preparation of a novel block copolymer consisting of a statistical copolymer N-(2-hydroxypropyl) methacrylamide-s-N-(3-aminopropyl) methacrylamide and a short terminal 3-guanidinopropyl methacrylamide block is reported. This polymer structure forms neutral but water-soluble nanosized complexes with siRNA. The siRNA block copolymer complexes are first analyzed using agarose gel electrophoresis and their size is determined with fluorescence correlation spectroscopy. The protective properties of the polymer against RNA degradation are investigated by treating the siRNA block copolymer complexes with RNase V1. Heparin competition assays confirm the efficient release of the cargo in vitro. In addition, the utilization of microscale thermophoresis is demonstrated for the determination of the binding strength between a fluorescently labeled polyanion and a polymer molecule.
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25th anniversary article: high-mobility hole and electron transport conjugated polymers: how structure defines function.
Adv. Mater. Weinheim
PUBLISHED: 02-06-2014
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The structural organization of three different families of semicrystalline ?-conjugated polymers is reported (poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis-alkyl-4H-cyclopenta-[2,1-b;3,4-b0]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)](cyclopentadithiophene-benzothiadiazole) (CDT-BTZ) and poly(N,N"-bis-2-octyldodecylnaphtalene-1,4,5,8-bis-dicarboximide-2,6-diyl-alt-5,5-2,2-bithiophene (P(NDI2OD-T2))). These have triggered significant interest for their remarkable charge-transport properties. By performing molecular mechanics/dynamics simulations with carefully re-parameterized force fields, it is illustrated in particular how the supramolecular organization of these conjugated polymers is driven by an interplay between the length and nature of the conjugated monomer unit and the packing of their alkyl side chains, and to what extent it impacts the charge-carrier mobility, as monitored by quantum-chemical calculations of the intermolecular hopping transfer integrals. This Progress Report is concluded by providing generic guidelines for the design of materials with enhanced degrees of supramolecular organization.
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Synthesis of structurally well-defined and liquid-phase-processable graphene nanoribbons.
Nat Chem
PUBLISHED: 01-24-2014
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The properties of graphene nanoribbons (GNRs) make them good candidates for next-generation electronic materials. Whereas 'top-down' methods, such as the lithographical patterning of graphene and the unzipping of carbon nanotubes, give mixtures of different GNRs, structurally well-defined GNRs can be made using a 'bottom-up' organic synthesis approach through solution-mediated or surface-assisted cyclodehydrogenation reactions. Specifically, non-planar polyphenylene precursors were first 'built up' from small molecules, and then 'graphitized' and 'planarized' to yield GNRs. However, fabrication of processable and longitudinally well-extended GNRs has remained a major challenge. Here we report a bottom-up solution synthesis of long (>200 nm) liquid-phase-processable GNRs with a well-defined structure and a large optical bandgap of 1.88 eV. Self-assembled monolayers of GNRs can be observed by scanning probe microscopy, and non-contact time-resolved terahertz conductivity measurements reveal excellent charge-carrier mobility within individual GNRs. Such structurally well-defined GNRs may prove useful for fundamental studies of graphene nanostructures, as well as the development of GNR-based nanoelectronics.
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Exciton-dominated optical response of ultra-narrow graphene nanoribbons.
Nat Commun
PUBLISHED: 01-21-2014
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Narrow graphene nanoribbons exhibit substantial electronic bandgaps and optical properties fundamentally different from those of graphene. Unlike graphene--which shows a wavelength-independent absorbance for visible light--the electronic bandgap, and therefore the optical response, of graphene nanoribbons changes with ribbon width. Here we report on the optical properties of armchair graphene nanoribbons of width N=7 grown on metal surfaces. Reflectance difference spectroscopy in combination with ab initio calculations show that ultranarrow graphene nanoribbons have fully anisotropic optical properties dominated by excitonic effects that sensitively depend on the exact atomic structure. For N=7 armchair graphene nanoribbons, the optical response is dominated by absorption features at 2.1, 2.3 and 4.2?eV, in excellent agreement with ab initio calculations, which also reveal an absorbance of more than twice the one of graphene for linearly polarized light in the visible range of wavelengths.
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Functional Layers for Zn(II) Ion Detection: From Molecular Design to Optical Fiber Sensors.
J Phys Chem B
PUBLISHED: 12-20-2013
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We report on the synthesis of a novel perylene monoimide derivative that shows high response and selectivity for zinc ion detection. The complexation of Zn(2+) by the dye is followed by FD-MS, (1)H NMR, UV-vis spectroscopy, and isothermal titration calorimetry. Quantum chemical calculations are performed to gain further insight into the electronic processes responsible for the spectroscopic changes observed upon complexation. Finally, the perylene dye is incorporated in a sol-gel silica layer coated on optical fibers that are then used for Zn(2+) detection in aqueous solution.
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Green emitting photoproducts from terrylene diimide after red illumination.
J. Am. Chem. Soc.
PUBLISHED: 12-10-2013
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The spectral properties of emissive photoproducts, formed upon 633 nm irradiation of a terrylene diimide dye, were investigated. Ensemble and single-molecule level experiments were conducted by immobilizing the TDI dye molecules in a polystyrene film. In the bulk experiments, green emission could be observed from the photobleached areas (photobleached with 633 nm light) when excited with 480 or 514 nm light. Similar phenomena were also observed at the single-molecule level. On the basis of the single-molecule experiments, a conversion efficiency of about 5% was estimated for the formation of emissive spectrally blue-shifted photoproducts. These green emissive photoproducts have spectral properties that resemble those of lower rylene homologues, e.g. perylene diimide or perylene monoimide. Our results indicate that the formation of blue-shifted emissive photoproducts can have implications for analyzing single-molecule FRET experiments or multiple color-labeled fluorescent systems.
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Ultrafast photoconductivity of graphene nanoribbons and carbon nanotubes.
Nano Lett.
PUBLISHED: 11-07-2013
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We present a comparative study of the ultrafast photoconductivity in two different forms of one-dimensional (1D) quantum-confined graphene nanostructures: structurally well-defined semiconducting graphene nanoribbons (GNRs) fabricated by a "bottom-up" chemical synthesis approach and semiconducting carbon nanotubes (CNTs) with a similar bandgap energy. Transient photoconductivities of both materials were measured using time-resolved terahertz spectroscopy, allowing for contact-free measurements of complex-valued photoconductivity spectra with subpicosecond time-resolution. We show that, while the THz photoresponse seems very different for the two systems, a single model of free carriers experiencing backscattering when moving along the long axis of the CNTs or GNRs provides a quantitative description of both sets of results, revealing significantly longer carrier scattering times for CNTs (ca. 150 fs) than for GNRs (ca. 30 fs) and in turn higher carrier mobilities. This difference can be explained by differences in band structures and phonon scattering and the greater structural rigidity of CNTs as compared to GNRs, minimizing the influence of bending and/or torsional defects on the electron transport.
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Sensor Arrays Based on Polycyclic Aromatic Hydrocarbons: Chemiresistors versus Quartz-Crystal Microbalance.
ACS Appl Mater Interfaces
PUBLISHED: 11-06-2013
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Arrays of broadly cross-reactive sensors are key elements of smart, self-training sensing systems. Chemically sensitive resistors and quartz-crystal microbalance (QCM) sensors are attractive for sensing applications that involve detection and classification of volatile organic compounds (VOCs) in the gas phase. Polycyclic aromatic hydrocarbon (PAH) derivatives as sensing materials can provide good sensitivity and robust selectivity towards different polar and nonpolar VOCs, while being quite tolerant to large humidity variations. Here, we present a comparative study of chemiresistor and QCM arrays based on a set of custom-designed PAH derivatives having either purely nonpolar coronas or alternating nonpolar and strongly polar side chain termination. The arrays were exposed to various concentrations of representative polar and nonpolar VOCs under extremely varying humidity conditions (5-80% RH). The sensor arrays classification ability of VOC polarity, chemical class and compound separation was explained in terms of the sensing characteristics of the constituent sensors and their interaction with the VOCs. The results presented here contribute to the development of novel versatile and cost-effective real-world VOC sensing platforms.
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Mesoporous Metal-Nitrogen-Doped Carbon Electrocatalysts for Highly Efficient Oxygen Reduction Reaction.
J. Am. Chem. Soc.
PUBLISHED: 10-17-2013
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A family of mesoporous non-precious metal (NPM) catalysts for oxygen reduction reaction (ORR) in acidic media, including cobalt-nitrogen-doped carbon (C-N-Co) and iron-nitrogen-doped carbon (C-N-Fe), was prepared from vitamin B12 (VB12) and the polyaniline-Fe (PANI-Fe) complex, respectively. Silica nanoparticles, ordered mesoporous silica SBA-15, and montmorillonite were used as templates for achieving mesoporous structures. The most active mesoporous catalyst was fabricated from VB12 and silica nanoparticles and exhibited a remarkable ORR activity in acidic medium (half-wave potential of 0.79 V, only ~58 mV deviation from Pt/C), high selectivity (electron transfer number > 3.95), and excellent electrochemical stability (only 9 mV negative shift of half-wave potential after 10,000 potential cycles). The unprecedented performance of these NPM catalysts in ORR is attributed to their well-defined porous structures with a narrow mesopore size distribution, high Brunauer-Emmett-Teller surface area (up to 572 m2/g), and homogeneous distribution of abundant metal-Nx active sites.
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Surface pretreatment boosts the performance of supramolecular affinity materials on quartz crystal microbalances for sensor applications.
Anal. Chem.
PUBLISHED: 10-16-2013
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A Teflon-like coating is the key for the boost in sensitivity of quartz microbalances for the tracing of airborne analytes. Since the undesired signals for the interfering compounds are suppressed and the ones for the targeted compounds (e.g., peroxide explosives) are enhanced, the PCA output is improved.
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Magnetic exchange coupling of a synthetic Co(II)-complex to a ferromagnetic Ni substrate.
Chem. Commun. (Camb.)
PUBLISHED: 10-10-2013
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On-surface assembly of a spin-bearing and non-aromatic porphyrin-related synthetic Co(II)-complex on a ferromagnetic Ni thin film substrate and subsequent magnetic exchange interaction across the interface were studied by scanning tunnelling microscopy (STM), X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and density functional theory +U (DFT + U) calculations.
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The right way to self-fuse bi- and terpyrenyls to afford graphenic cutouts.
Chem. Commun. (Camb.)
PUBLISHED: 10-05-2013
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In this work, we subject bi- and terpyrenyls to selective fusion for formation of extended polycyclic aromatic hydrocarbons (PAHs). Connecting the pyrene units at 4-4- or 1-4-positions led to smooth formation of extended PAHs, achieved via cyclodehydrogenation. This is far more difficult if pyrene is coupled in the 1,1-position.
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Surface Supported Gold-Organic Hybrids: On-Surface Synthesis and Surface Directed Orientation.
Small
PUBLISHED: 09-17-2013
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The surface-assisted synthesis of gold-organic hybrids on Au (111) and Au (100) surfaces is repotred by thermally initiated dehalogenation of chloro-substituted perylene-3,4,9,10-tetracarboxylic acid bisimides (PBIs). Structures and surface-directed alignment of the Au-PBI chains are investigated by scanning tunnelling microscopy in ultra high vacuum conditions. Using dichloro-PBI as a model system, the mechanism for the formation of Au-PBI dimer is revealed with scanning tunnelling microscopy studies and density functional theory calculations. A PBI radical generated from the homolytic C-Cl bond dissociation can covalently bind a surface gold atom and partially pull it out of the surface to form stable PBI-Au hybrid species, which also gives rise to the surface-directed alignment of the Au-PBI chains on reconstructed Au (100) surfaces.
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Tuning Polarity of Polyphenylene Dendrimers by Patched Surface Amphiphilicity-Precise Control over Size, Shape, and Polarity.
Macromol Rapid Commun
PUBLISHED: 08-30-2013
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In the ideal case, a precise synthesis yields molecules with a constitutional as well as a conformational perfectness. Such a case of precision is demonstrated by the synthesis of semi-rigid amphiphilic polyphenylene dendrimers (PPDs). Polar sulfonate groups are precisely placed on their periphery in such a manner that patches of polar and non-polar regions are created. Key structural features are the semi-rigid framework and shape-persistent nature of PPDs since the limited flexibility introduces a nano-phase-separated amphiphilic rim of the dendrimer. This results in both attractive and repulsive interactions with a given solvent. Frustrated solvent structures then lead to a remarkable solubility in solvents of different polarity such as toluene, methanol, and water or their mixtures. Water solubility combined with defined surface structuring and variable hydrophobicity of PPDs that resemble the delicate surface textures of proteins are important prerequisites for their biological and medical applications based upon cellular internalization.
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Assembly and fiber formation of a gemini-type hexathienocoronene amphiphile for electrical conduction.
J. Am. Chem. Soc.
PUBLISHED: 08-27-2013
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We report the synthesis, characterization, and self-assembly of a new gemini-type amphiphilic hexathienocoronene (HTCGemini), which owes its amphiphilicity to two hydrophobic dodecyl chains on one side of the HTC core and two hydrophilic triethylene glycol (TEG) chains on the other. Bearing a "softer" aromatic HTC core than the conventional hexa-peri-hexabenzocoronenes (HBC), and being more planar than contorted-hexabenzocoronenes (c-HBC), HTCGemini is demonstrated to yield various well-ordered assemblies in solution, at the liquid-solid interface, and in solid state by the use of different processing techniques. Regular fibers, helices, and tubes can be formed simply by processing from different solvents. At the liquid-solid interface, as visualized by scanning tunneling microscopy (STM), pairs of molecules adsorb very close to each other and arrange in the p2 plane group, driven by packing constraints and weak van der Waals interactions between adjacent molecules. HTCGemini also exhibits phase forming behavior in the bulk upon thermal treatment, resulting in a crystalline, herringbone-like columnar structure. Owning to an electron enriched aromatic core with respect to other reported coronenes, HTCGemini easily forms a stable radical cation, both in solution and in the bulk, upon oxidative doping with nitrosonium tetrafluoroborate (NOBF4). Furthermore, light irradiation of the blend film of HTCGemini and phenyl-C61-butyric acid methyl ester (PCBM) generates a prominent photocurrent which can be switched repeatedly with a large on/off ratio (6.0 × 10(4)). The self-assembled structures obtained from HTCGemini at different length scales have potential applications in optoelectronic devices, solar cells, and redox sensors.
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High-Performance Electrocatalysts for Oxygen Reduction Derived from Cobalt Porphyrin-Based Conjugated Mesoporous Polymers.
Adv. Mater. Weinheim
PUBLISHED: 08-17-2013
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A cobalt-nitrogen-doped porous carbon that exhibits ribbon-shape morphology, high surface area, mesoporous structure, high nitrogen and cobalt content is fabricated for high-performance self-supported oxygen reduction electrocatalytsts through template-free pyrolysis of cobalt porphyrin-based conjugated mesoporous polymer frameworks.
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Titania Nanosheet-Mediated Construction of a Two- Dimensional Titania/Cadmium Sulfide Heterostructure for High Hydrogen Evolution Activity.
Adv. Mater. Weinheim
PUBLISHED: 07-31-2013
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Two-dimensional titania nanosheets have been utilized to fabricate 2D titania-based mesoporous silica through a controlled sol-gel method, which can further serve as an robust and versatile template to construct various 2D heterostructures via a nanocasting technology. 2D titania-based CdS has been fabricated. This heterostructure manifests an excellent H2 -production rate of 285 ?mol·h(-1) under visible-light irradiation and an apparent quantum yield of 6.9% at 420 nm.
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A universal scheme to convert aromatic molecular monolayers into functional carbon nanomembranes.
ACS Nano
PUBLISHED: 07-09-2013
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Free-standing nanomembranes with molecular or atomic thickness are currently explored for separation technologies, electronics, and sensing. Their engineering with well-defined structural and functional properties is a challenge for materials research. Here we present a broadly applicable scheme to create mechanically stable carbon nanomembranes (CNMs) with a thickness of ~0.5 to ~3 nm. Monolayers of polyaromatic molecules (oligophenyls, hexaphenylbenzene, and polycyclic aromatic hydrocarbons) were assembled and exposed to electrons that cross-link them into CNMs; subsequent pyrolysis converts the CNMs into graphene sheets. In this transformation the thickness, porosity, and surface functionality of the nanomembranes are determined by the monolayers, and structural and functional features are passed on from the molecules through their monolayers to the CNMs and finally on to the graphene. Our procedure is scalable to large areas and allows the engineering of ultrathin nanomembranes by controlling the composition and structure of precursor molecules and their monolayers.
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Natural product and material chemistries--separated forever?
J. Am. Chem. Soc.
PUBLISHED: 06-07-2013
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Chemistry research is an eloquent, yet extremely complex discipline consisting of a diverse range of topics. The complexity of every sub-discipline requires extensive focus, which can limit cross-talk between fields, thus leading to their isolation. In particular, natural product and material chemistries have experienced this trend, and it has led to an ever growing separation between them. Yet by looking at the fundamental aspect of the relationship between molecular design and the resulting properties, it is possible to remind chemists of their ability to bridge these research areas. It is intradisciplinary collaborations that can provide a path toward collectively addressing the many challenges of chemistry.
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Processable Rylene Diimide Dyes up to 4?nm in Length: Synthesis and STM Visualization.
Chemistry
PUBLISHED: 05-31-2013
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Long and planar: Facile syntheses of soluble hexarylene diimides (HDI) and octarylene diimides (ODI) are described. They are stable in both solution and the solid state, and exhibit broad and intense NIR absorption. Scanning tunneling microscopy (STM) reveals that HDI, after deposition from solution, forms a unique herringbone bilayer or stable multilayers depending on the concentration.
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Manifestations of non-planar adsorption geometries of lead pyrenocyanine at the liquid-solid interface.
Chem Asian J
PUBLISHED: 05-20-2013
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In this work, we provide evidence for multiple non-planar adsorption geometries of a novel pyrenocyanine derivative at the liquid-solid interface under ambient conditions. When adsorbed at the organic liquid-solid interface, lead pyrenocyanine forms well-ordered monolayers that exhibit peculiar non-periodic contrast variation. The different contrast of the adsorbed molecules is attributed to dissimilar adsorption geometries which arise from the non-planar conformation of the molecules. The non-planarity of the molecular backbone in turn arises due to a combination of the angularly extended pyrene subunits and the presence of the large lead ion, which is too big to fit inside the central cavity and thus is located out of the aromatic plane. The two possible locations of the lead atom, namely below and above the aromatic plane, could be identified as depression and protrusion in the central cavity, respectively. The manifestation of such multiple adsorption geometries on the structure of the resultant monolayer is discussed in detail. The packing density of these 2D arrays of molecules could be tuned by heating of the sample wherein the molecular packing changes from a low-density, pseudo six-fold symmetric to a high-density, two-fold symmetric arrangement. Finally, a well-ordered two-component system could be constructed by incorporating C60 molecules in the adlayer of lead pyrenocyanine at the liquid-solid interface.
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Rational design of benzotrithiophene-diketopyrrolopyrrole-containing donor-acceptor polymers for improved charge carrier transport.
Adv. Mater. Weinheim
PUBLISHED: 05-07-2013
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Two new donor-acceptor polymers containing benzo[2,1-b:3,4-b:5,6-c]trithiophene (BTT) as donor and diketopyrrolopyrrole (DPP) as acceptor are synthesized and applied in OFETs. By tuning the alkyl substituents of the polymers, a striking difference in packing order, thin-film arrangement, and charge carrier transport is observed. The polymer without substituents at the BTT exhibits a hole mobility two orders of magnitude higher than that with alkyl chains therein.
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Atomically precise edge chlorination of nanographenes and its application in graphene nanoribbons.
Nat Commun
PUBLISHED: 05-04-2013
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Chemical functionalization is one of the most powerful and widely used strategies to control the properties of nanomaterials, particularly in the field of graphene. However, the ill-defined structure of the present functionalized graphene inhibits atomically precise structural characterization and structure-correlated property modulation. Here we present a general edge chlorination protocol for atomically precise functionalization of nanographenes at different scales from 1.2 to 3.4?nm and its application in graphene nanoribbons. The well-defined edge chlorination is unambiguously confirmed by X-ray single-crystal analysis, which also discloses the characteristic non-planar molecular shape and detailed bond lengths of chlorinated nanographenes. Chlorinated nanographenes and graphene nanoribbons manifest enhanced solution processability associated with decreases in the optical band gap and frontier molecular orbital energy levels, exemplifying the structure-correlated property modulation by precise edge chlorination.
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Fluorescent nanoparticle delivered dsRNA toward genetic control of insect pests.
Adv. Mater. Weinheim
PUBLISHED: 03-15-2013
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A fluorescent cationic core-shell nanoparticle efficiently enters into cells with high transfection efficacy. A FNP/CHT10-dsRNA complex is orally fed to insect pests and knocks down a midgut-specific chitinase gene of the Asian corn borer, which leads to death. This is the first report on the genetic control of insect pests through a non-viral gene delivery system to knock down key developmental gene expression.
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Energy transfer at the single-molecule level: synthesis of a donor-acceptor dyad from perylene and terrylene diimides.
Chemistry
PUBLISHED: 02-04-2013
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In 2004, we reported single-pair fluorescence resonance energy transfer (spFRET), based on a perylene diimide (PDI) and terrylene diimide (TDI) dyad (1) that was bridged by a rigid substituted para-terphenyl spacer. Since then, several further single-molecule-level investigations on this specific compound have been performed. Herein, we focus on the synthesis of this dyad and the different approaches that can be employed. An optimized reaction pathway was chosen, considering the solubilities, reactivities, and accessibilities of the building blocks for each individual reaction whilst still using established synthetic techniques, including imidization, Suzuki coupling, and cyclization reactions. The key differentiating consideration in this approach to the synthesis of dyad 1 is the introduction of functional groups in a nonsymmetrical manner onto either the perylene diimide or the terrylene diimide by using imidization reactions. Combined with well-defined purification conditions, this modified approach allows dyad 1 to be obtained in reasonable quantities in good yield.
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Graphene-based in-plane micro-supercapacitors with high power and energy densities.
Nat Commun
PUBLISHED: 02-01-2013
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Micro-supercapacitors are important on-chip micro-power sources for miniaturized electronic devices. Although the performance of micro-supercapacitors has been significantly advanced by fabricating nanostructured materials, developing thin-film manufacture technologies and device architectures, their power or energy densities remain far from those of electrolytic capacitors or lithium thin-film batteries. Here we demonstrate graphene-based in-plane interdigital micro-supercapacitors on arbitrary substrates. The resulting micro-supercapacitors deliver an area capacitance of 80.7??F?cm?² and a stack capacitance of 17.9?F?cm?³. Further, they show a power density of 495?W?cm?³ that is higher than electrolytic capacitors, and an energy density of 2.5?mWh?cm?³ that is comparable to lithium thin-film batteries, in association with superior cycling stability. Such microdevices allow for operations at ultrahigh rate up to 1,000?V?s?¹, three orders of magnitude higher than that of conventional supercapacitors. Micro-supercapacitors with an in-plane geometry have great promise for numerous miniaturized or flexible electronic applications.
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Heterogeneous diffusion in thin polymer films as observed by high-temperature single-molecule fluorescence microscopy.
J. Am. Chem. Soc.
PUBLISHED: 12-19-2011
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Single-molecule fluorescence microscopy was used to investigate the dynamics of perylene diimide (PDI) molecules in thin supported polystyrene (PS) films at temperatures up to 135 °C. Such high temperatures, so far unreached in single-molecule spectroscopy studies, were achieved using a custom-built setup which allows for restricting the heated mass to a minimum. This enables temperature-dependent single-molecule fluorescence studies of structural dynamics in the temperature range most relevant to the processing and to applications of thermoplastic materials. In order to ensure that polymer chains were relaxed, a molecular weight of 3000 g/mol, clearly below the entanglement length of PS, was chosen. We found significant heterogeneities in the motion of single PDI probe molecules near T(g). An analysis of the track radius of the recorded single-probe molecule tracks allowed for a distinction between mobile and immobile molecules. Up to the glass transition temperature in bulk, T(g,bulk), probe molecules were immobile; at temperatures higher than T(g,bulk) + 40 K, all probe molecules were mobile. In the range between 0 and 40 K above T(g,bulk) the fraction of mobile probe molecules strongly depends on film thickness. In 30-nm thin films mobility is observed at lower temperatures than in thick films. The fractions of mobile probe molecules were compared and rationalized using Monte Carlo random walk simulations. Results of these simulations indicate that the observed heterogeneities can be explained by a model which assumes a T(g) profile and an increased probability of probe molecules remaining at the surface, both effects caused by a density profile with decreasing polymer density at the polymer-air interface.
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Discotic hexa-peri-hexabenzocoronenes with strong dipole: synthesis, self-assembly and dynamic studies.
Chem. Commun. (Camb.)
PUBLISHED: 12-01-2011
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Strong dipole moments have been built into two hexa-peri-hexabenzocoronene (HBC) derivatives (1 and 2) originating from the push-pull structure of the molecules with one electron-donating and one electron-withdrawing substituent. The influence of dipole moment on the self-assembly of HBCs in solution and in bulk has been investigated.
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Dynamics of structure formation in a discotic liquid crystal by infrared spectroscopy and related techniques.
J Phys Chem B
PUBLISHED: 11-29-2011
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The dynamics of the herringbone structure formation have been studied in a monobromo hexa-peri-hexabenzocoronene derivative by infrared spectroscopy and complementary techniques. Selective probing of the vibration modes corresponding to the aromatic core and the alkyl chains, allowed investigation of their role in the phase transformation dynamics over an extraordinarily broad time-window (1-10(5) s). Identical kinetics were found suggesting that both the core and the alkyl chains simultaneously drive the system from the undercooled liquid crystalline to the crystalline phase with the herringbone structure.
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Deep blue polymer light emitting diodes based on easy to synthesize, non-aggregating polypyrene.
Opt Express
PUBLISHED: 11-24-2011
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Thorough analyses of the photo- and devicephysics of poly-7-tert-butyl-1,3-pyrenylene (PPyr) by the means of absorption and photoluminescence emission, time resolved photoluminescence and photoinduced absorption spectroscopy as well as organic light emitting devices (OLEDs) are presented in this contribution. Thereby we find that this novel class of polymers shows deep blue light emission as required for OLEDs and does not exhibit excimer or aggregate emission when processed from solvents with low polarity. Moreover the decay dynamics of the compound is found to be comparable to that of well blue emitting conjugated polymers such as polyfluorene. OLEDs built in an improved device assembly show stable bright blue emission for the PPyr homopolymer and further a considerable efficiency enhancement can be demonstrated using a triphenylamine(TPA)/pyrene copolymer.
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Making benzotrithiophene a stronger electron donor.
Org. Lett.
PUBLISHED: 10-27-2011
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A new member of the benzotrithiophene family, benzo[2,1-b:-3,4-b:5,6-c?]trithiophene (3a), and its alkyl substituted derivatives (3b-e) were synthesized and characterized. Their photophyscial, electrochemical, crystallographic, and self-assembly properties were described. Thin film structures varied widely with the exact nature of the alkyl substitution pattern, with decreasing self-assembly propensity with increasing alkyl chain length. The high HOMO levels and the coplanarity of these molecules show their potential as organic semiconductors and as donor components in donor-acceptor copolymers.
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Assembly and separation of semiconductor quantum dot dimers and trimers.
J. Am. Chem. Soc.
PUBLISHED: 10-26-2011
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Repeated precipitation of colloidal semiconductor quantum dots (QD) from a good solvent by adding a poor solvent leads to an increasing number of QD oligomers after redispersion in the good solvent. By using density gradient ultracentrifugation we have been able to separate QD monomer, dimer, and trimer fractions from higher oligomers in such solutions. In the corresponding fractions QD dimers and trimers have been enriched up to 90% and 64%, respectively. Besides directly coupled oligomers, QD dimers and trimers were also assembled by linkage with a rigid terrylene diimide dye (TDI) and separated again by ultracentrifugation. High-resolution transmission electron micrographs show that the interparticle distances are clearly larger than those for directly coupled dots proving that the QDs indeed are cross-linked by the dye. Moreover, energy transfer from the QDs to the TDI "bridge" has been observed. Individual oligomers (directly coupled or dye-linked) can be readily deposited on a substrate and studied simultaneously by scanning force and optical microscopy. Our simple and effective scheme is applicable to a wide range of ligand stabilized colloidal nanoparticles and opens the way to a detailed study of electronic coupling in, e.g., QD molecules.
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Chemical methods for the generation of graphenes and graphene nanoribbons.
Angew. Chem. Int. Ed. Engl.
PUBLISHED: 10-25-2011
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Synthetic carbon allotrope chemistry is currently among the most rapidly growing topics in materials chemistry. The youngest and at the same time probably the most promising representative of new carbon allotropes is graphene. In this article we outline our recent contributions to chemical graphene formation and functionalization.
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Vibrational excitations in molecular layers probed by ballistic electron microscopy.
Nanotechnology
PUBLISHED: 09-29-2011
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We demonstrate the information on molecular vibrational modes via the second derivative (d(2)I(B)/dV(2)) of the ballistic electron emission spectroscopy (BEES) current. The proposed method does not create huge fields as in the case of conventional derivative spectroscopy and maintains a zero bias across the device. BEES studies carried out on three different types of large polycyclic aromatic hydrocarbon (PAH) molecular layers show that the d(2)I(B)/dV(2) spectra consist of uniformly spaced peaks corresponding to vibronic excitations. The peak spacing is found to be identical for molecules within the same PAH family though the BEES onset voltage varies for different molecules. In addition, injection into a particular orbital appears to correspond to a specific vibrational mode as the manifestation of the symmetry principle.
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Core-extended terrylene tetracarboxdiimide: synthesis and chiroptical characterization.
Org. Lett.
PUBLISHED: 09-28-2011
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The novel 1,16:6,7:8,9:14,15-tetranaphthoterrylene tetracarboxdiimide was synthesized via a straightforward route, yielding optically active atropisomers with a high racemization barrier. Absorption, fluorescence, and circular dichroism measurements revealed high absorption coefficients and fluorescence quantum yields and enabled the stereochemical assignment in combination with quantum mechanical calculations.
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What is Visualize?

JoVE Visualize is a tool created to match the last 5 years of PubMed publications to methods in JoVE's video library.

How does it work?

We use abstracts found on PubMed and match them to JoVE videos to create a list of 10 to 30 related methods videos.

Video X seems to be unrelated to Abstract Y...

In developing our video relationships, we compare around 5 million PubMed articles to our library of over 4,500 methods videos. In some cases the language used in the PubMed abstracts makes matching that content to a JoVE video difficult. In other cases, there happens not to be any content in our video library that is relevant to the topic of a given abstract. In these cases, our algorithms are trying their best to display videos with relevant content, which can sometimes result in matched videos with only a slight relation.