Search

General chemistry are key gateway courses for many Science, Technology, Engineering, and Mathematics (STEM) majors. To improve student outcomes, it is critical both to identify at-risk students and...

The synthesis of 1,3,6,8-tetraaryl substituted pyrenes is described. The substituents on the aryl groups influence the optoelectronic properties of the pyrene core indicating a strong coupling between the aryl groups and the pyrene core. Electronic absroption spectra of the cation radicals generated in solution by the single electron oxidation of pyrenes reaffirmed the coupling of the aryl rings with the pyrene. The aryl groups at 1,3,6,8-positions completely inhibit the π-stacking of pyrene core in solution. Crystal structure of the cation radical salt of tetraphenyl shows charge delocalization between neutral pyrene unit and two cationic pyrene cores. Complete inhibition of the pyrene core stacking is observed in the solid state of cation radical when the dendritic pentaphenyl groups surround the pyrene core.

The electrochemical splitting of water holds promise for the storage of energy produced intermittently by renewable energy sources. The evolution of hydrogen currently relies on the use of platinum as a catalyst—which is scarce and expensive—and ongoing research is focused towards finding cheaper alternatives. In this context, 2D polymers grown as single layers on surfaces have emerged as porous materials with tunable chemical and electronic structures that can be used for improving the catalytic activity of metal surfaces. Here, we use designed organic molecules to fabricate covalent 2D architectures by an Ullmann-type coupling reaction on Au-(111). The polymer-patterned gold electrode exhibits a hydrogen evolution reaction activity up to three times higher than that of bare gold. Through rational design of the polymer on the molecular level we engineered hydrogen evolution activity by an approach that can be easily extended to other electrocatalytic reactions., Facultad de Ciencias Exactas, Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas

The surface chemistry of 1,6-dibromo-3,8-diiodopyrene (Br2I2Py) is comparatively studied on Au(111) versus Ag(111) surfaces under ultrahigh vacuum conditions by a combination of high-resolution scanning tunneling microscopy (STM) and X-ray spectroscopy. The chemical state of the molecular networks, that is, the dehalogenation and the possible formation of organometallic intermediates, is assessed by X-ray photoelectron spectroscopy. In addition, pyrene tilt angles are quantified by carbon K-edge near edge X-ray absorption fine structure experiments. Upon room-temperature (RT) deposition of Br2I2Py onto Au(111), only partial deiodination was found, and STM revealed the coexistence of ordered arrangements of both intact Br2I2Py molecules and organometallic dimers as well as few larger aggregates. Further annealing to 100 °C triggered full deiodination followed by the formation of organometallic chains of otherwise still brominated molecules. By contrast, on Ag(111), iodine is fully and bromine is partly dis...

Many 2D covalent polymers synthesized as single layers on surfaces show inherent disorder, expressed for example in their ring-size distribution. Systems which are expected to form the thermodynamically favored hexagonal lattice usually deviate from crystallinity and include high numbers of pentagons, heptagons, and rings of other sizes. The amorphous structure of two different covalent polymers in real space using scanning tunneling microscopy is investigated. Molecular dynamics simulations are employed to extract additional information. We show that short-range correlations exist in the structure of one polymer, i. e. that polygons are not tessellating the surface randomly but that ring neighborhoods have preferential compositions. The correlation is dictated by the energy of formation of the ring neighborhoods.

Solar fuel generation has attracted vast research interest as an environmentally benign means of producing energy from sunlight for catering to the ever growing world energy demands. As an alternative to inorganic semiconductors, organic polymers have entered the stage as promising photocatalytic systems offering a yet unprecedented scope for molecular engineering and precise tuning of optoelectronic properties. This perspective presents an overview of the development, state-of-the-art, and growth perspectives of this emerging field and highlights recent advances in photocatalyst design with a particular focus on structure–property–activity relationships in structurally well-defined 2D polymers for hydrogen evolution.

Surface-confined self-assembled molecular networks are commonly described as crystalline structures with short- and long-range order. Few exceptions to this trend are found in the literature reporting on amorphous and glassy networks and discussing the origins of their disordered nature at the atomic scale. Here, we show that an amorphous two-component metal–organic network can be synthesized on a crystalline metal surface at room temperature by vapor deposition of organic molecules and metal atoms. Scanning tunneling microscopy reveals the transformation of a crystalline close-packed organic layer into a disordered porous layer formed through the self-assembly of an organic semiconductor functionalized with two cyano groups upon introduction of iron atoms on Ag(111). In contrast to the commonly observed preference for trigonal crystal symmetry in 2D systems that form glassy networks, our metal–organic network favors rhombic or square tessellation of the surface. Statistical analysis of the network’s morp...

The optical gap of the organic semiconductor [1]benzothieno[3,2-b]benzothiophene and its 2,7-dibrominated analogue is measured in solution and in the crystalline state by means of UV-vis and emission spectroscopy. Bromination leads to a change in molecular packing from herringbone to π-stacked, resulting in a marked shift in the absorption and emission spectra which is found to be in accordance with TDDFT calculations.

Covalent organic frameworks (COFs) are a new class of nanoporous polymeric vector showing promise as drug-delivery vehicles with high loading capacity and biocompatibility. The interaction between the carrier and the cargo is specifically tailored on a molecular level by H-bonding. Cell-proliferation studies indicate higher efficacy of the drug in cancer cells by nanocarrier delivery mediated by the COF.

A simple synthesis of soluble tetrakis (poly -p- phenylene ) ethylenes (TPEs), where poly- p -phenylene units contain up to 4 phenylene moieties, is accomplished using an easily available tetrakis (4-bromophenyl)ethylene and phenyl, biphenyl, and p- terphenylboronic acids, containing solubilizing iso -alkyl groups as the end-capping substituents. Electrochemical oxidation of TPEs (i.e., T1–T4 ) showed that they undergo reversible electrochemical oxidations and form stable cation-radical salts in solution. The evaluation of absorption and emissions characteristics of T1 – T4 in solution indicated that unlike parent tetratolylethylene ( T1 ), the higher homologues (i.e., T2 – T4 ) show emission centered at 520 nm and the quantum efficiency of emission increases with the increasing number of phenylene moieties in poly- p -phenylene units in various TPEs.

Hydrogen evolution from photocatalytic reduction of water holds promise as a sustainable source of carbon-free energy. Covalent organic frameworks (COFs) present an interesting new class of photoactive materials, which combine three key features relevant to the photocatalytic process, namely crystallinity, porosity and tunability. Here we synthesize a series of water- and photostable 2D azine-linked COFs from hydrazine and triphenylarene aldehydes with varying number of nitrogen atoms. The electronic and steric variations in the precursors are transferred to the resulting frameworks, thus leading to a progressively enhanced light-induced hydrogen evolution with increasing nitrogen content in the frameworks. Our results demonstrate that by the rational design of COFs on a molecular level, it is possible to precisely adjust their structural and optoelectronic properties, thus resulting in enhanced photocatalytic activities. This is expected to spur further interest in these photofunctional frameworks where rational supramolecular engineering may lead to new material applications., Production of hydrogen via the photocatalytic reduction of water is an attractive source of energy, but the catalysts are often expensive and possess little room for modification. Here, the authors show that covalent organic frameworks can be tuned for optimal photocatalytic performance.

Porous polymers have joined the ranks of light-activated catalysts that split water into hydrogen, a carbon-free alternative to fossil fuels. Their properties are easily tuned — a big plus for the development of practically useful catalysts.

The spontaneous assembly of aromatic cation radicals (D+•) with their neutral counterpart (D) affords dimer cation radicals (D2+•). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assembl...

The spontaneous assembly of aromatic cation radicals (D(+•)) with their neutral counterpart (D) affords dimer cation radicals (D(2)(+•)). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography.

Soluble tetrakis(pentaphenylphenyl)ethylene (1), prepared in three steps, allows non-emissive tetraphenylethylene (TPE) to turn into an emitter (Phi(F) = 0.13) owing to the steric bulk of the four large hexaphenylbenzene propellers which hamper =C-Ph and C=C bond rotations in the TPE core.

A practical synthesis of 1,3,6,8-tetraisopropylpyrene and the isolation and X-ray structural characterization of its monomeric cation radical salt are described.

economists acquired a place of pride among social scientists their country and also among their peers in other parts of the world. They have distinguished themselves by their contribution to the country's agricultural and rural development policies. Two factors have contributed to the growth of this discipline ; (a) a very reputed professionnal association I. e. the Indian Society of Agricultural Economics (b) the Government's willingness to involve agricultural economists in the policy-making process. Three distinct streams can be distinguished among agricultural economists : the scholar-administrators during colonial and post-colonial period, the economists specialized in agricultural economics and the agricultural scientists trained in the agricultural universities created in the 1950's. The Indian Society of Agricultural Economics has succeeded in providing a strong intellectual leadership, in keeping together disparate groups of scholars. The high quality of its journal, the Indian Journal of Agricultural Economics, has spurred young professional to reach a level of proficiency which can be compared to the contributions in very good international journals., L'Inde a su se doter d'une économie rurale bien développée et organisée, reconnue par tous ses pairs pour sa contribution au développement rural du pays. L'émergence de l'économie rurale comme une discipline indépendante a pu se faire grâce à l'action de la Société indienne d'économie rurale et à l'attitude positive du gouvernement qui a su reconnaître les économistes ruraux et les utiliser. Trois courants ont participé au développement de cette discipline : les administrateurs, de la période coloniale et post-coloniale, les économistes qui se sont spécialisés en économie rurale et les agronomes qui, à partir des années cinquante, ont été formés dans les universités agronomiques du pays. La Société indienne d'économie rurale a su fédérer ces différents courants et a permis par sa revue, l'Indian Journal of Agricultural Economies, la parution d'études de recherches de grande qualité., S. Vyas Vijay. L'économie rurale en Inde : orientations et réalisations ..... In: Économie rurale. N°200, 1990. pp. 54-56.

Two-dimensional covalent organic frameworks (COFs) are crystalline porous materials composed of organic building blocks that are connected via covalent bonds within their layers, but through non-covalent interactions between the layers. The exact stacking sequence of the layers is of paramount importance for the optoelectronic, catalytic and sorption properties of these polymeric materials. The weak interlayer interactions lead to a variety of stacking geometries in COFs, which are both hard to characterize and poorly understood due to the low levels of crystallinity. Therefore, detailed insights into the stacking geometry in COFs is still largely elusive. In this work we show that the geometric and electronic features of the COF building blocks can be used to guide the stacking behavior of two related 2D imine COFs (TBI-COF and TTI-COF), which either adopt an averaged "eclipsed'' structure with apparent zero-offset stacking or a unidirectionally slip-stacked structure, respectively. These structural features are confirmed by XRPD and TEM measurements. Based on theoretical calculations, we were able to pinpoint the cause of the uniform slip-stacking geometry and high crystallinity of TTI-COF to the inherent self-complementarity of the building blocks and the resulting donor-acceptor-type stacking of the imine bonds in adjacent layers, which can serve as a more general design principle for the synthesis of highly crystalline COFs.