286 research outputs found

    Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers

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    We present femtosecond transient absorption measurements on π\pi-conjugated supramolecular assemblies in a high pump fluence regime. Oligo(\emph{p}-phenylenevinylene) monofunctionalized with ureido-\emph{s}-triazine (MOPV) self-assembles into chiral stacks in dodecane solution below 75∘^{\circ}C at a concentration of 4×10−44\times 10^{-4} M. We observe exciton bimolecular annihilation in MOPV stacks at high excitation fluence, indicated by the fluence-dependent decay of 111^1Bu_{u}-exciton spectral signatures, and by the sub-linear fluence dependence of time- and wavelength-integrated photoluminescence (PL) intensity. These two characteristics are much less pronounced in MOPV solution where the phase equilibrium is shifted significantly away from supramolecular assembly, slightly below the transition temperature. A mesoscopic rate-equation model is applied to extract the bimolecular annihilation rate constant from the excitation fluence dependence of transient absorption and PL signals. The results demonstrate that the bimolecular annihilation rate is very high with a square-root dependence in time. The exciton annihilation results from a combination of fast exciton diffusion and resonance energy transfer. The supramolecular nanostructures studied here have electronic properties that are intermediate between molecular aggregates and polymeric semiconductors

    Spatial layout planning in sub-surface rail station design for effective fire evacuation

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    The London Underground network is a crucial part of the transportation system in one of only four ‘Alpha’ world cities. The other three – Paris, New York and Tokyo – also have such sub-surface railway transport systems that may benefit from this shape grammar station design process in a future research proposal. In London’s case, the passenger flow rates are the underlining factor in sizing infrastructure where passengers have access – it is therefore this criterion that provides the basis for the shape grammar formulation for the largest, oldest and one of the most complex underground systems in the world. The research aims to improve passenger fire evacuation times, with due cognisance of the growth of numbers using the system, and its present susceptibility to terrorist attacks taken into account. The proposed shape grammar approach will provide for generation of spatial layouts, based upon visual rules of shape recognition, replacement / union, their connectivity and spatial relationships. The paper concentrates on definition and implementation of novel shape grammar design rules that incorporate station planning design knowledge, and in particular also discusses designers’ fire risk assessment approach and related knowledge that is also needed to produce credible station design solutions. Development, to date, of the proposed artificially intelligent CAD environment is also described along with parallel theoretical research. The proposed CAD interface provides familiarity to the designer and avoids incompatibility issues regarding drawing exchange format between various software systems. The shape grammar layouts produced will be tested in SIMULEX, a commercially available evacuation package, and be compared against ‘traditionally’ designed layouts to demonstrate improvements of preliminary ‘reference’ designs, which follow the standard London Underground design process as a later stage of this research

    Beyond the Woodward-Hoffman rules: what controls reactivity in eliminative aromatic ring-forming reactions?

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    The Mallory (photocyclization) and Scholl (thermal cyclohydrogenation) reactions are widely used in the synthesis of extended conjugated π systems of high scientific interest and technological importance, including molecular wires, semiconducting polymers and nanographenes. While simple electrocyclization reactions obey the Woodward-Hoffman rules, no such simple, general and powerful model is available for eliminative cyclization reactions due to their increased mechanistic complexity. In this work, detailed mechanistic investigations of prototypical reactions reveal that there is no single rate-determining step for thermal oxidative dehydrogenation reactions, but they are very sensitive to the presence and distribution of heteroatoms around the photocyclizing ring system. Key aspects of reactivity are correlated to the constituent ring oxidation potentials. For photocyclization reactions, planarization occurs readily and/or spontaneously following photo-excitation, and is promoted by heteroatoms within 5-membered ring adjacent to the photocyclizing site. Oxidative photocyclization requires intersystem crossing to proceed to products, while reactants configured to undergo purely eliminative photocyclization could proceed to products entirely in the excited state. Overall, oxidative photocyclization seems to strike the optimal balance between synthetic convenience (ease of preparation of reactants, mild conditions, tolerant to chemical diversity in reactants) and favourable kinetic and thermodynamic properties

    Novel Design of Electron Spectrograph Instrument for Particle Correlation Studies

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    Supramolecular organization in block copolymers containing a conjugated segment: a joint AFM/molecular modeling study

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    The solid-state supramolecular organization of block copolymers containing one π-conjugated block and one non-conjugated block is elucidated with a joint experimental and theoretical approach. This approach combines atomic force microscopy (AFM) measurements on thin polymer deposits, which reveal the typical microscopic morphologies, and molecular modeling, which allows one to derive the models for chain packing that are most likely to explain the AFM observations. The conjugated systems considered in this study are based on aromatic building blocks (i.e. phenylene, phenylene ethylene, fluorene, or indenofluorene), substituted with alkyl groups to provide solubility; they are attached to non-conjugated blocks such as polydimethylsiloxane, polyethylene oxide, or polystyrene. Films are prepared from solutions in solvents which are good for both blocks, in order to prevent aggregation processes in solution. Therefore, the morphology observed in the solid state is expected to result mostly from the intrinsic self-assembly of the chains, with little specific influence of the solvent. In such conditions, the vast majority of compounds show deposits made of fibrilar objects. Closer examination of single fibrils on the substrate surface indicates that the objects are ribbon- like, i.e. their width is significantly larger than their height, with typical dimensions of a few tens of nanometers and a few nanometers, respectively. These results suggest that a single type of packing process, governed by the π-stacking of the conjugated chains, is at work in those block copolymers. This prevalence of such a type of packing is supported by the theoretical simulations. Molecular mechanics/dynamics calculations show that the conjugated segments tend to form stable π-stacks. In these assemblies, the block copolymer molecules can organize in either a head-to-tail or head-to-head configuration. The former case appears to be most likely because it allows for significant coiling of the non-conjugated blocks while maintaining the conjugated blocks in a compact, regular assembly. Such supramolecular organization is likely responsible for the formation of the thin, 'elementary' ribbons, which can further assemble into larger bundles. The issue of chain packing in fluorene-based systems has been modeled separately, since in these compounds, the alkyl groups attached to sp3-hybridized sites inherently accommodate out of the plane of the conjugated backbone, which can disturb the chain packing. Various possibilities of chain packing have been explored, starting from short alkyl substituents and extending the size of the side groups to n-octyl. The calculations indicate that, when in zig-zag planar conformation, linear alkyl side groups can orient in such a way that close π-stacking of the conjugated chains is preserved. In contrast, branched alkyl groups are too bulky to allow close packing of the conjugated backbones to take place. This difference is consistent with the presence or absence of fibrilar structures observed in thin deposits of the corresponding polymers; it can also account for the differences observed in the optical properties. © 2002 Elsevier Science Ltd. All rights reserved

    Rodlike Bimetallic Ruthenium and Osmium Complexes Bridged by Phenylene Spacers. Synthesis, Electrochemistry, and Photophysics.

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    In the search for light-addressable nanosized compounds we have synthesized 10 dinuclear homometallic trisbipyridyl complexes of linear structure with the general formula [M(bpy)3-BL-M(bpy)3]4+ [M = Ru(II) or Os(II); BL = polyphenylenes (2, 3, 4, or 5 units) or indenofluorene; bpy = 2,2′-bipyridine]. By using a "chemistry on the complex" approach, different sizes of rodlike systems have been obtained with a length of 19.8 and 32.5 Å for the shortest and longest complex, respectively. For one of the ruthenium precursors, [RUbpy-ph2-Si(CH3) 3][PF6]2, single crystals were obtained by recrystallization from methanol. Their photophysical and electrochemical properties are reported. All the compounds are luminescent both at room and low temperature with long excited-state lifetimes due to an extended delocalization. Nanosecond transient absorption showed that the lowest excited state involves the chelating unit attached to the bridging ligand. Electrochemical data indicated that the first reduction is at a slightly more positive potential than for the reference complexes [M(bpy)3]2+ (M = Ru, Os). This result confirms that the best acceptor is the bipyridine moiety connected to the conjugated spacers. The role of the tilt angle between the phenylene units, in the two series of complexes, for the ground and excited states is discussed
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