A network flow approach to a common generalization of Clar and Fries numbers

Clar number and Fries number are two thoroughly investigated parameters of plane graphs emerging from mathematical chemistry to measure stability of organic molecules. We consider first a common generalization of these two concepts for bipartite plane graphs, and then extend it to a framework on general (not necessarily planar) directed graphs. The corresponding optimization problem can be transformed into a maximum weight feasible tension problem which is the linear programming dual of a minimum cost network flow (or circulation) problem. Therefore the approach gives rise to a min-max theorem and to a strongly polynomial algorithm that relies exclusively on standard network flow subroutines. In particular, we give the first network flow based algorithm for an optimal Fries structure and its variants

Discotic materials for organic electronics

Discotic Materials for Electronic Applications Marcel Kastler The modulation of the pronounced aggregation propensity of alkyl substituted hexa-perihexabenzocoronenes (HBCs, Figure 1) has been achieved by a reduction of the intermolecular attractive forces. Sterically demanding branched, racemic alkyl chains, with the branching site in close proximity to the aromatic core, have been introduced in a synthetically straight forward fashion. A striking feature is the ability to melt these novel HBCs with the bulkiest side chains at temperatures, which are below the thermal decomposition threshold. This opened the possibility to apply cheap melt-processing, such as zone-crystallization to obtain uniaxially organized molecules over macroscopical areas. Depending on the steric demand excerted by the side-chain, different molecular orientations with respect to the support have been achieved, which is very important for the successful implementation in devices, which require different orientations of the charge carrier pathways. Another important prerequisite for the implementation of materials into organic electronic devices is a high charge carrier mobility to allow an unperturbed migration of free charge carriers in the device structure. Time-resolved pulse-radiolysis microwave conductivity (TR-PRMC) and time-of-flight (TOF) measurements revealed good charge carrier mobilities for the novel materials together with very long charge carrier life-times. Implemented in organic heterojunction photovoltaics, good performing devices were obtained. The variation of the polycyclic aromatic hydrocarbon (PAH) perimeter has been theoretically predicted to influence strongly the electronic properties. Going from the “all-arm chair” HBC to PAHs with one, two and three “zigzag” sites (Figure ), the electronic spectroscopy revealed pronounced differences between the derivatives. However, the symmetry and the size of the aromatic core component in these derivatives excerted a strong influence onto molecular properties, such as the number of transitions in the UV/vis spectrum. The tuning of the electronic levels of materials is a fundamental task in material science to optimize the injection of charge carriers from electrodes with a specific work function. HBC Figure 1: „zigzag“ PAHs. Finally, the supramolecular order revealed a dependence upon the substitution pattern of the alkyl chains attached in the corona of the PAHs. Higher symmetry substitution patterns with fewer chains stabilized the intracolumnar organization, due to a better crystallization. In summary, next to the synthesis of new materials holding promise for organic electronics, novel concepts have been developed which allow a simpler functionalization of the PAH based discotic materials both in the periphery and at the aromatic core. This functionalization permits a fine-tuning of molecular and supramolecular properties, which is important to obtain promising materials for the application in organic electronic devices. Diskotische Materialien für elektronische Anwendungen Marcel Kastler Die Kontrolle der ausgeprägten Aggregationsfähigkeit von alkylsubstituierten Hexa-perihexabenzocoronenen (HBC) wurde durch die Reduktion der intermolekularen Wechselwirkungen erreicht. Sterisch anspruchsvolle, verzweigte Alkylketten, mit einem Verzweigungspunkt naher des aromatischen Kerns, wurden in die Corona der aromatischen Scheiben eingebracht und verleihen den Derivaten Schmelzbarkeit ohne thermische Zersetzung. Dies erlaubte eine kostengünstige Verarbeitungstechniken direkt aus der Schmelze wie z.B. Zonenschmelzen, um uniaxial organisierte makroskopische Filme zu erhalten. Abhängig von dem sterischen Anspruch, der durch die Seitenkette erzeugt wird, wurden unterschiedliche molekulare Orientierungen auf Oberflächen erhalten, was eine wichtige Voraussetzung ist, um diskotische Materialien in elektronische Bauteile zu implementieren. Eine weitere Voraussetzung sind hohe Ladungsträgerbeweglichkeiten und Ladungsträgerlebenszeiten in den Halbleitermaterialien, die mit time-resolved pulseradiolysis microwave conductivity (TR-PRMC) und time-of-flight (TOF) auch für die synthetisieren Materialien bestimmt wurden. Die neuen Materialien zeigten bereits in organischen Solarzellen gute Leistungen. Den Einfluss des Perimeters auf die elektronischen Eigenschaften der polyzyklischen aromatischen Kohlenwasserstoffe (PAKs) wurde theoretisch vorhergesagt und in dieser Arbeit durch die Synthese einer homologe Serie von PAKs experimentell bestätigt. Geht man von der „arm-chair“ Peripherie des HBC sukzessive zu einer partiellen „zickzack“ Peripherie, so findet man eine Abhängigkeit der elektronischen Banden von Symmetrie und Größe des aromatischen Systems. Die spontan ausgebildete Überstruktur dieser Derivate zeigte eine Abhängigkeit von Substitutionsmuster und der Natur der Alkylketten. Zusammenfassend wurden neben der Synthese von neuartigen Materialien für den Einsatz in der organischen Elektronik Synthesen entwickelt, die eine vereinfachte Funktionalisierung von ausgedehnten PAKs ermöglicht. Diese Konzepte erlauben eine Justierung der molekularen und supramolekularen Eigenschaften, eines der wichtigsten Voraussetzungen für den Einsatz von Materialien in elektronischen Bauelementen

Bowdoin Orient v.87, no.1-26 (1957-1958)

https://digitalcommons.bowdoin.edu/bowdoinorient-1950s/1008/thumbnail.jp

Maritime expressions:a corpus based exploration of maritime metaphors

This study uses a purpose-built corpus to explore the linguistic legacy of Britain’s maritime history found in the form of hundreds of specialised ‘Maritime Expressions’ (MEs), such as TAKEN ABACK, ANCHOR and ALOOF, that permeate modern English. Selecting just those expressions commencing with ’A’, it analyses 61 MEs in detail and describes the processes by which these technical expressions, from a highly specialised occupational discourse community, have made their way into modern English. The Maritime Text Corpus (MTC) comprises 8.8 million words, encompassing a range of text types and registers, selected to provide a cross-section of ‘maritime’ writing. It is analysed using WordSmith analytical software (Scott, 2010), with the 100 million-word British National Corpus (BNC) as a reference corpus. Using the MTC, a list of keywords of specific salience within the maritime discourse has been compiled and, using frequency data, concordances and collocations, these MEs are described in detail and their use and form in the MTC and the BNC is compared. The study examines the transformation from ME to figurative use in the general discourse, in terms of form and metaphoricity. MEs are classified according to their metaphorical strength and their transference from maritime usage into new registers and domains such as those of business, politics, sports and reportage etc. A revised model of metaphoricity is developed and a new category of figurative expression, the ‘resonator’, is proposed. Additionally, developing the work of Lakov and Johnson, Kovesces and others on Conceptual Metaphor Theory (CMT), a number of Maritime Conceptual Metaphors are identified and their cultural significance is discussed