1,721,025 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Radicals in Organometallic Coordination Complexes
No full textUntersucht wurden organometallische Komplexe mit Dithienylethan/Dithienylethen als Ligand, die direkt über die Heteroatome Sauerstoff oder Stickstoff an das Metallzentrum gebunden sind. Besonderes Augenmerk lag hierbei auf einer möglichen Photoschaltung sowie den stereochemischen Gegebenheiten - und deren Konsequenzen in Bezug auf die Chiralität - der Komplexe
Design und Synthese Moebius-topologischer und Moebius-aromatischer Kohlenwasserstoffe
Full text linkDie vorliegende Arbeit befasst sich mit den Design und der Synthese Moebius-topologischer und Moebius-aromatischer Kohlenwasserstoffe mit einer Linking Number größer als Eins. Moebius-Molekuele waren bisher nur mit einer Linking Number von Eins bekannt. Die Zielverbindung dieser Arbeit repräsentiert die erste Moebius-topologische Struktur mit einer Linking Number von Drei
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Chemoselective Cross-Coupling Reactions as Tools in Synthesis and Applications for Polymer Chemistry
Full text linkA nucleophile-selective cross-coupling reaction with vinyl and alkynyl bromides on a dinucleophilic thiophene bearing two metal groups, a boronic ester and a stannyl group, has been developed. The first nucleophilic site to react is the stannyl group, and subsequently, a Suzuki–Miyaura cross-coupling takes place after adding a second electrophile, base and water. The generality of the reaction was shown with various electron rich, electron neutral, electron poor vinyl bromides and alkynyl bromides. The roles typical in Sonogashira couplings or Heck reactions of the aromatic ring as the dielectrophile coupling to vinyl and alkynyl metal species are reversed, which presents a new tool for organic synthesis.
Four different difunctional thienylpyrrole monomers with a bromide group on the thienyl moiety and a metal group (Mg, B, Sn or Zn) on the pyrrole moiety were developed. Based on those monomers, π-conjugated poly(thiophene-alt-pyrrole)s PTPs were firstly synthesized via nickel and palladium catalyzed Kumada polycondensation from this A-B type monomer. The polymers were analyzed by NMR, IR, GPC, MALDI-MS, UV-vis, PL and TGA measurements
Photo- und mechanoresponsive Polymersysteme
Full text linkPolymere und Polymermaterialien haben seit der Entdeckung von Makromolekülen vor fast einhundert Jahren vielfältige Anwendungen gefunden. Diese Arbeit trägt zur Weiterentwicklung zweier Hauptforschungsgebiete im Bereich Chemie und Materialwissenschaften von Polymeren bei: verstärkte Polymerkomposite und photoschaltbare Systeme aus Polymeren oder Makrozyklen.
Keramik- oder faserverstärkte Kompositmaterialien werden insbesondere in großen, stark belasteten Strukturen wie Windrädern oder Flugzeugen eingesetzt. In dieser Arbeit wird die Entwicklung eines Kompositsystems vorgestellt, das die Exposition des Materials gegen die Stimuli Temperaturänderung, UV-Strahlung und mechanische Belastung anzeigt. Das keramisch verstärkte Kompositmaterial zeigte reversible Thermo- und Photochromie. Gleichzeitig stellt dieses Komposit das erste Material dar, in dem Spiropyrane in einer Polymermatrix mechanisch induziert schalten, ohne kovalent an das Polymer gebunden zu sein. Das Konzept wurde in glasfaserverstärkten Kompositen fortgeführt, wobei ein sichtbarer Farbumschlag vor dem Materialversagen demonstriert werden konnte. Im Rahmen von Variationen des Matrixpolymers wurde eine bisher unbeschriebene Zersetzungsreaktion von Spiropyranen entdeckt.
Die außergewöhnlich unterschiedliche Färbung der zwei Schaltzustände in Spiropyranen – Spiropyran und Merocyanin – zeugt von großen geometrischen Änderungen begleitete einschneidende Veränderungen der beteiligten π-Systeme. Während der Farbumschlag und der Größenunterschied im Konzept der mechanisch schaltbaren Komposite angewandt wurden, lässt sich die Ausdehnungsänderung der π-Konjugation nutzen, um die Leitfähigkeit konjugierter Polymere zu schalten. Als Machbarkeitsnachweis wurde ein Spiropyran mit Hexylthiophensubstituenten – ein häufiges Strukturelement in Polymeren für organische Elektronik – funktionalisiert. Das synthetisierte Molekül zeigte schaltbare Konjugation, wie mittels schaltbarer Fluoreszenz nachgewiesen wurde. Dichtefunktionaltheoretische Berechnungen stützen diesen Befund.
Im Forschungszweig photomechanischer Polymere werden Materialien verwandt, die mit
makroskopischen Änderungen auf Stimuli reagieren. Wir konnten den Einbau von Azobenzolschaltern in lineare Polymere zeigen. Hierbei stellen Siloxan-Brücken genügend geometrische Flexibilität bereit, um effiziente Photoschaltung zu ermöglichen. Die Schalteigenschaften der Polymere wurden in Hinblick auf die Schaltdynamik und Größenänderung sowohl in Lösung als auch im Film analysiert. Zusätzlich wurde ein makrozyklisches Azobenzoldimer mit Siloxanbrücken synthetisiert, das effiziente Photoschaltbarkeit bei langsamer thermischer Rückisomerisierung zeigte.Polymers and materials made thereof have found manifold applications since the discovery of macromolecules almost a century ago. This thesis contributes to the development of two main research areas in the field of chemistry and materials science of polymers: reinforced polymer composites and photoswitchable systems of polymers or macrocycles.
Polymer composites with reinforcements such as ceramics or fibers are particularly applied in large, highly stressed structures such as wind turbines. Within this thesis, the development of a composite system that indicates exposure of the material to the stimuli temperature change, UV irradiation and mechanical stress is demonstrated. The ceramic reinforced composite showed reversible thermo- and photochromism. This composite represents the first example to show mechanically induced switching of spiropyran molecules in a polymer matrix without being covalently bound to the matrix. The concept was advanced in glass fiber reinforced composites, demonstrating visible color changes before failure of the material. In the context of variation of the polymer matrix, a formerly undescribed degradation reaction of spiropyrans was discovered.
The exceptional color difference of the two switching states of spiropyran – spiropyran and merocyanine – indicates drastic changes of the involved π systems, that are accompanied by large geometrical changes. While the changes in color and size were utilized in the concept of mechanoswitchable composites, the change in the expanse of the conjugation can be used to switch the conductivity of conjugated polymers. In a proof of concept, hexylthiophene substituents – a common structural motif in polymers for organic electronics – were attached to a spiropyran. The synthesized molecule showed switchable conjugation, as was demonstrated by switchable fluorescence and supported by density functional theory calculations.
The field of photomechanical polymers covers polymer based materials that respond to stimuli with macroscopical changes. We demonstrated the incorporation of azobenzene switches in linear polymers. Siloxane linkers provided sufficient geometrical flexibility to allow efficient photoswitching. The switching properties of the polymers were analyzed with respect to the switching dynamics and size changes in solution and film. Additionally, a macrocyclic azobenzene dimer with siloxane linkers was synthesized, that showed efficient photoswitching and slow thermal backisomerization
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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