1,720,964 research outputs found
Synthesis of trans-A2B2 and A2BC dipyridylporphyrins as new building-blocks for the preparation of H-bonded supramolecular structures.
No full textMetal mediated self-assembled porphyrin metallacycles: Synthesis and multipurpose applications
No full textOver the last two decades the metal-mediated directional-bonding has established as an efficient tool for the construction of complex and functional supramolecular architectures. The success of this approach has strongly benefited from design principles based on the known and fixed ligand and metal coordina- tion geometry, thus ensuring a high level of control on the topology and energetics of the supramolecular adducts. Among them, self-assembled pyridylporphyrin metallacycles have attracted considerable atten- tion due to their symmetric structure, to their peculiar optical and redox properties, to their large inner cavity, that can be easily addressed by axial ligation to metal ions inserted in the porphyrin ring. This review is focused on this particular subset of metal–organic architectures and in particular on discrete metallacyclic system with a 4 + 4 and 2 + 2 metal/ligand stoichiometry formed by coordination of 4-pyr- idylporphyrins with different metal ions (Pd(II), Pt(II), Ru(II), Re(I)). Their synthesis, characterization and applications in the field of catalysis, molecular recognition, sensing and ionophoric behavior will be reviewed with the aim to stimulate further interest on these versatile supramolecular systems
Synthesis and characterization of a hydrophilic conjugated 4+4 Re(I)-porphyrin metallacycle
Full text linkThe preparation and the full characterization, including the X-ray structure determination, of a polar trans-dipyridylporphyrin functionalized with two short polyoxyethylene chains is reported. Reaction of the porphyrin with a Re(I) complex yielded a 4+4 metallacycle showing an improved solubility and a lower tendency to aggregate with respect to analogous porphyrin cyclic derivatives. These properties allowed a full NMR characterization of the metallacycle including VT 1H DOSY-NMR experiments and, for the first time, the recording of a 13C-NMR spectrum giving further insight into the structural definition of these type of metallacycles. Spray deposition of the metallacycle on a heated mica substrate shows the formation of regular ring-like nano-structures which are not formed by the parent porphyrin
Synthesis and characterization of trans-di-(4-pyridyl)porphyrin dimers
Full text linkPreparation and characterization of a small library of symmetric trans-di(4-pyridyl) porphyrin dimers, obtained by either Glaser–Hay or Sonogashira coupling reactions from appropriately prepared trans-di-4-pyridylporphyrin precursors, is presented. The porphyrin dimers are differentiated by a phenyl-alkynyl bridge of increasing length at one meso-position, while for all the derivatives the two remaining opposite meso-positions are tailored with a phenyl moiety bearing a short polyether chain. Coordination of the four pyridyl groups with appropriate metal fragments may be exploited to construct tubular hollow structures, with varied internal sizes, depending on the choice of the porphyrin dimer
component
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
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
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
- …
