1,720,989 research outputs found
High-Pressure Optical Properties and Chemical Stability of Picene
No full textPicene is a polycyclic aromatic hydrocarbon belonging to the class of phenacenes which have been recently found to behave as high-temperature superconductors upon alkali metal doping. The electronic properties of organic crystals can be finely and largely modified by the density changes obtained by the application of an external pressure. In this work, the role of pressure in tuning the optical properties of crystalline picene has been investigated from room conditions up to 15 GPa through the measurement of UV-visible absorption spectra, two-photon excitation profiles, and one- and two-photon excited fluorescence spectra in a diamond anvil cell. The pressure dependence of the optical band gap was determined, and the frequencies of several vibronic bands belonging to electronic transitions from the ground state (S0) to the four lowest-energy excited singlet states (S1 to S4) were determined as a function of pressure. We evidence a very different density dependence of the transition energy of S0 → S1, which undergoes a remarkable red shift of ∼400 cm-1/GPa, and of the transitions from S0 to the higher excited states, which remain constant in the whole investigated range. This is consistent with a S 1 state of 1La character in solid picene. The high-pressure chemical stability of solid picene was investigated through visible absorption and Fourier transform infrared spectroscopy (FTIR). A chemical transformation involving the bulk picene crystal occurs above ∼23 GPa, giving rise to a disordered material similar to the amorphous hydrogenated carbon obtained in the pressure-induced reactivity of benzene. The combination of electronic and vibrational data allows us to identify the presence of reaction intermediates at ∼10 GPa, preferentially forming at crystal defects. © 2013 American Chemical Society
Topochemical Polymerization of Phenylacetylene Macrocycles under Pressure
No full textSelf-assembly
of organic macrocycles has been exploited as a preliminary
step in the synthesis of soluble and tailorable carbon-based nanostructures.
Functionalized nanotubes have been prepared using, as core building
blocks, nearly planar ring structures containing several alkyne units,
exploiting the geometry achieved in the spontaneous preassembling
step driven by π interaction. Covalent cross-linking between
these units was achieved by thermal or photochemical activation with
UV light. Here, we apply a moderate pressure in a sapphire anvil cell
(1.0 GPa) to facilitate the preassembling and induce the cross-linking
under pressure either with visible light, absorbed by two-photon absorption,
or thermally. We observe a high yield of enhanced quality cross-linked
nanotubes in a sample, showing, at ambient pressure, only side-chain
decomposition. These results show that moderate pressures, easily
achievable in large volume cells, are able to selectively favor topochemical
reactions in such complex organic systems
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
HOMO-LUMO transitions in solvated and crystalline picene
No full textThe optical properties of picene at ambient conditions have been investigated through the measurement of UV/Vis absorption and fluorescence spectra and of excitation profiles, using one-and two-photon excitation, in solution and in the crystal phase. For solvated picene an assignment of the vibronic structure of the transitions to the four lowest-energy excited singlet states (S-1-S-4) has been obtained from the absorption data, and the vibronic structure of the fluorescence spectra has been assigned. The absorption and fluorescence spectra of the solid phase can be interpreted according to the single molecule analysis. Nevertheless, the strong increase of the optical density in the spectral region of the lowest HOMO-LUMO transitions and the frequency shift of absorption and fluorescence bands may be explained by a mixing of the states of adjacent molecules in the crystal. Moreover, peculiar emission features depending on the crystal dimensions (10(-1) to 10(2) mu m) are observed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770265
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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