1,720,976 research outputs found
Understanding the mechanism of refractive index modulation in materials undergoing photo-Fries rearrangement
No full textMolecules and polymers that undergo a photo-Fries rearrangement are studied by using DFT calculation in order to predict the refractive index modulation which accompanies this light induced process. In particular, monomeric units of polystyrene derivatives (such as poly(p-formyloxystyrene) and poly(4-acetoxystyrene))
are considered and the refractive index calculated starting from the molecular polarizability and following the Lorentz–Lorenz approach. The results demonstrate that the conversion of ester groups to ketone/alcohol provides just a small modulation of the refractive index that does not fit the experimental results reported in the literature. A change in material density is therefore considered as main source of the modulation of the refractive index. Theoretical and experimental evidences are reported to support the picture
Photosensitive Polymers Undergoing Photo-Fries reaction: Understanding the Origin of Refractive Index Modulation
No full textPhotosensitive polymers undergoing photo-Fries reaction for volume holography: understanding the mechanism of refractive index modulation
No full textPolymers that undergo to photo-Fries reaction show a remarkable modulation of the refractive index that makes them interesting in the field of volume holography. In order to understand the mechanism of the change in the refractive index we performed theoretical calculations (DFT) on model molecules. The results show that the modulation can be ascribed to a change in the polymer density upon photo-reaction whereas the contribution due to a change in molecular polarizability is small. The theoretical conclusions have been supported by experimental results on two polymers: Polyformyloxide styrene (PFS) and polyacetoxystyrene (PAS)
Tuning the Quinoid versus Biradicaloid Character of Thiophene- Based Heteroquaterphenoquinones by Means of Functional Groups
No full textA series of quinoidal bithiophenes (QBTs) with controlled variations in steric hindrance and electron activity of the substituents has been synthesized. Evidence of their quinoidal versus biradicaloid groundstate electronic character has been experimentally detected and coherently identified as fingerprints by spectroscopic methods such as NMR, UV−vis, multiwavelength Raman. From this analysis, alkoxy groups have been shown to strongly affect the electronic structure and the ground-state energy and stability of QBTs. Quantum-chemical calculations correctly predict the experimental spectroscopic response, even while changing the alkyl on phenone from a tertiary carbon atom to secondary to primary toward an unsubstituted phenone, further confirming the validity of the approach proposed. A control of the electronic structure accompanied by negligible variations of the optical gap of the molecules has thus been demonstrated, extending the potential use of quinoidal species in fields ranging from photon harvesting to magnetic applications
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
New photosensitive systems for volume phase holography
No full textVolume phase holographic elements are becoming attractive thanks to the large efficiency and good optical quality. They are based on photosensitive materials where a modulation of the refractive index is induced. In this paper, we highlight the strategies to obtain a change in the refractive index in a dielectric material, namely a change in the material density and/or in the molecular polarizability. Moreover, we show the results achieved for materials that undergo the photo-Fries reaction as function of the molecular structure and the illumination conditions. We also report the results on a system based on the diazo Meldrum's acid where volatile molecules are produced upon light exposure
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
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