1,721,071 research outputs found
Synthesis and Chiroptical Activity of π‐Expanded Electron‐rich Heterohelicenes Based on the 1,4‐Dihydropyrrolo[3,2‐b]pyrrole core
Full text linkHerein, we report the synthesis and chiroptical characteristics of the first (double) helicenes possessing the 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) moiety as their central core. We have developed a three-step synthesis with 6π-electrocyclization accompanied by HBr elimination as its key step. We found that, whereas for smaller peripheral arms double 6π-electrocyclization occurs smoothly forming a double helicene, in the case of longer polycyclic aromatic hydrocarbons the reaction becomes less efficient and mono-helicenes are the only products. The electron density distribution analysis of LUMO explains the different regioselectivity of 6π-electrocyclization. The synthesized heterohelicenes are characterized by greenish-blue emission, distinct solvatofluorochromism and good fluorescence quantum yields (up to 42 %). Moreover, the chiroptical measurements reveal that unsymmetrical double heterohelicene exhibits excellent circularly polarized luminescence brightness (BCPL) reaching 30 M−1 cm−1. The combined experimental and computational study has revealed that a charge-transfer state is responsible for the observed emission (hence the solvatochromism), while low-energy absorption derives from multiple transitions
Excited-State Symmetry Breaking in an Aza-nanographene Dye
Full text linkThe photophysics of a structurally unique aza-analogue of polycyclic aromatic hydrocarbons characterized by 12 conjugated rings and a curved architecture is studied in detail. The combined experimental and computational investigation reveals that the lowest excited state has charge-transfer character in spite of the absence of any peripheral electron-withdrawing groups. The exceptionally electron-rich core comprised of two fused pyrrole rings is responsible for it. The observed strong solvatofluorochromism is related to symmetry breaking occurring in the emitting excited state, leading to a significant dipole moment (13.5 D) in the relaxed excited state. The anomalously small fluorescence anisotropy of this molecule, qualitatively different from what is observed in standard quadrupolar dyes, is explained as due to the presence of excited states being close in energy but having different polarization directions
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
All-Polymer Microcavities for the Fluorescence Radiative Rate Modification of a Diketopyrrolopyrrole Derivative
Full text link[Image: see text] Controlling the radiative rate of emitters with macromolecular photonic structures promises flexible devices with enhanced performances that are easy to scale up. For instance, radiative rate enhancement empowers low-threshold lasers, while rate suppression affects recombination in photovoltaic and photochemical processes. However, claims of the Purcell effect with polymer structures are controversial, as the low dielectric contrast typical of suitable polymers is commonly not enough to provide the necessary confinement. Here we show all-polymer planar microcavities with photonic band gaps tuned to the photoluminescence of a diketopyrrolopyrrole derivative, which allows a change in the fluorescence lifetime. Radiative and nonradiative rates were disentangled systematically by measuring the external quantum efficiencies and comparing the planar microcavities with a series of references designed to exclude any extrinsic effects. For the first time, this analysis shows unambiguously the dye radiative emission rate variations obtained with macromolecular dielectric mirrors. When different waveguides, chemical environments, and effective refractive index effects in the structure were accounted for, the change in the radiative lifetime was assigned to the Purcell effect. This was possible through the exploitation of photonic structures made of polyvinylcarbazole as a high-index material and the perfluorinated Aquivion as a low-index one, which produced the largest dielectric contrast ever obtained in planar polymer cavities. This characteristic induces the high confinement of the radiation electric field within the cavity layer, causing a record intensity enhancement and steering the radiative rate. Current limits and requirements to achieve the full control of radiative rates with polymer planar microcavities are also addressed
Novel Method for the Synthesis of Merocyanines: New Photophysical Possibilities for a Known Class of Fluorophores
Full text linkA new, transformative methodology for the preparation of rhodols and other merocyanines from readily available tetrafluorohydroxybenzaldehyde and aminophenols has been developed. It is now possible to prepare merocyanines bearing three fluorine atoms and additional conjugated rings and the whole one-pot process occurs under neutral, mild conditions. Three heretofore unknown merocyanine-based architectures were prepared using this strategy from aminonaphthols and 4-hydroxycoumarins. The ability to change the structure of original rhodol chromophore into π-expanded merocyanines translates to a comprehensive method for the modulation of photophysical properties such as shifting the absorption and emission bands across almost the entire visible spectrum, reaching a huge Stokes shift i.e. 4800 cm-1, brightness ca. 80,000 M-1 cm-1, two-photon absorption cross-section above 150 GM and switching-on/off solvatofluorochromism. A detailed investigation allowed to rationalize the different spectroscopic behavior of rhodols and novel merocyanines, addressing solvatochromism and two-photon absorption
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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