105,172 research outputs found
Photoinduced Radical Cations Enable Anti‐Kasha Emission in a Pyrene‐Based Azacationic Ladder Polymer
No full textConjugated ladder polymers are a unique class of macromolecules, characterized by their rigid and thermally stable structures. This work presents the synthesis and characterization of a pyrene‐based azacationic ladder polymer (polymer A). Spectroscopic analysis points to the generation of radical cationic units during photoreduction, while cationic species are retained in the polymer backbone, thereby enabling in situ n‐type doping. Unconventional anti‐Kasha emission, with a maximum at 490 nm, appears to originate from these radical species in solution. In toluene, the lower dipole moment of the solvent leads to dual emission: anti‐Kasha emission from radical cations and an S1 → S0 transition from polycationic units at 780 nm. This interpretation is supported by density functional theory/time‐dependent density‐functional theory calculations, which indicates that the large energy gap between the D3 and D2 states of the radical cationic units may inhibit internal conversion, allowing anti‐Kasha behavior. Despite their potential reactivity, the cationic and radical cationic species remain stable in solution in the dark for over 110 h. To the authors’ best knowledge, polymer A is the only ladder‐type conjugated polymer reported to exhibit anti‐Kasha emission together with light‐induced n‐type doping behavior
Unusual Non-Kasha Photophysical Behavior of Aggregates of Push–Pull Donor–Acceptor Chromophores
No full textUnlike the Frenkel exciton model,
the essential state model (ESM)
for aggregates of donor–acceptor (DA) chromophores accounts
self-consistently for the effect of the reaction field on the ground
and excited states of the individual chromophores, an effect that
can lead to divergences from Kasha’s H- and J-aggregate classification
schemes for highly polarizable molecules. In this work, two slip-stack
dimer geometries of DA chromophores are considered, one with parallel
permanent dipole moments (Geometry I) and the other with antiparallel
dipole moments (Geometry II). Based on the ESM, it is shown that in
both GI and GII dimers the aggregation-induced spectral shifts characteristic
of H- and J-aggregates agree exactly with the Kasha/Frenkel model,
but only in the cyanine limit, where both the ground and excited states
of an isolated chromophore consist of equal admixtures of the neutral
and charge-separated diabatic states D0A0 and
D+A–. The agreement is also limited to
the perturbation regime where intermolecular interactions are relatively
weak. However, in the polyene limit, where the ground state of the
monomer is dominated by the D0A0 component,
the agreement with the Kasha/Frenkel model breaks down. In this limit,
GI dimers exhibit exaggerated spectral shifts, as much as twice as
large as the Kasha shifts, while GII aggregates exhibit shifts which
can be much smaller than the Kasha shifts. Hence, in the latter case,
one can obtain practically unshifted H- and J-aggregates. When vibronic
coupling is incorporated into the ESM via a Holstein-like Hamiltonian,
it is further revealed that the vibronic signatures derived from the
Frenkel exciton theory carry over to the ESM in the weak coupling
regime, providing an unambiguous means for determining H- and J-aggregation.
In the strong coupling limit, the divergences from the Kasha model
are even more pronounced with the emergence of strongly blue-shifted
J-aggregates with enhanced vibronic coupling. Moreover, when the intermolecular
coupling V becomes equal to −2η, where
η is the monomer’s diabatic excitation energy (with η
> 0), the dimer behaves as if no vibronic coupling were present
at
all. In other words, the strong vibronic progressions characteristic
of a monomer in the polyene limit vanish when V =
−2η. Applications are made to a merocyanine dimer, which
abides by the GII configuration
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe 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
No full textWe 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
The construction of Karen Karnak: The multi-author-function
No full textThis thesis is situated within the comparatively recent developments of Web 2.0 and the emergence of interactive WikiMedia, and explores the mode of authorship within a Read/Write culture compared to that of a Read/Only tradition. The hypothesis of this study is that the role of the audience has become merged with the author, and as such, represents new functions and attributes, distinct from a more conventional concept of authorship, in which the roles of audience and author are more separate. Read/Write and participatory culture, as defined by this study, is focused on collaboration, and includes the influences of D.I.Y. culture, Open-Source practices and the production of text by multiple authors. Multi-authorship presents a re-thinking of several concepts which support the notion of the individual author, since the focus of multi-authorship is not on attribution and ownership of a finished text, but on the continued malleability of a text. Modes of multi-authorship, demonstrated in the use of the pseudonyms Alan Smithee and Karen Eliot, represent declarative authors whose names signify multiple origins, whilst concurrently indicating a distinct body of work. The function of these names form an important context to this study, since primary research involves the construction of an experimental mode of multi-authorship utilising WikiMedia technology and the interaction of thirty nine participants, who are invited to create a body of work under the collective pseudonym Karen Karnak. The data generated by this experiment is analysed using aspects of Michel Foucault's author-function to identify and determine power structures inherent in the WikiMedia context. The interplay of power structures, including concepts such as identity, ownership and the body of work, affect the resulting mode of authorship and contribute to the construction of Karen Karnak, suggesting further areas of research into the emerging multi-author
Contribution of Information and Communication Technology (ICT) in Country’S H-Index
No full textThe aim of this study is to examine the effect of Information and Communication Technology (ICT) development on country’s scientific ranking as measured by H-index. Moreover, this study applies ICT development sub-indices including ICT Use, ICT Access and ICT skill to find the distinct effect of these sub-indices on country’s H-index. To this purpose, required data for the panel of 14 Middle East countries over the period 1995 to 2009 is collected. Findings of the current study show that ICT development increases the H-index of the sample countries. The results also indicate that ICT Use and ICT Skill sub-indices positively contribute to higher H-index but the effect of ICT access on country’s H-index is not clear
Fully Turbulent Mean Velocity Profile for Purely Viscous non-Newtonian Fluids
No full textThe characteristic near wall behavior of turbulent flow of purely-viscous non-Newtonian fluids is discussed for both power-law (P.-L.) and Herschel-Bulkley (H.-B.) rheological models. A proper scaling is presented for H.-B. fluids to establish an analogy with power-law fluids with same flow index. To provide reference data for turbulent flow of non-Newtonian fluids, DNS simulations of power-law fluids are conducted in a rectangular channel for a large range of power-law indices ( = 0.5, 0.69, 0.75, 0.9, 1, 1.2). The DNS data show that the mean velocity profile in the viscous and logarithmic layers follow expressions of the form and respectively, where shows a logarithmic dependency on the flow index.Comparison with some experimental data shows the above formulation to be valid for Reynolds numbers (based on shear velocity) as high as 1000
H-index and research evaluation: A suggested set of components for developing a comprehensive author-level index
No full textThe H-index has been investigated in various studies; this index has many strengths that have made it popular. However, it also has weaknesses, due to which other indicators have been developed. This study aims to identify the strengths and weaknesses of the H-index and provide the minimum set of necessary components for developing a comprehensive author-level index. In this systematic literature review, Scopus, PubMed, Web of Science, Emerald, and ProQuest databases were searched to identify relevant studies. From the number of 14,253 retrieved studies, after two stages of screening, 81 studies were selected according to the eligibility criteria for data extraction. The findings of the study led to the identification of 15 strengths in the three categories of Quality Features, Simplicity, and Suitability, and 13 weaknesses in the six categories of Publications, Citations, Academic Age, Author Credit Allocation, Variety of Fields, and mathematical calculation for H-index. Finally, 28 components were identified as the minimum set of necessary components to develop a comprehensive author-level index to help evaluate researchers more realistically and fairly. The minimum components that need to be considered in developing a comprehensive author-level index can be proposed as follows: Quality Features, Simplicity, Suitability, Publications, Citations, Academic Age, Author Credit Allocation, Variety of Fields, and mathematical calculation
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