1,720,967 research outputs found
Intramolecular Electronic Interactions in Photon Upconversion and Singlet Fission [Elektronisk resurs]
No full textThe sun provides our planet with an abundance of energy in the form of solar light – a continuous stream of photons. Increasing our direct utilization of solar energy will constitute a necessary part in our ongoing replacement of fossil fuels with energy sources free from greenhouse gas emissions. The sunlight contains a broad spectrum of photon energies, but only a fraction of the photons in this spectrum can be harvested by a solar energy device. In fact, the energy harvesting efficiency of solar energy technologies is mainly limited by the mismatch between the solar spectrum and the photon energies that the solar energy device can utilize efficiently. Photon energy conversion techniques provide a way to circumvent this mismatch by converting incoming photons of too high or too low energy to photons with energy that matches the absorption of the solar energy device, thus enabling utilization of a larger part of the solar spectrum. Photon energy conversion includes both upconversion and downconversion. In this thesis, the photophysical processes of photon upconversion (PUC) by triplet-triplet annihilation and exciton downconversion by singlet fission (SF) have been investigated. The work presented in this thesis focuses on gaining knowledge and in-depth mechanistic understanding of the electronic interactions and excitation energy transfer events between chromophores that govern PUC and SF. More specifically, this thesis presents results from an investigation of intra molecular electronic interactions between chromophores within a molecular construct designed for PUC or SF. The mechanisms of intra molecular energy transfer between chromophores used for PUC have been investigated with respect to rate and efficiency. Intra molecular SF in a molecular dimer has been investigated in a detailed study of how the relative orientation of the chromophores and molecular conformational flexibility influence the kinetics of SF. The results presented in this thesis show how the relative orientation of chromophores as well as the moiety connecting the chromophores, control the nature and magnitude of the intra molecular electronic coupling. This insight highlights the importance of controlling molecular orientations and conformation flexibility as a design parameter in the development of novel molecular systems for photon energy conversion. Further, it has been shown that the overall process of PUC or SF is faster in an intra molecular system compared to a corresponding intermolecular system. Finally, the work presented in this thesis has shown that careful design of molecular frameworks could enable efficient intra molecular PUC and SF materials, which have potential to increase the energy harvesting efficiency of solar energy technologies
Intramolecular Electronic Interactions in Photon Upconversion and Singlet Fission
Get PDFThe sun provides our planet with an abundance of energy in the form of solar light – a continuous stream of photons. Increasing our direct utilization of solar energy will constitute a necessary part in our ongoing replacement of fossil fuels with energy sources free from greenhouse gas emissions. The sunlight contains a broad spectrum of photon energies, but only a fraction of the photons in this spectrum can be harvested by a solar energy device. In fact, the energy harvesting efficiency of solar energy technologies is mainly limited by the mismatch between the solar spectrum and the photon energies that the solar energy device can utilize efficiently. Photon energy conversion techniques provide a way to circumvent this mismatch by converting incoming photons of too high or too low energy to photons with energy that matches the absorption of the solar energy device, thus enabling utilization of a larger part of the solar spectrum.Photon energy conversion includes both upconversion and downconversion. In this thesis, the photophysical processes of photon upconversion (PUC) by triplet-triplet annihilation and exciton downconversion by singlet fission (SF) have been investigated. The work presented in this thesis focuses on gaining knowledge and in-depth mechanistic understanding of the electronic interactions and excitation energy transfer events between chromophores that govern PUC and SF. More specifically, this thesis presents results from an investigation of intramolecular electronic interactions between chromophores within a molecular construct designed for PUC or SF. The mechanisms of intramolecular energy transfer between chromophores used for PUC have been investigated with respect to rate and efficiency. Intramolecular SF in a molecular dimer has been investigated in a detailed study of how the relative orientation of the chromophores and molecular conformational flexibility influence the kinetics of SF.The results presented in this thesis show how the relative orientation of chromophores as well as the moiety connecting the chromophores, control the nature and magnitude of the intramolecular electronic coupling. This insight highlights the importance of controlling molecular orientations and conformation flexibility as a design parameter in the development of novel molecular systems for photon energy conversion. Further, it has been shown that the overall process of PUC or SF is faster in an intramolecular system compared to a corresponding intermolecular system. Finally, the work presented in this thesis has shown that careful design of molecular frameworks could enable efficient intramolecular PUC and SF materials, which have potential to increase the energy harvesting efficiency of solar energy technologies
Approaching the Spin-Statistical Limit in Visible-to-Ultraviolet Photon Upconversion
Get PDFTriplet-triplet annihilation photon upconversion (TTA-UC) is a process in which triplet excitons combine to form emissive singlets and holds great promise in biological applications and for improving the spectral match in solar energy conversion. While high TTA-UC quantum yields have been reported for, for example, red-to-green TTA-UC systems, there are only a few examples of visible-to-ultraviolet (UV) transformations in which the quantum yield reaches 10%. In this study, we investigate the performance of six annihilators when paired with the sensitizer 2,3,5,6-tetra(9H-carbazol-9-yl)benzonitrile (4CzBN), a purely organic compound that exhibits thermally activated delayed fluorescence. We report a record-setting internal TTA-UC quantum yield (φUC,g) of 16.8% (out of a 50% maximum) for 1,4-bis((triisopropylsilyl)ethynyl)naphthalene, demonstrating the first example of a visible-to-UV TTA-UC system approaching the classical spin-statistical limit of 20%. Three other annihilators, of which 2,5-diphenylfuran has never been used for TTA-UC previously, also showed impressive performances with φUC,g above 12%. In addition, a new method to determine the rate constant of TTA is proposed, in which only time-resolved emission measurements are needed, circumventing the need for more challenging transient absorption measurements. The results reported herein represent an important step toward highly efficient visible-to-UV TTA-UC systems that hold great potential for driving high-energy photochemical reactions
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
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
- …
