1,721,556 research outputs found
Photoluminescent energy transfer from PPEs to a series of dyes
No full textThesis (S.M.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Vita.Includes bibliographical references (p. 45-49).Photoluminescent energy transfer in conjugated polymer-dye blend films was studied by means of steady-state fluorescence spectroscopy in order to identify efficient energy acceptors with red-shifted emissions relative to donor emission. A dialkoxy substituted poly(phenylene- ethylene) (PPE) and its pentiptycene derivative were used as the energy donor, and xanthene, cyanine, phenoxazine, and squaraine dyes were utilized as energy acceptors. Energy transfer efficiency was investigated by exciting the polymer and monitoring the quenching of polymer emission, along with the enhanced dye emission. Squaraine dyes exhibited up to 110 times greater emission than the intrinsic dye emission, even though they have little spectral overlap with the polymer donor. Moreover, even after normalization for the degree of spectral overlap, the sensitized emission intensity of squaraine was unusually high relative to those of the other dyes. This result suggested that an alternative Dexter mechanism substantially contributes to efficient energy transfer beyond the "through-space" dipolar mechanism of the Forster theory. This process is additionally assisted by the internal free volume within specific polymer hosts that prevent phase separation and self-aggregation of dyes, as well as preserving the dye emissive within the interstitial cavity.(cont.) Lastly, other factors that influence energy transfer, such as optimum doping concentration and the specific affinity of dyes to polymers, were investigated. The results provide crucial insights into the identification or design of competent energy acceptors and donors.by Inja Song.S.M
Design and synthesis of molecular actuators and sensors
Full text linkThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references.To date, the most successful conducting polymer actuators are based on polypyrrole, which operates through incorporating and expelling counterions and solvent molecules to balance the charges generated by electrochemical stimuli (swelling mechanism). Although significant progress has been made, there still exists a need for developing new materials that would overcome the intrinsic limitations in the swelling mechanism, such as slow diffusion rate, limited expansion volume, etc. Our group has contributed this area with a different approach -- lecular mechanisms, which utilize a dimensional change of a single polymer chain. We propose two types of molecular mechanisms: contracting and expanding. We proposed earlier a calix[4]arenebased molecular actuator for the contracting mechanism, in which p-dimer formation was proposed as a driving force. In this dissertation, we first confirm by model studies that p-dimer formation can indeed be a driving force for the calix[4]arene-based system. We propose another molecular hinge, binaphthol moiety, for the contracting model. The syntheses of polymers with binaphthols and their characterization, including signatures of oligothiophene interactions, are described. Due to its chirality, we examined the possibilities of the binaphthol polymer as a chiral amine sensor. To create actuators that make use of the expanding model, we propose new conjugated seven-membered ring systems with heteroatoms (thiepin with sulfur and azepine with nitrogen) and their syntheses and characterization will be described. Inspired by the fact that sulfoxide has very low extrusion barrier in the related system, we applied the thiepin molecules to create a peroxide sensor.(cont.) In addition, during the investigation of phenol functional groups in conducting polymers, we found interesting properties that strategic positioning of phenol groups can render a conjugation-broken meta-linked system just as conductive as a fully conjugated para-linked isomeric system.by Changsik Song.Ph.D
Controlling the architectures and optical properties of conjugated polymer aggregates and films
Full text linkThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Vita.Includes bibliographical references.The semiconducting properties of conjugated polymers are finding use in various optoelectronic applications, including chemical sensors and light-emitting diodes. In this thesis, we investigate aggregation in conjugated polymers and how it affects the optical properties of these organic materials. We discuss how aggregation enhances exciton transport properties in fluorescent polymers, thereby increasing the probability of excitons reaching low-energy sites in the polymer. A consequence of this aggregation-enhanced exciton migration is that low-energy defect sites in a conjugated polymer can dramatically alter the polymer's fluorescence properties when it is in an aggregated state. In a poly(pphenylene ethynylene) (PPE) that was previously proposed to form green-emitting excimers, we found that a small concentration of anthryl defects in the polymer emitted green fluorescence that was only noticeable when the polymer was in an aggregated state (otherwise the polymer was fluorescent blue). After elucidating the origin of the green fluorescence, we purposely added more emissive anthryl units into the polymer to enhance the blue-to-green fluorescence color change that accompanied polymer aggregation.(cont.) Using this anthryl-doped conjugated polymer, we developed aggregationbased chemical sensors that exhibited a visually noticeable fluorescence color change upon addition of poor solvents or biologically relevant, nonquenching, multicationic analytes (e.g., polyamines, neomycin) to the polymer solution. We also studied the effects of aggregation on the optical properties of a chiral poly(p-phenylene vinylene) (PPV) derivative in solutions and in films. We found that the organizations and functional properties existing in aggregated polymer solutions can be transferred to the film state by controlling the processing conditions. Using the same polymer, we were able to obtain films with different architectures and luminescence properties simply by adjusting the spin-casting solvent and film annealing conditions. Controlling the organizations and functional properties of conjugated polymer films is important in the fabrication of conjugated polymer-based optoelectronic devices.by Andrew Satrijo.Ph.D
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
Molecular design of conjugated polymers for the control of conformation, electronics and self-assembly
Full text linkThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Vita.Includes bibliographical references.The design, synthesis and characterization of organic electronic materials, in particular luminescent conjugated polymers, with structural motifs that allow for the controlled modulation of their photophysical properties are presented. In Chapter 1, the synthesis of new heterocyclic oligo(phenylene) analogues based on soluble, nonaggregating 1,2-diazines is reported. Palladium-catalyzed reductive coupling methods were developed to allow for the preparation of large quantities of iptycene-derived bipyridazines and biphthalazines, and the controlled synthesis of well-defined oligomers up to sexipyridazine. Crystallographic, spectroscopic and computational evidence indicate that in these analogues, hindrance at the ortho position is relaxed relative to poly(phenylene)s, which can favor planarization and extension of conjugation. New poly(aryl ether)s that incorporate iptycene-derived pyridazines have been prepared by three different synthetic routes, including a novel reductive polymerization of poly(aryl alkyl ether)s (Chapter 2). Some of these polymers exhibit low dielectric constants due to the introduction of internal free volume. Iptycene-derived pyridazines were also incorporated in conjugated organic materials that show thermotropic liquid crystalline properties and in platinum-based monomers for conjugated phosphorescent polymers that exhibit high luminescence efficiency under ambient conditions (Chapter 3). The luminescent properties of these platinum complexes are attributed to the introduction of steric bulk that prevents the formation of quenched aggregates. The donor-acceptor strategy to bandgap engineering was applied to the preparation of a highly selective fluorescent probe for thiol bioimaging (Chapter 4), and to the synthesis of conjugated luminescent polymers that incorporate new electron deficient monomers based on 2,1,3-benzothiadiazole and 2,1,3-benzooxazole bearing solubilizing side-chains (Chapter 5).(cont) The exquisite control over conformation and surface density offered in Langmuir monolayers of amphiphilic poly(phenylene ethynylene)s at the air-water interface was exploited in spectroscopic studies of energy transfer to perylene monoimide end-groups (Chapter 6). In these, planarization increases the efficiency of intramolecular energy transfer at low surface densities, and intermolecular energy transfer pathways become predominant at high surface densities. The modulation of energy transfer at dipolar interfaces provides a transduction mechanism that makes these polymers attractive platforms for the development of new biosensors for surface interactions.by Jean Bouffard.Ph.D
Manipulation of ring strain and antiaromaticity in the design and synthesis of unique optoelectronic materials
Full text linkThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2012.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections."June 2012." Vita. Cataloged from student-submitted PDF version of thesis.Includes bibliographical references.Polycyclic aromatic hydrocarbons (PAHs) and fully-conjugated ladder polymers are leading candidates for organics electronics, as their inherent conformational rigidity encourages electron delocalization. Many of these systems consist of fused benzenoid or heterocyclic aromatic rings. Less frequently, however, PAHs are reported with character that alternates between the aromaticity of benzene fragments and the antiaromaticity of a nonbenzenoid moiety. Due to its high degree of unsaturation and ring strain, 3,4-bis(methylene)cyclobutene presents an intriguing building block for a variety of polycyclic macromolecules. The syntheses of several derivatives of 3,4-bis(benzylidene)cyclobutene are reported. Previously unknown 1,2-dibromo-3,4-bis(benzylidene)cyclobutene was obtained through in situ generation of 1,6-diphenyl-3,4-dibromo-1,2,4,5-tetraene followed by electrocyclic ring closure. Ensuing reduction and metal-catalyzed cross-coupling provided additional derivatives. The effects of ring strain on the geometry and electronics of these derivatives were examined. The synthesis of new class of fully unsaturated ladder structures, phenylene-containing oligoacenes (POAs), using 3,4-bis(methylene)cyclobutene as a building block for sequential Diels-Alder reactions is described. The electronic effects of strain and the energetic cost of antiaromaticity can be observed via the optical and electrochemical properties of the reported compounds. The resulting shape-persistant ladder structures contain neighboring chromophores that are partially electronically isolated from one other while still undergoing a reduction in the band gap of the material. Singlet fission, a phenomenon in which two triplet excitons are generated from a single photon of light, has the potential to improve the efficiency of organic solar cells by increasing the theoretical quantum efficiency. Singlet fission is observed for the first time in dithienylsubstituted pentacene and tetracene. Dissociation of the triplets at the donor-acceptor interface in a solar cell constructed with 6,13-di(thien-2-yl)pentacene is demonstrated. The synthesis of a POA containing pentacene is also investigated. The chain-growth mechanism of polymerization allows for greater control of molecular weight and polydispersity than does the step-growth mechanism, however is currently limited to only a few reactions. Due to its unique Diels-Alder reactivity, 3,4-bis(methylene)cyclobutene and related structures are investigated as monomers for chain-growth Diels-Alder polymerization.by Rebecca R. Parkhurst.Ph.D
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
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