1,721,114 research outputs found
Polycatenanes Formed of Self-Assembled Metal-Organic Cages
No full textPoly-[n]-catenanes (PCs) self-assembled of three-dimensional (3D) metal organic cages (MOCs) (hereafter referred to as PCs-MOCs) are a relatively new class of mechanically interlocked molecules (MIMs) that combine the properties of MOCs and polymers. The synthesis of PCs-MOCs is challenging because of the difficulties associated with interlocking MOCs, the occurrence of multiple weak supramolecular electrostatic interactions between cages, and the importance of solvent templating effects. The high density of mechanical bonds interlocking the MOCs endows the MOCs with mechanical and physical properties such as enhanced stability, responsive dynamic behavior and low solubility, which can unlock new functional properties. In this Minireview, we highlight the benefit of interlocking MOCs in the formation of PCs-MOCs structures as well as the synthetic approaches exploited in their preparation, from thermodynamic to kinetic methods, both in the solution and solid-states. Examples of PCs-MOCs self-assembled from various types of nanosized cages (i.e., tetrahedral, trigonal prismatic, octahedral and icosahedral) are described in this article, providing an overview of the research carried out in this area. The focus is on the structure-property relationship with examples of functional applications such as electron conductivity, X-ray attenuation, gas adsorption and molecular sensing. We believe that the structural and functional aspects of the reviewed PCs-MOCs will attract chemists in this research field with great potential as new functional materials in nanotechnological disciplines such as gas adsorption, sensing and photophysical properties such as X-ray attenuation or electron conductivity.This article reviews the most relevant examples of polycatenanes in which the catenating subcomponents forming the mechanical bonds are interlocked three-dimensional metal organic cages (MOCs). We emphasize the beneficial aspects of mechanical bond formation among MOCs, synthetic approaches, supramolecular topologies, and structure-function correlations. The experimental and theoretical aspects of polycatenanes made of MOCs have also been investigated. imag
Host-Guest Chemistry of M12L8 Poly-[n]-catenanes: Inclusion Process by Switchable ''losed-Open'' ynamic Channels
No full textDirect guest exchange reactions in one-dimensional (1D) poly-[n]-catenanes self-assembled by elusive M12L8 large nanocages (similar to 2600 angstrom(3) of internal volume) have never been documented. Herein, we report for the first time the guest behavior of a TPB poly-[n]-catenane (1) self-assembled from M12L8 icosahedral nanocages in the absence of permanent communicating voids. High-resolution synchrotron X-ray data obtained by means of crystal-to-crystal guest exchange reactions unambiguously demonstrate the uptake of o-dichlorobenzene and p-chloroanisole. Density functional theory calculations aimed at studying local interaction energy variation among 1D chains of catenated M12L8 cages provided insights that help us further understand the intrinsic dynamic behavior of the 1D porous rods. The guest uptake by the icosahedral M12L8 nanocages is rationalized via a switchable closed-open process considering both the weak interchain interactions and the stronger intrachain mechanical bond. Mechanistic aspects of the molecular exchange consider both the sliding motion of the 1D chains of interlocked M12L8 nanocages and the dynamic aspects of single M12L8 cages within the 1D chains (i.e., cage compression/extension) in a concerted cooperative dynamic behavior. Thermal treatment of the ZnBr2-TPB poly-[n]-catenane demonstrates that the mechanical bond remains up to similar to 500 K. The exceptional structural properties of 1 have been studied for potential applications such as selectivity of chlorobenzenes. The labile nature of the Zn-N coordination bond allows the recyclability of the TPB ligand in water, thus making these materials candidates in green chemistry applications
Modeling of Poly(3-hexylthiophene) and Its Oligomer's Structure and Thermal Behavior with Different Force Fields: Insights into the Phase Transitions of Semiconducting Polymers
Full text linkThe polymorphism of poly(3-hexylthiophene) (P3HT), one of the reference systems in fundamental studies of polymer semiconductors, is explored by molecular dynamics modeling of selected 3-hexylthiophene (3HT) oligomers, comparing structural and thermal behavior simulation results with rare monodisperse oligomer experimental data. The relative stability of the two crystalline polymorphs and the mechanism of interconversion between them, as the degree of polymerization grows in (3HT)n oligomers (n = 10, 16, 20) to the polymer, can be investigated in infinite periodic oligomer crystals without implicitly imposing infinite molecular weights, as inevitable for polymers. To evaluate the impact of different descriptions of molecular interactions, simulations were performed by using three different force fields specifically adapted to poly(3-alkylthiophenes) (P3ATs). Our results show that MD may adequately describe the key features and relative stability of the different crystal phases and suggest plausible interconversion mechanisms for very rapid solid-solid or melting transitions, albeit with complementary differences among different force fields, which become substantial modeling highly disordered crystal structures or mesophases
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
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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