1,720,956 research outputs found
Guest-Shape-Directed Structural Switching between Two Isomers of a Pd6 Host and Its Structural Adaptability for Selective Photodimerization
No full textThe structure and functions of metal–organic cages are heavily dependent on the nature of the building blocks. Herein, a dimethyl-substituted propane diamine blocked cis-Pd(II) acceptor (A) was designed, which upon self-assembly with the tri-imidazole ligand (L), generated an unusual A6L4 octahedral cage (M1O) instead of the expected isomeric double-square architecture that was obtained from the tetramethyl-substituted ethylene diamine blocked cis-Pd(II) acceptor in water. Interestingly, in the presence of planar guests (Gn), M1O showed a transformation to a transient double-square architecture (M1DS), forming host–guest complexes with two such guests, (Gn)2@M1DS. The transient double-square cage (M1DS) readily converts back to the parent octahedral structure, M1O upon the removal of the guests. On the contrary, tetrahedral guests (G5/G6) stabilized the octahedral isomer of the host (M1O) by acting as suitable templates. Additionally, tetrahedral guests could induce the reverse transformation of M1DS to M1O by driving out planar guests from metastable M1DS. The specific antiparallel orientation and proximity of two anthracene derivatives within M1DS enabled them to be selectively transformed to the trans isomers of their respective dimers under photoirradiation. Upon dimer formation, the nonplanar product was expelled readily from the cavity of M1DS, and the host switched back to its original octahedral form (M1O), which functionally and structurally imitates enzymatic activity. Thus, a multifunctional supramolecular host was obtained that showed unique guest-shape-driven reversible structural switching and acted as an adaptive host for selective photodimerization
Formation of a Pd16 Molecular Basket Architecture of Reduced Symmetry and Angular Deviation in a Fluorenone Scaffold to Govern the Host–Guest Chemistry of Pd6 Trifacial Tubes
No full textThe employment of flexible ligands with significant conformational freedom in coordination-driven self-assembly enables the formation of unique and intricate structures. In this study, the self-assembly of such a fluorenone-appended ligand (L1) with a sterically demanding acceptor, [Pd(tmed)(ONO2)2] (M1), generated a new and unique molecular basket architecture, (M1)16(L1)8 (B), featuring a large hollow cavity. B possesses an unusual twisted architecture of low symmetry, consisting of 16 Pd(II) centers arranged as four tetrahedra connected by eight flexible ligands, representing a structurally complex system reminiscent of biological architectures. Designing such entropically disfavored, large architectures of reduced symmetry is challenging but desirable, since they can act as ideal models to study complicated natural systems. The host–guest property of supramolecular hosts is governed by the confined cavities and noncovalent interactions, which are dictated by the angular disposition of ligand coordination sites. To explore this, the fluorenone scaffold was used to synthesize two other tetradentate ligands (L2 and L3) that differed in the spatial distributions of their coordination vectors. The self-assembly of these ligands with [Pd(en)(ONO2)2] (M2) resulted in the formation of water-soluble (M2)6(L1/L2/L3)3 trifacial tubes of different geometries with varying internal cavity dimensions. These angular variations further altered the orientation of the fluorenone carbonyl groups within the cavities, thereby modulating their guest binding abilities and highlighting the importance of tailoring supramolecular hosts for specific guest binding
Stimuli‐Mediated Structural Interchange Between Pd6 and Pd12 Architectures: Selective Recognition of E‐Stilbene by the Pd6 Architecture and its Photoprotection
Full text linkThe dynamic behaviour of metal-ligand bonding cultivates stimuli-mediated structural transformations in self-assembled molecular architectures. The propensity of synthetically designed self-assembled systems to interchange between higher-order architectures is increased multi-fold when the building blocks have higher conformational degrees of freedom. Herein, we report a new ligand, (2,7-bis(di(pyridin-4-yl)amino)-9H-fluoren-9-one) (L), which, upon self-assembly with a cis-[(ethylene-1,2-diamine)Pd(NO3)2] acceptor (M), resulted in the formation of a M6L3 trifacial barrel (C1) in water. Interestingly, during crystallization, a rare M12L6 triangular orthobicupola architecture (C2) was generated along with C1. C2 could also be generated in solution via the application of several stimuli. C1 in aqueous media could stabilize one trans-stilbene (tS) or cis-stilbene (cS) molecule in its cavity, with a selectivity for the former from their mixture. Moreover, C1 acted as an effective host to prevent the otherwise facile photoisomerization of tS to cS inside its hydrophobic cavity under UV irradiation. Conversely, the visible-light-induced reverse isomerization of encapsulated cS to encapsulated tS could be achieved readily due to the better stabilization of tS within the cavity of C1 and its transparency to visible light. A multi-functional system was therefore designed, which at the same time is stimuli-responsive, shows isomer selectivity, and photo-protects trans-stilbene
Postassembly Modification of a Pd6 Host and C70 Encapsulation to Enhance Its ROS-Mediated Terpene Oxidation Ability under Visible Light
No full textThe properties of supramolecules can be modulated by post-assembly modification (PAM) of their building blocks or via guest encapsulation. This work demonstrates a largely uncharted approach to property modulation that integrates both PAM and guest encapsulation in a single system to boost photocatalytic activity. Self-assembly of a “phenothiazine”-functionalized ligand (L) with a cis-blocked Pd(II) acceptor (A) generated an A6L3 trifacial tube (T). Postassembly, T could be modified via irradiation with violet light, leading to the sulfoxidation of the “phenothiazine” moieties in T and thereby generating an oxidized tube (TO). Both TO and T could stabilize a C70 molecule within their cavities, forming C70@TO and C70@T, respectively. Although T showed relatively poor photocatalytic performance mediated by reactive oxygen species (ROS) with respect to oxidation of terpenes (S1–S4) under visible light, the modified TO was much better in that regard. Expectedly, C70@T showed better photocatalytic performance than T due to the presence of photosensitizing C70. While PAM or guest encapsulation alone led to reasonable improvements in photocatalytic ability, their combination within C70@TO led to a significant improvement. Catalytic amounts of C70@TO could instantly oxidize terpenes. Thus, we report here a new host that integrates the effects of both PAM and photosensitizer encapsulation for synergistically boosting its photocatalytic activity
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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