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Beta-cyclodextrin ammonium glycoclusters: convenient synthesis of potential drug/gene-delivery vehicles
No full textMolecular meccano. 29 - The synthesis and spectroscopic properties of a [2]catenane incorporating an anthracene chromophoric unit
No full textA modified bis-p-phenylene-34-crown-10 ring in which one of the 1,4-dioxybenzene units has been replaced by a 9,10-dioxyanthracene unit has been employed as a template for the formation of cyclobis(paraquat-p-phenylene). The [2]catenane which results has been shown by H-1 NMR spectroscopy to exist in solution exclusively in the translationally isomeric form in which (a) only the 1,4-dioxybenzene ring occupies the central cavity of the tetracationic cyclophane and (b) the crown ether ring is prevented from circumrotating through the cyclophane by the large 9,10-dioxyanthracene unit. The absorption spectrum and luminescence properties of this new [2]catenane and of its crown ether component in its free state have been investigated and compared with those of 1,4-dimethoxybenzene and a model anthracene derivative, carrying methylated triethylene glycol chains on the 9 and 10 positions of the anthracene ring. While the absorption spectrum of the crown ether is the sum of the spectra of the two component chromophoric moieties, its emission spectrum shows only the fluorescence band of the 9,10-dioxyanthracene-type unit. The excitation spectrum shows that the disappearance of the 1,4-dioxybenzene type emission in the crown ether is due to a very efficient (k(q) greater than or equal to 4 x 10(10) s(-1)) energy-transfer process from the 1,4-dioxybenzene to the 9,10-dioxyanthracene type unit. The absorption spectrum of the [2]catenane is noticeably different from the sum of the spectra of its two cyclic components, particularly as far as the presence of a very broad charge-transfer (CT) band in the visible spectral region (lambda(max) = 545 nm, epsilon(max) = 615 M(-1)cm(-1)) cm is concerned. Comparison with the CT band of a model compound shows that the very broad CT band of the [2]catenane is in fact the result of two component bands originating from the interaction of the two different electron-donor units (1,4-dioxybenzene and 9,10-dioxyanthracene type) present in the crown ether with the electron-acceptor bipyridinium-type units of the cyclobis(paraquat-p-phenylene). The emission spectrum of the [2]catenane does not show any band because of the quenching action (rate constant k(q) greater than or equal to 5 x 10(10) s(-1)) of the low-energy non-luminescent charge-transfer levels on the higher energy, potentially luminescent levels of the crown ether
A redox-active reverse donor-acceptor bistable [2]rotaxane
No full textThe synthesis and the dynamic behavior of a bistable [2]rotaxane, based on a reverse donor-acceptor motif containing naphthalene diimide (NpI) and 4,4'-bipyridinium (BIPY2+) as two electron-deficient stations and bis-1,5-dioxynaphthalene[38] crown-10 (BDNP38C10) as the electron-rich ring, is described. A functionalized tetraarylmethane moiety has been incorporated between the two stations in order to control the free energy barrier for the shuttling of the BDNP38C10 on the dumbbell component. The bistable [2]rotaxane was synthesized using the so-called "threading-followed-by-stoppering'' approach and characterized by NMR spectroscopy and mass spectrometry. Initially, the BDNP38C10 ring resides on the NpI station on account of the synthetic approach employed in the synthesis of the bistable [2]rotaxane. H-1 NMR spectroscopy was used to follow the equilibration process between the two translational isomers of the bistable [2]rotaxane-namely, NpI subset of BDNP38C10 and BIPY2+ subset of BDNP38C10. After 72 h, equilibrium was reached with a 3 : 2 ratio of the two translational isomers in favor of the NpI subset of BDNP38C10 co-conformation in CD3CN. The rate of relaxation of the crown ether from NpI subset of BDNP38C10 back to BIPY2+ subset of BDNP38C10 was associated with a rate constant of 2.2 +/- 0.3 +/- 10(-5) s(-1) (t(1/2) = 3.4 h), corresponding to a free energy of activation of 23.8 +/- 0.1 kcal mol(-1). Cyclic voltammetry (CV) reveals that the BDNP38C10 ring can be enticed to pass over the speed bump onto the neutral BIPY0 unit upon the generation of the NpI2- dianion, even although the neutral BIPY0 has presumably little or no affinity for the BDNP38C10 ring.
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Artificial Molecular Machines
No full textThe miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties—namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology
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
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