1,720,966 research outputs found
Tunable photophysical properties of phenyleneethynylene based bipyridine ligands
No full textA bipyridine-based system with phenyleneethynylene at the 4,4' positions (1) and its p-methyl (2) and p-methoxy (3) substituted derivatives were synthesized via Sonogashira coupling reactions. The photophysical properties of 1-3 and their related H+ and Zn2+ adducts (1:H+-3:H+ and 1:Zn2+-3:Zn2+) were investigated, as a function of solvent polarity, by using steady-state and time-resolved spectroscopic techniques. Molecular systems 1-3 exhibit trans conformation, whereas adducts with H+ and Zn2+ are conformationally locked cis species. The unsubstituted compound 1 emits at 360 nm with low fluorescence quantum yield (phi(fl) = 0.2%) regardless of the solvent polarity. Fluorescence spectra of 2 and 3 are bathochromically shifted in polar solvents, and the p-methoxy (3) derivative possesses phi(fl) as high as 12%. Complexation of 1-3 with H+ or Zn2+ in acetonitrile causes red-shift of the lowest energy absorption bands, whereas dramatic changes of the emission properties are found as a function of the electron donating ability of the substituents on the phenyleneethynylene moiety (-CH3 or -OCH3), suggesting a charge-transfer character of the lowest electronic transition of 1-3. 1:H+, 1:Zn2+, 2:H+ and 2:Zn2+ exhibit intense fluorescence with phi(fl) up to 33% (1:Zn2+) whilst 3:H+ and 3:Zn2+ are found to be weakly emissive. The singlet radiative and non-radiative rate constants of compounds and complexes were determined, along with triplet parameters, via phosphorescence and transient absorption spectroscopy. More conclusive evidence regarding the protonation of bipyridine nitrogen atoms of compounds 1-3 were obtained through 1H NMR titration studies. These studies indicate that the conjugate molecular systems based on 2,2'-bipyridine and phenyleneethenylenes possess tunable optical properties which can be further utilized for preparing organic and inorganic luminophores with potential application in optoelectronic systems
Tunable photophysical properties of phenyleneethynylene based bipyridine ligands
No full textA bipyridine-based system with phenyleneethynylene at the 4,4’ positions (1) and its p-methyl (2) and p-methoxy (3) substituted derivatives were synthesized via Sonogashira coupling reactions. The photophysical properties of 1-3 and their related H+ and Zn2+ adducts (1:H+-3:H+ and 1:Zn2+-3:Zn2+) were investigated, as a function of solvent polarity, by using steady-state and time-resolved spectroscopic techniques. Molecular systems 1-3 exhibit trans conformation, whereas adducts with H+ and Zn2+ are conformationally locked cis species. The unsubstituted compound 1 emits at 360 nm with low fluorescence quantum yield (phi(fl) = 0.2%) regardless of the solvent polarity. Fluorescence spectra of 2 and 3 are bathochromically shifted in polar solvents, and the p-methoxy (3) derivative possesses phi(fl) as high as 12%. Complexation of 1-3 with H+ or Zn2+ in acetonitrile causes red-shift of the lowest energy absorption bands, whereas dramatic changes of the emission properties are found as a function of the electron donating ability of the substituents on the phenyleneethynylene moiety (-CH3 or -OCH3), suggesting a charge-transfer character of the lowest electronic transition of 1-3. 1:H+, 1:Zn2+, 2:H+ and 2:Zn2+ exhibit intense fluorescence with phi(fl) up to 33% (1:Zn2+) whilst 3:H+ and 3:Zn2+ are found to be weakly emissive. The singlet radiative and non-radiative rate constants of compounds and complexes were determined, along with triplet parameters, via phosphorescence and transient absorption spectroscopy. More conclusive evidence regarding the protonation of bipyridine nitrogen atoms of compounds 1-3 were obtained through 1H NMR titration studies. These studies indicate that the conjugate molecular systems based on 2,2’-bipyridine and phenyleneethenylenes possess tunable optical properties which can be further utilized for preparing organic and inorganic luminophores with potential application in optoelectronic systems
Axial chirality-induced rigidification in aminoboranes enhances persistent room-temperature phosphorescence and circularly polarized luminescence
No full textLong-lived triplet exciton harvesting materials are of immense interest for applications in bioimaging, optoelectronics, anticounterfeiting, and sensing. However, achieving persistent room-temperature phosphorescence (pRTP) in metal-free systems remains a significant challenge. Herein, we present purely organic axially chiral aminoboranes (R/S-(BN)2) with enhanced pRTP properties and circularly polarized luminescence (CPL). By introducing axial chirality, the dual-core (R/S-(BN)2) system achieves steric-hindrance-caused rigidity, which restricts molecular motions, leading to superior phosphorescence properties. Notably, R-(BN)2 demonstrates a phosphorescence quantum yield (ΦP) of 9.2% (S-(BN)2 : ΦP = 8.7%) and an extended lifetime of 0.9 sec at room temperature, significantly outperforming its mono-core counterpart (BN)1 (ΦP = 3.0% and τP = 0.6 s). Theoretical analysis corroborates the observed improvements, revealing the synergistic role of borylation and axial chirality in stabilizing triplet states. Furthermore, the axially chiral aminoboranes exhibited CPL in dichloromethane solutions with a dissymmetry factor of ~10-3. These findings highlight the potential of axially chiral frameworks in designing efficient metal-free pRTP materials, as demonstrated in the security writing application, further paving the way for their use in bioimaging, anti-counterfeiting technologies, and next-generation organic electronics
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
Plasmon-enhanced optical chirality through hotspot formation in surfactant-directed self-assembly of gold nanorods
No full textPlasmonically enhanced optical dichroism has attracted substantial interest for its application in optical sensing, where the interplay between chirality emanating from both molecules and plasmon-supporting structures has been regarded as a critical ingredient. Here, we experimentally demonstrate that suitably self-assembled achiral plasmonic nanostructures produce a high degree of enhancement in the optical dichroism observed from chiral molecules placed in their vicinity. Specifically, we identify a near-field enhancement associated with plasmonic hotpots as the mechanism enabling our observation of visible-NIR circular dichroism emanating from small amounts of chiral molecules. Our structures consist of linear arrays of gold nanorods obtained by introducing chiral anionic surfactants, such as modified bile salts, which lead to selective destabilization of a cetyltrimethylammonium bromide coating layer on Au nanorods, thereby promoting a tip-to-tip oriented assembly. The proposed mechanism of plasmonically-enhanced circular dichroism is supported by deriving a simple, yet general theoretical formalism that confirms the observed results, revealing the role of optical hotspots at the gaps of linear tip-to-tip nanorod assemblies as the origin of enhancement in the dichroism from chiral molecules. Importantly, it is the refractive rather than the absorption-mediated chiral response of the molecules that produces dichroism in the visible-NIR plasmonic regime, far from their UV absorption resonances. The observed self-assembly mechanism suggests that chiral analytes not directly interacting with the nanorod surfaces, but just able to induce tip-to-tip aggregation, can be revealed by a CD signature in the plasmonic region, thereby supporting potential applications in ultrasensitive 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
- …
