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Photon-pair generation via bound states in the continuum in nonlinear metasurfaces
No full textGeneration of photon pairs in nonlinear materials enables the creation of non-classical entangled photon states.
With ultra-thin metasurfaces, composed of optical nano-resonators, one can enable the ultimate in miniaturising
nonlinear photon sources along with unprecedented configurability. We present a novel design of a nonlinear
metasurface incorporating AlGaAs nanodisks with oligomer-holes, which features symmetry protected bound
states in the continuum. It enables enhanced photon-pair generation at non-degenerate photon frequencies via
spontaneous parametric down-conversion. This opens the potential for quantum-entanglement between photons
at ultra-short time-scales across the visible and infrared regions, leading to new opportunities for quantum
spectroscopy, sensing, and imaging
Mechanistic Studies on the Decomposition of Photoactive Nitrosyl Hydride (HNO) Precursors
No full textNitroxyl hydride (HNO) is a biologically relevant, highly reactive molecule and is a redox cousin of NO. HNO shows extremely promising pharmacological properties, including treating congestive heart failure. The overarching motivation for this research is to facilitate kinetic and mechanistic studies of HNO with biologically relevant molecules. As HNO rapidly dimerizes in aqueous solution, HNO studies require compounds that release HNO upon demand. However, the release of HNO from thermally decomposing molecules is slow (typically minutes to hours), making kinetic and mechanistic studies challenging.
There is increasing interest in the development of photoactive HNO donor molecules because the use of light to activate HNO release enables a high degree of temporal and spatial control. In an effort to meet this need, a Piloty’s acid derivative comprising an HNO-generating trifluoromethanesulfonamidoxy moiety tethered to the (6-hydroxynaphthalene-2-yl)methyl (6,2-HNM) photocage (4) is evaluated in order to establish if it can rapidly and cleanly generate HNO upon light activation. The photodecomposition of 4 was probed under a range of solvent conditions using a combination of characterization techniques, including ¹H NMR spectroscopy, ¹⁹F NMR spectroscopy, high resolution mass spectrometry (HRMS), steady state fluorescence spectroscopy, and time-resolved transient absorption spectroscopy.
Photodecomposition of donor 4 occurred via two pathways: via concerted C-O and N-S bond cleavages to release ¹HNO, the 6,2-HNM carbocation, and trifluorosulfinate, CF₃SO₂⁻; or via heterolytic N-O bond cleavage to generate the sulfonamide CF₃SO₂NH₂, and 6-hydroxynaphthalene-2-carbaldehyde. The selectivity between the two pathways is highly responsive to solvent. Donor 4 is shown to decompose to selectively release HNO upon excitation in a solvent mixture of 80:20 v/v MeCN to 5 mM phosphate buffer (pH 7.0). HNO characterization was achieved using two trapping molecules, aquacobalamin and glutathione, and from observation of its dimerization product, N₂O. Evidence for concerted heterolytic C-O and N-S bond cleavages versus elimination of CF₃SO₂NHOH was obtained by ¹⁹F NMR spectroscopy under pH conditions where CF₃SO₂NHOH is stable.
¹H NMR and UV-vis spectroscopic titration experiments showed that donor 4 exists in three protonation states (pKa(NH) = 4.39 ± 0.06 and ₚKₐ(NH) = 9.73 ± 0.01, 25.0 ºC, aqueous solution). Importantly, HNO photorelease occurs from donor 4 under conditions where the naphtholic OH is protonated and the nitrogen atom is deprotonated. Despite exhibiting both photoacidity and photobasicity, release of HNO from donor 4 does not require water and hence does not involve excited state proton transfer(s). Indeed, deprotonation of the photoacidic site of donor 4 decreases pathway selectivity for the generation of HNO following photolysis. The time-resolved spectroscopic studies indicate that HNO release may occur on the picosecond timescale following excitation of donor 4 and occurs via the short-lived ¹NO⁻ species (¹HNO/¹NO⁻ ₚKₐ ~23).
Studies were also completed towards determining the rate constant for the reaction between HNO and hydroxycobalamin. A thermally decomposing HNO donor (Piloty’s acid) was used; this HNO donor releases HNO in alkaline conditions. Under alkaline conditions, a pH-dependent mixture of ¹HNO and ³NO⁻ was formed from the decomposition of Piloty’s acid. ³NO⁻ itself is a good reductant, and a reaction of ³NO⁻ and hydroxycobalamin was observed. Furthermore, ³NO⁻ reacts with the product of the reaction between hydroxycobalamin and ¹HNO/³NO⁻ namely nitroxylcobalamin
Mechanistic Studies on the Decomposition of Photoactive Nitrosyl Hydride (HNO) Precursors
No full textNitroxyl hydride (HNO) is a biologically relevant, highly reactive molecule and is a redox cousin of NO. HNO shows extremely promising pharmacological properties, including treating congestive heart failure. The overarching motivation for this research is to facilitate kinetic and mechanistic studies of HNO with biologically relevant molecules. As HNO rapidly dimerizes in aqueous solution, HNO studies require compounds that release HNO upon demand. However, the release of HNO from thermally decomposing molecules is slow (typically minutes to hours), making kinetic and mechanistic studies challenging.
There is increasing interest in the development of photoactive HNO donor molecules because the use of light to activate HNO release enables a high degree of temporal and spatial control. In an effort to meet this need, a Piloty’s acid derivative comprising an HNO-generating trifluoromethanesulfonamidoxy moiety tethered to the (6-hydroxynaphthalene-2-yl)methyl (6,2-HNM) photocage (4) is evaluated in order to establish if it can rapidly and cleanly generate HNO upon light activation. The photodecomposition of 4 was probed under a range of solvent conditions using a combination of characterization techniques, including ¹H NMR spectroscopy, ¹⁹F NMR spectroscopy, high resolution mass spectrometry (HRMS), steady state fluorescence spectroscopy, and time-resolved transient absorption spectroscopy.
Photodecomposition of donor 4 occurred via two pathways: via concerted C-O and N-S bond cleavages to release ¹HNO, the 6,2-HNM carbocation, and trifluorosulfinate, CF₃SO₂⁻; or via heterolytic N-O bond cleavage to generate the sulfonamide CF₃SO₂NH₂, and 6-hydroxynaphthalene-2-carbaldehyde. The selectivity between the two pathways is highly responsive to solvent. Donor 4 is shown to decompose to selectively release HNO upon excitation in a solvent mixture of 80:20 v/v MeCN to 5 mM phosphate buffer (pH 7.0). HNO characterization was achieved using two trapping molecules, aquacobalamin and glutathione, and from observation of its dimerization product, N₂O. Evidence for concerted heterolytic C-O and N-S bond cleavages versus elimination of CF₃SO₂NHOH was obtained by ¹⁹F NMR spectroscopy under pH conditions where CF₃SO₂NHOH is stable.
¹H NMR and UV-vis spectroscopic titration experiments showed that donor 4 exists in three protonation states (pKa(NH) = 4.39 ± 0.06 and ₚKₐ(NH) = 9.73 ± 0.01, 25.0 ºC, aqueous solution). Importantly, HNO photorelease occurs from donor 4 under conditions where the naphtholic OH is protonated and the nitrogen atom is deprotonated. Despite exhibiting both photoacidity and photobasicity, release of HNO from donor 4 does not require water and hence does not involve excited state proton transfer(s). Indeed, deprotonation of the photoacidic site of donor 4 decreases pathway selectivity for the generation of HNO following photolysis. The time-resolved spectroscopic studies indicate that HNO release may occur on the picosecond timescale following excitation of donor 4 and occurs via the short-lived ¹NO⁻ species (¹HNO/¹NO⁻ ₚKₐ ~23).
Studies were also completed towards determining the rate constant for the reaction between HNO and hydroxycobalamin. A thermally decomposing HNO donor (Piloty’s acid) was used; this HNO donor releases HNO in alkaline conditions. Under alkaline conditions, a pH-dependent mixture of ¹HNO and ³NO⁻ was formed from the decomposition of Piloty’s acid. ³NO⁻ itself is a good reductant, and a reaction of ³NO⁻ and hydroxycobalamin was observed. Furthermore, ³NO⁻ reacts with the product of the reaction between hydroxycobalamin and ¹HNO/³NO⁻ namely nitroxylcobalamin
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
Measuring Vibrational Dynamics at the Quantum-classical Interface with Resonance Raman Spectroscopy
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