1,721,006 research outputs found
Azobenzene cis-trans Photoisomerization Mechanism: Characterization of the Decay Ways from the Lowest pipi* Absorbing Singlet State
No full textIn this paper, we analyze the photoisomerization processes of azobenzene after its excitation in the bright S(ππ*). By state of the art/ ab initio / Complete Active Space calculations followed by perturbative corrections (CASPT2//CASSCF) we have identified the critical structures, the Minimum Energy Paths originating on the bright S(ππ*) and on other relevant excited states including the state S1(nπ*). The seams of conical intersections that are important in guiding the photoreaction are determined. We aim at establishing the mechanism of decay and of photoisomerization for the S(ππ*) state and at explaining the difference between the quantum yields found for the two lowest energy S1(nπ*) and S(ππ*) excited states. We found that an excited state based on the πN = NπN = N→π*N = Nπ*N = N configuration is a photoreaction intermediate that plays a very important role in the decay the bright S(ππ*). This doubly excited state, by driving the photoisomerization along the torsion path and by inducing a fast internal conversion to the S1(nπ*) occurring in a variety of geometries, explains all the most important features of the S(ππ*) azobenzene photoisomerization
Double Thionated Pyrimidine Nucleobases: Molecular Tools with Tunable Photoproperties
Full text linkSulfur-substitutednucleobases are DNA and RNA base derivativesthat exhibit extremely efficient photoinduced intersystem crossing(ISC) dynamics into the lowest-energy triplet state. The long-livedand reactive triplet states of sulfur-substituted nucleobases arecrucial due to their wide range of potential applications in medicine,structural biology, and the development of organic light-emittingdiodes (OLEDs) and other emerging technologies. However, a comprehensiveunderstanding of non-negligible wavelength-dependent changes in theinternal conversion (IC) and ISC events is still lacking. Here, westudy the underlying mechanism using joint experimental gas-phasetime-resolved photoelectron spectroscopy (TRPES) and theoretical quantumchemistry methods. We combine 2,4-dithiouracil (2,4-DTU) TRPES experimentaldata with computational analysis of the different photodecay processes,which are induced by increasing excitation energies along the entirelinear absorption (LA) ultraviolet (UV) spectrum. Our results showhow the double-thionated uracil (U), i.e., 2,4-DTU, appears as a versatilephotoactivatable instrument. Multiple decay processes can be initiatedwith different ISC rates or triplet-state lifetimes that resemblethe distinctive behavior of the singly substituted 2- or 4-thiouracil(2-TU or 4-TU). We obtained a clear partition of the LA spectrum basedon the dominant photoinduced process. Our work clarifies the reasonsbehind the wavelength-dependent changes in the IC, ISC, and triplet-statelifetimes in doubly thionated U, becoming a biological system of utmostimportance for wavelength-controlled applications. These mechanisticdetails and photoproperties are transferable to closely related molecularsystems such as thionated thymines
An ab initio Study of Decay Mechanism of Adenine: the Facile Path of the Amino NH Bond Cleavage
No full textA comprehensive study of the radiationless decay processes of the lowest excited singlet states in the isolated 9H‐Adenine has been performed at the CASPT2//CASSCF level. The minimum energy paths of the La, Lb and nπ* singlet states along different skeletal distortions have been computed and the Conical Intersections (CIs) involving these states have been determined. The fast deactivation path of La along a skeletal deformation, which leads to a S0/La CI, as previously discussed, is confirmed. Moreover, low‐lying CIs between S0 and πσ* singlet states have been characterized, where σ* is the antibonding orbital localized on a N‐H bond of the amino (πσNH2*) or of the azine group (πσN9H*). We have found that the repulsive πσNH2* state associated with an amino N‐H bond can be populated through a barrierless way. Therefore, the decay path shows a bifurcation leading to two possible ways of radiationless deactivation: on one hand a non‐photochemical decay through the S0/La or S0/nπ* CIs and on the other hand a photochemical process via the possible access to the S0/πσNH2* CI that produces N‐H cleavage. In this way, we can explain the H atom loss found upon UV excitation. We have considered also the decay of higher energy bright states. We have found that these states can decay also by converting to the repulsive πσN9H* state associated with the azine NH bond. This new channel suggests an increase of H‐atom photoproduction yield by excitating Adenine with lower wavelength radiations. The study of the decay processes of an Adenine molecule in the double strand d(A)10⋅d(T)10 in water solvent is currently underway: Adenine is treated by the Quantum Mechanical (QM) approach and the remaining molecules are described at the Molecular Mechanics (MM) level. We use the COBRAMM program that is a tunable QM/MM approach to complex molecular architectures developed by our research group
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
Ultrafast Spectroscopy of Photoactive Molecular Systems from First Principles: Where We Stand Today and Where We Are Going
Full text linkComputational spectroscopy is becoming a mandatory tool for the interpretation of the complex, and often congested, spectral maps delivered by modern non-linear multi-pulse techniques. The fields of Electronic Structure Methods, Non-Adiabatic Molecular Dynamics, and Theoretical Spectroscopy represent the three pillars of the virtual ultrafast optical spectrometer, able to deliver transient spectra in silico from first principles. A successful simulation strategy requires a synergistic approach that balances between the three fields, each one having its very own challenges and bottlenecks. The aim of this Perspective is to demonstrate that, despite these challenges, an impressive agreement between theory and experiment is achievable now regarding the modeling of ultrafast photoinduced processes in complex molecular architectures. Beyond that, some key recent developments in the three fields are presented that we believe will have major impacts on spectroscopic simulations in the very near future. Potential directions of development, pending challenges, and rising opportunities are illustrated
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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