1,721,058 research outputs found
Time-resolved electron paramagnetic resonance of photoinduced ion pairs in blends of polythiophene and fullerene derivatives
No full textSubstituted polythiophene and triethylenglycolpyrrolidino-C60 blends are examined by time-resolved electron paramagnetic resonance (TR-EPR) at different temperatures. TR-EPR spectra recorded on the microsecond time scale after a short laser pulse are assigned to polythiophene and fullerene radical ion pairs, generated by electron transfer from the excited state of polythiophene to fullerene. At low temperatures, TR-EPR spectra show polarized lines with an antiphase emission/absorption pattern. The origin of the polarization pattern is described in the frame of spin correlated radical pair theory, in which two unpaired electron spins (on radical cation and anion, respectively) interact through isotropic spin exchange and anisotropic dipolar interactions. The polarization pattern is accounted for assuming a singlet excited state as the precursor of the chargeseparated state. Spectral simulations yield dipolar and spin exchange coupling constants between unpaired electrons of the radical ion pair. Their values correspond to a mean distance between opposite charges of 22 Å. When the temperature is increased, the spectra gradually loose their antiphase character and eventually consist of a signal totally in emission. This behavior is explained by a polarization mechanism involving a spin-selective charge recombination (ST-1 mixing). The polarization pattern at different temperatures is examined in detail, and its generating mechanism is discussed
Simulation of EPR and Time Resolved EPR lineshapes in partially ordered phases
No full textSimulation of magnetic resonance spectra of probes in partially ordered glasses requires in principle a numerical integration on the full set of three Euler angles Ohm=(alpha beta gamma) from a laboratory fixed to a molecule fixed reference frame. It is shown that it is possible to manage efficiently this problem by using the algebraic properties of the Wigner matrix elements. This analysis is applied to time resolved EPR (TREPR) spectra of a series of bis-adducts of C-60 in the ordered glass of a nematic liquid crystal solvent. A paramagnetic triplet state is created by light excitation and TREPR spectra are obtained with the external magnetic field set parallel or perpendicular to the director n of the mesophase. The preferred orientation in the mesophase of the triplet state zero field tensor is determined
Ontology-based database access with dig-mastro and the obda plugin for protégé (demo)
No full textIn Ontology Based Data Access (OBDA), the aim is to use an ontology to mediate access to data. The main contribution of this work is to demonstrate two key components of an OBDA system, whose combination allows ontology practitioners to finally realize end-to-end OBDA systems. The first component is an OBDA-enabled reasoner, named DIG-MASTRO. The second component is the OBDA Plugin for Protégé, that in conjunction with the ontology editing features of Protégé, functions as an OBDA system designer
Spin polarization in fullerene derivatives containing a nitroxide group. Observation of the intermediate photoexcited quartet state in radical triplet pair interaction
No full textA series of C-60 fullerene derivatives containing a nitroxide group has been photoexcited by short LASER pulses in the microwave cavity of a cw-EPR spectrometer. Strongly spin polarized signals have been observed, in glassy matrix as well as in liquid solution, for both the ground electronic state and the excited quartet state. In the quartet state the excitation resides in the fullerene part and the molecule constitutes a triplet-radical pair with the partner covalently linked. The absorptive or emissive character of the transitions is explained in terms of the mechanism of radical-triplet interaction producing spin polarization. Opposite initial sign and polarization patterns are observed for molecules with different spacer between nitroxide and fullerene. The time evolution of the relevant sublevel populations is fitted by a kinetic model taking into account quartet decay constants, quartet and doublet spin-lattice relaxation rates and branching ratios
Nitroxide paramagnet-induced para-ortho conversion and nuclear spin relaxation of H-2 in organic solvents
No full textThe kinetics of para-ortho conversion and nuclear spin relaxation of H-2 in chloroform-d(1), were investigated in the presence of nitroxides as paramagnetic catalysts. The back conversion from parahydrogen (p-H-2) to ortho-hydrogen (o-H-2) was followed by NMR by recording the increase in the intensity of the signal of o-H-2 at regular intervals of time. The nitroxides proved to be hundreds of times more effective at inducing relaxation among the spin levels of o-H-2 than they are in bringing about transitions between p-H-2 and the levels of o-H-2. The value of the encounter distance d between H-2 and the paramagnetic molecule, calculated from the experimental bimolecular conversion rate constant k(0), using the Wigner theory of para-ortho conversion, agrees perfectly with that calculated from the experimental relaxivity R-1 using the force free diffusion theory of spin-lattice relaxation
Ontology-based data access with MASTRO (Demo)
No full textand the Semantic Web [4], clients need to access the services exported by the system, and hence require a representation of the intensional level of the application domain in terms of which they can specify the access to the exported services. One of the most interesting usages of such a shared conceptualization is ontology-based data access, where a conceptual layer is exported to the client, abstracting away from how actual data is maintained. Therefore, an ontology-based data access system has to adress the “impedance mismatch ” issue, existing between data actually present at the sources, and objects represented at the intensional level. In this demo, we present MASTRO, an ontology-based data access system that ex-tends the QuOnto system [1] with new capabilities and uses the QuOnto engine at its core. Specifically, MASTRO provides tractable reasoning over an ontology, laying over an autonomous relational DBMS managing the data layer. In what follows, we introduce the main services provided by MASTRO, namely (i) ontology specification, (ii) query answering, (iii) ontology satisfiability, and (iv) meta-query answering. The present work is one of the outcomes of the European project TONES1
Synthesizing extensional constraints in Ontology-based Data Access
No full textSeveral recent techniques and tools for Ontology-based Data Access (OBDA) make use of the so-called extensional constraints (a.k.a. ABox dependencies). So far, extensional constraints have been mainly considered in a setting where data are represented in an ABox, instead of external data sources connected to the ontology through declarative mappings. Moreover, the issue of how to generate extensional constraints in practice has not been addressed yet. In this paper we first provide a formal account of the notion of extensional constraints in a full-fledged OBDA setting, where an ontology is connected to the data sources of the information system by means of mappings, and then present an approach to the automatic generation of extensional constraints in such a setting. The technique we propose is based on the use of a first-order theorem prover that checks validity of relevant formulas built over the mapping views. The experiments we have carried out in real-world OBDA projects show the effectiveness of our approach in discovering large collections of extensional constraints entailed by the OBDA specification
Photoinduced Electron Spin Polarization in a Fullerene-Azothiophene dyad.
No full textSummary : Time Resolved EPR spectroscopy was applied to the investigation of photoinduced electron spin polarization in a molecular dyad. This is composed of a fullerene unit linked to a azothiophene dye. Compared to the separated components, spin polarization yield and lifetime were found to be strongly increased. This is discussed in terms of multiple energy transfer processes involving singlet and triplet excited states of the dyad components.
Abstract : La spèctroscopie de RPE a été appliquée à l'étude de la polarisation generèe par la lumière dans une composé moléculaire contenant une unitée de fullerene connectée à une azocolorant. Par rapport à ses composantes considérées séparement, on a trouvé que la durée et l’intensité de la polarisation ont étées fortement augmentés. On analyse ce fenomène en termes de procès multiples de transfer d'energie faisant intervenire les états excités de singulet e de triplet des composantes
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