Carrier recombination dynamics in InGaN/GaN multiple quantum wells

We have mesured the carrier recombination dynamics in InGaN/GaN multiple quantum wells over an unprecedented range in intensity. We find that at times shorter than 30\,ns, they follow an exponential form, and a power law at times longer than 1\,$\mu$s. To explain these biphasic dynamics, we propose a simple three-level model where a charge-separated state interplays with the radiative state through charge transfer following a tunneling mechanism. We show how the distribution of distances in charge-separated states controls the dynamics at long time. Our results imply that charge recombination happens on nearly-isolated clusters of localization centers.Comment: 11 pages, 3 figures, accepted for publication in PHYSICAL REVIEW

Computational development of rubromycin-based lead compounds for HIV-1 reverse transcriptase inhibition

The binding of several rubromycin-based ligands to HIV1-reverse transcriptase was analyzed using molecular docking and molecular dynamics simulations. MM-PBSA analysis and examination of the trajectories allowed the identification of several promising compounds with predicted high affinity towards reverse transcriptase mutants which have proven resistant to current drugs. Important insights on the complex interplay of factors determining the ability of ligands to selectively target each mutant have been obtained

Computational exploration of the reaction mechanism of the Cu+- catalysed synthesis of indoles from N-aryl enaminones

We have studied the role of Cu+-phenantroline as a catalyst in the cyclization of N-aryl-enaminones using density-functional theory computations. The catalyst was found to bind the substrate upon deprotonation of its eneaminone, and to dramatically increase the acidity of the carbon adjacent to the ketone functionality. The deprotonation of this carbon atom yields a carbanion which attacks the aryl moiety, thereby closing the heterocycle in the rate-determining step. This C–C bond forming reaction was found to proceed much more rapidly when preceded by re-protonation of the substrate N-atom (which had lost H+ in the initial step). Hydride transfer to the catalyst then completes the indole synthesis, in a very fast step. The influence of Li+ and K+ on the regioselectivity of the cyclization of bromo-substituted analogues could not, however, be reproduced by our model. Alternative pathways involving either single-electron transfer from the catalyst to the substrate or ring cyclization without previous carbon α-deprotonation were found to be kinetically or thermodynamically inaccessible

Unveiling the reaction mechanism of the das/chechik/marek synthesis of stereodefined quaternary carbon centers

The reaction mechanism of the Cu+‐catalyzed introduction of two all‐carbon‐substituted stereocenters in an ynamide system using a Grignard reagent, a zinc carbenoid, and an aldehyde, was investigated using density‐functional theory. In contrast to the formation of an organocopper(I) compound and subsequent carbocupration reaction, previously postulated as the initial step, the reaction proved to instead proceed through an initial complexation of the substrate alkyne bond by the Cu+‐catalyst, which primes this bond for reaction with the Grignard reagent. Subsequent addition of the zinc carbenoid then enables the nucleophilic attack on the incoming aldehyde, which is revealed as the rate‐limiting step. Our computations have also identified the factors governing the regio‐ and setereoselectivity of this interesting reaction, and suggest possible paths forits further development.info:eu-repo/semantics/publishedVersio

Transnational banking supervision and resilience: the SSM case

In this letter we assess the impact of adopting a transnational supervisor on the resilience of large and complex banks, exploring the establishment of the Single Supervisory Mechanism (SSM) in 2014. Using a differences-in-differences approach, we compare the performance of SSM banks vis-à-vis other banks with a similar size and complexity. Our results suggest that the adoption of a transnational supervisor can improve the resilience of large and complex banks, particularly for those operating in countries with larger banking sectors, higher market concentration and higher supervisory discretion.info:eu-repo/semantics/publishedVersio

Mesoscopic order and the dimentionality of long-range resonance energy transfer in supramolecular semiconductors

We present time-resolved photoluminescence measurements on two series of oligo-p-phenylenevinylene materials that self-assemble into supramolecular nanostructures with thermotropic reversibility in dodecane. One set of derivatives form chiral, helical stacks while the second set form less organised, frustrated stacks. Here we study the effects of supramolecular organisation on the resonance energy transfer rates. We measure these rates in nanoassemblies formed with mixed blends of oligomers and compare them with the rates predicted by Foerster theory. Our results and analysis show that control of supramolecular order in the nanometre lengthscale has a dominant effect on the efficiency and dimentionality of resonance energy transfer.Comment: 17 Pages, 5 Figures, Submitted to J. Chem. Phy

Evaluation of density functional methods on the geometric and energetic descriptions of species involved in Cu+-promoted catalysis

We have evaluated the performance of 15 density functionals of diverse complexity on the geometry optimization and energetic evaluation of model reaction steps present in the proposed reaction mechanisms of Cu(I)-catalyzed indole synthesis and click chemistry of iodoalkynes and azides. The relative effect of the Cu(+) ligand on the relative strength of Cu(+)-alkyne interactions, and the strong preference for a π-bonding mode is captured by all functionals. The best energetic correlations with MP2 are obtained with PBE0, M06-L, and PBE1PW91, which also provide good quality geometries. Furthermore, PBE0 and PBE1PW91 afford the best agreement with the high-level CCSD(T) computations of the deprotonation energies of Cu(+)-coordinated eneamines, where MP2 strongly disagrees with CCSD(T) and the examined DFT functionals. PBE0 also emerged as the most suitable functional for the study of the energetics and geometries of Cu(+) hydrides, while at the same time correctly capturing the influence of the Cu(+) ligands on the metal reactivity.info:eu-repo/semantics/publishedVersio

ACTION COLLECTIVE ET DÉVELOPPEMENT DURABLE

International audienceLe cas étudié rapporte l´expérience du « Collectif Régional du Cariri, Seridó et Curimataú », un réseau d´environ 300 communautés rurales de dix communes situées dans la région semi-aride de la Paraíba (Brésil nord-est), que nous accompagnons dès le début des années 1990, et dans lequel les familles agricultrices expérimentent, em dialogue avec leurs conseillers, diverses techniques simples et peu coûteuses, souvent adaptées ou inspirées par les savoirs ancestraux et répondant aux nécessités qu´ils rencontrent (réservoirs d´eaux, clotures, utilisation de la végétation locale pour l´alimentation animale ou plantes médicinales etc.). Ces communautés exercent également la gestion collective des ressources, découvrent leurs capacités, ce qui renforce leur auto-estime et, à travers les échanges de réflexions, se libèrent de la dépendance de l´oligarchie dominante. Il s´agit du segment le plus pauvre de la société (les agriculteurs familiaux) de la région la plus problématique du Brésil (le nord-est semi-aride) ; problématique par ses conditions naturelles et par son histoire, marquée par la concentration foncière et la domination de l´oligarchie locale qui maintient son pouvoir en distribuant ses « faveurs » (comme la distribution d´eau par camion-citerne en cas de sécheresse) en échange de votes. Le fait que les propositions aient été construites sur la base d´expériences bien réussies a été fondamental pour la durabilité du développement ainsi amorcé. Ces expériences ont été proposées au gouvernement et assumées comme politiques publiques. Ces expériences bien réussies renforcent l´auto-estime des agriculteurs et leurs initiatives