Versatile transamination in quinonediimine chemistry: Towards a novel class of water soluble UV/violet chromophores

The transamination reaction of 2,5-diaminobenzoquinonediimine (QDI) with ethylenediamine gave fluorescent 1,2,3,4-tetrahydropyrazino[2,3-g]quinoxaline (1). When the same reaction was carried out with N,N’-bis(aminoethyl)-1,3-propanediamine, a novel cationic quinoxalinium species (2) was isolated, which can be further condensed with p-cyanobenzaldehyde to afford a benzimidazolo-fused quinoxaline dye (3) that is a water-soluble fluorophore in the UV–visible range.     

Metathese d'acetates d'alcools ω-insatures : Synthese de pheromones d'insectes

Esters of alkenoic acids, including insect pheromones as examples, have been synthesized using alkene metathesis reactions.     

Time-resolved excitation spectra of xenon vapor in the 1500 Å region

Time-integrated and time-resolved excitation spectra, as well as fluorescence lifetimes, have been measured for xenon vapor as a funtion of pressure (for pure xenon as well as with different collision partners: krypton and helium), monitoring the 1700 Å second continuum afterglow. Three very different decay components have been observed: (a) A short component with lifetimes of the order of 2 ns, which is attributed to emission from the upper vibrational levels of the O⁺_u(¹Σ⁺_u) state of Xe*₂. (b) A long component with τ ≈ 60 ns, corresponding to emission from thermally relaxed vibrational levels of the 1_u,O^?_u(³Σ⁺_u) states of Xe*₂. No intervention of any Xe atomic excited species is invoked to explain the excitation and deactivation processes of Xe₂ molecules for these two components of the fluorescence. (c) A very long component with lifetimes in the 150–500 ns range, having a broad ? 1000 cm^?1 - excitation band, centered at ≈ 1470 Å and showing a striking screening and self-absorption effect as well as a strong effect when the partial pressure of a collision partner (Kr, He) is increased. The mechanism of this last excitation is not yet very well understood.     

Strengthening the Murty–Simon conjecture on diameter 2 critical graphs

A graph is diameter-2-critical if its diameter is 2 but the removal of any edge increases the diameter. A well-studied conjecture, known as the Murty–Simon conjecture, states that any diameter-2-critical graph of order $n$ has at most $?n^2∕4?$ edges, with equality if and only if $G$ is a balanced complete bipartite graph. Many partial results about this conjecture have been obtained, in particular it is known to hold for all sufficiently large graphs, for all triangle-free graphs, and for all graphs with a dominating edge. In this paper, we discuss ways in which this conjecture can be strengthened. Extending previous conjectures in this direction, we conjecture that, when we exclude the class of complete bipartite graphs and one particular graph, the maximum number of edges of a diameter-2-critical graph is at most $?{(n?1)}^2∕4?+1$. The family of extremal examples is conjectured to consist of certain twin-expansions of the 5-cycle (with the exception of a set of thirteen special small graphs). Our main result is a step towards our conjecture: we show that the Murty–Simon bound is not tight for non-bipartite diameter-2-critical graphs that have a dominating edge, as they have at most $?n^2∕4??2$ edges. Along the way, we give a shorter proof of the Murty–Simon conjecture for this class of graphs, and stronger bounds for more specific cases. We also characterize diameter-2-critical graphs of order $n$ with maximum degree $n?2$: they form an interesting family of graphs with a dominating edge and $2n?4$ edges.     

Functionalization of 2D materials by intercalation

Since the discovery of graphene many studies focused on its functionalization by different methods. These strategies aim to find new pathways to overcome the main drawback of graphene, a missing band-gap, which strongly reduces its potential applications, particularly in the domain of nanoelectronics, despite its huge and unequaled charge carrier mobility. The necessity to contact this material with a metal has motivated a lot of studies of metal/graphene interactions and has led to the discovery of the intercalation process very early in the history of graphene. Intercalation, where the deposited atoms do not stay at the graphene surface but intercalate between the top layer and the substrate, may happen at room temperature or be induced by annealing, depending of the chemical nature of the metal. This kind of mechanism was already well-known in the earlier Graphite Intercalation Compounds (GICs), particularly famous for one current application, the Lithium-ion Battery, which is simply an application based on the intercalation of Lithium atoms between two sheets of graphene in a graphite anode. Among numerous discoveries the GICs community also found a way to obtain graphite with superconducting properties by using intercalated alkali metals. Graphene is now a playground to “revisit” and understand all these mechanisms and to discover possible new properties of graphene induced by intercalation. For example, the intercalation process may be used to decouple the graphene layer from its substrate, to change its doping level or even, in a more general way, to modify its electronic band structure and the nature of its Dirac fermions. In this paper we will focus on the functionalization of graphene by using intercalation of metal atoms but also of molecules. We will give an overview of the induced modifications of the electronic band structure possibly leading to spin-orbit coupling, superconductivity, …We will see how this concept of functionalization is also now used in the framework of other 2D materials beyond graphene and of van der Waals heterostructures based on these materials.     

Calcul de l'emission infrarouge de milieux laser CO2

Vibrationally-excited CO₂ present with inverted population emits infrared radiation at 4·26 μm. For an emitter consisting of optically-thick isolated lines, calculations allow determination of the emissivity and radiancy of a homogeneous layer of gas. Infrared emission is a useful tool for measurement of the energy contained in the upper laser level. A very thin layer of cold gas strongly alters the spectral distribution of the intensity and causes the total radiancy to decrease.     

How does geographical distance translate into genetic distance?

Geographic structure can affect patterns of genetic differentiation and speciation rates. In this article, we investigate the dynamics of genetic distances in a geographically structured metapopulation. We model the metapopulation as a weighted directed graph, with $d$ vertices corresponding to $d$ subpopulations that evolve according to an individual based model. The dynamics of the genetic distances is then controlled by two types of transitions — mutation and migration events. We show that, under a rare mutation–rare migration regime, intra subpopulation diversity can be neglected and our model can be approximated by a population based model. We show that under a large population-large number of loci limit, the genetic distance between two subpopulations converges to a deterministic quantity that can asymptotically be expressed in terms of the hitting time between two random walks in the metapopulation graph. Our result shows that the genetic distance between two subpopulations does not only depend on the direct migration rates between them but on the whole metapopulation structure.     

Limits of structures and the example of tree semi-lattices

The notion of left convergent sequences of graphs introduced by Lovász et al. (in relation with homomorphism densities for fixed patterns and Szemerédi’s regularity lemma) got increasingly studied over the past $10$ years. Recently, Ne?et?il and Ossona de Mendez introduced a general framework for convergence of sequences of structures. In particular, the authors introduced the notion of $QF$-convergence, which is a natural generalization of left-convergence. In this paper, we initiate study of $QF$-convergence for structures with functional symbols by focusing on the particular case of tree semi-lattices. We fully characterize the limit objects and give an application to the study of left convergence of $m$-partite cographs, a generalization of cographs.     

Synthese de la cyclo-5'-8 desoxy-5' adenosine: stereochimie et mecanisme de la cyclisation d'un derive de l'iodo-5' adenosine par le zinc

N,N'-Dibenzoyl 2', 3'-O-isopropylidene 5'-deoxy 5'-iodo adenosine can efficiently transformed into the corresponding 5',8-cyclo-7,8-dihydronucleoside by zinc in pyridine. Only one diastereoisomer is obtained. By spin decoupling and NOE techniques at 250 MHz, the R-configuration at C-8 was established. This compound showed typical dihedral angles of about 90° for the vicinal protons H-1', H-2' and H-3', H-4'. Using adenosine derivatives stereospecifically labelled at C-5', it was demonstrated that the cyclisation occurs with racemisation at that position.