Optical Properties of Lanthanide Doped Hybrid Organic-Inorganic Materials

Sol-gel matrices have been investigated for some years as potential matrices for rare earth luminescence. Sm³⁺, Eu³⁺, Dy³⁺, Er³⁺ and Tm³⁺ optical properties in siloxane-zirconium hybrid matrices prepared at room temperature have been investigated. Even if luminescence efficiency is governed by non-radiative relaxation linked to hydroxyl groups, in these matrices, rare earth fluorescence is always observed but lifetimes and quantum yields are dependent on the elaboration procedure. Eu³⁺ is used as a probe of the local surrounding around the dopant and the emission and decay profiles measurements in rare earth doped and undoped matrices are presented.     

Ab initio molecular dynamics study of the keto-enol tautomerism of acetone in solution.

We have studied the keto-enol interconversion of acetone to understand the mechanism of tautomerism relevant to numerous organic and biochemical processes. Applying the ab initio metadynamics method, we simulated the keto-enol isomerism both in the gas phase and in the presence of water. For the gas-phase intramolecular mechanism we show that no other hydrogen-transfer reactions can compete with the simple keto-enol tautomerism. We obtain an intermolecular mechanism and remarkable participation of water when acetone is solvated by neutral water. The simulations reveal that C deprotonation is the kinetic bottleneck of the keto-enol transformation, in agreement with experimental observations. The most interesting finding is the formation of short H-bonded chains of water molecules that provide the route for proton transfer from the carbon to the oxygen atom of acetone. The mechanistic picture that emerged from the present study involves proton migration and emphasizes the importance of active solvent participation in tautomeric interconversion.     

Europium(III) and its halides in anhydrous room-temperature imidazolium-based ionic liquids: a combined TRES, EXAFS, and molecular dynamics study

Combining spectroscopic techniques (TRES and EXAFS) and molecular dynamics simulations, we have investigated the state of trivalent europium dissolved in room-temperature ionic liquids (RTILs), as a function of the RTIL anion and in the presence of added chloride anions. The studied RTILs are based on the 1-butyl-3-methyl-imidazolium (Bumim+) cation and differ by their anionic counterparts: BF4-, PF6-, Tf- (triflate, CF3SO3-), and Tf2N- [(CF3SO2)2N-]. The results show the strong influence of the RTIL nature on the first solvation shell of europium and on its complexation with chloride. Depending on the RTIL, europium(III), which was introduced in solution as a triflate salt, is found to be solvated either by RTIL anions only or as neutral undissociated EuTf3 moieties completed by solvent anions. Kinetic effects, related to the viscosity of the RTIL and the nature of the europium salt, also markedly influence the coordination of added Cl- or F- anions to the metal.     

Organocatalyzed Aldol Reaction between Pyridine‐2‐carbaldehydes and α‐Ketoacids: A Straightforward Route towards Indolizidines and Isotetronic Acids

Enantioselective aldol reactions between substituted pyridine carbaldehydes and α‐ketoacids were shown to provide isotetronic acids or their corresponding pyridinium salts, depending on the nature of the substituents on the pyridine ring. The pyridinium salts were generated through nucleophilic attack of the pyridine nitrogen atom onto the reactive keto functional group. Moderate‐to‐good yields of both compounds were typically obtained and high levels of enantioselectivity were observed by using benzimidazole pyrrolidine I as a catalyst. Hydrogenation of the resulting pyridinium salts led to new indolizidines with high ee values and diastereocontrol. X‐ray diffraction studies allowed the determination of the relative configuration of the products. Finally, DFT calculations were performed to rationalize the divergent pathway as a function of the pyridine substituents.     

Hydrogen oxidation reaction at the Ni/YSZ anode of solid oxide fuel cells from first principles

By means of ab initio simulations we here provide a comprehensive scenario for hydrogen oxidation reactions at the Ni/zirconia anode of solid oxide fuel cells. The simulations have also revealed that in the presence of water chemisorbed at the oxide surface, the active region for H oxidation actually extends beyond the metal/zirconia interface unraveling the role of water partial pressure in the decrease of the polarization resistance observed experimentally.     

Synthesis and Photophysical Properties of 1,1,4,4-Tetracyanobutadienes Derived from Ynamides Bearing Fluorophores**

1,1,4,4-Tetracyanobutadienes (TCBDs) bearing a large diversity of fluorophores were prepared following a multi-step synthesis. In a crucial last step, all compounds were obtained from the corresponding ynamides, which were particularly suitable for the formation of the TCBDs in the presence of tetracyanoethylene via a [2+2] cycloaddition/retroelectrocyclization step (CA-RE). Several fluorenyl derivatives in addition to phenanthrenyl and terphenyl ones provided ynamide-based TCBDs affording remarkable emission properties covering a large range of wavelengths. Those compounds emit both in solid state and in solution from the visible region to the NIR range, depending on the molecular structures. Quantum yields in cyclohexane reached unforeseen values for such derivatives, up to 7.8 %. A huge sensitivity to the environment of the TCBDs has also been unraveled for most of the compounds since we observed a dramatic fall of the quantum yields when changing the solvent from cyclohexane to toluene, while they are almost non-emissive in dichloromethane.