Ultrafast Decay of the Excited Singlet States of Thioxanthone by Internal Conversion and Intersystem Crossing

The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high‐level ab initio CASPT2 calculations of the singlet‐ and triplet‐state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin–orbit coupling terms. The initially populated singlet ππ* state is shown to decay through internal conversion and intersystem crossing processes via intermediate nπ* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet–triplet crossing near the singlet ππ* minimum and the large spin–orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity.     

Reversibly Switchable Fluorescent Molecular Systems Based on Metallacarborane–Perylenediimide Conjugates

Icosahedral metallacarboranes are θ-shaped anionic molecules in which two icosahedra share one vertex that is a metal center. The most remarkable of these compounds is the anionic cobalt-based metallacarborane [Co(C₂B₉H₁₁)₂]⁻, whose oxidation-reduction processes occur via an outer sphere electron process. This, along with its low density negative charge, makes [Co(C₂B₉H₁₁)₂]⁻ very appealing to participate in electron-transfer processes. In this work, [Co(C₂B₉H₁₁)₂]⁻ is tethered to a perylenediimide dye to produce the first examples of switchable luminescent molecules and materials based on metallacarboranes. In particular, the electronic communication of [Co(C₂B₉H₁₁)₂]⁻ with the appended chromophore unit in these compounds can be regulated upon application of redox stimuli, which allows the reversible modulation of the emitted fluorescence. As such, they behave as electrochemically-controlled fluorescent molecular switches in solution, which surpass the performance of previous systems based on conjugates of perylendiimides with ferrocene. Remarkably, they can form gels by treatment with appropriate mixtures of organic solvents, which result from the self-assembly of the cobaltabisdicarbollide-perylendiimide conjugates into 1D nanostructures. The interplay between dye π-stacking and metallacarborane electronic and steric interactions ultimately governs the supramolecular arrangement in these materials, which for one of the compounds prepared allows preserving the luminescent behavior in the gel state.     

Synthesis and characterization of an amethyrin-uranyl complex displaying aromatic character

Abstract

The reaction between amethyrin and non-aqueous uranyl silylamide (UO₂[N(SiMe₃)₂]₂) under anaerobic conditions affords a bench-stable uranyl complex. UV–vis spectroscopy, cyclic voltammetry, as well as proton NMR spectroscopic analyses provide support for the conclusion that all six pyrrole subunits participate in coordination of the uranyl dication and that, upon complexation, the amethyrin-core undergoes a 2-electron oxidation to yield a formal 22 π-electron aromatic species.     

Synthesis of (S)-(+)-sotalol and (R)-(−)-isoproterenol via a catalytic enantioselective Henry reaction

A unified approach for the synthesis of (S)-(+)-sotalol and (R)-(?)-isoproterenol has been developed. The enantioselective Henry reaction of the appropriate aldehyde in the presence of a camphor-derived amino pyridine–Cu(II) complex was the key step of the synthesis. The reduction of the nitro group to give the corresponding amino alcohols followed by reductive alkylation of the amine provided the target products with high enantiomeric excesses.     

The hausdorff dimension of the harmonic class on negatively curved surfaces

We study the regularity of the Hausdorff dimension of the harmonic class of a surface M of negative curvature as a function of the riemannian metric. We prove that it is a C^{r− 3} function of the metric in the Banach manifold of C^r riemannian metrics on M. We also prove regularity results for some asymptotic quantities associated to the Brownian motion on M.     

Determination of parathion in the presence of paraoxon and p-nitrophenol by flow-injection analysis with amperometric detection

The amperometric determination of parathion in the presence of its metabolites paraoxon and p-nitrophenol in a flow-injection system is reported. The method is based on the measurement of the oxidation signal yielded by p-nitrophenol on hydrolysis of parathion in the presence of Pd(II) or Hg(II). The oxidation of the metabolite was preferred to its reduction as it avoids the removal of oxygen, which poses major problems in flow-through configurations.     

Electrochemical Immunosensor for the Determination of Total Ghrelin Hormone in Saliva

An electrochemical immunosensor for ghrelin (GHRL) determination in saliva is reported. Anti‐GHRL was immobilized onto Protein G‐magnetic beads and a competitive immunoassay involving biotinylated GHRL and alkaline phosphatase‐streptavidin was implemented. Once conjugate was magnetically captured on a screen‐printed carbon electrode, GHRL quantization was accomplished by DPV of 1‐naphtol formed upon addition of 1‐naphtyl phosphate. A linear range between 10^?3 and 10³ ng/mL GHRL, and a LOD of 7 pg/mL, much smaller than those from commercial ELISA kits, were found. The usefulness of the immunosensor was demonstrated by analyzing human saliva spiked with GHRL at 0.01, 0.1, 1 and 10 ng/mL.     

Can Enantioselectivity be Computed in Enthalpic Barrierless Reactions? The Case of CuI‐Catalyzed Cyclopropanation of Alkenes

An extensive computational study has been carried out on different catalytic systems for cyclopropanation reactions based on copper. Most DFT schemes used present drawbacks that preclude the calculation of accurate absolute kinetic properties (energy barriers) of such systems, excepting the M05 and M06 suites of density functionals. On the other hand, there is a wide range of DFT methods capable of reproducing relative energy values, which can be easily translated into selectivities. Most of the theoretical levels used tend to overestimate activation barriers, allowing the location of the transition state (TS) on the potential-energy surface (PES) of the most reactive systems, which are probably artifacts of the method. However, after a thorough analysis of the calculated PES, and the origin of the energy differences obtained for the different alkene approaches in chiral systems, it is found that energy differences are almost constant over a wide range of geometries covering the reaction channel zone in which the true TS on the Gibbs free-energy surface (GFES) lies. Therefore, many computational schemes can still be used confidently to explain and predict enantioselectivities in these systems.