VO2+ complexation by bioligands showing keto-enol tautomerism: a potentiometric, spectroscopic, and computational study

The interaction of VO(2+) ion with ligands of biological interest that are present in important metabolic pathways--2-oxopropanoic acid (pyruvic acid, pyrH), 3-hydroxy-2-oxopropanoic acid (3-hydroxypyruvic acid, hydpyrH), oxobutanedioic acid (oxalacetic acid, oxalH(2)), (S)-hydroxybutanedioic acid (l-malic acid, malH(2)), and 2,3-dihydroxy-(E)-butanedioic acid (dihydroxyfumaric acid, dhfH(2))--was described. Their complexing capability was compared with that of similar ligands: 3-hydroxy-2-butanone (hydbut) and 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid, squarH(2)). All of these ligands (except l-malic acid) exhibit keto-enol tautomerism, and the presence of a metal ion can influence such an equilibrium. The different systems were studied with electron paramagnetic resonance (EPR) and UV-vis spectroscopies and with pH potentiometry. Density functional theory (DFT) methods provide valuable information on the relative energy of the enol and keto forms of the ligands both in the gas phase and in aqueous solution, on the geometry of the complexes, and on EPR and electronic absorption parameters. The results show that most of the ligands behave like α-hydroxycarboxylates, forming mono- and bis-chelated species with (COO(-), O(-)) coordination, demonstrating that the metal ion is able to stabilize the enolate form of some ligands. With dihydroxyfumaric acid, the formation of a non-oxidovanadium(IV) complex, because of rearrangement of dihydroxyfumaric to dihydroxymaleic acid (dhmH(2)), can be observed. With 3-hydroxy-2-butanone and 3,4-dihydroxy-3-cyclobutene-1,2-dione, complexation of VO(2+) does not take place and the reason for this behavior is explained by chemical considerations and computational calculations.     

Synthesis of 3-aryl-4-methyl-1,2-benzenedisulfonimides, new chiral Brønsted acids. A combined experimental and theoretical study

We have recently reported the use, in catalytic amounts, of 1,2-benzenedisulfonimide as a safe Brønsted acid in some acid-catalyzed organic reactions. With the design of new and chiral acid organocatalysts with the structure of 1,2-benzenedisulfonimide in mind, we herein propose a synthesis of 1,2-benzenedisulfonimide derivatives bearing an aryl group in the 3-position with good overall yields. The chirality of these compounds is due to the hindered rotation of the aryl group (atropisomerism). We resolved the atropisomers of one of these compounds.     

Asymmetric route to spirotetracyclic (1S)-5′,11′-dihydro-3H-spiro[cyclopentane-1,10′-dibenzo[a,d]cyclohepten]-3-one derivatives

A new synthetic pathway to the final compound 1-[(1S)-5’,11’-dihydrospiro[cyclopentane-1,10’-dibenzo[a,d]cyclohepten]-3-yl]-3-azetidinecarboxylic acid (1) was set up. The two preparative chiral HPLC separations needed in the first route were successfully avoided and replaced by two diastereomeric crystallizations of intermediate compounds 5 and 10.     

Photogenerated cumulenic structure of adamantyl endcapped linear carbon chains: an experimental and computational investigation based on infrared spectroscopy

The infrared (IR) spectrum of an adamantyl endcapped α, ω-polyyne (the hexayne, Ad-C(12)-Ad) is investigated both experimentally and computationally. A new IR band is observed upon UV photoexcitation of the compound (embedded in a poly methyl methacrylate matrix at 78 K), thus, revealing the existence of new photogenerated molecular structure trapped at low temperature. Complete reversibility is found, thus, demonstrating that the photoexcitation is responsible for the generation of metastable excited states of the molecule. Density functional theory and time dependent density functional theory calculations indicate that these metastable states result from the forbidden singlet (S(1)) or triplet (T(1)) excited states, and geometry optimizations of the polyyne trapped in either S(1) and/or T(1) states demonstrate that the carbon chain takes on a cumulenic structure. Comparison of the experimental and the computed IR spectra for the molecule trapped in the forbidden states confirms that the new IR features are clear markers of cumulenic species. The temperature and time dependent behavior of the new IR band is analyzed, while the experimentally determined value of the activation energy highlights the low stability of these molecular structures.     

Self-assembly and two-dimensional spontaneous resolution of cyano-functionalized [7]helicenes on cu(111)

Birds of a feather flock together: STM and DFT studies provide the first example of spontaneous chiral resolution of a helicene on a surface. Racemic 6,13-dicyano[7]helicene forms fully segregated domains of pure enantiomers (2D conglomerate) on Cu(111). The propensity of the system to optimize intermolecular CN???HC(Ar) hydrogen bonding and CN???CN dipolar interactions translates into chiral recognition with preferential assembly of homochiral molecules.     

Optical modulation of amplified emission in a polyfluorene-diarylethene blend

A novel system for the modulation of amplified emission based on a polyfluorene/diarylethene (namely F8BT/DTP) blend is shown. The high sensitivity of amplified spontaneous emission (ASE) is exploited to achieve efficient emission modulation with a low-intensity control signal. Modulation is then characterized by photoluminescence (PL) lifetime measurements, photocurrent experiments, and density functional theory calculations. This system can also act as a photocurrent switch based on the same principle. This technique may represent a useful tool for fluorescence quenching and sensing as well as find application in organic photonics.     

A computational investigation on singlet and triplet exciton couplings in acene molecular crystals

Quantum chemical calculations (DFT, TDDFT and ZINDO/S) of singlet and triplet exciton couplings are presented and discussed for some acene derivatives (such as anthracene, tetracene, 9,10-di(phenyl)anthracene and 9,10-bis(phenylethynyl)anthracene). An accurate excited state single molecule characterization has been carried out followed by an analysis of the inter-molecular excitonic interactions, taking place in the crystalline phase. These have been correlated to exciton coupling terms obtaining guidelines for the choice of molecular materials with large exciton couplings. Such organic systems are likely to show multiexciton processes such as singlet fission (SF) and triplet-triplet annihilation (TTA) which are useful in energy conversion phenomena to be exploited in photonic and optoelectronic devices.     

Towards laboratory produced relativistic electron–positron pair plasmas

We review recent experimental results on the path to producing electron–positron pair plasmas using lasers. Relativistic pair-plasmas and jets are believed to exist in many astrophysical objects and are often invoked to explain energetic phenomena related to Gamma Ray Bursts and Black Holes. On earth, positrons from radioactive isotopes or accelerators are used extensively at low energies (sub-MeV) in areas related to surface science positron emission tomography and basic antimatter science. Experimental platforms capable of producing the high-temperature pair-plasma and high-flux jets required to simulate astrophysical positron conditions have so far been absent. In the past few years, we performed extensive experiments generating positrons with intense lasers where we found that relativistic electron and positron jets are produced by irradiating a solid gold target with an intense picosecond laser pulse. The positron temperatures in directions parallel and transverse to the beam both exceeded 0.5 MeV, and the density of electrons and positrons in these jets are of order 10¹⁶ cm⁻³ and 10¹³ cm⁻³, respectively. With the increasing performance of high-energy ultra-short laser pulses, we expect that a high-density, up to 10¹⁸ cm⁻³, relativistic pair-plasma is achievable, a novel regime of laboratory-produced hot dense matter.