Molecular switches of fluorescence operating through metal centred redox couples

Three-component molecular systems (redox active subunit)-spacer-(light-emitting fragment) can operate as fluorescence switches, following the alternate addition of an oxidizing agent and a reducing agent (or the adjustment of the potential of the working electrode in an electrolysis experiment). The redox active subunit typically consists of a metal centred redox couple (M⁽ⁿ⁺¹⁾⁺/Mⁿ⁺), encircled by a macrocyclic receptor, and switching efficiency requires that one of the two oxidation states quenches the proximate fluorophore and the other does not. Four ON/OFF systems, based on either the Cu^II/Cu^I or Ni^III/Ni^II couple, will be discussed. The nature of the quenching process responsible for the OFF state, either electron transfer or energy transfer, is related to the length and to the flexibility-rigidity of the spacer.     

Low-temperature IR and NMR studies of the interaction of group 8 metal dihydrides with alcohols

The reactions of the octahedral dihydrido complexes [MH(2)(PP(3))] [M=Fe, Ru, Os; PP(3)=P(CH(2)CH(2)PPh(2))(3)] with a variety of weak ROH acids have been studied by IR and NMR methods in either CH(2)Cl(2) or THF in the temperature range from 190 to 290 K. This study has allowed the determination of the spectral and thermodynamic properties associated with the formation of dihydrogen bonds (DHB) between the terminal hydrides and the OH group. Both the DHB enthalpy values and the hydride basicity factors (E(j)) have been found to increase in the order Fe < Ru < Os. The proton transfer process, leading to the DHB complexes, and eventually to eta(2)-H(2) products, has been found to depend on the acidic strength of the alcohol as well as the nature of the solvent. Low temperature IR and NMR techniques have been used to trace the complete energy profile of the proton transfer process involving the osmium complex [OsH(2)(PP(3))] with trifluoroethanol.     

Allopurinol and its interactions with Cu<Superscript>2+</Superscript> ions: radical reactions and complex structure

Allopurinol (ALP), an inhibitor of the xanthine oxidase enzime, is reported to provide protection against free-radical mediated damage by various mechanisms, including free-radical scavenging and metal chelation (i.e. Cu(II)). To obtain a wider insight into the molecular aspects of the beneficial action of ALP, free ALP and the Cu(II)- ALP system were investigated by radiation chemical and spectroscopic studies. Pulse radiolysis experiments show that ALP is a good · OH scavenger (1.8 × 10⁹ and 5.4 × 10⁹ M^-1 s^-1 at pH 6.0 and 11.0, respectively), leading to · OH-adducts and transient semi-oxidized species, such as phenoxyl radicals. The latter are also formed by the reaction of ALP with some specific one-electron oxidizing radicals (i.e. N₃ and SO₄ ^-). The semi-oxidized species are stabilized by their resonance properties and scarcely react with oxygen. In addition, the chelation of Cu(II) by ALP does not significantly affect the reactivity of the drug towards ·OH (2.5×0⁹ M^-1 s^-1). Raman and the IR spectra support the good chelating ability of ALP, indicating the formation of two Cu(II)- ALP complexes with a slightly different structure. Depending on the metal/ligand ratio, pyrimidine nitrogens may take part to the Cu(II) co-ordination in addition to the N pyrazolic atoms and the C O groups of some ALP molecules. These results suggest that ALP may inhibit oxidative damage both through the direct radical scavenging and the copper-chelation mechanism. In fact, both the conversion of a harmful radical, such as ·OH, into a less reactive transient species, and the capture of copper ions, which play a relevant role in metal-catalysed generation of reactive oxygen species, will prove beneficial for the cell protection.     

Modulating charge-transfer interactions in topologically different porphyrin-C60 dyads

Control over the interchromophore separation, their angular relationship, and the spatial overlap of their electronic clouds in several ZnP-C(60) dyads (ZnP=zinc porphyrin) is used to modulate the rates of intramolecular electron transfer. For the first time, a detailed analysis of the charge transfer absorption and emission spectra, time-dependent spectroscopic measurements, and molecular dynamics simulations prove quantitatively that the same two moieties can produce widely different electron-transfer regimes. This investigation also shows that the combination of ZnP and C(60) consistently produces charge recombination in the inverted Marcus region, with reorganization energies that are remarkably low, regardless of the solvent polarity. The time constants of electron transfer range from the mus to the ps regime, the electronic couplings from a few tens to several hundreds of cm(-1), and the reorganization energies remain below 0.54 eV and can be as low as 0.16 eV.     

Isomeric alkyl cation/arene complexes in the gas phase

The kinetics and the stereochemistry of the protonation-induced unimolecular isomerization of (S)-(+)-1-D(1)-3-(p-tolyl)butane have been investigated in the gas phase in the 100-160 degrees C range. The process leads to the almost exclusive formation of the relevant meta isomer with complete racemization and partial 1,2-H shift in the migrating sec-butyl group. These results, together with the relevant activation parameters, point to the occurrence of low-energy, tightly bound isomeric sec-butyl cation/toluene complexes of defined structure and stability along the isomerization coordinate. The existence and the eta(1)-type structure of these low-energy intermediate species are confirmed by ab initio calculations on closely related systems at the MP2(full)/6-311++G**//HF/6-31+G** level of theory. Their role in the relevant energy surface clearly emerges from the comparison of the present results with those concerning sec-butylation of toluene carried out under comparable experimental conditions.     

Experimental and theoretical study of the interaction of CO2 with alpha-Al2O3

Density functional molecular cluster calculations are combined with X-ray photoelectron spectroscopy (XPS), quadrupolar mass spectrometry (QMS), and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy to investigate the interaction of CO2 with alpha-Al2O3 and partially reduced alpha-Al2O3. The electronic structure of the stoichiometric and partially reduced substrate, adsorbate geometries, chemisorption enthalpies, and adsorbate vibrational parameters are computed and discussed. Theoretical results agree quite well with experimental data and previous theoretical investigations. As far as the adsorbate-substrate interaction is concerned, the results of our calculations indicate that CO2 forms bidentate-chelating carbonate species. The bonding scheme of this surface complex implies a significant substrate-->adsorbate transfer of charge (from the occupied dangling bond of a surface Lewis base site into one component of the CO2 2 pi u LUMO) assisted by a definitely weaker adsorbate-->substrate donation (from one component of the CO2 1 pi g HOMO into an empty dangling bond of a surface Lewis acid site). Our estimate of the chemisorption enthalpy (-15 kcal/mol) agrees quantitatively with calorimetric data reported for CO2 adsorbed on high surface area alpha-alumina (-16.0 kcal/mol). [Mao, C.-F.; Vannice, M. A. Appl. Catal. A 1994, 111, 151.] According to XPS and QMS outcomes, theoretical results predict that the interaction of CO2 with partially reduced alpha-Al2O3 gives rise to the reduction of the adsorbate to CO and to the concomitant substrate reoxidation.     

Free-radical selective functionalization of 1,4-naphthoquinones by perfluorodiacyl peroxides

Perfluoroalkyl radicals, generated by thermal decomposition of perfluorodiacyl peroxides, react selectively with quinone rings of 1,4-naphthoquinones. In the presence of a non-conjugated alkene such as 1-hexene, perfluoroalkyl radicals add to the double bonds of the olefin forming a radical adduct, which selectively adds to the naphthoquinone ring. Several perfluorodiacyl peroxides have been synthesized and used for the direct and alkene-mediated functionalization of naphthoquinones. Geometrical parameters and electron density topology of all perfluorodiacyl peroxides have been calculated by the density functional formalism and quantum theory of atoms in molecules to attempt a rationalization of the experimental reactivity.     

(Co)oxidation/cyclization processes upon irradiation of triphenylamine

Irradiation of triphenylamine (Ph₃N) in nitrogen-flushed solution leads to 9-phenylcarbazole and two tetrahydroderivatives (1,2,3,4- and 1,2,7,8-) via disproportionation of the corresponding 4a,4b-dihydrocarbazole. In oxygen-equilibrated solution oxidative cyclization occurs through the intermediacy of a triplet peroxy diradical, which either abstracts a hydrogen atom intramolecularly or (mainly) cleaves back to the reagents. The role of the key intermediates is supported by DFT calculations and by trapping by triarylphosphines (that are thus efficiently oxidized, while preventing the cyclization of Ph₃N). The hydroperoxide, on the other hand, causes inefficient co-oxidation of sulfides.     

Role of the solvent on the stability of cycloserine under ESI‐MS conditions

The effects of methanol (M) and acetonitrile (A) on the stability of cycloserine (1) have been studied. InfraRed Multiphoton PhotoDissociation (IRMPD) spectroscopy of the ionic species from electrospray ionization tandem mass spectrometry (ESI‐MS) of 1/M and 1/A solutions points to extensive dimerization of 1 to cis‐3,6‐bis(aminooxymethyl)‐2,5‐piperidinedione (2), while the same process is not observed in the ESI‐MS of 1/M solutions. 1D and 2D nuclear magnetic resonance experiments confirmed these findings by showing that partial dimerization of 1 actually takes place at room temperature in acetonitrile even before ESI‐MS analysis. Comparison of nuclear magnetic resonance and IRMPD spectroscopic data from the same 1/A solution suggests that dimerization of cycloserine is enhanced in the ESI source. Copyright © 2014 John Wiley & Sons, Ltd.