Influence of ion pairing on the oxidation of iodide by MLCT excited states

The oxidation of iodide to diiodide, I(2)˙(-), by the metal-to-ligand charge-transfer (MLCT) excited state of [Ru(deeb)(3)](2+), where deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, was quantified in acetonitrile and dichloromethane solution at room temperature. The redox and excited state properties of [Ru(deeb)(3)](2+) were similar in the two solvents; however, the mechanisms for excited state quenching by iodide were found to differ significantly. In acetonitrile, reaction of [Ru(deeb)(3)](2+*) and iodide was dynamic (lifetime quenching) with kinetics that followed the Stern-Volmer model (K(D) = 1.0 ± 0.01 × 10(5) M(-1), k(q) = 4.8 × 10(10) M(-1) s(-1)). Excited state reactivity was observed to be the result of reductive quenching that yielded the reduced ruthenium compound, [Ru(deeb(-))(deeb)(2)](+), and the iodine atom, I˙. In dichloromethane, excited state quenching was primarily static (photoluminescence amplitude quenching) and [Ru(deeb(-))(deeb)(2)](+) formed within 10 ns, consistent with the formation of ion pairs in the ground state that react rapidly upon visible light absorption. In both solvents the appearance of I(2)˙(-) could be time resolved. In acetonitrile, the rate constant for I(2)˙(-) growth, 2.2 ± 0.2 × 10(10) M(-1) s(-1), was found to be about a factor of two slower than the formation of [Ru(deeb(-))(deeb)(2)](+), indicating it was a secondary photoproduct. The delayed appearance of I(2)˙(-) was attributed to the reaction of iodine atoms with iodide. In dichloromethane, the growth of I(2)˙(-), 1.3 ± 0.4 × 10(10) M(-1) s(-1), was similar to that in acetonitrile, yet resulted from iodine atoms formed within the laser pulse. These results are discussed within the context of solar energy conversion by dye-sensitized solar cells and storage via chemical bond formation.     

Adamantanes as spherical nanosondes in adducts with a chiral dirhodium complex-discriminating enantiomers and probing spatial proximities

Three different kinds of substituted chiral adamantane molecules—adamantanones, dioxolanoadamantanes and dithiolano—adamantanes—were studied in the dirhodium experiment (NMR measurement with 1:1 molar mixtures with Rh^(II)₂[(R)‐(+)‐MTPA]₄ in CDCl₃). Their different behavior in adduct formation is described, and the possibility of determining enantiomeric purities and absolute configurations is explored. Detailed inspection of one‐ and two‐dimensional NMR experiments allowed for an interpretation of steric and electronic intra‐adduct interaction showing that the phenyl groups of Rh* tend to enwrap the bound adamantane ligand so that through‐space effects over a range of 6–7 Å away from the binding rhodium atom can be observed. Even slight differences in the relative orientation of phenyl groups can be monitored when comparing diastereomeric adducts via NMR signal dispersion. Copyright © 2011 John Wiley & Sons, Ltd.     

One- and two-dimensional translational energy distributions in the iodine-loss dissociation of 1,2-C2H4I2+ and 1,3-C3H6I2+: what does this mean?

Threshold photoelectron photoion coincidence (TPEPICO) has been used to study the sequential photodissociation reaction of internal energy selected 1,2-diiodoethane cations: C(2)H(4)I(2)(+) → C(2)H(4)I(+) + I → C(2)H(3)(+) + I + HI. In the first I-loss reaction, the excess energy is partitioned between the internal energy of the fragment ion C(2)H(4)I(+) and the translational energy. The breakdown diagram of C(2)H(4)I(+) to C(2)H(3)(+), i.e., the fractional ion abundances below and above the second dissociation barrier as a function of the photon energy, yields the internal energy distribution of the first daughter, whereas the time-of-flight peak widths yield the released translational energy in the laboratory frame directly. Both methods indicate that the kinetic energy release in the I-loss step is inconsistent with the phase space theory (PST) predicted two translational degrees of freedom, but is well-described assuming only one translational degree of freedom. Reaction path calculations partly confirm this and show that the reaction coordinate changes character in the dissociation, and it is, thus, highly anisotropic. For comparison, data for the dissociative photoionization of 1,3-diiodopropane are also presented and discussed. Here, the reaction coordinate is expected to be more isotropic, and indeed the two degrees of freedom assumption holds. Characterizing kinetic energy release distributions beyond PST is crucial in deriving accurate dissociative photoionization onset energies in sequential reactions. On the basis of both experimental and theoretical grounds, we also suggest a significant revision of the 298 K heat of formation of 1,2-C(2)H(4)I(2)(g) to 64.5 ± 2.5 kJ mol(-1) and that of CH(2)I(2)(g) to 113.5 ± 2 kJ mol(-1) at 298 K.     

Bonding in a borylene complex investigated by photoionization and dissociative photoionization

The borylene complex [(OC)(5)Cr=B=N(SiMe(3))(2)] has been investigated by using threshold photoelectron-photoion coincidence spectroscopy with synchrotron radiation. The ionization energy of the parent complex and the 0?K appearance energies of the sequential CO loss channels have been determined. The derived bond-dissociation energies are used to discuss bonding and energetics in this compound.     

Grating coupled interferometry for optical sensing

An interferometric sensor based on gratings on a planar waveguide is introduced. The device combines the advantages of known interference-based waveguide sensors with the simplicity of grating couplers. In the presented configuration, two parallel and coherent light beams, laterally separated in the direction of mode propagation, are coupled into a planar waveguide through a grating. One of the coupled beams is phase modulated using a periodically relaxing liquid crystal modulator, resulting in a time varying intensity signal at the end face of the waveguide. Refractive index changes within the waveguide section between the two coupling regions are monitored by observing characteristic changes in the intensity signal.     

Dye-sensitized SnO2 electrodes with iodide and pseudohalide redox mediators

Dye-sensitized mesoporous nanocrystalline SnO2 electrodes and the pseudohalogen redox mediator (SeCN)2/SeCN- or (SCN)2/SCN- or the halogen redox mediator I3-/I- were implemented for regenerative solar cell studies. Adsorption isotherms of the sensitizers Ru(deeb)(bpy)2(PF6)2, Ru(deeb)2(dpp)(PF6)2, and Ru(deeb2(bpz)(PF6)2, where deeb is 4,4'-diethylester-2,2'-bipyridine, dpp is 2,3-dipyridyl pyrazine, and bpz is bipyrazine, binding to the SnO2 surface were well described by the Langmuir model from which the saturation coverage, Gamma0 = 1.7 x 10(-8) mol/cm2, and surface-adduct formation constant, Kad = 2 x 10(5) M(-1), were obtained. Following excited-state interfacial electron transfer, the oxidized sensitizers were reduced by donors present in the acetonitrile electrolyte as shown by transient absorption spectroscopy. With iodide as the donor, a rate constant k > 10(8) s(-1) was measured for sensitizer regeneration. In regenerative solar cells, it was found that the incident photon-to-current conversion efficiencies and open circuit voltages (Voc) were comparable for (SeCN)2/SeCN- and I3-/I- for all three sensitizers. The Voc varied linearly with the logarithm of the short circuit photocurrent densities (Jsc), with typical correlations of approximately 50-60 mV/decade. Capacitance measurements of the SnO2 electrode in the presence of I3-/I-, (SeCN)2/SeCN- or (SCN)2/SCN- are reported.     

Isothiocyanate controlled architecture,spectroscopic, and magnetic behavior of copper(II) l–arginine complexes

Abstract

We synthesized an l–arginine complex with the formula [Cu(l–Arg)₂(NCS)]·(NCS)·H₂O (1) (l–Arg = l–arginine). Two cis-chelated l–arginine zwitterions form the basal plane, while the weakly N-bonded isothiocyanate is located at the apex of the distorted square pyramidal structure (τ?=?0.143). The non-coordinated NCS^? anions held layers together in a 3-D supramolecular network. The crystal structure, spectroscopic (FT–IR, Raman, NIR–Vis–UV, EPR) and magnetic properties of 1 have been compared with [Cu(l–Arg)(NCS)₂] (2). For 1, two absorptions are observed for ν(C?=?N) stretching vibrations, corresponding to NCS^? ions N-bonded to the central Cu(II) (2077?cm^?1) and in the lattice (2057?cm^?1). In 2 a single band is observed at 2102?cm^?1, indicating equivalent NCS^? ions in the structure. The EPR spectra of complexes show anisotropic signal with g_⊥ and g_|| 2.062, 2.235 (1), and 2.08, 2.225 (2) characteristic for cis-N₂O₂ and N₃O donor sets in the xy plane, respectively. The unpaired electron mainly occupies the d_x2–y2 orbital, also confirmed by the single envelope of d–d bands at ca. 16,000?cm^?1 for 1 and 16,500?cm^?1 for 2. The magnetic properties ofcompounds are characteristic of a very weak antiferromagnetic interaction with J?=??0.055?cm^?1 and J?=??0.096?cm^?1 for 1 and 2, respectively.     

Determination of alpha-tocopherol in infant foods by liquid chromatography combined with atmospheric pressure chemical ionisation mass spectrometry

A novel, sensitive and specific method for the quantification of alpha-tocopherol in two infant foods (milk and cereals) using liquid chromatography on-line with positive atmospheric pressure chemical ionisation mass spectrometry detection (LC/APCI-MS) has been developed. The samples were first saponified in order to eliminate fats and to transform tocopherol esters into free tocopherol, followed up by a liquid-liquid extraction of the analyte in petroleum benzine/diisopropyl ether (75:25, v/v) prior to injection onto the LC system. For the quantification, deuterium-labelled tocopherol was used as internal standard and the samples were monitored in selected ion monitoring (SIM) mode. Calibration curves between 1-40 microg/mL of alpha-tocopherol showed a good linear correlation (r(2) = 0.99994), and the detection limit was determined to be 2.5 ng/mL. The within-day and between-day precision were determined for several dietetic infant formulae and certified reference samples, and found to be below 3.5%. The accuracy determined on a Nestlé reference sample (milk powder) was calculated to be 115.2 +/- 1.2%, which confirms the robustness of the proposed method. This study shows that single quadrupole LC/MS can be applied for the quantification of vitamins in food and the method offers better sensitivity and selectivity than traditional method such as LC-UV. This would simplify the preparation of the food samples and consequently enhance the vitamin analysis throughput in the food area.     

Targeted borate complex formation as followed with electrospray ionization Fourier transform ion cyclotron mass spectrometry: monomolecular model system and polyborate formation

Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) was applied to identify boric acid(B degrees)/borate(B-) complexes in a monomolecular model system, composed of aqueous caffeic acid and boric acid/borate solutions in various concentration ratios at pH 9.2. Using negative mode electrospray ionization as a 'soft' ionization technique, clusters of polyborate were detected beside the well-known BL degrees , BL- and BL2- complexes. An algorithm for the detection of boron complexes, based on their accurate mass and isotope patterns, is proposed which enabled the assignment of elemental compositions/structural formulae of boron/ligand complexes. We present experimental evidence of self-oligomerization of up to six borate units with caffeic acid, resulting in stable covalently bound polyborate-polyol complexes.