Structural changes induced on strontium barium niobate glass by femtosecond laser irradiation

Localized modification of the optical properties of erbium doped strontium barium niobate (SBN) glass has been performed using femtosecond laser irradiation. The samples, with composition SrO–BaO–Nb₂O₅–B₂O₅ and doped with 5%mol of Er³⁺, were fabricated using a melt-quenching method. The samples were irradiated with different number of pulses per spot (1–50 pulses) at two different laser fluences (2.6 and 5.6 J/cm²) by using an fs laser amplifier operating at 800 nm and generating pulses with a duration of 120 fs. Micro-luminescent microscopy, using an Ar⁺ laser as excitation source, has been used to analyze the modifications of the luminescent properties of the sample upon fs laser exposure. The emissions of the Er³⁺: ⁴I_11/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2 transitions allow appreciating the structural modifications caused by femtosecond laser exposure. The lifetimes of the levels involved in these transitions were measured inside and outside the laser irradiated region. These measurements have been compared with those obtained in bulk glass ceramic sample, which is obtained from the glass precursor by a thermal treatment in order to estimate the optimal conditions to produce nanocrystals in a localized region by ultrafast laser irradiation.     

Titanium Dioxide/Electrolyte Solution Interface: Electron Transfer Phenomena

Electron transfer between a titanium dioxide/electrolyte solution interface has been studied. As found by other researchers of similar interfaces (TiO(2)- and ZnO-electrolyte solution), a slow consumption of OH(-) ions takes place in this type of interface. A theoretical model has been developed for calculating the change in the Fermi energy of both electrolyte solution and semiconductor, showing that ion consumption from the solution is favoured by the decrease of the difference between their Fermi energies. A kinetic constant (upsilon) is found to characterise the consumption process, its value increasing with electrolyte and semiconductor mass concentrations. Furthermore, this process may be used to estimate the point of zero charge of a titanium dioxide colloidal dispersion. Copyright 2000 Academic Press.     

Synthesis of paramagnetic tetranuclear rhodium and iridium complexes with the 2,6-pyridinedithiolate ligand. Redox-induced degradation to diamagnetic triiridium compounds

The tetranuclear complexes [M4(mu-PyS2)2(diolefin)4] [PyS2 = 2,6-pyridinedithiolate; M = Rh, diolefin = cod (1,5-cyclooctadiene) (1), tfbb (tetrafluorobenzo[5,6]bicyclo[2.2.2]octa-2,5,7-triene) (2); M = Ir, diolefin = cod (3), tfbb (4)] exhibit two one-electron oxidations at a platinum disk electrode in dichloromethane at potentials accessible by chemical reagents. The rhodium tetranuclear complexes were selectively oxidized to the monocationic complexes [Rh4(mu-PyS2)2(diolefin)4](+) (1(+), 2(+)) by mild one-electron oxidants such as [Cp2Fe](+) or [N(C6H4Br-4)3](+) and isolated as the PF6(-), BF4(-), and ClO4(-) salts. Silver salts behave as noninnocent one-electron oxidants for the reactions with the rhodium complexes 1 and 2 since they give sparingly soluble coordination polymers. The complex [Ir4(mu-PyS2)2(cod)4](+) (3(+)) was obtained as the tetrafluoroborate salt by reaction of 3 with 1 molar equiv of AgBF4, but the related complex 4(+) could not be isolated from the chemical oxidation of [Ir4(mu-PyS2)2(tfbb)4] (4) with AgBF4. Oxidation of 3 and 4 with 2 molar equiv of common silver salts resulted in the fragmentation of the complexes to give the diamagnetic triiridium cations [Ir3(mu-PyS2)2(diolefin)3](+). The molecular structure of [Ir3(mu-PyS2)2(cod)3]BF4, determined by X-ray diffraction methods, showed the three metal atoms within an angular arrangement. Both 2,6-pyridinedithiolate tridentate ligands bridge two metal-metal bonded d(7) centers in pseudo octahedral environments and one d(8) square-planar iridium center. An interpretation of the EPR spectra of the 63-electron mixed-valence paramagnetic tetranuclear complexes suggests that the unpaired electron is delocalized over two of the metal atoms in the complexes 1(+)-3(+).     

The complete characterization of a rhodium Lewis acid-dipolarophile complex as an intermediate for the enantioselective catalytic 1,3-dipolar cycloaddition of C,N-diphenylnitrone to methacrolein

The 1,3-dipolar cycloaddition reaction of C,N-diphenylnitrone with methacrolein is efficiently catalyzed by the rhodium diphosphine compound (SRh,RC)-[(eta5-C5Me5)Rh(R-Prophos)(H2O)](SbF6)2 [R-Prophos = (R)-(+)-1,2-bis(diphenylphosphino)-propane, 1.SbF6]; the asymmetric catalytic process occurs with reversal of regioselectivity, perfect endo selectivity, and up to 92% ee. The complete (NMR and X-ray analysis) characterization of the involved intermediate (SRh,RC)-[(eta5-C5Me5)Rh(R-Prophos)(methacrolein)](SbF6)2 (7.SbF6) allows us to interpret the observed selectivity.     

Controlled Synthesis of Early–Late Zr-Ir-Rh Heterotrimetallic Compounds by Metal Exchange Reactions

Sulfido ligands meet the coordination and electronic requirements for stabilizing compounds containing divergent metal centers in close proximity, as shown by the controlled syntheses of early–late bimetallic (ZrIr₂ and ZrIr) and trimetallic complexes (ZrIrRh). The slow rotation of one 1,3-di-tert-butylcyclopentadienyl (Cp^tt) ring in 1 (structure shown) allows the observation of two rotamers at room temperature. cod=1,5-cyclooctadiene. [Cp^tt₂Zr(μ₃-S)₂{Ir(CO)₂}{Rh(cod)}] 1     

Rhodium and Iridium Pyrazolato Blues

The staggered arrangement of the two dimers in 1 facilitates the formation of an almost linear chain of four metal centers that is held together by a nonsupported metal–metal bond. The intense color of the compounds and the fractional oxidation states of the metal atoms can be explained by electron delocalization along the chain. M=Rh, Ir.     

Binuclear Oxidative Addition of Hydrogen in Diamidonaphthalene-Bridged Diiridium Complexes

Binuclear iridium(ii) complexes readily activate H ₂ , in contrast to the behavior of related diiridium(I ) species. The reaction occurs at the pocket of the binuclear complex to produce dihydridoiridium(III ) complexes containing a bridging and a terminal hydride. Due to their Brønsted acidity, these dihydrido complexes can react with a second molecule of H₂ to form trihydrido complexes. This process implies the heterolytic activation of hydrogen.