Harry D. Gafney

Excited-state acid-base chemistry of coordination complexes

Abstract Photoactivation of Ru(bpy)2dpp2+ (dpp=2,3-bis(2-pyridyl)pyrazine) in the presence of MCl6n− (M=Pt(IV), Rh(III), Pd(IV)) leads to quantitative formation of the corresponding bimetallic * Ru ( bpy ) 2 ( dpp ) 2+ + MCl 6 n − → [ Ru ( bpy ) 2 ( dpp ) MCl 4 ] 4 − n +2 Cl − Formation of the bimetallic does not occur by either electron- or energy-transfer processes. Instead, population of the MLCT state in Ru(bpy)2(dpp)2+, localized on the dpp ligand, increases the electron density on peripheral nitrogens as evidenced by an order of magnitude greater than six, increase in their Bronsted basicity relative to that of the ground state. The increased electron density promotes an excited-state coordination chemistry, where changes in coordination, rather than energy or electron transfer, leads to emission quenching and subsequent formation of the bimetallic. The enhanced basicity exists only in the excited state, and coordination of the second peripheral nitrogen, i.e. the chelate effect, competitive with relaxation of the excited state achieves the observed, thermodynamically stable product. This article summarizes the excited-state proton-transfer chemistry of Ru(II) diimine complexes, and discusses the application of this chemistry to excited-state coordination chemistry and the formation of bimetallic complexes.

Synthesis and spectroscopic properties of (Cr4+) doped sol-gels

The low temperature sol-gel process has been used for the preparation of Cr 4 -doped multicomponent gels and olivine group ceramic powders. Although absorption spectra indicate the presence of tetrahedrally coordinated Cr 4+ in the silicate-aluminate and germanate glasses, none exhibit near-IR emissions at room temperature. Cr(III) doping of Mg 2 SiO 4 , Ca 2 GeO 4 and the aluminates MAlO 2 (M = Li, Na and K) yield transparent materials with absorption spectra indicative of octahedrally coordinated Cr(III). Heating these transparent materials produces powders that exhibit absorption and near-IR emissions spectra characteristic of tetrahedrally coordinated Cr(IV).

Effect of photodeposited iron oxide and tin oxide on the consolidation of porous Vycor glass

Iron oxide and tin oxide have been photodeposited in porous Vycor glass and examined before and after consolidation of the glass. Scanning electron microscopy reveals that the iron oxide particles are larger than the tin oxide particles. However, small‐angle x‐ray scattering and Rutherford backscattering show that the glass consolidates about the iron oxide but not about the tin oxide. Photodeposition of tin oxide, which appears to chemically modify the glass surface and to prevent its consolidation, offers a means of producing highly resolved regions of porosity in the otherwise consolidated glass.

Iron and iron oxide particle growth in porous Vycor glass; correlation with optical and magnetic properties

Abstract The growth of elemental iron and iron oxide particles derived from the photolysis of Fe(CO) 5 adsorbed onto porous Vycor glass has been characterized and correlated with the optical and magnetic properties of the particles. Photolysis does not result in particle growth per se. Photolysis causes extensive diffusion of the precursor, which leads to a high surface coverage in the outer volumes of the glass and the formation of small, ⩽1 nm in diameter particles consisting of an elemental iron core with an Fe 2 O 3 coating. Particle growth occurs during the subsequent heating with the majority of growth occurring at temperatures of ⩽700 °C. The resultant particle size in the consolidated glass is equivalent to initial pore size in the glass, 10±1 nm, and the inter-particle spacing equivalent to the correlation length of the porous glass, 24±1 nm implying particle size and spacing are defined by the morphology of the porous glass. The 10±1 nm diameter particles consist of an ≈8 nm diameter Fe 0 core surrounded by 1 nm thick α -Fe 2 O 3 cladding. Particle growth is accompanied by a decline in optical transparency and a conversion from superparamagnetism to ferromagnetism with the particles exhibiting a large coercivity. The latter is attributed to an exchange anisotropy between the Fe 0 core and the oxide which becomes distributed throughout the body of the 10±1 nm diameter particles.

Cr K edge x-ray absorption study of Cr dopants in Mg2SiO4 and Ca2GeO4

X-ray absorption above the Cr K edge has been used to study two Cr-doped olivine-structure materials. For Mg2SiO4, the extended fine structure shows that Cr resides in octahedral and tetrahedral sites with Cr–O distances of 1.98±0.03 and 1.68±0.03 A, while the near edge structure indicates that Cr is in 3+ and 4+ oxidation states. For Ca2GeO4, site-size constraints limit Cr to the tetrahedral site with a Cr–O distance of 1.745±0.02 A, and this Cr is only in the 4+ oxidation state. This study illustrates the utility of x-ray absorption for probing and relating the oxidation states and sites of constituent elements. In this specific case of Cr in olivine hosts, such understanding is critical to the development of these materials as tunable infrared solid-state lasers: Cr in the 4+ state provides the desired emission centered near 1.3 μm.

Reactions of pentacarbonyltungsten adsorbed onto porous Vycor glass with various ligands

La photolyse UV de W(CO) 6 physisorbe sur du verre poreux Vycor, PVG, conduit au compose pentacarbonyle correspondant, qui reagit en presence de coordinats L bidentes, gazeux ou en solution, avec formation de W(CO) 5 L ou W(CO) 4 L. Explication des mecanismes du processus

EXCITED STATE ACID-BASE CHEMISTRY: A NEW QUENCHING MECHANISM

Abstract Most photochemical reactions degrade a metal complex, but recent experiments in this laboratory reveal a series of unusual photochemical reactions that increase the metal content of the complex. A 450 nm excitation of bis(2,2′-bipyridine)(2,3-bis(2-pyridyl)pyrazine)ruthenium(II), Ru(bpy) 2 dpp 2+ , complexes in the presence of PtCl 6 2− , PdCl 6 2− and RhCl 6 3− results in quantitative formation of the corresponding bimetallic [Ru(bpy) 2 (dpp)MCl 4 ] 2+ . The excited state energies and thermal redox potentials of the chlorides and the specificity of the reaction suggest that formation of the bimetallic is not the consequence of energy transfer or electron transfer quenching. Instead, we propose that these reactions are the result of an excited state acid-base chemistry that manifests itself as changes in coordination and nucleophilicity. The emissive MLCT state in Ru(bpy) 2 (dpp) 2+ is localized on dpp, and H + quenching indicates that population of the emissive MLCT state increases the basicity of at least one of the peripheral nitrogens by at least four orders of magnitude relative to that in the ground state. Population of the MLCT state (bpy) 2 Ru(CN) 2 , on the other hand, shifts charge density to the bpy ligand thereby decreasing the basicity of the cyanide ligand by ca. four orders of magnitude. The pH dependence of the quantum yield of decomposition of trans -[(bpy) 2 Ru(CN)(μ-CN)Rh(NH 3 ) 4 Br] 2+ suggests that this immense change in basicity leads to a dissociative excited state. In complexes that exhibit excited state acid-base properties, the data presented here suggest that quenching is not necessarily limited to electron or energy transfer, but also occurs by an excited state acid-base chemistry that leads to changes in coordination and/or nucleophilicity.

High coercivity single‐domain particles in glass matrix

Impregnating transition metallic compounds into porous glass is a simple way to form ultrafine particles in a glass matrix. In our studies, ultrafine particles were prepared by impregnating Fe compounds into porous glasses (pore size 70±21 A) and annealing. After annealing, the samples were studied at different temperatures with a vibrating sample magnetometer. In the annealedglasses two types of particles were formed: (a) superparamagnetic particles and (b) ferromagnetic particles with exceedingly high coercivity (Hc=13 kOe at 298 K). The ratio between superparamagnetic and high coercivity particles depends on the annealing temperature and time. In samples annealed at lower temperatures or at high temperature for short times, only superparamagnetic particles were found. In those annealed at high temperatures after certain times, only high coercivity particles were found. It was also found that the blocking temperature T B of the high coercivity particles depends on the annealing process. Moreover it was found that the Curie temperature of the high coercivity particles is below 520 K.

Microstructure of photodeposited Fe in porous Vycor glass

The products derived from the UV photolysis of Fe(CO)5 physisorbed onto porous Vycor glass have been examined by x‐ray microprobe analysis, Mossbauer spectroscopy, and extended x‐ray absorption fine structure spectroscopy (EXAFS). Although the photolysis and subsequent heatings are carried out in air, spectroscopic data reveal two surprisingly different forms of iron. Isomer shift, pre‐edge, and EXAFS data indicate that one product is similar to α‐Fe2O3 and consists of an Fe3+ ion octahedrally surrounded by six oxygen atoms at a distance of 1.8 A. Isomer shift, EXAFS, and pre‐edge feature analysis show that the second compound, which comprises ∼50% of the reaction product, is mainly elemental iron, in which a central Fe atom is surrounded by approximately eight other Fe atoms at a distance of 2 A. X‐ray microprobe analysis shows that aggregation occurs during photolysis due to the diffusion of Fe(CO)5 from the interior into exterior photodepleted volumes of glass. Heating has little effect on product rati...

Influence of amorphous silica matrices on the formation, structure, and chemistry of iron and iron oxide nanoparticles.

Fe(CO)(5) physisorbs onto Cornings code 7930 porous Vycor glass (PVG) and dried (<or=200 degrees C), base-catalyzed (NH(3)) tetramethoxysilane/methanol/water xerogels. Although chemically and structurally similar matrices, 488-nm photolysis of the physisorbed complex yields ca. equal amounts of Fe(0) and Fe(2)O(3) in PVG, but only Fe(2)O(3) in the xerogel. Mossbauer, EXAFS, and XANES results give no indication the photoproducts bind to either silica matrix, and consolidation of the PVG matrix leads to Fe(0)-Fe(2)O(3) nanoparticle formation with little change in the Fe(0)/Fe(III) ratio. PVG serves as a template defining the particle diameter and interparticle spacing, whereas consolidation of the xerogel does not result in nanoparticle formation. Instead, ca. 20% of the octahedrally coordinated Fe(III) converts to tetrahedral coordination during consolidation. The photoproducts within these porous silica matrices reflect a competition between aggregation and oxidation, where the extent and most likely the rate of aggregation are functions of the correlation lengths of these amorphous matrices. With a correlation length of 22 +/- 1 nm, aggregation exceeds oxidation in PVG and limits oxidation to the outer periphery, thereby creating particles whose Fe(0)/Fe(III) ratio is unaffected by air or water released during consolidation of the silica matrix. The correlation length of the xerogel, <or=1 nm, limits aggregation of the primary photoproduct and favors smaller particles. As a result, the primary photoproducts in the xerogel do not achieve sufficient size to limit oxidation to the outer periphery of the particle, and the primary photoproduct oxidizes, forming only Fe(2)O(3). Desorption of decomposition products derived from the xerogel precursors creates a dynamic surface that limits nanoparticle growth during annealing. Desorption also disrupts the growing silicate matrix, creating sites that facilitate the change from octahedrally to tetrahedrally coordinated Fe(III) in the xerogel.