Time-dependent Raman analysis of metal-to-ligand charge transfer excited states: Application to radiative and nonradiative decay

The photophysical properties of the emitting metal-to-ligand charge transfer (MLCT) excited states of the complexes, [Os(bpy)3]2+, [Os(bpy)2(py)2]2+, and [Os(bpy)(py)4]2+ (bpy = 4-4′-bipyridine, py = pyridine) have been characterized in aqueous solution at room temperature by absorption, emission, and Raman spectroscopies and by emission lifetimes and emission quantum yields. A spectroscopic model has been developed by using the time-dependent theory of Raman scattering, taking into account interference effects on resonance Raman profiles arising from interactions between the different ligands. A model based on the cylindrical model of Sension and Strauss provides a good fit to the data. The mode-specific vibrational parameters obtained from the spectroscopic analysis are used to calculate the vibrational contributions to the radiative and nonradiative decay rate constants for each of the complexes. These results and the experimental rate constants were used to calculate vibrationally induced electronic coupling matrix elements (Vk) for nonradiative decay and also transition moments, M, for radiative decay. For radiative decay, the average transition moment for the three complexes was 0.05Å, and for nonradiative decay, the average value of Vk was 910 cm-1. Within a reasonable margin of error, the Franck-Condon contributions are in agreement with values obtained in a previous study that used the single mode approximation and a Franck-Condon analysis of emission spectra

Ratchet growth in recycled PBX 9502

PBX 9502 is a plastic-bonded high explosive (PBX) containing 95 weight% TATB (triaminotrinitrobenzene) crystals in a polymer binder. TATB crystals are graphitic in nature, with a sheet-like structure and anisotropic CTE. Although the mechanism is not understood, solid-pressed TATB composites have been observed to undergo irreversible volume change ('ratchet growth') upon thermal cycling . This phenomenon has been studied but many aspects remain elusive and uncharacterized. Engineering or performance changes associated with ratchet growth have often been attributed to changes in density alone. We propose that the density changes which accompany ratchet growth involve a unique form of micro-damage distinguishable from the pore structure associated with low-pressed density. We have performed ratchet growth studies on Recycled PBX 9502 between -54 to 80{sup o}C with density changes of about 1.5%. Specimens of the same density were obtained using a lower pressure in the manufacturing process. Comparative measurements were made using quasi-static uniaxial tension tests, as well as micro x-ray computed tomography and ultra-small angle neutron scattering experiments. Through these measurements we have shown that ratchet grown PBX 9502 has properties quite different from predictions based on density alone. The pore size distribution of ratchet grown specimens is unique and easily distinguished from parts pressed to an equivalent density

Electronic structure of the water oxidation catalyst cis, cis -[(bpy) 2(H 2O)Ru IIIORu III(OH 2)(bpy) 2] 4+, the blue dimer

The first designed molecular catalyst for water oxidation is the "blue dimer", cis,cis-[(bpy) 2(H 2O)Ru IIIORu III(OH 2)(bpy) 2] 4+. Although there is experimental evidence for extensive electronic coupling across the μ-oxo bridge, results of earlier DFT and CASSCF calculations provide a model with magnetic interactions of weak to moderately coupled Ru III ions across the μ-oxo bridge. We present the results of a comprehensive experimental investigation, combined with DFT calculations. The experiments demonstrate both that there is strong electronic coupling in the blue dimer and that its effects are profound. Experimental evidence has been obtained from molecular structures and key bond distances by XRD, electrochemically measured comproportionation constants for mixed-valence equilibria, temperature-dependent magnetism, chemical properties (solvent exchange, redox potentials, and pK a values), XPS binding energies, analysis of excitation-dependent resonance Raman profiles, and DFT analysis of electronic absorption spectra. The spectrum can be assigned based on a singlet ground state with specific hydrogen-bonding interactions with solvent molecules included. The results are in good agreement with available experimental data. The DFT analysis provides assignments for characteristic absorption bands in the near-IR and visible regions. Bridge-based dπ → dπ* and interconfiguration transitions at Ru III appear in the near-IR and MLCT and LMCT transitions in the visible. Reasonable values are also provided by DFT analysis for experimentally observed bond distances and redox potentials. The observed temperature-dependent magnetism of the blue dimer is consistent with a delocalized, diamagnetic singlet state (dπ 1*) 2 with a low-lying, paramagnetic triplet state (dπ 1*) 1(dπ 2*) 1. Systematic structural-magnetic-IR correlations are observed between ν sym(RuORu) and ν asym(RuORu) vibrational energies and magnetic properties in a series of ruthenium-based, μ-oxo-bridged complexes. Consistent with the DFT electronic structure model, bending along the Ru-O-Ru axis arises from a Jahn-Teller distortion with Ru-O-Ru dictated by the distortion and electron-electron repulsion. © 2012 American Chemical Society.Fil: Jurss, Jonah W.. University of North Carolina; Estados UnidosFil: Concepcion, Javier J.. University of North Carolina; Estados UnidosFil: Butler, Jennifer M.. Los Alamos National Laboratory; Estados UnidosFil: Omberg, Kristin M.. Los Alamos National Laboratory; Estados UnidosFil: Baraldo Victorica, Luis Mario. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Los Alamos National Laboratory; Estados UnidosFil: Thompson, Darla Graff. Los Alamos National Laboratory; Estados UnidosFil: Lebeau, Estelle L.. Los Alamos National Laboratory; Estados UnidosFil: Hornstein, Brooks. Los Alamos National Laboratory; Estados UnidosFil: Schoonover, Jon R.. Los Alamos National Laboratory; Estados UnidosFil: Jude, Hershel. Los Alamos National Laboratory; Estados UnidosFil: Thompson, Joe D.. Los Alamos National Laboratory; Estados UnidosFil: Dattelbaum, Dana M.. Los Alamos National Laboratory; Estados UnidosFil: Rocha, Reginaldo C.. Los Alamos National Laboratory; Estados UnidosFil: Templeton, Joseph L.. University of North Carolina; Estados UnidosFil: Meyer, Thomas J.. University of North Carolina; Estados Unido

Electronic Structure of the Water Oxidation Catalyst <i>cis</i>,<i>cis</i>-[(bpy)₂(H₂O)Ru^IIIORu^III(OH₂)(bpy)₂]⁴⁺, The Blue Dimer

The first designed molecular catalyst for water oxidation is the “blue dimer”, <i>cis</i>,<i>cis</i>-[(bpy)₂(H₂O)­Ru^IIIORu^III(OH₂)­(bpy)₂]⁴⁺. Although there is experimental evidence for extensive electronic coupling across the μ-oxo bridge, results of earlier DFT and CASSCF calculations provide a model with magnetic interactions of weak to moderately coupled Ru^III ions across the μ-oxo bridge. We present the results of a comprehensive experimental investigation, combined with DFT calculations. The experiments demonstrate both that there is strong electronic coupling in the blue dimer and that its effects are profound. Experimental evidence has been obtained from molecular structures and key bond distances by XRD, electrochemically measured comproportionation constants for mixed-valence equilibria, temperature-dependent magnetism, chemical properties (solvent exchange, redox potentials, and p<i>K</i>_a values), XPS binding energies, analysis of excitation-dependent resonance Raman profiles, and DFT analysis of electronic absorption spectra. The spectrum can be assigned based on a singlet ground state with specific hydrogen-bonding interactions with solvent molecules included. The results are in good agreement with available experimental data. The DFT analysis provides assignments for characteristic absorption bands in the near-IR and visible regions. Bridge-based dπ → dπ* and interconfiguration transitions at Ru^III appear in the near-IR and MLCT and LMCT transitions in the visible. Reasonable values are also provided by DFT analysis for experimentally observed bond distances and redox potentials. The observed temperature-dependent magnetism of the blue dimer is consistent with a delocalized, diamagnetic singlet state (dπ₁*)² with a low-lying, paramagnetic triplet state (dπ₁*)¹(dπ₂*)¹. Systematic structural–magnetic–IR correlations are observed between ν_sym(RuORu) and ν_asym(RuORu) vibrational energies and magnetic properties in a series of ruthenium-based, μ-oxo-bridged complexes. Consistent with the DFT electronic structure model, bending along the Ru–O–Ru axis arises from a Jahn–Teller distortion with ∠Ru–O–Ru dictated by the distortion and electron–electron repulsion