Electronic structure of the water oxidation catalyst cis,cis-[(bpy)2(H2O)Ru(III)ORu(III)(OH2)(bpy)2]4+, the blue dimer.

The first designed molecular catalyst for water oxidation is the "blue dimer", cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(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.

Humidity-dependent dynamic infrared linear dichroism study of a poly(ester urethane).

Fourier transform infrared techniques, infrared difference spectroscopy and dynamic infrared linear dichroism (DIRLD), have been utilized to explore the effects of humidity and water absorption on a poly(ester urethane). An environmental infrared microbalance cell was used to measure the infrared spectra as a function of humidity and accompanying weight change for the absorption-desorption processes. The infrared difference data indicate that exposure to humidity affects the hydrogen-bonding interactions in the polymer. Dynamic infrared linear dichroism studies in tensile deformation mode as a function of humidity demonstrate how changes in water content affect the orientational response of functional groups. Complex behavior as a function of humidity for functional groups involved in hydrogen bonding indicates that water absorbed by the polymer affects the micro-environments near these functional groups.

Elemental and molecular characterization of aged polydimethylsiloxane foams.

The application and integration of micro X-ray fluorescence (MXRF) and Fourier transform infrared (FT-IR) imaging to polydimethylsiloxane (PDMS) foam aging issues have been applied to cross-sectional images. Previous work has shown the tin in the stannous 2-ethylhexanoate catalyst to be highly mobile and it typically migrates to the PDMS foam upper surface. The current paper discusses a method for the integration of full spectral MXRF and FT-IR imaging of aged foams. Solvent extractions have also been performed on both fresh and aged foams to further examine aged foam properties. Combining elemental and molecular imaging techniques and applying them to PDMS aging provides synergistic information that aids in understanding the sample composition and distribution of components. Application of chemometric analysis to the full spectral elemental and molecular maps demonstrates correlations within the foams of the residual tin, organo-tin functional group moieties, and the presence of nitroplasticizer from an exogenous source.

Dynamic infrared linear dichroism study of the temperature-dependent, viscoelastic behavior of a poly(ester urethane).

In the study reported here, of the poly(ester urethane), Estane 5703, simultaneous dynamic mechanical analysis (DMA) and dynamic infrared linear dichroism (DIRLD) measurements have been carried out at continuously variable temperatures from -50 to +30 degrees C. Multivariate curve resolution-alternating least squares (MCR-ALS) analysis of the spectral data has been correlated with the thermo-mechanical properties. Spectral changes, analyzed as a function of temperature, are compared with the storage and loss moduli to provide insight into viscoelastic behavior at the molecular level. In addition, the data for pure Estane have been compared to those for plasticized Estane samples, which contain 10 and 30% plasticizer by weight. These comparisons show a strong and consistent correlation between the macroscopic rheological properties and the microscopic (molecular, inter-molecular, and sub-molecular) responses of this block co-polymer.

Resonance Raman Spectra of [M(6)X(8)Y(6)](2)(-) Cluster Complexes (M = Mo, W; X, Y = Cl, Br, I).

Resonance Raman spectra of the cubic metal-halide complexes having the general formula [M(6)X(8)Y(6)](2)(-) (M = Mo or W; X, Y = Cl, Br, or I) are reported. The three totally symmetric fundamental vibrations of these complexes are identified. The extensive mixing of the symmetry coordinates that compose the symmetric normal modes expected in these systems is not observed. Instead the "group-frequency" approximation is valid. Furthermore, the force constants of both the apical and face-bridging metal-halide bonds are insensitive to the identity of either the metal or the halide. Raman spectra of related complexes with methoxy and benzenethiol groups as ligands are reported along with the structural data for [Mo(6)Cl(8)(SPh)(6)][NBu(4)](2). Crystal data for [Mo(6)Cl(8)(SPh)(6)][NBu(4)](2) at -156 degrees C: monoclinic space group P2(1)/c; a = 12.588(3), b = 17.471(5), c = 20.646(2) Å; beta = 118.53(1) degrees, V = 3223.4 Å(3); d(calcd) = 1.664 g cm(-)(3); Z = 2.

Mid-Infrared Spectrum of [Ru(phen)(3)](2+).

Time-resolved infrared spectra in the fingerprint region (1300-1700 cm(-)(1)) are reported for the metal-to-ligand charge-transfer (MLCT) excited state(s) of [Ru(phen)(3)](2+) and [Os(phen)(DAS)(2)](2+) (phen is 1,10-phenanthroline; DAS is 1,2-bis(diphenylarsino)ethane) in acetonitrile-d(3) at 298 K. The spectra are assigned by comparison to electrochemically generated [Ru(III)(phen)(3)](3+) and [Ru(II)(phen(*)(-)())(phen)(2)](+). The data provide clear evidence for the localized description [Ru(III)(phen(*)(-)())(phen)(2)](2+) on the approximately 100 ns time scale. They also give insight into electronic distribution in the excited state, aid in the interpretation of the time-resolved resonance Raman spectrum of [Ru(phen)(3)](2+), and suggest why measuring ground- and excited-state resonance Raman spectra of phen complexes is difficult.

Excited-State Electronic Structure in Polypyridyl Complexes Containing Unsymmetrical Ligands.

Step-scan Fourier transform infrared absorption difference time-resolved (S(2)FTIR DeltaA TRS) and time-resolved resonance Raman (TR(3)) spectroscopies have been applied to a series of questions related to excited-state structure in the metal-to-ligand charge transfer (MLCT) excited states of [Ru(bpy)(2)(4,4'-(CO(2)Et)(2)bpy)](2+), [Ru(bpy)(2)(4-CO(2)Et-4'-CH(3)bpy)](2+), [Ru(bpy)(4,4'-(CO(2)Et)(2)bpy)(2)](2+), [Ru(4,4'-(CO(2)Et)(2)bpy)(3)](2+), [Ru(bpy)(2)(4,4'-(CONEt(2))(2)bpy)](2+), [Ru(bpy)(2)(4-CONEt(2)-4'-CH(3)bpy)](2+), and [Ru(4-CONEt(2)-4'-CH(3)bpy)(3)](2+) (bpy is 2,2'-bipyridine). These complexes contain bpy ligands which are either symmetrically or unsymmetrically derivatized with electron-withdrawing ester or amide substituents. Analysis of the vibrational data, largely based on the magnitudes of the nu(CO) shifts of the amide and ester substituents (Deltanu(CO)), reveals that the ester- or amide-derivatized ligands are the ultimate acceptors and that the excited electron is localized on one acceptor ligand on the nanosecond time scale. In the unsymmetrically substituted acceptor ligands, the excited electron is largely polarized toward the ester- or amide-derivatized pyridine rings. In the MLCT excited states of [Ru(bpy)(2)(4,4'-(CO(2)Et)(2)bpy)](2+) and [Ru(bpy)(2)(4,4'-(CONEt(2))(2)bpy)](2+), Deltanu(CO) is only 60-70% of that observed upon complete ligand reduction due to a strong polarization interaction in the excited state between the dpi(5) Ru(III) core and the excited electron.

Excited-State Electron Transfer in a Chromophore-Quencher Complex. Spectroscopic Identification of a Redox-Separated State.

In the chromophore-quencher complex fac-[Re(Aqphen)(CO)(3)(py-PTZ)](+) (Aqphen is 12,17-dihydronaphtho[2,3-h]dipyrido[3,2-a:2',3'-c]-phenazine-12,17-dione; py-PTZ is 10-(4-picolyl)phenothiazine), Aqphen is a dppz derivative, containing a pendant quinone acceptor at the terminus of a rigid ligand framework. This introduces a third, low-lying, ligand-based pi acceptor level localized largely on the quinone fragment. Laser flash excitation of fac-[Re(Aqphen)(CO)(3)(py-PTZ)](+) (354.7 nm; in 1,2-dichloroethane) results in the appearance of a relatively long-lived transient that decays with tau(298K) = 300 ns (k = 3.3 x 10(6) s(-)(1)). Application of transient absorption, time-resolved resonance Raman, and time-resolved infrared spectroscopies proves that this transient is the redox-separated state fac-[Re(I)(Aqphen(*)(-)())(CO)(3)(py-PTZ(*)(+)())](+) in which the excited electron is localized largely on the quinone portion of the Aqphen ligand.

Pressure-dependent Fourier transform infrared spectroscopy of a poly (ester urethane).

The effects of hydrostatic pressure upon (1) a segmented poly (ester urethane), (2) a hydrolytically degraded sample of the same polymer, and (3) models for the polyurethane and polyester segments in this polymer have been studied by Fourier transform infrared spectroscopy using high-pressure diamond anvil cells (DACs). The pressure responses of the vibrational frequencies of specific functional groups of the poly (ester urethane) in the 0-100-kbar range are compared with data for individual segment models and the partially degraded sample. The results indicated that the polymer is highly stable in this pressure regime, with no measurable degradation or phase changes. Differences in the pressure dependency of specific infrared bands between the poly (ester urethane) sample and the partially degraded sample are slight and consistent with changes in hydrogen-bonding interactions and shorter chain lengths in the degraded sample.

Reactivity of an adsorbed Ru(VI)-oxo complex: oxidation of benzyl alcohol.

The phosphonated ruthenium complex, [Ru(tpy-PO(3)H(2))(OH(2))(3)](2+) (1) (tpy-PO(3)H(2) = 4'-phosphonato-2,2':6',2' '-terpyridine), was synthesized and attached to glass|ITO or glass|ITO|TiO(2) electrodes. After attachment to the metal oxide surface through the phosphonate linkage, 1 can be oxidized (either chemically or electrochemically) to the reactive Ru(VI)-dioxo complex, glass|ITO|[((HO)(2)OP)tpy)RuVI(O)(2)(OH(2))](2+), which remains attached to the surface. The attached Ru(VI) complex reacts with benzyl alcohol through mechanisms similar to those proposed for the solution analog. More specifically, Ru(VI) is reduced in a stepwise fashion to Ru(IV) and then finally to Ru(II). The reduction of Ru(VI) is accompanied by a rate-limiting insertion to the C-H bond of benzyl alcohol, followed by solvolysis of the aldehyde hydrate. In addition, the surface-bound Ru(VI) acts as an electrooxidation catalyst which carries out approximately 130 (2e(-)) turnovers before deactivation.

Synthesis and characterization of oligoproline-based molecular assemblies for light harvesting.

Helical oligoproline arrays provide a structurally well-defined environment for building photochemical energy conversion assemblies. The use of solid-phase peptide synthesis (SPPS) to prepare four such arrays, consisting of 16, 17, 18, and 19 amino acid residues, is described here. Each array contains the chromophore [Rub'(2)m](PF(6))(2) (b' = 4,4'-diethylamidocarbonyl-2,2'-bipyridine; m = 4-methyl-2,2'-dipyridine-4'-carboxylic acid) and the electron transfer donor PTZ (phenothiazine). The arrays differ systematically in the distance between the redox-active metal complex and PTZ sites. They have been used in photophysical studies to provide insight into the distance dependence of electron transfer. (J. Am. Chem. Soc. 2004, 126, 14506-14514). This work describes the synthesis, purification, and characterization of the oligoproline arrays, including a general procedure for the synthesis of related arrays.

Ultrafast energy transfer between the 3MLCT state of [Ru(II)(dmb)2(bpy-an)]2+ and the covalently appended anthracene.

The time scale for triplet-triplet energy transfer (EnT) between a Ru(II) chromophore and a ligand bound anthracene acceptor in [Ru(II)(dmb)2(bpy-an)]2+ (dmb = 4,4'-dimethyl-2,2-bipyridine; bpy-an = 4-(9-anthrylethylene), 4-methyl-2,2-bipyridine) has been measured using femtosecond transient absorption spectroscopy. The appearance of the anthracene excited state is monitored following photoexcitation to a metal-to-ligand charge transfer (MLCT) state via the pi pi* absorption of the triplet excited state of anthracene. Our time-resolved experiments show the presence of fast, sub-100 ps energy transfer to the anthracene occurring on two characteristic time scales of 23 and 72 ps.

Raman study of aluminum speciation in simulated alkaline nuclear waste.

The chemistry of concentrated sodium aluminate solutions stored in many of the large, underground storage tanks containing high-level waste (HLW) at the Hanford and Savannah River Nuclear Reservations is an area of recent research interest. Not only is the presence of aluminate in solution important for continued safe storage of these wastes, the nature of both solid and solution aluminum oxyhydroxides is important for waste pretreatment. Moreover, for many tanks that have leaked high aluminum waste in the past, little is known about the speciation of Al in the soil. In this study, Raman spectroscopy has been used to investigate the speciation of the aqueous species in the Al2O3-Na2O-H2O system over a wide range of solution compositions and hydration. A ternary phase diagram has been used to correlate the observed changes in the spectra with the composition of the solution and with dimerization of aluminate that occurs at elevated aluminate concentrations (>1.5 M). Dimerization is evidenced by growth of new Al-O stretching bands at 535 and 695 cm(-1) at the expense of the aluminate monomer band at 620 cm(-1). The spectrum of water was strongly influenced by the high concentrations of Na+ and OH- (>17 M). Upon increasing the concentration of NaOH in solution, the delta-(H-O-H) bending band of water (v2 mode) increased in frequency to 1663 cm(-1), indicating that the water contained in the concentrated caustic solution was more strongly hydrogen bonded at the higher base content. In addition, the sharp, well-resolved band at 3610 cm(-1), assigned to the v(O-H) of free OH-, increased in intensity with increasing NaOH. Analysis of the v(O-H) bands in the 3800-2600 cm(-1) region supported the overall increase in hydrogen bonding as evidenced by the increase in relative intensity of a strongly hydrated water band at 3118 cm(-1). Taking into consideration the activity of water, the molar concentrations of the monomeric and dimeric aluminate species were estimated using the relative intensities of the Al-O stretching bands from the Raman spectra. A constant apparent log Kdimer value was obtained at aluminate concentrations >1.5 M with a value of 0.97+/-0.04 at approximately 25 degrees C. This study represents the first spectral-based estimation of a thermodynamic equilibrium constant for the Al2O3-Na2O-H2O system.

Application of time-resolved infrared spectroscopy to electronic structure in metal-to-ligand charge-transfer excited states.

Infrared data in the nu(CO) region (1800-2150 cm(-1), in acetonitrile at 298 K) are reported for the ground (nu(gs)) and polypyridyl-based, metal-to-ligand charge-transfer (MLCT) excited (nu(es)) states of cis-[Os(pp)2(CO)(L)](n)(+) (pp = 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy); L = PPh3, CH(3)CN, pyridine, Cl, or H) and fac-[Re(pp)(CO)3(4-Etpy)](+) (pp = phen, bpy, 4,4'-(CH3)2bpy, 4,4'-(CH3O)2bpy, or 4,4'-(CO2Et)2bpy; 4-Etpy = 4-ethylpyridine). Systematic variations in nu(gs), nu(es), and Delta(nu) (Delta(nu) = nu(es) - nu(gs)) are observed with the excited-to-ground-state energy gap (E(0)) derived by a Franck-Condon analysis of emission spectra. These variations can be explained qualitatively by invoking a series of electronic interactions. Variations in dpi(M)-pi(CO) back-bonding are important in the ground state. In the excited state, the important interactions are (1) loss of back-bonding and sigma(M-CO) bond polarization, (2) pi(pp*-)-pi(CO) mixing, which provides the orbital basis for mixing pi(CO)- and pi(4,4'-X(2)bpy)-based MLCT excited states, and (3) dpi(M)-pi(pp) mixing, which provides the orbital basis for mixing pipi- and pi(4,4'-X(2)bpy*-)-based MLCT states. The results of density functional theory (DFT) calculations on the ground and excited states of fac-[Re(I)(bpy)(CO)3(4-Etpy)](+) provide assignments for the nu(CO) modes in the MLCT excited state. They also support the importance of pi(4,4'-X2bpy*-)-pi(CO) mixing, provide an explanation for the relative intensities of the A'(2) and A' ' excited-state bands, and provide an explanation for the large excited-to-ground-state nu(CO) shift for the A'(2) mode and its relative insensitivity to variations in X.