Recursos de colección

Caltech Authors (157.532 recursos)

Repository of works by Caltech published authors.

Group = CCI Solar Fuels

Mostrando recursos 1 - 20 de 87

  1. Two-photon spectroscopy of tungsten(0) arylisocyanides using nanosecond-pulsed excitation

    Takematsu, Kana; Wehlin, Sara A. M.; Sattler, Wesley; Winkler, Jay R.; Gray, Harry B.
    The two-photon absorption (TPA) cross sections (δ) for tungsten(0) arylisocyanides (W(CNAr)6) were determined in the 800–1000 nm region using two-photon luminescence (TPL) spectroscopy. The complexes have high TPA cross sections, in the range 1000–2000 GM at 811.8 nm. In comparison, the cross section at 811.8 nm for tris-(2,2′-bipyridine)ruthenium(II), [Ru(bpy)_3]^(2+), is 7 GM. All measurements were performed using a nanosecond-pulsed laser system.

  2. Two-photon spectroscopy of tungsten(0) arylisocyanides using nanosecond-pulsed excitation

    Takematsu, Kana; Wehlin, Sara A. M.; Sattler, Wesley; Winkler, Jay R.; Gray, Harry B.
    The two-photon absorption (TPA) cross sections (δ) for tungsten(0) arylisocyanides (W(CNAr)6) were determined in the 800–1000 nm region using two-photon luminescence (TPL) spectroscopy. The complexes have high TPA cross sections, in the range 1000–2000 GM at 811.8 nm. In comparison, the cross section at 811.8 nm for tris-(2,2′-bipyridine)ruthenium(II), [Ru(bpy)_3]^(2+), is 7 GM. All measurements were performed using a nanosecond-pulsed laser system.

  3. Two-photon spectroscopy of tungsten(0) arylisocyanides using nanosecond-pulsed excitation

    Takematsu, Kana; Wehlin, Sara A. M.; Sattler, Wesley; Winkler, Jay R.; Gray, Harry B.
    The two-photon absorption (TPA) cross sections (δ) for tungsten(0) arylisocyanides (W(CNAr)6) were determined in the 800–1000 nm region using two-photon luminescence (TPL) spectroscopy. The complexes have high TPA cross sections, in the range 1000–2000 GM at 811.8 nm. In comparison, the cross section at 811.8 nm for tris-(2,2′-bipyridine)ruthenium(II), [Ru(bpy)_3]^(2+), is 7 GM. All measurements were performed using a nanosecond-pulsed laser system.

  4. The mechanism for catalytic hydrosilylation by bis(imino)pyridine iron olefin complexes supported by broken symmetry density functional theory

    Lam, Yan Choi; Nielsen, Robert J.; Goddard, William A., III; Dash, Aswini K.
    Density functional theory (DFT, B3LYP-D3 with implicit solvation in toluene) was used to investigate the mechanisms of olefin hydrosilylation catalyzed by PDI(Fe) (bis(imino)pyridine iron) complexes, where PDI = 2,6-(ArN CMe)_2(C_5H_3N) with Ar = 2,6-R_2-C_6H_3. We find that the rate-determining step for hydrosilylation is hydride migration from Et3SiH onto the Fe-bound olefin to form (PDI)Fe(alkyl)(SiEt_3). This differs from the mechanism for the Pt Karstedt catalyst in that there is no prior Si–H oxidative addition onto the Fe center. (PDI)Fe(alkyl)(SiEt_3) then undergoes C–Si reductive elimination to form (PDI)Fe, which coordinates an olefin ligand to regenerate the resting state (PDI)Fe(olefin). In agreement with...

  5. The mechanism for catalytic hydrosilylation by bis(imino)pyridine iron olefin complexes supported by broken symmetry density functional theory

    Lam, Yan Choi; Nielsen, Robert J.; Goddard, William A., III; Dash, Aswini K.
    Density functional theory (DFT, B3LYP-D3 with implicit solvation in toluene) was used to investigate the mechanisms of olefin hydrosilylation catalyzed by PDI(Fe) (bis(imino)pyridine iron) complexes, where PDI = 2,6-(ArN CMe)_2(C_5H_3N) with Ar = 2,6-R_2-C_6H_3. We find that the rate-determining step for hydrosilylation is hydride migration from Et3SiH onto the Fe-bound olefin to form (PDI)Fe(alkyl)(SiEt_3). This differs from the mechanism for the Pt Karstedt catalyst in that there is no prior Si–H oxidative addition onto the Fe center. (PDI)Fe(alkyl)(SiEt_3) then undergoes C–Si reductive elimination to form (PDI)Fe, which coordinates an olefin ligand to regenerate the resting state (PDI)Fe(olefin). In agreement with...

  6. Membranes for artificial photosynthesis

    Chabi, Sakineh; Papadantonakis, Kimberly M.; Lewis, Nathan S.; Freund, Michael S.
    Membrane-based architectures enable optimization of charge transport and electrochemical potential gradients in artificial photosynthesis. Spatial integration of the membrane-bound components reduces the impact of charge recombination and can reduce electrical resistances associated with ionic and electronic transport processes. In addition to eliminating the need for external electrical circuits, a membrane-based architecture also ensures separation of energetic products, thereby preventing the formation of potentially dangerous fuel/oxidant mixtures. Membrane-based structures may also be coupled with other devices, such as perovskite-based solar cells, to further benefit solar fuel production. This review discusses the key roles that various different types of membranes play in...

  7. Conservation of vibrational coherence in ultrafast electronic relaxation: The case of diplatinum complexes in solution

    Monni, Roberto; Auböck, Gerald; Kinschel, Dominik; Aziz-Lange, Kathrin M.; Gray, Harry B.; Vlček, Antonín; Chergui, Majed
    We report the results of ultrafast transient absorption studies of tetrakis(μ-pyrophosphito)diplatinate(II), [Pt_2(μ-P_2O_5H_2)_4]^(4−) (Pt(pop)) and its perfluoroborated derivative [Pt_2(μ-P_2O_5(BF_2)_4]^(4−) (Pt(pop-BF_2)) in water and acetonitrile upon excitation of high lying (<300 nm) UV absorption bands. We observe an ultrafast relaxation channel from high lying states to the lowest triplet state that partly (Pt(pop) in H_2O, Pt(pop-BF_2)) or fully (Pt(pop) in MeCN) bypasses the lowest singlet excited state. As a consequence, vibrational wave packets are detected in the lowest triplet state and/or the lowest excited singlet of both complexes, even though the electronic relaxation cascade spans ca. 2 and 1.3 eV, respectively. In...

  8. Electronic structures and photophysics of d^8-d^8 complexes

    Gray, Harry B.; Záliš, Stanislav; Vlček, Antonín
    Research on d^8-d^8 complexes is being actively pursued, owing, in part, to newly developed time-resolved optical, IR, and X-ray methods that directly interrogate bonding changes upon excitation. Our review covers work on the ground- and electronic excited states, as well as the oxidized and reduced forms, of these complexes. Recent experimental and theoretical results add a new chapter to the rich history of d^8-d^8 spectroscopic and chemical behavior.

  9. A Mechanistic Study of the Oxidative Reaction of Hydrogen-Terminated Si(111) Surfaces with Liquid Methanol

    Plymale, Noah T.; Dasog, Mita; Brunschwig, Bruce S.; Lewis, Nathan S.
    H–Si(111) surfaces have been reacted with liquid methanol (CH_3OH) in the absence or presence of a series of oxidants and/or illumination. Oxidant-activated methoxylation of H–Si(111) surfaces was observed in the dark after exposure to CH_3OH solutions that contained the one-electron oxidants acetylferrocenium, ferrocenium, or 1,1’-dimethylferrocenium. The oxidant-activated reactivity toward CH_3OH of intrinsic and n-type H–Si(111) surfaces increased upon exposure to ambient light. The results suggest that oxidant-activated methoxylation requires that two conditions be met: (1) the position of the quasi-Fermi levels must energetically favor oxidation of the H–Si(111) surface and (2) the position of the quasi-Fermi levels must energetically favor...

  10. Intersystem Crossing in Diplatinum Complexes

    Lam, Yan Choi; Gray, Harry B.; Winkler, Jay R.
    Intersystem crossing (ISC) in solid [(C_4H_9)_4N]4[Pt_2(μ-P_2O_5(BF_2)_2)_4], abbreviated Pt(pop-BF2), is remarkably slow for a third-row transition metal complex, ranging from τ_(ISC) ≈ 0.9 ns at 310 K to τ_(ISC) ≈ 29 ns below 100 K. A classical model based on Boltzmann population of one temperature-independent and two thermally activated pathways was previously employed to account for the ISC rate behavior. An alternative we prefer is to treat Pt(pop-BF_2) ISC quantum mechanically, using expressions for multiphonon radiationless transitions. Here we show that a two-channel model with physically plausible parameters can account for the observed ISC temperature dependence. In channel 1, ^1A_(2u) intersystem...

  11. Intersystem Crossing in Diplatinum Complexes

    Lam, Yan Choi; Gray, Harry B.; Winkler, Jay R.
    Intersystem crossing (ISC) in solid [(C_4H_9)_4N]4[Pt_2(μ-P_2O_5(BF_2)_2)_4], abbreviated Pt(pop-BF2), is remarkably slow for a third-row transition metal complex, ranging from τ_(ISC) ≈ 0.9 ns at 310 K to τ_(ISC) ≈ 29 ns below 100 K. A classical model based on Boltzmann population of one temperature-independent and two thermally activated pathways was previously employed to account for the ISC rate behavior. An alternative we prefer is to treat Pt(pop-BF_2) ISC quantum mechanically, using expressions for multiphonon radiationless transitions. Here we show that a two-channel model with physically plausible parameters can account for the observed ISC temperature dependence. In channel 1, ^1A_(2u) intersystem...

  12. Electrocatalysis of CO_2 Reduction in Brush Polymer Ion Gels

    McNicholas, Brendon J.; Blakemore, James D.; Chang, Alice B.; Bates, Christopher M.; Kramer, Wesley W.; Grubbs, Robert H.; Gray, Harry B.
    The electrochemical characterization of brush polymer ion gels containing embedded small-molecule redox-active species is reported. Gels comprising PS–PEO–PS triblock brush polymer, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIm-TFSI), and some combination of ferrocene (Fc), cobaltocenium (CoCp_2^+), and Re(bpy)(CO)_3Cl (1) exhibit diffusion-controlled redox processes with diffusion coefficients approximately one-fifth of those observed in neat BMIm-TFSI. Notably, 1 dissolves homogeneously in the interpenetrating matrix domain of the ion gel and displays electrocatalytic CO_2 reduction to CO in the gel. The catalytic wave exhibits a positive shift versus Fc^(+/0) compared with analogous nonaqueous solvents with a reduction potential 450 mV positive of onset and 90% Faradaic efficiency...

  13. Proton–hydride tautomerism in hydrogen evolution catalysis

    Aguirre Quintana, Luis M.; Johnson, Samantha I.; Corona, Sydney L.; Villatoro, Walther; Goddard, William A., III; Takase, Michael K.; VanderVelde, David G.; Winkler, Jay R.; Gray, Harry B.; Blakemore, James D.
    Efficient generation of hydrogen from renewable resources requires development of catalysts that avoid deep wells and high barriers. Information about the energy landscape for H_2 production can be obtained by chemical characterization of catalytic intermediates, but few have been observed to date. We have isolated and characterized a key intermediate in 2e^– + 2H^+ → H_2 catalysis. This intermediate, obtained by treatment of Cp*Rh(bpy) (Cp*, η^5-pentamethylcyclopentadienyl; bpy, κ^2-2,2′-bipyridyl) with acid, is not a hydride species but rather, bears [η^4-Cp*H] as a ligand. Delivery of a second proton to this species leads to evolution of H_2 and reformation of η^5-Cp* bound...

  14. Proton–hydride tautomerism in hydrogen evolution catalysis

    Aguirre Quintana, Luis M.; Johnson, Samantha I.; Corona, Sydney L.; Villatoro, Walther; Goddard, William A., III; Takase, Michael K.; VanderVelde, David G.; Winkler, Jay R.; Gray, Harry B.; Blakemore, James D.
    Efficient generation of hydrogen from renewable resources requires development of catalysts that avoid deep wells and high barriers. Information about the energy landscape for H_2 production can be obtained by chemical characterization of catalytic intermediates, but few have been observed to date. We have isolated and characterized a key intermediate in 2e^– + 2H^+ → H_2 catalysis. This intermediate, obtained by treatment of Cp*Rh(bpy) (Cp*, η^5-pentamethylcyclopentadienyl; bpy, κ^2-2,2′-bipyridyl) with acid, is not a hydride species but rather, bears [η^4-Cp*H] as a ligand. Delivery of a second proton to this species leads to evolution of H_2 and reformation of η^5-Cp* bound...

  15. Mechanistic investigation of proton reduction by cobaloximes: Insight from H2 oxidation kinetics

    Del Ciello, Sarah A.; Winkler, Jay R.; Peters, Jonas C.; Gray, Harry B.
    Cobaloximes are a class of earth- abundant proton redn. catalysts that have been known since the 1980's. Previous studies have come to conflicting conclusions about the mechanism of proton redn. by these catalysts, with evidence existing for both a homolytic and heterolytic mechanism. In this work, kinetic anal. of the reverse reaction is used to gain further insight.

  16. Effect of interlayer anions on [NiFe]-LDH nanosheet water oxidation activity

    Hunter, Bryan M.; Winkler, Jay R.; Gray, Harry B.; Mueller, Astrid M.
    Powering the planet with sustainable, carbon-neutral fuels affects every aspect of human life. Sunlight-driven water splitting is an attractive soln. to provide environmentally benign hydrogen fuel. Global scalability demands that all photoelectrode and catalyst materials consist of earth- abundant elements. The water oxidn. half reaction requires four coupled electron and proton transfer steps, for which robust and efficient electrocatalysts are needed. We have shown previously that [NiFe] - layered double hydroxide (LDH) nanosheets are highly active water oxidn. catalysts [Hunter, Blakemore, Deimund, Gray, Winkler, Mueller, J. Am. Chem. Soc. 2014, 136, 13118]. They were synthesized by pulsed laser ablation in liqs. (PLAL), a medium-throughput method...

  17. Effect of interlayer anions on [NiFe]-LDH nanosheet water oxidation activity

    Hunter, B. M.; Hieringer, W.; Winkler, J. R.; Gray, H. B.; Müller, A. M.
    We synthesized nickel–iron layered double hydroxide ([NiFe]-LDH) nanosheets with different interlayer anions to probe their role in water oxidation catalysis. In alkaline electrolyte in ambient air, carbonate rapidly replaced other interlayer anions and catalytic activity was highest. Electrocatalytic water oxidation in virtually carbonate-free alkaline electrolyte revealed that activity was a function of anion basicity. Our [NiFe]-LDH nanosheets, prepared by pulsed laser ablation in liquids, were regenerated in carbonate-containing aqueous KOH. Anion binding motifs were assessed by X-ray photoelectron spectroscopy in combination with density functional theory calculations, suggesting that nitrite species bound to edge-site Fe in the precatalyst correlated with higher...

  18. In-situ spectroscopies of mixed-metal nanosheet water oxidation catalysts made by pulsed laser ablation in liquids

    Hunter, Bryan; Winkler, Jay; Gray, Harry; Mueller, Astrid
    Our ability to utilize sustainable resources will be crit. in meeting the ever-increasing global energy demand in an eco-friendly manner. The most reliable and available source of sustainable energy is the sun. The biggest challenges to its practical utilization are its intermittency on earth and unequal local energy needs. As a consequence, we must find a way to convert solar energy into storable, transportable fuels. We envision a solar water splitting device that produces hydrogen fuel. Subsequent functionalization with carbon dioxide will produce liq. fuels that are carbon neutral upon combustion. Water oxidn., the more demanding half-reaction in the water splitting process, is a key component...

  19. Computational Study of Fluorinated Diglyoxime-Iron Complexes: Tuning the Electrocatalytic Pathways for Hydrogen Evolution

    Harshan, Aparna Karippara; Solis, Brian H.; Winkler, Jay R.; Gray, Harry B.; Hammes-Schiffer, Sharon
    The ability to tune the properties of hydrogen-evolving molecular electrocatalysts is important for developing alternative energy sources. Fluorinated diglyoxime-iron complexes have been shown to evolve hydrogen at moderate overpotentials. Herein two such complexes, [(dAr^FgBF_2)_2Fe(py)_2], denoted A, and [(dAr^Fg_2H-BF_2)Fe(py)_2], denoted B [dAr^Fg = bis(pentafluorophenyl-glyoximato); py = pyridine], are investigated with density functional theory calculations. B differs from A in that one BF_2 bridge is replaced by a proton bridge of the form O–H–O. According to the calculations, the catalytic pathway for A involves two consecutive reduction steps, followed by protonation of an Fe^0 species to generate the active Fe^(II)-hydride species. B...

  20. Thermally Tunable Dual Emission of the d^8–d^8 Dimer [Pt_2(μ-P_2O_5(BF_2)_2)_4]^(4–)

    Hofbeck, Thomas; Lam, Yan Choi; Kalbáč, Martin; Záliš, Stanislav; Vlček, Antonín, Jr.; Yersin, Hartmut
    High-resolution fluorescence, phosphorescence, as well as related excitation spectra, and, in particular, the emission decay behavior of solid [Bu_4N]_4[Pt_2(μ-P_2O_5(BF_2)_2)_4], abbreviated Pt(pop-BF_2), have been investigated over a wide temperature range, 1.3–310 K. We focus on the lowest excited states that result from dσ^*pσ (5d_z2–6p_z) excitations, i.e., the singlet state S_1 (of ^1A_2u symmetry in D_(4h)) and the lowest triplet T_1, which splits into spin–orbit substates A_(1u)(^3A_(2u)) and E_u(^3A_(2u)). After optical excitation, an unusually slow intersystem crossing (ISC) is observed. As a consequence, the compound shows efficient dual emission, consisting of blue fluorescence and green phosphorescence with an overall emission quantum yield...

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