Recursos de colección

Caltech Authors (160.918 recursos)

Repository of works by Caltech published authors.

Group = CCI Solar Fuels

Mostrando recursos 1 - 20 de 110

  1. Triphenylsulfonium topophotochemistry

    Despagnet-Ayoub, E.; Kramer, W. W.; Sattler, W.; Sattler, A.; LaBeaume, P. J.; Thackeray, J. W.; Cameron, J. F.; Cardolaccia, T.; Rachford, A. A.; Winkler, J. R.; Gray, H. B.
    The products from the 193 nm irradiation of triphenylsulfonium nonaflate (TPS) embedded in a poly(methyl methacrylate) (PMMA) film have been characterized. The analysis of the photoproduct formation was performed using chromatographic techniques including HPLC, GPC and GC-MS as well as UV-vis and NMR spectroscopic methods. Two previously unreported TPS photoproducts, triphenylene and dibenzothiophene, were detected; additionally, GPC and DOSY-NMR spectroscopic analyses after irradiation suggested that TPS fragments had been incorporated into the polymer film. The irradiation of acetonitrile solutions containing 10% w/v PMMA and 1% w/v TPS in a 1 cm-path-length cuvette showed only a trace amount of triphenylene or...

  2. Ultrafast Wiggling and Jiggling: Ir_2(1,8-diisocyanomenthane)_4^(2+)

    Pižl, Martin; Hunter, Bryan M.; Greetham, Gregory M.; Towrie, Michael; Záliš, Stanislav; Gray, Harry B.; Vlček, Antonín
    Binuclear complexes of d^8 metals (Pt^(II), Ir^I, Rh^I, ) exhibit diverse photonic behavior including dual emission from relatively long-lived singlet and triplet excited states, as well as photochemical energy-, electron-, and atom transfer. Time-resolved optical spectroscopic and X-ray studies have revealed the behavior of the dimetallic core, confirming that M-M bonding is strengthened upon dσ *→pσ excitation. We report the bridging ligand dynamics of Ir_2(1,8-diisocyanomenthane)_4^(2+) (Ir(dimen)), investigated by fs-ns time-resolved IR spectroscopy (TRIR) in the region of C≡N stretching vibrations, ν(C≡N), 2000-2300 cm^(-1). The ν(C≡N) IR band of the singlet and triplet dσ*pσ excited states is shifted by -22 and...

  3. Photoelectrochemical Performance of BiVO_4 Photoanodes Integrated with [NiFe]-Layered Double Hydroxide Nanocatalysts

    Sinclair, Timothy S.; Gray, Harry B.; Müller, Astrid M.
    We immobilized laser-made nickel iron layered double hydroxide ([NiFe]-LDH) nanocatalysts on BiVO_4 photoanodes. We compared photoelectrochemical performance of integrated [NiFe]-LDH-BiVO_4 photoanodes in sulfite-free aqueous electrolyte with photocurrent generation of neat BiVO_4 photoanodes in aqueous electrolyte with sulfite added as sacrificial hole acceptor. We optimized catalyst mass loading, which is a tradeoff between most efficient depletion of photogenerated holes that drive catalytic turnover and parasitic light absorption by the catalyst particles. We also mitigated nanocatalyst aggregation on the BiVO_4 surface by a surfactant that selectively ligated the catalysts or by dispersing the catalyst suspension more rapidly on the photoanode surface. Our...

  4. Electrochemistry in ionic liquids: Case study of a manganese corrole

    McNicholas, Brendon J.; Blumenfeld, Carl; Kramer, Wesley W.; Grubbs, Robert H.; Winkler, Jay R.; Gray, Harry B.
    Voltammetry of [5,10,15-tris(pentafluorophenylcorrole)]Mn(III) was investigated in four different ionic liquids (ILs): 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIm-TFSI); 1-ethyl-3-methylimidazolium ethylsulfate (EMIm-EtOSO_3); 1-ethyl-3-methylimidazolium triflate (EMIm-OTf); and 1-ethyl-3-methylimidazolium tetracyanoborate (EMIm-TCB). We found that Mn^(IV/III) E_(1/2) values depend on IL counter anion: OTf–< EtOSO_3− < TFSI− < TCB−. In EMIm-TCB and BMIm- TFSI, reversible, diffusion-controlled MnIV/III reactions occurred, as evidenced in each case by the ratio of anodic to cathodic diffusion coefficients over a range of scan rates. Axial coordination was evidenced by a cathodic to anodic diffusion coefficient ratio greater than one, an increasing cathodic to anodic peak current ratio with increasing scan rate, and a...

  5. 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.

  6. 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.

  7. 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.

  8. 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...

  9. 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...

  10. 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...

  11. 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...

  12. 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.

  13. 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...

  14. 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...

  15. 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...

  16. 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...

  17. 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...

  18. 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...

  19. 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.

  20. 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...

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