Recursos de colección

Caltech Authors (160.918 recursos)

Repository of works by Caltech published authors.

Group = Resnick Sustainability Institute

Mostrando recursos 1 - 20 de 131

  1. 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...

  2. Design, Synthesis, and Self-Assembly of Polymers with Tailored Graft Distributions

    Chang, Alice B.; Lin, Tzu-Pin; Thompson, Niklas B.; Luo, Shao-Xiong; Liberman-Martin, Allegra Lauren; Chen, Hsiang-Yun; Lee, Byeongdu; Grubbs, Robert H.
    Grafting density and graft distribution impact the chain dimensions and physical properties of polymers. However, achieving precise control over these structural parameters presents long-standing synthetic challenges. In this report, we introduce a versatile strategy to synthesize polymers with tailored architectures via grafting-through ring-opening metathesis polymerization (ROMP). One-pot copolymerization of an ω-norbornenyl macromonomer and a discrete norbornenyl co-monomer (diluent) provides opportunities to control the backbone sequence and therefore the side chain distribution. Toward sequence control, the homopolymerization kinetics of 23 diluents were studied, representing diverse variations in the stereochemistry, anchor groups, and substituents. These modifications varied the homopolymerization rate constants over...

  3. Design, Synthesis, and Self-Assembly of Polymers with Tailored Graft Distributions

    Chang, Alice B.; Lin, Tzu-Pin; Thompson, Niklas B.; Luo, Shao-Xiong; Liberman-Martin, Allegra Lauren; Chen, Hsiang-Yun; Lee, Byeongdu; Grubbs, Robert H.
    Grafting density and graft distribution impact the chain dimensions and physical properties of polymers. However, achieving precise control over these structural parameters presents long-standing synthetic challenges. In this report, we introduce a versatile strategy to synthesize polymers with tailored architectures via grafting-through ring-opening metathesis polymerization (ROMP). One-pot copolymerization of an ω-norbornenyl macromonomer and a discrete norbornenyl co-monomer (diluent) provides opportunities to control the backbone sequence and therefore the side chain distribution. Toward sequence control, the homopolymerization kinetics of 23 diluents were studied, representing diverse variations in the stereochemistry, anchor groups, and substituents. These modifications varied the homopolymerization rate constants over...

  4. Optimal Load-side Control for Frequency Regulation in Smart Grids

    Mallada, Enrique; Zhao, Changhong; Low, Steven
    Frequency control rebalances supply and demand while maintaining the network state within operational margins. It is implemented using fast ramping reserves that are expensive and wasteful, and which are expected to become increasingly necessary with the current acceleration of renewable penetration. The most promising solution to this problem is the use of demand response, i.e. load participation in frequency control. Yet it is still unclear how to efficiently integrate load participation without introducing instabilities and violating operational constraints.

  5. Van der Waals Materials for Atomically-Thin Photovoltaics: Promise and Outlook

    Jariwala, Deep; Davoyan, Artur R.; Wong, Joeson; Atwater, Harry A.
    Two-dimensional (2D) semiconductors provide a unique opportunity for optoelectronics due to their layered atomic structure, electronic and optical properties. To date, a majority of the application-oriented research in this field has been focused on field-effect electronics as well as photodetectors and light emitting diodes. Here we present a perspective on the use of 2D semiconductors for photovoltaic applications. We discuss photonic device designs that enable light trapping in nanometer-thickness absorber layers, and we also outline schemes for efficient carrier transport and collection. We further provide theoretical estimates of efficiency indicating that 2D semiconductors can indeed be competitive with and complementary...

  6. Ruthenium Olefin Metathesis Catalysts Featuring a Labile Carbodicarbene Ligand

    Liberman-Martin, Allegra L.; Grubbs, Robert H.
    Ruthenium benzylidene complexes containing a carbodicarbene (CDC) ligand are reported. Mechanistic studies indicate that the CDC ligand can dissociate under relatively mild conditions to afford active olefin metathesis catalysts. These catalysts were found to be effective at ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) reactions.

  7. Ruthenium Olefin Metathesis Catalysts Featuring a Labile Carbodicarbene Ligand

    Liberman-Martin, Allegra L.; Grubbs, Robert H.
    Ruthenium benzylidene complexes containing a carbodicarbene (CDC) ligand are reported. Mechanistic studies indicate that the CDC ligand can dissociate under relatively mild conditions to afford active olefin metathesis catalysts. These catalysts were found to be effective at ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) reactions.

  8. Directed Evolution: Bringing New Chemistry to Life

    Arnold, Frances H.
    In this competitive age, when new industries sprout and decay in the span of a decade, we should reflect on how a company survives to celebrate its 350th anniversary. A prerequisite for survival in business is the ability to adapt to changing environments and tastes and to sense, anticipate and meet needs faster and better than the competition. This requires constant innovation as well as focused attention to execution. A company that continues to provide meaningful and profitable solutions to human problems has a chance to survive, even thrive, in a rapidly changing and highly competitive world.

  9. Directed Evolution: Bringing New Chemistry to Life

    Arnold, Frances H.
    Tailor-made: Discussed herein is the ability to adapt biology's mechanisms for innovation and optimization to solving problems in chemistry and engineering. The evolution of nature's enzymes can lead to the discovery of new reactivity, transformations not known in biology, and reactivity inaccessible by small-molecule catalysts.

  10. Hot Carrier Dynamics in Photoexcited Gold Nanostructures: Role of Interband Excitations and Evidence for Ballistic Transport

    Tagliabue, Giulia; Jermyn, Adam S.; Sundararaman, Ravishankar; Welch, Alex J.; DuChene, Joseph S.; Davoyan, Artur R.; Narang, Prineha; Atwater, Harry A.
    Harnessing short-lived photoexcited electron-hole pairs in metal nanostructures has the potential to define a new phase of optoelectronics, enabling control of athermal mechanisms for light harvesting, photodetection and photocatalysis. To date, however, the spatiotemporal dynamics and transport of these photoexcited carriers have been only qualitatively characterized. Plasmon excitation has been widely viewed as an efficient mechanism for generating non-thermal hot carriers. Despite numerous experiments, conclusive evidence elucidating and quantifying the full dynamics of hot carrier generation, transport, and injection has not been reported. Here, we combine experimental measurements with coupled first-principles electronic structure theory and Boltzmann transport calculations to provide...

  11. Hot Carrier Dynamics in Photoexcited Gold Nanostructures: Role of Interband Excitations and Evidence for Ballistic Transport

    Tagliabue, Giulia; Jermyn, Adam S.; Sundararaman, Ravishankar; Welch, Alex J.; DuChene, Joseph S.; Davoyan, Artur R.; Narang, Prineha; Atwater, Harry A.
    Harnessing short-lived photoexcited electron-hole pairs in metal nanostructures has the potential to define a new phase of optoelectronics, enabling control of athermal mechanisms for light harvesting, photodetection and photocatalysis. To date, however, the spatiotemporal dynamics and transport of these photoexcited carriers have been only qualitatively characterized. Plasmon excitation has been widely viewed as an efficient mechanism for generating non-thermal hot carriers. Despite numerous experiments, conclusive evidence elucidating and quantifying the full dynamics of hot carrier generation, transport, and injection has not been reported. Here, we combine experimental measurements with coupled first-principles electronic structure theory and Boltzmann transport calculations to provide...

  12. Hot Carrier Dynamics in Photoexcited Gold Nanostructures: Role of Interband Excitations and Evidence for Ballistic Transport

    Tagliabue, Giulia; Jermyn, Adam S.; Sundararaman, Ravishankar; Welch, Alex J.; DuChene, Joseph S.; Davoyan, Artur R.; Narang, Prineha; Atwater, Harry A.
    Harnessing short-lived photoexcited electron-hole pairs in metal nanostructures has the potential to define a new phase of optoelectronics, enabling control of athermal mechanisms for light harvesting, photodetection and photocatalysis. To date, however, the spatiotemporal dynamics and transport of these photoexcited carriers have been only qualitatively characterized. Plasmon excitation has been widely viewed as an efficient mechanism for generating non-thermal hot carriers. Despite numerous experiments, conclusive evidence elucidating and quantifying the full dynamics of hot carrier generation, transport, and injection has not been reported. Here, we combine experimental measurements with coupled first-principles electronic structure theory and Boltzmann transport calculations to provide...

  13. A Statistical Graphical Model of the California Reservoir System

    Taeb, A.; Reager, J. T.; Turmon, M.; Chandrasekaran, V.
    The recent California drought has highlighted the potential vulnerability of the state's water management infrastructure to multi-year dry intervals. Due to the high complexity of the network, dynamic storage changes in California reservoirs on a state-wide scale have previously been difficult to model using either traditional statistical or physical approaches. Indeed, although there is a significant line of research on exploring models for single (or a small number of) reservoirs, these approaches are not amenable to a system-wide modeling of the California reservoir network due to the spatial and hydrological heterogeneities of the system. In this work, we develop a...

  14. A Statistical Graphical Model of the California Reservoir System

    Taeb, A.; Reager, J. T.; Turmon, M.; Chandrasekaran, V.
    The recent California drought has highlighted the potential vulnerability of the state's water management infrastructure to multiyear dry intervals. Due to the high complexity of the network, dynamic storage changes in California reservoirs on a state-wide scale have previously been difficult to model using either traditional statistical or physical approaches. Indeed, although there is a significant line of research on exploring models for single (or a small number of) reservoirs, these approaches are not amenable to a system-wide modeling of the California reservoir network due to the spatial and hydrological heterogeneities of the system. In this work, we develop a...

  15. Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation

    Reed, Christopher J.; Agapie, Theodor
    A new series of tetranuclear Fe clusters displaying an interstitial μ_4-F ligand was prepared for a comparison to previously reported μ_4-O analogues. With a single nitric oxide (NO) coordinated as a reporter of small-molecule activation, the μ_4-F clusters were characterized in five redox states, from Fe^(II)_3{FeNO}^8 to Fe^(III)_3{FeNO}^7, with NO stretching frequencies ranging from 1680 to 1855 cm^(–1), respectively. Despite accessing more reduced states with an F– bridge, a two-electron reduction of the distal Fe centers is necessary for the μ_4-F clusters to activate NO to the same degree as the μ_4-O system; consequently, NO reactivity is observed at more...

  16. Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation

    Reed, Christopher J.; Agapie, Theodor
    A new series of tetranuclear Fe clusters displaying an interstitial μ_4-F ligand was prepared for a comparison to previously reported μ_4-O analogues. With a single nitric oxide (NO) coordinated as a reporter of small-molecule activation, the μ_4-F clusters were characterized in five redox states, from Fe^(II)_3{FeNO}^8 to Fe^(III)_3{FeNO}^7, with NO stretching frequencies ranging from 1680 to 1855 cm^(–1), respectively. Despite accessing more reduced states with an F– bridge, a two-electron reduction of the distal Fe centers is necessary for the μ_4-F clusters to activate NO to the same degree as the μ_4-O system; consequently, NO reactivity is observed at more...

  17. Anti-Markovnikov alkene oxidation by metal-oxo–mediated enzyme catalysis

    Hammer, Stephan C.; Kubik, Grzegorz; Watkins, Ella; Huang, Shan; Minges, Hannah; Arnold, Frances H.
    Catalytic anti-Markovnikov oxidation of alkene feedstocks could simplify synthetic routes to many important molecules and solve a long-standing challenge in chemistry. Here we report the engineering of a cytochrome P450 enzyme by directed evolution to catalyze metal-oxo–mediated anti-Markovnikov oxidation of styrenes with high efficiency. The enzyme uses dioxygen as the terminal oxidant and achieves selectivity for anti-Markovnikov oxidation over the kinetically favored alkene epoxidation by trapping high-energy intermediates and catalyzing an oxo transfer, including an enantioselective 1,2-hydride migration. The anti-Markovnikov oxygenase can be combined with other catalysts in synthetic metabolic pathways to access a variety of challenging anti-Markovnikov functionalization reactions.

  18. Nitrogen Fixation via a Terminal Fe(IV) Nitride

    Thompson, Niklas B.; Green, Michael T.; Peters, Jonas C.
    Terminal iron nitrides (Fe≡N) have been proposed as intermediates of (bio)catalytic nitrogen fixation, yet experimental evidence to support this hypothesis has been lacking. In particular, no prior synthetic examples of terminal Fe≡N species have been derived from N_2. Here we show that a nitrogen-fixing Fe-N_2 catalyst can be protonated to form an Fe(NNH_2) hydrazido(2−) intermediate, which, upon further protonation, heterolytically cleaves the N–N bond to release [Fe^(IV)≡N]^+ and NH_3. These observations provide direct evidence for the viability of a Chatt-type (distal) mechanism for Fe-mediated N_2-to-NH_3 conversion. The physical oxidation state range of the Fe complexes in this transformation is buffered...

  19. Nitrogen Fixation via a Terminal Fe(IV) Nitride

    Thompson, Niklas B.; Green, Michael T.; Peters, Jonas C.
    Terminal iron nitrides (Fe≡N) have been proposed as intermediates of (bio)catalytic nitrogen fixation, yet experimental evidence to support this hypothesis has been lacking. In particular, no prior synthetic examples of terminal Fe≡N species have been derived from N_2. Here we show that a nitrogen-fixing Fe–N_2 catalyst can be protonated to form a neutral Fe(NNH_2) hydrazido(2−) intermediate, which, upon further protonation, heterolytically cleaves the N–N bond to release [Fe^(IV)≡N]^+ and NH_3. These observations provide direct evidence for the viability of a Chatt-type (distal) mechanism for Fe-mediated N_2-to-NH_3 conversion. The physical oxidation state range of the Fe complexes in this transformation is...

  20. Catalytic Reduction of Alkyl and Aryl Bromides Using Propan-2-ol

    Haibach, Michael C.; Stoltz, Brian M.; Grubbs, Robert H.
    Milstein's complex, (PNN)RuHCl(CO), catalyzes the efficient reduction of aryl and alkyl halides under relatively mild conditions by using propan-2-ol and a base. Sterically hindered tertiary and neopentyl substrates are reduced efficiently, as well as more functionalized aryl and alkyl bromides. The reduction process is proposed to occur by radical abstraction/hydrodehalogenation steps at ruthenium. Our research represents a safer and more sustainable alternative to typical silane, lithium aluminium hydride, and tin-based conditions for these reductions.

Aviso de cookies: Usamos cookies propias y de terceros para mejorar nuestros servicios, para análisis estadístico y para mostrarle publicidad. Si continua navegando consideramos que acepta su uso en los términos establecidos en la Política de cookies.