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couple of an N-heterocyclic carbene dinuclear FeIFeI complex, (μ-pdt) [FeI(CO)2(PMe3)][FeI(CO)2(IMes)] (IMes= 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), complex D, has led to the isolation of the mixed-valent cationic complex Dox as a biomimetic of the 2Fe2S subsite of the oxidized H cluster in [FeFe]hydrogenase. This is a rare example of FeIFeII paramagnetic H2ase model complex studied by X-ray diffraction. As compared with complex D, a remarkable reorientation of the IMes NHC ligand enables the (μ-pdt)[Fe(CO)2(PMe3)] [Fe(CO)2(IMes)]+ cation, Dox, to exist as a “rotated” structure, with structural and spectroscopic similarities to the diiron unit of Has isolated or Hox (Nicolet et al., 200; Roseboom et al., 2006). The structural makeup of the model includes a Fe-Fe distance that matches that of the enzyme, a semi-bridging CO group, and a pseudo-octahedral iron with open site blocked by a strategically positioned arene group from the bulky NHC carbene ligand (Peters et al., 1998; Nicolet et al., 1999). Other asymmetric disubstituted diiron complexes, (μ-pdt)[FeI(CO)2(P)][FeI(CO)2L)] with a selection of P-donor and NHC ligands designed to illustrate the principles that govern stability and function of the FeIFeII redox level are being studied. The reactivity of the mixed valent FeIFeII species is being explored.
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Bioinspired Chemistry for Energy: A Workshop Summary to the Chemical Sciences Roundtable
Appendix C
Poster Abstracts
ARTIFICIAL HYDROGENASE SYSTEMS
Redox Reactivity of Amine Hydrides of Iridium
Zachariah M. Heiden and Thomas B. Rauchfuss
Department of Chemistry, University of Illinois, Urbana, IL 61801
Metal hydrido-amine complexes (metal = Ru, Rh, and Ir) popularlized by Noyori et al. are highly active and enantioselective transfer hydrogenation catalysts. Much of their reactivity beyond use as transfer hydrogenation catalysts remains relatively unexplored. The metal diamido complexes behave as a dehydrogenase-related catalyst toward alcohol/organic substrates. We have found that protonation of the unsaturated diamido derivatives affords an unusual class of soft Lewis acids that will be described. Furthermore, the hydrido-amines act as an oxygenase, and homogeneous fuel cell, catalyzing the unusual reduction of dioxygen with hydrogen similar to knall gas bacteria, resulting in water as the only byproduct.
Mixed Valent, Fe(II)Fe(I), Diiron Complexes Reproduce the Unique Rotated State of the [FeFe]Hydrogenase Active Site
Tianbiao Liu and Marcetta Y. Darensbourg
Department of Chemistry, Texas A&M University, College Station, TX 77843
The reversible couple of an N-heterocyclic carbene dinuclear FeIFeI complex, (μ-pdt) [FeI(CO)2(PMe3)][FeI(CO)2(IMes)] (IMes= 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), complex D, has led to the isolation of the mixed-valent cationic complex Dox as a biomimetic of the 2Fe2S subsite of the oxidized H cluster in [FeFe]hydrogenase. This is a rare example of FeIFeII paramagnetic H2ase model complex studied by X-ray diffraction. As compared with complex D, a remarkable reorientation of the IMes NHC ligand enables the (μ-pdt)[Fe(CO)2(PMe3)] [Fe(CO)2(IMes)]+ cation, Dox, to exist as a “rotated” structure, with structural and spectroscopic similarities to the diiron unit of Has isolated or Hox (Nicolet et al., 200; Roseboom et al., 2006). The structural makeup of the model includes a Fe-Fe distance that matches that of the enzyme, a semi-bridging CO group, and a pseudo-octahedral iron with open site blocked by a strategically positioned arene group from the bulky NHC carbene ligand (Peters et al., 1998; Nicolet et al., 1999). Other asymmetric disubstituted diiron complexes, (μ-pdt)[FeI(CO)2(P)][FeI(CO)2L)] with a selection of P-donor and NHC ligands designed to illustrate the principles that govern stability and function of the FeIFeII redox level are being studied. The reactivity of the mixed valent FeIFeII species is being explored.
(1) Nicolet, Y. L., B. J. Lemon, J. C. Fontecilla-Camps, and J. W. Peters. Trends Biochem. Sci. 2000, 25, 138-143.
(2) Roseboom, W. D. L., A. L. De Lacey, V. M. Fernandez, E. C. Hatchikian, and S. P. J. Albracht, J. Biol. Inorg. Chem. 2006, 11, 102−118.
(3) Peters, J. W., W. N. Lanzilotta, B. J. Lemon, and L. C. Seefeldt, Science 1998, 282, 1853 −1858.
(4) Nicolet Y., C. Piras, P. Legrand, C. E. Hatchikian, and J. C. Fontecilla-Camps, Structure 1999, 7, 13− 23.
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Toward Understanding the Way Hydrogen Is Formed and Consumed at the Catalytic Center in the Ni-Fe Hydrogenase Enzymes
Michelle Millar, Dao Nguyen, Harmony Voorhies, and Susan Beatty, Department of Chemistry, SUNY Stony Brook, Stony Brook, New York 11794-3400
The Ni-Fe containing hydrogenases are multicomponent enzymes that catalyze the reversible production and consumption of H2. Beyond the biological significance, these enzymes have been heralded as models for potentially low-cost, efficient electrode replacements for the unique Pt electrode systems in fuel cells. We have acquired a number of special nickel-thiolate compounds that replicate some of the unusual structural and electronic and redox properties of the Ni center in hydrogenases, including series of Ni(II), NI(III) and Ni(IV) redox levels, Ni-H and Ni-CO interactions, as well as Ni-Fe compounds that replicate a portion of the Ni-Fe centers in hydrogenase. Attempts to acquire species that display electrocatalysis will be presented. The ligands developed for this chemistry contain the PS3 and PS2 coordinating entities, as well as related derivatives.
New Concepts in Hydrogen Processing: Modeling the Hmd Cofactor and Redox Active Ligands with Platinum Metals
Aaron Royer, Swarna Kokatam, Zachariah Heiden, Thomas B. Rauchfuss
The enzyme H2-forming methylenetetrahydromethanopterin dehydrogenase, Hmd, is associated with a central step in methanogenesis by Ni-starved archaea. The active site contains an Fe(II) bound organic 3,5-dimethylpyrid-2-one-6-acetic acid group conjugated to a nucleotide. While the Fe complexation in the native enzyme is yet unknown, we have examined the coordination and reactivity of a similar organic ligand, 6-carboxymethyl-4-methyl-2-hydroxypyridine, with Cp*Rh(III). In order to probe the role of the cofactor, dehydrogenation of secondary alcohols and interligand hydrogen bonding will be discussed.
Transition metal ions with organic radicals exist in the active sites of metalloproteins. The best understood example is galactose oxidase, which features a single Cu(II) ion coordinated to a modified tyrosyl radical. Many combined experimental and theoretical studies have focused on electronic properties of metal complexes with redox active ligands, yet reactivity beyond characterization has been limited. We will demonstrate the influence of the metal complex redox state on H2 activation by anilino-phenolate noninnocent ligands.
Biomimetic Efficiency: A Structural and Electronic Investigation of Rotational Barriers Found in DFT-inspired Fe-hydrogenase Models
Michael Singleton, Roxanne Jenkins, and Marcetta Y. Darensbourg
Department of Chemistry, Texas A&M University, College Station, TX 77843
While the literature is filled with structural models of Fe-hydrogenases, a truly efficient functional model for the uptake or production of hydrogen gas has yet to be realized. This deficiency is often blamed on the fact that most structural models do not contain the unique “rotated” or entatic state that is the consensus structure of the enzyme active site (eas) in its resting state.1 As demonstrated by 13C VT NMR studies the minimal model of the eas, (μ-pdt)[FeI(CO)3]2 shows mobility in the FeI(CO)3 units via apical/basal intramolecular CO exchange and in the 3-atom S to S linker.2 Density functional theory computations have suggested that an electronic effect engendered by the substitution of a CO by a better donor ligand, (μ-pdt)[FeI(CO)3][FeI(CO)2L], lowers the barrier to rotation of the nonsubstituted Fe(CO)3 unit.3 The computations also suggest that a steric effect in the μ-SRS bridge promotes rotation. In an effort to verify the computational results, we have prepared a series of sterically bulky (μ-SRS)[Fe(CO)3]2 complexes such as the (μ-SCH2C(Me)2CH2S)[FeI(CO)3]2 shown left and characterized them by various X-ray diffraction as well as other spectroscopies, including 13C VT NMR. The prospective application of functional biomimetic models toward the development of cost-effective fuel cells has also led to the evaluation of the all CO compounds as well as the L-substituted derivatives as electrocatalysts for H2 production.
(1) Nicolet, Y. L., B. J. Lemon, J. C. Fontecilla-Camps, and J. W. Peters. Trends Biochem. Sci. 2000, 25, 138-143.
(2) Lyon, E. J., I. P. Georgakaki, J. H. Reibenspies, and M. Y. Darensbourg, J. Am. Chem. Soc. 2001, 123, 3268-3278.
(3) Tye, J. W., M. Y. Darensbourg, and M. B. Hall, Inorg. Chem. 2006, 45, 1552-1559.
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ARTIFICIAL PHOTOSYNTHETIC SYSTEMS
Multifrequency Pulsed EPR Studies of the Manganese Cluster of PSII
Greg Yeagle, Richard Debus, R. David Britt
We are completing the construction of our CalEPR center at UC-Davis (http://brittepr.ucdavis.edu) with five research-grade EPR instruments. Of particular note here are two pulsed EPR instruments working at the intermediate microwave frequencies of 31 and 35 GHz that are providing new high-resolution data on amino acid coordination of the important water-splitting manganese cluster of Photosystem II (PSII).
Previous lower-frequency electron spin echo envelope modulation (ESEEM) studies showed a histidine nitrogen interaction with the Mn cluster in the S2 state, but the amplitude and resolution of the spectra were relatively poor at these low frequencies. With the intermediate frequency instruments we are much closer to the “exact cancellation” limit, which optimizes ESEEM spectra for hyperfine-coupled nuclei such as 14N and 15N. We will report the results on 14N and 15N labeled PSII at these two frequencies, along with simulations constrained by both isotope datasets at both frequencies, with a focus on high-resolution spectral determination of the histidine ligation to the cluster in the S2 state.
Photochemical Production of Hydride Donor with Ruthenium Complexes with an NAD+ Model Ligand
Etsuko Fujita,1 Dmitry Polyansky,1 Diane Cabelli,1 Koji Tanaka,2 and James T. Muckerman1
1Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA
2Institute for Molecular Science and CREST, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan
NAD+/NADH is one of the most important redox mediators in biological systems, including photosystem I, and acts as a reservoir/source of two electrons and a proton. A polypyridylruthenium complex with an NAD+ functional model ligand investigated here is the first example that an NAD+/NADH model complex works as a catalytic hydride donor for chemical reactions such as the electroreduction of acetone to 2-propanol (Koizumi and Tanaka, 2005). Herein we report clear evidence (Polyanski et al., in press) of photochemical formation of a hydride donor that can transfer a hydride or its equivalent to acetone, and ultimately to C1 species derived from CO2 reduction as nature does. These results open a new door for photocatalytic hydride (or proton-coupled-electron) transfer reactions originating from metal-to-ligand charge-transfer (MLCT) excited states of metal complexes with a bioinspired NADH-like ligand, and point to a new path for generating fuels from solar energy.
The research carried out at Brookhaven National Laboratory was supported under contract DEAC02-98CH10886 with the U.S. Department of Energy.
(1) Koizumi, T., and K. Tanaka, Angew. Chem. Int. Ed. 2005, 44, 5891-5894.
(2) Polyansky, D., D. Cabelli, J. T. Muckerman, E. Fujita, T. Koizumi, T. Fukushima, T. Wada, and K. Tanaka, Angew. Chem. Int. Ed. 2007, in press.
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Pathway to the Artificial Photosynthetic Unit
Elias Greenbaum,1 Barbara R. Evans,1 Hugh M. O’Neill,1 and Ida Lee2
1Chemical Sciences Division, Oak Ridge National Laboratory
2Department of Electrical Engineering, University of Tennessee
A key objective in the field of bioinspired chemistry for energy production is a comprehensive understanding of light-driven electron transfer and the generation of a rational model to serve as a template for future synthetic nanomaterials for solar fuels production. Research at Oak Ridge National Laboratory is aimed at integration of fundamental molecular structural, kinetic, and mechanistic understanding of the conversion of solar energy into chemical energy. The critical science problems of this area of research are the harvesting of solar photons throughout the visible region of the solar emission spectrum and the photocatalytic formation of small fuel molecules, such as hydrogen, methanol, or methane. Using natural photosynthesis as our inspiration combined with biological-synthetic (“soft-hard”) catalyst structures, we will couple biomimetic light-activated energetic reactions to nanoscale photocatalytic chemistry to drive the fuel-forming reactions and use water as the source of electrons. This research program will produce the first artificial photosynthetic units and artificial photosynthetic membranes. Success in this area will have a significant impact on the larger picture of a fossil-fuel-free future in which renewable fuels are produced by bioinspired photocatalytic systems.
Linker Controlled Energy and Charge Sharing Chlorophyll A Assemblies
Richard F. Kelley,1 Michael J. Tauber,2 and Michael R. Wasielewski1
1Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208
2Department of Chemistry and Biochemistry, University of California, San Diego, CA 92093
The ability of chlorophyll molecules to act as donors and acceptors for both energy and charge transfer in natural photosynthetic systems makes the incorporation of these chromophore/redox centers into artificial photosystems highly desirable. Here we present the first Suzuki and Sonogashira cross-coupling to the 20-meso position of chlorophyll a. This methodology was used to rigidly incorporate chlorophyll a molecules into several arrays using both covalent and noncovalent interactions. The rigid linkers allow efficient energy transfer among neighboring chlorophylls, efficient charge transfer between chlorophylls in the covalent arrays, and unhindered self-assembly of the arrays in nonpolar media. Small-angle X-ray scattering (SAXS) measurements using the high-flux synchrotron radiation of the Advanced Photon Source at Argonne National Laboratory was used to elucidate the structures of the noncovalent assemblies. The picosecond lifetimes of energy hopping between chromophores in each array were determined using femtosecond transient absorption spectroscopy. Solution-phase electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) studies on chemically oxidized arrays reveal that an unpaired electron is shared between the redox centers in the covalent arrays on a timescale faster than 107 Hz. Future work involves characterization of charge migration in the noncovalently assembled systems.
Bioinspired Supramolecular Device and Self-Assembly for Artificial Photosynthetic Reaction Center
Oh-Kil Kim,1 Mike Pepitone,1 Sungjae Chung,1 Joseph Melinger,2 Glenn Jernigan,2 and Daniel Lowry3
1Chemistry Division.
2Electronics Science & Technology Division.
3Center for Biomolecular Science & Engineering, and Institute for Nanoscience
A unique supramolecular device is architectured for artificial photosynthetic reaction center based on helical amylose, which is a linear chain polymer of 1,4-α D-glucose and capable of encapsulating various guest molecules as long as the size and interaction forces are compatible with each other. A photo/electro-active donor-acceptor (D-A) pair chromophore is included and rigidified inside the helix, and the helical surface is templated by an array of cyanine dye J-aggregates (super-helix). Such integration of the supramolecular entity occurs by spontaneous self-organization processes in the presence of amylose and the resulting nanodevice becomes water soluble.
A close photonic/electronic communication takes place across the helix between the J-aggregates (antenna) and the chromophore (inside the helix) such that very efficient exciton/electron-transfer proceeds unidirectionally along the helical axis. Energy-transfer (ET) and electron-transfer (eT) from the antenna to D, and from D to A in the confined chromophore, respectively, were investigated based on fluorescence quenching and excited-state lifetime measurements with respect to the helical encapsulation, D-A distance, D/A strength. A remarkably efficient (> 95 percent) ET and eT over D-A distance >20 Å were observed with distinct distance dependence and directionality for the encapsulated chromophores in clear contrast with the encapsulation-free counterparts. It was also found that the helical encapsulation is a powerful means to develop a highly ordered self-assembly of chromophores onto a substrate. This was proved by a fast redox reaction in cyclic voltammetry and oriented thin films often as helical bundles (AFM) upon casting aqueous solution. These were not observable with the encapsulation-free chromophores under the conditions employed.
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Light Energy Conversion by Photosynthetic Proteins at Inorganic Electrodes
Nikolai Lebedev
U.S. Naval Research Laboratory, 4555 Overlook Ave., Washington, DC 20375
The photosynthetic reaction center (RC) is one of the most advanced light-sensing and energy-converting materials developed by nature. Its coupling with inorganic surfaces is attractive for the identification of the mechanisms of interprotein electron transfer (ET) and for the possible applications for the construction of protein-based innovative photoelectronic and photovoltaic devices. Using genetically engineered bacterial RC proteins and specifically synthesized organic linkers, we were able to construct self-assembled and aligned biomolecular surfaces on various electrodes, including gold, carbon, indium tin oxide (ITO), highly ordered pyrrolytic graphite (HOPG), and carbon nanotube (CNT) arrays. Our results show that after immobilization on the electrodes, the photosynthetic RC can operate as a highly efficient photosensor, optical switch, and photovoltaic device.
WATER OXIDATION
Bioinspired Manganese Complexes for Solar Energy Utilization
Gary W. Brudvig, Sabas G. Abuabara, Clyde W. Cady, Jason B. Baxter, Charles A. Schmuttenmaer, Robert H. Crabtree, and Victor S. Batista
Department of Chemistry, Yale University, PO Box 208107, New Haven, CT 06520-8107
Manganese complexes that catalyze the evolution of oxygen from water, inspired by the oxygen-evolving complex of photosystem II (McEvoy and Brudvig, 2006; Spoviero. et al., 2007), have been extensively investigated by our group (Limburg et al., 1999; Chen et al., 2007). With the goal of using water-oxidation catalysts for solar energy utilization, we have studied the photochemistry of TiO2 nanoparticles to which a Mn(II)-terpy complex is covalently attached (terpy = 2,2′:6,2″-terpyridine). These TiO2 nanoparticles exhibit visible-light sensitization and charge separation as evidenced by UV-visible, terahertz, and EPR spectroscopy of colloidal thin films and aqueous suspensions. Photoexcitation of [MnII(H2O)3(catechol-terpy)]2+/TiO2 surface-attached complex leads to Mn(II)→Mn(III) photooxidation within 300 fs, as indicated by terahertz spectroscopic measurements and computational simulations of interfacial electron transfer. The half-time for regeneration of the Mn(II) complex is ca. 23 sec (at 6 K), as monitored by time-resolved measurements of the Mn(II) EPR signal. These results are expected to be particularly relevant to photocatalytic applications of Mn(III) complexes, which are known to be effective catalysts for a wide range of oxidation reactions because Mn(III) is only well known as a stoichiometric oxidant. Our aim is to make the reaction catalytic.
(1) “Water-Splitting Chemistry of Photosystem II”, James P. McEvoy and Gary W. Brudvig (2006) Chem. Rev. 106, 4455-4483.
(2) “Quantum Mechanics/Molecular Mechanics Structural Models of the Oxygen-Evolving Complex of Photosystem II”, Eduardo M. Sproviero, José A. Gascón, James P. McEvoy, Gary W. Brudvig and Victor S. Batista (2007) Curr. Opin. Struct. Biol. 17, 173-180.
(3) “A Functional Model for O-O Bond Formation by the O2-Evolving Complex in Photosystem II”, Julian Limburg, John S. Vrettos, Louise M. Liable-Sands, Arnold L. Rheingold, Robert H. Crabtree and Gary W. Brudvig (1999) Science 283, 1524-1527.
(4) “Speciation of the Catalytic Oxygen Evolution System: [MnIII/IV2(μ-O)2 (terpy)2(H2O)2](NO3)3 + HSO5−”, Hongyu Chen, Ranitendranath Tagore, Gerard Olack, John S. Vrettos, Tsu-Chien Weng, James Penner-Hahn, Robert H. Crabtree and Gary W. Brudvig (2007) Inorg. Chem. 46, 34-46.
Bioinspired Water Oxidation Catalysts for Renewable Energy Production
Greg A. N. Felton,1 Robin Brimblecombe,2 Johanna Scarino,1 John Sheats,3 Gerhard F. Swiegers,4 Leone Spiccia,2 G. Charles Dismukes.1
1Department of Chemistry and the Environmental Institute, Princeton University.
2School of Chemistry Monash University, Australia.
3Science Faculty, Rider University.
4Division of Molecular Science Commonwealth Scientific and Industrial Research Organisation, Australia.
The capture of light energy to drive water splitting is considered key to future renewable energy production. Studies of the natural photosynthetic water oxidation complex (WOC) of photosystem II (PSII) have led to a series of bioinspired model compounds. These compounds contain [Mn4O4]7+ cubic cores. Presently, conditions have been discovered that enable these manganese-oxo cubanes to catalyze the sustained photo-assisted oxidation of water, for several thousand turnovers. These conditions are based on the doping of these cubane compounds into a Nafion® film. The properties of these compounds, along with the nature of the conditions use in their incorporation into photoanodes, are being vigorously explored.
Fine-Tuning the Redox Potential of Mn4O4L6 Cubes by Use of Substituted Diarylphosphinic Acids
John E. Sheats,1 G. Charles Dismukes,2 Paul Lucuski,1 Marlena Konieczynska,1,3 Eric Sellitto,1,4 Esteban Alverado, 1,3 Matthew Vecchione,1,4 and Arren Washington1,4
1 Department of Chemistry, Biochemistry, and Physics, Rider University, Lawrenceville, NJ 08648.
2 Department of Chemistry, Princeton University, Princeton, NJ 08544.
3Project SEED Student
4Undergraduate Student, Rider University.
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Dismukes and coworkers have demonstrated that Mn4O4L6 (L=Ar2PO2−), when bound to a Nafion-coated electrode, can oxidize H2O to produce H2 and O2 in a catalytic cycle for up to 50,000 turnovers. The voltage required is 1.20 V, the same as needed for photosynthetic oxygen evolution. The Mn cubes have been isolated in four oxidation states: Mn4 (III) 4O2(OCH3)2L6 to Mn4(III)(IV)3O4L6+. The redox potential for the oxidation of Mn4O4L6 (L=(C6H5)2PO2) to Mn4O4L6+, 1.20 V, can be reduced by 0.15 V by using (4−CH3O−C6H4)2PO2− and increased substantially by use of (3−NO2−C6H4)2PO2−. Experiments are underway to test stronger electron donors such as 4−(CH3)2N−C6H4 and 4−t−C4H9O−C6H4 and weaker acceptors such as 4−CF3−C6H4 and 3−Cl−C6H4. Methods for covalently anchoring the Mn cubes to the surface of an electrode are also being investigated.
SOLAR CELLS
Self-Assembled Biomimetic Multifunctional Coatings
Nicholas C. Linn, Chih-Hung Sun, Peng Jiang
Department of Chemical Engineering, University of Florida, Gainesville, FL 32611
We report a simple bioinspired self-assembly technique for fabricating multifunctional optical coatings that mimic both unique functionalities of antireflective motheye and superhydrophobic cicada wing. Wafer-scale, non-close-packed colloidal crystals with remarkably large hexagonal domains are created by a spin-coating technology, which is based on shear-aligning colloidal silica particles suspended in nonvolatile triacrylate monomers. The resulting polymer-embedded colloidal crystals exhibit highly ordered surface modulation and can be used directly as templates to cast poly(dimethylsiloxane) (PDMS) molds. Moth-eye antireflection coatings with adjustable reflectivity can then be molded against the PDMS master. The specular reflection of replicated nipple arrays matches the theoretical prediction using a thin-film multilayer model. The microstructures of the replicated films also lead to the formation of hydrophobic surfaces, even though the native material is inherently hydrophilic. These biomimetic materials are of great technological importance in developing self-cleaning antireflection optical coatings for crystalline silicon solar cells.
Nanopyramid Arrays for Solar Cells
Chih-Hung Sun, Nicholas C. Linn, Peng Jiang
Department of Chemical Engineering, University of Florida, Gainesville, FL 32611
Current production of solar cells is dominated by crystalline silicon modules; however, due to the high refractive index of silicon, more than 30 percent of incident light is reflected back, which greatly reduces the conversion efficiency of photovoltaic devices. Surface texturing has become a common practice for Si solar cells and, in combination with vacuum deposited antireflection coatings (ARCs), reduces reflection losses a few percent. Unfortunately, the high cost of vacuum deposition of ARCs is a big challenge for economic production of large photovoltaic panels. Inspired by the antireflection properties of moth eyes, we have developed subwavelength ARCs for crystalline silicon solar cells. Wafer-scale, crystalline arrays of inverted pyramids, which directly function as efficient ARCs, are anisotropically etched in silicon substrates by a cheap yet scalable non-lithographic technique. The inverted pyramid array on Si dramatically reduces the specular reflectivity of the surface and consequently has the potential to increase the conversion efficiency of silicon solar cells.
X-ray Fingerprinting Bioinspired Supramolecular Structure and Dynamics in Solution
D. M. Tiede,1 X. Zuo,1 L. X. Chen,1 and K. Attenkofer2
1Chemistry Division and 2Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, 60439
Bioinspired, self-assembling supramolecular materials are increasingly being designed for applications in solar energy conversion and storage. However, the dynamic features of these molecular materials typically preclude structural analyses using crystallographic techniques. This makes in situ structural characterization a critical challenge. We have developed techniques that combine wide-angle solution X-ray scattering (WAXS) measured to better than 2 Å spatial resolution with atomistic simulation to provide a new experimental approach for the characterization of supramolecular solution state structure. Comparisons between experimental scattering patterns measured for a range of proteins, DNA, metal coordination complexes, and host-guest assemblies show WAXS and corresponding pair distribution function (PDF) patterns to be sensitive to supramolecular conformation, dynamics, and solvation. For example, a comparison of experimental scattering and PDF patterns for γ-cyclodextrin show features characteristic of the host structure, configurational broadening, and solvation. In current work we are testing the ability of WAXS to serve as a benchmark for quantitative evaluation of molecular dynamics simulations. The ability to provide an experimental marker for supramolecular dynamics and solvation that is directly connected to coordinate models represents a new opportunity for resolving structural dynamics coupled to light-induced charge separation in natural and artificial host matrices. Toward this end we are extending the WAXS technique to include pump-probe techniques at the Advanced Photon Source. Future work is planned for combining 100 ps time-resolved WAXS, X-ray spectroscopy, and magnetic resonance data to achieve a more complete picture of structural reorganization resolved during the time-course of solar energy conversion function.
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Experimental PDF measured for γ-cyclodextrin with 1.0 Å spatial resolution (top) compared to PDF calculated from a coordinate model with resolution varying from 6 Å (1) to 0.4 Å (7).
BIOLOGICAL TRANSFORMATIONS
Electrobiocatalytic Reduction of CO2 to Formate: Whole Cell and Isolated Enzyme Systems
Boonchai Boonyaratanakornkit,1 Rolf J. Mehlhorn,1 Robert Kostecki,1 Douglas S. Clark1,2
1Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
2Department of Chemical Engineering, University of California, Berkeley, CA 94720
Enzymatic reduction of CO2 to formate and ultimately to methanol can occur via concurrent electrochemical regeneration of reduced cofactors. By using photovoltaic energy this bioelectrochemical reduction can provide transportable, energy-dense, carbon-neutral liquid fuels. The enzymes involved in fuel production are formate dehydrogenase (FDH) and methanol dehydrogenase, which use the cofactors methyl viologen and pyrroloquinoline quinine (PQQ), respectively. Two issues addressed are the O2-lability of FDH and electron transfer to the enzyme’s redox center. Whole-cell biocatalysis is explored by demonstrating that reduced cofactor is permeable to the cell membrane. Cells provide an intracellular environment that stabilizes FDH against O2 inactivation. Furthermore, we are connecting FDH to a graphite electrode via a PQQ-FAD linker to enable direct electron transfer from the electrode to the enzyme. This will obviate diffusion of cofactor into the enzyme’s redox center and should increase the rate of CO2 reduction.
BIOINSPIRED POLYMERS
Bioinspired Polymers for Nanoscience Research
Ronald Zuckermann
Lead Scientist, Biological Nanostructures Facility
The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California
Peptoids are a novel class of non-natural biopolymer based on an N-substituted glycine backbone that are ideally suited for nanomaterials research. This bioinspired material has many unique properties that bridge the gap between proteins and bulk polymers. Like proteins, they are a sequence-specific heteropolymer, capable of folding into specific shapes and exhibiting potent biological activities; and like polymers, they are chemically and biologically stable and relatively cheap to make. Peptoids are efficiently assembled via automated solid-phase synthesis from hundreds of chemically diverse building blocks, allowing the rapid generation of huge combinatorial libraries. This provides a platform to discover nanostructured materials capable of protein-like molecular recognition and function.
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