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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Appendix C Poster Abstracts ARTIfICIAL HYDROGENASE SYSTEMS [FeFe]hydrogenase. This is a rare example of FeIFeII para- magnetic H2ase model complex studied by X-ray diffraction. Redox Reactivity of Amine Hydrides of Iridium As compared with complex D, a remarkable reorientation of Zachariah M. Heiden and Thomas B. Rauchfuss the IMes NHC ligand enables the (μ-pdt)[Fe(CO)2(PMe3)] Department of Chemistry, University of Illinois, Urbana, [Fe(CO)2(IMes)]+ cation, Dox, to exist as a “rotated” struc- IL 61801 ture, with structural and spectroscopic similarities to the diiron unit of Has isolated or Hox (Nicolet et al., 200; Roseboom Metal hydrido-amine complexes (metal = Ru, Rh, et al., 2006). The structural makeup of the model includes and Ir) popularlized by Noyori et al. are highly active and a Fe-Fe distance that matches that of the enzyme, a semi- enantioselective transfer hydrogenation catalysts. Much of bridging CO group, and a pseudo-octahedral iron with open their reactivity beyond use as transfer hydrogenation cata- site blocked by a strategically positioned arene group from lysts remains relatively unexplored. The metal diamido com- the bulky NHC carbene ligand (Peters et al., 1998; Nicolet et plexes behave as a dehydrogenase-related catalyst toward al., 1999). Other asymmetric disubstituted diiron complexes, alcohol/organic substrates. We have found that protonation (μ-pdt)[FeI(CO)2(P)][FeI(CO)2L)] with a selection of P-donor of the unsaturated diamido derivatives affords an unusual and NHC ligands designed to illustrate the principles that class of soft Lewis acids that will be described. Furthermore, govern stability and function of the FeIFeII redox level are the hydrido-amines act as an oxygenase, and homogeneous being studied. The reactivity of the mixed valent FeIFeII fuel cell, catalyzing the unusual reduction of dioxygen with species is being explored. 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 A-1.eps (1) Nicolet, Y. L., B. J. Lemon, J. C. image bitmap Fontecilla-Camps, and J. W. Peters. The reversible Fe Fe D Fe Fe couple of an N- I I I II Trends Biochem. Sci. 2000, 25, 138-143. heterocyclic carbene dinuclear FeIFeI complex, (μ-pdt) (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. [FeI(CO)2(PMe3)][FeI(CO)2(IMes)] (IMes= 1,3-bis(2,4,6- (3) Peters, J. W., W. N. Lanzilotta, B. J. Lemon, and L. C. Seefeldt, Science trimethylphenyl)imidazol-2-ylidene), complex D, has led to 1998, 282, 1853 –1858. the isolation of the mixed-valent cationic complex Dox as a (4) Nicolet Y., C. Piras, P. Legrand, C. E. Hatchikian, and J. C. Fontecilla- biomimetic of the 2Fe2S subsite of the oxidized H cluster in Camps, Structure 1999, 7, 13– 23. 5

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 APPENDIX C Toward Understanding the Way Hydrogen Is Formed Biomimetic Efficiency: A Structural and Electronic and Consumed at the Catalytic Center in the Ni-Fe Investigation of Rotational Barriers Found in DFT- Hydrogenase Enzymes inspired Fe-hydrogenase Models Michelle Millar, Dao Nguyen, Harmony Voorhies, and Susan Michael Singleton, Roxanne Jenkins, and Marcetta Y. Beatty, Department of Chemistry, SUNY Stony Brook, Stony Darensbourg Brook, New York 11794-3400 Department of Chemistry, Texas A&M University, College Station, TX 77843 The Ni-Fe containing hydrogenases are multicomponent enzymes that catalyze the reversible production and con- While the literature is filled with structural models of sumption of H2. Beyond the biological significance, these Fe-hydrogenases, a truly efficient functional model for the enzymes have been heralded as models for potentially uptake or production of hydrogen gas has yet to be realized. low-cost, efficient electrode replacements for the unique Pt This deficiency is often blamed on the fact that most struc- electrode systems in fuel cells. We have acquired a number tural models do not contain the unique “rotated” or entatic of special nickel-thiolate compounds that replicate some state that is the consensus structure of the enzyme active site of the unusual structural and electronic and redox proper- (eas) in its resting state.1 As demonstrated by 13C VT NMR ties of the Ni center in hydrogenases, including series of studies the minimal model of the eas, (μ-pdt)[FeI(CO)3]2 Ni(II), NI(III) and Ni(IV) redox levels, Ni-H and Ni-CO shows mobility in the FeI(CO)3 units via apical/basal intra- interactions, as well as Ni-Fe compounds that replicate a por- molecular CO exchange and in the 3-atom S to S linker.2 tion of the Ni-Fe centers in hydrogenase. Attempts to acquire Density functional theory computations have suggested species that display electrocatalysis will be presented. The that an electronic effect engendered by the substitution of ligands developed for this chemistry contain the PS3 and PS2 a CO by a better donor ligand, (μ-pdt)[FeI(CO)3][FeI(CO coordinating entities, as well as related derivatives. )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 New Concepts in Hydrogen Processing: Modeling the to verify the computational results, we have prepared a Hmd Cofactor and Redox Active Ligands with Platinum series of sterically bulky (μ-SRS)[Fe(CO)3]2 complexes Metals such as the (μ-SCH2C(Me)2CH2S)[FeI(CO)3]2 shown left Aaron Royer, Swarna Kokatam, Zachariah Heiden, Thomas and characterized them by various X-ray diffraction as B. Rauchfuss well as other spectroscopies, including 13C VT NMR. The prospective application of functional biomimetic models The enzyme H 2 -forming methylenetetrahydro- toward the development of cost-effective fuel cells has methanopterin dehydrogenase, Hmd, is associated with also led to the evaluation of the all CO compounds as well a central step in methanogenesis by Ni-starved archaea. as the L-substituted derivatives as electrocatalysts for H2 The active site contains an Fe(II) bound organic 3,5- production. dimethylpyrid-2-one-6-acetic acid group conjugated to a (1) Nicolet, Y. L., B. J. Lemon, J. C. Fontecilla-Camps, and J. W. Peters. nucleotide. While the Fe complexation in the native enzyme Trends Biochem. Sci. 2000, 25, 138-143. is yet unknown, we have examined the coordination and (2) Lyon, E. J., I. P. Georgakaki, J. H. Reibenspies, and M. Y. Darensbourg, reactivity of a similar organic ligand, 6-carboxymethyl-4- J. Am. Chem. Soc. 2001, 123, 3268-3278. methyl-2-hydroxypyridine, with Cp*Rh(III). In order to (3) Tye, J. W., M. Y. Darensbourg, and M. B. Hall, Inorg. Chem. 2006, 45, probe the role of the cofactor, dehydrogenation of second- 1552-1559. ary 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 coor- dinated to a modified tyrosyl radical. Many combined experi- mental 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.

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 APPENDIX C Previous lower-frequency electron spin echo envelope Cys modulation (ESEEM) studies showed a histidine nitrogen S S [4Fe4S] S interaction with the Mn cluster in the S2 state, but the Fe Fe amplitude and resolution of the spectra were relatively poor OC C C N Hδ+ C C at these low frequencies. With the intermediate frequency N O O δ+ H instruments we are much closer to the “exact cancellation” limit, which optimizes ESEEM spectra for hyperfine-coupled enzyme active site “entatic” state nuclei such as 14N and 15N. We will report the results on N and 15N labeled PSII at these two frequencies, along 14 with simulations constrained by both isotope datasets at both frequencies, with a focus on high-resolution spectral S CO determination of the histidine ligation to the cluster in the S S2 state. Fe Fe OC C CO C OC O O Photochemical Production of Hydride Donor with transition state Ruthenium Complexes with an NAD+ Model Ligand structure of mode Etsuko Fujita,1 Dmitry Polyansky,1 Diane Cabelli,1 Koji Tanaka,2 and James T. Muckerman1 Chemistry Department, Brookhaven National Laboratory, Upton, NY 1 11973-5000, USA S OC O Institute for Molecular Science and CREST, 5-1 Higashiyama, Myodaiji, 2 C S Okazaki, Aichi 444-8787, Japan Fe Fe OC CO OC CO NAD+/NADH is one of the most important redox ground state mediators in biological systems, including photosystem I, structure of mode and acts as a reservoir/source of two electrons and a proton. A polypyridylruthenium complex with an NAD+ functional A-2.eps 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 photo- chemical 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 A-3.eps metal complexes with a bioinspired NADH-like ligand, and bitmap image—enlarged only 110% to preserve resolution to a new path for generating fuels from solar energy. point The research carried out at Brookhaven National Labo- ratory was supported under contract DEAC02-98CH10886 ARTIfICIAL PHOTOSYNTHETIC SYSTEMS with the U.S. Department of Energy. Multifrequency Pulsed EPR Studies of the Manganese Cluster of PSII (1) Koizumi, T., and K. Tanaka, Angew. Chem. Int. Ed. 2005, , 5891-5894. Greg Yeagle, Richard Debus, R. David Britt (2) Polyansky, D., D. Cabelli, J. T. Muckerman, E. Fujita, T. Koizumi, T. Fukushima, T. Wada, and K. Tanaka, Angew. Chem. Int. Ed. 2007, We are completing the construction of our CalEPR in press. 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 provid- ing new high-resolution data on amino acid coordination of the important water-splitting manganese cluster of Photo- system II (PSII).

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 APPENDIX C Pathway to the Artificial Photosynthetic Unit lifetimes of energy hopping between chromophores in each Elias Greenbaum,1 Barbara R. Evans,1 Hugh M. O’Neill,1 array were determined using femtosecond transient absorp- and Ida Lee2 tion spectroscopy. Solution-phase electron paramagnetic Chemical Sciences Division, Oak Ridge National Laboratory resonance (EPR) and electron-nuclear double resonance 1 Department of Electrical Engineering, University of Tennessee 2 (ENDOR) studies on chemically oxidized arrays reveal that an unpaired electron is shared between the redox centers in A key objective in the field of bioinspired chemistry the covalent arrays on a timescale faster than 107 Hz. Future for energy production is a comprehensive understanding of work involves characterization of charge migration in the light-driven electron transfer and the generation of a rational noncovalently assembled systems. model to serve as a template for future synthetic nano- materials for solar fuels production. Research at Oak Ridge National Laboratory is aimed at integration of fundamental Bioinspired Supramolecular Device and Self-Assembly molecular structural, kinetic, and mechanistic understand- for Artificial Photosynthetic Reaction Center ing of the conversion of solar energy into chemical energy. Oh-Kil Kim,1 Mike Pepitone,1 Sungjae Chung,1 Joseph The critical science problems of this area of research are the Melinger,2 Glenn Jernigan,2 and Daniel Lowry3 harvesting of solar photons throughout the visible region of Chemistry Division. 1 the solar emission spectrum and the photocatalytic forma- Electronics Science & Technology Division. 2 Center for Biomolecular Science & Engineering, and Institute for tion of small fuel molecules, such as hydrogen, methanol, 3 Nanoscience or methane. Using natural photosynthesis as our inspiration combined with biological-synthetic (“soft-hard”) catalyst A unique supramolecular device is architectured for structures, we will couple biomimetic light-activated ener- artificial photosynthetic reaction center based on helical getic reactions to nanoscale photocatalytic chemistry to amylose, which is a linear chain polymer of 1,4-α D-glucose drive the fuel-forming reactions and use water as the source and capable of encapsulating various guest molecules as of electrons. This research program will produce the first long as the size and interaction forces are compatible with artificial photosynthetic units and artificial photosynthetic each other. A photo/electro-active donor-acceptor (D-A) pair membranes. Success in this area will have a significant chromophore is included and rigidified inside the helix, and impact on the larger picture of a fossil-fuel-free future in the helical surface is templated by an array of cyanine dye which renewable fuels are produced by bioinspired photo- J-aggregates (super-helix). Such integration of the supra- catalytic systems. molecular entity occurs by spontaneous self-organization processes in the presence of amylose and the resulting nanodevice becomes water soluble. Linker Controlled Energy and Charge Sharing Chloro- A close photonic/electronic communication takes place phyll A Assemblies across the helix between the J-aggregates (antenna) and Richard F. Kelley,1 Michael J. Tauber,2 and Michael R. the chromophore (inside the helix) such that very efficient Wasielewski1 exciton/electron-transfer proceeds unidirectionally along Department of Chemistry, Northwestern University, 2145 Sheridan 1 the helical axis. Energy-transfer (ET) and electron-transfer Road, Evanston, IL 60208 Department of Chemistry and Biochemistry, University of California, (eT) from the antenna to D, and from D to A in the confined 2 San Diego, CA 92093 chromophore, respectively, were investigated based on fluo- rescence quenching and excited-state lifetime measurements The ability of chlorophyll molecules to act as donors and with respect to the helical encapsulation, D-A distance, acceptors for both energy and charge transfer in natural pho- D/A strength. A remarkably efficient (> 95 percent) ET and tosynthetic systems makes the incorporation of these chro- eT over D-A distance >20 Å were observed with distinct mophore/redox centers into artificial photosystems highly distance dependence and directionality for the encapsulated desirable. Here we present the first Suzuki and Sonogashira chromophores in clear contrast with the encapsulation-free cross-coupling to the 20-meso position of chlorophyll a. This counterparts. It was also found that the helical encapsulation methodology was used to rigidly incorporate chlorophyll a is a powerful means to develop a highly ordered self-assembly molecules into several arrays using both covalent and non- of chromophores onto a substrate. This was proved by a fast covalent interactions. The rigid linkers allow efficient energy redox reaction in cyclic voltammetry and oriented thin films transfer among neighboring chlorophylls, efficient charge often as helical bundles (AFM) upon casting aqueous solu- transfer between chlorophylls in the covalent arrays, and tion. These were not observable with the encapsulation-free unhindered self-assembly of the arrays in nonpolar media. chromophores under the conditions employed. 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

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 APPENDIX C Light Energy Conversion by Photosynthetic Proteins at for a wide range of oxidation reactions because Mn(III) is Inorganic Electrodes only well known as a stoichiometric oxidant. Our aim is to Nikolai Lebedev make the reaction catalytic. U.S. Naval Research Laboratory, 4555 Overlook Ave., Wash- (1) “Water-Splitting Chemistry of Photosystem II”, James P. McEvoy and ington, DC 20375 Gary W. Brudvig (2006) Chem. Rev. 0 4455-4483. . . 0, (2) “Quantum Mechanics/Molecular Mechanics Structural Models of the The photosynthetic reaction center (RC) is one of Oxygen-Evolving Complex of Photosystem II”, Eduardo M. Sproviero, the most advanced light-sensing and energy-converting José A. Gascón, James P. McEvoy, Gary W. Brudvig and Victor S. materials developed by nature. Its coupling with inorganic Batista (2007) Curr. Opin. Struct. Biol. , 173-180. (3) “A Functional Model for O-O Bond Formation by the O2-Evolving surfaces is attractive for the identification of the mecha- Complex in Photosystem II”, Julian Limburg, John S. Vrettos, Louise nisms of interprotein electron transfer (ET) and for the M. Liable-Sands, Arnold L. Rheingold, Robert H. Crabtree and Gary possible applications for the construction of protein-based W. Brudvig (1999) Science , 1524-1527. innovative photoelectronic and photovoltaic devices. Using (4) “Speciation of the Catalytic Oxygen Evolution System: [MnIII/IV2(μ-O)2 genetically engineered bacterial RC proteins and specifi- (terpy)2(H2O)2](NO3)3 + HSO5-”, Hongyu Chen, Ranitendranath Tagore, Gerard Olack, John S. Vrettos, Tsu-Chien Weng, James Penner-Hahn, cally synthesized organic linkers, we were able to construct Robert H. Crabtree and Gary W. Brudvig (2007) Inorg. Chem. , self-assembled and aligned biomolecular surfaces on vari- 34-46. ous electrodes, including gold, carbon, indium tin oxide (ITO), highly ordered pyrrolytic graphite (HOPG), and carbon nanotube (CNT) arrays. Our results show that after Bioinspired Water Oxidation Catalysts for Renewable immobilization on the electrodes, the photosynthetic RC can Energy Production operate as a highly efficient photosensor, optical switch, and Greg A. N. Felton,1 Robin Brimblecombe,2 Johanna Scarino,1 photovoltaic device. John Sheats,3 Gerhard F. Swiegers,4 Leone Spiccia,2 G. Charles Dismukes.1 WATER OxIDATION Department of Chemistry and the Environmental Institute, Princeton 1 University. School of Chemistry Monash University, Australia. 2 Bioinspired Manganese Complexes for Solar Energy Science Faculty, Rider University. 3 Division of Molecular Science Commonwealth Scientific and Industrial 4 Utilization Research Organisation, Australia. Gary W. Brudvig, Sabas G. Abuabara, Clyde W. Cady, Jason B. Baxter, Charles A. Schmuttenmaer, Robert H. Crabtree, The capture of light energy to drive water splitting and Victor S. Batista is considered key to future renewable energy production. Department of Chemistry, Yale University, PO Box 208107, Studies of the natural photosynthetic water oxidation com- New Haven, CT 06520-8107 plex (WOC) of photosystem II (PSII) have led to a series of bioinspired model compounds. These compounds contain Manganese complexes that catalyze the evolution of [Mn4O4]7+ cubic cores. Presently, conditions have been oxygen from water, inspired by the oxygen-evolving complex discovered that enable these manganese-oxo cubanes to of photosystem II (McEvoy and Brudvig, 2006; Spoviero. et catalyze the sustained photo-assisted oxidation of water, for al., 2007), have been extensively investigated by our group several thousand turnovers. These conditions are based on (Limburg et al., 1999; Chen et al., 2007). With the goal of the doping of these cubane compounds into a Nafion® film. using water-oxidation catalysts for solar energy utilization, The properties of these compounds, along with the nature of we have studied the photochemistry of TiO2 nanoparticles the conditions use in their incorporation into photoanodes, to which a Mn(II)-terpy complex is covalently attached are being vigorously explored. (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 Fine-Tuning the Redox Potential of Mn4O4L6 Cubes by of colloidal thin films and aqueous suspensions. Photoexcita- Use of Substituted Diarylphosphinic Acids tion of [MnII(H2O)3(catechol-terpy)]2+/TiO2 surface-attached John E. Sheats,1 G. Charles Dismukes,2 Paul Lucuski,1 complex leads to Mn(II)→Mn(III) photooxidation within Marlena Konieczynska,1,3 Eric Sellitto,1,4 Esteban Alverado, 1,3 300 fs, as indicated by terahertz spectroscopic measurements Matthew Vecchione,1,4 and Arren Washington1,4 and computational simulations of interfacial electron trans- Department of Chemistry, Biochemistry, and Physics, Rider University, 1 fer. The half-time for regeneration of the Mn(II) complex is Lawrenceville, NJ 08648. ca. 23 sec (at 6 K), as monitored by time-resolved measure- Department of Chemistry, Princeton University, Princeton, NJ 08544. 2 Project SEED Student ments of the Mn(II) EPR signal. These results are expected 3 Undergraduate Student, Rider University. 4 to be particularly relevant to photocatalytic applications of Mn(III) complexes, which are known to be effective catalysts

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50 APPENDIX C Dismukes and coworkers have demonstrated that ciency of photovoltaic devices. Surface texturing has become Mn4O4L6 (L=Ar2PO2-), when bound to a Nafion-coated elec- a common practice for Si solar cells and, in combination with trode, can oxidize H2O to produce H2 and O2 in a catalytic vacuum deposited antireflection coatings (ARCs), reduces cycle for up to 50,000 turnovers. The voltage required is reflection losses a few percent. Unfortunately, the high 1.20 V, the same as needed for photosynthetic oxygen evo- cost of vacuum deposition of ARCs is a big challenge for lution. The Mn cubes have been isolated in four oxidation economic production of large photovoltaic panels. Inspired states: Mn4 (III) 4O2(OCH3)2L6 to Mn4(III)(IV)3O4L6+. The by the antireflection properties of moth eyes, we have devel- redox potential for the oxidation of Mn4O4L6 (L=(C6H5)2PO2) oped subwavelength ARCs for crystalline silicon solar cells. to Mn4O4L6+, 1.20 V, can be reduced by 0.15 V by using Wafer-scale, crystalline arrays of inverted pyramids, which (4–CH3O–C6H4)2PO2– and increased substantially by use directly function as efficient ARCs, are anisotropically of (3–NO2–C6H4)2PO2–. Experiments are underway to test etched in silicon substrates by a cheap yet scalable non- stronger electron donors such as 4 –(CH 3) 2N –C 6H 4 and lithographic technique. The inverted pyramid array on Si 4–t–C4H9O–C6H4 and weaker acceptors such as 4–CF3-C6H4 dramatically reduces the specular reflectivity of the surface and 3 –Cl –C 6H 4. Methods for covalently anchoring the and consequently has the potential to increase the conversion Mn cubes to the surface of an electrode are also being efficiency of silicon solar cells. investigated. X-ray Fingerprinting Bioinspired Supramolecular Struc- SOLAR CELLS ture and Dynamics in Solution Self-Assembled Biomimetic Multifunctional Coatings D. M. Tiede,1 X. Zuo,1 L. X. Chen,1 and K. Attenkofer2 Chemistry Division and 2Advanced Photon Source, Argonne National Nicholas C. Linn, Chih-Hung Sun, Peng Jiang 1 Laboratory, Argonne, Illinois, 60439 Department of Chemical Engineering, University of Florida, Gainesville, FL 32611 Bioinspired, self-assembling supramolecular materials are increasingly being designed for applications in solar We report a simple bioinspired self-assembly technique energy conversion and storage. However, the dynamic fea- for fabricating multifunctional optical coatings that mimic tures of these molecular materials typically preclude struc- both unique functionalities of antireflective motheye and tural analyses using crystallographic techniques. This makes superhydrophobic cicada wing. Wafer-scale, non-close- in situ structural characterization a critical challenge. We packed colloidal crystals with remarkably large hexagonal have developed techniques that combine wide-angle solution domains are created by a spin-coating technology, which X-ray scattering (WAXS) measured to better than 2 Å spatial is based on shear-aligning colloidal silica particles sus- resolution with atomistic simulation to provide a new experi- pended in nonvolatile triacrylate monomers. The resulting mental approach for the characterization of supramolecular polymer-embedded colloidal crystals exhibit highly ordered solution state structure. Comparisons between experimental surface modulation and can be used directly as templates to scattering patterns measured for a range of proteins, DNA, cast poly(dimethylsiloxane) (PDMS) molds. Moth-eye anti- metal coordination complexes, and host-guest assemblies reflection coatings with adjustable reflectivity can then be show WAXS and corresponding pair distribution function molded against the PDMS master. The specular reflection (PDF) patterns to be sensitive to supramolecular conforma- of replicated nipple arrays matches the theoretical prediction tion, dynamics, and solvation. For example, a comparison of using a thin-film multilayer model. The microstructures of experimental scattering and PDF patterns for γ-cyclodextrin the replicated films also lead to the formation of hydrophobic show features characteristic of the host structure, configura- surfaces, even though the native material is inherently hydro- tional broadening, and solvation. In current work we are test- philic. These biomimetic materials are of great technological ing the ability of WAXS to serve as a benchmark for quan- importance in developing self-cleaning antireflection optical titative evaluation of molecular dynamics simulations. The coatings for crystalline silicon solar cells. ability to provide an experimental marker for supramolecular dynamics and solvation that is directly connected to coor- dinate models represents a new opportunity for resolving Nanopyramid Arrays for Solar Cells structural dynamics coupled to light-induced charge separa- Chih-Hung Sun, Nicholas C. Linn, Peng Jiang tion in natural and artificial host matrices. Toward this end we Department of Chemical Engineering, University of Florida, are extending the WAXS technique to include pump-probe Gainesville, FL 32611 techniques at the Advanced Photon Source. Future work is planned for combining 100 ps time-resolved WAXS, X-ray Current production of solar cells is dominated by crys- spectroscopy, and magnetic resonance data to achieve a more talline silicon modules; however, due to the high refractive complete picture of structural reorganization resolved during index of silicon, more than 30 percent of incident light is the time-course of solar energy conversion function. reflected back, which greatly reduces the conversion effi-

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5 APPENDIX C suited for nanomaterials research. This bioinspired material has many unique properties that bridge the gap between pro- teins 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 chemi- cally diverse building blocks, allowing the rapid generation of huge combinatorial libraries. This provides a platform to discover nanostructured materials capable of protein-like for γ-cyclodextrin with 1.0 Å spatial Experimental PDF measured A-4.eps molecular recognition and function. resolution (top) compared toonly 110% to preserve resolution bitmap image—enlarged PDF calculated from a coordinate model with resolution varying from 6 Å (1) to 0.4 Å (7). R4 R2 O O N N OH BIOLOGICAL TRANSfORMATIONS HN N N Electrobiocatalytic Reduction of CO2 to Formate: Whole O O O R1 R3 R5 Cell and Isolated Enzyme Systems Boonchai Boonyaratanakornkit,1 Rolf J. Mehlhorn,1 Robert Peptoid Oligomer Kostecki,1 Douglas S. Clark1,2 A-5.eps Environmental Energy Technologies Division, Lawrence Berkeley 1 enlarged only 110% for consistency National Laboratory, Berkeley, CA 94720 Department of Chemical Engineering, University of California, 2 Berkeley, CA 94720 Enzymatic reduction of CO2 to formate and ultimately to methanol can occur via concurrent electrochemical regen- eration 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

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