Fundamental Interactions Program

Raman Spectroscopy of Model Photosynthetic Systems

Investigator(s)	Cotton, T.M.; Chumanov, G.D.			$106,000
Phone	515-294-9887
E-mail	tmcotton@ameslab.gov

The objective of this project is to study heterogeneous electron transfer between different molecular assemblies at metal surfaces with the ultimate goal to prepare model photosynthetic systems. Monolayers and multilayers composed of donor acceptor species are fabricated by self-assembly and Langmuir-Blodgett techniques. This approach allows the control of structure and composition of these molecular assemblies. The distinguishing feature of this project is the use of metal substrates as a means of enhancing different optical phenomena as well as photoinduced electron transfer. Such enhancement occurs due to excitation of the plasmon resonances on specially prepared metal surfaces. Recently, a completely new concept was developed based on the coupling of the plasmon resonances of individual metal particles in highly organized two-dimensional arrays. The coupling produces nonradiative energy transfer between particles resulting in highly efficient light collection. Such substrates (termed Colloidal Metal Films) were successfully prepared using surface chemistry to immobilize small silver and gold particles on glass surfaces. Currently, their properties are under investigation with the goal of developing an artificial light antenna system. In the future, this artificial antenna will be coupled to different model photosynthetic systems in order to increase significantly their efficiency.

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Fundamental Interactions Program

Laser Spectroscopy and Electronic Energy Transfer of Light Conversion Systems

Investigator(s)	Small, G. J.; Hayes, J. M.; Reddy, N. R. S.; Jankowiak, R.			$226,000
Phone	515-294-3859
E-mail	gsmall@ameslab.gov

The primary events of electronic excitation and electron transport in photosynthesis are studied in order to determine and characterize the key aspects of nature's highly evolved and efficient solar energy conversion processes. The problem is important because the understanding of natural photosynthesis will provide design criteria for artificial solar energy conversion systems. To identify the critically important features (structure-function relationships) of photosynthesis, the light harvesting and reaction center protein-chlorophyll complexes of green plants, bacteria and algae are studied. An adequate understanding of natural photosynthesis requires understanding the excited electronic states of interacting chlorophylls bound to protein, how chlorophyll and protein vibrations mediate the transport processes and how the inherent glass-like structural disorder affects the rates of excitation and electron transfer. The high resolution spectroscopic technique of spectral hole burning is employed to obtain relevant data which are used to test theoretical models and, if necessary, develop new ones. Hole burning is combined with high pressure to investigate the pressure and temperature dependence of the kinetics of primary events. This unique facility has led to valuable new insights, e.g. the mechanism of the 1 picosecond B800B850 energy transfer process in the LH2 antenna complex of purple bacteria.

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Fundamental Interactions Program

Ultrafast Spectroscopy and Energy Transfer

Investigator(s)	Struve, W.S.; Savikhin, S.			$308,000
Phone	515-294-4276
E-mail	wstruve@ameslab.gov

Ultrafast laser spectroscopy is applied to studies of electronic energy transfer, vibrational cooling, and charge separation on the femtosecond time scale in natural and artificial photosynthetic systems. Recent areas of emphasis have been energy transfer studies of light-harvesting antennae from purple photosynthetic bacteria, bacteriochlorophyll-protein complexes from the green bacterium Chlorobium tepidum, and oligomeric bacteriochlorophyll antennae in chlorosomes from green bacteria. This program has recently achieved the first-characterization of level-to-level energy transfers in a strongly coupled antenna, a major milestone in the study of structure-function relationships in natural antennae. A biomimetic project is being pursued in a collaboration with other investigators in the Ames Laboratory. This project has focussed on phthalocyanine-based artificial antennae using two approaches. The first deals with synthetic pigment-protein and pigment-polymer arrays, engineered for efficient excitation delivery to reaction centers. The second approach, based on Coulomb tetraazaphthalocyanine-phospholipid bilayers in Langmuir-Blodgett transfer films, has been successfully used to prepare two-dimensional antennae with high optical density, efficient energy transfer, and minimal pigment aggregation.

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Chemistry Division

Time-Resolved Molecular Structure Determination in Photoinduced Electron Transfer Processes

Investigator(s)	Chen, L.X.; Thurnauer, M.C.; Wasielewski, M.R.			$70,000 Operating $230,000 Equipment
Phone	630-252-3533
E-mail	lchen@anlchm.chm.anl.gov

The objective of the research is to determine transient molecular structures in photochemical reactions, in particular, those related to photoinduced electron transfer and solar energy conversion and storage, with time-domain X-ray absorption spectroscopy (TRXAS) using synchrotron radiation. Many photoinduced charge separation processes are achieved via excited states or a series of intermediate states having electronic structures distinctively different from those of the ground state molecules. The determination of the structures of these transient species is the key to understanding the fundamental chemistry of photoinduced charge separation. The TRXAS experiments will implement "pump-probe" X-ray absorption spectroscopy (XAS) at the Advanced Photon Source at Argonne, where a short laser pulse is used as a "pump" for creation of photoinduced intermediates, and an X-ray pulse, as a "probe" for monitoring the structural changes. The XAS spectra of a particular intermediate state of electron donor-acceptor molecules will be collected at a certain delay time from the laser "pump" pulse based on the kinetic information obtained with optical and magnetic resonance measurements. The information on structural changes of the electron donor-acceptor molecules following photoinduced electron transfer will reveal new insights into the correlation of nuclear motion with electron transfer, that, in turn, will provide new grounding for theoretical modeling of electron transfer and guidance in designing new molecular systems for solar energy conversion and storage.

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Chemistry Division

Electron Transfer and Energy Conversion

Investigator(s)	Miller, J.R.; Meisel, D.; Curtiss, L.A.			$1,066,000
Phone	630-252-3481
E-mail	jrmiller@anlchm.chm.anl.gov

This project involves fundamental research on intramolecular electron transfer and charge transfer at interfaces in microheterogeneous environments. Recent efforts concern: (a) exploration of how electron transfer between molecules is controlled by distance, energy, and molecular structure; (b) electron transfer at surfaces of semiconductor nanoparticles and between such particles; (c) the nature and dynamical characteristics of excited states of radical ions and free radicals. Experimental methods to couple high-speed lasers to an electron linear accelerator are under development for the study of radical species and for the possible extension to light-driven electron transfer reactions of these species. The pulsed electron radiolysis technique is uniquely suited to investigate the role of important variables such as distance, energy, ion pairing, and solvent polarity on electron transfer, because it enables experiments which clearly delineate the role of each variable. These capabilities are augmented in this program by coupling to theory at the ab initio level.

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Chemistry Division

Hierarchical Photosynthetic Systems for Photochemical Energy Conversion

Investigator(s)	Thurnauer, M.C.; Tiede, D.M.; Tang, J.			$778,000
Phone	630-252-3570
E-mail	thurnauer@anlchm.chm.anl.gov

This program investigates mechanisms for linking ultrafast photoinduced charge separation to the production of chemical and electrochemical products. Natural photosynthesis is being studied as an example of a photochemical system in which ultrafast photophysical processes are nearly optimally linked to chemical energy conversion. The natural systems have a hierarchical design, consisting of discrete chemical subsystems that break photosynthesis up into a sequence of individual photochemical and chemical reaction steps. This program examines how linkages between different photosynthetic subsystems are achieved, and delivers strategies for the design of novel, hierarchical artificial systems for enhanced photochemical energy conversion. The biological systems under investigation include the bacterial photosynthetic reaction center, and the photosystems I and II from algae and plants. Surface-modified, semiconductor colloid photocatalysts are being synthesized and investigated as models of synthetic hierarchical photochemical systems. Novel pulsed EPR, time-resolved EPR, CW EPR, neutron and X-ray diffraction experimental techniques and theoretical modeling are being developed to investigate structure and photochemical activity in hierarchical photochemical systems. Notable contributions from this program have been the resolution of structures and sequences of electron transfer events in natural photosynthesis, and following from this, the design and synthesis of surface-modified semiconductor colloids that exhibit photoinduced sequential electron transfer steps leading to charge separation.

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Chemistry Division

Stable Isotope Labeling of Photosynthetic Materials and Structural Analysis

Investigator(s)	Tiede, D.M.			$65,000
Phone	630-252-3539
E-mail	tiede@anlchm.chm.anl.gov

This project produces photosynthetic materials enriched with nonnatural, stable isotopes that offer unique opportunities for structural and photochemical analyses of complex natural and synthetic photochemical systems using magnetic resonance and neutron scattering techniques. A central problem for unraveling mechanisms for solar energy conversion lies in detecting photochemistry and structure of individual components buried within larger molecular arrays. The isotopically labeled materials produced by this program provide an elegant, nonperturbative solution to this problem. Primary emphasis is on the production of 99.7% deuterium enriched algae and photosynthetic bacteria. Deuteration greatly simplifies the electron paramagnetic properties of free-radical intermediates produced by light-induced chemistry. Neutron scattering experiments also benefit from deuteration by amplifying the coherent neutron scattering signal from the labeled molecules and suppressing incoherent scattering. These features are being exploited for the determination of photochemistry and structure in natural photosynthesis using new, high-resolution, state-of-the-art pulsed and CW magnetic resonance spectroscopies and neutron scattering techniques. In addition, the photosynthetic materials are being used as sources for the isolation of fully deuterated chlorophylls, bacteriochlorophylls, chlorins, quinones, carotenoids and lipids. These molecules are used for the reconstitution of selectively labeled natural photosynthetic assemblies, and for the synthesis of artificial assemblies with selected isotopic composition. This program is providing new materials for probing the components, structure, and mechanisms of charge separation as performed by natural and artificial photosynthetic systems.

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Chemistry Division

Reactive Intermediates in the Condensed Phase: Radiation and Photochemistry

Investigator(s)	Trifunac, A.D.; Bartels, D.M.; Jonah, C.D.; Shkrob, I.A.; Werst, D.W.			$1,773,000
Phone	630-252-3483
E-mail	trifunac@anlchm.chm.anl.gov

Fundamental studies of chemical effects of ionizing radiation focus on the early time events in radiolysis and photoionization. The purpose is to provide molecular details of the energy and charge transport phenomena in liquids and in solids subjected to ionizing radiation. The knowledge developed in the course of this research will benefit diverse areas of technology such as radioactive waste management, polymer processing, treatment of hazardous wastes, and the broad use of silicon-containing materials. The study of charge transport, ion reactions, and newly discovered transient species encompasses polar and nonpolar liquids and crystalline and amorphous solids. The role of solvation in controlling reactivity of charged species is analyzed experimentally and theoretically. Solid-state studies of ions provide insights into control of chemistry via matrix-ion interaction and have led to development of new analytical methods for probing catalytic effects of silicon-based solids like zeolites. The role of excited states of neutrals and of ions is explored in photoionization studies of water and of hydrocarbon solutions. Ultrafast studies of water photoionization yield new details on the role of excited states of water. Studies of aromatic solutes in hydrocarbon solutions via two-color laser photoionization reveal the occurrence of hole transfer into the solvent manifold when highly excited aromatic radical ions are produced. These studies are carried out using state-of-the-art time domain tools which include a picosecond linear accelerator, a Van de Graaff accelerator, subpicosecond and nanosecond UV lasers coupled to specialized detection equipment.

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Chemistry Division

Molecular Systems for Photochemical Energy Conversion and Storage

Investigator(s)	Wasielewski, M.R.; Chen, L.X.; Gosztola,D.; Wiederrecht, G.P.			$843,000
Phone	630-252-3538
E-mail	wasielewski@anlchm.chm.anl.gov

The principal goal of this project is to design, prepare, and study the fundamental properties of molecular systems that will efficiently convert light energy into useful chemical energy. The picosecond, high quantum yield photochemical charge separation that occurs in natural photosynthesis serves as a conceptual model for the systems studied in this project. Artificial photosynthetic charge separation systems are designed to yield photocatalysts that will perform well in practical chemical environments. New supramolecular systems that consist of covalently linked arrays of electron donors and acceptors have been synthesized. These systems utilize visible light to separate charge with 80% efficiency and separation lifetimes that exceed 1 millisecond. The organic electron donors and acceptors within these supermolecules maintain well-defined structural, solvation, and electronic relationships among themselves. Current research focuses on issues that are fundamental to optimizing charge separation and storage efficiencies in the solid state. This includes the interplay between the properties of the organized donor-acceptor array and the molecular organization of the surrounding medium. Ultrafast laser spectroscopy is used to probe the molecular interactions that occur in these ordered systems.

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Department of Applied Science

Porphyrin Chemistry

Investigator(s)	Fajer, J.; Barkigia, K.M.; Renner, M.W.			$598,000
Phone	516-344-4521
E-mail	fajerj@bnluxl.bnl.gov

This program focuses on bioenergetic reactions mediated by porphyrins with particular emphasis on the mechanisms by which light is harvested and converted into chemical energy in photosynthesis. The expanding body of structural, theoretical, and experimental data evolving from the photoconversion modeling is readily extended to the multielectron catalyses of nitrogen assimilation, regiospecific reactions, and carbon dioxide conversion mediated by the broad generic classes of porphyrins. To model these energy and electron transfer reactions, the project uses photochemistry, spectroelectrochemistry, magnetic resonance, X-ray, and synchrotron radiation techniques which are further supported by theoretical methods. Recent results with conformationally designed models have provided particularly simple synthetic avenues to controlling optical, redox, and excited state properties as well as sites of reactions in vitro. In combination with theoretical calculations, the synthetic models offer useful insights into photochemical and catalytic reactions that often occur on picosecond time scales in vivo. The cumulative thrust of the combined theoretical, structural, and experimental approaches is to characterize transients and mechanisms in bioenergetic conversion, and to provide specific guidelines for the development of synthetic photocatalytic systems.

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Department of Applied Science

Electrochemistry and Photoelectrochemistry

Investigator(s)	Feldberg, S.W.; Smalley, J.F.			$352,000
Phone	516-344-4480
E-mail	feldberg@bnl.gov

The objective of this program is to provide new insights into the mechanisms of electrochemical and photoelectrochemical phenomena. The experimental objective is to understand the role of interfacial structure and organization in a variety of interfacial processes e.g., double-layer relaxations, charge (ion or electron) transfer between the solution and the electrode, electron transfer between electrode and immobilized or adsorbed redox moieties. The current focus is on heterogeneous electron transfer between a metal-electrode and redox species attached to the electrode, generally in the form of a self-assembled monolayer. The determination of the distance dependence of heterogeneous electron transfer is of particular interest. The experimental approach utilizes an indirect laser-induced interfacial temperature-jump (ILIT) technique. At its present level of development, ILIT can access interfacial rate processes occurring in the nanosecond time domain, arguably a leading-edge experimental approach exceeding the capabilities of other state-of-the-art electrochemical approaches. Theoretical analyses and computer simulations attack a broad range of electrochemical and photoelectrochemical problems that may be generally described as heterogeneous electron (or hole) transfer coupled with homogeneous chemical reactions.

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Chemistry Department

Thermal, Photo- and Radiation-Induced Reactions in Solution

Investigator(s)	Sutin, N.; Brunschwig, B.S.; Cabelli, D.; Castner, E.W.; Creutz, C.; Fujita, E.; Holroyd, R.A.; Newton, M.D.; Schwarz, H.A.; Seltzer, S.; Wishart, J.F			$2,770,000
Phone	516-344-4358
E-mail	sutin@bnl.gov

This program addresses issues fundamental to the efficient capture and storage of light energy: excited-state formation, chemistry, and photophysics; energy transduction by electron-transfer reactions; and energy storage through chemical transformations. Theoretical and experimental efforts are elucidating the factors controlling excited-state lifetimes and electron-transfer rates; the roles of electronic configuration, donor/acceptor separation, bridging groups, nuclear-configuration and free-energy changes, as well as the role of solvent dynamics are being investigated through studies of transition-metal complexes and other donor/acceptor systems. Electron pulse radiolysis and flash photolysis techniques are being used to generate and characterize transient species important in solar energy conversion, including the preparation and properties of transition-metal complexes in unusual oxidation states and their ability to bind and activate small molecules, and to determine bimolecular and intramolecular electron-transfer rates. The properties and reactions of electrons and other ions in dielectric fluids are being studied utilizing both X-ray and high energy electron sources. The long-term storage of solar energy as fuels or valuable chemicals requires efficient coupling of light absorption and chemical transformation processes. Mechanistic studies of systems which couple photoinduced electron-transfer processes to the bond-forming reactions required in the photogeneration of hydrogen and the photoreduction of carbon dioxide to useful chemicals are a major focus.

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Structural Biology Division

Chemistry with Near Infrared Photons

Investigator(s)	Frei, H.M.			$325,000
Phone	510-486-4325
E-mail	hmfrei@lbl.gov

The objective of this project is to establish useful chemistry that can be accomplished with the sun's most abundant, long-wavelength photons. Current focus is on reactions that suggest new concepts in red and near-infrared light-assisted synthesis of organic building blocks, high-value compounds, and fuels from abundant chemicals. Photochemistry of cage reactant pairs is being explored for controlled synthesis in zeolite matrices. Highly selective partial oxidation of small alkenes, alkanes, and substituted aromatics by molecular oxygen has been achieved with visible light at room temperature. Examples include conversion of toluene to benzaldehyde, propylene to acrolein, and cyclohexane to cyclohexanone. This is the first selective method for hydrocarbon oxidation by oxygen at high conversion. A central aspect of the new method is the exploitation of the very high electrostatic field inside the zeolite cage. It stabilizes the excited hydrocarbon-oxygen charge-transfer state by 1.5-3.0 electron volt, thus allowing initiation of the reaction with low-energy visible photons. Time-resolved FT-infrared absorption spectroscopy (step-scan technique) at ten nanosecond resolution has been developed as a new tool for mechanistic studies. The first transient infrared spectrum of a molecule in a zeolite matrix has been obtained. Elucidation of elementary steps of photochemical reactions in zeolites on the nano-to-millisecond time scale is in progress.

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Basic Sciences Division

Surface-Modified Semiconductor Electrodes and Novel Electrolyte-Semiconductor Interfaces

Investigator(s)	Frank, A.J.			$280,000
Phone	303-384-6262
E-mail	afrank@nrel.gov

This research addresses basic issues in the photochemical conversion of light energy to electrical or chemical energy relating to understanding charge transfer processes at the semiconductor/redox electrolyte interface as a basis for tailoring the semiconductor surface or redox electrolyte to control photocorrosion, charge recombination, and electron transfer catalysis. The work includes the synthesis, characterization, and use of advanced surface modifying reagents for semiconductor photoelectrodes. The current research concerns the development of a simple quantitative model for understanding interfacial charge transfer, charge recombination studies of dye-modified nanocrystalline TiO₂ photoelectrochemical (PEC) solar cells, studies of multiple bandgap photoelectrodes for water splitting, transient and steady-state studies of the light-induced charge generation/recombination mechanism of sexithiophene films for photoelectrodes, and the development of a new solid-state PEC cell based on a plasticized polymer-redox electrolyte.

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Basic Sciences Division

Photoconversion Processes in Liquid Crystal Porphyrin Films and Other Molecular Semiconductors

Investigator(s)	Gregg, B.A.			$205,000
Phone	303-384-6635
E-mail	bgregg@nrel.nrel.gov

This research examines fundamental aspects of photoconversion processes in molecular semiconductors. Current emphasis is on understanding the dynamics of exciton motion and dissociation, since these processes control the production and initial separation of charge carriers. Steady state and time-resolved luminescence measurements have shown that exciton motion is extremely rapid in films of perylene diimides, with intermolecular hopping times on the order of 100 fs. Exciton dissociation by interfacial electron transfer must be able to compete with this hopping process in order to generate charge carriers. Various contacting films are being employed to elucidate the factors that control exciton dissociation or reflection at the interface. Some conducting polymer films have been found to promote rapid exciton dissociation at interfaces with perylene diimide films; the dynamics of the electron transfer process at such interfaces are being investigated. The conducting polymer/perylene diimide interface is one of the few known systems in which exciton dynamics occur on the same time scale as in the natural photosynthetic system. A detailed exploration of exciton dynamics in molecular semiconductor films will contribute to understanding natural photosynthesis and to the ultimate development of organic-based solar energy conversion systems.

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Basic Sciences Division

Synthesis and Characterization of Novel III-IV and II-VI Semiconductor Quantum Dot Structures for Solar Photochemical Conversion

Investigator(s)	Mii, O.I.			$126,000
Phone	303-384-6626
E-mail	micico@tcplink.nrel.gov

The photochemical and photophysical properties of quantum dot structures are being investigated for solar photochemical conversion. Quantum dots of InP, GaP and GaInP₂ with the diameters ranging from 20 to 65 Å were synthesized as well-crystallized nanoparticles with bulk zinc blende structure. X-ray powder diffraction patterns of GaInP₂ quantum dots exhibit the zinc blende structure with lattice spacings which are approximately the average of the diffraction peaks of InP and GaP bulk materials. Colloidal solutions of InP, GaP and GaInP ₂ quantum dots show optical absorption and emission spectra blue-shifted compared to bulk material. The high sample quality of the InP and GaP quantum dots results in excitonic features in the absorption spectra. For 20 Å- and 30 Å-GaP quantum dot preparations, the absorption spectra indicate that GaP has not undergone a transition from indirect to direct semiconductor. It seems that the GaP quantum dots remain an indirect semiconductor, but the selection rules are relaxed to allow indirect transitions because of symmetry-breaking effects produced by small quantum dot size. The GaP and GaInP₂ quantum dot colloids exhibited very intense (quantum yields of 15%) visible photoluminescence at room temperature. The photoluminescence for InP preparations showed highly efficient band-edge photoluminescence (lowest energy HOMO-LUMO transition) after the particles were etched. The quantum yield is 30% at room temperature and 60% at 10 K which is tunable with particle size.

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Basic Sciences Division

Carrier Dynamics and Quantization Effects in Photoelectrochemistry

Investigator(s)	Nozik, A.J.			$664,000
Phone	303-384-6603
E-mail	nozik@stripe.colorado.edu

The dynamics of photoinduced charge transfer at semiconductor-liquid interfaces, for both macroscopic and size-quantized structures, is being investigated both theoretically and experimentally. New, rigorous ab initio calculations of electron transfer kinetics at model semiconductor-liquid redox systems have been performed which treat, for the first time, the full electronic structure of the coupled semiconductor-solvent-redox couple system. Experimental studies of electron transfer dynamics at semiconductor-liquid junctions are being conducted in the ns to fs time scales using transient photoluminescence spectroscopy based on upconversion and time-correlated single photon counting; the theoretical predictions are being tested against experiment. A model to extract the heterogeneous rate constants from the photoluminescence decay data has been developed. Results for p-GaAs electrodes capped with a thin GaInP₂ surface barrier show nearly ideal kinetic behavior and an electron transfer rate that is apparently electric field-dependent; this electrode also shows an ideal concentration dependence of the current-voltage characteristic. Studies are being made of the electron cooling and electron transfer dynamics in excellent quality InP quantum dots. These quantum dots exhibit highly efficient band-edge emission and an apparent "dark" excitonic ground state; they show much faster electron transfer to methyl viologen cations compared to cobaltocenium ions.

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Basic Sciences Division

Fundamental Studies of Electron Injection Dynamics at Dye-Sensitized Nanostructured Semiconductors in Photochemical Solar Cells

Investigator(s)	Nozik, A.J.			$150,000
Phone	303-384-6603
E-mail	nozik@stripe.colorado.edu

The kinetics of electron injection from dye molecules adsorbed on nanostructured TiO₂ films into the TiO₂ conduction band is being investigated in the fs to ns time regime using fs to ns transient absorption spectroscopy and ns to ps transient photoluminescence spectroscopy. These spectroscopic systems have been constructed and are operational. An experimental procedure to reproducibly synthesize excellent quality TiO₂ colloidal particles that can be subsequently formed into nanostructured electrodes has been developed. This capability is critical since the electron injection dynamics in nanostructured films will be very sensitive to the morphology, size, and surface properties of the TiO₂ particles. We will attempt to help resolve a major controversy in this field related to the magnitude of the electron injection rate, which has been reported to vary from fs to ns. This project is part of an integrated effort of basic and applied research to develop and evaluate the potential of photochemical (i.e., dye-sensitized semiconductor) solar cells as a viable technology. The basic research contained in this project will underpin the applied/development effort.

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Radiation Laboratory

Sulfur-Centered and Other Heteroatom-Based Organic Radicals

Investigator(s)	Asmus, K.-D.; Hug, G.L.; Carmichael, I.; Tripathi, G.N.R.; Schuler, R.H.			$697,000
Phone	219-631-5561
E-mail	asmus.1@nd.edu

This project is concerned with the properties of radicals derived from heteroatom-containing organic compounds, particularly those with the radical site located at the heteroatom itself. The emphasis on sulfur-organic molecules reflects the significance of this group of compounds in radiation-induced biological damage. Of primary interest are odd-electron bonded species in which antibonding electrons exert a bond-weakening effect on existing or newly formed bonds. Other systems which can participate in this type of bond and are important for comparison to the sulfur systems are radicals containing the elements N, P, O, Se, and the halides. These studies have fundamental implications for understanding the formation and breakage of chemical bonds. Current studies deal with SN, SeSe, SS and SP 2/1* three-electron-bonded radical cations and anions, resulting from an intramolecular interaction of an unpaired electron on the oxidized heteroatom with a free electron pair of another heteroatom. Conformational preferences have been theoretically determined for cyclic SN radical cations, and optical and vibrational frequencies have been calculated in excellent agreement with experimental measurements obtained by electron pulse radiolysis and Raman spectroscopy. The optical absorption spectra of these systems respond very sensitively to small changes in electronic and structural parameters.

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Radiation Laboratory

Reactive Intermediates and Elementary Chemical Processes in Solution

Investigator(s)	Asmus, K.-D.; Schuler, R.H.; Fessenden, R.W.; Hug, G.L.; Madden, K.P.			$607,000
Phone	219-631-5561
E-mail	asmus.1@nd.edu

The primary aim of this project is the study of chemical reaction mechanisms which are initiated by or involve free radicals. These basic studies are a prerequisite for the optimization and control of radical reaction systems. The investigations rely on time-resolved radiation chemical techniques using optical and electron spin resonance (ESR) spectroscopic detection, complemented by photochemistry, steady-state radiolysis, electrochemistry, mass spectrometry and chromatographic analysis. Free radical and redox-initiated degradation of halogenated organic compounds often proceeds through halogenated peroxyl radicals. It has now been established that these species undergo an overall two-electron transfer process with various organic sulfur and selenium compounds and inorganic iodide, involving radical adduct intermediates and solvent participation. Substantial progress has been made in understanding the decarboxylation mechanism of sulfur-containing amino acids and the role of this process in their overall redox chemistry. The unusual ESR behavior of e_aq^- with oxidizing radicals has been explained by correlating the energetics of the reaction with the excited state energies of the products. Electron transfer quenching of the triplet excited state of 2,2'-biphenol by fumarate has been established. The kinetic behavior has been determined with high accuracy for the reaction of methyl radicals with iodine species in aqueous solution, a system of potential significance in nuclear reactor accidents. Detailed investigation of the ^·OH induced oxidation of thiocyanate has established the existence of very short-lived SCN(OH)^· and ^·SCN radicals as precursors of (SCN)₂^{· -}.

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Radiation Laboratory

Structure and Chemical Properties of Radiation-Produced Intermediates

Investigator(s)	Chipman, D.M.; Tripathi, G.N.R.; Fessenden, R.W.; Carmichael, I.; Bentley, J.			$881,000
Phone	219-631-5562
E-mail	chip@smallv.rad.nd.edu

Highly reactive free radical and molecular excited state intermediates produced by radiolysis or photolysis are studied to determine their often unusual structures and bonding characteristics and to relate these to the chemical properties, kinetics, and mechanisms of the reactions they undergo. This will enhance our ability to design new chemical systems that store, convert, or liberate energy in desired ways. Time-resolved experimental approaches include resonance Raman spectroscopy, electron spin resonance, and microwave absorption, which are complemented with theoretical calculations of electronic structure based on modern methods of quantum chemistry. Recent work on aminobenzoic acid radicals has established the role of excited states in intramolecular electron transfer leading to bond dissociation and accounted for the marked pH dependence of the nature and yields of the reaction products. A structural explanation has been found for the greatly enhanced proton reactivities of carboxylic and amide groups when bonded to semireduced pyridine, and unambiguous evidence has been obtained for protonation of the amide group at oxygen, rather than at nitrogen. New quantum mechanical operators have been developed that improve efficiency of theoretical hyperfine coupling calculations. Accurate hyperfine coupling constants have been determined for HNCN radical to aid interpretation of the observed laboratory spectra. Dipole moments of the lowest singlet excited states of coumarin and aminophthalimide donor-acceptor molecules have been measured to facilitate their use as probes of solvent polarity and dielectric relaxation behavior.

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Radiation Laboratory

Mechanisms of Redox Processes in Coordination Compounds

Investigator(s)	Ferraudi, G.; Guldi, D.M.			$283,000
Phone	219-631-7676
E-mail	ferraudi.1@nd.edu

This project involves determination of the contributions from the electronic and molecular structures of metal complexes to the rates of redox reactions in which they participate. Such reactions are essential components of the overall processes of carbon dioxide fixation, the splitting of water, and the destruction of chemical contaminants. Magnetokinetic effects, which probe electronic interactions in reactions of a transition metal complex with a radical or another metal complex, have been investigated by experimental and theoretical means. The Zeeman mechanism, spin-orbit coupling and hyperfine coupling, among other perturbations to the Hamiltonian, are being unravelled by comparing several series of compounds of the transition metal ions Ni(III), Co(II) and Mn(II). Recent studies have explained the roles of the orbital angular momenta and the nuclear and electronic spins, whether interacting with each other or with a magnetic field. Ongoing work involves the determination of the role of the dynamic probability factor, and its convolution with the spin evolution, on magnetokinetic effects in radical-ion reactions. Mechanistic studies are aimed toward promoting the activation of small molecules such as carbon dioxide, nitrate or sulfur dioxide by transition metal complexes in unusual oxidation states. Such studies have demonstrated structural contributions to the charge localization, either in a ligand or at the metal center, in porphynoid, phthalocyanine, and simple macrocyclic complexes.

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Radiation Laboratory

Photochemical and Photoelectrochemical Processes for Energy Conversion

Investigator(s)	Kamat, P.V.; Guldi, D.M.			$537,000
Phone	219-631-5411
E-mail	kamat.1@nd.edu

Mechanistic and kinetic aspects of excited state and redox behavior of organic dyes, functionalized fullerene derivatives and semiconductor nanoclusters are being studied. Electron pulse radiolysis and laser flash photolysis are used to investigate the primary chemical events in these systems. Surface modification of large-bandgap semiconductors such as TiO₂, ZnO and SnO₂ with either squaraine and oxazine dyes or small-bandgap semiconductors such as CdS and CdSe provides a convenient method to extend the semiconductor photoresponse into the infrared and to improve the efficiency of charge separation. Photoelectrochemical, microwave conductivity and spectroelectrochemical measurements of dye-modified semiconductor films indicate that electron injection between an excited sensitizer and the semiconductor nanocrystallite can be controlled by suitable choice of experimental conditions. The back electron transfer between an injected electron and the sensitizer cation radical as well as the role of redox couple in sensitizer regeneration will be a major focus of future work. Charge-separated products have been confirmed in mechanistic studies on intramolecular electron- and energy transfer reactions of fullerenes bearing covalently attached photo- or electroactive substituents. Variation of the active centers and modification of the spacing between donor and acceptor sites have differentiated electron transfer from excimer formation involving singlet excited fullerene states.

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Radiation Laboratory

Heavy Ion Radiolysis and Track Structure

Investigator(s)	LaVerne, J.A.; Pimblott, S.M.; Mozumder, A.; Schuler, R.H.			$455,000
Phone	219-631-5563
E-mail	laverne.1@nd.edu

The effects of particle track structure on radiation chemical processes are examined using experimental and theoretical techniques in order to provide basic knowledge on the variation of radiation damage with different types of ionizing particles. Single photon counting studies with heavy ions from the Notre Dame Nuclear Structure Laboratory show, for the first time, that the yields and lifetimes of the excited states in neat liquid benzene are dependent on particle type. Examination of the Fricke dosimeter with ⁵⁸Ni and ²³⁸U ions suggests that even for very high linear energy transfer particles there is a significant escape of radicals from the track which has important radiation chemical and biological implications. Detailed theoretical studies using complete Monte Carlo track simulation methods demonstrate that the spatial distribution of energy deposition by electrons in water changes from a centered sphere through an ellipsoid to a teardrop shape as the energy increases. Kinetic simulations reveal the effects of electron energy and track structure on the yield of the Fricke solution and aid in the reconciliation of the time-dependent kinetics and scavenger yield studies of the hydrated electron in electron radiolysis of water. Energy partition between the core and penumbra of heavy-ion tracks in liquid argon slightly favors the core.

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Radiation Laboratory

Radiation Chemistry Data Center

Investigator(s)	Madden, K.P.; Helman, W.P.			$175,000
Phone	219-631-6528
E-mail	madden.1@nd.edu

Chemical property data for reactive intermediates in solution are being compiled, evaluated and collected into databases, focusing on free radicals and excited states produced in radiation chemical studies. Electronic publication of these databases on-line facilitates easy retrieval of timely kinetic data. Scientists and engineers modeling chemical systems involving free-radical intermediates use these values as input to their calculations. Complementary data from the photochemical literature are also included in this information retrieval effort. A compilation of rate constants for reactions of aliphatic carbon-centered radicals in aqueous solution has been completed and published. The center provides on-line resources for searching the Radiation Chemistry Data Center (RCDC) bibliographic database, and numeric databases containing condensed-phase triplet-triplet absorption spectral data, and rate constants for inorganic radicals in aqueous solution. A world wide web site (http://allen.rad.nd.edu) has been developed for the RCDC, containing an updated critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (OH / O-) in aqueous solution, and a compilation of kinetic data on the decay and reactivity of singlet molecular oxygen in fluid solution.

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