|
SUMMARIES OF FY 1996
RESEARCH IN THE CHEMICAL
SCIENCES |
Photochemical and Radiation Sciences:
Offsite Projects
University of Akron
Akron, OH 44325
Department of Chemistry
Dynamics of Charge-Transfer Excited States Relevant to Photochemical Energy
Conversion
| Investigator(s) |
Lim, E.C.
|
$100,000 |
| Phone | 330-972-5297 |
| E-mail |
elim@uakron.edu |
The primary objective of the research is to gain a fundamental understanding of the factors
governing the efficiency of charge and energy transfer processes in molecular systems of interest,
or pertinence, to photochemical energy conversion. The major focus of the current study is on the
excited-state intermolecular interactions between two moieties that are brought together in close
proximity, either by a short covalent linkage or by ground-state intermolecular association.
Excited-state interactions ranging from strong charge transfer to weak van der Waals forces are
being investigated in solution and in supersonic free jets, using laser-based techniques. Where
appropriate, and feasible, quantum chemical methods are also used to gain theoretical
understanding of the charge and energy transfer processes.
University of Alabama
Tuscaloosa, AL 35487
Department of Chemistry
Magnetic Resonance and Optical Spectroscopic Studies of Carotenoids
The current goal is to evaluate the role of polar media in the mechanism of carotenoid cation
radical formation and decay and to determine the special properties of carotenoids bound to
pigment-protein complexes in photosynthetic membranes that enable them to serve both as
antennae and as photoprotective agents and as a possible component of electron transfer
processes. Simultaneous electrochemical and electron spin resonance measurements, simultaneous
electrochemical and optical measurements, and simultaneous electrochemical and resonant Raman
measurements have been carried out. From these studies, the reason has been deduced for the
observation of carotenoid radicals in some photosystems and not others. In the solid state, the
energy of the cis isomers falls close enough to that of the all trans isomers that the solid host can
stabilize higher energy cis isomers. All trans or cisoidal carotenoid cation radicals can exist on
solid supports and possibly in solution. The reason for the preference of the higher energetic
twisted solid state configurations of the carotenoids in reaction centers has been determined.
Semiempirical molecular orbital (RHF-INDO/SP) calculations of the canthaxanthin cation radical
in solution are in excellent agreement with the electron nuclear double resonance measurements.
The host matrix is being manipulated in such a manner as to understand the carotenoid function
and to ultimately develop predictive mechanisms for directing the outcome of photochemical
events.
Arizona State University
Tempe, AZ 85287
Department of Chemistry
Supramolecular Structures for Photochemical Energy Conversion
| Investigator(s) |
Gust, J. D. Jr.; Moore, T.A.; Moore, A.L.
|
$195,000 |
| Phone | 602-965-4547 |
| E-mail |
gust@asu.edu |
Photosynthetic solar energy conversion is the ultimate energy source for essentially all life and is
one of the most durable and efficient solar conversion "technologies." The goal of this
project is to synthesize artificial photosynthetic reaction centers that employ the basic chemistry
and physics of photosynthesis to help meet energy needs. Specifically, the research involves the
preparation and study of photochemically active multicomponent molecules that functionally
mimic photosynthetic light harvesting, photoprotection from light-initiated singlet oxygen
damage, and, most importantly, photoinduced multistep electron transfer to generate long-lived
charge-separated states with a quantum yield close to unity. One current project involves the
preparation of molecular triads and tetrads featuring new linkages between the donor and
acceptor moieties that lead to ultrafast electron transfer both in fluid solution and in glassy solids
at low temperature. Another investigation deals with new methods for the stabilization of charge
separation through intramolecular proton transfer. Recently, we have expanded our mimicry of
photosynthetic processes to include the conversion of intramolecular redox potential to chemical
energy manifest as a proton potential gradient across a bilayer lipid membrane. This has been
accomplished by the assembly of molecular triad-based artificial reaction centers and collateral
quinones into a liposome-based model system that uses light energy to translocate protons across
the bilayer. Upon excitation, electron transfer processes in the triad generate reduction potential
near the outer surface of the bilayer and oxidation potential near the inner surface. In response to
this vectorial redox potential gradient, a freely-diffusing quinone alternates between its oxidized
and semiquinone forms to transport protons across the bilayer. These experiments demonstrate
the conversion of light energy to transmembrane proton motive force in a purely synthetic,
biomimetic system.
Boston University
Boston, MA 02215
Department of Chemistry
Photoinduced Electron Transfer in Ordered Polymers
Investigations involve the design and characterization of systems capable of photochemical
electron transfer between electron donor and acceptor groups that are bound to polymer or
biopolymer chains. Objectives of the research include the observation of the effects of polymer,
peptide, or protein microenvironments on the efficiency and rate of electron transfer between
groups that are separated by distances that can be controlled within the macromolecular domain.
For these systems the (bio)polymer acts as a template or scaffolding for assembly of
chromophores and photoactive species. Of special interest is charge migration among groups
located at the ends of short peptide chains or within the domain of a peptide [alpha] helix. For use
in these polymer-based arrays, a family of new chromophores based on the acridinium ion has
been synthesized. On photoexcitation these structures engage in intramolecular charge separation
within picoseconds of an excitation pulse. Decay of the electron transfer intermediates (the time
scale for molecular "switching") is variable over at least four orders of magnitude,
reaching the 10-nanosecond time domain. Domain-forming vinyl polymers and glasses are
especially effective in extending the lifetime of charge separation. Current work also includes the
synthesis of amphipathic helices that will potentially provide protein "bundles" for
assembly of synthetic reaction centers. Methods that are employed in these investigations include
peptide synthesis, laser flash photolysis and fast kinetics, fluorescence probes, circular dichroism,
and molecular modeling. In these studies, emphasis is placed on the opportunities for construction
of highly functionalized synthetic polymer materials in which reactive groups are held in relatively
rigid arrays that provide controls at the molecular level of charge separation and photochemical
energy storage. The work is important to the understanding of charge transport in both natural
and biomimetic systems and the development of energy conversion devices based on reversible
electron transfer.
Brandeis University
Waltham, MA 02254
Department of Chemistry
Mechanistic Studies of Excited State Chemical Reactions
This program aims to elucidate fundamental mechanisms of endergonic redox reactions in
solution, so that general factors governing the efficiency of such processes can be understood and
optimized. Kinetics and primary radical yields are studied by flash photolysis and other techniques
to provide data and basic parameters for testing theories of electron transfer.For example,
reductive quenching of triplet C60 by halides is a reaction involving spherical
centers and therefore uniquely appropriate for treatment by Marcus-Hush theory. Recent new
directions in our work concern the characterization of the important class of reactions in which
electron transfer is concerted with proton movement. These processes are studied in
self-assembled model systems in which rates and radical yields are greatly enhanced by
hydrogen-bonding of reductive or oxidative quenching agents to a third molecule. Thus, quenching
by phenols is enhanced by addition of pyridines,and quenching by quinones is enhanced by
alcohols. Such structurally imposed proton coupling provides means for controlling both the free
energies and reorganization energies of endergonic processes, as well as the rates of dissipative
back reactions that involve heavy-particle movement. Kinetic behavior of such systems is
correlated with key parameters, including hydrogen bonding equilibria, redox potentials and
acid-base properties of incipient radical products. Deuterium isotope effects are also studied to
help characterize the form of the proton-binding potential. Collaborative sub-picosecond flash
experiments are being done to establish the relative phasing of electron and proton movement.
California Institute of Technology
Pasadena, CA 91125
Department of Chemistry
Picosecond Dynamic Studies of Electron Transfer Rates at III-V Semiconductor/Liquid
Interfaces
Photoelectrochemical cells based on semiconductors as working electrodes have been shown in
numerous cases to yield stable, efficient current-voltage properties at a lower cost than solid-state
devices. However, several fundamental questions about the semiconductor surface remain
unanswered at present. For example, the rate constants for charge transfer across a
semiconductor/liquid interface are largely unknown. Also, little is known about the surface
reactivity of semiconductors typically used in solar energy applications. The answers to these
questions are needed to gain insight into how to improve the efficiency of desirable
electrode/electrolyte combinations and the stability of photoelectrodes, especially in aqueous
systems. Indium phosphide is an ideal candidate for study because of its importance in
optoelectronic applications, its optimal bandgap for solar energy conversion, and its well-behaved
current-voltage characteristics in non-aqueous solvents. Charge transfer rate constants were
measured for four n-InP/electrolyte systems which exhibited exchange currents dominated by the
rate of electron transfer across the semiconductor/liquid interface. To determine the expected rate
constants, the Marcus theory expression for liquid-liquid interfaces was modified for the
semiconductor/liquid interface In all four experimental cases, the measured rate constants ranged
from 10-17-10-16 cm4 s-1, very
close to the calculated maximum possible rate constant, supporting the theory. Next, the
photoluminescence (PL) decay of n-type InP in contact with ferrocenes in nonaqueous electrolytes was monitored as a
function of the applied voltage for a series of low and intermediate excitation intensities using the
time-correlated single photon counting technique. The PL-response reflects the total decay
kinetics of both the delocalized minority carriers in the bulk semiconductor and those at the
surface. It was observed that the voltage dependence of the PL-decay varies significantly with the
excitation intensity. Computer simulations using the Two-Dimensional Semiconductor Analysis
package ToSCA demonstrate how the voltage dependence of the PL-decay reflects the influence
of the charge transfer rate constant. Recently, long-chain alkanes have been covalently attached to
InP surfaces. Work is currently being done to attach redox active molecules to the surfaces by the
same means, so that the effect of distance on the electron transfer rate constant can be
determined.
University of California, Berkeley
Berkeley, CA 94720
Department of Chemistry
Theoretical Studies of Electron Transfer in the Photosynthetic Reaction Center
The structure, dynamics, and free energies pertaining to electron-transfer in complex systems are
analyzed through large-scale numerical simulations and through analytical methods. The research
on electron transfer is concerned with (1) the mechanism of charge transfer in photosynthetic
systems; (2) derivation of analytical theories of electrostatics and solvation, tested by numerical
simulation and used to explain measured free energetics for electron-transfer reactions; and (3)
derivation of simplified quantum dynamical theories for electron transfer processes. These
dynamical theories will be used to interpret and guide current simulation studies and to suggest
new experimental work.
University of California, Berkeley
Berkeley, CA 94720-1460
Department of Chemistry
Femtosecond Photoinduced Dynamics in Transition Metal Complexes: Probing the
Elementary Processes of Excited-State Relaxation
The proposed research involves the application of femtosecond excited-state spectroscopy for
studying the photoinduced dynamics of transition metal complexes. The overall goals are to
understand at a fundamental level the nature of excited-state relaxation immediately
following photon absorption but prior to excited-state thermalization. Tailored chemical synthesis
is being coupled to these ultrafast studies in order to systematically examine the roles of electronic
energy, and solvent in influencing wave packet dynamics as well as ultrafast electron and energy
transfer processes. Preliminary results have revealed dynamics which are inconsistent with widely
accepted models of excited-state behavior, suggesting that significant conceptual advances for
understanding the photoinduced properties of molecular systems are possible.
University of California, San Diego
La Jolla, CA 92093-0358
Department of Chemistry and Biochemistry
Energy and Charge Transfer Processes at Nanocrystalline Silicon Interfaces
The objective of the proposed work is to understand fundamental energy and charge-transfer
processes at nanocrystalline silicon interfaces. Energy- and charge-transfer rates between
luminescent porous Si and solutions of organic dye molecules, molecular donors or molecular
acceptors will be measured. The processes will be studied with steady-state and time-resolved
photoluminescence quenching, photoaction spectroscopy, and infrared and Raman experiments.
The experiments should provide a detailed picture of the energetics of Si quantum crystallites. In
particular, the studies should provide insight into the fundamental relationship between the bulk
band structure, the size-quantized states, and surface states in this material.
University of Chicago
Chicago, IL 60637
Department of Chemistry
Exploring Energetics of Photoinduced Electron Transfer in Integral Membrane Proteins
The goal of this work is to understand better the primary events of natural photosynthesis for
practical implementation. The proposed work is divided into two experimental areas: 1) the
exploration of the energetics of backward primary charge recombination for comparison with
forward primary charge separation using a series of reaction centers modified by site directed
mutagenesis and 2) the investigation of reorganization energy and electron transfer rates in
previously unexplored integral membrane proteins. The work has three specific goals: 1) the
measurement of the intermediate energy gap and the reorganization energy for both forward
charge separation and backward charge annihilation in the bacterial reaction center with the aim of
clarifying the mechanism of forward primary electron transfer in photosynthetic bacteria, 2) the
determination of reorganization energy of new integral membrane proteins to see if the bacterial
reaction center is unique in having low reorganization energy for charge separation, and 3) the
development of a new electron transfer model system based on the light harvesting antenna of
bacterial photosynthesis.
Colorado State University
Fort Collins, CO 80523-1872
Department of Chemistry
Electron Transfer Dynamics at Semiconductor Nanocluster Interfaces
This research program deals with the synthesis, characterization and electron/hole dynamics in
semiconductor nanoclusters. Specifically the research focuses on electron transfer across the
nanocluster/solution and nanocluster/solid interfaces. These electron transfer reactions are of
great importance in the development of photocatalysts. Most of our studies involve layered metal
chalcogenide semiconductors, such as MoS2, WSe2,
PtS2, etc. The interest in these systems is due to their great photostability.
Dyes, electron donors and electron acceptors have been absorbed on the nanocluster surfaces and
the interfacial electron transfer rates measured by time-resolved optical spectroscopy. The
dependence of these rates on the energetics, solvent polarity, and nature of the nanocluster trap
states may be determined and understood in terms of modern theories of electron transfer. Most
of the research thus far has been on MoS2 nanoclusters with adsorbed electron
acceptors. These MoS2 nanoclusters consist of a single S-Mo-S trilayer with
diameters of 2.5-4.5 nm. Variation of the nanocluster size changes the extent of quantum
confinement of the electron/hole pair, and thus the electron transfer energetics. The electron
acceptors are substituted 2,2'-bipyridines, and the nature of substituents affects their reduction
potentials. By varying the size of the nanocluster and the substituents on the electron acceptor, a
wide range of electron transfer driving forces may be obtained.
Marcus "normal" and "inverted" behavior of the electron transfer rates
have been obtained. Further studies will include examining solvent effects and other electron transfer
reactions.
Colorado State University
Fort Collins, CO 80523
Department of Chemistry
Studies of High Quantum Yield Sensitization Processes at
Semiconductor Electrodes
Dye sensitization has the potential to increase the light utilization of large band gap
semiconductors. The excited state of a dye molecule adsorbed onto the surface of the
semiconductor electrode can inject electrons into the conduction band of an n-type
semiconductor. These electrons can then be detected as a energies less than the band gap of the
semiconductor. The quantum yield for electrons collected per photon absorbed by the dye at
single crystal oxide electrodes was always less than a few percent. The quantum yield per
absorbed photon can approach 100% when two dimensional chalcogenides are used as
photoelectrodes. Recently methods for increasing the surface area of SnS2
photoelectrodes have been developed. By photoelectrochemically etching the surface in either
acid or basic solutions, increases in the quantum yield for electron flow per incident photon have
been obtained. The adsorption isotherm for methylene blue on etched and unetched surfaces has
the same shape but shows an increase of 20 times more in the quantum yield. An in situ scanning
tunneling microscopy (STM) method is being developed for detecting the position and energy
levels of dye molecules adsorbed on these surfaces with molecular resolution. This is
accomplished by modulating a light source at the wavelength of the dye absorption maximum and
extracting the photoinduced contribution to the tunneling current via a lock-in amplifier. A
simultaneous picture of photocurrent response and topography can then be obtained. Questions
such as the state of dye aggregation on the surface and whether dye molecules are adsorbed on
special sites could then be answered. The photo-STM technique has already been applied to
semiconductor surfaces with bandgap light. Methods are also being developed to produce
organized layers of dye molecules on these surfaces. Two dimensional layers of perylene dyes
have been formed and imaged with the STM and squarilium dyes have been doped into two
dimensional liquid crystal matrices on layered compound surfaces.
University of Colorado
Boulder, CO 80309
Department of Chemistry and Biochemistry
Mechanistic Aspects of Photoconversion at Semiconductor-Liquid Junctions and in Facilitated
Transport Membranes
The goal of this research is understanding how solar energy can be utilized in
photoelectrochemical cells (PEC's) and facilitated transport membranes. Reactions of
"hot" electrons that are created by light absorption in a semiconductor electrode have
the potential to increase the energy conversion efficiency or to alter the product distribution in
PEC's. Current investigations involve studying the photoreduction of organobromide compounds
in the presence of oxidized metallocenes. Rotating ring disk electrochemical methods are used to
determine the products of the photoreduction reactions. These data can be used to infer relative
rate constants for thermalized and "hot" electron reactions. The materials being
investigated include p-InP and p-GaAs capped with GaInP2. Photochemistry in
membranes can allow chemical species to be transported against their concentration gradients. In
principle, this process can be used in a variety of contexts ranging from environmental restoration
to energy storage. Membranes that contain photochemically active carriers are prepared and
characterized. Illumination of these membranes allows ions or molecules to be selectively
separated and concentrated. A mathematical model based on molecular parameters (reaction
rates, excited state lifetimes, interfacial kinetics, etc.) that describes this type of membrane process
is being developed. This model will be used to guide subsequent experiments aimed at improving
the selectivity, productivity and photoefficiency of photochemically controlled membrane
transport.
Columbia University
New York, NY 10027
Department of Chemistry
Charge Generation and Separation at Liquid Interfaces
Polarity is an important parameter affecting chemical and physical processes in bulk solution as
well as at liquid interfaces. While this quantity is well studied in bulk liquids, interface polarity is
less well understood. We have developed a novel means of investigating interface polarity based
on surface-specific second harmonic (SH) spectroscopy of polarity indicator molecules. We have
used this approach to determine the polarity of the air/water and organic/water interfaces, using
two polarity indicator molecules; n,n-diethyl-p-nitroaniline (DEPNA) and Reichardt's Dye
(ET(30)). Our experiments on these two different molecules, characterized by
distinct solvatochromic behavior, yield the same polarity value for air/water interface. This
supports the generality of our approach using SH spectroscopy of solvatochromic molecules as an
interfacial polarity probe. We have recently demonstrated that second harmonic generation (SHG) can be obtained from the
surfaces of spherical particles in bulk isotropic solution. Although SHG is generally described as
being electric-dipole forbidden in centrosymmetric media such as liquids, this implicitly assumes
that there is centrosymmetry on a length scale orders of magnitude less that the wavelength of
light. Our observation of SHG from the surfaces of centrosymmetric particles in bulk solution
results from the separation of adsorbed molecules of opposite orientation. We have observed not
only SHG due to molecules on the particle surfaces but also SHG due to the polarization of
solvent species by charged surface groups. This promising discovery provides a powerful
spectroscopic method for the investigation of physical and chemical processes on the surfaces of
small particles in centrosymmetric environments.
Columbia University
New York, NY 10027
Department of Chemistry
Theoretical Studies of Electron Transfer and Optical Spectroscopy
This project involves development of new theoretical methods for studying electron transfer and
optical spectroscopy with particular applications to small semiconductor particles and long range
electron transfer through intervening media. For semiconductor particles, an
empirical pseudopotential model has been developed that predicts bandgaps to ~0.1eV as a function of particle size for
a significant number of semiconductors, e.g. CdS, CdSe, Si, and GaP. For electron transfer,
a Redfield relaxation model capable of treating multilevel electronic systems with
intervening bridges has been developed, with an initial application to long distance electron transfer through a
molecular bridge. A novel mechanism, virtually independent of distance, was obtained and
possible relevant to electron transfer through a DNA bridge was discussed. Finally,
ab initio electronic structure methods have been applied to the computation of electron transfer matrix
elements. Using novel numerical methods, large systems are tractable, for example
two bacteriochlorophyll molecules, in modest CPU times on a single workstation, at the
Hartree-Fock level. This approach will be applied to a variety of systems,
such as molecule/solid electron transfer and transfer through
various complex molecules. Future work will also include using ab initio methods to determine
structures and electronic states at the surface of semiconductor particles.
Columbia University
New York, NY 10027-6948
Department of Chemistry
Photo-CIDNP of Photosynthetic Reaction Centers
The Chemically Induced Dynamic Nuclear Polarization (CIDNP) process is used to observe the
nuclear magnetic resonance (NMR) spectra of 15N-labeled photosynthetic
reaction centers. In simple echo-detected Bloch decay spectra collected under magic-angle
spinning with or without proton decoupling, nuclear polarization has been detected for the
nitrogens in the tetrapyrroles of the bacteriochlorophyll special pair ("P"), associated
imidazoles, and the primary acceptor pheophytin ("I") that are far from Bolzmann
equilibrium. The resulting NMR lines are emissive and 300 times the intensity of the thermally
relaxed nuclei. The polarization is not observed if the quinones are present and preoxidized. The
signals presumably result from a transient nonequilibrium mixing of the singlet and triplet states of
the initially formed charge transfer pair, P+I-. Selectively labelled
samples are used to assign the signals. Ongoing efforts aimed at extensions to the plant reaction
centers have identified polarized signals from photosystem I of cyanobacteria.
Columbia University
New York, NY 10027
Department of Electrical Engineering
Translational-Energy-Resolved Studies of Photogenerated Carrier-Induced Reactions on
UHV Semiconductor Surfaces
Studies of the photodissociation of methyl halides (CH3X, X=Br,Cl,I) on GaAs
(110) using time-of-flight (TOF), temperature programmed desorption (TPD), and
photoluminescence (PL) in ultrahigh vacuum (UHV) has revealed the operation of several
photochemical processes on semiconductor surfaces. For example, photogenerated substrate
electron-hole pairs promote adsorbate dissociation via an electron attachment mechanism
to form a negative ion resonance with subsequent cleavage of the C-X bond.
The angular variation of
the photoyield suggests that the adsorbate adopts a strongly tilted geometry with respect to the
surface normal; a hypothesis confirmed by a "direct" measurement of the adsorbate
geometry via the NEXAFS technique and by modeling of the magnitude of the
adsorbate-adsorbate repulsive interaction manifested in TPD spectra. Also, recent theoretical
modeling by our group using ab initio calculations have also confirmed this geometry for the case
of CH3Br interacting with a GaAs cluster. Extension of these studies to the
similar case of CH3CH2X [( X=Br, Cl, I)] also reveals strikingly
anisotropic photofragment angular distributions which reflect the orientation of the bond
undergoing fission.
Other recent experiments have
applied a similar experimental approach to an investigation of the more complex dissociation
dynamics of methylated metal-organic chemical molecules
[(CH3)2X, X=Zn, Cd] on model III-V and II-VI semiconductor
surfaces. While complex dissociative thermal chemistry is found to dominate the adsorption
process on GaAs surfaces, molecular adsorption/desorption phenomena are the dominant thermal
processes for the case of adsorption on the CdTe(110) surface. In this system, photodissociation
of the molecular adsorbate with TOF analysis of the resulting photofragments reveals that the
photodissociation process is in this case dominated by direct photon absorption by the adsorbate.
The observed photofragment dynamics are indicative of a complex curve crossing between
excited states. A photon stimulated desorption process is also observed which results from the
deposition of large amounts of vibrational energy in the overlayer following the surface mediated
quenching of electronically photoexcited molecules.
Dartmouth College
Hanover, NH 03755
Department of Chemistry
Photoinduced Dipoles and Charge Pairs in Condensed Media
The objective is to understand photoionization in liquids and solids comprising organic molecules.
One goal is to understand the formation and recombination or separation of hole-electron pairs
formed by absorption of visible photons in electron donor-acceptor materials. Fast current
measurements are used to follow the separation of the resulting holes and electrons (or ions). The
fast photocurrent measurements led to measurement of the dipole moments of excited molecules
in solution with the same techniques. The dipole moment technique has proved useful in the study
of both intramolecular and intermolecular charge transfer. A dipole moment of 89 D
(corresponding to a charge separation of 1.8 nm) has been measured for a donor-acceptor triad
molecule, synthesized at Argonne National Laboratory. Charge separation of 0.7 nm is seen in the
triplet exciplex formed by C60 and a substituted benzene donor. While in the
C60 exciplex the charge transfer is complete; in many other exciplexes it is not.
Smaller dipoles and partial charge transfers are being studied in weaker donor-acceptor pairs.
Current investigations include the solvent polarity dependence of intramolecular dipoles, exciplexes, contact
ion pairs, and solvent-separated ion pairs.
Georgia Institute of Technology
Atlanta, GA 30332
School of Chemistry and Biochemistry
Time-Resolved Laser Studies of the Proton Pump Mechanism of Bacteriorhodopsin
There are two basic systems in nature that convert solar energy into chemical energy, i.e.,
undergo photosynthesis. The first is the chlorophyll-based system present in green plants and the
other is bacteriorhodopsin (bR) present in Halobacterium Salinarium. In both systems, solar
energy is first converted into electric energy and then into chemical energy stored in the chemical
bonds of adenosine triphosphate (ATP). The final step in the solar to electric energy conversion
involves the formation of proton gradients. In both systems, the mechanism of the conversion of
the proton gradients into ATP is the same while the molecular mechanism of the conversion of the
solar energy into proton gradient is very different. In chlorophyll, it involves electron pumps while
in bR the absorption of light leads to very rapid (450 femtosecond) retinal isomerization,
separation of positive and negative charges, and protein conformation changes that finally lead to
pumping protons from inside the cell to the membrane surface, thus creating the proton gradients;
thus bR is a solar proton pump. This pump requires metal cations for its function. Our present
research is focused on trying to answer two fundamental questions regarding the proton pump:
(1) What are the molecular mechanisms by which the protein catalyzes the retinal
photoisomerization in bR and (2) What role do metal cations play in the proton pump? We are
presently determining the femtosecond time and quantum yield of retinal photoisomerization in bR
and in its modified derivatives, e.g. mutants in which charged and hydrogen bonding residues in
the retinal cavity are individually replaced by neutral nonhydrogen bonding ones. Studies at
different pH and temperature are also being carried out. The observed results are examined in
terms of the electronic and steric effects on the retinal excited state potential energy surface in bR.
In order to understand the role of metal cations in the bR function, their location must first be
determined. In this effort, their binding constants have been determined in bR and in a number of
its mutants. The two high affinity cations, one of which is vital to the function, are electrostatically
coupled to the charged residues within the retinal cavity. Now attempts are being made to locate
the position of these two metal cations by use of anomalous X-ray and extended X-ray absorption
fine structure techniques using the Synchrotron Radiation Source at Brookhaven National
Laboratory.
University of Houston
Houston, TX 77204
Department of Chemistry
Charge Separation in Photoredox Reactions
| Investigator(s) |
Kevan, L.
|
$125,000 |
| Phone | 713-743-3250 |
| E-mail |
kevan@uh.edu |
This research is directed toward a better molecular understanding of the structural aspects
controlling charge separation in photoredox reactions in organized molecular assemblies
especially vesicles and microporous silicas. Control of the location of an electron donor will be
achieved by attachment of variable length alkyl chains to porphyrins. Control of the electron
acceptor location is also being initiated. Photoyields will be monitored by electron spin resonance
and the photoproduced cation location will be assessed by deuterium electron spin echo
modulation. Molecular electron acceptors will also be studied. The results will help determine the
structural requirements for optimizing photoinduced charged separation for the storage of light
energy.
Johns Hopkins University
Baltimore, MD 21218
Department of Chemistry
Electron Transfer Dynamics in Efficient Molecular Solar Cells
Regenerative solar cells based on dye sensitization of wide band gap semiconductors have
recently experienced an order of magnitude increase in light-to-electrical energy conversion.
Results in this laboratory and others have shown high efficiency and excellent stability, indicating
an economically competitive approach to solar energy conversion. This remarkable breakthrough
marks the first time that devices which operate on a molecular level are competitive with
traditional solid state photovoltaics. With further optimization, significant improvements in
efficiency and stability are expected. In this program the use of novel sensitizers wil be explored
for dye-mediated light-to-electricity conversion in photoelectrochemical cells based on
nanostructured wide band gap materials. Specifically, Ru(II) and Os(II)
polypyridyl sensitizers will be attached to nanostructured TiO2 photoanodes fabricated using
sol-gel processing techniques and explore their photoelectrochemical and electron transfer
characteristics. The materials processing and synthesis will be carried out in conjunction with
characterization of cell performance, detailed investigation of operating mechanisms, and
modeling of charge transfer and transport in these systems.
Marquette University
Milwaukee, WI 53233
Department of Chemistry
Organized Photochemical Assemblies Based on Y-Zeolite
Supports
The essential goal of this project is the preparation and characterization of molecular assemblies
entrapped within the supercage network of Y-zeolite. Current efforts are focused on the synthesis
of assemblies based on polypyridine complexes of divalent ruthenium. Extensive photophysical
characterization of zeolite-entrapped tris-bipyridine ruthenium
(Ru(bpy)32+) preparations, at various loading levels, document
strong interactions between excited states of complexes occupying adjacent supercages. Synthetic
methods have been developed to generate isolated adjacent-cage dyads of related complexes,
wherein the two complexes are strongly coupled with respect to energy and electron transfer
interactions. Photoredox studies of such particles, which have been loaded with an excess of
common acceptors, such as methylviologen or related species, document a significant increase in
photoinduced net charge separation relative to zeolite based systems which contain randomized
loadings of the sensitizer complexes. Such increases in charge separation efficiency are most
reasonably attributed to minimization of the (reduced)acceptor-to-(oxidized)sensitizer back
electron transfer reaction via rapid reduction of the oxidized sensitizer by the adjacent cage
(donor) complex.
University of Massachusetts-Boston
Boston, MA 02125
Department of Chemistry
Magnetic Resonance Studies of Photoinduced Electron Transfer Reactions
Research focuses on the application of time-resolved Electron Paramagnetic Resonance
(TR-EPR) techniques in the study of photoinduced electron transfer reactions. The application of
TR-EPR in this area of research is of interest for several reasons. First, high spectral resolution
makes it possible to identify paramagnetic molecules and to obtain information on their interaction
with the environment. Second, the time development of the spectra generally is affected by
chemically induced dynamic electron polarization (CIDEP). These CIDEP effects provides unique
mechanistic insights. Third, relaxation data can be used to study the effect of temperature and
medium on molecular motion. Fourier transform EPR has been used to investigate oxidative and
reductive electron transfer quenching of C60 triplets
(3C60)in homogeneous and heterogeneous media as well as
photochemical reactions in which C60 acts as a light harvester and
photosensitizer. The method is also applied in studies of the mechanism and kinetics of
photoinduced redox chemistry at the surface of semiconductor particles.
University of Minnesota
Minneapolis, MN 55455
Department of Chemistry
Femtosecond Time-Resolved Experiments on the Solvated Electron and Intermolecular
Charge Transfer in Solution
The photophysical properties of the solvated electron have been studied using a unique
20 fs transient absorption spectrometer. The technique involves three separate laser pulses. The
first pulse generates solvated electrons, the second pulse excites the electron from the ground
state to the first excited state, and the third pulse probes the dynamics of the ground state
repopulation and the ground and excited state relaxation. The first results using this spectrometer
have resolved the inertial dynamics of the excited state relaxation of the electron. This observation is critical
in evaluating theoretical models for the solvated electron relaxation dynamics. In addition, the
excited state proton transfer kinetics of the solvated electron in alcohols has been resolved for the
first time. Future studies will concentrate on the comparison of theory and experiment for the
early time dynamics of the solvated electron in various solvents. Photoinduced charge separation and reverse
electron transfer will be examined in charge transfer complexes.
University of Minnesota
Minneapolis, MN 55455
Department of Chemistry
The Photophysical Properties of Organic Liquids at High Excitation Energies
The absorption and excitation spectra of concentrated solutions of simple aromatics (benzene,
toluene, etc) have been examined from 220 to 150 nm. The intense S0
S3 absorption,
which is the dominant excitation in this region, is observed to dramatically redistribute its
oscillator strength with increase in the aromatic concentration. Dispersion theory with
Lorentz-Lorenz corrections to the local field predict this spectral behavior and also predict well
the spectral positions of the extrema in the refractive index of the neat liquid and the spectral
position of the maximum of the energy loss function.
Calculations suggest that the terminal states of the
S0S3 absorption in these concentrated solutions may have a
significant collective component. Parallel studies on the effect of concentration to alter the
fluorescence excitation spectrum in the region from 220 to 150 nm, appear to support this
assignment. The recovery of the aromatic fluorescence that develops at ca. 195-185 nm, and only
in concentrated solutions, correlates well with the predicted development of collective excitations
in this region of very high spatial and spectral oscillator strength density. The effect is attributed
to a very efficient non-radiative coupling of the fluorescent state to the higher-lying collective
states due to the effect of spatial diffusion of the excitation density to reduce nuclear distortions.
Further manifestations of collective effects are being examined. In a separate study, the general
theory of hyperfine-driven spin evolution of a geminate pair of ions diffusively recombining has
been found to predict the shapes and positions of observed resonances in the effect of a magnetic
field on the quantum yield of geminate recombination fluorescence of isooctane cations with
hexafluorobenzene anions generated by photoionization of the isooctane at 124nm. Determination
of the magnetic fields at which the resonances occur provide a simple technique for determination
of hyperfine constants of short-lived anions. Additionally, the magnetic field dependence provides
a new technique for examining the range distribution of ionized electrons.
National Institute of Standards and Technology, Gaithersburg
Gaithersburg, MD 20899
Chemical Kinetics and Thermodynamics Division
Electron Transfer Reactions of Metalloporphyrins
The pulse radiolysis technique is applied to the study of electron transfer processes involving
metalloporphyrins. Reactive intermediates are produced in solution by electron pulse irradiation
and the kinetics of their reactions are followed by time resolved absorption spectrophotometry.
Complementary experiments are carried out with laser flash photolysis and supportive product
analyses are done with various techniques following photolysis or gamma radiolysis. The studies
focus on the unique ability of pulse radiolysis to provide absolute rate constants for many fast
reactions of metalloporphyrins, which permits evaluation of these strongly light absorbing
molecules as sensitizers and intermediates in solar energy conversion. Metalloporphyrins react
with free radicals via electron transfer, involving the ligand or the metal center, or via bonding to
the metal, leading to a variety of chemical species whose behavior is also investigated. One of the
important potential applications of metalloporphyrins under investigation is as catalysts for
reduction and binding of carbon dioxide.
University of New Orleans
New Orleans, LA 70148
Department of Chemistry
Electronic and Nuclear Factors in Intramolecular Charge and Excitation Transfer
| Investigator(s) |
Piotrowiak, P.
|
$130,000 |
| Phone | 504-286-6840 |
| E-mail |
ppcm@uno.edu |
The research focuses on the molecular level control of the rate and efficiency of
photoinduced electron and excitation transfer processes. Carefully selected model systems
relevant to energy conversion and design of molecular electronic devices are being synthesized
and investigated. The current projects include the study of: 1) the interplay between the MO
symmetry and the density of states in "symmetry forbidden" vibronically coupled
excitation transfer; 2) the liquid medium contribution to the electronic coupling between rigidly
linked donors and acceptors; 3) the role of the low frequency medium modes in intramolecular
charge transfer; 4) preferential solvation and ionic aggregation in the vicinity of charge separated
species in mixed media. The work involves time resolved spectroscopic measurements in liquids,
glasses and jets. New transient Raman and subpicosecond transient emission/absorption
capabilities are being implemented.
North Carolina State University
Raleigh, NC 27695-8204
Department of Chemistry
Biomimetic Porphyrin Light Harvesting Arrays
A molecular building block approach is being used to synthesize biomimetic
multiporphyrin light-harvesting (LH) arrays containing from 2 to 20 pigments. The major
objective of this work is to elucidate molecular design issues for efficient light-harvesting in
synthetic systems. The versatility of these synthetic systems will complement studies of the more
complex natural systems.
The model systems are being characterized spectroscopically in
collaboration with Prof. David Bocian (UC-Riverside), Prof. Graham Fleming (U-Chicago), and
Prof. Dewey Holten (Washington U). Energy migration rates are determined by measurement of
fluorescence lifetimes including polarization studies, and time-resolved donor-acceptor experiments.
Temperature and medium effects will be examined. Exciton annihilation experiments will be
attempted in order to characterize exciton mobilities in various arrays. Resonance Raman
experiments of neutral complexes, and EPR studies of various oxidized arrays will be performed
to assess electronic communication in the arrays. Other experiments such as time-resolved Raman
spectroscopy may be attempted as required to identify mechanisms of energy migration. The data
from these spectroscopic studies will be used to guide the molecular design of more efficient
light-harvesting model systems.
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599
Department of Chemistry
Excited State Processes in Transition Metal Complexes. Redox Splitting in Soluble
Polymers
Polypyridyl complexes of Ru(II), Os(II), and Re(I) are being investigated for applications in
photochemical energy conversion. These molecules have well understood light absorption and
excited state properties. They undergo facile electron and energy transfer, and in molecular
assemblies they show promise in molecular conversion. This chemistry is being extended through
new synthetic procedures for a family of black absorbers that absorb light efficiently throughout
the near-UV visible region. These molecules emit in the near infrared, are photochemically stable,
and have excited states whose lifetimes are sufficiently long to be accessible to energy conversion
processes. Emission and resonance Raman spectroscopies are being used to explore electronic
structure, coupled vibrations, and the role of medium on properties and lifetimes. Synthetic
methods have been developed based on ether or amide links for preparing assemblies based on
soluble polymers. These assemblies contain both light absorbers and electron or energy transfer
relays. Photophysical studies on the resulting materials have demonstrated photoinduced electron
or energy transfer on single polymeric strands. This has led to the design of efficient
"antenna" polymers for collecting and storing visible light.
Northwestern University
Evanston, IL 60208
Department of Chemistry
Dynamic Structural Studies of Light-Induced Charge Transfer and Electronic
Localization/Delocalization Phenomena in Metal-Based Molecular Systems
This project involves (1) time-dependent scattering studies of vibrational structural changes
accompanying chemically important photoredox processes; (2) collaborative studies of
femtosecond charge-transfer kinetics in fully vibrationally characterized systems; (3) electronic
Stark effect studies of one-electron transfer distances; and (4) resonant vibrational
characterization of delocalized intervalence transitions. Recent work in area (3) has now yielded
unprecedented orbital-specific charge transfer distances, solvent reorganization energies and
nonadiabatic electronic coupling energies. Work in area (4) has yielded a quantitative
experimental description of the vibrational and electronic factors that drive electronic localization
events in strongly interacting systems.
Northwestern University
Evanston, IL 60208
Department of Chemistry
Light-Driven Charge Separation in Face-to-Face Donor-Spacer-Acceptor Supramolecular
Systems
The objective of this project is to investigate the kinetics of charge separation and recombination
in donor-spacer-acceptor systems of well-defined geometry in which the spacer is an aromatic
molecule or an array of aromatic molecules. The unique molecular architecture of tertiary
aromatic amides and ureas will be used to construct supramolecular systems in which the donor
and acceptor are held either in an edge-to-face or face-to-face geometry with respect to the
aromatic spacer. The urea structure can be elaborated to place two or more spacers between the
donor and acceptor. The analogous secondary amide and ureas have extended structures,
providing a basis for comparison of extended and folded structures. Base pairing in duplex DNA
provides a well-defined geometry in which the aromatic bases are held in a parallel stack. Charge
separation will also be investigated in synthetic DNA hairpins and dumbbells which possess
acceptor and donor molecules at one or both ends of a region of duplex DNA. The unique feature
of these systems is the location of the donor and acceptor on opposite faces of the aromatic
spacer(s). The shortest distance for electron transfer in these molecules is through the aromatic
spacer rather than the sigma-bonded framework connecting the donor and acceptor.
Northwestern University
Evanston, IL 60208
Department of Chemistry
Vibrational Dynamics in Photoinduced Electron Transfer
Theory and experiment suggest that molecular vibrations and distortions are important controlling
elements for electron transfer. The objectives of the project are to develop a new molecular
understanding of electron transfer processes. The unique method of picosecond infrared
absorption spectroscopy is being used to monitor electron transfer kinetics. The first case of
vibrational state dependent electron transfer has been experimentally determined. The electron
transfer is between two cobalt metal atoms interacting in a solvent-caged, contact ion pair. The
compound has a cobaltacinium cation and a cobalt tetracarbonyl anion with a visible absorbing
charge-transfer band. The neutral pair created by a pulse of visible light has a rate of return
electron transfer that is dependent on vibrational quantum number. The vibrational excitation in
the ion pair formed after the electron transfer also was measured. Recently a
similar ion pair complex with a vandium hexacarbonyl anion was found that also shows vibrationally resolved
electron transfer rates with over a two-fold change in rate for each quantum of vibration in a CO
stretching vibration. This molecular structure is amenable to detailed modeling so that quantum
calculations of molecular structure and vibrations as well as electron transfer models are being
developed to understand these results. Additional spectroscopic and kinetic measurements are
being done on these and other molecules.
Ohio State University
Columbus, OH 43210
Department of Chemistry
Energy and Electron Transfer Properties of Photochemical Assemblies in Zeolites
| Investigator(s) |
Dutta, P.K.
|
$105,000 |
| Phone | 614-292-4532 |
| E-mail |
dutta.1@osu.edu |
In designing artificial photosynthetic assemblies, the choice of molecules, their spatial arrangement
and their environment is fundamental to the success of the process as evidenced in nature.
Zeolitic microporous frameworks are being investigated as a means to spatially arrange molecules for
efficient energy and electron transfer processes. Azobenzene molecules are being aligned in
zeolite channels, and spectroscopic properties of these aggregates, including absorption,
fluorescence, Raman and second harmonic generation are being investigated to explore the
possibility of energy migration along these dipole chains. Zeolites are also being investigated as
supports for photochemical redox assemblies. Efficient photoelectron transfer from intrazeolitic
Ru(bpy)32+ to a neutral viologen in solution is possible by
mediation of an intrazeolitic viologen in the absence of a sacrificial electron donor. Choice of the
intermediary viologen is critical in promoting rapid forward electron transfer but slowing down
the back electron transfer reaction. This depends on several factors, including the reduction
potential of the viologen. The dynamics of the electron transfer process is being examined for a
series of viologens whose reduction potential vary over a range of 500 mV. The reaction of water
with the photochemically generated intrazeolitic Ru(bpy)33+ to
make dioxygen is being studied. This species is unstable in aqueous solution at neutral to basic pH
because of intermolecular reactions leading to degradation. Because of the lack of translational
mobility in the zeolites, the reaction of Ru(bpy)33+ with water
leads to dioxygen formation. The route this reaction follows involves hydroxide ion attack on the
bipyridyl ligand followed by release of hydroxyl radicals. These radicals attack the viologen
molecules. Thus, Mn and Ru based oxides as well as their bipyridyl complexes are being investigated
as multielectron catalysts which, in a four-electron concerted step, can convert water to oxygen.
University of Oregon
Eugene, OR 97403
Department of Chemistry
Photochemical Water-Splitting Using Organometallic Oxides as Sensitizers
The project objective is to split water photochemically into hydrogen and oxygen using
homogeneous molybdenum oxide catalysts.
NMR experiments and manometric measurements showed that molybdocene reacts quantitatively
with water to form molybdocene oxide and hydrogen. (The molybdocene was generated by
irradiation of molybdocene dihydride.) However, the water-splitting
cycle could not be completed because earlier experiments could not be reproduced in which visible light
irradiation of molybdocene oxide formed oxygen and molybdocene. One possible explanation is
that the oxygen that is formed is reacting with the molybdocene oxide. Control experiments
showed that the Mo-containing products of the photoreactions were indeed the same as those that
form when oxygen reacts with molybdocene oxide. Attempts to sweep out the oxygen gas
product before it could react with the starting material were unsuccessful, however. Efforts in the
forthcoming year will focus on finding ways to remove the oxygen gas from the system before it
can react with the starting material. Other, less oxygen-sensitive, sensitizers will also be studied.
Mechanistic studies of the hydrogen and oxygen producing reactions are underway. This system
represents one of the first homogeneous systems in which both hydrogen and oxygen are formed
from the same catalyst in solution.
Pennsylvania State University, University Park
University Park, PA 16802
Department of Chemistry
Electron Transfer Reactions in Microporous Solids
| Investigator(s) |
Mallouk, T.E.
|
$170,000 |
| Phone | 814-863-9637 |
| E-mail |
tom@chem.psu.edu |
This project uses microporous solids and surface assemblies as organizing media for light-induced
intermolecular electron and energy transfer reactions. The role of the solid is to control the
juxtaposition of redox-active and photoactive molecules in space, and also, in some cases, to
participate as an active component of the electron or energy transfer cascade. One aspect of the
project involves the sensitization of layered oxide semiconductors, such as alkali niobates and
titanoniobates, with ruthenium polypyridyl photosensitizers. When loaded with interlamellar metal
catalysts, these materials are catalysts for photogeneration of hydrogen from water and
non-sacrificial electron donors, such as iodide. Surface modification of these materials with
polyelectrolytes increases the efficiency of charge separation and hydrogen evolution, and this
strategy is now being applied to improving the efficiency of dye-sensitized photoelectrochemical
cells. A new technique has been devised for exfoliating lamellar semiconductors and restacking
them on high surface area substrates, with control over the stacking sequence at the monolayer
level. This method is now being used to prepare multicomponent electron and energy transfer
chains, in which individual electroactive and photon harvesting polymer monolayers are separated
by single sheets of oxide semiconductors or insulating metal phosphates. The kinetics of
light-induced electron transfer reactions are being studied by flash photolysis techniques, and
heterostructures designed for the visible light photolysis of water are being synthesized.
Pennsylvania State University, University Park
University Park, PA 16802
Department of Chemistry
Polar Solvation, Dielectric Friction, and Electron Transfer
| Investigator(s) |
Maroncelli, M.
|
$233,000 |
| Phone | 814-865-0898 |
| E-mail |
mpm@chem.psu.edu |
The focus of this project is on understanding how polar solvents influence electron and other
charge transfer reactions in solution. Of special interest are dynamical aspects of the solvation
process and the role that such dynamics play in determining charge transfer rates. A number of
recent theories have predicted a proportionality between the rate of electron transfer and solvent
reorganization rates. Time-resolved fluorescence studies on simple intramolecular charge transfer
reactions are used to explore this connection. An important part of the work currently in progress
also involves obtaining a prerequisite understanding of the dynamics of solvation in nonreactive
systems. Results obtained to date indicate that the time scales of solvation in polar liquids are
poorly predicted by simple continuum models of solvation, but it is just such models that have
been used to theoretically study the connection between solvation, and electron transfer. In order
to build a more adequate understanding of charge transfer in solution it is first necessary to better
understand and model the dynamics of solvation in simple, nonreactive situations. Time-resolved
experiments and molecular dynamics computer simulations are being used to probe the dynamics
of solvation in a variety of solvents and solvent mixtures. The goal of this work is to develop and
test simple models of the static and dynamic aspects of polar solvation of relevance to the
solvent-reaction coupling.
University of Pennsylvania
Philadelphia, PA 19104
Department of Chemistry
Evaluation of Electronic Coupling In Photoinduced Electron Transfer Reactions
The primary objective of this project is to delineate the relative and absolute importance of the
factors that determine the dimension of the electronic coupling matrix element
(Hab) for photoinduced and thermal charge recombination electron transfer
processes. These studies are being carried out with several families of donor-spacer-acceptor
(D-Sp-A) complexes that utilize electronically excited porphyrin donors and quinone acceptors. A
key feature of this effort lies in the detailed focus on the role played by the tunneling medium in
such reactions and the parameters that should affect the dimension of Hab such as
absolute D-A energetics, medium topology, and medium electronic structure. Experimental work
in the progress focuses on probing the nature of charge tunneling interference phenomena as well
as how medium band energetics, 
-
electronic coupling,
-manifold orientation, and medium excited
electronic states establish the magnitude of Hab. The fundamental information
obtained in these studies will be useful in directing and controlling the electron and energy transfer
processes essential to effective photoconversion.
University of Pittsburgh
Pittsburgh, PA 15260
Department of Chemistry
Experimental Studies of Photoinduced Charge Carier Processes at Semiconductor/Electrolyte
Interfaces
This project aims to develop a quantitative understanding of the kinetics of photogenerated
carriers in semiconductor electrodes and at the semiconductor/electrolyte interface. This goal is
being pursued on three fronts. First, an understanding of bandgap emission
and its utility for monitoring the transport and relaxation of charge carriers will be established. An analytical model
and computer algorithm are being developed to model the fluorescence decay of semiconductor
band gap emission. Experimental studies are proceeding on the fluorescence decay of InP. These
studies are probing its voltage and dopant level dependence. Second, self-assembled monolayers
of octadecylthiols on InP electrodes have been prepared, and these insulating barriers are being
used to probe the nature of the electron tunneling through the layer. The third area is investigating
how functionalized monolayer films can be used to modify the interfacial recombination rate and
electron transfer rate processes at these interfaces.
Portland State University
Portland, OR 97207
Department of Chemistry
Asymmetric Polymeric Porphyrin Films for Solar Energy Conversion
| Investigator(s) |
Wamser, C.C.
|
$90,000 |
| Phone | 503-725-4261 |
| E-mail |
WamserC@pdx.edu |
This project involves the synthesis and characterization of thin films of polymeric porphyrins,
where such films are potentially useful as components of solar energy conversion methods. Two
different approaches have been used to create polymeric films of substituted
tetraphenylporphyrins on transparent electrodes: (a) interfacial polymerization of a pair of reactive
monomers into a thin film, later deposited onto an electrode, or (b) oxidative
electropolymerization of electron-rich porphyrins directly onto an electrode. For interfacial
polymerization, an aqueous solution of either tetra(4-hydroxyphenyl)porphyrin (THPP) at pH 11
or tetra(4-aminophenyl)porphyrin (TAPP) at pH 3 is layered atop a dichloromethane solution of
tetra(4-chlorocarbonylphenyl) porphyrin (TCCPP), creating a thin polyester or polyamide film at
the interface. Such films are highly crosslinked, with a distinctive asymmetry of functional groups
that creates a gradient of porphyrin redox potentials across the film. Observed photopotentials are
directional, with the charge separation following the predicted direction of the redox potential
gradient. Continued work in this area is aimed at characterizing the unique structural asymmetry
and developing a model of the directional charge transport processes within the film.
Electropolymerized films of TAPP are also being studied as possible conductive mediators for
dye-sensitized semiconductor solar cells.
University of Rochester
Rochester, NY 14627
Department of Chemistry
Photochemistry of Platinum Group Element Complexes: Applications to Energy Conversion and Bond Activation
Diimine dithiolate complexes of platinum(II) and other d8 metal ions are being
investigated for their potential use as chromophores in the conversion of light-to-chemical energy.
Studies in previous years on Pt diimine dithiolate complexes have shown that these solution
emissive compounds have a directional charge transfer excited state involving a mixed Pt(d)/S(p)
orbital as the filled donor function and a * orbital of the diimine as the acceptor function. A detailed study of
the Pt diimine dithiolate chromophore published during the year reveals that excited state
properties such as emission energy, lifetime, redox potentials, electron transfer quenching rates
and relaxation dynamics can be systematically tuned by ligand variation. Current efforts are
focussing on the development of a supramolecular photochemical system for light-driven
hydrogen generation. Such a system would consist of a platinum diimine dithiolate chromophore,
a dark reaction catalyst and a redox center. The components are connected by ligand bridges. The
simplest of these bridges is dipyridocatecholate (dpcat) which has been made in high yield and has
been used to link two metal centers together. Characterization of new dpcat complexes is in
progress. The use of Pt(diimine)(dithiolate) chromophores in new multi-component systems builds
on the understanding that has been achieved regarding their luminescence and excited state
properties. The dark catalyst in systems will be either a noble metal colloid or a macrocyclic
complex and will have two or more chromophores attached to it. Finally, new Schiff base
complexes of rhodium have been synthesized and characterized regarding their electronic
structures and reaction chemistry. Square pyrimidal Rh(III) complexes exhibit weak solution
luminescence and undergo photolysis to generate metalloradical species that are important in
substrate activation reactions. Particular emphasis in ongoing studies is being given to
synthesizing Schiff base complexes of rhodium and iridium having
N2S2 donor sets that are capable of binding to d7
metalloradicals more strongly.
University of Rochester
Rochester, NY 14627
Department of Chemistry
Photoinduced Electron Transfer Processes in Homogeneous and Microheterogeneous
Solutions
This project is comprised of a study of light-induced electron transfer reactions in solution, the
solid state, and thin films. It focuses on potentially useful net chemical conversions that can occur
following single electron transfer quenching of excited states. Typically, these reactions are
initiated by photoexcitation of visible or near ultraviolet light absorbing electron donors (or
acceptors) and subsequent quenching by single electron transfer. In the cases examined in these
studies either the acceptor or donor or both contain a potentially fragmentable bond. The
compounds are stable as the even electron precursors and hence the fragmentation is only
accessible in one or both of the photogenerated radical ions due to a drastic and selective
reduction of bond dissociation energies in these species. Such a change in bond dissociation
energies has been found to be quite general for a number of different species upon one electron
oxidation or reduction. Donors that can fragment from their cation radicals include amines,
diamines, aminoketones, aminoalcohols and pinacols. Acceptors that can undergo corresponding
fragmentations include organic halides, ethers and esters. Polymeric systems containing
photoexcitable acceptors and fragmentable donors that can be reacted in solid state, solution, and
thin films have been designed, synthesized, and studied. Amphiphilic donors and acceptors that
can be incorporated into various organized media are also under investigation. Co-fragmentation
reactions involving excited pinacols reacting with various organic halides are also under
investigation. In some cases, especially with oxygen present, the co-fragmentation reactions can
include a chain process resulting in highly efficient generation of acid and potentially useful
radicals.
Rutgers, The State University of New Jersey
Piscataway, NJ 08855
Department of Chemistry
Electron Transfer in Constrained Helical, Constrained Peptides and Hydrogen Bonding Networks
This project is an investigation of the electron mediating properties of peptide bonds, amino acid
side chains, and hydrogen bonds in the control of long-range electron transfer in proteins. In this
research, well-defined, rigid peptide systems where photoactive metal donors and metal acceptors
are covalently attached at the peptide terminals and amino acid side chains, resulting in the
formation of linear, cyclic and poly-cyclic peptide networks possessing specific secondary
structures. Peptide and/or hydrogen bonding networks are designed to emphasize special features
such as the number of peptide bonds, H-bonding networks, connectivity of redox centers to main
chain and side chain of the peptide, as well as the overall distance dependence in rigid molecules.
Results from metal derivatized peptide networks show that electron transfer rate through a
solvent stabilized helix (Pro)n bridge (n>4) shows unusually small distance
dependence in comparison to saturated hydrocarbons. This implies that specific secondary
structures may be promoting these efficient electron transfer pathways. Also the effect of
hydrogen-bonding networks in cross-linked 
-helical peptides and guest-host assemblies may also be implicated. Overall,
results of this research have shown that the number and the nature of connectivity between the
donor and the acceptor, the secondary structure of the bridge and solvent structure around the
complex all play an important role in increasing the distance across which rapid long-range
electron transfer can be observed.
Stanford University
Stanford, CA 94305
Department of Chemistry
Photoinduced Electron Transfer and Electronic Excitation Transport in Complex Systems
The problems of photoinduced electron transfer and geminate recombination in liquids and in
restricted geometry systems, e. g., micelles, are being addressed both experimentally and
theoretically. A major advance in the theory is the inclusion of realistic finite-volume solvent
effects. The finite volume of solvent molecules gives rise to a non-uniform distribution of particles
around an electron donor. The non-uniform particle distribution significantly affects the electron
transfer rates and the distribution of ion pairs formed by forward electron transfer. In addition,
finite solvent size affects the rate of relative diffusion between donor-acceptor pairs. These
"hydrodynamic effects" slow down the interparticle diffusion rates when near contact,
resulting in a major change in the long time behavior of photoexcited electron transfer systems.
Experiments were perform and analyzed with the full theory to determine the dynamics of
photoinduced electron transfer. Fluorescence measurements were made with picosecond time
correlated single photon counting. These experiments, combined with the theoretical analysis,
yield the first realistic description of through-solvent photoinduced electron transfer. In addition,
pump-probe experiments measuring geminate recombination were performed on the same donor,
acceptor, solvent systems. The new theory was used to analyze the geminate recombination data.
The first rigorous calculation of photoinduced electron transfer and geminate recombination in
finite volume, complex geometry systems were performed. The method was applied to the
problem of donors and acceptors diffusing on the surface of micelles. Ion spatial distributions as a
function of time were calculated and used to understand the possibility of achieving long-term ion
separation. The validity of the theory was confirmed by comparison to Monte Carlo. The first
studies of photoinduced electron transfer for donors and acceptors on micelle surfaces have been
conducted on three micelle systems. Very efficient electron transfer occurs because the acceptors
are concentrated near the donor. The data analysis uses the new restricted geometry electron
transfer theory. New theoretical descriptions of excitation transport in complex geometry systems
have been developed. The work has focused on systems of clustered chromophores, in particular
diblock copolymers which can form polymer micelles. The sensitivity of the rate of excitation
transfer to the structure of the polymer micelles was investigated.
University of Tennessee at Knoxville
Knoxville, TN 37996
Department of Chemistry
Studies of Radiation-Produced Radicals and Radical Ions
The objective of this project is to characterize both the structure and reactivity of selected organic
free radical and radical ion intermediates generated by irradiation of molecular systems. Of
particular interest is the study of the radical ions that are generated in the primary chemical
processes resulting from the absorption of high-energy radiation, since these charged species play
an important role in the mechanisms of both radiation and photochemical effects. Specific projects
include structural and reactivity aspects of novel species including (a) 1,3- and 1,4-cycloalkanediyl
radical cations where one electron is delocalized over two nonadjacent carbon centers, (b) twisted
structures in olefin (1,2-diyl) radical cations, (c) bisallylic (5 electrons) radical cations involving through-space interactions, (d) distonic radical
cations where spin and charge are separated in the same molecule, and (e) studies of thermal and
photoinduced rearrangements of radical cations in rigid matrices proceeding by hydrogen transfer,
ring opening, ring closure, and sigmatropic shifts. Many of these thermal radical cation
rearrangements can be interpreted in terms of the role of vibronic coupling between the ground
state and the first excited state of the radical cation. There is also a special focus on the
photochemistry of radical cations in the visible region of the solar spectrum, with emphasis on the
role of orbital, configuration, and state symmetry in going from the photoexcited state of the
reactant to the ground state of the product.
University of Texas at Arlington
Arlington, TX 76019
Department of Chemistry and Biochemistry
Interfacial Chemistry at the Chalcogenide Semiconductor/Electrolyte Junction
This research involves two projects: (1) new electrosynthetic routes to the fabrication of
chalcogenide semiconductors at support electrode surfaces; and (2) real-time insitu correlations of
the interfacial chemistry and the electrical performance of a semiconductor electrode/electrolyte
junction. Proof-of-concept experiments have been completed and have opened a new route to the
electrosynthesis of chalcogenide semiconductors that is based on a chemically-modified support
electrode. Specifically, a sulfur-modified gold electrode is electroreduced in a medium containing
a targeted metal ion (or ions) to yield immobilized metal sulfide particles at the gold surface. The
utility of this approach for molecular-level ordering of the immobilized semiconductor particles
and the role of the sulfur layer as a "template"is being investigated. A second topic is
the development and use of new families of real-time/in situ probes for the characterization of
semiconductor/electrolyte interfaces. These include the complementary probes, laser Raman
spectroscopy and electrochemical quartz crystal microgravimetry. A photoelectrochemical version
of the latter technique has been developed to facilitate microgravimetry measurements on
metal/semiconductor interfaces under illumination. Experiments have been completed on the
growth and characterization of copper sulfide layers. Composite photoelectrodes containing
semiconductor particles in a nickel or polymer matrix have also been prepared and characterized.
Finally, reversible photochromism at a titanium dioxide/methylene blue dye interface has been
observed.
University of Texas at Austin
Austin, TX 78712
Department of Chemistry and Biochemistry
Vectorial Electron Transfer in Spatially Ordered Arrays
Chemically modified semiconductor surfaces are being studied mechanistically as sites for
controlled photomediated oxidation and reduction reactions. New methods for synthetic
manipulation of the surfaces are being explored, along with new methods for preparation and
characterization of coordination polymers and solar light harvesting polymeric layers. These
polymeric coatings are then associated with optically transparent electrodes, producing a
multicomponent system that can be used to probe quantitative aspects of electrocatalysis,
electrosorption, and chemisorption. The use of new polymerization methods for preparing
rectifying mono- and bilayer electrode coatings are being explored, with the attainment of
vectorial migration of excitons and electrons being an ultimate project goal.
University of Texas at Austin
Austin, TX 78712
Department of Chemistry and Biochemistry
Photophysics and Photoredox Processes at Polymer-Water Interfaces
This research has focused on the use of chromophores covalently attached to amphiphilic polymers in
order to enhance the yield of ion-pairs produced in photoredox reactions. Using anionic
polymerization methods triblock polymers of polystyrene, vinyldiphenylanthracene (vDPA) and
methacrylic acid have been prepared. The block length of the vDPA segment is relative short, 3-4
units, while the other blocks are on the order of 200 units. These triblock polymers self-assemble
into micelles. Excited state electron transfer has been studied using a zwitterionic viologen (SPV)
as a quencher. The fraction of singlet state quenching events that lead to ion-pair separation is
excellent (ca. 0.5-1.0) and the lifetime of the SPV anion radical is exceptional, with almost no
decay over a 10 ms. In fact it is possible to build up a steady state populations of SPV anion
radical that persists for hours in the absence of oxygen. However the corresponding anthracene
cation radical is not observed on any time scale, from ps to minutes. Evidently some component
of the polymer micelle system is acting as a sacrificial reagent to reduce the cation radical.
Alternatively the unusual environment of the cation radical may be broadening or distorting the
absorption spectrum such that it is not detected.It was found that C60 or C70 and styrene would
copolymerize with standard free-radical methods. The photophysical properties of the fullerene
are strongly modified after copolymerization The resulting polymers have the normal solution
properties of polystyrene and one can easily prepare deeply colored solutions or films. C60 can be
polymerized with water soluble monomers (e.g. polymethacrylic acid) or reacted with
amine-terminated polyethylene glycol, thereby producing a water soluble fullerene derivative. The
latter material appears to have unusual surfactant properties with respect to unreacted C60, such
that a water emulsion of C60 can be prepared. Studies of the photophysics of these fullerene
derivatives in water are under way.
Tulane University
New Orleans, LA 70118
Department of Chemistry
Photochemical Studies of Two Component Organic Systems within the Restricted Spaces of
Zeolites
Because of lack of knowledge on the location and mobility of organic guest molecules within
zeolites, photochemical studies in this medium have largely been restricted to one component
materials. As a result, our ability to control photoprocesses through a second molecule within
zeolites has been restricted. The present project addresses this problem.
Within zeolites, more than one type of organic molecule will be assembled within contact distances and the
'supramolecular assembly' will be used to bring about changes on photochemical processes of guests that may
be otherwise difficult to achieve. In this context,
energy and electron transfer will be examined. With respect to energy transfer, the aim is to establish that
two components can be assembled within zeolites in such a way that energy transfer and a chemical
reaction resulting from energy transfer will take place. As an illustration of the concept of energy
transfer, singlet oxygen will be generated, characterized, and reacted within zeolites. A part of
the program focuses on exploring the use of zeolite matrices to carry out electron transfer-mediated
photoreactions. The rate of back electron transfer in the case of a few donor-acceptor pairs has
been shown to be at least five orders of magnitude slower on zeolite surfaces than in solution.
This feature will be utilized to generate radical ions within zeolites. Interest in radical ions
within zeolites is centered around performing quantum chain reactions. In the proposed
investigation, the medium plays the central role. Mechanistically well-studied
reactions and experimentally well-examined donor-acceptor pairs will be used as probes to explore in what
way zeolites are different from isotropic solvents as media for photochemical processes which
involve more than one molecule.
Tulane University
New Orleans, LA 70118
Department of Chemistry
Photoinduced Energy and Electron Transfer Reactions in Light Harvesting Arrays of
Transition Metal Complex Chromophores
Recent results have shown that multimetallic transition metal complexes can serve as effective
sensitizers for photoelectrochemical cells based on wide band gap semiconductors. Photon to
current efficiencies approaching unity have been achieved using a trimetallic Ru (II) diimine
complex adsorbed on nanocrystalline TiO2 deposited on optically transparent tin
oxide electrodes. This research has focused on (1) understanding excitation energy migration in
multimetallic transition metal complexes and (2) devising new light harvesting arrays capable of
absorbing a higher fraction of incident photons per particle. Through systematic investigations of
intramolecular energy transfer in bimetallic complexes, bridging diimine ligands
(linked bis-2,2'-bipyridyl ligands) have been found that are capable of mediating energy migration between adjacent metal
centers with 100% efficiency. In addition, simple synthetic strategies have been developed for
preparing the most effective of these ligands. Using these ligands,
arrays of chromophores can be prepared at interfaces via sequential reaction of complimentary
components. For example, films having alternating Ru (II) and Fe (II) centers can be prepared by
alternate reaction of aqueous Fe (II) and the complex [Ru (tpht)
2]2+ with a surface initially treated to have an attached terpyridine
ligand. Dendrimeric surfaces can be prepared by alternate reaction of a 2,2'-bipyridine modified
surface with [Ru (II) (DMSO)4Cl2] and bphb. These surfaces
have light harvesting arrays of eight or more chromophores linked by a bridge capable of
efficiently mediating energy migration to the reactive surface bound complex.
New ways to make multimetallic arrays are currently being explored which do not involve ligand chelation at a
chromophoric center.
Washington State University
Pullman, WA 99164-4630
Department of Chemistry
Investigations of Charge-Separation Processes in Metal Complexes
This research is directed toward the elucidation of the excited states of metal complexes. Species
containing Rh(III), Pt(II), Re(I), bimetal species [M1,M2 =
Au(I), Pt(II), Rh(I), Ir(I)] and trimetal species containing Rh(I) and Ir(I) are under investigation.
The intent is to synthesize stable complexes capable of utilizing incident radiation for chemical
reactions, energy transfer, and storage. Analyses of luminescence decay times as a function of
temperature, the measurement of relative polarizations of absorption and emission bands, and the
determinations of the optical effects of intense magnetic fields (0-5 Tesla) are the principal tools
of investigation. Recent data are providing values for the energy gaps separating states of
disparate orbital parentages, the Arrhenius activation energies for photochemical reactions, and
the paths of energy migration in excited systems. The ultimate goal of the research is to provide
fundamental information on charge-separation processes in well-defined metal complexes.
Washington State University
Pullman, WA 99164
Department of Chemistry
Membrane-Organized Chemical Photoredox Systems
This research is designed to improve our conceptual understanding of reaction mechanisms in two
general areas related to solar photoconversion: (1) catalysis of water oxidation to
O2 and (2) transmembrane separation of photoredox products across artificial
bilayer membranes. Both areas are critical to developing membrane-based integrated chemical
systems for photogeneration of fuels. The immediate objective in the catalysis studies is to identify
the oxygen-evolving species formed by oxo-bridged ruthenium dimers in the presence of strong
oxidants. These studies entail structural analyses by resonance Raman and electron paramagnetic
resonance methods, made in conjunction with steady-state kinetic measurements of
O2 evolution rates. The focus of the transmembrane product separation studies
will be upon developing multifunctional molecules that can act both as oxidative quenchers of
photoexcited dyes and as transmembrane cotransporters of electrons and protons. The conceptual
basis for these studies is our recent demonstration that a prototypic compound,
N-methyl-4-cyanopyridinium, can function as a highly efficient oxidative quencher/transmembrane
charge carrier in vesicle-containing systems.
University of Washington
Seattle, WA 98185
Department of Biochemistry
Femtosecond Spectroscopy of Energy Transfer Dynamics in Photosynthetic Bacterial Antennas
Photosynthetic antennas are pigment/protein complexes that collect sun light and pass the
excitation energy on to a "reaction center" where, consequently, a chemical potential
develops. This, in a nutshell, is how sunlight is stored. There are different kinds of antenna
complexes in a given system and, in the purple photosynthetic bacteria, there are two major types
of complexes and these have distinct absorption spectra in the near-infrared. The
mode and mechanism of the transfer of the electronic excitation is being studied within and between the antenna
complexes of the purple bacterium Rhodobacter sphaeroides. Excitation transfer is very fast,
occurring in a trillionth of a second or less. In the experiment, a laser pulse lasting 50
femtoseconds or less (1 femtosecond = one thousandth of a trillionth of a second) excites one of
the complexes selectively and the changes in the absorption spectrum are monitored as a function
of time. The evolution of the absorption spectrum reveals details about the transfer of excitation
from one type of complex to another. The knowledge gained from these experiments allows for the postulation of
the spatial and energetic arrangement of the antenna complexes in the photosynthetic
membrane, which information is essential for fabricating efficient synthetic antenna systems.
Wayne State University
Detroit, MI 48202
Department of Chemistry
Photoinduced Charge and Energy Transfer Processes in Molecular Aggregates
The major goals of this research project involve the systematic investigation of models that
describe various aspects of the photoinduced transfer of charge, or the migration of energy
between donor and acceptor transition metal-complexes. Research involves the design and
synthesis of molecular systems to be used as mechanistic probes and the characterization of
photochemical transients using very sensitive detection techniques. Work in progress varies from
studies of the general problem of electronic coupling in donor-acceptor systems to specific
problems relating to the pathways for intramolecular energy transfer from the lowest energy
excited state of chromium(III). Considerable electronic coupling of donor and acceptor seems to
be an important characteristic of polynuclear transition-metal complexes with CN-
bridging groups, and the effects of this coupling are manifested in excited-state electron-transfer
rates, ground-state spectroscopic behavior, and electrochemical behavior. In complexes for which
the donor is (3CT) Ru (bpy) 32+ and the acceptor
is a covalently linked metal complex, the donor and acceptor centers usually behave reasonably
independently. The systematic comparison of the properties of some homologous series of
complexes has indicated that: (1) the donor-acceptor coupling inferred from electrochemical
measurements is consistently much larger than that inferred from a Hush/Mulliken interpretation
of the spectroscopic measurements; and (2) there is an unusual shift to lower energies of the
bridging cyanide stretching frequency. This shift of the CN-stretch is proportional to the D/A
coupling, and it is symmetry dependent. These and some other observations on this class of
complexes are accommodated by a simple vibronic model in which the electronic and nuclear
motions are coupled. Studies in progress are designed to examine implications of this model.
Wichita State University
Wichita, KS 67260
Department of Chemistry
Mixed-Metal, Multielectron Photocatalysts for Solar Energy Conversion
The design, synthesis, and examination of the photophysical properties of potential solar energy
photocatalysts is the focus of this research. The photophysical properties of complexes of
ruthenium, rhenium and platinum have been investigated. The platinum systems have examined
the dependence of emission lifetimes on the type of bidentate ligand and the type of bis(diphenyl
phospine) ligand coordinated to the platinum center. Attention in ruthenium chemistry has
centered on the chemical behavior of the diazafluorenone ligand which undergoes an unusual ring
opening reaction. The rhenium systems investigated have involved pyridyl-pyrimidine complexes
which emit with lifetimes in the low nanosecond time regime.
Last updated by Harry J. Dewey, (hd@lanl.gov) on December 17,
1996.