Research

The Wasielewski Group’s research centers on light-driven charge transfer and transport in molecules and materials, photosynthesis, nanoscale materials for solar energy conversion, spin dynamics of multi-spin molecules, molecular materials for optoelectronics and spintronics, and time-resolved optical and electron paramagnetic resonance spectroscopy.

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Artificial Photosynthesis: Solar Fuels

Because solar radiation is the largest renewable energy source — with potential to satisfy the expected tripling of global energy needs by 2100 — the storage of solar energy in chemical bonds via generation of reduced, renewable fuel sources such as hydrogen or methanol is a critical research challenge. [1]  Artificial photosynthetic systems for practical solar fuels production must integrate the functions of light harvesting, charge separation, and catalysis, with water as the source of electrons for reductive fuel-forming chemistry. [2]  The design of a light-driven water-splitting system based on molecular catalysts requires a fundamental understanding of the individual electron transfer steps involved in multielectron catalyst activation.  To this end, we are covalently integrating water-oxidation, proton-reduction, and CO2-reduction catalysts into organic donor-acceptor systems, using ultrafast transient absorption and vibrational spectroscopies to probe the effects of structural and energetic factors on photoinduced electron transfer and, ultimately, fuel generation.
We have recently demonstrated the photoinduced oxidation of an iridium-based water oxidation catalyst [3] and reduction of a diirion hydrogenase mimic for hydrogen generation [4, 5].  These projects have been collaborative Argonne-Northwestern Solar Energy Research (ANSER) Center efforts with the Brudvig and Crabtree groups at Yale University and the Rauchfuss group at University of Illinois at Urbana-Champaign, respectively.

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Artificial Photosynthesis: Solar Electricity

Producing electricity directly from sun light using organic photovoltaics (OPVs) is an important step away from fossil fuel energy sources.  OPV efficiency must increase in order to become economically viable.  There are four fundamental processes within every OPV that need improvement (Figure 1):

  1. Photon absorption – absorb light from the sun to form an excited molecular state (exciton)
  2. Charge separation – transfer an electron from the excited molecule to a nearby, unexcited molecule
  3. Charge transport – provide a pathway for the electron and hole to quickly diffuse apart
  4. Charge collection – have that pathway end at an electrode where the electron (hole) may be injected and used to do work

Our work focuses on improving all four of these processes.  Our primary research thrust involves designing and synthesizing covalently bound electron donor-acceptor molecules which 1) strongly absorb solar radiation, 2) efficiently separate charges intramolecularly, 3) readily assemble to form charge conduits perpendicular to the molecule which 4) extend to both electrodes.  A newer research thrust approaches the challenge from a different direction.  By harnessing the process of singlet fission, where a singlet exciton spontaneously decays to a pair of triplet excitons, the theoretical maximum efficiency of OPVs may be dramatically increased (Figure 2).  However, this process only occurs in a few molecules, none of them well suited to OPVs, so we are actively searching for more promising candidates.

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Molecular Electronics

Designing molecular systems for molecular electronics or for solar energy conversion that are capable of moving charge efficiently over long distances through molecular bridges requires a fundamental understanding of electron transport in donor-bridge-acceptor (D-B-A) systems. Charge transport has been studied in covalently linked D-B-A systems with various bridge molecules, including proteins, DNA, porphyrins, saturated and unsaturated hydrocarbons. Nevertheless, in many of these systems multiple charge transport mechanisms and pathways exist and the factors that favor particular mechanisms remain poorly understood. Continuing efforts toward understanding how the electronic structure and composition of the bridge governs the charge transport mechanism, and thus the lifetimes of photogenerated radical ion pairs (RPs), are important for the rational design of “molecular wires.”

We are currently trying to answer three specific questions with our research:

  • How do the molecular structure and electronic properties of the bridge determine the transition from the superexchange (tunneling) regime to the charge hopping regime?
  • What are the structural and electronic requirements for efficient photodriven electron or hole transport by the charge hopping mechanism?  How does the degree of hole or electron localization on discrete, energetically degenerate bridge sites affect this process?
  • How can new theoretical insights into charge transport in cross-conjugated molecules impact the design of optimized molecular systems for photodriven charge transport?

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Molecular Spintronics

Controlling the spin dynamics of complex multi-spin systems is a major goal in the quest for molecule-based spintronics.1 Photoinitiated picosecond electron transfer within covalently-linked organic donor-acceptor (D-A) molecules having specific D-A distances and orientations results in formation of highly polarized spin correlated radical pairs (SCRPs) in which the initial spin state is well defined. These organic RPs display coherent spin motion for microseconds at room temperature and longer at low temperatures, which makes it possible that this coherence can provide the basis for new organic information processing devices.2 Our group utilizes modern pulsed magnetic resonance techniques to develop new schemes for manipulating such spin systems for potential spintronics applications.

The specific goals of this project are to investigate four questions that are important for understanding spin coherence in organic molecules and the development of organic spintronics:

Can the spin dynamics of SCRPs be manipulated to control spin polarization transport?
Can coherent spin states be transported (“teleported”) over long distances using SCRPs?
Can spin coherence be transferred by optical excitation of a SCRP?
Can a SCRP be used to gate spin coherence transport between two radicals?

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The Wasielewski Group
at Northwestern University
2190 Campus Dr., Ryan Hall, Room 1010
Evanston, IL 60208
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