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Biography of Michael R. Wasielewski Michael R. Wasielewski was born on the south side of Chicago, Illinois, in 1949, and spent his earliest years in a neighborhood on the Lake Michigan shore, which, at the time, was sandwiched between the University of Chicago on the north and the steel industry on the south. Both of these “neighbors” would play a role in sparking his interest in chemistry and fostering that interest through his education. Mike was fascinated by the sights, sounds, and scents of steel making he experienced during evening drives south along the lakeshore with his parents. Some of his earliest memories of scientific curiosity are of wondering how that process worked, and of speculating about what went on inside the many stately gray buildings of the University, where the Illinois Central commuter train that Mike took downtown would stop to board a parade of “very seriouslooking” young people. The singular event that demonstrated the magic of science and led Mike, like many of his peers in the baby-boom generation, to pursue it as a career was the dawn of the space age. He distinctly remembers the personal excitement that he felt when Sputnik was launched in 1957. It sparked a lifelong interest in astronomy and space exploration, and enhanced his already well-developed interest in finding out how things work. Mike remembers a fondness for dismantling his grandparents’ short-wave radios and putting them back together, something that, much to their dismay, did not always go as planned. A desire to understand rocket propulsion led to an interest in chemistry. Mike’s parents moved to the suburbs and an obligatory part of having a new house for him was to have a basement chemistry laboratory. The experiments that he performed at home compensated for the lack of formal science in much of the elementary school curriculum at that time. This strong interest in science, and chemistry in particular, continued through high school and helped Mike earn acceptance as an undergraduate into the University of Chicago. Being at Chicago with its broad core curriculum opened a wide range of intellectual possibilities, including an opportunity to perform undergraduate research under one of the first NSF-sponsored programs for that purpose. This research led Mike to develop a strong interest in the physical properties of organic molecules, especially under the guidance of Leon Stock, Gerhard Closs, and N. C. Yang. He was equally fascinated by the work of Clyde Hutchison, Jr., a pioneer in electron paramagnetic resonance spectroscopy, and remembers that Clyde would patiently explain complex magnetic resonance concepts to him, an undergraduate who had barely enough background at the time to understand them. Mike elected to stay at Chicago for graduate school to fulfill his military obligation as a U.S. Army Reservist. This was a fortuitous circumstance because he was able to develop further in the areas he was most interested. Mike did his Ph.D. research under the guidance of Leon Stock, whose patience, guidance, and kindness he will always appreciate. The Stock group’s interests at the time focused on the delocalization of electron spin through bonds. They were interested in how the electronnuclear hyperfine splittings of hydrogen, carbon-13, and fluorine atoms depend on the structure of the paramagnetic molecules. Large splittings were determined using EPR spectroscopy, while smaller ones were determined indirectly using NMR chemical shifts in paramagnetic Ni complexes. Little did Mike know at
the time that, years later, he would return to this problem in a completely different context. After about three years, Mike left Chicago with a Ph.D. and began a postdoctoral appointment at Columbia University with Ronald Breslow. His group at the time was divided into three subgroups, and Mike was part of the “4n” group, which dealt with the topic of antiaromaticity. Breslow had developed some very informative approaches to determining the pKa values of very weak hydrocarbon acids using thermodynamic cycles based on reversible electrochemical potentials. Mike worked largely on using this technique to put the capstone on the cyclopropenyl anion antiaromaticity problem by measuring the pKa of the parent cyclopropene. The technical issues were significant, and he learned a great deal of electrochemistry along the way. What he learned most of all, however, is the wonderfully dynamic, enthusiastic way in which Ron Breslow interacts with his research group. Mike could not help getting excited about chemistry in the presence of Breslow’s infectious energy and enthusiasm. In 1975, Mike returned (somewhat unexpectedly) to the Chicago area when a position opened up at the Argonne National Laboratory. As was historically the case, many new scientists at Argonne were hired at the postdoctoral level, and then competitively vied for staff positions. One of his inspirations from the Breslow group was the emerging field of biomimetic chemistry. Ron’s lab was very active in developing models from hydrolytic enzymes based on a cyclodextrin scaffold. Although Mike had not directly participated in that work while at Columbia, he was inspired by the fact that Argonne had a worldclass group of scientists trying to understand the molecular basis of photochemical energy transduction by photosynthetic organisms. In addition, some of the most well-recognized scientists in the fields of electron transfer and photochemistry were also members of the Chemistry Division at Argonne. Mike joined the group of Joseph J. Katz and quickly got to work trying to develop biomimetic molecular systems that could help to understand some of the primary photochemical and electron transfer pathways in the so-called reaction center proteins from both bacterial and green plant photosynthesis. The Katz group at Argonne was an exciting place to be in the mid-1970s, as the group consisted of a number of enthusiastic young scientists along with senior colleagues who had extensive knowledge of photosynthesis and related fields. Joe Katz had a wealth of scientific expertise and breadth dating back to his work on boron and fluorine chemistry in the Manhattan project to using stable isotopes to probe photosynthesis. Apart from his broad scientific interests, Joe was widely read, and lunchtime conversations ran the gamut from politics to important new literature. Once again, Mike learned some valuable lessons about how to conduct scientific research, especially in a larger team context, that have proven invaluable in running his own research group over the years. One lesson was that part of doing first-rate science is selecting important problems that are incredibly challenging, and in facilitating the development of new fields. Progress in understanding photosynthesis over the past 40 years certainly fits this description. After one year as a postdoctoral fellow at Argonne, Mike accepted an offer to join the scientific staff there. During the late 1970s the nature of the primary electron donor in photosynthetic reaction center proteins was of great impor-
10.1021/jp106711n 2010 American Chemical Society Published on Web 11/11/2010
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14113 tance. On the basis of EPR experiments, Jim Norris at Argonne had concluded that photoexcitation of the primary donor resulted in electron transfer that produced a cation radical in which the charge (spin) was shared between two chlorophylls. One of the first problems that Mike addressed was what the properties of such a chlorophyll dimer would be, and how such a species would transfer an electron to a nearby electron acceptor. At that time, prior to the advent of the X-ray structure of the reaction center protein, there was much speculation as to the precise placement of the redox cofactors within the protein. He prepared a number of modified chlorophyll derivatives and began collaborations with Norris using EPR to probe the radical properties of these systems and with Ken Kaufmann, then at the University of Illinois, Urbana-Champaign, to explore their excited state properties using the then relatively new picosecond transient absorption technique. In Ken’s lab is where Mike’s fascination with lasers and ultrafast spectroscopy was born. They had a great deal of fun taking days to do experiments that now routinely take us a few minutes with 100× the signal-to-noise! The Nd-glass picosecond laser system could fire once every 5 min or so on a good day, so that they spent many hours in the lab gathering data. Laser safety consisted of covering your eyes with your hand as the whine of the capacitor banks reached a crescendo just before the laser shot. In the early 1980s Mike’s interests turned to the electron transfer problem itself, both in the primary charge separation chemistry in photosynthetic reaction centers and as a general chemical problem. The work of the Argonne group had already demonstrated that the ultrafast multistep charge separation within these proteins results in the formation of spin-correlated radical pairs. These radical pairs are born in highly non-Boltzmann spin states; i.e., they are “spin-polarized” and can be directly observed by time-resolved EPR techniques, many of which were developed at Argonne by Jim Norris, Marion Thurnauer, Mike Bowman, and Alex Trifunac. Moreover, time evolution of the spin states results in formation of triplet radical pairs, so that there is often a significant charge recombination channel leading to the lowest neutral triplet state of the system. In the case of the reaction center proteins, the triplet state resides on the primary chlorophyll special pair donor. This triplet is also spinpolarized with a unique sublevel population that is characteristic of the radical pair intersystem crossing mechanism. Gerhard Closs’s seminal work on chemically induced dynamic nuclear polarization (CIDNP) also played an important role in modeling the corresponding chemically induced dynamic electron polarization (CIDEP) observed in reaction centers. In the early 1980s Mike and his colleagues carried out a number of optically detected time-resolved EPR experiments that were designed to obtain additional information about the nature of this mechanism in the proteins, but all through this time his ultimate goal was to produce a structurally well-defined electron donor-acceptor model system that mimicked all of the features of the reaction center electron transfer and spin dynamics. The rationale was simple: The spin dynamics of the system depend on radical pair interactions that reflect subtle features of the molecular structure that are strongly tied to the details of the electron transfer mechanism. Duplicating the spin dynamics in detail would thus yield an understanding of the relationship of electron transfer rates to molecular structure, a truly fundamental feature of this ubiquitous chemical reaction. It quickly became clear that little was known about how photochemical charge separation and recombination reactions depended on reaction free energies, electronic coupling between the donor and acceptor, solvation, and a variety of other factors.
In short, all of physical parameters that had gone into electron transfer theory as developed by Marcus, Hush, Jortner, Levich, and Dogonadze came into play in addressing this problem. In the early 1980s, many in the field were fascinated by the predictions of these theories, especially that of the so-called “Marcus inverted region”, where reaction rates would slow down if their free energies of reaction were large. It became clear that one way to investigate this problem would be to eliminate diffusive encounters between donors and acceptors that can complicate kinetic analyses of very fast reactions. In 1983, Mike obtained the resources to establish his own picosecond transient absorption apparatus based on mode-locked dye laser technology. The repetition rate of the laser had increased to an astounding 10 Hz! He used ultrafast transient absorption measurements to measure the kinetics of photoinduced charge separation and radical ion pair recombination for a series of fixed distance porphyrin-quinone systems that provided unequivocal confirmation of Marcus theory for photoinduced charge separation and recombination. This work is especially important in the context of photosynthesis and is complementary to the important Closs-Miller experiments on nonphotochemical charge shift reactions. Mike and his group quickly discovered that the “rules of the game” were different for reactions in which charges were created or destroyed versus those in which charge is conserved, such as in charge shift reactions. This difference largely has to do with ionic solvation energies that contribute to the overall energetics of electron transfer reactions. In fact, at that time it was somewhat puzzling as to why some photodriven charge separation reactions that proceed at reasonable rates in low polarity fluid media are uncompetitive with excited state decay in glassy solids (usually at low temperatures). On the basis of their experiments using a large series of porphyrin-based fixed distance donor-acceptor molecules, they found that the ion pair energies of most molecular donor-acceptor molecules increase by about 0.75 eV in glassy solvents at low temperature relative to their values in polar fluids. Thus, the dramatic decreases in charge separation rates observed upon freezing a solvent are explained by the inability of the immobile solvent to stabilize the charges. For Mike, this was an important observation because it gave him a clue as to why it had been so difficult to produce a reaction center model donor-acceptor molecule that would efficiently separate charge at low temperature. Why was a low temperature of such interest? First of all, he was keenly interested in mimicking all of the reaction center spin dynamics in a model system. Following charge separation, the electronic coupling between the oxidized donor and reduced acceptor in the reaction center is very small, as reflected in the weak spin-spin exchange interaction between these radical ions. The overall spin state of this pair evolves coherently in time to form a mixed state with both singlet and triplet character. Charge recombination from this mixed state results in a significant yield of neutral triplet state, whose spin sublevels bears the non-Boltzmann population carried over from the radical pair. The observed EPR spectrum of this triplet state was unique to the reaction center at that time and is the signature of the radical pair intersystem crossing mechanism. Thus, observing this spin-polarized triplet state in a model system would lead to an understanding of how the electronic coupling between the radicals depends on the donor-acceptor structure and its surrounding environment. Second, any donor-acceptor system that separates charge efficiently in the solid state is potentially important as a material for technologically important devices such as solar cells.
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Seeking this ideal system led Mike’s group to prepare and characterize the photophysics of an ever-increasingly complex array of multistep donor-acceptor molecules, some of which took more than 20 steps to prepare. An important collaborator and co-worker in his lab at that time and for a total of 23 years was Prof. Mark Niemczyk from Wheaton College in nearby Wheaton, IL. Mark worked enthusiastically and tirelessly during summers, part-time during the academic year, and over three sabbatical years to produce many of the interesting systems that were studied. Walter Svec, Mike’s long-time technician at Argonne, also played an important role in the preparation of many of these complex molecules. Most of these molecules remained faithful to the biomimetic motif by employing chlorophylls and porphyrins as electron donors and quinones as electron acceptors. They were able to learn a great deal about the fundamentals of electron transfer reactions and were able to duplicate many of the features of reaction center protein radical pair spin dynamics in the most recent of these systems. However, none of these systems were able to produce the unusual spin-polarized charge recombination triplet. In the early 1990s, Mike and his group became interested in the emerging field of molecular electronics and began to look for simpler electron donors and acceptors around which to design robust molecules that could function as optical switches. They quickly discovered that 4-aminonaphthalimides (ANIs) were simple, useful chromophores that could act as both electron donors and acceptors. Moreover, the development of modelocked Ti-sapphire lasers that could easily produce pulses having 100 fs duration or less, as well as chirped-pulse amplification, made it possible to do high repetition rate transient absorption experiments using a 400 nm pump wavelength, conveniently near the absorption maximum of ANI. They soon decided that this new laser technology would be advantageous for all of their work and built a system that was up and running by early 1993. They quickly targeted a donor-acceptor triad system based on an ANI chromophore that upon photoexcitation at 400 nm carried out two-step charge separation across 20 Å to produce a radical ion pair with about a 200 ns lifetime. The great advantage of this system was that it could be prepared in high yield in five simple steps that usually took no more than 2 or 3 days. This made it possible to examine a wide variety of structural variations to investigate how electron transfer depends on structure. Over the years, Haim Levanon from the Hebrew University of Jerusalem visited Argonne for a number of extended periods of time. Mike and Haim collaborated on examining the photogenerated radical pairs of the first ANI donor-acceptor in liquid crystals. Haim had shown earlier that the potential associated with maintaining order in liquid crystalline phases sometimes would extend the lifetime of radical pairs, placing their lifetimes in a convenient window for time-resolved EPR measurements, which are for the most part restricted to times >10 ns. In the case of the ANI derivatives, they already had long-lived radical pairs, but importantly, nematic liquid crystals readily aligned the rodlike derivatives and simplified the analysis of the time-resolved spectra. Working with Haim, Mike found immediately that the ANI system displayed both a strong spinpolarized radical pair signal and the long sought after spinpolarized triplet state resulting from charge recombination. The irony was not lost on them that by simplifying the systems to what has now come to be called a “bio-inspired” approach rather than restricting them to a fully biomimetic approach, the goal was achieved. Using simple aromatic imides and bis(imides) as redox chromophores blossomed in Mike’s laboratory and has
provided the basis for probing in detail a wide range of electron transfer and spin dynamics problems. For example, the small electronic couplings characteristic of nonadiabatic electron transfer reactions are important determinants of electron transfer rates, yet are very difficult to measure and are nearly impossible to calculate accurately. They have been able to directly measure spin-spin exchange interactions in photogenerated radical ion pairs using a combination of magnetic field effects on the triplet yield following charge recombination and direct time-resolved EPR measurements of radical ion pair dynamics within structurally well-defined molecules. This has allowed them to examine how significant changes in electronic coupling depend on subtle changes in molecular structure. All through these years Mike continued to have a great interest in the electron transfer dynamics of photosynthetic reaction centers. For example, during the late 1980s he engaged in a particularly satisfying collaboration with Govindjee from the University of Illinois, Urbana-Champaign, and Mike Seibert from the National Renewable Energy Laboratory. The D1-D2 protein complex from photosystem II from green plants, which is responsible for photoinduced charge separation on the oxygenevolving side of photosynthesis, became available. Unfortunately, the protein complex was somewhat unstable and very difficult to work with until Seibert discovered how to actively scrub oxygen from the solution. With a stabilized preparation in hand, they set out to determine the kinetics of the primary charge separation. This was a daunting task, given the strong spectral overlap and electronic coupling between the chlorophyll and pheophytin redox cofactors within the protein. Nevertheless, they were able to develop a model in which the primary charge separation dynamics could be determined. This investigation proved to be fun from both a scientific and personal perspective. The three collaborators would get together for several days at a time and the long days (and nights) they spent getting the data allowed them ample time to enjoy and appreciate each other’s expertise and views. Other noteworthy collaborations involved examining the “dark” lowest excited singlet state of carotenoids. The experiments began with Lowell Kispert from the University of Alabama in 1986, whose main interests at that time were radicals generated from carotenoids, but with whom Mike’s group mainly investigated carotenoid excited states. In that year they measured for the first time the 8 ps lifetime of the A1g lowest excited singlet state of β-carotene, whose symmetry disallows direct transitions between it and the ground state. This experiment was a heroic effort from the ultrafast laser standpoint in 1986, given that they needed to employ two separate multistep dye amplifiers and a colorful variety of laser dyes to generate picosecond laser pulses in the blue-green spectral region. The development of Ti-sapphire laser technology made subsequent experiments more straightforward and efficient, so that they were able to collaborate with Harry Frank from the University of Connecticut for many years working out the excited state dynamics of a broad range of carotenoids. Beginning in the early 1980s, Mike made regular trips to Japan, formed a number of collaborations and friendships with Japanese colleagues, and gained keen appreciation for and enjoyment from Japanese culture. His visits to Japan frequently bring him to Kyoto and Osaka, where in the early 1980s, he particularly enjoyed his discussions with Noboru Mataga and, in more recent years, with Hiroshi Masuhara at Osaka University. Early collaborations with Yoshiteru Sakata at Osaka proved fruitful, and over the past 25 years collaborations with Atsuhiro Osuka at Kyoto have been very enjoyable.
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14115 During Mike’s early career at Argonne, several career opportunities arose to move his research to a university setting, but none of these approaches had enticed him away. This changed abruptly in 1994, when he was offered a joint appointment in the Chemistry Department at Northwestern. Many of the faculty there had research interests that both paralleled and complemented his own. Another attractive and important feature of the scientific culture at Northwestern was and still is the highly collaborative and collegial nature of the chemistry faculty there. In the fall of 1994 Mike established his group at Northwestern with three graduate students and one postdoc. Space was a problem at that time because the building that housed the chemistry department was undergoing an extensive renovation. Nevertheless, with a small laser lab and a synthesis lab, the work at Northwestern got off to a fast start. Mike immediately began close collaborations with Mark Ratner and Fred Lewis; these partnerships are still flourishing. Maintaining two research groups at locations 40 miles apart presented some logistical challenges (“Now, which office did I leave that in?”), but eventually most of these problems were resolved and progress was made at both locations in a highly collaborative fashion within his entire group of co-workers. The work that Mike began at Northwestern addressed aspects of electron transfer that related to new organic materials for solar cells, wires, switches, and transistors. For example, his first Northwestern graduate student, Bill Davis, who was a joint student with Mark Ratner, developed new molecules based on conjugated oligomers that yielded the first demonstration of the energetic and electronic coupling criteria necessary to make long organic molecules function as molecular wires. Bill was able to demonstrate that the mechanism of electron transfer through conjugated oligomers changes from the strongly distance dependent superexchange mechanism in short oligomers to the weakly distance dependent charge hopping mechanism in longer oligomers. This change makes it possible to design molecules that transport charge over long distances. Over the past 16 years, Mike has collaborated with his colleague Fred Lewis on unraveling the fundamental mechanisms of charge transport within DNA in which they employed both femtosecond transient absorption and time-resolved EPR spectroscopy. These studies have been performed largely on DNA hairpins in which the chromophore linking the two single strands of DNA that comprise the duplex serves as a photooxidant. This work proved unequivocally that charge transport rates over short distances within DNA are strongly distance dependent and adhere to a superexchange mechanism, whereas a weakly distance dependent charge hopping mechanism takes over only at longer distances. In the mid-1990s, again with a focus on molecular electronics, Mike’s group began to exploit a wide variety of derivatives based on the chemically robust chromophore, perylenediimide, and its related “rylenes”. They recognized early on that these molecules could be readily manipulated to modify their excited state and redox properties, at the same time maintaining their robust nature. Another important feature of this class of molecules is their ability to noncovalently bind to one another using π-π interactions to form large aggregates. Much of the chemistry of these molecules that they explored during the intervening years targets control of these aggregation phenomena to self-assemble well-ordered, functional materials that have tailored photochemical as well as nonphotochemical charge transport properties. In the late 1990s the group focused on several new approaches to organic molecular switches involving control of electron
transport through arrays of organic donor-acceptor molecules by the application of multiple ultrafast laser pulses. For example, when a complex molecule having multiple donor-acceptor components is excited with a femtosecond laser pulse of one color to produce an ion pair, the electric field generated by the ion pair changes the electronic environment of neighboring donor-acceptor pairs. This causes significant changes in the rates of electron transfer within these neighboring pairs when they are subsequently excited by a second femtosecond pulse of a different color. This behavior provides the basis for logical operations based solely on ultrafast photonic control of electron movement within arrays of organic molecules. In 1999, Mike transitioned to a full-time faculty position at Northwestern and rapidly expanded his group there. He combined his ongoing interests in fundamental electron transfer processes in both photosynthesis and chemical systems with expanding interests in molecular electronics and materials into a broad-based program. He also took on the duties of Chair of the Chemistry Department in 2001 and was engaged in the development of large team proposals for a variety of opportunities. Mike’s group moved to the new Nanofabrication (Ryan Hall) building in the autumn of 2002, where they now reside. In the last several years, the group has focused on developing covalent building blocks whose size and shape control their selfassembly into large photofunctional arrays that function as hybrid photoconversion materials for both artificial photosynthesis and organic electronics. These materials are based on photochemically robust aryleneimide and diimide chromophores and carry out photochemical charge separation in a manner similar to photosynthetic systems, yet transport charge in a manner similar to that of semiconductors. Since 2003 they have applied synchrotron-based X-ray scattering at the Advanced Photon Source at Argonne to elucidate the structures of organic molecules that self-assemble into these supramolecular systems. Mike is grateful to his Argonne colleague Dave Tiede for a number of important collaborative ventures in this area. The structures of these supramolecular systems cannot be determined readily using NMR techniques, so that X-ray scattering studies provide a unique portal to obtaining structural information in solution. This method allows them to obtain molecular structures under the same conditions that other time-resolved spectroscopic techniques are used to probe their photochemical electron transfer mechanisms. During the past decade Mike has collaborated extensively with Tobin Marks on developing new materials for solar cells and organic electronics based on the semiconductor properties of rylenes and their related derivatives. For example, one of their joint graduate students recently developed the first high performance, air-stable n-type organic semiconductor, which not only has high electron mobility but also shows excellent performance in prototype fully organic circuitry. Over the past few years, the Wasielewski group has embarked on a “radically” new program to develop molecular systems that can make use of spin-entangled statesssuch as the spincorrelated radical pairs found in photosynthetic organisms and in many of the electron donor-acceptor systems that they have designed and studiedsto transport information while maintaining quantum coherence. This is a challenging problem whose potential rewards are not only a deeper understanding of coherence phenomena in molecular systems but also potentially new ways to process information. In the past three years, Mike’s portfolio of energy-related activities has expanded greatly by the establishment of the Argonne-Northwestern Solar Energy Research (ANSER) Center,
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which he directs. The ANSER Center is now a DOE-sponsored Energy Frontier Research Center focused on fundamental scientific questions relating to solar fuels and solar electricity production and is emblematic of the collegial and collaborative spirit that originally attracted him to Northwestern. This spirit has extended to the scientific staff at Argonne National Laboratory in recent years, and a growing number of Northwestern faculty members and Argonne staff have joint appointments at these institutions that are bringing new opportunities and talent to bear on important energy-related problems of great societal impact. In fact, some things in Mike’s life have come full circle. In conjunction with the ANSER Center directorship and his own collaborative interests, he now again holds a joint appointment in the Center for Nanoscale Materials at Argonne.
Mike looks forward to many more years of exciting scientific endeavors and wants to express his “heartfelt appreciation to all my colleagues, co-workers, and collaborators who have made my career in chemistry both exciting and fulfilling”.
Malcolm Forbes Leif Hammarstrom Josh Vura-Weis Emily Weiss JP106711N
