Special Issue Preface pubs.acs.org/JPCB
Tribute to John R. Miller and Marshall D. Newton
T
he charge-transfer community has made a series of impressive advances over the past 50 years, building on the pioneering work of Marcus, Hush, and Levich and Dogonadze. Their work highlighted the issues and questions that would consume the discipline as it strove to test theory’s predictions and make the best use of these new insights. Many groups developed ingenious experiments and novel theoretical methods to probe the inverted region, inner-sphere versus outer-sphere rates, diffusion effects, the detailed role of solvent in charge transfer, nuclear and electron tunneling, and the impacts of various media on the tunneling rate. The implications of this body of work for fields ranging from biology to nanoscale-fabricated devices are immense, and the emerging applications point to its critical importance for future technologies. No individual or single research group can claim responsibility for all of these advances, but it is fair to say that without the prodigious efforts of John R. Miller and Marshall D. Newton the field would be significantly behind where it is at present. This issue of the Journal of Physical Chemistry B honors the work of two extraordinary scientists in the way most apropos their careers: namely, with new scholarly works authored by long-time and new colleagues which advance the field John and Marshall have served and loved. Marshall and John each carved out his own wide ranging sphere of research and influence, but there are delightful parallels in their work and lives. Each began his chemical life as an experimentalist. While this may not be a surprise for those who know John, it is a little known fact that Marshall’s first two publications were in Tetrahedron Letters and The Journal of Organic Chemistry. Each took his first position at a National Laboratory, John at Argonne and Marshall at Brookhaven, and each by 1980 was working on charge-transfer problems. Each began his research on charge transfer by taking on the pivotal question of electron tunneling; John’s thesis (1972) concerned long-range tunneling in glasses, while Marshall’s first published foray (1980) was his ground-breaking work on the Fe3+/Fe2+ self-exchange reaction where quantum chemistry was first applied in a decisive way to understand the size of the electronic coupling element. In the mid-1990s each published important theoretical treatments of superexchange effects in tunneling (John with Larry Curtiss and Marshall with Congxian Liang). Each continued to be a respected authority on charge transfer, and (with John’s move in the late 1990s) each has ended up at Brookhaven National Laboratory. Beyond all of these examples, however, by far the most important commonalities are their willingness to take on hard problems and their ability to solve them and advance the field. John’s autobiography nicely outlines the trajectory of his work, but there are several studies that deserve particular mention here. John’s very first publication on long-range tunneling in low-temperature organic glasses, which appeared in 1972, had suddenly thrust him to the forefront of electron-transfer studies
Photo Credit: Richard Backofen
Special Issue: John R. Miller and Marshall D. Newton Festschrift Published: June 18, 2015 © 2015 American Chemical Society
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DOI: 10.1021/jp5113069 J. Phys. Chem. B 2015, 119, 7117−7119
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The Journal of Physical Chemistry B
original concept to correlate the single-electron two-orbital exchange processes (ET and HT) with the two-electron fourorbital exchange (TT) that drove this collaborative effort with the Closs group. The outcome of this experimental work, which has shown that in homologous systems βTT ≈ 2βET, has been confirmed, challenged, and reconfirmed by theorists as recently as 2014. Starting from the late 90s John has been developing new collaborations, both in the U.S. and abroad, and shifting his attention to electron, hole, and energy transfer in more complex systems. A new major thrust of work has been devoted to the study of polaron localization and electron transport in conjugated polymers and organometallic oligomers. Among other topics, he has been exploring the excited-state dynamics of radical anions, and his most recent scientific adventure involves singlet fission (nearly energy lossless splitting of a singlet exciton into two triplet excitons) in organic materials. If anything, John’s output has been steadily accelerating in recent years. It is worthwhile to note that throughout John’s career his experimental work relied on pulse radiolysis, a singularly powerful yet at times inflexible and awkward tool. Ever since the 70s, pulse radiolysis and radiation chemistry have struggled under the increasing pressure from the more modern and far more affordable ultrafast laser technology, and many facilities worldwide have been shuttered. Bucking the trend, John continued to use pulse radiolysis to produce creative, cutting edge science, first with the help of the 20 MeV LINAC at Argonne and then the 5 MeV LEAF electron accelerator at Brookhaven. Indeed, when it comes to the study of electron transfer, pulse radiolysis holds a distinct advantage: it allows one to study the reaction kinetics of ground-state species. As a result, the crucial thermodynamic parameters were much easier to determine than is the case for photoinduced processes. Furthermore, electron transfer in John’s ground-state ions was much more amenable to high-level theory, while dealing with excited states constituted a significant additional challenge−a challenge which in the 80s was often insurmountable. This is one more reason why John’s work continues to be of such high interest to theorists. Marshall’s autobiography also does a wonderful job of surveying the many areas in which he has worked, but we offer a selective summary here with less reserve than Marshall applies to his own prodigious accomplishments. Marshall’s involvement in the rapidly developing field of electronic structure theory (in the Lipscomb, Coulson, and Pople groups) provided him with a set of tools that had yet to be applied to charge transfer for condensed-phase reactions. Those tools were used to great effect in his position at Brookhaven National Laboratory, where pioneering work was going on to understand solution-phase transition-metal charge-transfer reactions. His work with J. Jafri, Jean Logan, Balachandran Tembe, and Harold Friedman provided extraordinary detail about the Fe3+/ Fe2+ hexaquo self-exchange reaction and yielded high-quality quantitative comparisons with experiment. He extended this work to consider tunneling through ammonia ligands and treated the complex kinetics of the Co3+/Co2+ hexamine reaction, considering spin−orbit coupling effects on the overall rate. Marshall also worked with colleagues at Brookhaven (Bruce Brunschwig and Norman Sutin) to systematize variants of electron-transfer theories and understand the transitions between classical, semiclassical, and quantum regions, yielding among other things an invaluable review with Norman Sutin in Annual Reviews of
and defined his future scientific path. The clarity and simplicity of these early papers are remarkable. John’s gift for the design of logical, often sparse experiments that can demonstrate complex concepts became the trademark of his research style. Once on his own, John quickly realized that his position at Argonne National Laboratory provided the optimum environment and resources for his fascination with moving electrons in an insulating medium across the distance of many angstroms. In 1975, already at Argonne yet still almost fresh out of the graduate school, John published his single author Science paper on intermolecular electron transfer mediated by quantum mechanical tunneling. Three years later the first paper on the distance dependence of electron tunneling in glasses appeared. Shortly afterward, in 1979, the groundbreaking work on the exothermic rate restriction in electron-transfer reactions done with Jim Beitz brought a conclusive demonstration of the existence of the “inverted region” predicted theoretically by Rudy Marcus and others. The year 1982 marked the beginning of the fruitful collaboration with Gerhard Closs. It was John who convinced Closs to turn his interest and synthetic chemistry skills to study intramolecular electron and energy transfer. In 1984, the widely cited paper on the driving force dependence of electron-transfer rate between donors and acceptors covalently attached to a rigid steroid bridge appeared in JACS. To most physical chemists this was a logical reconfirmation of John’s seminal 1979 work with Beitz; however, in terms of ultimate validation and acceptance of the back-then still controversial concept of the “inverted region” by a much broader audience of organic and inorganic chemists, it was an important milestone. In the following years the iconic kET versus ΔG0 figures from the Miller and Beitz and Miller and Closs papers appeared in numerous reviews and on the covers of books and journals. Many researchers embarked on designing their own donor− acceptor systems to reproduce the parabolic dependence of the reaction rate on the driving force. Today, one can find these familiar and important plots in the kinetics section of many undergraduate chemistry textbooks. The covalently linked compounds provided by the Closs group enabled John to map out the distance dependence of the through-bond electron-transfer rate and determine the corresponding tunneling attenuation factor β for saturated hydrocarbons. The cyclic bridging units allowed him to probe some of the more subtle aspects of electron tunneling, such as the role of the stereochemical attachment as well as the constructive and destructive interference effects between competing superexchange coupling pathways. The distinct temperature dependence of electron transfer in the “normal” and “inverted” thermodynamic region, which can serve as the litmus test for the correct positioning of the donor−acceptor pair on the ΔG0 curve, was also demonstrated. It was during this period that the research interests of John and Marshall came inseparably close together. While they have never formally collaborated with one another, the commonality of scientific goals and the choice of similar targets were evident. Seeing their talks at conferences and the annual DOE Solar Photochemistry “contractor’s meetings”, one would be readily convinced that they worked as a team. The beautiful interplay between John’s experiments and Marshall’s calculations was a stimulating element for the entire field. Having charted the ΔG0 and the distance dependence of electron and hole transfer mediated by the superexchange mechanism, John proceeded to his next major contribution, the quantitative correlation between long-range electron (ET), hole (HT), and triplet energy transfer (TT). As before, it was John’s 7118
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The Journal of Physical Chemistry B Physical Chemistry. From this point on the primary focus of his scientific work was charge transfer. Marshall has been a major contributor to understanding electron tunneling and solvent effects in a wide array of systems. He studied nonequilibrium solvent behavior in dielectric continuum and molecular solvent models with a number of collaborators, including Harold Friedman, Fernando Ranieri, Yi-Ping Liu, Misha Basilevsky, and the Voth group. He has studied long-distance tunneling through covalent bridges both to test the assumptions of simple superexchange models (with Congxin Liang) and to understand tunneling effects in electrochemical systems (with a host of experimental collaborators including Chris Chidsey, Steve Feldberg, and John Smalley). He also retained a focus on development of methods for the accurate calculation of the electronic coupling element for both electron and electronic excitation transfer. Throughout all this work Marshall has been a major conversation partner with many experimental groups, and his work and thinking have had a profound influence on many beyond his own research group. He also had a major impact on the field through his 1991 review in Chemical Reviews entitled “Quantum Chemical Probes of Electron Transfer Kinetics: The Nature of Donor−Acceptor Interactions.” Here he carefully defined the states of interest, unraveled the origins of superexchange effects and their perturbative treatments, carefully examined orthogonalization effects in the calculation of donor−acceptor coupling, discussed in detail the impact and extent of many-electron effects, and provided a wealth of experimental examples that illustrated the complexity and range of electronic effects on the rates of charge transfer. It was the first comprehensive treatment of the electronic coupling in the literature, and it is no exaggeration to say it is still the best place to go to understand the details of electron tunneling. For newcomers to the field (experimentalists or theorists) it should be required reading. Beyond his published work, two unique facets of Marshall’s approach to science deserve mention. First, he reads the literature voraciously. It is rare that someone else is able to bring new results to his attention that he has not already seen, unless the results are unpublished. Once he has read the work, Marshall does not just know the basics, he knows the details at a deep level and often has a host of questions the work generated for him. It is one of the things that makes him such a stimulating collaborator; he takes the time to understand his collaborators’ work as well as they do. Second, Marshall does not leave “experiments to the experimentalists”, at least when it comes to probing their assumptions and testing their conclusions. Tenacious and dogged (meant in the best of ways) are a pair of words that come to mind when Marshall does not understand or believe someone’s explanation for a result. Anyone who has interacted with him knows that their science has been made better by his interest and questions. No better example of his dedication to the field can be given than by the role Marshall plays at symposia or conferences. Whatever the method or topic, Marshall is able to ask a thoughtful question, citing other work in the literature, relating it to the speaker’s work. The audience learns not only from the talk but also from the connections Marshall makes; he exemplifies exactly the sort of attitude one hopes for in attendees to scientific meetings, but few people have the breadth and insight to be able to pull it off. Several years ago Marshall was unable to attend the Electron Donor−Acceptor Interactions Gordon Research Conference held biannually in Newport Rhode Island. The conference was excellent, but the
energy in the discussion and poster sessions was noticeably lower than that of other years. Within a day of the conference’s start the “hot” question was, “Where is Marshall?”, in acknowledgment of what all his colleagues know: science is better and more fun when done with Marshall around. If one had to search for differences between Marshall and John, Marshall’s East Coast reserve could be easily contrasted with John’s more rugged West Coast image. Indeed, while John would never resist an opportunity for an impromptu tennis match or a spur of the moment swim (typically in a body of water and at a temperature which would discourage any more timid or merely reasonable souls), Marshall always seemed decidedly more at ease in the comfort of a lounge, holding a glass of scotch and relaying in picturesque detail a memorable visit to a Russian banya in the company of his KGB minders. While Marshall cherished his ancestral flintlock mounted above the mantelpiece, John was as deeply attached to his trusty surfboard. Yet these differences are only skin deep and misleading - Marshall was a varsity wrestler in high school and John’s knowledge of the Cold War era politics and history is second to none. Both of them are not only outstanding scientists but also exceptionally bright, perceptive, and complete human beings, whose company and mentorship were always among the best benefits enjoyed by the authors of this Perspective. In another important common trait, both John and Marshall have shown that to leave a lasting scientific imprint one does not have to grow an enormous research group. Nowadays, their compact teams of two, perhaps at most four, associates would be easily dwarfed by the groups of many beginning assistant professors, experimentalists and theorists alike. In fact, some of the best contributions of both honorees of this special issue were single author papers. Despite this seeming disadvantage, John’s and Marshall’s thoughtful, exquisitely well focused work has already proven to have a deeper and more durable impact on the field than most of us can aspire to in the race for funding and large numbers of publications. Lastly, we would be remiss if we did not highlight the important role played by the National Laboratories in fostering the careers of these two remarkable people. Argonne and Brookhaven created cultures that nurtured important basic science and allowed John’s and Marshall’s careers to blossom in ways one might not have been able to predict at their starts; however, that freedom, coupled to their hard work, has multiplied the impact of these laboratories far beyond the students and postdocs trained by Marshall and John. Budgets and program objectives do not have the flexibility that once seemed a given at the National Laboratories (or in academic or the elite industrial laboratories) and this may be a fact of life. However, the profound impact of (relatively) unfettered research on important problems by creative people is clearly illustrated by the careers of John Miller and Marshall Newton. The charge-transfer community is truly fortunate to have them in our midst.
Robert J. Cave
Harvey Mudd College
Piotr Piotrowiak
Rutgers University, Newark
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