Special Issue Preface pubs.acs.org/JPCB
Autobiography of John R. Miller
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GROWING UP “Get B’s” was my Dad’s advice. He told me that when he was in college if there was one A to be had in a class, he went after it and usually got it. But looking back it left too little time for fun and friends. So his words were “get B’s”, but his example was hard work. As for many of us the tension between throwing oneself into science and living loomed large. Harry Crosby, my high school chemistry teacher, and Josefina Trujillo, who gave me summer jobs in her biochemistry lab, gave me early inspirations for science and a sense of research. I went to Oregon State, an outstanding place for the upcoming field of food science and technology but soon discovered the deductive reasoning in chemistry to be much more appealing. Still, Oregon State was a great place. I came away with much of the knowledge I needed and the growth and enjoyment of knowing several faculty members who nurtured my sense of self. At Wisconsin for graduate school I joined John Willard’s group because his trapped electrons in rigid glasses were appealing. John had written a review chapter describing, in his Socratic way, the collision of two ideas in the field. On one hand, capture of electrons by minute concentrations of electron scavenger molecules seemed to say that the electrons moved thousands of angstroms through the matrix before they became trapped. On the other hand, he noted evidence that most trapped electrons were close to the positive ions they were born with, so after ionization the electrons must not have traveled far. Enjoying this puzzle he presented I wondered if electrons might not be efficiently captured while mobile, but once trapped they might reach out to distant electron acceptor molecules by tunneling to them. After building my own vacuum line, as every student in John’s group did, I took some data and concocted a few experiments that might distinguish between the conventional wisdom of cross sections for capture of mobile electrons and this new idea of tunneling by trapped electrons. When I showed John Willard a research report including a few different tests of the “tunneling hypothesis” and conclusions that trapped electrons tunneled about 50 Å to react, he was very pleased indeed. All the experimental tests supported the new hypothesis; tunneling was fun and easy to love! Most of John’s pleasure was derived from seeing one of his students develop and test a hypothesis and finding interesting, even compelling results. What I did not realize at the time was that a small part of his pleasure may have been derived from seeing a challenge to the established theory of a distinguished professor at another university, who, I learned later, was sometimes a rival to my mentor John. When John proposed that I submit a paper about the results, I gained an early lesson in scientific disagreements. My “new” tunneling hypothesis was welcomed by some, but by at least a few it was bitterly criticized, something to be resisted, and if possible even suppressed. Tunneling was seductive and lovable, but science was only mostly that way. New was in quotation marks because tunneling was not new. Tunneling over long distances was known in solid-state physics. Its role in chemistry was somewhat new because most, although © 2015 American Chemical Society
not all, chemists had experience that led them to believe that molecules had to come into contact to react, even just to transfer an electron. I’m grateful for the many supporters of tunneling in those early years and hope to come to understand that the strongest detractors were split between those who believe it to be fanciful and nice but impossible and those who had toyed with the idea without bringing it to fruition. I was young then. In advanced spectroscopy class Professor Cornwell spent 2 weeks filling the backboard with irreducible tensor operators, only to finish by sketching, for application, a large organic molecule. At the sight of that molecule the class erupted in a spontaneous chorus of boos, and I joined in. I think that large molecule filled many of us with fear. I, and my classmates, did not have Marshall’s youthful facility with organic chemistry. Marshall’s knowledge of synthesis has always enhanced my already great respect of him as a chemist of unusual breadth, or to use one of his terms, a renaissance man. I suspect he would not have taken part in the youthful, but narrow, view I shared with my classmates. How short of vision was I then, with no sense of how important organic synthesis would become to the science I loved.
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A CAREER AT ARGONNE AND BROOKHAVEN When I left Wisconsin I took knowledge, respect for John Willard along with other professors from whom I’d learned from there, and the example John set as a man of integrity in science and in life. I hope I’ve taken a bit of John and incorporated it into myself. While I saw a postdoc as a great chance to go abroad, taking a suggestion from John I applied to Argonne National Laboratory. The tunneling hypothesis predicted that after their creation, trapped electrons would react with electron acceptors, displaying an unusual time dependence. Tunneling was fascinating for me. When asked what direction I wanted to head, I answered that I hoped to work to better harness solar energy. Of course I wondered if and hoped that tunneling might have a role in solar. For the tunneling hypothesis the electron accelerators at Argonne enabled wonderful tests of the predicted time dependence that arose from a random distribution of distances and an exponential dependence of rate on distance. There were also people capable with electronics, including Ken Johnson and Gene Clifft, who helped to create an instrument that could measure transient absorption over nine decades in time to observe the curious time dependence predicted by the tunneling picture. Chuck Jonah, whose genius made him a great collaborator, even lent me some of his desk. A natural extension of the tunneling work was to seek and find very similar behavior when the electron donor was the anion of a molecule; the long distance transfers were not limited to electrons in physical traps. While that yielded a paper in Science, my measurements were still in rigid glasses, the value of which Special Issue: John R. Miller and Marshall D. Newton Festschrift Published: June 18, 2015 7120
DOI: 10.1021/jp5113075 J. Phys. Chem. B 2015, 119, 7120−7122
The Journal of Physical Chemistry B
Special Issue Preface
quantized vibrations by Levich, Jortner, Ulstrup, Van Duyne, and Fischer and others, using rigid glasses to hold electron donors and acceptors at a fixed distribution of distances so we could see the inverted region, preventing the diffusioncontrolled limit from hiding much of the interesting chemical physics. While he claimed not to remember it, I recall that Jim Norris suggested that I should fix donor−acceptor distances with a rigid framework. Three colleagues kindly tried to help me, but the synthetic chemistry proved too difficult until I asked Gerhard Closs to get a postdoc to work full time on it. Gerhard, who was serving as Group Leader of Radiation Chemistry then said the budget at Argonne could not support it, but he was interested and would use some of his funds if he could be part of the project. We hired Tessie Calcaterra, who first succeeded with a synthesis I, of all people, proposed after some reading. My goal was to see fast electron transfer at a fixed distance and at room temperature. Adding to the joy was a bet of a good bottle of wine from a colleague in Germany who was sure that the electron would “never” transfer through the rigid steroid frame we planned to use. When I asked him to define never, he just said “never,” but was delighted to accept my suggestion that he would be the winner of our bet if the electron transfer rate was slower than 1 ms. I think he had centuries in mind. When the electron transfer was faster than 1 ns he agreed and procured a nice bottle. Gerhard was enthusiastic to move on to better characterize the inverted region. I felt that Jim, Kurt, and I had already done it but was glad for the expanded horizon. With advice from Fried, a steroid expert at Chicago, we figured out a scheme that could attach a range of acceptor groups to the same frame. When the synthesis encountered problems Gerhard went into the lab himself to find the solutions. One of his graduate students asked me, “What going on? Gerhard is in the laboratory?!” The series of molecules Tessie and Gerhard made gave a beautiful tour of the normal and inverted regions with distances fixed by the steroid frame in liquid THF. These results were much easier to understand than the results in glasses with distributions of distances and rates, but these results confirmed Rudy’s inverted region for most. To me, those experiments in THF seemed mainly a visualization, so everyone could understand what Jim, Kurt, and I had already observed, but the best was the effect of a nonpolar solvent. I found that I could dissolve enough of the molecules in nonpolar isooctane to see spectacular changes in the rates that examined another prediction of the Marcus theory. The spectacular accelerations of rates for weakly exoergic reactions and nearly as spectacular reductions in rates of highly exoergic reactions in isooctane were the most exciting results for me. Soon after Mike Wasielewski observed normal and inverted electron transfer by photoexcitaiton. I gained loves of knowing structures and energies to complement my romance with tunneling. One special part of my time at Argonne was collaboration with Larry Curtiss to understand propagation of electronic interactions through rigid molecules. We worked together with Conrad Naleway to develop an intuitive picture of that propagation using natural bond orbitals, an effort in which Marshall and Ken Jordan joined. Larry was a terrific colleague who taught me a lot. Another was a visit to my lab by a young JSPS student, Tomokazu Iyoda, who did beautiful work and became a special friend.
was not apparent to at least one of the leaders in the group in which I was a part of. Nevertheless, after I joined the staff at Argonne, a young division director, John Unik, decided that a budget priority should be to get a postdoc to assist this crazy young scientist, who had recently joined the staff and pursued different ideas. What a blessing to have Jim Beitz as my first postdoc. A talented experimentalist and a deep scientist, Jim set about to improve our data collection, often done at 77 K, to reach 4 K and temperatures in between and to just do everything better. The times he led me came early and were frequent. Jim and I also investigated how the rates for the tunneling reactions depended on the nature of the electron acceptor. While the role of the Marcus theory, with its relaxing polar molecules, was not as apparent to us yet, Jim was especially keen to investigate predictions of slowed highly exoergic reactions, even when high-frequency, quantized vibrations were involved. Jim read and led me to papers by Joshua Jortner, Jens Ulstrup, and their colleagues, who generalized understandings of nonradiative decay of excited states to describe electron-transfer reactions. When we presented our results at a meeting in Philadelphia and in our first paper exploring the role of energy, some rates of highly exoergic reactions were indeed slow, but others were very fast. While we could utilize the spectra of radical anions to explain each of the fast rates by electron transfer to create an excited product, our scattered plot led many to be suspicious about what Jim and I were reporting as rate reductions because the reactions were too exoergic. Jim and I investigated tunneling from neutral molecules to radical cation in rigid glasses. To our surprise, the rates were not slower, although the “tunneling barriers” estimated from the ionization potentials of neutral molecules were far larger than the barriers for tunneling from anions. Earlier at a conference Joshua Jortner had poured Jim and me some of his Scotch and urged us (pronounced really) to think of what we called electron tunneling as a “superexchange” process described by Harden McConnell. McConnell’s CH2 orbitals could be the LUMO’s of solvent molecules between our donor anion and acceptor. It occurred to me that the transfer of positive charge could occur if the hole donor, a radical cation, could take an electron from the HOMO of a solvent molecule between it and the neutral electron donor. The much lower energy barriers for this “hole tunneling” process made it easier to understand the long distance transfer of positive charge. Unlike electron tunneling, hole tunneling occurs through molecules but cannot occur through a vacuum. Joshua also urged us to think of the role of “nuclear tunneling” in the temperature dependence of electron transfer. He discounted the term electron tunneling, seeing it as ignoring the essential role of nuclear motions. While valuing all his advice, I still claim that both electrons and nuclei can tunnel, and the name “electron tunneling” (or hole tunneling) is valuable in highlighting the propagation of electronic wave functions over long distances. When Jim Beitz moved to another group to get a permanent job our project to investigate the free energy dependence of electron transfer from molecular anions in rigid glasses was not complete, but Kurt Huddleston took it up. The finished data set clearly showed rates decreasing in the inverted region. Because the free energy changes were smaller compared with reactions of trapped electrons, few reactions deviated due to low-lying excited states. The data provided wonderful confirmation of both the Marcus and Hush theory and the additions of 7121
DOI: 10.1021/jp5113075 J. Phys. Chem. B 2015, 119, 7120−7122
The Journal of Physical Chemistry B
Special Issue Preface
Another gift from Gerhard was that I got to know an extremely bright and gifted student, Piotr Piotrowiak, who finished his Ph.D. with Gerhard and came to Argonne as a postdoc, where we did fascinating work together and continue to be colleagues and friends to this day, as he and Bob, one of our most respected theoretical colleagues, have chosen to honor Marshall and me in this way. When I moved to Brookhaven it was much easier to talk with Marshall. When we had been 800 miles distant, we regularly met at scientific meetings to chew over interesting scientific issues aided by Marshall’s library. Marshall normally traveled to meetings with a box of papers, some theoretical, but more experimental, that could serve as a basis for discussions. His interest in and knowledge of experiments is a special aspect of Marshall as a scientist. In moving to Brookhaven, being able to stay in the same building and chat with Marshall as well as Norman Sutin and Carol Creutz, a recent loss, was one of the big positives. The development of conjugated polymers, which might act as “molecular wires” offered special possibilities for tunneling. They seemed even more interesting to many scientists, including myself, for possible transport of charges over very long distances, invoking images that can make one dream. Pulse radiolysis offers a unique opportunity to inject electrons or holes into conjugated chains. Tomokazu Iyoda sent a young Sadayuki Asaoka to our lab for a year. The polymers Sada made here then and our subsequent collaboration have been wonderful. Our work at Brookhaven is now aided by special capabilities like the LEAF accelerator built by Jim Wishart and the optical fiber single shot detection system by Andy Cook and the new LEAF IR system by David Grills. Other benefits at Brookhaven are the presence of Dick Holroyd, whose very fast electrons offer special new possibilities to examine the limits of the inverted region, technical help from Bobby Layne and Jack Preses, and great young people like Matt Bird, Tomo Mani, and Matt Sfeir in the Nanocenter. We also have microwave conductivity constructed by Matt in collaboration with Garry Rumbles and Obadiah Ried of NREL. We’ve had a wonderful collaboration with Lori Zaikowski of Dowling College and her students. Conjugated polymers are indeed used in organic solar cells, but their abilities to transport charges and excitons over long distances are not clear, so we are exploiting our new capabilities to learn about them.
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CLOSING NOTES I’ve received many gifts starting with my parents and early mentors, continuing with a number of great colleagues, some mentioned above and many more, and people like Dick Kandel, Mary Gress, Alan Laufer, and Mark Spitler and their colleagues who have worked to provide funding and often wisdom. For all of these I’m grateful, and to all who contribute to this issue, especially Piotr and Bob. Late in my life I’m sometimes amazed at how busy I still am and that tension between life and work remains. Freud is sometime credited with saying, “Man needs to love and work.” I still struggle a bit, but in the last 18 years with my wife Laura, each just seems to find its place in the whole. John R. Miller
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DOI: 10.1021/jp5113075 J. Phys. Chem. B 2015, 119, 7120−7122
