Index of Memorial Resolutions and Biographical Sketches
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PAUL F. BARBARA
“I have this crazy idea!” exclaimed Paul Barbara as he walked into his colleague’s office next door. “This was often the greeting from Paul,” recalled this close colleague. Boundless ideas, infectious enthusiasm, and complete openness to sharing the most creative and deepest thoughts—these were just some of the traits that made Paul Barbara such a successful and influential scientist. When Paul had a new idea on the molecular world and how to test his idea with careful and rigorous experiments, his excitement could be child-like, and he was often oblivious to the surroundings. His colleague told a story recounting a shared one hundred-mile ride from a Gordon conference to the airport with Paul driving. A few minutes into the journey, Paul started to share his most recent discovery on how his students could resolve charge movement in polymer molecules with sub-nanometer resolution (one nanometer = one billionth of a meter) and how he could explain the discovery with a new model. The discussion became so heated that Paul’s hands were alternating at a high frequency between illustrating the molecules and steering the car, not realizing the potential side effects until he had to use two hands to illustrate a key point. Fortunately, Paul realized the danger just in time and pulled over to the side of the road to finish the discussion. They arrived late at the airport.
Paul’s deep desire to understand the molecular world began in childhood in New York City. At an interview after his election to the National Academy of Sciences, he was asked what brought him to science. “The first book I ever got was a book on molecules, and I think that was in fourth grade,” he recalled. “I remember going to school with my mother and then going to the library where they had a few books you could buy. And my mother said I could pick one. I picked one called Molecules and Atoms.” This was perhaps the beginning of his life-long love affair with molecules. “I constantly imagined that they were there and that if you had an ideal microscope, you’d be able to see them, and they’d be everywhere around you.” It is bittersweet that one of his last papers, published after his death, was on direct observation of the charges in individual polymer molecules (Science 2011, 331, 564-567), which was the subject of the hazardous discussion in the car ride to the airport mentioned before.
After high school in New York City, Paul took the first step toward realizing his childhood dream of seeing molecules. He attended Hofstra University on Long Island, N.Y., majoring in chemistry, with a focus on physical organic chemistry. Paul had great memories of his undergraduate years, immersed in the “pursuit of true knowledge,” as he later would call his goal in college. Paul went on to Brown University for his graduate education under the mentorship of his Ph.D. supervisor, Ron Lawler. They applied the latest technology, nuclear magnetic resonance (NMR) spectroscopy, to study electron transfer reaction mechanisms in solutions. Paul was never satisfied with a “black box.” He soon opened up the brand new machine, figured out how it ticked, and made important modifications to improve experimental measurements. Here was Paul, the “instrument jock,” tasting his first sweet success. Paul’s first paper, co-authored with Lawler and Jacobs, published in the Journal of the American Chemical Society, already showed signs of a successful scientific approach: the application of a novel tool and rigorous experimental measurements to probe the most fundamental questions in molecular science.
After completing his Ph.D. in 1978, Paul carried out postdoctoral research at Bell Laboratories (Murray Hill, N.J.), which, at the time, was one of the most eminent research labs in the world of physical sciences. He worked under the mentorship of two brilliant pioneers, Dr. Peter Rentzepis, an expert on ultrafast spectroscopy, and Dr. Louis Brus, an innovator in molecular spectroscopy and nanoscience. Bell Labs was well known not only for technological innovations but also for fast-moving research in fundamental science on the cutting edge. A high concentration of some of the world’s brightest minds in one place meant exceptionally exciting opportunities for aspiring young scientists. Paul had very fond memories of his two years at Bell Labs and particularly of learning how to identify exciting research opportunities. “In choosing good problems, you really have to have a bottom-up and top-down strategy at the same time and then find that perfect connection. You’ve got to know what your toolset is capable of,” he recalled. This view really shaped his research philosophy as he moved on to start his independent and distinguished career.
Paul Barbara was widely recognized as one of the visionaries of physical chemistry, although he earned his Ph.D. in physical organic chemistry, applying the methods available in the late 1970s to elucidate particle spin states to study the transfer of electrons between organic molecules in solution. His postdoctoral work at Bell Labs propelled him to the forefront of the pioneering field of “ultrafast” spectroscopy. He carried out frontier studies of the dynamics of molecules in solution, resolving processes that occur in only a picosecond (one millionth of one millionth of one second). This work was particularly focused on reactions involving the transfer within and between molecules of the very lightest and most common atomic nucleus, the proton. Paul’s chemical background provided him with a great appreciation for molecular structure and the basis for selecting exceptional molecular candidates for study. As a result, his later research combined both the elegance of state-of-the-art physical chemical methods in time-resolved spectroscopy and the relevance of interesting molecular species and reactions.
Fresh from his exciting experience at Bell Labs, he joined the University of Minnesota (Minneapolis, M.N.) in 1980 as an assistant professor of chemistry and set up a laboratory to conduct ultrafast spectroscopic experiments on the dynamics of chemical processes in the solution phase. During the eighteen years (1980-97) as a member of the chemistry faculty at Minnesota, Paul made critical and lasting contributions to the understanding of the molecular motions comprising reactions in chemical contexts that are essential in both solution chemistry and biological chemistry. These included his continued focus on the ubiquitous proton and electron transfer reactions that are the essential steps in the electrochemical processes in batteries and solar cells, as well as the most common elementary chemical steps underlying the basic biological processes of photosynthesis and many other metabolic steps in living species.
During this period, Paul also developed remarkably detailed experimental insight into individual solvated electron dynamics, a fundamental reactant associated with radiation-induced chemistry. All of this was achieved with the clever development of new ways to use ultrashort light pulses that could resolve the elementary steps in the exceedingly fast processes taking place at the atomic level. His exceptional success in these challenging frontier ventures cannot be attributed to any single element in his approach. His creativity in experimental design, his appreciation for molecular behavior, and his close interaction with members of the active theoretical community who were trying to understand the role of a liquid environment in these processes, all came together in his unique style, which moved science forward in unforeseen new directions.
In 1998, Paul moved to the The University of Texas at Austin as the R.J.V. Johnson-Welch Regents Chair in Chemistry. Because of insufficient laboratory space for the size of his scientific team in Welch Hall, he was given a renovated suite of offices and labs in the Engineering-Science Building (ENS). While these quarters had the advantage of being new and spacious, they were located in the basement of ENS and a block away from his colleagues in chemistry. Paul made up for this by obtaining an office in Welch Hall, spending a lot of time talking with colleagues in the chemistry and biochemistry department, and arranging for them to visit his new space for meetings and conferences.
Just at this time, spurred by the discovery of new instruments that could examine surfaces at the atomic level and of particles of nanometer size (the most famous being the 60-atom clusters of carbon, known as C60 or buckyballs), an intense worldwide interest in the field of nanoscience arose. Paul immediately recognized the potential importance of this field and began an effort to gather together scientists and engineers at UT Austin, who were working in related areas, to set up collaborations and joint projects and to seek funding to provide the needed instrumentation for this work. This culminated in the establishment of the Center for Nano- and Molecular Science and Technology (CNM) as an organized research unit within the College of Natural Sciences, and Paul almost immediately started seeking space for the instrumentation facility, meeting rooms, offices, and laboratories that could house the CNM. After much effort and campaigning, he eventually convinced the UT Austin administration to erect a building in the back of the original Experimental Science Building. This was completed in 2006 and was first designated the Nanoscience and Technology Building (NST) and then renamed in 2010 for UT Austin President Larry R. Faulkner, who had been instrumental in getting the building approved. Paul was always proud of his efforts to organize and house the CNM. At the opening of the new building, he said, “Nano is the science with the greatest potential to affect society. Our children will be proud of what we’ve started here at UT Austin.”
Paul also assembled a group of graduate students and postdoctoral fellows at UT Austin (self-named “the Barbarians”), who continued some of the research from Minnesota, but the group also started several new lines. One line of research begun at the University of Minnesota was on the single-molecule spectroscopy of conjugated polymers. These macromolecules are important for their potential applications in organic electronic devices such as solar cells, flat panel displays, and field effect transistors. The ability to look at the chemistry of single molecules provided the possibility of learning things about molecular behavior that is not possible by the traditional chemical approach of studying large numbers (ensembles) and averaging the properties of a huge number of individual molecules. Paul studied conjugated polymers spectroscopically, i.e., as they absorb and emit light. He began his research on the photochemistry and photophysics of conjugated polymers by studying a popular material, MEH-PPV. His fascination with understanding the structure/property relationship of conjugated molecules lasted for the rest of his career. Single isolated MEH-PPV molecules show large fluorescence intensity fluctuations. For a polymer with hundreds of independently emitting chromophores, this was an observation that was difficult to rationalize. In a single isolated conjugated polymer chain, Paul discovered that the electronic energy transfer among chromophores is, in general, very fast. Consequently, a single quenching defect can quench a large fraction of the total chromophores. His group also discovered that single MEH-PPV molecules display polarized fluorescence excitation and emission. This was a puzzling observation because it was difficult to understand how a polymer with hundreds of chromophores can form an ordered structure that would produce polarized emission. Paul, however, was determined to solve this puzzle. With the help of many discussions with colleagues, postdoctoral researchers, and students, as well as the discovery of a publication describing simulation of a stiff polymer chain, he realized that simulations combined with experimental work would be the path forward. Eventually, Monte Carlo simulations, considering the polymer’s backbone stiffness and solvent effects, indicated that MEH-PPV molecules could form highly ordered structures. The simulated structures explained the optical polarization results perfectly.
Paul, who expressed child-like enthusiasm with every piece of experimental information, would exclaim, “Isn’t that amazing!” His associates said he could see things most people would miss, because he had a unique way of always trying to link the experimental data to his molecular level vision. Combining experimental data, simulations, and, more importantly, his vision, the structure and properties of MEH-PPV at the single-molecule level were revealed in enormous detail. His ideas and determination produced a series of new discoveries that shaped the field of conjugated polymer. These scientific discoveries were on-going in the Energy Frontiers Research Center that Paul was directing at the time of his death, which was largely supported by the U.S. Department of Energy.
Another interesting aspect of the spectroscopy of conjugated polymers is that, unlike ensembles, which show a constant emission (fluorescence) under excitation, single molecules blink on and off and sometimes turn off and stay off (photobleaching). Paul had already shown that the blinking of MEH-PPV was a reversible process under the right conditions, as opposed to permanent photobleaching. Since what was happening in this process was poorly understood, Paul invented a method, that he called fluorescence-voltage single-molecule spectroscopy (F-V/SMS), to understand this behavior. With F-V/SMS, in which single conjugated polymer molecules are embedded in capacitor-like devices and subjected to an electrical field, charges can be injected into or removed from single MEH-PPV molecules. In 2004, he got the experiment to work and found that the reversible blinking of conjugated polymers was due to a reversible electron transfer process between the conjugated polymer in the excited state and traces of oxygen present in the samples. By controlling the applied bias to the device, it became also possible to control the oxidation state of the single polymer molecules. Paul’s invention of F-V/SMS once and for all proved that reversible blinking in conjugated polymers involves photoinduced charge transfer in a complex between the conjugated polymer and oxygen. From there, Paul developed the experiment further to show that holes destroy triplet excitons much more efficiently than singlet excitons in conjugated polymers.
Paul had an uncanny talent to program code in Matlab. He would have his students and postdoctoral researchers watch him write the code in the lab or his office, which proved helpful to seek out the abundant typing mistakes that would keep crashing the scripts Paul wrote on the fly. During the F-V/SMS work, Paul got so excited that the experiment was working that he decided to sit in the student office (in the basement of ENS) writing Matlab code to average the fluorescence data as a function of time as well as device bias. He wrote, tested, and revised his program, while data were literally coming out in the lab in real-time on the first day the experiment started working.
After moving his labs into the new CNM, it became easier for him to collaborate with colleagues across the street in Welch Hall. This led to work combining his spectroscopic techniques with electrochemical ones and his development of a new experimental method, termed single-molecule spectroelectrochemistry (SMS-EC), which combined the capabilities of single-molecule fluorescence spectroscopy and electrochemistry. The technique was used to determine the distribution (not just the ensemble average) of key electrochemical variables, such as the half-wave potential, E1/2, of single conjugated polymer molecules. Paul found that when positive charges are injected into nanometer-size particles of these polymers, they underwent a reversible transformation of their electrical properties, creating “charge-traps.” These “traps” slowed the movement of charges in the material and consequently had a strong effect on the performance of organic/electronic devices. These studies also led to measurement, for the first time, of the electrogenerated chemiluminescence (ECL) of single immobilized sub-40 nm particles of conjugated polymers. This work later led to a detailed description of the break-in phenomena associated with the electrochemical injection of charges into thin polymer films. His studies of the ECL in polymer films, a complicated phenomenon that produced fascinating, changing patterns of ECL emission, continued to 2010. In his usual quest to achieve a molecular-level understanding, Paul developed a detailed semiquantitative mechanism to describe the phenomena. The mechanism took into account the formation of point defects on the films and the propagation of the double layer during the electrochemical charging of the film.
Paul loved to intellectually challenge and motivate his students, postdoctoral researchers, and colleagues. One of his favorite catch phrases was, “Stop, think!” Usually these words were followed by an animated discussion for several hours about the new discoveries.
Following the SMS-EC work and building on the F-V/SMS work, Paul focused on all solid-state hole injection devices to study the interaction of charge on single chains of conjugated polymers. This work revealed the reversibility for charge injection and the nature of light interactions with the charge. Polymer chains exhibit substantially higher energetic barriers to charge injection on the single-molecule level as compared to a bulk film. Light acts as a mediator for charge injection into the molecule, overcoming the barrier. Once the charge is on the chain, it localizes at a trap site and remains there. The trapped charge results in fluorescence quenching of the polymer chain due to energy transfer. By monitoring the positional change of the fluorescence spot on the CCD image, the range of energy transfer can be visualized; then by repeatedly cycling the quenching process, the trap site was shown to be at the same location on the polymer chain, resulting in a reproducible shift in the fluorescence spot. From these measurements, it can be surmised that energy transfer in single polymer chains can occur over long distances (50 nm or more) and that the trap sites for charges are stationary and most likely the result of a particular conformation of the polymer chain.
Very early, Paul recognized that he could make contributions in a very different field involving biological macromolecules. He saw that the ability to visualize biomolecules at the single-molecule level had the potential of providing unique insights into the molecular structure and dynamics of proteins in action. Therefore, part of his group at the Laboratory for Spectroscopic Imaging focused on the study of complex biological problems, specifically related to the mechanism behind HIV replication in the early 2000s. Paul applied single-molecule Förster resonance energy transfer (SM-FRET) methodologies and developed fluorescence cross correlation analysis of single-molecule intensity-time trajectories and multistep single-molecule spectroscopy kinetic methods. Armed with these tools, his group was able to explore the conformational distribution and dynamics of HIV-1 transactivation response (TAR) DNA hairpins and hairpin mutants complexed with HIV-1 nucleocapsid proteins (NC). This work provided the first time-resolved SM-FRET measurements on TAR DNA hairpins and hairpin mutants complexed with NC. These data were analyzed to determine the effect of NC complexation on the DNA end-to-end dynamics and end-to-end equilibrium distribution. Important intermediates in the nucleocapsid-protein-chaperoned minus-strand transfer step in HIV-1 reverse transcription were individualized. Altogether, his single-molecule biophysical work provided a unique mechanistic insight onto the chaperoning action of NC in interplay with DNA and RNA.
Those who worked with Paul remember that his enthusiasm for science was infectious and his passion unmatched. His group reported that, on many occasions, when Paul would visit the lab, he would get carried away discussing the progress and new ideas. What would begin as a brief chat would extend for hours, with an outpouring of new concepts and experiments, as well as the needed control experiments to perform. Only when he was interrupted by a call did Paul realize that he was late for an outing or an activity with his family.
Paul lived and breathed as a true pioneer in the molecular world, educated a whole generation of young scientists (the Barbarians), and influenced many colleagues who were fortunate to have had the opportunity of collaborating with him. His words will forever echo in the memory of his students, colleagues, and friends. “Isn’t that amazing!”<signed>
William Powers Jr., President
The University of Texas at Austin
Sue Alexander Greninger, Secretary
The General Faculty
This memorial resolution was prepared by a special committee consisting of Professors Allen J. Bard (chair), Peter J. Rossky, and Xiaoyang Zhu.Acknowledgment: We are indebted to former Barbara group members Dehong Hu, Andre Gesquiere, Rodrigo Palacios, Josh Bollinger, Gonzaolo Cosa, and Pat Kambhampati for their assistance in preparation of this resolution.