Biography of George W. Flynn

When molecules collide, energy is thrown into flux, shifting modes, converting types, or becoming excited enough to rupture the strongest of chemical bonds. Current knowledge of energy transfer in molecules has been revolutionized by the study of such quantum state kinetics, and physical chemist George W. Flynn has contributed greatly to this area with his inquiries into vibrational energy modes. Flynn, Eugene Higgins Professor of Chemistry at Columbia University (New York, NY) and a member of the National Academy of Sciences since 2001, examines the molecular dynamics and assembly of halogens in his Inaugural Article in this issue of PNAS (1). Flynn and his coworkers show that the self-assembly of haloethane arrays is controlled by the nature of the halogen attached, which in turn changes the van der Waal's forces between molecules, as well as the forces between molecules and surface. For more than a decade, Flynn has used scanning tunneling microscopy to spy on molecules, learning how they spontaneously arrange themselves into highly ordered patterns, and, throughout his career, he has systematically followed the kinetic properties of energy flow within individual molecules.

Pure Start to Science

Flynn was born in Hartford, CT, in 1938. Growing up, he loved to read science fiction and would often walk down the block to the local library branch to read such books. Initially, Flynn wanted to be an engineer because, to him, it meant blending science and technology. But in high school, an impressionable young Flynn chose science over engineering. “An older classmate mentioned, `I happen to like pure science,' and I decided right at that moment that I liked pure science, too,” he says. “`Pure science' sounded much more noble than being an engineer.”

Flynn credits his senior-year high school chemistry teacher, Harold Coburn, with piquing his interest in chemistry. A “very skilled teacher,” Coburn encouraged thinking over memorizing and ran an inquiry-based class, says Flynn. He had taken physics the year before, but he found chemistry more to his liking. In chemistry, “everything was reasonable and obvious,” he says. Although neither of his parents' education extended past middle school, they were supportive of Flynn's academics, and he did well. In his senior year, he was accepted to Yale University (New Haven, CT). When his father passed away in April of that same year, Flynn wrote to Yale explaining that the little money his family had been planning to contribute was no longer available. Yale responded with full room, board, and tuition scholarships for all four of Flynn's undergraduate years. It was “an amount of money beyond my imagining,” he says. “It really changed my life.”

Figure 1.

George W. Flynn

At Yale, Flynn declared a chemistry major, but he says, “I became more interested in physics when I was a sophomore. I discovered how you learn physics in a rigorous way.” In the summer after his junior year, Flynn participated in what he remembers as one of the first National Science Foundation (NSF) Research Experience for Undergraduates (REU) programs. In the laboratory of Julian Sturtevant at Yale, Flynn isolated the enzyme acetylesterase from orange peels and studied its kinetic catalytic activity by using optical techniques. This work eventually became the subject of Flynn's senior thesis. Two decades later, when Flynn returned to Yale to give a talk, Sturtevant gave him a copy of the unpublished 1960 thesis.

“I thought I wanted to go to graduate school before I walked in the door at Yale,” Flynn says, reflecting on his freshman year, but “after the first week, I just wanted to make it through the first term.” He was amazed by the caliber of students surrounding him, but he grew more confident and found he “could stay with the crowd.” After graduating from Yale in 1960, Flynn chose to remain near his family and entered graduate school at Harvard University (Cambridge, MA) with a NSF fellowship. Upon joining the chemistry program, Flynn realized that he did not want to focus on only one project. “I was the victim, or rather the very fortunate recipient, of a liberal education, so I didn't want to get too narrow a focus.” He chose to work with E. Bright Wilson, Jr., because Wilson was the only researcher working on the two projects that Flynn wanted to study: nuclear magnetic resonance (NMR) and microwave spectroscopy. Flynn laid his multiple interests out before Wilson, whom Flynn says was game. “The important thing is the science,” he recalls Wilson saying.

Wilson suggested that Flynn speak with a new chemistry faculty member, John Baldeschwieler, for the NMR portion of his research, and Flynn eventually split his thesis credits between Wilson and Baldeschwieler. Under Baldeschwieler, Flynn studied gas-phase molecules using NMR. After rummaging around the Harvard chemistry department looking for a gas with fluorine and hydrogen atoms for his first project, Flynn located an unused tank of 1,1-difluoroethane in the laboratory of Bill Klemperer. “I saw the most spectacular spectra from this randomly chosen gas sample that anybody had ever seen before” (2). For his other research project, Flynn used microwave spectroscopy to investigate “what happens when a molecule is subjected to one very intense electromagnetic radiation field and a second weak field.” He used microwaves to bombard the gas molecule OCS (carbonyl sulfide) with intense electromagnetic fields to study the molecule's relaxation after excitation (3). With the dual focus on NMR and microwaves, Flynn graduated with his doctorate in 4 years, which was about a year longer than many of his peers. This extension did not bother Flynn. “I didn't particularly want to leave graduate school. It was too much fun.”

Pulse Switch

Toward the end of his doctoral studies, Flynn felt the need to choose a new focus for his postdoctoral research. “Microwave spectroscopy was a mature technique, so I didn't think that was a good place for a young scientist to make his mark.” And though he found the NMR field exciting, it was receiving a flood of organic chemists. Flynn thus turned to the emerging field of lasers. Wilson suggested that he call Ali Javan at the Massachusetts Institute of Technology (MIT, Cambridge, MA). Although Javan's laboratory was a physics group, they were looking for a chemist. Flynn joined the group in 1964 with a NSF postdoctoral fellowship in hand.

At the time, Javan's laboratory was building gas lasers, including a special type that used carbon dioxide and produced “gobs and gobs of photons,” according to Flynn. Javan, Flynn, and their colleagues received a patent for developing a Q-switched CO₂ gas laser that increased the peak power of the device by 10,000-fold (4). By rotating a mirror rapidly and lining it up with a second mirror, the molecules in the upper level are able to build up and release their stored energy simultaneously. “You can think of it like a water bucket,” explains Flynn. “You pour water in'til the bucket is full, and then you rip the bottom out getting all the stored water—energy—at once.”

Flynn also used the new pulsed laser to study the time dependence of the laser itself. Based on an idea of Javan's, he placed a cell that “looks like a short piece of laser” into the laser cavity to act as a probe for CO₂ behavior and pumped in gas molecules, measuring the infrared emissions during a laser pulse. This allowed for measurement of the lifetime of the molecules. Flynn then made an accidental discovery: after forgetting to turn on the discharge in the cell, he still observed an energy emission (5). “It was emission without excitation,” he says. This finding led to one of Flynn's first independent projects as a faculty member, investigating the movement of energy within molecules.

Vibrational Energy in New York City

Flynn joined Columbia University in 1967 as an assistant professor of chemistry. As much as the science was appealing at Columbia, so was the city of New York, which offered a plethora of social options for Flynn. During his time in New York, he met his future wife, Jean Pieri, on a blind date. Pieri, who has a doctorate in nursing education, and Flynn were married in 1970. The day after Flynn returned from his delayed honeymoon in March 1971, his laboratory made one of its biggest discoveries. Flynn and his graduate students Eric Weitz and Tom Knudtson were studying how vibrational energy moves within a molecule. Flynn wanted to know, “If you put energy into one vibrational mode, how long does it take to get into the other modes?” His group investigated this phenomenon by using lasers to excite a variety of molecules (6, 7). The surprising finding that day in 1971 resulted from an examination of the methylfluoride molecule. Flynn and his team timed emissions from methylfluoride to assess the number of collisions necessary for a vibrational energy shift and found it took “about 100 whacks” for a molecule to change modes (8, 9).

At the same time as this research, Flynn began a collaboration with Norman Sutin at Brookhaven National Laboratories (Upton, NY) to develop a laser temperature jump apparatus for studying the kinetics of chemical reactions. Thanks to a suggestion by Columbia undergraduate student James Beitz, now a staff member at Argonne National Laboratory (Argonne, IL), Flynn and Sutin used Raman shifting of a neodymium YAG laser to produce infrared light pulses where water absorbs strongly (10). “The technique is now one of the key methods used for following the kinetics of protein folding in solution,” says Flynn.

“When you can see something on the atomic level, that is an incredibly powerful concept.”

In the mid-1970s, Flynn collaborated with another Brookhaven scientist, Ralph Weston, to study heat dissociation and its effect on energy states. Flynn and Weston used excimer lasers to rupture the chemical bonds of hydrogen chloride, and such dissociations occurred at energy levels 2 eV above the natural dissociation energy. This results in “a huge amount of excess energy,” says Flynn. “The hydrogen comes flying off at tremendous speed,” calculated by Flynn at approximately 1 million cm/s. He considered these findings as a window into a fundamental question in chemistry: how does heat dissociate molecules? “Heat drives molecules to vibrate so strongly, they actually rupture a bond, but it is collisions that drive the molecules up into these energetic vibrational states,” says Flynn.

By using high-resolution infrared fluorescence and laser diode techniques, Flynn and Weston visualized the collision process between these hot hydrogen atoms and a given molecule (11, 12). When CO₂ is bombarded with hot hydrogen atoms, for example, the molecule rotates and recoils. “We were able to follow all of these motions: translational, vibrational, and rotational,” says Flynn (13). He continued to study the movement of energy to other molecules, such as nitrous oxide, through the 1980s, which he dubs “the hot atom era” (14, 15).

Scanning and Tunneling for Self-Assembly

The 1990s brought Flynn into a new area of research. In the early part of the decade, he contemplated using lasers to do the same work in fluids as gases. He decided, however, that “it was not going to take me as far afield intellectually as I wanted.” Flynn thus branched into scanning tunneling microscopy (STM) because it was a relatively new field and relatively simple to set up. “I thought [STM] had the potential in 1990 that NMR had in 1960,” he says. “It literally allows you to image a molecule. When you can see something on the atomic level, that is an incredibly powerful concept.”

Utilizing STM technology, Flynn began to study self-assembly of molecules at interfaces. The ordered arrays that molecules form when a drop of liquid is added on top of a solid is “completely determined by forces between the molecules, and between the molecules and the surface.” Adding a new set of molecules results in a new set of forces to be studied, and Flynn investigated the interactions of chiral and achiral molecules, which possess remarkable self-assembly patterns and properties (16–18).

In his PNAS Inaugural Article, Flynn and his team describe how the functional group on a haloethane affects the assembly of the molecules into a monolayer (1). Using STM, Flynn looked at the interactions of molecules in an ultra-high vacuum. A vacuum provides a simplified experimental setup because the interactions of the molecules themselves can be studied in the absence of solvent. In the case of haloethanes, each molecule forms a slightly different ordered array, a feature that Flynn's research shows is due to the nature of the substituted halogen. For instance, the small electrostatic forces generated by bromine are different from those of fluorine and thus affect the orientation of the molecules in the array. The combination of rigorous theoretical modeling coupled with careful experiments here has allowed the relevant forces and their role in self-assembly to be identified. Understanding how molecules self-assemble into ordered arrays is becoming increasingly important for work on the nanoscale, where structures are being built from the molecular level upward.

Going with the Flow

In the future, Flynn plans to use STM to understand the fundamentals of molecular electronics. Currently, he is working on what he calls “macro applications,” studying the interactions of pollutants with iron oxide in the soil (19). His laboratory has also begun investigating the surface interactions of graphite, such as those that form soot (20). Such research could have potential applications for understanding air pollution and associated health risks.

Flynn's research projects are likely to keep him busy over the next decade, at which point he says he will think about retirement. For now, this once-serious golfer and swimmer has little time to pursue his hobbies. “Retirement might be appealing just for a chance to do those things again.” He also would like to spend even more time than he does with his family, which now includes two grandchildren.

Looking back at his past research achievements, Flynn sees a trajectory in which he systematically followed the flow of energy within molecules. He is most proud of the work that was planned but is always in awe of what is brought about by accidental discovery. Upon further reflection, Flynn is also proud that 14 years ago, at the age of 52, he had the courage to switch to a new research field in STM. The STM work has brought Flynn great satisfaction and success. “A few people thought I was nuts,” he says. “I think we've been exonerated.”