photoisomerization reaction mechanismsstate in photochemistry and convincing regina ro to be-come...

Volume 18Issue 2

Summer 2005

© Center for Photochemical Sciences

Photoisomerization Reaction Mechanisms

Photochemistry in the Middle of an Ocean

An Interview with Robert Liu

Center for Photochemical SciencesBowling Green State UniversityBowling Green, Ohio 43403419.372.2033

FacultyExecutive Director D. C. Neckers

Biological SciencesGeorge S. Bullerjahn

ChemistryPavel AnzenbacherJohn R. CableFelix N. CastellanoMichael Y. OgawaVladimir V. PopikMichael A. J. RodgersDeanne L. SnavelyR. Marshall Wilson

Physics and AstronomyBruno Ullrich

Scientifi c Advisory BoardEugene G. ArthursAllen J. BardRobert E. BlankenshipLisa DharMiguel A. Garcia-GaribayHarry B. GrayGeorge S. HammondRong-Chang LiangGeorge R. NewkomeNorman NitschkeKenneth G. SpearsJohn L. WestDavid G. Whitten

The SpectrumEditor Pat GreenContributing Editor Michael WoodsProduction Editor Alita Frater

The Spectrum (ISSN 1044-5536) is published four times yearly at no charge. For subscriptions e-mail [email protected]

The Spectrum is available on the web at www.bgsu.edu/departments/photochem

The Spectrum is published by the Center for Photochemical Sciences at Bowling Green State University, Bowling Green, Ohio. The mission of The Spectrum is to disseminate the latest research and news in the photochemical sciences to a general audience worldwide.

TheSpectrumA A

On the coverThe top half of the cover is the photoisomerization reaction mechanisms. Top, torsional relaxation or one-bond-fl ip (OBF), involving turning over of one-half of the molecule, giving a one-bond isomerized product. Middle, Bicycle-pedal (BP), involving rotation of two double-bonds simultaneously, or the turning over of two CH units, giving a two-bond isomerized product. Bottom, Hula-twist (HT), involving simultaneous rotation of two adjacent bonds, or the turning over of one CH unit only, giving a double-bond and a single-bond isomerized product. The Hula-girl cartoon fi gures emphasize the change of sidedness of one half of the body in OBF and no change in HT. Instead, the latter shows a healthy twisting hip motion. The molecule merrily pedals along in the middle cartoon fi gure.On the bottom half of the cover Dr. Robert Liu happily paddles along in the ocean in a photograph created by Tomas Gillbro.

special featureperspective on 4 photochemistry in the middle of an ocean an interview with Robert S. H. Liu

viewpoint3 D. C. Neckers

in this issue12 structure and dynamics of macromolecular free radicals

Malcolm D. E. Forbes, Vanessa P. McCaffrey & Elizabeth J. Harbron

18 shining light on the photodecomposition of beer Kevin Huvaere & Denis De Keukeleire

30 news from the center for photochemical sciences at bowling green state university

focus on26 news of interest in the photochemical sciences

publicationa quarterly

The July 1, 2005, issue of Science magazine celebrates the journal’s 125th anniversary. In a special collection of articles Science will look forward “at the most compelling puzzles and questions facing scientists today.” The series will explore one hundred and twenty-fi ve big questions that face scientifi c inquiry over the next quarter century. The top twenty-fi ve ques-tions include, “What is the Universe Made Of”, “Can the Laws of Physics be Unifi ed,” and “How Hot will the Greenhouse World Be?” One wonders if the 25th anniversary issue (1905) anticipated Einstein’s preeminence and/or the signifi cance of the quantum postulate. I have not, however, researched what the Science editorial board might have said about such things at that time.

Scientifi c brainstorming about hard problems is fun. In the photosciences we surely know a number of them. Bob Liu’s (Bob is our featured interview in this issue) beautiful work on the organic chemistry of vision certainly suggests how little we really know about how man, animals and even insects see. And we’ve yet to unravel just how that fi rst cellular aber-ration that leads to a photoinduced skin cancer happens. Even though Solar Cells, Inc., benefactor Harold McMaster’s last company, has just expanded its manufacturing facilities to 160,000 square feet, photovoltaic solar energy conversion remains less than 15% effi cient. And the storage of solar energy, either as a high energy chemical intermediate from which heat or power can be released or a stable compound that can be burned at night to produce energy, is still far away. The Grätzel cell, for all of its promise, is far from being practical. So all of the hype about hydrogen powered autos makes one ask what is the source of the energy to produce the hydrogen?

A correlative question that Science editors might have posed is not what hard problems remain, but whether someone is willing to spend money solving them? In fact, hard problems are a dime a dozen; bright people can solve hard problems.But unless the political community wants to know their answers, they’ll remain hard problems waiting for a solution. A good example of this, and one in which photoscientists have a stake, is the expected shortages in the supply of fossil fuels.The political community, at least in America, has yet to realize the implications of a shortage of, or increased competition for, the products of fossil fuels. Our energy policy remains largely based on the assumption that the supply of oil is infi nite. It’s easy to see why. The average citizen in America worries about such things when fueling the car, or heating the home.If one assumes a wage of $15 an hour, a 40 hour work week, and a commute of 20 miles a day, the cost of fuel for the aver-age worker’s commute is less than one hour’s work a week. In 1965, when the cost of fuel for the average worker’s car was 25 cents per gallon, for the same commute the $1.50 per hour worker would need to work more than an hour to fuel the car because fuel effi ciency was signifi cantly lower then. So even though the general public complains about fuel prices, the complaints largely fall on deaf ears because the supply seems to remain almost limitless.

The cost of heating oil and natural gas is a different matter. Particularly in the northern parts of the U.S., large facilities like churches, many very old, are increasingly stressed by it. In the absence of new ways to heat old buildings, many of these gorgeous facilities may in the next twenty-fi ve years be torn down or sold to developers to turn into condominiums.

The question is just what is it worth to the persons who pay the bills? In case of center city churches, those persons are the ever dwindling supply of center city parishioners. And where are those former parishioners? Living in the suburbs com-muting to their jobs and subtracting one hour’s worth of wage each week to pay for the commute. That’s a long way from being a serious enough expenditure for the average person to be worried about it.

So, in the immortal words of Senator Hillary Clinton of New York in describing the current president of the United States,

“What, me worry?”

The Spectrum Volume 18 Issue 2 2005

viewpoint

The University of Hawaii at Manoa launched a special lecture series a few years ago, with top faculty members making lunch-time presentations to the down-town business community. Those elite professors picked fl ashy titles to grab the attention of non-scientists.

One Egyptologist who used ancient mummies to study life and health in ancient times chose: “Mummy Talks: The Egyptian Mummy Project.” A women’s studies professor topped that with “Princess Di of the North” for her reassessment of Catherine the Great’s role in Russia. One chemistry professor, however, took the cake with “Can Dogs See Ghosts?”

Perhaps more than any other photoscientist today, Robert S. H. Liu may be uniquely qualifi ed to tackle that question.

Many researchers have delved into the basic chemistry of vision, arguably the most important human sense. Harvard biochemist George Wald, who won the 1967 Nobel Prize, spent a career on the topic. Bob Liu may be the only re-search photoscientist who devoted his professional life to vision at the most fundamental level. His work has been signifi cant, involving highly unstable compounds that are horrendously diffi cult to separate, yet has received all-too-little attention.

Such may be the fate of a scientist on an isolated archi-pelago in the middle of the Pacifi c Ocean.

For more than three decades, Liu has focused on vitamin A (retinol) and the visual pigment rhodopsin. Rhodopsin is a protein in the membrane of the rod photoreceptor cells in the retina of the eye that catalyzes the only light-sensitive step in vision. It consists of the protein, opsin, linked to 11-cis retinal (formed after the body oxidizes retinol to an aldehyde). When photons strike the retina, retinal changes from the 11-cis form to the all-trans form. It triggers a chain of events that results in transmission of impulses through the optic nerve to the brain.

an interview with Robert S. H. Liu

special feature

perspective on photochemistry in the middle of an ocean

Be it resolved, that the House of Representatives of the Fourteenth Legislature of the State of Hawaii, Regular Session of 1988, commends Dr. Robert S. H. Liu for his excellence in teaching, his outstanding accomplishments in organic chemical research, and his overall contribution to science and the advancement of knowledge.

House of Representatives, State of Hawaii

Volume 18 Issue 2 2005 The Spectrum

Liu worked with Ramamurthy to synthesize the previous-ly unknown 7-cis isomers of retinal, the visual chromophore. During a sabbatical in George Wald’s lab, Liu succeeded for the fi rst time in binding several of the 7-cis isomers to opsin. He later did pioneering research on the binding site require-ments of rhodopsin and bacteriorhodopsin, using an inge-nious series of retinal analogs as probes.

The “Hula Twist” mechanism of photoisomerization, of course, is among Liu’s other contributions, which he has re-ported in more than 220 papers. Bob tells colleagues that he is especially proud of four of those publications. They appear in the Journal of Chemical Education, and refl ect how he teaches introductory organic chemistry courses, which are invariably over-subscribed. Noted on campus for putting on a good show, Bob sometimes lectures in a Superchemist shirt, or a Mandarin outfi t—perfect for using Chinese noo-dles to demonstrate polymerization. How can you help stu-dents understand the polarizability of atoms? Bob compares it to body features of Konishiki, a gargantuan sumo wrestler famous throughout Hawaii.

Liu received an undergraduate degree from Howard Payne College and Ph.D. from Caltech under George Hammond. He spent four years in the Central Research Department at E. I. duPont de Nemours, where Bob modestly cites two ma-jor achievements: Discovery of the role of the second triplet state in photochemistry and convincing Regina Ro to be-come Mrs. Robert Liu. In 1968, he joined the University of Hawaii. He has been an Alfred P. Sloan Fellow, a John Simon Guggenheim Fellow, and received the University of Hawaii Regents’ medals for excellence in research and in teaching.

Bob was born in 1938 in Shanghai, China, into a west-ernized family. Both grandfathers were offi cials in Chinese embassies in Europe. Bob’s father got a master’s degree in banking in the United States and his mother learned to speak French and English before she learned how to write in Chinese. As the Communists took control after World War II, the Liu family fl ed to Hong Kong.

In this interview, Bob refl ects on his youth, initiation into the “Hammond Mafi a,” transitions into industry and

Courtesy of Bob Liu

Robert S. H. Liu


back, and research leading to the the Hula Twist. Liu notes that his research does suggest ways to improve human vi-sion, and discusses other milestones in an extraordinary career in the photosciences. Bob even reveals how he got George Hammond to do the Hula Twist in Aloha.

The Spectrum: How did high school in Hong Kong lead to Howard Payne College in Texas?

Robert Liu: There was nothing logical about it. It was a matter of necessity. Howard Payne was the only college that gave me suffi cient fi nancial assistance—a small scholarship and a guarantee of a sizable workship.

The Spectrum: What did you think when you arrived in Brownwood, Texas—a foreign student in a small town?

Liu: Brownwood was a town of 10,000 in tumbleweed country, without a single Chinese restaurant. In fact, when I walked downtown, I got the impression that people across the street whispered, “Look! A yel-low face!” I had no time for culture shock. After paying the immediate required items on registration day, the $250 in my pocket had dwindled to $54. Nothing more was go-ing to come from my parents. I immediately signed on to work and to fi nd any odd job on campus that fi tted into my schedule. But no cash was paid for those jobs, only credit to-ward my expenses. I had to fi nd odd jobs off campus for the minimum wage of $1 an hour—double what the on-campus jobs paid. That seemed heavenly then.

The Spectrum: You majored in chemistry?

Liu: Dr. Daniels, the only chemistry professor at Howard Payne, convinced me to switch from chemical engineering to chemistry. In the fi rst place, there was no engineering of any sort offered at HPC. Also, Dr. Daniels himself had switched from chemical engineering to chemistry when he was a student.

The Spectrum: How did you fi nd your way to George Hammond?

Liu: Caltech was the obvious pie-in-the-sky for me. In Hong Kong, Caltech was known as the MIT of the west. My

fi rst two quarters at Caltech were sheer panic. Everyone in my class seemed to know so much more and had wide ranges of research experience. My only “research experience” was reading Werner’s book and review articles on coordination chemistry. I took the intermediate organic chemistry class taught by George Hammond. I was so impressed by the fact that there was logical, deductive reasoning in organic chem-istry rather than all brutal memorization. And I also heard through the grapevine that his research group was doing ex-citing new things. So, I asked to be a member in his group. I knew it must have been a surprise to him because I was supposed to be an inorganic chemist. George had just ac-cepted three new Ivy leaguers into his group. One more per-

son from Timbuktu could not possibly be a welcome addition. But he put on a pretty good poker face and said yes.

The Spectrum: Who was there in 1961?

Liu: Angelo Lamola was the obvious star student in my year. He already had fi ve or six papers on his record and seemed to know everything. Ahead of us were two other stars: Nick Turro and Jack Saltiel. All three helped me greatly in my early venture into research. I learned every basic photochemical technique from Nick. From Angelo I learned how to do independent research. At fi rst, when I ran into a blank wall, I waited for the next opportunity to talk to the master.

Angelo would tell me not to wait for words from the top but rather to keep on trying on my own. Soon, I stumbled across the gold mine of photosensitized dimerization of isoprene. Jack showed me the need to be thorough in one’s thought process. His stilbene work was simply trail-blazing and no doubt, he was sharp and imposing especially to a dumb fi rst year student. But Jack, at that time, also had the habit of reminding you, “How stupid can you be!” At least I had enough sense not to fold in front of him, but rather learned to be more thorough in my thought process.

The Spectrum: Why isoprene dimerization?

Liu: It was an ideal project for a starter. At fi rst, it was a challenging separation problem when I obtained a mixture of seven dimers. Then it became a learning experience to assign all seven dimers using primarily NMR spectra with

Bob with George Hammond at an outing at an IAPS meeting in Iguazu Brazil.

Courtesy of Bob Liu

Robert S. H. Liu


a 60 MHz spectrometer. It was also an analytical problem to obtain quantitative numbers of dimer composition as I changed the triplet photosensitizers. Toward the end of the fi rst summer, I typed out my fi rst progress report for George, 35 pages long. I remembered it was in red because only the red ribbon on the typewriter in the lab was any good. I turned in the report on a Friday.

The Spectrum: And probably held your breath all weekend.

Liu: Monday morning George charged into my lab with that silly grin on his face. He started to tell me the idea about stereoisomeric triplets of conjugated dienes, how the rapidly interconverting ground state conformers of the di-enes becoming non-interconvertible isomeric triplets in the excited state and how the relative amounts of the isomeric triplets varied depending on the triplet excitation energy of the sensitizer. It was non-stop talking for 10-15 minutes from a very excited boss. The following day, in the library I found a paper by Evans on singlet-triplet absorption spectra under high pressure of oxygen. In it was the triplet excita-tion energy of transoid butadiene (60 kcal/mole) which was indeed higher than the cisoid 1,3-cyclohexadiene (53 kcal/mole)—the needed hard evidence for George’s postulate. I happily showed those numbers to George.

The Spectrum: George seldom wasted time getting a discovery into print.

Liu: The next Friday, George visited my lab again to show me a hand-written copy of a communication intended for JACS with me being the sole co-author. Even though dur-ing the library search I began to understand some of the sig-nifi cance of the work, I was still surprised by the quick turn of events. Could it be right that this very green student, still unsure of his future, would have a paper in his fi rst year of study? I began to entertain the thought that they might not fl unk me out after all!

The Spectrum: You certainly joined an exceptional group of people destined to shape modern organic photochemistry.

Liu: That was a unique group of excited graduate students. We learned from each other, grilled each other, but we never got mad at each other. We were the hardest working group, but we never felt that we were working. George never was over-bearing but we all knew that he was a big part of the

success. And the research program itself had a lot to do with the unusual atmosphere that existed. I don’t think any of the original Hammond Mafi a nor anyone else was able to fully reproduce that atmosphere when we later established our own research groups.

The Spectrum: What did you learn from George and the rest of the group that proved most important later on?

Liu: The episode of isomeric diene triplets showed me George’s uncanny ability to grasp the central point of a narrow experimental observation and generalize it into a concept of broad impact. George never taught us how to do experiments, but he showed us how to think. I learned by watching and listening to George’s reactions in Monday night seminars and I relished the opportunities to listen to him talk about our work to a general audience. Suddenly, it seemed that without my work on photodimerization of isoprene, the world would be poorer in our understanding of radiationless transitions of electronically excited molecules. I am sure some of the generalizations turned out to be mostly bull later on. But, oh! It felt so good listening to him. And that ability to generalize is something I always tried to imi-tate when I was on my own.

The Spectrum: And that fi rst paper led you forward.

Liu: The isoprene project was the beginning of a rather in-volved investigation of chemical properties of diene triplets in general. I wanted to collect and synthesize every diene that came into my mind in order to examine their triplet chemistry. The next two years of my stay in Caltech were the most enjoyable years of my life.

The Spectrum: What attracted you to DuPont?

Liu: Partly, necessity. During my fi nal year at Caltech, my father suffered a debilitating stroke. I felt obligated to con-tribute to the fi nancial need facing our family. So, I spurned the chance to postdoc in Paul Bartlett’s lab and took an in-dustrial job instead. I was lucky to be on the last leg of the CRD philosophy that publication in fi rst rate journals was good for the company. I worked under Howard Simmons, the top organic chemist there, who gave me a free hand to do what I considered important. I spent four years there.

The Spectrum: Was it common then for new CRD chemists to make a career decision after three to fi ve years—to stay or test the waters in academe?

Robert S. H. Liu


Liu: I belonged to the second group. When I told George, he mentioned that there was an opening in Hawaii that would be ideal for me. Hawaii turned out to be the only place that offered me a position and it was an associate professor posi-tion. Even at that level, I had to take a 30% cut in salary. But I was eager to try out the academic life. So, with my new bride, we packed up and moved west, way west.

The Spectrum: Was there any concern about isolation due to the distance from the mainland?

Liu: Perhaps isolation was a problem. But I think I used it to my advantage. I never considered myself very good in com-peting in popular areas of research. Instead, I often worked in areas that were of little interest to others. For the T2-project, I used to comment that quantum yields were so low (<10-3) that any academician would perish long before fi nd-ing any interesting results to publish. As an industrial chem-ist, I never felt the need to rush things. Even later on when my group became known for the vitamin A work, people wondered why I continued to work in such a diffi cult area with highly unstable and diffi cult-to-separate compounds. I did not mind the loneliness as we plodded slowly ahead.

The Spectrum: You did sabbaticals with two Nobel laureates, George Wald and George Porter. What are your recollections of Wald, who became quite controversial as a political activist, and Lord Porter?

Liu: When I joined George Wald’s group, he already ac-quired many outside interests and devoted only a fraction

of his time in the lab. Only on one occasion, he called me over to help him inject vitamin A into bullfrogs for his study of A1 to A2 conversion. I am not a politically minded per-son. We never discussed his many political views while I was there. But I kept him informed of my progress in the lab. When the time came for me to prepare a draft for our fi rst paper on isomeric rhodopsin analogs, I naturally included Wald as a co-author. But he said, “No!” He read it and of-fered generous suggestions but he declined to be a co-author, saying that he was only a reader.

The Spectrum: And Lord Porter?

Liu: The environment at the Royal Institution when I joined George Porter was unique. He was the director, and a big part of that job involved presenting science to the general public. There were constantly groups of high school students marching in and out of the building. Thursday eve-nings were the usual black tie events attended by people from all walks of life, including royalties of the highest level. The presentation given by Porter himself in one evening while I was there was simply out of this world. What a showman! At 9 p.m. sharp, “Ding!” the main doors to the lecture hall opened and in came Professor Porter in splendid tails. At 10 p.m., “Ding!” Out went the Professor. In between there was a non-stop performance of at least 30 chemical demonstra-tions. I remembered telling myself that I must borrow a little bit of that fl air and showmanship into my lectures.

The Spectrum: How did you start on the preparation of new stereoisomers of vitamin A?

Liu: In my third year at Hawaii, Ramamurthy came on board. He was a “mistreated” physical chemistry graduate student in India and decided to restart his graduate study in Hawaii. Quickly I noticed his drive and effi ciency in some simple warm up project that I gave him. So I suggested that he should work on truncated vitamin A analogs, starting with dienes then working his way up to trienes, tetraenes and eventually with the pentaene vitamin A. In spite of his physical chemistry background, he immediately jumped in to synthesize all the starting materials needed for the pho-tochemical studies.

The Spectrum: And he started on the ring-containing diene fragment of vitamin A?

Liu: Yes, β-ionol. In no time, he came back with a stun-ning result. Not only could he carry out the trans-to-cis

With V. Ramamurthy in front of Bilger Hall, the chemistry building at the University of Hawaii.

Courtesy of Bob Liu

Robert S. H. Liu


isomerization, but under the selective triplet sensi-tization condition he was able to achieve the con-version in 100%. Apparently “non-vertical” excita-tion was not an impediment in this case, even though it was known to divert stilbenes and piperylenesaway from quantitative trans to cis conversion. When I did a little literature research later on, I was surprised to fi nd that such 7-cis geometry was unknown in the vitamin A and carotenoid fi eld. In fact, in a 1939 paper Linus Pauling predicted that such 7-cis and even the less sterically hin-dered 11-cis isomers of vitamin A were too unstable forexistence. No serious attempts were made to prepare olefi ns containing such a hindered geometry. The obvious conclu-sion was that Linus Pauling was simply too convincing. His paper must have dissuaded chemists from trying.

The Spectrum: Sounds like one case when people took the literature too seriously.

Liu: Yes, it seemed so, but we did not know better. And it was, no doubt, very reassuring when we had gram quantities of the 7-cis isomer sitting on our bench when we found out that they were not supposed to exist. Subsequently, Murthy demonstrated that the tri-ene analogs also gave the 7-cis isomers in near quan-titative yield. But the luck stopped there. Any longer polyenes failed to give a trace amount of the hin-dered 7-cis isomers. Soon, however, Ramamurthy departed for a postdoc position in Paul de Mayo’s lab. Before his de-parture he completed partially a total synthesis of four new isomers of vitamin A. I was able to convince the Tetrahedron editor that we had suffi cient information to merit a paper in that journal.

The Spectrum: But your new synthetic adventure continued?

Liu: Thanks to a paper by Nakanishi on 14-methylretinal in JACS in which the use of a Corasil hplc column was

mentioned for isolating their retinal isomers. It was some hplc column, 7 feet long with 20 micron silica gel (fi nest particle available then). I duplicated the condition and tried it out on the only hplc unit in the department. At that moment I had no reliable student to work on this project, so I did all the work by myself. Walla. There were the 7-cis and 7,9-dicis isomers, all cleanly separated, with a partial separation of the two 13-cis isomers. Later I showed Murthy the 100 MHz H NMR spectra of the purifi ed isomers. I think that was the only time Murthy thought that I was a compe-tent experimentalist.

The Spectrum: Murthy’s work pointed you in a new direction.

Liu: It demonstrated synthetic utility of a very effi cient photochemi-cal transformation, and opened the door to many new things for my group. I managed to create an assembly line for our bud-ding bioorganic program: Synthesis by Al Asato, photoisomerization by Marlene Denny, protein analog studies by Hiro Matsumoto and paper writing by me. It was an effi cient operation. With that jump start, I was able to line up several collaborative research groups to carry out more detailed biochemical and biophysical studies of the

rhodopsin analogs prepared in this laboratory. Along the way, I decided that there was a need to introduce methods of our own for examining protein substrate interactions particularly in cases like rhodopsin where the 3D protein structure was unknown. The crystal structure eventually came out in 2000. So, I decided to start our own molecular modeling and our own protein NMR studies using F-19 as the reporter. Eventually, we were armed with data to discuss topics such as stereospecifi city of the binding site of rho-dopsin and regiospecifi c protein perturbation on the retinyl chromophore in a more specifi c and somewhat more quan-titative manner.

Discussing with Al Asato, a long time associate, on azulenic chromophores.

Courtesy of Bob Liu

Robert S. H. Liu


The Spectrum: Did the rhodopsin system focus your attention on isomerization within confi ned cavities and get you thinking about those hula dancers?

Liu: The Hula-twist, yes. When I was ready to reveal this new mechanistic process, I mischievously included the name Hula-twist (HT) in a footnote.

The Spectrum: And it jumped out and caught on...

Liu: The hula-twist was designed to answer an obvious ques-tion that arose in the mid-1970s about the photochemistry of rhodopsin: How can the very volume-demanding tor-sional relaxation process, the traditional mechanism for cis to trans isomerization, take place within the rigid binding cavity of rhodopsin? It was clearly a question on medium-directed photochemical reactivity asked before supramolec-ular photochemistry became popular among organic photo-chemists. Arieh Warshel fi rst proposed the volume conserv-ing Bicyle-Pedal mechanism of isomerization in which two alternating bonds rotate simultaneously. I found the idea fascinating because it was the type of conformational and mechanistic photochemical reasoning that I was supposed to know something about.

The Spectrum: How did you work it out?

Liu: With the help of a simple set of molecular models. I soon found a different mode of volume-conserving isom-erization. Instead of rotating two alternating double bonds, if one rotates two adjacent bonds simultaneously, the same cis-to-trans isomerization can be achieved. For a conju-gated system, this means rotating a double and a single bond. Hence, only the middle H-atom fl ips in-and-out of the plane of the molecule, obviously a volume conserving process. It has the advantage of isomerizing only one dou-ble bond. Therefore, it was not in confl ict with the known one-photon-one-bond isomerization (11-cis to all-trans) of rhodopsin. The additional single bond isomerization would produce a high energy ground state molecule—that did not seem to be a problem because many dark intermediates of rhodopsin were known.

The Spectrum: So you had a mechanism. What about the supporting evidence?

Liu: It turned out to be diffi cult. Practically nothing hap-pened during the next 13 years. It was not until 1998, a paper appeared in Angew. Chem. in which Werner Fuss and

coworkers described photoisomerization of pre-vitamin A in an organic glass at liquid nitrogen temperature showing simultaneous isomerization of a pair of adjacent double and single bonds, the expected stereochemical consequence for the HT process. Fuss kindly communicated with me before the paper appeared in print. This paper not only revived my own interest in the HT mechanism but also made it clear to me the exact conditions for fi nding new examples of HT. In the literature I found many “unexplained” photoisomeriza-tion examples conducted under confi ned media that could be successfully accounted for by the HT process. I decided that future studies should not be limited to my lab. So, I proceeded to lay out all my ideas in two papers in Proceedings of the National Academy of Sciences and Accounts of Chemical Research.

The Spectrum: The response?

Liu: Well, the world was not eager to pick up the island music and follow the HT beat. So, instead, I redirected my limited resources on the search of new examples of HT. Subsequently, a new source of money came from the National Science Foundation. Three consecutive postdoc-toral researchers brought to fruition all the predicted exam-ples of HT. Hula-twist as a viable photochemical reaction mechanism appears to be beyond doubt.

The Spectrum: Some chemists still don’t hear the island music, do they?

Liu: Well, people tend to be comfortably stuck with the old tune. The traditional one-bond-fl ip, OBF (or torsional relaxation) mechanism of isomerization is deeply ingrained. The need to think about the HT-processes one vinyl hydro-gen at a time seems to escape the minds of quite a few people (or deemed too complicated to others). I suspect that is be-cause there is no equivalent regioselective OBF reactions that lead to uniquely different chemical consequences. But such mistakes are really not quite excusable for those who chose to march into this mechanistic project on their own while disregarding what have been clearly delineated in the literature.

The Spectrum: Tell us how you got George Hammond doing the hula twist in Aloha.

Liu: When I was completing the second HT paper in 2000, it became obvious that the theme overlapped with that of isomeric triplets of conjugated dienes. So, I searched for my

old boss, and found George was happily residing in the re-tirement community of—of all places—Aloha, Oregon! I told him about my project and invited him to be co-author. He responded positively within a week. So there it was: George started to gyrate the hula-twist in Aloha! That also turned out to be the start of another round of collaboration between George and myself. At this moment our fi fth pa-per since the turn of this century just appeared in print. My fi rst publication ever was published in 1963 with George. Forty-two years later I am again publishing with him. The only difference is that I have since boldly put my name in the front. Also, I can proudly brag in front of other ex-Hammond members about this record of longevity in joint publication with one’s mentor. Hey, Dick Weiss, over there at Georgetown University! Can you beat that?

The Spectrum: Does research on the chemistry of vision have a suffi ciently high priority at NIH?

Liu: I am glad you asked this question. The answer is not so clear cut. There is a National Eye Institute (NEI), one of the smallest institutes under NIH. My fi rst two years of the NIH grant was picked up by NEI. But NEI is more interested in curing diseases of the eye. Instead, soon it became obvi-ous that our bioorganic program had a broader impact than the visual process alone. The larger DK (Digestive Diseases and Kidney) Institute became interested in our fi ndings and generously supported our research program for the next 26

years and in between the National Cancer Institute (NCI). They bought us some large preparative hplc instruments for scaling up of the new vitamin A isomers for cancer preven-tive studies. I guess the answer to your question is: for any long term support, one will have to be fl exible.

The Spectrum: Then, NIH has been your primary source of support.

Liu: Our photochemistry program also received generous support from the Army Research Offi ce (ARO) and the National Science Foundation (NSF). The ARO’s interest was largely in the near infrared absorbing materials includ-ing non-linear optical materials and unusually long wave-length absorbing visual pigments and bacteriorhodopsin analogs. The NSF support allowed us to probe for informa-tion of excited state properties of polyenes without having to worry about their immediate applications.

The Spectrum: What’s keeping you busy now?

Liu: HT launched us into thinking about other general pho-tochemical issues such as how distribution of the ultrafast relaxation processes of the initially electronically excited molecules (the Franck-Condon species) can affect the even-tual chemical fate of the molecule. Excited singlet state re-actions of olefi ns are particularly rich in different relaxation pathways. It became clear to us that such ultrafast processes should be extremely sensitive to external perturbations in-cluding minor effects not normally considered important in perturbing chemical reactivity (of the thermalized excited species). Such thoughts necessitated a reorganization in my own mind of medium perturbation effects on trapped sub-strates. Supramolecular effects on protein bound substrates must be related to those effects already established for sub-strates trapped in many man-made host systems. While this is still a developing story, some new thoughts have come for-ward. For example, the supramolecular effects of organized hosts (crystals, zeolites or any other defi ned structures) can be very different from that of amorphous organic glasses. In organized hosts, the immediate space available for reactions could be larger than that encountered in solution, and re-activity may be less inhibited. But in an organic glass, the collapsing host molecules surrounding the substrate as the solution freezes mean less space, making the reactivity of the trapped molecule more inhibited than that in solution.

The Spectrum: Looking back on those 44 years in chem-istry, what was the single most important key to success?

Robert S. H. Liu

The Superchemists of Organic Chemistry II. Liu (middle) with his students.

Courtesy of Bob Liu


Liu: If there is one, it would be that I was quite lucky in having avoided any situation that required lengthy periods of professional training. Caltech allowed me to learn and produce simultaneously throughout the 38 months there. DuPont did not require postdoctoral training. I was green but I observed and learned how to navigate under a very capable boss while moving ahead on the scientifi c front. I even learned in the mandatory semiannual performance reviews that required me to justify continued support from upper management. It was a useful preparation for proposal writing. In Hawaii, I was spared the usual induction or ob-servation period of an assistant professor. That trust perhaps only served to give me more confi dence to learn and explore new projects as we moved along. I never felt that my rela-tively short period of formal training was a handicap in my later life. In fact, it might have worked the opposite way. My youthful exuberance, or naiveté, probably did lead to some mistakes. But it also opened the doors to new areas not visited by others.

The Spectrum: And the long training periods today?

Liu: To me, they are excessive. They can easily stifl e enthu-siasm and motivation. I am therefore very supportive of the recent drive to shorten national postdoctoral fellowships to a maximum period of three years. Similarly, any program or policy to discourage Ph.D. tenure to six years or beyond should be applauded.

The Spectrum: Before closing we must ask whetheryour research really suggested routes to the kind ofimproved human vision you alluded to in that talk tothe downtown business folks?

Liu: I think you are sneakily baiting for the answer to the question: “Can dogs see ghosts?” Sorry, I am not going to let the cat out of the bag. By the way, I am hoping to take the show to the ACS Speaker Circuit. But I can tell you that after some discussion on the background information on the chemistry of vision, I did return and gave an answer to that very question. At the same time I also mentioned that we are on the hot trail of providing an answer to the question why the rhodopsin pigment is so much more sensi-tive than the same vitamin A chromophore in solution. It is a question that has puzzled researchers in the vision fi eld for some time. The situation is identical to putting a roll of ASA 200 fi lm into a magical camera and fi nding that it suddenly behaves like ASA 800, a four-fold increase in sensitivity. Understanding the mechanism of enhancement

Robert S. H. Liu


of this magical camera is clearly very important. We believe that not only we have some understanding of the enhance-ment in our eyes but also with some defi ned fi ne-tuning (chemically), we might turn that roll of fi lm into one of ASA 1200. Is that important research? I thought it was. But that was when my NIH program ran into an unceremonious termination. Apparently I had stepped on the sensitive toes of those controlling the purse string, those believing that no one could improve on Nature!

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structure and dynamics ofmacromolecular free radicals

Malcolm D. E. Forbes*, Vanessa P. McCaffrey†, & Elizabeth J. Harbron‡

*University of North Carolina at Chapel Hill, †Albion College, ‡The College of William and Mary


Introduction The degradation of polymers continues to be a subject

of intense interest to chemists, physicists and materials sci-entists.1–3 From the environmental and medical impact of biodegradable macromolecules,4 to the increasing expecta-tions for the stability of polymers in industrial applications such as architectural coatings5 and lithography,6–7 mechanis-tic information about degradation processes is highly sought after in many fi elds of research. The thermal and photo-chemical degradation mechanisms of macromolecules often involve free radical reactions,8–11 therefore knowledge of the structure, molecular dynamics and chemical reactivity of these intermediates is highly desirable. Electron paramag-netic resonance (EPR) spectroscopy is a powerful tool for such studies, and has contributed much to our understand-ing of polymer degradation and dynamics over the past 50 years.12–13

The photodegradation of poly(methyl methacrylate) (PMMA), depicted in Scheme 1, has been studied exten-sively in our laboratory14–17 and others.18–23 The EPR tech-nique can be used to detect free radicals directly, that is, to characterize primary degradation products such as main chain radical 1 and oxo–acyl radical 2. Degradation mecha-nisms can also be studied indirectly by EPR spectroscopy, for example when a secondary radical results from a rear-rangement. An example, shown at the bottom of Scheme 1, is the β–scission reaction leading to propagating radical

3 and a terminal olefi n.24 It is often advantageous to run the EPR experiment in time–resolved mode to observe primary processes,25–26 and to use the steady–state method to observe equilibrated species. Steady–state EPR can also be used with spin probes or spin labels (e.g., nitroxides) to study swelling

and void volumes in thin polymer fi lms or to investigate the dynamics of polymer chains.27–30 This can be informative be-cause of the sensitivity of the nitroxide spectral shape to mo-lecular motion over a wide range of correlation times.31–33

During the past several years our research group has car-ried out a detailed study on the photochemical degradation of acrylic polymers in liquid solution using time–resolved EPR (TREPR) spectroscopy with continuous wave micro-wave excitation and pulsed (laser) radical production. There are several advantages of the time–resolved technique over conventional steady–state EPR methods.34 For example, the fast time response of the experiment (~50 ns) allows for ob-servation, in most cases, of the primary paramagnetic inter-mediates produced in a photochemical degradation event, that is, before any rearrangements have occurred. In regard to the chemistry outlined in Scheme 1 this means that the TREPR experiment detects only the main chain acrylic rad-ical 1 (and often also the oxo–acyl counter radical 2) over a time scale of a few microseconds at room temperature and above. This is in stark contrast to the results of steady–state EPR studies on the same photochemical reaction, where only spectra assigned to the propagating radical 3 have been reported.35–38 In fact until our fi rst paper on this topic,14 EPR characterization data for main chain acrylic radicals, in the form of isotropic hyperfi ne coupling constants and g–fac-tors, had not been reported even though these structures had long been suspected as the primary reactive intermedi-ates involved in the acrylate photodegradation process.

In 2000 we published the fi rst high resolution EPR spectra of main chain acrylic radicals from photolysis of poly(ethylacrylate) (PEA) and PMMA, shown for different temperatures in Figure 114 (for PEA, R = H, R’ = CH3CH2– in Scheme 1). This initial study helped establish and con-fi rm several principles regarding acrylate photodegradation. The presence of emissive chemically induced dynamic elec-tron spin polarization (CIDEP)39–40 in the TREPR spectra of these radicals confi rmed that the excited state precursor was a triplet state of the side chain ester functionality. Computer simulations (Figure 1, right side) of both the polymeric radi-cal and the side chain oxo–acyl radical allowed unambigu-ous assignment of the signal carriers and provided defi nitive evidence that side chain cleavage of the ester group was the primary photochemical event after laser excitation of ac-rylates at 248 nm. Kinetic results allowed the upper limit of the β–scission reaction depicted in Scheme 1 to be esti-mated at 5 x 104 s–1. The oxo–acyl radical is normally only observed below 40 °C in our experiments as it undergoes rapid spin relaxation due to spin-rotation interaction, a pro-cess that increases in rate with increasing temperature.

Scheme 1. Photodegradation of PMMA. R = R’ = -CH3.

Forbes, McCaffrey & Harbron


From the perspective of magnetic resonance spectros-copy, a very interesting discovery from our previous work is that the β–methylene hyperfi ne coupling constants in the polymeric main chain radical 1 enjoy a symmetry relation-ship due to the stereochemical arrangement on every other carbon atom in the polymer. This relationship is expressed graphically in Figure 2 for three tacticities of PMMA

(isotactic: all methyl groups face the same direction, as do all ester groups; syndiotactic: methyl and ester groups have alternating orientations going down the chain; atactic: ori-entations of both groups are random). This is analogous to the 1H NMR assignments for the methyl protons (i.e., meso and racemic dyads) in acrylic polymers.41 The free radical created in the center of the polymer chain by loss of the

Figure 1. X–band TREPR spectra of polymeric main chain radicals from A) poly(ethylacrylate) (PEA) at room temperature, B) PEA at 115 °C, C) PMMA at room temperature, D) PMMA at 93 °C. Radical structures are shown to the right, with protons giving hyperfi ne interaction in red and the side chain of the ester in brown. Note that the oxo–acyl radical (structure 2 in Scheme 1) only appears in lower temperature spectra. High temperature spectra were acquired in propylene carbonate solution at a delay time of 0.8 µs and with a sweep width of 150 Gauss. Low temperature data was obtained in THF solution. In these and all subsequent spectra, lines below the baseline are emissive due to triplet mechanism CIDEP (see text for details). Left side contains experimental spectra, right side shows simulations using hyperfi ne coupling and g-factor parameters taken from ref 14. No simulation is currently possible for the spectrum in Figure 1C due to the dynamic effects discussed below.

Figure 2. Stereochemistry, polymer tacticity, symmetry relationships and magnetic equivalence of the β–methylene protons in main chain radicals from PMMA. Note that magnetic equivalence in the “meso” radical (mirror plane symmetry element) is between H

a and

Ha’ (and H

b and H

b’), but the assignment is reversed for the “racemic” radical (C

2 axis symmetry element). In both cases the hyperfi ne

splitting pattern consists of a quartet from interaction with the methyl protons, each line of which is further split into a triplet of triplets by the two sets of diastereotopic β–methylene protons.



ester group can exist as a “meso” radical in the case of the isotactic and syndiotactic polymers, and either a “meso” or “racemic” radical if the polymer is atactic. The magnetic in-equivalence of the β–methylene protons leads to a triplet of triplets rather than a quintet at fast motion. The steri-cally hindered conformations of the polymeric radicals are the reason why high temperatures are required to obtain fast motion hyperfi ne coupling constants.

As noted in Figure 2, the splitting pattern for PMMA at fast motion is expected to show 36 lines (quartet of triplets of triplets, or 4 x 3 x 3 lines). In general only 21 lines are ob-served due to accidental degeneracies, although we have not ruled out spin diffusion as a cause of these degeneracies so this phenomenon may not be a complete coincidence. This process is currently under investigation in our laboratory. A very interesting observation is that the coupling constants in each tacticity of polymer converge to fast motion limit values at different temperatures, as shown in Figure 3. This

is in line with previous observations regarding chain stiff-ness and stereochemistry in acrylic macromolecules. Even more interesting to us is that the coupling constants, listed in the caption to Figure 3, are approximately the same for all three polymer tacticities at fast motion. This tells us that while long range macromolecular forces are clearly impor-tant in establishing the conformational energies and low temperature hyperfi ne coupling constants, they have little or no effect once the fast motion limit has been reached.

Recently, we reported TREPR spectra from photolysis of four additional acrylate polymers, which led to characteriza-tion data for three new main chain radicals at high tempera-ture (Figure 4).15 This dataset showed that the ester cleav-age mechanism is a general one for acrylates, and that the onset of fast motion occurs at about 100 °C for most of these structures. For the fl uorinated side chain analog of PMMA, poly (fl uorooctyl methacrylate), (PFOMA), the polymeric radical did not exhibit fast motion spectra even at 150 °C; only the oxo–acyl counter–radical was resolved.

Another 2005 paper from our laboratory reported analy-sis of dynamic effects due to conformational motion in these bulky radicals.16 We developed a two-site jump model, out-lined in Figure 5, for the β–methylene hyperfi ne coupling constant modulation that took advantage of the symmetry relationships discussed above. With the standard Freed–Fraenkel equations,42 we were able to successfully simulate the temperature dependences of two of the polymers shown in Figure 4. Our analysis led to Arrhenius plots for the in-verse correlation time versus inverse temperature and then the extraction of activation barriers for the motion. A repre-sentative set of simulations for PMMA–d3 is shown in Figure 6 and an Arrhenius plot from this dataset is shown in Figure 7. The temperature dependent dynamic effects observed previously in the TREPR spectra of acrylic polymers mani-fest themselves as alternating line widths in the lower tem-perature spectra (see, e.g., Figure 1C and Figure 4).16 Similar effects have been observed in EPR spectra of the PMMA propagating radical and analogs35–38 (structure 3 in Scheme 1), and debate is still somewhat heated as to the correct

Figure 3. A) Experimental and B) simulated high temperature (fast motion) TREPR spectra for the PMMA main chain radical. Hyperfi ne values for each simulation are: 3 a

H(CH

3) = 22.9 G,

2 aH(CH

2) = 16.4 G, 2 a

H(CH

2) = 11.7 G for isotactic PMMA; 3

aH(CH

3) = 22.9 G, 2 a

H(CH

2) = 16.2 G, 2 a

H(CH

2) = 11.7 G for

syndiotactic PMMA; and 3 aH(CH

3) = 23.0 G, 2 a

H(CH

2) = 16.4 G,

2 aH(CH

2) = 11.3 G for atactic PMMA. The magnetic fi eld sweep

width for all spectra is 150 G. The three spectra were acquired at temperatures of (from top to bottom) 93 °C, 126 °C, and 135 °C, respectively.

Figure 4. X-band TREPR spectra of A) poly(ethyl methacrylate), B) PMMA–d

3, C) poly(cyanoethyl acrylate), D) poly(fl uoroctyl

methacrylate) (PFOMA) at high temperatures in liquid solution. Sweep width is 150 Gauss for all spectra. The simulation for PFOMA shows only the oxo-acyl counter radical (similar to radical 2 in Scheme 1). Parameters for simulations are taken from ref 15. All spectra have sweep widths of 150 Gauss.



spectroscopic analysis of those systems.43–48 The two–site jump model outlined in Figure 5 was able to account for line broadening effects in some main chain radicals we have investigated, but it did not reproduce the temperature de-pendence for all of them. This may be a defi ciency in the model, which relies heavily on the symmetry relationships between the nuclear spin sub-ensembles of the two confor-mations shown. Previously, we assumed that the modulation occurred between two symmetric structures, each giving a triplet of triplets which were rapidly interchanging in the TREPR spectrum. It is possible that instead of this symmet-ric situation, the two triplets are modulated into four dou-blets, each of which has a different coupling constant.42 It is also possible that different acrylic polymers require different motional models to describe their chain dynamics around the radical center. There are in fact very few reliable meth-ods for the accurate determination of activation barriers for local conformational motion in polymers49–50 and/or poly-meric radicals.35–38 A current interest in our laboratory is how these barriers change with chain length and position of the radical center on the chain. These radicals can be thought of as the smallest possible “spin label” for dynamic studiesin polymers.

Another recent publication from our laboratory reported long range stereochemical effects on the TREPR spectra of PMMA radicals,17 and provided more detailed analyses of solvent effects on the spectra, particularly with regard to electron spin relaxation times. For example, Figure 8 shows how small changes in polymer tacticity can affect the fast motion coupling constants for PMMA radicals, and how sensitive these hyperfi ne couplings are to temperature. This can be contrasted with spectra of radicals from PFOMA and poly(adamantyl acrylate) (PAMA), shown in Figure 9. Here we see that fast motion limit spectra of the polymeric radicals are not attainable even at very high temperatures, and only the oxo–acyl counter radical is observed with high resolution and intensity in each case. The oxo–acyl radical from PFOMA is of special interest because it does not show the fast spin relaxation at high temperatures common to its alkyl analogs. Further examination reveals that the triplet observed in Figure 9B is in fact triplet of triplets, indicating that there is very long range hyperfi ne coupling to fl uorines in this system. Figure 10 shows an expansion of this spec-trum and a simulation.

In summary, the TREPR technique is mechanistically powerful, with high spectral resolution and fast time re-sponse. We can carry out the experiment at multiple fre-quencies (L–, X– and Q–band), and with several different sources of excitation (excimer lasers at 157 nm, 193 nm,

Figure 5. Two–site jump model for hyperfi ne modulation in PMMA main chain radicals. The methyl group hyperfi nes are omitted for clarity. Symmetric rotation of the bonds indicated causes interchange of the coupling constants between H

a

and Ha’, and H

b and H

b’. Transitions that interchange from this

process are connected by dashed lines. Transitions are identifi ed by their nuclear spin orientations (quantum numbers m

i). Thick

lines become broadened and thin lines remain sharp due to symmetry considerations, leading to the alternating line width pattern ultimately observed.

Figure 6. Experimental and simulated X–band TREPR spectra of the main chain radical from PMMA–d

3 at the temperatures

indicated. A two-site jump model for hyperfi ne modulation as used for the simulations.

Figure 7. Arrhenius plot from the TREPR data in Figure 6, giving an activation barrier of 22 kJ mol–1.



248 nm and 308 nm, Nd3+:YAG lasers at 266 nm, 355 nm, and a YAG–pumped OPO for excitation across the visible spectrum if necessary). We can study conventional liquid solutions, pressurized liquids,51–52 supercritical fl uids, and thin polymer fi lms or melts.43 In addition to our recent work on acrylates, we have previously studied the degradation of alternating styrene–CO copolymers in liquid solution by TREPR.54–55 Using steady–state spin probe methods we have investigated the swelling of acrylic and acetate poly-mers with CO2.

51 We can simulate and model time–resolved and steady–state EPR spectra as a function of time, mag-netic fi eld or temperature (including CIDEP phenomena and dynamic effects). This versatility gives our laboratory a unique modern arsenal for the elucidation of degradation mechanisms and free radical dynamics in macromolecular systems.

References 1. Schnabel, W. Polymer Degradation: Principles and

Applications; Hansen & Gardner, 1982. 2. Rabek, J. E. Mechanisms of Photophysical Processes and

Photochemical Reactions in Polymers; Wiley: New York, 1987.

3. Guillet, J. Polymer Photophysics and Photochemistry: An Introduction to the Study of Photoprocesses in Macromolecules; Cambridge University Press: New York, 1985.

4. Swift, G. Acc. Chem. Res. 1993, 26, 105–110. 5. Ryntz, R. A. Plastics & Coatings: Durability,

Stabilization, Testing; Hansen & Gardner, 2001. 6. Wallraff, G. M.; Hinsberg, W. D. Chem. Rev. 1999,

99, 1801–1822. 7. Kamat, P. V. Chem. Rev. 1993, 93, 267 –300.

Figure 8. Temperature dependence of the TREPR spectra of the polymeric radical of PMMA with differing degrees of isotacticity. Left hand side: Experimental spectra: A) 91% mm, 7% mr and 2% rr dyads at 94 °C, delay time of 0.6 µs. B) 72% mm, 19% mr and 9% rr dyads at 90 °C, delay time of 0.3 µs. C) and D): expansion of boxed regions of A) and B), respectively. E) Same spectrum as D but at 110 °C, F) at 120 °C, G) at 130 °C. Right hand side: Simulations of experimental data. Sweep widths of the spectra are both 150 G.

Figure 9. TREPR spectra of the polymeric and oxo-acyl radical of PFOMA (2) in FC - 70 (1.72 g in 40 mL). Delay time is 0.3 µs for all spectra. Sweep width is 150 G. A) 51 °C. B) 110 °C. C) TREPR spectrum of the main chain polymeric radical of PAMA (3) in methylene chloride at 25 °C. Delay time is 1.0 ms, sweep width is 150 G.

Figure 10. Experimental (top) and simulated (bottom) TREPR spectra of the oxo–acyl radical from PFOMA in the perfl uorinated solvent FC–70 (3M company). Sweep width is 50 G. Simulation parameters: a

F(γ-CF

2) = 3.2 G, a

F(∆-CF

2) = 0.8 G, LW = 0.8 G.



8. Grassie, N.; Scott, G. Polymer Degradation and Stabilization; Cambridge University Press: New York, 1985.

9. Grossetête, T.; Rivaton, A.; Gardette, J. L.; Hoyle, C. E.; Ziemer, M.; Fagerburg D. R.; Clauberg, A. H. Polymer 2000, 41, 3541–3554.

10. Nagai, Y.; Nakamura, D.; Miyake, T.; Ueno, H.; Matsumoto, N.; Kaji, A.; Ohishi, F. Polym. Degrad. Stab. 2005, 88, 251–255.

11. Hughes O. R.; Coard L. C. J. Polym. Sci., Part A-1 1969, 7, 1861–1872.

12. Moore, J. A.; Choi, J. O. In Radiation Effects On Polymers; Clough, R. L., Shalaby, S. W., Eds.; American Chemical Society: Washington DC, 1991; pp 156–192.

13. Carswell, T. G.; Garrett, R. W.; Hill, D. J. T.; O’Donnell, J. H.; Pomery, P. J.; Winzor, C. L. In Polymer Spectroscopy; Fawcett, A. H., Ed.; Wiley: Chicago, 1996; pp 253–274.

14. Harbron, E. J.; McCaffrey, V. P.; Xu, R.; Forbes, M. D. E. J. Am. Chem. Soc. 2000, 122, 9182 –9188.

15. McCaffrey, V. P.; Forbes, M. D. E. Macromolecules 2005, 38, 3334–3341.

16. McCaffrey, V. P.; Harbron, E. J.; Forbes, M. D. E. Macromolecules 2005, 38, 3342–3350.

17. McCaffrey, V. P.; Harbron, E. J.; Forbes, M. D. E. J. Phys. Chem. B 2005, 109, 10686–10694.18. Wochnowski, C.; Shams Eldin, M. A.; Metev, S.

Polym. Degrad. Stab. 2005, 89, 252-264.19. Ranby, B.; Rabek, J. F. In Photodegradation,

Photooxidation and Photostabilization of Polymers – Principles and Applications; Wiley, 1975; pp 153 ff.

20. Hardy, W. B.; In Developments in Polymer Photochemistry – 3; Allen, N. S., Ed.; Applied Science Publishers: London/New Jersey, 1980; pp 322 ff.

21. Kopietz, M.; Lechner, M. D.; Steinmeier, D. G. Polym. Photochem. 1984, 5, 109–119.

22. Jónsson, S.; Schaeffer, W.; Sundell, P.–E.; Shimose, M.; Owens, J.; Hoyle, C. E. In Proceedings of the Conference of RadTech International North America, 1994, 1,

pp 194 ff.23. Garnett, J. L. Radiat. Phys. Chem. 1995, 46, 925–930. 24. Beck, G.; Lindenau, D.; Schnabel, W. Macromolecules

1977, 10, 135–138.25. Forbes, M. D. E. The Spectrum 1995, 8, 3 –8.26. Forbes, M. D. E.; Peterson, J.; Breivogel, C. S. Rev. Sci.

Instrum. 1991, 66, 2662–2665.27. Tsay, F.-D.; Gupta, A. J. Polym. Sci., Part B: Polym.

Phys. 1987, 25, 855–881.

28. Szajdzinska-Pietek, E.; Wolszczak, M.; Plonka, A.; Schlick, S. Macromolecules 1999, 32, 7454–7460.

29. Pilar, J.; Labsky, J.; Schlick, S. J. Phys. Chem. 1995, 99, 12947–12951.

30. Szajdzinska-Pietek, E.; Schlick, S.; Plonka, A. Langmuir 1994, 10, 1101–1109.

31. Schneider, D. J.; Freed, J. H. Calculating Slow Motional Magnetic Resonance Spectra: A User’s Guide. In Spin Labeling: Theory and Applications, Vol III, Biological Magnetic Resonance; Berliner, L. J., Reuben, J., Eds.; Plenum: NY, 1989; Vol. 8, pp 1–76.

32. Kivelson, D. J. Chem. Phys. 1960, 33, 1094–1106.33. Hudson, A.; Luckhurst, G. R. Chem. Rev. 1969, 69,

191–225.34. Forbes, M. D. E. Photochem. Photobiol. 1997, 65, 73–81.35. Abraham, R. J.; Melville, H. W.; Ovenall, D. W.;

Whiffen, D. H. Trans. Faraday Soc. 1958, 54, 1133–1139.36. Doetschman, D. C.; Mehlenbacher, R. C.; Cywar, D.

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Polym. J. 1978, 10, 69–75.38. Tian, Y.; Zhu, S.; Hamielec, A. E.; Fulton, D. B.;

Eaton, D. R. Polymer 1992, 33, 384–390.39. Atkins, P. W.; Evans, G. T. Chem. Phys. Lett. 1974,

25, 108–110.40. Wong, S. K.; Hutchinson, D. A.; Wan, J. K. S. J. Chem. Phys. 1973, 58, 985–989.41. Tonelli, A. E. NMR Spectroscopy and Polymer

Microstructure: The Conformational Connection; Wiley: New York, 1989; Chapter 6.

42. Freed, J. H.; Fraenkel, G. K. J. Chem. Phys. 1963, 39, 326–348.

43. Sugiyama, Y. Bull. Chem. Soc. Jpn. 1998, 71, 1019–1023.44. Harris, J. A.; Hinojosa, O.; Arthur, J. C. J. Polym. Sci.,

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Continued on page 24

shining light on thephotodecomposition of beer

Kevin Huvaere & Denis De KeukeleireGhent University


Beer, with a global consumption of 60 mL per capita per day, even exceeds the intake of a most nourishing bever-age such as milk. Lager beer is an excellent thirst quencher, while connoisseurs appreciate the enormous variety of fl a-vors that characterize top-fermented beers. Although state-of-the-art beer brewing has become a sophisticated process, a barley-derived alcoholic beverage, known as sikaru, was al-ready prepared by the Sumerians at the dawn of civilization (ca. 7000 years BC).1,2 The rather primitive brewing skills were greatly improved by Egyptians centuries later. Beer was considered a gift from the gods, as the intoxicating effect was experienced as a divine feeling. Gradually, cultivation of barley (an essential raw material in beer brewing) spread throughout Europe, in particular in rather cool regions that were not suitable for large-scale viniculture. During the Middle Ages, brewing techniques were refi ned by European monks, who introduced the use of hops (Humulus lupulus L.).3 Initially added as a natural preservative to keep beer for a longer period of time, hops soon became an essential beer fl avoring agent. The use of hops was enforced by the famous “Reinheitsgebot” (Law of Purity) issued by Duke Wilhelm IV of Bavaria (1516), in which was stated that the only permit-ted ingredients in beer were malt, water, hops, and yeast.

Initial Perceptions of the Light-induced Deterioration of Beer

In the early days, beer was mostly kept in stoneware bottles, but, during the 18th century, glass bottles became the package material of choice. Glass was hygienic, easy to clean and reusable, while, at that time, green glass could be cheaply manufactured. Hence, most beers came in green bottles thereby allowing inspection of the content before consumption. It was, however, not realized that, with respect to the beer quality, the transparency of glass was to become a nightmare to brewers, who, until today, are condemned to worry about this unsolved problem. Lintner (1875) was the fi rst to report on the formation of an offending taste and ob-noxious odor in beer exposed to light, which he called “light-struck fl avor” (LSF).4 It took more than a century to confi rm the light-induced formation of the compound, which is largely contributing to LSF, namely 3-methylbut-2-ene-1-thiol (MBT),5 one of the most powerful fl avor substances known (threshold of ca. 4 ng per liter beer).6 MBT is gener-ally referred to as “skunky thiol”, because the odor resembles that of secretions of the anal glands of skunks (Mustelavison L.).

In 1963, Kuroiwa (then head of the research depart-ment of the Kirin brewery in Japan) and collaborators sug-gested a formal mechanism for MBT formation.7 Since the

organoleptic anomalies could not be observed in unhopped beers, hop-derived compounds had to be essential in LSF for-mation.8 From experiments in model systems, it was demon-strated that the presence of isohumulones is pivotal. These bittering principles are not extracted from hops as such, but they are the result of a thermal isomerization of humulones (important secondary metabolites that prevail in the lupulin glands of hops) during boiling of hops (1.5-2 h) with wort (a sugar solution derived from enzymic breakdown of starch in barley malt) in the brewing kettle (Scheme 1). An epi-meric mixture of cis-isohumulones and trans-isohumulones is formed, the descriptors cis and trans referring to the rela-tive position of the hydroxyl group at C(4) and the prenyl substituent at C(5). Since humulones consist of a series of three main components, any beer analysis reveals the pres-ence of six isohumulones (Figure 1). Concentrations vary from few mg/L in weakly hopped beers to more than 100 mg L-1 in some American imperial pale ales. The organolep-tic properties of isohumulones determine typical beer char-acteristics including bitter taste (threshold of few mg L-1), hoppy aroma, stability of the beer foam,9 and bacteriostatic activity.10 Moreover, health-benefi cial properties due to par-ticular compounds present in hops confer to beer a unique position in the appreciation of (low-)alcoholic drinks. The only unfavorable property of hops with respect to beer is the vulnerability of isohumulones to light leading to photode-composition of beer.

Scheme 1. Thermal isomerization of humulones to cis- and trans-isohumulones, and structures of dihydroisohumulones and tetrahydroisohumulones.

Huvaere & De Keukeleire


Photodecomposition of Isohumulones and Tetrahydroisohumulones on Direct UV Irradiation

Comprehensive studies on the identifi cation of reac-tive intermediates formed on direct irradiation of isohu-mulones, as well as product analyses, led to detailed dis-closure of the photoreactivity of the substrates.11-13 Since the enolized β-tricarbonyl chromophore has an absorption maximum at 255 nm, with a shoulder around 270-280 nm(ε275 nm > 104), isohumulones are excited on exposure to UV-B light (280-320 nm). Ensuing dissipation of the excitation energy induces cleavage of the α-hydroxyketone moiety yielding, after decarbonylation, a 3-methylbut-2-enyl radi-cal, which must be considered the key intermediate in the development of LSF. Trapping of this species by a suitable sulfur source in the beer matrix (e.g., sulfur-containing pro-teins or constituents of the hop essential oil) should lead to MBT. Tetrahydroisohumulones (Scheme 1) cannot give rise to formation of MBT (the double bonds in the side chains of the isohumulones are hydrogenated), hence, these compounds were for a long time considered photostable. However, we showed that decomposition on UV-irradia-tion occurred readily, as the absorbing enolized β-tricarbon-yl chromophore is identical both in isohumulones and intetrahydroisohumulones, and we reported on this in a previ-ous issue of The Spectrum.14 Thus, irradiation of beers con-taining tetrahydroisohumulones furnishes an obnoxious off-fl avor, which evidently is not due to MBT, but most likely to other sulfur-containing compounds that have not been iden-tifi ed yet. On the other hand, dihydroisohumulones (Scheme 1), which are formed by reduction of the α-hydroxyketone to a vicinal diol, were found to resist decomposition on

exposure to UV-B light. This review provides an update of our continuing studies on the photodecomposition of beer, in particular when exposed to visible light.

Visible Light to Blame for the Photodecomposition of Beer

Notwithstanding the evidence that UV-B light leads to the development of precursors of MBT on interaction with isohumulones, LSF appears mainly when beer is exposed to visible light. Indeed, UV-B light accounts for only 0.5% of the solar spectrum15 and the exposure of beer to UV-leak-ing fl uorescent lamps (UV-A) in shops and supermarkets is rather limited. Moreover, UV light is not transmitted through colored glass bottles, although it is a general ob-servation that formation of LSF is quite evident when beer in green-colored bottles is exposed to visible light.16 In this respect, brown-colored glass that has a wavelength cut-off around 500 nm offers a better protection than green-col-ored glass, which transmits visible light down to 400 nm. This fi nding is in accordance with reports by Kuroiwa et al., who fi rst revealed that LSF was most evident on ex-posure of beer to visible light in the 350-500 nm range.7 Since isohumulones are transparent in this wavelength region,17 some sort of photosensitization must be involved in provoking light-induced reactions of these compounds. Ribofl avin (RF; present in several hundreds of micrograms per liter beer)18 shows absorption maxima at 375 nm and at 445 nm and is converted to its triplet-excited state (3RF*) on visible-light irradiation.19 The Japanese group proposed a reaction mechanism based on energy transfer from 3RF* to isohumulones, thereby initiating a decomposition pathway,

Figure 1. HPLC-analysis of isohumulones in beer (peaks 1 and 2: R: CH(CH3)

2, peaks 3 and 4: R: CH

2CH(CH

3)

2, peaks 5 and 6: R:

CH(CH3)CH

2CH

3; major peaks are cis-isohumulones, minor peaks are trans-isohumulones).



which results in the formation of MBT. This formal mecha-nism was compromised a few decades later, when Hastingset al. showed that the triplet-state energy of the isohumulo-nes (ca. 300 kJ mol-1) is signifi cantly higher than the triplet-state energy of RF (ca. 210 kJ mol-1). 20 Consequently, energy transfer must be an unfavorable endothermic process. By re-considering excited-state properties of RF, it is evident that 3RF* is a strong electron acceptor,21 which is able to oxidize various organic substrates.22,23 As also isohumulones may be appropriate substrates, RF-induced photooxidation would be feasible as the key reaction step in the pathway towards formation of LSF.

A Digression to ElectrochemistryAlthough redox properties of 3RF* (E=1.7 V) have been

profoundly investigated, information on electron-donating properties of isohumulones (or hydrogenated derivatives) in the interaction with 3RF* was not available. In this re-spect, values of the redox potentials of the compounds are essential in determining the thermodynamic feasibility of photooxidation by 3RF*. We, therefore, embarked on an electrochemical study including cyclic voltammetry and bulk electrolysis.24 Isohumulones, dihydroisohumulones, and tetrahydroisohumulones in their undissociated forms were subjected to electrochemical oxidation in acetonitrile, but, surprisingly, none of the compounds was electroactive. In contrast, we found that the corresponding anions (result-ing from proton abstraction from the enolized β-tricarbonyl chromophore, which is common to all substrates) were readily oxidized and showed prominent oxidation waves in the cyclic voltammograms (Ep of ca. 1.4 V) (Figure 2). This feature has profound consequences, since, indeed, isohumu-lones or hydrogenated derivatives with pKa values around 3 occur largely as salts in lager beers (pH usually between 4.2 and 4.4).25 Furthermore, the one-electron oxidized intermediates proved to be highly reactive, as the oxida-tions were irreversible. The striking similarity of the cyclic

voltammograms of various substrates strongly suggested electron abstraction from a common functionality, which is, in fact, the ionized β-tricarbonyl chromophore.

Confi rmation was sought in the identifi cation (by elec-tron paramagnetic resonance (EPR) spectroscopy) of the structures of incipient radicals resulting from electrochemi-cal oxidation. Bulk electrolysis in the cavity of the EPR spectrometer failed to generate radicals in detectable con-centrations. Therefore, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) or 2-methyl-2-nitrosopropane (MNP) were added as spin traps prior to electrolysis in order to convert the highly reactive radicals to more stable EPR-active adducts. Computer simulations of the signals in the EPR spectra per-mitted calculation of hyperfi ne coupling constants (Figure 3), which are characteristic for the nature of the initial

radicals. Apparently, a radical with low-electron density had been trapped after one-electron oxidation of isohumulones and similar observations were made for dihydroisohumulo-nes and tetrahydroisohumulones. The formation of analo-gous radical adducts is in agreement with the notion that a common functional group is oxidized and, from analyzing the values of the coupling constants of radical adducts with DMPO and MNP, it was concluded that a triacylmethyl radical had been trapped.26,27

Figure 2. Cyclic voltammograms of the anionic forms of trans-isohumulones (left), dihydroisohumulones (middle), and tetrahydroisohumulones (right).

Figure 3. Experimental (upper trace) and simulated (lower trace) spin patterns of radicals derived from trans-isohumulones after electrolysis and trapping by DMPO under nitrogen.



Electron Transfer: Key to the Photodecomposition of Beer

While the electrochemical experiments demonstrated that anions of isohumulones and hydrogenated derivatives may act as electron donors, the next step in our research was to provide evidence for such interaction with 3RF*. By laser fl ash photolysis, followed via monitoring the formation and the disappearance of short-lived intermediates by transient absorption spectroscopy,28 the necessary kinetic data were procured for the redox interaction of 3RF* and isohumu-lones, dihydroisohumulones, and tetrahydroisohumulones. The transient absorption traces of ribofl avin after a laser fl ash at 355 nm show 3 maxima (Figure 4), the absorption

at 720 nm being exclusively attributed to 3RF*. Addition of varying concentrations of the potassium salts of isohu-mulones gradually affected the decay of 3RF*. The bimo-lecular rate constants for the reaction of isohumulones and hydrogenated derivatives were determined from the slope of the linear plot of the pseudo-fi rst-order rate constant for decay of 3RF*, observed at 720 nm, as a function of the concentration of isohumulones or hydrogenated derivatives(Figure 5).

It is quite obvious that all hop-derived bitter com-pounds exhibit very comparable reaction kinetics (Table 1), hence, it was suggested that photooxidation exclusively involved the enolized and ionized β-tricarbonyl chromo-phore. Conclusive evidence for electron transfer towards 3RF* followed from observation of a transient species with an absorption maximum around 520 nm, which was formed concurrently with the decay of 3RF*. Time-resolved EPR (TR-EPR) analysis of model systems consisting of RF and isohumulones (or hydrogenated derivatives) led to unequiv-ocal identifi cation of the intermediate as the reduced ribo-fl avin radical (RFH•) originating from electron transfer and accompanying proton exchange (Figure 6).29

Radicals derived from isohumulones and hydrogenated derivatives escaped detection by laser fl ash photolysis fol-lowed by transient absorption spectroscopy. In order to gain structural information about these intermediates, a mixture of isohumulones (or hydrogenated derivatives) and ribofl avin was irradiated inside the continuous-wave EPR spectrometer cavity.30 Still, no steady-state radical concentrations could be reached and spin traps (DMPO and MNP) were added to scavenge the highly reactive radicals. On DMPO spin trapping under a nitrogen atmosphere, a similar signal was

Figure 4. Transient absorption spectra of ribofl avin in acetoni-trile/water (1:1, v/v) following excitation by a 355-nm laser fl ash at different time intervals (a: 0.1 µs; b: 1 µs; c: 10 µs; d: 50 µs).

Figure 5. Linear plot of the pseudo-fi rst-order rate constant (k

obs) for decay of triplet-excited ribofl avin, observed at 720

nm, as a function of the concentration of the potassium salts of isohumulones (at pH 4.6 (°) and at pH 7.0 (p)).

Figure 6. Time-resolved EPR spectrum of the reduced ribofl avin radical.

Table 1. Bimolecular rate constants (k / dm3 mol-1 s-1)for the interaction of isohumulones and hydrogenatedderivatives with triplet-excited ribofl avin in model systems (MeCN/buffer, v/v, 1:1)

Substrate pH 4.6 pH 7.0

Isohumulones 1.7 x 108 1.7 x 108

Dihydroisohumulones 1.8 x 108 1.7 x 108

Tetrahydroisohumulones 2.3 x 108 1.9 x 108



observed for all substrates (isohumulones, dihydroisohumu-lones, and tetrahydroisohumulones), thereby reassuring that radicals arose from a common functionality on interaction with 3RF* (Figure 7). Moreover, as radical adducts were very

similar to adducts observed after electrochemical oxidation, we inferred that a triacylmethyl radical (i) (Scheme 2) had been trapped. Interestingly, after prolonged irradiation, we were able to identify an adduct originating from trapping of an alkoxy radical (ii), which, very likely, was the result of an inter- or intramolecular hydrogen abstraction from the hydroxyl group at C(4) by the triacylmethyl radical. Ensuing α-cleavage should give rise to a 4-methylpent-3-enoyl radical (iii), which, however, escaped detection due to ready decarbonylation to a stabilized 3-methylbut-2-enyl radical.31 Direct spectroscopic evidence for the existence of the latter species followed from the observation of a broad superimposed signal on the spectrum of the reduced ribofl a-vin radicals in the TR-EPR experiments of model systems.29 On the other hand, as a 4-methylpentanoyl radical, result-ing from decomposition of tetrahydroisohumulones, should only slowly be decarbonylated,12 a major spin adduct of this radical was observed.

To confi rm the occurrence of the radical intermediates, detailed product analyses after photooxidation of isohumu-lones (and hydrogenated derivatives) by 3RF* were carried out.32 In order to detect primary, non-volatile reaction prod-ucts, reagents were loaded in a transparent syringe, which was exposed to visible light concurrent with fl ow injection into the electrospray ionization source of a quadrupole time-of-fl ight mass spectrometer (ESI-MS). Thus, product forma-tion could be monitored instantaneously. Volatile fragments were investigated by analyzing the headspace by gas chroma-tography, coupled to a mass spectrometer (GC-MS). Results

for isohumulones nicely corroborated the proposed α-cleav-age reaction pathway, as dehydrohumulinic acids were the main non-volatile degradation products (Scheme 2). After decarbonylation of the 4-methylpent-3-enoyl radical, the resulting 3-methylbut-2-enyl radical abstracts hydrogen to form 2-methylbut-2-ene, the most prominent reaction product detected by GC-MS. Tetrahydroisohumulones gave rise to hydrogenated dehydrohumulinic acids, while volatile reaction products were identifi ed as 2-methylbutane and 4-methylpentanal. Indeed, slow decarbonylation of a 4-meth-ylpentanoyl radical should be competitive with hydrogen abstraction from a suitable donor. Dihydroisohumulones, which were shown to be photostable on direct UV irradia-tion, proved to be photoreactive in the presence of 3RF*, since dehydrohumulinic acids were identifi ed as the main non-volatile degradation products. The ketyl radical result-ing from α-cleavage should be readily converted to 4-meth-ylpent-3-enal, which was, indeed, the major compound de-tected in the headspace.

Figure 7. Experimental (upper trace) and simulated (lower trace) spin patterns of radicals derived from dihydroisohumulones after photooxidation by triplet-excited ribofl avin and subsequent spin trapping by DMPO under a nitrogen atmosphere.

Scheme 2. Photooxidation mechanism of anions of isohumulones by triplet-excited ribofl avin.



ConclusionsLight exposure is harmful to the beer quality and pro-

tection is necessary against photodecomposition of hop-de-rived bitter compounds. Prevention of the lightstruck fl avor has mainly focused on physical protection; for example, cans and dark-colored bottles should prevent light from interact-ing with the beer. Still, beer is prone to undergo photode-composition during consumption from a glass and the fi nal reaction product, 3-methylbut-2-ene-1-thiol, is the main cause of the so-called “skunky fl avor”.

Our studies have resulted in clear insights into the mechanisms that govern the photodecomposition of beer. Direct absorption of UV light by isohumulones and tetrahy-droisohumulones leads to energy transfer from the excited triplet state of the enolized β-tricarbonyl chromophore to the α-hydroxyketo group, which subsequently undergoes α-cleavage to a 4-methylpent-3-enoyl radical and a 4-methylpentanoyl radical, respectively. These radicals react further to unpleasant volatiles, such as 3-methylbut-2-ene-1-thiol, which arises by decarbonylation of the 4-methyl-pent-3-enoyl radical followed by trapping by a thiyl radical. Dihydroisohumulones lacking the α-hydroxyketo group can not undergo α-cleavage and the excitation energy is dis-sipated, hence, the compounds are lightstable, at least on direct irradiation.

Photodecomposition in visible light involves the in-termediacy of a light-absorbing species, which is, in beer, mainly ribofl avin. The excited triplet-state of ribofl avin abstracts an electron from the anionic forms of isohumu-lones, dihydroisohumulones, and tetrahydroisohumulones, as these are predominant in beer. The resulting radicals are stabilized by inter- or intramolecular hydrogen abstraction leading to an alkoxy radical, which provokes α-cleavage to the same radicals as identifi ed on direct irradiation of iso-humulones and tetrahydroisohumulones. The light-insta-bility of dihydroisohumulones, on the other hand, delivers 4-methylpent-3-enal as the main reaction product. Thus, all hop-derived beer bitter compounds are photodecomposed on exposure to visible light by a redox reaction originating in the common enolized and ionized β-tricarbonyl function-ality. Ribofl avin does not function as a true photosensitizer, rather as a photogenerated oxidant.

Our fi ndings wipe out a general misconception by hop processors and brewers. Dihydroisohumulones and tetra-hydroisohumulones are currently applied by a number of brewers worldwide to brew so-called lightstable beers that can be packaged in clear bottles. The error resides in the fact that photodecomposition of these hydrogenated isohu-mulones cannot give rise to formation of the skunky thiol,

3-methylbut-2-ene-1-thiol. However, lightstruck fl avor should not solely be attrributed to this compound, since other sulfur-containing volatiles and volatile unsaturated aldehydes are reputed for causing objectionable off-fl avors.

Since hydrogenated isohumulones obviously do not pro-tect beer against the deleterious infl uence of light, more so-phisticated solutions should be elaborated based on a thor-ough knowledge of the mechanistic details of the photode-composition of isohumulones. Our research group possesses by now full expertise to realize the ultimate dream of brew-ing a fully lightstable beer, a thought that is undoubtedly cherished by both brewers and beer enthusiasts.

AcknowledgementsWe wish to acknowledge the great contributions to our

beer research program made by Dr. Arne Heyerick (Ghent University, Belgium), Prof. Dr. Malcolm Forbes (University of North Carolina, Chapel Hill, NC, USA), Prof. Dr. Mogens Andersen, Prof. Dr. Karsten Olsen, and Prof. Dr. Leif Skibsted (Royal Veterinary and Agricultural University, Frederiksberg, Denmark). KH is indebted to the Institute for the Promotion of Innovation and Technology in Flanders (IWT - Flanders, Brussels) for providing a postdoctoral fel-lowship, which is supported by the Ghent University and by InBev (Leuven, Belgium).

References 1. Glover, B. The World Encyclopedia of Beer; Anness

Publishing: London, UK, 1997. 2. Hardwick, W. A. Handbook of Brewing; Marcel Dekker:

New York, NY, USA, 1994. 3. Moir, M. J. Am. Soc. Brew. Chem. 2000, 58, 131. 4. Lintner, C. Lehrbuch der Bierbrauerei; Verlag Vieweg

und Sohn: Braunschweig, Germany, 1875. 5. Gunst, F.; Verzele, M. J. Inst. Brew. 1978, 84, 291. 6. Irwin, J.; Bordeleau, L.; Barker, R. L. J. Am. Soc. Brew.

Chem. 1993, 51, 1. 7. Kuroiwa, Y.; Hashimoto, N.; Hashimoto, H.; Kobuko,

E.; Nakagawa, K. Proc. Am. Soc. Brew. Chem. 1963, 181.

8. Kuroiwa, Y.; Hashimoto, H. Rep. Res. Lab. Kirin Brew. Co. 1961, 4, 35.

9. Clark, D. C.; Wilde, P. J.; Wilson, D. R. J. Inst. Brew. 1991, 97, 169.

10. Simpson, W. J. J. Inst. Brew. 1993, 99, 405.11. Blondeel, G. M. A.; De Keukeleire, D.; Verzele, M. J.

Chem. Soc., Perkin Trans. I 1987, 1, 2715.12. Burns, C. S.; Heyerick, A.; De Keukeleire, D.; Forbes,

M. D. E. Chem.–Eur. J. 2001, 7, 4553.


13. Heyerick, A.; Zhao, Y.; Sandra, P.; Huvaere, K.; Roelens, F.; De Keukeleire, D. Photochem. Photobiol. Sci. 2003, 2, 306.

14. De Keukeleire, D. The Spectrum 2001, 14(1), 1.15. Suppan, P. Chemistry and Light; The Royal Society of

Chemistry: Cambridge, UK, 1994.16. Sakuma, S.; Rikimaru, Y.; Kobayashi, K.; Kowaka, M.

J. Am. Soc. Brew. Chem. 1991, 49, 162.17. Maye, J. P.; Mulqueen, S.; Weiss, S.; Xu, J.; Priest, M. J.

Am. Soc. Brew. Chem. 1999, 57, 55.18. Duyvis, M. G.; Hilhorst, R.; Laane, C.; Evans, D. J.;

Schmedding, D. J. M. J. Agric. Food. Chem. 2002, 6, 1548.

19. Weber, G.; Teale, W. J. Trans. Faraday Soc. 1957, 53, 646.

20. Hastings, J. D.; McGarrity, M. J.; Bordeleau, L.; Thompson, J. D. Abstracts of the Brewing Congress of the Americas, September 1992, St. Louis, Missouri, USA.

21. Heelis, P. F. Chem. Soc. Rev. 1982, 11, 15.22. Heelis, P. F.; Parsons, B. J.; Phillips, G. O.; McKellar, J.

F. Photochem. Photobiol. 1978, 28, 169.23. Kino, K.; Saito, I.; Sugiyama, H. J. Am. Chem. Soc.

1998, 120, 7373.24. Huvaere, K.; Andersen, M. L.; Olsen, K.; Skibsted,

L. H.; Heyerick, A.; De Keukeleire, D. Chem.–Eur. J. 2003, 9, 4693.

25. Simpson, W. J. J. Inst. Brew. 1993, 99, 317.26. Buettner, G. R. Free Rad. Biol. Med. 1987, 3, 259.27. Duling, D. R. J. Magn. Reson., Ser. B 1994, 104, 105.28. Huvaere, K.; Olsen, K.; Andersen, M. L.; Skibsted,

L. H.; Heyerick, A.; De Keukeleire, D. Photochem. Photobiol. Sci. 2004, 4, 337-340.

29. Heyerick, A.; Huvaere, K.; De Keukeleire, D.; Forbes, M. D. E. Photochem. Photobiol. Sci. 2005, 4, 412.

30. Huvaere, K.; Andersen, M. L.; Skibsted, L. H.; Heyerick, A.; De Keukeleire, D. J. Agric. Food Chem. 2005, 5, 1489.

31. Vollenweider, J. K.; Paul, H. Int. J. Chem. Kinet. 1986, 18, 791.

32. Huvaere, K.; Sinnaeve, B.; Van Bocxlaer, J.; De Keukeleire, D. Photochem. Photobiol. Sci. 2004, 3, 854.

About the AuthorsDr. Kevin Huvaere received a B.Sc. in Chemistry (1999)

and a Ph.D. in Pharmaceutical Sciences (2004) from Ghent University. His doctoral thesis was on “Studies on the Mechanism of Formation of the Lightstruck Flavor in Beer” and, currently, he is a postdoctoral research fellow of the IWT - Flanders (Institute for the Promotion of Innovation

and Technology in Flanders) at Ghent University under the supervision of Prof. Dr. Denis De Keukeleire. His research project deals with a novel approach to inhibit the light-struck fl avor in order to produce lightstable beers.

Dr. Denis De Keukeleire (Ph.D. in chemistry at Ghent University, Belgium, 1971) was a postdoctoral research fel-low with George Hammond at the California Institute of Technology, Pasadena, CA, USA, during 1971-1972. Since 1992, he is a full professor at the Faculty of Pharmaceutical Sciences, Ghent University. His research interests are on health-benefi cial properties of medicinal plants, in particu-lar hops, and on various properties of hops that contribute to the beer fl avor including light-induced changes. Prof. De Keukeleire holds several research-related responsible posi-tions at Ghent University, the Fund for Scientifi c Research – Flanders, and the Belgian Ministry of Health. His address is Ghent University, Faculty of Pharmaceutical Sciences, Laboratory of Pharmacognosy and Phytochemistry, Harelbekestraat 72, B-9000 Ghent, Belgium (+3292648055); e-mail: [email protected]; http://allserv.ugent.be/~ddkeukel.

Forbes Continued from Page 1750. Spyros, A.; Dais, P.; Heatley, F. Macromolecules 1994,

27, 6207–6215.51. Dukes, K. E.; Harbron, E. J.; Forbes, M. D. E.;

DeSimone, J. M. Rev. Sci. Instrum. 1997, 68, 2505–2510.52. Avdievich, N. I.; Dukes, K. E.; Forbes, M. D. E.;

DeSimone, J. M. J. Phys. Chem. A 1997, 101, 617–621.53. Harbron, E. J.; Bunyard, W. C.; Forbes, M. D. E. EPR

Spin Probe Study of Carbon Dioxide Induced Polymer Plasticization. J. Poly. Sci., B 2005, in press.

54. Forbes, M. D. E.; Barborak, J. C.; Dukes, K. E.; Ruberu, S. R. Macromolecules 1994, 27, 1020–1026.

55. Forbes, M. D. E.; Ruberu, S. R.; Nachtigallova, D.; Jordan, K. D.; Barborak, J. C. J. Am. Chem. Soc. 1995, 117, 3946–3951.

About the AuthorsMalcolm D. E. Forbes completed B.S. degrees in Chemistry

and Mathematics with Computer Science at the University of Illinois at Chicago in 1983. He obtained his Ph.D. degree at the University of Chicago in 1988, working with the late



Forbes, McCaffrey & Harbron Continued

G. L. Closs. After a National Science Foundation postdoc-toral fellowship at Caltech with N. S. Lewis, he joined the faculty at the University of North Carolina at Chapel Hill, where he is now Professor of Chemistry. His research inter-ests are focused on the structure, dynamics and reactivity of free radicals. Current major interests include polymer degra-dation and amino acid photo-oxidation. His e-mail address is [email protected].

Vanessa P. McCaffrey studied chemistry at McNeese State University in Lake Charles, Louisiana, where she earned a B.S. in Chemistry in 1996. She received her Ph.D. from the University of North Carolina at Chapel Hill in 2001, working with Prof. Forbes on the photodegradation of polymers in solution. After postdoctoral work at Michigan State University with Profs. J. Dye and N. Jackson, Vanessa is now an Assistant Professor of Chemistry at Albion College in Albion, Michigan. Her current research interests are in the area of molecular magnetism. Her e-mail address is [email protected].

Elizabeth J. Harbron earned a B.A. in Chemistry from Grinnell College, Grinnell, Iowa, in 1995. She studied un-der the direction of Prof. Forbes at the University of North Carolina, graduating with a Ph.D. in 1999. She was an NIH postdoctoral fellow with Prof. Paul F. Barbara at the University of Texas at Austin prior to becoming Assistant Professor of Chemistry at The College of William and Mary in Williamsburg, Virginia. Her current research interests include conjugated polymers and single molecule spectros-copy. Her e-mail address is [email protected].

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Chemists: “Get Involved in Biophotonics and Nanophotonics”

Biophotonics and nanophotonics are hot fi elds that intersect with chem-istry but require knowledge and skills traditionally within the realm of physi-cists and biologists. How much biol-ogy or physics must a chemist know to branch out into those fi elds?

Less than might be expected, ac-cording to Paras N. Prasad, who urged that more chemists get involved in those fi elds during an American Chemical Society lecture entitled “Novel Directions in Photonics: Nanophotonics and Biophotonics.” He heads the Institute for Lasers, Photonics and Biophotonics at the University of Buffalo.

Prasad defi ned nanophotonics as the science behind light and matter inter-acting on the nanoscale, and biopho-tonics as the science behind the ways that light and biological matter inter-act. Both fi elds sometimes are mistak-enly viewed as falling more within the purview of physicists than chemists, he noted.

“Since the beginnings of both nano-photonics and biophotonics, chemists have been making major contributions to these fi elds, but their contributions often go unnoticed,” said Prasad. “We need to recognize how much chem-ists have done in these fi elds already and to encourage younger chemists to continue that work. I think chem-ists may get intimidated because they feel they have to learn a lot of phys-ics to start working in nanophotonics or a lot of biology to start working inbiophotonics.”

Prasad cited himself as an illustra-tion of how quickly chemists can ac-quire the knowledge and skills impor-tant for success in the fi elds, noting that eight years ago he knew “very little” about biology.

By forging research relationships with scientists in other departments, he began to learn how his background in chemistry could be instrumental in solving some of the important prob-lems in biophotonics. By 2004, he had written Introduction to Biophotonics and Nanophotonics, two comprehen-sive books on the topics published by John Wiley & Sons, and done widely recognized research on quantum dots and novel biophotonic materials with applications in photodynamic cancer therapy and bioimaging.

“There are huge opportunities for chemists in these areas,” Prasadconcluded.

Needed: Scientists for ShanghaiCiba Specialty

Chemicals has bol-stered its photochemistry and oth-er R&D programs

in China with a new $20-million fa-cility located in Shanghai. It expands Ciba’s global research network to 23 locations.

“It is a signifi cant step in the imple-mentation of our strategy to strength-en our presence in China,” said Armin Meyer, chairman and CEO of Ciba. “It also refl ects Ciba Specialty Chemicals’ belief that innovation is the key to profi table and sustainable business growth, everywhere in the world.”

In addition to photochemistry, the Shanghai center will house Chinese R&D activity in polymer chemistry, organic synthesis, physical chemistry, analytical chemistry, and application and formulation sciences. It will oc-cupy about 65,000 square feet of space in the Caohejing High-Tech Park.

Although the R&D will create new products and solutions for Ciba cus-tomers globally, the Shanghai center will have a special focus on the fast-growing Asian market.

Ciba already had 3 trading compa-nies, 6 branch offi ces and 12 produc-tion sites in China, which accounted for 7 percent of its $5.8 billion world-wide sales in 2004. Asia and the Pacifi c region accounts for 28 percent of Ciba’s global sales. Ciba has other R&D cen-ters in Asia, located in Amagasaki, Japan and Mumbai, India.

The company plans to recruit 100 scientists for the Shanghai operation. “In a fast-growing market like China, there is always a need for more talented people than there are,” Meyer said.

About 1,600 scientists work in Ciba’s global research network. The fi rm spends about $24 million annually on R&D.

Toward the Perfect LensConventional lenses produce im-

ages by capturing light waves propagat-ing from an object and then bending them. The angle of the bend, deter-mined by the index of refraction, has always been positive.

Objects also emit “evanescent” waves that carry a great deal of detail but decay exponentially and never reach the image plane—an optics

At top (A) is the higher resolution image of the word NANO created with a silver superlens. Below that (B) is an image created during a control experiment in which the superlens is replaced by spacer layer. The averaged line width is 89 nanometers in image A with the superlens, and 321 nanometer in image B without the superlens. The scale bar in both images is 2 micrometers.

Courtesy of Cheng Sun, UC Berkeley

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threshold known as the diffraction limit. Breaking the diffraction limit and capturing evanescent waves is the key to creating a “perfect” representa-tion of an object, regarded as the Holy Grail in optics.

Scientists at the University of California, Berkeley, have reported development of a superlens that can overcome the diffraction limit and produce much sharper images.

“Optics is involved in much of today’s technology, including imaging and photolithography,” noted Xiang Zhang, principal investigator in the study. “Our work has a far reaching

impact on the development of de-tailed biomedical imaging, higher den-sity electronic circuitry and ever-fasterfi ber optic communications.”

One of the fi rst applications may be in the development of medical imaging devices that could reveal never-before-seen details with optical microscopy, said Nicholas Fang, who was a member of the research team.

Current optical microscopes can resolve only relatively large structures in a cell, such as the nucleus and mi-tochondria. Optical microscopes with a superlens could reveal movements

of individual proteins traveling along the microtubules that make up a cell’s skeleton.

The researchers emphasized that optical microscopes do have advan-tages over higher-resolution scanning electron and atomic force microscopes, which resolve details down to a few nanometers. Such microscopes typi-cally limited to non-living samples, and image capture times can take up to several minutes.

Optical microscopes, on the other hand, can capture an entire frame with a single snapshot in a fraction of a sec-ond, opening the way to nanoscale im-aging to living materials.

Zhang and associates used a thin fi lm of silver as the lens and ultraviolet (UV) light to record images of an array of nanowires and the word “NANO” onto an organic polymer at a resolution of about 60 nanometers. In compari-son, current optical microscopes can only make out details down to about 400 nanometers.

It was the fi rst demonstration of optical imaging with a superlens. “We did not create a perfect image in our experiment,” said Fang. “But it’s clear that our image is dramatically better than the one created without the sil-ver superlens.”

Software: The New ChemDraw

Since the release of ChemDraw 1.0 in the mid-1980s, CambridgeSoft’s

chemical drawing and analysis pro-grams have become the worldwide standard for scientists and students in industry and academic settings.

The introduction marked the start of a continuous evolution and im-provement that changed the program from a molecular drawing tool into a powerful productivity tool with a host of tools.

Chemists using an older version of ChemDraw should take a look at ChemDraw 9.0 and ChemOffi ce Pro 2005, the suite that includes ChemDraw 9.0 and an array of other tools. There are additions and im-provements galore, which increases the power and versatility without reducing reliability or user friendliness.

Free trial versions are available at www.cambridgesoft.com.

Patent It YourselfSelf-patenting may sound like a

throwback to another era, when lone scientists working by themselves car-ried a good idea all the way from labo-ratory to marketplace with little out-side assistance.

The Internet, however, has made self-patenting a more realistic option for inventors without the personal fi -nancial resources or support network of a university or corporation. You can do-it-yourself, and fi le patents online, if you know how. One of the newest and best sources of advice is Filing Patents Online: A Professional Guide.

For approximately $65, it walks in-ventors through each step of fi ling and prosecuting patents via the Internet at a fraction of the usual cost, including a primer on the art of writing winning claims.

Imaging Skin DeeperResearchers at the National

Institute of Standards and Technology (NIST) and Johns Hopkins University have reported development of a new optical method for imaging structures below the surface of the skin. It shows promise for eventual use in medical ap-plications such as helping to diagnose or determine the extent of certain types of skin cancer.

Based on differences in the way sur-face and subsurface features of various materials scatter light, the method is

Schematic drawing of nano-scale imaging using a silver superlens that achieves a resolution beyond the optical diffraction limit. The red line indicates the enhancement of “evanescent” waves as they pass through the superlens.

Courtesy of Cheng Sun, UC Berkeley

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an extension of scattering techniques NIST scientists originally developed to image surface and subsurface features in inorganic materials such as silicon wafers, mirrors, and paint coatings.

The process involves illuminating a sample with polarized light and using a digital camera with a rotating polariza-tion fi lter to image the light scattered from the sample. Researchers manipu-lated the polarization to minimize light scattered from the rough skin surface, and positioned the light source inmultiple locations to separate out, and delete, light scattered more than one time from deeper sample layers. They generate an image of subsurfacestructure by varying the polarization settings and combining two images made with the light source in different positions.

So far, they have demonstrated it with small pieces of pigskin, and are working to improve it.

Dermatologists long have used po-larized light to identify the edges of lesions. The new method minimizes the effects of unwanted light scattering at once, and might offer the potential for producing more detailed images of deeper layers of skin.

Laser Control of BehaviorLaser control of the behavior of

chemical reactions is a well-estab-lished research fi eld. Scientists at Yale University, however, have broken new ground, using laser light to control the behavior of fruit fl ies that have geneti-cally designed triggers in their brains.

Exposure to laser light changed the way the fl ies jumped, beat their wings, and fl ew in an escape response. In other experiments, researchers used light to activate dopamine neurons that stimu-lated walking and affected the types of paths the fl ies chose to follow. Loss of dopamine cell activity in humans un-derlies Parkinson’s disease.

“The ability to control brain func-tions non-invasively opens many new possibilities for the analysis of neural circuits, the search for the cellular substrates of behavior, and, possibly even restoring function after injury or disease,” said Gero Miesenböck, who headed the study. “This is a signifi cant step toward moving neuroscience to active and predictive manipulation of behavior.”

Unfocused laser light was used to “broadcast” a signal to genetically en-gineered “phototriggers” that were ex-pressed only in specifi c groups of cells. Changing conditions of the light puls-es altered the activity level of the fl ies and the direction of their fl ight. These responses to laser light demonstrated a direct link between specifi c neurons and specifi c behaviors.

Miesenböck noted that the photo-receptors are ion channels that spark action potentials when illuminated. Depending on which neurons are light-sensitive, the remote-controlled fl ies jump, fl y or change their walking patterns on command.

Miesenböck and Susana Lima, a graduate student, said that a future re-mote control system may help reveal how neural circuits are wired and how they function, as well as how cell ac-tions and connections are related to more complex behaviors like learning, aggression and even abstract thought. The optical controls might also permit construction of “bionic” computers—hybrid devices in which neural circuits are interfaced with electronic circuits.

Ringing a Bell With Photoionization

Numerous electronic structure stud-ies of photoexcited ions have been conducted over the years, including carbon, nitrogen, oxygen, fl uorine, neon, scandium, titanium, manganese, nickel, and other atomic and molecular species. Ronald Phaneuf, University of Nevada-Reno, and colleagues now are tackling the electronic structure of photoexcited carbon-60.

Phaneuf’s group has used Beamline 10.0.1 at the Advanced Light Source

Colorized photos show two images of pigskin taken under different lighting conditions (top and middle) that were combined to reveal greater subsurface detail (bottom).

Courtesy of NIST/Johns Hopkins University

Laser light activates fruit fl y motion behavior.

Courtesy of Yale University

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(ALS) synchrotron radiation lab in Berkeley, which is specially designed to study the interaction of ions and pho-tons. It is equipped with a specialized research facility that collides opposing beams of photons and ions, an ECR (for electron-cyclotron-resonance) ion source.

ECR sends a beam of singly or mul-tiply charged ions to interact with an extraordinarily bright beam of pho-tons. The energy of the photon beam, produced by the beamline’s associated undulator magnet, can be tuned from

ultraviolet to soft x-rays with very high spectral resolution.

Fred Schlachter, one of the group of collaborators on the ALS staff, said the C-60 photoexcitation breaks new ground. “For a molecule made from a single element, C-60 is very large,” he explained. “It marks the transition from atoms to solids.”

Research in the 1990s showed that an excitation energy of about 22 elec-tron volts caused the four valence elec-trons in each of buckyball’s 60 carbon atoms to act collectively, resulting in a “giant resonance.” That collective mo-tion is called a plasmon.

The collective motion fi rst identi-fi ed in C-60 was a surface plasmon, a

undergoing photoionization. The same thing happens when a 40-electron volt photon hits a charged buckywall, except that the electron cloud wobbles in and out, penetrat-ing the cage—a phenomenon unique to charged buckminsterfullerene.

Phaneuf compared it to making buckyballs ring, almost like hitting a bronze bell with a clapper.

Jobs on DisplayThe fl at-panel display industry is

growing rapidly, with Market Analysts predicting that it will reach $70 billion an-nually by the end of 2005. Companies are hiring to keep pace with the growing market. The U.S. Display Consortium (USDC), a public/private partnership, offers a comprehensive database of job openings in the fi eld. (See www.usdc.org/resources/resources_jobs.html.)

Dozens of fi rms and their employ-ment opportunities can be accessed at headquarters in San Jose, CA. USDC funds projects and shares the results with its member companies. USDC also serves as a communication chan-nel among industry, government, and the fi nancial communities for display issues; sponsors workshops to broaden the impact of technological develop-ments; and educates consumers on the importance of displays in providingaccess to information technology. More information is available at www.usdc.org.

back-and-forth normal-mode oscil-lation of the whole cloud of valence electrons, relative to the effectively rigid cage of carbon cores.

In the new experiments, research-ers evaporated soot containing C-60 in the ECR ion source, producing a fi ne beam of buckyball ions. The beam was accelerated from the ion source and turned 90 degrees to collide head-on with a similarly fi ne beam of ultravio-let photons.

The new experiments have found evidence of a second resonance at an

energy of 40 eV. This second type of collective excitation is considered a “volume plasmon” since the shape of the collective electron ensemble is thought to be oscillating with respect to the center of the molecule.

Collective motion would be impos-sible if C-60 were a solid sphere instead of a hollow charged shell; for this rea-son it has not been observed in metal clusters like gold nanoparticles, where surface plasmons are common.

When a 22-electron volt photon hits a singly charged buckyball hav-ing 239 outer electrons, the whole spherical cloud surrounding the cage structure oscillates with enough energy to eject another electron,

When stimulated by photons at an energy of about 20 electron volts, a buckyball dis-plays collective electron motion as a surface plasmon. But when stimulated by photons of about 40 eV, the result is a different mode of collective electron motion, a volume plasmon.

Courtesy of Ronald Phaneuf, University of Nevada-Reno

news from the center for photochemical sciences at bowling green state university

New Building for Center to Feature Historic Library

On April 29, 2005, Bowling Green State University and the Center for Photochemical Sciences embarked on a major fundraising campaign for a new building for the photochemical sciences. A Boston area architectural fi rm has been engaged to assist us with planning and develop-ment documents, and we are looking forward to new world- class facilities to enable research in the various fi elds of the photosciences. This is coupled with other University commitments specifi cally to increase our faculty size and the number of student positions in the Center. The Center for Photochemical Sciences is poised to make a huge im-pact in photoscience research and education in the decadesto come.

Almost more than any other fi eld, the photosciences have provided the basic intellectual rubric for the quantum revolution and the understanding of the structure of the atom. One of my dreams for this new facility is to house a library to display historical items, books and other memo-rabilia in the photosciences. Surely historical items related to the many important developments surrounding the fi eld exist around the world. We would like to gather them in one spot to provide a historical context and guarantee that they are preserved for generations to come.

I would welcome any inquiries about our building proj-ect including plans for the library as well as from anyone who may have something to contribute to our efforts either through a fi nancial donation or a gift of memorabilia for the library.

D. C. NeckersExecutive Director, Center for Photochemical [email protected]

Center Publications540. Magda, D. J.; Wang, Z.; Gerasimchuk, N.; Wei,

W.; Anzenbacher, P., Jr.; Sessler, J. L. Synthesis of Texaphyrin Conjugates. Pure Appl. Chem. 2004, 76, 365-374.

541. Ullrich, B.; Erlacher, A.; Danilov, E. Switch performance and electronic nature of photonic laser digitizing thin GaAs fi lms on glass. Semicond. Sci. Technol. 2004, 19, L111-L114.

542. Kharenko, O.; Ogawa, M. Y. Metal-induced folding of a designed metalloprotein. J. Inorg. Biochem. 2004, 98 (11), 1971-1974.

543. Kozlov, D. V.; Castellano, F. N. Photochemically Reversible Luminescence Lifetime Switching in

Metal-Organic Systems. J. Phys. Chem. A 2004, 108, 10619-10622.

544. Kozlov, D. V.; Castellano, F. N. Anti-Stokes Delayed Fluorescence from Metal-Organic Bichromophores. Chem. Commun. 2004, 24, 2860-2861.

545. Ivanikova, N. V.; McKay, R. M. L.; Bullerjahn, G. S. Construction and characterization of a cyanobacterial bioreporter capable of assessing nitrate assimilatory capacity in freshwaters. Limnol. Oceanogr. Methods 2004, 3, 86-93.

546. Montes, V. A.; Pohl, R.; Li, G.; Shinar, J.; Anzenbacher, P., Jr. Effective Color Tuning in OLEDs based on Aluminum Tris(5-Aryl-8-hydroxyquinoline) Complexes. Adv. Mater. 2004, 16, 2001-2003.

547. Anzenbacher, P., Jr.; Jursíková, K.; Aldakov, D.; Marquex, M.; Pohl, R. Materials chemistry approach to anion-sensor design. Tetrahedron 2004, 60 (49), 11163-11168.

548. Polyansky, D. E.; Neckers, D. C. Photodecomposition of Organic Peroxides Containing Coumarin Chromophore: Spectroscopic Studies. J. Phys. Chem. A, 2005, 109, 2793-2800.

549. Hua, F.; Kinayyigit, S.; Cable, J. R.; Castellano, F. N. Green Photoluminescence from Platinum(II)

Complexes Bearing Silylacetylide Ligands. Inorg. Chem. 2005, 44, 471-473.

550. Danilov, E.; Pomestchenko, I. E.; Kinayyigit, S.; Gentili, P. L.; Hissler, M.; Ziessel, R.; Castellano, F. N. Ultrafast Energy Migration in Platinum(II) Diimine Complexes Bearing Pyrenylacetylide Chromophores. J. Phys. Chem. A 2005, 109,

2465-2471.551. Wex, B.; Kaafarani, B.; Neckers, D. C. Synthesis of

the anti and syn Isomers of Thieno[f,f’]bis[1] benzothiophene. Comparison of the Optical and

Electrochemical Properties of the anti and syn Isomers. J. Org. Chem. 2005, 70, 4502-4505.

552. Yang, J.; Neckers, D.C. Cobaltic accerlerator for the methylene blue photoinitiation system in aqueous acrylate solution. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3836-3841.

553. Golovkova, T. A.; Kozlov, D. V.; Neckers, D. C. Synthesis and Properties of Novel Fluorescent Switches. J. Org. Chem. 2005, in press.

For any reprints, please write or e-mail the Center for Photochemical Sciences and refer to the reprint by number.


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