Designs on C60
Cath O'Driscoll takes a look at what's in store for the evolving field of fullerene chemistry, and talks to one of its founders, Sir Harold Kroto.
'What's the worst thing that could possibly happen?' Rick Smalley recalled asking his research group at Rice University in Texas in the late summer of 1985. 'Harry's coming' came back the chorus in reply. Three days later, using money borrowed from his wife's bank account to pay for the flight, Harry Kroto of Sussex University was on his way to Texas. Though he had no way of knowing it at the time, it was a journey that would also lead him to discover an entirely new form of carbon fleeting glimpses of C60 in a laser beam and the start of a five year quest to prove unequivocally its existence. Over a decade later, and six years after physicists Donald Huffman and Wolfgang Krätschmer first isolated tangible amounts of reddish C60 crystals, there can be no doubts. Besides C60 itself (dubbed buckminsterfullerene by Kroto), there is a now a whole galaxy of fullerenes, including magic species C70, C72, C76 etc, extending all the way to C100 or more, as well as a range of bucky tubes and cages.
Kroto's role as one of the founding fathers of the field was acknowledged in the last New Year's Honours list, when he became one of only a handful of chemists of his generation to receive a knighthood. It is an honour that came as a complete surprise, he says; as a fundamental scientist, he never contemplated such recognition of his work.
Kroto, who originally trained as a spectroscopist, admits that: 'Microwave chemists in particular have a chip on their shoulder. People glibly use all of the fundamental constants on small molecules such as formaldehyde and acetaldehyde bond lengths, internal rotation barriers and dipole moments etc but have no idea where these values come from. Most of them didn't come from crystallography as some people think, they came from microwave spectroscopy'.
It was Kroto's interest in microwave spectroscopy, as well as in radioastronomy, that led to the chance discovery of C60. He is well aware of the irony over the timing of his knighthood: he believes that cutbacks in funding over the past 15 years make it unlikely that such purely curiosity motivated research would receive any funding today.
Such constraints only add to Kroto's conviction that, given the choice again now, he would 'almost certainly not choose a career in chemistry'. In fact, Kroto never planned to become a chemist. He describes his career path as 'a course of least resistance. I was good at science and I was also good at design and graphics'.
After graduating in chemistry in 1961, he stayed at the University of Sheffield to work with Richard Dixon on the high resolution electronic spectra of free radicals. Choosing to do a PhD wasn't solely motivated by his interest in chemistry: 'I also wanted to stay at university for another three years because I played a lot of tennis and was involved in doing the design and graphics of the university magazine'.
Graphic design is a continuing passion; in 1994 Kroto won the prestigious Moët Hennessy/Louis Vuitton Science Pour L'Art prize for his designs, inspired by the discovery of C60, on the theme From chaos to symmetry. Only a lack of obvious career tracks had prevented him from taking up a full-time career in design after his PhD: 'If it had not been for a prestigious postdoc fellowship in the world famous spectroscopy group at the National Research Council [NRC] in Canada, I would almost certainly have gone that way', he says. After two years at the NRC in Ottawa, Kroto spent one year at Bell Laboratories in New Jersey studying liquid phase interactions by Raman spectroscopy and carrying out studies in quantum chemistry.
Kroto returned to the UK to begin his academic career in 1967. 'When I came here to Sussex I gave myself five years and at the end of that time things started moving', he says. 'I was most proud of having initiated methods for making C=P and C(integral)P and other multiply bonded compounds. From that breakthrough whole new areas of carbon- phosphorus chemistry have evolved'.
Today, Kroto's role in founding the now active fields of phosphaalkene and phosphaalkyne chemistry is generally unknown: 'If you leave a field your contribution tends to be forgotten; few people tend to remember exactly how the field was born', he says.
Kroto puts his good fortune with C60 down largely to serendipity: 'It doesn't matter how good a scientist you are. The likelihood of a discovery like that is like winning the pools'. He compares the course of his research with the way he explores a foreign city: 'I just get lost in it. I follow the way that the city roads go. I don't have a plan (for science or life). Whatever I do is whatever interests or puzzles me at that time'.
Following his nose in whatever interests him has proved successful, but Kroto acknowledges that he couldn't have done it on his own. One of his most successful collaborations - and the work that ultimately put him on the path to C60 - was with friend and colleague David Walton at Sussex. Walton rekindled Kroto's interest in multiply bonded carbon chains molecules whose unusual spectroscopic characteristics had already been brought to Kroto's attention during his PhD work with Dixon, when he had carried out studies on carbon suboxide (O=C=C=C=O). Kroto likened such carbon chain molecules to 'very bendy batons' which he imagined being tossed high into the air by a 'microscopic quantum mechanical cheerleader'.
Years later, as radioastronomy techniques capable of detecting interstellar molecules began to be developed, Kroto's curiosity was aroused by the possibility that such molecules might occur in space. Following a procedure devised by Walton, undergraduate student Anthony Alexander successfully synthesised the first target cyanopolyyne, HC5N, in 1974. A year later, collaborations with former NRC colleague Takeshi Oka and other Canadian radioastronomers resulted in the successful detection of HC5N radio signals from Sgr B2, a giant cloud of molecules near the centre of the galaxy.
It was an exciting result and led to speculation about the occurrence of even longer carbon chains in space. Kroto was determined to pursue the next analogue in the series, HC7N. Undergraduate Colin Kirby was given the job of making the compound in the laboratory - a task that proved quite difficult. By the time Kirby had obtained the vital radio frequency of HC7N, Kroto and the team of Canadian astronomers had already begun their allotted session on the telescope at Algonquin Park in Ontario. In an article in Angewandte Chemie in 1992 Kroto recalled:
The next few hours were high drama. We dashed out to the telescope and tuned the receiver to the predicted frequency range as Taurus rose above the horizon (perfect timing). We tracked the extremely weak signals from the cold dark cloud throughout the evening . As the night wore on we became more and more excited, convinced that the signal was significantly more often high than low; we could hardly wait for Taurus to set. By 1.00 am we were too excited and impatient to wait any longer and shortly before the cloud vanished completely, Avery stopped the run and processed the data. The moment when the trace appeared on the oscilloscope was one of those that scientists dream about and which, at a stroke, compensate for all the hard work and disappointments which are endemic in life.
This, and the later discoveries of HC9N (by Oka et al) and HC11N (by Bell et al), were a revelation - at that time space was still considered far too inhospitable to accommodate such high molecular weight molecules. The search for the source of these molecules became something of an obsession. How and where, Kroto wondered, were such molecules being formed? Observations of red giant stars - spectacular infrared objects continually pumping out vast amounts of chains and grains - suggested the answer.
It was several years before Kroto was to pursue this thought any further, while visiting fellow microwave spectroscopist Bob Curl in Texas in 1984. During his stay, Curl persuaded Kroto to look in on the laboratory of his colleague Rick Smalley. At that time Smalley was producing some impressive results with SiC2 clusters using a laser vaporisation apparatus that he and his coworkers had developed. But what excited Kroto 'was the thought that by simply replacing silicon carbide by graphite, it should be possible to simulate the type of chemistry that takes place in a red giant carbon star'. If his conjecture over the origin of interstellar carbon chains was correct, the experiment should produce these molecules, he reasoned.
Smalley had other priorities; Kroto had to wait 17 months before he was invited back to try out his idea. When he did, however, the results were almost immediately successful. Kroto began his experiments on Sunday 1 September 1985, working alongside research students Jim Heath, Sean O'Brien and Yuan Liu. In the Angew. Chem. article, he recalls 'As the experiments progressed it gradually became clear that something quite extraordinary was taking place. As we varied 720 amu (corresponding to a C60 species) behaved in a most peculiar fashion. Sometimes it was completely off-scale; at other times it was quite unassuming'. The omnipresence of the cluster (which Kroto dubbed the 'Godwadge') was intriguing, but even more puzzling was the question of what it could be. During the group's discussions, a general consensus gradually emerged that it had to be a cage network of some kind.
Kroto's enthusiasm for design was to prove extremely useful. For him, the cage concept evoked memories of Buckminster Fuller's geodesic dome at Expo '67 in Montreal. 'I had actually been inside this remarkable structure at that time and remembered pushing my small son in his pram along the ramps and up the escalators, high up among the exhibition stands and close to the delicate network of struts from which the edifice is primarily constructed'. Even more significant was the memory of a polyhedral cardboard stardome that he had made for his children some years before. Kroto remembered that he had needed to cut out not only hexagons, but also pentagons, to assemble the structure.
Could it be that C60 was also made up of such units, Kroto wondered. His conjecture, which he discussed with S malley, was to prove correct. The model that Smalley produced soon afterwards 'seemed identical to the stardome as I remembered it' Kroto later recalled. Still, it took a mathematician to appreciate the true significance of C60, when he rang to 'tell Rick it's a soccer ball'.
The resulting paper, which appeared in the 14 November 1985 issue of Nature, caused a sensation. Within weeks dozens of papers had been published worldwide speculating on the properties and characteristics of C60. Not everyone was convinced though.
What Kroto needed was to extract enough C60 from the laser soot to characterise it fully in the laboratory. It would be what Kroto describes as 'five years in the desert' before that goal was realised, and to everyone's amazement the accolade was to go to two physicists.
Just days after Kroto's postdoctoral student Jonathan Hare succeeded in isolating a red solution, which he confidently labelled C60, Kroto received a call from the editor of the journal Nature, asking him to referee a paper by Krätschmer, Huffman and colleagues. The fax that arrived later that day confirmed his worst fears.
Not only had Krätschmer and Huffman obtained a red solution in benzene, but they had extracted beautiful orange-brown crystals of C60from it, and observed them under the microscope.
It was a bleak moment for Kroto. The irony was that, following his work with Ken McKay, he and Jeff Cloke had submitted funding proposals for a mass spectrometer to investigate in detail promising results obtained by exactly the same arc discharge method. 'What no one realised was that C60would be so abundant by this method - I thought I would need a more sensitive technique (mass spectrometry) to detect it. Had I been able to continue that experiment I would have realised that C60 was coming out of its ears, and we would have had that paper two year earlier'.
Despite such regrets, however, Kroto denies suggestions that the synthesis of C60 was a race: 'It was tough luck for Sussex, but philosophically I think "what the heck". Jonathan had extracted C60 before that paper arrived and I consider that one of the greatest pieces of science my group has done'. Moreover there was great consolation in the finding, with colleague Roger Taylor, that Hare's solution could be separated into a magenta solution of C60 and a red solution of C70. This allowed the group to prove conclusively the structure of C60 by NMR - 'an independent achievement that is very satisfying', Kroto says.
Nevertheless the race metaphor was hard to shake off. Talking about Horizon's exciting 1992 documentary Molecules with sunglasses Kroto says: 'The film is a retrospective view of what happened, it is not my view. I have often said that we lost a race that we did not know we were running in. I always like to think of Rashömon, a film directed by Akira Kurosawa, which basically describes a crime. You listen to each person describing their version of events and you wonder whether it's the same crime because they all have a different view of what happened'.
At least, however, Horizon succeeded in telling the story essentially as it was. Much of the other media coverage had little to do with the work actually done. Kroto recalls a story that appeared in his local Brighton Evening Argus, running under the headline 'Life's key may lie among the stars' (below which one of Kroto's students handwrote 'That's Showbiz' on his copy). This related how 'Sussex University boffins' had discovered evidence to suggest that 'the very first forms of life could have been created in outer space'. What these boffins had found, the story continued, was 'organic chemicals' which could be transformed into the 'building blocks of life' in only 'one simple step'.
Such misinterpretations are no laughing matter. 'People I know who read this paper think that this is the work I am actually doing', Kroto says. A similar article that made the headlines in 1991 prompted him to say 'Is a scientist to laugh or cry. He's made the front cover of The Times, but it has very little to do with his work. It is a serious problem, because these things that drive us are actually very difficult to explain to non-scientists'.
Scientific ignorance is not restricted to the uninterested layperson. A report in 10 December 1991 volume of Hansard concerned a debate on the future of buckyballs in the House of Lords, initiated by Lord Errol of Hale who asked Her Majesty's Government: 'What steps they are taking to encourage the use of buckminsterfullerene in science and industry? The debate rambled on for some time, amid growing confusion, until Baroness Seear felt it incumbent upon herself to ask, perplexedly, 'My Lords, forgive my ignorance, but can the noble lord say whether this thing is animal, vegetable or mineral?' Lord Reay provided an eloquent explanation, describing the football-like shape and constitution of this molecule 'known to chemists as C60 '. His response, however, had Lord Renton baffled: 'My Lords, is it the shape of a rugger football or a soccer football?', he enquired.
And so the debate went on, until at last Lord Campbell of Alloway ventured to ask that all-important question: 'My Lords, what does it do?', to which Lord Reay responded 'My Lord, it is thought it may have several possible uses: for batteries; as a lubricant; or as a semi-conductor. All that is speculation. It may turn out to have no uses at all. At that, Earl Russell finally had the last word: 'My Lords, can one say that it does nothing in particular and does it very well'. It is a sentiment with which Sir Harold Kroto might very well agree.
Source: Chemistry in Britain
- J. Baggott, Perfect symmetry: the accidental discovery of buckminsterfullerene. Oxford: OUP, 1996.
- H. Aldersey Williams, The most beautiful molecule. London: Aurum press, 1995.
- P. Ball, Designing the molecular world. US: Princeton University Press, 1994.
- H. W. Kroto, Chem. Br., January 1986, p 11.
- H. W. Kroto, Chem. Br., January 1990, p 40.
- H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl and R. E. Smalley, Nature (London), 1985, 318, 162.
- W. Krätschmer, L. Lamb, K. Fostiropoulos and D. Huffman, Nature (London), 1990, 347, 354.
- H. W. Kroto, Angew. Chem. Int. Ed. Engl., 1992, 31(2), 111.
A round up of carbon chemistry
The discovery of C60 , and shortly afterwards a whole series of other fullerenes, was greeted with near disbelief. Not until Huffman and Krätschmers' paper was published did chemists finally agree to rewrite the textbooks. From then on, they have had to rethink radically their ideas about carbon chemistry.
The first step was to try to understand the chemistry of these molecules: how do they react; what compounds can be made from them; and, equally important, what applications might they have? To begin with, chemists tried to draw analogies with conventional carbon chemistry. For C60 the obvious comparison was with the archetypal ring molecule benzene. It was a poor fit. Unlike the aromatic, stabilising, ring current of benzene, C60 was found to contain two opposing ring currents that cancel each other out. Rather, C60 is an electron deficient electron acceptor, which rapidly decomposes in the presence of light and trace amounts of ozone in the air.
On the face of it, this was good news. As a 'super alkene', chemists reckoned, C60 should be a precursor for a wealth of potentially useful compounds. The problem of targeting addition across a 60-atom structure was a challenge they couldn't resist.
Several groups set about the task of taming these reactions. The Sussex group focused on selective halogenations. Independently, researchers at DuPont also made significant advances in this area and the resulting halides became important intermediates in a variety of chemical reactions, paving the way for a whole series of fullerene derivatives based on substitutions of bromine by a variety of other groups such as phenyl and methoxy.
Work by John Holloway at the University of Leicester, along with several other groups, to produce selectively the potentially more readily substituted, low fluorine content, fluorofullerenes proved more difficult. Only recently have researchers begun to overcome some of the problems inherent in such reactions, with the publication earlier this year of a paper by Roger Taylor at Sussex and Olga Boltalina at Moscow State University, describing the preparation of C60F36-38.
Polymer chemists were quick to get in on the game. In the early 1990s chemists at Sandia in New Mexico successfully prepared the first C60 copolymer, by reacting the diradical xylylene with C60 . Although there are no foreseeable applications for such fullerene polymers in the short term, one polymer comprising C60 and Pd atoms is showing promise for catalysing hydrogenation reactions.
Organometallic chemists have also had a field day with the fullerenes, producing osmium, platinum and iridium derivatives among others. And while conventional chemistry might have limited them to attaching metals to the outside, the unique spherical shape of the fullerenes has also enabled them to prepare a series of metal-encapsulating derivatives, including La @ C60 , La @ C82 and Sc2 @ C84 . Work is also under way to try to encapsulate small molecules such as carbon monoxide and there is speculation about the possibility of using such cages as delivery vehicles for drugs and radioactive nucleides, or as miniature sensors.
There is an interesting reverse scenario. Researchers at the University of California at Santa Barbara led by Fred Wudl have discovered that the soluble fullerene derivative di(phenylethylaminosuccinate) fulleroid inhibits the enzyme HIV-1 protease, a key enzyme in the human immunodeficiency virus, by sitting snugly in the enzyme's active site. Even more startling, researchers at Emory University in Atlanta have since found that the same derivative is also active against another HIV enzyme, reverse transcriptase. Unlike the anti-Aids drug AZT, which is only effective against acutely infected cells, the C60 derivative also inhibits reproduction of HIV in cells chronically infected with HIV.
But perhaps the biggest surprise of the newly emerging field of fullerene chemistry was the finding, by Robert Haddon and colleagues at AT & T Bell Laboratories (Chem. Br., September 1994, p 746), that alkali metal salts of C60 are superconductors. In particular, the group reported in a March 1991 issue of Nature, the potassium fulleride salt K3C60 has the high superconducting transition temperature (Tc) of 18 K. Soon afterwards, Katsumi Tanigaki in Japan raised the Tc for superconducting fullerides even higher, with Cs2RbC60 having a Tc of 33 K. The current record, set by Otto Zhou and colleagues of Bell Laboratories in the US, stands at 40 K for Cs3C60 - ever nearer to the 77 K target desirable for commercial applications. Theoretically, the potential of such superconductors is enormous - with possible uses ranging from Maglev trains to MRI magnets. Before fullerene superconductors become commercially viable, however, chemists must also overcome problems of reactivity in air; K3C60 is pyrophoric and the thin films quickly lose their conductivity. Nevertheless, there is growing confidence that chemical modification will lead to more robust materials that may find some novel applications as lightweight electric motors and electromagnets.
Equally promising is the potential of so-called bucky tubes - elongated pipelines of hexagonal carbon faces capped at both ends with the requisite 12 pentagons needed for curvature. Physicist Roger Bacon first observed these unusual 'carbon whiskers' in the 1960s, but it was Sumio Iijima in Japan who first appreciated their significance in terms of fullerene structures in 1991. Others have also been quick to appreciate their potential. Carbon nanotubes are predicted to be stronger than any known material (even diamond), with potential applications in numerous nanoscale architectures such as in microelectronics. In 1992 Iijima's NEC colleagues Thomas Ebbesen and Pulickel Ajayan brought this prospect a step closer by perfecting a way of making bulk quantities of single-walled nanotubes. Using this method, Iijima and Ajayan later found a way of filling the tubes with molten lead through a mechanism not unlike that of capillary action. The possibility of using such filled tubes as molecular scale wires is attracting considerable interest, fuelled by the observation that at below about 7 K lead becomes superconducting.
And the explosion of interest in fullerene chemistry does not stop there. Chemists at DuPont, for example, have recently produced even more novel forms of carbon: carbon 'sea urchins' comprising central cores of gadolinium carbide from which radiate carbon nanotubes; and 'nanoworms' made of a palladium crystal head and a segmented tail of carbon tubes.
Like their counterparts in the world of conventional carbon chemistry, some of these fullerene compounds have also been found to occur as isomers. The first example of a chiral fullerene, C76, was reported in 1991 - a joint effort between Patrick Fowler and David Manolopoulos at the University of Nottingham and François Diederich and Robert Whetten at the University of California in Los Angeles. Shortly after Manolopoulos had predicted the chirality of C76, Diederich produced the corresponding 19-line 13C NMR spectrum. Since then chemists have discovered one chiral form of C78 and have found that C84 occurs as two different forms, though these have yet to be isolated.
But perhaps even more surprising is the revelation that C60 has been around on earth since the end of the Mesozoic era - formed among the black soot that enveloped the earth as a result of whatever global catastrophe wiped out the dinosaurs 65 million years ago. The question is of course how it got there.
Recent investigations of meteorites are providing us with some clues. Since 1994, researchers claim to have been observing the characteristic signatures of C60+ and C70+ in meteorites, and earlier this year researchers at Scripps Institute of Oceanography in California reported finding C60 entrapped He in a 2000m year-old meteor impact crater at Sudbury in Ontario. The ratios of He-3 to He-4 in these samples match the interstellar ratios found in meteorites and interplanetary dust particles, lending support to the extraterrestrial origins of these organic molecules.
The other explanation (though the isotopic ratios are inconsistent) is that the fullerenes may have been formed during the intense heat resulting from impact. Whatever the answer, making fullerenes is a lot simpler than we first thought. Forget the expensive lasers, or even the bell jar apparatus. More recent research shows that we have probably all made C60 in the soot of a Bunsen burner flame at some time or another - only its reactivity with air has prevented us from finding it any sooner .