Science researchers at Memorial have developed the first X-ray structural evidence that C60-fullerene can co-crystallize with corannulene. This finding could prove to be significant to those studying the assembly of fullerene into other arrays for energy applications, such as photovoltaic cells for solar energy, or into fullerene-based vehicles for drug delivery.
Dr. Paris Georghiou of Memorial’s Department of Chemistry has been working with C60-fullerene or “buckyballs”, a spherical molecule composed of sixty carbon atoms. The molecules are the namesake of the late Buckminster Fuller, who has been recognized for his architectural designs involving geodesic domes.
Corannulene is an important structural fragment of a buckyball. It is bent into a bowl shape and is sometimes called a buckybowl. This particular component has held a lot of interest for theoretical and synthetic chemists with many working on different aspects and uses for it.
“Together with Professor Larry Scott and some of his students at Boston College in the United States who have pioneered synthetic work involving corannulene, we have been trying since the ’90s to determine whether corannulene could form complexes with fullerene through supramolecular bonds, but we couldn’t get any direct evidence,” explained Dr. Georghiou. “Supramolecular bonds are a “third type” of chemical bond which are neither covalent nor ionic.”
Later attempts conducted by HuuAnh Tran, a former graduate student of Dr. Georghiou’s, were also inconclusive. However, the reaction mixtures he created to test the hypothesis were meticulously catalogued and retained, and a number of years later while looking through that student’s samples for something unrelated, Dr. Georghiou noticed that red crystals had formed.
Dr. Louise Dawe, a crystallographer with the Department of Chemistry and the Centre for Chemical Analysis, Research and Training (C-CART), studied the crystals and was successful in determining an X-ray model of the structure, but noted the data quality was poor. Other attempts at data collection on this batch of crystals failed to yield a more satisfactory result.
“It wasn’t easy to get these complexes to crystalize in the first place,” explained Dr. Georghiou. “It took six or seven years and then time to study them and get information back and yet at the end of the day there was nothing that could be published."
Still, Dr. Georghiou says there is no doubt as to what the results indicated.
Fortunately, crystals from another sample, prepared this time by Tayel Al Hujran, a current graduate student of Dr. Georghiou’s, provided convincing data for a paper that was recently accepted in Chemical Communications, a prestigious journal of the Royal Society of Chemistry (Chem. Commun., 2012, 48, 5563-5565). These crystals, however, presented another problem.
“Because they’re spherical, it doesn’t matter how you twist or turn it, they still occupy the same volume,” said Dr. Dawe. “So unless they are frozen in one orientation through intermolecular interactions with some other type of molecule, they twist and turn in every possible orientation.”
To come up with a satisfactory model of the fullerene, Dr. Dawe called upon the skills of Jason Mercer, a graduate student in the Department of Computer Science.
“Dr. Dawe was able to find strong signatures of where many carbons might be,” explained Mr. Mercer. “She was able to determine which ones were part of a buckyball at one particular orientation, while others were a buckyball in another orientation.”
He then used Monte Carlo methods, a class of computational algorithms that rely on repeated random sampling, to calculate where the missing atoms would fit with respect to the atoms they had already identified and was able to use this technique to shift the model into position.
“It had been visualized through theoretical modelling by many people, but getting the experimental evidence of what has been predicted by theory, in reality, is the big deal here,” said Dr. Dawe.
Based on their paper, the researchers were invited to do the cover illustration for the journal. This work showed the power of a multi-disciplinary and collaborative approach that was successful.