Portal de Periódicos da CAPES

The Legacy of Sir Harold W. Kroto: Fullerenes and Beyond

2019; Elsevier BV; Volume: 5; Issue: 4 Linguagem: Inglês

10.1016/j.chempr.2019.03.015

2451-9308

Nazario Martı́n,

Nanotechnology research and applications

Professor Sir Harold W. Kroto, one of the discoverers of fullerenes (the first molecular allotrope of carbon), passed away in 2016 at the age of 76. This paper is dedicated to him, one of the most groundbreaking scientists who applied his artistic vocation to science. Harry W. Kroto was born in Wisbech, a small town of Cambridgeshire in England, on October 7, 1939, just 1 month after the start of the Second World War. However, despite these circumstances, Kroto’s parents always looked after the education of their son, who always showed great interest in art and science and, particularly, chemistry. He studied chemistry at Sheffield University and carried out his doctorate thesis at the University of Sussex (1961–1964). After postdoctoral positions at the National Research Council in Ottawa, Canada, and a further year at Bell Laboratories in New Jersey (1966–1967), he became an expert in molecular spectroscopy, which was eventually critical for the discovery of C60. Kroto was always one of those men concerned by everyone else. Because of this, he became very popular by having a great number of friends inside and outside his profession. Actually, he was appointed “Sir” by Queen Isabel II in 1996. In 2004, Kroto retired and moved from Sussex to become the Francis Eppes Professor of Chemistry at the University of Florida, where he worked until 2015, when he moved back to London. On April 30, 2016, Sir Harold W. Kroto, one of the greatest scientists of the last decades, passed away in the historical city of Lewes in the county of East Sussex, England, at the age of 76. Kroto was ahead of his time, and he gave us some clear messages to take into account. In 1996 he claimed that “without scientific education, mankind would not last [beyond the 21st] century.” Iconoclastic and disruptive and a firm believer in science and in favor of dispelling myths, he defined himself as a decent man who was not worried about the way in which he would like to be remembered in the future. No doubt, Harry Kroto will be remembered for his important legacy in science as well as his infinite humanism.Without scientific education, mankind would not last [beyond the 21st] century. The discovery of fullerenes in 1985 by Robert Curl and the late Richard E. Smalley, as well as their then students Jim Heath and Sean O’Brien, represented a scientific revolution not only in the chemistry of carbon but, most importantly, also in global science. Actually, this discovery is considered one of the landmarks in the development of nanoscience and nanotechnology.The discovery of fullerenes is one of those famous examples of serendipity in science. The discovery of fullerenes is one of those famous examples of serendipity in science. Fullerenes were discovered during carbon nucleation studies simulating the experimental conditions found in red giant stars. Just 11 years later, in 1996, the aforementioned scientists received the Nobel Prize in Chemistry for “the discovery of fullerenes.”1Kroto H.W. Symmetry, space, stars, and C60.Angew. Chem. Int. Ed. 1997; 36: 1578-1593Crossref Scopus (80) Google Scholar One year earlier, the new molecule C60 had been declared the “molecule of the year” by the scientific journal Science. In addition to the two known allotropes of carbon, diamond (constituted by sp3Iijima S. Helical microtubules of graphitic carbon.Nature. 1991; 354: 56-58Crossref Scopus (39514) Google Scholar carbon atoms) and graphite (formed by sp2Kroto H.W. Heath J.R. O’Brien S.C. Curl R.F. Smalley R.E. C60: Buckminsterfullerene.Nature. 1985; 318: 162-163Crossref Scopus (14011) Google Scholar carbon atoms), that show reticular 3D structures, a new allotrope emerged in 1985 with the advent of fullerenes. It became the third and only molecular allotropic form of carbon, and it is formed by highly symmetric closed cages containing a precise number of carbon atoms.2Kroto H.W. Heath J.R. O’Brien S.C. Curl R.F. Smalley R.E. C60: Buckminsterfullerene.Nature. 1985; 318: 162-163Crossref Scopus (14011) Google Scholar After the finding of fullerenes, other forms of carbon were discovered; in chronological order, the most famous and well studied are multi-wall3Iijima S. Helical microtubules of graphitic carbon.Nature. 1991; 354: 56-58Crossref Scopus (39514) Google Scholar and single-wall4Iijima S. Ichihashi T. Single-shell carbon nanotubes of 1-nm diameter.Nature. 1993; 363: 603-605Crossref Scopus (7733) Google Scholar, 5Bethune D.S. Kiang C.H. de Vries M.S. Gorman G. Savoy R. Vazquez J. Beyers R. Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layer walls.Nature. 1993; 363: 605-607Crossref Scopus (3575) Google Scholar carbon nanotubes and graphene,6Novoselov K.S. Geim A.K. Morozov S.V. Jiang D. Zhang Y. Dubonos S.V. Grigorieva I.V. Firsov A.A. Electric field effect in atomically thin carbon films.Science. 2004; 306: 666-669Crossref PubMed Scopus (52498) Google Scholar and all have significantly modified the scenario of the so-called carbon nanostructures. Other less-common nanoforms of carbon—such as nanohorns, nanoonions, nanotori, nanobuds, and peapods—have also been found or synthesized, and their properties and chemical reactivity have been studied less so far.7Delgado J.L. Herranz M.A. Martín N. The nanoforms of carbon.J. Mater. Chem. 2008; 18: 1417-1426Crossref Scopus (206) Google Scholar Furthermore, pristine empty fullerenes have been formally combined with other elements allocated in their inner cavity, giving rise to an already huge and less-common type of molecules known as endohedral fullerenes.8Akasaka T. Nagase S. Endofullerenes: A New Family of Carbon Cluster. Kluwer, 2002Crossref Google Scholar All known fullerenes are closed-cage carbon allotropes containing 2 (10 + H) carbon atoms, where H is the number of hexagons, or the number of pentagons fixed to 12. The simplest and most abundant is C60, which has 60 carbon atoms distributed in 12 pentagons and 20 hexagons, followed by C70. Fullerene C60 has icosahedral symmetry with a diameter of 7.8 Å. In contrast to diamond and graphite, which are sparingly soluble in organic solvents, fullerenes are soluble in some organic solvents. They undergo a variety of chemical reactions in solution to afford a huge number of fullerene derivatives, which generally preserve the original chemical, physical, electrochemical, and photophysical properties of pristine fullerenes. An important landmark in fullerene science occurred when Wolfgang Krätschmer and Donald Huffman (both astrophysicists) obtained [60]fullerene in multigram amounts in 1990.9Krätschmer W. Lamb L.D. Fostiropoulos K. Huffman D.R. Solid C60: a new form of carbon.Nature. 1990; 347: 354-358Crossref Scopus (7097) Google Scholar This important finding paved the way for the chemical modification of fullerenes to go beyond a simple academic curiosity. The significance of this event was noted by Nobel Laureate Richard E. Smalley, who claimed that “had there not been a method to make it in measurable amounts, it would not have had an impact.” In addition, Nobel Laureate Robert Curl considered this scientific contribution by saying, “Huffman's work took it from mass spectrometers to the laboratory. It must have been a close decision by the Nobel Committee over who should get it.” It is important, however, to go one step back in the history of fullerenes and mention some important related events. In this regard, the possible existence of the C60 molecule was mentioned 15 years before its discovery by Eiji Osawa from Kyoto University. However, Kroto was not aware of this work before the discovery, partly because of its publication in Japanese. Nevertheless, Kroto gave credit to this Japanese scientist afterward. The existence of cages formed by carbon atoms was predicted by David E.H. Jones in 1966. He claimed that large empty carbon cages could be formed from a planar net of hexagonal carbons (a vision of graphene?) by the addition of impurities. However, he never explained how it could be performed. Remarkably, in situ transmission electron microscopy experiments supported by quantum chemical calculations have shown that graphene sheets undergo a direct transformation into fullerene cages under an 80-keV electron-beam irradiation.10Chuvilin A. Kaiser U. Bichoutskaia E. Besley N.A. Khlobystov A.N. Direct transformation of graphene to fullerene.Nat. Chem. 2010; 2: 450-453Crossref Scopus (292) Google Scholar Finally, other romantic chemists have seen the first molecular modeling of C60 in Leonardo da Vinci’s illustration of the truncated icosahedron coined “Ycocedron Abscisus Vacuus” in Luca Pacioli’s book De Divina Proportione, published in Venice in 1509. [60]Fullerene is not only the first obtained fullerene but also the most abundant and studied of all of them. A former and intriguing question was why the cage containing 60 atoms is favored among the many possible cages. Furthermore, among the 1,812 possible isomers that can be formed from 60 carbon atoms, why is only the icosahedral symmetry Ih-C60 molecule (soccer-ball shape) formed? These questions were answered by Kroto, who found that the local strain increases with the number of bonds shared by two pentagons (pentalene unit), leading to significantly less-stable molecules. This rule, known as the isolated pentagon rule (IPR), states that all pentagons must be surrounded by hexagons, which thus forms the well-known corannulene moiety.11Kroto, H.W. (1987). Nature 329, 529–531.Google Scholar The resonance destabilization resulting from the adjacent pentagons (8π electrons that do not satisfy the Hückel rule) and the reduction of the overlapping π-orbital as a result of the cage curvature account for the poorer stability of the so-called “non-IPR fullerenes.” A head-to-tail exclusion rule has also been proposed to account for the higher stability of those fullerenes obeying the IPR. For a defined number of carbon atoms forming a cage, the number of theoretically possible non-IPR fullerene isomers is amazingly larger than those obeying the IPR. Furthermore, in addition to doubly fused pentagons found in the simplest non-IPR fullerenes, triply directly fused pentagons and triply sequentially fused pentagons (as well as quadruply fused pentagons) have also been described. Considering the aforementioned aspects, there is a lot of interest in the potential huge number of possible non-IPR fullerenes whose chemical reactivity and properties are significantly less explored. It is interesting to remark that carbon cages in endohedral fullerenes are different from those observed in empty fullerenes. Currently the two main strategies for increasing the stability of less-stable non-IPR fullerenes consist of endohedral and exohedral chemical derivatization. The ultimate goal of both approaches is the stabilization of the non-IPR fullerenes by means of releasing the strain of the fused pentagons. In 2010, two groups simultaneously reported the first example of the higher stability of a non-IPR fullerene than of its related IPR isomer for exohedrally functionalized C72Cl4.12Tan Y.-Z. Zhou T. Bao J. Shan G.-J. Xie S.-Y. Huang R.-B. Zheng L.-S. C72Cl4: a pristine fullerene with favorable pentagon-adjacent structure.J. Am. Chem. Soc. 2010; 132: 17102-17104Crossref Scopus (49) Google Scholar, 13Ziegler K. Mueller A. Amsharov K.Y. Jansen M. Disclosure of the elusive C2v-C72 carbon cage.J. Am. Chem. Soc. 2010; 132: 17099-17101Crossref Scopus (41) Google Scholar Surprisingly, these experimental findings violate the “universal” IPR for fullerenes. However, these results confirm the validity of “strain-release” and “local-aromaticity” principles in predicting the stability of fullerene derivatives. The singular electronic properties of fullerenes—namely remarkable electron accepting ability, low reorganization energy, outstanding electrochemical and photophysical properties, and solubility in organic solvents—are reasons enough that fullerenes are considered appealing materials for application in a variety of electronic devices. In particular, their outstanding performance in organic photovoltaics14Meng L. Zhang Y. Wan X. Li C. Zhang X. Wang Y. Ke X. Xiao Z. Ding L. Xia R. et al.Organic and solution-processed tandem solar cells with 17.3% efficiency.Science. 2018; 361: 1094-1098Crossref PubMed Scopus (2031) Google Scholar and a variety of bio-medical applications has also been reported in recent years.15Muñoz A. Sigwalt D. Illescas B.M. Luczkowiak J. Rodríguez-Pérez L. Nierengarten I. Holler M. Remy J.S. Buffet K. Vincent S.P. et al.Synthesis of giant globular multivalent glycofullerenes as potent inhibitors in a model of Ebola virus infection.Nat. Chem. 2016; 8: 50-57Crossref Scopus (218) Google Scholar Soon after the discovery of the fullerene C60, the encapsulation of chemical species in the inner cavity was a challenge that eventually became a reality that is currently one of the most active fields in fullerene science. As expected, the properties of endohedral fullerenes strongly depend on the chemical species encapsulated in the inner cavity. As representative examples, endohedral fullerenes synthesized by “molecular surgery” are shown in the last image in this article.16Krachmalnicoff A. Bounds R. Mamone S. Alom S. Concistrè M. Meier B. Kouřil K. Light M.E. Johnson M.R. Rols S. et al.The dipolar endofullerene [email protected]60.Nat. Chem. 2016; 8: 953-957Crossref Scopus (137) Google Scholar As previously mentioned, the synthesis of endo- and exohedrically functionalized non-IPR fullerenes is currently a synthetic challenge. Considering the potential number of non-IPR fullerenes, the revival of fullerenes could become a reality and have a strong impact on new technologies on the basis of these new carbon nanoforms. Thus, Kroto’s legacy goes beyond the discovery of fullerenes, and it could be considered one of the landmarks of nanoscience. No doubt, the scientists who discovered the first molecular carbon settled down a spotlight for the coming generations of scientists.Chemical structure of C60, a single-walled carbon nanotube, and graphene.View Large Image Figure ViewerDownload Hi-res image Download (PPT)The isolated pentagon rule (IPR) states that stable fullerenes are those in which all pentagons must be surrounded by hexagons, similarly to a soccer ball.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Representative examples of endohedral [60]fullerenes prepared by molecular surgery.View Large Image Figure ViewerDownload Hi-res image Download (PPT) N.M. thanks his magister, friend, and colleague Harry Kroto for providing him with a new and singular research field where many scientists have dedicated a great amount of time to enjoying this unprecedented spherical molecule that eventually paved the way to the science of nanocarbons.