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Home , Fullerene chemistry, Supramolecular chemistry

thermal treatment of 1,6-fullerenynes (1) affords cyclobutene adducts (3) without the presence of a catalyst; in a reaction this is the first example of a thermal [2+2] cyclization involving a double bond as the moiety of the reactive 1,6-enyne7 (Fig. 1). A recent example of an unknown chemical reactivity has been found in fulleropyrrolidines, which are among the most studied fullerene derivatives used for many applications in as well as in the search for biological properties.8 In contrast to Advances in Molecular other labile fullerene cycloadducts such as those prepared from Diels-Alder or and Supramolecular Bingel reactions,1 fulleropyrrolidines have been considered to be stable fullerene derivatives. However, the Fullerene thermal quantitative retro- of fulleropyrrolidines to obtain the by Nazario Martín, Nathalie Solladié, and Jean-François Nierengarten pristine fullerene together with its typical magenta color in solution (Fig. 2) n 1996 Sir Harold W. Kroto, Robert discipline, a wide variety of important has been reported only recently.9 This F. Curl, and the late Richard E. reactions involving and reaction reveals that the understanding I Smalley received the for have not been studied previously on of the reactivity of fullerene derivatives the discovery of the . After the fullerene surface. One example is still far from the level where it is a decade, these round-shaped carbon of the most successful reactions in possible to predict the reaction pathway allotropes still are a major hot topic is the Pauson- reliably. This reaction constitutes a in chemical research and additional Khand (PK) reaction which has been protection-deprotection protocol which findings are continuously being reported used extensively for the construction has allowed, for example, the efficient for applying these three-dimensional of biologically active five-member separation of two isomers (Ih and D5h) of for practical purposes.1 carbocycles in a convergent approach. It endohedral Sc3N@C80. Since the discovery of the fullerenes involves the [2+2+1] cycloaddition of an One of the most successful in 1985,2 other important breakthroughs , an alkene, and carbon monoxide applications of fullerenes is related to in fullerene science occurred in 1990,3 mediated or catalyzed by a transition electron/energy transfer processes.10 when Wolfgang Krätschmer and Donald . Recently, this reaction has been Photoinduced electron transfer is a carried out on fullerene C60 acting as fundamental process in nature because Huffman prepared the fullerene, C60, for the first time in multigram amounts, the alkene component; a highly efficient it governs photosynthesis in plants and thus starting in motion, the chemical and regioselective intramolecular PK bacteria. This process occurs through modification advances and, therefore, reaction has afforded a type of fullerene rapid charge separation at the reaction the preparation of sophisticated derivative (2) showing three (or five) centers with quantitative quantum , fullerene architectures. Iijima made fused pentagonal rings on the same thus enabling the transformation of 6 another major development in 1991 hexagon of the fullerene surface. sunlight into chemical energy.11 with the discovery of carbon nanotubes The transition metal catalyzed In contrast to other well- (NT),4 a perfect material to be used cyclization of enynes represents an known electron acceptors such as in the emerging active research area which has been p-benzoquinone derivatives used by discipline. The above outstanding studied extensively as a powerful nature in the photosynthetic process, findings received scientific and social method for the construction of carbo- the C60 accelerates charge cachet in 1996 when Kroto et al. were and heterocyclic molecules. The unique separation during photoinduced electron awarded the .5 geometry of fullerenes has allowed us to transfer processes to form a charge This short article highlights some of observe a different chemical reactivity separated state and simultaneously the more remarkable advances occurring to that found for related 1,6-enynes. (Continued on next page) during the last few years in fullerene Recently, it was reported that the science and, particularly, on modified fullerenes related to molecular and H R supramolecular aspects. H COOEt O R R N N Recent Advances in Molecular N O n R1 Chemistry of Fullerenes R1 Unlike most organic molecules, the convex surface of fullerenes offers possibilities for the study of reactions and mechanisms under severe geometrical constraints. Although the chemistry of fullerenes is nowadays 2 1 3 considered to be an established FIG. 1. 1,6-Fullerenynes (1) are versatile building blocks.

The Electrochemical Society Interface • Summer 2006 29 Martín, et al. (continued from previous page) slows the charge recombination process in the absence of light. This behavior has been rationalized by the smaller reorganization energy (λ) of 12 C60 compared with other acceptors. These unique electrochemical and photophysical properties of fullerenes have allowed the design of a wide variety of donor-acceptor systems in which the electroactive units are connected by covalent or supramolecular bonds.13 [Editor’s note: This aspect is also addressed in the third feature article in this issue.] To improve the charge separation processes in artificial photosynthetic model systems, photoinduced electron/energy transfer from suitable electron donors, namely, , phthalocyanins, tetrathiafulvalenes FIG. 2. Pristine fullerene C60 can be recovered quantitatively from (TTFs), ferrocene, and other the corresponding fulleropyrrolidine. metallocenes, amines, or π-conjugated oligomers, and , to electron accepting fullerenes have been studied.14

The design of C60-based photosynthetic mimics has been directed to the formation of long-lived charge separated states in a highly efficient way. For this purpose, the different factors which influence the competition between energy vs. electron transfer have been analyzed, and factors such as nature of the photo- and electroactive units, , donor-acceptor distance, molecular topology in which donor and acceptor are connected and arranged in space, and the nature of the spacer connecting them, have a strong influence on the photophysical results. Unusually slow charge recombination dynamics has been reported in some molecular dyads which, despite the proximity between the donor and acceptor units, appear to FIG. 3. Molecular modeling of photo- and electroactive exTTF-oPPV-C60 (top) and ZnP-oPPV-C60 (bottom) triads. be electronically isolated due to their singular geometry.14 As a representative example, lifetimes of 230 µs in C12H25O benzonitrile at 25°C, an exceptionally long-lived intermediate for a simple O C12H25O O molecular dyad, have been reported O recently.15 O Recently, Martín and co-workers have shown the molecular wire behavior of NH CF CO oligo-p-phenylenevinylene (oPPV) as 3 3 2 π-conjugated oligomers in a series of supermolecules of the type C60-oPPV- donor [donor = TTF or zinc porphyrinate (ZnP)] ranging from the monomer to the heptamer (Fig. 3). Photophysical NH N studies revealed an exceptionally low O O N HN attenuation factor (β ~ 0.01 Å-1) and a O O strong electronic coupling element (V ~ O O 5.5 cm-1) even at distances as large as 5 nm.16 These examples clearly show the high FIG. 4. In addition to the -crown recognition, π-stacking interactions between the C60 sphere and the moiety have been observed in a supramolecular complex obtained from a versatility of fullerenes for the study porphyrin- conjugate and a fullerene derivative bearing an ammonium unit. (continued on page 32)

30 The Electrochemical Society Interface • Summer 2006 C60

C60 + + "click" + +

OR RO

O RO OR O O O O O O O O O O O O O

O OR RO

O O O O O O O O O O O O H3N NH3

R = C8H17 CF3CO2 CF3CO2 R = C12H25

FIG. 5. Schematic representation of the supramolecular principle and calculated structure of the clicked supramolecular complex resulting from the association of a bis-crown ether derivative and a bis-ammonium fullerene .

0,9 y t i s n e t n e I 0,6

v i e t c a l n e

R 0,3 oresce u l F 0,0 600 650 700 750 Wavelength (nm)

FIG. 6. A fullerene derivative bearing two pyridine subunits has been designed to allow the assembly of a macrocyclic clicked edifice with a bis-Zn(II)- phorphyrinic receptor, the K values deduced from the emission data reveal an increased stability of the macrocyclic complex as a result of the simultaneous coordination of the two Zn centers by the two pyridine moieties.

The Electrochemical Society Interface • Summer 2006 31 Martín, et al. (Fig. 4).25 In this particular case, the the formation of a single complex (continued from page 30) association constant (Ka) for the complex with log K1 = 5.09 ± 0.07. The sizeable is increased by two orders of magnitude increase of stability compared to that of artificial photosynthetic systems when compared to Ka values found of C60 derivatives bearing a single in which it is possible to control the previously for the complexation of the pyridine moiety and metalloporphyrins lifetime of the photogenerated charge same ammonium cation with other through coordination to the metal separated states by different strategies. crown ether derivatives. More recently, emphasized the peculiar structure the preparation and the characterization of the complex in which the bis- Recent Advances in Supramolecular of a stable supramolecular complex pyridyl fullerene substrate is clicked Chemistry of Fullerenes obtained from a bis-crown ether receptor on the ditopic receptor (Fig. 6). Similar In 1987 Jean-Marie Lehn together and a bis-ammonium fullerene ligand increases in the association constants with two other supramolecular have been described.26 Owing to the have been reported by D’Souza and chemistry pioneers, Donald J. Cram perfect complementarity of the two co-workers for supramolecular systems and Charles J. Pedersen, received components, the bis-cationic substrate resulting from the axial coordination the Nobel Prize for the development can be clicked on the ditopic crown of the C60-pyridine ligand to Zn(II)- and utilization of molecules with ether derivative thus leading to a non- porphyrins and possessing an additional highly selective structure-specific covalent macrocyclic 1:1 complex (Fig. recognition element.29 interactions. 5). Concluding Remarks and fullerenes crossed each other to Comparison with thermodynamic give rise to an interdisciplinary field in data available in the literature for Fullerenes have shown a wide which the imagination of has closely related systems (crown ether range of unique physical and chemical facilitated the design and construction and ammonium derivatives) shows properties that make them attractive for of unprecedented fullerene-based coordination stronger by more than the preparation of advanced materials. 17 supramolecular architectures. three orders of magnitude. This is Applications encompass a wide range of 18 19 Examples are , , associated mainly to the two-center technologies and many developments self-assembled coordination host-guest topography. Similar to the rely on new materials and our abilities 20 compounds, liquid crystalline click chemistry concept of Sharpless,27 to understand their intimate properties. 21 inclusion complexes, and photoactive the supramolecular click chemistry Modern organic synthesis is the starting 22 supramolecular devices. principle described herein is a powerful point of this research, as compounds As part of this research, and selective process for the preparation must first be made and studied and then supramolecular donor-fullerene systems of stable macrocyclic non-covalent modified to improve their properties toward projected applications. Novel based on the self-assembly of C60 arrays. This approach is modular and derivatives bearing an ammonium appears applicable to a wide range of synthetic methodologies for the unit with crown have been functional groups for the preparation preparation of new fullerene derivatives reported.23,24 The ammonium- of supramolecular architectures with have still to be discovered and remain crown ether interaction itself is weak. tunable structural and electronic a key step toward the elaboration of Consequently, the binding constants are properties. In particular, Trabolsi et al. fullerene-based molecules and materials low and only a small fraction of the two have shown that supramolecular click displaying unique properties. Despite components are effectively associated chemistry is perfectly suited for the some remarkable recent achievements, in solution. This prompted several preparation of a stable non-covalent clearly, the examples discussed above groups to design systems with additional fullerene-porphyrin hybrid system.28 represent only the tip of the iceberg recognition elements to increase the The bis-Zn(II)-phorphyrinic receptor in the design of fullerene-based binding constants. For example, π- has been selected as a platform with two molecular assemblies which can display functionality at the macroscopic level. stacking interactions between the C60 equivalent Zn binding sites separated by sphere and the porphyrin moiety have about 20 Å. A fullerene ligand bearing More research in this area is needed to been evidenced in a supramolecular two pyridine subunits has been designed fully explore the possibilities offered array obtained from a porphyrin-crown to allow the assembly of a macrocyclic by these materials, for example, in ether conjugate and a fullerene clicked edifice with the bis-porphyrinic nanoscience or in photovoltaics.  derivative bearing an ammonium unit receptor. Binding studies revealed

References 1. For some recent books: (a) A. Hirsch, 4. S. Iijima, Nature, 354, 56 (1991). 9. N. Martín, M. Altable, S. Filippone, The Chemistry of Fullerenes, Wiley-VCH, 5. Nobel Lectures: (a) R. E. Smalley, Angew. A. Martín-Domenech, L. Echegoyen, and Weinheim, Germany (2005); (b) Fullerenes: Chem. Int. Ed., 36, 1594 (1997); (b) C. M. Cardona, Angew. Chem. Int. Ed., 45, From Synthesis to Optoelectronic Properties, H. W. Kroto, Angew. Chem. Int. Ed., 36, 110 (2006). D. M. Guldi and N. Martín, Editors, Kluwer 1578 (1997); (c) R. F. Curl, Angew. Chem. 10. (a) Special issue on Functionalised Fullerene Academic Publishers, Dordrecht, The Int. Ed., 36, 1566 (1997). Materials, M. Prato and N. Martín, Editors, Netherlands (2002); (c) R. Taylor, Lecture 6. (a) N. Martín, M. Altable, S. Filippone, and J. Mater. Chem., 12, 1931 (2002); (b) D. Notes on : A Handbook A. Martín-Domenech, Chem. Commun., Gust, T. A. Moore, and A. L. Moore, Acc. for Chemists, Imperial College Press, 1338 (2004); (b) N. Martín, M. Altable, Chem. Res., 34, 40 (2001). London (1999); (d) S. Reich, C. Thomsen, S. Filippone, A. Martín-Domenech, 11. Energy Harvesting Materials, D. I. Andrews, and J. Maultzsch, Carbon Nanotubes: Basic A. Poater, and M. Solá, Chem. Eur. J., 11, Editor, World Scientific Publishing Co., Concepts and Physical Properties, Wiley-VCH, 2716 (2005). Singapore (2005). Weinheim, Germany (2004). 7. N. Martín, M. Altable, S. Filippone, 12. D. M. Guldi, Chem. Commun., 321 (2000). 2. H. W. Kroto, J. R. Heath, S. C. O’Brien, A. Martín-Domenech, M. Güell, and 13. N. Martín, L. Sánchez, B. Illescas, and R. F. Curl, and R. E. Smalley, Nature, 318, M. Solá, Angew. Chem. Int. Ed., 45, 1439 I. Pérez, Chem. Rev., 98, 2527 (1998). 162 (1985). (2006). 14. A. Harriman, Angew. Chem. Int. Ed., 43, 3. W. Krätschmer, L. D. Lamb, 8. (a) M. Prato and M. Maggini, Acc. Chem. 4985 (2004). K. Fostiropoulos, and D. Huffman, Nature, Res., 31, 519 (1998); (b) N. Tagmatarchis 15. K. Ohkubo, H. Kotani, J. Shao, Z. Ou, 347, 354 (1990). and M. Prato, Synlett, 768 (2003). K. M. Kadish, G. Li, R. K. Pandey,

32 The Electrochemical Society Interface • Summer 2006 M. Fujitsuka, O. Ito, H. Imahori, and and L. Echegoyen, J. Am. Chem. Soc., 126, Hahn, A.-M. Albrecht-Gary, and S. Fukumuzi, Angew. Chem. Int. Ed. Engl., 3388 (2004). J.-F. Nierengarten, Chem. Eur. J., 11, 4793 43, 853 (2004). 19. (a) P. R. Ashton, F. Diederich, M. Gómez- (2005). 16. (a) F. Giacalone, J. L. Segura, N. Martín, López, J.-F. Nierengarten, J. A. Preece, 25. N. Solladié, M. E. Walther, M. Gross, and D. M. Guldi, J. Am. Chem. Soc., 126, F. M. Raymo, and J. F. Stoddart, Angew. T. M. Figueira Duarte, C. Bourgogne, and 5340 (2004); (b) F. Giacalone, J. L. Segura, Chem. Int. Ed. Engl., 36, 1448 (1997); J.-F. Nierengarten, Chem. Commun., 2412 N. Martín, J. Ramey, and D. M. Guldi, (b) Y. Nakamura, S. Minami, K. Iizuka, and (2003). Chem. Eur. J., 11, 4819 (2005); (c) G. De la J. Nishimura, Angew. Chem. Int. Ed., 42, 26. U. Hahn, M. Elhabiri, A. Trabolsi, Torre, F. Giacalone, J. L. Segura, N. Martín, 3158 (2003). H. Herschbach, E. Leize, A. van Dorsselaer, and D. M. Guldi, Chem. Eur. J., 11, 1267 20. (a) T. Habicher, J.-F. Nierengarten, A.-M. Albrecht-Gary, and J.-F. Nierengarten, (2005). V. Gramlich, and F. Diederich, Angew. Angew. Chem. Int. Ed., 44, 5338 (2005). 17. N. Martin and J.-F. Nierengarten, Chem. Int. Ed., 37, 1916 (1998); ( b) F. 27. The concept of click chemistry has been Tetrahedron Symposium-in-Print, No. 117, Cardinali, H. Mamlouk, Y. Rio, introduced by K. B. Sharpless, see: Supramolecular fullerene chemistry, N. Armaroli, and J.-F. Nierengarten, Chem. H. C. Kolb, M. G. Finn, and K. B. Sharpless, Tetrahedron, 62, 1917 (2006). Commun., 1582 (2004). Angew. Chem. Int. Ed., 40, 2004 (2001). 18. (a) F. Diederich, C. O. Dietrich-Buchecker, 21. D. Felder, B. Heinrich, D. Guillon, 28. A. Trabolsi, M. Elhabiri, M. Urbani, J.-F. Nierengarten, and J.-P. Sauvage, Chem. J.-F. Nicoud, and J.-F. Nierengarten, Chem. J. L. Delgado de la Cruz, F. Ajamaa, Commun., 781 (1995); (b) N. Armaroli, Eur. J., 6, 3501 (2000). N. Solladié, A.-M. Albrecht-Gary, and F. Diederich, C. O. Dietrich-Buchecker, 22. (a) F. Diederich and M. Gómez-López, J.-F. Nierengarten, Chem. Commun., L. Flamigni, G. Marconi, J.-F. Nierengarten, Chem. Soc. Rev., 28, 263 (1999); 5736 (2005). and J.-P. Sauvage, Chem. Eur. J., 4, 406 (b) D. M. Guldi and N. Martin, J. Mater. 29. F. D’Souza, R. Chitta, S. Gadde, (1998); (c) T. Da Ros, D. M. Guldi, Chem., 12, 1978 (2002). M. E. Zandler, A. S. D. Sandanayaka, A. F. Morales, D. A. Leigh, M. Prato, and 23. M. Gutiérrez-Nava, H. Nierengarten, Y. Araki, and O. Ito, Chem. Commun., R. Turco, Org. Lett., 5, 689 (2003); P. Masson, A. van Dorsselaer, and 1279 (2005). (d) N. Watanabe, N. Kihara, Y. Furusho, J.-F. Nierengarten, Tetrahedron Lett., 44, T. Takata, Y. Araki, and O. Ito, Angew. 3043 (2003). Chem. Int. Ed., 42, 681 (2003); (e) K. Li, 24. M. Elhabiri, A. Trabolsi, F. Cardinali, U. D. I. Schuster, D. M. Guldi, M. A. Herranz,

About the Authors

NAZARIO MARTÍN is a graduate of the and as a member of the international She is involved in the organization of University Complutense of Madrid (UCM), editorial advisory board of The Journal of the symposium on “Porphyrins and where he obtained the PhD in 1984. He Materials Chemistry. He is the Regional Supramolecular Assemblies” of the ECS worked as postdoctoral fellow (1987-1988) Editor for Europe of the journal Fullerenes, Fullerenes, Nanotubes, and Carbon with Michael Hanack on electrically Nanotubes and Carbon . Nanostructures Division. She may be conducting organic materials at the Institut He is a fellow of The Royal Society of reached at [email protected]. für Organische Chemie der Universität Chemistry, and currently he is the President Tübingen. In 1994, he was a visiting of the Spanish Royal Society of Chemistry. JEAN-FRANÇOIS NIERENGARTEN studied professor at the Institute for Polymers and He is involved in the organization of and chemistry at the Organic (IPOS) at the University the symposium on “Molecular and Université Louis Pasteur of Strasbourg, of California, Santa Barbara (UCSB) Supramolecular Chemistry of Fullerenes” France, and received his doctoral degree working with Fred Wudl on fullerenes. of the ECS Fullerenes, Nanotubes, and under the supervision of Jean-Pierre Sauvage He is currently full professor of organic Carbon Nanostructures Division. He may be and Christiane Dietrich-Buchecker in chemistry at the UCM. Professor Martín’s reached at [email protected]. Strasbourg in 1994. After postdoctoral work research interests span a range of targets with François Diederich at the ETH-Zürich, with emphasis on the chemistry of carbon NATHALIE SOLLADIÉ studied chemistry at Switzerland, during 1994-1996, he returned nanostructures such as fullerenes and the Université Louis Pasteur of Strasbourg, to Strasbourg as a CNRS researcher. In June carbon nanotubes, π-conjugated systems France, and received her doctoral degree 2005, he joined the Laboratoire de Chimie as molecular wires, and electroactive under the supervision of Jean-Pierre Sauvage de Coordination in Toulouse. His current molecules, namely tetrathiafulvalenes in Strasbourg in 1996. After postdoctoral scientific interests range from covalent (TTFs), in the context of electron transfer work with Andrea Vasella at the ETH- chemistry of fullerenes to dendrimers and π- processes, photovoltaic applications Zürich, Switzerland, during 1996-1997, conjugated systems with unusual electronic and nanoscience. Professor Martín has she returned to Strasbourg as a CNRS and optical properties. He is involved in been visiting professor at UCLA (USA) researcher. In June 2005, she joined the the organization of the symposium on and Angers (France) universities. He is Laboratoire de Chimie de Coordination in “Molecular and Supramolecular Chemistry currently a member of the Editorial Board Toulouse. Her current scientific interests of Fullerenes” of the ECS Fullerenes, of Chemical Communications, and he range from covalent chemistry of porphyrins Nanotubes, and Carbon Nanostructures has served as General Editor of the Spanish to fullerene-porphyrin systems with Division. He may be reached at journal Anales de Química (2000-2005) unusual electronic and optical properties. [email protected].

The Electrochemical Society Interface • Summer 2006 33