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Advances in Molecular and Supramolecular Fullerene Chemistry

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Advances in Molecular and Supramolecular Fullerene 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 fullerene double bond as the alkene 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 materials science 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 Bingel reactions,1 fulleropyrrolidines and Supramolecular have been considered to be stable fullerene derivatives. However, the Fullerene Chemistry thermal quantitative retro-cycloaddition 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 alkenes and alkynes reaction reveals that the understanding I Smalley received the Nobel Prize for have not been studied previously on of the reactivity of fullerene derivatives the discovery of the fullerenes. 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 organic synthesis 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 molecules 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 alkyne, 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 metal. 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 yield, 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 nanotechnology 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 molecule accelerates charge cachet in 1996 when Kroto et al. were and heterocyclic molecules. The unique separation during photoinduced electron awarded the Nobel Prize in chemistry.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, porphyrins, 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 dendrimers, 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, solvent, 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 ammonium-crown ether recognition, π-stacking interactions between the C60 sphere and the porphyrin moiety have been observed in a supramolecular complex obtained from a versatility of fullerenes for the study porphyrin-crown ether 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 click chemistry principle and calculated structure of the clicked supramolecular complex resulting from the association of a bis-crown ether derivative and a bis-ammonium fullerene ligand. 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, ion 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.
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