FEATURE ARTICLE [60]Fullerene chemistry for materials science applications Maurizio Prato Dipartimento di Scienze Farmaceutiche, Universita` di T rieste, Piazzale Europa 1, 34127 T rieste, Italy Since their first detection and bulk production, the fullerenes have gained a primary role on the scientific scene, reaching their climax when the 1996 Nobel Prize for Chemistry was awarded to Kroto, Curl and Smalley for their seminal discovery.The unique physical and chemical properties of these new forms of carbon led many scientists to predict several technological applications. This created a heavy disappointment when it was clear that fullerene-based materials would not soon be ready for the market. However, the fullerenes have so far delighted several dozens of researchers who found that C60 and its relatives undergo a variety of chemical reactions. In most cases, the new derivatives retain the main properties of the original fullerene, and it is now not unlikely that some functionalized fullerenes may find useful applications in the field of materials science and technology. In this Article we summarize the basic principles of the organic chemistry of fullerenes, together with a description of the physicochemical properties that have made these carbon cages popular in materials science, and review the most recent achievements in the functionalization of fullerenes aimed at the production of new molecular materials.[60]Fullerene, the most abundant representative of the fuller- Initially hypothesized as a ‘super aromatic’ molecule, C60 was ene family,1 was produced for the first time on a preparative rather found to possess a polyenic structure, with all the double scale in 1990, by resistive heating of graphite.2 The availability bonds inside the six-membered rings.31 X-Ray crystal structure of milligram quantities of C60 generated an extraordinary determinations on C60 and on some of its derivatives have outburst of academic and industrial research that led to the proved the existence of two dierent types of bonds: ‘short discovery of several interesting physical properties, along with bonds’ or 6,6 junctions, shared by two adjacent hexagons (ca. a careful definition of the chemical reactivity of the fuller- 1.38 A° long) and ‘long bonds’, or 5,6 junctions, fusing a enes.3–10 Among the most spectacular findings, C60 was found pentagon and a hexagon (ca. 1.45 A° long). The geometric to become a superconductor in M3C60 species (M=alkali demand of the spherical cage is such that all the double bonds metal),11–14 an organic soft ferromagnet in TDAE+VC60-V in C60 deviate from planarity.13 This pyramidalization of the (TDAE=tetrakisdiethylaminoethylene),15 a relatively stable sp2-hybridized carbon atoms confers an excess of strain to C60 hexaanion in cyclic voltammetry,16,17 and an interesting mate- which is responsible for the enhanced reactivity of the fullerene.rial with non-linear optical properties.18,19 A release of strain is in fact associated with the change of It was immediately clear that a new molecular material had hybridization from sp2 to sp3 that accompanies most chemical been discovered with enormous potential in several dierent reactions.32 disciplines.Especially in materials science, the rich electronic The chemical reactivity of C60 is typical of an electron- and electrochemical behaviour generated great expectations. deficient olefin. C60, in fact, reacts readily with nucleophiles However, the dicult processibility of the fullerenes has rep- and is a reactive 2p component in cycloadditions.33 The vast resented a major problem in the hectic search for practical majority of reactants will attack the 6,6 ring junctions of C60, applications.C60, in fact, is insoluble or only sparingly soluble which possess more electron density. Insertions into 5,6 bonds in most solvents and aggregates very easily, becoming even have been reported only as rearrangements following a 6,6 less soluble.20 This serious obstacle could be, at least in part, junction attack (see below).surmounted with the help of the ‘functionalization chemistry The main objective of fullerene chemistry is the production of the fullerenes’.21–29 The organic derivatization of C60 has of well-defined, stable and characterizable adducts. In this put forth an increasingly high number of compounds which, respect, several dierent approaches have given excellent while retaining most of the original properties of the fullerene, results.The reaction types can be of widely dierent nature, become much easier to handle. but the single-addition products can be classified into a few In this review we will focus on the use of fullerene chemistry broad categories, based on the structure which is obtained. In to produce compounds useful in materials science and technol- particular, with relation to the geometrical shape built on a ogy.An exhaustive review of all the literature produced so far 6,6 ring junction of C60, there can be: an open structure; a on this argument is beyond the scope of the present work.An three-membered ring, which also includes carbon or nitrogen attempt will be made to give an idea of the potential of the insertion into a 5,6 ring junction; a four-membered ring; a five- fullerene materials in practical applications. membered ring; a six-membered ring (Fig. 1). First, we will briefly review the basic principles of the In general, the word dihydrofullerenes has been coined to chemical reactivity of C60, as the chemistry of fullerenes has specifically indicate a monofunctionalized fullerenes, or else already been reviewed in detail by several authors.21–29 We the word organofullerene more widely indicates a fullerene shall then illustrate the main physicochemical properties that derivative containing an organic appendage.have made fullerenes popular materials. We will finally address the main subject of this review, considering only materials derived from functionalized C60. For unmodified C60-based Open structures thin films and materials, the reader is referred to a recent, Adducts can be obtained by careful hydrogenation (Nu=E= excellent review.30 H),34–38 or by addition of a nucleophile followed by quenching with acid or an electrophile (Scheme 1).Basic Principles of C60 Chemistry Usually a 1,2-addition is observed, but 1,4-additions have been reported in a few cases where hindrance between sterically The C60 surface contains 20 hexagons and 12 pentagons. All the rings are fused, all the double bonds are conjugated. demanding addends becomes relevant.Nucleophiles success- J. Mater. Chem., 1997, 7(7), 1097–1109 1097Fig. 1 Geometrical shapes built onto a 6,6 ring junction of C60: (a) open, (b) three-membered ring, (c) four-membered ring, (d) five-membered ring and (e) six-membered ring Nu Nu E Nu– E+ – 1 Scheme 1 fully employed include Grignardreagents,33,39,40 organolithium lead to azafulleroids.75–77 If nitrenes are generated instead, azamethanofullerenes are formed.77–83 derivatives,39–41 cyanide ion,42 etc.43–55 A case which illustrates the utility of this approach to produce interesting materials is It is interesting to note that, among the many families of organofullerenes, fulleroids and azafulleroids are the only reported in Scheme 1.Cyanide addition to C60, followed by quenching with toluene-p-sulfonyl cyanide, led to the synthesis derivatives which maintain the 60p electron configuration typical of C60.of a dinitrile derivative 1 (Nu=E=CN).42 As detected by cyclic voltammetry, 1 and also other monocyano-dihydrofullerenes display interesting properties, allowing a fine-tuning of Four-membered rings the electron-accepting capacity of cyanodihydrofullerenes.Cyclobutanofullerene derivatives are typically obtained by [2+2] cycloadditions. Benzyne addition was reported Three-membered rings first,84,85 followed by addition of electron-poor alkenes,86–88 electron-rich alkenes and alkynes.89–92 [2+2+2] Cyclo- This category represents one of the most fascinating and thoroughly investigated classes of functionalized fullerenes.addition of quadricyclane to C60 gave rise to a norbornene derivative93 which was used for polymer preparation.94 The addition of diazomethane derivatives to C60, pioneered by the Wudl group,21,25,56 can lead to two dierent structures, commonly called fulleroids 3 and methanofullerenes 4. Five-membered rings These are usually prepared by [3+2] cycloadditions. A variety of carbocyclic or heterocyclic systems have been reported, which include cyclopentane derivatives,95,96 pyrrolidines, 81,97–101 isoxazolines,102–104 pyrazolines,105 furans,106 etc.107,108 The addition of azomethine ylide to C60, leading to fulleropyrrolidines, is becoming increasingly popular (see below).The reason for such success is probably due to the simple approach, as starting materials are usually commercially available or easily prepared, and a single product of monoaddition across a 6,6 junction of the fullerene is obtained The first step of the reaction is a 1,3-dipolar cycloaddition (Scheme 2).of diazomethane to C60, yielding a pyrazoline derivative 2, isolated only in the case of diazomethane,57 but not in other Six-membered rings cases. Extrusion of nitrogen leads typically to a mixture of The classical [4+2] cycloaddition to C60 produces six-mem- fulleroids and methanofullerenes.58,59 Conversion of fulleroids bered rings fused to 6,6 junctions.95,109–122 This is also a very to methanofullerenes can be achieved (in most cases, but not for R=R¾=H60) thermally,25,58 electrochemically61 or photochemically. 62 It can also be acid-catalysed.63 A wide variety of diaryl, aryl-alkyl and dialkyl fulleroids and methanofullerenes21,25 have been prepared so far, providing materials for potential applications in many fields (see below).Three-membered rings fused on 6,6 junctions of C60 can be produced cleanly (without formation of fulleroids) electrochemically64 or by addition of nucleophiles,65–67 diazirines,68 carbenes,69–72 sulfonium ylides.73 Azide additions to C60 follow closely the reaction course of diazo compounds.In this case, triazoline derivatives can be R1 N+ CH2 HC N R1 R2 R2 –CO2 –H2O C60 – R1NHCH2CO2H + R2CHO heat Scheme 2 isolated and characterized,74 which, after extrusion of nitrogen, 1098 J. Mater. Chem., 1997, 7(7), 1097–1109popular reaction, utilized by many groups, which has oered bonds, all sharing the same reactivity.Typically, the addition of a nucleophile to a 6,6 bond of C60 in a stoichiometric entries to a wide variety of functionalized fullerenes (see below). amount leads to a complex mixture containing one product of monoaddition together with several multiple addition prod- Holes ucts. Usually, the monoadduct is separated by chromatography Ever since the fullerenes were discovered, the idea of trapping and the multiple adducts discarded.This is because a mixture atoms, molecules or ions inside the carbon cage has fascinated of diadducts (from a symmetrical reagent) can contain up to the scientific community.1,123 It is in fact believed that novel eight dierent positional isomers, with the number of possible materials with peculiar properties may be produced.Whereas isomers increasing with the number of additions. Isomers some fullerenes containing transition metals inside have been inside each family of adducts (monoadducts, diadducts, triad- isolated,124 the preparation of bulk quantities of inclusion ducts, etc.) tend to possess the same chromatographic proper- assemblies would require a chemical modification of C60 such ties, and this makes the separations a very complex operation.that a hole is opened on the fullerene surface by breaking one Therefore, addition conditions are usually optimized for the or more double bonds. Then the atom, molecule or ion must maximum yield of the monoaddition product, with little atten- be forced inside the cage and trapped by restoring the carbon– tion to more highly functionalized fullerenes.carbon bonds. Conceptually very simple, the opening of a hole Recently, the chemistry of multiple additions to the fullerene on the fullerene cage proved very dicult. The first orifice core has become a fundamental issue in the design of useful generated was obtained by oxidative light incision.125 The C60 derivatives, and several research groups have taken the sample, however, was prepared on an analytical scale, and was challenge of isolating and characterizing diadducts as well as only characterized by IR spectroscopy.An eleven-membered higher adducts. Investigations of polyadditions have been ring hole was produced on the fullerene surface by means of carried out using osmylation,139 g2-metal complexation,140–142 regioselective diaddition of azides.76 Other holey spheres have hydrogenation,143,144 cyclopropanation,145–150 azide been obtained on preparative scales, and the resulting com- addition,76,77,151,152 [4+2] cycloaddition,113,148,149,153,154 pounds fully characterized.126–128 A very interesting result has [3+2] cycloaddition,155 azomethine ylide cycloaddition,156 been disclosed recently by Hirsch and collaborators, who silylation,157 epoxidation158,159 and amine addition.50,160 reported the first example of chemical modification of fullerenes In particular, the Hirsch group and the Diederich group that allows the synthesis of open and closed valence isomers have engaged in a systematic study aimed at determining the with the same addition pattern.They found that diadducts 5 factors that govern the regiochemistry of these additions and and 6 formed by addition of azides to C60 possess dierent at gaining control over multiple additions. The German team, structures, depending on the substituent on the nitrogen atom. after isolating and characterizing all diadducts formed in the Unsubstituted diadduct 5 is locked in the ring-closed form, base-catalysed addition of bromomalonates,145 reached the whereas a carbamate functionality gives rise to the open form, conclusion that the addition of a second nucleophile does not which has a relatively large hole.occur randomly, but is controlled by the frontier molecular orbitals of the monofunctionalized fullerene.146 Furthermore, in a decisive step toward the synthesis of hexakis adducts, the same group employed the reversible addition of 9,10-dimethyl anthracene to produce the Th-symmetrical hexakis adduct of cyclopropanation of C60 with an octahedral addition pattern in an astonishing 48% yield.161 On the other hand, the strategy used by the ETH group has been the tether-directed functionalization of C60, which produces exclusively adducts derived by equatorial addition.29,148 This approach has allowed the Swiss group to achieve outstanding results, like two Saunders and collaborators have demonstrated that at very solubilized representatives of a new class of carbon allotropes, high temperatures (650 °C) and pressures (3000 atm) the noble C195 and C260.162 A unique case of topochemically controlled gases helium, neon, argon, krypton and xenon can be intro- fullerene difunctionalization has been recently reported.153 In duced inside the cage in one in every 1000 molecules of C60.129 the solid state, when heated at 180 °C for 10 min, the crystalline A temporary bond breaking of the cage has been proposed to monoadduct of C60 and anthracene evolved to the antipodal explain the process.For obvious reasons this methodology diadduct in a quantitative way. cannot be extended easily to other guests but noble gases. In Polyhydroxylated C60 derivatives, fullerenols C60(OH)10–12, addition, Saunders warned that conventional chemical syn- can be obtained by dierent methods.163–166 The hydroxy thesis at ambient pressures, though elegant, may not succeed, groups are randomly distributed on the fullerene surface, due to the small free volume inside the fullerene sphere.generating mixtures of isomeric structures, but providing a high density of reactive sites useful for practical applications Heterofullerenes (see below). Although complete control over the addition chemistry of Another objective of fullerene chemistry relates to the possi- the fullerenes has yet to be reached, the encouraging results bility of substituting one or more carbon atoms of the cage reported so far give way to the hope that, in the future, the with heteroatoms.130–132 This substitution leads to hetero- fullerenes, and in particular C60, may be used as building fullerenes, which may possess properties dierent from the blocks in the construction of very complex molecular parent fullerenes.The most popular heteroatom so far incor- assemblies. porated in fullerenes is nitrogen,133,134 and the C60 homologue has been isolated in bulk quantities as its dimer is (C59N)2.135,136 The chemical and physical properties of these Physicochemical properties of fullerene derivatives new compounds are the object of intense current investigations.137,138 Electrochemical properties From the early days of the functionalization chemistry, the Multiple additions electrochemistryof fullerene derivativeshas been systematically studied by the Wudl group at the University of California at Without taking into account 5,6 bonds (which can undergo insertions, see above), C60 possesses 30 equivalent double Santa Barbara.In fact, a striking feature of C60, as shown by J. Mater. Chem., 1997, 7(7), 1097–1109 1099cyclic voltammetry, is that, in solution, this fullerene can accept the visible region up to 650 nm, are retained in most derivatives. In addition, dihydrofullerenes extend their absorptions reversibly up to six electrons. The UCSB group found that both fulleroids and methanofullerenes essentially retain the throughout the entire visible region, with a weak maximum at ca. 700 nm. This additional feature makes excitation possible electronic properties of C60.21,56,61,75,95,167–171 The same behaviour has been observed for most C60 derivatives, whose cyclic by means of irradiation at very low energy. Analogously to C60, dihydrofullerenes are excited to a short- voltammograms are typically characterized by a small shift to more negative values of the reduction potentials. This is lived singlet which converts rapidly into a long-lived triplet, with quantum yields slightly lower than C60.179,180 Whereas expected on considering that saturation of a double bond in C60 causes a partial loss of conjugation. Due to this eect, at C60 exhibits a triplet–triplet absorption at 750 nm, the same peak is shifted to ca. 700 nm in C60 derivatives.180–185 This most five reduction peaks for the C60 moiety in fullerene derivatives have been detected so far in the accessible potential triplet–triplet transition is characterized by a higher absorption coecient than the ground state, and may be responsible for range.172–176 An extensive investigation of the redox properties of several its non-linear behaviour.Accordingly, solutions of C60 and C70 exhibit optical limiting (OL) properties, which compare very variously functionalized organofullerenes has been reported by Suzuki et al., who studied the influence of the groups attached well with those of materials currently in use.18 This feature holds great promise for practical applications, such as incorpor- directly to C60 on CV potentials.177 A small inductive eect was found, revealed by changes in the reduction and oxidation ation of fullerene derivatives in proper transparent matrices for protection against high-energy laser pulses. As compared (where possible) potentials and mainly related to the electronegativity of the atoms attached.to C60, fullerene derivatives show a lower singlet–triplet quantum yield,179,180 so that they are expected to exhibit a lower A more incisive control of the electronic properties of the fullerenes might still be a relevant issue. The conjugated p- OL eciency. However, it has been demonstrated that the optical limiting properties of C60 and its derivatives depend system of C60 seems ideally suited for non-linear optical (NLO) applications.18 Molecules with large NLO properties are often on the excitation wavelength.186,187 When the latter is closer to the triplet–triplet absorption maximum of the fullerene characterized by an electron-donating group and an electronwithdrawing group at opposite ends of a conjugated p system.derivative (700 nm) than to the equivalent transition of C60 (750 nm), the OL performance of the organofullerene becomes In principle, attachment of donors and acceptorsin conjugation with the fullerene p system should result in an interesting more ecient.Dierences in the ground-state absorption can also play a role. push–pull assembly. However, most reactions of the fullerenes lead to derivatives in which the addends are attached to C60 via sp3 carbons, an event that breaks the conjugation.This Multiple adducts problem was ingeniously faced by Wudl and co-workers, who The physicochemical properties of the multiple adducts are used fluorenyl systems spiro-linked to a methanofullerene largely dependent on the number of addends.56,146,188 In a moiety. An interaction through a ‘periconjugation’ mechanism systematic electrochemical investigation, it was found that the between the fluorene group and the fullerene spheroid was reduction and oxidation characteristics depend very heavily detected by cyclic voltammetry.168,171 It was found that the on the number and pattern of the addends in fullerene deriva- electrochemical behaviour of compounds 7 is relatively sensitives. As a general trend, the fullerene derivatives become tive to the presence of substituents on the fluorene moiety.harder to reduce going from mono to hexakis adducts. This With electron-donating groups (7b) the reduction potentials has been attributed to the reduced conjugation occurring in are shifted to more negative values. On the other hand, the multiply functionalized fullerene compounds, which leads to a first reduction potential becomes less negative if strong eleccorresponding increase of the energy of the LUMO.188 The tron-attracting groups are placed in the 9-fluorenyl moiety same trend is observed in the study of UV–VIS absorption (7c).The electronic properties of spiromethanofullerenes 7 can features, where changes due to loss of conjugation are be attributed to their peculiar geometry, as the fluorenyl planar observed.146,188 skeleton is held rigidly perpendicular to the surface of the spheroid.This unique arrangement may be responsible for ‘through-space’ interactions between the fluorenyl moiety and Spin-labelled derivatives the spheroid, which thus becomes sensitive to electronic A series of C60 derivatives incorporating a nitroxide unit has changes in the fluorenyl counterpart.The promising NLO been synthesized (Fig. 2).189–192 These compounds possess a molecules 8 were also synthesized as a mixture of diadducts paramagnetic probe useful for investigating the electronic (R=electron-withdrawing and electron-donating groups). A properties of the fullerenes. Indeed, they were successfully linear free energy relationship of the reduction potentials on employed for the study of the anions192 and the excited triplet the Hammett sm of the substituent inside a family of methanostates189 of the C60 moiety.fullerenes was also reported by Wudl and collaborators.178 Applications Polymers As we have already seen, the fullerenes possess several outstanding properties.The incorporation of fullerenes in polymers would potentially endow the polymer of most of the fullerene properties.193 Thus, electroactive polymers can be obtained, or polymers with optical limiting properties.194 On the other hand, fullerenes embedded in polymers become more easily processible. The resulting materials can be used for surface coating, photoconducting devices, and also to create new molecular networks.Optical properties There may be several ways to combine polymers with fullerenes. The simplest way is the plain mixing of the two The ground state absorption properties of C60, characterized by strong bands in the UV region and weaker absorptions in components, either as a solid mixture, or as a solution in a 1100 J. Mater. Chem., 1997, 7(7), 1097–1109N x y N3 x y C60, heat –N2 Scheme 4 (d) A dendrimer can be built on a fullerene nucleus (Fig. 3).211,212 In addition, three-dimensional, starburst polyurethane networks have been prepared using fullerenols as molecular cores and condensing them with isocyanate prepolymers. Highperformance elastomers with enhanced thermal stability are thus obtained.213,214 A comparison between dierent ways of producing C60 polymers has been reported recently.215 Free radical polymeriz- Fig. 2 A series of C60 derivatives incorporating a nitroxide unit ation of methyl methacrylate (MMA) was carried out in the absence (PMMA) and in the presence (PMMA-9) of derivatives 9, and compared to simple embedding compounds 9 in common solvent which is then evaporated.The latter mixing preformed PMMA (emb-9). It was found that samples from produces more homogeneous samples. This practice usually dierent preparations dier significantly. PMMA-9 clearly leads to non-covalent interactions between the two shows cross linking of the polymer chains, which leads to an components. increase in Tg of ca. 8°C with respect to plain PMMA. In The chemical linking of polymers and fullerenes can be addition, the cross linked material did not dissolve in chloro- obtained by four main ways.form, a solvent in which PMMA and emb-9 were readily (a) Fullerenes that are present during the polymerization of a soluble. Cross linking was not observed when C60 was used in monomer can react and be attached to the polymer place of 9 during the polymerization process.This was attri- chain.194–201 Typically, this happens in anionic and free radical buted to the lower solubility of C60 in MMA as compared polymerizations, where species are generated that react ran- to 9. domly with the double bonds of the fullerene. In this case there is no chemical control: multiple additions to the fullerene double bonds occur, so that the fullerene structure is not welldefined.Cross linked materials are usually obtained (Scheme 3). (b) A preformed polymer is treated under conditions that favour the chemical linking to the fullerenes.202–209 This is generally obtained by generating nucleophilic polymeric species. Also in this case the chemical attack to the fullerene double bonds is indiscriminate, and mixtures of isomeric fullerene species can be obtained (Scheme 4).(c) A monomer containing a fullerene unit is polymerized or Information on the structure of the cross linked species was co-polymerized.94,210 In this case, if the fullerene monomer is obtained from the analysis of the EPR spectra recorded for a well-defined monoaddition product and if the conditions the lowest excited triplet of PMMA-9.An unusually large employed are chemically inert to the fullerene double bonds, electron dipolar splitting D parameter of positive sign was the final polymer contains a fullerene species with an almost observed. The spectrum was simulated using a simple model intact electronic configuration, in which only one double bond calculation which considers C60 and derivatives as a collection (or none, in the case of fulleroids or azafulleroids) of the of fully localized double bonds. A positive sign of D is expected pristine fullerene has been saturated (Scheme 5).The electronic for PMMA-9 in which cross linking has occurred in the and electrochemical properties of C60 were shown to be equatorial belt of the molecule.215 retained in the polymers.94,210 A peculiar type of fullerene polymers (all-carbon polymers) has been obtained by irradiation of oxygen-free films or solutions of C60216,217 as well as by heating AC60 crystals (A= K, Rb, Cs).14 A quasi-linear structure, derived from [2+2] cycloadditions of C60 double bonds leading to four-membered rings, has been proposed for these polymers.218,219 Fullerenes, and C60 in particular, show very limited solubility in any medium, especially polar solvents.For any molecule that contains more than one C60 sphere, this experimental problem is amplified. Therefore, when preparing fullerene- Scheme 3 based polymers, one of the main issues that needs to be J. Mater. Chem., 1997, 7(7), 1097–1109 1101N Mo Ar But RO RO m [Mo] = [Mo] + n Scheme 5 Fig. 3 A dendrimer built on a fullerene nucleus addressed is the solubility of the material.The typical result is to the diminished solubility of 10 in most solvents, Wudl warned that polymers containing C60 should be expected to that only oligomers are obtained with relatively low molecular weights. Wudl and co-workers reported the synthesis of deriva- be insoluble and intractable, unless a solubilizing group is attached.The same conclusions were reached by other groups, tives containing two C60 units (10),167 which were shown to retain the original electronic fullerene properties. But, owing during the synthesis of compounds 11 and 12.110,220 1102 J. Mater. Chem., 1997, 7(7), 1097–1109polymerization degree, the authors reported that both components, namely the conjugated polymer and C60, retain their original electrochemical properties, and that some new properties may be expected from their interactions. Thin films Thin films containing fullerenes are of current high interest, owing to the possibility of transferring the interesting fullerene properties to bulk materials by simple surface coating.In this respect, self-assembled monolayers (SAM)30,229,230 and Langmuir films are being increasingly used, as controlled organized structures can be obtained.In a very stimulating experiment, Echegoyen and Kaifer used molecular recognition to induce the formation of molecular monolayers of the 18-crown-6 functionalized fullerene 13.231 A gold surface was modified using a thiol-terminated ammonium salt (Scheme 6). When the modified gold layer was immersed into a CH2Cl2 solution of 13, surface coverage was obtained which corresponds to a compact monolayer of C60, as found by OSWV measurements.The attachment of 13 to the ammonium salt, and thus to the gold surface, was demonstrated to be reversible in a CH2Cl2 solution. A major problem encountered during the preparation of Langmuir films of fullerenes is related to the high hydrophobicity of the carbon cage compounds.Eorts have been aimed at the preparation of fullerene derivatives which present a hydrophilic end.111,232–244 In these cases, monomolecular layers with an area per molecule of approximately 10 A° 2 have 10 11 12 been often obtained. Langmuir–Blodgett transfers to solid substrates, however, proved very dicult, and only a few However, even when solubilizing hexyloxy chains were intro- successful cases have been reported.Only two recent represen- duced in the polymer, the number of fullerene moieties involved tative examples of successful transfers will be discussed here was still low, and precipitation of oligomers (n=0–5) occurred (for a more detailed discussion on Langmuir films of fullerenes, due to cross linking.221 The facile cross linking of C60-contain- see ref. 30). ing polymers represents, in fact, another, strictly connected An extensive investigation on the Langmuir behaviour of problem. The number of reactive double bonds in C60 is such several C60 derivatives was reported recently.235 The amphi- that up to eight or ten chains can radiate from a fullerene philic fullerene monoadducts studied include carboxylic acid nucleus.This leads to a very tight, cross linked structure, and amine derivatives, a bis-phenol, a crown ether and a whose solubility and processibility become problematic. A cryptate, together with some protected and deprotected sugars. higher Tg is usually observed upon addition of C60 or organof- Monomolecular layers were obtained for the cryptate deriva- ullerenes to a polymer,94,207,215 which suggests that fullerenes tive, but were not very stable as assessed by compression– may be used as additives for increasing the thermal stability expansion cycles.Langmuir–Blodgett transfer of films derived of a material. Eventually, when cross linking was avoided, a from some sugar derivatives was only possible using highly soluble polymer with Mw of ca. 80000 was prepared. The hydrophobic, phenyl-functionalized glass or quartz substrates. improvement was obtained using a mixture of two dierent o- Spreading behaviour independent of concentration in the quinodimethanes, one of which helped avoid cross linking range 0.1–2.0 mM and area/molecule of 96 A° 2 , with thickness while increasing the solubility.221 of 7±3 A°, in excellent agreement with theory, was obtained An interesting example of a C60 end-capped polystyrene star for methanofullerene 14.233 The monolayers were transferred has been recently reported (Fig. 4). The attachment to C60 was to solid substrates (quartz or mica) with transfer ratios close obtained via azide addition, and the resulting polymer was to unity.shown to retain the basic C60 electrochemical properties.222 Rotello and co-workers took advantage of the reversibility of the addition of cyclopentadiene to C60 for the temporary attachment of the fullerene to a modified Merrifield resin.223 Addition of the cyclopentadiene-modified resin to C60 was achieved at room temperature, whereas the fullerene was released at 180°C upon addition of maleic anhydride as a CO2CH2CH2OCH2CH2OCH2CH2OCH3 CO2CH2CH2OCH2CH2OCH2CH2OCH3 14 cyclopentadiene trap.The authors proposed the methodology for a non-chromatographic purification of the fullerenes.224 Thin films useful for laser protection can be obtained by A few examples of electrochemical polymerization of C60 incorporation or covalent attachment of fullerenes to transderivatives have been reported. Starting from a dialkynylated parent solid matrices.The optical limiting properties of C60, methanofullerene, Diederich et al. observed formation of an originally detected in toluene solutions (see above),18 can be electrically conducting film on the surface of the platinum transferred to solid substrates without significant activity cathode.225 A redox-active fullerene polymer with interesting loss.245 mechanical and electrical properties was also obtained by Whereas polymeric substrates are damaged by high power electrochemically polymerizing the fullerene oxide C60O.226,227 laser pulses,245 glasses show very high damage thresholds, A monomer unit, having a cyclopentadithiophene moiety which makes them ideal for OL purposes.Sol–gel processing attached to C60 was electrochemically polymerized, leading to provides an excellent means for the preparation of glassy a conjugated polymer that contains C60 covalently attached.228 matrices at reasonable temperatures, compatible with the stability of most organic compounds.246–250 However, C60 has Although some solubility problems arose, leading to a low J.Mater. Chem., 1997, 7(7), 1097–1109 1103Fig. 4 A C60 end-capped polystyrene star attach the monofunctionalized fullerene to the silicon matrix. This can be achieved by introducing a silicon alkoxide functionality in the diene or the 1,3-dipole that will add to C60. To this aim, the derivatives shown in Fig. 5 were synthesized, whose OL properties in solution have been reported.186 Their chemical attachment to silicon matrices has been obtained and the OL properties of the resulting materials are under investigation.252 Electrooptical devices The combination of the rich electronic and electrochemical properties of C60 with those of other electroactive species is currently a field under intense investigation.It is in fact believedthat chemically modified fullerenes may play a relevant role in the design of novel molecular electronic devices, and in particular for applications in artificial photosynthesis.To this end, a number of electron-rich groups have been covalently attached to C60, which acts as an electron acceptor, for the O O O O O O S NH3 + S NH3 + O O O O O O Au + Au 13 creation of a large variety of dyads. Donor units used to this Scheme 6 end include aromatics,119,182,183,253–255 porphyrins181,256–259 and phthalocyanines,260 a rotaxane,261 tetrathiafulvalene,176,262 a carotene unit,263 Ru–bipy264 and Ru–terpy265 complexes, no or very low solubility in the polar solvents typically used as well as ferrocene.172,176 Some of these dyads have been during the sol–gel process.In addition, C60 has a high tendency studied with respect to photoinduced charge separ- to form clusters, thus making it very dicult to prepare opticalation. 181,182,263,266–268 quality films. When solubilized in the form of organofullerene, Both energy and electron transfer processes between the the optical properties of C60 can be transferred to sol–gel donor and the acceptor (C60 moiety) have been reported. For materials.251 The best way to pursue the preparation of homoinstance, intramolecular quenching of C60 singlet excited state geneous thin films of optical quality, with tunable amounts of dihydrofullerene for applications in theOL field is to covalently was detected, from electron transfer by the ferrocene moiety 1104 J.Mater. Chem., 1997, 7(7), 1097–1109blends is the high tendency of the fullerene to form clusters and to crystallize.This results in poor homogeneity and low optical quality of the films. These problems have been partially overcome with the use of soluble fullerene derivatives, such as the methanofullerenes 20 and 21. Ecient charge transfer in composite films of poly(bis-2,5-epi-cholestanoxy-1,4-phenylene vinylene) and 20 or 21 showed that the increased miscibility of the functionalized fullerene with the conjugated polymer can represent an important prerequisite for the construction of electrooptical devices.273–276 Fig. 5 Silicon-functionalised fullerene derivatives in dyads 15–19.269 The nature of the spacer was found to play a role: through bond electron transfer was shown for dyads 15–17, whereas formation of a transient intramolecular exciplex was observed for compounds 18 and 19.While in 15–17 fast charge recombination probably prevents sucient stabilization, the saturated hydrocarbon bridge in dyads 18 and 19 is able to avoid charge recombination and long-lived charge separated states are detected in polar solvents (t1/2=1.8 and 2.5 ms in benzonitrile). 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