Small carbon nano-onions (CNOs, 6–8 shells) were prepared in high yield and functionalized with carboxylic groups by chemical oxidation. After functionalization these nanostructures were soluble in aqueous solutions. 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2 tetrazolium (MTS) tests showed excellent cytocompatibility of all CNOs analyzed at 30 and 300 μg mL−1, so these carbon nanostructures can be safely used for biological applications. The first covalent functionalization of oxidized CNOs (ox-CNOs) with biomolecules, by using biotin–avidin interactions is reported here. Multilayers were prepared on a gold surface by layer-by-layer assembly and the process was monitored by surface plasmon resonance (SPR) spectroscopy and atomic force microscopy (AFM). Covalent binding of molecules to the short amine-terminated organosulfur monolayers was assessed by Fourier transform infrared spectroscopy using total attenuated reflactance mode (FT-IR/HATR).
Bingel–Hirsch derivatives of the trimetallic nitride template endohedral metallofullerenes (TNT-EMFs) Sc3N@Ih-C80 and Lu3N@Ih-C80 were prepared by reacting these compounds with 2-bromodiethyl malonate, 2-bromo-1,3-dipyrrolidin-1-ylpropane-1,3-dionate bromide, and 9-bromo fluorene. The mono-adducts were isolated and their 1H NMR spectra showed that the addition occurred with high regioselectivity at the [6,6] bonds of the Ih-C80 fullerene cage. Electrochemical analysis showed that the reductive electrochemistry behavior of these derivatives is irreversible at a scan rate of 100 mV s−1, which is comparable to the behavior of the pristine fullerene species. The first reduction potential of each derivative is either cathodically or anodically shifted by a different value, depending on the attached addend. Bis-adducts containing EtOOC-C-COOEt and HC-COOEt addends were isolated by HPLC and in the case of Sc3N@Ih-C80 the first reduction potential exhibits a larger shift towards negative potentials when compared to the mono-adduct. This observation is important for designing acceptor materials for the construction of bulk heterojunction (BHJ) organic solar cells, since the polyfunctionalization not only increases the solubility of the fullerene species but also offers a promising approach for bringing the LUMO energy levels closer for the donor and the acceptor materials.
The regioselectivity of 1,3-dipolar cycloaddition reactions between N-(4-thiocyanatophenyl)glycine and tetrakis[bis(ethoxycarbonyl)methylene]-C60 (7) was studied. The mono-addition resulted in two penta-adduct isomers (8 and 9) in a 1:1 ratio. A second addition to each compound led to, in the case of 8, two hexa-adduct isomer products (10 and 11), whereas addition to 9 led to 11 and 12. Finally, reaction of 11 and the glycine afforded a third addition, giving rise to hepta-adduct isomers 13 and 14. Preparative TLC and column chromatography were used to separate and purify these compounds. The structures were assigned based on their molecular symmetries as analyzed by 1H, 13C, and 2D NMR spectra and MALDI-TOF MS.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
Seit dem ersten Fund von endohedralen Metallofullerenen (EMFs) hat sich die Suche nach Kohlenstoffkäfigen mit eingeschlossenen Metallen oder kleinen Molekülen rasch zu einem sehr aktiven Forschungsgebiet entwickelt. EMFs weisen besondere elektronische und Struktureigenschaften auf und finden vielfache Anwendungsmöglichkeiten. Noch mehr Potenzial bieten funktionalisierte EMFs, die besser löslich sind und durch Röntgenkristallographie und andere Techniken leicht charakterisiert werden können. Wir geben hier eine Übersicht über das allgemeine Gebiet der EMFs und insbesondere der funktionalisierten EMFs. Wir befassen uns in diesem Aufsatz mit der Struktur und den (elektrochemischen) Eigenschaften dieser faszinierenden Verbindungen und werden auch auf Anwendungen eingehen.
Ever since the first experimental evidence of the existence of endohedral metallofullerenes (EMFs) was obtained, the search for carbon cages with encapsulated metals and small molecules has become a very active field of research. EMFs exhibit unique electronic and structural features, with potential applications in many fields. Furthermore, functionalized EMFs offer additional potential applications because of their higher solubility and their ease of characterization by X-ray crystallography and other techniques. Herein we review the general field of EMFs, particularly of functionalized EMFs. We also address their structures and their (electrochemical) properties, as well as applications of these fascinating compounds.
The first pyrrolidine and cyclopropane derivatives of the trimetallic nitride templated (TNT) endohedral metallofullerenes Ih-Sc3N@C80 and Ih-Y3N@C80 connected to an electron-donor unit (i.e., tetrathiafulvalene, phthalocyanine or ferrocene) were successfully prepared by 1,3-dipolar cycloaddition reactions of azomethine ylides and Bingel–Hirsch-type reactions. Electrochemical studies confirmed the formation of the [6,6] regioisomers for the Y3N@C80-based dyads and the [5,6] regioisomers in the case of Sc3N@C80-based dyads. Similar to other TNT endohedral metallofullerene systems previously synthesized, irreversible reductive behavior was observed for the [6,6]-Y3N@C80-based dyads, whereas the [5,6]-Sc3N@C80-based dyads exhibited reversible reductive electrochemistry. Density functional calculations were also carried out on these dyads confirming the importance of these structures as electron transfer model systems. Furthermore, photophysical investigations on a ferrocenyl–Sc3N@C80-fulleropyrrolidine dyad demonstrated the existence of a photoinduced electron-transfer process that yields a radical ion pair with a lifetime three times longer than that obtained for the analogous C60 dyad.
Three new families of trimetallic nitride template endohedral metallofullerenes (TNT EMFs), based on cerium, praseodymium, and neodymium clusters, were synthesized by vaporizing packed graphite rods in a conventional Krätschmer–Huffman arc reactor. Each of these families of metallofullerenes was identified and characterized by mass spectroscopy, HPLC, UV/Vis-NIR spectroscopy, and cyclic voltammetry. The mass spectra and HPLC chromatograms show that these larger metallic clusters are preferentially encapsulated by a C88 cage. When the size of the cluster is increased, the C96 cage is progressively favored over the predominant C88 cage. It is also observed that the smaller cages (C80–C86) almost disappear on going from neodymium to cerium endohedral metallofullerenes. The UV/Vis-NIR spectra and cyclic voltammograms confirm the low HOMO–LUMO gap and reversible electrochemistry of these M3N@C88 metallofullerenes.
While the trimetallic nitrides of Sc, Y and the lanthanides between Gd and Lu preferentially template C80 cages, M3N@C80, and while those of Ce, Pr and Nd preferentially template the C88 cage, M3N@C88, we show herein that the largest metallic nitride cluster, La3N, preferentially leads to the formation of La3N@C96 and to a lesser extent the La3N@C88. This is the first time that La3N is successfully encapsulated inside fullerene cages. La3N@C2n metallofullerenes were synthesized by arcing packed graphite rods in a modified Krätschmer–Huffman arc reactor, extracted from the collected soot and identified by mass spectroscopy. They were isolated and purified by high performance liquid chromatography (HPLC). Different arcing conditions were studied to maximize fullerene production, and results showed that yields have a high La2O3/C dependence. Relatively high yields were obtained when a 1:5 ratio was used. Three main fractions, La3N@C88, La3N@C92, and La3N@C96, were characterized by UV/Vis-NIR and cyclic voltammetry. Unlike other trimetallic nitride metallofullerenes of the same carbon cage size, La3N@C88 exhibits a higher HOMO–LUMO gap and irreversible reduction and oxidation steps.
A novel series of biaryl-type, meso,meso-linked and planar, triply fused diporphyrin derivatives was prepared and fully characterized together with the corresponding monoporphyrin control compounds. They feature peripheral meso-3-cyanophenyl and meso-3,5-cyanophenyl groups, which have previously been shown to undergo transformation into malonates without perturbation of the porphyrin core and subsequent Bingel addition to fullerene C60. The tetrapyrrolic metal binding sites in the diporphyrin arrays are either complexed to two ZnII or CuII ions, or, in a mixed coordination, to one CuII and one ZnII ion; alternatively, one or both sites remain unoccupied. The interaction between the differentially metallated porphyrin rings was systematically investigated by UV/Vis spectroscopy and electrochemistry. Cyclic voltammetry and differential pulse voltammetry reveal that electronic communication in the diporphyrin arrays varies strongly with the mode of connection (meso,meso-linked or triply fused), the nature of the bound metal ion, and the number of peripheral cyano groups. The electrochemical HOMO–LUMO gap in both series of diporphyrins is strongly but differentially affected by the choice of the inserted metal ions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)
Room for expansion: A versatile protocol for the synthesis of a new generation of strained cross-conjugated macrocycles (expanded radialenes) has been achieved. This method uses the Sonogashira cross-coupling reaction to prepare expanded radialenes, bisradialenes, and radiaannulenes. Preliminary electronic, redox, and structural characterization of the macrocycles are presented.
Nd3N pushes the limits: Unlike all the other encapsulated trimetallic nitride clusters (Sc3N, Y3N, Dy3N, Gd3N, …), which prefer a C80 cage, the Nd3N cluster is preferentially encapsulated in a larger C88 cage (see the HPLC trace in the picture). UV/Vis/NIR spectroscopy and electrochemistry performed on the isolated Nd3N@C88 fraction show a very small band gap for this compound.
The carbon nanoparticles obtained from either arcing of graphite under water or thermal annealing of nanodiamonds are commonly called carbon nano onions (CNOs), or spherical graphite, as they are made of concentric fullerene cages separated by the same distance as the shells of graphite. A more careful analysis reveals some dramatic differences between the particles obtained by these two synthetic methods. Physicochemical methods indicate that the CNOs obtained from nanodiamonds (N-CNOs) are smaller and contain more defects than the CNOs obtained from arcing (A-CNOs). These properties explain the enhanced reactivity of the N-CNOs in cycloaddition and oxidation reactions, as well as in reactions involving radicals. Given the easier functionalization of the N-CNOs, they are the most obvious choice for studying the potential applications of these multi-shelled fullerenes.
Mixed bis-adduct derivatives of C60 containing a pyrrolidine and a malonate methano group were synthesized. Three regioisomers, the e′, the trans-2, and the trans-3, were isolated and characterized. In-depth NMR studies of these methano-pyrrolidinofullerenes showed that the nitrogen inversion on the pyrrolidine moiety is not a fast event in the 1H NMR time scale as previously regarded. Solvent effects, variable temperature experiments, and protonation of the pyrrolidine nitrogen are addressed.
Raum zum Wachsen: Ein vielseitiges Syntheseprotokoll für neuartige gespannte Makrocyclen mit Kreuzkonjugation ermöglicht den Aufbau von erweiterten Radialenen, Bisradialenen und Radiaannulenen durch eine Sonogashira-Kreuzkupplung. Erste Ergebnisse zu den elektronischen und Redoxeigenschaften sowie zur Struktur der Makrocyclen werden vorgestellt.
Mehr Platz: Im Unterschied zu allen anderen trimetallischen Nitridclustern (Sc3N, Y3N, Dy3N, Gd3N, …), die einen C80-Käfig bevorzugen, verkapselt sich der Nd3N-Cluster bevorzugt in einem größeren C88-Käfig (siehe die HPLC-Spur). UV/Vis/NIR-Spektroskopie und elektrochemische Studien an der isolierten Nd3N@C88-Fraktion weisen auf eine sehr kleine Bandlücke dieser Verbindung hin.
Retro ist angesagt: Pyrrolidinofullerene reagieren durch Retrocycloaddition mit einem Überschuss an Dipolarophilen (Maleinsäureanhydrid oder N-Phenylmaleimid) in hohen Ausbeuten zu den entsprechenden Fullerenen (C60, C70 oder einem endohedralen C80-Metallofulleren; siehe Schema). Die Reaktion verläuft mit höheren Fullerenen effizient und führte zur Isolierung eines der Konstitutionsisomere von Sc3N@C80.
The photophysical, electrochemical, and self-assembly properties of a novel triply fused ZnII–porphyrin trimer were investigated and compared to the properties of a triply fused porphyrin dimer and the analogous monomer. The trimer exhibited significantly red-shifted absorption bands relative to the corresponding monomer and dimer. Electrochemical investigations indicated a clear trend in redox properties amongst the three porphyrin structures, with the lowest oxidation potential and the lowest HOMO–LUMO gap exhibited by the triply fused trimer. This electrochemical behavior is attributed to the extensive π-electron delocalization in the trimeric structure relative to the monomer and dimer. Additionally, it was found that the trimer forms extremely strong and nearly irreversible supramolecular interactions with single-walled carbon nanotubes (SWNTs), resulting in stable solutions of porphyrin–nanotube complexes in THF. Formation of these complexes required the addition of trifluoroacetic acid (TFA) to the solvent. This allowed the oligomers to make close contact with the nanotubes, enabling the formation of stable supramolecular assemblies. Atomic force microscopy (AFM) was used to observe the supramolecular porphyrin–nanotube complexes and revealed that the porphyrin trimer formed a uniform coating on the SWNTs. Height profiles indicated that nanotube bundles could be exfoliated into either individual tubes or very small bundles by exposure to the porphyrin trimer during sonication.
Going retro: Controlled-potential electrolysis (CPE) is employed as a synthetic tool in the selective removal of addends from fullerene adducts. Reductive CPE has been extensively used in the retro-cyclopropanation of fullerenes without affecting pyrrolidine addends also present. Now oxidative CPE for the selective removal of pyrrolidine addends in the presence of methano adducts is reported (see scheme).
Two C60 hexakis-adducts (2 and 3) were synthesized by using a protection–deprotection strategy. The symmetric fullerene tetrakis-adduct 8 was obtained by anthracene removal from the hexakis-adduct 7. Reaction of 8 with terpyridylglycine or pyridylglycine afforded two hexakis-adducts, 2 and 3. By using the retro-cyclopropanation reaction, the four malonate addends located on the equatorial belt of the hexakis-adducts were removed to afford two trans-1 bis-adducts, 4 and 5, with terpyridyl- or pyridylpyrrolidine groups. The structures of 2 and 3 were confirmed by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, and 1H, 13C, and COSY NMR, and UV-visible spectroscopy. The cyclic voltammograms of fullerene multiadducts 2, 3, and 9 show irreversible reductions. Self-assembled monolayers (SAMs) of 1 and 3 were formed on gold surfaces through nitrogen adsorption. SAMs of 3 represent the first example of a fullerene hexakis-adduct formed on gold surfaces through nitrogen adsorption. Controlled potential electrolyses (CPE) were conducted to prepare trans-1 bis-adducts 4 and 5 modified with terpyridyl and pyridyl groups.
Carbon nano-onions (CNOs) represent a still largely unexplored carbon allotrope. Promising properties of these unique carbon structures are driving the research efforts in this area, but many technical problems remain in their preparation, derivatization, separation and characterization. In this article, we report the preparation, partial purification, and multiple functionalization and solubilization of CNOs. With only one priorly published short communication1 describing CNO functionalization, the present work is the most comprehensive description of CNO functionalization and characterization to date.
We report the synthesis and physical properties of novel fullerene–oligoporphyrin dyads. In these systems, the C-spheres are singly linked to the terminal tetrapyrrolic macrocycles of rod-like meso,meso-linked or triply-linked oligoporphyrin arrays. Monofullerene–mono(ZnII porphyrin) conjugate 3 was synthesized to establish a general protocol for the preparation of the target molecules (Scheme 1). The synthesis of the meso,meso-linked oligopophyrin–bisfullerene conjugates 4–6, extending in size up to 4.1 nm (6), was accomplished by functionalization (iodination followed by Suzuki cross-coupling) of the two free meso-positions in oligomers 21–23 (Schemes 2 and 3). The attractive interactions between a fullerene and a ZnII porphyrin chromophore in these dyads was quantified as ΔG=−3.3 kcal mol−1 by variable-temperature (VT) 1H-NMR spectroscopy (Table 1). As a result of this interaction, the C-spheres adopt a close tangential orientation relative to the plane of the adjacent porphyrin nucleus, as was unambiguously established by 1H- and 13C-NMR (Figs. 9 and 10), and UV/VIS spectroscopy (Figs. 13–15). The synthesis of triply-linked diporphyrin–bis[60]fullerene conjugate 8 was accomplished by Bingel cyclopropanation of bis-malonate 45 with two C60 molecules (Scheme 5). Contrary to the meso,meso-linked systems 4–6, only a weak chromophoric interaction was observed for 8 by UV/VIS spectroscopy (Fig. 16 and Table 2), and the 1H-NMR spectra did not provide any evidence for distinct orientational preferences of the C-spheres. Comprehensive steady-state and time-resolved UV/VIS absorption and emission studies demonstrated that the photophysical properties of 8 differ completely from those of 4–6 and the many other known porphyrin–fullerene dyads: photoexcitation of the methano[60]fullerene moieties results in quantitative sensitization of the lowest singlet level of the porphyrin tape, which is low-lying and very short lived. The meso,meso-linked oligoporphyrins exhibit 1O2 sensitization capability, whereas the triply-fused systems are unable to sensitize the formation of 1O2 because of the low energy content of their lowest excited states (Fig. 18). Electrochemical investigations (Table 3, and Figs. 19 and 20) revealed that all oligoporphyrin arrays, with or without appended methano[60]fullerene moieties, have an exceptional multicharge storage capacity due to the large number of electrons that can be reversibly exchanged. Some of the ZnII porphyrins prepared in this study form infinite, one-dimensional supramolecular networks in the solid state, in which the macrocycles interact with each other either through H-bonding or metal ion coordination (Figs. 6 and 7).
A series of subphthalocyanine–phthalocyanine dyads has been prepared by means of palladium-catalyzed cross-coupling reactions between a monoalkynylphthalocyanine and different monoiodosubphthalocyanines. Electronic coupling between the two photoactive units is ensured by a rigid and π-conjugated alkynyl spacer. In addition, the electronic characteristics of the subphthalocyanine moiety were modulated by the introduction of different peripheral substituents. Cyclic and Osteryoung square-wave voltammetry experiments revealed that the reduction potential of this subunit can be decreased by about 400 mV on going from thioether or no substituents to nitro groups. As a consequence, the energy level of the charge-transfer state could be fine-tuned so as to gain control over the fate of the photoexcitation energy in each subunit. The diverse steady-state and time-resolved photophysical techniques employed demonstrated that, when the charge-transfer state lies high in energy, a quantitative singlet–singlet energy-transfer mechanism from the excited subphthalocyanine to the phthalocyanine takes place. On the contrary, stabilization of the radical pair by lowering the redox gap between electron donor and acceptor results in a highly efficient photoinduced electron-transfer process, even in solvents of low polarity such as toluene (ΦET≈0.9). These features, together with the extraordinary absorptive cross section that these molecular ensembles display across the whole UV/Vis spectrum, make them model candidates for application in situations where broadband light sources are needed.
Se han preparado una serie de díadas de subftalocianina-ftalocianina a través de reacciones de acoplamiento cruzado catalizadas por paladio entre una monoalquinilftalocianina y distintas monoiodosubftalocianinas. El empleo de un espaciador rígido y π-conjugado de tipo alquinilo asegura el acoplamiento electrónico entre ambas unidades. Además, las características electrónicas de la unidad de subftalocianina han sido moduladas mediante la introducción de diferentes sustituyentes periféricos. De hecho, los experimentos de voltametría cíclica y de onda cuadrada de Osteryoung han revelado que el potencial de reducción de esta unidad puede disminuirse en aproximadamente 400 mV al ir desde sustituyentes tioéter o ningún sustituyente, a grupos nitro. Como consecuencia, el nivel de energía del estado de transferencia de carga puede ajustarse con el fin de alcanzar un control sobre el destino de la energía de fotoexcitación en cada subunidad. Las diversas técnicas fotofísicas empleadas han demostrado que, en aquellos casos en los que el estado de separación de cargas posee una energía elevada, se produce una transferencia de energía cuantitativa desde el estado singlete de la subftalocianina al singlete de la ftalocianina. Por el contrario, la estabilización del par radical mediante la disminución del gap redox entre el dador y el aceptor de electrones da lugar a un proceso de transferencia electrónica fotoinducida muy eficiente, incluso en disolventes de polaridad moderada como el tolueno (ΦET∼0.9). Estas características, junto con la intensa absorción que presentan estas moléculas en todo el espectro de UV/Vis, las convierte en excelentes candidatos para su aplicación en aquellos casos en los que se necesiten fuentes de luz de banda ancha.
The electrochemical and photophysical properties of molecular architectures consisting of oligomeric meso,meso-linked oligoporphyrin rods linked at both extremities to methanofullerene moieties are presented in comparison to those of model systems. Cyclic voltammetry data evidence the presence of a strong intramolecular electronic coupling along the porphyrin oligomers that varies slightly with their length. This interaction affects the redox potentials of both fullerene and porphyrin moieties. The electronic coupling between the two chromophores is confirmed by comparing the redox potentials of porphyrin arrays before and after attachment of the carbon sphere. Electronic absorption, fluorescence, and phosphorescence spectra of the porphyrin oligomers in toluene are reported, which provide the energy of the lowest singlet and triplet electronic excited states. In the fullerene–porphyrin conjugates, ground-state charge-transfer (CT) interactions are evidenced by low-energy absorption features above 750 nm. These systems also exhibit near-infrared (NIR) CT luminescence in toluene with lifetimes shorter than 1000 ps. On increasing the solvent polarity (from toluene to Et2O and THF), CT emissions become progressively weaker, red-shifted, and shorter lived, which reflects the energy-gap law and Marcus inverted region effects. Luminescence is not detected in benzonitrile. Picosecond transient absorption spectroscopy of the porphyrin–fullerene conjugates allows detection of the porphyrin cation as a clear fingerprint for electron transfer. The rate of charge recombination is in agreement with CT luminescence lifetimes, which confirms the occurrence of NIR radiative back-electron transfer.
The synthesis and self-assembled monolayer (SAM) formation of three conformational isomers of p-tert-butylcalix[4]crown-6 derivatives 3a−3c and one p-tert-butylcalix[4]arene derivative (3d) are reported. The SAMs of these compounds were prepared and characterized by electrochemistry, reflection-absorption infrared spectroscopy (RAIRS) and contact-angle measurements. The recognition properties of these SAMs with metal cations were systematically investigated using electrochemical techniques and impedance spectroscopy. The monolayers exhibit selective binding and sensing abilities with alkaline earth metal cations such as Ca2+ and Ba2+ over alkali metal ions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
The efficient removal of methano addends from fullerenes using reductive electrochemistry is a versatile and useful reaction, which was initially called the retro-Bingel reaction since it was observed for the first time on bis(alkoxycarbonyl)methano adducts, commonly known as Bingel adducts. Since its initial discovery, this reaction has been successfully employed in the separation of enantiomers and constitutional isomers of fullerenes, which in some cases were otherwise not accessible. Even more interesting than the retro-Bingel reaction was the electrochemically induced isomerization that bis-methano adducts of C60 exhibit, and which was referred to as the “shuffle”. The combination of both reactions gave interesting results during the electrolysis of tetrakis-, pentakis-, and hexakis-Bingel adducts of C60. The selective removal of Bingel-type addends, while leaving other addends undisturbed, was also investigated. More recently, it was observed that in addition to bis(alkoxycarbonyl)methano adducts, electrochemical reduction of other methano adducts, such as spiromethanofullerenes, could also result in removal of the methano addends, thus the reaction was more generally denoted as retro-cyclopropanation. In some of the spiromethanofullerenes investigated, and depending on the solvent used, an electrochemically induced intermolecular addend transfer was observed, which provided a regioisomeric distribution that differed significantly from that obtained by a synthetic route. We have recently shown that singly bonded dimers are formed as intermediates during retro-cyclopropanation reactions. In this microreview, we describe the general observations that led to the initiation of these exciting electrochemical investigations, review the differences between the chemical and electrochemical methodologies in retro-cyclopropanation reactions and explore the mechanisms involved during addend removal. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
Up to fifteen electrons are reversibly accommodated in a triply fused porphyrin dimer conjugated to two [60]fullerene moieties. Its photophysical properties differ completely from those of the many known porphyrin–fullerene dyads: Photoexcitation of the C60 moieties results in quantitative sensitization of the low-lying (about 1 eV) and very short lived lowest singlet level of the porphyrin sheet (see scheme).
Ein dreifach verbrücktes Porphyrindimer, das mit zwei [60]Fulleren-Einheiten konjugiert ist, kann bis zu fünfzehn Elektronen reversibel aufnehmen. In seinen photophysikalischen Eigenschaften unterscheidet es sich grundlegend von den bislang bekannten Porphyrin-Fulleren-Konjugaten: Photochemische Anregung der C60-Einheiten führt zur quantitativen Sensibilisierung des tiefliegenden (ca. 1 eV) und sehr kurzlebigen niedrigsten Singulett-Zustands des Porphyrinteils (siehe Schema).
Three C60 derivatives, 1, 2 and 3, have been studied by cyclic voltammetry (CV) under high vacuum in anhydrous tetrahydrofuran (THF). The CV behavior was essentially similar to that already observed for other cyclopropanated fullerene derivatives. After the second reduction processes all compounds undergo a chemical reaction that generates another electroactive species. This “new” chemical species is likely to be the compound with the cyclopropane ring open. Differences in CV behavior were observed for the different addends. Electrochemical data obtained at different scan rates for a given potential window, were fit with the BAS digital simulation program, DigiSim. The purpose of this study was to probe the proposed mechanisms and to obtain reliable estimations of the kinetic constants for the homogeneous chemical reactions taking place during the CV experiments. Calculations at the PM3 level lend additional support to the conclusions derived from digital simulations. The proposed mechanism is similar for all the compounds and involves two main chemical reactions in a reversible square scheme.
Four methanofullerene derivatives, with phosphonate or sulfone groups attached to a C60 core through a Bingel procedure, were synthesized to probe their stability upon electrolytic reduction. Derivatives 1 and 2 are the most stable upon electroreduction and do not exhibit retro-cyclopropanation reactions until more than three electrons per C60 derivative are transferred. The cyclopropane ring is then removed and C60(>CH2)n (n=1–3) products result from reactions of the trianion of C60 with the solvent, CH2Cl2. The situation with diphosphonate 3 or phosphonatecarboxylate 4 is dramatically different. For 3, quantitative retro-cyclopropanation occurs when 2.8 e− per molecule are transferred. In the case of 4, when more than two electrons per molecule are transferred, there is evidence of the reversible formation of a very stable intermediate, which is oxidized at a potential 500 mV more positive than the first fullerene-based reduction of the parent compound. Electrolysis of a simple C70-Bingel monoadduct (5) also exhibits the formation of a similar intermediate. On the basis of cyclic voltammetry, ESR spectroscopy, and MALDI analysis of products, the intermediate observed during the electrolysis of compounds 4 and 5 is assigned to a dimeric structure.
