Anionic Fullerene Research Articles

Superhalogens inside fullerenes X@C2n (X = BO2, BeF3; 2n = 60, 70).

The exploration of endohedral fullerenes has garnered significant attention recently due to their distinctive chemical, electrochemical, and optoelectronic properties. Charge transfer, which usually occurs from encapsulated species to fullerenes, importantly affects the structures and properties of endohedral fullerenes. In this study, we theoretically investigated endohedral superhalogen fullerenes X@C2n (X = BO2, BeF3; 2n = 60, 70), in which the charge is reversely transferred from the fullerene to the superhalogen, by using density functional theory calculations and ab initio molecular dynamics simulations. Both natural population analysis and the quantum theory of atoms in molecules confirm about one electron transfer from the fullerene to the superhalogen, resulting in the formal valence state of X-@C2n+. Energy decomposition analysis on the interaction between the superhalogen and fullerene revealed that electrostatic energy contributes predominantly to the total interaction energy. These endohedral superhalogen fullerenes with cationic fullerenes were predicted to be able to serve as building blocks for one dimensional fullerene-based nanowires when combined with endohedral alkali-metallofullerenes with anionic fullerenes.

Modifications in the angular photoemission time delay in Ar@C60q=−1 : Coulomb confinement resonance as an amplifier of the spin-orbit-interaction-activated interchannel coupling effect

Coulomb confinement resonances (CCRs) are the main features of the photoionization spectra of an atom inside an anionic fullerene. These CCRs can act as an amplifier to spin-orbit-interaction-activated interchannel coupling (SOIAIC) and can lead to discernible signatures of relativistic effects even for low $Z$ confined atomic systems. The present paper demonstrates this intriguing feature by studying angular photoemission time delay of the $2{p}_{3/2}$ subshell of $\mathrm{Ar}@{\mathrm{C}}_{60}^{\ensuremath{-}1}$ in the vicinity of the $2{p}_{1/2}$ ionization threshold. Here $2{p}_{1/2}$ CCR functions as an amplifier to the SOIAIC and, thereby, induces significant modifications in the $2{p}_{3/2}$ angular time delay through the interchannel coupling. Moreover, the effects of the SOIAIC for the spin-up and spin-down photoelectrons are found to be different. All of these suggest that angle-resolved time-delay measurements can reveal the relativistic effects even for the low Z confined atomic systems, and $\mathrm{Ar}@{\mathrm{C}}_{60}^{\ensuremath{-}1}$ is a potential candidate for such experimental scrutiny. The impact of the model parameters on the predicted feature is also critically examined by using different sets of parameters. All of those calculations showed the existence of SOIAIC enabled structures in the angular time-delay profile.

Development of Bio-active Fullerene Derivatives Suitable for Drug

Fullerene (C60) and fullerene derivatives are attractive novel compounds not only for carbon materials of nanotechnology but also for medical fields because of its unique chemical and physical properties. We intend to develop fullerene derivatives as novel lead compounds for drug discovery. At first, we synthesized many types of water-soluble fullerene derivatives to investigate their biological activities because of their poor solubility in water. We found that anionic fullerene derivatives possess anti-oxidant activities, whereas di-cationic fullerene derivatives exhibited antiproliferative activities against various cancer cell lines including drug-resistant cells. Proline-type fullerene derivatives showed inhibitory activities against human immunodeficiency virus (HIV) reverse transcriptase, HIV protease, hepatitis C virus (HCV) NS5B RNA polymerase, and HCV NS3/4A protease. These activities may strongly inhibit virus replication via a synergistic effect and fullerene derivatives may be used as novel multi-target drugs for the treatment of AIDS and hepatitis C in the future.

(Invited) Reactivity of Endohedral Metallofullerene Anions

Anionic fullerenes are nucleophilic reagents that can be easily generated by either chemically or electrochemically reductive methods. Electrochemical reduction is more convenient than chemical reduction since it is easy to generate the desired anions by controlling the potential and the charge transferred to the fullerene cages. Anionic C60 and C70 have been extensively used as nucleophilic reagents for fullerene functionalizations. The results clearly show the potential importance of fullerene anions as reagents to prepare functionalized fullerenes which are difficult to prepare in other ways. It is worth noting that different fullerene anionic states can lead to different products even under the same reaction conditions as a result of their different nucleophilicities. The chemistry of anionic fullerenes is an important complement to that of neutral species. The reactivity of anionic empty fullerenes is reasonably well developed, while the corresponding endohedral metallofullerene anions remain relatively less explored. The electrosynthetic method has not been widely applied to EMFs, and the reactivities of anionic EMF species have not been extensively studied. This prompted us to study the reactivity of EMF anions.

Dithiolation of [70]Fullerene with Aliphatic Primary Thiols in the Presence of n-Butylamine via Aerobic Oxidation Reaction.

Aerobic oxidation reaction of [70]fullerene with aliphatic primary thiols in the presence of n-butylamine was studied. The reaction afforded both the fullerene dithiol (2) and dithiol epoxide (3) compounds, which were characterized by 1H and 13C NMR, HRMS and UV-vis spectroscopies. In situ vis-NIR spectral measurement showed absorption bands due to anionic fullerene species, indicating that the reaction was likely initiated with the addition of thiolate anions to C70. Computational calculations were performed to rationalize the structure of the dithiol epoxide compounds. The electrochemical property of 2,5-( n-C5H11S)2C70 (2a) was compared with two analogue compounds of 2,5-Bn2C70 and 2-MeO-5-BnC70 that had identical addition pattern and number of addends. The result showed that the dithiol compound was more electron deficient than the other two derivatives.

(Invited) Aromaticity, Cage Structure, and Relative Abundancy of Endohedral Metallofullerenes

The synthesis of endohedral metallofullerenes (EMFs) from a carbon soot sample of an arc discharge leads to a variety of EMFs that are obtained in different relative abundances. Several different guiding principles as the relative size of the metallic cluster respect to fullerene cage, the number of electrons that are formally transferred from the metallic cluster to the fullerene cage and electrostatic repulsion between charged five-carbon rings have been used to explain the choice of the EMF most stable cages.1 However, no single factor was capable to predict the most stable isomer for all EMFs. Our group has proposed that maximum aromaticity is the driving force that determines the most suitable hosting carbon cage for a given metallic cluster.2 The most stable anionic fullerene isomer is the one whose total aromaticity is maximized. Our results show that maximum aromaticity criterion (MARC) successfully predicts the most suitable C2n cage isomer for the endohedral metallofullerenes with carbon cages from C66 to C104. Furthermore, we have used electronic-based aromaticity measures to show that relative abundances of Sc3N-based EMFs correlate with the aromaticity measures. Our results show that, for the Sc3N-based EMFs, the most abundant EMFs are the most aromatic. This result reinforces the idea that aromaticity plays a key role in determining the stability of EMFs. 1. Garcia-Borras, M.; Osuna, S.; Luis, J. M.; Swart, M.; Sola, M. Chem-ical Society Reviews 2014, 43, 5089.2. M. GarciaBorràs, S. Osuna, M. Swart, J.M. Luis, and M. Solà, Angew. Chem. Int. Ed. 2013, 52, 9275. Figure 1

Vis-Near-IR Spectroscopic and Time-Dependent DFT Study of Reduced Singly Bonded C60 Species.

Reduced fullerenes and fullerene derivatives exhibit intense absorptions in the vis-near-IR (vis-NIR) region. The absorptions are sensitive toward the addition pattern, number of addends, and oxidation state of the fullerene species and are used as an important benchmark for identifying anionic fullerene species. Similar absorptions are also shown for the reduced singly bonded C60 species, which are electronically different from reduced fullerene derivatives. However, much less work has been carried out on the vis-NIR spectroscopic study of the anionic singly bonded C60 species, likely due to the difficulty in controlled production of these species. Herein, we report the vis-NIR spectroscopic study of the mono- and dianions of the singly bonded C60 species (RC60(-), R(-)C60(-), and RC60(2-•)), which are selectively generated by controlled-potential bulk electrolysis (CPE) of the singly bonded C60 dimer and C60 oxazolino heterocycles. Time-dependent density functional theory (TD-DFT) calculations were performed to rationalize the experimental results.

(Invited) DAN Assisted Formation of Charge Transfer Complex from Highly Charged Fullerenes and Conjugated Polymer

Water-soluble, same charged, cationic conjugated poly(phenylenevinylene) (PPV) and cationic fullerenes were complexed with negatively charged single stranded DNA and double stranded DNA via electrostatic and pi-pi interactions. The photoinduced charge transfer complex (PPV/DNA/C60) has a charge transfer and separation efficiencies at least comparable to those observed from oppositely charged, cationic PPV and anionic fullerene. DNA serves as a template that straightens the PPV chain conformation which leads to an enhanced quantum yield. Meanwhile, DNA allows incorporation of C60 by overcoming the electrostatic repulsion between PPV and C60. While oppositely charge donor and acceptor molecules feature charge transfer in the form of static quenching, for DNA-assisted charge transfer complexes, the charge transfer proceeds as a combination of static and dynamic quenching for ssDNA and purely dynamic for dsDNA.

Magnetic Coupling and Optical Properties of theS6-Dodecakis(trifluoromethyl)fullerene Radical Anions in the Layered Salt (PPN+)[C60(CF3)12.−

Poly(trifluoromethyl)fullerene S6-C60(CF3)12 was reduced by sodium fluorenone ketyl in the presence of (PPN)Cl (PPN = bis(triphenylphosphine)iminium) to afford the salt (PPN)[C60(CF3)12] (1), which contains C60(CF3)12(·-) radical anions. In the crystal structure of 1, C60(CF3)12(·-) layers alternate with the PPN(+) cations. There are short F⋅⋅⋅F contacts between C60(CF3)12(·-) radical anions within the layers but no C⋅⋅⋅C contacts. DFT calculations revealed that the negative charge on C60(CF3)12(·-) is distributed mainly between sp(2) carbon and fluorine atoms, whereas spin density is localized mainly on the fullerene-cage sp(2) carbon atoms. IR and UV/Vis/NIR spectra in the solid state and solution showed characteristic changes relative to those of neutral S6-C60(CF3)12 due to the formation of radical anions. The solid-state electronic spectrum of 1 exhibits a single broad band at 738 nm attributed to C60(CF3)12(·-). Crystals of 1 show a narrow EPR signal with g = 2.0025 (ΔH = 0.45 mT) at 300 K. The temperature dependence of the integral intensity follows the Curie-Weiss law with a negative Weiss temperature of -11.8 K (30-300 K) indicating antiferromagnetic interaction of spins. This dependence was approximated by the Heisenberg model for one-dimensional chains of antiferromagnetically interacting spins with exchange interaction J/kB = -9.1 K. It was assumed that magnetic interaction between the C60(CF3)12(·-) spins in the layers is mediated by short F⋅⋅⋅F contacts.

Water-soluble PPV and C60 nanocomposite with enhanced miscibility and enhanced photo-induced charge transfer through ground state electrostatic interactions

We report the preparation of highly charged nanocomposites comprised of water-soluble, anionic fullerene and cationic poly-phenylenevinylene (PPV) derivatives. The nanocomposites display high fluorescence quenching efficiency (99%) presumably due to enhanced miscibility between cationic PPV and anionic C60 via electrostatic interactions. We show that complexation between the cationic PPV and anionic C60 derivatives leads to formation of nanocomposites with optical and electronic properties distinct from individual components without preferential electrostatic interactions. Photo-induced charge transfer quenches fluorescence from the PPV component is consistent with the frontier energy offsets of PPV and C60, and cyclic voltammetry and UV–Vis spectroscopy measurements. This result confirms high miscibility between donor and acceptor and resonance Raman spectra indicate a conformational changes of the PPV backbone upon complex formation.

Synthesis, Structural and Magnetic Properties of Ternary Complexes of (Me4P+)·{[Fe(I)Pc(−2)]−}·Triptycene and (Me4P+)·{[Fe(I)Pc(−2)]−}·(N,N,N′,N′-Tetrabenzyl-p-phenylenediamine)0.5 with Iron(I) Phthalocyanine Anions

Ternary complexes of (Me4P(+))·{[Fe(I)Pc(-2)](-)}·TPC (1) and (Me4P(+))·{[Fe(I)Pc(-2)](-)}·(TBPDA)0.5 (2) containing iron(I) phthalocyanine anions, tetramethylphosphonium cations (Me4P(+)), and neutral structure-forming triptycene (TPC) or N,N,N',N'-tetrabenzyl-p-phenylenediamine (TBPDA) molecules have been obtained as single crystals. In contrast to previously studied ionic compounds with monomeric [(Fe(I)Pc(-2)](-) anions, the anions form coordination {[Fe(I)Pc(-2)](-)}2 dimers both in 1 and 2, in which a nitrogen atom of one phthalocyanine anion weakly coordinates to the iron(I) atom of neighboring [Fe(I)Pc(-2)](-). The Fe···N distances in the dimers are 3.08(1) and 3.12(1) Å in 1 at 280 K and 2.986(5) (100 K) and 3.011(5) Å (180 K) in 2. The {[Fe(I)Pc(-2)](-)}2 dimers are packed in the layers in 1 arranged parallel to the ac plane and in isolated chains in 2 arranged along the a axis. Extended Hückel based calculation of intermolecular overlap integrals showed stronger and weaker π-π interactions within and between phthalocyanine dimers, respectively, both in 1 and 2. EPR signals of both complexes manifest two components. An major low-field asymmetric component is attributed to the Fe(I) atoms with the d(7) configuration. An origin minor narrow signal with g-factor close to the free-electron value (g = 2.0018-2.0035) is assigned to partial electron density transfer from the iron(I) center to the phthalocyanine macrocycle and the formation of the [Fe(II)Pc(-3)](-) species. Effective magnetic moments of the complexes of 1.69 (1) and 1.76 μB (2) correspond to the contribution of about one S = ½ spin per formula unit in accordance with low-spin state of [Fe(I)Pc(-2)](-). Negative Weiss temperatures of -7.6 K (1) and -13 K (2) in the 30-300 K range indicate antiferromagnetic interaction of spins in the phthalocyanine dimers. The multicomponent approach was previously proposed for the anionic fullerene complex formation. It also seems very promising to design and synthesize anionic phthalocyanine complexes with one- and two-dimensional macrocycle arrangements.

Behavior of octa(benzo-15-crown-5)- and tetrasulfophthalocyanines in the presence of water-soluble C60 compounds

Octa[(4′-benzo-15-crown-5)oxy]phthalocyanine was shown to dissolve in the presence of water-soluble fullerene compounds C60Cl(NHCH2CH2NH3+)5(−OOCCF3)5 or C60Cl(C6H4CH2CH2COONa)5. The solubilization of phthalocyanine was attributed to the possibility of formation of the hydrogen bonds N-H...O(CH2) and coordination of the sodium cations inside the cavities of the crown ether fragments for the cationic and anionic fullerene derivatives, respectively. Solubilization of the crown-containing phthalocyanine with participation of water-soluble fullerene compounds proceeded with the formation of aggregated forms. A monomeric form of nickel tetrasulfonated phthalocyaninate in the presence of an equimolar amount of C60Cl(NHCH2CH2NH3+)5(−OOCCF3)5 in 50% ethanol is transformed into a dimeric form. The structures of the fullerene-containing compounds and their state in solution facilitate the stacking aggregation of phthalocyanines.

Efficient light harvesting anionic heptamethine cyanine–[60] and [70]fullerene hybrids

New anionic fullerene–heptamethine conjugates absorbing as far as 900 nm have been synthesized and fully characterized by physicochemical means. In terms of electron transfer, we have succeeded in realizing the rarely occurring scenario of a charge transfer commencing with a higher excited state. Kinetic studies reveal that charge separation and charge recombination dynamics occur in the normal and inverted Marcus region, respectively.

Chemical redox recovery of giant, small-gap and other fullerenes

A simple method for extracting otherwise insoluble fullerenes into organic solvents is presented. The fullerenes are reduced to anionic charge states by contact with zinc in the presence of an alkylphosphonium salt in tetrahydrofuran. The anionic fullerenes become soluble, and the non-fullerene carbon matrix is separated by filtration. The anionic fullerenes can then be precipitated from solution by the action of a chemical oxidant such as iodine. It is also shown that there are approximately as many fullerenes left behind in carbon arc soot after solvent extraction as are extracted by the solvent. In contrast, more than twice as many fullerenes are left behind in combustion-produced fullerene soot as are solvent-extracted. Thus, approximately 70% of the total recovered carbon product from large-scale combustion synthesis is fullerenes. Conversely, no detectable amount of fullerenes can be recovered from conventional carbon blacks under the same conditions. Analysis by single photon ionization mass spectrometry suggests that C 60 and C 70 account for almost half of the fullerenes that were not solvent-extractable, suggesting that they are readily incorporated into toluene-insoluble fullerene polymers. The redox process presented here is easily up-scaled, and we routinely recover 100 g of heretofore insoluble fullerenes in 8 h.

Fabrication and Conductive Properties of Multilayered Ultrathin Films Designed by Layer-by-Layer Assembly of Water-Soluble Fullerenes

Fullerene ultrathin films were fabricated by layer-by-layer (LbL) assembly of an anionic fullerene C(61)(COO(-))(2) (FDCA) and poly(diallyldimethylammonium chloride) (PDDA) or a cationic fullerene C(60)C(2)H(4)N(CH(3))(2)(+) (FMAC) and poly(sodium 4-styrenesulfonate) (PSS). The dynamic light scattering and zeta-potential measurements revealed that both water-soluble fullerenes are stably dispersed as polyelectrolytes with a diameter of 20-70 nm for FDCA and 60-180 nm for FMAC in aqueous solutions. In spite of such large fullerene aggregates, the thickness of fullerene LbL films increased regularly by a few nanometers with each deposition, and the resultant LbL films were homogeneous. For FMAC/PSS LbL films, the monolayer thickness was evaluated to be 4 nm for FMAC and 0.5 nm for the PSS layer. In other words, the volume fraction of the fullerene moiety is as high as approximately 80 vol %. The conductivity of fullerene LbL films was comparable to that of a C(60)-dispersed polystyrene film with a similar fullerene fraction, suggesting that there exist effective percolating networks due to the high fullerene fraction in the LbL films. The electron mobility of FMAC/PSS LbL films was as high as 3 x 10(-5) cm(2) V(-1) s(-1), which is comparable to the hole mobility of poly(p-phenylenevinylene) LbL films reported previously.

Stability of multiply charged fullerene anions and cations

We present a systematic study of the stability of highly charged cationic and anionic fullerenes whose most stable neutral counterparts follow the isolated pentagon rule (IPR). In agreement with recent studies, we have found that, for many highly charged fullerenes, non-IPR isomers are significantly more stable than the IPR ones. To understand this behavior, we compare the results of elaborate density-functional theory (DFT) calculations to those of a simple H\uckel molecular-orbital theory in which the DFT energies of the corresponding neutral systems are used as a reference. The model leads to a reasonable estimate of the relative stability of the IPR and non-IPR isomers as a function of charge, which can be used to identify, among the thousands of possible isomers and charge states, the non-IPR species that are likely more stable than the IPR isomers.

Biological activities of water-soluble fullerene derivatives

Three types of water-soluble fullerene derivatives were synthesized and their biological activities were investigated. C60-dimalonic acid, an anionic fullerene derivative, showed antioxidant activity such as quenching of superoxide and relief from growth inhibition of E. coli by paraquat. C60-bis(7V,7V-dimethylpyrrolidinium iodide), a cationic fullerene derivative, has antibacterial activity and antiproliferative effect on cancer cell lines. The mechanism is suggested to be respiratory chain inhibition by reactive oxygen species produced by the cationic fullerene derivative. Proline-type fullerene derivatives showed strong inhibition activities on HIV-reverse transcriptase. The IC50 values were remarkably lower than nevirapine, a clinically used anti-HIV drug. Fullerene derivatives have a big potential for a new type of lead compound to be used as medicine.

Electrophoresis of electrostatically assembled fullerene–porphyrin conjugates

The formation of electrostatically coupled assemblies of a series of anionic dendritic fullerene derivatives and cationic porphyrins in buffered aqueous media was studied with gel electrophoresis. Of central interest in these investigations was the variation of the amount of charge carried by the molecules, their size, shape and self-aggregation. Ferric cytochrome c 1 and the more rigid zinc porphyrin served as octacationic species. The two new anionic fullerene derivates 3 and 6 were synthesized. The formation of electrostatic complexes of the fullerene polyelectrolytes 3 and 4 with 1and 2 was clearly evident in the gel electrophoresis experiment. Compounds and showed a similar behaviour towards 2. The electrophoresis experiments confirmed previous results obtained with other techniques on a qualitative level and gave new insights into aggregation phenomena.

An octa-cationic core-shell dendrimer as a molecular template for the assembly of anionic fullerene derivatives

An assembly comprising the non-covalent binding of eight mono-anionic fullerene subunits to a polyaryl ether dendrimer with an octa-cationic tetra[bis(benzylammonium)aryl]silane core has been prepared via a straightforward anion exchange reaction. The assembled octa-fullero-dendrimer has been characterized by NMR and UV–vis spectroscopy and its molecular weight determined by size-exclusion chromatography coupled with low angle light scattering (LALLS) techniques.

Anionic Fullerenes, Calixarenes, Coronenes, and Pyrenes as Activators of Oligo/Polyarginines in Model Membranes and Live Cells

We report that the efflux of 5(6)-carboxyfluorescein anions from neutral egg yolk phosphatidylcholine vesicles is mediated by oligo/polyarginines only in the presence of activating amphiphilic anions. Screening of anion activators reveals best synergism for amphiphilic carboxylates (fullerene > calix[4]arene approximately coronene > pyrene > calix[6]arene > alkyl), whereas amphiphilic sulfates show less satisfactory activation despite often lower effective concentrations. The analogous alcohols and one calix[4]arene diphosphate were inactive. These results are discussed in the context of a tentative anion carrier mechanism, where interactions with bilayer (interface-directed translocation) and carrier (arene-templated carboxylate-guanidinium pairing) contribute to activator efficiencies. Applied to HeLa cells, pyrenebutyrate is shown to significantly increase the uptake of a fluorescently labeled octaarginine in a concentration-dependent manner.