Soluble Fullerene Research Articles

The nature of the noncovalent interactions between fullerene C60 and aromatic hydrocarbons

To better understand the solubility of fullerenes in common aromatic organic solvents such as benzene, toluene, tetralin, 1,2,4-trimethylbenzene and 1-methylnaphthalene, the structures, energies and electronic properties of the complexes formed by fullerene C60 with benzene, toluene, tetralin, 1,2,4-trimethylbenzene and 1-methylnaphthalene, respectively, have been investigated by the density functional theory with long-range dispersion correction. It was found that the solubility of C60 in these solvents is generally proportional to the intermolecular force between solute molecule and solvent molecule. In order to further understand the nature of the noncovalent interactions between C60 and these aromatic hydrocarbons, interaction energy decomposition analyses were performed with the symmetry adapted perturbation theory. The results clearly show that the noncovalent interactions between C60 and the aromatic hydrocarbons are mainly dependent on both electrostatic and dispersion components, with dispersion playing the dominant role. The other important finding is that the nature of the noncovalent interactions between C60 and alkylbenzene or alkylnaphthalene is almost the same as the nature of the noncovalent interactions between C60 and benzene.

Simultaneous Top and Bottom Perovskite Interface Engineering by Fullerene Surface Modification of Titanium Dioxide as Electron Transport Layer.

Optimization of the interface between the electron transport layer (ETL) and the hybrid perovskite is crucial to achieve high-performance perovskite solar cell (PSC) devices. Fullerene-based compounds have attracted attention as modifiers on the surface properties of TiO2, the archetypal ETL in regular n-i-p PSCs. However, the partial solubility of fullerenes in the aprotic solvents used for perovskite deposition hinders its application to low-temperature solution-processed PSCs. In this work, we introduce a new method for fullerene modification of TiO2 layers derived from nanoparticles (NPs) inks. Atomic force microscopy characterization reveals that the resulting ETL is a network of TiO2-NPs interconnected by fullerenes. Interestingly, this surface modification enhances the bottom interface of the perovskite by improving the charge transfer as well as the top perovskite interface by reducing surface trap states enhancing the contact with the p-type buffer layer. As a result, rigid PSCs reached a 17.2% power conversion efficiency (PCE), while flexible PSCs exhibited a remarkable stabilized PCE of 12.2% demonstrating the potential application of this approach for further scale-up of PSC devices.

A theoretical study on the C30X15Y15 (X = B, and Al; Y = N, and P) heterofullerenes

Using density functional theory calculations, we investigated the stability, solubility, structural, electronic, and optical properties of C30X15Y15 (X=B, and Al; Y=N, and P) heterofullerenes. The C30B15N15 is found to be the most stable heterofullerene with standard formation enthalpy (ΔH°f) of 222.2kcal/mol, followed by C30Al15N15 (ΔH°f=891.6kcal/mol). The relative stability of fullerenes are as follows: BN>C60>AlN∼BP>AlP, indicating that the BN doping makes the C60 more stable. The XY doping process significantly increases and decreases the solubility of C60 fullerene in water and benzene solvents, respectively. The standard Gibbs free energy of solvation (ΔG°solv) of C60 in water is increased from −0.7 to −33.5kcal/mol by AlN doping process. The dopant X atoms are mainly contributed in generation of occupied states but the Y ones largely create the virtual states. Compared to the pristine C60, the electrical conductivity of the XY doped fullerenes are higher, and the order is as follows: AlP>BP>BN∼AlN>>C60. This indicates that the Al atoms destabilize and P ones stabilize the HOMO and LUMO levels, respectively. The optical gap of these fullerenes is smaller than the HOMO-LUMO gap by about 0.44–0.55eV.

Pressure dependence of the solubility of light fullerenes in n-nonane

Solubility of light fullerenes (C60 and C70) in n-nonane was investigated in the ranges of pressure form 0.1MPa up to 100MPa and temperature from 298.3K to 353.3K. Under isothermal conditions, the solubility, expressed as weight fraction of the fullerene in the solution, increases monotonously with increasing pressure. At ambient pressure, we have found that the temperature dependence of the solubility of C60 in n-nonane is non-monotonic: the solubility diagram consists of two branches corresponding to crystallization of different solid phases and one invariant point corresponding to simultaneous saturation of both phases. At 0.1MPa, the solubility diagram of the binary system C70 – n-nonane in the analysed temperature range consists of only one branch corresponding to crystallization of non-solvated C70.

Structural investigation of the stable isotope <sup>13</sup>C labeled fullerenol

Fullerenes are novel carbon nanomaterials that hold great promise in diverse areas, but the limited solubility make them hardly dispersible in aqueous environment and the applications in hydrophilic systems are restricted, in particular for biomedical areas. Currently, chemical functionalization is the main approach to increase the hydrophilicity of fullerenes, where the soluble fullerene derivatives show lower toxicity and interesting bio-effects. Among these fullerene derivatives, hydroxylated fullerene (fullerenol) has good biocompatibility, antitumor activity, antivirus activity, antioxidation activity and photosensitive activity, thus is widely studied and applied. The structure and functionalization of fullerenol determine the bio-effects in biomedical applications. However, the structure of fullerenol is very complicated and the precise structure of fullerenol has not been achieved yet, such as the hydroxyl group number, the position of functional groups and the chemical forms of surface groups. Herein, we adopted 13C staple isotope labeling to investigate the chemical structure of fullerenol. 13C-skeleton labeled fullerene C60 was prepared by arc discharge method. Then, 13C-C60 was hydroxylated by tetrabutylammonium hydroxide (TBAH) amine alkali catalysis-oxidation to obtain 13C-fullerenol. The 13C-fullerenol was characterized by TOF-MS, IR, X-ray photoelectron spectroscopy (XPS), NMR to analyze the structure of fullerenol. The results indicated that 13C atoms were labeled on the skeleton of fullerene cage. Hydroxylation did not break the carbon cage, which was reflected by the strong C60 peak in the TOF-MS spectrum. The Poisson distribution of mass spectra indicated the isotopic effects and the skeleton labeling by 13C. Based on the TOF-MS, there were about five 13C atoms on each fullerene cage. The higher mass signals were assigned to the oxidized fullerene cage and the lack of very large mass signal suggested the detachment of functional groups during the laser irradiation. The oxygen containing groups were confirmed by IR spectrum. A 3340, 1372 and 1074 cm - 1 peaks were attributed to be C−O/C−OH groups. XPS analyses indicated the chemical components of fullerenol as C 76%−82%, O 14%−18% and Na 0.6%−4%. The C 1s XPS spectra were divided into three major components, namely 284.56−284.86 eV for pristine carbon, 286.07−286.38 eV for C−O and 288.57−288.82 eV for C=O. Based on these, the chemical formulation of fullerenol was defined as Na n +[C60O x (OH) y ] n - , where the oxidation degree of fullerenol was regulated by the alkali concentration. The 1H NMR spectrum showed signals at δ 1.23, 2.50 and 3.34. Only the weak peak at δ 1.23 was due to hydroxyl groups of fullerenol, the rest signals were due to water and dimethyl sulphoxide (DMSO). Enhanced 13C NMR signals were observed at δ 175, 137 and 75−80, which were assigned to C=C−O, C=C and C−OH. Other weak signals were found at δ 160 for O=C−OH, δ 100 for RO−RCH−OH or RO−R1CR2−OH and δ 54 for C atoms in epoxy structure. In conculsion, the structure of 13C-fullerenol was explore to reach the formula of Na n +[C60O x (OH) y ] n - and the exact numbers depended on the reaction parameters during the hydroxylation. 13C isotope labeling largely enhanced the 13C-NMR signals and revealed the intact fullerene cage during the hydroxylation according to the TOF-MS. The complicated surface functional groups were relfected by IR, XPS and NMR, where multiple forms were identified as hemiacetal/hemiketal groups, epoxy groups, carboxylic ester, carbonyl groups, and so on. The results would definitely benifit the ongoing exploration of fullerenol bio-effects/bioapplications and structure-activity relationship. The 13C labeled fullerene and fullerenol could be used for the quantification of carbon nanomaterials in biological systems.

Investigation of donor–acceptor copolymer films and their blends with fullerene in the active layers of bulk heterojunction solar cells by Raman microspectroscopy

Thin films made of three low-band gap donor–acceptor copolymers (CDTF, CDTDP and CDTDOP) composed of 4,6-bis(3′-dodecylthiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole-5′,5′-diyl as an electron-acceptor structural unit and various electron-donor structural units, such as 9,9-bis(2-ethylhexyl)fluorene-2,7-diyl, 2,5-didodecyl-1,4-phenylene and 2,5-didodecyloxy-1,4-phenylene, respectively, and thin films of their blends with various ratios of a soluble fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester ([60]PCBM) as an active layer for bulk heterojunction solar cells were studied by means of UV–vis absorption spectroscopy and Raman microspectroscopy. The molecules of CDTDP and CDTDOP possess the same main chains; they differ in the side-chain oxygen only, which changes the donor strength of the donor units. UV–vis and Raman studies allow us to show differences in the hindering of molecule planarization and aggregation in the blends. Absorption of the polymer films covered the whole visible spectral region and extended up to near infrared for CDTDOP. The absorption behavior of the CDTDP blend films qualitatively differed from the absorption behavior of the blend films of CDTF or CDTDOP. The Raman measurements were performed at two different laser excitation wavelengths (633 and 785 nm), which enabled the photoluminescence of both components in the Raman spectra to be distinguished. The Raman study was performed in different parts of the films, including the separated areas. It was proven that the separated areas in the blend films had higher contents of [60]PCBM than the rest of the films.

Search for arc-produced heterofullerenes

Replacing one or more of the carbon atoms in a fullerene cage with one or more non-carbon atoms is expected to result in a structure (heterofullerene) with significantly altered and potentially useful properties. While (C59N)- and (C69N)-based compounds can be synthesized via solution-phase modification of C60 and C70, respectively, a general, high-yield method for producing heterofullerenes has not been identified. We report the results of a search for heterofullerenes using atmospheric pressure photo-ionization mass spectrometry on samples of fullerene soot produced in nitrogen and carbon monoxide via the Krätschmer-Huffman arc technique. Contrary to earlier reports, we found no evidence of the presence of heterofullerenes in any of the samples we examined. X-ray photoelectron spectroscopy of fullerene soot produced in nitrogen indicates that while nitrogen is incorporated into the carbon matrix, the observed nitrogen-carbon bonding configurations do not permit the formation of fullerenes. We also report the discovery of production conditions under which argon can be used as a buffer gas and the yield of soluble fullerenes is comparable to the best yield achievable using helium. These findings may shed light on the general problem of the fullerene formation process.

(Invited) Colloidal Gold Nanoparticles Stabilized By Hydroxylated Fullerenes

Since bare gold nanoparticles are unstable, they have to be stabilized by protecting with ligands, stabilizing with polymers or immobilizing on solids. Properties of gold nanoparticles depends on the design of their protecting ligands. Fullerene C60, a polyhedral carbon allotrope, possesses unique physical and chemical properties such as electron accepting character, high reactivity to radical species, low electrical conductivity, high thermal stability and so on, therefore utilization of fullerene derivatives to metal particles might be interesting, indeed, there are some reports on Au nanocomposites with C60. In order to prepare the colloidal dispersion of the Au and fullerene composites, soluble fullerene derivatives should be used. We have developed a series of polyhydroxylated fullerenes, C60(OH)n, and among them, C60(OH)36exhibits high water solubility up to 58.9 mg/mL and high dispersion on a molecular level. Au:C60(OH)36 was synthesized by reduction with excess amount of NaBH4 from HAuCl4 aqueous solution in the presence of C60(OH)36. The colloidal particles are homogeneously dispersed in water and are stable for months. Its core size (measured by Transmission Electron Microscope) and colloid size (measured by Induced Grating Method) are 3.7 nm and 12.2 nm, respectively. X-rays photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements revealed that the nanogold is negatively charged in total, but rather positively charged on the surface due to the interface interaction with C60(OH)36. In this presentation, the catalytic activity of the thus-prepared gold nanoparticles will also be discussed.

Hierarchical Fullerene Assembly: Seeded Coprecipitation and Electric Field Directed Assembly.

Hierarchical C60 colloidal films are assembled from nanoscale to macroscale. Fullerene molecular crystals are grown via seeded cosolvent precipitation with mixed solvent [tetrahydronaphthalene (THN)/trimethylpyridine (TMP)] and antisolvent 2-propanol. The fullerene solutions are aged under illumination, which due to the presence of TMP reduces the free monomer concentration through fullerene aggregation into nanoparticles. The nanoparticles seed the growth of monodisperse fullerene colloids on injection into the antisolvent. Diverse colloidal morphologies are prepared as a function of injection volume and fullerene solution concentration. The high fullerene solubility of THN enables C60 colloids to be prepared in quantities sufficient for assembly (5 × 108 ). Electric fields are applied to colloidal C60 platelets confined to two dimensions. The particles assemble under dipolar forces, dielectrophoretic forces, and electrohydrodynamic flows. Frequency-dependent phase transitions occur at the critical Maxwell-Wagner crossover frequency, where the effective polarizability of the particles in the medium is substantially reduced. Structures form as a function of field strength, frequency, and confinement including hexagonal, oblique, string fluid, coexistent hexagonal-rhombic, and tetratic.

Magnetic Fullerene-DNA/Hyaluronic Acid Nanovehicles with Magnetism/Reduction Dual-Responsive Triggered Release.

We created the dual-responsive nanovehicle that can effectively combine and abundantly utilize magnetic and glutathione (GSH)-reductive triggers to control the drug delivery and achieve more intelligent and powerful targeting. In the nanovehicles, paramagnetic fullerene (C60@CTAF) was prepared via one-step modification of fullerene with magnetic surfactant CTAF by hydrophobic interaction for the first time. The perfect conjugation of C60 and CTAF increased the solubility or dispersity of fullerenes and qualified CTAF with more powerful assembly capability with DNA. DNA molecule in the nanovehicles acted as an electrostatic scaffold to load anticancer drug Dox as well as the important building block for assembly with C60@CTAF into C60@CTAF/DNA. The further combination of deshielding and targeting functions in reduction-responsive disulfide modified HA-SS-COOH coating on C60@CTAF/DNA complexes could reduce the agglomeration and regulate the morphology of C60@CTAF/DNA complexes from irregular microstructures to more uniform ones. More importantly, the introduction of HA-SS-COOH provided a response to a simulating reductive extra-tumoral environment by efficient cleavage of disulfide linkages by GSH and site-specific drug delivery to HepG2 cells. Amazingly, the final nanovehicles presented an increased magnetic susceptibility compared with paramagnetic CTAF, and they "walked" under an applied magnetic field. Because of their facile fabrication, rapid responsiveness to extra tumoral environment, and external automatic controllability by external magnet, the drug delivery nanovehicles constructed by magnetic fullerene-DNA/hyaluronic acid might be of great interest for making new functional nucleic-acid-based drug carriers.

High-fidelity self-assembly pathways for hydrogen-bonding molecular semiconductors

The design of molecular systems with high-fidelity self-assembly pathways that include several levels of hierarchy is of primary importance for the understanding of structure-function relationships, as well as for controlling the functionality of organic materials. Reported herein is a high-fidelity self-assembly system that comprises two hydrogen-bonding molecular semiconductors with regioisomerically attached short alkyl chains. Despite the availability of both discrete cyclic and polymeric linear hydrogen-bonding motifs, the two regioisomers select one of the two motifs in homogeneous solution as well as at the 2D-confined liquid-solid interface. This selectivity arises from the high directionality of the involved hydrogen-bonding interactions, which renders rerouting to other self-assembly pathways difficult. In thin films and in the bulk, the resulting hydrogen-bonded assemblies further organize into the expected columnar and lamellar higher-order architectures via solution processing. The contrasting organized structures of these regioisomers are reflected in their notably different miscibility with soluble fullerene derivatives in the solid state. Thus, electron donor-acceptor blend films deliver a distinctly different photovoltaic performance, despite their virtually identical intrinsic optoelectronic properties. Currently, we attribute this high-fidelity control via self-assembly pathways to the molecular design of these supramolecular semiconductors, which lacks structure-determining long aliphatic chains.

Water-Gated n-Type Organic Field-Effect Transistors for Complementary Integrated Circuits Operating in an Aqueous Environment.

The first demonstration of an n-type water-gated organic field-effect transistor (WGOFET) is here reported, along with simple water-gated complementary integrated circuits, in the form of inverting logic gates. For the n-type WGOFET active layer, high-electron-affinity organic semiconductors, including naphthalene diimide co-polymers and a soluble fullerene derivative, have been compared, with the latter enabling a high electric double layer capacitance in the range of 1 μF cm–2 in full accumulation and a mobility–capacitance product of 7 × 10–3 μF/V s. Short-term stability measurements indicate promising cycling robustness, despite operating the device in an environment typically considered harsh, especially for electron-transporting organic molecules. This work paves the way toward advanced circuitry design for signal conditioning and actuation in an aqueous environment and opens new perspectives in the implementation of active bio-organic interfaces for biosensing and neuromodulation.

Exploring Simple, Interpretable, and Predictive QSPR Model of Fullerene C60 Solubility in Organic Solvents

Buckminsterfullerene (C60) and its derivatives have currently been used as promising nanomaterial for diagnostic and therapeutic agents. They are applied in pharmaceutical industry due to their nanostructure characteristics, stability and hydrophobic character. Due to its sparingly soluble nature, the solubility of C60 has been of enormous attention among carbon nanostructure investigators owing to its fundamental importance and practical interest in nanotechnology and medical industry. In order to study the diverse role of C60 and its derivatives the dependence of fullerene's solubility on molecular structure of the solvent must be understood. Current study was dedicated to the exploration of the solubility of fullerene C60 in 156 organic solvents using simple, interpretable and predictive 1D and 2D descriptors employing quantitative structure-property relationship (QSPR) technique. The authors employed genetic algorithm followed by multiple linear regression analysis (GA-MLR) to generate the correlation models. The best performance is accomplished by the four-variable MLR model with internal and external prediction coefficient of Q2 = 0.86 and R2pred = 0.89. The study identified vital properties and structural fragments, particularly valuable for guiding future synthetic as well as prediction efforts. The model generated with the highest number of organic solvents to date with simple descriptors can be reproduced in no time to predict the solubility of C60 in any new or existing organic solvents. This approach can be used as an efficient predictor for fullerenes' solubility in various organic solvents.

Limitations with solvent exchange methods for synthesis of colloidal fullerenes

We have found demonstrated that the use of tetrahydrofuran (THF) as a solvent to produce dispersed, water soluble fullerenes results in significant oxidation and degradation of the fullerene cage, which has not been reported previously. We also report a new finding that the use of N, N-dimethylformamide (DMF) can also generate stabilised fullerene (C60) nanoparticle dispersions in aqueous solutions including water and phosphate buffered saline (PBS) buffer. We compare this new DMF method with the well-known THF method following an extensive chemical and physical analysis of the resulting nanoparticles. The exact mechanism of action behind this oxidation and degradation is unknown, however, the role of peroxides is likely. The method of solvent exchange based on the use of DMF results in the formation of fullerene nanoparticle agglomerates that are highly stable in PBS and water, while the THF agglomerates are only stable in water. However, caution should be applied when using these approaches due to the significant degradation of the fullerene cage observed when using various techniques such as dynamic light scattering (DLS), matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1H NMR) and cryo transmission electron microscopy (cryo-TEM). Our results show that the solvent exchange technique using THF results in partial oxidation and degradation of C60, interestingly, the DMF evaporative method results in greater oxidation and degradation of C60 but significantly enhanced colloidal stability in buffer.

Supramolecular Self-Organization of Fullerene Derivatives in Solutions Studied by Pulsed Field Gradient NMR Technique

Self-organization of a series of differently functionalized fullerene derivatives in solvents of different polarity was investigated by pulsed field gradient nuclear magnetic resonance on 1H, 19F and 31P nuclei. The hydrodynamic diameters were calculated for each fullerene derivative-solvent system on the basis of Stokes–Einstein model. It was shown that fullerene derivatives comprising no polar groups do not undergo any noticeable aggregation in carbon bisulfide, deutereted chloroform and toluene-d8. The particle diameters of 1.2–1.4 nm revealed for these solutions were very close to the van der Waals diameter of the fullerene derivative molecules. The fullerene derivatives comprising polar –COOH, –COOK and –P(O)(OH)2 groups appended to the carbon cage undergo self-assembling in solutions. The diameters of the formed clusters varied from 2.2 to 9.6 nm depending on the solvent and the temperature. The strongest tendency to self-assembling was revealed for water soluble fullerene derivatives in aqueous solutions where, presumably, water molecules are also incorporated in the structure of the clusters.

Toxic and DNA damaging effects of a functionalized fullerene in human embryonic lung fibroblasts

Water-soluble fullerenes have been studied as potential nanovectors and therapeutic agents, but their possible toxicity is of concern. We have studied the effects of F-828, a soluble fullerene [C60] derivative, on diploid human embryonic lung fibroblasts (HELFs) in vitro. F-828 causes complex time-dependent changes in ROS levels. Inhibition of Nox4 activity by plumbagin blocks F-828-dependent ROS elevation. F-828 induces DNA breaks, as measured by the comet assay and γH2AX expression, and the activities of the transcription factors NF-kB and p53 increase. F-828 concentrations>25μM are cytotoxic; cell death occurs by necrosis. Expression levels of TGF-β, RHOA, RHOC, ROCK1, and SMAD2 increase following exposure to F-828. Our results raise the possibility that fullerene F-828 may induce pulmonary fibrosis in vivo.

Influence of Blend Ratio and Processing Additive on Free Carrier Yield and Mobility in PTB7:PC71BM Photovoltaic Solar Cells.

Charge separation and extraction dynamics were investigated in high-performance bulk heterojunction solar cells made from the polymer PTB7 and the soluble fullerene PC71BM on a broad time scale from subpicosecond to microseconds using ultrafast optical probing of carrier drift and the integral-mode photocurrent measurements. We show that the short circuit current is determined by the separation of charge pairs into free carriers, which is strongly influenced by blend composition. This separation is found to be efficient in fullerene-rich blends where a high electron mobility of >0.1 cm2 V–1 s–1 is observed in the first 10 ps after excitation. Morphology optimization using the solvent additive 1,8-diiodooctane (DIO) doubles the charge pair separation efficiency and the short-circuit current. Carrier extraction at low internal electric field is slightly faster from the cells prepared with DIO, which can reduce recombination losses and enhance a fill factor.

Spectroscopic and theoretical insights on non-covalent interaction between fullerenes and Xantheno-linked benzo-15-crown-5 receptor in solution

This paper envisages the spectroscopic insights behind supramolecular interaction between xantheno-linked benzo-15-crown-5 (1) and fullerenes in solution. Estimation of binding constants (K) in various solvents gave rise to the highest binding affinity for C70 complex of 1 in toluene, i.e., K=133.000dm3⋅mol−1. Studies of binding as a function of solvent (K1,2-dichlorobenzene<Kchlorobenzene<Ktoluene) correlates inversely with fullerene solubility, indicating that desolvation of fullerene is a major factor for determining the magnitude of K. DFT calculations provide convincing support in favor of high value of K for C70-1 system.

Remanent magnetization measurements of polymeric materials containing fullerol C60(OH)24 before and after exposure to external magnetic field

ABSTRACTThe problem of low solubility of fullerene is surpassed by its functionalization with –OH groups making a new molecule called fullerol C60(OH)24. Materials used in this study are synthesized by incorporating fullerol into commercial soft contact lens material Definitive74 made by Contamac, UK. Three different mass concentrations of fullerol are used: 0.003%, 0.01% and 0.015%. Hence, three materials are produced and the fourth material, the standard one, was used as a reference sample. The purpose of incorporating fullerol into polymer was to develop new materials for soft contact lenses with improved optical and mechanical characteristics. The magnetic properties of polymers change due to fullerol presence therefore the aim was to show the differences in magnetic properties of the materials before and after the exposure to four different external influences. The measurements of a remanent magnetization of these materials are done by spinner magnetometer, JR-6A.

(Invited) Charge Generation in Bulk-Heterojunctions Based on Fullerene Heterodimers

Organic photovoltaic (OPV) cells remain to be one of the most attractive approaches for utilization of solar energy showing gradually increasing conversion efficiency of 11%. Significant efforts were directed to the improvement of the electron donating (polymer) part of the bulk-heterojunction (BHJ) solar cells. On the other hand, less advancement has been made on the acceptor side. Recently, a few examples of soluble fullerene hetero- and homodimers based on C60 and C70 linked through chemical bridges have been developed and were shown to competitively perform in solar cells. However, the electronic structure of these fullerene dimers is not well understood. Thus, it is not known how those two fullerene molecules are electronically coupled in the dimer, i.e., whether the excited neutral state or anion state of the dimer is delocalized over the whole dimer or still localized only on one fullerene cage. We have experimentally shown that fullerene molecules in the C60-C70-heterodimer in solid films are strongly electronically coupled and the spin densities of the anion and excited triplet states are delocalized over the whole molecular dimer. However, in the frozen diluted solutions the fullerene cages in heterodimers are instead weakly coupled. The anion and triplet states of the dimers show the signature of individual C60 or C70 molecules. We explain this phenomenon by the presence of two different conformers in the solid-state films and in frozen solutions. Since the electronic coupling between fullerene molecules in the dimers is influenced by their packing in the blends, our observation provides insight into how to tune the electronic properties of the fullerene acceptors by proper adjustment of molecular bridge structures between fullerene cages to fix the desired conformation, which in turn may have an impact on the charge carrier generation efficiency in organic solar cells.