Rheological Behavior of Poly(vinylpyrrolidone)/Fullerene C60 Complexes in Aqueous Medium

Aqueous solutions of complexes formed between poly(vinylpyrrolidone) (PVP), as a matrix polymer, and fullerene C₆₀ were investigated. The effect of the external hydrodynamic field on the supermolecular assemblies formed by the complexes was analyzed. Despite the low content in the complexes (1.5 mass%), fullerene significantly modified the viscosity of aqueous PVP. Thus, the dynamic viscosity of the PVP/C₆₀ complexes grew faster than that of the pure PVP upon increasing the PVP/C₆₀ concentration. The difference in viscosities is especially pronounced for semidilute solutions. As a possible explanation, it is assumed that fullerenes act as crosslinks, in addition to the physical entanglements of the PVP macromolecules, which appear in the vicinity of the crossover concentration. Shear flow corresponding to the high shear rates destroyed fullerene‐induced intermolecular crosslinks in PVP/C₆₀ solutions.     

A theoretical study on the interaction between well curved conjugated systems and fullerenes smaller than C60 or larger than C70

A large body of data exists about the interaction between curved π systems and C₆₀ or C₇₀. However, little is known about the interaction with fullerenes smaller than C₆₀ or larger than C₇₀. To fill that gap, we studied, by means of density functional theory (M06‐2X), the interaction between corannulene, pentaindenocorannulene, C₆₀H₂₈ buckycatcher and the following fullerenes: C₄₄, C₅₀, C₈₀, C₉₀, C₁₀₀, C₁₈₀ and C₂₄₀. For fullerenes smaller than C₆₀, their high reactivity facilitated the covalent addition to the hosts assayed. Yet, the reaction energies determined for the covalent addition were comparable to those calculated for the formation of supramolecular complexes. Thus, the receptor may host a fullerene and at least have another one attached. As expected, for fullerenes larger than C₇₀, supramolecular complexes were preferred over covalent assemblies. The binding energies with bowls increased with the size of the fullerenes in a non‐monotonic fashion since they depended on the shape of the fullerene. Indeed, for one C₈₀ isomer, it is possible to find a region which forms a complex with corannulene that is stronger than C₆₀@corannulene, while another region exists whose interaction with corannulene is weaker. As the size of the fullerene becomes larger, ball–socket interactions are weakened, and CH–π interactions become important, accounting for the large interaction determined for corannulene and graphene. Finally, for the buckycatcher, the maximum encapsulation energy among the fullerenes assayed was displayed by C₉₀. The fullerenes C₈₀, C₉₀ and C₁₀₀ formed complexes with the buckycatcher which are stronger than in C₆₀@buckycatcher. Copyright © 2014 John Wiley & Sons, Ltd.     

Photoinduced quenching of the luminescence of singlet oxygen in fullerene solutions

The photoinduced quenching of the luminescence of singlet oxygen in solutions of C₆₀ and C₇₀ fullerenes in CCl₄ is studied. It is shown that intense pumping of the solutions by ultraviolet and visible radiation leads to formation of relatively long-lived fullerene-oxygen complexes, which are effective quenchers of the ¹Δ_g excited state of singlet oxygen. The behavior of these complexes depending on the experimental conditions (pumping intensity, concentration of fullerenes in the solution, and temperature) is studied. Decomposition of complexes with time is investigated. It is shown that, upon formation and gradual degradation of these complexes, the absorption spectra of the solutions change with time. The C₆₀ fullerene is shown to be more stable to the intense pumping, its concentration being completely restored with the decomposition of the complexes. At the same time, C₇₀ fullerene is partly irreversibly lost and the solution does not reach its initial concentration after decomposition of complexes.     

On the encapsulation of azafullerenes inside the single-walled carbon nanotubes: Density-functional theory based treatments

We theoretically studied the encapsulation of azafullerene (C₅₉N) inside the single-walled carbon nanotubes (SWCNTs) from the first-principles. Adsorption energy is calculated, and the azafullerene affinities for the typical semiconducting and metallic nanotubes are investigated and compared with those of pure C₆₀ fullerene. It has been found that the azafullerene as well as the fullerene affinity for the semiconducting nanotubes is stronger than that for the metallic ones, and the energy values and binding distances are typical for the physisorption. Our first-principles results indicate that the interaction between SWCNTs and azafullerenes is comparable with the nanotubes-C₆₀ system. The charge analysis shows, however, that the charges have been transferred from the cage to the tube in the azafullerene peapods, while in the fullerene peapods the charges were found to be transferred from the tube to the fullerene nanocage. Furthermore, it was found that the interaction between the considered fullerenes and host nanotubes strongly depends on the tube diameters.     

Impact of a physiological medium on the aggregation state of C<Subscript>60</Subscript> and C<Subscript>70</Subscript> fullerenes

The C₆₀ and C₇₀ fullerene-cluster size distribution in aqueous solutions and a physiological medium is studied via dynamic light scattering. The initial aqueous solutions of fullerenes obtained via different methods are found to contain clusters with a characteristic size of about 100 nm. The additional aggregation of fullerenes is observed after their transfer into a physiological medium (0.9% NaCl) and is established to depend on the preparation method. The cluster-size distribution in a fullerene–pectic-acid mixture is found to vary in water and a physiological medium. The results reveal the need for additional studies of the structure and properties of C₆₀ and C₇₀ molecules, as well as their complexes with medicines, in a physiological medium for medical applications.     

Adsorption Mechanism of Hydrogen on Boron-Doped Fullerenes

The C35BH-H2 complex and two other possible isomers, C34BCaH-H2 and C34BCbH-H2, are investigated using the local-spin-density approximation （LSDA） method. The results indicate that a single hydrogen molecule could be strongly adsorbed on two isomers, C34BCaH and C34BCbH, with binding energies of 0.42 and 0.47eV, respectively, and that these calculated binding energies are suitable for reversible hydrogen adsorption/desorption near room temperature. However, it is difficult for the H2 molecule to be firmly adsorbed on C35BH. We analyze the interaction between C34BCxH （x= a, b） and the H2 molecule using dipole moments and molecular orbitals. The charge analysis showed there was a partial charge （about 0.32e） transfer from 1-12 to the doped fullerenes. These calculation results should broaden our understanding of the mechanisms of hydrogen storage using borondoped fullerenes.     

Adsorption of helium on isolated C60 and C70 anions

Adsorption of helium on free, negatively charged fullerenes is studied in this work. Helium nanodroplets have been doped with fullerenes and ionised by electron attachment. For suitable experimental conditions, C^?₆₀ and C^?₇₀ anions are found to be complexed with a large number of helium atoms. Prominent anomalies in the ion abundances indicate the high stability of the commensurate 1×1 phase in which all hollow adsorption sites are occupied by one atom each. The adsorption energy for an additional helium atom is about 40% less than for atoms in the commensurate layer, similar to our previous findings for fullerene cations and in agreement with theoretical dissociation energies. Similarly, an anomaly in the adsorption energy occurs when 60 helium atoms are attached to C^?₆₀ or 65 to C^?₇₀. For C₆₀, the anomaly coincides with the one observed for cationic complexes but for C₇₀ it does not. Implications of these features are discussed in light of several theoretical studies of neutral and positively charged helium–fullerene complexes.     

Effect of C60 fullerene, fullerene-containing soot, and other carbon materials on the sliding edge friction of metals

A comparative study of different carbon materials (C₆₀ fullerene; soot, both with and without fullerenes; graphite; and industrial carbon black) as additives to industrial lubricating oils has been carried out for copper-steel and steel-steel sliding couples. The soot containing fullerene and the powder of pure fullerene produce a noticeable improvement in the antifriction and antiwear properties of steel-steel and steel-copper couples, especially under heavy loads and pressures at the contact. The greatest improvement was observed for the steel-steel couple. Structural-mechanical studies were carried out for copper riders and it has been demonstrated by several methods that the addition of the C₆₀ fullerene (pure fullerene or as a fullerene-containingsoot) creates a fullerene-polymer film on the frictional surface about 1000 Å thick, which has a protective effect.     

Quasi-one-dimensional fullerene-nanotube composites: Structure, formation energetics, and electronic properties

Carbon nanotubes coated with close-packed C₆₀ (or C₇₀) fullerenes, which are “attached” to the nanotubes by van der Waals forces, are considered and classified as a new class of nanocomposites. Semiempirical and molecular-dynamics calculations reveal the most energetically stable systems and show that a topological (Stone-Wales) defect on a nanotube can promote a more favorable “attachment” of fullerene to the nanotube. It has been shown that the molecular interaction of the fullerene coating with the nanotube leads to a significant change in its electronic spectrum, namely, to the formation of minibands including a large number of branches associated with the lift of the degeneracy of levels of C₆₀ and to the consolidation of branches of the carbon nanotube into the Brillouin zone smaller than that in the carbon nanotube. This fact should strongly change the interaction of light with such a nanocomposite as compared to carbon nanotubes and fullerenes, which provides prospect of its application in photovoltaics.     

Phase Transitions in a Mixture of Amorphous C<Subscript>60</Subscript> and C<Subscript>70</Subscript> Fullerene Phases at High Temperatures and Pressures

Phase transitions in two types of amorphous fullerene phases (C₆₀–C₇₀ (50/50) mixtures and an amorpous C₇₀ fullerene phase) are studied via neutron diffraction at pressures of 2–8 GPa and temperatures of 200–1100°C. Fullerenes are amorphized by grinding in a ball mill and sintered under quasi-hydrostatic pressure in a toroidal-type chamber. Diffraction studies are performed ex situ. It is shown that the amorphous phase of fullerenes retains its structure at temperatures of 200–500°C, and amorphous graphite is formed at 800–1100°C with a subsequent transition to crystalline graphite. This process is slow in a mixture of fullerenes, compared to C₇₀ fullerene. According to neutron diffraction data, the amorphous graphite formed from amorphous fullerene phases has anisotropy that is much weaker in a fullerene mixture.     

Functionalization of pentagon–pentagon edges of fullerenes by cyclic polysulfides: A DFT study

We have performed a computational study to investigate the cyclosulfurization of the pentagon–pentagon (p–p) junctions in the non-IPR fullerenes C₆₀(D₃) and C₇₀(C_2v), and also Stone-Wales defective C₆₀ fullerene. Our results indicate the exothermic character of cyclosulfurization processes which can be related to the increase of pyramidalization angle (spherical excesses) and p characters of natural hybrid orbitals of C atoms at the p–p junctions. In fact these lead to the structural strain relief and stability of the cyclosulfurization derivatives of the non-IPR fullerenes. Moreover, the cyclosulfurization reaction of p–p bonds on the C₇₀(C_2v) is more energetically favorable than that of C₆₀(D₃), due to the higher curvature of carbon sites and the larger values of the p characters of natural hybrid orbitals in the C₇₀(C_2v). On the other hand, localization of the excess electrons on the C atoms at the p–p junctions leads to the low tendency of the charged non-IPR fullerenes to cyclosulfurization process. The desulfurization pathway of the exohedral derivatives of C₇₀(C_2v) indicates that it is energetically unfavorable for the functionalized fullerenes to break into individual fullerene and sulfur molecules. HOMO–LUMO gaps almost are independent of the number of pentathiepin rings while sensitive to the type of parent fullerene.     

Theoretical investigation on the encapsulation of atomic hydrogen into heterofullerene nanocages

We have investigated the structural and electronic configurations of the H@X-doped C₆₀ fullerene (X = B, Si, P, O, S) as the novel materials for quantum bit (qubit) application by using density functional theory with the generalized-gradient approximation. Our results show that incorporated hydrogen atom exhibits significantly different interaction strengths and the calculated binding energies follow the hierarchy H@C₅₉O < H@C₅₉Si < H@C₆₀ < H@C₅₉B < H@C₅₉S < H@C₅₉P. In the considered complexes the binding energy is negative and the incorporated ¹H atom resides at the center of heterofullerene nanocages. The obtained results also reveal that for the H@C₅₉P complex the binding energy is four times higher than that of the traditional H@C₆₀ fullerene, thus the H@C₅₉P seems to be a promising material for the solid state quantum computers. Furthermore, the electronic and magnetic structures of the considered complexes at their ground state are discussed within the context.     

Nanoencapsulation of Fullerenes in Organic Structures with Nonpolar Cavities

The formation of supramolecular structures, assemblies, and arrays held together by weak intermolecular interactions and non-covalent binding mimicking natural processes has been used in applications being anticipated in nanotechnology, biotechnology and the emerging field of nanomedicine. Encapsulation of C₆₀ fullerene by cyclic molecules like cyclodextrins and calixarenes has potential for a number of applications. Similarly, biomolecules like lysozyme also have been shown to encapsulate C₆₀ fullerene. This poster article reports the recent trends and the results obtained in the nanoencapsulation of fullerenes by biomolecules containing nonpolar cavities. Lysozyme was chosen as the model biomolecule and it was observed that there is no covalent bond formed between the bimolecule and the C₆₀ fullerene. This was confirmed from fluorescence energy transfer studies. UV–Vis studies further supported this observation that it is possible to selectively remove the C₆₀ fullerene from the nonpolar cavity. This behavior has potential in biomedical applications     

Ultraviolet Photoelectron Spectroscopy of Fullerenes C60 and C70:A Model Study

Geometrical optimizations of two fullerenes, C₆₀ and C₇₀, have been performed by means of density-functional theory techniques. Based on the Gelius model, ultraviolet photoelectron spectra (UPS) of C₆₀ and C₇₀ have been simulated. We have shown how the different local arrangements of carbon atoms of C₇₀ are responsible for the spectra. Our calculated spectra are in good agreement with the experimental counterparts.     

Influence of defects in the carbon network on the static polarizability of fullerenes

The static polarizability of the C₆₀, C₇₀, C₈₀, and C₁₈₆ fullerenes has been calculated within the semiempirical MNDO approximation implemented in the MOPAC quantum-chemical program package. It is demonstrated that the results obtained are comparable with experimental data and the results of the ab initio B3LYP method using the 6–31G(d, p) basis set. The influence of topological defects (five-, seven-, and eight-membered rings), vacancies produced by removing pentagons, and nitrogen and boron atoms on the geometric parameters and the polarizability of the C₆₀, C₂₄₀, and C₅₄₀ fullerenes has been investigated by the MNDO method. It is revealed that the polarizability of the fullerene with topological defects is higher than the polarizability of the perfect icosahedral fullerene. The formation of vacancies in the carbon cage leads to a linear decrease in the polarizability of the fullerene and an increase in the specific polarizability. The polarizability of the heterofullerene with nitrogen or boron atoms spaced apart in the carbon cage is higher than that of the fullerene with heteroatoms located adjacent to each other.     

Fast degradation of fullerenes by ultraviolet laser radiation

The first observation of fullerene C₆₀ ultraviolet photolysis in hexane solution was published two years ago [1]. Similar further experiments realized with an ultraviolet lamp and solar light gave inconsistent results with ambiguous interpretations. We report the unexpectedly fast and efficient degradation of the fullerenes in n-hexane solutions, induced by an XeCl-excimer laser. Well-defined experimental conditions and good reproducibility in these experiments allow us to estimate the minimal value of the quantum yield of fullerene photolysis.     

Optical limiting with higher fullerenes

Optical limiting has been investigated for higher fullerenes and compared with C₆₀. The transmission through an aperture placed after solutions of C₇₆, C₇₈, and C₈₄ in tetrahydronaphthalene was measured using Q-switched laser pulses with a wavelength of 532 nm and a pulse width of 8 ns FWHM. Unlike C₆₀, the transmission for these higher fullerene solutions decreased linearly with increasing optical pulse energy. We attribute the linearized optical limiting response to self-defocusing of the optical beam and the absence of excited-state absorption. The ground state absorption spectra for the higher fullerenes suggest their use for optical limiting in the near infrared, and the C₈₄-tetrahydronaphthalene solution was found to be an optical limiter at 1.064 m.     

Polarizability exaltation of endofullerenes X@C<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 20, 24, 28, 36, 50, and 60; X is a noble gas atom)

The polarizability exaltation in molecules of endohedral complexes of C₂₀, C₂₄, C₂₈, C₃₆, C₅₀, and C₆₀ fullerenes with He, Ne, Ar, and Kr noble gas atoms has been revealed and studied by the density functional theory method. It has been found that the sign of the Δα polarizability exaltation depends on the number of atoms in a fullerene molecule.     

Electronic and optical properties of fullerene nanostructures

Two new types of molecular/electronic fullerene nanostructures are considered: 1) highly stable hydrated clusters (I _h symmetry group) and microcrystals (T _h symmetry group) of fullerene C₆₀ in water solution and 2) the single-walled carbon nanotube from C₆₀ fullerenes. The vibrational spectra of these fullerene nanostructures are calculated using molecular dynamics. The electronic properties of a single-walled fullerene nanotube are investigated using the tight-binding method. The theoretical results obtained were compared with available experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 885–887 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.