C<Subscript>60</Subscript> fullerene as an η<Superscript>6</Superscript> ligand in π complexes of transition metals

The possible existence of complexes formed by the C₆₀ fullerene or its derivatives with transition metals interacting with the carbon cage via η⁶−π type bonding is discussed. The derivatives C₆₀ R ₆ of the C₆₀ fullerene (R = −, H, F, Cl, Br, CN) are analyzed using the density functional method within the Perdew-Burke-Ernzerhof approximation. In these molecules, the R groups are attached to carbon atoms located in the α positions with respect to the common hexagon of the C₆₀ fullerene. The structure and electron configuration of complexes formed by these molecules with Cr(C₆H₆), Cr(CO)₃, MoC₆H₆, and Mo(CO)₃ particles are modeled. The “dimer” systems C₆₀R₆-M-R ₆C₆₀ (M = Cr, Mo, R =-, H, F) are investigated in which two fullerene molecules interact via a transition-metal atom. It is found that the introduction of six R groups in the α sites with respect to the common hexagon of C₆₀ favors the formation of complexes of these derivatives of the C₆₀ fullerene with the Cr(C₆H₆), Cr(CO), Mo(C₆H₆), and Mo(CO)₃ particles in which η⁶-π type bonds arise between the metal and the atoms of the hexagon fringed with the R groups. It is also demonstrated that analogous complexes with a “bare” C₆₀ fullerene are possible, but they are significantly less stable. The (C₆H₆) M-R ₆C₆₀ R ₆-M (C₆H₆) complexes of particles M(C₆H₆) (M= Cr, Mo) and derivatives R ₆C₆₀ R ₆ (R =-, H, F, Cl, Br) are studied. In the R ₆C₆₀ R ₆ molecule, six R groups are located in the α sites with respect to the common hexagon of the C₆₀ fullerene and six other groups fringe the opposite hexagon. The obtained results can be applied to planning synthesis of new complexes that C₆₀ fullerene derivatives can form with transition metals. Original Russian Text ￠ E.G. Gal’pern, A.R. Sabirov, I.V. Stankevich, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 12, pp. 2220–2223.     

Absorption Spectra of C<Subscript>60</Subscript> Fullerene Monomolecular Films

The absorption spectra of C₆₀ fullerene thin (from submonolayer to 5–6 monolayers) films deposited on different substrates have been studied. It has been shown that the spectra of the submonolayer and thick (more than 100 monolayers) films virtually coincide. The behavior of fullerene on the surface of different polymers can be judged from changes in the absorption spectra.     

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.     

Study of the Initial Stage of Fluorinated C<Subscript>60</Subscript>F<Subscript>18</Subscript> Fullerene Adsorption on the Cu(001) Surface

The dynamics of the adsorption and evolution of fluorinated C₆₀F₁₈ fullerene molecules on the Cu(001) surface are studied by real-time ultra-high vacuum scanning tunneling microscopy. Fluorinated fullerene molecules are shown to decompose with time on the Cu(001) surface transforming to C₆₀ molecules. The decay rate depends on the initial molecular coverage. The rapid decay of fluorinated fullerene molecules is observed when the coverage is no higher than 0.2 single layers. As a result, two-dimensional islands consisting of pure C₆₀ molecules are formed on the Cu(001) surface. 2D islands consisting of fluorinated fullerene molecules are formed when the initial molecular coverage is higher than 0.5 single layers. The molecules inside these islands also tend to decompose with time. It is found experimentally that fluorine atoms are removed completely from the initial C₆₀F₁₈ molecules adsorbed on the Cu(001) surface after 250 h when the initial molecular coverage is 0.6 single layers.     

Collision dynamics of He@C<Subscript>60</Subscript>+He@C<Subscript>60</Subscript> at low energies

A semi-empirical molecular dynamics model is developed. The central collisions of C₆₀+C₆₀ and He@C₆₀+He@C₆₀ at different incident energies are investigated based on this model. It is found that the dimer structures have been produced at proper incident energies and these fullerene dimers could be formed by a self-assembly of C₆₀ fullerene and He@C₆₀. The He atom has a significant effect at higher incident energy and this embedded He atom can enhance the stability of the dimer structure.     

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.     

Production of anti-fullerene C<Subscript>60</Subscript> polyclonal antibodies and study of their interaction with a conjugated form of fullerene

The aim of this study was to produce anti-fullerene C₆₀ antibodies for the development of detection systems for fullerene C₆₀ derivatives. To produce anti-fullerene C₆₀ antibodies, conjugates of the fullerene C₆₀ carboxylic derivative with thyroglobulin, soybean trypsin inhibitor, and bovine serum albumin were synthesized by carbodiimide activation and characterized. Immunization of rabbits by the conjugates led to the production of polyclonal anti-fullerene antibodies. The specificity of the immune response to fullerene was investigated. Indirect competitive immunoenzyme assay was developed for the determination of conjugated fullerene with detection limits of 0.04 ng/mL (calculated for coupled C₆₀) and 0.4 ng/mL (accordingly to total fullerene–protein concentration).     

Increase of photoluminescence from fullerenes-doped poly (alkyl methacrylate) under laser irradiation

Increase of photoluminescence (PL) from fullerenes (C₆₀ and C₇₀)-doped poly(alkyl methacrylate), such as poly(ethyl methacrylate) (PEMA), poly(isopropyl methacrylate) (PiPMA) and poly(isobutyl methacrylate) (PiBMA), have been studied under laser irradiation with wavelength of in air. After laser irradiation, PL peaks of all fullerenes doped-polymers are broadened and blue-shifted. This PL increase depends on the fullerene concentrations. By comparing with fullerenes-doped PMMA, fullerenes-doped PEMA have the greatest PL increase among the four kinds of polymers, including PEMA, PiPMA, PiBMA and PMMA. PL intensity of C₇₀-doped polymers increases much more quickly than the corresponded C₆₀-doped polymers at the initial stage of laser irradiation. Great change on their UV-visible absorption spectra before and after laser irradiation indicate some great variation on chemical structure of fullerene molecules dispersed in polymer matrix under laser irradiation. This great PL increase may be attributed to formation of fullerene oxide-polymer and oxidized fullerene-polymer adducts due to laser-induced photochemical reactions among fullerene, oxygen and polymer.     

Molecular dynamics simulation of the low-energy interaction between Cu<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>@C<Subscript>60</Subscript> endofullerenes and the surface of a copper crystal

A molecular dynamics simulation of the low-energy interaction of C₆₀ fullerenes and Cu₁@C₆₀, Cu₆@C₆₀, and Cu₁₃@C₆₀ endofullerenes with a Cu(100) surface was performed. The effects of a copper cluster encapsulated in a fullerene and of a fullerene’s translational motion and rotation energy on its penetration into a surface were investigated. It was shown that the presence of an encapsulated cluster has a positive effect on fullerene penetration into a surface with preservation of the fullerene’s structure. The optimal conditions for fullerene penetration into a copper crystal surface were determined.     

Thermally stimulated desorption of C<Subscript>60</Subscript> and C<Subscript>70</Subscript> fullerenes from rigid-chain polyimide films

Polyimide-fullerene composite thin coatings are investigated using thermal desorption mass spectrometry in the temperature range 20–800°C. It is found that, at temperatures below the temperature of decom-position of the polymer matrix, thermally stimulated desorption of fullerene molecules is limited by the diffusion of fullerene molecules in the matrix. The diffusion coefficients and activation energies of diffusion of C₆₀ and C₇₀ fullerene molecules are determined from the experimental data on thermally stimulated desorption in the framework of several approaches. It is revealed that the diffusion of C₇₀ molecules in the polyimide matrix is more hindered than the diffusion of C₆₀ molecules in the same matrix.     

New property of the fullerenes: the anomalously effective quenching of electronically excited states owing to energy transfer to the C<Subscript>70</Subscript> and C<Subscript>60</Subscript> molecules

The quenching of the electronically excited states of various energy donors—Tb³⁺; 9,10-anthracene dibromide; and adamantanone—by C₇₀ fullerene has been detected and analyzed. The quenching is characterized by anomalously high biomolecular quenching rate constants, which are obtained from the Stern-Volmer dependences of the energy-donor photoluminescence intensity on the concentration of the C₇₀ molecules. It has been shown that the high efficiency of quenching by the C₇₀ fullerene as compared to the C₆₀ fullerene is due to the higher polarizability of the C₇₀ molecule and large overlap integrals of the energy-donor photoluminescence spectra with the absorption spectrum of the C₇₀ fullerene.     

Polymerization of solid C<Subscript>60</Subscript> under C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack> cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C₆₀⁺ cluster ions is reported. The Raman spectra of the irradiated C₆₀ films reveal a new peak rising at 1458 cm⁻¹ with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C₆₀ induced by the cluster ion impacts. The aligned C₆₀ polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C₆₀ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C₆₀ polymeric phase indirectly confirms the dominant role of shock waves in the detected C₆₀ phase transformation.     

Thermal desorption states of C<Subscript>60</Subscript> fullerene molecules in polymer matrices

Polymer-C₆₀ fullerene composite coatings are studied using thermal desorption mass spectrometry. It is found that thermal desorption spectra of C₆₀ fullerene molecules can exhibit several resolved peaks (at a specified heating rate) corresponding to thermal desorption states. The relative intensity of the thermal desorption peaks depends on the procedure used for preparing the composite coatings, in particular, on the time of sedimentation of the polymer-fullerene suspension. The occurrence of different stages in thermally stimulated desorption of C₆₀ fullerene molecules is explained by the fact that the fullerene molecules can exist in several phase states characterized by different densities and degrees of ordering in the polymer matrix.     

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.     

Effect of heating of the electronic subsystem on the thermal stability of the C<Subscript>60</Subscript> and C<Subscript>20</Subscript> fullerenes and (C<Subscript>20</Subscript>)<Subscript>2</Subscript> cluster molecule

The effect of heating of the electronic subsystem on the thermal stability of C₆₀ and C₂₀ fullerenes and a (C₂₀)₂ cluster molecule is investigated theoretically. It is demonstrated that the excitation of electrons to upper energy levels in accordance with the Fermi-Dirac distribution function does not lead to a substantial change in the activation energy E _a for decay of the C₂₀ fullerene. The stability of the C₆₀ fullerene and the (C₂₀)₂ cluster molecule likewise does not change radically. However, the inclusion of corrections associated with the finite sizes of the heat bath leads to the activation energy E _a which is in better agreement with the calculated height of the potential barrier preventing the cluster decay.     

The oxidation of fullerenes (C<Subscript>60</Subscript>, C<Subscript>70</Subscript>) with various oxidants under ultrasonication

The reaction of C₆₀, under ultrasonication, with various oxidants, such as 3-chloroperoxy benzoic acid (Fluka 99%), 4-methyl morpholine N-oxide (Aldrich 97%), chromium (VI) oxide (Aldrich 99.9%), and the oxone® monopersulfate compound, causes the oxidation of fullerenes at room temperature. The FAB-MS spectra and HPLC profile confirmed that the products of fullerene oxidation were [C₆₀(O)_n] (n=1～3 or n=1). C₇₀ also reacted, under ultrasonication, with various oxidants, but the reaction rate of C₇₀ was lower than that of C₆₀.     

Modeling of the structure and electronic structure of condensed phases of small fullerenes C<Subscript>28</Subscript> and Zn@C<Subscript>28</Subscript>

A comparative analysis of the stability factors and electronic structure of two possible crystalline forms of small fullerene C₂₈ and endohedral fullerene Zn@C₂₈ with diamond and lonsdaleite structures is performed using a cluster model. Atoms of elements that, when placed inside C₂₈ cages, have no significant effect on the stability of free small-fullerene molecules are shown to be able to dramatically change the electronic properties and reactivity of the C₂₈ skeleton and to be favorable for forming small-fullerene crystalline modifications, which are covalent crystals. In contrast, if the presence of foreign atoms inside C₂₈ cages stabilizes the isolated nanoparticles, then molecular crystals (such as C₆₀ fullerites) are formed due to weak van der Waals forces.     

Withdrawal of Electrodynamical Forbiddance and Peculiarities of the SERS Spectra in Fullerene C<Subscript>70</Subscript>

It is demonstrated that in fullerene C₇₀, which can be considered as a deformed fullerene C₆₀ in some mean sense there is a withdrawal of an Electrodynamical forbiddance of a strong quadrupole light-molecule interaction, which is realized in the fullerene C₆₀. This situation occurs because of the reduction of symmetry of C₆₀ from the icosahedral symmetry group Y_h to the group D_5h. The withdrawal results in appearance of the lines in the SERS spectra of C₆₀, which are forbidden in usual Raman scattering and are active in the infrared absorption spectra. The experimentally measured SERS spectra of C₇₀ demonstrates existence of such lines that strongly confirms our ideas about the dipole-quadrupole SERS mechanism.     

Processing of C<Subscript>60</Subscript> thin films by Matrix-Assisted Pulsed Laser Evaporation (MAPLE)

Thin films of fullerenes (C₆₀) were deposited onto silicon using matrix-assisted pulsed laser evaporation (MAPLE). The deposition was carried out from a frozen homogeneous dilute solution of C₆₀ in anisole (0.67 wt%), and over a broad range of laser fluences, from 0.15 J/cm² up to 3.9 J/cm². MAPLE has been applied for deposition of fullerenes for the first time and we have studied the growth of thin films of solid C₆₀. The fragmentation of C₆₀ fullerene molecules induced by ns ablation in vacuum of a frozen anisole target with C₆₀ was investigated by matrix-assisted laser desorption/ionization (MALDI). Our findings show that intact fullerene films can be produced with laser fluences ranging from 0.15 J/cm² up to 1.5 J/cm².