On the Supermolecular Structure of Fullerene C60 Solutions

Small angle X‐ray scattering (SAXS) and wide angle X‐ray scattering (WAXS) techniques were used for investigation of fullerene C₆₀ solutions in toluene and p‐xylene. On all SAXS curves, intensity decreases to some constant value of I_C with increase of scattering angle. The value of I_C depends on concentration non‐monotonically: it first slightly increases, then drops sharply to some minimal value, and then increases again. A qualitative explanation of such dependence is offered. It is supposed that the presence of fullerene C₆₀ in solution suppresses thermal fluctuations of density in the solvent. In combination with the X‐ray data the results obtained for solutions of fullerene C₆₀ by various others techniques (calorimetry, densimetry, etc.) are discussed. Possible models of a supermolecular structure of fullerene C₆₀ solutions in good solvents are considered.     

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.     

A DFT study on Diels–Alder cycloadditions of trans‐1,3‐butadiene to C60 and C70

Investigation of the relative reactivity of bonds in fullerenes will provide fundamental theory for the design of fullerene‐based materials. We have theoretically investigated the reactivity of the Diels–Alder (DA) cycloaddition of cis‐1,3‐butadiene to all types of bonds in C₆₀ and C₇₀ using the M06‐2X hybrid density functional theory (DFT) calculations (J. Phys. Org. Chem. 2012, 25 850–855) and have pointed out that the DA cycloadditions of cis and trans forms of 1,3‐butadiene to ethylene have a specially intimate relationship (J. Phys. Org. Chem. 2014, 27 652–660). For the aim of telling a whole story of the DA cycloaddition concerning C₆₀ and C₇₀, the DA cycloadditions of trans‐1,3‐butadiene to all types of bonds in C₆₀ and C₇₀ were explored at the same theoretical level as those of the cis‐1,3‐butadiene. The calculated results related with the trans‐ and cis‐1,3‐butadienes were compared. The potential energy curves of DA cycloadditions of trans‐ and cis‐1,3‐butadiene to C₆₀ and C₇₀ were discussed. The distortion–interaction energy model was employed to elucidate the origin of different reactivity of all kinds of C?C bonds. The solvent effects were examined using the continuum solvent model. These current results, along with our previous research, will help to obtain an overall view of the DA cycloadditions of 1,3‐butadiene to C₆₀ and C₇₀. Copyright © 2014 John Wiley & Sons, Ltd.     

A comparative study of ion-induced damages in C60 and C70 fullerenes

The stability of fullerenes (C₆₀ and C₇₀) under swift heavy ion irradiation is investigated. C₆₀ and C₇₀ thin films were irradiated with 120 MeV Ag ions at fluences from 1×10¹² to 3×10¹³ ions/cm². The damage cross-section and radius of damaged cylindrical zone were found to be higher for C₆₀ than C₇₀ as evaluated by Raman spectroscopy, which shows that the C₇₀ molecule is more stable under energetic ion impact. The higher damage cross-section of the C₆₀ molecule compared with that of the C₇₀ molecule is explained on the basis of thermal conductivity in the framework of the thermal spike model. The surface morphology of pristine C₆₀ and C₇₀ films is studied by atomic force microscopy. UV-visible absorption studies revealed that band gap for C₆₀ and C₇₀ fullerenes thin films decreases with increasing ion fluence. Resistivity of C₆₀ and C₇₀ thin films decreases with increasing ion fluence but the decrease is faster for C₆₀ than C₇₀, indicating higher damage in C₆₀. Irradiation at a fluence of 3×10¹³ ions/cm² results in complete damage of fullerenes (C₆₀ and C₇₀) into amorphous carbon.     

A theoretical study of the mechanisms for 1,3‐dipolar cycloadditions of diphenyldiazomethane to C60 and C70

The 1,3‐dipolar cycloaddition (1,3‐DPCA) reaction plays a crucial role during the functionalization of fullerenes, which have broad applications in the materials and pharmaceutical fields. In concert with previous experiments, we theoretically investigated the mechanisms of 1,3‐DPCA of diphenyldiazomethane (DDMf) to two fullerenes (C₆₀ and C₇₀) using the M06‐2X density functional method under vacuum and in solvents. To understand the influence of the dipolarophile on these reactions, the 1,3‐DPCA of DDMf to three common acceptors, specifically tetracyanoethylene (TCNE), 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ), and chloranil (CA), was also studied at the same computational level. The substituent effects on the five reactions were investigated by modeling 1,3‐DPCA reactions with 12 different substituted DDMf (DDMs) with five dipolarophiles, totaling 60 reactions. Including the five unsubstituted DDMf reactions, 65 1,3‐DPCA reactions were studied. The stereoselectivity, relative reactivity, solvent effects, and distortion/interaction energy model were carefully considered and analyzed based on their corresponding electronic structures, electrostatic potential surfaces, interaction models, solvent models, and thermodynamic data. An intermediate was identified for each of the 65 reactions. A possible biradical pathway for the reactions between DDMf and the two fullerenes was also investigated. The calculated results corroborate and enrich the experimental observations. The conclusion and detailed discussion are generally important for understanding the 1,3‐DPCA reactions to fullerenes. Copyright © 2014 John Wiley & Sons, Ltd.     

Spectroscopic and atomic force microscopy investigations of hybrid materials composed of fullerenes and 3-aminopropyltrimethoxysilane

Fullerene based materials may open a new horizon in many fields of science. In this study we fabricated thin films of the hybrid materials formed as a result of interactions between C₆₀ fullerenes and 3-aminopropyltrimethoxysilane (APTMS). The deposition technique was a combination of spin-coating and evaporation methods. Interactions within the films were investigated by means of X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy (NEXAFS). Surface morphology was measured by atomic force microscopy (AFM). We found that there are strong chemical reactions between the nucleophilic nitrogen atoms from APTMS and electrophilic fullerene molecules. Results of NEXAFS investigations suggest that due to direct interactions between APTMS and C₆₀ the electronic structure of the fullerene molecules changes while at the same time AFM proved that the C₆₀ molecule diameter is not altered.     

Chemical reactivity of lithium‐doped fullerenes

The addition of free radicals and the 1,3 dipolar cycloaddition onto pristine and lithium‐doped C₆₀ were studied by means of the Perdew–Burke–Ernzerhof (PBE) and M06‐2X density functionals. In all cases, lithium increased the reactivity even though for the 1,3 dipolar cycloaddition onto C₆₀ the change observed with respect to bare C₆₀ was minimal. Both functionals employed gave similar encapsulation energies for Li@C₆₀ namely, 33.1 and 38.2 kcal/mol at the PBE/6‐31G* and M06‐2X/6‐31G*, respectively. However, the increased reactivity because of lithium doping determined at the PBE level is smaller as compared with that computed with the M06‐2X functional, whereas that determined at the second‐order Møller–Plesset (MP2) level is the largest one. For example, using the M06‐2X functional the binding energy of fluorine to Li@C₆₀ is 28.5 kcal/mol larger than that determined for C₆₀, whereas at the PBE/6‐31G* level it is predicted to be increased by 24.7 kcal/mol. The results clearly suggest that Li@C₆₀ is a much better free radical scavenger than C₆₀. Finally, the complex hindered rotations of lithium inside C₆₀ are expected to be strongly inhibited because lithium doping increases the well depth between the cage center and the equilibrium position near the addition site of the lithium atom. Copyright © 2011 John Wiley & Sons, Ltd.     

Densification of sol–gel silica thin films induced by hard X‐rays generated by synchrotron radiation

In this article the effects induced by exposure of sol–gel thin films to hard X‐rays have been studied. Thin films of silica and hybrid organic–inorganic silica have been prepared via dip‐coating and the materials were exposed immediately after preparation to an intense source of light of several keV generated by a synchrotron source. The samples were exposed to increasing doses and the effects of the radiation have been evaluated by Fourier transform infrared spectroscopy, spectroscopic ellipsometry and atomic force microscopy. The X‐ray beam induces a significant densification on the silica films without producing any degradation such as cracks, flaws or delamination at the interface. The densification is accompanied by a decrease in thickness and an increase in refractive index both in the pure silica and in the hybrid films. The effect on the hybrid material is to induce densification through reaction of silanol groups but also removal of the organic groups, which are covalently bonded to silicon via Si—C bonds. At the highest exposure dose the removal of the organic groups is complete and the film becomes pure silica. Hard X‐rays can be used as an efficient and direct writing tool to pattern coating layers of different types of compositions.     

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.     

Synthesis of multishell fullerenes by laser vaporization of composite carbon targets

Multishell fullerenes are the smallest among multishell carbon clusters, such as the larger graphitic onions or multishell nanotubes. Unlike classical fullerenes, which have a cage structure and are known to have been synthesized in a variety of sizes (C₆₀, C₇₀, C₈₄, C₁₀₂, etc.), multishell fullerenes have a cage-inside-cage concentric structure, such as double-shell C₆₀@C₂₄₀ or triple-shell C₆₀@C₂₄₀@C₅₆₀. We report on the synthesis of multishell fullerenes by laser vaporization of C₆₀-containing composite carbon targets. Transmission electron microscopy, Raman scattering spectroscopy, and other methods were used for characterization of the product. The yield of the process reaches up to 40%, which permits production in gram amounts even in laboratory conditions.     

Hydrogen storage in hydrofullerides

Highly reduced fullerenes containing nearly 5 wt.% of dihydrogen can be synthesized using readily available reagents. Hydrides prepared from C₆₀ by dissolving metal reductions give complex mixtures of unstable isomers of C₆₀H₃₆. High temperature syntheses using transfer hydrogenation reagents such as dihydroanthracene provide mainly two stable isomers of C₆₀H₃₆ that have been characterized spectroscopically. Dihydrogen can be removed thermally from these materials at 500 °C. The experimentally determined activation energy for the thermal removal of dihydrogen from C₆₀H₂ is 61.4 kcal/mol. This value is in good agreement with the value determined computationally for a multi step radical reaction. The 92 kcal/mol barrier determined computationally for the concerted loss of dihydrogen reflects the symmetry-imposed barrier to dehydrogenation. PACS 81.05.Tp; 82.30.Cf; 82.56.Ub     

Enhancement of measurement accuracy of X‐ray PIV in comparison with the micro‐PIV technique

The X‐ray PIV (particle image velocimetry) technique has been used as a non‐invasive measurement modality to investigate the haemodynamic features of blood flow. However, the extraction of two‐dimensional velocity field data from the three‐dimensional volumetric information contained in X‐ray images is technically unclear. In this study, a new two‐dimensional velocity field extraction technique is proposed to overcome technological limitations. To resolve the problem of finding a correction coefficient, the velocity field information obtained by X‐ray PIV and micro‐PIV techniques for disturbed flow in a concentric stenosis with 50% severity was quantitatively compared. Micro‐PIV experiments were conducted for single‐plane and summation images, which provide similar positional information of particles as X‐ray images. The correction coefficient was obtained by establishing the relationship between velocity data obtained from summation images (V_S) and centre‐plane images (V_C). The velocity differences between V_S and V_C along the vertical and horizontal directions were quantitatively analysed as a function of the geometric angle of the test model for applying the present two‐dimensional velocity field extraction technique to a conduit of arbitrary geometry. Finally, the two‐dimensional velocity field information at arbitrary positions could be successfully extracted from X‐ray images by using the correction coefficient and several velocity parameters derived from V_S.     

Buckminsterfullerene, higher fullerenes, and their endohedral and fluorine derivatives

This paper presents an overview of recent works concerned with determination of the electron affinity (EA) and the ionization energy (IE) for higher fullerenes and their endohedral and fluorine derivatives. The numerical values of the electron affinity are analyzed for higher fullerenes up to C₁₀₆ and lanthanum, gadolinium, and scandium endohedral fullerenes, including Sc₃N@C₈₀. Most attention is concentrated on two methods for producing fluorofullerenes, namely, direct fluorination of fullerenes with molecular fluorine in a manganese difluoride matrix and solid-phase reactions between fullerenes and fluorinating agents capable of donating fluorine to fullerene. The structures of three fluorofullerenes (C₆₀F₁₈, C₆₀F₂₀, and C₆₀F₄₈) characterized by a distortion of the carbon cage due to attachment of functional groups are discussed.     

Vibrational spectra of lanthanide endofullerenes with different symmetries of molecular structure

Quantum-chemical calculations of the geometric structure and vibrational spectra of lanthanide endofullerenes have been carried out. The vibrational frequencies of lanthanide atoms depend substantially on the symmetry of the molecular structure of the endofullerene and on the distance between the metal atom and the carbon cage. The infrared spectra of the endofullerenes M@C₆₀ contain vibrations that are forbidden by symmetry for the empty fullerene C₆₀. A change in the vibrational spectra due to the encapsulation of a metal atom in fullerenes with a C₆₀ cage is considerably more pronounced than that of the higher fullerenes. In the vibrational spectra, there are lines not characteristic of C₆₀, which indicates the presence of M@C₆₀ endofullerenes in a mixture with C₆₀ fullerenes.     

Atomic force microscopy of C60/C70 single-crystal fullerenes under ethanol

C₆₀/C₇₀ crystal surfaces were imaged by atomic force microscopy under ethanol with resolution of single molecules. Spherical and elongated elliptical fullerenes can be distinguished corresponding most likely with C₆₀, respectively C₇₀. Determination of the maximum diameter for a large number of molecules confirms the presence of two species of fullerenes, one with 9.4 Å, the other with 11.2 Å. The measured ratio C₆₀:C₇₀ is 81:19 which resembles the spectroscopical data. The molecules are arranged either in hexagonal (hcp) or cubic (fcc) packing, in some areas the two arrangements alternate within a few nm. Elongated fullerenes apparently prefer the hexagonal packing.     

Specific features of the stone-wales transformation in the C20 and C36 fullerenes

The initial stage of the defect formation in the C₂₀ and C₃₆ fullerenes has been investigated by the ab initio method and in terms of tight-binding models. A comparison with the Stone-Wales transformation in the C₆₀ fullerene has revealed the presence of two independent stages in this process, the first of which is an “incomplete” Stone-Wales transformation. At this stage, the C₂₀ and C₃₆ fullerenes transform into metastable defect configurations with two adjacent “windows” on their surface, whereas a similar configuration of the C₆₀ fullerene is unstable and corresponds to a saddle stationary point of the potential energy of the cluster. A new mechanism of plastic deformation due to the Stone-Wales transformation has been predicted for the (C₃₆) _n fullerites.     

Optical performance of materials for X‐ray refractive optics in the energy range 8–100 keV

A quantitative analysis of the crucial characteristics of currently used and promising materials for X‐ray refractive optics is performed in the extended energy range 8–100 keV. According to the examined parameters, beryllium is the material of choice for X‐ray compound refractive lenses (CRLs) in the energy range 8–25 keV. At higher energies the use of CRLs made of diamond and the cubic phase of boron nitride (c‐BN) is beneficial. It was demonstrated that the presence of the elements of the fourth (or higher) period has a fatal effect on the functional X‐ray properties even if low‐Z elements dominate in the compound, like in YB₆₆. Macroscopic properties are discussed: much higher melting points and thermal conductivities of C and c‐BN enable them to be used at the new generation of synchrotron radiation sources and X‐ray free‐electron lasers. The role of crystal and internal structure is discussed: materials with high density are preferable for refractive applications while less dense phases are suitable for X‐ray windows. Single‐crystal or amorphous glass‐like materials based on Li, Be, B or C that are free of diffuse scattering from grain boundaries, voids and inclusions are the best candidates for applications of highly coherent X‐ray beams.     

Modulation of the work function of fullerenes C60 and C70 by alkali-metal adsorption: A theoretical study

The impact of alkali-metal (Li/Na/Cs) adsorption on work function of fullerenes C₆₀ and C₇₀ was investigated by first-principles calculations. After adsorption, the work functions of fullerenes C₆₀ and C₇₀ decrease distinctly and vary linearly with the electronegativity of the alkali metal elements, and the positions where the alkali atoms are adsorbed considerably influence the work functions. On the contrary, a vacancy defect elevates the work functions of the fullerenes C₆₀ and C₇₀. The variation in the work functions rests with variation in Fermi level (which are attributed to charge transfer) and variation in vacuum levels (which are attributed to the induced dipole moments). Moreover, alkali-metal adsorption can also improve the electric conductivity of a fullerene mixture of C₆₀ and C₇₀.     

A study of carbon nanosystems using the Hubbard model

The Hubbard model is used as a framework for analyzing carbon nanosystems: the fullerenes C₆₀ and C₈₀ and open-ended carbon nanotubes with chiralities (5, 5) and (10, 10) of various lengths. In the strong-correlation limit, the model predicts that open carbon nanotubes have a lower energy per atom as compared to C₆₀ and C₈₀ fullerenes. This finding contradicts the conventional view that dangling bonds increase the energy of a system. However, the increase, if any, is due to the presence of five-member carbon rings in fullerenes. The energy per atom should be higher for the five-member carbon ring compared to the six-member one, because the former cannot exist in a lower energy singlet state. Carbon nanotube growth is explained. The ionization energies and electron affinities of C₆₀ and C₈₀ fullerenes are calculated and found to agree well with experimental data.     

Effect of fullerenes on spectral and nonlinear optical properties of <Emphasis Type="Italic">N</Emphasis>-(4-nitrophenyl)-(<Emphasis Type="Italic">L</Emphasis>)-prolinol structure

The influence of fullerenes C₆₀ and C₇₀ on the optical characteristics of a promising organic system, N-(4-nitrophenyl)-(L)-prolinol (NPP), is studied. The UV and blue absorption bands are found that can be controlled by introducing fullerenes. These bands are caused by the change in the electronic structure of the organic system induced by fullerenes and indicate the presence of an ordering in the arrangement of macromolecules. The observed changes in the IR absorption result from the structural rearrangement of the system and may be related to a weak complexing between a donor fragment of the NPP molecule and fullerene. The effect of restriction of the nanosecond laser radiation at 532 nm is considered and characteristics of nonlinear transmission of laser radiation at 337 nm are presented. The levels of attenuation and incident energy are estimated, which determines the possibility of using results of these studies in the development of optical gates in a broad spectral range.