Separation Of Fullerenes Research Articles

In English

Fullerenes are interesting objects of research in view of their promising use as a nano-sized additive to coatings, films, adsorbents, as well as active components in medicine, pharmacy, cosmetology. This paper considers the possibility of modifying commercial adsorbents - silica gel and cation-exchanger based on activated carbon with available and inexpensive linear carbon radicals. Behentrimonium chloride (C22H45(CH3)3N+Cl-) and cetyl alcohol (C16H33OН) were chosen as the latter. The obtained adsorbents were compared with a commercial sample of silica gel modified with a linear hydrocarbon radical –C18Н37. The adsorbents were described by the methods of IR spectroscopy, thermogravimetry, the main characteristics of the porous structure were determined by the method of low-temperature adsorption-desorption of nitrogen. Modification of adsorbents based on silica gel leads to a decrease in the specific surface area to 202.4 and 236.5 m2/g and the total pore volume to 0.32 and 0.39 cm3/g, which is almost 2 half source material. Increasing the fraction of micropores contributes to better separation rates of large molecules. Treatment of cation-exchanger based on activated carbon by cation modifier leads to a significant decrease in the specific surface area, as well as to an increase in the size of pores with the predominant formation of mesopores. It is shown that the modification of silica gel is carried out both through silanol and siloxane groups, in contrast to the commercial sample of silica gel. The approximate number of modifier groups on the surface of the original SiO2 matrix was estimated, which is one modifier group ~ per 11 SiO2 molecules. To study the behavior of adsorption of C60, C70 fullerenes and their mixtures on adsorbents, column experiments were carried out with changes in the geometrical parameters of the column and the initial concentrations of the solutions. The results showed that the modified silica gels are potential adsorbents for the separation of a mixture of fullerenes with a ratio of 65/25. Silica gel modified with cetyl alcohol C16H33OH showed the best separation efficiency. By the help of an adsorbent, it is possible to separate C60 - 90.52 % from a less concentrated solution and 87.26 % from a more concentrated solution. To increase the purity of the product, it is necessary to pass the solution through the sorbent 2–3 times. The competitive capability of the proposed modified silica gels, together with economic efficiency, ease of modification, and the possibility of fullerene separation characterize the proposed adsorbents as potential materials for practical application. Due to the significantly lower cost and simpler manufacturing methods, the proposed adsorbents can be used for separation at a large scale.

A π-extended molecular belt with selective binding capability for fullerene C70.

A molecular belt incorporating naphthalene moieties, featuring an ellipsoidal cavity, was precisely engineered through bottom-up synthesis. Its pre-arranged geometry exhibits excellent complementarity to fullerene C70, resulting in remarkable selective binding ability (K = 1.3 × 106 M-1) for C70 compared to C60 (K = 176 M-1), forming a 1 : 1 complex. This superiority was unequivocally demonstrated by the single crystal structure of the complex, which revealed outstanding concave-convex shape complementarity between the two components. This highlights the potential application of molecular belts in the purification and separation of fullerenes.

Reversible encapsulation and release of fullerenes using calix[n]phenoxazines.

This investigation presents the synthesis of butyl-decorated calix[n]phenoxazines of varying sizes by kinetic control and the ring-expansion of calix[3]phenoxazine, which uniquely exhibits distinct binding affinities for fullerenes C60 and C70. Calix[3]phenoxazine demonstrates a higher binding affinity for cationic ammonium, which can be reversibly deprotonated and protonated, enabling the reversible release and reloading of fullerenes. This system holds potential for applications in fullerene extraction and separation.

Fullerene-Functionalized Halogen-Bonding Heteroditopic Hosts for Ion-Pair Recognition.

Despite their hydrophobic surfaces with localized π-holes and rigid well-defined architectures providing a scaffold for preorganizing binding motifs, fullerenes remain unexplored as potential supramolecular host platforms for the recognition of anions. Herein, we present the first example of the rational design, synthesis, and unique recognition properties of novel fullerene-functionalized halogen-bonding (XB) heteroditopic ion-pair receptors containing cation and anion binding domains spatially separated by C60. Fullerene spatial separation of the XB donors and the crown ether complexed potassium cation resulted in a rare example of an artificial receptor containing two anion binding sites with opposing preferences for hard and soft halides. Importantly, the incorporation of the C60 motif into the heteroditopic receptor structure has a significant effect on the halide binding selectivity, which is further amplified upon K+ cation binding. The potassium cation complexed fullerene-based receptors exhibit enhanced selectivity for the soft polarizable iodide ion which is assisted by the C60 scaffold preorganizing the potent XB-based binding domains, anion-π interactions, and the exceptional polarizability of the fullerene moiety, as evidenced from DFT calculations. These observations serve to highlight the unique properties of fullerene surfaces for proximal charged guest binding with potential applications in construction of selective molecular sensors and modulating the properties of solar cell devices.

In-situ carbonizing of coal pitch on the surface of silica sphere as quasi-graphitized carbon stationary phase for liquid chromatography

A novel chromatography stationary phase with a quasi-graphitized carbon modified shell has been developed. Coal pitch was directly carbonized on the surface of porous silica with in-situ carbonization. The carbonized coal pitch coating exhibits some degree of graphitization with a 78 nm-thick layer on the surface of silica and a 0.5 nm-thick layer on the inner surface of the mesopores. Based on the special structure of the graphitized carbon coating, the novel stationary phase can provide multiple interactions such as hydrophobic interaction, π-π interaction and dipole-dipole interaction. The novel composite material exhibited unique separation selectivity and excellent separation efficiency for polar compounds, including imidazoles, nucleosides and pesticides. Besides, the packed column also exhibited great repeatability with the RSDs of the retention time of nucleosides between 0.07%-0.50% (n = 5). Finally, satisfied result was achieved in the separation of fullerenes on the new column, suggesting the great potential in the industrial-scale purification of fullerenes.

Capturing the Long-Sought Dy@C2v(5)-C80 via Benzyl Radical Stabilization.

Endohedral metallofullerenes (EMFs) are one type of intriguing metal/carbon hybrid molecule with the molecule configuration of sphere cavity-encapsulating metal ions/metal clusters due to their unique physicochemical properties and corresponding application in the fields of biological materials, single molecule magnet materials and energy conversion materials. Although the EMF family is growing, and versatile EMFs have been successfully synthesized and confirmed using crystal structures, some expected EMF members have not been observed using the conventional fullerene separation and purify strategy. These missing EMFs raise an interesting scientific issue as to whether this is due to the difficulty in separating them from the in situ formed carbon soot. Herein, we successfully captured a long-sought dysprosium-based EMF bearing a C2v(5)-C80 cage (Dy@C2v(5)-C80) in the form of Dy@C2v(5)-C80(CH2Ph)(Ph = −C6H5) from carbon soot containing versatile EMFs using simple benzyl radical functionalization and unambiguously confirmed the molecule structure using single crystal X-ray diffraction characterization. Meanwhile, the crystal structure of Dy@C2v(5)-C80(CH2Ph) showed that a single benzyl group was grafted onto the (5,6,6)-carbon, suggesting the open-shell electronic configuration of Dy@C2v(5)-C80. The theoretical calculations unveiled that the benzyl radical addition enables the modulation of the electronic configuration of Dy@C2v(5)-C80 and the corresponding stabilization of Dy@C2v(5)-C80 in conventional organic solvents. This facile stabilization strategy via benzyl radical addition exhibits the considerable capability to capture these missing EMFs, with the benefit of enriching the endohedral fullerene family.

Molecular Tweezer Based on Perylene and Crown Ether for Selective Recognition of Fullerenes.

A molecular tweezer trans-di(perylene-3-ylmethanaminobenzo)-18-crown-6 (DP-18C6) incorporating two perylene subunits in a single crown ether core was designed and synthesized as a host for fullerenes. Through the cooperative effect of the perylene subunits and the crown ether moiety, DP-18C6 can efficiently recognize fullerenes including C60, C70, and C76. 1H NMR titration and fluorescence titration experiments demonstrated that DP-18C6 can effectively grasp the fullerene molecule to form a 1:1 host–guest complex. Density functional theory calculations revealed the presence of intermolecular π–π interactions between the perylene subunits of DP-18C6 and the fullerene molecule. More importantly, DP-18C6 exhibited remarkably high binding selectivity for higher fullerenes over C60, revealing potential application for the separation of fullerenes by means of host–guest interactions.

Structural Studies of Giant Empty and Endohedral Fullerenes.

Structure elucidations of giant fullerenes composed of 100 or more carbon atoms are severely hampered by their extremely low yield, poor solubility and huge numbers of possible cage isomers. High-temperature exohedral chlorination followed by X-ray single crystal diffraction studies of the chloro derivatives offers a practical solution for structure elucidations of giant fullerenes. Various isomers of giant fullerenes have been determined by this method, specially, non-classical giant fullerenes containing heptagons generated by the skeletal transformations of carbon cages. Alternatively, giant fullerenes can be also stabilized by encapsulating metal atoms or clusters through intramolecular electron transfer from the encapsulated species to the outer fullerene cage. In this review, we present a comprehensive overview on synthesis, separation and structural elucidation of giant fullerenes. The isomer structures, chlorination patterns of a series of giant fullerenes C2n (2n = 100-108) and heptagon-containing non-classical fullerenes derived from giant fullerenes are summarized. On the other hand, giant endohedral fullerenes bearing different endohedral species are also discussed. At the end, we propose an outlook on the future development of giant fullerenes.

Selective extraction of fullerene C60 by in situ interweaved AgNO3 complex encapsulated C60 molecular cages

With an in situ synthesized AgNO3 complex molecular cage, icosahedral C60 can be selectively extracted from fullerene mixture by a facial liquid–liquid interfacial precipitation, resulting in exclusive C60 organometallic microcrystal (C60/C70 = 6/1). By exploiting its intrinsic insolubility against nonpolar solvent, the C60 can be separated from the homologues, C70, which contrarily is assembled through merely hydrophobic interaction and π-π stacking. The AgNO3 complex encaged C60 crystal was morphologically and structurally characterized by SEM, Raman and XRD spectra to revealing the underlying separation mechanism. The selective extraction efficiency (89.7%) was semiquantitatively characterized by combined optical spectroscopy and mass spectrometry, indicating a decent specificity of spontaneously formed molecular cage toward C60 extraction. Thus, this discovery of a template-specific molecular cavity would pave the way to advance molecular imprinting technology using supramolecular array for fullerene separation, prevailing over the conventional “design-synthesis-performance” strategy by substantially reducing the laborious work and economic cost.

Complete Dynamic Reconstruction of C60, C70, and (C59N)2 Encapsulation into an Adaptable Supramolecular Nanocapsule.

The dynamic adaptability of tetragonal prismatic nanocapsule 18+ in the selective separation of fullerenes and endohedral metallofullerenes (EMFs) remains unexplored. Therefore, the essential molecular details of the fullerene recognition and binding process into the coordination capsule and the origins of fullerene selectivity remain elusive. In this work, the key steps of fullerene recognition and binding processes have been deciphered by designing a protocol which combines 1H-1H exchange spectroscopy (2D-EXSY) NMR experiments, long time-scale Molecular Dynamics (MD) and accelerated Molecular Dynamics (aMD) simulations, which are combined to completely reconstruct the spontaneous binding and unbinding pathways from nanosecond to second time-range. On one hand, binding (k'on) and unbinding (koff) rate constants were extracted from 1H-1H exchange spectroscopy (EXSY) NMR experiments for both C60 and C70. On the other hand, MD and aMD allowed monitoring the molecular basis of the encapsulation and guest competition processes at a very early stage under nonequilibrium conditions. The receptor capsule displays dynamical adaptability features similar to those observed in the process of biomolecular recognition in proteins. In addition, the encapsulation of bis-aza[60]fullerene (C59N)2 within a supramolecular coordination capsule has been studied for the first time, showcasing the pros and cons of the dumbbell-shaped guest in the dynamics of the encapsulation process and in the stability of the final bound adduct. The powerful combination of NMR, MD, and aMD methodologies allows to obtain a precise picture of the subtle events directing the encapsulation and is thus a predictive tool for understanding host-guest encapsulation and interactions in numerous supramolecular systems.

Synthesis and chromatographic evaluation of poly(pentabromostyrene)-silica composite: A versatile stationary phase for separating both polar and non-polar aromatic compounds

Polymer-silica composite materials have been witnessed as one of the most promising chromatographic packing materials. The demands of novel stationary phases are always existing due to the requirements for the separation of special samples. In our present work, the novel poly(pentabromostyrene)-silica composite stationary phases were successfully prepared by surface initial free-radical polymerization. The polymer chains bonding on the surface of the poly(pentabromostyrene) contain plenty of phenyl moiety, which provide hydrophobic and π-π interactions between the stationary phase and analytes. Besides, the bromide atoms, existing in the polymer, provide extra polar interaction with polar or easily polarized analytes. Compared with the typical C18 stationary phase, the novel stationary phase showed special separation selectivity. Samples, including polycyclic aromatic hydrocarbons, phenols, anilines, amino acids, halogenated benzenes, acetophenone derivatives, benzene homologous compounds, were all well separated by the new stationary phase. Also, the great stability (0.13% ≤ RSDs ≤ 0.98%, n = 5) of packed columns and the good synthesis repeatability were demonstrated. Finally, the new columns were applied to the separation of fullerenes. The loading ratio reached 2.67% (36 mg of fullerenes was loaded onto the column packed with 1.35 g stationary phase), with high yield (86.3%) of high pure C60 (>95%). This novel stationary phase has a great potential to be a complementary to traditional C18 column and a special column for fullerenes separation.

Quantitative Evaluation of Fullerene Separation by Liquid Chromatography

Although a huge number of facile synthesis methods for nanoparticles (NPs) do exist, the products usually exhibit a distribution in particle size and shape. Thus, a separation step needs to be applied to adjust the particle properties according to the needs of the later application. Chromatography is a potentially scalable separation method which is well-established for separating molecules and promising for classifying NPs by size. Herein, we lay the foundations of particle chromatography by studying the separation of a C60/C70 fullerene mixture as well-defined particle probes using a pyrene-functionalized silica stationary phase. C60 fullerenes are perfect model particles as they are spherical, roughly 1 nm in diameter, interacting via van der Waals interactions only. First, we extract the Henry coefficients for C60 and C70 fullerenes from the elution behavior of the fullerene mixture. Using a particle-wall as well as particle-particle model for the van der Waals interaction potential, we determine the ...

Thermally activated isomeric all-hydrocarbon molecular receptors for fullerene separation

A new all-hydrocarbon macrocycle, cyclo[8](1,3-(4,6-dimethyl)benzene) (CDMB-8) has been reported. As prepared, it exists in Cs symmetry and shows no interaction with fullerenes (e.g., C60 or C70). High temperature (573 K) treatment induces thermal conversion of the material to an isomeric conformer with D4d symmetry as a receptor for fullerene separation.

Selective Extraction of C70 by a Tetragonal Prismatic Porphyrin Cage.

Along with the advent of supramolecular chemistry, research on fullerene receptors based on noncovalent bonding interactions has attracted a lot of attention. Here, we present the design and synthesis of a cationic molecular cage: a cyclophane composed of two tetraphenylporphyrins, bridged face-to-face by four viologen units in a rhomboid prismatic manner. The large cavity inside the cage, as well as the favorable donor-acceptor interactions between the porphyrin panels and the fullerene guests, enables the cage to be an excellent fullerene receptor. The 1:1 host-guest complexes formed between the cage and both C60 and C70 were characterized in solution by HRMS and NMR, UV-vis and fluorescence spectroscopies, and confirmed in the solid state by single-crystal X-ray diffraction analyses. The results from solution studies reveal that the cage has a much stronger binding for C70 than for C60, resulting in a selective extraction of C70 from a C60-enriched fullerene mixture (C60/C70 = 10/1), demonstrating the potential of the cage as an attractive receptor for fullerene separation.

Examining Structure–Property–Function Relationships in Thiophene, Selenophene, and Tellurophene Homopolymers

Heavy atom main group element-containing conjugated polymers have attracted increasing attention in recent years. The synthesis of these compounds is generally involved, and little is known about their optoelectronic device performance. Here we examine the relationship between polymer structure and optoelectronic behavior in a series of chalcogenophene homopolymers of thiophene, selenophene, and tellurophene with well-matched molecular weights, dispersity, and regioregularity. We employ fast and slow drying device preparations to study the effect of polymer–fullerene separation on charge separation and collection in canonical bulk heterojunction photovoltaic cells. In both preparations, increasing heteroatom size leads to larger proportions of finely mixed polymer–fullerene domains. Differences in polymer–fullerene separation between preparations result in the formation of optimal morphologies in selenophene and tellurophene devices with little impact on thiophene devices. We then use planar heterojunction devices to directly examine the effects of heteroatom substitution on charge transport and charge generation and find that in the absence of polymer–fullerene mixing, devices exhibit similar diode behavior. We further demonstrate that ultrafast decay pathways unique to heavy heteroatom-containing polymers are apparent in both planar and bulk heterojunctions and thus not dependent on polymer–fullerene mixing or polymer assembly. This work directly examines the role of heteroatom substitution in defining the photovoltaic performance of conjugated homopolymers. Through single-atom substitution we are able to significantly modify polymer assembly, mixing, and optoelectronic properties. Specific emphasis on tellurophene polymers reveals relationships between polymer structure and properties that are not apparent in more traditional light-atom chalcogenophenes such as thiophene and selenophene.

A Shape-Adaptable Organometallic Supramolecular Coordination Cage for the Encapsulation of Fullerenes.

The development of three-dimensional supramolecular coordination complexes (SCCs), with cavities suitable for guest binding, is of great interest because these materials are useful in a large number of applications. Herein, a nickel-conjoined organometallic molecular prism is described, which has been fully characterized. The X-ray diffraction structure of the molecule reveals that the cage possesses an internal cavity with a volume of 1028 Å3 , thus suitable for the encapsulation of large 3D-molecules, such as fullerenes. This cage shows highly selective complexation of C70 over C60 , thus being potentially useful for fullerene separation and purification. The combined experimental and computational studies suggest that the complexation process is largely entropically-driven, and DFT calculations suggest that the cage is flexible and can adapt the size and the shape of the cavity to maximize the face-to-face interaction with the fullerenes.

Platinum(II)-Based Convex Trigonal-Prismatic Cages via Coordination-Driven Self-Assembly and C60 Encapsulation.

The development of three-dimensional (3D) supramolecular coordination complexes is of great interest from both fundamental and application points of view because these materials are useful in molecular catalysis, separation and purification, sensing, etc. Herein, we describe the synthesis of two Klärner's molecular-clip-based tetrapyridyl donors, which possess a C-shaped structure as shown by X-ray analysis, and subsequently use them to prepare four convex trigonal-prismatic cages via coordination-driven self-assembly with two 180° diplatinum(II) acceptors. The cages are fully characterized by multinuclear NMR (31P and 1H) analysis, diffusion-ordered spectroscopy, electrospray ionization time-of-flight mass spectrometry, and UV/vis absorption spectroscopy. Moreover, the incorporation of molecular-clip-based ligands provides these cages with free cavities to encapsulate fullerene C60 via aromatic interactions, which may be useful for fullerene separation and purification. The studies described herein enlarge the scope of the platinum(II)-based directional bonding approach in the preparation of curved 3D metallacages and their host-guest chemistry.

Silica based click-dibenzo-18-crown-6-ether high performance liquid chromatography stationary phase and its application in separation of fullerenes

Crown ether is a type of typical macrocyclic polyether compounds, which can produce strong interactions with cationic species, such as metal ions and protonated amines due to its cavity structure and the strong electronegative effect of heteroatoms on the crown ether ring. In this paper, a type of silica based crown ether stationary phase was prepared by covalently bonding dibenzo-18-crown 6-ether (DBCE) on silica gel via copper (I) catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC). The tetraazido DBCE was rapidly prepared by bromomethylation and subsequent azido substitution of DBCE. Finally, this key intermediate was covalently bonded to silica beads by click chemistry to get click-DBCE. Its structure was confirmed by FT-IR and elemental analysis, and the structures of all related intermediates were confirmed by NMR, IR and MS spectra. The crown ether based stationary phase showed good chromatography characteristics and column efficiency (up to 43,553 plates m-1 theoretical plate number is obtained in the case of fluoranthene) under reverse phase liquid chromatographic (RPLC) model. The primary chromatographic evaluation showed that click-DBCE displayed better chromatography efficiency than commercial C18 stationary phase (THERMO BDS HYPERSIL), when they were applied in the separation of regio-isomers of benzene, polycyclic aromatic hydrocarbons (PAHs) and macrolide pharmaceuticals. Furthermore, this stationary phase could also be used to separate fullerenes under normal phase mode, and in this case, 1-chloronaphthalene was suitable additive for improving chromatographic separation ability.

Importance of Solvent Removal Rate on the Morphology and Device Performance of Organic Photovoltaics with Solvent Annealing.

Solvent vapor annealing has been widely used in organic photovoltaics (OPV) to tune the morphology of bulk heterojunction active layer for the improvement of device performance. Unfortunately, the effect of solvent removal rate (SRR) after solvent annealing, which is one of the key factors that impact resultant morphology, on the morphology and device performance of OPV has never been reported. In this work, the nanoscale morphology of small molecule (SM):fullerene bulk heterojunction (BHJ) solar cell from different SRRs after solvent annealing was examined by small-angle neutron scattering and grazing incidence X-ray scattering. The results clearly demonstrate that the nanoscale morphology of SM:fullerene BHJ especially fullerene phase separation and concentration of fullerene in noncrystalline SM was significantly impacted by the SRR. The enhanced fullerene phase separation was found with a decrease of SRR, while the crystallinity and molecular packing of SM remained unchanged. Correlation to device performance shows that the balance between pure fullerene phase and mixing phase of SM and fullerene is crucial for the optimization of morphology and enhancement of device performance. Moreover, the specific interfacial area between pure fullerene phase and mixing phase is crucial for the electron transport and thus device performance. More importantly, this finding would provide a more careful and precise control of morphology of SM:fullerene BHJ and offers a guideline for further improvement of device performance with solvent annealing.

Non-Chromatographic Purification of Endohedral Metallofullerenes.

The purification of endohedral metallofullerenes by high performance liquid chromatography is very time-consuming and expensive. A number of rapid and inexpensive non-chromatographic methods have thus been developed for large-scale purification of metallofullerenes. In this review, we summarize recent advances in non-chromatographic purification methods of metallofullerenes. Lewis acid-based complexation is one of the most efficient and powerful methods for separation of metallofullerenes from empty fullerenes. The first oxidation potential of metallofullerenes is a critical factor that affects the separation efficiency of the Lewis acid-based method. Supramolecular methods are effective for separation of fullerenes and metallofullerenes that are different in size and shape. Chemical/electrochemical reduction and exohedral functionalization are also utilized to separate and purify metallofullerenes on a large scale.