内嵌富勒烯的研究进展

内嵌富勒烯由于其结构新颖以及独特而优异的性质在国际上引起持续而广泛的关注,成为近年来的研究热点之一.目前已经研究发现的内嵌富勒烯多达近百种,从惰性气体到碱土金属再到稀土元素都已被成功地嵌入到不同尺寸的碳笼中.其中金属离子或含金属的离子簇内嵌入富勒烯碳笼形成的内嵌金属富勒烯,以其种类丰富、结构多样成为内嵌富勒烯的主要研究对象.本文就近年来研究报道的种类繁多的内嵌富勒烯按其内嵌物类型进行归纳阐述,为今后开发更多新型的内嵌富勒烯提供一定的参考.     

Quantum-chemical study of endohedral fullerenes

State-of-the-art in quantum-chemical research on endohedral fullerenes is considered. Separate chapters are devoted to complexes with noble gases, main group elements, and methals and their nitrides. The structures and the mechanisms of formation and decomposition of these compound sare highlighted. Applicability of various quantum-chemical methods for endohedral fullerene complexes is discussed. Problems associated with the use of the ROHF and UHF methods are revealed. The DFT method with hybrid functionals and effective core potentials for heavy atoms is shown to hold the greatest promise.     

Vibrational properties of noble gas endohedral fullerenes

Analysis of IR and Raman spectra of Ar@C(60) and Kr@C(60) shows that the incorporation of noble gas atoms causes a blue shift of low energy vibrations, which have radial character, and a red shift of higher energy ones which have a tangential character movement. The mechanism of these phenomena is explained on the basis of ab initio numerical experiments with DFT and MP2 procedures. Methodological discussions are advanced, altogether with a scheme for the estimation of the van der Waals interaction between fullerene and noble gas, based on the frequency shifts.     

Dipole polarizabilities of noble gas endohedral fullerenes

First-principles calculations were performed on the polarizabilities of noble gas endohedral complexes, X@C₆₀ (X = He–Kr). The polarizability of X@C₆₀ increases from He–Kr, but is distinctly smaller than the sum of the X atoms and C₆₀, unlike many other non-bonded systems for which an additive rule holds. Compared to C₆₀, the increased polarizabilities in X@C₆₀ are mainly from the enhanced charge transfer contribution of the cage and the local polarization of the interior X atoms, though the interior atoms are found less polarized than the corresponding free ones.     

Functionalised endohedral fullerenes in single-walled carbon nanotubes

Atomically thin carbon nanotubes serve as transparent-test tubes for individual molecules of functionalised endohedral fullerenes. Aberration-corrected transmission electron microscopy reveals the complex dynamic behaviour of these molecules at the atomic level, and it sheds light on the mechanism of their encapsulation into nanotubes.     

Template ordered open-grid arrays of paired endohedral fullerenes

Developing useful molecular systems, such as planar networks for novel molecular electronics, requires the ability to control the way molecules assemble at surfaces. Here we report how an oxide crystal surface can be used as a template to controllably order endohedral fullerenes, Er3N@C80, into two-dimensional (2D) open-grid arrays. The crystal surface is made of highly ordered oxide nanostructures which self-assemble on the surface of SrTiO3(001). This method of molecular ordering can be applied to other fullerenes and has the potential to provide a basis for developing a wide range of molecular architectures.     

Metallic nitride endohedral fullerenes: synthesis and electrochemical properties

Metallic nitride endohedral fullerenes (MNEFs) are one of the most interesting types of metallofullerenes due to their high yields and interesting electronic properties. The synthesis of these compounds allows the encapsulation of different metals inside fullerenes making it possible to combine the properties of the metal with those of the fullerene. Their interesting electrochemical and optical features make them potential candidates for several applications such as molecular electronics, biomedical imaging, non-linear optical devices, and MRI agents. We report herein the synthesis, isolation, and electrochemical properties of the larger MNEFs including Gd₃N@C₈₂ and Gd₃N@C₈₆, reported here for the first time.     

Calculations and assignments of endohedral helium-3 chemical shifts of open-cage fullerenes and higher fullerenes

The endohedral ³He NMR chemical shifts of open-cage fullerene compounds and higher fullerenes ³He@C _n (n = 82, 84, 86) have been calculated at the GIAO-B3LYP/3-21G//AM1 level. The predicted ³He NMR chemical shifts of open-cage fullerene compounds agree well with the experimental data. More importantly, the challenging peak assignments in the two ³He NMR spectra of higher fullerenes have been successfully achieved by our computed endohedral ³He chemical shifts in combination with experimental results.     

Th@C76. Computational characterization of larger actinide endohedral fullerenes

The actinide endohedral fullerene Th@C₇₆ was successfully prepared in a very recent experiment (Wang et al., J. Am. Chem. Soc. 2017 , 139, 5110) yet without any structural information. In this work, density functional theory calculations revealed that this novel molecule bears a T_d(19151)‐C₇₆ cage obeying the isolated pentagon rule. The internal Th atom is off‐centered and resides under a sumanene‐type hexagonal ring, formally donating 4e to the carbon cage. The metal position was rationalized by the structure and charge distribution of the negatively charged cage. Interestingly, an octahedron‐like dynamic trajectory of metal inside the C₇₆ cage at high temperature was found based on the cage symmetry and located transition state structures. Furthermore, the infrared, NMR, and UV/vis spectra of Th@C₇₆ were simulated to assist future experimental characterization.     

Radiochemical challenges in the study of endohedral fullerenes and MD simulation

The formation of atom-doped fullerenes has been investigated by using several types of radionuclides produced by nuclear reactions. From the trace of the radioactivities after high performance liquid chromatography (HPLC), it was found that formation of endohedral fullerenes (or heterofullerene) with small atoms (Be, Li), noble-gas atoms (Kr, Xe) and 4B–6B elements (Ge, As, Se, Sb, Te etc.) is possible by a recoil process following the nuclear reaction. In order to show the possibility of creating endohedral fullerenes by inserting a foreign atom with a suitably high kinetic energy into C₆₀, we have carried out large-scale ab initio molecular dynamics simulations on the basis of the all-electron mixed-basis approach with atomic orbitals and plane waves for Li, Be, N, O, Na, S, Cl, K, V, Cu, As, Se, Sb, Te, Kr, Xe. This revised version was published online in July 2006 with corrections to the Cover Date.     

Triplet exciton generation in bulk-heterojunction solar cells based on endohedral fullerenes

Organic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu(3)N@C(80) (Lu(3)N@C(80)-PCBEH) show an open circuit voltage (V(OC)) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid methyl ester (PC(61)BM). To fully exploit the potential of this acceptor molecule with respect to the power conversion efficiency (PCE) of solar cells, the short circuit current (J(SC)) should be improved to become competitive with the state of the art solar cells. Here, we address factors influencing the J(SC) in blends containing the high voltage absorber Lu(3)N@C(80)-PCBEH in view of both photogeneration but also transport and extraction of charge carriers. We apply optical, charge carrier extraction, morphology, and spin-sensitive techniques. In blends containing Lu(3)N@C(80)-PCBEH, we found 2 times weaker photoluminescence quenching, remainders of interchain excitons, and, most remarkably, triplet excitons formed on the polymer chain, which were absent in the reference P3HT:PC(61)BM blends. We show that electron back transfer to the triplet state along with the lower exciton dissociation yield due to intramolecular charge transfer in Lu(3)N@C(80)-PCBEH are responsible for the reduced photocurrent.     

Purification of endohedral trimetallic nitride fullerenes in a single, facile step

A major hurdle hampering the development of fullerenes, endohedral metallofullerenes, and nanotubes has been the difficulty of obtaining high purity samples. Soots prepared in the usual manner via a Kr?tschmer-Huffman electric-arc generator consist of mixtures of insoluble carbonaceous materials and soluble fullerenes: C60, C70, C76, C78, C84, etc. When metals are introduced as endohedral species the complexity of the resultant soot is even greater because of the presence of multiple isomers of both the empty fullerenes and the endohedral metallofullerenes. Here, for the first time, we report that lanthanide trimetallic nitride endohedral metallofullerenes, A3N@C80 (A = lanthanide atom, e.g., Er, Gd, Ho, Lu, Sc, Tb, Tm, Y), can be obtained in pure form directly from as-prepared soots in a single facile step by taking advantage of their extraordinary kinetic chemical stability with respect to the other fullerenes in Diels-Alder reactions with a cyclopentadiene-functionalized resin. We show that careful control of conditions (stoichiometry, time, temperature) allows separation of fullerenes with different cage sizes, as well as isomeric species. Furthermore, the Diels-Alder reaction is thermally reversible, and we demonstrated that the bound empty-cage fullerenes and classical endohedral metallofullerenes can be recovered by displacement with maleic anhydride.     

Energy of compressed endoatoms and the energy capacity of small endohedral rare-gas fullerenes

The excess energies of the endoatoms within endofullerenes X@C_n (X = He, Ne, Ar; n = 20–50) as compared with the energies of the free atoms X were estimated using the simplest Thomas-Fermi model and density functional calculations at the B3LYP/6-311G* level. The energy capacities of the endofullerenes under study are primarily determined by the contributions of the compressed electronic systems of the endoatoms rather than the stretched fullerene cages. Dedicated to Academician A. L. Buchachenko on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1939–1942, September, 2005.     

Structural interconnections and the role of heptagonal rings in endohedral trimetallic nitride template fullerenes

Recent experiments indicate that fullerene isomers outside the classical definition can also encapsulate metallic atoms or clusters to form endohedral metallofullerenes. Our systematic study using DFT calculations, suggests that many heptagon‐including nonclassical trimetallic nitride template fullerenes are similar in stability to their classical counterparts, and that conversion between low‐energy nonclassical and classical parent cages via Endo–Kroto insertion/extrusion of C₂ units and Stone–Wales isomerization may facilitate the formation of endohedral trimetallic nitride fullerenes. Close structural connections are found between favored isomers of trimetallic nitride template fullerenes from C₇₈ to C₈₂. It appears that the lower symmetry and local deformations associated with introduction of a heptagonal ring favor encapsulation of intrinsically less symmetrical mixed metal nitride clusters. © 2016 Wiley Periodicals, Inc.     

Investigation of the fragmentation and oxygen reactivity of endohedral metallofullerenes M@C60

The stability, fragmentation, and chemical reactivity of endohedral metallofullerenes M@C₆₀ (M = Ca, Sr, La, and Gd) were examined with laser desorption/ionization mass spectroscopy. A laser ablation/sublimation procedure was used to generate and purify M@C₆₀ samples, which were then characterized by both time-of-flight and Fourier transform ion cyclotron resonance mass spectrometries. Collisional dissociation of the ionized endohedral fullerenes listed above revealed multiple C₂ loss (either as sequential C₂ loss or larger C_2n eliminations) to generate similar terminal fragmentation products (i.e. MC_44,46), which might be predicted due to the similar ionic radii of the encapsulated metals. The reactivity of the endohedral compounds with oxygenated compounds was found to be dependent on the charge state of the metallofullerene. Singly charged cations of the metallofullerenes (M@C_n⁺) were completely unreactive in the gas phase with oxygenated compounds such as ethylene oxide; however, the corresponding neutral species appear to react readily with oxygenated species. This point was demonstrated in that exposure of solid phase M@C₆₀ to ambient conditions (laboratory air at room temperature) leads to the ultimate destruction of the metallofullerenes concomitant with the formation of metal oxides and carbonates.     

Synthesis,extraction, and chromatographic purification of higher empty fullerenes and endohedral gadolinium metallofullerenes

Higher empty and endohedral gadolinium metallofullerenes were prepared by electric arc synthesis, isolated, and separated chromatographically to high degree of purity (99 wt %). The samples obtained were characterized by chromatography, optical spectroscopy, and mass spectrometry. According to small-angle neutron scattering data, the endohedral metallofullerenes in o-xylene form a molecular solution in which the molecules interact to form molecular pairs.     

Production of endohedral <Superscript>133</Superscript>Xe-higher fullerenes by ion implantation

Endohedral ¹³³Xe-higher fullerenes (¹³³Xe@C₇₆ and ¹³³Xe@C₈₄) were produced by implantation of ¹³³Xe ions using an isotope separator. A high performance liquid chromatography (HPLC) analysis showed that the peak of endohedral ¹³³Xe-higher fullerenes shifted backward from that of empty fullerenes, suggesting a possibility of the separation of endohedral ¹³³Xe-higher fullerenes from empty fullerenes. The yields of endohedral ¹³³Xe-fullerenes were in the order of ¹³³Xe@C₇₆<¹³³Xe@C₈₄<¹³³Xe@C₆₀<¹³³Xe@C₇₀.     

Dissociative Photoionization of m-Xylene

The photoionization and dissociative photoionization of m-xylene (C₈H₁₀) were researched by using synchrotron radiation vacuum ultraviolet (SR-VUV) and supersonic expanding molecular beam reflectron time-of-flight mass spectrometer (RFTOF-MS) system. The photoionization efficiency spectra (PIEs) of parent ion C₈H₁₀⁺ and main fragment ions C₈H₉⁺ and C₇H₇⁺ were observed, and the ionization energy (IE) of m-xylene and appearance energies (AEs) of main fragment ions C₈H₉⁺ and C₇H₇⁺ were determined to be 8.60 ± 0.03 eV, 11.76 ± 0.04 eV and 11.85 ± 0.05 eV, respectively. Structures of reactant, transition states (TSs), intermediates (INTs), and products involved in two dominant dissociation channels were optimized at the B3LYP/6-311++G(d,p) level, and the relative energies were calculated at the G3 level. Based on the results, two major dissociative photoionization channels, C₇H₇⁺+CH₃ and C₈H₉⁺+H were calculated at the B3LYP/6-311++G(d,p) level. On the basis of theoretical and experimental results, the dissociative photoionization mechanisms of m-xylene were proposed. The C–H or C–C bond dissociation and hydrogen migration are the main processes in the dissociation channels of m-xylene cation.

     

Photoionization and Dissociative Photoionization Study of Cholesterol by I R Laser Desorption/Tunable Synchrotron VUV Photoionization Mass Spectrometry

Elementary cholesterol was analyzed with IR laser desorption/tunable synchrotron vacuum ultraviolet photoionization mass spectrometry. An exclusive molecular ion of cholesterol is observed by near threshold single-photon ionization with high efficiency. Fragments are yielded with the increase of photon energy. The structures of various fragments are determined with commercial electron ionization time-of-flight mass spectrometry. Dominant fragmentation pathways are discussed in detail with the aid of ab initio calculations.