Structure of Endohedral Fullerenes: Comparison
Please note this is a comparison between Version 3 by Jessie Wu and Version 2 by Jessie Wu.

Fullerenes have a unique structure, capable of both encapsulating other molecules and reacting with those on the exterior surface. Fullerene derivatives have also been found to have enormous potential to address the challenges of the renewable energy sector and current environmental issues, such as in the production of n-type materials in bulk heterojunction solar cells, as antimicrobial agents, in photocatalytic water treatment processes, and in sensor technologies. Endohedral metallofullerenes, in particular, can possess unpaired electron spins, driven by the enclosed metal atom or cluster, which yield valuable magnetic properties. These properties have significant potential for applications in molecular magnets, spin probes, quantum computing, and devices such as quantum information processing,, atomic clocks, and molecular magnets. 

  • endohedral metallofullerenes
  • EMFs
  • arc discharge
  • bottom-up synthesis
  • in situ doping

1. Introduction

Carbon, the building block of life and the fourth most abundant element in the universe, is one of the most indispensable materials known to humankind. In addition to the two naturally occurring allotropes, graphite and diamond, carbon is also capable of taking several remarkable physical forms on the nanoscale. One such distinct allotrope in the family of carbon nanomaterials is the fullerene. Its discovery [1] sparked a great deal of curiosity and interest amongst the scientific community due to its unique closed, cage-like structure, composed solely from sp2 hybridized carbon [2][3]. Within the first few years after the discovery of buckyball C60, the term ‘fullerene’ achieved a uniform acceptance as the general name for these carbon cages. Fullerenes can have different numbers of carbon atoms, shapes, and symmetry; hence, the largely accepted notation system [4] is to subscript the number of carbon atoms to state the class of the fullerene. The structure of C60, the most abundant buckyball, has been verified to consist of 12 five-membered rings and 20 six-membered rings [2]. Most of the fullerenes obey the isolated pentagon rule (IPR), where pentagons are exclusively surrounded by hexagons, which increases their stability. However, several fullerenes, especially a number of endohedral fullerenes (see next section), do not follow the IPR rule [5]. Instead they follow the maximum pentagon separation rule, where they exhibit maximum pentagon separation for maximum stability [6].
Fullerenes are known to have different isomers, with C60 having 1812 theoretically different isomers [7][8]. These isomers can have different symmetry, form, and energetic stability. An example of the different structures of the six most stable isomers of C80 is shown in Figure 1, with D5d and D2 being the most stable and abundantly found isomers [9]. Fullerene isomers are also known to have different oxidation potential and chemical reactivity. For instance, the D3 isomer of C78 is easier to oxidize than the C2v isomer of the same fullerene by about 0.25 V [10]. Another very intriguing feature of the fullerenes is that they have a hollow space, which is large enough to encapsulate other atoms, ions, molecules, and various kinds of clusters, giving rise to a new class of hybrid materials called endohedral fullerenes.
Figure 1. (af) Schematic representation of the different isomers of the C80 cage.

2. Structure 

The ability of different sizes of fullerene cages to incorporate different types of metals and clusters can lead to the formation of materials with novel solid-state properties. The robust nature of the carbon cage plays a very important role in isolating the atoms from the exterior environment, while the electronic properties of the cage can be tuned by the selection of the atoms inside. La@C82 was among the first discovered endohedral fullerene, where the symbol @ denotes the inclusion of a La atom inside a C82 cage. Since then, fullerenes encapsulating metallic [11], carbide [12][13], nitride [14], and many methano- [15] and oxide- [16] clusters have been synthesized and isolated over the years. In fact, fullerenes containing single atoms of elements that otherwise are not stable, such as noble gases [17][18][19] and non-metals [20] such as nitrogen [21], have also been synthesized. Transition metals and actinides were amongst the challenging elements to encapsulate within fullerenes, primarily due to their reactivity and the difficulties associated with their incorporation into the graphite rod for arc-discharge synthesis. However, over the past decade, significant progress has been made in also encapsulating various transition metals within fullerenes. [22] In these cases, while describing the diversity of the fullerene cages, prefixes and cage symmetries are also used. For instance, Sc43-O)3@Ih(7)-C80 represents the endohedral cluster group, cage structure, and point-group symmetry, following the nomenclature of Fowler and Manolopoulos [4].
In 1991, researchers succeeded in encapsulating metal atoms, which opened a complete new set of possibilities in the field of material development, generally referred to as endohedral metallofullerenes [23]. Among the endohedral fullerenes, the trimetallic nitride-containing cages of the type M3N@C2n (where M = Sc, Gd; 2n = 68–96), shown in Figure 2, have been one of the most studied materials since their discovery in 1999 [14] due to their high yields during synthesis and their appropriate HOMO-LUMO levels, desired for applications in solar cells. However, despite the comparable high yields, empty fullerenes still constituted as major product during the synthesis process. The yields of M3N@C2n were further improved by Dunsch and Yang [24], who introduced ammonia during fullerene synthesis and produced an endohedral fullerene, Sc3N@Ih-C80, as the majority product for the first time. This family of fullerenes has an especially high yield because of the charge transfer from the encaged cluster, which results in a stable ion pair and stabilizes the higher fullerene cages of various isomeric structures. In addition, the large HOMO-LUMO gaps give further kinetic stability to the hybrid molecule.
Figure 2. A representative figure of two endohedral metallofullerenes, (a) monometallic in a C82 cage and (b) trimetallic nitride encapsulated in a C80 fullerene cage. The red and the yellow balls correspond to the metal ion encapsulated in the carbon cage and the green to nitrogen.

2.1. Endohedral Metallofullerenes: Endohedral Metallofullerenes

In the years following the discovery of C60, the study of empty cage fullerenes heavily dominated the field of nanomaterials. The focus on empty cage buckyballs often resulted in the blind assumption that endohedral metallofullerenes (denoted as EMFs) possessed the same structural or chemical properties as the corresponding empty cages. However, over the years, after more studies were conducted on individual EMFs, it became established that they exhibit novel structural, chemical, and electronic properties, which were completely different to those of their parent empty cages. For instance, carbon cages of EMFs have different isomeric structures as compared to their parent fullerene cage, and they also show different solubility in individual solvents [25]. As for their electronic and chemical behaviour, their chemical reactions with radicals [26] and many other cyclo-addition [27] reactions show significantly different patterns and redox potentials [28]. For instance, the reduction potentials of La@C82 and La2@C80 are 0.70 V and 0.81 V lower than that of C60 [29]. The reason for such a huge difference is largely attributed to the covalent interaction between the enclosed species and the carbon cage as well as the charge transfer phenomenon from the metallic species to the fullerene cage.
The most studied metallofullerenes involve rare earth metals, such as La, Sc, Dy, and Tm [23][30][31][32][33][34][35], and main group metals such as Li and Ca [36]. There is an extensive array of EMFs that have one, two, three, or even four metal atoms trapped inside them [37]. Alternatively, they can also contain M3N [14] or M2C2 units [12], where M refers to metals. The key to understanding the electronic properties of these hybrid materials is through studying the degree of charge transfer from the metal species to the exterior cage, which alters its electronic state. While the number of electrons transferred from a majority of metal atoms to the cage is known and verified using X-ray diffraction, absorption, and other spectroscopic methods [31][32][33][34][35], researchers observed that contradictory results still exist in the literature regarding the effect of the various parameters on the arc synthesis and the formation mechanism of these fullerenes. For instance, Dunk et al. claimed that the formation mechanism is governed by a charge transfer between the metal and the cage [38]. Through detailed mechanistic studies, they revealed that the C2 insertion during the bottom-up synthesis becomes more difficult as the oxidation state of the metal increases. This charge transfer and the nature of the cluster inside the cage also dictates a series of properties, such as electrochemical behaviour and light emission. In fact, Ito et al. studied Er2@C82 and Er2C2@C82, which showed nearly identical cage properties with a 6- negatively charged C826−, using fluorescence spectroscopy [39]. Most notably, this cage charge contradicted an earlier publication by Wang et al. claiming that the Er2@C82 and Er2C2@C82 compounds have a 4- negatively charged cage, i.e., C824− [40]. Furthermore, Ito et al. showed that the characteristic emission of the erbium ions at 1520 nm was influenced by the symmetry of the cage, an observation later echoed by Plant et al. [41] Until as recently as 2020, previously unknown factors, such as enantiomers and enthalpy, were discovered to play vital and evolving roles in the synthesis mechanism for fullerenes C2n (50 ≤ 2n ≤ 70) [42]. After a critical analysis of research articles [43][44], it can be concluded that the complete mechanism of their formation, either through a bottom-up or top-down approach, and the charge transfer itself remains elusive and highly controversial [45]. In fact, the insights derived from experimental results have also been contradictory to theoretical results in the field. For instance, it has been suggested that factors such as the C2 concentration and temperature may dictate the more favourable direction of the reversible C2 injection/ejection reaction during the synthetic process [46]. However, theoretical studies have highlighted that the C2 injection is more energetically favourable than its C2 ejection counterpart, contradicting the experimental data on the importance of temperature and concentration [47][48]. The missing key links and low synthesis yield of fullerenes make it difficult to investigate the fullerene formation mechanism. Utilizing both experimental and theoretical frameworks is therefore necessary to answer some of the fundamental questions of this field.
As briefly mentioned in the previous section, the isomeric structures of the carbon cages encapsulating metals differ from their empty counterparts [34]. Further study of the positions and movements of the encapsulated atoms is also important in determining the chemical and physical properties of EMFs, including their magnetic behaviour, electron paramagnetic resonance (EPR), and electrochemical properties [49]. The structure of the EMFs themselves had long been speculated, but the first crucial evidence came through in 1995 when Shinohara et al. experimentally confirmed that the metal atom is indeed encapsulated within the fullerene cage [31]. The synchrotron X-ray powder diffraction patterns of Y@C82 and the maximum entropy method (MEM) revealed that the yttrium atom was displaced from the centre and instead tightly bound to the cage [50]. The study also confirmed strong dipole–dipole and charge transfer interaction between the different molecules [49]. MEM/Rietveld analysis has long been used to determine the structures of metallofullerenes [31][50]. However, the results (structure and position of metals inside the carbon cage) obtained by this method have been in question in recent years. This started when Wang et al. [12] discovered that the scandium carbide endohedral fullerene, Sc3C2@C80, was, in fact, what had long been considered to be a tri-metallofullerene Sc3@C82 [51]. These incoherences emphasize the importance of 13C NMR and synchrotron X-ray structural analysis in determining the structures and encapsulated cluster inside the fullerene cage and suggest the unreliability of the decades of results obtained exclusively using MEM/Rietveld analysis when determining the structures of metallofullerenes [52][53][54]. Consequently, the structures and positions of metals inside a fullerene have been re-examined in the past few years [55][56][57][58]. Furthermore, it was also observed that these metal clusters had motions and trajectories inside the cage [59], which also changed positions depending on the temperature [60][61].

2.2. Environmental Application

A massive number of publications in fullerene-related research show the possibility of using fullerenes for a variety of environmental and energy applications. The characteristics of fullerenes that give them an advantage over other materials include their robust carbon structure, absorption of light in the visible range, electron-accepting capabilities, and more recently, superconductivity [62]. All these properties make fullerenes especially sought-after materials for energy applications, especially organic solar cells [63]. They have become, and remain, an important part of high-performance organic solar cells.

2.3. Synthesis

Over the past few decades, EMFs have been proposed for a number of applications, including energy harvesting in organic solar cells [64] and MRI contrast agents [65][66][67]. EMFs have been synthesized through a range of different methodologies, such as laser ablation of graphite [23][68], arc discharge [30][33], ion bombardment [36][69], resistive heating [70], and electron initiation inside carbon nanotubes (CNTs) within aberration-corrected high-resolution transmission electron spectroscopy (AC-HRTEM) (CNT acts as a nanoreactor container to facilitate the formation of endohedral fullerenes from metals and the amorphous carbon atoms trapped inside. The process is energetically enabled under the presence of an electron beam inside an AC-HRTEM.) [71]. Some of these popular methods are summarized in Table 1 below.
The Kratschmer–Huffman arc method has been widely used and accepted as the most efficient and cost-effective technique to produce EMFs.
While the Kratschmer–Huffman method immediately became the most popular choice of EMF synthesis, it was not exempt from certain limitations. Most notably, only 60–70% of the graphite electrode was transferred to the carbon condensate (CC) containing the fullerenes and EMF, while the remaining 30–40% was lost as graphite build-up on the cathode. This disadvantage has now been eliminated by utilizing a high frequency arc discharge to supply the arc. The symmetrical setup of the electrodes allowed reaching up to a 100% conversion of the electrode material into the fullerenes and EMF containing CC [74]. Additionally, high-frequency arc discharge can also be used for further mechanistic investigation of fullerene synthesis.
A metal oxide/graphite composite is used as the anode, whereas graphite is used as the cathode, and both of them are arced in the D.C. mode in the presence of an inert gas (usually helium). Despite three decades of extensive research and various proposed pathways, a well-established formation mechanism for the synthesis of fullerenes and EMFs is still missing. Their ambiguous reaction pathways, alongside the violent conditions involving the plasma formation, arc, and chemical ionization that are required for the synthesis, are a major drawback in studying the synthesis mechanisms of these unique carbon nanostructures. Furthermore, a few other drawbacks hinder the scaling-up of their synthesis and consequently the applications of endohedral metallofullerenes. The overall yield of the arc-discharge synthesis of EMFs is low, with the ratio of EMF-to-empty-cage typically at around 1% [49]. As a result, a mixture of empty cage and endohedral fullerenes is typically produced, thus limiting the applications of the existing chromatographic methodologies for their fast and efficient separation. This results in the use of excessive amounts of solvents to efficiently separate them, which is neither ecological nor environmentally friendly. Therefore, it becomes important to review the synthesis of fullerenes. Shinohara published the first detailed scientific review of EMF research in 2000 [75], followed by an extensive update on endohedral fullerenes by Popov et al. in 2012 [76]. Many other groups have addressed the synthesis and properties of endohedral fullerenes [76][77][78][79], including nitride fullerenes [24], carbide cluster fullerenes [80], and oxide cluster fullerenes. Herein, wresearchers focus primarily on critically discussing the key findings in the field for optimizing of synthesis parameters of endohedral metallofullerenes and the new chemically activated method, with a particular emphasis on the post-2000 science along with comparisons to the early methodologies. The yield of the EMF has been observed to be highly dependent on the pressure of He gas, the arc current, the arc gap, and the composition of anodes, among other factors [51][81]. Liu [82] and Bandow [83] have argued that other parameters, such as the presence of catalysts (Cu, Fe, etc.), the back-burning of anodes, and in situ activation during arc discharge, also have an effect on the EMF yield. However, the latter two parameters would not increase the yield by a huge fraction because they do not modify the arc conditions drastically or change/influence the synthesis chemistry, e.g., lower the activation energy of the formation of the endohedral metallofullerenes. However, in order to maximize the efficiency of the method, a balance between the time spent during the synthesis and increase in yield needs to be established. Predominant aspects of overcoming the above challenge are anode composition, ionization potential, catalysts, and helium pressure in synthesis [74].
In recent years, the Kratschmer–Huffman arc design has also been modified mechanically in several ways to make it more environmentally friendly. For instance, the use of demineralized coal electrodes, minimizing the use of current and using direct current instead of high alternating current are a few ways in which the process has been made more efficient and less energy intensive [84]. Recently, Kyesmen et al. showed scaled-up production yield of fullerenes by employing such methods and including efficient energy usage by using resistive heating of one of the electrodes along its length [85]. Such green methodologies should be further explored for future research purposes and industrial-scale production of fullerenes to ensure an environment friendly approach towards using nanomaterials.

References

  1. Kroto, H.W.; Heath, J.R.; O Brien, S.C.; Curl, R.F.; Smalley, R.E. C60: Buckminsterfullerene. Nature 1985, 318, 162–163.
  2. Hawkins, J.M.; Meyer, A.; Lewis, T.A.; Loren, S.; Hollander, F.J. Crystal Structure of Osmylated C60: Confirmation of the Soccer Ball Framework. Science 1991, 252, 312–313.
  3. Wudl, F. The chemical properties of buckminsterfullerene (C60) and the birth and infancy of fulleroids. Accounts Chem. Res. 1992, 25, 157–161.
  4. Akasaka, T.; Lu, X. Structural and electronic properties of endohedral metallofullerenes. Chem. Rec. 2012, 12, 256–269.
  5. Kobayashi, K.; Nagase, S. A stable unconventional structure of Sc2@C66 found by density functional calculations. Chem. Phys. Lett. 2002, 362, 373–379.
  6. Rodríguez-Fortea, A.; Alegret, N.; Balch, A.L.; Poblet, J.M. The maximum pentagon separation rule provides a guideline for the structures of endohedral metallofullerenes. Nat. Chem. 2010, 2, 55–961.
  7. Schwerdtfeger, P.; Wirz, L.N.; Avery, J. The topology of fullerenes. WIREs Comput. Mol. Sci. 2015, 5, 96–145.
  8. Sure, R.; Hansen, A.; Schwerdtfeger, P.; Grimme, S. Comprehensive theoretical study of all 1812 C60 isomers. Phys. Chem. Chem. Phys. 2017, 19, 14296–14305.
  9. Rios, C.; Salcedo, R. Computational Termochemistry Study of the C80 Isomers and Their Endo Lanthanum Complexes by Applying Homodesmotic and Isodesmic Reactions. Molecules 2012, 17, 14588–14601.
  10. Yang, Y.; Arias, F.; Echegoyen, L.; Chibante, L.P.F.; Flanagan, S.; Robertson, A.; Wilson, L.J. Reversible Fullerene Electrochemistry: Correlation with the HOMO-LUMO Energy Difference for C60, C70, C76, C78, and C84. J. Am. Chem. Soc. 1995, 117, 7801–7804.
  11. Takata, M.; Nishibori, E.; Sakata, M.; Inakuma, M.; Yamamoto, E.; Shinohara, H. Triangle Scandium Cluster Imprisoned in a Fullerene Cage. Phys. Rev. Lett. 1999, 83, 2214–2217.
  12. Wang, C.R.; Kai, T.; Tomiyama, T.; Yoshida, T.; Kobayashi, Y.; Nishibori, E.; Shinohara, H. A Scandium Carbide Endohedral Metallofullerene: (Sc2C2)@C84. Angew. Chem. Int. Ed. 2001, 40, 397–399.
  13. Wang, T.-S.; Chen, N.; Xiang, J.-F.; Li, B.; Wu, J.-Y.; Xu, W.; Jiang, L.; Tan, K.; Shu, C.-Y.; Lu, X.; et al. Russian-Doll-Type Metal Carbide Endofullerene: Synthesis, Isolation, and Characterization of Sc4C2@C80. J. Am. Chem. Soc. 2009, 131, 16646–16647.
  14. Stevenson, S.; Rice, G.; Glass, T.; Harich, K.; Cromer, F.; Jordan, M.R.; Craft, J.; Hadju, E.; Bible, R.; Olmstead, M.M.; et al. Small-bandgap endohedral metallofullerenes in high yield and purity. Nature 1999, 401, 55–57.
  15. Krause, M.; Ziegs, F.; Popov, A.A.; Dunsch, L. Entrapped Bonded Hydrogen in a Fullerene: The Five-Atom Cluster Sc 3 CH in C 80. ChemPhysChem 2007, 8, 537–540.
  16. Stevenson, S.; Mackey, M.A.; Stuart, M.A.; Phillips, J.P.; Easterling, M.L.; Chancellor, C.J.; Olmstead, M.M.; Balch, A.L. A Distorted Tetrahedral Metal Oxide Cluster inside an Icosahedral Carbon Cage. Synthesis, Isolation, and Structural Characterization of Sc4(μ3-O)2@Ih-C80. J. Am. Chem. Soc. 2008, 130, 11844–11845.
  17. Syamala, M.S.; Cross, R.J.; Saunders, M. 129Xe NMR Spectrum of Xenon Inside C60. J. Am. Chem. Soc. 2002, 124, 6216–6219.
  18. Saunders, M.; Jiménez-Vázquez, H.A.; Cross, R.J.; Poreda, R.J. Stable Compounds of Helium and Neon: He@C60 and Ne@C60. Science 1993, 259, 1428–1430.
  19. Saunders, M.; Jimenez-Vazquez, H.A.; Cross, R.J.; Mroczkowski, S.; Gross, M.L.; Giblin, D.E.; Poreda, R.J. Incorporation of helium, neon, argon, krypton, and xenon into fullerenes using high pressure. J. Am. Chem. Soc. 1994, 116, 2193–2194.
  20. Larsson, J.A.; Greer, J.C.; Harneit, W.; Weidinger, A. Phosphorous trapped within buckminsterfullerene. J. Chem. Phys. 2002, 116, 7849–7854.
  21. Mauser, H.; Hirsch, A.; van Eikema Hommes, N.J.; Clark, T.; Pietzak, B.; Weidinger, A.; Dunsch, L. Stabilization of Atomic Nitrogen Inside C60. Angew. Chem. Int. Ed. Engl. 1997, 36, 2835–2838.
  22. Cai, W.; Zhang, M.; Echegoyen, L.; Lu, X. Recent Advances in Endohedral Metallofullerenes. Fundam. Res. 2023; in press.
  23. Chai, Y.; Guo, T.; Jin, C.; Haufler, R.E.; Chibante, L.P.F.; Fure, J.; Wang, L.; Alford, J.M.; Smalley, R.E. Fullerenes with metals inside. J. Phys. Chem. 1991, 95, 7564–7568.
  24. Dunsch, L.; Yang, S. Metal Nitride Cluster Fullerenes: Their Current State and Future Prospects. Small 2007, 3, 1298–1320.
  25. Tsuchiya, T.; Wakahara, T.; Lian, Y.; Maeda, Y.; Akasaka, T.; Kato, T.; Mizorogi, N.; Nagase, S. Selective Extraction and Purification of Endohedral Metallofullerene from Carbon Soot. J. Phys. Chem. B 2006, 110, 22517–22520.
  26. Sato, S.; Maeda, Y.; Guo, J.D.; Yamada, M.; Mizorogi, N.; Nagase, S.; Akasaka, T. Mechanistic Study of the Diels–Alder Reaction of Paramagnetic Endohedral Metallofullerene: Reaction of La@C82 with 1,2,3,4,5-Pentamethylcyclopentadiene. J. Am. Chem. Soc. 2013, 135, 5582–5587.
  27. Bickelhaupt, F.M.; Solà, M.; Fernández, I. Understanding the Reactivity of Endohedral Metallofullerenes: C78 versus Sc3N@C78. Chem. A Eur. J. 2015, 21, 5760–5768.
  28. Yutaka, M.; Akasaka, T.; Nagase, S. Advances in Carbon Nanomaterials; Jenny Stanford Publishing: Sigapore, 2012; Volume 269.
  29. Suzuki, T.; Maruyama, Y.; Kato, T.; Kikuchi, K.; Achiba, Y. Electrochemical properties of La@C82. J. Am. Chem. Soc. 1993, 115, 11006–11007.
  30. Kikuchi, K.; Suzuki, S.; Nakao, Y.; Nakahara, N.; Wakabayashi, T.; Shiromaru, H.; Saito, K.; Ikemoto, I.; Achiba, Y. Isolation and characterization of the metallofullerene LaC82. Chem. Phys. Lett. 1993, 216, 67–71.
  31. Takata, M.; Umeda, B.; Nishibori, E.; Sakata, M.; Saitot, Y.; Ohno, M.; Shinohara, H. Confirmation by X-ray diffraction of the endohedral nature of the metallofullerene Y@C82. Nature 1995, 377, 46–49.
  32. Yannoni, C.S.; Hoinkis, M.; de Vries, M.S.; Bethune, D.S.; Salem, J.R.; Crowder, M.S.; Johnson, R.D. Scandium Clusters in Fullerene Cages. Science 1992, 256, 1191–1192.
  33. Johnson, R.D.; de Vries, M.S.; Salem, J.; Bethune, D.S.; Yannoni, C.S. Electron paramagnetic resonance studies of lanthanum-containing C82. Nature 1992, 355, 239–240.
  34. Kirbach, U.; Dunsch, L. The Existence of Stable Tm@C82 Isomers. Angew. Chem. Int. Ed. Engl. 1996, 35, 2380–2383.
  35. Shinohara, H.; Sato, H.; Ohkohchi, M.; Ando, Y.; Kodama, T.; Shida, T.; Kato, T.; Saito, Y. Encapsulation of a scandium trimer in C82. Nature 1992, 357, 52–54.
  36. Campbell, E.E.; Fanti, M.; Hertel, I.V.; Mitzner, R.; Zerbetto, F. The hyperpolarisability of an endohedral fullerene: Li@C60. Chem. Phys. Lett. 1998, 288, 131–137.
  37. Deng, Q.; Junghans, K.; Popov, A.A. Carbide clusterfullerenes with odd number of carbon atoms: Molecular and electronic structures of Sc4C@C80, Sc4C@C82, and Sc4C3@C80. Theor. Chem. Accounts 2015, 134, 10.
  38. Dunk, P.W.; Mulet-Gas, M.; Nakanishi, Y.; Kaiser, N.K.; Rodríguez-Fortea, A.; Shinohara, H.; Poblet, J.M.; Marshall, A.G.; Kroto, H.W. Bottom-up formation of endohedral mono-metallofullerenes is directed by charge transfer. Nat. Commun. 2014, 5, 5844.
  39. Ito, Y.; Okazaki, T.; Okubo, S.; Akachi, M.; Ohno, Y.; Mizutani, T.; Shinohara, H. Enhanced 1520 nm Photoluminescence from Er3+ Ions in Di-erbium-carbide Metallofullerenes (Er2C2)@C82 (Isomers I, II, and III). ACS Nano 2007, 1, 456–462.
  40. Wang, J.; Zhao, Y.; Lee, P.-H.; Irle, S. Er3+ Photoluminescence in Er2@C82 and Er2C2@C82 Metallofullerenes Elucidated by Density Functional Theory. Inorg. Chem. 2017, 56, 6576–6583.
  41. Plant, S.R.; Dantelle, G.; Ito, Y.; Ng, T.C.; Ardavan, A.; Shinohara, H.; Porfyrakis, K. Acuminated fluorescence of Er3+ centres in endohedral fullerenes through the incarceration of a carbide cluster. Chem. Phys. Lett. 2009, 476, 41–45.
  42. Li, M.-Y.; Zhao, Y.-X.; Han, Y.-B.; Yuan, K.; Nagase, S.; Ehara, M.; Zhao, X. Theoretical Investigation of the Key Roles in Fullerene-Formation Mechanisms: Enantiomer and Enthalpy. ACS Appl. Nano Mater. 2020, 3, 547–554.
  43. Yang, S.; Zalibera, M.; Rapta, P.; Dunsch, L. Charge-Induced Reversible Rearrangement of Endohedral Fullerenes: Electrochemistry of Tridysprosium Nitride Clusterfullerenes Dy3N@C2n (2n =78, 80). Chem. A Eur. J. 2006, 12, 7848–7855.
  44. Popov, A.A.; Avdoshenko, S.M.; Cuniberti, G.; Dunsch, L. Dimerization of Radical-Anions: Nitride Clusterfullerenes versus Empty Fullerenes. J. Phys. Chem. Lett. 2011, 2, 1592–1600.
  45. Zhang, J.; Bowles, F.L.; Bearden, D.W.; Ray, W.K.; Fuhrer, T.; Ye, Y.; Dixon, C.; Harich, K.; Helm, R.F.; Olmstead, M.M.; et al. A missing link in the transformation from asymmetric to symmetric metallofullerene cages implies a top-down fullerene formation mechanism. Nat. Chem. 2013, 5, 880–885.
  46. Crichton, R.; Zhang, J. Handbook of Fullerene Science and Technology; Springer Nature: Singapore, 2022.
  47. Mulet-Gas, M.; Abella, L.; Dunk, P.W.; Rodríguez-Fortea, A.; Kroto, H.W.; Poblet, J.M. Small endohedral metallofullerenes: Exploration of the structure and growth mechanism in the Ti@C2n (2n = 26–50) family. Chem. Sci. 2014, 6, 675–686.
  48. Yao, Y.R.; Chen, Z.C.; Chen, L.; Zheng, S.Y.; Yang, S.; Deng, S.L.; Zheng, L.S. Two Metastable Endohedral Metallofullerenes Sc2C2@C1(39656)-C82 and Sc2C2@C1(51383)-C84:Direct-C2-Insertion Products from Their Most Stable Precursors. J. Am. Chem. Soc. 2023, 145, 16778–16786.
  49. Yamada, M.; Akasaka, T.; Nagase, S. Endohedral Metal Atoms in Pristine and Functionalized Fullerene Cages. Acc. Chem. Res. 2009, 43, 92–102.
  50. Takata, M. The MEM/Rietveld method with nano-applications—Accurate charge-density studies of nano-structured materials by synchrotron-radiation powder diffraction. Acta Crystallogr. A 2008, 64, 232–245.
  51. Shinohara, H.; Inakuma, M.; Hayashi, N.; Sato, H.; Saito, Y.; Kato, T.; Bandow, S. Spectroscopic Properties of Isolated Sc3@C82 Metallofullerene. J. Phys. Chem. 1994, 98, 8597–8599.
  52. Senapati, L.; Schrier, J.; Whaley, K.B. Reply to Comment on “Electronic Transport, Structure, and Energetics of Endohedral Gd@C82 Metallofullerenes”. Nano Lett. 2005, 5, 2341.
  53. Kobayashi, K.; Nagase, S. Theoretical study of structures and dynamic properties of Sc3@C82. Chem. Phys. Lett. 1999, 313, 45–51.
  54. Yamada, M.; Wakahara, T.; Lian, Y.; Tsuchiya, T.; Akasaka, T.; Waelchli, M.; Mizorogi, N.; Nagase, S.; Kadish, K.M. Analysis of Lanthanide-Induced NMR Shifts of the Ce@C82 Anion. J. Am. Chem. Soc. 2006, 128, 1400–1401.
  55. Guo, Y.-J.; Zheng, H.; Yang, T.; Nagase, S.; Zhao, X. Theoretical Insight into the Ambiguous Endohedral Metallofullerene Er3C74: Covalent Interactions among Three Lanthanide Atoms. Inorg. Chem. 2015, 54, 8066–8076.
  56. Lu, X.; Akasaka, T.; Nagase, S. Carbide Cluster Metallofullerenes: Structure, Properties, and Possible Origin. Accounts Chem. Res. 2013, 46, 1627–1635.
  57. Popov, A.A.; Zhang, L.; Dunsch, L. A Pseudoatom in a Cage: Trimetallofullerene Y3@C80 Mimics Y3N@C80 with Nitrogen Substituted by a Pseudoatom. ACS Nano 2010, 4, 795–802.
  58. Zhao, P.; Zhao, X.; Ehara, M. Theoretical Insight into Sc2C76: Carbide Clusterfullerene Sc2C2@C74 versus Dimetallofullerene Sc2@C76. Inorg. Chem. 2017, 56, 10195–10203.
  59. Jin, P.; Hao, C.; Li, S.; Mi, W.; Sun, Z.; Zhang, J.; Hou, Q. Theoretical Study on the Motion of a La Atom Inside a C82 Cage. J. Phys. Chem. A 2006, 111, 167–169.
  60. Liu, F.; Velkos, G.; Krylov, D.S.; Spree, L.; Zalibera, M.; Ray, R.; Samoylova, N.A.; Chen, C.-H.; Rosenkranz, M.; Schiemenz, S.; et al. Air-stable redox-active nanomagnets with lanthanide spins radical-bridged by a metal–metal bond. Nat. Commun. 2019, 10, 571.
  61. Hao, Y.; Wang, Y.; Dubrovin, V.; Avdoshenko, S.M.; Popov, A.A.; Liu, F. Caught in Phase Transition: Snapshot of the Metallofullerene Sc3N@C70 Rotation in the Crystal. J. Am. Chem. Soc. 2021, 143, 612–616.
  62. Speller, E.M. The significance of fullerene electron acceptors in organic solar cell photo-oxidation. Mater. Sci. Technol. 2017, 33, 924–933.
  63. Ganesamoorthy, R.; Sathiyan, G.; Sakthivel, P. Review: Fullerene based acceptors for efficient bulk heterojunction organic solar cell applications. Sol. Energy Mater. Sol. Cells 2017, 161, 102–148.
  64. Ross, R.B.; Cardona, C.M.; Guldi, D.M.; Sankaranarayanan, S.G.; Reese, M.O.; Kopidakis, N.; Peet, J.; Walker, B.; Bazan, G.C.; Van Keuren, E.; et al. Endohedral fullerenes for organic photovoltaic devices. Nat. Mater. 2009, 8, 208–212.
  65. Li, T.; Xiao, L.; Yang, J.; Ding, M.; Zhou, Z.; LaConte, L.; Jin, L.; Dorn, H.C.; Li, X. Trimetallic Nitride Endohedral Fullerenes Carboxyl-Gd3N@C80: A New Theranostic Agent for Combating Oxidative Stress and Resolving Inflammation. ACS Appl. Mater. Interfaces 2017, 9, 17681–17687.
  66. Lu, J.; Sabirianov, R.F.; Mei, W.N.; Gao, Y.; Duan, C.G.; Zeng, X. Structural and Magnetic Properties of Gd3N@C80. J. Phys. Chem. B 2006, 110, 23637–23640.
  67. Zhang, J.; Ye, Y.; Chen, Y.; Pregot, C.; Li, T.; Balasubramaniam, S.; Dorn, H.C. Gd3N@C84(OH)x: A New Egg-Shaped Metallofullerene Magnetic Resonance Imaging Contrast Agent. J. Am. Chem. Soc. 2014, 136, 2630–2636.
  68. Weaver, J.; Chai, Y.; Kroll, G.; Jin, C.; Ohno, T.; Haufler, R.; Guo, T.; Alford, J.; Conceicao, J.; Chibante, L.; et al. XPS probes of carbon-caged metals. Chem. Phys. Lett. 1992, 190, 460–464.
  69. Murphy, T.A.; Pawlik, T.; Weidinger, A.; Höhne, M.; Alcala, R.; Spaeth, J.M. Observation of Atomlike Nitrogen in Nitrogen-Implanted Solid C60. Phys. Rev. Lett. 1996, 77, 1075–1078.
  70. Funasaka, H.; Yamamoto, K.; Sakurai, K.; Ishiguro, T.; Sugiyama, K.; Takahashi, T.; Kishimoto, Y. Preparation of Fullerene Derivatives by Resistive Heating with Graphite Crucible. Full- Sci. Technol. 1993, 1, 437–448.
  71. Sinitsa, A.S.; Chamberlain, T.W.; Zoberbier, T.; Lebedeva, I.V.; Popov, A.M.; Knizhnik, A.A.; McSweeney, R.L.; Biskupek, J.; Kaiser, U.; Khlobystov, A.N. Formation of Nickel Clusters Wrapped in Carbon Cages: Toward New Endohedral Metallofullerene Synthesis. Nano Lett. 2017, 17, 1082–1089.
  72. Popov, A.A. Synthesis and Molecular Structures of Endohedral Fullerenes. In Nanostructure Science and Technology; Lockwood, D., Ed.; Springer: Berlin/Heidelberg, Germany, 2017; pp. 1–34.
  73. Bloodworth, S.; Whitby, R.J. Synthesis of endohedral fullerenes by molecular surgery. Commun. Chem. 2022, 5, 121.
  74. Churilov, G.; Popov, A.; Vnukova, N.; Dudnik, A.; Samoylova, N.; Glushenko, G. Controlled synthesis of fullerenes and endohedral metallofullerenes in high frequency arc discharge. Full-Nanotub. Carbon Nanostructures 2016, 24, 675–678.
  75. Shinohara, H. Endohedral metallofullerenes. Rep. Prog. Phys. 2000, 63, 843–892.
  76. Chen, N.; Ortiz, A.L.; Echegoyen, L. Fullerenes: Principles and Applications, 2nd ed; The Royal Society of Chemistry: London, UK, 2011; Chapter 2; pp. 12–65.
  77. Yang, S. Synthesis, Separation, and Molecular Structures of Endohedral Fullerenes. Curr. Org. Chem. 2012, 16, 1079–1094.
  78. Lu, X.; Feng, L.; Akasaka, T.; Nagase, S. Current status and future developments of endohedral metallofullerenes. Chem. Soc. Rev. 2012, 41, 7723–7760.
  79. Lu, X.; Bao, L.; Akasaka, T.; Nagase, S. Recent progress in the chemistry of endohedral metallofullerenes. Chem. Commun. 2014, 50, 14701–14715.
  80. Jin, P.; Tang, C.; Chen, Z. Carbon atoms trapped in cages: Metal carbide clusterfullerenes. Coord. Chem. Rev. 2014, 270–271, 89–111.
  81. Shinohara, H.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Ohkohchi, M.; Ando, Y.; Saito, Y. Isolation and spectroscopic properties of scandium fullerenes (Sc2@C74, Sc2@C82, and Sc2@C84). J. Phys. Chem. 1993, 97, 4259–4261.
  82. Liu, B.-B.; Xu, W.-G.; Liu, Z.-Y.; Yang, H.-B.; Li, M.-H.; Liu, S.-Y.; Zou, G.-T. High yield synthesis and extraction of La@C2n. Solid State Commun. 1996, 97, 407–410.
  83. Bandow, S.; Shinohara, H.; Saito, Y.; Ohkohchi, M.; Ando, Y. High yield synthesis of lanthanofullerenes via lanthanum carbide. J. Phys. Chem. 1993, 97, 6101–6103.
  84. Lu, W.; Baek, J.B.; Dai, L. (Eds.) Carbon Nanomaterials for Advanced Energy Systems; Wiley: Hoboken, NJ, USA, 2015.
  85. Kyesmen, P.I.; Onoja, A.; Amah, A.N. Fullerenes synthesis by combined resistive heating and arc discharge techniques. SpringerPlus 2016, 5, 1323.
More
ScholarVision Creations