C60 Cage Research Articles

Tunneling Motion and Antiferroelectric Ordering of Lithium Cations Trapped inside Carbon Cages

Dielectric and X-ray diffraction measurements of [Li@C60](PF6) single crystals reveal the motion of the Li+ cations inside the C60 cages at low temperature. An increase in the dielectric permittivity below 100 K is consistent with a combined tunneling and hopping motion of the Li+ cation between two positions inside the C60 cage. A phase transition accompanied by a decrease in the dielectric permittivity at TC = 24 K is explained by an antiferroelectric ordering of the Li+ cations. The Li+ ordering is caused by interactions among electric dipole moments formed between the Li+ cations inside and the PF6− anions outside the C60 cages. The electric dipole moments that are switched by the Li+ tunneling and interact with each other are potential qubits in a quantum computer using electric dipole moments.

Graphene nanobuds: Synthesis and selective organic derivatisation

Herein, the formation and selective organic derivatisation of graphene nanobud, an all carbon nanohybrid comprised by C60 and graphene, are presented. C60 cages are directly attached to the graphenic surface with stable covalent bonds. Importantly, prepared graphene nanobud shows remarkable dispersibility, decreased number of defects and highly aromatic character that are associated to improved electrical conductivity compared to pristine graphene. The high chemical reactivity of the curved sp2 carbon atoms of C60 led to a selective covalent attachment of organic groups onto C60 avoiding damage of the graphene aromatic system.

Fullerene‐Based Macro‐Heterocycle Prepared through Selective Incorporation of Three N and Two O Atoms into C60

AbstractA 14‐membered heterocycle is created on the C60 cage skeleton through a multistep procedure. Key steps involve repeated PCl5‐induced hydroxylamino N−O bond cleavage leading to insertion of nitrogen atoms, and also piperidine‐induced peroxo O−O bond cleavage leading to insertion of oxygen atoms. The hetero atoms form one pyrrole, two pyran, and one diazepine rings in conjunction with the C60 skeleton carbon atoms. The fullerene‐based macrocycle showed unique reactivities towards fluoride ion and copper salts.

Fullerene-Based Macro-Heterocycle Prepared through Selective Incorporation of Three N and Two O Atoms into C60.

A 14-membered heterocycle is created on the C60 cage skeleton through a multistep procedure. Key steps involve repeated PCl5 -induced hydroxylamino N-O bond cleavage leading to insertion of nitrogen atoms, and also piperidine-induced peroxo O-O bond cleavage leading to insertion of oxygen atoms. The hetero atoms form one pyrrole, two pyran, and one diazepine rings in conjunction with the C60 skeleton carbon atoms. The fullerene-based macrocycle showed unique reactivities towards fluoride ion and copper salts.

Electronic property and charge carrier mobility of one-dimensional nanowires consisting of C20 cages

This work presented theoretical studies on the one-dimensional (1D) nanowires constructed from fullerene C20 cages based on first-principle calculations. The relative energies, electronic, charge transport, and mechanical properties of the 1D nanowires were investigated systemically and in detail. It is found that formations of the C20 nanowires built from isolated cages were all energetically favorable. They also exhibit high kinetic stability according to molecular dynamics simulations. Although they were all constructed with C20 cages as building blocks, NW-2–NW-6, and NW-9 are semiconductors, whereas NW-1, NW-7, and NW-8 exhibit metallic property. Thus the metallic/semiconducting properties of the 1D C20 nanowires can be mainly determined by the connecting patterns. High charge mobility was revealed for the 1D C20 nanowires based on the deformation potential theory and effective mass approach. Further understanding of the charge mobility is achieved with the aid of crystal orbital analyses. Moreover, the mechanical property of the 1D C20 nanowires was also studied based on the results of Young’s modulus.

The inner-induced effects of YCN in C76 on the structures and nonlinear optical properties

Very recently, an unprecedented novel monometallic cluster of fullerenes entrapping a yttrium cyanide (YCN) cluster inside a popular C82 cage YCN@Cs(6)-C82 was synthesized and characterized. Inspired by this investigation, four non-IPR YCN@C1(17459)-C76, YCN@C2v(19138)-C76, YCN@C2(17646)-C76, and YCN@C1(17894)-C76 (1, 2, 3, and 4) containing a pair of adjacent pentagons are designed to explore the encapsulated molecular effect on their interaction energies and nonlinear optical properties. The interaction energy (E int) values of 1, 2, 3, and 4 are -481.35 (1), -477.91 (2), -482.04 (3), -482.69 (4) kcal mol(-1), respectively, which shows that the E int value of 4 is the largest. Furthermore, the electron-transfer is mainly from the YCN to C76 cage. When YCN is encapsulated into C76 cage, we can find that the α0 values of the four molecules are very close, ranging from 6.50 × 10(2) to 6.65 × 10(2) au. Significantly, the first hyperpolarizabilities are in relation to the encapsulated molecular: 1.63 × 10(3) (1) > 8.03 × 10(2) (2) > 7.76 × 10(2) (4) > 4.86 × 10(2) au (3), the results show that the βtot value of 1 is the largest. Besides this, the encapsulation of the YCN to C76 cage brings some distinctive changes in its UV-vis spectra along with its other electronic properties that might be used by the experimentalists to develop the potential nonlinear optical nanomaterials based on endohedral metallofullerenes.

Isolation and Crystallographic Characterization of the Labile Isomer of Y@C82 Cocrystallized with Ni(OEP): Unprecedented Dimerization of Pristine Metallofullerenes

AbstractAlthough the major isomers of M@C82 (namely M@C2v(9)‐C82, where M is a trivalent rare‐earth metal) have been intensively investigated, the lability of the minor isomers has meant that they have been little studied. Herein, the first isolation and crystallographic characterization of the minor Y@C82 isomer, unambiguously assigned as Y@Cs(6)‐C82 by cocrystallization with Ni(octaethylporphyrin), is reported. Unexpectedly, a regioselective dimerization is observed in the crystalline state of Y@Cs(6)‐C82. In sharp contrast, no dimerization occurs for the major isomer Y@C2v(9)‐C82 under the same conditions, indicating a cage‐symmetry‐induced dimerization process. Further experimental and theoretical results disclose that the regioselective dimer formation is a consequence of the localization of high spin density on a special cage‐carbon atom of Y@Cs(6)‐C82 which is caused by the steady displacement of the Y atom inside the Cs(6)‐C82 cage.

Isolation and Crystallographic Characterization of the Labile Isomer of Y@C82 Cocrystallized with Ni(OEP): Unprecedented Dimerization of Pristine Metallofullerenes.

Although the major isomers of M@C82 (namely M@C2v (9)-C82 , where M is a trivalent rare-earth metal) have been intensively investigated, the lability of the minor isomers has meant that they have been little studied. Herein, the first isolation and crystallographic characterization of the minor Y@C82 isomer, unambiguously assigned as Y@Cs (6)-C82 by cocrystallization with Ni(octaethylporphyrin), is reported. Unexpectedly, a regioselective dimerization is observed in the crystalline state of Y@Cs (6)-C82 . In sharp contrast, no dimerization occurs for the major isomer Y@C2v (9)-C82 under the same conditions, indicating a cage-symmetry-induced dimerization process. Further experimental and theoretical results disclose that the regioselective dimer formation is a consequence of the localization of high spin density on a special cage-carbon atom of Y@Cs (6)-C82 which is caused by the steady displacement of the Y atom inside the Cs (6)-C82 cage.

Theoretical survey on M@C80 (M = Ca, Sr, and Ba): Behavior of different alkaline earth metal impacting the chemical stability and electronic properties

Structures of mono-metallofullerenes M@C80 (M=Ca, Sr, and Ba) that separated in early experiment are determined owning the C2v(31920)-C80 cage. The change rule of properties for M@C80 (M=Ca, Sr, and Ba) influenced by different inner metal are discussed. As the trapped metal changes from calcium to barium, performance of thermodynamic stabilities for M@C2v(31920)-C80, M@C2v(31922)-C80, and M@D5h(31923)-C80 are significantly different. Orbital analysis suggests that the lowest unoccupied molecular orbitals (LUMOs) of Ca@C2v(31920)-C80 and Ca@D5h(31923)-C80 are mostly located on the trapped metal, whereas reduction reactions of Ca@C2v(31920)-C80 and Ca@D5h(31923)-C80 occur on the fullerene cage. Natural electron configuration analyses demonstrates that the decentralized electron back-donation of Ba@C2v(31920)-C80 would take responsible for the instability of itself. Electronic properties such as electron affinities and ionization potentials are significantly affected by encapsulated metal are also found. Computational UV–visible–NIR spectra for M@C2v(31920)-C80 (M=Ca, Sr, Ba) are in perfect accord with the spectra obtained experimentally.

A comparative study of the radial distribution of hydrogen on C20, C19Si, and C19B cage fullerenes: a Monte Carlo simulation

The radial distribution of hydrogen on C20(cage) and C19Si(cage), and C19B(cage) fullerene structures is investigated at different temperatures (273 K, 293 K, 320 K, and 400 K) for the pressure range between 1 MPa and 30 MPa using the (N,V,T) Monte Carlo simulation. The gravimetric storage capacity and radial distribution function parameters show that, under the identical temperature and pressure conditions, the magnitude of the hydrogen radial distribution on the C19B surface is larger than that on C19Si and C20. The calculated maximum of the gravimetric storage capacity for C19B at 273 K and 30 MPa is 7.6%.

Fullerene-Assisted Photoinduced Charge Transfer of Single-Walled Carbon Nanotubes through a Flavin Helix.

One of the greatest challenges with single-walled carbon nanotube (SWNT) photovoltaics and nanostructured devices is maintaining the nanotubes in their pristine state (i.e., devoid of aggregation and inhomogeneous doping) so that their unique spectroscopic and transport characteristics are preserved. To this effect, we report on the synthesis and self-assembly of a C60-functionalized flavin (FC60), composed of PCBM and isoalloxazine moieties attached on either ends of a linear, C-12 aliphatic spacer. Small amounts of FC60 (up to 3 molar %) were shown to coassembly with an organic soluble derivative of flavin (FC12) around SWNTs and impart effective dispersion and individualization. A key annealing step was necessary to perfect the isoalloxazine helix and expel the C60 moiety away from the nanotubes. Steady-state and transient absorption spectroscopy illustrate that 1% or higher incorporation of FC60 allows for an effective photoinduced charge transfer quenching of the encased SWNTs through the seamless helical encase. This is enabled via the direct π-π overlap between the graphene sidewalls, isoalloxazine helix, and the C60 cage that facilitates SWNT exciton dissociation and electron transfer to the PCBM moiety. Atomistic molecular simulations indicate that the stability of the complex originates from enhanced van der Waals interactions of the flexible spacer wrapped around the fullerene that brings the C60 in π-π overlap with the isoalloxazine helix. The remarkable spectral purity (in terms of narrow E(S)ii line widths) for the resulting ground-state complex signals a new class of highly organized supramolecular nanotube architecture with profound importance for advanced nanostructured devices.

(Invited) Photoinduced Charge Transfer Reactions and Excited State Properties in Triphenylamine C60 Donor-Acceptor Conjugates

The molecular heterojunction triphenylamine/C60 is a potential material to construct high-efficient dye-sensitized solar cells. We have carried out a combined molecular dynamics and time-dependent density functional theory study to analyze charge separation and exciton delocalization in excited states of two constitutional isomers of this interface.[1] Our results reveal the presence of exciton states characterized by partial charge-transfer character (hybrid states); which are suggested to promote the direct charge separation process by excitation. These states may have a significant impact on the efficiency of the light-harvesting ensembles. That is to say, a greater amount of such hybrid states is found at short distances between the triphenylamine fragment and the C60 cage; as a result, charge transfer reactions can occur faster. This trend resembles the experimental evidence since rate constants for photoinduced charge transfer reactions kct are bigger for the constitutional isomer with the shortest distance between the triphenylamine fragment and the C60 cage.[2] We have also evaluated the performance of U-CAM-B3LYP/6-31G* and TD-CAM-B3LYP/6-31G* level of theories to compute the different parameters affecting kct ; that is the reorganization energy λ, the electronic coupling Vij , and free energies ΔG.[3] Our study is done under the theory of nonadiabatic electron transfer. By using the standard Marcus formulation, our results show that the outer-sphere contribution to λ and ΔG for the recombination reaction are the parameters that more uncertainty introduces to the estimation of kct . We implemented a low-cost methodology based on U-DFT for the evaluation of the outer-sphere part of λ and ΔG; wherein Franck-Condon and relaxed charge-transfer states are conveniently modeled, including properly the solvent reorganization energy, by U-CAM-B3LYP/6-31G* coupled to continuum solvation models. Finally, it is worth to mention that the replacement of the C60 cage by Ih -Sc3N@C80 generates longer-lived charge-transfer states; which is attributed to the lower ΔG of the charge recombination reaction.[2] Our group is currently analyzing this trend for the whole family of endohedral metallofullerenes; for which dynamics is expected to be in the normal region of the Marcus parabola. [1] J. P. Martínez, S. Osuna, M. Solà, A. Voityuk, Theor. Chem. Acc. 2015, 134, 12. [2] J. R. Pinzón, D. C. Gasca, S. G. Sankaranarayanan, G. Bottari, T. Torres, D. M. Guldi, L. Echegoyen, J. Am. Chem. Soc. 2009, 131, 7727–7734. [3] J. P. Martínez, M. Solà, A. Voityuk, 2015, in preparation. Figure 1

(Invited) Regioselective Bis-Prato Addition on TNT-EMF (M3N@Ih -C80, M = Y, Gd) Controlled By Endohedral Metal Cluster

Aiming the development of stable and controlled way of the chemical functionalization of fullerenes, there have been many reactions so far reported. The Prato reaction1 (1,3-dipolar cycloaddition reaction) has been used by many groups as one of the efficient chemical conversion methods in the preparation of functional fullerene derivatives. This method usually provides sufficiently stable fullerene derivatives in reasonable synthetic yields. Addressing the preparation of biocompatible fullerene derivatives, we are using this Prato reaction for the controlled functionalization of C60, C70 and TNT-EMF2 (M3N@Ih -C80 or M3N@C80). C60 usually produces the mono-Prato adduct selectively on the [6,6]-junction. However, as reported by the groups of Dorn3 and Echegoyen,4 M3N@C80 provides the mono-Prato adduct on both [6,6]- and [5,6]-junctions as a results of [6,6]-to-[5,6] isomerization via a [1,5]-sigmatropic rearrangement. The ratio of the thermodynamic regioisomers ([6,6]-mono and [5,6]-mono) was dependent on the size of metal cluster (M3N) inside C80 5 and was not affected by the exohedral functional groups outside C80 cage. 6 Although the bis-Prato addition of C60 proceeded uncontrolled manner, interestingly, bis-Prato addition of M3N@C80 proceeded in a regioselective manner especially in the presence of larger size of endohedoral metal clusters such as Y3N and Gd3N.7 Thermal treatment of bis-Prato adduct of Y3N@C80 caused random isomerization, but the bis-Prato adduct of Gd3N@C80 stayed rather stable presumably by the effect of larger endohedral metal cluster. (1) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115, 9798. (2) Stevenson, S.; Rice, G.; Glass, T.; Harich, K.; Cromer, F.; Jordan, M. R.; Craft, J.; Hadju, E.; Bible, R.; Olmstead, M. M.; Maitra, K.; Fisher, A. J.; Balch, A. L.; Dorn, H. C. Nature 401, 898. (3) Cai, T.; Slebodnick, C.; Xu, S.; Harich, L.; Glass, T. E.; Chancellor, C.l Fettinger, J. C.; Olmstead, M. M.; Balch, A. L.; Gibson, H. W.; Dorn, H. C. J. Am. Chem. Soc. 2006, 128, 6486. (4) Cardona, C. M.; Eliott, B.; Echegoyen, L. J. Am. Chem. Soc. 2006, 128, 6480. (5) Aroua, S.; Yamakoshi, Y. J. Am. Chem. Soc. 2012, 134, 20242. (6) Aroua, S.; Garcia-Borras, M.; Osuna, S.; Yamakoshi, Y. Chem. Eur. J. 2014, 20, 14032. (7) Aroua, S.; Garcia-Borràs, M.; Bölter M. F.; Osuna, S.; Yamakoshi, Y. J. Am. Chem. Soc. 2015, 137, 58. Figure 1

(Invited) Chemical Purification of M3N Clusters in the Larger C88-C102 Cage

Since the discovery of Sc3N@C80 in 1999, much literature has focused on isolating metallic nitride clusters (M3N) entrapped within the smaller C80 cage. Over time, scientists (e.g., Echegoyen) have explored the same M3N clusters, but with bigger cage sizes, to expand the family of M3N@C2n. While HPLC has been the dominant method to isolate the larger M3N@C88-112, our research group is investigating non-chromatographic methods to purify M3N@C88-112 with the goal of disseminating these larger species to the scientific community. In this presentation, we demonstrate selective reactions of metallic nitride endohedrals present in soot extracts. Successful reagents include electron-poor Lewis acids and electron-rich amines. Combining these two reaction classes now permits the chemical purification of mixed-metal CeLu2N@C80 as an example of a C80 cage. Our most recent example of chemically isolating a metallic nitride in a larger cage is Gd3N@C88. Currently, we are spreading this purification method to transition metal and other rare-earth M3N clusterfullerenes.

Fullerene-Assisted Quenching of Single-Walled Carbon Nanotubes Photoluminescence through a Flavin-Helix

Maintaining single-walled carbon nanotubes in their pristine state (i.e. devoid of aggregation and inhomogeneous doping) within nanostructured photovoltaic devices is a major challenge. In this contribution, we report on the synthesis and self-assembly of a C60-functionalized flavin (FC60), composed of PCBM and isoalloxazine moieties attached on either ends of a linear aliphatic spacer. Small amounts of FC60 (up to 3 molar %) were shown to co-assembly with an organic soluble derivative of flavin (FC12) around SWNTs and impart effective dispersion and individualization. Steady state and transient absorption spectroscopy illustrate that 1% or higher incorporation of FC60 allows for an effective photo-induced charge transfer quenching of the encased SWNTs through the seamless helical encase. This is enabled via the direct π-π overlap between the graphene sidewalls, isoalloxazine helix and the C60 cage, that facilitates SWNT exciton dissociation and electron transfer to the PCBM moiety. Atomistic molecular simulations elucidate how the fullerene is brought in the vicinity of the isoalloxazine helix to quench the nanotube. The narrow ES ii linewidth for the resulting ground state complex indicates a new class of highly organized supramolecular nanotube architecture with profound importance for advanced nanostructured devices.

(Invited) Near Infrared Photoluminescence of Endohedral Metallofullerenes

Physical properties of endohedral metallofullerenes can be tuned by changing the fullerene size as well as the metal atom(s) inside the fullerene cage. One of the most interesting characteristic properties of the metallofullerenes encapsulating lanthanide metal atoms is the near-infrared photoluminescence (NIR-PL) due to the f-f transition of M3+ (or M2+) in fullerene cages. Previously, we reported that (Er2C2)@C82 (III) has exhibited the strongest PL in isomer-separated Er2@C82 (I, II, III) and (Er2C2)@C82 (I, II, III) in CS2 solution [1]. The encapsulated C2 molecule enhances the PL intensity by increasing the efficiency of the energy transfer from the C82 cage to the encapsulated Er3+. In this talk, we will report the NIR-PL from Tm atom(s) in Tm metallofullerenes for the first time. The PL spectra and their intensities strongly depend on the electronic states of Tm ions and the fullerene cages. The detailed mechanisms will be discussed. [1] Y. Ito, T. Okazaki, S. Okubo, M. Akachi, Y. Ohno, T. Mizutani, T. Nakamura, R. Kitaura, T. Sugai and H. Shinohara, ACS Nano, 1, 456-462 (2007).

(Invited) Entrapping a Group-VB Transition Metal, Vanadium, within an Endohedral Metallofullerene

So far the entrapped metals for the isolated endohedral metallofullerenes (EMFs) are primarily limited to rare earth metals, whereas except group-IVB metals whether or not it is possible to entrap other d-block transition metals remains unclear. In this talk, we report the successful entrapment of the group-VB transition metal vanadium (V) into fullerene cage, affording the heretofore unknown V-containing EMFs. Two novel V-containing EMFs — VxSc3-xN@C80 (x=1, 2) — were isolated, and their molecular structures were unambiguously determined by X-ray crystallography to be Ih (7)-C80 cage entrapping the planar VSc2N/V2ScN clusters. VxSc3-xN@Ih (7)-C80 (x=1, 2) were further characterized by UV-vis-NIR and ESR spectroscopies and electrochemistry, revealing that the electronic and magnetic properties of VxSc3-xN@Ih (7)-C80 (x=1, 2) are tunable upon varying the number of entrapped V atoms (i.e., x value). The molecular structures and electronic properties of VxSc3-xN@Ih (7)-C80 (x=1, 2) were further compared with those of the reported analogous EMFs based on lanthanide metals and the adjacent group-IVB transition metal Ti, revealing the peculiarity of the group-VB transition metal V-based EMFs.

Targeting Type-I Eicosacationic [70]Fullerene-(Light Harvesting Antenna) PDI Agents for Efficient Photokilling of Multiantibiotics-Resistant Bacteria

We are particularly interested in mediating broad-spectrum killing of pathogenic Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacteria targets. Systematic evaluation was taken to evaluate a structure–function relationship of fullerene-(light harvesting antenna) conjugative diads and triads on the influence of their biological activity, aiming to achieve the optimized photodynamic inactivation (PDI) efficiency against multiantibiotics-resistant bacteria. Accordingly, we found a rational design of effective fullerene-based nano-photosensitizers by chemical modification of the carbon cage with two cell-targeting moieties each with a well-defined high number of cationic charges that allowed us to demonstrate the feasibility of these compounds giving cytotoxicity toward both Gram-positive and Gram-negative species. We also uncovered the large enhancement of PDI efficiency in photokilling of infectious bacteria by replacing the C60 cage with a C70 leading to novel type-I eicosacationic [70]fullerenes as potential nano-PDI agents.

PET Imaging of Tumor Uptake of a Biocompatible C60 Fullerene Drug Delivery Vector

Traditional chemotherapy in the form of cytotoxic drugs is one of the standards in cancer treatment, but it is hindered by various barriers to drug delivery throughout the cancer microenvironment. Recently, nanoparticles have been of great interest as drug delivery vectors because they can be tailored for specific functions to increase the amount of administered drug in solid tumors. In particular, biocompatible C60 fullerene has potential as a drug delivery scaffold since it has been shown to penetrate very restrictive physiological membranes (e.g. blood-brain-barrier and nuclear membrane)1. In addition, radiofrequency (RF) irradiation can also be combined with C60 fullerene-based drug delivery to increase the permeability of solid tumors to improve material uptake in cancerous tissue2. Our long-term goal is to determine the biodistribution of a biocompatible C60 material, as well as other pharmacokinetic data in both small and large animal models in order to evaluate its potential as a clinical drug vector for anti-cancer drugs. In this work, we have synthesized a C60 derivative with a metal chelating agent covalently bound to the C60 cage (Figure 1). The chelating agent of this material can be easily radiolabeled with 64Cu2+ (β+ emitter; t1/2 = 12 h) and tracked in vivo using positron emission tomography (PET) imaging. Because of its extreme sensitivity and routine clinical use, PET is an ideal imaging technique for studying pharmacokinetic properties and tumor accumulation of a C60 fullerene delivery vector, for both preclinical and possible future clinical studies. The material has been synthesized following an optimized six-step synthesis and the stability of the 64Cu2+ ion in the chelate has been evaluated by challenge with biological media. Animal biodistribution data will be determined using PET imaging in tumor- and non-tumor-bearing murine models and possibly porcine models as well. Our primary focus is on pancreatic cancer, since clinical treatment of this disease is presently ineffective3. Future studies include examining changes in biodistribution and exploring the potential to achieve increased selectivity for cancerous tissue with RF irradiation, as well as expanding to other tumor models. Acknowledgements: We gratefully acknowledge The Welch Foundation (C-0627) and the Kanzius Cancer Research Foundation for support of this work, as well as the Department of Chemistry and the Smalley-Curl Institute at Rice University and the Department of Surgery at Baylor College of Medicine. Additionally, the work is supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. 1450681 (N.G.Z.).

Photoemission study of Li@C60

Ultraviolet and X-ray photoelectron spectra (UPS and XPS) of thin films prepared by either depositing or applying [Li@C60]+(PF6)− on the substrates are presented. The UPS and XPS of [Li@C60]+(PF6)− applied films suggest that PF6− anions come out from the surface by annealing at 250°C. The UPS and XPS of the deposited thin films indicate that the film does not contain PF6− anion but is composed of only Li@C60. Changing the sublimation temperature reveals that encapsulated Li cations begin to escape from the C60 cage when heated above 550°C.