Electrochemical reduction of cationic Li+@C60 to neutral Li+@C60˙-: isolation and characterisation of endohedral [60]fulleride

Synthesis and Characterization of Ionic Li+ @C70 Endohedral Fullerene

Ion-endohedral-fullerene has attracted growing interest due to the unique electronic and structural characteristics arising from its distinctive ionic nature. Although there has been only one reported ion-encapsulated fullerene, Li+ @C60 , a significant number of fundamental and applied studies have been conducted, making a substantial impact not only in chemistry and physics but also across various interdisciplinary research fields. Nevertheless, studies on ion-endohedral fullerenes are still in their infancy due to the limitations in variety, and hence, it remains an open question how the size and symmetry of fullerene, as well as the motion and position of the encapsulated ion, affect their physical/chemical properties. Herein, we report the synthesis of lithium-ion-endohedral [70]fullerene (Li+ @C70 X- , X=PF6 - and TFSI- ), a novel ionic endohedral fullerene. X-ray crystallography confirmed the encapsulation of Li+ by C70 cage as well as its ion-pair structure stabilized by external TFSI- counter anion. The encapsulated Li+ drastically lowered the orbital energy of the C70 cage by Coulomb interactions but did not affect the orbital energy gap and degeneracy. DFT studies were also performed, which supported the experimentally observed electronic effects caused by the encapsulated Li+ .

The Various Packing Structures of Tb@C82 (I, II) Isomers in Their Cocrystals with Ni(OEP)

Soot-containing terbium (Tb)-embedded fullerenes were prepared by evaporation of Tb₄O₇-doped graphite rods in an electric arc discharge chamber. After 1,2,4-trichlorobenzene extraction of the soot and rotary evaporation of the extract, a solid product was obtained and then dissolved into toluene by ultrasonication. Through a three-stage high-pressure liquid chromatographic (HPLC) process, Tb@C₈₂ (I, II) isomers were isolated from the toluene solution of fullerenes and metallofullerenes. With the success of the growth of cocrystals of Tb@C₈₂ (I, II) with Ni(OEP), the molecular structures of Tb@C₈₂ (I) and Tb@C₈₂ (II) were confirmed to be Tb@C_2v(9)-C₈₂ and Tb@Cs(6)-C₈₂, respectively, based on crystallographic data from X-ray single-crystal diffraction. Moreover, it was found that Tb@C₈₂ (I, II) isomers demonstrated different packing behaviors in their cocrystals with Ni(OEP). Tb@C_2v(9)-C₈₂ forms a 1:1 cocrystal with Ni(OEP), in which Tb@C_2v(9)-C₈₂ is aligned diagonally between the Ni(OEP) bilayers to form zigzag chains. In sharp contrast, Tb@Cs(6)-C₈₂ forms a 2:2 cocrystal with Ni(OEP), in which Tb@Cs(6)-C₈₂ forms a centrosymmetric dimer that is aligned linearly with Ni(OEP) pairs to form one-dimensional structures in the a-c lattice plane. In addition, the distance of a Ni atom in Ni(OEP) to the Cs(6)-C₈₂ cage is much shorter than that to the C_2v(9)-C₈₂ one, indicative of a stronger π-π interaction between Ni(OEP) and the C₈₂ carbon cage in the cocrystal of Tb@C_S(6)-C₈₂ and Ni(OEP). Density functional theory calculations reveal that the regionally selective dimerization of Tb@C_S(6)-C₈₂ is the result of a dominant unpaired spin existing on a particular C atom of the C_S(6)-C₈₂ cage.

Synthesis of neutral Li-endohedral PCBM: an n-dopant for fullerene derivatives

Li@PCBM, the first neutral Li@C₆₀ derivative, was synthesized. The Li@PCBM exists in a monomer-dimer equilibrium in solution but as a monomer in the PCBM matrix. The fully dispersed Li@PCBM n-doped the surrounding empty PCBM, raising the Fermi level by 0.13 eV compared with the undoped PCBM film. The hybrid films were utilized as an ETL for PSCs, promoting the efficiency of the device.

Designing stable binary endohedral fullerene lattices

Nanoparticle lattices and endohedral fullerenes have both been identified as potential building blocks for future electronic, magnetic and optical devices; here it is proposed that it could be possible to combine those concepts and design stable nanoparticle lattices composed from binary collections of endohedral fullerenes. The inclusion of an atom, for example Ca or F, within a fullerene cage is known to be accompanied by a redistribution of surface charge, whereby the cage can acquire either a negative (Ca) or positive (F) charge. From calculations involving a combination of van der Waals and many-body electrostatic interactions, it is predicted that certain binary combinations, for example a metal (A) and a halogen (B), could result in the formation of stable nanoparticle lattices with the familiar AB and AB₂ stoichiometries. Much of the stability is due to Coulomb interactions, however, charge-induced and van der Waals interactions, which always enhance stability, are found to extend the range of charge on a cage over which lattices are stable. Some lattice types are shown to be three or four times more stable than an equivalent neutral C₆₀ structure. An extension of the calculations to the fabrication of structures involving endohedral C₈₄ is also discussed.

Heterospin frustration in a metal-fullerene-bonded semiconductive antiferromagnet

Lithium-ion-encapsulated fullerenes (Li⁺@C₆₀) are 3D superatoms with rich oxidative states. Here we show a conductive and magnetically frustrated metal–fullerene-bonded framework {[Cu₄(Li@C₆₀)(L)(py)₄](NTf₂)(hexane)}_n (1) (L = 1,2,4,5-tetrakis(methanesulfonamido)benzene, py = pyridine, NTf₂⁻ = bis(trifluoromethane)sulfonamide anion) prepared from redox-active dinuclear metal complex Cu₂(L)(py)₄ and lithium-ion-encapsulated fullerene salt (Li⁺@C₆₀)(NTf₂⁻). Electron donor Cu₂(L)(py)₂ bonds to acceptor Li⁺@C₆₀ via eight Cu‒C bonds. Cu–C bond formation stems from spontaneous charge transfer (CT) between Cu₂(L)(py)₄ and (Li⁺@C₆₀)(NTf₂⁻) by removing the two-terminal py molecules, yielding triplet ground state [Cu₂(L)(py)₂]⁺(Li⁺@C₆₀^•−), evidenced by absorption and electron paramagnetic resonance (EPR) spectra, magnetic properties and quantum chemical calculations. Moreover, Li⁺@C₆₀^•− radicals (S = ½) and Cu²⁺ ions (S = ½) interact antiferromagnetically in triangular spin lattices in the absence of long-range magnetic ordering to 1.8 K. The low-temperature heat capacity indicated that compound 1 is a potential candidate for an S = ½ quantum spin liquid (QSL).

Conductive and magnetically frustrated solids may enable the development of high-performance molecule-based spintronic devices. Here the authors report a conductive and magnetically frustrated metal–fullerene-bonded framework prepared from a redox-active dinuclear copper complex and lithium ion-encapsulated fullerenes.

An Adamantane Capsule and its Efficient Uptake of Spherical Guests up to 3 nm in Water

Efficient uptake of small to large guests, with a large difference in relative size, is quite rare for synthetic host compounds. Herein we designed and prepared a micellar capsule, composed of bent amphiphiles bearing two adamantyl groups, as a new host with a well-defined nanostructure. Unlike previous covalent, coordination, and hydrogen-bonding hosts, the adamantane-based capsule displays unusual uptake abilities toward spherical molecules with small (∼0.6 nm in diameter; e.g., adamantane) to medium size (∼1 nm; e.g., fullerene) as well as large size (∼3 nm; i.e., metal-organic polyhedra (MOP)), where the size differences are up to 5-fold, in water. Moreover, the resultant MOP-uptaking capsule incorporates medium-sized molecules (e.g., perylene and eosin Y) into the polyhedral cavity to generate ternary core-shell structures.

Endocircular Li Carbon Rings

By employing accurate state-of-the-art many-electron quantum-chemistry methods, we establish that monocyclic carbon rings can accommodate Li guest atoms. The low-lying electronic states of these endocircular systems are analyzed and found to include both charge-separated states where the guest Li atom appears as a cation and the ring as an anion and encircled-electron states where Li and the ring are neutral. The electron binding energies of the encircled-electron states increase drastically at their highly symmetric equilibrium geometries with increasing size of the ring, and in Li@C24 , this state becomes the ground state. Li is very weakly bound vertical to the rings in the low-lying encircled-electron states, hinting to van-der-Waals binding. Applcations are mentioned.

Fully Conjugated Pyrene-BODIPY and Pyrene-BODIPY-Ferrocene Dyads and Triads: Synthesis, Characterization, and Selective Noncovalent Interactions with Nanocarbon Materials

Several pyrene-boron-dipyrromethene (BODIPY) and pyrene-BODIPY-ferrocene derivatives with a fully conjugated pyrene fragment appended to the α-position(s) of the BODIPY core have been prepared by Knoevenagel condensation reaction and characterized by one-dimensional (1D) and two-dimensional (2D) nuclear magnetic resonance (NMR), UV-vis, fluorescence spectroscopy, high-resolution mass spectrometry as well as X-ray crystallography. The redox properties of new donor-acceptor BODIPY dyads and triads were studied by electrochemical (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)) and spectroelectrochemical approaches. Formation of weakly bonded noncovalent complexes between the new pyrene-BODIPYs and nanocarbon materials (C₆₀, C₇₀, single-walled carbon nanotube (SWCNT), and graphene) was studied by UV-vis, steady-state fluorescent, and time-resolved transient absorption spectroscopy. UV-vis and fluorescent spectroscopy are indicative of the much stronger and selective interaction between new dyes and (6,5)-SWCNT as well as graphene compared to that of C₆₀ and C₇₀ fullerenes. In agreement with these data, transient absorption spectroscopy provided no evidence for any significant change in excited-state lifetime or photoinduced charge transfer between pyrene-BODIPYs and C₆₀ or C₇₀ fullerenes when the pyrene-BODIPY chromophores were excited into the lowest-energy singlet excited state. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations suggest that the pyrene fragments are fully conjugated into the π-system of BODIPY core, which correlates well with the experimental data.

Rotation of fullerene molecules in the crystal lattice of fullerene/porphyrin: C60 and Sc3N@C80

The temperature driven rotation of the encapsulated Sc₃N cluster in a C₈₀ fullerene cage was unraveled by variable temperature X-ray diffraction, which is significantly different from its analogues (Ho₂LuN/Lu₃N).

Molecular spinning top: visualizing the dynamics of M3N@C80 with variable temperature single crystal X-ray diffraction

Variable temperature X-ray diffraction studies on two single crystals containing M3N@C80, i.e., Ho2LuN@C80·NiOEP·2(C6H6) and Lu3N@C80·NiOEP·2(C6H6), (NiOEP = Nickel octaethylporphyrin) unravelled the temperature dependent rotation of the M3N cluster and C80 cage on the static NiOEP molecule.

Charge separated states of endohedral fullerene Li@C20

We report on high-level coupled-cluster calculations of electronic states of the neutral endohedral fullerene Li@C₂₀. All computed states of neutral Li@C₂₀ are found to be the charge separated states of the Li⁺@C₂₀ ^- type. Using the state-of-the-art EA-EOM-CCSD method, we found that neutral Li@C₂₀ (D_3d) possesses several valence and superatomic charge separated states with considerable electron binding energies, the strongest bound state of Li⁺@C₂₀ ^- being the 1²E_u state (6.73 eV). The valence charge separated states correspond to two sets of states of C₂₀ ^-. The states 1²E_u, 1²A_2u, 2²E_u, and 2²A_2u correspond to the respective bound states of C₂₀ ^-, and the states 2²A_2g, 1²E_g, 1²A_1g, and 4²E_u correspond to the unbound states of C₂₀ ^-. There are eight superatomic states with electron binding energy higher than 1.0 eV, being much stronger bound than the single weakly bound superatomic state of the parent fullerene anion. The analysis of the radial density distribution of the excess electron on the carbon cage indicates the important role of the inner part of the superatomic states in forming the charge separated states.

Li@C60 endohedral fullerene as a supraatomic dopant for C60 electron-transporting layers promoting the efficiency of perovskite solar cells

C₆₀:Li@C₆₀ hybrid n-type semiconducting films were first fabricated.

Endohedral alkali cations promote charge transfer transitions in complexes of C60 with [10]cycloparaphenylenes

The endohedral alkali cations in M⁺@C₆₀⋯[10]CPP complexes boost the near infrared absorption bands associated with charge transfer from the nanoring to the fullerene.

Crystalline functionalized endohedral C60 metallofullerides

Endohedral metallofullerenes have been extensively studied since the first experimental observation of La@C60 in a laser-vaporized supersonic beam in 1985. However, most of these studies have focused on metallofullerenes larger than C60 such as (metal)@C82, and there are no reported purified C60-based monomeric metallofullerenes, except for [Li@C60]+(SbCl6)- salt. Pure (metal)@C60 compounds have not been obtained because of their extremely high chemical reactivity. One route to their stabilization is through chemical functionalization. Here we report the isolation, structural determination and electromagnetic properties of functionalized crystalline C60-based metallofullerenes Gd@C60(CF3)5 and La@C60(CF3)5. Synchrotron X-ray single-crystal diffraction reveals that La and Gd atoms are indeed encapsulated in the Ih-C60 fullerene. The HOMO-LUMO gaps of Gd@C60 and La@C60 are significantly widened by an order of magnitude with addition of CF3 groups. Magnetic measurements show the presence of a weak antiferromagnetic coupling in Gd@C60(CF3)3 crystals at low temperatures.

Structure of [60]fullerene with a mobile lithium cation inside

The structure of crystalline [60]fullerene with a lithium cation inside (Li⁺@C₆₀) was determined by synchrotron radiation X-ray diffraction measurements to understand the electrostatic and thermal properties of the encapsulated Li⁺ cation. Although the C₆₀ cages show severe orientation disorder in [Li⁺@C₆₀](TFPB^-)·C₄H₁₀O and [Li⁺@C₆₀](TFSI^-)·CH₂Cl₂, the Li⁺ cations are rather ordered at specific positions by electrostatic interactions with coordinated anions outside the C₆₀ cage. The Li⁺@C₆₀ molecules in [Li⁺@C₆₀](ClO₄^-) with a rock-salt-type cubic structure are fully disordered with almost uniform spherical shell charge densities even at 100 K by octahedral coordination of ClO₄^- tetrahedra and show no orientation ordering, unlike [Li⁺@C₆₀](PF₆^-) and pristine C₆₀. Single-bonded (Li⁺@C₆₀^-)² dimers in [Li⁺@C₆₀^-](NiOEP)⋅CH₂Cl₂ are thermally stable even at 400 K and form Li⁺-C bonds which are shorter than Li⁺-C bonds in [Li⁺@C₆₀](PF₆^-) and suppress the rotational motion of the Li⁺ cations.

Synthesis and Crystal Structure of Li+@Fluoreno[60]fullerene: Effect of Encapsulated Lithium Ion on Electrochemistry of Spiroannelated Fullerene

The reaction of [Li⁺@C₆₀]TFSI^- (TFSI = bis(trifluoromethanesulfonyl)imide) with 9-diazofluorene directly produced a [6,6]-adduct of lithium-ion-containing fluoreno[60]fullerene, [6,6]-[Li⁺@C₆₀(fluoreno)]TFSI^-, which was crystallographically characterized. Cyclic voltammetry of the compound showed a reversible one-electron reduction wave at -0.51 V (vs Fc/Fc⁺) and an irreversible reduction wave for the second electron. The latter was attributed to opening of the three-membered ring due to strong stabilization of the resulting sp³-carbanion by the encapsulated Li⁺ and formation of a 14π-electron aromatic fluorenyl anion.

Crystallographic Structure Determination of Both [5,6]- and [6,6]-Isomers of Lithium-Ion-Containing Diphenylmethano[60]fullerene

Organic functionalization of lithium-ion-containing [60]fullerene, Li⁺@C₆₀, was performed by using diphenyl(diazo)methane as a stable, readily available diazo compound to obtain lithium-ion-containing [5,6]- and [6,6]-diphenylmethano[60]fullerenes, Li⁺@C₆₁Ph₂. The bis(trifluoromethanesulfonyl)imide (TFSI) salts of [5,6]- and [6,6]-Li⁺@C₆₁Ph₂ were successfully separated by using a cation exchange column with eluent containing LiTFSI. Improved separation protocol and high crystallinity of ionic components in less polar solvents enabled separate crystallization of each isomer. Both [5,6]-open and [6,6]-closed structures of Li⁺@C₆₁Ph₂ were determined by synchrotron radiation X-ray crystallography. Elucidating the [5,6]-open methano[60]fullerene (fulleroid) structure will contribute to materials research on fulleroids.

A crystalline anionic complex of scandium nitride endometallofullerene: experimental observation of single-bonded (Sc3N@Ih-C80(-))2 dimers

Reduction of scandium nitride clusterfullerene, Sc3N@Ih-C80, by sodium fluorenone ketyl in the presence of cryptand[2,2,2] allows the crystallization of the {cryptand[2,2,2](Na(+))}2(Sc3N@Ih-C80(-))2·2.5C6H4Cl2 (1) salt. The Sc3N@Ih-C80˙(-) radical anions are dimerized to form single-bonded (Sc3N@Ih-C80(-))2 dimers.