Redox-tuning endohedral fullerene spin states: from the dication to the trianion radical of Sc(3)N@C(80)(CF(3))(2) in five reversible single-electron steps

Functionalized Fullerenes and Their Applications in Electrochemistry, Solar Cells, and Nanoelectronics

Carbon-based nanomaterials have rapidly advanced over the last few decades. Fullerenes, carbon nanotubes, graphene and its derivatives, graphene oxide, nanodiamonds, and carbon-based quantum dots have been developed and intensively studied. Among them, fullerenes have attracted increasing research attention due to their unique chemical and physical properties, which have great potential in a wide range of applications. In this article, we offer a comprehensive review of recent progress in the synthesis and the chemical and physical properties of fullerenes and related composites. The review begins with the introduction of various methods for the synthesis of functionalized fullerenes. A discussion then follows on their chemical and physical properties. Thereafter, various intriguing applications, such as using carbon nanotubes as nanoreactors for fullerene chemical reactions, are highlighted. Finally, this review concludes with a summary of future research, major challenges to be met, and possible solutions.

Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer

We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y _x Sc_3-x N@C₈₀ (x = 0-3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S₁-T₁ gap decrease in this row. For Sc₃N@C₈₀, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin-lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with ¹⁴N, ⁴⁵Sc, and ⁸⁹Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y₃N to Sc₃N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.

Unusually large hyperfine structure of the electron spin levels in an endohedral dimetallofullerene and its spin coherent properties

We report the synthesis, ESR spectroscopic and spin coherent properties of the dimetallofullerene Sc2@C80(CH2Ph). The single-electron metal-metal bond of the Sc2 dimer inside the fullerene's cage is stabilized with the electron spin density being fully localized at the metal bond. This results in an extraordinary strong hyperfine interaction of the electron spin with the 45Sc nuclear spins with a coupling constant a = 18.2 mT (∼510 MHz) and yields a fully resolved hyperfine-split ESR spectrum comprising 64 lines. The splitting is present even at low temperatures where the molecular dynamics are completely frozen. The large extent and the robustness of the hyperfine-split spectra enable us to identify and control the well-defined transitions between specific electron-nuclear quantum states. This made it possible to demonstrate in our pulse ESR study the remarkable spin coherent dynamics of Sc2@C80(CH2Ph), such as the generation of arbitrary superpositions of the spin states in a nutation experiment and the spin dephasing times above 10 μs at temperatures T < 80 K reaching the value of 17 μs at T ≤ 20 K. These observations suggest Sc2@C80(CH2Ph) as an interesting qubit candidate and motivate further synthetic efforts to obtain fullerene-based systems with superior spin properties.

Molecular Structure and Magnetic and Optical Properties of Endometallonitridofullerene Sc3 N@Ih -C80 in Neutral, Radical Anion, and Dimeric Anionic Forms

A series of compounds with Sc₃ N@I_h -C₈₀ in the neutral, monomeric, and dimeric anion states have been prepared in the crystalline form and their molecular structures and optical and magnetic properties have been studied. The neutral Sc₃ N@I_h -C₈₀ ⋅3 C₆ H₄ Cl₂ (1) and (Sc₃ N@I_h -C₈₀ )₃ (TPC)₂ ⋅5 C₆ H₄ Cl₂ (2, TPC=triptycene) compounds both crystallized in a high-symmetry trigonal structure. The reduction of Sc₃ N@I_h -C₈₀ to the radical anion resulted in dimerization to form diamagnetic singly bonded (Sc₃ N@I_h -C₈₀ ^- )₂ dimers. In contrast to {[2.2.2]cryptand(Na⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2.5 C₆ H₄ Cl₂ (3) with strongly disordered components, we synthesized new dimeric phases {[2.2.2]cryptand- (K⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2 C₆ H₄ Cl₂ (4) and {[2.2.2]cryptand- (Cs⁺ )}₂ (Sc₃ N@I_h -C₈₀ ^- )₂ ⋅2 C₆ H₄ Cl₂ (5) in which only one major dimer orientation was found. The thermal stability of the (Sc₃ N@I_h -C₈₀ ^- )₂ dimers was studied by EPR analysis of 3 to show their dissociation in the 400-460 K range producing monomeric Sc₃ N@I_h -C₈₀ ^.- radical anions. This species shows an EPR signal with a hyperfine splitting of 5.8 mT. The energy of the intercage C-C bond was estimated to be 234±7 kJ mol^-1 , the highest value among negatively charged fullerene dimers. The EPR spectra of crystalline (Bu₃ MeP⁺ )₃ (Sc₃ N@I_h -C₈₀ ^.- )₃ ⋅C₆ H₄ Cl₂ (6) are presented for the first time. The salt shows an asymmetric EPR signal, which could be fitted by three lines. Two lines were attributed to Sc₃ N@I_h -C₈₀ ^.- . Hyperfine splitting is manifested above 180 K due to the hyperfine interaction of the electron spin with the three scandium atoms (a total of 22 lines with an average splitting of 5.32 mT are observed at 220 K). Furthermore, each of the 22 lines is additionally split into six lines with an average separation of 0.82 mT. The large splitting indicates intrinsic charge and spin density transfer from the fullerene cage to the Sc₃ N cluster. Both the monomeric and dimeric Sc₃ N@I_h -C₈₀ ^- anions show an intrinsic shift of the IR bands attributed to the Sc₃ N cluster and new bands corresponding to these species appear in the NIR range of their UV/Vis/NIR spectra, which allows these anions to be distinguished from neutral species.

Lewis Acid-Base Adducts of Sc2C2@C3v(8)-C82/N-Heterocyclic Carbene: Toward Isomerically Pure Metallofullerene Derivatives

The addition of a bulky N-heterocyclic carbene (NHC) to Sc₂C₂@C_3v(8)-C₈₂ affords two monoadducts (2a and 2b) quantitatively and regioselectively, representing the first examples of Lewis acid-base pairs of metal carbide cluster fullerenes. 2b is likely a kinetically favorable labile product that cannot be isolated from the solution. The crystallographic results of 2a unambiguously demonstrate that one polarized C-C single bond is formed between the normal carbene site C2N of the NHC and a specific [5,6,6]-carbon atom out of 17 types of nonequivalent cage carbon atoms of Sc₂C₂@C_3v(8)-C₈₂. Theoretical calculations demonstrate that the high regioselectivity, the unexpected addition pattern, and the quantitative formation of monoadducts are synergistic results from the cage geometry and electron distribution on the cage.

Electronic Structures, and Optical and Magnetic Properties of Quadruple-Decker Phthalocyanines

For applications of magnetic devices with operating nuclear-spin-based quantum bits in quantum computing, electronic structures, and magnetic and optical properties of quadruple-decker phthalocyanines with 3d transition metals, such as scandium, yttrium, and lanthanum atoms (M3Pc4: M = Sc, Y, and La), were studied by quantum calculation using density function theory. Electron density distributions at the highest occupied molecular orbital and lowest unoccupied molecular orbital were considerably delocalized on the phthalocyanine ring with considerable bias of the electrostatic potential. The wide energy gaps and the ultraviolet-visible-near infrared spectra of the systems were based on the phthalocyanine ring-ring interactions with overlapping π-orbitals on the phthalocyanine rings. The chemical shift behavior of 13C and 14N-NMR of Sc3(Pc)4, Y3(Pc)4, and La3(Pc)4 depended on the deformation of their structures owing to Jahn-Teller splitting of the d-orbital in the metal ligand field, the considerable perturbation of the metal ligand crystal field on the phthalocyanine ring, the electronic structure based on the electron density distribution, and the magnetic interaction of the nuclear quadrupole interaction. The magnetic parameters of the principle g-tensor, the V-tensor of the electronic field gradient, and the asymmetric parameters were influenced by the deformed structures of the complex with the considerable deviation of the charge density distribution. The quadruple-decker metal phthalocyanines using 3d transition metals have an advantage in controlling the electronic structure and magnetic parameters based on the nuclear spin interaction in spin lattice relaxation with respect to applications of single-molecular magnets.

Awaking N-hyperfine couplings in charged yttrium nitride endohedral fullerenes

Charged yttrium nitride endohedral fullerenes show particular N-hyperfine couplings that are sensitive to the outer carbon cage.

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.

Synthesis and Isolation of the Titanium-Scandium Endohedral Fullerenes-Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 and Sc2 TiC2 @Ih -C80 : Metal Size Tuning of the Ti(IV) /Ti(III) Redox Potentials

The formation of endohedral metallofullerenes (EMFs) in an electric arc is reported for the mixed-metal Sc-Ti system utilizing methane as a reactive gas. Comparison of these results with those from the Sc/CH4 and Ti/CH4 systems as well as syntheses without methane revealed a strong mutual influence of all key components on the product distribution. Whereas a methane atmosphere alone suppresses the formation of empty cage fullerenes, the Ti/CH4 system forms mainly empty cage fullerenes. In contrast, the main fullerene products in the Sc/CH4 system are Sc4 C2 @C80 (the most abundant EMF from this synthesis), Sc3 C2 @C80 , isomers of Sc2 C2 @C82 , and the family Sc2 C2 n (2 n=74, 76, 82, 86, 90, etc.), as well as Sc3 CH@C80 . The Sc-Ti/CH4 system produces the mixed-metal Sc2 TiC@C2 n (2 n=68, 78, 80) and Sc2 TiC2 @C2 n (2 n=80) clusterfullerene families. The molecular structures of the new, transition-metal-containing endohedral fullerenes, Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 , and Sc2 TiC2 @Ih -C80 , were characterized by NMR spectroscopy. The structure of Sc2 TiC@Ih -C80 was also determined by single-crystal X-ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc2 TiC- and Sc2 TiC2 -containing clusterfullerenes have Ti-localized LUMOs. Encapsulation of the redox-active Ti ion inside the fullerene cage enables analysis of the cluster-cage strain in the endohedral fullerenes through electrochemical measurements.

Crystallographic Evidence for Direct Metal-Metal Bonding in a Stable Open-Shell La2 @Ih -C80 Derivative

Endohedral metallofullerenes (EMFs) have novel structures and properties that are closely associated with the internal metallic species. Benzyl radical additions have been previously shown to form closed-shell adducts by attaching an odd number of addends to open-shell EMFs (such as Sc3 C2 @Ih -C80 ) whereas an even number of groups are added to closed-shell EMFs (for example Sc3 N@Ih -C80 ). Herein we report that benzyl radical addition to the closed-shell La2 @Ih -C80 forms a stable, open-shell monoadduct instead of the anticipated closed-shell bisadduct. Single-crystal X-ray diffraction results show the formation of a stable radical species. In this species, the La-La distance is comparable to the theoretical value of a La-La covalent bond and is shorter than reported values for other La2 @Ih -C80 derivatives, providing unambiguous evidence for the formation of direct La-La bond.

Anion radicals of isomeric [5,6] and [6,6] benzoadducts of Sc3N@C80: remarkable differences in endohedral cluster spin density and dynamics

The anion radicals of isomeric [5,6] and [6,6] Sc3N@C80 benzoadducts were studied by electron spin resonance spectroscopy, density functional theory computations, and molecular dynamics. In both compounds the rotation of the Sc3N cluster is frozen and the spin density distribution of the cluster is highly anisotropic, with hyperfine coupling constants of 9.1 and 2 × 33.3 G for the [5,6] adduct and ∼0.6 and 2 × 47.9 G for the [6,6] adduct. Remarkably, the subtle variation of the position of the exohedral group on the surface of the cage results in very pronounced changes in the spin density distribution and the dynamics of the encapsulated Sc3N cluster.

Solution-Phase Perfluoroalkylation of C60 Leads to Efficient and Selective Synthesis of Bis-Perfluoroalkylated Fullerenes

A solution-phase perfluoroalkylation of C₆₀ with a series of R_FI reagents was studied. The effects of molar ratio of the reagents, reaction time, and presence of copper metal promoter on fullerene conversion and product composition were evaluated. Ten aliphatic and aromatic R_FI reagents were investigated (CF₃I, C₂F₅I, n-C₃F₇I, i-C₃F₇I, n-C₄F₉I, (CF₃)(C₂F₅)CFI, n-C₈F₁₇I, C₆F₅CF₂I, C₆F₅I, and 1,3-(CF₃)₂C₆F₃I) and eight of them (except for C₆F₅I and 1,3-(CF₃)₂C₆F₃I) were found to add the respective R_F groups to C₆₀ in solution. Efficient and selective synthesis of C₆₀(R_F)₂ derivatives was developed.

Redox-active scandium oxide cluster inside a fullerene cage: spectroscopic, voltammetric, electron spin resonance spectroelectrochemical, and extended density functional theory study of Sc4O2@C80 and its ion radicals

The clusterfullerene Sc(4)O(2)@C(80) with a mixed redox state of scandium was found to be an exciting molecule for endohedral electrochemistry as demonstrated by means of an in situ electron spin resonance (ESR) spectroelectrochemical study of the spin density distribution in its electrochemically generated cation and anion radicals. The compound exhibits two reversible reduction and oxidation steps with a relatively small electrochemical gap of 1.10 V. The ESR spectra of the ion radicals have a rich hyperfine structure caused by two pairs of equivalent Sc atoms. The Sc-based hyperfine structure with large hyperfine coupling constants shows that both oxidation and reduction of Sc(4)O(2)@C(80) are in cavea redox processes, which is the subject of endohedral electrochemistry. The assignment of the experimentally determined a((45)Sc) values to the two types of Sc atoms in the Sc(4)O(2) cluster was accomplished by extended density functional theory and molecular dynamics simulations. Sc atoms adopting a divalent state in the neutral Sc(4)O(2)@C(80) exhibited an especially large coupling constant of 150.4 G in the cation radical, which is the record high a((45)Sc) value for Sc-based endohedral metallofullerenes. Such a high value is explained by the nature of the highest occupied molecular orbital (HOMO) localized on the six-atom Sc(4)O(2) cluster. This HOMO is a Sc-Sc bonding MO and hence has large contributions from the 4s atomic orbitals of Sc(II). We claim that ESR spectroelectrochemistry is an invaluable experimental tool in the studies of metal-metal bonding in endohedral metallofullerenes and in endohedral electrochemistry.

Poly(perfluoroalkylation) of metallic nitride fullerenes reveals addition-pattern guidelines: synthesis and characterization of a family of Sc3N@C80(CF3)n (n = 2-16) and their radical anions

A family of highly stable (poly)perfluoroalkylated metallic nitride cluster fullerenes was prepared in high-temperature reactions and characterized by spectroscopic (MS, (19)F NMR, UV-vis/NIR, ESR), structural and electrochemical methods. For two new compounds, Sc(3)N@C(80)(CF(3))(10) and Sc(3)N@C(80)(CF(3))(12,) single crystal X-ray structures are determined. Addition pattern guidelines for endohedral fullerene derivatives with bulky functional groups are formulated as a result of experimental ((19)F NMR spectroscopy and single crystal X-ray diffraction) studies and exhaustive quantum chemical calculations of the structures of Sc(3)N@C(80)(CF(3))(n) (n = 2-16). Electrochemical studies revealed that Sc(3)N@C(80)(CF(3))(n) derivatives are easier to reduce than Sc(3)N@C(80), the shift of E(1/2) potentials ranging from +0.11 V (n = 2) to +0.42 V (n = 10). Stable radical anions of Sc(3)N@C(80)(CF(3))(n) were generated in solution and characterized by ESR spectroscopy, revealing their (45)Sc hyperfine structure. Facile further functionalizations via cycloadditions or radical additions were achieved for trifluoromethylated Sc(3)N@C(80) making them attractive versatile platforms for the design of molecular and supramolecular materials of fundamental and practical importance.

[2 + 2] cycloaddition reaction to Sc3N@I(h)-C80. The formation of very stable [5,6]- and [6,6]-adducts

The [2 + 2] cycloaddition reaction of Sc(3)N@I(h)-C(80) with benzyne was successfully conducted for the first time. The reaction affords both the [5,6]- and [6,6]-monoadducts with a four-membered ring attached to the cage surface on 5,6- and 6,6-ring fusions, respectively. The compounds were characterized by MALDI-TOF, NMR, UV-vis-NIR spectroscopy and single-crystal X-ray structure determination. The electrochemical behavior of both monoadducts was investigated. The [5,6]-regioisomer displays reversible cathodic behavior similar to that observed for the fulleropyrrolidines with a 5,6-addition pattern. Surprisingly, the [6,6]-regioisomer also exhibits reversible cathodic behavior. The interconversion reaction of the isomers was also explored, and the results showed that both monoadducts are thermally very stable.