Y.F.Wang

Carbon‐based nanocages have emerged as a new platform for advanced energy storage and conversion owing to their hollow interior cavity with microchannels across the shells, their high specific surface area with defective outer surface, and their tunable electronic structure. Up‐to‐date progress on the synthesis, encapsulating/supporting of carbon‐based nanocages and their applications is presented, along with the research challenges and trends.

Abstract

Energy storage and conversion play a crucial role in modern energy systems, and the exploration of advanced electrode materials is vital but challenging. Carbon‐based nanocages consisting of sp² carbon shells feature a hollow interior cavity with sub‐nanometer microchannels across the shells, high specific surface area with a defective outer surface, and tunable electronic structure, much different from the intensively studied nanocarbons such as carbon nanotubes and graphene. These structural and morphological characteristics make carbon‐based nanocages a new platform for advanced energy storage and conversion. Up‐to‐date synthetic strategies of carbon‐based nanocages, the utilization of their unique porous structure and morphology for the construction of composites with foreign active species, and their significant applications to the advanced energy storage and conversion are reviewed. Structure–performance correlations are discussed in depth to highlight the contribution of carbon‐based nanocages. The research challenges and trends are also envisaged for deepening and extending the study and application of this multifunctional material.

Clusters in fullerenes: A series of compounds with neutral, monomeric, and dimeric anion states of Sc₃N@I_h ‐C₈₀ have been prepared and their molecular structures and optical and magnetic properties have been studied. It is shown herein that the (Sc₃N@I_h ‐C₈₀ ⁻)₂ dimers dissociate in the 400–460 K range to produce Sc₃N@I_h ‐C₈₀ ^.−. The EPR spectra of Sc₃N@I_h ‐C₈₀ ^.− show a hyperfine splitting of 5.32 mT due to the interaction of the spin with the three scandium atoms and an additional hyperfine splitting of 0.82 mT.

Abstract

A series of compounds with Sc₃N@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 Sc₃N@I_h ‐C₈₀ ⋅3 C₆H₄Cl₂ (1) and (Sc₃N@I_h ‐C₈₀)₃(TPC)₂ ⋅5 C₆H₄Cl₂ (2, TPC=triptycene) compounds both crystallized in a high‐symmetry trigonal structure. The reduction of Sc₃N@I_h ‐C₈₀ to the radical anion resulted in dimerization to form diamagnetic singly bonded (Sc₃N@I_h ‐C₈₀ ⁻)₂ dimers. In contrast to {[2.2.2]cryptand(Na⁺)}₂(Sc₃N@I_h ‐C₈₀ ⁻)₂ ⋅2.5 C₆H₄Cl₂ (3) with strongly disordered components, we synthesized new dimeric phases {[2.2.2]cryptand‐ (K⁺)}₂(Sc₃N@I_h ‐C₈₀ ⁻)₂ ⋅2 C₆H₄Cl₂ (4) and {[2.2.2]cryptand‐ (Cs⁺)}₂(Sc₃N@I_h ‐C₈₀ ⁻)₂ ⋅2 C₆H₄Cl₂ (5) in which only one major dimer orientation was found. The thermal stability of the (Sc₃N@I_h ‐C₈₀ ⁻)₂ dimers was studied by EPR analysis of 3 to show their dissociation in the 400–460 K range producing monomeric Sc₃N@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 (Bu₃MeP⁺)₃(Sc₃N@I_h ‐C₈₀ ^.−)₃ ⋅C₆H₄Cl₂ (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 Sc₃N@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 Sc₃N cluster. Both the monomeric and dimeric Sc₃N@I_h ‐C₈₀ ⁻ anions show an intrinsic shift of the IR bands attributed to the Sc₃N 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.

The molecular structure of Ho₃N@C ₂ (22010)‐C₇₈ is shown in the cover artwork, along with the two molecules of nickel octaethylporphyrin (Ni(OEP) that surround it in the co‐crystal, Ho₃N@C ₂(22010)‐C₇₈ ⋅2 Ni(OEP)⋅2 toluene. Ho₃N@C ₂(22010)‐C₇₈ contains a planar Ho₃N unit surrounded by a chiral carbon cage. That cage violates the isolated pentagon rule by having two sites where two pentagons share a common edge. Color code: nitrogen, blue; holmium, orange; nickel, green; carbon, gray. More information can be found in the Full Paper by S. Stevenson, H. C. Dorn, M. M. Olmstead, A. L. Balch, et al. on https://doi.org/10.1002/chem.201902559page 12545.

Excited and exciting: Photoexcited triplet fullerenes can be successfully used for nanoscale distance measurements in biomolecules by pulsed dipolar electron paramagnetic resonance spectroscopy. They feature outstanding spectroscopic properties as spin labels, provide drastic signal enhancement compared to all known alternatives, and are applicable up to near room temperatures.

Abstract

Precise nanoscale distance measurements by pulsed electron paramagnetic resonance (EPR) spectroscopy play a crucial role in structural studies of biomolecules. The properties of the spin labels used in this approach determine the sensitivity limits, attainable distances, and proximity to biological conditions. Herein, we propose and validate the use of photoexcited fullerenes as spin labels for pulsed dipolar (PD) EPR distance measurements. Hyperpolarization and the narrower spectrum of fullerenes compared to other triplets (e.g., porphyrins) boost the sensitivity, and superior relaxation properties allow PD EPR measurements up to a near‐room temperature. This approach is demonstrated using fullerene–nitroxide and fullerene–triarylmethyl pairs, as well as a supramolecular complex of fullerene with nitroxide‐labeled protein. Photoexcited triplet fullerenes can be considered as new spin labels with outstanding spectroscopic properties for future structural studies of biomolecules.

Beautiful curves: Double‐heptagon‐containing C₇₀ with negative curvature was captured in the form of dihept‐C₇₀Cl₆ (see structure). Chlorination at the intersection of a heptagon and two adjacent pentagons greatly enlarges the HOMO–LUMO gap, which made dihept‐C₇₀Cl₆ isolable by chromatography. The synthesis of dihept‐C₇₀Cl₆ offers precious clues with respect to the fullerene formation mechanism in the carbon‐clustering process.

Abstract

All previously reported C₇₀ isomers have positive curvature and contain 12 pentagons in addition to hexagons. Herein, we report a new C₇₀ species with two negatively curved heptagon moieties and 14 pentagons. This unconventional heptafullerene[70] containing two symmetric heptagons, referred to as dihept‐C₇₀, grows in the carbon arc by a theoretically supported pathway in which the carbon cluster of a previously reported C₆₆ species undergoes successive C₂ insertion via a known heptafullerene[68] intermediate with low energy barriers. As identified by X‐ray crystallography, the occurrence of heptagons facilitates a reduction in the angle of the π‐orbital axis vector in the fused pentagons to stabilize dihept‐C₇₀. Chlorination at the intersection of a heptagon and two adjacent pentagons can greatly enlarge the HOMO–LUMO gap, which makes dihept‐C₇₀Cl₆ isolable by chromatography. The synthesis of dihept‐C₇₀Cl₆ offers precious clues with respect to the fullerene formation mechanism in the carbon‐clustering process.

The endohedral fullerene CH₄@C₆₀ has been synthesized for the first time using photochemical desulfinylation of an open fullerene as the key step. Methane is a much larger molecule than has been previously encapsulated in the closed C₆₀ cage and is amongst the largest of possible guests.

Abstract

The endohedral fullerene CH₄@C₆₀, in which each C₆₀ fullerene cage encapsulates a single methane molecule, has been synthesized for the first time. Methane is the first organic molecule, as well as the largest, to have been encapsulated in C₆₀ to date. The key orifice contraction step, a photochemical desulfinylation of an open fullerene, was completed, even though it is inhibited by the endohedral molecule. The crystal structure of the nickel(II) octaethylporphyrin/ benzene solvate shows no significant distortion of the carbon cage, relative to the C₆₀ analogue, and shows the methane hydrogens as a shell of electron density around the central carbon, indicative of the quantum nature of the methane. The ¹H spin‐lattice relaxation times (T ₁) for endohedral methane are similar to those observed in the gas phase, indicating that methane is freely rotating inside the C₆₀ cage. The synthesis of CH₄@C₆₀ opens a route to endofullerenes incorporating large guest molecules and atoms.

Nature Energy, Published online: 17 June 2019; doi:10.1038/s41560-019-0406-2

Nature Energy, Published online: 17 June 2019; doi:10.1038/s41560-019-0400-8

An all‐fullerene electron donor–acceptor conjugate was designed, synthesized, and characterized. In their Communication on https://doi.org/10.1002/anie.201901863page 6932, M. Solà, D. M. Guldi, N. Martín et al. describe the C₆₀‐Lu₃N@I_h‐C₈₀ dumbbell and demonstrate that photoinduced electron transfer leads to the formation of a long‐lived charge‐separated triplet state.