Solution Dynamics of Covalent Open-[60]Fullerene Dimers.

The translational diffusivity of covalent open-[60]fullerene dimers in an organic solvent was found to be well describable by a prolate ellipsoid model while a monomeric open-[60]fullerene behaves like a sphere model. The water association dynamics were examined for two open-[60]fullerene dimers, showing a higher water affinity for the sp3-linked dimer relative to sp2-linked dimer owing to an effective orbital-orbital overlap identified by π(fullerene)→σ*(H2O) interactions as suggested by theoretical calculations.

Molecular CO2 Storage: State of a Single-Molecule Gas.

CO2 evolution is one of the urgent global issues; meanwhile, understanding of sorptive/dynamic behavior is crucial to create next-generation encapsulant materials with stable sorbent processes. Herein, we showcase molecular CO2 storage constructed by a [60]fullerenol nanopocket. The CO2 density reaches 2.401 g/cm3 within the nanopore, showing strong intramolecular interactions, which induce nanoconfinement effects such as forbidden translation, restricted rotation, and perturbed vibration of CO2. We also disclosed an equation of state for a molecular CO2 gas, revealing a very low pressure of 3.14 rPa (1 rPa = 10-27 Pa) generated by the rotation/vibration at 300 K. Curiously enough, the CO2 capture enabled to modulate an external property of the encapulant material itself, i.e., association of the [60]fullerenol via intercage hydrogen-bonding.

Water Adsorption on π-Surfaces of Open-Fullerenes.

Understanding the water adsorptive behavior of fullerenes is of particular importance for their material application in aqueous media. The conventional fullerenols usually provide complex physical pictures of water adsorption due to their uncontrollable hydroxylation degree and substitution pattern. Herein, we focus on poorly hydroxylated fullerenes with well-defined structures. The water adsorptive behavior was examined by synchrotron IR spectroscopy and computational studies. As a result, three types of IR bands were observed for adsorbed water. The population of the three states was considerably altered by the orientational difference of the hydroxy groups. Nevertheless, water adsorption could not occur for 9-fluorenol and catechol. This indicates that the Lewis acidic fullerene π-surface plays a prominent role in water adsorption, while the rather Lewis basic π-surface of 9-fluorenol is unable to attract much water at a boundary with humid air.

Phosphine-Mediated Dimerization of Open-[60]Fullerenes.

By a reaction of trimethylphosphine with an open-[60]fullerene, corresponding dimers could be generated via two-fold deoxygenation processes even though the formation of ß-oxo-phosphorous ylide is inevitable, a part of which is hydrolyzed to yield an α-methylene carbonyl derivative. Nevertheless, Wittig reaction and aldol condensation did not proceed well, indicating the presence of an unknown dimerization pathway. In the ylide formation, 1-phosphonium-3-carbabetaine was previously proposed as a key intermediate. Upon assuming that the betaine also participates in the dimerization process, we examined a possible reaction pathway computationally. As the results, the betaine formed by a reaction with the first phosphine was suggested to undergo nucleophilic addition to an unreacted molecule of the open-[60]fullerene, yielding an epoxide dimer which is then deoxygenated by the second phosphine to furnish the desired open-[60]fullerene dimer.

Direct Through-Space Substituent-π Interactions in Noncovalent Arene-Fullerene Assemblies.

The arene-arene interactions between electron-rich and deficient aromatics have been less understood. Herein, we focus on a [60]fullerene π-surface as an electron-deficient aromatics. Using a 1H signal of H2O@C60 as a magnetic probe, the presence of benzene-fullerene interactions was confirmed. To investigate substituent effects on the noncovalent arene-fullerene interactions, NMR titration experiments were carried out using an open-[60]fullerene and a series of substituted benzenes, i. e., PhX (X=NO2, CN, Cl, OMe, H, CH3, and NH2), demonstrating a 1 : 2 stoichiometry with a positive correlation between stabilization energies upon the first association (ΔG1) and Hammet constants (σm). The destabilization of the self-assembled structure for X=OMe with a σ-withdrawing nature clearly showed direct through-space substituent-π interactions describable by the Wheeler-Houk model while the second association was suggested to be considerably perturbed by the secondary effects.

An open-cage bis[60]fulleroid as an electron transport material for tin halide perovskite solar cells.

An open-cage bis[60]fulleroid (OC) was applied as an electron transport material (ETM) in tin (Sn) halide perovskite solar cells (PSCs). Due to the reduced offset between the energy levels of Sn-based perovskites and ETMs, the power conversion efficiency (PCE) of Sn-based PSCs with OC reached 9.6% with an open-circuit voltage (VOC) of 0.72 V. Additionally, OC exhibited superior thermal stability and provided 75% of the material without decomposition after vacuum deposition. The PSC using vacuum-deposited OC as the ETM could afford a PCE of 7.6%, which is a big leap forward compared with previous results using vacuum-deposited fullerene derivatives as ETMs.

Synthesis of inter-[60]fullerene conjugates with inherent chirality.

Coalescence of [60]fullerenes potentially produces hypothetical nanocarbon assemblies with non-naturally occurring topologies. Since the discovery of [60]fullerene in 1985, coalesced [60]fullerene oligomers have only been observed as transient species by transmission electron microscopy during an oligomerization process under a high electron acceleration voltage. Herein, we showcase the rational synthesis of covalent assemblies consisting of inherently chiral open-[60]fullerenes. The crystallographic analyses unveiled double-caged structures of non-conjugated and conjugated inter-[60]fullerene hybrids, in which the two [60]fullerene cages are bounds to each other through a covalent linkage. The former one further assembles via a heterochiral recognition so that four carbon cages are arranged in a tetrahedral manner both in solution and solid state. Reflecting radially-conjugated double π-surface nature, the inter-[60]fullerene conjugate exhibits strong electronic communication in its reduced states, intense absorption behavior, and chiroptical activity with a dissymmetry factor of 0.21 (at 674 nm) which breaks the record for known chiral organic molecules.

Open-[60]fullerenols with water adsorbed both inside and outside.

The water affinity on [60]fullerenols was found to be governed by surface electrostatic potential while water aggregation is initiated by the hydroxy groups attached on the carbon surface. The molecular water adsorption at the internal sphere caused a significnat inhibition of water adsorption at the external carbon surface.

CH3 CN@open-C60 : An Effective Inner-Space Modification and Isotope Effect Inside a Nano-Sized Flask.

Despite several small molecules being encapsulated inside cage-opened fullerene derivatives, such species have not considerably affected the structures and properties of the outer carbon cages. Herein, we achieved an effective inner-space modification for an open-cage C60 derivative by insertion of a neutral CH3 CN molecule into the cavity. The CH3 CN@open-C60 thus obtained showed an enhanced polarity, thus affording an easy separation from a mixture containing the empty cage by column chromatography on silica gel, without the preparative HPLC that was needed for previous cases. The less negative reduction potentials with respect to those of empty cage reflect the decreased energy level of the LUMO, which is supported by the DFT calculations. NMR spectroscopy, single-crystal X-ray analysis, and theoretical calculations revealed that both the presence of the encapsulated CH3 CN and cage deformation caused by the CH3 CN play an essential role in the change of the electronic properties. Furthermore, the favored binding affinity of deuterated acetonitrile CD3 CN with internal C60 surface is discussed.

Construction of a 21-Membered-Ring Orifice on [60]Fullerene.

Open-[60]fullerenes possessing a huge orifice with a ring-atom count exceeding 19 have been confined to only a few examples. Herein, we report a 20-membered-ring orifice which enables for a guest molecule such as H2 , N2 , and CH3 OH to be encapsulated inside the [60]fullerene cavity. In addition, a 21-membered-ring orifice was prepared via a reductive decarbonylation, in which one of the carbon atoms was moved out of the [60]fullerene skeleton as an N,N-dimethylamide group. At a low temperature of -30 °C, an Ar atom was encapsulated with an occupation level up to 52 %. At around room temperature, the amide group on the orifice rotates along with the C(amide)-C(fullerene) bond axis, realizing a self-inclusion of the methyl substituent on the amide group as confirmed NMR spectroscopically and computationally.

Bilateral π-extension of an open-[60]fullerene in a helical manner.

The conventional π-elongation of open-[60]fullerenes could only give unilaterally π-extended derivatives. Herein, we report the further π-elongation at another site to achieve bilateral π-elongation via a consecutive nucleophilic addition of 4,5-dimethyl-o-phenylenediamine. The thus-formed π-extended open-[60]fullerene bears two-fold diaza[n]helicene (n = 5 and 6) motifs in its skeleton. The crystallographic analysis revealed the characteristic helicene-fullerene interactions with close contacts of 3.09 and 3.14 Å.

Synthesis of open-[70]fullerenes bearing huge orifices.

Orifices with ring-atom counts of up to 13 have been created on [70]fullerenes as an isolable form. Herein, we report orifice-enlargement reactions providing huge orifices on [70]fullerene with ring-atom counts of 17, 18, and 20. These orifices allow H2O, N2, and Ar to be encapulated in situ inside the open-[70]fullerene cavity.

Open-[60]fullerene-aniline conjugates with near-infrared absorption.

Two open-[60]fullerene-aniline conjugates were synthesized, in which the two-fold addition of diamine gave a thiazolidine-2-thione ring on the [60]fullerene cage in the presence of CS2. By increasing the number of N,N-dimethylaniline moieties, the absorption edge was considerably shifted up to 1200 nm owing to effective acceptor-donor interactions.

π-Extended Open-[70]Fullerenes with a Fused Azaacene.

By the reaction with aromatic diamines, π-extended open-[60]fullerenes were synthesized, in which the nucleophilicity of the diamine switched annulation and orifice-cutting modes, thus generating a fused pyrazine or imidazole. Employing this method for [70]fullerene analogues, we synthesized the first π-extended [70]fullerenes exhibiting strong absorption behavior owing to the fused azaacene and 10π-elongation from the [60]fullerene core. The thus-formed 20-membered ring orifice realized the encapsulation of argon within the [70]fullerene cavity.

Synthesis of Hydrogen-Bonded Open-[60]Fullerenol Dimers.

Two open-[60]fullerenols were synthesized through a selective bond cleavage by reactions with N-oxide or singlet oxygen. Both open-[60]fullerenols having a bis(hemiketal) moiety were aggregated via intra- and intermolecular hydrogen-bondings to show a dimeric configuration in a face-to-face fashion where the hydroxy groups adopt a rhomboidal or chair-like arrangement as confirmed by X-ray diffraction analysis. The dimerization constant of one of the two open-[60]fullerenols was measured to be 22.2±4.7 M-1 (300 K, CDCl3 ) which is twice as large as that of a previously-reported open-[60]fullerenol bearing three hydroxy groups. According to theoretical calculations, an averaged interaction energy was estimated to be ca. -1.95 kcal/mol per an H-bonding. By a loss of a water molecule from the bis(hemiketal) moiety, these open-[60]fullerenols are transformed into tetracarbonyl derivatives with a 16-membered-ring orifice. Despite being the same ring-atom count, a larger tilt angle of one of the carbonyl groups reduces the effective orifice size as suggested by theoretical calculations.

C2-insertion into a fullerene orifice.

The embedment of a Cn-unit into a carbon network constituting fullerene(s) potentially enables a cage-expansion. Herein, we report a C2-insertion into a fullerene orifice in which the mechanism was examined computationally. The C2-embedded open-[60]fullerene possesses an orifice enlarged from an octagon to a decagon, while the inner space was notably expanded as confirmed by the dynamic motion of the incarcerated H2O molecule.

Near-Infrared-Absorbing Chiral Open [60]Fullerenes.

Though [60]fullerene is an achiral molecular nanocarbon with Ih symmetry, it could attain an inherent chirality depending upon a functionalization pattern. The conventional chiral induction of C60 relies mainly upon a multiple addition affording a mixture of achiral and chiral isomers while their chiral function would be largely offset by the existence of pseudo-mirror plane(s). These are major obstacles to proceed further study on fullerene chirality and yet leave its understanding elusive. Herein, we showcase a carbene-mediated synthesis of C1 -symmetric chiral open [60]fullerenes showing an intense far-red to near-infrared absorption. The large dissymmetry factor of |gabs |=0.12 was achieved at λ=820 nm for circular dichroism in benzonitrile. This is, in general, unachievable by other small chiral organic molecules, demonstrating the potential usage of open [60]fullerenes as novel types of chiral chromophores.

π-Extended Fullerenes with a Reactant Inside.

Fullerene-graphene hybrids potentially exhibit unprecedented properties owing to interactive communication between the two units through a linkage. However, most of their discrete molecular structures have been still undisclosed thus far. With the recent rise in the awareness of facile access to molecular nanocarbon hybrids, we showcase novel π-extended fullerenes with a fused pyrazine or imidazole. Owing to the effective planar-curved π-conjugation, their absorption coefficients significantly increased in the visible region. Curiously enough, during the formation of π-extended fullerenes, an in situ generated NH3 molecule was spontaneously encapsulated inside the fullerene cavity. The NH3 molecule then underwent a timed orifice-expansion triggered by its sustained release. This is the first demonstration that fullerene captures a reactant inside, suggesting their potential usage for a sustained dosing and/or material delivery toward postfunctionalization of fullerene-graphene hybrids.

Phosphorus ylides of cage-opened sulphide [60]fullerene derivatives.

The partial structural change in cage-opened C60 derivatives by replacing a carbonyl group with a sulphide moiety significantly lowers the chemical stability of the corresponding phosphorus ylides. Thus they readily undergo in situ hydrolysis to give an α-methylene carbonyl compound. Owing to the elevated LUMO level, the near-infrared absorption band was hypsochromically shifted from λedge 880 to 800 nm.

Chiral Open-[60]Fullerene Ligands with Giant Dissymmetry Factors.

The optical resolution of open-[60]fullerenes has been limited to only one example since 1998, while the recent advances revealed the excellence of fullerenes as revisited chiral functional materials. Different from conventional chiral induction on [60]fullerene by a multiple-functionalization, a random disruption of the spherical π-conjugation is avoidable for open-[60]fullerenes. Moreover, the macrocyclic orifices enable a metal coordination which endows modulated electronic structures on chiral chromophores. Herein, we showcase Li+-coordination behavior and optical resolution of three chiral open-[60]fullerene ligands, showing a giant dissymmetry factor up to 0.20 owing to a congenital topology of the spherical π-conjugation.