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.

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.

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.

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.

An H2 O2 Molecule Stabilized inside Open-Cage C60 Derivatives by a Hydroxy Stopper.

An H2 O2 molecule was isolated inside hydroxylated open-cage fullerene derivatives by mixing an H2 O2 solution with a precursor molecule followed by reduction of one of carbonyl groups on its orifice. Depending on the reduction site, two structural isomers for H2 O2 @open-fullerenes were obtained. A high encapsulation ratio of 81 % was attained at low temperature. The structures of the peroxosolvate complexes thus obtained were studied by 1 H NMR spectroscopy, X-ray analysis, and DFT calculations, showing strong hydrogen bonding between the encapsulated H2 O2 and the hydroxy group located at the center of the orifice. This OH group was found to act as a kinetic stopper, and the formation of the hydrogen bonding caused thermodynamic stabilization of the H2 O2 molecule, both of which prevent its escape from the cage. One of the peroxosolvates was isolated by HPLC, affording H2 O2 @open-fullerene with 100 % encapsulation ratio, likely due to the intramolecular hydrogen-bonding interaction.

Hydrogenation of cage-opened C60 derivatives mediated by frustrated Lewis pairs.

Multiply-carbonylated fullerene derivatives were found to work as one component in frustrated Lewis pairs which caused an Si-H bond activation in the presence of B(C6F5)3, leading to the carbonyl hydrogenation in up to 99% yield. The Lewis acid-mediated reductive arylation also took place to furnish a corresponding ketal derivative.

Inelastic Electron Transport and Ortho-Para Fluctuation of Water Molecule in H2O@C60 Single Molecule Transistors.

Light molecules such as H2O are the systems in which we can have access to quantum mechanical information on their constituent atoms. Here, we have investigated electron transport through H2O@C60 single molecule transistors (SMTs). The H2O@C60 SMTs exhibit Coulomb stability diagrams that show multiple tunneling-induced excited states below 30 meV. Furthermore, we have performed terahertz (THz) photocurrent spectroscopy on H2O@C60 SMTs and confirmed the same excitations. From comparison between experiment and theory, the excitations observed below 10 meV are identified to be the quantum rotational excitations of the water molecule. Surprisingly, the quantum rotational excitations of both para- and ortho-water molecule are observed simultaneously even for a single water molecule, indicating that the fluctuation between the ortho- and para-water states takes place in a time scale shorter than our measurement time (∼1 min), probably by the interaction between the encapsulated water molecule and conducting electrons.

Cage-Expansion of Fullerenes.

Despite the first proposal on the cage inflation of fullerenes in 1991, the chemical expansion of fullerenes has been still a formidable challenge. Herein, we provide an efficient methodology to expand [60] and [70]fullerene cages by the inclusion of totally C5N unit, giving nitrogen-containing closed structures as C65N and C75N with double fused heptagons. This method consists of two steps commenced with the construction of an opening by the reaction with triazine as a C3N source, followed by the cage reformation using N-phenylmaleimide as a C2 source. We also synthesized endohedral cages, demonstrating that the encapsulated H2O molecule inside the C75N cage prefers the orientation which maximizes the intramolecular interaction with the carbon wall. Additionally, we revealed the existence of a through-space magnetic dipolar interaction between the encapsulated H2 molecule and the embedded N atom.

Reactions of C60 with Pyridazine and Phthalazine.

Cage-opened bisfulleroids are one of suitable building blocks for making a large hole on fullerenes. This work focuses on the Diels-Alder reaction of C60 with azines, among synthetic methods developed thus far, to provide bisfulleroids. Surprisingly, the computational study predicted that the reaction proceeds with normal electron demand in contrast to hitherto considered inverse-electron-demand pathway. The benzoannulation to the pyridazine ring, i. e., phthalazine, resulted in the remarkably shortened reaction time due to the better interaction between the HOMO of phthalazine and the LUMO of C60 as well as stronger 2,3-diaza-1,3-butadiene character in the phthalazine as confirmed crystallographically. Contrary to expectations, the benzobisfulleroid was converted into corresponding orifice-enlarged derivative via the photooxygenation slightly faster than the fulleroid derived from pyridazine.

Reactions on a 1,2-Dicarbonyl Moiety of a Fullerene Skeleton.

A 1,2-dicarbonyl moiety on a cage-opened fullerene skeleton is one of suitable building blocks for the further derivatization. Herein, we discuss the chemical transformation of a 1,2-dicarbonyl compound into ß-oxo-phosphorus ylide, acid anhydride, and α-methylene carbonyl derivatives. Despite possessing a sterically small methylene unit in the last one, the release of an encapsulated water molecule was significantly supressed whereas the ß-oxo-phosphorus ylide bearing three bulky p-tolyl groups on the P-atom enabled the faster insertion/release dynamics, implying the flexibility of the phosphonium substituent. The replacement of the carbonyl group with phosphorus ylide and methylene units largely varied electrochemical properties of the fullerene skeleton, likely arising from the anionic charge delocalized over the entire molecule and removal of an electron-withdrawable carbonyl group, respectively.

Nonclassical Abramov Products Formed on Orifices of Cage-Opened C60 Derivatives.

By nucleophilic addition of phosphite P(OMe)3 to a cage-opened C60 derivative, α-hydrophosphate and enol phosphate were obtained as kinetic and thermodynamic products, respectively. Different from classical Abramov products bearing a phosphorus-carbon bond, these products have a phosphorus-oxygen bond. The observed anomaly originates from the fully conjugated π system, which significantly stabilizes zwitterionic intermediates bearing a phosphorus-oxygen bond. The thus formed enol phosphate was found to exhibit an intense absorption band that extended to 730 nm, reflecting the intramolecular charge-transfer transitions. We also report domino phosphorylation reactions, which gave a cage-opened C60 derivative bearing a direct P-C bond.

Dynamics and magnetic properties of NO molecules encapsulated in open-cage fullerene derivatives evidenced by low temperature heat capacity.

Low-temperature heat capacity analyses for an NO-encapsulated fullerene derivative revealed (i) low-energy motion and (ii) strong magnetic anisotropy of the NO molecule due to its orbital angular momentum. The low-energy motion was attributed to reorientational motions of the NO molecules, in which only a small number (n ∼ 0.04) of NO molecules were found to participate. The NO molecules were confirmed to be paramagnetic even at 1 K. Ab-initio calculation indicated that the magnetic properties of the NO unit strongly depended on its surroundings, allowing the conformation of the fullerene cage to be estimated.

Precise Fixation of an NO Molecule inside Carbon Nanopores: A Long-Range Electron-Nuclear Interaction.

[60]Fullerene-based carbon nanopores were synthesized to enable the placement of two molecules of nitric oxide (NO) at an accurate distance from one another. A linear orientation of the two NO molecules inside the assembled nanopores was confirmed crystallographically. Theoretical studies suggested possible free rotation inside the carbon nanopore, while the two conformations of NO in which its long axis was oriented toward the orifice of the nanopore were predicted to be dominant. The paramagnetic shifts caused by NO showed a major contribution from the Fermi contact mechanism. The Solomon-Bloembergen theory was found to describe well the paramagnetic relaxation enhancement of a water molecule in a paired nanopore even under equilibrium as a result of fixing of the NO molecule with a distance of approximately 12 Å, thus demonstrating a long-range bimolecular magnetic interaction.

Double-Holed Fullerenes.

Fully-fused caged nanocarbons with multiple orifices are segmental structures of porous carbon frameworks long envisioned as synthetic targets of interest. Conventional bottom-up approaches, however, could not overcome the high strain energies required for graphitic precursors to be rounded up. Herein, we report a top-down approach to produce fully-fused carbon nanoelbows as double-holed fullerenes derived from strained C60. The concise one-pot synthesis featuring unique selectivity enabled the isolation of six compounds, while orifice sizes were modifiable from 8- to 12-membered rings and vice versa. The crystallographic analysis confirmed their elbow-shaped structures with different curvatures. Within the crystal, cylindrical nanoporous arrangement were found with the inclusion of solvent guests, reminiscent of hypothetical fullerene sponges.

H2O/Olefinic-π Interaction inside a Carbon Nanocage.

The H2O/CH2═CH2-type hydrogen-bonding (H-bonding) model was experimentally constructed using a water complex of an open-cage C60 derivative, in which an olefinic double bond and a single molecule of H2O are geometrically confined. To investigate OH/π-type H-bonding, that is, H2O···(C═C) interaction, we performed 1H NMR spectroscopic studies that demonstrated the monotonic downfield shift of the proton signal corresponding to H2O with remarkable rotational perturbation by lowering the temperature. From the temperature dependence of the angular momentum correlation time (τJ), the interaction energy was quantitatively estimated to be ca. 0.3 kcal/mol. The computational studies were thoroughly conducted to clarify its inherent nature. As a consequence, the orientation of H2O was found to play a prominent role in varying the bonding strength as well as contribution from the electrostatic attraction and orbital-orbital interaction significantly driven by the favorable orbital overlap identified as π(C═C) → σ*(OH) interaction.

Probing the Regioselectivity with Encapsulated H2 : Diels-Alder Reaction of an Open-Cage C60 Derivative with Anthracene.

The regioselectivity in the Diels-Alder reaction of an unsymmetrical open-cage C60 derivative with anthracene was studied. By using an encapsulated H2 molecule as a magnetic probe, the product population was successfully evaluated in detail, indicating the formation of approximately ten compounds as major components. The nucleus-independent chemical shift (NICS) calculations showed a close resemblance to the observed 1 H NMR spectrum, which allowed for a facile characterization of the products. Theoretical studies on the formation of all 29 possible anthracene adducts were also performed. The results indicated that the regioselectivity is strongly governed by steric factors, values of the frontier orbital coefficients, and thermodynamic stabilities. Single-crystal X-ray analysis of the dominant compound revealed the supramolecular architecture between the anthracene moiety and the π-sphere of a neighboring molecule.

Tuneable single-molecule electronic conductance of C60 by encapsulation.

It has been demonstrated that the single-molecule transport properties of fullerene C60 can be modulated by encapsulating endohedral species, i.e. Li+ and H2O, which exhibit different degrees of van der Waals interactions with the C60 cage. Single-molecule junctions were prepared between the gaps of Au electrodes using a break junction technique. Encapsulation of H2O inside the cage caused a slight decrease in the electronic conductivity relative to that of pristine C60. This is in sharp contrast to Li+ encapsulation, which results in a twofold-to-fourfold increase in the conductivity. The electronic couplings between the C60 cage and the Au electrodes were weakly dependent on the endohedral species in the cage, though the molecular orbital energy levels were remarkably modulated upon encapsulation.

Construction of a Metal-Free Electron Spin System by Encapsulation of an NO Molecule Inside an Open-Cage Fullerene C60 Derivative.

A reactive radical species, nitric oxide (NO), was encapsulated in a unimolecular form inside an open-cage fullerene derivative under high-pressure conditions in the solid state. Surprisingly, the molecular complex showed sharp 1 H NMR signals despite the existence of the paramagnetic species inside the carbon cage. Owing to the paramagnetic shifts, the escape rate of the NO was determined experimentally. After constructing a stopper on the rim of the opening, the NO was found to stay inside the cage even at 50 °C. The ESR measurements of the powdery sample showed paramagnetic properties at low temperature. The single-crystal X-ray structure analysis clearly demonstrated the existence of the encapsulated NO molecule, suggesting rapid rotation inside the cage. The 1 H NMR chemical shifts displayed a large temperature dependence owing to the paramagnetic effects.

Facile Access to Azafullerenyl Cation C59N+ and Specific Interaction with Entrapped Molecules.

The facile preparation of azafullerenyl cation C59N+ has been achieved by the assistance of trifluoromethanesulfonic acid. The thus formed C59N+ was quite stable in solution over 1 month and can be used as an intermediate for the electrophilic reaction. Applying this method to endohedral azafullerenes, corresponding cations (H2@C59N+ and H2O@C59N+) were prepared and the dynamic behavior of entrapped molecules was studied on the basis of 1H NMR relaxation time measurements. The results indicated that there is strong intramolecular C59N+···Oδ-H2 interaction in H2O@C59N+, which stands in contrast to isoelectronic H2O@C60 with no electrostatic interaction. We also demonstrated that the magnetic shielding environment inside the C59N+ cage closely resembles that for isoelectronic C60.