Yoshifumi Hashikawa

Synthesis of open-cage ketolactam derivatives of fullerene C60 encapsulating a hydrogen molecule.

A novel open-cage fullerene C60 derivative having a bis(hemiketal) moiety was synthesized by the reaction of C60-N-MEM-ketolactam (MEM: 2-methoxyethoxymethyl) with N-methylmorpholine N-oxide in the presence of water. The structure was clearly determined by single crystal X-ray analysis. Further enlargement of the opening was performed by treatment with trifluoroacetic anhydride to give the tetraketo derivative having a 15-membered ring opening. For H2-insertion into the cage, the derivative was exposed to a high pressure of H2. After the encapsulation, the opening size was reduced to the original one while keeping the hydrogen molecule inside the cage. This compound can be a possible precursor for endohedral azafullerenes encapsulating a hydrogen molecule.

Water Entrapped inside Fullerene Cages: A Potential Probe for Evaluation of Bond Polarization

The concept of the bond polarization is a useful tool to understand chemical reactions and fundamental properties of compounds. However, experimental considerations are limited owing to its difficulty of reliable description. We demonstrated that geometrically isolated H2 O inside the cage of fullerene C60 is a possible probe to evaluate the polarization degree of covalent bonds C(C60 )-X (X=heteroatom) on the C60 cage. The 1 H NMR relaxation times of entrapped H2 O have been systematically measured at variable temperatures for H2 O@C60 X (X=CR2 , NR, O, and O2 ). The results followed in the order of electronegativities of C (2.55), N (3.04), and O (3.44), indicating that entrapped H2 O can sensitively respond to the degree of the bond polarization.

Solvent-Coordinated Tin Halide Complexes as Purified Precursors for Tin-Based Perovskites

A series of solvent-coordinated tin halide complexes were prepared as impurity-free precursors for tin halide perovskites, and their structures were determined by single-crystal X-ray diffraction analysis. Using these precursors, the tin halide perovskites, MASnI3 and FASnI3, were prepared, and their electronic structures and photophysical properties were examined under inert conditions by means of photoelectron yield spectroscopy as well as absorption and fluorescence spectroscopies. Their valence bands (MASnI3: −5.02 eV; FASnI3: −5.16 eV) are significantly higher than those of MAPbI3 or the typical hole-transporting materials 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene and poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine). These results suggest that to develop the solar cells using these tin halide perovskites with efficient hole-collection properties, hole-transporting materials should be chosen that have the highest occupied molecular orbital levels higher than −5.0 eV.

Orientation of a Water Molecule: Effects on Electronic Nature of the C59N Cage

A hydrogen-bonding network is a key impelling force for an assembly in bulk water. The fullerene cage can incarcerate a water molecule without hydrogen-bonding. Herein, we focused on spin system H2O@C59N·. The 1H NMR relaxation time of entrapped H2O was significantly reduced by the paramagnetic effect. Interestingly, the electron affinity and ionization energy were suggested to vary depending on the orientation of entrapped H2O owing to the degree of the partial charge transfer from entrapped H2O to C59N·.

Single Molecular Junction Study on H2O@C60: H2O is "Electrostatically Isolated"

A water molecule exhibits characteristic properties on the basis of hydrogen bonding. In the past decade, single water molecules placed in non-hydrogen-bonding environments have attracted growing attention. To reveal the fundamental properties of a single water molecule, endohedral fullerene H2 O@C60 is an ideal and suitable model. We examined the electronic properties of H2 O@C60 by performing single-molecule measurements. The conductance of a single molecular junction based on H2 O@C60 was found to be comparable to that of empty C60 . The observed values were remarkably higher than those obtained for conventional molecular junctions due to the effective hybridization of the π-conjugated system to the metal electrode. Additionally, the results undoubtedly exclude the possibility of electrostatic contact of entrapped H2 O with the carbon wall of C60 . We finally concluded that H2 O entrapped inside a C60 cage can be regarded as an electrostatically isolated molecule.

Palladium-Catalyzed Cyclization: Regioselectivity and Structure of Arene-Fused C60 Derivatives

The palladium-catalyzed cyclization on the fullerene C60 cage has been achieved using several aryl halides and C60. This reaction was found to be accelerated by the addition of pivalic acid, which can be rationally explained by the computational study based on the concerted metalation-deprotonation mechanism. We also demonstrated the regioselective π-functionalization using prefunctionalized designed molecules possessing the same substructure on the C60 cage. The single crystal X-ray analysis and electrostatic potential map revealed that the orientation of entrapped H2O inside the naphthalene-fused open-cage C60 derivative is electrostatically demanded due to the naphthalene-fusion and construction of the opening.

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.

High-Resolution Photoelectron Imaging of Cryogenically-Cooled C59N– and (C59N)22– Azafullerene Anions

We report a photoelectron imaging study of cryogenically cooled C59N- and (C59N)22- anions produced from electrospray ionization. High-resolution photoelectron spectra are obtained for C59N- for the first time, allowing seven vibrational frequencies of the C59N azafullerene to be measured. The electron affinity of C59N is determined accurately to be 3.0150 ± 0.0007 eV. The observed vibrational features are understood on the basis of calculated frequencies and compared with those of C60 and C59HN. The photoelectron image of (C59N)22-, which has the same mass/charge ratio as C59N-, is also observed, allowing the second electron affinity of the (C59N)2 azafullerene dimer to be measured as 1.20 ± 0.05 eV. The intramolecular Coulomb repulsion of the (C59N)22- dianion is estimated to be 1.96 eV and is investigated theoretically using the electron density difference between (C59N)22- and (C59N)2.

Synthesis and properties of open-cage fullerene C60 derivatives: impact of the extended π-conjugation

We have developed a method for the synthesis of open-cage fullerene C60 derivatives with extended π-conjugation bearing thienyl groups. By applying this method to an asymmetric diketo derivative, the symmetric form can be obtained without changing the molecular formula. To investigate the structure–property relationship for the asymmetric and symmetric forms, we conducted electrochemical and photophysical measurements. The UV-vis absorption edge was shifted by 210 nm upon changing from the asymmetric to the symmetric form due to the narrower HOMO–LUMO gap, which was also demonstrated by electrochemical analyses. From theoretical calculations, the major contribution of the longest wavelength absorption for the symmetric form is assignable to unusual intramolecular charge transfer transitions whereas π–π* transitions are dominant for the asymmetric form.