(Invited) Synthesis of Nitrogen-Embedded Buckybowl and Its Strong Association Behavior with C60

Abstract

Curved π-conjugated molecules have attracted considerable interest because of their unique properties originating from their curved π surface. Besides their figurative beauty, the curved psurface generates unique functions such as chiroptical properties, anisotropic electron transitions, and dynamic motion in solution and solid states. However, the synthesis of such distorted molecules requires harsh conditions, which hamper an easy access to heteroatom-containing curved π-systems. Buckybowls represent important curved π-conjugated molecules, which can be precursors for the bottom-up synthesis of fullerenes and nanotubes. The nitrogen-embedded bowl-shaped molecules have been sought as model compounds for azafullerenes and nitrogen-doped carbon nanotubes. However, the synthesis of buckybowls with internal nitrogen atoms has been still challenging. Here we report the synthesis of a π-extended azacorannulene with nitrogen in its center. The oxidation of 9-aminophenanthrene provides tetrabenzocarbazole, which is converted to the azabuckybowl through palladium-catalyzed intramolecular coupling reaction. The structure was unambiguously elucidated by X-ray diffraction analysis, showing its bowl-shaped conformation. The electron-donating nature and curved π surface of the azabuckybowl enable its tight association with C60 in solution and solid states. The titration experiments with C60 by electronic absorption and emission spectroscopic analysis indicate the presence of intermolecular charge-transfer interaction between them. In addition, high charge carrier mobility was observed for the azabuckybowl/C60 assembly in solid state. This new compound would be a novel molecular entity in the field of curved π systems as fullerene hosts, anisotropic π donors, and precursors to nitrogen-containing nanocarbon materials. Figure 1