C₃-Symmetric Borabuckybowl Featuring a Pyramidal-like sp² Boron Atom for Advanced Electronics and Optical Functions

Heterobuckybowls containing tricoordinate boron atoms are promising electron-transporting and stimuli-responsive materials owing to their low-lying LUMO levels and enhanced Lewis acidity arising from curved π-conjugated systems. However, their development has been limited to BN-substituted analogues due to synthetic difficulties and poor stability. In this study, we report the synthesis and characterization of a C₃-symmetric heterobuckybowl (BS₃-bb) incorporating three peripheral sulfur atoms and a central boron atom without B–heteroatom bonds. Theoretical calculations revealed that BS₃-bb possesses an unusual pyramidal-like sp² boron, a structural motif generally considered unstable yet realized here in a stable molecule. BS₃-bb exhibited four-stage amphoteric redox behavior, comprising two reversible processes in both the reductive and oxidative regions. Notably, even weak Lewis bases, such as THF, were found to coordinate with the boron center, a feature that is rarely observed in previously reported boron-containing polyaromatic hydrocarbons. X-ray crystallography revealed a polar columnar crystal structure formed by strong concave–convex π–π interactions. This crystalline assembly exhibited second harmonic generation activity under 800 nm pulsed laser irradiation, along with moderate photoconductivity as confirmed by flash-photolysis time-resolved microwave conductivity analysis. These multifaceted properties, along with the new synthetic approach demonstrated here, provide a foundation for the future development of advanced borabuckybowl materials.