Azabuckybowl-Based Molecular Tweezers as C₆₀ and C₇₀ Receptors

We designed and synthesized molecular tweezers consisting of nitrogen-embedded buckybowl subunits. The judicious choice of the covalent linkers modulated their binding strength with C₆₀ or C₇₀ in solution. Titration studies by optical and ¹H NMR analyses revealed a 1:1 composition of the resulting complexes. X-ray diffraction analysis elucidated their solid-state structures, in which two azabuckybowl units surround one fullerene molecule. The large association constants stabilize the complexes toward redox reactions and the purification process on silica-gel column chromatography. The linker enabled tuning of the cavity size for binding of fullerenes, achieving complementary fullerene hosts for C₆₀ and C₇₀: the carbazole-bridged dimer preferentially associates with C₇₀ over C₆₀, while the phenanthrene-bridged dimer interacts with C₆₀ more strongly than C₇₀. Electrochemical analysis in combination with density functional theory calculations indicated the existence of intermolecular charge-transfer interactions between the buckybowl units and the fullerenes. Nonlinear optical measurements showed that the two-photon absorption cross sections of the molecular tweezers are enhanced upon association with fullerenes.