Alleno-acetyllenic scaffolding for the construction of axially chiral C60 dimers

The preparation of enantiopure dumbbell-type dimeric fullerenes consisting of two C₆₀ units connected by axially chiral alleno-acetylenic spacers is reported for the first time. As a key step, the attachment of the terminal alkyne moiety of the spacers to C₆₀ was efficiently accomplished by employing an in situ C₆₀-ethynylation methodology. In addition to spectral analyses, single-crystal X-ray crystallographic studies allowed for the unambiguous structural assignment of two C₆₀–alleno-acetylene conjugates. Circular dichroism measurements showed that the axial chirality of the allene moieties linked to the fullerene sphere is able to perturb the intrinsic symmetry of the fullerene π-system. Large characteristic Cotton effects were observed for two bisfullerenes in the 200–350 nm spectral region. UV/Vis absorption spectroscopic studies showed improved molar absorptivity of these dimeric fullerenes, but no strong evidence for a significant through-space electronic communication between the two C₆₀ spheres; electrochemical investigations further confirmed this conclusion.