All-Benzene Carbon Nanocages: Size-Selective Synthesis, Photophysical Properties, and Crystal Structure

The
design and synthesis of a series of carbon nanocages consisting solely
of benzene rings are described. Carbon nanocages are appealing molecules
not only because they represent junction unit structures of branched
carbon nanotubes, but also because of their potential utilities as
unique optoelectronic π-conjugated materials and guest-encapsulating
hosts. Three sizes of strained, conjugated [n.n.n]carbon nanocages (1, n = 4; 2, n = 5; 3, n = 6) were synthesized with perfect size-selectivity.
Cyclohexane-containing units and 1,3,5-trisubstituted benzene-containing
units were assembled to yield the minimally strained bicyclic precursors,
which were successfully converted into the corresponding carbon nanocages
via acid-mediated aromatization. X-ray crystallography of 1 confirmed the cage-shaped structure with an approximately spherical
void inside the cage molecule. The present studies revealed the unique
properties of carbon nanocages, including strain energies, size-dependent
absorption and fluorescence, as well as unique size-dependency for
the electronic features of 1–3.