Aromatic Nanosandwich Obtained by σ‑Dimerization of a Nanographenoid π‑Radical

A 139-π-electron
nanographenoid radical was obtained by expanding
the periphery of a naphthalimide–azacoronene hybrid with a
methine bridge. The radical was isolated in the form of its σ-dimer,
which was shown to possess a conformationally restricted two-layer
structure both in the solid state and in solution. The dimer is cleaved
into its parent radicals when exposed to ultraviolet or visible radiation
in toluene solutions but is resistant to thermally induced dissociation.
Under inert conditions, the radicals recombine quantitatively into
the σ-dimer with observable kinetics, but they are oxidized
into a ketone derivative in the presence of atmospheric oxygen. Combined
structural, spectroscopic, and theoretical evidence shows that the
σ-dimer contains a weak C­(sp3)–C­(sp3) bond, but is stabilized against thermal dissociation by a very
strong dispersive interaction between the overlapping π surfaces.