Stable Diradical on the Dimethyldihydropyrene Scaffold

The diradical character in a molecular architecture can
be customized
primarily in two ways: first, by employing a quinoidal pro-aromatic
system with net energy gained by aromatization that compensates for
the energy required to generate the diradical species and, second,
by employing an antiaromatic system having easily accessible triplet
states that impart a diradical character. We have chosen a 14π
aromatic framework, Boekelheide’s dimethyldihydropyrene, and
perturbed its aromaticity through the construction of its quinoidal
form. The perturbed aromaticity was evident from the bond alteration
in the X-ray diffraction structure, 1H nuclear magnetic
resonance chemical shifts, and quantum chemical calculations. The
aromaticity was restored as the system underwent a transition to the
biradical structure centered on two exocyclic carbons. In addition,
upon photoexcitation and without using an external reducing reagent,
the diradical could be converted to a radical anion and dianion form
easily when dimethylformamide was used as a solvent.