A Clar-type regularity for dianions of benzenoid hydrocarbons

Phenanthreno-annelated anthracenes as well as their dianions are studied by means of molecular-orbital theory (at the AMI level). The charge distribut ion in the dianions is highly non-uniform: the anthracene fragment bears ro ughly 50% more negative charge than the phenanthrene fragment(s). These fin dings point at the possibility to apply the concepts of Clar aromatic sexte t theory to benzenoid dianions. Within the framework of this Clar-type mode l, phenanthreno-annelated anthracene dianions may be viewed as charged anal ogs of all-benzenoid hydrocarbons.