Π-Conjugated Non-Kekule Molecules and the Limits of Hund's Rule

Whether certain theoretical restrictions on the applicability of Hund's rule must be kept in mind for the proper design of high-spin organic molecules as potential building blocks for ferromagnetic materials is a subject of intense current concern. It has been almost fifty years since E. Hückel pointed out that a biradical constituted by union of inactive sites of two monoradicals might have a singlet ground state. Experimental tests of this conjecture (in its recent more sophisticated formulations by Borden and Davidson and by Ovchinnikov) now have led to the synthesis and characterization of several new ground state singlet biradicals.