Single crystals of an ionic anthracene aggregate with a triplet ground state

Crystalline supramolecular aggregates consisting of charged organic molecules, held together through metal-cluster-mediated Coulomb interactions, have attracted interest owing to their unusual structural, chemical and electronic properties1,2,3. Aggregates containing metal cation clusters ‘wrapped’ by lipophilic molecular anions have, for example, been shown4,5 to be kinetically stable and soluble in nonpolar liquids such as saturated hydrocarbons. The formation of supramolecular aggregates can even be exploited to generate aromatic hydrocarbons that carry four negative charges and crystallize in the form of organic poly(metal cation) clusters6,7 or helical polymers8. Here we report the anaerobic crystallization of an ionic organic aggregate—a contact ion septuple consisting of a fourfold negatively charged ‘tripledecker’ of three anthracene molecules bridged by four solvated potassium cations. Its electronic ground state is shown experimentally, using temperature-dependent electron paramagnetic resonance spectroscopy, to be a triplet. Although the spins in this biradical ionic solid are separated by a considerable distance, density functional theory calculations9 indicate that the triplet ground state is 84 kJ mol-1 more stable than the first excited singlet state. We expect that the successful crystallization of the ionic solid we report here, and that of a covalent organic compound with a triplet ground state10 at room temperature, will stimulate further attempts to develop new triplet-ground-state materials for practical use.