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

Crystalline supramolecular aggregates consisting of charged organic molecul es, held together through metal-duster-mediated Coulomb interactions, have attracted interest owing to their unusual structural, chemical and electron ic properties(1-3). Aggregates containing metal cation dusters 'wrapped' by lipophilic molecular anions have, for example, been shown(4,5) to be kinet ically stable and soluble in nonpolar liquids such as saturated hydrocarbon s. The formation of supramolecular aggregates can even be exploited to gene rate aromatic hydrocarbons that carry four negative charges and crystallize in the form of organic poly(metal cation) clusters(6,7) or helical polymer s(8). Here we report the anaerobic crystallization of an ionic organic aggr egate-a contact ion septuple consisting of a fourfold negatively charged 't ripledecker' of three anthracene molecules bridged by four solvated potassi um cations, Its electronic ground state is shown experimentally, using temp erature-dependent electron paramagnetic resonance spectroscopy, to be a tri plet, Although the spins in this biradical ionic solid are separated by a c onsiderable distance, density functional theory calculations(9) indicate th at the triplet ground state is 84 kJ mol(-1) more stable than the first exc ited singlet state. We expect that the successful crystallization of the io nic solid we report here, and that of a covalent organic compound with a tr iplet ground state(10) at room temperature, will stimulate further attempts to develop new triplet-ground-state materials for practical use.