Resonating valence bond ground state in oxygen-functionalized phenalenyl-based neutral radical molecular conductors

We report the preparation, crystallization, and solid-state characterization of the first two members of a new family of spiro-bis(1,9-disubstituted phenalenyl)boron neutral radicals based solely on oxygen functionalization, and we show that this strategy significantly lowers the electrochemical disproportionation potentials (Delta E), in comparison with other spiro-bis(1,9-disubstituted phenalenyl) boron salts. In the solid state, these radicals pack in a continuous array of pi-pi-stacked phenalenyl units with very short intermolecular carbon center dot center dot center dot carbon contacts. These two radicals are among the most highly conducting neutral organic solids, with room temperature conductivities reaching 0.3 S/cm. Magnetic susceptibility measurements show that the radicals do not exist as isolated free radicals, and there is significant spin-spin interaction between the molecules in the solid state as expected from the crystal structures and the calculated band structures; the solid-state properties are best rationalized in terms of the resonating valence bond model.