Conducting charge-transfer salts based on neutral π-radicals

MOST molecular conductors rely on charge transfer to create carriers. For example, the ET salts¹ are hole-doped whereas the C₆₀ salts² are electron-doped. Neutral radical species in which bands are formed by π-orbital overlap would be expected to have half-filled bands and thus to be conducting³, but no such metals have yet been reported. Here we report the synthesis and characterization of a molecular conductor which combines both of these approaches: energy bands are formed from one-dimensional stacks of neutral π-radicals, and the material is rendered conducting by electron transfer from the conduction band following doping with an acceptor. The radical species is the l,4-phenylene-bis(dithiadiazolyl) diradical 1,4-[(S₂N₂C)C₆H₄(CN ₂S₂)] (2 in Fig. 1), reaction of which with iodine vapour leads to crystals of [2][I]. At low temperatures this compound is essentially a diamagnetic insulator, but above 200 K the conductivity and magnetic susceptibility increase markedly, and at room temperature the conductivity reaches l00 S cm^-1, which is comparable to that shown by conventional molecular charge-transfer salts.