Conducting Charge-Transfer Salts Based on Neutral Pi-Radicals
by Bryan, C. D.; Cordes, A. W.; Fleming, R. M.; George, N. A.; Glarum, S. H.; Haddon, R. C.; Oakley, R. T.; Palstra, T. T. M.; Perel, A. S.; Schneemeyer, L. F.; Waszczak, J. V.
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 pi-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 pi-radicals, and the material is rendered conducting by electron transfer from the conduction band following doping with an acceptor. The radical species is the 1,4-phenylene-bis(dithiadiazolyl) diradical 1,4-[(S2N2C)C6H4(CN2S2)] (2 in Fig. 1), reaction of which with iodine vapour leads to crystals of [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 100 S cm-1, which is comparable to that shown by conventional molecular charge-transfer salts.