MOST molecular conductors rely on charge transfer to create carriers.
For example, the ET salts1 are hole-doped whereas the C60 salts2 are e
lectron-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 whi
ch combines both of these approaches: energy bands are formed from one
-dimensional stacks of neutral pi-radicals, and the material is render
ed 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), reac
tion of which with iodine vapour leads to crystals of [2][I]. At low t
emperatures this compound is essentially a diamagnetic insulator, but
above 200 K the conductivity and magnetic susceptibility increase mark
edly, and at room temperature the conductivity reaches 100 S cm-1, whi
ch is comparable to that shown by conventional molecular charge-transf
er salts.