An Alternating π-Stacked Bisdithiazolyl Radical Conductor

A general synthetic route to the resonance-stabilized pyrazine-bridged bisdithiazolyl framework,
involving the reductive deprotection of 2,6-diaminopyrazine-bisthiocyanate and cyclization with thionyl
chloride, has been developed. An N-methyl bisdithiazolyl radical, 4-methyl-4H-bis[1,2,3]dithiazolo[4,5-b:5‘,4‘-e]pyrazin-3-yl, has been prepared and characterized in solution by electron paramagnetic resonance
spectroscopy and cyclic voltammetry. Its crystal structure has been determined at several temperatures.
At 295 K, the structure belongs to the space group Cmca and consists of evenly spaced radicals π-stacked
in an alternating ABABAB fashion along the x-direction. At 123 K, the space group symmetry is lowered by
loss of C-centering to Pccn, so that the radicals are no longer evenly spaced along the π-stack. At 88 K,
a further lowering of space group symmetry to P21/c is observed. Extended Hückel Theory band structure
calculations indicate a progressive opening of a band gap at the Fermi level in the low-temperature structures.
Magnetic susceptibility measurements over the range 4−300 K reveal essentially diamagnetic behavior
below 120 K. Variable-temperature single-crystal conductivity (σ) measurements indicate that the conductivity
is activated, even at room temperature, with a room-temperature value σRT = 0.001 S cm-1 and a thermal
activation energy Eact = 0.19 eV. Under an applied pressure of 5 GPa, σRT is increased by 3 orders of
magnitude, but the conductivity remains activated, with Eact being lowered to 0.11 eV at 5.5 GPa.