Preparation and characterization of a neutral π-radical molecular conductor

The synthesis and solid-state characterization of the heterocyclic pi-radical 1,2,5-thiadiazolo[3,4-b]-1,2,3-dithiazolo[3,4-b]pyrazin-2-yl, 1,2,3-TDTA, is described. The ESR spectrum of 1,2,3-TDTA (in CH2Cl2, 293 K, g = 2.009) confirms a highly delocalized spin distribution, with observable hyperfine coupling to all five nitrogen atoms of the tricyclic molecule (a(N) = 0.514, 0.343, 0.109, 0.051, and 0.035 mT). While chemical and electrochemical oxidation (E-1/2(ox) = 1.14 V vs SCE) of 1,2,3-TDTA requires relatively harsh conditions, reduction is extremely facile (E-1/2(red) = 0.15 V vs SCE). More importantly both the observed cell potential E-cell and computed (MNDO) gas-phase enthalpy Delta H-disp for the disproportionation of this and other 1,2,3-dithiazolyls are significantly lower than those observed for their 1,3,2-isomers. Crystals of 1,2,3-TDTA are monoclinic P2(1)/n, with a = 6.6749(16) Angstrom, b = 11.7178(14) Angstrom, c = 8.6148(14) Angstrom, beta = 103.297(16)degrees, and Z = 4. The crystal structure consists of slipped stacks of head-to-tail (centrosymmetric) pi-dimers. The closest intradimer S- - -S contact (S2- - -S3) is 3.2331(15) Angstrom. Variable-temperature magnetic susceptibility measurements establish that 1,2,3-TDTA is essentially diamagnetic at room temperature. The magnetic data, along with the results of variable-temperature single-crystal conductivity measurements (1,2,3-TDTA exhibits a room-temperature conductivity sigma = 1 x 10(-4) S cm(-1)), are interpreted in terms of one-dimensional hopping mechanism for charge transport.