Preparation and Characterization of a Neutral π-Radical Molecular Conductor

The synthesis and solid-state characterization of the heterocyclic π-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 CH 2 Cl 2, 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 0.045 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 Δ 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 P 2 1 / n, with a = 6.6749(16) Å, b = 11.7178(14) Å, c = 8.6148(14) Å, β = 103.297(16)°, and Z = 4. The crystal structure consists of slipped stacks of head-to-tail (centrosymmetric) π-dimers. The closest intradimer S- - -S contact (S2- - -S3) is 3.2331(15) Å. 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 σ = 1 × 10 - 4 S cm - 1 ), are interpreted in terms of one-dimensional hopping mechanism for charge transport.