Semiquinone-Bridged bis-Dithiazolyls as Neutral Radical Conductors
Radicals are potential building blocks to prepare conductive and magnetic materials. In order to achieve high conductivity, materials displaying a large bandwidth W and a low on-site Coulomb repulsion energy U must be generated. Semiquinone-bridged bis-1,2,3-dithiazolyl radicals (R = Cl, Ph, Me and the MeCN adduct of R = Cl) represent a new family of resonance stabilized neutral radical for use in the design of single-component conductive materials were prepared and fully characterized. In solid state these radicals remain as unassociated (monomers) in the solid state and typically form superimposed alternating π-stacks or slipped π-stacks, arranged in several different space groups. The predominate intermolecular interactions are S•••N′ and / or S•••O′ contacts, which increase the dimensionality from one dimensional π-stacked systems (i.e., poor lateral overlap) to two dimensional systems in the solid state. Thus the semiquinone-bridged bis-dithiazolyl radicals exhibit a significant decrease in activation energy (ca. 0.1 – 0.2 eV) and the conductivity is two to three orders of magnitude (ca. σ ≈ 1E-5 – 1E-2 S / cm) higher in comparison to the previously reported pyridine based systems. This high conductivity is attributed to the low on-site Coulomb repulsion energies (U) which were estimated from the solution cell potentials (EPC) obtained from CV measurements and improved bandwidth (W) from the S•••N′ and / or S•••O′ interactions. Furthermore, the all sulphur containing semiquinone-bridged bis-dithiazolyls have the lowest activation energies and the highest conductivity under ambient conditions compared with other all sulfur nitrogen based radicals known to date. The semiquinone-bridged bis-dithiazolyl (R = Cl) orderes as spin-canted antiferromagnets, TN = 8 K, and displayed large coercivity (80 Oe). The ZFC-FC measurement at low field (i.e., H = 100 Oe) established the phase transition temperatures and the spontaneous magnetization was used to estimate the spin canting angles (~ 0.14°). In the case of R = Ph, the antiparallel alignment of the ferromagnetic coupled chains leads to a spin-canted antiferromagnet (TN = 4.5 K), which undergo a unique field induced spin flop transition. The MeCN solvated of R = Cl behaves as a simple paramagnet at room temperature with bulk antiferromagnetic interactions, but no observed magnetic ordering from 2-300 K.