Semiquinone-Bridged Bisdithiazolyl Radicals as Neutral Radical Conductors

Semiquinone-bridged bisdithiazolyls 3 represent
a
new class of resonance-stabilized neutral radical for use in the design
of single-component conductive materials. As such, they display electrochemical
cell potentials lower than those of related pyridine-bridged bisdithiazolyls,
a finding which heralds a reduced on-site Coulomb repulsion U. Crystallographic characterization of the chloro-substituted
derivative 3a and its acetonitrile solvate 3a·MeCN, both of which crystallize in the polar orthorhombic space
group Pna21, revealed the importance of
intermolecular oxygen-to-sulfur (CO···SN) interactions
in generating rigid, tightly packed radical π-stacks, including
the structural motif found for 3a·MeCN in which
radicals in neighboring π-stacks are locked into slipped-ribbon-like
arrays. This architecture gives rise to strong intra- and interstack
overlap and hence a large electronic bandwidth W.
Variable-temperature conductivity measurements on 3a and 3a·MeCN indicated high values of σ­(300 K) (>10–3 S cm–1) with correspondingly low
thermal activation energies Eact, reaching
0.11 eV in the case of 3a·MeCN. Overall, the strong
performance of these materials as f = 1/2 conductors is attributed to a combination of low U and large W. Variable-temperature magnetic
susceptibility measurements were performed on both 3a and 3a·MeCN. The unsolvated material 3a orders as a spin-canted antiferromagnet at 8 K, with a canting angle
ϕ = 0.14° and a coercive field Hc = 80 Oe at 2 K.