Benzoquino-bis-1,2,3-dithiazole 5 is a closed shell, antiaromatic 16π-electron zwitterion with a small HOMO-LUMO gap. Its crystal structure consists of planar ribbon-like molecular arrays packed into offset layers to generate a "brick-wall" motif with strong 2D interlayer electronic interactions. The spread of the valence and conduction bands, coupled with the narrow HOMO-LUMO gap, affords a small band gap semiconductor with σ = 1 × 10 S cm and E = 0.14 eV for transport within the brick-wall arrays. Closure of the band gap to form an all-organic molecular metal with σ > 10 S cm can be achieved by the application of pressure to 8 GPa.

A critical feature of the electronic structure of oxobenzene-bridged bisdithiazolyl radicals 2 is the presence of a low-lying LUMO which, in the solid state, improves charge transport by providing additional degrees of freedom for electron transfer. The magnitude of this multiorbital effect can be fine-tuned by variations in the π-electron releasing/accepting nature of the basal ligand. Here we demonstrate that incorporation of a nitro group significantly stabilizes the LUMO, and hence lowers U, the effective Coulombic barrier to charge transfer. The effect is echoed, at the molecular level, in the observed trend in E, the electrochemical cell potential for 2 with R = F, H and NO. The crystal structures of the MeCN and EtCN solvates of 2 with R = NO have been determined. In the EtCN solvate the radicals are dimerized, but in the MeCN solvate the radicals form superimposed and evenly spaced π-stacked arrays. This highly 1D material displays Pauli-like temperature independent paramagnetic behavior, with χ = 6 × 10 emu mol, but its charge transport behavior, with σ near 0.04 S cm and E = 0.05 eV, is more consistent with a Mott insulating ground state. High pressure crystallographic measurements confirm uniform compression of the π-stacked architecture with no phase change apparent up to 8 GPa. High pressure conductivity measurements indicate that the charge gap between the Mott insulator and metallic states can be closed near 6 GPa. These results are discussed in the light of DFT band structure calculations.

N-Oxide incorporation into thiazole-containing conjugated materials can have substantial effects on both the electronic and physical properties. A notably strong non-covalent S–O interaction is observed between the N-oxide and neighbouring sulfur.