Fine Tuning the Performance of Multiorbital Radical Conductors by Substituent Effects

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_eff, the effective Coulombic barrier to charge transfer. The effect is echoed, at the molecular level, in the observed trend in E_cell, the electrochemical cell potential for 2 with R = F, H and NO₂. The crystal structures of the MeCN and EtCN solvates of 2 with R = NO₂ 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_TIP = 6 × 10^-4 emu mol^-1, but its charge transport behavior, with σ_RT near 0.04 S cm^-1 and E_act = 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.