Pressure enhanced conductivity in bis-1,2,3-thiaselenazolyl dimers

A synthetic sequence to salts of N-alkylated pyridine-bridged 1,2,3-thiaselenazolo-1,2,3-thiaselenazolylium cations [2](+) (R-1 = Me, Et; R-2 = H) is described. The corresponding radicals 2 (R-1 = Me, Et; R-2 = H) can be generated from the cations by chemical or electrochemical reduction. Crystals of the two radicals are isostructural and consist of interpenetrating pi-stacked arrays of closed-shell Se-Se sigma-bonded dimers [2](2) laced together with numerous short intermolecular Se---Se, Se---S, and Se---N contacts. Variable-temperature magnetic, conductivity, and near-infrared optical measurements indicate that the bulk materials behave as small band gap semiconductors with room-temperature conductivities sigma(RT) near 10(-6) S cm(-1) and thermal activation energies E-a of 0.32 eV (R-1 = Me) and 0.36 eV (R-1 = Et). LMTO band structure calculations on both compounds are consistent with this interpretation. The application of external pressure leads to dramatic increases in conductivity; at 4 GPa sigma(RT) reaches a value near 10(-1) S cm(-1) for R-a = Me and 10(-2) S/cm for R-1 = Et. The conductivity remains activated for both compounds, but for R-1 = Me the activation energy E-a is reduced to near 0.03 eV at 5 GPa, suggestive of a weakly metallic state.